WILLIAM ESTY Head of Department of Electrical Engineeiing, Lehigh University. ALTERNATING-CURRENT MACHINERY- A PRACTICAL TREATISE ON ALTERNATING-CURRENT PRINCIPLES AND SYSTEMS, COMMERCIAL TYPES OF ALTERNATORS, SYNCHRONOUS MOTORS, TRANSFORMERS, CONVERTERS, INDUCTION MOTORS, SWITCHBOARD AND STATION APPLIANCES, ETC. By WILLIAM ESTY, S.B., M.A. > i HEAD OF DEPARTMENT OF ELECTRICAL ENGINEERING, LEHIGH UNIVERSITY ILLUSTRATED CHICAGO AMERICAN SCHOOL OF CORRESPONDENCE 1912 COPYRIGHT 1912 BY AMERICAN SCHOOL OP CORRESPONDENCE Entered at Stationers 7 Hall, London All Rights Reserved CONTENTS ALTERNATING=CURRENT PRINCIPLES ALTERNATING ELECTROMOTIVE FORCES AND CURRENTS Page Simple alternator 1 Variations of electromotive force 3 Relations between speed and frequency 3 Advantages and disadvantages of alternating currents 4 Comparison of direct- and alternating-current problems 7 Physical basis for the differences between D. C. and A. C. calcula- tions 10 Graphical representation of alternating electromotive forces and cur- rents 12 Average and effective values of E. M. F 14 Instantaneous and average power 15 Harmonic electromotive forces and currents 16 Relation between maximum and effective values 24 Inductance 27 Capacity 30 Capacity of condensers 31 Fundamental equations of the A. C. circuit 32 Electrical resonance 41 Miscellaneous considerations . . 47 MEASURING INSTRUMENTS INDICATING INSTRUMENTS Hot wire ammeter and voltmeter 60 Electrostatic voltmeter, 64 Electrodynamometers 72 Induction instruments 75 Wattmeter 77 INTEGRATING INSTRUMENTS Thomson Watt-hour meter 82 Induction Watt-hour meter 85 Directions for reading Watt-hour meter dials 93 Calculations of customers' bills 95 Spark gauge 95 257953 2 CONTENTS ALTERNATORS Page Fundamental equation of alternator 97 Armature reaction 104 Armature inductance 106 Electromotive force los" in armature drop 107 Alternator regulation 120 Field excitation 109 POLYPHASE ALTERNATORS AND SYSTEMS Limitations of single-phase system 118 Two-phase alternator 118 Three-phase alternator 121 Y-connected armatures 124 A-connected armatures 125 Receiving circuits to three-phase mains 126 Summary of electromotive-force and current relations for A and Y connections 127 MEASUREMENT OF POWER Balanced systems 129 Unbalanced systems 131 ARMATURE WINDINGS Classification 135 Single-phase windings. 142 Two-phase windings 143 Three-phase windings 143 Commercial Types REVOLVING-ARMATURE ALTERNATORS Fort Wayne single-phase 147 Westinghouse uni-coil armature 150 Westinghouse armature with distributed winding 151 Field structure for Westinghouse 180-kw. alternator 152 General Electric three-phase alternator 152 REVOLVING-FIELD ALTERNATORS Construction 154 Water-wheel-driven alternators 165 Steam-turbine-driven alternators 169 Economy Factors CONDITIONS AFFECTING COST Speed 179 Voltage 179 Regulation Frequency 180 CONTENTS 3 POWER LOSSES EFFICIENCY Page Practical and ultimate limits of output 182 Influence of power factor upon output 183 RATING AND OVERLOAD CAPACITIES American Institute rules 184 TESTING Alternating-current testing in general 188 Insulation testing * 188 Determination of resistance of armature 198 Regulation 200 Heat test 205 Core loss and friction test 209 Calculation of efficiency . 214 SYNCHRONOUS MOTORS Any alternator a synchronous motor 216 Advantages 218 Disadvantages : 219 Comparison of synchronous and D. C. motors 219 Starting the motor 220 Hunting action 223 Torque and power output 225 Field excitation and power factor 226 Use as a condenser 227 TESTING Phase characteristic 229 Pulsation test 230 Break-down test 231 Self-starting test 231 TRANSFORMER Description 234 Physical action 234 Automatic action of the transformer 236 Ideal and practical transformer 236 Maximum core flux 237 Ideal transformer action graphically represented 239 Influence of coil resistance and magnetic leakage 240 Performance 241 CONNECTIONS Parallel-constant-voltage transformers 242 Autotransformer. . .... 251 4 CONTENTS IN POLYPHASE SYSTEMS Page Two-phase system 255 Three-phase system 255 Transformers with compound magnetic circuits 257 Phase transformation 257 PRACTICAL CONSIDERATIONS Transformer losses 262 Transformer efficiency 263 Transformer regulation 266 Rating of transformers 267 COMMERCIAL TYPES Core type 271 Shell type 283 Three-phase transformers 289 Cooling of transformers '. . . 292 Series or current transformers 297 Constant-current transformers 299 Transformer fuse blocks 305 Mounting of outdoor transformers 308 TESTS Heat test 308 Core-loss and exciting-current test 310 Resistance of coils 310 Impedance 311 Regulation 313 Efficiency calculation 314 Polarity test 315 CONVERSION OF ALTERNATING INTO DIRECT CURRENT Rectifying commutator 317 Aluminum valve rectifier 318 Mercury-vapor arc rectifier 320 Rotary or Synchronous Converter Comparison with direct-current dynamo 326 To make a direct-current dynamo into a rotary converter 327 Three-ring converter 329 Four-ring converter 329 Six-ring converter 330 Multipolar rotary converters 330 E. M. F. relations for rotary converter 330 Current relations for rotary converter 336 CONTENTS 5 ROTARY CONVERTERS IN PRACTICE Page Uses of the rotary converters 333 Starting rotary converters 339 Oscillators for rotary converters 342 Characteristic types of rotary converters 343 Hunting of rotary converter 345 Inverted rotaries 347 Control of direct-current voltage 349 Field excitation -7 . . 353 Rotary converter with Edison three-wire system 355 Six-phase converter 357 Transformer connections for rotary converters 357 TESTING ROTARY CONVERTERS Standard tests 360 Heat run 361 Motor Generators Comparison with the rotary converter 365 Use of motor generators 366 Types 367 INDUCTION MOTOR Constructive elements 371 Stator windings and their action 372 Action of 375 Torque and speed 375 Starting resistance in the rotor windings 376 Efficiency and speed 376 Ratio of mechanical to electrical energy in rotor 377 Ratio of rotor voltages to stator voltages 378 Efficiency and rotor resistance 379 Structural details of a typical induction motor 380 Types of rotors for constant and variable speed 381 Behavior at starting and in operation 385 Typical induction motor installations 389 Single-phase induction motor 393 Induction generator 397 Frequency changer 397 Comparison of Synchronous Motor and Induction Motor INDUCTION MOTOR TESTS Breakdown test : . 408 Starting torque test 409 Core loss test . . 410 6 CONTENTS Impedance test > 412 Efficiency test. 413 Slip test 414 Performance curves of an induction motor 416 SWITCHBOARD AND STATION APPLIANCES SWITCHBOARDS Typical single-phase switchboard 422 Polyphase switchboard 425 Feeder panels 430 High voltage panels 433 SPECIAL SWITCHBOARD APPARATUS Lincoln synchronizer 433 CIRCUIT-INTERRUPTING DEVICES Fuses 435 Air-break switches 436 Circuit breakers 438 Oil-break switches 440 Feeder or voltage regulators 444 Voltmeter compensator 450 LIGHTNING ARRESTERS Effects of lightning 452 Multi-gap non-arcing arrester . 454 Multi-path arrester 455 Electrolytic-cell lightning arrester 456 Combination of a condenser with a choke coil 459 APPENDIX PARALLEL OPERATING OF ALTERNATORS Necessary conditions 462 Determination of relative frequency and phase coincidence 462 Directions for connecting one alternator in parallel with another alter- nator 466 Directions for cutting out an alternator which is running in parallel with one or more alternators 466 Index , 499 INTRODUCTION THIS treatise has been prepared with the special object of giv- ing the beginner and the so-called practical electrician a work- ing knowledge of alternating-current apparatus in order that he may know how to install and operate it intelligently. In most cases no argument is needed to convince the practical station man of the advantage of getting this working knowledge. But unfortun- ately there seems to be a general impression that no one who is not an expert mathematician or a college graduate can get much use- ful information out of a book on alternating currents. Knowing only the rudiments of algebra, geometry, and trigonometry, the beginner soon gets beyond his depth in a sea of advanced mathe- matics, becomes discouraged, and finally gives up in disgust. It is precisely for him that this book is written. It contains no mathe- matics beyond the simplest trigonometry; and great pains have been taken to make the descriptions, explanations, and proofs simple and clear. The reader is assumed to have some acquaintance with the elementary laws of electricity and magnetism. C| Graphical, or geometric, methods rather than analytical, or alge- braic, methods have been adopted wherever possible, as will be seen in the repeated application of the "clock" diagram in deter- mining the relations between electromotive forces and currents in the various types of apparatus discussed. Abstract and arbitrary statements have been reduced to a minimum; and on the other hand, a large variety of examples and numerical illustrations have been used throughout the text to reinforce the principles by concrete applications. ljln the opening articles the reader is made acquainted with the essential features of the source of alternating currents the alter- nator and, through it, is introduced to the new terms, definitions, and conceptions which form the alphabet of the alternating-cur- rent language. Attention is early called to the characteristic fea- tures of alternating-current problems which cause them to differ from direct-current problems. Indeed, the first fifty pages may be considered the foundation upon which the rest of the book is built. After the fundamental principles of alternating-current working have been explained and illustrated, a description and discussion of the instruments used to measure alternating current, electro- motive force, and power, follow in natural sequence. The general plan which has been adopted in the treatment of the subsequent sections dealing successively with alternators, synchronous motors, transformers, rotary converters, and induction motors, is as follows: (a) The physical theory of each class of apparatus is explained; (6) the applications, behavior, and operation are discussed; (c) the structural details of commercial types are illustrated; (d) tl\e meth- ods used in making the usual tests, and the calculations based thereon are explained. t][ In conclusion the author expresses the hope that the reader who has thoughtfully followed the elements of the subject as here pre- sented, may be encouraged to pursue further the study of alternating- current machinery as discussed in the more advanced text-books. WILLIAM ESTY. 0,3 Bl M JjCf- as- ALTERNATING=CURRENT MACHINERY PART I ALTERNATING ELECTROMOTIVE FORCES AND CURRENTS Simple Alternator. The alternator is an arrangement by means of which mechanical energy is used to cause the magnetic flux from a magnet to pass through the opening of a coil of wire first in one and then in the opposite direction. This varying magnetic flux induces in the coil, first in one direction and then in the other, what is called an alternating electromotive force, which in turn produces an alternating current in the coil, and in the circuit which is con- nected to the terminals of the coil. In the common type of alternator, the above-mentioned magnet and coil move relatively to each other. Fig. 1 shows the essential features of such an alternator. The poles N, S, N, S, etc., of a multipolar magnet called the field magnet, project radially inwards toward the passing teeth a a a of a rotating mass A of laminated iron; and upon these teeth are wound coils of wire c c, in which the alternating electromotive force is induced. The rotating mass of iron with its windings of wire is called the armature. At one end of the armature (not shown in the figure) are mounted two insulated metal rings r r, called collecting rings. These metal rings are con- nected to the ends of the armature winding, and metal brushes b b rub on these rings, thus keeping the ends of the armature winding in continuous contact with the terminals of the external circuit to which the alternator supplies alternating current. No external circuit is shown in the figure. The electromotive forces induced in adjacent armature coils are in opposite directions at each instant, and the coils are so con- nected together that these electromotive forces do not oppose each Copyright, 1912, by American School of Correspondence. 2'*' * A-LTRNATING-CURRENT MACHINERY other. This is done by reversing the connections of every alternate coil, as indicated by the dotted lines connecting the coils in Fig. 1. The electromagnetic action of this type of alternator depends only upon the relative motion of field magnet and armature, and large machines are usually built with stationary armature and revolving field magnet. In the type of machine illustrated in Fig. 1, the arma- ture revolves while the field magnet is stationary. This type, called the revolving-armature type, is generally adopted in small alternators. The field magnet of an alternator is usually an electromagnet which is excited by a continuous electric current supplied by an independent generator, generally by an auxiliary continuous-current dynamo, called the exciter. The exciting current flows through coils Fig. 1. Diagram of Armature and Field of Simple Alternator of wire wound on the projecting poles N, S, N, S of the field magnet. These coils are not shown in Fig. 1. The type of armature core shown in Fig. 1 is called the toothed armature core; and the armature winding is said to be concentrated, that is, the armature conductors are grouped in a few heavy bunches. Armature cores are also made with many small slots, in which the armature conductors are grouped in small bunches. This type of core is called a multi-toothed core, and the winding is said to be dis- tributed. In some of the earlier types of alternators the armature core consisted of a smooth, cylindrical mass of laminated iron, upon ALTERNATING-CURRENT MACHINERY 3 the face of which the conductors were arranged in bands side by side, one layer or more in depth. This type of armature is called the smooth-core armature; it has been superseded by the toothed core type. Variations of Electromotive Force. Cycle. The electromotive force of an alternator passes through a set of positive values while a given coil of the armature is passing from a south to a north pole of the field magnet, and through a similar set of negative values while the coil is passing from a north to a south pole. The complete set of values, including positive and negative is called a cycle. Frequency. Frequency is equal to the number of cycles per second; it is sometimes expressed by stating the number of alterna- tions or reversals per minute. For example, an alternator having a frequency of 133 cycles per second has 266 reversals or alternations per second, or 15,960 alternations per minute. Frequencies are sometimes specified in alternations per minute, but specification in cycles per second is the more usual practice and is preferable. alternations per min. cycles per sec. = 2XQO Period. The fractional part of a second occupied by one cycle is called the periodic time, or period, of the alternating electromotive force or current. Let / be the frequency in cycles per second, and T the period expressed as a fraction of a second. Then f=jr 0)' Therefore, if an alternating current has a frequency of 60 cycles per second, the period T of one cycle is one-sixtieth of a second. Relations Between Speed and Frequency. Let p be the num- ber of poles of the field magnet of an alternating-current machine; let n be the speed of its armature in revolutions per minute; and let / be the frequency of its electromotive force in cycles per second. Then 60 4 ALTERNATING-CURRENT MACHINERY Examples. 1. A certain alternator has 10 poles, and runs at 1,500 revolutions per minute. What is its frequency? SOLUTION. Substituting 10 for p, and 1,500 for n, in equation (2), we have 10 1500 /= X = 125 cycles per second 2. An alternator is to run at 600 revolutions per minute and is to give a frequency of 60 cycles per second. What number of poles is required? SOLUTION. Solving equation (2) for p, we have _2X60X/ from which, substituting / = 60, and n 600, we have 2X60X60 Advantages and Disadvantages of Alternating Currents. The electric transmission of a given amount of power may be accom- plished by a large current at low electromotive force, or by a small current at high electromotive force. In the first case very large and expensive transmission wires must be used, or the loss of power in the transmission line will be excessive. In the second case com- paratively small and inexpensive transmission wires may be used. Thus it is a practical necessity to employ high electromotive forces in long-distance transmission of power. Example. It is desired to transmit 1,000 kilowatts of power over a distance of 10 miles, supposing that a loss in the line of 10 per cent of the power delivered is considered permissible. This corresponds to a loss of 100 kilowatts. SOLUTION. Case 1. Suppose that the electromotive force at the re- ceiving end of the line is to be 100 volts. Then the current would be 1,000,000 watts divided by 100 volts, or 10,000 amperes. The resistance of the line must be such that the watts lost in the line namely 100,000 watts would W be equal to I 2 R, so that the resistance R of the line must be 5-, or 0.001 ohm. This would require two transmission wires each 10 miles long and 33 inches in diameter, or a total weight of 175,000 tons of copper, which would cost about $52,500,000. Case 2. Suppose that the electromotive force at the receiving end of the line is to be 1,000 volts. Then the current would be 1,000,000 watts divided by 1,000 volts, or 1,000 amperes. The resistance of the line must be such that the watts lost in the line namely, 100,000 watts would be equal to PR y so that the resistance R of the line must be 0.1 ohm. This would require two transmission wires, each 10 miles long and 3.3 inches in diameter, or a total weight of 1,750 tons of copper, which would cost about $525,000. ALTERNATING-CURRENT MACHINERY TABLE I Size and Cost of Copper Wire Two=Wire System To transmit 1,000 kilowatts a distance of lO^miles (one way) with a line loss equal to 10 per cent of the power delivered, for_three different values of electromotive force Volts at Amperes Ohms Diameter Weight "Cost of Receiving End of Line in Line in Line of Wire, of Wire, Line Copper, in E 7 R in Inches in Tons Dollars 100 10,000 0.001 33 175,000 52,500,000 1,000 1,000 0.1 3.3 1,750 525,000 10,000 100 10.0 0.33 15.5 5,250 Case 3. Suppose that the electromotive force at the receiving end of the line is to be 10,000 volts. Then the current would be 1,000,000 watts divided by 10,000 volts, or 100 amperes. The resistance of the line must be such that the watts lost in the line namely 100,000 watts would be equal to I 2 R, so that the resistance R of the line must be 10 ohms. This would require two transmission wires, each 10 miles long and 0.33 inch in diameter, or a total weight of 17.5 tons of copper, which would cost about $5,250. These results are summarized in Table I. Transformation of High E. M. F.'s. High electromotive forces are dangerous under the conditions that ordinarily obtain among users of electric light and power; and many types of apparatus, such as incandescent lamps, operate satisfactorily only with medium or low electromotive forces. Therefore, means must be provided, at a receiving station, for transforming the power delivered, from high electromotive force and small current to low electromotive force and large current, if long-distance transmission is to be success- ful. This is called step-down transformation. The advantage of the alternating current over the direct current lies almost wholly in the cheapness of construction and of operation, and in the high efficiency of the alternating-current apparatus as compared with the direct-current apparatus that is required for transformation. In step-down transformation of direct current, a motor takes a small current from the high-electromotive-force transmission mains, and drives a dynamo which delivers large current to service mains at low electromotive force. This apparatus, or its equivalent, the dynamotor, is expensive to construct; it requires attention in operation; and its efficiency is never, perhaps, above 90 per cent. 6 ALTERNATING-CURRENT MACHINERY The step-down transformation of alternating currents is accom- plished by means of the alternating-cur tent transformer, which is described later on. The alternating-current transformer is very much cheaper than a dynamo and motor of the same output; it requires no attention in operation; and its efficiency under full load is usually greater than 97 per cent, especially in large sizes. Simple Construction of A. C. Machines. The alternating cur- rent has some minor advantages over the direct current on account of the fact that alternating-current machines are frequently simpler in construction than direct-current machines. In particular, the commutator is not an essential part of an alternating-current gen- erator. Again, in the case of the inductor alternator and the induc- tion motor, the rotating part may not have any sliding electrical contacts whatever. Miscellaneous A. C. Machines. The simple single-phase alter- nating current is not well adapted to general power service. The single-phase alternating-current induction motor does not start satisfactorily under load, in the case of large machines, although self-starting single-phase motors up to perhaps 20 horse-power are in commercial use, where neither direct-current nor polyphase alternating-current machines are available. The single-phase series commutator motor within a few years has been developed especially for electric railway service both for trolley cars and for electric locomotives. Its operating characteristics, resembling closely those of the direct-current series motor, however, are not suitable for general power requirements. This type of motor is used on the electric locomotives of the New York, New Haven and Hartford Railroad. For uninterrupted service the synchronous motor is frequently used, the starting being effected by an auxiliary engine or other independent mover. The synchronous motor is not satisfactory when frequent starting is necessary, for such service the induction motor being used. The simple induction motor, to start satisfac- torily, must be supplied with two or more distinct alternating currents transmitted to .the motor over separate lines. This is called the polyphase system of transmission, and is reserved for full treatment in later pages. For some purposes, especially for the electrolytic processes ALTERNATING-CURRENT MACHINERY 7 used on a large scale in electro-chemical works, only direct current can be used. When power transmitted by alternating current is to be delivered in the form of direct current, the conversion is effected by means of the rotary converter, motor generator, or mercury rectifier. Comparison of Direct= and Alternating=Current Problems. Direct Current. In direct-current work the electrical engineer is concerned with the relations between electromotive force, resistance, current, and power. These relations are determined by the applica- tions of the following laws: POWER LAW for direct-current circuits P=EI (3) in which P is the total power in watts delivered to a circuit by a generator of which the terminal electromotive force is E volts, when it produces a current of I amperes. OHM'S LAW for direct-current circuits in which I amperes is the steady current produced by E volts acting on a circuit of R ohms resistance. JOULE'S LAW for direct-current circuits P=PR (5) in which P is the power in watts expended in heating a circuit of R ohms resistance, when a current of / amperes is forced through the circuit. KIRCHOFF'S LAWS for direct-current circuits. 1. When a circuit branches, the current in the main circuit is equal to the sum of the currents in the separate branches. 2. (a) When two or more sources of electromotive force are connected in series, the total electromotive force is the sum of the individual electromotive forces. 3. (b) When an electromotive force acts on a number of elements or things in series, it is subdivided into parts, each of which acts upon one of the elements, and the sum of these parts is equal to the total electromotive force. For example, an arc-light dynamo of which the terminal electromotive force is 3,000 volts, 8 ALTERNATING-CURRENT MACHINERY acts on 60 similar arc lamps connected in series. Neglecting the resistance of the connecting wires, each lamp is acted upon by one- sixtieth of the total electromotive force, or by 50 volts. Alternating Current. In alternating-current work the electrical engineer is likewise concerned with the relations between electro- motive force, resistance, current, and power. These relations are determined by the application of the same fundamental laws as in the case of direct currents, but in more or less modified forms.* A summary of the fundamental laws of alternating currents is here given simply for purposes of comparison. POWER LAW for alternating-current circuits P= El cos (6) in which P is the power in watts delivered to a circuit by an alter- nator of which the "effective" terminal electromotive force is E volts, when it produces an "effective" current of I amperes in a circuit, and cos 6 is what is called the "power factor" of the circuit. OHM'S LAW for alternating-current circuits E = in which 7 is the "effective" current in amperes produced by an "effective" electromotive force of E volts acting on a circuit of which the resistance is R ohms, and the "reactance" is X ohms. The expression i/R 2 XX 2 in equation (7) is called the impedance, and it is expressed in ohms. JOULE'S LAW for alternating-current circuits P=PR (5) in which P is the power in watts expended in heating a circuit of R ohms resistance when an "effective" alternating current of / amperes is forced through the circuit. KIRCHOFF'S LAWS for alternating-current circuits 1. When an alternating-current circuit branches, the "effective" current in the main circuit is the "geometric"! (or "vector") sum of the "effective" currents in the separate branches. *The student is not expected to understand fully the reasons for the statements here given, until he has completed Parts I and II. {Alternating electromotive forces and alternating currents are added in the same way that forces are added, that is, by means of the principle known as the "parallelogram of forces." ALTERNATING-CURRENT MACHINERY 9 Example. An alternator A, Fig. 2, supplies an effective current of / amperes in the main circuit, which divides into two branches. The' effective currents in the two branches are 7 : and 7 2 amperes, respectively. The -rela- tion between 7, 7^ and 7 2 is shown in Fig. 3. The angles 6 l and 2 depend Fig. 2. Diagram of a Branched Alternating Circuit Fig. 3. Vector Diagram of Branched Circuit upon the relative values of the resistance and reactance of the respective branches, as is explained later. It is to be particularly noticed that the arithmetical sum of 7 X and 7 2 is in general greater than 7. 2. (a) When two or more alternators (or transformer second- aries) are connected in series, the total effective electromotive force is the "geometric" (or "vector") sum of the effective electromotive forces of the individual alternators. MAfHS MA//YS Fig. 4. Two Alternators in Series . Fig. 5. Vector Diagram of E. M. for Two Alternators in Series Example. Two alternators A v and A t , Fig. 4, of which the effective electromotive forces are E v and E 2 , respectively, are connected in series to supply mains. Then the effective electromotive force E, between mains, is the geometric sum of E l and E 2 , as shown in Fig. 5. The angle 6 depends upon the positions, relatively to the field magnets, of the armature coils on the respective machines. 10 ALTERNATING-CURRENT MACHINERY (b) When an alternating electromotive force E acts upon a number of elements or things in series, it is subdivided into parts, each of which acts upon one of the elements, and the "geometric" (or "vector") sum of these parts is equal to E. Example. Two coils b and c, Fig. 6, are connected in series between mains supplied from an alternator, of which the effective electromotive force is E. Then the total effective electromotive force E is subdivided into two parts Ei and Ea, which act upon the respective coils, as indicated in Fig. 6; and the "geometric," or in Fig. 7. The angles 61 actance of the respective coils. It is to be particularly noticed that the arith- metical sum of Ei and Ez is in general greater than E. Physical Basis for the Differences between D. C. and A. C. Calculations. The above mentioned differences between direct- current and alternating-current calculations are due to the fact that an alternating current changes rapidly in value from instant to instant, while a direct current is steady and does not change its 1 Vector" sum of Ei and E z is equal to E, as shown and 62 depend upon the relative resistance and re- Fig. 6. Alternating Current Through Two Coils in Series Fig. 7. Vector Diagram of E. M. F. for Conditions Shown in Fig. 6. direction of flow. A clear idea of the efforts of the rapid changes of an alternating current may be obtained as follows : Fig. 8 represents an alternator producing alternating current in a circuit of wire; and Fig. 9 represents a valveless pump, of which the piston oscillates rapidly up and down, producing an alternating current of water in a circuit of pipe. The electromotive force of the alternator A not only has to overcome the resistance of the wire in order to cause an alternating current to surge back and forth through the circuit, but it also has to overcome the electrical inertia of the circuit first, in getting a pulse of current started ; and second, in stopping this pulse of current and starting another in the reverse direction. The pressure developed by the pump P not only has to ALTERNATING-CURRENT MACHINERY 11 overcome the frictional resistance of the pipe in order to cause an alternating current of water to surge back and forth through the pipe, but it also has to overcome the inertia of the water in the pipe P/PE W/ffE Simple Alternating Circuit Fig. 9. Water Analogy for an Alter- nating Circuit first, in getting a pulse of water current started; and second, in stopping this pulse of water current and starting another in the reverse direction. Fig. 10 represents an alternator producing alternating current in a circuit of wire which contains a condenser C; and Fig. 11 rep- resents a valveless pump producing an alternating current of water in a circuit of pipe, which leads to a chamber H H, across which is stretched an elastic diaphragm DD. In this case the pressure developed by the pump has to overcome the frictional resistance of the pipe, the inertia of the water, and the elastic reaction of the diaphragm DD. Similarly the alternating electromotive force of WIRE W/RE Fig. 10. Alternating Circuit Containing Condenser Fig. 11. Water Analogy for Alternating Circuit with Condenser the alternator A, Fig. 10, has to overcome the electrical resistance of the wire, the electrical inertia of the wire circuit, and the electro- elastic reaction of the insulating material, or dielectric, between the plates of the condenser. 12 ALTERNATING-CURRENT MACHINERY The electrical inertia of a circuit is called its inductance and the electro-elasticity of a co idenser is called its capacity, and it is to inductance and capacity that the peculiar features of alternating- current calculations are due. The effect of capacity is strikingly shown by the fact that an alternating current may be made to flow through a circuit which for direct currents would be an open circuit like that shown in Fig. 10. Thus an alternator connected to long transmission lines, which are disconnected at the distant end and perfectly insulated from the ground, will send a considerable alternating current into the lines, which current can be measured by an alternating-current ammeter. In such a case the current is called the charging current of the line; . POSITIVE VALUES , _ E / OF E.M.F. \ E I AXIS OF TIME \| NEGATIVE VALUES/ M OF E.M.F. / jEGATIVE VALUES/ F E.M.F. / _J ONE CYJ^LE OR TWO ALTERNATIONS Fig. 12. Development of Three Field Magnet Poles and E. M. F. Curve for One Cycle and it may amount to several amperes, according to the length of the line, the distance apart and size of wires, and the electromotive force of the alternator. Graphical Representation of Alternating Electromotive Forces and Currents. When an armature conductor of an alternator ap- proaches a north pole of the field magnet, the electromotive force of the machine rises in value as the conductor enters the strong field under the pole; and the electromotive force falls in value as the conductor passes from under the pole. As the conductor passes the point midway between two adjacent poles, the electromotive force of the machine falls to zero, since no lines of the force are cut ALTERNATING-CURRENT MACHINERY 13 at this point. As the armature continues to revolve and the con- ductor approaches the next (the south) pole of the field magnet, the electromotive force of the machine again increases in value, but in a direction opposite to that of the previous electromotive force; and it falls again to zero as the conductor passes from under the south pole and reaches the point midway between the next pair of poles. In Fig. 12 are represented the development of three successive poles S, N, S of the field magnet of an alternator, from which the lines of magnetic flux are emanating, and spreading out more or less as they enter the armature core. The armature core, also a developed view, is shown as having only one slot A, which contains a number of armature conductors. The ordinates of the curve E E E E E E represent the successive instantaneous values of the electromotive force induced in the armature conductors as the slot moves from left to right. The duration of one cycle is indicated in the figure, and this cycle repeats itself as the conductors pass by successive pairs of field poles. Thus, in a ten-pole alternator, there would be five complete cycles, or five complete waves of the elec- tromotive curve for each Fi g- 13 - Typical .f e ^;. ( ^^ e e forAIternator ~ revolution. When a wave repeats itself after a definite time interval, it is called a periodic wave. The curve E E E E E E is called the electromotive curve or electro- motive force wave of the alternator. A curve of which the ordinates represent the successive instan- taneous values of the alternating current and of which the abscissas represent time, is called an alternating-current curve or alternating- current wave. Figs. 13, 14, and 15 show typical forms of electromotive force curves given by commercial alternators. Fig. 13 shows what is called a "peaked" wave; Fig. 14 shows a "flat-topped" wave; and Fig. 15 shows a "sine" or "sinusoidal" wave. All three waves are of 14 ALTERNATING-CURRENT MACHINERY course periodic. The exact shape of the electromotive force wave given by an alternator depends upon the relations between pole pitch (distance from center to center of adjacent poles), width, and shape of pole faces, width of armature coils, and dis- tribution of coils on the ar- W mature. Alternators which give electromotive force waves approximating a sine Fig. 14. Typical E. M. F. Curve for Alternator "Fiat Topped" Wave wave, are preterrcd lor power transmission. Average and Effective Values of E. M. F. The average value of an alternating electromotive force or current during a complete cycle is zero, inasmuch as similar sets of positive and negative values occur. The average value of an electromotive force or current during the positive (or negative) part of a cycle is usually spoken of briefly as the "average value" or "mean value," and is not zero. Consider now an alternating current, of which the instantaneous value is i. The rate at which heat is generated in a circuit through which the current flows is i 2 R } where R is the resistance of the circuit; and the average rate at which heat is generated in the circuit is R multiplied by the average value of i 2 . A continuous current which would produce the same heating effect would be one of which the square is equal to the average value of i 2 , or of which the actual value is equal to V average i 2 . This square root of the average square of an alternating current is called the effective value of the alternating current. Similarly, the square root of the average square of an alternating electro- motive force is called the effective value of the alter- nating electromotive force. Fig. 15. Typical E. M. F. Curve for Alternator Voltmeters and ammetei'S "Sine" Wave used for measuring alter- nating electromotive force or current always give effective values irrespective of wave form; and in specifying an alternating elec- tromotive force or current, its effective value is always used. ALTERNATING-CURRENT MACHINERY 15 Example. Ten successive instantaneous values of an alternating elec- tromotive force during half a cycle are 0, 30, 60, 80, 90, 95, 90, 80, 60, and 30 volts. The sum of these values is 615 volts, which, divided by the num- ber of values, namely ten, gives 61.5 volts, which is the average value of this electromotive force during half a CYCLE-*)' E.M.F. Fig. 16. Rectangular Form of E. M. F. Curve c y cle - Squaring each of the above values, adding the squares together, and dividing their sum by their number, namely ten, gives the average value of the square of the electromotive force, which is 4,702.5 volts 2 ; and the square root of this aver- age square is 68.57 volts, which is the effective value of the given electro- motive force. Form Factor. The ratio effective value -r- average value, depends upon the shape of the electromotive force wave, and is called the "form factor" of the wave. Example. The form factor in the above case is or 1.115. The 68.57 61.5 form factor of the electromotive force curve given in Fig. 15, which is a sine wave, is 1.11. The more peaked the wave the greater the value of its form factor. The rectangular electromotive force shown in Fig. 16 has a form factor equal to unity, which is the least possible value of the form factor. This rectangular wave, however, is never realized in commercial alternators. Instantaneous and Average Power. Let e be the value, at a given instant, of the electromotive force of an alternator and let i be the value of the current at the same instant. Then ei is the power in watts which is delivered by the alternator at the given instant; and the average value of ei during a complete cycle is the average power delivered by the alternator. Fig. 17. E. M. F., -Current, and Power Curves for an Alternator in a Circuit Containing Inductance In Fig. 17 the full-line curve represents the electromotive force of an alternator and the heavy-dotted curve represents the current delivered by the alternator to a receiving circuit having 16 ALTERNATING-CURRENT MACHINERY inductance, such as an induction motor, for instance. The ordinates of the light-dotted curve represent the successive instantaneous values of the power ei. As shown in the figure, the power has both Fig. 18. E. M. F., Current, and Power Curves for Alternating Circuit with Large Inductance positive and negative values; the alternator does work on the cir- cuit when ei is positive, or is above the horizontal axis of time; and the circuit returns power to the alternator when ei is negative, or is below the horizontal axis of time; and this means of course, that while ei is negative, the dynamo is momentarily a motor and will be for the moment returning power to the fly wheel of the driving engine or turbine. When the inductance of the receiving circuit is very large, the electromotive force and current curves are related as shown in Fig. 18; the instantaneous power ei passes through approximately similar sets of positive and negative values, as shown by the light-dotted curve; and the average power is approx- imately zero. This case would be very closely exemplified by an alternator con- nected to a transformer whose secondary was open-circuited, that is, supplying no current. Harmonic Electromotive Forces and Currents. A line OP, Fig. 19, revolves, at a uniform rate,/ revolutions per second about a point 0, in the direction of the arrow gh. Since the length of OP is fixed, the path or locus of the point P will be a circle about as a center. Consider the projection Ob of this rotating line upon the fixed line AB, this projection being considered positive when above and negative when below 0. Fig. 19. Diagram of Harmonic E. M. F.s ALTERNATING-CURRENT MACHINERY 17 A harmonic electromotive force (or current) is an ekctromotive force (or current) which is at each instant proportional to the line Ob. The line Ob represents at each instant the actual value e of the harmonic electromotive force to a definite scale, and the length of the line OP, which is the maximum length of Ob, represents the maximum value E of the harmonic electromotive force to the same scale. The line Ob passes through a complete cycle of values during one revolution of OP, and so also does the harmonic electromotive force e. Therefore, the revolutions per second / of the line OP is the frequency of the harmonic electromotive force e. The rotating lines E and I, Fig. 20, of which the projections on a fixed line (not shown in the figure) represent the actual instantaneous values e and i of a harmonic electromotive force and a harmonic current, are said to "represent" the harmonic electromotive force and cur- rent, respectively. Of course, the rotation of the lines E and I is a thing merely to be imagined. The rotation is understood to be in a counter-clockwise direction, as indicated in Fig. 19. Clock Diagram Representation. A diagram in which a number of electromotive forces or currents, or both, are represented by lines im- agined to be revolving, is called a clock diagram. Simple problems involving relations between a number of harmonic electromotive forces and currents of the same frequency , are most easily treated by means of the clock diagram. The ~ ig> 20 . C iock Diagram proper representation of alternating electro- motive forces and currents in a clock diagram, requires that (a) The given electromotive forces and currents be harmonic, and be of the same frequency. (b) The lengths of the lines represent their maximum value to a suitable scale although the scales chosen for volts and for amperes may be different. (c) The direction of the electromotive forces and currents be indicated by arrow heads. (d) The relative position or phase of the electromotive forces and currents be constant and indicated by the angle between the various lines rep- resenting the given quantities. When an electromotive force (or current) wave is not a sine curve, the electromotive force (or current) is not harmonic, and can- 18 ALTERNATING-CURRENT MACHINERY not properly be represented by a line in a clock diagram, because the projection of the rotating line is not at each instant proportional to the electromotive force (or current). The approximate represen- 270 Fig. 21. Method of Plotting Sine E. M. F. Curve tation, by lines in a clock diagram, of non-harmonic electromotive forces (or currents) such, for example, as those represented by the curves in Figs. 13, 14, and 16, depends upon the finding of harmonic electromotive forces (or currents) which for the particular purpose in view are approximately equivalent to the actual given electro- motive forces (or currents). Graphical Representation. A harmonic electromotive force or current is represented by a sine wave as shown in Fig. 15. The rela- tion between the rotating line OP in Fig. 19 and the sine-wave curve of electromotive force is shown as follows : Divide the circumference of the circle in Fig. 21 into equal parts, and lay off a horizontal line divided into the same number of equal parts. Draw horizontal dotted lines through each division on the cir- cumference of the circle, and vertical dotted lines through the correspond- ing divisions on the horizonta" line. The points of the intersection of these pairs of dotted lines are points on a curve which is a curve of sines. A flat loop of wire with its,ter- minals connected to two collect- ing rings gives a harmonic elec- tromotive force when it is rotated at constant speed in a uniform magnetic field. This arrangement is shown in Fig. 22. Fig. 22, Simple Dynamo Diagram ALTERNATING-CURRENT MACHINERY 19 Algebraic Representation. The line OP, Fig. 19, revolves uni- formly / revolutions per second and, therefore, it turns through 27rf radians* per second, since there are 2n radians in a revolution; that is W=27T/ (8) in which aj is the angular velocity of the line OP in radians per second. Let time be reckoned from the instant that OP coincides with Oa; then, after t seconds, OP will have turned through the angle /? (=^t); and from Fig. 19 we have Ob = OP sin /? = OP sin cut since Ob is the projection of OP on the line AB. But Ob represents the actual value e of the harmonic electromotive force at the time t, and OP represents its maximum value E; therefore e = E sin ut (9) is an algebraic expression for the actual value e of a harmonic electro- motive force at time t, E being the maximum value of e, and being the frequency according to equation (8). Similarly , . /ln . i = I sm cot (10) is an algebraic expression for the actual value i of a harmonic cur- rent at time t, I being the maximum value of i. If time is reckoned from the instant that OP, Fig. 19, coincides with the line 06, then equations (9) and (10) become e = E cos cot i=I cos cot Synchronism. Two alternating electromotive forces or currents are said to be in synchronism when they have the same frequency. Two alternators are said to run in synchronism when their electro- motive forces and frequencies are similar. Phase Difference. Consider two harmonic electromotive forces represented by the ordinates of the curves EI and E v Fig. 23. The electromotive force represented by the curve E t reaches its maximum value before the electromotive force represented by the curve E r The electromotive force EI is said to lead, or to be ahead of, the *The unit of angle chiefly used in mechanics and in all theoretical work is the radian. It is the angle of which the arc is numerically equal to the radius (of a circle). There are, therefore, 27T radians in one circumference. 20 ALTERNATING-CURRENT MACHINERY electromotive force E 2 in phase. Conversely, the electromotive force E 2 is said to lag behind or to follow the electromotive force EI in phase. The same two Ei ^xp v T-.E S electromotive forces E l and E 2 are also represented by the lines OE^ and OE 2 in the clock diagram, Fig. 24. Here the line OE 2 is behind OE lt since the imagined rotation about as a center is counter-clock- TIME Fig. 23. Curves of Two Related E. M. F.s wse. The phase difference is the time interval 6 in Fig. 23, or the angle between OE l and OE 2 in Fig. 24. If according to equation (9) the actual value of the harmonic electromotive force .Eiis e\E\ sin cot, then the actual value of the electromotive force E 2t which lags de- grees behind E lf is e 2 =E 2 sin (cot 6). Similarly, if E 2 were taken as the reference line in the diagram, its actual value would be e 2 = E 2 sin cot, and the value of EI would then be e\ E\ sin (cot-{-0). When the angle Q, Fig. 24, is zero, as shown in Fig. 25, the electromotive forces E l and E 2 are said to be in phase. In this case the electromotive forces increase together and decrease together; Fig. 24. Clock Diagram of E. M. F.s for Fig. 23fl Fig. 25. Clock Diagram for Curves in Fig. 23 with E. M. F.s in Phase that is, when E l is zero, E 2 is also zero ; and when E 1 is at its maximum value, so also is E 2> etc. Therefore, e l = E l sin cot and e 2 = E 2 sin cot. When 6 =90, as shown in Fig. 26, the two electromotive forces are said to be in quadrature. In this case one electromotive force is zero when the other is a maximum, etc., or e 1 =E 1 sin cot and e 2 =E 2 sin H-f). ALTERNATING-CURRENT MACHINERY 21 Fig. 26. E. M. F.s of Fig. 23 / Diagram of s of Fig. f ' in Quadrature When 6 = 180, as shown in Fig. 27, the two electromotive forces are said to be in opposition. In this case they are at each instant opposite in sign; and when one is at its positive maximum, the other is at its negative maximum, etc. In this case #1 = EI sin cot and e 2 = E 2 sin (cot TT). It is to be particularly noted that the principle of phase difference which has been illustrated in Figs. 22-25 for the case of two harmonic electromotive forces EI and E 2} applies equally to the case of two harmonic currents /i and 7 2 and to the case of an electromotive force EI and a current /i. Thus if in the clock diagram, Fig. 20, E represented a harmonic electromotive force having a maximum value of 1,000 volts, and I represented a harmonic current having a maximum value of 10 amperes with a phase dif- ference of 0=30 between them, the instantaneous values of E and / would be e = 1000 sin cot i= 10 sin '(cot -) Addition of Harmonic Electromotive Forces and Currents. Consider two har- monic electromotive forces of which the successive instantaneous values e and e 2 are represented by the pro- jections of the lines EI and E 2) Fig. 28, which are imagined to be revolving about the point 0. These elec- tromotive forces being of the same fre- quency, the lines EI and E 2 revolve at the same speed, so that the angle between EI and E 2 remains unchanged in value. The ordinary arithmetical sum of e and e 2 , namely e -f- e 2 , is a harmonic electro- motive force of the same frequency as e\ and e 2 - y and this electromotive force (ei-\-e 2 ) is represented by the projection of the line E, Fig. 28, which revolves at the same speed as EI and E 2 . Ths is evident when we consider that the projection, on any line, of the diagonal of a parallelogram is equal to the sum of the Fig. 27 Diagram of E. M. F.s of Fig. 23 When in Opposition Fig. 28. Vector Diagram Show- ing Two Harmonic E. M. F.s 22 ALTERNATING-CURRENT MACHINERY Fig. 29. Diagram Showing Addition of Harmonic E. M. F.s projections of two adjacent sides of the parallelogram, as shown in Fig. 29. The projection of E l is Oc, which represents ej and the projection of E 2 is equal to cd, which represents e 2 . The projec- tion of the diagonal Ob of the parallelogram is Od, which is the sum of Oc and cd. The two lines marked E 2 in Fig. 29 are equal and parallel and have, therefore, the same projected length on the ver- tical line. As a corollary to the above, it may be stated that the ordinary arithmetical sum (e. + + e + 123 etc.) of the instantaneous values of any number of harmonic electromotive forces (or currents) is another harmonic electromotive force (or current) of the same fre- quency; it is represented in magnitude and phase by a line that is the geometric (or vector) sum of the lines representing the given individual electromotive forces (or currents). This is evident when we consider that e 1 -}-e 2 is a harmonic electromotive force (or current) according to the above discussion; and this, added to e 3 , gives an electromotive force (or current) which is harmonic and of the same frequency as e l9 e 2 , and e 3 . The geometric, or vector, sum of a num- ber of lines is obtained as follows : Given three lines OE' l9 OE 2 , and OE 3 , Fig. 30. Find the diagonal OA of the parallelo- gram constructed on OE and OE 2 as sides. This gives the vector sum of OE 1 and OE 2 . Next construct a parallelogram on OA and OE 3 as sides; the diagonal OE of this par- allelogram is the vector sum of the three given lines. This line OE is the closing side of the polygon formed by drawing OE lf then drawing OE 2 from the extremity of OE l (this giving the point A), and then drawing OE 3 from A. This method is called addition by means of the vector polygon. For example, two alternators A and B running in synchronism Fig. 30. Addition of E. M. F.s by Vector Polygon ALTERNATING-CURRENT MACHINERY 23 are connected in series between mains as shown in Fig. 31. If the electromotive forces of A and B are in phase, the electromotive force between the mains will be simply the numerical sum of the electromotive forces of A and B. If, on the other hand, the electro- motive forces of A and B differ in phase, the state of affairs will be as represented in Fig. 32, in which the lines A and B represent the mom man Fig. 31. Two Alterna- tors Running in Syn- chronism Fig. 32, Vector Diagram of Con- ditions in Fig. 31 if E. M. F.a Differ in Phase electromotive forces of the alternators A and B, respectively, is the phase difference of A and B, and the line E represents the electro- motive force between the mains. The line E is the vector sum or resultant of A and B, and as shown is the diagonal of a parallelo- gram constructed on A and B as sides. Again, alternators A and B running in synchronism are con- nected in parallel between the mains as shown in Fig. 33. Let the lines A and B, Fig. 34, represent the currents given by the alter- nators A and B, respectively, the phase difference between the cur- rents being 0; then the current in the main line is represented by /. Consider the case of two circuits A and B, Fig. 35, connected in mo/n mom Fig. 33. Two Alternators Running in Synchronism Connected in Parallel Fig. 34. Vector Diagram of Currents for the Condition of Fig. 33 series between the mains of an alternator. The line E, Fig. 36, represents the electromotive force between the mains; the line A 24 ALTERNATING-CURRENT MACHINERY represents the electromotive force between the terminals of the circuit A; and the line B represents the electromotive force be- tween the terminals of the circuit B. The circuits A and B are ma/n mam -* Fig. 35. Diagram of Two Coils in Series with an Alternator Fig. 36. Diagram of E. M. F.s for Conditions Shown in Fig. 35 supposed to have inductance. If either of the circuits contains a condenser, then the electromotive forces A and B, Fig. 36, may be nearly opposite to each other in phase, and A and B may each be indefinitely greater than the electromotive force E between the mains. Again, two circuits A and B, Fig. 37, are connected in parallel across the terminals of an alternator as shown. The current / from the alternator is related to the currents A and B as shown in Fig. 38. If either of the circuits A or B contains a condenser, then the cur- rents A and B may be nearly opposite to each other in phase, and the currents A and B may each be indefinitely greater than the current I from the alternator. Subtraction of Harmonic Electromotive Forces and Currents. One harmonic elec- tromotive force (or current) is subtracted from another by reversing the direction of the line that represents it in the clock dia- gram, and then adding the reversed line (or vector) to the other. An example of the subtraction of harmonic electromotive forces Fig. 37. Coils in Parallel with . , . ,. .,, ., ,. an Alternator will be given in connection with the discus- sion of three-phase electromotive force. Relation between Maximum and Effective Values. The effective value E or I of a harmonic, electromotive force, or current that is, main ALTERNATING-CURRENT MACHINERY 25 one whose graph is a sine wave is equal to the maximum value E or I divided by the square root of 2. That is E 1/2 (11) (12) in Fig. 37 1/2 This may be shown as follows: Let e (=E sin tot) be a harmonic electromotive force. To find the average value of e 2 (= E 2 sin 2 tot], it is necessary to find the aver- age value of the square of the sine of the uniformly variable angle tot. We have the general relation (a) sin 2 tot + cos 2 tot = 1 so that (b) Av. sin 2 tot + Av. cos 2 tot = 1 Now, during a cycle, the cosine of a uniformly variable angle passes similarly through the same set of values as the sine; hence Av. sin 2 tot and Av. cos 2 tot are equal, so that from equation (b) above, we have 2 Av. sin 2 tot = 1 or Av. The average value of e 2 is Av. e 2 = E 2 Av. sin 2 tot or and In Fig. 19 the length of the revolving line OP was understood to represent the maximum value of the harmonic electromotive force (or current). When, however, a number of harmonic electro- motive forces (or currents) are represented to scale by lines in a clock diagram, the lengths of the lines may be interpreted as giving 26 ALTERNATING-CURRENT MACHINERY not maximum but effective values, since there is a constant ratio V 2 between the effective and the maximum values of each of the electromotive forces (or currents) represented in the diagram. NOTE. It is desirable to interpret the lines in a clock diagram in terms of effective values rather than maximum values, because effective values are always given by measuring instruments and are nearly always used in nu- merical calculation. Therefore, unless it is expressly stated to the contrary, the lines in clock diagrams are always understood to represent effective values. For example, a certain harmonic alternating current gives a reading of 100 amperes on an alternating-current ammeter, and its effective value is, therefore, 100 amperes. This harmonic current actually pulsates between zero and a maximum value of i/ 2 X 100 amperes, or 141.4 amperes. Again, a certain harmonic alternating electromotive force gives a reading of 1,000 volts on an alternating-current voltmeter, and its effective value is, therefore 1,000 volts. This electromotive force actually varies between zero and a maximum value of V 2 X 1,000 volts, or 1.414 volts. The above simple relation between 'maximum and effective values is true only for harmonic (that is, sine-wave) electromotive forces and currents. In general, the maximum values of alternating electromotive forces or currents cannot be inferred from effective values as measured by voltmeters or ammeters. Thus, an alter- nating electromotive force which is known to have a peaked-wave form might have a maximum value very greatly in excess of V 2 times its effective value. Expression for Power, (a) When the current is in phase with the electromotive force, that is, when the circuit is non-inductive, then the power (average ei, see page 15) is P=EI (13) in which P is the power in watts, E is the effective value of the electromotive force in volts, and / is the effective value of the cur- rent in amperes. Equation (13) is identical with the power equation for direct-current circuits. (b) If the phase difference between current and electromotive force in a given circuit were 90 degrees, which can never actually occur, then the power (average value of ei) would be equal to zero, as explained on page 16. ALTERNATING-CURRENT MACHINERY 27 (c) When the phase difference between current and electro- motive force is 0, as shown in Fig. 39, then P= El cos 6 (14) in which P is the power (average ei) in watts, E is the effective value of the electromotive force in volts, and / is the effective value of the current in amperes. For example, the given current I shown in Fig. 39 may be thought of as resolved into two components, as shown in Fig. 40. One of these components 7 cos 6 is parallel to (that is, in phase with) E; and the other 7 sin 6 is at right angles to E. The power corre- sponding to the actual current I may be thought of as the sum of the powers corresponding to its two components, respectively. But the component I sin 6 is at right angles to E, as in (b) above; hence the power corresponding to it is zero. This component is, therefore, frequently called the wattless component or better the reactive com- ponent of the given current. On the other hand, the com- ponent I cos is parallel to (that is, in phase with) E, as in (a) above; hence the power corre- sponding to this component is equal to EX I cos 6. The com- ponent / cos of the given cur- rent / is frequently Called the Fi S- 39 - Diagram Shying Phase Difference Between E. M. F. and Current power component of 7; and the factor cos 6 is called the power factor of the circuit. Inductance. It has been pointed out, page 11, that an electric circuit has a certain kind of inertia analogous to the inertia of water in a circuit of pipe, and it was there noted that this inertia of an electric circuit is called inductance. If an electric current in a cir- cuit is made to change in value, a portion of the electromotive force acting upon the circuit must be used to cause the current to change. In the same way a force over and above that required to over- come frictional resistance must act upon a moving body to accelerate it, that is, to make its speed increase. The inertia of a body is meas- ured by the force required to accelerate it at the rate of unit change in speed per second; and the inductance of a circuit is measured by the electromotive force required to cause a current in the circuit 28 ALTERNATING-CURRENT MACHINERY to change at the rate of one ampere per second. A circuit is said to have an inductance of one henry* when one volt (over and above the electromotive force required to overcome the electrical resist- ance) will cause the current to change at the rate of one ampere per second. Let x be the rate in amperes per second at which the current in a circuit is increasing in value. Then the electromotive force E (over and above that required to overcome the resistance of the circuit) required to cause the current to increase at this rate is E=Lx (15) in which L is the inductance of the circuit in henrys, E being ex- pressed in volts. Example. The coil of a certain large electromagnet has 2.5 henrys of inductance and 5 ohms of resistance. At a given instant this coil is connected to 110- volt direct-current mains. At the instant of connecting the coil, the current is zero, and all of the 110 volts is used to cause the current in the coil to increase, so that, according to E equation (15), x is equal to 7-, or Fig. 40. Resolution of the I of Fig. 39 into Two Components 110 volts -s- 2.5 henrys, or 44 amperes per second. That is, the current in the magnet coil begins to increase at the rate of 44 amperes per second. When the current in the magnet coil has reached the value of 10 amperes, 50 volts ( = 5 ohms X 10 amperes) of the total 110 volts are used in overcoming the resist- ance of the coil, so that 60 volts ( = 110 volts 50 volts) are used to make the current increase. Therefore, as the current in the coil passes the value E , or 60 volts -5- 2.5 henrys, or 24 of 10 amperes, it is increasing at the rate amperes per second. The inductance of a coil wound on a given spool is proportional to the square of the number of turns N of wire. For example, a given spool wound with No. 16 wire has 500 turns and an induc- tance of, say, 0.0025 henry; the same spool wound with No. 28 wire would have about eight times as many turns, and its induc- tance would be about 64 times as great, or 0.16 henry. *The henry is a very large inductance, and the inductances usually met with in practice are expressed in thousandths of a henry, that is, in milli-henrys. ALTERNATING-CURRENT MACHINERY 29 The inductance of a coil of given shape is proportional to its linear dimensions, the number of turns of wire being unchanged. For example, a given coil has an inductance of 0.022 henry; and a coil three times as large in length, diameter, etc., but having the same number of turns of wire, has an inductance of 0.066 henry. Formulas for Inductance. The inductance in henrys of a coil of wire wound in a thin layer on a long wooden cylinder of length / centimeters and of radius r centimeters, is 47r 2 r 2 N* in which N is the total number of turns of wire in the coil. This equation is strictly true for very long coils wound in a thin layer; but the equation is also very useful in calculating the approxi- mate inductance of even short, thick coils. Thus, a coil 25 centi- meters long and 2| centimeters mean radius, containing 150 turns of wire, has an approximate inductance of L= 4^(2.5)*X150* =0000 '-.. 25 X 10 9 If a coil of N turns of wire is wound on a long rod of iron, in- stead of wood or other non-magnetic material, the inductance in henrvs is given by the equation JX10- in which L, r, N, and / are the same as in equation (16), and t*. is the permeability of the iron core at the particular flux density produced in the iron. This equation applies to any iron rod of length / centi- meters and radius r centimeters, wound with N turns of wire, whether in the form of a long, straight rod or bent into a closed ring. The permeability /* of iron varies from 500 to 1,000 or more; and, therefore, the effect* of placing an iron core in a coil is to in- crease greatly the inductance of the coil. The iron core of an inductance coil to be used with alternating currents should be laminated to reduce eddy currents and the con- *The permeability fj, of a given sample of iron is not constant, but decreases in value as the magnetizing force increases. Therefore, the inductance L of a coil having an iron core is not a definite constant quantity as is the inductance of a coil without an iron core. 30 ALTERNATING-CURRENT MACHINERY sequent loss of energy, and to prevent excessive heating of the core. For example, the inductance of the field coil of a certain shunt- wound dynamo is 7.5 henrys. The inductance of a pair of No. 0, B. & S. copper line wires carried at a distance of 18 inches apart on a pole line is 0.0035 henry per mile. The inductance of the secondary coil of a large induction coil (X-ray coil) having 200,000 turns of wire, is 2,000 henrys. Series and Parallel. The inductance of two or more coils in series is equal to the sum of the individual inductances. The equivalent inductance of two or more similar coils in parallel, such as the similar coils on an armature, is equal to ft of the inductance of one coil, n being the number of coils connected in parallel. Capacity. It has been pointed out, page 11, that a charged condenser has an elastic-like reaction analogous to the elastic reaction of a distorted diaphragm. The elasticity of a diaphragm might be measured by the pressure required to distort it to the extent of producing one unit of increase of volume in the space on one side of the diaphragm and one unit of decrease of volume in the space on the other side of the diaphragm. Similarly, the capacity of a condenser may be, and in fact is, measured by the electromotive force required to force one unit of charge of electricity into one plate of the condenser, and at the same time to withdraw one unit of charge from the other plate. A condenser is said to have a capacity of 1 farad* when one volt of electromotive force pushes one coulomb of electric charge into the condenser. The charge Q pushed into a condenser by a steady electro- motive force E is Q=CE (18) in which C is the capacity of the condenser in farads, Q is the charge in coulombs, and E is the electromotive force in volts. The electro- motive force required to hold a given charge Q in the condenser is of course equal to -^r" Condensers, to have a large capacity (as much as a microfarad), are usually made up of alternate sheets of tinfoil and waxed paper, *The farad is an exceedingly 'large capacity, and capacities encountered in practice are usually expressed in millionths of a farad, that is, in microfarads. ALTERNATING-CURRENT MACHINERY 31 TABLE II Inductivitics of Dielectrics Referred to Air as Unity Glass 3.00 to 10.00 Mica 4.00 to 8.00 Vulcanite 2.50 Shellac 2.93 to 3.3 Paraffin 1.68 to 2.30 Turpentine 2.15 to 2.43 Beeswax 1.86 Petroleum 2.02 to 2.19 Gutta-percha 3.3 to 4.9 Rubber (pure India) 2.12 to 2.24 or mica, as indicated in Fig. 41. Alternate metal sheets are connected together giving two terminals as shown, thus practically forming two metallic plates of large area. A condenser having a capacity of 1 microfarad contains about 3,600 square inches of tinfoil. Inductivity of Dielectric. The capacity of a condenser of given dimen- sions depends upon the material used between the plates, called the dielectric. The quotient, capacity of a condenser with given dielectric -r- its capacity with air as the dielectric, is called the inductivity or the specific induc- tive capacity of the given dielectric. The values of the inductivity for the most commonly used dielectrics are given in Table II. Capacity of Condensers. The capacity of a condenser is given by the equation = 885 k* Fig. 41. Diagram of Condenser in which C is the capacity in microfarads, k is the inductivity of the dielectric used, d is the distance in centimeters between plates, i. e. y the thickness of the dielectric, and A is the area in square centi- meters of both sides of all the inner plates plus the area of the inner surfaces of the two outside plates. NOTE. If the total number of plates =n, A will be the area of both sides of (n-1) plates; or, in other words, it is necessary to provide one more plate than is necessary, using both sides, to equal an area of A square cen- timeters. Example. It is required to design a plate condenser to have a capacity of 1.5 microfarads using a dielectric of oiled paper 0.0043 inch thick and tin- foil 0.0007 inch thick. 32 ALTERNATING-CURRENT MACHINERY Assuming for the oiled paper an inductivity of 2.67, equation (19) gives for the total active plate area 1.5X10 10 X0.0043X2.54 A = - - = 69400 square centimeters = 10759 square inches 885 X2.67 If 65 plates be used, the area of one side of each plate will be 10759 ^-^=84 square inches BO that the dimensions of one active plate may be 10|"X8" and the area of tinfoil needed will be 84 X 65 = 5460 square inches. The capacity of an ordinary 2-quart Ley den jar is about 0.005 microfarad. The capacity of an average submarine telegraph cable is about 0.4 microfarad per nautical mile. The capacity of a pair of transmission lines of No. 0, B. & S. wires placed 18 inches apart be- tween centers on poles, is 0.036 microfarad per mile. Series and Parallel. The capacity of a number of condensers in parallel is equal to the sum of the individual capacities. The capacity of a number of condensers in series is C- - (20) in which C v C y C y etc., are the individual capacities, and C is the joint capacity. Fundamental Equations of the A. C. Circuit. An alternator A f Fig. 42, delivers an alternating current of / amperes, effective, to a circuit consisting of a resistance of R ohms, an inductance of L henry s, and a condenser of which the capacity is C farads, all connected in series. It is to be remem- bered that any coil having inductance has resistance also; that is, inductance and resist- ance are practically inseparable. Neverthe- Fi g . 42. Alternating circuit less inductance and resistance are essentially Containing Resistance, In- . ductance, and Capacity J different in nature and in their effects, and they are always considered separately, so that it is helpful to think of them as actually separated in a circuit, as indicated in Fig. 42. A resistance is conventionally represented thus, -AA/W- ; an inductance thus, 'CJOQOCOO^- ; and a conden- ser thus, or RI sfe ALTERNATING-CURRENT MACHINERY 33 The current in the circuit, Fig. 42, is assumed to be harmonic, that is, to be a sine-wave current, and this current is represented by the line 01 in Fig. 43. A portion of the electromotive force of the alternator is used to overcome the resistance of the circuit. The portion of the electromotive force so used is an alter- nating electromotive force of which the effective value is RI; it is in phase with the current, and is represented by the line RI in Fig. 43. A portion of the electromotive force of the alternator is used to overcome the in- Fig. 43. vector Diagram of Con- . , 1 , . . t . . ditions in Fig. 42 ertia or inductance or the circuit in causing the current to increase and decrease. The portion of the electromotive force so used is an alternating electromotive force of which the effective value is cuLI; it is 90 degrees ahead of I in phase, and is represented by the line (oLI in Fig. 43. The quantity a> is equal to 2/r times the frequency of the current /. A portion of the electromotive force of the alternator is used to overcome what we have previously called the electro-elasticity of the condenser, or, in other words, to hold electric charge on the condenser plates at each instant. The portion of the electromotive force so used is an alternating electromotive force of which the effective value is ; it is 90 degrees behind I in phase, and is rep- resented by the line in Fig. 43. OJ\j The total electromotive force E of the alternator is equal to the geometric (or vector) sum of the parts RI, (oLI, and . This C0\^/ vector sum is formed by subtracting from ojLI, since it is opposite &>G to uLI in direction, and then adding RI and (coLI -) geometric- al ally, as shown in Fig. 44, in which the line Oa represents (tuLI , 34 ALTERNATING-CURRENT MACHINERY and the line E represents the geometric sum of Oa and RI. From Fig. 44 we have, by geometry: or or I = E (21) (The quantity a)L -- ^ is called the reactance of the circuit. The &>C term a>L is often called inductance reactance; and the term - is cuC often called capacity reactance. Inductance reactance is always positive, and capacity reactance is always negative. It is convenient to represent the reactance a>L -- of a circuit by the single letter o)C X; that is X=L-1- C (22) Therefore, writing X for coL -- -in equation 21, we have cuC I = E (23) VR* + X 2 Furthermore, from the right triangle in Fig. 44 we have or tan 8 =4 (24) K in which is the angle of phase lag of the current I behind the electromotive force E; X is the reactance of the circuit; and R is the resistance of the circuit. ALTERNATING-CURRENT MACHINERY 35 RI coC Resistance, Reactance, and Impedance. Consider a harmonic alternating electromotive force E which produces a harmonic alter- nating current 7 in a circuit. This electromotive force may be resolved into two compo- nents, one parallel and the other perpendicular to I, as * shown, for example, in Fig. ^ 43. The component of E parallel to 7 is equal to RL The resistance of an al- ternating-current Circuit is Fig 44 complete Diagram of Conditions sometimes defined as that factor which, multiplied by the current, gives the component (of the elec- tromotive force) which is parallel to I. The component of E perpendicular to 7 is equal to cuLI -> or is equal to XL The reactance of an alternating-current circuit may be defined as that factor which, multiplied by the current, gives the component (of the electromotive force) which is perpendicular to 7. The factor i/7?- 2 + A' 2 , which, when multiplied by the current 7, gives the total value of the electromotive force E, is called the impedance (denoted by Z) of the alternating-current circuit. Of course, E divided by Z gives the value of the current 7. NOTE. Resistance, reactance, and impedance are all expressed in ohms; we may, for example, speak of 10 ohms of resistance, 10 ohms of reactance, or 10 ohms of impedance. Thus, ohms are used in alternating-current work to express the three essentially different things resistance, reactance, and impedance; and a specification of a certain number of ohms is not intelligible unless it is stated whether it is ohms of resistance, ohms of reactance, or ohms of impedance. The reactance and the impedance of a circuit depend upon the frequency of the alternating current, as well as upon the physical constants L and C of the circuit, since the factor a> is equal to 2n times the frequency. The reactance of a circuit may be positive or negative, accord- ing as coL is larger than or less than When reactance is posi- 36 ALTERNATING-CURRENT MACHINERY tive, the inductance reactance coL exceeds the capacity reactance , and the current is behind the electromotive force in phase, as ct)C shown in Fig. 44. When the total reactance is negative, however, the capacity reactance exceeds the inductance reactance, and the current is ahead of the electromotive force in phase, as shown in Fig. 45. The same results may be obtained from equation (24). If the total reactance X is negative, then tan 6 is negative, which means that 6 is a negative angle, or that the electromotive force is behind the current in phase, or that the current is ahead of the elec- tromotive force. Special Cases of Electromotive Force and Current Relations. A clear understanding of the following examples as special cases of the general relations of electromotive force and current as discussed on pages 34 and 35, depends upon the following facts: (a) That the effect of inductance in an alternating-current circuit be- comes negligible when the inductance is very small, for then the reactance is equal to 2n X 133, or 835.7 radians per second; the reactance of the coil is 835. 7 X 0.3, or 250.7 ohms; the impedance is V2 2 + 250.7 2 or 250.7 ohms; the current is , or 3. 989am- 250.7 peres; the current lags about 89| behind the electromotive force; and the power delivered to the coil is 1,000 volts X 3.989 amperes X cos 89| (0.008), which is equal to 31.9 watts ( = I 2 R). The product El, sometimes called ap- parent watts, is equal to 3,989 volt amperes. This example illustrates one remarkable feature of alternating currents namely, the very small amount of actual power that is delivered to a circuit of large reactance even though the electro- motive force is large and the current considerable. In the case of a direct current, 3.989 amperes taken from 1,000-volt mains would mean an actual delivery of 3,989 watts of power, while in the above case the actual power delivered is only 31.9 watts. The ratio true watts -=- apparent watts is called the power factor of a circuit; and U>LI or XI RI Fig. 49. Diagram of E. M. F.s and Current Relations for Condition in Fig. 48 in case of the coil here under discussion, this ratio is equal to about 0.008 (=cos 0). One never encounters in practice a circuit in which the reactance is so large compared to the resistance as in the above example; that is, one never encounters one in which the power factor is so small as 0.008. Cases are often met with, however, where the reactance is from two to ten times as large as the resistance. Thus, one of the primary windings of a certain 110-volt induction motor has a resist- ance of 0.7 ohm and a reactance of 4.2 ohms. With zero load this 110 110 circuit, equation (23), takes . = A OKQ , or 25.83 amperes; 4.258 : the angle of phase lag of the current, according to equation (24), is about 80J; and the power factor of the circuit is 0.164. 40 ALTERNATING-CURRENT MACHINERY A circuit which contains a coil wound on an iron core takes more power than is expended in the mere heating of the wire, namely, I 2 R, for some power is con- sumed in the iron core on account of magnetic hyster- esis and eddy currents. In the above examples this con- sumption of power in an iron core is neglected for the sake of simplicity. The term equivalent resist- Fig. 50.. Circuit Containing Resistance and Capacity aU f / S USed * Designate a fictitious resistance which when multiplied by 7 2 gives the actual power consumed by such a circuit, including both the power consumed in heating the wire, and that consumed by core loss. The equivalent resistance of such circuits has a value larger than the mere resistance of the copper winding. CASE C. Circuits containing resistance and a condenser. In this case the reactance ^T (= -) is negative, and the current leads the electromotive force in phase, as before pointed out. The V tangent of the angle of lead is equal to according to equation (24). H Therefore, the angle of lead of the current is small when X is small compared with R, that is, when a>C is large; while the angle of lead of the current approaches 90 when X is large compared with R, that is, when coC is small. Fig. 50 represents a circuit containing resistance and a condenser connected to an alternator A; and Fig. 51 shows the relation between the electromo- tive force and current. Example. A condenser with a capacity of 2 microfarads (which is large, as condensers go) is connected to alternating-current mains through a resistance coil of 200 ohms. The effective electromotive force between the Ri XI or coC Fig. 51. Diagram of E. M. F. and Current Relations for Conditions of Fig. 50 ALTERNATING-CURRENT MACHINERY 41 mains is 1,000 volts, and the frequency is 133 cycles per second, so that the factor CD is equal to 2nX 133 or 835.7 radians per second; the reactance of the 10 6 condenser is = 598.3 ohms (negative) ; the impedance of the circuit is 2 X 835.7 /^_^_o ' "^ ~ " 3 ^200 -f 598.3 =630.85 ohms; the current, according to equation (23), is 1.585 amperes; the current, according to equation (24), is 71 31' ahead of the electromotive force in phase; and the power delivered to the circuit is 1,000 volts X 1.585 amperesXcos 71 31', which is equal to 502.5 watts (=/ 2 #). If the above condenser is connected to the 1,000-volt, 133-cycle mains through a wire of negligible resistance, then the current will be - 598.3 ohms or 1.671 amperes; the current will be very nearly 90 ahead of the electromo- tive force in phase; the power factor cos d, will be nearly zero; and of course the power delivered to the condenser will be nearly zero. A circuit containing a condenser takes a little more power than is expended in the mere heating of the wire, namely, I 2 R, for some power is consumed in the insulating material between the condenser plates. This power consumed in the dielectric is said to be due to dielectric hysteresis. In the above examples, this consump- tion of power in the insulating material of a condenser is neglected for the sake of simplicity. CASE D. Circuit in which the inductance reactance a>L is balanced by the capacity reactance . In this case equation (21) reduces to /= E' ; that is, the electromotive force acting upon the circuit has to R overcome resistance only, as in the case of the non-reactive circuit. This case in which a>L is equal to zero is considered again in (JL)\J the following article on resonance. Electrical Resonance. Consider a circuit, like the one shown in Fig. 42, containing a given resistance R, a given induction L, and a given capacity C. Suppose that the alternator A is at first run at very slow speed so as to give very low frequency, and is then gradually increased in speed so as to cause the frequency to increase. This gradual increase of frequency will cause a gradual increase in the value of the factor a> (equal to 2n times the frequency) ; and as w increases, the following relations between inductance reactance coL and capacity reactance - - will obtain: 42 ALTERNATING-CURRENT MACHINERY (a) At first, when the frequency is very low (few cycles per second), the value of to is small. Therefore, the inductance reactance toL is small; the capacity reactance is large; and the total net toC reactance toL is negative, and very nearly the same as - , toC toC alone. (6) As the frequency increases, the value of to increases. There- fore, the inductance reactance toL increases; the capacity reactance - decreases; and the total net reactance toL increases in toC toC value on account of the increase of toL, and also on account of the decrease of . For a certain critical value of the frequency, toL toL> becomes equal to , so that the total net reactance is then zero. That is wL =0 or or or VLC or, since to equals 2-nf', we have /' = ~ (26) in which/' is the critical value of the frequency for which inductance reactance is balanced by capacity in the given circuit, L being the inductance of the circuit and C the capacity of the condenser. See Fig. 42. ALTERNATING-CURRENT MACHINERY 43 MU 100 20 n V -_ Fig. 52. Graphical Relation of Current and Fre- quency; E. M. F. Constant (c) As the frequency increases beyond the critical value /', the inductance reactance coL continues to increase; the capacity reactance continues to decrease; and the net reactance ajL ~, now positive in value, continues to increase in value. Now, imagine the electromotive force of the alternator to be constant in value, although increasing in frequency as the alter- nator is speeded up.* The current in the circuit will at first increase with increasing frequency until the critical frequency /', equa- tion (26), is reached, and then the current will decrease in value as the frequency in- creases, a maximum value of current being produced at the critical frequency /'. This production of a maximum cur- rent at the critical frequency /' is called electrical resonance. At critical'frequency the reactance ajL 1 J? ^ is zero; and the general equation (21) reduces to I = , toC ft as explained in CaseD, page 41. That is, the value of the current at the critical frequency is determined solely by the resistance of the circuit and for a given resistance has its maximum value. The variation of current in a circuit like that shown in Fig. 42, with increasing frequency, electromotive force being kept constant in value, is shown graphically in Fig. 52, which is calculated from the following data: E=200 volts (effective); R=2 ohms; L= 0.352 henry; and =20 microfarads. The critical frequency corresponding to these values of L and C is 60 cycles per second, according to equation (26). The maxi- mum point of the curve is not a cusp, as would appear from the figure; but the curve is rounded at the top, the figure being drawn on too small a scale to show it. It should be remarked that the conditions for complet reeson- ance can be obtained only when C and L are constant and con- *In practice this condition could be realized by adjusting the field rheostat of the alternator, or of the exciter, so as to reduce the exciting current as the speed of the alter- nator is increased. 44 ALTERNATING-CURRENT MACHINERY centrated (not distributed), and when the electromotive force is harmonic. If the electromotive force is non-harmonic, it means that it is composed of a number of sine waves having different fre- quencies. It is evident, therefore, that ioL can never be made exactly equal to the fraction = unless there is but one fundamental CO\j (i) frequency equal to /= . 2n Multiplication of E. M. F. by Resonance. When resonance exists in a circuit containing an inductance and a condenser in series, the alternating electromotive force coLI between the terminals of the inductance, and the alternating electromotive force - between wC the terminals of the condenser, may each be much greater than the alternating electromotive force RI which acts upon the circuit. This fact is easily understood by means of the mechanical analogue. If even a very / weak periodic force act upon a weight which is suspended from a spiral spring, the weight will be set into violent vibra- ?/=" / tion, provided the frequency of the force is the same as the proper frequency of oscillation of the body. The forces acting on the spring may reach enormously greater values than the periodic force Fig 53. Diagram showing Muitipii- which maintains the motion of the sys- cation of E. M. F.s by Resonance * tern. Moreover, the forces which act up- ori the weight to produce its up-and-down acceleration may reach values very much larger than the periodic force which maintains the motion. Example. A coil having an inductance of 0.352 henry and a resistance of 2 ohms, and a condenser of 20 microfarads capacity, are connected in series between alternating-current mains. The critical frequency of this circuit is 60 cycles per second, according to equation (26). The electromotive force between the mains is 200 volts, and its frequency is 60 cycles per second. The current in the circuit is . or 100 amperes, according to equation 2 ohms (21); the effective electromotive force between the condenser terminals is 13,270 volts effective ( = ~): and the electromotive force between the ajC nals of the inductance is also 13,270 volts effective ( = ^L7). ALTERNATING-CURRENT MACHINERY 45 The multiplication of electromotive force by resonance may be clearly understood with the help of the clock diagram, Fig. 53. a a a b b b Fig. 54. An Alternator Delivering Current to a Long Transmission Line The electromotive force toLI required to overcome inductance reactance is equal and opposite to the electromotive force re- quired to overcome capacity reactance, as shown in Fig. 53, so that the geometric sum of coLI, , and #7 is, simply, RI. A transmission line has both inductance and capacity and, there- fore, electrical resonance may occur on a transmission line. The phenomena of a transmission line, however, are very greatly com- plicated by the fact that the capacity is distributed; and a simple explanation of line reasonance can be given only by approxi- mation. For example, an alternator A, Fig. 54, delivers current to a long transmission line, The resonance effects are nearly independent 000000000000 Fig. 55. Diagram of Conditions Shown in Fig. 54 with Transmission Lines Insulated from Each Other of whether the receiving apparatus at the end of the line, viz, at B, is connected into the circuit or not. We shall, therefore, consider that the receiving apparatus is disconnected and, furthermore, that the ends of the two transmission lines are insulated from each other. 46 ALTERNATING-CURRENT MACHINERY Fig. 56. Circuit Showing Conden- ser and Inductance in Parallel A first approximation to the behavior of the line may be ob- tained by looking upon the distant end of the line b b b as a con- denser purely and simply, while the near end of the line aaa is looked upon simply as an inductance. The transmission line shown in Fig. 54 is then equivalent to the combination shown in Fig. 55, which is identical with the combination shown in Fig. 42. The value of L may be taken as the induc- tance of, say, half the length of the line; and the capacity C may be taken as the capacity of the distant half of the transmission line. Then, if the alternator A gives a frequency equal to the critical value of these values of L and C, as per equa- tion (26), we shall have resonance, and the electromotive force be- tween the lines at the distant end (between the terminals of (7, Fig. 55) may be greatly in excess of the electromotive force of the alternator A. This condition actually occurs in the practical opera- tion of long transmission lines; and it is not an uncommon thing to have as much as 11,000 volts at the receiving end of a long transmission line when the electromotive force of the generator is but 10,000 volts. Multiplication of Current by Resonance. An alternator A , Fig. 56, delivers current to a circuit which divides at the points a and 6 into two branches, one branch containing an inductance L and the other branch con- taining a capacity (7, as shown. The two branches constitute a closed circuit in and of themselves; and if the fre- quency of the alternator is equal to the critical frequency of the circuit constitu- ted by the two branches, that is, if the frequency of the alternator is equal to =, as per equation (26), then the Fig. 57. Vector Diagram for Multipli- cation of Current by Resonance / - , small current I from the alternator will divide into two currents /i and 7 2 in the respective branches, and the currents 7 1 and 7 2 may each be very much larger in value than the undivided cur- ALTERNATING-CURRENT MACHINERY 47 main 550 vo/ts 133 cyc/es per second main rent 7. The fact is that, because of resonance, a very large current is made to surge back and forth around the closed circuit formed by the two branches. The multiplication of cur- rent by resonance may be clearly understood with the help of the clock diagram, Fig. 57. The line OE represents the electromotive force between the branch points and a b, Fig. 56 ; the line 7 X rep- Fig. 58. Diagram of circuit showing Muiti- . . plication of Current by Resonance resents ttye lagging current which the electromotive force E produces in the branch containing the inductan fjfc the line 7 2 represents the leading current which the electromom^e force E produces in the branch containing the con- denser; and the line 7, which is the geometric sum of 7 X and 7 2 , represents the total current in the undivided part of the circuit in Fig. 56. For example, three similar 32-candle-power incandescent lamps A, B, and 7), Fig. 58, each having 100 ohms resistance, are con- nected as shown, to 550-volt mains; L is an inductance of 0.597 henry; and C } a capacity of 2.49 microfarads. Then the current flowing through the lamp A is not quite 0.4 ampere, while one ampere of current flows through each of the lamps B and D. Miscellaneous Considerations. Condenser as Compensator for Lagging Current. An alternator may be designed to develop a cer- tain effective electromotive force E, and to deliver a certain effective current 7, at full load. The U permissible power output of such an alternator would be El watts to a non-reactive circuit having unity power factor (cos 0=1); but if the receiving circuit is reactive, the permissible power output of the alternator is only El cos 6, where the power factor (cos 6) may in practice have a value of .8 or less. If a condenser C of sufficiently large capacity, Fig. 59, is connected across the terminals of an alternator A in parallel with Fig. 59. Diagram of Condenser Compen- sating for Lagging Current 48 ALTERNATING-CURRENT MACHINERY an inductive receiving circuit RL, the effect of L will be neutralized; the current delivered by A will be in phase with the electromotive force of A; and the permissible power output will be EL The condenser is said to compensate for the lagging current taken by the inductive receiving circuit. The compensation produced by a condenser is due to the fact that the alternating current taken by it is ahead of the electromotive force in phase, while that taken by the reactive receiving circuit is behind the electromotive force in phase. Another advantage, aside from the increase of the permissible power output of the generator, that would result from this com- pensating of lagging current by means of a condenser, is that the electromotive force of the alternator would not fall JMSO much with increase of load as is the case when lagging cil^rent is not compensated for. The cost, however, of large condensers is so great that their use for compensation of lagging current is not commercially prac- ticable, as may be seen from the follow- ing discussion: Let I r be the current delivered to the receiving circuit RL, Fig. 59. Let I be the current delivered to the condenser C; this current is 90 degrees ahead of E in Fig -dLnI e sto^\ a n er r1g.5 f 9 Con - .phase. Let I a be the current delivered by the alternator A. It is desired that I a be in phase with E, as shown in Fig. 60. Let cos be the power factor of the receiving circuit RL. From Fig. 60 it is evident that I is equal and opposite to that component of I r which is at right angles to E, namely, I r sin 0. Therefore I c = 7 P sin A Now, 7. is equal to 1 (= uCE), that is, is equal to the electro- wC motive force between the condenser terminals divided by the reactance of the condenser. The value of sin is /- J and I r = ALTERNATING-CURRENT MACHINERY 49 E ^; so that, substituting the values of 7 P and sin 6, the above equation for I becomes coLE CO C E = R 2 + w 2 L 2 whence C = L R 2 +w 2 L 2 (27) in which R is the resistance (in ohms) of the receiving circuit; L is the inductance (in henry s) of the receiving circuit; a; is a factor equal to 2rc times the frequency in cycles per second; and C is the capacity (in farads) of the condenser required to compensate for the lagging current delivered to the receiving circuit. Suppose an alternator having an electromotive force of 1,100 volts and a fre- quency of 60 cycles per second, delivers 102.4 amperes of current to a receiving circuit of which the power factor is 0.871 (9.35 ohms resistance and 0.014 henry inductance). The capacity of a con- denser, which will compensate for the lagging current in this case, may be calculated from equation (27) as follows : C= 0-014 9.35 2 +(27rX60) 2 X0.014 2 = 0.0001214 farad = 121.4 microfarads This condenser would take from the mains a current of 50.34 am- peres, which would be 90 degrees ahead of the e. m. f. in phase, and this current would be equal and opposite to the wattless com- ponent of the 102.4 amperes of current delivered to the inductive circuit. Such a condenser would require about 114,000 leaves of tinfoil 8 inchesXIO inches, separated by 114,000 leaves of paraffined paper, each 0.03 inch in thickness. This would give a stack of condenser leaves of about 400 feet total thickness; and the cost of material and labor would be at least $10 per microfarad. Such a condenser would be impracticable but a synchronous motor with over-excited field magnets behaves like a large con- denser in that it takes an armature current which is ahead of the electromotive force in phase. The synchronous motor is often used in practice to compensate for lagging line current and when so operated is called a rotary condenser. 50 ALTERNATING-CURRENT MACHINERY Circuits in Series. An alternator A, Fig. 61, delivers current to two coils (or elements) in series as shown. Let R^ be the resist- Let RZ be the resistance ance and X^ the reactance of coil 1. ma/n Fig. 61. Two Elements in Series in Alternating Circuit E, Fig. 62. Diagram of E. M. F. Conditions for Fig. 61 and X 2 the reactance of coil 2. Let 7 be the current flowing through the circuit; E the electromotive force between the mains; EI the electromotive force between the terminals of coil 1; and E 2 the electromotive force between the terminals of coil 2. Let be the phase difference between E and /; d l the phase difference between E t and /; and 2 the phase difference between E 2 and 7, as shown in Fig. 62. V Of course 0\ is the angle whose tangent is ; #2 is the angle HI \7 whose tangent is 2 ; and 6 is the angle whose tangent is Rz according to equation (24). / Y [ Y \ Example. Two impedance coils have resistances of 5 and 8 ohms and inductances of 0.01 and 0.2 henry, respectively. If these coils are connected in series across 220- volt, 60-cycle mains, find: (a) the current; (b) the volt- ages impressed across the coils; and (c) the phase relations between the cur- rent and the voltages impressed across the coils. SOLUTION. We have JBi=5, #2 = 8, # = 220, / = 60, Li=0.01, L 2 =0.2. Then w=2^/=27rX 60 = 377 radians per second. The reactance of coil 1 is Xi = ojLi = 377 X 0.01 = 3.77 ohms, and the reactance of coil 2 is X 2 = ojLi = 377 X 0.2 = 75.4 ohms. The total impedance of the circuit is Z== >/(#! + # 2 ) 2 + (Xi+ X 2 ) 2 = V(5+ 8)2 + (3.774- 75.4) 2 = 80.23 ohms, so that the current is 220 8O23 2.74 amperes ALTERNATING-CURRENT MACHINERY 51 To find the voltages E i and E 2. (See Figs. 61 and 62.) The impedance of coil 1 is Z\ = -v/5 2 + 3.77 2 = 6.262 ohms and the impedance of coil 2 is Z 2 = -v/8 2 + 75. 4 2 = 75.82 ohms. The magnitude of the voltage Ei is Ei= 7Zi= 2.74 X 6.262 = 17.16 volts, and the magnitude of the voltage E 2 is E* = IZ 2 = 2.74 X 75.82 = 207.7 volts The phase angle 0i between the current and E\ is obtained from the V Q 77 relation tan Oi= - - - = 0.754, whence Ri 5 01 = 37 1' Similarly the phase angle 2 between / and E 2 is found to be 2 = tan" 1 = tan -1 9.425, or 02 =83 56' and tan = - ^- 8 = ' =6.090, whence -fil+ XV2 = 80 41' Referring to Fig. 62 it is evident that by taking the line (or vector) representing the current 7 as the axis of reference, the lines representing the electromotive forces, E\, E 2 , and E may each be resolved into two components, one parallel to or in phase with / and the other perpendicular to or in quad- rature with /. Then the sum of the components of E\ and E parallel to / will be equal to the component of E parallel to 7, and similarly the sum of the components of Ei and E 2 perpendicular to 7, will be equal to the vertical component of E. Expressing these statements in equation form and substi- tuting the values, we have the following: (Ei cos 0i) + (E* cos 2 ) (E cos 0) (17.16 cos 37 1')+ (207.7 cos 83 56') = (220 cos 80 40..3') (13.71) + (21.98) = (35.69 appro*. ) and (Ei sin 0i) + (Ei sin 2 ) = (E sin 0) (17.16 sin 37 1') +(207.7 sin 83 56') = (220 sin 80 40.5') (10.34) . + (206.7) = (217approx.) Circuits in Parallel. An alternator A, Fig. 63, delivers current to two coils (or elements) in parallel as shown. Let RI be the resistance, and Xi the reactance of coil 1; let R 2 be the resistance, and X 2 the reactance of coil 2; let E be the electromotive force between the coil terminals; let /i be the current in coil 1, 7 2 the current in coil 2, and / the total current; and let 6 be the angle of phase difference between E and /; 0\ the 52 ALTERNATING-CURRENT MACHINERY phase angle between 7 X and E; and 2 the angle between 7 2 and E, as shown in Fig. 64; The angles 0i, 2 , are known from the relations tan 0!= X, E and/ 2 = The branch currents are: 7i= but since they are not in phase, they cannot be added algebraically to obtain the resultant current 7, but must be added geometric- ally as is done in Fig. 64. The magnitude of 7 may be found by trigonometry thus: 7= V I \ + l + 27i 7 2 cos (0 2 - In the case of circuits in parallel the voltage across each branch is the same, and it is, therefore, convenient to determine all currents by their magnitudes and their relations with respect to this common Fig. 63. Two Elements in Parallel in Alternating Circuit voltage. Thus in Fig. 64 draw the line OE to represent the voltage in magnitude; it is convenient to draw it horizontal since the current vectors in the clock diagram are to be referred to this vector voltage as the axis of reference. The vectors Oh and 07 2 are then drawn from to represent the currents /i and 7 2 in magnitude and with the proper directions as determined by the phase angles 61 and 2 . The magnitude and phase of the resultant current 7 may be obtained graphically from the clock diagram, but this method is not to be recommended where accuracy is sought. To obtain by calculation the phase angle between the main current 7 and the voltage E, it is necessary to resolve 7 into its components parallel to and in quadrature with E. ALTERNATING-CURRENT MACHINERY 53 The power component of the current /i is a RJi E RI cos 0i= = X- =Eg T) here gi= -^ is called the conductance of circuit 1. *i Similarly the power component of 7 2 is J /2 COS 2 = ~ * The "reactive" or "wattless" component of the current /i is where 61 =^ is called the susceptance of circuit 1. Fig. 64. Vector Diagram of Current Conditions for Fig. 63 Similarly the reactive component of the current 7 2 is Since / is the vector sum of the two branch currents The power component of the resultant current is the sum of the power components of the two branch currents, or I cos 6= Ii cos 0i+ 7 2 cos 2 = Eg 1 -{-Eg 2 = E (gi+g z ) and the reactive componert of I is I sin 0= /i sin 1 +7 2 sin 2 = &i+E6 2 = E (&i+6 2 ) 54 ALTERNATING-CURRENT MACHINERY Therefore tan#= 7cos# and the magnitude of the vector 7 being the hypotenuse of a right- angle triangle, having a base 7 cos 6, and an altitude 7 sin 6, is By drawing a semicircle on OE as a diameter and extending the current vectors 7i, 7 2 , and 7, till they meet the semicircle at B, A, and C in Fig. 64, some important relations may be deduced. Join the points A, B, and C with E, thus completing the triangles OAE, QBE, and OCE. They are all right-angle triangles because inscribed in semicircle. A careful study of Fig. 64 will show that OA=RJ 2} AE=X Z I 2 , (see page 35), and that <\[ OA 2 +^AE 2 = OE = E from which R 2 = OA AE OE , X 2 = -p- , and Z 2 = . ^2 7 2 7 2 Similarly OB= RJ l} BE= XJi, and OB + BE = 0E= E and #7= OC, CE=XI, andC 2 + 2 = ^ in which # is called the "equivalent resistance" of the total circuit, and X its "equivalent react- ance." In other words, if the branched circuits were replaced by a single coil having a resistance equal to 72 and a reactance equal to X, the coil would take the same current from the mains as before, with the same angle of lag, and would absorb the same total power.- The impedance of this single "equivalent coil" is not the sum of the F impedances of the separate coils, but is . Example. An impedance coil having a resistance of 19.05 ohms and a reactance (at a frequency of 60 cycles) of 11 ohms is connected in parallel with a second impedance coil having a resistance of 22 ohms and a reactance of 38.1 ohms across 220-volt, 60-cycle mains. Find (a) the current in each coil and the total (or line) current; (b) the phase relation of the currents; (c) the power factor of the entire circuit and the power expended in it; and (d) the "equivalent" resistance and reactance of the circuit. ALTERNATING-CURRENT MACHINERY 55 Solu:ion. (a) To find the currents, we have E __ 220 __ 220 = Vi?T*f " V(19.Q5) +(!!) " "2* = E 220 220 / 2 = . = . = -- = 5 amperes V(22) 2 + (38.1) 2 44 and The above values are simply the magnitudes of the branch currents. Before they can be represented in a clock diagram, their directions must be found. e l = -30 The minus sign means a lagging current. X 38 1 tan 2 = --- = -4- =1.732, or /to 22 6 2 = - 60 The angle of phase difference between 7j and 7 2 is 2 - O l = -60 - ( -30) = - 30 The vector currents may now be drawn to scale as is done in Fig. 64. The resultant or vector sum of the currents 7j and 7 Z may be found by draw- ing the diagonal of the parallelogram on 7 X and 7 2 as sides. The main current is then 01. Its magnitude may be found by trigonometry; thus , 7 2 cos (d 2 -d = VlO 2 + 5 2 + (2 X 10 X 5 X 0.866) = V 211.6 = 14.55 amperes. To find the phase of 7, we have tan where b > - " " . - =- 0227; 6 '-' " - 01968 ; 56 ALTERNATING-CURRENT MACHINERY 0.0227 + 0.01968 0.04238 tan 9 = 0.0394+0.0114 - 5^5T or 6 = 39 53' which is the angle by which I lags behind E. (c) The power factor of coil 1 is cos 6 l cos 30 =0.86603 and the power expended in it is I\ flj = 10 2 X 19.05 = 1905 watts, or it is E 2 g 1 = (220) z X 0.0394 = 1905 watts. The power factor of coil 2 is cos 2 = cos 60 =0.5 and the power ex- pended in it is I\ R 2 = (5) 2 X 22 = 550 watts, or it is E*g t = (220) 2 X 0.01 136 = 550 watts. The power expended in coils 1 and 2 =1905 + 550 = 2455 watts. The power factor of the entire circuit (two branches) is cos#= cos 39 53' = 0.76735 and the power expended in it is El cos 0=220X14.55X0.76735 = 2455 watts (d) The "equivalent resistance" R is OC 220 cos 39 53' 168.81 T -HiT" H55" 11 - 6ohmB The power absorbed in R is 7 2 # = (14.55) 2 X11.6=2455 watts The "equivalent reactance" X is CE 220 sin 39 53 / 14.55 9.7 ohms The current / lags behind the voltage E, by an angle 6 given by X 97 tan = or tan d = - =0.836 from which = -39 53' the same as found K ll.o above. Electromotive Force Losses in Transmission Lines. An alter- nator of which the electromotive force is E, delivers current over a transmission line of which the resistance is RI and the reactance (inductance reactance) is X\, to a receiving circuit of which the resistance is R 2 and the reactance is X 2 . The total electromotive ALTERNATING-CURRENT MACHINERY 57 force used to overcome the resistance and the reactance of the transmission line is E\, and the electromotive force between the Fig. 65. Diagram of E. M. F. Losses in Transmission Lines Receiving Circuit Non-Reactive terminals of the receiving circuit is E 2 . The current delivered is 7. Then the general relation between E, EI, and E 2 is as shown in Fig. 62, except that E 2 is usually wry much larger than EI in value. There are three interesting and simple special cases of electromo- tive force losses in transmission lines, as follows:* CASE 1. When the receiving circuit is non-reactive. In this case the electromotive force E 2 between the terminals of the re- ceiving circuit is in phase with 7, the power factor of the receiving circuit is unity, and the general diagram of Fig. 62 takes the form shown in Fig. 65. The total electromotive force E 1 con- sumed in the line is sometimes called the impedance loss or drop and its two components 7^7 and XJ, as shown in Fig. 65, are called the resistance loss and the reactance loss, respectively. Now, a careful inspection of Fig. 65 makes it evident that the numerical difference be- tween the values of E and E 2 is very nearly equal to the resistance loss in the line 7^7; and that the reactance loss in the line XJ has little to do with the difference between the values of E and E 2 . Therefore, when the receiving circuit is non-reactive, the difference in value be- tween generator electromotive force E and receiver electromotive force E 2 is sensibly equal to the resistance loss of electromotive force in the line, and sen- sibly independent of the reactance loss of electromotive force in the line. *This discussion applies to comparatively short lines, ten miles or less in length, inasmuch ad the capacity of the line is not here taken into account. Fig. 66. E. M. F. Losses When the Receiving Circuit is Highly Reactive 58 ALTERNATING-CURRENT MACHINERY CASE 2. When the receiving circuit is highly reactive. In this case the electromotive force E 2 between the receiving circuit ter- minals is nearly 90 degrees ahead of / in phase, and the general diagram of Fig. 62 takes the form shown in Fig. 66. A careful inspection of Fig. 66 makes it evident that the difference in value of E and E 2 is very nearly equal to the reactance loss in the line XJ; and that the resistance loss in the line RJ has little to do with the difference between the values of E and E 2 . Therefore, when the receiving circuit is highly reactive, the numerical difference in value between generator electromotive force E and receiver electro- motive force E 2 is sensibly equal to the re- actance loss of electromotive force in the line, and sensibly independent of the resist- ance loss of electromotive force in the line. CASE 3. When the receiving circuit has large capacity reactance. In this case the electromotive force E% between the receiving circuit terminals is nearly 90 de- grees behind 7 in phase, and the general diagram of Fig. 62 takes the form sliown in Fig. 67. A careful inspection of Fig. 67 makes it evident that the difference between the values of E and E 2 is very nearly equal to the reactance loss in the line XJ, and that this reactance loss is added to the generator electromotive force E to give the receiver electromotive force E%. Inspection of Fig. 67 shows further- more that the resistance loss in the line RJ has little to do with the difference in value of E and E 2 . There- fore, when the receiving circuit has a high capacity reactance, the re- actance loss in the line is sensibly equal to the rise in value of the elec- tromotive force between generator and receiver, and this rise in value is sensibly independent of the resistance loss of electromotive force in the line. It is somewhat confusing to speak of the electromotive force XJ as reactance loss when the receiving circuit has a high capacity reactance; it would be better in this case to speak of XJ as the reactance gain of electromotive force in the line. /E, Fig. 67. E. M. F. Losses with Large Capacity Reactance ALTERNATING-CURRENT MACHINERY 59 Electromotive Force Losses 'in Alternator Armatures. Let E be the total induced electromotive force in the armature of an alterna- tor. A portion EI of this electromotive force is used to overcome the resistance RI and the reactance Xi of the armature; and the remainder E 2 is available at the terminals of the alternator for producing current in the outside circuit, of which the resistance is R 2 and the reactance is X 2 . The general relation between E, E v aneL.Ea is as shown in Fig. 62, except that E 2 is usually wry much larger than EI in value. There are three interesting and simple special cases of elec- tromotive force losses in alternator armatures, as follows: CASE 1. When the receiving circuit is non-reactive. In this case the electromotive force E 2 between the terminals of the alter- nator is in phase with the current I delivered by the machine, and the general diagram of Fig. 62 takes the form shown in Fig. 65, from which it is evident that when the receiving circuit is non- reactive, the numerical difference in value between the total induced electromotive force E and the terminal electromotive force E 2 of the machine is sensibly equal to the resistance loss of electro- motive force Ril in the armature, and sensibly independent of the reactance loss of electromotive force XJ in the armature. CASE 2. When the receiving circuit is highly reactive. In this case the electromotive force E 2 between the terminals of the alter- nator is nearly 90 degrees ahead of I in phase, and the general diagram of Fig. 62 takes the form shown in Fig. 66, from which it is evident that when the receiving circuit is highly reactive the numerical difference in value between the total induced electromotive force E and the terminal electromotive force E 2 of the machine is sensibly equal to the reactance loss of electromotive force XJ in the arma- ture, and sensibly independent of the resistance loss of electro- motive force Ril in the armature. CASE 3. When the receiving circuit has large capacity react- ance. In this case the electromotive force E 2 between the ter- minals of the alternator is nearly 90 degrees behind I in phase, and the general diagram of Fig. 62 takes the form shown in Fig. 67, from which it is evident that when the receiving circuit has a high capacity reactance the difference in value between E and E 2 is sensibly equal to XJ, E 2 being larger than E, and sensibly independent of the resistance loss of electromotive force RJ in the armature. 60 ALTERNATING-CURRENT MACHINERY MEASURING INSTRUMENTS Electrical measuring instruments may be divided into three classes, as follows: (a) Indicating instruments which give the value of an elec- trical quantity at the time of observation, or which may be so manipulated as to give this value. (b) Integrating instruments which combine the element of time with the element of electrical quantity. For instance, a watt- hour meter gives a reading proportional to the product of the average power in watts and the time in hours, or in other words, it measures the electrical energy expended in the circuit to which it is connected ; it is not, therefore, a power meter. (c) Recording instruments which trace a curve or other graphic record showing the variation of some electrical quantity, such as voltage, with time. It should be carefully noted that the name "recording wattmeter" is very commonly but incorrectly applied to the integrating watt-hour meter. The latter is not a recording instrument, nor is it a wattmeter. A true recording wattmeter would draw a curve showing the variation of the watts with time. Electrical instruments may be also classified according to con- struction and method of use into: switchboard, portable, and semi- portable or laboratory types. INDICATING INSTRUMENTS Indicating electrical instruments may be divided into three groups, as follows: (1) Those adapted for direct currents only, a consideration of which is beyond the scope of this text; (2) those adapted for both direct- and alternating-current circuits, which class comprises hot-wire, electrostatic, and electromagnetic instru- ments, and electrodynamometers ; (3) instruments operating only on alternating current, which depend upon the interaction of induced and inducing currents, and are usually described as induction instru- ments. Hot=Wire Ammeter and Voltmeter.* Instruments of the hot- wire type depend upon the expansion of a stretched wire when heated *A11 voltmeters except fthe electrostatic voltmeter are essentially ammeters; that is. the electromotive force to be measured produces a current which actuates the instrument, The scale over which the pointer moves, may be arranged to indicate either the value of the current flowing through the instrument, or the value of the electromotive force acting between the terminals of the instrument. ALTERNATING-CURRENT MACHINERY 61 by the passage of a current, which actuates a pointer moving over a divided scale. These instruments are adapted for both direct and alternating current for, when calibrated by continuous currents or electromotive forces, they indicate effective values of alternating cur- rents or electromotive forces. This may easily be proved true by considering an alternating current and a continuous current C which give the same reading. These currents generate heat in the wire- at the same average rate, which is C 2 R for the continuous current, and average i 9 XR for the alternating current,^ being the instantaneous value of the alternating current. Therefore, C 2 R= average i 2 XR; or C 2 = average i 2 ; or C= V average i z . The proof for electromotive forces is similar to this proof for currents. Recent instruments of the hot-wire type have a working wire from 3 1 to 8 inches in length, of small size for voltmeters and of larger size for ammeters. The voltmeters have a non-inductive resistance connected in series, and the ammeters usually have the working wire connected in parallel with several sections in order to reduce the required drop of voltage in the shunt. Hot-wire instruments are comparatively little used in this country in practical work, their disadvantages being relatively large power consumption, uncertainty of zero (on the scale), and errors due to change of surrounding temperature and to heating when left long in circuit; furthermore, to secure sensibility, the working wire must be operated at a fairly high temperature and may thus be easily damaged by sudden overloads which in other types of in- struments would hardly do more damage than the bending of a pointer. The advantages of hot-wire instruments which cause its con- tinued use for certain classes of work are its independence of fre- quency, wave form, and stray magnetic fields; the fact that it may be calibrated on direct current; and the fact that shunts may be used with the ammeter on alternating current. For use in labora- tories with unusual frequencies or wave forms, and where there are facilities for calibration on direct current, the hot-wire type has marked advantages. It should be noted, however, that for very high frequencies, such as those employed in wireless telegraphy, shunted hot-wire ammeters are not reliable. This is due to the fact that for 62 ALTERNATING-CURRENT MACHINERY such high-frequency currents the effective resistance of the shunt including the so-called "skin effect" is much greater than for direct current, and that the current in the shunt will lag more behind the electromotive force than the current in the working wire. For these extreme frequencies, therefore, it is necessary to use a hot-wire ammeter so constructed as to permit the whole current to pass through the working wire which shall have its effective resistance at the frequency used practically equal to its resistance for direct current, thus eliminating the shunt altogether. Fig. 68 is a general view of the hot-wire voltmeter of the Roller- Smith Company, and Fig. 69 is a diagram of its essential mechanism. In Fig. 69, a wire a, called the working wire, because it alone carries Fig. 68. General View of a Hot-wire Voltmeter Roller-Smith Company current, is fastened at one end to a plate c, passed around a pulley d secured to a shaft e, and its free end brought back again and me- chanically attached to, though electrically insulated from, the same plate c. The wires a and b are kept under tension by the spring / attached to the plate c, which is being constantly pulled in a direction at right angles to the axis of the shaft e, and is so guided that it can be moved in that one direction only. To the shaft e is secured an arm g which is forked at its lower end and counterweighted at its upper end. Between the forked ends of the arm g is another shaft h on which there is a small pulley and to which is attached the light ALTERNATING-CURRENT MACHINERY 63 pointer i; a fine silk fiber passes around the pulley and is secured to the ends of the fork arms which are springy and keep the silk fiber taut. The current to be measured passes through the wire a only, entering and leaving as shown by the arrow heads on the terminals near c and d . When a is heated by the current it expands, causing its tension to be less than the tension of b. The result is that the pulley d is rotated in a clockwise direction until the tensions in a and b ^r again balanced. When the pulley d is rotated, g is moved to the left and this causes the silk fiber to rotate the shaft h and the pointer i. From this construction and the ratio of the lever arms, it is evident that a very slight elongation of the wire a suffices to produce a considerable movement of the pointer i. One of the objections to hot-wire instruments, viz, that the working wire is affected by changes in the temperature of the air thereby intro- ducing an error in the measurements, is successfully overcome in this make of instrument by simple compensation. Thus, if the temperature of the sur- rounding air changes, the wires a and b are affected alike, both either con- tracting or expanding by the same amount, which causes a movement of the plate c back or forth in its path, but without any tendency to rotate the pulley d. The entire moving system illustrated in Fig. 69 is mounted on a single base plate and, by means of a lever projecting through the instrument case, may be rotated slightly about a heavy shaft whose axis is in line with the axis of the pointer shaft h. The scale with its support being stationary, this device permits of correction for a bent pointer, or adjustment to zero on the scale without interfering with the mechanism in any way. While the hot-wire instruments of the Roller-Smith Company are nearly dead-beat, an auxiliary damping device is usually fur- nished consisting of an aluminum disk swinging between the poles Fig. 69. Diagram of Hot-wire Instrument 64 ALTERNATING-CURRENT MACHINERY of a stationary permanent magnet. Hot-wire voltmeters are specially suited for measuring low electromotive forces up to about 75 volts. The switchboard hot-wire ammeters, reading from 25 up to 1,000 amperes, have separate shunts to which the instrument is connected by flexible leads. Electrostatic Voltmeter. Instruments of the electrostatic type depend upon the attraction of oppositely charged bodies, and repul- sion of similarly charged ones. Two metallic plates connected to the terminals of a battery, or any other source of electromotive force, attract each other with a force which is exactly proportional to the square of the electromotive force. As these forces are relatively small, instruments of this type are not w^ell adapted for use as am- meters, and it is difficult to construct satisfactory voltmeters on the electrostatic principle for the ordinary low voltages of 110 to 220 volts. The electrostatic principle is especially adapted to the meas- urement of high voltages from about 10,000 up to 200,000 volts. The voltmeter consists essentially of a fixed metal plate and a mov- able plate delicately mounted on a jewelled pivot. The movable plate carries a pointer, which plays over a divided scale. The electro- motive force to be measured is connected between the fixed plate and the movable plate, and the electrical attraction between the plates causes the movable plate to turn about its supporting pivot and move the pointer. Such an instrument) when calibrated by con- tinuous electromotive force, indicates effective values of alternating electromotive force, as may be seen from the following discussion: A given deflection of the movable plate depends upon a definite average or constant force acting between the two plates. The force due to a constant electromotive force E is kE 2 , that is, the force is proportional to E 2 ; and the average force due to an alternating electromotive force e, is kX average e 2 . If these electromotive forces give equal deflections, the constant force kE 2 must be equal to the average force kX average e 2 ; that is kE 2 = k average e 2 or E 2 = average e 2 or V average e 2 =E ALTERNATING-CURRENT MACHINERY 65 The great advantage of this type of voltmeter is that it takes no current when used on direct-current circuits, and an extremely small current when used on alternating-current circuits. Its other advantages, like those of the hot-wire instruments, are that it is independent of changes in frequency, wave form, and stray mag- netic fields. Furthermore, for very high voltages up to several hundred thousand volts the construction is relatively simple -and cheap, and no auxiliary "potential" transformers are required for reducing the voltage to be measured. It has the disadvantage of small ratio of torque to weight of moving parts, so that errors due to friction of the moving element are relatively large and difficult to avoid. For this reason low-range electrostatic voltmeters are often made with the moving element suspended by a wire or strip instead of rotating on pivots. Fig. 70. Diagram of Westinghouse Electrostatic Voltmeter Excepting the electrostatic ground detector, which is essen- tially a voltmeter, this type of instrument has as yet been little used in commercial work outside of the laboratory. The adoption, however, of increasingly high voltages for long-distance transmission of power carries with it a demand for a reliable commercial form of this instrument. Great progress has lately been made towards putting the electrostatic voltmeter on a commercial basis. Fig. 70 shows a diagrammatic view of the principle and arrange- ment of parts of the electrostatic voltmeter brought out by the West- inghouse Electric Company. Fig. 71 (a) shows the moving element. The meter element consists of two stationary curved aluminum plates BI and J5 2 , between which is suspended a movable vane MI M% ALTERNATING-CURRENT MACHINERY controlled by a light spiral spring so adjusted that the pointer P remains at zero with no voltage on the meter, and gives the full scale deflection at the proper voltage for which the instrument is designed. The curved plates BI and B 2 are connected, as shown, to the inner plates of the condensers Ci and C 2 , and so arranged with respect to MI MI that when a voltage difference exists between the terminals TI and T 2 , the induced charge on MI and the + charge on M 2 being attracted by the + charge on BI and the charge on B 2) respectively, the moving element MI M 2 rotates counter-clockwise into the new position shown by the dotted lines in Fig. 70. The condensers C\ and C 2 are in series with the other parts of the instrument, the inner plate of each being connected to the fixed V (a) Old Form of Moving Element (b) Modern Moving Element Fig. 71. Westinghouse Electrostatic Voltmeter plates BI and B 2 , and the outer plate of each to the terminals TI and T 2 . For high voltage readings the instrument is connected as a shunt across one of the two (or more) condensers in series, thus impressing any desired fraction of the total voltage upon the instru- ment terminals. For reading lower voltages, one or more of the condensers are short-circuited, thus permitting the same instrument to be used over a wide range. Fig. 71 (b) shows a modern element. The meter is placed in a sheet-iron tank rilled with transformer oil. This is necessary because oil has a far greater dielectric strength than air, and in this meter the distance between the parts, between ALTERNATING-CURRENT MACHINERY 67 which there is a high voltage difference, is less than the distance at which arcing across in air would occur. The oil also acts to dampen the moving element, thus making the instrument dead-beat and easy to read. The electrostatic ground detector is a modified electrostatic volt- meter. Its essential features are shown in Fig. 72. Two metal plates A and B are connected to the two mains a and b (usually through two high resistances RR); and a light movable metal plate m, suspended between A and B, is connected to ground (usually through a high resistance R). If both lines are equally well insulated from ground, the plates A and B are each at the same electrical pressure or potential, attracting the plate m equally so that it hangs midway between them. If one of the mains, say a, is grounded, its pressure or potential becomes equal to the potential of m, so that plate A no longer attracts m, and the plate m is, therefore, pulled to the right by the attraction of B, the movement being indicated by a pointer p. The same results may be accom- plished, when the plate m is entirely insulated from the ground, by having a grounded auxiliary stationary plate near m. The essential features and mode of connection of the General Electric Com- pany's electrostatic ground detector are shown in Fig. 73 ; and a general view of the instrument, with cover removed, is shown in Fig. 74. Electromagnetic Ammeters and Volt= meters. Instruments of this type depend upon the action of a coil car- rying the current to be measured, upon one or more pieces of soft iron. Such instruments are also called "moving-iron", "soft-iron", and "magnetic- vane" types. Instruments of this general type have been used for many years, the earlier ones having been constructed with heavy soft iron cores which were drawn into solenoids energized by the current to be measured. The pull on the plunger due to the current was balanced by a weight attached to a lever arm. The earlier design Fig. 72. Diagram of Essentials of Electrostatic Ground Detector 68 ALTERNATING-CURRENT MACHINERY sometimes referred to as the "plunger " type, was faulty, and has been since superseded by the modern designs which involve the GROUND GROUND Fig. 73. Connecting Circuits for an Electro- static Ground Detector use of one or two thin vanes of soft iron mounted on a pivoted staff within the coil. The soft-iron type of instrument is calibrated by the use of direct electromotive forces or currents, but it does not indicate accurately effective values of alternating electromotive forces, or Fig. 74. General Electric Ground Detector with Cover Removed currents. A soft-iron type meter (ammeter or voltmeter) should be calibrated, using alternating current of the same frequency as ALTERNATING-CURRENT MACHINERY 69 that for which the meter is afterwards to be used, and of the same wave shape. Thus, if an instrument of the electromagnetic type is to be used as an ammeter for alternating currents of a given wave shape and frequency, it should be calibrated by currents of this wave shape and frequency, these currents, for the purpose of the calibration, being measured by a standard alternating-current am- meter, such as an electrodynamometer. The indications of an instrument of the electromagnetic type, however, do not vary greatly with wave shape and frequency, and such instruments are practically correct for any ordinary wave shape and frequency. The ammeters are but slightly affected by even large changes in frequency; the voltmeters, on account of their relatively large inductance, are more affected than the ammeters. The error in the Fig. 75. Mounted Coil of a Thomson Inclined-Coil Meter voltmeter is not large over the ordinary range of frequencies, and may be computed for extreme frequencies from the measured values of resistance and inductance. A valuable feature of this type of instrument is its very small temperature coefficient. They are well adapted for commercial measurements on alternating current circuits, and for direct current when only approximate results (within 2 or 3 per cent) are needed. The electromagnetic volt- meter or ammeter cannot be checked accurately on direct currents, as even the average of reversed readings does not give an accurate test of the performance on alternating current. This is due to the hysteresis effect in the soft-iron vanes, thereby causing the instru- ment to read higher for decreasing direct currents than for increas- ing values. This difference may amount to several per cent. An- other advantage of these instruments is their moderate price, and simple construction. 70 ALTERNATING-CURRENT MACHINERY Thomson Inclined-Coil Meter. The Thomson inclined-coil meter of the General Electric Company is an example of the electromag- netic type. The working parts of this instrument are shown in Fig. 75. A coil A, through which flows the current to be measured, is mounted with its axis inclined as shown. A vertical staff mounted in jewel bearings and controlled by a hair spring, passes through the coil, and to this staff are fixed a pointer b and a vane of thin soft sheet iron a. This vane of iron is mounted obliquely to the staff. When the pointer is at the zero point of the scale, the iron vane a lies nearly across the axis of the coil; and when a current passes through the coil, the vane tends to turn until it is parallel to the axis of the coil, position a' in Fig. 75, thus turning the staff and moving the attached pointer over the calibrated scale. The vane Fig. 76. Thomson Inclined-Coil Ammeter Fig. 77. Working Parts of Thomson Inclined-Coil Ammeter tends to turn into a position which makes the reluctance of the magnetic circuit a minimum. Fig. 76 is a general view of a Thomson inclined-coil ammeter, and Fig. 77 is a view of the working parts of the instrument. The structural details of the inclined-coil voltmeter are identical with those of the ammeter, except that the voltmeter has fine wire in the inclined coil and usually an auxiliary non-inductive resistance in series with the inclined coil. Roller-Smith Repulsion Ammeter. Another example of an am- meter of the electromagnetic type using the repulsion principle is illustrated in Figs. 78, 79, 80, which give a plan, sectional elevation, and detail, respectively. It is manufactured by the Roller-Smith Company. The current to be measured is led by cables to the ALTERNATING-CURRENT MACHINERY 71 terminals a and i and thence to the coil through heavy copper straps shown in dotted outline underneath the scale in Fig. 78. The terminal ai is thus electrically connected to the brass spool m to which is soldered the conducting strip j, Fig. 79, whose carrying capacity is adapted to the current for which the instrument is de- signed. The conductor j is wound about the spool upon a layer of insulating material to give the required number of turns "and is finally brought to the post r, Fig. 78, which passes down through Fig. 78. Plan of Roller-Smith Repulsion Ammeter the insulating bushing /, and connects underneath with the copper strap terminating at a. The same method of winding is used in the case of voltmeters, except that instead of the copper straps, flexible leads are used to connect the spool to binding posts covered with hard rubber, and located outside of the case. The moving element c is carried and supported by the structure shown at g, which is secured to the brass spool by the two hexagon- headed bolts as shown in Fig. 78. This construction enables the moving elements as well as the coils to be made up separately as occasion demands, and independently of the rest of the mechanism, and thus reduces the manufacturing cost. The* structure g consists of a casting into which is inserted a piece of brass tubing, having a solid bottom, for holding the lower 72 ALTERNATING-CURRENT MACHINERY jewel screw. The side of the tube is used to support the fixed magnetic vane b. The moving iron vane c is fastened to the staff i, which also carries the pointer e, which moves over the divided scale. The vibrations of e are dampened by the vane /, Fig. 79, which is forced to move in the nearly air-tight box which forms an integral part of the casting. The controlling spring d is adjusted so that the pointer e is normally held at zero on the scale. If the pointer fails to return to zero, it may be brought there by means of the lever h, which is connected to a button on the outside of the case. The action of the instrument is as follows: when a current is passed through the coil wind- ing j, it magnetizes both the fixed and movable vanes within the coil. Both vanes being made SECTION A-B ^ ver y so ^ ^ ron > are similarly magnetized and, Fig. 79. sectional Elevation therefore, repel each other. The movable vane of Repulsion Ammeter . . i i T c thus rotates in a clockwise direction carrying the pointer e to the right along the scale. This movement of the vane is opposed by the action of the controlling spring d resulting in the pointer coming to rest at a point depending upon the strength of the current passing in the coil. The design and arrangement of the movable and fixed soft iron vanes is such as to give the scale shown in Fig. 78, which is reproduced from an actual cali- brated scale. The pivots of the moving system are of steel, tem- pered and highly polished, and the bearings are of highly polished sapphires. Electrodynamometers. Of the four types of instruments for use on both direct and alter- nating currents, the electrodynamometer is on the whole the most valuable. This instrument depends upon the force exerted by one circuit F of R?puis?o e n a Amme g ter m carrying a current upon another, or by a por- tion of a given circuit upon another portion of the same circuit. Instruments of this type usually contain one or more fixed coils which set up a magnetic field directly proportional ALTERNATING-CURRENT MACHINERY 73 to the strength of the current flowing through them. Within this field is arranged a moving coil, or system of coils, through which current may be passed. If the two setc of coils are connected in series, the torque exerted upon the moving system, for a given relative position of the coil sys- tems, is proportional to the square of the current, and is not dependent upon the direction of the current. Such an instru- ment, therefore, constitutes an ammeter which is equally correct on direct cur- rent, and on alternating or pulsating current of any frequency or wave form. The value of this type consists largely in this inherent accuracy on these differ- ent kinds of current. Since the funda- mental electrical standards and precision methods of testing involve direct current ' y ' only, the electrodynamometer type, Fig . l . sectional Elevation of sie- calibrated on direct current, may be used as a precision instrument for alternating current. Electrodynamometer Used as an Ammeter. The electrodyna- mometer consists of a fixed coil and a movable coil connected in series, through both of which the current to be measured flows. The current causes the fixed coil to exert a certain force upon the mova- ble ceil; and the value of the current is determined (a) by observing the angle <$> through which a helical spring must be twisted by hand in order to bal- ance the above-mentioned force; or (b) by allowing the force to turn the mov- able coil, thus moving a pointer over a divided scale. In the Siemens electro- dynamometer method (a) is used; while / in many commercial forms of electrody- x^ namometer method (b) is used. The essential features of the Siemens electrodynamometer are shown in Figs. 81 and 82. The stationary coil A is supported by a clamp attached to the standard S; and Ele tr " 74 ALTERNATING-CURRENT MACHINERY the movable coil B is hung by a thread, the plane of coil B being at right angles to the plane of coil A. The terminals of the mov- able coil dip into cups of mercury a a, and the current to be measured is sent through both coils in series. The force acting between the coils is balanced by carefully turning the torsion head c by hand, thus twisting a helical spring b, one end of which is attached to the coil B and the other to the torsion head c. The observed angle of twist necessary to bring the swinging coil to its zero position, is read off by means of the pointer d and the grad- uated scale e. The pointer / attached to the coil shows when it has been brought to its zero position. The observed angle of twist of the helical spring affords a measure of the force acting between the coils, and the current is proportional to the square root of this angle of twist. That is I=kV~$ (28) in which 7 is the effective value of the alternating current; (f> is the observed angle of twist of the helical spring b; and k is a con- stant called the reduction factor of the instrument. For example, a twist of 220 is required to balance the force due to 18.8 amperes in a certain Siemens electrodynamometer; the reduction factor k of the instrument is, therefore, by equation (28), equal to 18.8 amperes -f- i/220~, or 1.267. A certain current to be i measured requires a twist of the torsion head of 165, hence the value of the current is equal to 1.267 X 1/165, or 16.28 amperes. The electrodynamometer, when standardized by direct currents, indicates effective values of alternating currents; thus, a given de- flection of the suspended coil depends upon a definite average or constant force acting between the coils. The constant force due to a constant current C is kC 2 (proportional to C 2 ) ; and the average force due to an alternating current is &X average i 2 ; so that if these currents give equal deflections, we have kC 2 =kXaveragei 2 ; or C 2 average i 2 ; or C = V average i 2 . Electrodynamometer Used as a Voltmeter. When used as a volt- meter the coils of the electrodynamometer are made of fine wire, and an auxiliary non-inductive resistance is usually connected in series with the coils. When the inductance of the electrodynamometer coils is small, such an instrument, when calibrated by continuous electro- ALTERNATING-CURRENT MACHINERY 75 motive forces, indicates effective values of alternating electromotive forces. When it is certain that the inductance of an electrodyna- mometer is negligibly small, the instrument may be used in precise alternating electromotive force measurements. In order to deter- mine this it will be necessary to find the inductance error of the electro- dynamometer when used as a voltmeter. An electrodynamometer which has been calibrated by continuous electromotive forces in- dicates less than the effective value of an alternating electromotive force. Let E be the reading of an electrodynamometer voltmeter when an alternating electromotive force (harmonic), of which the effective value is E, is connected to its terminals. That is, E is the continuous electromotive force which gives the same deflection as E; and, since E gives the same deflection as E, it follows that the effective current produced by E is equal to the continuous current produced by E; that is JL- E R in which R is the total resistance of the instrument; L its inductance; and a> = 2nf, where / is the frequency of the alternating electromotive force. Solving the above equation for E, we have That is, the reading of the instrument must be multiplied by the factor - - - to give the true effective value of a harmonic H alternating electromotive force. Induction Instruments. Instruments suitable for alternating current only, depend upon the interaction of inducing and induced currents, and are commonly described as "induction" instruments. They are usually designed with a laminated iron core around which one or more coils of wire are wound. An alternating magnetic flux is produced in the air gap of this core when current passes through the coils. The effect of a rotating magnetic field is secured, either by having currents differing in phase pass through two groups of coils, or by using a single inducing coil with fixed copper plates or 76 ALTERNATING-CURRENT MACHINERY bands in which induced currents are generated. The resultant action of the flux due to the coil and that due to the induced currents in the fixed copper pieces, produces the effect of a rotating magnetic field. An aluminum disk or drum pivoted in the field tends to rotate with the rotating field. The motion of the drum or disk is opposed by a suitable spring in the case of indicating instruments, whereas retarding magnets and registering mechanism to read the total quantity that has passed through the meter are provided in the case of integrating instruments, such as the watt-hour meter. Indicating or direct-deflection induction meters are made in portable form for commercial testing, but are more generally used for switchboard instruments, including ammeters, voltmeters, watt- meters, frequency meters, and power factor meters. The advantages possessed by port- able induction indicating instruments are that they are not sensitive to external stray fields, that they have long (over 300) open scales, that the moving element is simple, light, and strong, and has no windings and, therefore, needs no provision for lead- ing current in and out. The disadvantages of this type of meter are that its readings tend to vary more or less with changes in fre- quency and temperature, although in instruments of the latest design these errors have been greatly reduced. These instrument's should be calibrated under conditions (espe- cially frequency) as nearly as possible like those under which they are to be used. The construction and principle of induction indicating instru- ments is illustrated by the "Type F" induction voltmeter made by the Westinghouse Electric Company for switchboard service, shown in Fig. 83 with its case and dial removed. The view shows the fine wire primary coils of a current transformer wound about the two legs of an inverted U-shaped structure of laminated iron. The primary coils are connected in series with a high-resistance wire of Fig. 83. Westinghouse Induction Voltmeter ALTERNATING-CURRENT MACHINERY 77 zero temperature coefficient, the object of which is to make the readings of the voltmeter more nearly independent of frequency. The secondary coil of the current transformer is wound directly under the primary coils and connected to the few turns of coarse wire marked "auxiliary coils," Fig. 83, on the pole pieces. The entire secondary circuit forms a closed winding. The poles constitute in effect a two-phase bipolar rotating field produced by two magnetic fluxes which are nearly at right angles to each other. One of these fluxes is due to the auxiliary coils carrying the secondary current and the other to the magnetizing component of the primary current. To vary the capacity of the meter, the number of primary turns and the size of wire, only, need to be changed. In the air gap between the poles is the moving element which is a very light aluminum drum mounted on a shaft having highly polished and hardened pivots rest- ing in polished sapphire jewels. This shaft also carries the indi- cating pointer and control spring which opposes the torque by the moving element. Type F voltmeters are made for circuits ranging in voltage from 150 up to 750 volts. For higher voltages, voltage transformers are required. Fig. 84 is a general view of an induction indicating ammeter also made by the Westinghouse Electric Company for switchboard service. It is identical in principle with the voltmeter above described, practically the only difference being in the primary coils of the current transformer which have fewer turns of larger wire. They are wound for 5 amperes and can be connected to circuits where smaller currents are to be measured and the voltage is less than 1,000 volts. When the current exceeds 5 amperes, series or current transformers are used, having their secondaries wound for 5 amperes. Wattmeter. A good wattmeter is the standard instrument for measuring power in alternating-current circuits. The wattmeter Fig. 84. Westinghouse Induction Indicating Ammeter 78 ALTERNATING-CURRENT MACHINERY is an electrodynamometer, of which one coil a, Fig. 85, made of fine wire, is connected to the terminals of the circuit CC, in which the power to be measured is expended. The other coil b, made of large wire, is connected in series with CC, as shown. The fine-wire coil a is movable, and carries the pointer which indicates on a divided scale the watts expended in CC. Such an instrument ivhen calibrated with continuous current and electromotive force indicates power accurately when used with alternating currents, provided the induc- tance of the circuit ar is small. This is true whatever the wave shape of the electromo- tive force or current, and whatever the ma/'n N| character of the receiving circuit CC. Fig. ss. Diagram of an Aiterna- Thus the circuit CC may have any react- Ci wai t tmet n e t r ain ance, as may be seen from the following discussion : A given deflection of the mov- able coil a depends upon a certain average or constant force action be- tween the coils. Consider a continuous electromotive force E, which produces a current in a, and a current C in CC and b. The force action between the coils is proportional to the product of the currents in a and b; that is, the force action is kX X C, where & is a constant. r Consider an^alternating electromotive force of which the instan- /> taneous value is e; this produces a current through a (provided the in- ductance of a is zero), and a current i in CC and b. The instantaneous p force action between the coils is kX ~Xi; and the average force action k is X average ei. If this alternating electromotive force gives the same deflection as the continuous electromotive force, then k k X aver age ei= EC or r r average ei= EC that is, the given deflection indicates the same power whether the currents are alternating or direct. ALTERNATING-CURRENT MACHINERY 79 Power Factor. Let P be the true power delivered to the circuit CC as measured by a wattmeter; let E be the effective electromotive force between the terminals of CC as measured by an alternating- current voltmeter; and let I be the effective current flowing in CC as measured by an alternating-current ammeter. P . Then the ratio is called the power factor of the circuit CC. hil Examples. 1. The primary coil of a certain 5-kilowatt transformer with secondary coil open-circuited takes 0.14 ampere from 1,000-volt mains; and the power as measured by a wattmeter is 100 watts. The power factor, therefore, is 100 watts 1,000 volts X 0.14 ampere 2. One of the stator circuits of a polyphase induction motor running unloaded has a power factor of 0.6. It takes 2 amperes from 200-volt mains. The true power, as would be indicated by a wattmeter, is P - 200 volts X 2 amperes X 0.6 = 240 watts Compensated Wattmeter. If a wattmeter is connected as shown in Fig. 85, it measures the power delivered to the circuit CC plus the power consumed in heating the circuit ar; hence the wattmeter reading is greater than the power delivered to the circuit CC. Again, if a wattmeter is connected as shown in Fig. 86, it meas- ures the power delivered to the circuit CC plus the power consumed in heating the coil b; hence, in this case also, the watt- meter reading is greater than the power delivered to the circuit CC. The compensated wattmeter, which is of the direct-deflection type, manufac- tured by the Weston Electrical Instru- ment Company, is designed to eliminate the above-mentioned sources of error in the following manner, the connections being shown in Fig. 85. Let C be the current in CC, and let a be the current in a and r. Then the current in b is C-\-a, and the force acting upon the movable coil is proportional to the product a (C4-a), instead of being proportional to the product aC. In the compensated wattmeter, the wire leading over to the coil a, con- ma/n ma/n Fig. 86. Diagram of Alternating Circuit Containing Wattmeter Improperly Connected 80 ALTERNATING-CURRENT MACHINERY nected as shown, is laid alongside each and every turn of wire in coil b. Then current C-\-a flows down through b; current a flows back alongside the wire of coil b; and the result is the same as if the current a were subtracted from the current (7+ a, so far as the mag- netic action of the coil b is concerned. Portable Torsion Wattmeter. A portable wattmeter of the torsion type manufactured by the Roller-Smith Company, Figs. 87 and 88, is of the electrodynamometer type but without the mercury cups of Fig. 87. Roller-Smith Portable Torsion Wattmeter the Siemens instrument, the current being led into and out of the fine wire movable (voltage) coil through the springs s and Si. Like the Siemens instrument, it is operated by rotating a torsion head through the nut k, Fig. 88, until the tension of the controlling springs s and Si balances the torque of the movable element a. The attain- ment of a balance between the two opposing forces is indicated when the needle h attached to the moving element registers with the zero mark on the scale. The amount of torsion required to bring the needle h back to zero is indicated by the scale reading of the index pointer e, which is secured to the torsion head j. Referring to Fig. 88, the terminals for the current (fixed) coils c are shown at p, and those for the voltage (movable) coil a at p\. ALTERNATING-CURRENT MACHINERY 81 The current coils a are clamped to and supported by a metal frame which is built up in sections, each section being carefully screwed and locked together, and the joints insulated to prevent the forma- tion of eddy currents in the frame. The moving or voltage coil a is clamped to and supported by the staff g. The lower controlling spring *i, is fastened to an arm which is rotated by the torsion head j by means of the rubber covered nut k projecting through the glass top of the wattmeter. The spring Si has its outer convolution secured to a lever which projects through the side of the case. By adjusting this lever, correction to zero may be made. The staff g is of brass with hardened and polished pivots resting in polished sapphire bear- ings. The improved construction of the torsion head ball bearing shown at / requires only the slightest turning effort, and unevenness Fig. 88. Vertical Section of Roller-Smith Portable Torsion Wattmeter in manipulation is obviated. Vibrations of the movable element are dampened by the vane which is enclosed in the nearly air-tight box b. On the side opposite the dampening vane and just counter- balancing its weight is the needle h which passes up through a slot in the brass scale pan and indicates the position of the moving system on the dial. The scales of these instruments are about 10 inches long covering an arc of over 300 degrees, a feature which is favorable to accuracy of reading. Another advantage of this type of wattmeter is that the movable element is always brought back to the same position with respect to the field produced by the fixed coils when readings are being taken. This feature eliminates the error caused by a varying angle between the movable and fixed coils, and is of special importance when measuring power in circuits having different power factors. 82 ALTERNATING-CURRENT MACHINERY A short scale shown at the top of Fig. 87 is drawn on either side of the zero line for the needle h. It is used when measuring the power of a fluctuating load and indicates the watts to be added or sub- tracted from the watts shown by the index pointer e. These torsion-type wattmeters are made for standard maximum voltages of 150, 300, and 600 volts. For higher voltages, multipliers (large non-inductive resistances) to be connected in series with the voltage coil of the wattmeter are furnished. These wattmeters are made either for one current or for two different currents. The two-current instruments are furnished with binding posts arranged for connecting the current coils either in series or in parallel. A torsion-type polyphase wattmeter, also made by the Roller- Smith Company, consists of two instrument mechanisms super- imposed and having the two movable coils secured to a common staff; it gives accurate readings on two- or three-phase circuits, whether the load is balanced or not, and is equally useful on single- phase and on three-wire circuits. INTEGRATING INSTRUMENTS The watt-hour meter is an instrument for summing up (inte- grating) the total electrical work or energy expended in a circuit in a given time. It registers on suitable dials the energy in terms of kilowatt hours. These meters are often referred to as "recording wattmeters," though incorrectly, for they are not wattmeters, nor do they record. Commercial watt-hour meters may be divided into two classes, according to the principle they employ: (a) elec- trodynamometer type, (b) induction type. They are all motor meters. Some are adapted to operate on direct current only, some on alternating current only (the induction type), and others on either kind of current. Thomson Watt=Hour Meter. The Thomson watt-hour meter, made by the General Electric Company, is a small electric motor without iron, the field and armature coils of which constitute an electrodynamometer. It is furnished with a small commutator and brushes, and though it may be used on both direct- and alternating- current circuits, it is principally used for direct-current work. For alternating currents the induction type of meter is usually employed. The field coils BB of this motor, Fig. 89, are connected in ALTERNATING-CURRENT MACHINERY 83 series with the circuit CC, in which the work to be measured is expended. The armature A, together with an auxiliary non- inductive resistance R, is connected between the terminals of the circuit CC, as shown. Current is led into the armature by means of the brushes dd pressing on a small silver commutator e. The stationary and movable coils are connected to the mains and receiving circuit, that is, to the load, exactly as are the stationary and movable coils of the indicating wattmeter; and the torque which the stationary (field) coils exert upon the movable coil (armature) is proportional to the watts delivered; that is, is proportional to the rate at which energy is being delivered to the receiving circuit. The instrument is so constructed that the speed of its armature is proportional to the torque that drives it. Therefore, the rate of Fig. 89. Diagrammatic View of Thomson Watt-Hour Meter turning of the armature is proportional to the rate at which energy is delivered to the circuit CC. Hence, the total number of revolu- tions turned by the armature in a given time is proportional to the total energy expended in the circuit CC. To make the armature speed proportional to the driving torque, the armature is mounted so as to be free, as nearly as possible, from ordinary friction; and an aluminum disk /, Fig. 89, is mounted on the armature spindle so as to rotate between the poles of permanent steel magnets MM. To drive such a disk requires a driving torque proportional to its speed. 84 ALTERNATING-CURRENT MACHINERY Starting Coil. In the above discussion it is assumed that the .orque which opposes the motion of the armature A, Fig. 89, is proportional to the speed of the armature. In fact, however, this opposing torque may be considered as consisting of two parts: (1) the torque required to overcome friction; and (2) the torque required to overcome the damping action of the magnets on the aluminum disk. The first part of the torque may be taken to be approximately con- stant, while the second part is accurately proportional to the speed. Therefore, an arrangement for exerting on the armature a constant torque, sufficient to overcome friction, would largely eliminate errors due to friction. This is accomplished in the Thomson meter by supplementing the field coils B, Fig. 89, with an auxiliary field Fig. 90. General View of Thomson Watt-Hour Meter coil, called a starting coil, connected in the armature circuit. So long as the electromotive force between the mains does not vary, the current in the starting coil is constant and it, therefore, exerts a constant torque upon the armature. If, however, the electromotive force between the mains varies, the torque, due to the starting coil, varies with the square of the electromotive force. Example. A certain watt-hour meter will not run, even if started by a slight impulse from the hand, until the delivered power reaches 37.5 watts; that is, the running friction of the watt-hour meter is equal to the driving torque produced by 37.5 watts of delivered power. This meter is adjusted (by moving the damping magnets) so as to read correctly when the delivered ALTERNATING-CURRENT MACHINERY 85 s^wwvnL b Res/sfance LA I 1 ; Line Load Fig. 91. Thomson Watt-Hour Meter Con- nected to a Two- Wire Current Supply power is 500 watts. What will the instrument indicate when run for four hours with constant delivery of 200 watts of power? Solution. Express driving torque in terms of watts of power delivered to the receiving circuit, since the driving torque is proportional to the watts delivered. Let us express speed in terms of watt hours indicated by the dials per hour. Now, the speed is proportional to that part of the driv- ing torque which is used to over- come the retarding action of the damping magnets. In the problem under consideration, the total driving torques are 500 watts and 200 watts, respectively; and since the running friction absorbs 37 watts of torque, the torques available for overcoming the retarding action of the damping magnets are, respectively, (50037^) watts and (200 -37) watts. The speed in the first case is 500 watt hours per hour. Let x be the speed (watt hours indicated per hour) in the second case. Then (500 -37*) : (200 -37) : : 500 : x This gives a value of 175.6 watt hours per hour for x, so that the watt hours recorded in four hours run will be 4X175.6 watt hours per hour, or 702.4 watt hours. The actual total of watt hours delivered is of course 4 hours X 200 watts, or 800 watt hours. The effect of running friction in a watt-hour meter is to cause the in- strument to read low for an amount of delivered power that is less than the delivered power for which the damping magnets are adjusted to give a correct reading. Fig. 90 shows a general view of a Thomson watt-hour meter. Fig. 91 shows the connections of a Thomson watt-hour meter to two-wire supply mains, and Fig. 92 shows the connections to three-wire supply mains. Induction Watt=hour Meter. The single-phase induction watt- hour meter has many advantages over the type using commutator and brushes, and in one form or another is very extensively used in Fig. 92. Thomson Watt-Hour Meter Con- nected to a Three-Wire Current Supply 86 ALTERNATING-CURRENT MACHINERY measuring electrical energy. In principle and operation it is merely a small single-phase induction motor having stationary shunt and series windings so related and arranged with respect to the lami- nated iron core as to produce a rotating field, which acting upon a closed movable secondary causes it to rotate. The shunt or volt- age winding, consisting of a large number of turns of fine wire wound on a laminated iron core, has a high inductance and, therefore, the current in it lags about 90 degrees behind the impressed line volt- age. The series, or current, winding composed of but a few turns of coarse wire, has a very low resistance and inductance and, there- fore, the current in it will be in phase with the impressed voltage, if the circuit in which the en ,rgy is being measured, is non-inductive. Thus the field produced by the shunt winding will lag approxi- mately 90 degrees behind that of the series winding, on a non- inductive load. When, therefore, the alternating current in the series coil has its maximum value, the current in the shunt coil has its minimum value. If it were not for the iron core loss and copper (PR) loss in the shunt coil, the angle of lag would be exactly 90. In commercial meters provision is made for making this angle 90, whatever the power factor of the circuit may be. The strength of the rotating field flux, that is, the resultant produced by the series and shunt coils together, assuming a lag of exactly 90, is proportional to the product of the currents in the two coils and, therefore, proportional to the product of the current and the voltage in the circuit being measured. At any power factor less than unity, the resultant field flux is proportional to this product multiplied by the sine of the angle of phase difference between the two meter currents. If the current in the voltage coil is exactly in quadrature with the voltage of the metered circuit, at any power factor the sine of the angle of phase difference between the currents in the meter circuits will be equal to the cosine of the angular dis- placement between the current and voltage in the metered circuit. Under these conditions, therefore, the strength of the shifting field is proportional also to the power factor of the circuit. In other words, the strength of the rotating field is proportional to the product of the volts, amperes, and power factor and is, therefore, a measure of the actual power. Single-Phase. A view of a typical single-phase watt-hour ALTERNATING-CURRENT MACHINERY 87 meter of the induction type manufactured by the Westinghouse Electric Company is shown with its cover removed in Fig. 93, and with its case removed in Fig. 94. The shape of its electromagnetic circuit is shown diagrammatically in Fig. 95. That the resultant field produced by the shunt and series coils is actually a shifting or rotating one may be seen by a careful study of Figs. 95 and 96. TERMINALS TERMINAL BUSHINGS DIAL AND REGISTERING MECHANISM \ RETARDING MAGNETS MAGNET CLAMP DISK . TERMINAL INSULATING PAD POWER FACTOR ADJUSTMENT MAGNET CLAMPING SCREWS A MAGNET SUPPORTING AND ADJUSTING FRAME ;LAMPING >CREWSB LIGHT LOAD ADJUSTING SCREWS C Fig. 93. Typical Single-Phase Westinghouse Watt-Hour Meter The dotted lines in Fig. 95 show the main paths of the magnetic flux produced by the two field windings, but the direction of the fluxes are constantly reversing owing to the alternations of the cur- rent in the coils. Referring to the shunt and series pole tips by the letters used in Fig. 95 the relation of the field fluxes at each quarter period (cycle) may be followed with the help of Fig. 96. The signs + and represent the instantaneous values of the poles indicated. 88 ALTERNATING-CURRENT MACHINERY Thus, at one instant the shunt pole tips A, C, and A' are maximum +> , and +, respectively, because the instantaneous value of the current is maximum, while the value of the series flux is zero. At J period later the shunt current is zero, giving zero magnetic poten- tial at the pole tips, while the series current has reached a maximum value, giving maximum and + at the pole tips B and Z). At the next J period the shunt current is again maximum but in a di- rection opposite to what it was at the beginning, making the pole tips A, C, and A' , +, and , respectively, while the series current TOP BEARING SC-REW .SERIES COILS REGISTERING MECHANISM DISK POWER FACTOR ADJUSTMENT LIGHT LOAD ADJUSTMENT LOOPS RETARDfNG MAGNET MOUNTING FRAME MAGNET CLAMPING SCREWS A SHUNT COIL LAMINATED ELECTRO MAGNET CORE Fig. 94. WestinghouselSingle-Phase Watt-Hour Meter with Case Removed j-gain is zero. Continuing, the other relations of + and poles shown in Fig. 96 are obtained. It will ,be observed from the table that both the + and signs move constantly in the direction from A' to A, indicating a shifting of the field in this direction, the process being repeated during each cycle. The losses in the meter coils are very low, particularly when compared with the high torque developed. The potential coils ALTERNATING-CURRENT MACHINERY 89 K / ^ J--T ^___|_l ^r "/" \ ** - x ^ .-' SHI/NT ELEMENT Fig. 95. Diagram of Electromagnetic Circuit of Westinghouse Single-Phase Watt- Hour Meter have a loss of 1.4 to 1.8 watts per phase and the series coil pro- duces a drop of only 0.4 volt at full load. The moving element consists of a thin aluminum disk which rotates about a vertical shaft in the air gap in which the rota- ting magnetic flux is produced. The disk acts like the squirrel- cage rotor (armature) of an in- duction motor. Currents are in- duced in it which combine with the rotating field to produce a torque proportional to the power in the circuit. This torque is counterbalanced by that due to the generator action of the permanent retarding magnets in inducing currents in the disk, so that the speed is exactly proportional to the torque. In this meter the disk performs two distinct functions. It serves not only as the armature of a motor, but also as the retarding or gen- erator element, the disk be- ing rotated by the meter field at one edge while it is retarded by the permanent magnet field at the opposite edge. The power factor adjust- ment, Fig. 94, consists of a short-circuited loop enclos- ing part or all of the shunt START / PERIOD / PERIOD - PERIOD fi/LL PERIOD, O - O -' Y field flux. The flux induces a current in the loop which, acting with the current in Fig. 96. the shunt coil, produces a slightly lagging field. By shifting the position, or changing the Diagram of Relation betwen Field Fluxes for One Cycle 90 ALTERNATING-CURRENT MACHINERY resistance, of this loop, the lag may be so adjusted that the shunt field flux is exactly 90 degrees behind the voltage in phase. This ad- justment, however, makes the meter correct at or near one particular frequency only. This type of meter should be calibrated and adjusted at or near the frequency at which it is to be used. When such a meter has been adjusted to read correctly at or near some particular frequency, such as 60 cycles, it will read too low at higher frequencies, as is shown by the lowest of the curves in Fig. 97. /o/ 99 \\ \-\\\\\\\\\\\\\\\\\\ LOAD CUffVE AT CONSTANT VOLTAGE AMD FREQUENCY % f?GISTff AT/ON H_l_$ vo L TAGE CL/ffVE A T C0/ySTAMT L OAD AMD rffEQUEMCY " r * __^ h *--^. _ """"- 1 . -* - 2O 40 6O 8O /t O /?O /4O f/TQt/ffCY CL/ffYE AT CONSTANT L OAD AHD^ VOL TAGE * -~. 1 -. "^ \ "N ^ "s s^ 2O 4O 6O 80 /OO /c O /4O is Fig. 97. Curves Showing Per Cent of Variation in Registration of Single-Phase Watt- Hour Meter for Variation in Load, Voltage, and Frequency The three curves of Fig. 97 show graphically the per cent of varia- tion in registration of these meters due to any per cent of variation in load, voltage, and frequency. All meters of this type have some initial bearing friction which if not compensated for would require a portion of the driving torque of the meter to overcome it, and would result in no rotation at very small loads, and in general the meter registration would be too low at light loads. Compensation for initial friction especially when the meter is running on light loads is, therefore, important and, consists in providing a constant torque adjustable to the exact value of the ALTERNATING-CURRENT MACHINERY 91 frictional torque, and one entirely independent of the load on the meter. The compensating torque, moreover, must not be so great as to cause the moving element to rotate or "creep" when no current is passing in the series coil. LIGHT LOAD ADJUSTING SCREWS C POWER FACTOR^ ADJUSTMENT foR UPPER ELEMENT .LAMPING SCREWS B UPPER RETARDING MAGNETS MAGNET SUPPORTING .ANDAOJUSTINfi FRAME 5PPER ISK TERMINAL BUSHINGS LOWER DISK MAGNET CLAMR TERMINALS MOUNTING FRAME MAGNET CLAMPING SCREWSA.' CASE L Fig. LIGHT LOAD ADJUSTING SCREWS C' Westinghouse "Type C" Polyphase Induction Watt-Hour Meter Compensation for meter friction, or "light load adjustment," is made by slightly unbalancing the magnetic fluxes in the two limbs of the shunt magnetic circuit. To do this a short-circuited loop is placed in each air gap and by moving either of the two screws shown at C, Vig. 93, and at C", Fig. 98, the position of the loops may be changed, so that one loop will enclose and choke back more of the (,.)watt Hrs. MULTIPLY BY Watt HrsQ, No.l= 1,1 11,100 No.2= 999,900 O 1,000.000 100X)00 10,000 j No.3= 1,000,100 ipoo.ooo IOQOOO 10,000 10,000.000 ipoo o. No. 4= 9,999,500 ID 1,000,000 100.000 10.000 01 6, 10,000,000 1,000 ip o. No.6= 99,700 1,000,000 100POO 10,000 10,000,000. o O Na7= 9,912,100 1,000.000 looooo iaooo 10,000,000 .0 !. ipoo No.8= 9,928,000 IPOO.OOO 100,000 10,000 2 3-1 U3 10,000,000 ipoo No.9= 9,918,100 ipoo.ooo 100,000 10,000 10.0C 0,000 ipoo pj KP,5 -909,100 No.lO=9 t 928,300 Fig. 99. Illustrative Watt-Hour Meter Readings fluv ALTERNATING-CURRENT MACHINERY 93 flux than the other loop, and thus produce a slight torque in the disk. This slight torque depends only on the voltage which is prac- tically constant, and is entirely independent of the load current. Polyphase. The polyphase induction watt-hour meter is vir- tually two independent single-phase meter elements having a common shaft. Each meter element exerts a torque on the shaft and the combined torque is, therefore, proportional to the sum of the elec- trical energy in the two phases being measured, regardless of the phase relation between the phases. These polyphase meters whefT properly connected indicate the true energy in a polyphase system, whatever the degree of unbalance between the phases may be, and for any power factor. Fig. 98 shows a Westinghouse "Type C" polyphase induction watt-hour meter with covers removed. It is evident that it is prac- tically two single-phase watt-hour meters combined to make one instrument. Directions for Reading Watt=Hour Meter Dials. To read correctly the dial of a watt-hour meter, some care is necessary. The figures marked under or over a dial (1,000, 10,000, etc.), are the amounts recorded by a complete revolution of the hand; therefore, one division on a dial indicates one-tenth of the amount indicated above or below. A complete revolution of the first hand (the one to the extreme right) in No. 6, Fig. 99, for example, indicates 1,000, and moves the second hand one division of the second dial. The first hand, in the position given, indicates 700 not 7,000. In deciding on the reading of a hand, the hand before it (to the right) must be consulted. Unless the hand before it has reached or passed the 0, or in other words completed a revolution, the other has not completed the division on which it may appear to rest. For this reason, ease and rapidity are gained by reading a meter from right to left. For example, in No. 2, the first dial (the extreme right) reads 900. The second apparently indicates 0; but, since the first has not completed its revolution, but indicates only 9, the second cannot have completed its division; hence the second dial also in- dicates 9. The same is true of the hand of the third dial; the second, being 9, has not quite completed its revolution; so the third has not completed its division and, therefore, we again have 9. The same holds true of the hand of the fourth dial. The last hand (the extreme 94 ALTERNATING-CURRENT MACHINERY left) appears to rest on 1 ; but since the fourth is only 9, the last has not completed its division and, therefore, indicates 0. Putting the figures down from right to left, the total reading is 999,900, though one might erroneously read 1,999,900, making a mistake of 1,000,000 units. ***] The hands sometimes become slightly misplaced, as shown in Nos. 8, 9, and 10. In No. 8 we have on the first dial (the extreme right) 0; we, therefore, put down three zeros thus, 000. The hand of the second dial is misplaced ; for inasmuch as the first registers 0, Fig. /ff eo e4 ^8 32 36 40 44 48 5 SG 6O 64 68 THOUSANDS OF WA TT-HOVKS 100. Graphical Chart for Calculating Customers' Bills the second should rest exactly on a division; therefore, we know that it should have reached 8, making 8,000. The remaining hands are correct, and make a total of 9,928,000. In No. 9 the second hand is misplaced; for since the first in- dicates 1, the second should have just passed a division, and, as it is nearest to the 8, we know it should have just passed that figure. The remaining three hands are approximately correct. The total reading is 9,918,100. In No. 10 the second hand is bekmd its correct position. The total indication is 9,928,300. ALTERNATING-CURRENT MACHINERY 95 By carefully following these directions one will find little difficulty in reading a meter even if the hands become misplaced. The above directions apply to watt-hour meters of all kinds induction type as well as Thomson meters. Calculations of Customers* Bills. The consumption of energy in watt hours being known, the exact amount of the bill can at once be read from a price chart, one of which is shown in Fig. 100. Dollars and cents are read from the vertical line at the left, while the horizontal line at the bottom of the chart indicates thousands of watt hours. To determine the amount to be charged for, say 28,000 watt hours at 12J cents per 1,000 watt hours, follow the Fig. 101. High Potential Testing Transformer of the General Electric Company vertical line marked 28 to its intersection with the diagonal line marked 12J cents. From the intersection of these two lines, follow the horizontal line to the left, and find $3.50, which is the amount of the bill for 28,000 watt hours at 12J cents per 1,000 watt hours. In practice, of course, the price chart would be drawn to a large scale for accuracy in reading. Spark Gauge. The high electromotive forces used in break- down tests are usually measured by means of the spark gauge. This consists of an adjustable air gap, which is changed until the electromotive force to be measured is just able to strike across 96 ALTERNATING-CURRENT MACHINERY in the form of a spark. The electromotive force is then taken from empirical tables such as Table VIII, Part III, page 192, which are based upon previous measurements of the electromotive force required to strike across various widths of gap. In the spark gauge of the General Electric Company, the spark gap is between metal points, one of which is attached to a micrometer screw whereby the gap space may be adjusted and measured. The striking distance in any spark gauge varies greatly with the condition of the points. It is, therefore, necessary to see that the points are sharp and well polished before taking measurements. Fig. 101 is a general view of the high-potential testing trans- former of the General Electric Company. The spark gauge is shown mounted on top of the instrument. While being used, the gauge is protected by a cover. The function of this cover is to keep the ob- server's fingers from the dangerous high-voltage points of the gauge; and the cover, moreover, acts as a switch which automatically dis- connects the gauge from the high-voltage terminals of the trans- former when the cover is removed. ALTERNATING-CURRENT MACHINERY PART II ALTERNATORS - - Fundamental Equation of Alternator. The equation express- ing the effective electromotive force of an alternator in terms of the useful magnetic flux per pole, the number of poles, the number of armature conductors, and the speed of the armature, is called, from its importance in calculations in designing, the fundamental equation of the alternator. This equation is olts (29) in which E is the effective electromotive force of the alternator; K is what we shall call the electromotive force factor of the machine and its value depends upon the ratio of breadth of pole face to pole pitch, and upon the distribution of windings upon the armature core ; p is the number of poles of the field magnet; is the useful magnetic flux per pole, that is, the number of lines of ^ "^ ^S^s^ magnetic flux that cross the gap from \- ___ y> one pole into the armature; n is the \s ^N s/ /^ speed of the armature in revolutions per Fig. 102. Concentrated or Uni-coii second;andZis the total number of con- ductors on the surface of the armature. We shall discuss this equation for the simplest case first, that is, when the armature conductors are concentrated in one slot per pole. This type of winding is called a concentrated, or uni-coil, winding, illustrated in part in Fig. 102. A given conductor cuts p$ lines of force in passing all of the poles in one revolution, and since the armature makes n revolutions per second, the given conductor cuts np$> lines of force per second. Now, by definition, the cutting of one line of force per second induces Copyright 1912, by American School of Correspondence. 98 ALTERNATING-CURRENT MACHINERY in a conductor one c. g. s. (centimeter-gram-second) unit of electro- motive force. Therefore, there is an average of npfy c. g. s. units of electromotive force induced in one armature conductor; but since there are Z armature conductors in series between the col- lector rings, the average electromotive force between collector rings is Znp3> Znp3> c. g. s. units, or - - volts. 10 8 The factor by which the average electromotive force must be multiplied to give the effective electromotive force is called the form factor of the electromotive force curve of the alternator. There- fore, if K is this form factor, we have KZnp$> effective E= - volts 10 8 & Since = T, or Z= 2 T and since pn= 2f, this equation may be written ft so as to give the electromotive force (effective) of the alternator in terms of the number of turns T of wire on the armature, and of the frequency /, as follows : volts (30) For example, an alternator has 200 turns of wire on its arm- ature and 1,000,000 lines of magnetic flux from each field pole. It is run to give a frequency of 125 cycles per second. The value of the factor K is 1.11, assuming a sine-wave electromotive force curve and concentrated winding. The effective electromotive force of this" alternator, therefore, is 4X 1.11 X 10 6 X 125 X 200 . - = 1,110 volts Electromotive Force Factor K in Equation (29). When the magnetic flux in the air gap is distributed in the ideal way explained below and represented in Fig. 103, the factor K is called the phase constant of the winding. When the winding is concentrated, the factor K is called the form factor of the electromotive force curve. This factor K depends in general upon the manner in which the magnetic flux is distributed ALTERNATING-CURRENT MACHINERY 99 in the air gap, and upon the manner in which the armature windings are distributed around the armature. CASE 1 . When a harmonic electromotive force is induced in each turn of the armature winding. This is the case never fully realized Fig. 103. Ideal Distribution of Magnetic Flux Fig. 104. Armature Winding Diagram, Four Slots per Pole in practice when the magnetic flux-density that is, the field intensity in the gap space between the pole faces and the iron of the armature core is zero at the points a, Fig. 103, and when this field intensity increases to a maximum at c and at c' in such a manner that the field intensity at any point b is proportional to the sine of the angle /?. Consider an armature rotating in a magnetic field distributed in the ideal way above specified. Suppose the winding to be arranged in slots spaced as shown in Fig. 104, four slots per pole. Fig. 105 shows one group abed of these slots drawn to a larger scale. Two wires on the armature, at a distance apart equal to the distance between adjacent north poles, and subtending the angle q, Fig. 105, have induced in them two electro- motive forces. These electromotive forces are to be thought of as differing in phase by 360, because of the fact that the electromotive force in a given con- ductor passes through a complete cycle while the conductor moves from the center of a given north pole to the center of the next north pole. Fig. 105. Enlarged Section of_Fig. 104 100 ALTERNATING-CURRENT MACHINERY Therefore, the phase difference of the electromotive forces induced in the wires placed in slot a, and those induced in the wires placed in o slot 6, is - - X 360; or, in other words, this angle is: width of tooth -f- width of slot circumference of armature -J- number of pairs of poles The lines A and B in the clock diagram, Fig. 106, represent in magnitude and phase the electromotive forces induced in the wires in slots a and b, respectively, Fig. 104. Similarly, the lines C and D in Fig. 106* represent the electromotive forces induced in the wires in slots c and d, re- spectively, Fig. 104. If, now, the windings in the slots a, b, c, and Fig. 106. Clock Diagram of Induced E. M. F.'s for Fig. 105 d t Fig. 104, are as in practice connected in series, the total electromotive force produced by all the windings will be represented by the line E in Fig. 106. The line E is the closing side of the polygon of which the sides A, E', C", and D' are drawn respectively, parallel and equal to the electromotive force lines A, B, C, and D. The value of K, which, in the case of the ideally distributed field flux here considered, is called the phase F constant of the winding, is equal to the ratio ; that is, the value 4./1 of K is equal to the ratio of the length of the line E to four times the length of one of the lines A, B, C, or D. CASE 2. When a harmonic electromotive force is not induced in each turn of the armature winding. We shall discuss, (a) the case of a concentrated winding in which case K is simply the form factor of the electromotive force wave; and (b) the case of a distributed winding. (a) Fig. 107 shows a developed view of a four-pole alternator having four armature conductors a, b, c, and d, represented by the symbols and depending upon whether the induced electro- motive forces are directed towards or away from the reader, respect- ively. Of course, these four conductors are connected in series be- tween the collecting rings (not shown in the figure) and in tracing ALTERNATING-CURRENT MACHINERY 101 the circuit from one collecting ring to the other, one would pass down along conductor a, then across to conductor 6, up b, then across to conductor c, down c, then across to conductor d, up d, and then to the other collecting ring. Let time be reckoned from the instant when conductor a is in the position shown; and let time be plotted as abscissas, and suc- cessive values of the induced electromotive force as ordinates, in the diagram AB, Fig. 107. Suppose that the intensity of the magnetic field in the gap space between pole faces and armature core is uni- form, and that it terminates sharply at the pole tips, that is, that there is no spreading of the lines of force such as is shown in Fig. 103; as a matter of fact, however, the field always does spread. The arm- ature conductors move with uniform velocity to the right, and the ratio of breadth of pole face, 6 inches, to pole pitch, 10 inches, is .6. Then, during each cycle the duration of which is 20 units of time, the successive instantaneous values of the induced electromo- tive force are: constant, positive, and equal to E for 6 units of time; -*- 6"-* N .. - N S 6 unite motion r/me | | Ax/'s of time B \\Qun/te fime \ f/'me time Fig. 107. Development and E. M. F. Curve for a Four-Pole Alternator zero for 4 units of time; constant, negative, and equal to E for 6 units of time; and again zero for 4 units of time. The average value of the electromotive force during the first half cycle is, therefore, equal to (EX 6) +(0X4) 10 = .6 E volts The squares of the successive instantaneous values of the in- duced electromotive force are: constant, positive, and equal to E 2 for 6 units of time; zero for 4 units of time; and so on. The average value of the squares of the successive instantaneous values of the bduced electromotive force during half a cycle is, therefore, equal to 102 ALTERNATING-CURRENT MACHINERY ( 2 X6)+(0 2 X4) 10 volts 2 = .6 E 2 volts 2 or effective value of induced electromotive force = V .6 X E volts The value of K (form factor), in this particular case of a con- centrated winding, is effective value _ 1/.6 E average value .6 E K 1.29 The value of K for a concentrated winding may be calculated, as in the above example, for any breadth of pole face. (b) The value of K, in the case of a distributed winding, is cal- Fig. 108. Development and E. M. F. Curve of Four-Pole Alternator with Two Conductors for Each Field Pole culated as follows, assuming, for the sake of clearness, a ratio of 12 pole breadth to pole pitch of = 0.6. Fig. 108 shows a four-pole alternator with two armature con- ductors a and b for each field pole. (Only two of these conductors are shown in the figure.) The curve of the electromotive force induced in all the a conductors is shown by the rectangular waves AB, This electromotive force has a constant, positive value E rt - p ALTERNATING-CURRENT MACHINERY 103 for 12 units of time; then a zero value for 8 units of time; then a constant, negative value E for 12 units of time; and so on. The curve of the electromotive force induced in all the b con- ductors is shown by the rectangular waves CD. This electromo- tive force rises from zero to the full value E at the instant when the conductors b are in the position of a, as shown in the figure; that is, the electromotive force induced in conductors b rises from zero to its full value E, 7 units of time before the corresponding rise of the electromotive force occurs in conductors a. The total electromotive force curve EF is found by adding corresponding ordinates of the curves AB and CD. A careful inspection and comparison of AB and CD shows that the total electromotive force of the alternator is zero for 1 unit of time, equal to E for 7 units of time, equal to 2E for 5 units of time, and equal to E for 7 units of time, during each half cycle of 20 units of time. Therefore, the average value of the electromotive force EF during half a cycle is (OX1)+(X7)+(2X5)+(EX7) 24 Referring to the curve EF, Fig. 108, it is evident that the squares of the successive instantaneous values of the total electro- motive force of the machine are as follows: O 2 for one unit of time, E 2 for seven units of time, (2E) 2 for five units of time, and E 2 for seven units of time, during each half cycle of twenty units of time. Therefore, the average value of the squares of the electromotive force EF during half a cycle, is (0 2 X 1) + (E 2 X 7) + (4E 2 X 5) + (# 2 X 7) _ 34 /\ Hi and the effective value of the electromotive force EF is |ix = 1.304 E \ 20 The value of K for the special. case under consideration, as shown in Fig. 108, is the ratio effective electromotive force _ 1 .304 _ average electromotive force 1.2 104 ALTERNATING-CURRENT MACHINERY which is simply the form factor of the electromotive force curve of the alternator. The form factor of a sine wave electromotive force has already been shown to be 1.11. NOTE. The factor K is the same thing as form factor whenever the distance ab, Fig. 108, between the remotest conductors of a group of con- ductors is less than the distance cd between the pole tips, on the assumption that the magnetic lines of force do not spread into the spaces between the pole tips. Armature Reaction. The current that circulates in an alter- nator armature has magnetizing action; and the actual useful flux per pole is due to the combined magnetizing action of the field coils and of the armature coils. This magnetizing action of the armature current with respect to its effect upon the useful flux cp, is called armature reaction. In case the current in the arma- ture lags behind the electromotive force when, for example, the outside receiving circuit has inductance, as when the alternator supplies current to induction motors the effect of armature reaction is to reduce the useful flux < from each pole. In case the current in the armature is ahead of the electromotive force in phase (a condition that obtains when the alter- nator supplies current to an over-excited synchronous motor or to any receiving apparatus over a long transmission line, or, in general, when the receiving appa- ratus acts like a condenser), the effect of armature reaction is to increase the useful flux from each pole. To state the matter in another way, it may be said that the effect of a lagging armature current is to oppose the magnetizing action of the field coils. On the other hand, the effect of a leading current in the armature is to help the magnetizing action of the field coils. In an alternator the invisible variations in phase difference between the armature current and the electromotive force, due to the varying character of the receiving apparatus, correspond, in their influence on armature reaction, to the visible variations in the position of the brushes of a direct-current generator. Fig. 109. Portion of Armature and Field Coil for Single- Phase Alternator ALTERNATING-CURRENT MACHINERY 105 Fig. 109 represents a single-phase alternator of the revolving armature type running in the direction indicated by the curved arrow. The electromotive force induced in the armature coil A is zero at the instant when the armature is in the position shown; and if the armature current is in phase with the induced electromotive force, the current also will be zero at this instant. In considering armature reaction we shall discuss three cases, as follows : 1. Armature current in phase with the electromotive force 2. Armature current lagging behind the electromotive force. 3. Armature current leading the electromotive force. CASE 1. As the armature tooth A, Fig. 109, passes by the field pole N, the current in the armature coil on A is reversed ii. direction. If the current is in phase with the electromotive force induced in the armature, this reversal of direction of current occur: at the instant when the tooth A is squarely under N. In thi case the armature current flowing in coil A just previous to the reversal, that is, when the tooth A is approaching N, opposes by its magnetizing action the flux from N; and after reversal the cur- rent in the coil A helps the flux from N. Therefore, the former or demagnetizing action of coil A is balanced by the subsequent magnetizing action; and the only effect of the armature current in A is to weaken the one side of the pole N, and to strengthen to an equal extent the other side, thus leaving the useful flux <$ un- changed. CASE 2. When the current lags behind the electromotive force, the reversal of current in the coil A occurs at a later instant than in Case 1, that is, when the coil A has passed beyond the center of the pole N. Hence, the demagnetizing action of the cur- rent in coil A before reversal lasts for a longer time than the mag- netizing action of the current in coil A after reversal, and the de- magnetizing action exceeds the magnetizing action. Therefore, the resultant effect of the armature reaction is to decrease the use- ful flux <|> from the pole N. CASE 3. When the current is in advance of the electromotive force in phase, the reversal of current in the coil A occurs at an instant earlier than in Case 1, that is, before the coil A has reached the center of the pole N. Hence, the demagnetizing action of the 106 ALTERNATING-CURRENT MACHINERY current in coil A before reversal lasts for a shorter time than the magnetizing action of the current in coil A after reversal, and the magnetizing action exceeds the demagnetizing action. Therefore, the resultant effect of the armature reaction is to increase the use- ful flux <|> from the pole N. Armature Inductance. The value of the inductance of an alternator armature varies with the position of the armature coils with respect to the field-magnet poles, so that the inductance of an armature increases and decreases at a frequency twice* as great as the frequency of the electromotive force of the alternator. It is helpful to remember that inductance is proportional to the product of the magnetic flux into the number of turns threaded by this flux, divided by the amperes passing through the turns. The armature of the alternator shown in Fig. 1, page 2, for example, has about three or four times as much inductance when the arma- ture teeth are squarely under the field poles as it has when the ar- mature teeth are midway between field poles. That is, the mag- netic flux produced through the armature teeth by a given current is three or four times as great in the first case as in the second case. This fluctuation of armature inductance makes it very difficult to predetermine the electromotive force and, in general, the be- havior of a machine. In the following discussion the armature inductance is assumed to be constant. The inductance of an alternator armature is proportional to the linear dimensions of the armature, other things being equal; and the inductance of an armature of given size and given total number of turns is much greater when the winding is concentrated than it is when the winding is distributed. A moderate amount of armature inductance is advantageous in alternators which are to be run in parallel; and in case of a short- circuit, the armature inductance keeps the current from becoming excessive. On the other hand, armature inductance is more or less ob- jectionable in an alternator which is to be used to supply current at constant electromotive force, on account of the electromotive force that is lost in the armature. *The electromotive forde of an alternator passes through a cycle when an armature coil passes from a north pole of the field to the next north pole. The inductance passes through a cycle of values when an armature coil passes from one field pole to the next field pole. ALTERNATING-CURRENT MACHINERY 107 The inductance of an armature is best determined by sending through it when at rest, from an outside source, a measured alter- nating current 7, and measuring the electromotive force E (volts drop) between the collecting rings. Then or, solving for L, we have 1 L = l/E 2 - (IR)* (31) Jco Knowing the armature resistance and the frequency , we can find L from equation (31). The value of L thus calculated depends greatly upon the position in which the armature is held, as explained above, and also upon the degree of field excitation. For example, the armature of a certain single-phase alternator has a resistance of 0.2 ohm measured between collector rings. An electromotive force of 100 volts at a frequency of 125 cycles per second (&>=785 radians per second) applied to the collecting rings of the armature at rest, produces an effective current of 100 am- peres. Therefore, the inductance of the armature, as calculated by use of equation (31), is L= l 100785 Electromotive Force Lost in Armature Drop. The electro- motive force between the collecting rings of an alternator with given load, is less than the electromotive force between rings at zero load, with given field excitation, because of two electromo- tive force losses that occur, and because of the effect of armature reaction. (a) The loss of electromotive force, or the drop, is due, in the first place, to the resistance of the armature. This loss is equal to IR; it is in phase with 7; and it is precisely analogous to the electromotive force lost in a direct-current armature due to the resistance of the armature. This IR drop is of relatively small value and importance. (b) The loss of electromotive force, or the drop, is due, secondly, to the inductance of the armature. This loss is equal to ajLI, and it is 90 degrees ahead of 7 in phase. 108 ALTERNATING-CURRENT MACHINERY (c) The demagnetizing action of the armature current on the field lessens the useful flux, and thus indirectly causes a falling-off in the induced electromotive force. The result of the actions (b) and (c) above is to cause a loss of electromotive force in the armature of the same character in each case in so far as phase relations with current are concerned. There- fore, it is convenient to attribute the total effect of (b) and (c) to a fictitious armature inductance L f , which is, of course, larger in value than the armature inductance L in equation (31). The inductance reactance coL f corresponding to this equivalent induct- ance L', is called the synchronous reactance of the armature. Alternator Regulation. Given an alternator, having constant field excitation. It has a certain electromotive force between col- lecting rings when its current output is zero. As the current out- put increases, the electromotive force between collector rings gen- erally decreases, because of the actions already described; and, conversely, as the current output decreases, the terminal electro- motive force rises. The increase of electromotive force from full load to zero load, with constant full-load field excitation and con- stant speed of driving, expressed as a percentage of the full-load terminal electromotive force, is called regulation of the alternator. For example, a certain alternator gives 1,100 volts between its collector rings at full-load current and full-load field excitation. When the current output is decreased to zero by opening the main switch, leaving the field excitation and speed unchanged, the ter- minal electromotive force rises to 1,166 volts. The regulation is, therefore, 1,166- 1,100 1,100 X 100 =6 per cent The regulation of a given alternator varies greatly with the character of the receiving circuit to which it delivers current. When the receiving circuit has large inductance reactance (as in the case of under-loaded transformers and induction motors), the terminal electromotive force, under increasing load, falls off very much more than when the receiving circuit is non-inductive (as,' for example, when the receiving circuit consists of incandescent lamps supplied through fully loaded transformers). In other words, the regulation of an alternator is larger (i. e., poorer) for in- ALTERNATING-CURRENT MACHINERY 109 ductive receiving circuits than for non-inductive receiving circuits. If the receiving circuit has large capacity reactance (as, for example, when the receiving circuit consists of over-excited synchronous motors), the terminal electromotive force of the alternator will rise with an increase of the current output; and the regulation of the alternator will be negative. In practice, the receiving circuit never as a whole has capacity reactance. For example, a given alternator having a regulation of 8 per cent on a non-inductive receiving circuit (unity power factor), has a regulation of about 21 per cent on an inductive receiving circuit having a power factor of 0.9 and a regulation of about 26 per cent at a power factor of 0.8 (lagging). Field Excitation. In most alternating-current systems the voltage at the points from which current is distributed is kept con- stant or approximately constant. This requires that the voltage at the terminals of the alternator be somewhat increased as the amount of current (or load) is increased, the amount of the increase in electromotive force depending on the volts lost in the line. If the field excitation of an alternator be kept constant while the current taken from the armature is increased, the voltage at the terminals will decrease, just as in the case of a direct-current shunt generator. Hence, in order to keep the voltage at the terminals constant, or to cause a rise of voltage with increasing current output, it becomes necessary to increase the field excitation with increasing current output. There are in general three methods in use for accomplishing this voltage control. (1) By varying the field excitation with the load. (a) Through control of the field-exciting current of the alter- nator by a rheostat operated either by hand or automatically from the alternator. (b) Through control of the exciter itself by the main current from the alternator with or without a rheostat. This is accomplished in one of the three following ways: first, compounding the exciter with rectified current supplied to its field circuit; second, by sup- plying the armature circuit of the exciter with alternating current from the alternator (compensated field method); and third, by an external regulator for varying the exciter field current by rapidly 110 ALTERNATING-CURRENT MACHINERY short-circuiting its field rheostat, the duration of the periods of short-circuit depending on the terminal voltage of the alternator (Tirrill regulator). (2) By interaction between the fields of the alternator and its exciter. This is Heyland's method which is used abroad but not in this country. Use is made of the stray flux from the alter- nator field which is arranged to strengthen the field of the exciter and thus obtain a compounding effect. The reaction on the field of the exciter is proportional to the armature reaction of the alter- nator and the terminal voltage of the exciter follows closely the variations in the load on the alternator. (3) By utilizing the magnetic flux due to the armature cur- rent so that the armature reaction of the alternator increases the total flux per pole, and thus increases the voltage of the alternator as the load in- creases. The exciting field current itself is not varied. This is Walker's method and it has been used by the Brit- ish Westinghouse Company. Of the above methods the first includes practically all that are used at present in this country. The tend- ency is to abandon the attempts to design alternators to be inherently self -regulating and to avoid as far as possible all special devices internal to the alterna- tor and its exciter for securing automatic voltage control, and to adopt instead an automatic regulator external to the alternator. Under method (1) will be described the three commonest ways of voltage control employed at present, namely, separate excitation; composite excitation, and the automatic regulator, external to the alternator. CASE!. Separate excitation. The simplest method is that illus- trated by the diagram, Fig. 110, in which A represents the armature winding, the terminals of which T\ and T< 2 are connected to the collector rings, which in turn are connected to the line wires through the brushes. Fig. 110. Diagram Showing Method of Separate Excitation ALTERNATING-CURRENT MACHINERY 111 The field of the alternator is excited by a set of coils on the pole pieces. These coils are represented by F; and current is sup- plied to these coils from a small direct-current dynamo E, called the exciter. This exciter is a small direct-current shunt-wound or compound-wound dynamo furnished with an adjustable rheostat r in series with its field /. An adjustable rheostat R is placed in the alternator field circuit also. When the electromotive force of the alternator decreases, its field may be strengthened by cutting out resistance in either R or r, or in both. Regulation by r alone is generally used in large machines, since the exciter's field current is relatively small, while the alter- nator field current is usually large and hence would cause a large I 2 R loss if passed through a rheostat. Separate excitation is still used in some of the older electric lighting stations. CASE 2. Composite excita- tion. The electromotive force of an alternator excited as in Case 1 falls off greatly with increasing current output ; and to counteract this tendency automatically, an auxiliary field excitation is some- times provided, which increases with the current output of the machine. For this purpose the whole or a portion of the current given out by the machine is recti- fied,* and sent through the auxiliary field coils. This arrangement is shown in Fig. 111. The field winding of the alternator has two sets of coils F and C. The coils F are separately excited as in Case 1. The coils C, known as the "series" or "composite coils," are excited by the main current from the alternator. . One terminal of the armature winding is connected directly to a collecting ring. The other arma- ture terminal connects to one set of alternate bars of the rectifying *Connections to field coils are reversed with every reversal of main current, so that, in the field coils, the current is uni-directional. Fig. 111. Diagram Showing Method of. Composite Excitation 112 ALTERNATING-CURRENT MACHINERY commutator B. From the rectifier the current is led through the winding C, thence back to the rectifier, and thence to the other collecting ring. The shunt S, within the commutator, may be used when it is desired to rectify only a part of the current. There is also a shunt S' which may be used to regulate the amount of current flowing through the coil C. The alternating-current rectifier is an arrangement for revers- ing the connections of the field circuit with each reversal of the current from the alternator, so that the current may flow always in the same direction in the field circuit. The rectifier is a commu- tator mounted on the armature ^shaft. This commutator has as many bars as there are poles on the field magnet of the alternator. These bars are wide, and are separated by quite narrow spaces filled with mica insulation. Let these bars be numbered in order around the commutator. The even-numbered bars are connect- ed together, and the odd-number- ed bars are connected together. The connecting wire leading from one terminal of the alternator armature to one of the collector rings is cut ; and the two ends thus formed are connected, one to the even-numbered bars (shown in full black in Fig. Ill) of the rec- tifying commutator, and the other to the odd-numbered bars (shown white in Fig. 111). The field circuit that is to receive the rectified current is connected to two brushes which rub on the rectifying commutator, these brushes being so spaced that one touches an odd-numbered bar when the other touches an even-numbered bar. The brushes are carried in a rocker arm, which is moved forwards or backwards until the brushes are passing from one bar to the next at the instant that the alternating current from the alternator is passing through the value zero. The proper adjustment of the brushes is indicated by a minimum of sparking. Fig. 112. Diagram Showing Composite! Excitation with Transformer ALTERNATING-CURRENT MACHINERY 113 Fig. 112 shows an alternator A with two sets of field coils F and C as before. One armature terminal is connected to a collect- Au*///ary Fie/cl fMMXlOQQOQOCXHHL -200O 20JOO- KOQ 1400 Cornmuta tor ^ Q pop QQ Series Transformer Armature A 2 B 2 Fig. 113. Two-Phase Alternator Diagram with Composite Field Excitation with Balanced Receiving Circuits ing ring; and the other armature terminal connects to the primary of a transformer T, and thence to the other collecting ring. The terminals of the secondary coil of T connect to the bars of the recti- fying commutator B, from which the composite field winding C is supplied. The transformer T is usually placed inside the armature. F/e/d 20 OQQQQOQQ Corn mut 01 tor Series Transformer Armature Q&flfi2Q Co//ecAor /T*//7< ABC Fig. 114. Three-Phase Alternator Diagram with Composite Field Excitation with B.alanced Receiving Circuits Composite field excitation is, however, not satisfactory in case 'of polyphase alternators, unless the receiving circuits supplied from the alternators are approximately balanced. Unbalancing of the receiv- 114 ALTERNATING-CURRENT MACHINERY ing circuits changes the electromotive forces generated in the dif- ferent phase windings of the armature, by different amounts; and composite excitation, applied to the magnetic field as a whole, cannot, properly, correct the different electromotive force variations of the several phases. In cases where the receiver circuits are approximately balanced, the current for the composite field excitation is taken through a rectifying commutator from the secondary coil of a series (or current) transformer which has two or three distinct primary coils, one for each phase. This arrangement applied to a two-phase alternator is shown in Fig. 113, and applied to a three-phase alternator in Fig. 1 14. The effect of the several primary coils on the series trans- former is to balance up the slight differences of the several poly- phase currents, in so far as their action upon the composite excita- tion is concerned. This method has been used by the Westinghouse Company in the case of alternators of small capacity. CASE 3. Automatic regulator. There are on the market a number of automatic devices which are designed to change the field strength of a generator in accordance with a change in generator voltage. The most successful of these devices is the Tirrill regulator manufactured by the General Electric Company. This regulator differs from other types of regulators in that it does not make use o? the principle of switching resistances in and out of the field circuit by the step-by-step method. The Tirrill regulator controls the gen- erator voltage by rapidly opening and closing a shunt circuit con- nected across the exciter field rheostat, the duration of such periods of short-circuit being varied automatically. The field rheostat is first turned in until the exciter voltage is much reduced and the regu- lator circuit is then closed. This short-circuits the rheostat through contacts in the regulator, causing the voltage of the exciter and the generator to immediately increase. At a predetermined point the regulator contacts are automatically opened which causes the field current of the exciter to again pass through the rheostat. The resulting decrease in voltage is quickly checked by another closing of the regulator contacts, which continue to vibrate to and fro thus keeping the generator voltage within the desired limits. Fig. 115 is a front view of a Tirrill regulator "form A2" designed for alternators having exciters of small capacities. Fig. 116 is a rear ALTERNATING-CURRENT MACHINERY 115 Fig. 115. Front View of Tirrill Regulator view of the regulator showing the resistance box and iron brackets for mounting it at the end of the switchboard if desired, although it is recommended that it be mounted di- rectly on the switchboard. A diagram of the electrical connections for a Tirrill regulator with an alternator and its exciter is shown in Fig. 117. The regulator has a direct-current control magnet, an alternating-current control magnet, and a relay. The direct-current control magnet is con- nected to the exciter bus bars. This magnet has a fixed stop-core in the bottom and a movable core in the top which is attached to a pivoted lever having at the opposite end a flexible contact pulled downward by four spiral springs. For clear- ness, however, only one spring is shown in the diagram. Opposite the direct-current control magnet is the alternating-current control magnet which has a potential winding connected by means of a potential transformer to the al- ternating-current generator or bus bars. There is an adjustable compensating winding on the alternating-current magnet con- nected through a current trans- former to the principal lighting feeder. The object of this wind- ing is to raise the voltage of the al- ternating-current bus bars as the load increases. The alternating- current control magnet has a movable core and a lever and con- tacts similar to those of the direct- Fig. 116. Back View of Tirrill Regulator current control magnet, and the two combined produce what is known as the "floating main contacts." 116 ALTERNATING-CURRENT MACHINERY The relay consists of a U-shaped magnet core having a differ- ential winding and a pivoted armature controlling the contacts which open and close the shunt circuit across the exciter field rheostat. One of the differential windings of the relay is permanently con- nected across the exciter bus bars and tends to keep the contacts open. The other winding is connected to the exciter bus bars through the floating main contacts and when the latter are closed neutralizes the effect of the first winding and allows the relay contacts to short- circuit the exciter field rheostat. Condensers are connected across the relay contacts to prevent severe arcing and possible injury. The cycle of operation is as follows: The circuit shunting the exciter field rheostat through the relay contacts is opened by means of a single-pole switch at the bottom of the regulator panel and the GSrtERATQR Fig. 117. Diagram of Electrical Connections for Tin-ill Regulator rheostat turned in until the alternating-current voltage is reduced 65 per cent below normal, which so weakens both of the control magnets that the floating main contacts are closed. This closes the relay circuit and demagnetizes the relay magnet, releasing the relay armature, and the spring closes the relay contacts. The single- pole switch is then closed and as the exciter field rheostat is short- circuited, the exciter voltage will at once rise and bring up the voltage of the alternator. This will strengthen the alternating-current and direct-current control magnets, and at the voltage for which the counterweight has been previously adjusted, the main contacts will open. The relay magnet will then attract its armature and by opening the shunt circuit at the relay contacts will throw the full resistance into the exciter field circuit tending to lower the exciter ALTERNATING-CURRENT MACHINERY 117 and the alternator voltage. The main contacts will then be again closed, the exciter field rheostat short-circuited through the relay contacts, and the cycle repeated. This operation is continued at a high rate of vibration due to the sensitiveness of the control magnets and maintains not a constant but a steady exciter voltage. One of the advantages of this regulator is that in controlling the voltage of the alternator by operating entirely on the field cir- cuit of the exciter, the heating losses are far smaller and the efficiency correspondingly higher than is the case with those regulators which operate directly on the alternator field. Another advantage is that several alternators may be operated in parallel using but one regulator, if all use the same exciter. On the other hand if, as is more usual, several exciters are used in parallel, one regulator and an equalizer rheostat for each additional exciter are necessary. If two or more exciters, not operating in parallel, are used, a separate regulator must be installed for each exciter. The Tirrill "form F" regulator is made for large installations, and is furnished with several relays varying from two to twelve, according to the size, the capacity, and the characteristics of the exciters used. While these "form F" regulators differ more or less in detail according to special conditions, the main features of operation are the same as in the "form A2" regulator. The standard voltage for exciters is 125 volts, and in some cases as high as 250 volts. Tirrill regulators are designed for a range of from 70 to 140 volts in the first case, and for a range of from 140 to 280 volts in the second case. With the growing use of these automatic regulators external to the alternator, it has been found desirable by manufacturers to minimize their efforts to design alternators of low inherent regula- tion, especially in the case of turbo-alternators and machines of large rated output. A low inherent regulation is today considered an expensive and unnecessary luxury, for by some sacrifice in this quality a relatively large gain in rated capacity becomes possible, and in many cases a higher efficiency. The advent of the automatic regulator has thus enabled de- signers to effect considerable improvement in alternators by relieving them of the troublesome question of low inherent regulation, and 118 ALTERNATING-CURRENT MACHINERY permitting them to give greater weight than ever to the important matters of increasing output and efficiency. POLYPHASE ALTERNATORS AND SYSTEMS SINGLE=PHASE SYSTEM Limitations. As long as alternating current was generated, transmitted, and used for electric lighting only, the single-phase system gave complete satisfaction, simplicity in the generating, transmitting, and receiving apparatus being its most striking and valuable feature. In the earlier days of the electric lighting industry, there was very little, if any, demand for current to operate motors for power purposes. Since that time, however, there has developed an ever- increasing demand for current for power purposes, fully equaling, if not exceeding, that for lighting work. With the advent of this new condition, the great obstacle to the use of the single-phase alternat- ing-current system became manifest. Single-phase constant-speed motors are difficult to make self-starting* under load, especially in units of large size; and hence the use of the single-phase system for general power purposes, with the apparatus now available, is not practicable. It was in 1888, in Italy, that Ferarris discovered the important principle of the production of a rotating magnetic field by means of two or more alternating currents displaced in phase from one another, and he thus made possible, by means of the induction motor, the use of polyphase currents for power purposes. The most important advantage of polyphase alternating currents over the simple single-phase system is that alternating-current motors can be satisfactorily operated by them. It was mainly the requirements of the induction motor that led to the development of the polyphase system, TWO=PHASE SYSTEM Two-Phase Alternator. The simplest form of polyphase gen- erator consists of two similar and independent single-phase armatures mounted rigidly on one and the same shaft, one beside the other, in such a manner that the electromotive forces at the terminals of *This statement does not include the single-phase series commutator motor which is especially adapted to railway motors. ALTERNATING-CURRENT MACHINERY 119 the respective armatures arrive at their maximum values 90 degrees, or one-fourth of a period, apart. The currents from such a machine are said to have a two-phase relationship. The two separate armatures are supposed to revolve inside the same crown of field magnet poles. Fig. 118 shows an end view of such an arrangement, but arma- ture B is here shown inside of armature A for the sake of clearness. As will be seen in the figure, armatures A and B are so mounted on the shaft that the slots of A are midway under the poles N, S when the slots of B are midway between the same poles. With this arrange- ment, the electromotive force generated in the armature coils of A and B are so related in their variations that the electromotive force of A is at its maximum when the electromotive force of B is zero. Or in other words, the two electromotive forces are 90 degrees apart in phase, or are in quadrature (at right-angles) to each other. A careful study of Fig. 118 will show that the electromotive forces induced in armatures A and B are 90 degrees apart in phase. Thus the figure shows the arma- ture A in the position in which its windings (in the slots) are cutting lines of magnetic flux from the field poles at a maximum rate, while the armature B is shown in the position in which its windings are midway between the field poles where they do not cut any magnetic flux at all. Therefore, the electro- motive force in the windings of the armature A is at its maximum value, while the electromotive force in the windings of armature B is zero at the same instant. That these electromotive forces generated in the windings of A and B differ in phase by 90 degrees, may also be shown as follows: The electromotive force generated by a con- ductor on armature A passes through a complete cycle of changes as it moves from the center of one north pole to the center of the next north pole. The interval between the centers of two adjacent north poles, a "double pole pitch," corresponds then to 360 "electrical" de- grees. Therefore, the phase (space) difference between the conductors Fig. 118. Diagram of Two-Phase Alter- nator with Two Separate Armatures 120 ALTERNATING-CURRENT MACHINERY on A and B is seen to be J of 360 degrees, or 90 electrical degrees. The two equal, but distinct and independent electromotive forces generated by such a two-phase alternator are generally used to supply two distinct and separate currents to two distinct and independent cir- cuits. When so used the system is called a two-phase, four-wire system. We shall see later that it is possible to in- terconnect the two circuits in such a manner that one of the four line wires may be omitted. In practice the actual two- phase alternator is constructed F &slparSe a Ka!fe^ h so C nr C - by placing both the armature windings of A and B upon one and the same armature core, instead of on separate cores. To accom- plish this the armature core has twice as many slots as either A or B in Fig. 118. Fig. 119 shows such an armature. The slots marked a\, a 2 , a 3 , etc., contain the conductors comprising phase A; whereas the slots marked 61, 6 2 , 6 3 , etc., contain the conductors comprising phase B. The A winding passes in slot ai from front to back of the armature core; then towards the reader (that is, from back to front) in slot a 2 ; then from front to back in a 3 , from back to front in a 4 , and so on. The various conductors in slots a\, a 2 , a 3 , etc., are joined in series by connectors (at front and back), and the two ends of the final series are connected to two collector rings. The B winding passes in slot 61, from front to back of the arma- ture core; then towards the reader, that is, from back to front, in slot 6 2 ; then from front to back in 6 3 . from back to front in 6 4 , and Fig. 120. Distributed or Multi-Coil Winding for Two-Phase Alternator ALTERNATING-CURRENT MACHINERY 121 ring \ main \ so on. The various conductors in slots &i, 6 2 , 6 3 , etc., are joined in series, and the two ends of the final series are connected to two collector rings, which rings are distinct from the pair of rings to which the A winding is connected. The armature windings A and B just described are of the concentrated or uni-coil type, page 100, having only one slot per pole for each winding, i. e., per phase. Distributed or multi-coil windings main 3 Fig. 121. Diagram of Collector Ring System for Two-Phase Alternator also are frequently used for two- phase alternators. Thus, Fig. 120 shows an end view of a portion of a two-phase armature with its A and B windings each distributed in two slots per pole. The coils belonging to winding A are lightly shaded, and those belonging to winding B are darkly shaded in the figure. The connections between the coils of the A winding are shown in the figure by the full lines, while the connections of the B winding are shown by the dotted lines. Two-phase alternators are usually provided with two sets of collector rings; one ring, however, may be made to serve as a com- mon connection for the two armature windings, as shown in Fig. 121. The lines A and B in the clock diagram, Fig. 122, represent the generator electromotive forces, a represents the current in main l,b represents the current in main 2, and c f which is the vector sum of a and b } represents the current in the common main 3. If a = 6, it is evident that c = Fig. 122. Clock Diagram of E. M. F.'s and Currents for Two-Phase Alternator THREE=PHASE SYSTEM Three = Phase Alternator. Consider three similar single- phase armatures A, B, and C, mounted side by side on the same shaft and revolved in the same field, each armature having as many slots as there are field poles . Fix the attention upon a certain ar- mature slot of A, and let time be reckoned from the instant that this slot is squarely under an TV' pole. Let t be the time which elapses as this 122 ALTERNATING-CURRENT MACHINERY Fig. 123. Clock Diagram of Three-Phase E.M.F. 'sand Currents armature slot passes from the center of one JV pole to the center of the next N pole. The armature B is to be so fixed to the shaft that its slots are squarely under the poles at the instant J t; and the armature C is to be so fixed that its slots are squarely under the poles at the in- stant f t. While a slot passes from the center of one N pole to the center of the next N pole, the electromotive force passes through one complete cycle. Hence, the electromotive forces given by three armatures arranged as above, will be 120 degrees apart in phase, as shown in Fig. 123, in which the lines A, B, and C represent the respective electromotive forces. The cur- rents given by the armatures to three similar receiving circuits lag equally behind the respective electromotive forces, and are repre- sented by the dotted lines a, b, and c. This combination of three single-phase alternators is called a three-phase alternator. In prac- tice the three distinct windings A, B, and C are placed upon one and the same armature body. For this purpose the armature core has three times as many slots as A, B, or C. Fig. 124 shows the arrange- ment of the slots for such a wind- ing. The slots belonging to phase A are drawn in heavy lines, and are marked ai, a 2 , etc. Those belonging to phase B are shown dotted, and those belonging to phase C are shown Fig. 124. Arrangement of Slots for Three- in light lines. The A winding Phase Alternator Armature , , 1,1 would pass up slot i, down a 2 , up a 3 , etc.; the B winding, up 61, down 6 2 , up 6 3 , etc.; and similarly for winding C. The windings A, B, and C here described are of the concen- trated type, having only one slot per pole for each winding. Dis- ALTERNATING-CURRENT MACHINERY 123 Fig. 125. Portion of Diagram of Windings for Three-Phase AlternatoV Armature tributed windings also are frequently used for three-phase alter- nators. Thus Fig. 125 shows a portion of a three-phase armature with its A, B, and C windings each distributed in two slots per pole. The coils belonging to windings A, B, and C, respectively, are differently shaded to distinguish them. The manner of connect- ing the coils of each winding is described on page 145. If the three circuits of a three-phase alternator are to be entirely independent, six collec- tor rings must be used, two for each winding; however, the cir- cuits may be kept practically independent by using four col- lector rings and four mains, as shown in Fig. 126. The main 4 serves as a common return wire for the independent currents, in mains 1, 2. and 3. When the three receiving circuits are equal in resist- ance and reactance, that is, when the system is balanced, the three currents are equal, and are 120 degrees apart in phase (each cur- rent lagging behind its electromotive force by the same amount as the others) ; and their sum is at each instant equal to zero. In this case, main 4> Fig. 126, carries no current. Therefore, main 4 an d the corresponding collector ring may be dispensed with, the three windings connected together at the point A 7 , called the common junction or neutral point. This arrangement, shown in the symmetrical diagram, Fig. 127, is called the "Y" or "star" scheme of connecting the three windings or phases .A, B, and C. Another scheme for connecting the three windings A, B, and C (also for balanced loads), called the "A" (delta) or "mesh" scheme, is illustrated in Fig. 128. Winding (or phase) A is connect- ed between rings 3 and 1; wind- ing (or phase) B between rings 1 and 2; and winding (or phase) C between rings 2 and 3. The direction in a circuit in which the electromotive force or current is considered as a positive electromotive force or current, Fig. 126. Collector Ring System for Three- Phase Alternator 124 ALTERNATING-CURRENT MACHINERY is called the positive direction through the circuit. This direction is chosen arbitrarily. The arrows in Figs. 127 and 128 indicate the positive directions in the mains and through the windings. It must be remembered that these arrows represent not the actual direc- tions of the electromotive forces or currents at any given instant, but merely the directions of positive electromotive forces or currents. Thus, in Fig. 127, the currents are considered positive when flowing from the common junction towards the collecting rings, and the currents are never all of the same sign. Y=Connected Armatures. Electromotive Force Relations. We shall consider the electromotive force between mains 1 and 2, Fig. 127, to be positive, when it tends to send current through a receiving circuit i from main 1 to main 2. Similarly, the electromotive force Fig. 127. The "Y" Scheme of Connecting Three Phases in Three-Phase Alternator Fig. 128. Diagram of "A" Scheme forlConnecting Phases in Three- Phase Alternator between mains 2 and 3 is considered positive, from main 2 to main 3; and the electromotive force between mains 1 and 3 is considered positive, from main 3 to main 1. Passing through the windings A and B from ring 2 to ring 1, Fig. 127 (which is the direction in which an electromotive force must be generated to give an electro- motive force acting upon a receiving circuit from main 1 to main 2), the winding A is passed through in the positive direction, and the winding B in the negative direction. Therefore, the electromotive force from main 1 to main 2 is AB. Similarly the electromotive force from main 2 to main 3 is B C, and the electromotive force from main 3 to main 1 is C A. These differences are shown in the clock diagram, Fig. 129. The electromotive force between mains 1 and 2 (namely, AB) is 30 degrees behind A in phase, and its effective ALTERNATING-CURRENT MACHINERY 125 value is 2E cos 30= V 3 E, where E is the common value of each of the electromotive forces A, B, and C. Similar statements hold concerning the electromotive forces between mains 2 and 3, and those between mains 3 and 1 . Hence, the electromotive force beticeen any pair of mains leading from a three-phase alternator with a Y '-connected armature is equal to the electromotive force generated per phase mul- tiplied by Vs. Current Relations. In the Y connection, the currents in the mains are equal to the currents in the respective windings or arma- ture phases, as is evident from Fig. 127. A=Connected Armatures. Electromotive Force Relations. In A-connected armatures the electromotive forces between the mains Fig. 129. Clock Diagram of E. M. F.s for Three-Phase "Y" Winding Fig. 130. Clock Diagram for Cur- rents in "A" Connected Three- Phase Armature or collector rings are equal to the electromotive forces of the respec- tive windings, as is evident from Fig. 128. Current Relations. Referring to Fig. 128, we see that a positive current in winding A produces a positive current in main 1, and that a negative current in winding B produces a positive current in main 1; therefore, the current in main 1 is a b, where a is the current in winding A, and b is the current in winding B. Similarly, the current in main 2 is bc, and the current in main 3 is c a. These differences are shown in Fig. 130. The current in main 1 (namely a b) is 30 degrees behind a in phase; and its effective value is V 37, where / is the common effective value of the currents, 126 ALTERNATING-CURRENT MACHINERY rrta/n a 'WOOOOO ma/n 3 Fig. 131. "Y" Method of Con- necting Receiving Circuits a, b, c, in the different phases. Similar statements hold for the currents in mains 2 and 3; so that the current in each main from a A-connected armature is V 3 times the current in each winding. Receiving Circuits to Three=Phase Mains. Dissimilar Circuits (Unbalanced System) . W T hen the receiv- ing circuits which take current from three-phase mains are dissimilar, that is, do not each have equal resistance and reactance, four mains should be employed, as indicated in Fig. 126; each receiv- ing circuit being connected from main 4 to one of the other mains. A com- mon example of an unbalanced system is where a mixed load of induction motors and incandescent lamps is connected unsymmetrically to three-phase receiving mains. It is, however, desirable to keep the three windings A, B, and C of the alternator almost equally loaded; and in practice the receiving cir- cuits are so disposed as to satisfy this condition as nearly as possible. Similar Circuits (Balanced System). When three-phase cur- rents are used to drive induction motors, synchronous motors, or rotary converters, each one of these machines takes current equally from the three mains; and since three-phase currents are utilized chiefly in the operation of the machines mentioned, the system is usually balanced. In this case three mains only are employed, and each receiving unit has three similar receiving circuits connected to the mains according to either the Y or the A method. The Y method of connecting receiving circuits is shown in Fig. 131. One terminal of each receiving circuit is connected to a main, and the other terminals are connected together at the neutral point N. In this case the current in each re- ceiving circuit is equal to the current in the main to which it is connected. The electromotive force between the termi- nals of each receiving circuit, as A, is equal to = , where E is the electromotive force between any pair ma/n \ nia/n Fig. 132. "A" Method of Connect- ing Receiving Circuits of mains. ALTERNATING-CURRENT MACHINERY TABLE HI A- and Y -Connection Data in Mains 127 A connection Y connection E. M. F. between Mains Current in Each Main Power Rating '*' VT/. The A method of connecting receiving circuits is shown in Fig. 132. Here the three receiving circuits are connected between the respective pairs of mains; the electromotive force acting on each receiving circuit is the electromotive force between the mains; and the current in each receiving circuit is ,- where I is the current in each main. * 3 Examples. 1. The three windings or phases of a three-phase induc- tion motor are Y-connected to three-phase mains. The voltage between mains is 500, and each main delivers 25 amperes to the motor. It is required to find the current in each phase of the motor, and the electromotive force acting on each phase of the motor. Solution. Since the windings are Y-connected, the current in each is the same as the current in each main, namely, 25 amperes; and the electro- 500 motive force acting on each phase of the motor winding is -^=. , or 288.7 volts. The power input is P = V 3 X 500 X 25 = 21650 watts. 3 2. The three phases of the above three-phase induction motor are A-con- nected to three-phase mains. The voltage between mains in 288.7, and the current in each main is 43.3 amperes. It is required to find the current in each phase of the motor, and the electromotive force acting on each phase of the motor. Solution. Since the windings are A-connected, the electromotive force acting on each phase is the same as the voltage between the mains, namely ^ 43 3 288.7 volts; and current in each phase of the motor is or 25 amperes _ V 3 The power input is P= V 3 X 288.7 X 43.3 = 21650 watts, the same as before. Summary of Electromotive Force and Current Relations for A and Y Connections. Let E w be the rated electromotive force of each winding, and /, the rated full-load current output of each phase of the winding of a three-phase alternator, then, for a generator with non-inductive load, the data is as given in Table III. Let E be the electromotive force between mains of a three- 128 ALTERNATING-CURRENT MACHINERY TABLE IV and Y=Connection Data in Receiving Circuits E. M. F. between Ter- minals of Each Re- ceiving Circuit Current in Each Receiving Circuit Total Power Input A connection E 7 V1T7?7 E VT Y connection. . _l~ 7 V 3 El "V3 phase system, and let / be the current in each main, then, for three receiving circuits, the data is as given in Table IV. The permissible power output or rating of a three-phase alter- nator is the same whether its armature windings are Y-connected or A-connected. The power output of a three-phase generator is V 3 X electro- motive force between mainsX current in one mainX power factor of the receiving circuits. MEASUREMENT OF POWER In alternating-current circuits, power cannot in general be measured by means of an ammeter and a voltmeter, as in the case of direct current, because the power expended is generally less than the product of effective electromotive force and effective current on account of the difference in phase between the electromotive force and the current. A well-designed wattmeter is the standard instrument for measuring power in alternating-current circuits, and the methods involving its use are the most generally satisfactory. NOTE. The discussion of the wattmeter given on page 78 applies pri- marily to the use of the wattmeter for the measurement of power delivered in single-phase systems. The several circuits of a polyphase system are often entirely sep- arate and independent; and in such cases the total power delivered to a receiving apparatus is found by measuring the power delivered to each separate receiving circuit. The total power delivered is the sum of the amounts delivered to the different receiving circuits. In order to measure the power delivered to one of the receiv- ing circuits of a polyphase system, the current coil of the watt- meter is to be connected in series with this receiving circuit, and ALTERNATING-CURRENT MACHINERY 129 the pressure (or voltage) coil of the wattmeter is to be connected between the terminals of this receiving circuit. In some cases this connection of the voltage coil cannot be made, because on ter- minal of the receiving circuit may be out of reach in th,e interior of the apparatus. Balanced Systems. In general, the several circuits which receive current and power from polyphase mains are more or less unlike in both resistance and reactance, and take different amounts of current and power from the mains. It is, however, desirable that the several receiving circuits be alike, so that they may take equal currents and equal amounts of power from the mains. When this condition is realized, the system is said to be balanced. For example, when independent groups of lamps are supplied from polyphase mains, each group taking current directly, or through a transformer, from one phase of the polyphase system, the system is said to be unbalanced when the number of lamps is not the same in the several groups. In general, the supply of power to several separate, independent, and unrelated receiving circuits, such as independent groups of lamps and single-phase motors, leads to the unbalancing of a polyphase system. Apparatus, such as polyphase induction motors, synchronous motors, and rotary converters which are especially designed to take power from polyphase mains, is always provided with two or three similar receiving circuits so as to take equal amounts of current and power from each phase of the system. When such polyphase apparatus takes power from polyphase mains only, the system is always very nearly balanced. If a polyphase system were exactly balanced it would be suffi- cient to measure the power delivered by one phase only; but since a balanced condition of a system is seldom exactly realized in prac- tice, there may be considerable error introduced by assuming that a system is balanced, and by calculating the total power from the wattmeter reading of power delivered by one phase only. In balanced or approximately balanced polyphase systems, the measurement of power by use of a single wattmeter is best accomplished by special arrangement of connections as follows: Three-Wire Two-Phase Systems. The current coil of the watt- meter should be connected in the middle main as shown in Fig. 133. After a reading is taken with the pressure coil connected between 130 ALTERNATING-CURRENT MACHINERY middle and lower mains, this connection is quickly changed to the upper main, as indicated by the dotted line, and the wattmeter again read. The sum of the two successive readings gives the total power. If this method is used, the system should not only be bal- anced, but no change in the load should occur between readings. 7C7S6 A rece/\ 7S But, from the figure, e' e"=e" f ; hence W'+W"= average (e' i')+average (e" i")+ average (e' n i'"} Although a formal proof of the principle of the two-wattmeter method has not been given for a Y-connected circuit, it is not neces- sary to show independently that it holds for both cases. A little consideration will show that if the electromotive forces acting be- tween the three wires, the currents flowing in them, and their phase relations are given, there is then a perfectly definite amount of power transmitted along the three lines, and it is quite immaterial whether this power is being delivered to circuits connected A or Y. ALTERNATING-CURRENT MACHINERY 135 ARMATURE WINDINGS In general, any direct-current armature winding may be used for the armature of an alternator; but the desirability of generating comparatively high voltages in the armature so as to avoid the use of step-up transformers, makes it necessary to abandon the styles of winding best suited to direct-current machines, and to use windings specially adapted to the conditions of alternating- current practice. Comparing the armature windings used for direct-current machines with those for alternators, we find, first, that all the re- entrant (or closed-coil) direct-current windings must necessarily be either two-circuit or multiple-circuit windings, i. e., they must have at least two paths in parallel through the armature between brushes; and second, that the armatures of alternators (and syn- chronous motors) may, and generally do, from practical considera- tions, have one-circuit windings, i. e., windings having one cir- cuit per phase. It follows, therefore, that any direct-current wind- ing may be used for alternating-current machines; but the con- verse statement, that any alternating-current winding may be used for direct-current machines, is not true in general. In other words, the windings of alternating-current armatures are essentially non- re-entrant (or open-circuit) windings. The only exceptions are the A-connected (or mesh-connected) polyphase windings, and the short-circuited windings of "squirrel-cage" induction motors, both of which are re-entrant (or closed-circuit) windings. The A-con- nected polyphase windings are, therefore, the only windings that can be used for the armatures of rotary converters. In the type of winding generally employed for alternators, a number of distinct coils are arranged on the armature; in these coils alternating electromotive forces are induced as they pass the field- magnet poles, and the several coils are connected in series between the collecting rings. Classification. According to Shape oj Core. Armatures for alternators, just as in the case of direct-current machines, may be divided into drum armatures; ring armatures; and disk armatures. Of these the ring and the disk armatures are seldom used in America although they are to some extent adopted in European practice. 136 ALTERNATING-CURRENT MACHINERY The ring and the disk types of the armature are mechanically less stable than the drum type; and the ring armature, moreover, other things being equal, requires more wire to be wound upon it for a given output than in the case of the drum armature, and pos- sesses, therefore, a greater inductance than the latter type. Drum armatures, whether the alternators are of the revolving or station- ary armature type, have laminated iron cores similar in construc- tion to the armature cores for direct-current machines. Disk armatures, on the other hand, are usually made up without iron, thus introducing constructional difficulties. According to Construction of Core. With reference to the con- struction of their cores, the armatures of alternators may be classi- fied, as in the case of direct-current machines, into smooth-core armatures and toothed-core armatures. In the smooth-core armature the conductors, arranged in flat coils, lie on the surface of the armature core, and the coils in some cases are bent down over the ends of the core, where they are fastened by end plates or by blocks of wood or fiber. The spaces in the centers of the coils are filled with wooden blocks either screwed to the cores or held in place by the binding wires. In other cases the coils are flat or "pancake" shaped, and of the same length as the armature core. In the latter case they are laid upon the cylin- drical surface of the armature core, and are securely bound with wire bands. The form of the wave of electromotive force produced by smooth-core armatures is very nearly harmonic (sinusoidal) or slightly flat-topped. The inductance of a smooth-core (or surface- wound) armature is considerably less than that of a toothed-core armature. Although much used in earlier designs, the smooth- core armatures owing largely to their comparatively weak mechan- ical structure, have been superseded in modern practice by the toothed-core constructions, One or another of the forms of toothed-core armature is now almost universally used in practice. The conductors are laid in slots or grooves, the sides and bottom of which are first carefully insulated by troughs of mica-canvas, micanite, or other suitable insulating material. The insulated conductors (cotton covered) are generally wound into coils on "formers," each coil being care- ALTERNATING-CURRENT MACHINERY 137 fully taped, and then impregnated with insulating compound (or varnish). The coils are then thoroughly dried by baking in ovens. The conductors being enclosed in slots between teeth which project more or less over the conductors, the toothed-core type is often called iron-clad. This construction has three great advantages over the smooth-core type: (a) It allows the length of air gap from iron of pole-face to iron of arma- ture core to be reduced to a minimum; just enough for mechanical clearance. Other things being equal, this means a saving in the cop- per required to magnetize the field. (b)It protects the embedded conductors from injury. (c) It affords an admirable way of supporting and securing the conductors firmly in place against the action of centrifugal force; and, further, it shields the conductors almost completely from the racking action of the magnetic drag due to the magnetic field. The shape and number of the slots in a toothed armature core have a marked effect on the shape of the electromotive force wave, and upon the regulation of the alternator. The shape of the wave is affected by the distribution of the magnetic flux in the air gap. The regulation is affected by the inductance of the armature coils, and the inductance depends on the number and shape of the arma- ture slots. Fig. 139 shows a portion of an armature- r "CA\ \J u //"? core disk or stamping for a 12-pole, uni-tooth \ ' (one slot per pole per phase), three-phase alternator. The armature winding adapted to this uni-tooth core is, of course, the uni- Fig. 139 Portion of stamp- t ing for Twelve-Pole Um- coil or concentrated winding. The armature Toot AiSnato? hase coils are held in place in the slots by wedges of wood or fiber driven in from the ends of the core and fitting into notches near the tops of the teeth, as shown in the figure. This construction, now almost universally adopted by manufact- urers, avoids the necessity for any binding wire on the arm- ature core. Alternators with uni-tooth armature cores are characterized by large armature inductance and by peaked-wave shapes of the induced electromotive forces; also by marked variations in the shape of the wave of induced electromotive force, according to the magnitude and power factor of the load. 138 ALTERNATING-CURRENT MACHINERY On account of their comparatively large inductance, uni-tooth armature constructions require a relatively large increase in the field-exciting current in passing from no load to full load output. In other words, regulation is poorer than for multi-tooth armature cores. According to present practice in design, the great majority of alternators are constructed with armature-core stampings having two or three or more slots per pole per phase. Fig. 140 shows a portion of an armature-core stamping for a 12-pole, three-phase alternator. It has three slots per pole per phase. The slots are open, which, together with the distributed (multi-coil) type of winding, results in a low armature inductance. This means that a relatively small increase in the field-exciting current is required in passing from no load to full load output. Alternators with multi-tooth armature cores are especially adapted for long-distance transmission where step-up transformers are used. The regulation is better than with the uni-tooth core construction; and the wave shape of the electromotive force gener- ated by the distributed winding approaches a sine wave, which is the best wave shape for the long-distance transmission of power. This is because of the fact that the nearer a f\t\T\I\fl/l/]/)/7/i gi yen curve of electromotive force approaches V^ / a sme curve, the less the likelihood of a dan- \ / gerous rise of voltage (resonance) occuring at V ->/ the distant end of a long transmission line be- cause of the capacity (condenser) effect. A long transmission line has a series of frequen- cies of electrical oscillation just as a stretched violin string has a series of frequencies of mechanical vibration. If the frequency of the current delivered by an alternator hap- pens to coincide with any of these "proper" frequencies of the transmission line, the line will have violent electrical oscillations set up in it, and excessive voltages will occur at certain points along the line. A sine wave of electromotive force has only one frequency. Any other kind of an alternating electromotive force is composed of a series of sine waves of ascending frequencies (the harmonics or over-tones in music), all combined into a resultant wave form. There is, therefore, more danger that one out of all ALTERNATING-CURRENT MACHINERY 139 the frequencies may coincide with one of the "proper" line fre- quencies than that the single frequency of a sine wave may so coincide. According to Progression of Winding. With reference to the progress of the winding from slot to slot, armature windings may be divided into spiral or ring windings; lap windings; and wave windings. These terms have the same meaning when applied to alternator windings as they do when applied to the windings of direct-current machines. According to Disposition of Coils. With reference to the dis- position of the coils around the periphery of the core, we have to distinguish between two general classes, viz, concentrated, or uni-coil t windings, and distributed, or multi-coil, windings. Concentrated, or uni-coil, windings, as the name implies, con- sist of one coil per pole per phase. The armature conductors are thus grouped in bundles, and usually placed in slots, there being one slot per pole for each phase. Examples of concentrated windings are illustrated in Figs. 1 and 102, in which the armature conductors are shown as lying in one slot per pole. Fig. 1 shows adjacent sides of two different armature coils lying in one slot. In some cases, each slot is filled by one side of a single armature coil, giving one slot per phase per pair of poles. Such windings are sometimes called "half-coiled" or "hemi-tropic." Distributed, or multi-coil, windings consist of several coils per pole per phase. The armature conductors are distributed in two or more slots per pole per phase. Examples of distributed windings are shown in Figs. 120 and 125. Fig. 120 shows an end view of a two-phase winding distributed in two slots per pole per phase. Fig.. 125 shows an end view of a three-phase winding distributed in two slots per pole per phase. Concentrated windings .are less expensive to make; and they give a greater effective electromotive force (at zero load) for a given number of conductors, other things being equal, than distributed windings. This is on account of the fact that all the conductors of a concentrated winding cut the field flux simultaneously, while the various conductors of a distributed winding do not cut the field flux simultaneously. 140 ALTERNATING-CURRENT MACHINERY Concentrated windings have greater inductance than distrib- uted windings for the same total number of conductors, and also cause a greater armature reaction for a given current than distrib- uted windings do. Consequently the terminal electromotive force of an alternator falls off more with a concentrated winding than with a distributed winding, when the current output is increased. Fig. 141. Completed Armature with Strap Winding, Four Slots Per Phase Therefore, an alternator with a concentrated winding has a poorer (higher) regulation than an alternator with a distributed winding; and although a concentrated winding may give a higher electro- motive force at zero load, it may actually give a lower electromo- tive force at full load. ALTERNATING-CURRENT MACHINERY 141 According to Form of Conductor. According to the form of the conductors used, armature windings may be divided into three classes, viz, wire winding; strap winding; and bar winding. Fig. 142. Portion of Bar-Wound Armature, One Bar Per Slot Wire ivinding, which is usually employed in high- voltage machines of low-current output, consists of machine-wound coils, which are entirely formed and insulated before being placed in the armature slots. 142 ALTERNATING-CURRENT MACHINERY Strap winding is used for machines of lower voltage and of greater current output, and it consists of copper strap, forged into the required shape and carefully insulated. Both the wire and strap windings are placed in the slots with- out any mechanical bending, thus preventing damage to the insu- lation. In armature core's having the slots partially closed, the winding is slipped in from the end; but in cores having open slots, wedges of hard fiber secure the coils in place. Fig. 141 is an illustration of strap winding distributed in four slots per pole per phase. The completed armature, ready for direct connection to a steam engine, is shown in the figure, and is intended to revolve inside of a stationary field-magnet structure. The four collector rings indicate that the armature is wound for two phases. It is manufactured by the Westinghouse Electric Company. Bar windings are held in place by the overhanging tips of the teeth. The bars, after being carefully insulated, are slipped into the slots from one end of the armature. The end connections of the bar winding are bolted and soldered to the bars after the con- ductors are in place. Bar windings are usually ar- ranged with either one or two bars per slot. There are no band wires on the armature core. Fig. 142 shows a portion of a bar- Fig. 143. Concentrated single-Phase wound revolving armature having Armature Winding one bar per slot. Single=Phase Windings. Fig. 1 shows a common type of single-phase winding having one coil per pole. Fig. 143 shows another type of concentrated single-phase winding, having one coil to each pair of poles or one slot per pole. The sketch b is a sectional view of a portion of the armature core, showing one of the slots containing the conductors forming one side of a single armature coil and standing opposite to a field pole. In the main diagram the heavy sector-shaped figures represent the coils, and the light lines represent the connections between the terminals of the coils. The radial parts of the sector-shaped figures represent the portions ALTERNATING-CURRENT MACHINERY 143 of the coils that lie in the slots, and the curved parts represent the ends of the coils. The circles at the center of the figure represent the collecting rings, one being shown inside the other for clear- ness. The arrows represent the direction of the current at a given instant. All electromotive forces induced under N poles are in one direction, and all electromotive forces induced under S poles are in the opposite direction. These remarks apply to Figs. 143 to 150 inclusive. Fig. 144 represents a single- phase winding distributed in two slots per pole, all the coils being connected in series. The sketch b, is a sectional view of a portion of the armature core, showing two slots. i Two=Phase Windings. The two-phase winding consists of two independent single-phase windings on the same armature, each being connected to a separate pair of collecting rings, as shown in Figs. 145 and 146. Fig. 145 shows a two-phase concentrated winding, one slot per pole for each phase. Fig. 146 shows a two-phase wind- ing distributed in two slots per pole for each phase. In each of the figures (145 and 146), the winding of one of the phases is shown by dotted lines, to distinguish it from that of the other. Fig. 144. Single-Phase Armature Winding Two Slots Per Pole A^fA Fig. 145. Two-Phase Concentrated Armature Winding Fig. 146. Two-Phase Armature Wind- ing, Two Slots Per Pole Per Phase Three=Phase Windings. The three-phase winding consists of three independent single-phase windings on the same armature, 144 ALTERNATING-CURRENT MACHINERY the terminals of the individual windings being connected accord- ing to the Y scheme or A scheme, as explained on page 145. Fig. Fig. 147. Three-Phase Concen- trated Winding Y-Connected Fig. 148. Three-Phase Concen- trated Winding A-Connected 147 shows a three-phase concentrated winding (one slot per pole for each phase), Y-connected. Fig. 148 shows the same winding A- connected. In Figs. 147 and 148 the winding for phase A is shown by heavy full lines; the winding for phase B is shown by light full lines; and the winding for phase C is shown by dotted lines. The Y connection gives I/ 3 times as much electromotive force between collecting rings as the A connection for the same total number of conductors per phase, and is the more suitable for high electromotive force machines. The A connection, on the other hand, is especially adapted for machines for large current out- put. The line current is ]/ 3 times as great as the current in each winding in a A-connected armature. Fig. 149 shows a three-phase bar winding distributed in two slots per pole for each phase. The sketch 6, is a sectional view of one slot containing a single con- ductor in the form of a rectang- ular bar. Fig. 150 shows a three-phase coil winding distributed in two slots per pole for each phase and arranged in two layers, there being as many coils on the armature as there are slots, so that portions of Fig. 149. Three-Phase Bar Winding, Two Slots Per Pole Per Phase ALTERNATING-CURRENT MACHINERY 145 two coils lie in each slot, one above the other. The portions of the coils represented by full lines lie in the upper parts of the slots, and the adjacent dotted portions lie in the bottoms of the same slots. The sketch b is a sectional view of one slot showing two half-coils one above the other, thus constituting a two-layer winding. The method of connecting up the separate windings of a three- phase alternator is as follows: Y CONNECTION. The terminals of the individual windings which~are to be connected to the common junction and to the collecting rings, may be determined as follows: Consider the instant when winding A is squarely under the poles, as shown in Fig. 147. The electromotive force in this wind- ing (and the current also, if the circuit is non-inductive) is a maximum, and the currents in the other two phases B and C are each half as great. If wind- ing A is connected so that its current is flowing away from /c, windings B and C must be connected so that their currents flow towards k. A CONNECTION. The three windings form a closed circuit when A connected. The total electromotive force around this circuit at any instant must be zero. Consider the instant when winding A is squarely under the poles, as shown in Fig. 148. The electromotive force in this wind- ing is a maximum, and the elec- tromotive forces in the other two windings are each half as great. Then winding A is connected to any pair of rings, say 1 and 2; winding B is connected to ring 3 and ring 1 (or 2); and winding C k is ^connected to ring 3 and ring 2 (or 1); these connections are made so that the electromotive forces at the given instant are in the directions indicated by the arrows in Fig. 148. F - 150 Three-Phase Coil Winding, Two Slots Per Pole Per Phase COMMERCIAL TYPES OF MACHINES Alternators are of three types, differing in the means employed for causing the conductors to cut the magnetic flux from the field magnet, viz, revolving-armature type; revolving-field type; and inductor type- Of these three types the one which experience has proven the fittest to survive is the revolving field type and it is today generally adopted by manufacturers as the standard. Alternators of the revolving Armature type are still manufactured to supply special 146 ALTERNATING-CURRENT MACHINERY demands and for small isolated electric plants, and they may be found in satisfactory operation in many of the older and smaller electric central stations. The inductor type is rarely if ever built today. In this type both the field winding and the armature (core and winding) are stationary, and the magnetic flux produced by the field winding is caused to move past the armature conductors by means of a revolving iron structure furnished with polar projections called the inductor. The advantages of this type, viz, absence of moving wires, collecting devices and brushes, with the consequent minimum cost of attend- ance are outweighed by the disadvantages of increased size, weight, and cost, and by the lower efficiency and poorer (higher) regulation especially when supplying current to inductive loads such as induc- tion motors or other apparatus having a power factor less than unity. On account of these disadvantages the manufacture and use of inductor alternators have been virtually discontinued. The revolving-armature type of alternator is limited to a general power and lighting distribution where only a moderate voltage is required. Machines of this type are comparatively cheap to build. They can be automatically compounded by the use of composite field windings without any compilation of parts, which is not the case with the revolving field type. They can be furnished with an aux- iliary armature winding and small commutator for exciting their fields, thus dispensing with any external exciter. The revolving-armature type, on the other hand, is not suit- able for generating either high or low voltages, on account of the difficulties of insulating the armature conductors and collecting rings in the first case (high voltage), and of collecting a large arma- ture current in the second case (low voltage). The advantages of the revolving field type over the revolving armature type are as follows : (1) The revolving field type gives more space for the armature winding, and thus permits the stationary armature to be easily insulated to withstand a testing pressure of over 30,000 volts. Large alternators with stationary armatures have been built to generate voltages up to 20,000; but it is doubtful whether, on the whole, it is economical to build them for voltages greater than about 13,000. ALTERNATING-CURRENT MACHINERY 147 (2) The insulation of the armature is relieved from the strains due to centrifugal force at high speeds. Furthermore, a revolving field can be made stronger and more compact than a revolving arma- ture and, therefore, the revolving-field type of alternator is much the better suited to the high speeds employed in alternators driven by steam turbines. (3) The number of collecting rings is reduced to a minimum, viz, two, and the amount of electrical energy transmitted through them is only about two per cent of that which would have to be trans- mitted through the collector rings of a revolving armature alternator of the same capacity. The voltage between the collector rings is also relatively small, being either 125 volts, or 250 volts in the larger machines. REVOLVING=ARMATURE ALTERNATORS In this type of alternator, the field magnet is stationary, while the armature is mounted on a shaft and is driven (by means of a belt or mechanical coupling) by the prime mover, which may be either a steam engine, a steam turbine, or a water wheel. The current induced in the armature conductors is delivered to the external circuit through collector rings on which brushes rub. The armature may be wound for a single-phase current (having two collector rings), for two-phase currents (having four collector rings), for three-phase currents (having three collector rings), etc. The revolving-armature type is used almost exclusively for alternators of small output and moderate voltage. Although single-phase alternators, and especially those of the revolving-armature type have been virtually superseded by poly- phase machines of the revolving-field type, still there are today a number of the older fashioned alternators in regular use in some of the smaller electric lighting stations. For this reason some of the typical features of these machines will be described before considering the more modern designs. Fort Wayne Single=Phase. Fig. 151 shows a 90-kilowatt 1,100- volt 8-pole single-phase belt-driven alternator manufactured by the Fort Wayne Electric Company, of Fort Wayne, Indiana. It is designed to be driven at a speed of 900 revolutions per minute; hence the frequency of its electromotive force is - ~ = 60 cycles per sec- 148 ALTERNATING-CURRENT MACHINERY ond. The figure shows the two collector rings adjacent to the arma- ture, also the rectifying commutator with its brushes for supplying s uni-directional current to the coarse wire coils of the composite field winding.* ALTERNATING-CURRENT MACHINERY 149 The exciter is a shunt-wound 4-pole 2-kilowatt direct-current generator running at a speed of 1,400 revolutions per minute. It is shown belted to a pulley on the alternator shaft. The current from this exciter is led to the fine wire coils of the composite field winding. Each field pole of the alternator is provided with two coils wound on one and the same spool one of coarse wire, sup- plied with current from the rectifying commutator; and the other of fine wire, supplied with current from the exciter. The field structure, base-plate, and pedestals are cast in one piece, and the whole machine rests upon a cast-iron sub-base. This sub-base is provided with slide rails along which the machine may be^moved by means of a screw turned by a ratchet and lever, as shown this for the purpose of adjusting the tension of the main driving belt. The field poles are "built up" of sheet-iron stamp- ings and are held together by long bolts. These field poles are ar- ranged in the mould in which the field frame is cast, and are thus cast-welded to the frame. The field coils are ma- chine wound on spools, with insulated copper wire, the coarse wire coils being wound on top of the fin^ \\nre* nnil TVif* fiplfl Fig. 152. German Silver Resistance Coils in Shunt with US. le/ltfi Coarse Wire Field Coils spools are held in position on the field poles by brass collars fixed to the outer ends of the poles. The coarse wire coils, connected in series, are supplied with uni-directional current from the rectifying commutator ; and the entire set is shunted with resistance consisting of a strip of German silver wound on an insulated form, Fig. 152, and mounted on an insulated block inside the hollow pedestal on the collector end of the machine. The armature is of the iron-clad ring type built up of small overlapping sheet-steel punchings, annealed and japanned to reduce the loss caused by hysteresis and eddy currents. The armature winding is of the concentrated or uni-coil type. The coils are wound by hand directly on each armature tooth, insulated copper ribbon being used. Armature coils are generally wound on formers, and afterwards sprung into place in the slots on the armature core. 150 ALTERNATING-CURRENT MACHINERY Westinghouse Uni=Coil Armature. Figs. 153 and 154 show a Westinghouse single-phase uni-coil armature very much like the one used in the alternator shown in Fig. 151. The coils of the West- Fig. 153. Westinghouse Single-Phase Uni-Coil Armature Core inghouse alternator, however, are machine- wound on formers; and, after being taped, varnished, and baked, they are spread out slightly so as to pass over the teeth, and are forced into place in the deep slots, being securely held there by wooden wedges. Fig. 153 shows the core unwound, and Fig. 154 the method of placing coils in this type of machine. The armature teeth are T shaped, and partially overhang the armature coils, thus protecting the coils from injury. There are, of course, the same number of armature teeth as field poles in the uni-coil armature. Thus Figs. 144 and 145 show an armature core for an eight-pole field. Fig. 155 shows the completed armature of which the core and coils are shown in Figs. 153 and 154. At the ends of the armature are two brass shields for protecting the ends of the armature coils. Fig. 154. Westinghouse Single-Phase Armature with Coils in Place The ring-oiling, spherical-seated, self -aligning bearings are shown on the shaft in Fig. 155; and at the end of the shaft are shown the two ALTERNATING-CURRENT MACHINERY 151 collecting rings and the rectifying commutator. The wires leading to the collector rings and to the rectifying commutator are led through the shaft, which is made hollow. Fig. 155. Westinghouse Single-Phase Uni-Coil Armature Complete Fig. 156. Westinghouse Single-Phase Armature Core and Coils. Distributed Winding Fig. 157. Westinghouse Single-Phase Distributed Winding Armature Complete Westinghouse Armature with Distributed Winding. Figs. 156 and 157 show a single-phase Westinghouse alternator armature with distributed winding. Three armature coils are shown in Fig. 156 152 ALTERNATING-CURRENT MACHINERY separately and also grouped together. The manner of grouping shown is carried out when the armature coils are assembled on the armature core; and there are as many of these groups of armature coils as there are poles in the field. Fig. 157 shows the finished armature with end shields, collecting rings, and rectifying commutator. In assembling the stampings of an armature core a stiff, cor- rugated stamping (or its equivalent) is inserted at intervals between groups of the flat sheet-iron stampings, thus leaving radial air ducts from the inside to the outside of the armature core. These are known as ventilating ducts, and their object is to permit of a free circulation of cool air through the arma- ture core and coils, thus preventing excessive rise of temperature. The motion of the armature causes air to be drawn in through the end shields to the interior of the armature, whence it is thrown out through the radial ducts, as in the case of a ventilating fan. Four of these spaces between stampings can be seen in Figs. 156 and 157; and the armature "spider" and end shields at each end, as shown, are provided with apertures for admitting cool air. Field Structure for Westinghouse 180=Kw. Alternator. Fig. 158 shows the field structure of a Westinghouse 180-kilowatt alter- nator before the field coils have been placed upon it. The field poles (laminated) are shown projecting radially inwards from the cast- iron, ring-shaped yoke. The yoke is cast in two pieces, and the upper half is bolted to the lower half, as shown in the figure. The yoke is divided in this way in order that the upper half may be unbolted and may be lifted off by means of the eye-bolt on the top of the machine, thus giving easy access to the armature for inspection and repairs. General Electric Three=Phase Alternator. Fig. 159 shows a view of a 25-kilowatt three-phase revolving-armature type of alternator Fig. 158. Field Structure of a Westinghouse 180-Kw. Alternator ALTERNATING-CURRENT MACHINERY 153 built by the General Electric Company for use in small isolated plants. The alternator is belt-driven at 1,800 r. p. m. and, there- fore, requires four poles to give a frequency of 60 cycles. The view shows three collector rings mounted on the armature shaft for the collection of the three-phase currents, and a com- mutator between the rings and the armature core. The special feature of this machine is that it requires no separate exciter, for the armature is wound with two distinct windings, one of which is con- Fig. 159. General Electric 25-Kw. Three-Phase Alternator nected to the commutator and supplies the exciting current to the field coils. The main armature winding generates the three-phase alternating currents and is connected to the three collector rings. The field structure consists of laminated pole pieces cast into a rigid stationary frame. The field-magnet cores are wound with coils which furnish a magnetic flux which is common to both alternator and exciter armature windings. These machines are built in three sizes and rated at 7.5, 15, and 25 kw. at unity power factor, and at 154 ALTERNATING-CURRENT MACHINERY 6, 12, and 20 kw., respectively, at 0.8 power factor. Standard voltages are 120, 240, 480, and 600 volts at 60 cycles, and their ratings are based on either two- or three-phase operation. For single-phase operation any one of the phases may be used, but the rating then is only 70 per cent of the polyphase rating on account of the heating. REVOLVING=FIELD ALTERNATORS In this type of alternator, the armature is stationary, and the armature windings are arranged in slots on the inner face of the armature structure, the latter forming a closed ring inside of which the multipolar field magnet revolves. The armature structure con- sists of an external frame of cast iron or steel supported on a bed- plate. The armature core proper consists of a ring built up of relatively small stampings of sheet iron dovetailed into the external frame and pressed together between two flanges by bolts. The ex- ternal frame in this type of alternator does not to any perceptible extent carry lines of force, that is, it does not form a part of the magnetic circuit, but serves only as a support for the laminated armature core. Construction. Frame. When a given type of alternator has been standardized by a manufacturer, it is customary to lay out a complete line of machines, which differ in weight by fifteen or twenty per cent between consecutive sizes. The capacity of a given frame is dependent upon speed, voltage, and specified performances. Frames are made in two general styles, one called the box type and the other the skeleton type. The box type consists of a single casting for the smaller sizes until an outside diameter of about 8 feet is reached. Above this diameter the frame castings are usually divided into upper and lower sections, split construction being neces- sary on account of the limitation imposed in handling and shipping. The skeleton type consists of two side castings between which substantial spacing rods are set at suitable intervals. For manu- facturing reasons the skeleton type is preferred for certain sizes of machines, as its construction readily permits of changes being made in the assembling of the armature-core laminations without neces- sitating a change in the patterns. The main point to be looked after in alternator frames is a design that will give the maximum of rigidity. The only function ALTERNATING-CURRENT MACHINERY 155 of the frames of rotating-field alternators isxtoff Fig. 187. Diagram of Connections for Dielectric Strength Test with High Potential Transformer The use of a water rheostat in series with the primary (low voltage) coil of the testing transformer, as shown in Fig. 187, for varying the impressed voltage, is open to some objection. Such a resistance is liable to seriously affect the wave form of the electro- motive force, thereby causing its maximum value to bear a different and unknown ratio to its effective value. The most approved way ' of securing voltage control is by adjusting the field excitation of the alternator supplying the testing transformer with current. Another method, though not as satisfactory, is by using a transformer with a variable ratio of primary to secondary turns. ALTERNATING-CURRENT MACHINERY 191 Tests of dielectric strength are made with voltages ranging from 1 \ to 10 times the rated terminal voltage of a piece of apparatus, according to the rated voltage and output of the apparatus. For example, a transformer of any output whose rated terminal vol- tage is 20,000 would, according to the recommendations of the American Institute of Electrical Engineers, be tested with 40,000 volts, whereas a 1,000-volt transformer would be tested with 3,500 volts. An induction motor under 10 h. p. rated at 110 volts would be tested with 1,000 volts. Break-Down. The break-down test is freauently applied in the 15 /3 S; / Sf/LOVOL 73, soMffftfoor or ME/MS QUAfff D/STAWCE /W//YC BTWt W3 MMZS. ^ f J> 10 JO 35 SO 50 bO /O 80 //O 120 /JO a< 07 /.(. fa ^l. P Ttt 77 -e5 ? ? es ? *5 15 5 55 35 15 9 75 85 95 -^ / y/ f / ^ / f / JL +t. J>< z/ 8. 9.1 /O /J. /3 /J f / A / f / f / j / ^ >^ s / ^ X ^X ^ /O ^0 JO 4O SO 6O 70 80 90 /OO //O MLO~VOLT3-SQUA/?C /TOOT /3O /4O /50 Fig. 188. Curve Showing Relation between Sparking Distance and Voltage testing of small samples of insulating material. For example, a sheet of fuller board, mica-canvas, oiled linen or cloth, would be clamped between sheets of metal connected to the terminals of the high- voltage coil of the testing transformer, and the voltage would be increased until the insulation was punctured, the voltage causing puncture being recorded. A basis is thus obtained for the acceptance or rejection by the purchaser of a lot of insulating material. Table VI gives, in inches and in centimeters, the sparking distances in air between opposed sharp needle-points, correspond- ing to various effective sinusoidal voltages. The voltages are ex- pressed in kilo volts (1 kilo volt = 1,000 volts). Fig. 188 is a curve 192 ALTERNATING-CURRENT MACHINERY TABLE VI Sparking Distances for Various Voltages Kilovolts Square Root of Mean Square Distance Kilovolts Square Root of Mean Square Distance Inches Cms. Inches Cms. 5 0.225 0.57 140 13.95 35.4 10 0.47 1.19 150 15.0 38.1 15 0.725 1.84 160 16.05 40.7 20 1.0 2.54 170 17.10 43.4 25 1.3 3.3 180 18.15 46.1 30 1.625 4.1 190 19.20 48.8 35 2.0 5.1 200 20.25 51.4 40 2.45 6.2 210 21.30 54.1 45 2.95 7.5 220 22.35 56.8 50 3.55 9.0 230 23.40 59.4 60 4.65 11.8 240 24.45 62.1 70 5.85 14.9 250 25.50 64.7 80 7.1 18.0 260 26.50 67.3 90 8.35 21.2 270 27.50 69.8 100 9.6 24.4 280 28.50 72.4 110 10.75 27.3 290 29.50 74.9 120 11.85 30.1 300 30.50 77.4 130 12.90 32.8 plotted from the data given in the table and shows graphically the relation between sparking distance and voltage. The voltage corre- sponding to a given sparking distance varies greatly with the sharp- ness of the needle-points, and with the shape of the electromotive force wave. Sometimes the value of the voltage applied to the apparatus is not measured by the spark gauge, but is inferred from the read- ing of a low-reading voltmeter connected between the points A and B in Fig. 187. Thus, if there are 100 times as many turns of wire in the secondary (high-voltage) coil of the testing trans- former as in the primary coil, then the readings of the voltmeter connected as specified must be multiplied by 100. The best prac- tice is to connect an electrostatic voltmeter in parallel with the spark gap and to check its readings against the sparking distances in accord- ance with Table VI. It should be noted, however, that the volt- meter gives effective values while the spark gap gives maximum values of the voltage. ALTERNATING-CURRENT MACHINERY 193 Characteristic Curves. Saturation Curve. The saturation curve of a generator shows the relation between the volts generated in the armature and the amperes of field current (or ampere-turns of the field) for a constant armature current. The armature current may be zero, in which case the curve is called the no-load saturation curve, or sometimes the open-circuit characteristic curve. Observa- tions for a saturation curve may be taken with full-load current in the armature; but this is rarely done, except in alternators of comparatively small output. If a full-load saturation curve is desired, it can be approximately calculated from the no-load satura- tion curve and the curve for synchronous impedance, as will be explained later. Fig. 189. Diagram of Connections for Saturation Curve Test of Alternator The diagram of connections for determining tne no-load satura- tion curve is shown in Fig. 189. The alternator is represented as a three-phase machine of the revolving-field type (armature is stationary). The field winding is connected through slip-rings d and e, and brushes / and g, to an ammeter A, and through an adjustable resistance r, to the exciter or other direct-current source. A voltmeter V\ is connected across the field-winding terminals to measure the voltage applied to the field winding. The volt- meter F 2 , for measuring the voltage generated by the machine, is connected directly to two of the armature terminals, a and c, in the case of a low- voltage machine. If the voltage generated is greater than the capacity of the voltmeter, a multiplying coil or a 194 ALTERNATING-CURRENT MACHINERY vo/fage oadf/eh step-down potential transformer may be used to reduce the electro- motive force to be measured. For very high voltages a potential transformer must be used. A series of observations of the electromotive force between the terminals of one of the phases, such as a and c, is made for different values of the field current. Eight or nine points along the curve are usually suf- ficient, the series ex- tending from zero elec- tromotive force to about fifty per cent above normal rated voltage. The points should be taken more closely together in the vicinity of normal volt- age than at other por- tions of the curve. Care must be taken that the generator is run at its rated speed, and this speed must be kept con- stant. Deviations from constant speed may be Fig. 190. Saturation Curve for 2000-Kw. Alternator most easily detected by the use of a tachometer. If the machine is two-phase or three-phase, the voltmeter may be connected to any one phase throughout a complete series of observations. The voltage of all the phases should be observed for normal full-load excitation by connecting the voltmeter to each phase successively, keeping the field current constant at normal voltage. This is done in order to see how closely the voltage of the different phases agree. The observations required for the determination of the satura- tion curve are volts at armature terminals, Vi', amperes in field, A; volts at field terminals, V\\ and speed. urar/on Cu for W A/rerr afor ALTERNATING-CURRENT MACHINERY 195 Fig. 190 shows a saturation curve taken, from a 2,000-kilowatt three-phase alternator of the revolving-field type, having 16 poles, and generating 2,000 volts and 576 amperes per phase, when run at 300 revolutions per minute. 3800- 3600- Hh 135 K.W. 2 PHASE GFNERATOR 2400 V- 900 P.P. M. 60 P. PS. 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 3840 ARMATURE CURREMT Fig. 191. Set of Curves for Two-Phase 135-Kw. Inductor Alternator Fig. 191 gives a number of curves for a two-phase 135-kilowatt 2,400-volt 60-cycle inductor alternator. In particular, the no-load saturation curve is shown giving the relation of the field current to the voltage between armature terminals of one phase. This curve shows that nearly 7 amperes of field current is required to give rated voltage, namely 2,400, at no load. The field current required to give rated voltage at full non-inductive load is 8.8 am- 196 ALTERNATING-CURRENT MACHINERY peres, as is explained on page 202. From the saturation curve it is evident that this full-load field current will produce about 2,625 volts at no load. Synchronous Impedance Curve.* The synchronous impedance curve shows the relation between armature voltage and armature current, the armature being j>hort-circuited so that the only condi- tion that limits the current for a given volt- age generated is the synchronous impedance of the armature. This is materially different from the impedance of the armature when the machine is standing still. The connections for this test are similar to those for the saturation curve, except that the voltmeter (or potential transformer) connected to the armature is re- placed by an ammeter. If the current is be- yond the capacity of the ammeters at hand, a current transformer may be connected in place of the ammeter, and the ammeter may be connected to its secondary. A series of observations is to be taken of the current in the armature, with the latter short-circuited through the ammeter, for different field currents, commencing at a very low value, and increasing the field current by successive steps until the armature current has reached a value of 100 per cent above its rated full- load value. The last few readings must be made quickly to pre- *See page 108. eoo / 1600 , / / 1400 / / 1200 / 1000 / eoo / / eoo / 400 / Synch ronous /mpecti nee D' *f> POO / / 300 o k \N A for ffernah / / 150 300 450 600 750 900 Amperes Fig. 192. Curve Showing Synchronous Impedance Drop for 2,000-Kw. Alternator ALTERNATING-CURRENT MACHINERY 197 vent undue heating of the armature. The armature winding should be at approximately normal temperature when the test is made. The speed should be kept approximately at the rated speed of the machine. It is not as essential to keep the speed constantly at rated value as when observations are being made for the determination of the saturation curve. The observations to be recorded are: amperes in the armature; amperes in field; volts at field terminals; and Fig. 192 shows a curve giving the relation between electromo- tive force induced in the armature and the current in the armature when short-circuited through an ammeter. This figure relates to the same 2,000-kilowatt alternator whose saturation curve was given in Fig. 190. The electromotive forces plotted in this figure are not observed values, but the field excitations required to pro- duce them are observed, and the electromotive forces correspond- ing to these field excitations are taken from the saturation curve. The total electromotive force induced in the armature for a given value of the field current may be read off from the no-load saturation curve of the machine, obtained as previously described. A curve may then be plotted with the electromotive force induced in the armature as ordinates, and the observed armature currents (on short-circuit) as abscissas. This curve is sometimes called the synchronous impedance curve, although it does not explicitly show the values of the synchronous impedance of the armature. The synchronous impedance of the armature for a given value of armature current may, however, be derived from this curve by divid- ing the total electromotive force induced in the armature (ordinate) by the corresponding value of the short-circuited armature current (abscissa). For example, the synchronous impedance correspond- ing to 576 amperes (which is the full-load current per phase of the machine) is 1,145 volts n . = 1.99 ohms 576 amperes Synchronous impedance is used as a basis for the predeter- mination of the regulation of the machine, thereby avoiding the trouble and expense of an actual test of regulation under full load. The synchronous impedance of an alternator armature is very nearly equal to the synchronous reactance of the armature, inas- 198 ALTERNATING-CURRENT MACHINERY much as the armature resistance is usually small. Moreover the electromotive force required to overcome synchronous reactance is very nearly equal to the electromotive force required to overcome synchronous impedance. Determination of Resistance of Armature. In the case of a three-phase armature, the resistance per phase cannot be measured directly between collector rings, since there are two phases in series between collector rings in a Y-connected arma- ture; while in case of a A-connected armature, two phases in series are in parallel with the third phase between any two collector rings. The resistance per phase can be measured either directly by Wheatstone Fig. 193. Diagram for Armature when bridge and galvanometer, or by the "fall of potential" method. In the case of a Y-connected armature, the resistance per phase is one- half the resistance between terminals, provided that the resistance of all the phases are alike. In case the resistances of the phases are unequal, the resistance of any phase may be deduced as follows: The resistance between terminals A and B, Fig. 193, is RAB a + b The resistance between terminals B and C is REC = 6 + c The resistance between terminals C and A is RCA = c+'a Then CL RAB b b= RBCC C = RCA a Substituting (m) in (v) we"obtain b = RBC RCA"}" O> Substituting (mi) in (iv) we obtain (n) (Hi) () () (n) (oil) ALTERNATING-CURRENT I MACHINERY 199 from which Similarly we find Ci == RAB RBC ~f~ RCA & RAB RBC H~ RCA a = RBC RCA + RAB and RCA R BC If R AB = RBC = RCA, then a= 6 = c = R AB MB -Ll nnn lx Fi S- 195 - Vector Diagram for Field phase, 2,000-KW., 16-pOle, 2,000-VOlt, Excitation in a Non-inductive . . _ , . . , Receiving Circuit revolving- field alternator having a speed of 300 r. p. m. The full-load armature current is 576 amperes per phase. The armature resistance per phase is 0.009239 ohms. Hence, the IR drop in the armature = (576 X 0.009239) = 5.3 volts. E t = 2,000; E t + IR = 2,000+5.3 = 2005.3 volts. From the saturation curve, Fig. 190, we find that it requires 83.5 amperes in the field winding to generate this 2,005.3 volts at no load. This represents the component OA in Fig. 195. From the synchronous impedance curve, Fig. 192, we find that 1,137 volts in the armature are required to force the full-load current of 576 amperes per phase through the armature. From the satura- tion curve, we find that 1,137 volts correspond to 43 amperes in the field winding. The component OB in Fig. 195 is, therefore, 43 amperes. The full-load field current OC is OC = VOA Z +OB Z = l/43 2 +83.5 2 = 93.8 amperes This field current would produce at zero load an electromotive 202 ALTERNATING-CURRENT MACHINERY force of 2,169 volts, as shown by the saturation curve. Therefore, according to the definition of regulation, we have Regulation = X 100 = X 100 = 8.45 per cent If the regulation had been desired at a power factor other than unity (for instance, at 85 per cent power factor), the field cur- rent OB, Fig. 195, would not be taken at right angles to OA to find the resultant field current OC at full load, but the angle BOA would be decreased by the angle whose cosine is 0.85, or by 31.75. The resultant field current OC is represented as before, by the diagonal of the parallelogram; but it is no longer equal to the square root of the sum of the squares of OA and OB. The angle B OA, Fig. 194, now is 58.25 and, therefore, OC 2 != OA 2 + 05 2 + 2 OA X OB cos 58.25 whence OC = 112 amperes By referring to the saturation curve, we find that a full-load field current of 112 amperes would produce 2,375 volts at no load. Hence- the regulation at 85 per cent power factor is This example illustrates the general fact already explained, that the electromotive force at the terminals of an alternator suffers a greater decrease in value on an inductive load (power factor less than unity) than on a non-inductive load (power factor unity) for the same current output. Regulation Curve. Fig. 191 shows the regulation curves of a two- phase, 135-kilowatt, 2,400- volt, 60-cycle inductor alternator. The lower regulation curve shows the regulation of the alternator at 100 per cent power factor, that is, on non-inductive load; and the upper regulation curve shows the regulation of the alternator at 70 per cent power factor. The abscissa of a" given point on one of these regulation curves represents a given current output per phase of the machine; and the ordinate of the point represents the voltage obtained at the armature terminals when this armature current ALTERNATING-CURRENT MACHINERY 203 is reduced to zero, the field current being kept constant at that value which gives the rated voltage of 2,400 volts with the given current output per phase. For example, with full-load current output, namely 28.1 am- peres per phase and 100 per cent power factor, the voltage rises from 2,400 to 2,625 (the ordinate of the regulation curve for 100 per cent power factor corresponding to the abscissa representing 28.1 amperes of armature current) when the load is thrown off (arma- ture current reduced to zero), the field current remaining unchanged. From these data the full load regulation of the machine on 100 per cent power factor is found to be 225 2400 X 100 = 9.4 per cent With half-load current output, namely 14.05 amperes per phase, [and 70 per cent power factor, [the voltage rises from 2,400 to 2,650 when the load is thrown off, the field current remaining unchanged. From these data the half-load regulation of the machine at 70 per cent power factor is found to be 250 2^X100 =10.4 per cent These curves show that the regulation of the machine is higher on inductive loads than on non-inductive loads. American Institute Rules. The following are the recommenda- tions of the American Institute of Electrical Engineers concerning the regulation of electrical apparatus and prime movers. DEFINITIONS 187 REGULATION. The regulation of a machine or apparatus in regard to some characteristic quantity (such as terminal voltage, current, or speed) is the ratio of the deviation of that quantity from its normal value at rated load to the normal rated load value. The term "regulation," therefore, has the same meaning as the term "inherent regulation," occasionally used. 188 CONSTANT STANDARD. If the characteristic quantity is intended to re- main constant (e.g., constant voltage, constant speed, etc.) between rated load and no load, the regulation is the ratio of the maximum variation from the rated load value to the no-load value. 189 VARYING STANDARD. If the characteristic quantity is intended to vary in a definite manner between rated load and no load, the regulation is 204 ALTERNATING-CURRENT MACHINERY the ratio of the maximum variation from the specified condition to the normal rated-load value. 190 NOTE (a) If the law of the variation (in voltage, current, speed, etc.) between rated load and no load is not specified, it should be assumed to be a simple linear relation; i. e., one undergoing uniform variation between rated load and no load. 191 NOTE (b) The regulation of an apparatus may, therefore, differ accord- ing to its qualification for use. Thus, the regulation of a compound- wound generator specified as a constant-potential generator, will be differ- ent from that which it possesses when specified as an over-compounded generator. 192 In CONSTANT-POTENTIAL MACHINES, the regulation is the ratio of the maximum difference of terminal voltage from the rated-load value (occur- ring within the range from rated load to open circuit) to the rated load terminal voltage. 193 In CONSTANT-CURRENT MACHINES, the regulation is the ratio of the maximum difference of current from the rated-load value (occurring within the range from rated-load to short-circuit, or minimum limit of operation), to the rated-load current. 194 In CONSTANT-POWER APPARATUS, the regulation is the ratio of maxi- mum difference of power from the rated-load value (occurring within the range of operation specfied) to the rated power. 195 In CONSTANT-SPEED DIRECT-CURRENT MOTORS and INDUCTION MO- TORS, the regulation is the ratio of the maximum variation of speed from its rated load value (occurring within the range from rated load to no load) to the rated-load speed. 196 The regulation of an induction motor is, therefore, not identical with the slip of the motor, which is the ratio of the drop in speed from syn- chronism, to the synchronous speed. 197 In CONSTANT-POTENTIAL TRANSFORMERS, the regulation is the ratio of the rise of secondary terminal voltage from rated non-inductive load to no-load (at constant primary impressed terminal voltage) to the secondary terminal voltage at rated load. 198 In OVER-COMPOUNDED MACHINES, the regulation is the ratio of the maximum difference in voltage from a straight line connecting the no-load and rated-load values of terminal voltage as function of the load current, to the rated-load terminal voltage. 199 In CONVERTERS, DYNAMOTORS, MOTOR-GENERATORS and FREQUENCY CONVERTERS, the regulation is the ratio of the maximum difference of terminal voltage at the output side from the rated-load voltage, to the rated-load voltage on the output side. 200 In TRANSMISSION LINES, FEEDERS, ETC., the regulation is the ratio of the maximum voltage difference at the receiving end, between rated non- inductive load and no load to the rated-load voltage at the receiving end (with constant voltage impressed upon the sending end). 201 In STEAM ENGINES, the regulation is the ratio of the maximum varia- tion of speed in passing slowly from rated-load to no-load (with constant steam pressure at the throttle) to the rated-load speed. ALTERNATING-CURRENT MACHINERY 205 202 In a HYDRAULIC TURBINE or OTHER WATER-MOTOR, the regulation is the ratio of the maximum variation of speed in passing slowly from rated- load to no-load (at constant head of water; i. e., at constant difference or level between tail race and head race), to the rated load speed. 203 In a GENERATOR-UNIT, consisting of a generator united with a prime- mover, the regulation should be determined at constant conditions of the prime-mover; i. e., constant steam pressure, head, etc. It includes the inherent speed variations of the prime-mover. For this reason the regu- lation of a generator-unit is to be distinguished from the regulation of either the prime-mover, or of the generator contained in it, when taken separately. CONDITIONS FOR AND TESTS OF REGULATION 204 SPEED. The regulation of generators is to be determined at constant speed, and of alternating apparatus at constant impressed frequency. 205 NON-INDUCTIVE LOAD. In apparatus generating, transforming, or transmitting alternating currents, regulation should be understood to refer to non-inductive load, that is, to a load in which the current is in phase with the e. m.f. at the output side of the apparatus, except where ex- pressly specified otherwise. 206 WAVE FORM. In alternating apparatus receiving electric power, regu- lation should refer to a sine wave of e. m.f., except where expressly speci- fied otherwise. 207 EXCITATION. In commutating machines, rectifying machines, and syn- chronous machines, such as direct-current generators and motors, alter- nating-current and polyphase generators, the regulation is to be deter- mined under the following conditions: (1) At constant excitation in separately excited fields. (2) With constant resistance in shunt-field circuits, and (3) With constant resistance shunting series-field circuits; i. e., the field adjustment should remain constant, and should be so chosen as to give the required full-load voltage at full-load current. 208 IMPEDANCE RATIO. In alternating-current apparatus, in addition to the non-inductive regulation, the impedance ratio of the apparatus should be specified; i. e., the ratio of the voltage consumed by the total internal im- pedance of the apparatus at full-load current, to its rated full-load voltage. As far as possible, a sinusoidal current should be used. 209 COMPUTATION OF REGULATION. When in synchronous machines, the regulation is computed from the terminal voltage and impedance voltage, the exciting ampere-turns corresponding to terminal voltage plus armature- resistance-drop, and the ampere-turns at short-circuit corresponding to the armature-impedance-drop, should be combined vectorially to obtain the resultant ampere-turns, and the corresponding internal e. m. f. should be taken from the saturation curve. Heat Test. The heat test is made by running the generator under full-load conditions until a constant temperature has been reached. When this condition has been attained the machine is 206 ALTERNATING-CURRENT MACHINERY shut down, and the temperature of the various parts is taken by thermometers placed against the heated surfaces. The resistances of the armature and field windings are also measured while hot. By comparing these "hot" resistances with the same resistances previously measured at room temperature, the temperature rise of the armature and field coils can be computed. The method recom- mended by the Standardization Committee of the American Institute of Electrical Engineers for making these computations is as follows: The fundamental relation between the increase of resistance in copper and the rise of temperature may be taken as R t =R (1+0.00420 where RQ is the resistance of the copper conductor at 0C. and R t is the cor- responding resistance at t C. This is equivalent to taking a temperature co- efficent of 0.42 per cent per degree C. temperature rise above C. For initial temperatures other than 0C., a similar formula may be used sub- stituting the proper coefficient corresponding to the actual initial temperature. The formula thus becomes at 25 C. p,, 0.3801 r ** r "* ( , + -~T5o~~ where R t is the initial resistance at 25 C. Ri + r the final resistance, and r the temperature rise above 25 C. In order to find the temperature rise in degrees centigrade from the initial resistance R { at the initial temperature iC. and the final resistance #, + we may use the formula I") r = (238.1 + i) ( - ^ 1) degrees C. Ri Example. The "cold" resistance of the armature of a generator is meas- ured and found to be 0.046 ohm at a temperature of 20.5C. After a heat run under full load the resistance is found to be 0.052 ohm. What is the rise in temperature? In this case the initial temperature i is 20.5C., the final resistance R{+ r is 0.052 ohm, and the initial resistance R t is 0.046 ohm. The temperature rise is, therefore, r- (238.1+20.5) ( - 1) = 33.72 centigrade degrees 0.046 which means that the final temperature of the armature is i + r = 20.5 + 33.72 = 54.22 C The temperatures of the following generator parts are usually re- corded : ALTERNATING-CURRENT MACHINERY 207 Armature coils Field coils Armature laminations Pole tip (leading) Armature ventilating ducts Pole tip (trailing) Frame Field yoke Bearings Room Small generators are usually run at full load, delivering their output to water rheostats. During the heat run, thermometers, placed on different parts of the machine, are read regularly^If the generator is of the revolving-armature type, a thermometer is placed against the field winding, and another thermometer is so placed that the hot air issuing from the ventilating ducts will come in contact with it. If the generator be of the revolving-field type, thermometers are placed against the armature coil, on the armature laminations, and in the ventilating ducts. When these thermometers do not show an increasing temperature, and the resistance of the field, as determined from the field ammeter and voltmeter, has become constant, the machine is considered as having attained its ultimate temperature, and is shut down for the application of thermometers, which should be ready for immediate application, as the machine cools rapidly. The time taken by a machine to reach constant temperature varies from 3 to 4 hours in the case of a small machine, and from 12 to 18 hours for very large ones. The following maximum values of temperature elevation have been recommended by the Standardization Committee of the Ameri- can Institute of Electrical Engineers: Field and armature, 50 C. by resistance Commutator and brushes, 55C. by thermometer Collector rings, 65C. by thermometer Bearings and other parts of machine, 40C. by thermometer The rise of temperature should be referred to the standard conditions of a room temperature of 25C., a barometric pressure of 760 mm., and normal conditions of ventilation; that is, the ap- paratus under test should neither be exposed to draft nor enclosed, except where expressly specified. If the room temperature during the test differs from 25C., the observed rise of temperature should be corrected by J per cent for each degree C. Thus, with a room temperature of 35C., the observed rise of temperature has to be decreased by 5 per cent; and with a room temperature of 15C., the observed rise of tern- 208 ALTERNATING-CURRENT MACHINERY perature has to be increased by 5 per cent. The thermometer in- dicating the room temperature should be screened from thermal radiation emitted by heated bodies or from drafts of air. When it is impracticable to secure normal conditions of ventilation on account of an adjacent engine, or other sources of heat, the thermometer for measuring the air temperature should be placed so as to fairly indicate the temperature which the machine would have if it were idle, in order that the rise of temperature determined shall be that caused by the operation of the machine. _ The temperature should be measured after a run of sufficient duration to reach practical constancy. This is usually from 6 to 18 hours, according to the size and construction of the apparatus. It is permissible, however, to shorten the time of the test by run- ning a lesser time on an overload in current and voltage, then reduc- ing the load to normal, and maintaining it thus until the temperature has become constant. In making a heat test of a large alternator it is customary to imitate full-load conditions, electrically and magnetically, so as to produce all the heating effects that would occur under actual full load, but without taking any actual electrical power from the alter- nator. To be able to do this is of great advantage from two stand- points: first, that of convenience; and second, that of economy. To accomplish this desirable result, the generator is usually run on short- circuit with a number of the field spools connected in opposition to (or "bucked" against) the remainder; or, in other words, with the effective number of poles reduced. To determine the proper number of field-magnet spools to be connected in opposition, we proceed in the following manner: It was previously explained that from the saturation and syn- chronous impedance curves of a generator, we could determine the field current required to produce rated voltage at full load. In the case of the 2,000-kw. generator, of which the curves are given in Figs. 190 and 192, the normal full-load field current was found to be 93.8 amperes. This field current would, if the armature were short-circuited, produce, according to Fig. 192, an armature cur- rent of more than 1,050 amperes, which is greatly in excess of rated full-load current. If the field current were reduced to 43 amperes, the armature current would be normal; but the field current and, ALTERNATING-CURRENT MACHINERY 209 therefore, the magnetic density in the field magnet and the arma- ture core, and the consequent iron losses in the armature, would be very much below normal. We may reduce the excessive current in the short-circuited armature to its normal full-load value by reducing the effective number of field poles in the ratio of 93.8 to 43 instead of by reducing the field current in this ratio. The alternator has 16 field poles, and to reduce this number in the ratio of 93.8 to 43, would give 16 X 43 = 7.34 poles; but inasmuch as there must be an integral 9o.o and even number of field poles, the closest approximation to the desired number is 8 effective poles. If, therefore, we reverse the connections of any four (adjacent) field coils, the four poles pro- duced by these reversed field coils will be reversed in polarity; and the electromotive forces induced in the armature conductors under these reversed poles will be reversed, and will balance the electromotive forces induced in the armature conductors under four of the unreversed field poles, so that the electromotive forces induced in the armature conductors under the remaining eight poles only, will be effective in producing current in the armature, that is, only eight field poles will be effective. If, under these conditions, we short-circuit the armature of the alternator, it will take approximately the full-load field cur- rent of 93.8 amperes to produce full-load current in the armature, and the heat test can then be made under full-load conditions, namely, full-load armature current, full-load field current, and full-load iron losses, while the machine is running on short-circuit and, therefore, delivering no power. Only the power represented by the losses occurring in the machine need be supplied to drive it. This is a method often used in practice. If the normal field current does not give normal armature current, after the spools are con- nected in opposition as described, the effect of a slight excess of armature current may be approximately balanced by a slight de- ficiency of field current, or vice versd. Core Loss and Friction Test. For this test the generator is driven by a motor at normal speed on open circuit. The power required to drive it, its field current being zero, is measured. The power so measured is the power lost in friction and windage. Then 210 ALTERNATING-CURRENT MACHINERY 2,000 K W. at no L pod the field current is increased, step by step, until the terminal electro- motive force has increased to from 25 to 50 per cent above its normal rated full-load value; the terminal voltage is observed at each step, the armature being on open circuit (main switch open). The power required to drive the machine at each of these observed voltages is determined. This power is used to supply friction and windage loss and iron (or core) loss at the observed voltage. The difference, therefore, between the power required to drive the machine at a given voltage on open circuit, and the amount of power required to drive the machine without field excitation, that is, to supply the friction and windage loss, represents the iron loss, or core loss, at the given voltage. Fig. 196 shows the no-load core-loss curve for the 2,000-kw. alternator of which the saturation curve and the synchronous impedance curve are given in Figs. 190 and 192, respectively. The most convenient way to measure the power delivered to the alternator is to drive it by a motor and measure the electrical input to the motor. If we determine the losses occur- ring in the motor the dif- ference between the input to the motor and these losses is the power deliv- ered to the alternator. Fig. 197 shows a diagram of complete connections for the carrying out of the core loss test. A is the alternator under test. Its voltage is measured by the voltmeter V\ connected to the secondary of the potential trans- former T. Its field current, supplied by the exciter E, is measured by the ammeter A\ and controlled by the resistance R E in the field of the exciter. The power is supplied to the motor M by the direct-current generator G. The power is measured by the ammeter A 2 and the voltmeter F 2 . The motor M is separately excited, its field current being measured by the ammeter A 3 . This field current is kept constant by adjusting the rheostat R F . Full 900 200 600 2000 Fig.:i96. Volts in Armature No-Load Core-Loss Curve for 2,000-Kw. Alternator ALTERNATING-CURRENT MACHINERY 211 As the field current and the voltage of the generator under test are increased, the load on the driving motor M is increased also, since the hysteresis and eddy current loss in the generator, which must be supplied by the motor , are increased. As the load on the motor increases, it will slow down if its field current and the voltage between its brushes remain constant. However, the alternator to be tested must be run at constant speed, and its speed is con- trolled by varying the voltage at the motor terminals by means~of the field rheostat R of the direct-current generator G. In the figure the field rheostat R of the generator G, is shown beside the alternator A. It is placed at this point for convenience, so that the observer stationed at the alternator may readily keep the speed under control. To oufs/de source of c/irecf current Fig. 197. Diagram of Connections for Core-Loss Test of Alternators The input to the motor armature in watts is equal to the pro- duct of the readings V% and A^ Part of this input is consumed in supplying the various losses in the motor armature, including friction and windage; and the remainder is converted into useful mechanical power, and is transmitted by the belt (not shown in Fig. 197) to the pulley of the alternator A, which is being tested. In other words, the watts input to A is equal to the input to the armature of M minus the various losses in the armature of M^ The losses in the armature of M consist of hysteresis, eddy current, 212 ALTERNATING-CURRENT MACHINERY friction, windage, and PR losses. Since the alternator is to be run at constant speed, the motor will also run at constant speed if there is no belt slip, which should be the case; and since the motor runs at constant speed and at a constant field excitation, all the losses in the motor armature will be constant in value, whatever its load, except the PR loss. This PR loss varies because the current I varies for different loads on the motor. If the speed of the motor were con- trolled by varying its field current, instead of varying the voltage applied at its terminals, the above-mentioned losses in the motor armature would not remain constant; hence the latter method is adopted. If the motor be run free, i. e., with belt off, the output of the motor will be zero and, therefore, the power input must be used up entirely in supplying the losses. Let E be the voltage at motor brushes, and 7 the current in the motor armature when it is run- ning free (or unloaded), then El stray poweT-\-PR, where "stray power" equals the sum of all the constant losses in the motor arma- ture, i.e., hysteresis, eddy current, friction, and windage losses. Hence, stray poweTEIPR. The mechanical or useful output of the motor at a given load, the voltage applied to its brushes being EI, and its armature current being /i, is output = EJi I\Ri stray power That is, the mechanical output is equal to the total electrical input minus the total losses. The stray power can be determined once for all by running the motor free, or at no load, at the speed and with the field current used in the test when E and I were observed. Thus, we may deter- mine the power required to drive A (the machine to be tested) with zero field excitation of A, in which case we obtain its friction and windage losses; and we may determine the power required to drive A at any given field excitation and voltage, in which case we obtain the sum of the friction, windage, and core (or iron) losses. In carrying out these tests, therefore, the following order is found most convenient: (1) The motor is made to drive the alternator with zero field current in the latter; and a number of observations are taken, as indicated in the tabular arrangement below. This is to determine the friction and windage loss of the alternator. Motors ALTERNATING-CURRENT MACHINERY 213 (2) The alternator is excited to produce a series of values of terminal voltage, for each of which a similar set of observations is taken; and so on, until the voltage of the alternator has been increased to about 25 per cent above normal rated voltage. (3) The motor is now shut down and its belt thrown off, and it is then run unloaded in order to determine its own stray-power losses. (4) Finally the motor is shut down, and the resistance of its armature is measured. For each set of readings the following observations should be recorded: Volts at brushe ( Volts at terminals ' Amperes in armature Alternator I Amperes in field Amperes in field ( Speed Speed The amperes in the field of the motor, and the speed of the motor and generator are recorded merely to show whether they have been kept constant during the test. For the best results the motor should have a rated capacity of from 15 to 20 per cent of the rated capacity of the machine to be tested. Example. In a core-loss test of the 2,000-kw. alternator above referred to, the following observations were taken: With zero field excitation of the alternator, the voltage applied to the motor armature and the current in the motor armature were 510 volts and 48 amperes, respectively. When the alternator was excited to give 2,070 volts between its terminals, the volts applied to, and the current in, the motor armature were 511 volts and 117 amperes, respectively. With the motor running unloaded r the voltage applied to, and the current in, the motor armature were 509 volts and 11.5 amperes, respectively. The resistance of the armature of the motor was 0.056 ohm. Find: (a) The stray-power loss of the driving motor, from the readings taken above when the motor was running unloaded. Solution. Motor input was 509 volts X 11.5 amperes = 5,853 watts. But by definition, stray power El PR. Hence stray power = 5,853 - 0.056 X (11.5) 2 = 5,846 watts (b) The friction and windage loss of the alternator. Solution. Power supplied to motor when driving alternator at full speed with zero field excitation was 510 volts X 48 amperes = 24, 480 watts; and the useful output of the motor or the power used to drive the alternator was the power input to the motor minus stray-power loss and minus PR loss in its armature. Therefore friction and windage loss = 24,480 -5,846- [(48) 2 X 0.056] = 18,505 watts (c) The core (or iron) loss of the alternator. Solution. Power supplied to motor when driving the alternator at full speed, field excited to give 2,070 volts between alternator terminals, was 511 volts X 117 amperes = 59, 787 watts. The useful output of the motor in this case was 59,787 watts-5,846 watts -[(117) 2 X 0.056] = 53,175 watts; and this is equal to the sum of friction and windage loss and iron loss. Therefore core loss of alternator = 53,175 - 18,505 = 34,670 watts I 214 ALTERNATING-CURRENT MACHINERY The core loss of the alternator was calculated for each value of the field excitation of the alternator in a manner similar to the above. These various core losses are plotted as ordinates of the curve in Fig. 196, the abscissas being the electromotive forces be- tween alternator terminals corresponding to the various field ex- citations. This curve shows that at the rated full-load voltage, namely, 2,000 volts, the core loss of the alternator is 30.5 kilowatts or 1.525 per cent of the rated full-load output of the alternator. Calculation of Efficiency. 2,000-Kw. Alternator. Since the efficiency of an alternator is the ratio of the output to the output plus the losses, as explained on page 181, we may now calculate the efficiency of the alternator, having made all necessary tests. In the case of the 2,000-kw. alternator referred to above, for which the curves have been given, the following losses occur: (1) Friction and windage loss. The loss was found under (b) in the example just given to be 18,505 watts. (2) PR loss in armature at full load. The resistance of each phase of the armature winding of the given alternator was measured and found to be 0.00924 ohm, and the full-load rated current is 576 amperes per phase. Therefore, the PR loss per phase is (576) 2 X 0.00924 = 3,065 watts Hence, the total PR loss for all three phases is 3 X 3,065 watts = 9,195 watts (3) PR loss in the field. The resistance of the field winding of the given alternator is 0.966 ohm, and the full-load field current is 93.8 amperes. Hence, PR loss in the field is 8,500 watts. (4) Core loss. The core loss on open circuit at full-load excita- tion was found to be 30,500 watts. This is somewhat less than the core loss would be with full-load excitation and with full-load arma- ture current. Therefore, the sum of all the losses is Friction and windage loss 18,505 PR loss in armature 9,195 PR loss in the field 8,500 Core loss . 30,500 Total loss - 66,700 watts ALTERNATING-CURRENT MACHINERY 215 The rated full-load output being 2,000 kilowatts, we have 2,000,000 efficiency at full load = 2 WQQ + 66?7QQ = 96.8 per cent If the efficiency at any fractional part of full load is desired, it may be calculated as follows: J rated full-load output ^ rated full-load output + losses at half load (1) Friction and windage loss is approximately the same at half load as at full load. (2) PR loss in the armature is one-fourth as great as at full load since the armature current is half as great. (3) PR loss in the field is slightly less at half load than at full load, inasmuch as field current is slightly less. (4) Core loss at half load is slightly less than core loss at full load. 135-Kw. Inductor Alternator. Fig. 191 shows the efficiency curve of a two-phase, 135-kilowatt, 2,400- volt, 60-cycle inductor alternator. The ordinates of this efficiency curve represent the efficiencies of this alternator corresponding to different current outputs per phase, the latter being plotted as abscissas. The efficiencies shown by this curve apply to the machine when it delivers power to non- inductive receiving circuits (100 per cent power factor). When the power factor is less than 100 per cent, the efficiencies are less than the efficiencies shown by the curve. At full-load current output of 28.1 amperes per phase and 100 per cent power factor, the efficiency as represented by the ordi- nate of the curve is 93 per cent. At half-load current output of 14.05 amperes per phase, the efficiency is 90 per cent. At full load the several losses in this machine are as follows: (1) Friction and windage loss (obtained by experiment), 2,000 watts. (2) PR loss in the armature (calculated from hot resistance of armature 1.46 ohm per phase, and full-load armature current 28.1 amperes per phase) is 2 X (28.1) 2 X 1.46 = 2,305 watts 216 ALTERNATING-CURRENT MACHINERY (3) PR loss in the field (calculated from hot resistance of field coil 9.35 ohms, and full-load field current 8.8 amperes) is 9.35 X (8.8) 2 - 724 watts (4) Core loss (determined by experiment using the method described on page 209), 5,000 watts. The total loss is, therefore Friction and windage loss 2,000 watts I 2 R loss in armature 2,305 watts PR loss in field 724 watts Core loss 5,000 watts Total loss 10,029 watts Therefore 135 000 efficiency at full-load - m ,m'+W,l = 93 per cent SYNCHRONOUS MOTORS Motors designed to be operated with alternating currents may be divided into two classes: synchronous motors and induction motors. Both kinds are in common use; and although there are a few other motors which do not come under the above classification, yet by far the larger part of all the motors run with alternating cur- rents belongs to one or the other of these classes. The induction motor, pages 371-420, having properties which adapt it to a much wider field of application than is possible with the synchronous motor, is much the more commonly used. Any Alternator a Synchronous Motor. When a given armature conductor of an alternating-current generator is under a north pole of the field magnet, the current in the conductor is in a direction such that the force which the field exerts on the wire opposes the motion of the armature. When the given conductor has moved sufficiently to be under a south pole of the field magnet, the cur- rent will have reversed in direction, and the force which the field exerts on the wire will still oppose the motion of the armature. The work done in driving the armature against these opposing electro- magnetic forces is the work that goes to maintain the alternating current delivered by the alternator. ALTERNATING-CURRENT MACHINERY 217 Consider an alternator driven by a small engine or auxiliary motor. Let an alternating current from an outside source be forced through the revolving armature of the alternator, the field magnet of which is supplied with a direct current from an exciter. Then the motion of the revolving armature will be helped by the alter- nating current if the following conditions are satisfied: (a) If the speed of the armature is such that a given armature con- ductor moves from the middle of a north pole to the middle of a south pole during the time of one alternation (half a cycle) of the supplied alternating current. (b) If the direction of the supplied alternating current, when the given conductor is under a north pole, is such that the force exerted upon the con- ductor by the field helpsv the motion of the armature. This is evident when we consider that the current reverses every time the given conductor passes from one field pole to the next' and that this reversed current will be^ffcted upon by the re- versed polarity of the next pole with a force always in the direction of the motion. When the alternator speed has been carefully adjusted so that the conditions (a) and (b) are satisfied, the driving engine or auxiliary motor may be disconnected; the armature of the alternator will continue to revolve at constant speed (the frequency of the sup- plied alternating current being constant), and the revolving arma- ture may deliver power by belt to drive machinery. An alt:rnator used in this way is called a synchronous motor. Any alternator may be used as a synchronous motor without alteration of any kind. Electrically and mechanically, the syn- chronous motor is the same as the alternating-current generator. In fact the same machine is often used indifferently as motor or generator according to circumstances. Composite field winding is never provided on an alternator which is to be used as a synchronous motor. Synchronous motors may be designed to operate either on single-phase or polyphase systems, and are called synchronous be- cause they always run in synchronism with, i.e., at the same fre- quency as, the alternator supplying the current to them. The speed of the motor cannot change unless the speed of the generator changes; but it is not necessarily the same speed as that of the gen- 218 ALTERNATING-CURRENT MACHINERY erator. The speed of the motor will be the same as the speed of the generator only when the motor happens to have the same num- ber of poles as the generator. The speed at which a synchronous motor will run when connected to an alternator supplying current at a frequency /, is 2X/X60 s P where s is speed of the motor in revolutions per minute; / is fre- quency of the alternating-current supply; and p is number of poles on the motor field. For example, if a 10-pole motor were run from a 60-cycle alternator, the speed of the motor would be 2 X 60 X 60 -JQ = 720r.-p.-ai Moreover, it follows that if the motor had the same number of poles as the alternator, the speed of the motor would be just the same as the speed of the alternator, and any variation in the speed of the latter would cause a corresponding change in the speed of the motor; or if the motor had half as many poles as the generator, its speed would be double that of the latter, and any change in the speed of the generator would cause a proportional change in that of the motor; in other words, the cyclic speeds must be the same. Advantages. The synchronous motor, especially in units of large output, possesses a number of features which make its use at times preferable to that of the induction motor. Its advantages may be briefly summed up as follows : (a) Unvarying speed at all loads. (b) Power factor, variable at will by change of the exciting current, can be made approximately unity at any load. (c) The current in the armature can be made to lead the electromotive force by over-exciting the field magnets, thus producing the same effect as a large condenser. The leading current in the armature can be used to neutralize the unfavorable effects of inductance (which causes lagging currents) in other parts of the system. (d) The synchronous motor is cheaper to build, especially at low speeds, than the induction motor. (e) Its efficiency is generally higher than that of the induction motor. (f) It is specially adapted to high-voltage winding. ALTERNATING-CURRENT MACHINERY 219 Disadvantages. The synchronous motor, on the other hand, has several disadvantages, as follows: (a) It is not adapted to work requiring variable speed, as no independ- ent speed regulation is possible. (b) It has small starting torque; hence it is not suitable for work re- quiring large starting torque, or frequent starting of the load. (c) It has a tendency to "hunt." (d) It requires an exciting current which must be supplied by an out- side source. (e) It requires the most skilful and intelligent attention. Synchronous motors are used where power is required in large amounts, and where the motor does not have to be started and stopped frequently. Comparison of Synchronous and D. C. Motors. Synchronous motors behave differently from direct-current motors. Thus, if the field of a direct-current motor be weakened, the motor will speed up in order to keep the counter-electromotive force at a proper value. But if the field strength of a synchronous motor be changed, the speed cannot change, because the motor must run in synchronism with the alternator that supplies it with current. What then does enable a synchronous motor to adjust itself to changes of load and field strength? It is the change of phase difference between the arma- ture current and the applied electromotive force. Suppose a synchronous motor is running light, i. e., unloaded, and suppose further that there is no friction, hysteresis, or eddy- current losses ; then, if this motor be run up to synchronism, and its field adjusted to such a strength that the counter-electromotive force of the motor is equal and opposite to that of the generator, no current will flow through the circuit when the circuit is closed. At any instant the electromotive force that causes current to flow in the circuit is the difference between the instantaneous electro- motive force of the alternator and the counter-electromotive force of the motor. On loading the motor, its armature will lag a small fraction of a revolution behind that of the alternator, so that the counter-electromotive force of the motor will no longer be in op-, position to that of the alternator. -The result is that there will be a current of a ^r*\gnitude such as to produce the torque necessary to enabk }he motor \o ~arry its load. The greater the load on the rootor, Aie greater the lag of its armature, and hence the greater 220 ALTERNATING-CURRENT MACHINERY the difference in phase between the applied and the counter-electro- motive forces; this, in turn, permits a larger current to flow to supply the additional torque required. If, however, too great a load is put on the motor, the slipping behind of the armature will become sufficiently great to throw the motor out of synchronism, or to cause the motor to "break down," when it will stop. In other words, the motor, under these conditions, cannot exert sufficient torque to handle the load. 1 Starting the Motor. Single-Phase Circuit. If a single-phase alternator be electrically connected to alternating-current supply mains, the machine will not start up and run as a motor, because the current in its armature is rapidly reversing, thus tending to turn the armature first in one direction and then in the other direc- tion in rapid succession. Such a synchronous motor, therefore, whether loaded or unloaded, must always be started and brought up to full speed by an engine or other outside source of power, since it develops no starting torque whatever when thrown in circuit at a standstill. Polyphase Circuit. If a polyphase alternator, on the other hand, is connected to polyphase supply mains, the machine will start and run up to full speed if it has little or no belt load. This self-starting property of the polyphase synchronous motor is ex- plained as follows: As one of the polyphase currents through the armature of the machine dies away, it leaves a slight amount of residual magnetism in the field-magnet structure if the field magnet is not excited by direct current. This residual magnetism acts upon the growing current of the other phase (or phases), and produces a torque tending to turn the armature. This action of the polyphase alternator is essentially the same as the action of the induction motor. A polyphase alternator which is to be used as a synchronous motor develops but little starting effort when connected directly to the supply mains, hence it is generally started, especially in larger sizes, by means of a small engine or auxiliary motor, and then thrown into circuit, after which its load is connected to it through a fric- tion clutch, thus allowing it to be thrown on gradually. In smaller sizes the machine is generally self-starting without load, the load being thrown on afterwards. Exciter. The exciter of a synchronous motor (a small direct- current dynamo) is usually belted to, or mounted upon, the synchro- ALTERNATING-CURRENT MACHINERY 221 nous motor shaft, so that when the synchronous motor has been brought up to full speed either by separate starting or by self-starting, the exciter is in full operation and is in readiness to supply current for exciting the field magnet of the synchronous motor. Separate Starting. In the case of the single-phase machine, the power for starting is always derived from a source entirely inde- pendent of the single-phase supply. In the case of the polyphase machine, the power for starting is usually developed by a small induction motor supplied with polyphase currents from the mains that supply currents to the synchronous motor itself. In other words, the method of separate starting is essentially the same for both single-phase and polyphase machines, the procedure being exactly the same as for starting and adjusting an alternator that is to be connected in parallel with another alternator already in opera- tion. Self-Starting of the Polyphase Type. In the self-starting of the polyphase synchronous motor, its armature terminals may be con- nected directly to the polyphase supply mains with its field unexcited. When the machine reaches synchronous speed (driving its exciter), the field is connected to the exciter. The machine is then in full operation as a synchronous motor, and its load may be gradually thrown on. The objection to this mode of starting is that the machine takes excessively large lagging currents at starting; and this gen- erally causes a drop in the supply voltage great enough to disturb seriously the general system of distributing mains from which the synchronous motor receives its currents. This excessive demand for current at starting is objectionable when the motor takes a large proportion of the generator output, or is used in connection with a lighting service, especially when the motor is started and stopped at frequent intervals. To avoid an excessive demand for current at starting, an auto- starter or compensator is frequently employed. The starting com- pensator for a two-phase synchronous motor consists of two trans- formers (three transformers for a three-phase machine) having their primaries connected across the respective phases of the supply mains, their secondaries being provided with a number of taps so that, at starting, a fraction of the full supply voltage can be applied 222 ALTERNATING-CURRENT MACHINERY to the armature terminals of the synchronous motor. This fraction is usually from 40 to 60 per cent of the full voltage; and a switch- ing device is provided by means of which the change from frac- tional to full voltage can be quickly made when the synchronous motor reaches full speed. This starting compensator is also used in connection with induction motors. The transformers used in the starting compensator are always autotransformers. The self-starting of a polyphase synchronous motor (by induc- tion-motor action) depends upon the magnetizing action on the unexcited field magnet poles by the armature currents. Therefore, a polyphase alternator having high armature reaction, that is, large magnetizing action on the field for a given armature current, as in the case of an armature with concentrated windings or a machine with small air gaps, will give a large starting torque when used as a self -starting synchronous motor. The revolving field-structures of self -starting synchronous motors are usually provided with squirrel- cage windings to increase the starting torque when used with start- ing compensators, and to reduce the tendency to hunt. Usually less than one minute is required to bring even large synchronous motors to full speed by either method of starting. At the time of starting, the armature and field windings of a synchronous motor are related to each other as are the primary and the secondary of an alternating-current transformer. The result is, that when the field coils have many turns of wire, a dangerously high electromotive force may be induced in them. This production of high voltages in the field coils of a self- started polyphase synchronous motor may be, to a great extent avoided, by using few turns of large wire in the field winding, thus necessitating the use of a low-voltage exciter. For this reason ex- citers giving an electromotive force as low as 50 volts are frequently used. Another method of obviating the danger referred to, is to provide short-circuited metal rings around the field poles. These rings limit the changes of magnetism in the pole-pieces, and thereby prevent the formation of excessively high induced voltages in the field coils. In synchronous motors of the stationary field type, the field circuit may be broken up into many separate parts so as to divide up the induced electromotive force. Thus, Fig. 198 shows the ALTERNATING-CURRENT MACHINERY 223 stationary field of an alternator (synchronous motor) with the ter- minals of each field spool brought out to convenient switches on the frame of the machine. During starting these switches are open, and when the machine has reached synchronous speed, they are all closed, thus connecting all the field spools in series to the exciter. Hunting Action. When the load on a synchronous motor is suddenly increased, the motor slows down momentarily and falls behind the generator in phase. When the motor has fallen behind sufficiently to take in power enough to enable it to carry its load, it is still running slightly below synchronism; it, therefore, falls still further behind, and takes an excess of power from the generator, Fig. 198. Diagrammatic View of Stationary Field of a Syn- chronous Motor which quickly speeds it above synchronism. It then gains on the generator in phase until it is taking in less power than is required for its load, when it again slows down, and so on. This oscillation of speed above and below synchronism, called hunting, is accompanied by great changes in the current supplied to the synchronous motor, and by rapid rise and fall of the electromotive force between the terminals of the motor. It is frequently a source of great annoy- ance, especially where several synchronous motors, or rotary con- verters, are run in parallel from the same mains. Hunting is frequently produced by the periodic changes in the speed of the engine that drives the generator. Thus the engine momentarily increases its speed as the steam acts upon the piston 224 ALTERNATING-CURRENT MACHINERY at each stroke, and diminishes its speed in the intervals between the strokes. The hunting of a synchronous motor is a phenomenon of the same nature as the hunting of a steam engine having an over-sen- sitive governor. When the load on the engine is suddenly increased, the engine slows down momentarily, causing the governor to admit more steam than is needed for the increased load. The result is that the engine quickly speeds up, causing the over-sensitive governor to shut off too much steam, so that the engine slows down again, and SQ on. Hunting is associated with more or less violent shifting of the resultant magnetic flux in the air gap under the poles. This is due to the fluctuations in the armature current, caused by the flow of the "corrective" current be- tween generator and alter- nator while hunting occurs. Reduction by Use of Dampers. Hunting is greatly reduced or entirely elimi- nated by the use of heavy copper frames or dampers in the neighborhood of the poles. One form of damper is shown in Fig. 199, which is a view of a portion of the revolving field of an alternator. The dampers consist of rectangular copper frames driven into place under the overhanging tips of two adjacent poles. A damper is provided between each pair of adjacent poles, all around the field, both in the alternator and in the synchronous motor. Another form of damper which has been found very effective is called the "squirrel-cage" damper. Heavy bars of copper are placed in slots at the surface of the poles and their ends bolted to two closed copper rings,so as to short-circuit all the bars. This cage- damper applied to a rotor for use as a synchronous motor is illus- trated in Fig. 200, which relates to a 200-kilowatt "type E" machine of the Westinghouse Company. The principle of the damping action is that the shifting magnetic Fig. 199. Portion of Revolving Field Showing Copper Frame Dampers ALTERNATING-CURRENT MACHINERY 225 field sets up induced currents in the short-circuited frames or bars of the damper, and these currents react on the magnetic field so as to oppose the shifting of the flux and thereby dampen the hunting oscil- lations. The electrical effect of the dampers is analogous to the mechanical effect produced by immersing the bob of a swinging pen- dulum in a heavy oil which resists the motion of the bob. Fig. 200. Rotor of a Synchronous Motor Provided with "Squirrel- Cage" Form of Damper Torque and Power Output. A given synchronous motor operat- ing with given applied voltage is capable of developing a definite maximum torque, or of delivering a definite maximum amount of power. An attempt to take more than this maximum power from the machine, causes the machine to fall out of step, that is, out of proper phase relation, with respect to the supply voltage; and the motor accordingly stops. A properly designed synchronous motor will, however, carry a reasonable overload befort, reaching the above-mentioned maximum at which the machine stops. 226 ALTERNATING-CURRENT MACHINERY The maximum power that can be delivered by a synchronous motor is greatly increased by increase of the applied voltage, and greatly decreased by a decrease of the applied voltage. In fact the maximum power. output is proportional to the square of the applied voltage; therefore, the voltage of supply should never be allowed to fall much below the normal or rated value. Field Excitation and Power Factor. While the power factor of a non-synchronous (induction) alternating-current motor is fixed by its design, and its current is always lagging behind the applied electromotive force, the current delivered to a synchronous motor may be made either lagging or leading at will. This remarkable control of the phase of the current is accomplished by varying the strength of the field exci- tation. An increase in the field excitation of a synchronous motor will cause a corre- sponding increase in the counter-electromotive force generated in the motor ar- mature. By properly ad- justing the field excitation, this counter-electromotive Fig. 201. Curve Showing Variation of Armature Current with Field Excitation current, force of the motor can be made considerably greater than the electromotive force applied at the motor terminals. The result is that an increased but leading current, that is, one ahead of the applied voltage, as in a condenser, flows in the armature. On the other hand, a field excitation below the normal amount produces an increased but lagging current, that is, one behind the applied voltage, as in an inductance coil. If the field excitation is normal, that is, of such a value that the current in the motor armature is exactly opposed in phase to the counter-electromotive force (unity power factor), then the effective value of this current will be a minimum; and hence the efficiency of the motor, generator, and transmission lines will be a maximum because the PR losses will be minimum. Considered simply as a motor, without reference to the trans- ALTERNATING-CURRENT MACHINERY 227 mission system as a whole, the most efficient point of operation is with unity power factor, or, in other words, with a field excitation which will make the armature current a minimum. The effect upon the armature current, produced by varying the field excitation, is shown by the curves in Fig. 201. Up to a certain point, as the excitation is increased, the armature current is lagging, and decreases to a minimum value. Further increase of the exciting current causes the armature to take more current, which is now ahead of the applied electromotive force in phase, that is, is now leading. There is one value of the exciting current for which the armature current is a minimum. In motors of good regulation this value of the exciting current varies but slightly with different loads. Use as a Condenser. A synchronous motor with its field mag- net over-excited takes a current which is ahead of the applied electro- motive force in phase. Such a machine may, therefore, be connected la Fig. 202. Diagram Showing Synchronous Motor Used as a Condenser across the terminals of an inductive receiving circuit, as shown in Fig. 202, so as to compensate for lagging current delivered to the receiving circuit, thus reducing the line current to the lowest value that will suffice to transmit the power taken by the receiving circuit. The clock diagram, Fig. 203, shows how the leading current 7 2 taken by the synchronous motor M, Fig. 202, gives, when combined with the current I\ delivered to the receiving circuit, a resultant line current /, which is in phase with the electromotive force E between the mains. A synchronous motor used primarily for compensating the lagging current delivered to an inductive receiving circuit is called a rotary condenser or a synchronous compensator. The rotary con- denser is especially useful when induction motors or lightly loaded 228 ALTERNATING-CURRENT MACHINERY transformers or both, are supplied over a long transmission line. In such cases the reduction of the line current to the smallest possible value effects considerable saving in the matter of power losses in the transmission line. The use of a rotary condenser is also an ad- vantage in that the regulation of voltage at the receiving end of the transmission line is improved. A given synchronous motor is most effective as a rotary con- denser when it is not required to deliver any mechanical power as a motor, that is, when it is run at zero load. When a synchronous motor is to be used to deliver mechanical power, as well as to take a leading current for the purpose of compensating the lagging current taken by the inductive receiving circuit, it is customary to limit the load on the motor to 70.7 per cent of its full-load rating, that is, its rating if it were to be used as a motor only, and not as a rotary condenser. When the motor takes in its full-load rated current, but only 70.7 per cent of its full-load rated power, its field being over-excited, then its current is 45 degrees ahead of the e. m. f.; the power factor of the motor is E 70.7 per cent; the power component of the current, that is, the compo- nent which is in phase with the sup- ply electromotive force, is 70.7 per ,-,. ono ni cent of the full current: and the Fig. 203. Clock Diagram of Cur- rent Relations^ Synchronous wattless component of the current (the component which is 90 degrees ahead of the supply electromotive force in phase) is also 70.7 per cent of the full-load current. For example, a synchronous motor rated at 100 kilowatts at 1,000 volts would have a full-load rated current of 100 amperes, the field of the machine being excited to give unity power factor. If this machine is to be operated as a rotary condenser, its field excitation will have to be increased. If the machine is to carry a load of 70.7 kilowatts, and at the same time to take as large a lead- ing wattless current as possible (without, however, exceeding the full-load rated current of 100 amperes), then the field will have to be over-excited to such an extent as to cause the machine to take its full rated current of 100 amperes at 70.7 kilowatts load. In this ALTERNATING-CURRENT MACHINERY 229 case the power component of the 100-ampere current will be 70.7 amperes; the wattless component of the 100-ampere current will be 70.7 amperes [lOO = 1/(70.7) 2 + (70.7) 2 ]; and the power factor of the over-excited motor will be 70.7 per cent. MOTOR TESTING The difference between a synchronous motor and an alternating- current generator consists mainly in the method of operating; any alternator will run as a synchronous motor, and vice versa. There- fore, all the tests described on pages 188-216, with reference to alter- nators, may be applied in a similar manner to synchronous motors. In making a heat test on a small synchronous motor, it is usually run at full load as a motor. In the case of large synchronous motors, the "heat-run" or test is usually made by running the machine as a generator on short- circuit, with a portion of its field coils connected in opposition, as described on page 205 on the heat test of alternators. The efficiency of a synchronous motor is calculated in the same manner as that of an alternator, page 214. Phase Characteristic. Regulation for alternators, as described on page 200, is not calculated in the case of a synchronous motor; but the determination of the "phase characteristic" of a synchro- nous motor corresponds to, and is substituted for, the regulation test. A "phase characteristic" is a curve showing the relation be- tween the armature current and the field current of a synchronous motor, the test being carried out under constant conditions with respect to voltage, frequency, and load. Phase characteristics are shown in Fig. 201. Phase characteristics are usually taken at no load, although it not infrequently happens that it is desired to obtain a phase char- acteristic at full load. To make the test at no load, it is simply necessary to run the motor unloaded, supplying alternating currents of the proper voltage and frequency to the armature terminals of the motor. The field current is then varied by successive steps,, both above and below its normal value, until the armature current has attained rated full-load value, both for leading and for lagging current. For each value of the field current, simultaneous obser- vations are made of the amperes in the armature, the volts supplied 230 ALTERNATING-CURRENT MACHINERY to the armature terminals, the amperes in the field, and the speed. The normal field current is that value for which the armature cur- rent has the lowest value. If a phase characteristic at full load is desired, the most con- venient w r ay of loading the motor is to cause it to drive a direct- current generator of known efficiency, by means of a belt connecting their respective pulleys. The power output of the direct-current generator can be accurately measured by means of an ammeter and a voltmeter. Knowing the output, and the efficiency of the direct-current generator at any output, the mechanical input, to the generator, that is, -^r-r .can be calculated. This input to the efficiency generator is evidently equal to the mechanical output of the syn- chronous motor, the power lost in the belting being negligible. The load on the motor must be kept constant throughout the test. The armature current for a full-load phase characteristic cannot be varied through such a wide range of values as at no load, owing to the inability of the armature windings of the motor to carry the excessive current without over-heating. Pulsation Test. This test is to determine whether or not the synchronous motor has a decided tendency to hunt. For this test the synchronous motor is supplied with alternating current or currents from a very steadily driven alternator over an "artificial transmission line." This "artificial line" consists simply of a resist- ance equal to the resistance* of the transmission line over which the synchronous motor is to be supplied with current when finally installed. The synchronous motor is driven at zero load, taking current through the artificial line, the tendency to hunt being greatest at zero load. The connections are the same as in the test for phase characteristic. The hunting action is indicated by the swinging to and fro of the pointers of the ammeters and voltmeters. The field current of the motor is varied from considerably below to consider- ably above rated full-load field current. For each value of the field current, the indications of the instruments are carefully observed; the pulsations of the instruments are noted. *Usually the transmission line has a resistance such as to give a 10 per cent drop of electromotive force when full-load current is delivered to the motor. ALTERNATING-CURRENT MACHINERY 231 NOTE. During this test, care must be taken that the alternator supply- ing the power to drive the motor under test, does not pulsate, for, if it does, the pulsations of the instrument pointers would not then give a reliable indi- cation of the performance of the motor -itself under normal working conditions. Break=Down Test. As its name implies, this test is to deter- mine the maximum power that a synchronous motor will deliver at its pulley, before falling out of synchronism and stopping. As in the case of the full-load phase characteristic, the power output of the motor is most conveniently absorbed and measured by belting the motor to a direct-current generator, and measuring the electrical output of this generator. For further description and details of the break-down test, the reader is referred to the article on the break- down test for induction motors. Self=Starting Test. The object of a starting test on a syn- chronous motor is to determine: (a) The voltage and current required to start the motor. (b) The time required for the motor to reach synchronous speed (syn- chronism). (c) The electromotive force induced in the field-magnet windings at the instant of starting. The starting test is made on polyphase motors only, for, as previously stated, single-phase synchronous motors are not inher- ently self -starting, but must, even in the smaller sizes, be provided with special starting devices. The synchronous motor to be tested is connected to mains supplying alternating currents of the proper frequency. Arrange- ment is made to adjust the voltage applied to the armature ter- minals of the motor by means of potential (voltage) regulators con- nected in the armature circuits between the supply mains and the armature terminals. An ammeter is connected in series with each phase winding of the armature, that is, two ammeters are needed for a two-phase motor, and three ammeters for a three-phase motor. A voltmeter is connected to the terminals of the secondary coil of a small step-down potential transformer, the primary coil of which is connected to the terminals of the field winding of the motor. A voltmeter is also connected between the supply mains. The test is made with no current in the field and the field cir- cuit open. * The voltage applied to the armature terminals is slowly *The primary of the potential transformer takes but little current, and the field cir- cuit, to all intents and purposes, is open. 232 ALTERNATING-CURRENT MACHINERY increased until the motor starts to revolve. At the instant that the motor starts, the readings of all the instruments are recorded as follows: volts between armature terminals; amperes in the arma- ture (each phase); and volts induced in field windings. After the motor starts, the voltage applied to the armature terminals is kept constant; and the time required for the motor to attain synchronous speed, reckoned from the instant it starts, is observed. The exact instant that synchronism is reached, is indi- cated by violent swings of the pointers of the ammeters as well as those of the voltmeters connected to the armature terminals. At synchronism, another set of observations is taken. In order to find the most unfavorable position of the armature and the corresponding starting current, and the time required to reach synchronous speed from this most unfavorable position, the above procedure is repeated for a series of initial positions of the armature, chosen as follows: The circumference of the armature between the centers of two adjacent field poles is divided into a number of equal parts, this number not being a multiple of the number of phases, nor of the number of slots per pole per phase; usually there are seven parts. The starting positions thus chosen will include every possible posi- tion that a magnet pole may have relative to an armature slot. These parts are marked by chalked lines, and the various starting positions of the armature relative to the field-magnet poles are deter- mined by setting each of the marks in succession into coincidence with a given field pole tip, ALTERNATING-CURRENT MACHINERY PART IV TRANSFORMER Description. The transformer consists of two separate and distinct coils of wire insulated from each other, and wound upon one and the same laminated iron core. Fig. 242, page 273, shows a sec- tional view of a commercial type of transformer. In practice, one of the coils receives alternating current from a high (or low) voltage source of supply; and the other coil delivers alternating current to a receiving system at a low (or high) voltage. When the transformer receives alternating current at high voltage and delivers it at low voltage, we have what is called step-down transformation; when the transformer receives alternating current at low voltage and delivers it at high voltage, we have what is called step-up transformation. The coil of a transformer which receives alternating current from a source of supply, is called the primary coil; and the coil which delivers alternating current is called the secondary coil. In Fig. 242, each limb of the core is wound with half of the secondary coil (coarse wire) next to the core, and with half of the primary coil (fine wire) over the secondary. The alternator, as we have already seen, is a machine in which an alternating electromotive force is produced by the cutting of a permanently established magnetic flux by wires on account of the motion of the flux relative to the wires. The transformer, on the other hand, is an arrangement whereby an alternating electromotive force is produced in a stationary coil of wire (secondary) by reversals of magnetic flux through a station- ary iron core, these reversals of flux being produced by alternating current supplied to the primary coil of the transformer. Copyright, 191%, by American School of Correspondence. 234 ALTERNATING-CURRENT MACHINERY Physical Action. Without Load. When the secondary of a transformer is on open circuit, it can of course, deliver no current; and the transformer is said to be operating at zero load. Under these conditions, only a small amount of alternating current flows through the primary coil. This current causes repeated reversals of magnetic flux through the iron core. These reversals of magnetic flux induce electromotive forces in both coils. The electromotive force thus induced in the primary coil is opposite in direction, and very nearly equal, to the electromotive force applied to the primary coil. Only the difference between the applied electromotive force and the opposing induced electromotive force is available for producing current through the primary coil; and since this difference is small, the primary current is small at zero load. The primary current at zero load is called the no-load current of the transformer. With Load. Whether the secondary coil of a transformer is delivering current or not, the reversals of magnetic flux in the trans- former core always induce an alternating electromotive force in the coil. When alternating current is taken from the secondary of a transformer, the transformer is said to be loaded. The action of this secondary current as it flows through the secondary coil, is to oppose the magnetizing action of the slight current already flowing in the primary coil, thus decreasing the maximum value reached by the alternating magnetic flux in the core, and thereby decreasing the induced electromotive forces in both coils. The amount of this decrease, however, is' very small, inasmuch as a very small decrease of the induced electromotive force in the primary coil greatly in- creases the difference between electromotive force applied to the primary coil and the opposing electromotive force induced in the primary coil, so that the primary current is greatly increased. In fact, the increase of primary current due to the loading of the trans- former is just great enough (or wry nearly) to exactly balance the mag- netizing action of the current in the secondary coil; that is, the flux in the core must be maintained approximately constant by the primary current whatever value the secondary current may have. Electromotive Force Relations. The electromotive forces in- duced in the respective coils of a transformer are proportional to the number of turns of wire in each; and from the above discussion, it is evident that the electromotive force induced in the primary coil ALTERNATING-CURRENT MACHINERY 235 is sensibly equal to the supply electromotive force, whether the transformer is loaded or not. Therefore ^-- Z/ (32) E" Z" in which E f is the electromotive force applied to the primary, E" is the electromotive at which the secondary coil delivers alternating current, Z f is the number of turns of wire in the primary coil, -and Z" is the number of turns of wire in the secondary coil. Current Relations. The magnetizing action of the primary current /' of a transformer having Z' turns of wire in its primary coil, may be expressed by the product Z'I f , that is, by ampere-turns; and similarly, the magnetizing action of the secondary current /" may be expressed by the product Z"l n ', where Z" is the number of turns of wire in the secondary coil. Therefore, since the magnetiz- ing actions of the two coils are equal (and opposite), we have Z f I'=Z"l" or r/ v/r T-T (33) in which Z f and Z" are the turns of wire in the respective coils; I" is the current delivered by the secondary coil; and I' is the increase of current taken by the primary coil over and above the no-load cur- rent, due to the fact that the secondary coil is delivering current. Now in most commercial transformers the no-load current is quite small; and, neglecting this current entirely, the only current in the primary coil would be the increase of primary current due to the fact that the secondary coil is delivering current. Therefore, equation (33) expresses, with sufficient accuracy for most purposes, the relation between the actual primary current /' and the secondary current I". Summary of Electromotive Force and Current Relations. A trans- former which delivers current I" to a receiving circuit, takes an amount Z" "' of current equal to I" y( (= I') from the source of supply. The A electromotive force of the source of supply is E f , and the electro- motive force at which the secondary delivers current is equal to E'X~ (=") 236 ALTERNATING-CURRENT MACHINERY Example. A certain transformer rated at 5j kilowatts has 660 turns of wire in its primary coil and 66 turns of wire in its secondary coil. The primary coil is connected between 1,100-volt supply mains. Therefore, the secondary electromotive force, by equation (32) is f7 f f A A E" = - - X E' = - - X 1,100 = 110 volts Z 660 If ten lamps, each taking half an ampere, are connected to the secondary coil, the secondary current will be 5 amperes; and the primary current, by equation (33) will be r *frX 7 "-J| X 5 = .5 ampere The power delivered to the lamps by the secondary coil is equal to E" I" since the lamp circuit is non-inductive. That is: Power delivered to lamps = 110 volts X 5 amperes = 550 watts. The power delivered to the primary coil in this case (non-inductive sec- ondary circuit), is equal to E' I'. That is: Power delivered to primary = 1,100 volts X 0.5 amperes = 550 watts. If 100 lamps, each taking half an ampere, are connected to the secondary coil, the secondary current will be 50 amperes, and the primary current will be 5 amperes; the power delivered to the lamps will be 5,500 watts, and the power delivered to the primary coil will also be 5,500 watts. The above calculations ignore the following actions which take place in an actual transformer: (a) losses of electromotive force in overcoming the resistances of primary and secondary coils; (b) losses of power (PR) in the primary and secondary coil; and (c) loss of power in the iron core, due to hysteresis and eddy currents. Automatic Action of the Transformer. When the load on a transformer is increased, the primary of the transformer automatically takes additional current and power from the supply mains in direct proportion to the load on the secondary. When the load on the secondary is reduced, for example, by turning off lamps, the power taken from the supply mains by the primary coil is automatically reduced in proportion to the decrease in the load. This automatic action of the transformer due to the balanced magnetizing action of the primary and secondary currents is illustrated in the above example. Ideal and Practical Transformer. The foregoing discussion of electromotive force and current relations in a transformer is based upon the following assumptions: (a) That the no-load current of the transformer is negligible, and that it represents no power taken from the supply mains; or, in other words, that eddy-current and hysteresis losses are absent. ALTERNATING-CURRENT MACHINERY 237 (b) That the resistance of the coils is negligible, so that the electro- motive force applied to the primary coil is wholly balanced by the opposing electromotive force in the primary coil, and so that the whole of the electro- motive force induced in the secondary coil is available at the terminals of that coil. (c) That all the magnetic flux which passes through the primary coil passes through the secondary coil also; or, in other words, that there is no magnetic leakage. A transformer that would meet these conditions would be~arr ideal transformer. A well-designed transformer operating on mod- erate load does approximate quite closely to the ideal transformer in its action; and equations (32) and (33) are much used in practical calculations. For some purposes, however, it is desirable to con- sider the action of the transformer, taking account of coil resist- ances, of eddy currents and hysteresis, and of the fact that some lines of magnetic flux pass through one coil without passing through the other (magnetic leakage). The extent to which a well-designed transformer deviates from an ideal, is exemplified by the following actual results obtained with the 5J-kilowatt transformer used in the example on the preceding page. At no load, the value of E" is 109.8 volts; the no-load current is 0.129 amperes; and the power taken from the mains by this no-load current (core loss) is 100 watts. This core loss is nearly constant at all loads. When 100 lamps, taking 50 amperes of current, are connected to the secondary, then E" is 107.2 volts. The PR loss in the primary coil is 65 watts; and the PR loss in the secondary is 65 watts. Therefore, the power delivered to the lamps is 5.36 Tdlo watts; the power taken from the supply mains is 5,360 + 100 + 65 + 65, which is equal to 5,590 watts; and the full-load efficiency of the transformer is 96 per cent. Maximum Core Flux. In the designing of transformers and in the predetermination of core loss, it is necessary to calculate the maximum value reached by the alternating magnetic flux through the transformer core. This maximum value of the core flux may be easily and accurately calculated in the following manner: Let < be the maximum value of the core flux (equal to the pro- duct EA of maximum flux density and sectional area of the core) E the effective value of the electromotive force applied to the primary coil, Z the number of turns of wire in the primary coil, and / the frequency of the applied electromotive force, in cycles per second. Now consider the instant when the core flux is at its maximum posi- 238 ALTERNATING-CURRENT MACHINERY live value . After a quarter of a cycle, or after second, the flux is reduced to zero. The average rate of change of the flux during this quarter of a cycle is equal to total change of flux _ <& . elapsed time 1 which is equal to the average electromotive force (in c. g. s. units) induced in each turn of wire in the primary coil. Therefore, the total electromotive force induced in the Z turns of wire in the primary coil is 4/Z c. g. s. units, or 4/Z-:-10 8 volts, since 10 8 c. g. s. units equal 1 volt. The average value of the induced electromotive force is equal (very nearly) to the average of the electromotive force applied to the primary coil; and it must be multiplied by the form factor* of the electromotive force curve to give the effective value E of the applied electromotive force. The form factor of a sine-wave electro- motive force is 1.11. Therefore E= 1.11 X average value = 4.44/$Z-^ 10 8 volts Hence, solving for 4>, we obtain the equation #V 10 8 Example. In the 5i-kilowatt transformer used as an illustration on page 236, there are 660 turns of wire in the primary coil, that is Z = 660. This primary coil is connected to alternating-current supply mains so that the electromotive force applied to the primary coil is 1,100 volts (effective). The frequency of the electromotive force is 125 cycles per second ( =/) . On the assumption that the electromotive force wave is a sine curve, we have, using equation (34), 1 100 X 10 8 4.44X660X125 This is the maximum value of the alternating magnetic flux in the transformer core. The cross-sectional area A of the transformer core is 15j square inches, so that the maximum value of the magnetic flux-density in the core is $ 300,000 = = 19,350 lines per square inch A 15$ This is equivalent to a flux-density of 3,000 lines per square centimeter. *See page 15. ALTERNATING-CURRENT MACHINERY 239 Ideal Transformer Action Graphically Represented. CASE (a) Without Load. The line 0$ in the clock diagram, Fig. 204, repre- sents the alternating magnetic flux in the core of a transformer, the line OE' represents the electromotive force applied to the primary coil, and the line OE" represents the electromotive force induced in the secondary coil. When the transformer is at zero load, the current in the primary coil (the no-load current) lags greatly behind the applied electromotive force E', as shown in the figure, in which the line O/o represents the no-load current. The electro- motive forces induced in both primary and Fig. 204. clock Diagram , ., f\r\ 1 i i i j.1 for Transformer with- secondary coils are 90 degrees behind the core out Load flux 0< in phase, and the electromotive force OE' applied to the primary coil, being at each instant opposite to the electromotive force induced in the primary, is 90 degrees ahead of 0$ in phase. CASE (b) With Load, Receiving Circuit Nearly Non-inductive. The lines 0, OE' OE", and OI Q in Fig. 205 represent alternating core flux, primary applied electromotive force, secondary induced electromotive force, and no-load current, exactly as in Fig. 204. The line 01" represents the secondary current lagging slightly behind the secondary electro- motive force OE"\ and the line OA represents the increase of primary current due to the loading of the transformer. The total pri- mary current is represented by the line 01', which is the vector (or geometric) sum of OA and 07 . The current OA is exactly opposite to 01" in phase; and the product of this current OA and the primary turns Z' balances the magnetizing action Z"I" of the secondary current. As is evident from Fig. 205, the loading of a transformer (non- inductive load) not only increases the value of the primary current, but reduces its angle of lag behind the primary applied electromotive force. Thus, at zero- rig. 205. Clock Diagram for Transformer with Load 240 ALTERNATING-CURRENT MACHINERY Fig. 206. Clock Diagram for Trans- former J with Load highly Induc- tive Receiving Circuit^ load the primary current is 0/ ; and when the transformer is loaded (non-inductive load) the primary current becomes OF. CASE (c) With Load, Receiving Circuit Highly Inductive. In Fig. 206 the line 01" represents the cur- rent delivered by the secondary coil of a transformer to a highly induc- tive receiving circuit; the line OA represents the increase of primary current due to the load; and OP represents the total primary current. In this case, also, the part OA of the primary current is exactly op- posite in phase to the secondary cur- rent or. Influence of Coil Resistances and Magnetic Leakage. The foregoing discussion takes account of the no-load current of a trans- former. This no-load current is the only factor that affects the ideal relation, equation (33), between primary and secondary currents in a transformer. Coil resistances and magnetic leakage are, on the other hand, the only things that affect perceptibly the ideal relations, equation (32), of primary and secondary electromotive forces. Magnetic leakage is equivalent, in its effect upon the action of a transformer, to an outside inductance (a choke coil) con- nected in series with the primary coil. Let L' be this inductance (in henrys) which is equivalent to the magnetic leak- age of a transformer. Then wL f (co equals 2n times the frequency) is the reactance (in ohms) of this inductance. The effects of coil resistances and mag- netic leakage upon the ideal relation be- tween E' and E" are shown in the clock Fig. 207. Clock Diagram Show- ing Effects of Coil Resistance diagram, Fig. 207. and Magnetic Leakage The total electromotive force OE f ap- plied to the primary coil is used (a) to overcome the resistance R f of the ALTERNATING-CURRENT MACHINERY 241 primary coil; (b) to overcome the electromotive force induced in the primary coil by the leakage flux; and (c) to balance the electromotive force induced in the primary coil by the magnetic flux 0$, which passes through both coils. The part (a) of OE' is equal to I'R', and it is in phase with /'. The part (b) of OE' is equal to coLT, and it is 90 degrees ahead of /' in phase. The part (c) of OE' is represented by the line OA'. The total electroinotive force induced in the secondary coil is equal Zi ff to OA' X ^7. This electromotive force is represented by the line Z* OB. A portion of this total induced electromotive force OB is used to overcome the resistance R" of the secondary coil; and the remainder OE" is available at the terminals of the secondary coil to force current through the secondary receiving circuit. E" From Fig. 207 it is evident that the ratio is less than its Z" OB ideal value ( = 77-77) because of the "resistance loss" I'R' of electromotive force in the primary coil, because of the "leakage loss" w L' F of electromotive force in the primary coil, and because of the "resistance loss" I" R" of electromotive force in the secondary coil. Performance. With Non-Inductive Load. When a transformer secondary delivers current to a non-inductive circuit, then 01", Fig. 207, is parallel to OB, and 01' is nearly parallel to OE', so that R'l' is nearly parallel to OE', and ajL'I' is nearly at right angles to OE'. Therefore, the difference in value between OE' and OA' is nearly equal to I'R', and nearly independent of a>L '/'. Therefore, the falling off of the secondary voltage E", with increase of load, is due almost wholly to I'R' and to I"R" when the receiving circuit is non-in- ductive, and is not due, to any perceptible extent, to magnetic leakage. With Highly Inductive Load. When a transformer secondary delivers current to a highly inductive circuit, then 01", Fig. 207, is nearly at right angles to OB, and 01' is nearly at right angles to OE', so that I'R' is nearly at right angles to OE'; cuLT is nearly parallel to OE'; and further, I"R" is nearly at right angles to OB. There- fore, the difference in value between OE' and OA' is nearly equal to coLT, and nearly independent of RT, while OB is nearly equal to OE". Therefore, the falling off of the secondary voltage E", with 242 ALTERNATING-CURRENT MACHINERY increase of load, is due chiefly to ajLT, that is, to magnetic leakage, when the receiving circuit is highly inductive, and is not due to any great extent to coil resistances. Example. The primary of a certain 10-kilowatt (1,000-volt : 100-volt) transformer has a resistance of 1.5 ohms, and the secondary coil has a resist- ance of 0.015 ohms. The secondary coil delivers 100 amperes to a non-inductive receiving circuit; and, ignoring no-load current, the primary takes 10 amperes from 1,000-volt mains. The IR loss of electromotive force in the primary coil is, therefore, 1.5 ohms X 10 amperes = 15 volts, so that the portion OA', Fig. 207, of the primary applied voltage is very nearly 1,000 15=985 volts. Therefore, the total electromotive force OB induced in the secondary coil is Z" ^-X'985 volts = 98. 5 volts. The IR loss of electromotive force in the second- Z ary coil is 0.015 ohms X 100 amperes = 1.5 volts, so that the electromotive force between the terminals of the secondary coil is 98.5 volts 1.5 volts, or 97 volts. In this case, the secondary receiving circuit is non-inductive, and the portion a>L'I r of the primary applied voltage is nearly at right angles to E 1 '. This loss of voltage ajL'I' has, therefore, no appreciable effect in lessening the value of the available part OA' of the primary applied voltage E'. The leakage reactance wL' of the above transformer is 5 ohms. If the secondary coil delivers 100 amperes of current to a very highly inductive re- ceiving circuit, then this 100 amperes is nearly 90 degrees behind OB, Fig. 207, in phase; and the primary current of 10 amperes is nearly 90 degrees behind OA' in phase. Therefore, the leakage voltage loss toL'I', which is equal to 50 volts, is nearly parallel to OA', so that OA' is very nearly equal to 1,000 Z" volts 50 volts, or 950 volts, and E" is very nearly equal to X 950 volts, or 7i 95 volts. In this case, I'R' and I"R" are nearly at right angles to OA' and OB, and these resistance losses of voltage do not have an appreciable effect in lessening the secondary terminal voltage E". The above discussion of the effects of coil resistances and of magnetic leakage shows that the ratio of E f and E" is very nearly Zi' equal to its ideal value -when the primary and secondary cur- ^ rents of a transformer are small, that is, when the load on the trans- former is zero. TRANSFORMER CONNECTIONS Parallel Constant=Voltage Transformers. In systems of dis- tribution where alternating currents are delivered to a number of units (groups of lamps, for example) all at constant voltage, each unit (each group of lamps) is supplied from the secondary of a separate and distinct transformer; and the primaries of the respective transformers are connected in parallel across the constant-voltage ALTERNATING-CURRENT MACHINERY 243 mains that lead out from the supply alternator. This arrangement is shown in Fig. 208, in which P, P', P", etc., are the transformer primaries; S, S f , S", etc., are the corresponding secondaries; and A, A', A", etc., are the separate receiving units or groups of lamps. For this kind of service, where the primary of a transformer is sup- plied at constant voltage, and it is desired that the transformer shall deliver current to a receiving unit at sensibly constant voltage irrespective of load, i. e. y irrespective of the number of lamps, the transformer must be designed so that a very slight decrease of in- duced electromotive force in the primary will permit the necessary current to flow through the primary coil. This requires that the coils of the transformer shall have as little resistance as possible, and that the primary and secondary coils shall be wound close together, so that no perceptible portion of the magnetic flux that is forced through the core by the magnetizing current may flow out of the core between the coils, instead of passing through the sec- ondary coil as well as through the primary coil. A transformer IF" I 5" IESTEB1IB A' A" Fig. 208. Diagram of Connections of Constant- Voltage Transformers in Parallel specially designed to realize these two conditions is sometimes called a constant-potential or constant-voltage transformer. A constant-voltage transformer is necessary if it is desired to transform a given voltage in a determinate ratio so as to be able to infer the value of the given voltage from -the measured value of the transformed voltage. Specially designed transformers, how- ever, are not always necessary for this purpose, for the reason that the voltmeter used for measuring the transformed voltage usually takes very little current, and it is the currents in a transformer that disturb the ideal ratio of voltage transformation. Multi-Coil Type. Most commercial transformers are now made with two (or more) primary coils, and with two (or more) secondary coils. Each of the primary coils of such a transformer may be 244 ALTERNATING-CURRENT MACHINERY adapted to direct connection to 1,100-volt mains, and be wound with wire large enough to carry, say, 10 amperes without undue heat- ing. In this case, if these two primary coils are properly connected in parallel they constitute in effect a single primary coil, suited for direct connection to 1,100-volt mains, and capable of taking 20 amperes without undue heating. On the other hand, if these two h m t (!/ 1 a. g b If III tfffi! T b Fig. 209. Proper Parallel Connections for Trans- formers Fig. 210. Improper Paral- lel Connections for Trans- formers primary coils are properly connected in series, they constitute in effect a single primary coil suited for direct connection to 2,200- volt mains, and capable of taking 10 amperes of current without undue heating. Each of the secondary coils of such a transformer may likewise be adapted to deliver 100 amperes at 110 volts, in which case the two secondaries, if properly connected in parallel, consti- tute in effect a single secondary coil adapted to deliver 200 amperes at 110 volts; whereas, if the two secondaries are properly connected in series, they constitute in effect a single secondary adapted to deliver 100 amperes of current at 220 volts. Two coils of a transformer (primary or secondary) are prop- erly connected in parallel when the current which divides between them flows around the core in the same direction in both coils, that tt Fig. 211. Proper Series Con- nections for Transformers Fig. 212. Improper Series Con- nections for Transformers is, so that both coils magnetize the core in the same direction. Proper and improper connections in parallel are shown diagrammatically in Figs. 209 and 210. See also "Polarity Test," page 315. Two coils of a transformer (primary or secondary) are properly connected in series when the current which flows through them flows around the core in the same direction in both coils. Proper and improper connections in series are shown diagrammatically in Figs. 211 and 212. ALTERNATING-CURRENT MACHINERY 245 When two primaries of a transformer improperly connected in parallel are connected to the supply mains, the currents in the coils oppose each other in their magnetizing action on the core. The result is that the core is not perceptibly magnetized; but little opposing electromotive force is induced in the coils (by leakage flux); and the two improperly connected coils constitute a short' circuit when they are connected to the supply mains. When two primaries of a transformer, improperly connected in series, are connected to the supply mains, the current which Primary Mams Fig. 213. Proper Transformer Connections to a Three- Wire System flows through the two coils will have equal and opposite magnet- izing actions in the two coils. The core flux will, therefore, be practically zero, and no counter-electromotive force will be induced in the windings to balance the applied electromotive force. The flow of current is, therefore, hindered only by the coil resistances and by the electromotive forces induced by the leakage flux. The result is that two coils improperly connected in series constitute a short-circuit when they are connected to the supply mains. Two secondary coils improperly connected in parallel give rise to short-circuit conditions. Two secondary coils improperly connected in series do not lead to short-circuit conditions, but give zero electromotive force between their terminals. 246 ALTERNATING-CURRENT MACHINERY Edison Three-Wire System Single-Phase. The Edison three- wire system, extensively used in direct-current distribution, is com- monly used in alternating-current distribution. Fig. 213 shows two transformers properly connected for supplying current to a three- wire system; and Fig. 214 shows two transformers improperly con- nected for supplying current to a three-wire system. In the proper connection, the middle secondary main c carries only the difference of the currents in the outside mains a and b; and in the improper connection, the current in the middle secondary main is the sum of the currents in the outside mains. The proper connection gives '/mnry WCti r o vo/rs -t Fig. 214. Improper Transformer Connections to a Three-Wire System double voltage between the outside secondary mains and the im- proper connection gives zero voltage between outside secondary mains. The Edison three-wire system must not be confused with the two-phase and three-phase three-wire systems. The advantages of the Edison three-wire system when used for the distribution of single-phase alternating currents, are exactly the same as the ad- vantages of this system when used for the distribution of direct currents, namely, a great saving in the copper required in the dis- tributing mains. This saving, in general, amounts to five-eighths of the copper that would be required for a two-wire system using ALTERNATING-CURRENT MACHINERY 247 one-half the total voltage between outside mains. As explained on page 5, the amount of copper required varies inversely as the square of the voltage used. Single-phase current may be supplied to Edison three-wire distributing mains by a single transformer having two secondary coils properly connected in series to the outside mains, the middle main being connected to the junction of the two secondary coils. An "autotransformer" or "balance coil" having a tap at the midcfte point of its single winding is often used. (See page 252.) Fig. 215. Connections for "Banked" Transformers Banking of Transformers. Two service mains may be supplied with current by two or more transformers with their primaries con- nected in parallel between the supply mains, and with their seconda- ries properly connected in parallel to the service mains. Fig. 215 shows two transformers properly "banked" so as to supply current to the two service mains a and 6. In many large transmission systems, such as the Niagara-Buffalo, a dozen or more large trans- formers may be banked (on each phase) for step-up or step-down transformation . It is very important to note, however, that proper connections alone will not insure satisfactory parallel operations. Perfect oper- 248 ALTERNATING-CURRENT MACHINERY ation of two or more transformers in parallel means that each of the separate units contributes to the total load an amount of power proportional to its rated output, and that the numerical sum of the currents in the separate units is equal to the line (total) current. To secure this desirable result, two conditions must be fulfilled : (a) the ratio of primary turns to secondary turns must be the same in all the units, and (b) the voltage drop from no load to full load must be the same, both in magnitude and phase, for all the units. If condition (a) is not fulfilled, there will be a difference at all loads in the secondary voltages, and the transformer having the highest voltage will carry the largest load. Condition (b) is ful- filled if all the transformers have the same impedance volts (ZI) and the same ratio of resistance to impedance. If the transformers in parallel have the same impedance volts, it follows that the mag- nitude of the current furnished by each will be inversely as its im- pedance. If further they have the same ratio of reactance to resist- ance, it follows that the currents delivered by each will have the same phase, and the total current will then be the numerical sum of the individual currents. Given two or more transformers, having the same ratio of pri- mary to secondary turns and having relatively small magnetizing currents, to be banked so as to operate in parallel; the division of the total load between them depends chiefly upon the total impe- dances between the bus bars, including with each transformer its connecting wires and any meters or relays through which the cur- rent may pass. In any case where transformers banked in parallel do not divide the total load according to their rated capacities, a proper division of the current may be effected by increasing the impedance in the circuit of the transformer which delivers more than its share of the total current. This may be easily done by inserting a suitable choke coil (a coil of wire wound on a laminated iron core) in series with either its primary or secondary lead wires. When the ratio of reactance to resistance is unequal in the transformers, the phases of the secondary currents are not the same, so that the transformers may deliver equal currents, and still not deliver equal amounts of power to the circuit. It follows that a wattmeter connected with its series (current) coil in circuit with only one of, say, two transformers of equal rating will not measure ALTERNATING-CURRENT MACHINERY 249 half of the total power. It will measure more than half or less than half according to the value of cos 6 in the expression El cos 0, where reactance tan = r- - . resistance From the above discussion it is evident that special care must be taken in banking transformers for parallel operation to see that the several units are delivering their proper share of the total cur- rent output. Furthermore, it is not safe to assume that trans- formers even of the same rated capacity and of the same make will share a given load equally when operated in parallel. The only safe procedure in such cases is to measure the voltage, current, and watts of the several transformers. Series=Current Transformers. In some of the older systems of distribution by alternating currents, it was desired that each P P 1 P" P"' vflg.pg.gj?/ ommuh vMgggg/ Fig. 216. Connections for Transformers in Series receiving unit (arc lamp, for example) should receive a constant current equal to the whole, or to a definite fractional part, of the constant current delivered by an alternator. This condition can be realized by supplying each unit or group of units from the sec- ondary of a separate and distinct transformer, the primaries of all the transformers being connected in series as shown in Fig. 216, in which P, P', P", P"', etc., are the primaries of the respective transformers; S, S', S", S'", etc., are the transformer secondaries; and A, A', A" , A'" y etc., are the receiving units. For this kind of service, where the primary of a transformer is supplied with a definite current, and it is desired that the transformer shall deliver to a receiving unit a current which is equal to an invariable fractional part of the primary current, irrespective of variations of resistance in this unit, the transformer must be designed to take as small a magnetizing current as possible, for, according to the discussion on page 235, it is the magnetizing current that disturbs the ideal relation of primary 250 ALTERNATING-CURRENT MACHINERY to secondary current in a transformer. A transformer which is specially designed to realize this condition is sometimes called a current transformer. The transformer used in connection with the composite field excitation of an alternator, as shown in Fig. 112, is a current trans- former which delivers a current equal to a definite fraction of the current output of the alternator to the rectifying commutator. The current transformer is frequently used for sending current equal to a definite fractional part of an alternating current through an ammeter from the reading of which, together with the known ratio of current transformation, the value of the whole alternating current is deduced. The current transformer here described must not be confused with the so-called constant-current transformer which receives variable Fig. 217. Diagram of Connections for Transformer in Series with a Number of Lamps current from a constant voltage supply, and delivers a constant current to a group of receiving units connected in series, the delivered current being constant irrespective of increase or decrease in the number of receiving units. The connection of a transformer primary in series in a circuit containing many other elements (lamps) so that the current pass- ing through the primary does not vary much with the varying re- sistance of the circuit to which the secondary of the transformer delivers current, gives rise to actions which are not very familiar to electrical engineers, for the reason that this arrangement is now seldom used in practice. The actions, however, which are inter- esting, are as follows: Fig. 217 represents a transformer primary P connected in series in a circuit containing many elements e', e", e'", etc., (lamps). ALTERNATING-CURRENT MACHINERY 251 The action will be described in two steps, viz, (a) "on the assumption that the magnetizing current of the transformer is always negligi- ble, and (b) without the aid of this simplifying assumption. (a) If the magnetizing current is always negligibly small, then the current in A is equal to a fixed fractional part of the sensibly constant current in the main circuit, so that any increase of the resistance or reactance of A must be accompanied by a correspond- ing increase of the voltage E", which is pushing current through ~A; and this must be accompanied by a corresponding increase of the voltage E f between the terminals of the primary coil P. Thus, if A has zero resistance and zero reactance, then E" is zero, and E' is zero. That is, the current in the main circuit flows through P with- out any opposition at all, just as if P were a connection of zero re- sistance. If the resistance or reactance of A is increased, E" (and also E') must increase, which means that the current in the main circuit encounters greater and greater opposition in flowing through P. If the circuit of A is opened (infinite resistance), then, on the above assumption of negligible magnetizing current, the opposition to the flow of current in P becomes infinite, so that breaking the circuit of A is equivalent to breaking the main circuit. (b) As a matter of fact, as the resistance or reactance of A is increased, causing an increase of E" and E', the magnetism of the transformer core must increase proportionally with E" and E' in order that these increased voltages may actually be induced in the transformer coils; and this increase of magnetism of the core requires more and more magnetizing current.* The magnetizing current, therefore, is not always negligible irrespective of the resistance of A. In fact, when the resistance of A is infinite (open circuit), there is no secondary current; all the current in the coil P is magnetizing current; and the voltage E', which opposes the flow of current through P, rises only to that value which corresponds to the degree of magnetism of the core that can be produced by the magnetizing action of the whole primary current. The transformer then becomes simply a choke coil. Autotransformer. Given a source of supply of alternating *It must be remembered that the magnetizing current is that part of the primary current whose magnetizing action is not balanced or annulled by the secondary current as explained on page 235. 252 ALTERNATING-CURRENT MACHINERY current. This current may be delivered to a receiving unit in three ways: (a) By connecting the unit directly to the supply mains. (b) By connecting the unit to the secondary of a transformer of which the primary is connected to the supply mains. (c) By a combination of methods (a) and (b). The combination method may be realized with any ordinary trans- former; which, when so used, is called an autotransformer, o*- com- pensator. NOTE. It will appear in the following discussion that when the voltage of the supply differs but little from the desired service voltage, the combina- tion method is much preferable to the method in which the transformer alone is used, because of the fact that a smaller transformer suffices, and because the combination method involves less energy loss. This combination or auto- transformer method is quite simple of treatment when attention is confined to a particular case; but it is complicated when attempt is made to give it a general discussion, that is, when attempt is made to discuss all the theoretically possible ways in which 4 a given ordinary transformer may be used as an auto- transformer. In Fig. 218 A and B are alternating-current supply mains, between which the voltage is, say 100; C and D are service mains, to which it is desired to deliver alternating current at 90 or 110 volts; P and S are the primary and secondary coils of an ordinary transformer. The primary P is connected to A and B as shown. Fig. 218. Connections for Auto step-Up The secondarv S has one-tenth as Transformer * many turns as P, therefore, the voltage induced in S is one-tenth of the voltage acting on P, or 10 volts. Let the dotted arrows represent the directions of the induced electromotive forces in the two coils at a given instant. Then the long heavy arrow will represent the direction of the voltage between the mains at the same instant, inasmuch as the induced voltage in the primary coil of a transformer is always opposed to the supply voltage. Auto Step-Up Transformation. The 10 volts induced in the coils, Fig. 218, will help push current into the service mains if we connect from supply main A, out of which the current at the given instant is tending to flow, to terminal g of coil S; connect from ALTERNATING-CURRENT MACHINERY 253 terminal / to service main C; and connect from service main D to supply main B, as shown by the dotted lines. In this case, the vol- tage induced in the coil S is added to the supply voltage, and the service voltage is, therefore, 110 volts. Auto Step-Down Transformation. The 10 volts induced in the coil S will oppose the flow of current into the service mains if we connect from supply main A to terminal/, connect from terminal g to service main C, and connect from service main D to supply main B, as shown by the dotted lines in Fig. 219. In this case the induced voltage in the coil S is subtracted from the supply voltage, and the service voltage is now 90. Current Relations. In Figs. 218 and 219 the directions of the induced voltages in P and S are shown by the dotted arrows. These induced voltages are in the same direction in the two coils. The currents in the two coils of a transformer are, on the other hand, always in opposite directions, inasmuch as they balance the mag- netizing action of each other. Suppose for the sake of concreteness that 10 amperes are delivered to the service mains. Then we have the following relations: (a) Ten amperes flow through S, Fig. 218, in the same direction as the in- duced electromotive force of ten volts, so that one ampere (one-tenth as much cur- rent, since there are ten times as many turns in P as in S) flows through P in opposition to the induced or counter- electromotive force 0f 100 volts. There- Fig. 219. Connections for an Auto fore, the COil P takes 100 watts from Step-Down Transformer the supply mains, which power is trans- ferred to the coil S by ordinary transformer action, whence it is given out (ten volts pushing ten amperes) in assisting the flow of current to the service mains. The total power delivered to CD is evidently 1,100 watts (10 amperes at 110 volts), and of course the total power taken from the supply mains is 1,100 watts (11 amperes at 100 volts). (b) Ten amperes flow through S, Fig. 219, in a direction opposite to the ten volts of induced electromotive force, so that one ampere flows through P in the same direction as the induced electromotive force of 100 volts. There- fore, of the 1,000 watts delivered by the supply mains in forcing the ten am- peres through S and through the service mains, 100 watts are delivered to the coil S, and 900 watts are delivered to the service mains (10 amperes at 90 volts). The 100 watts delivered to S are transferred by transformer action to the coil P, and delivered by P back to the supply mains. It is to be particularly noted that only 100 watts are involved in the 254 ALTERNATING-CURRENT MACHINERY above as genuine transformer action, although 900 or 1,000 watts are actually delivered to the service mains. The autotransformer is used commercially for many purposes. A common use is as a "balance coil" in a three-wire distribution from a two-wire supply, arid it is also used as a balancer in connection with rotary converters for supplying a three-wire direct-current service as explained on page 355. Compensators are also used for the operation of low-voltage tungsten lamps in parallel for house lighting, for electric signs, for arc lamps, as voltage regulators foir mercury-vapor rectifiers, as explained on page 324. Autotrans- formers are also widely used as "starting compensators" for alter- nating-current motors. In such cases they supply a reduced voltage (usually half voltage) to the motor circuits while the armature is accelerating from rest. Each autotransformer is usually provided with several taps so that a number of low voltages may be obtained at will. Autotransformers are used for supplying a varying voltage to single-phase commutator motors used on electric cars and locomo- tives, and thus for controlling the speed of the motors. TRANSFORMERS IN POLYPHASE SYSTEMS A polyphase transmission system is essentially the utilization of two or three entirely separate and distinct single-phase transmis- P" S" Fig. 220. Transformer Con- nections on a Two-Phase System o o o S" Fig. 221. Transformer Con- nections on a Three-Wire Two-Phase System sion systems of which the separate and distinct electromotive forces or currents are maintained in definite phase relations with each other by mechanical connections in the generator. Step-up or step-down trans- formation in a polyphase system is accomplished, in general, by a separate and distinct transformer of the ordinary type for each phase. ALTERNATING-CURRENT MACHINERY 255 Two=Phase System. Fig. 220 shows a two-phase system in which the current of each phase A and B is transmitted over an Fig. 222. Connections for Three Transformers with Primaries Con- nected to Three-Wire Three-Phase Mains entirely independent circuit, and the electromotive force of each phase is stepped down (or up) by an ordinary transformer P'/S' and P"S", respectively. Fig. 221 shows what is called the three-wire, two- phase system, in which one line wire is used as a common return wire for both phases, and where two ordinary transformers P'S' and P"S" are used for stepping the voltage down (or up). These two figures contain all that is essential in the step-down or step-up trans- formation of a two-phase system. Fig. 223. System Shown in Fig. 222 with One Transformer Removed Three=Phase System. The usual transmission line for a three- phase system consists of three wires, each wire being in effect a com- 256 ALTERNATING-CURRENT MACHINERY Fig. 224. Diagrammatic View of Two Separate Trans- formers for Step-Up or Step-Down Transforma- tion Two-Phase System mon return for currents that pass out over the other two. In this case, the usual arrangement for step-down (or step-up) transforma- tion is to connect the three primaries of three ordinary transformers to the supply mains, using either Y or A con- nections; and to connect the three secondaries to the service mains, using either Y or A connec- tions. Furthermore, the primaries may be Y-con- nected and the secondaries A-connected, or vice versd. The A connec- tions of both primaries and secondaries are preferred in practice, inas- much as with this [arrangement the complete three-phase, step-down (or step-up) transformation is still effected even though one trans- former may be entirely disconnected because of burn-out or break- down. In this case the two remaining transformers are said to be con- nected in V . In such a case, however, the two remaining transformers do not have two-thirds of the transforming capacity of all three, but only T Vir of | = 0.567 as much, or a little over one-half the capacity. Fig. 222 shows three ordinary transformers P'S', P"S", and P'"S'", with their primaries A-connected to three-wire, three-phase supply mains 1, 2, and 3; and with their secondaries A-connected to three-wire, three-phase service mains a, 6, and c. Fig. 223 shows the arrange- ment of Fig. 222 with one of the transformers P'"S'"omit- ted (any one of the three may be omitted), thus giving the V connection. This arrange- ment is operative for three- phase, three-wire step-up or step-down transformation, ex- cept that its power capacity is only about 57 per cent of the power capacity of the arrangement in which three similar transformers are used. pi II Fig. 225. Two Transformers for Two-Phase Sys- tem with Common Magnetic Return Circuit ALTERNATING-CURRENT MACHINERY 257 Transformers with Compound Magnetic Circuits. Let A and B, Fig. 224, be the two separate transformers to be used for step-up or step-down transfor- mation on a two-phase system. Each iron core may have its own re- turn circuit for the mag- netic flux; or a single magnetic return C may be used for the two, as Shown in Fig. 225. In Fig. 226. Three Transformers for Three-Phase System ,, i . i -i A wit h Common Magnetic Return Circuit the latter case only 1.4 as much iron need be used for the common return as would have to be used for each single magnetic return. Similarly,* three transformers A, B, and C, Fig. 226, used for step-up or step-down; transformation on a three-phase system, may be combined magnetically so that each transformer core is the magnetic return for the other two, as shown. Commercial examples of polyphase transformers are described on pages 292 and 293. Phase Transformation. If a two-phase* supply is connected to the similar primaries of two transformers, an electromotive force of any desired value and of any desired phase may be produced. Fig. 227 shows the two-phase supply mains connected to the similar primaries P' and P" of two separate transformers. On core A is wound a secondary coil a, and on core B is wound a secondary coil b; these two secondary coils are connected in I P' series, and the desired electromotive force E is produced by the two secondary coils in conjunction. In order that the desired elec- tromotive force E may Fig. 227. Connections for Phase Transformation from . .. ' Two-Phase Supply Mains be produced by coils a and b jointly, the following conditions must be fulfilled: *Or three-phase supply. The discussion of phase transformation is, however, much simpler with a two-phase supply. 258 ALTERNATING-CURRENT MACHINERY Fig. 228. Vector Diagram of E.M.F. Relations for Fig. 227 Let the vectors A and B in the clock diagram, Fig. 228, rep- resent the two two-phase electromotive forces; and let E represent the desired electromotive force. Then coil a, Fig. 227, must be wound with a sufficient number of turns of wire to produce the component a of E; and coil b must be wound with a sufficient number of turns of wire to produce the component b of E. See Fig. 228. If it is desired that the resultant elec- tromotive force E be in the second, third, or fourth quadrant, the coil a or the coil b, or both coils a and b must be reversed as indicated in Fig. 229. It, therefore, follows that by cor- rectly proportioning and connecting the coils a and b, any desired value and position of the resultant electromotive E may be produced. The general two-phase, three-phase transformer consists of two separate transformers with similar primary coils P r and P" which are connected to the respective phases of the two-phase supply mains, as shown in Fig. 227; and each of the three-phase electromotive forces is produced by a pair of properly proportioned and properly connected secondary coils (one coil on each trans- former) connected in series. Each pair of secondary coils constitutes one unit of the three-phase system, and these three units may be either Y-connected or A-connected to three-wire, three-phase service mains. The general two-phase, three-phase transformer is greatly simplified if we choose to have one of the three-phase electromotive forces in phase with one of the two-phase electromotive forces. Thus, if E, Fig. 228, is in phase with B, then E may be produced by a single sec- ondary coil on core B, instead of being produced by a pair of secondary coils, one on each core. Fig. 229. Vector Diagram of E.M.F. Relations for E in any Quadrant ALTERNATING-CURRENT MACHINERY 259 Fig. 230. Vector Diagram of E.M.F. Relations in Three Secondaries of Scott Transformer '000000 a Fig. 231. Diagram of Di- vided Secondary in Scott ^Transformer Scott Transformer. The two-phase, three-phase transformer per- mits of still further simplification if the A-connection of the three-phase units is excluded, that is, if the three-phase units are to be adapted only for Y-connection to the three-phase mains. This ultimate sim- plification is realized in the Scott transformer. The transformer shown in Fig. 227 may be converted into a Scott transformer by replacing its windings with the wind- ings shown in Fig. 234. The two similar primary coils A and B, which are connected to the respective phases of a two-phase system, are placed on cores A and B, respectively. Coils b, a, and c are the three secondaries _ which are Y-connected to the three-phase mains, b is placed on core B and gives the electromotive force b, Fig. 230. It is to be noticed that coils a and c are formed by bringing out a connection at the mid-point of one large coil, Fig. 231. a and c are placed on core A, Fig. 227, and give the electromotive forces a and c, Fig. 230. The number of turns on a and c are equal. The number of turns on b are made to equal V 3 times the turns on either a or c, i.e.,\ia and c each have 50 turns, b will have approximately 87 turns. The points 1, 2, and 3, Fig. 230, are at the angles of an equilat- eral triangle. The point repre- sents the common junction point of the three terminals, one from each of the coils a, 6, and c. The other terminals of these coils are connected to the three-phase mains represented by the figures 1, 2, and 3, Figs. 230 and 234. Review page 124, Book II, to get a clear idea of the electromotive force and current relations in three-phase systems Y-connected, Fig. 232. Vector Diagram of E.M.F. Relations in Scott Transformer 260 ALTERNATING-CURRENT MACHINERY JWQQMQfl From these relations and keeping in mind the ratios of the wind- ings of the coils in Fig. 234, it may be seen that the electromotive force a helps to push current in a receiving circuit from main 1 to main 2; while electromotive force b opposes a in this respect. Therefore, the electromotive force from main 1 to main 2 is a b, as shown in Fig. 232. Similarly the electromotive force b helps to push current in a receiving circuit from main 2 to main 3, while electromotive force c opposes b in this respect. Therefore, the electromotive force from main 2 to main 3 is bc, as shown in Fig. 232. Lastly the electromotive force c helps to push current in a receiving circuit from main 3 to main 1, while the electromotive force a opposes c in this respect. Therefore, the electromotive force from main 3 to main 1 is c a, as shown in Fig. 232. An inspection of Fig. 232 will show that ab=bc=ca. Therefore, if E is allowed to represent the value of each, this dis- cussion shows that the electromotive forces between mains 1 to 2, 2 to 5, and 3 to 1 have the common value E volts, and are 120 degrees apart in phase, provided Fig. 233. Y Connections for Three Secondaries of . Scott Transformer for Two-Phase System and b = E cos 30 =Esin 30 C But the ratios of the windings were so proportioned that this is true. Hence the e. m. f. s are 120 degrees apart and result in a balanced three-phase circuit. A clear idea of the Scott trans- former may now be obtained as follows: Two similar cores have similar primary coils, which are connected to the respective phases of a two-phase system. One of these cores has a secondary winding b, one end of which is connected to one of the three-phase mains (main 2, as shown in Fig. 234. Complete Connections of Scott Transformer ALTERNATING-CURRENT MACHINERY 261 ~\ Fig. 233), and the other end of which is connected to the middle point of the secondary winding a c, which is wound on the other core. The terminals of the winding a c are connected to the remaining two of the three-phase mains (mains 1 and 3, as shown in Fig. 233). 2 The entire secondary winding ac has = times as many V O turns of wire as the coil b, that is, 1.16 times as many turns._ The complete connections of the Scott transformer are shown in Fig. 234. The three-phase system requires less line copper than either the single-phase or the two-phase system to transmit a given amount of power with a given line voltage and with a given loss. Hence, for the long-distance transmission of electric power, the three- phase system is universally adopted in this country. For the local distribution of electric power, on the other hand, the two-phase system offers certain advantages. It is often the case, therefore, that two-phase alternators are used to generate alternating currents at a central station, and that two-phase currents are used for power and lighting purposes in the neighborhood of the station. When, however, power is to be transmitted to points fifteen or more miles distant, it becomes desirable, as explained above, to use the three- phase system. It is in such cases, especially, that phase tranforma- tion is used. The Niagara-Buffalo transmission is the earliest as well as one of the most extensive examples of this practice. Power is generated by large two-phase alternators at 2,200 volts. A large part of this power is distributed to factories and chemical works in the vicinity of the central power plant. A large amount of power also is transmitted to Buffalo, a distance of eighteen miles, by means of three-phase alternating currents derived from the two-phase alter- nators by two-phase, three-phase transformation. Scott trans- formers are used; and the two-phase currents at a voltage of 2,200 are stepped up to 22,000 volts, and at the same time are trans- formed to three-phase currents. At Buffalo, the three-phase, high- voltage currents are stepped down to about 2,200 volts, and are transformed back into two-phase currents, also by Scott three-phase two-phase transformers. The 2,200-volt two-phase currents are then distributed by feeders and service mains for lighting and power purposes throughout the city. 262 ALTERNATING-CURRENT MACHINERY PRACTICAL CONSIDERATIONS Transformer Losses. The power output of a transformer is less than its power intake because of the losses in the transformer. These losses are: (a) the iron or core losses due to eddy currents and hys- teresis ; and (b) the copper losses due to the resistances of the primary and secondary coils. (a) Iron Losses. The iron losses are practically the same in amount at all loads. They depend upon the frequency and range of the flux density B, upon the quality and volume of the iron, and upon the thickness of the laminations. Dr. C. P. Steinmetz has found by exhaustive experiments that for ordinary sheet steel the hysteresis loss may be expressed in watts as jr fc =?F/B"X10- 7 . (35) in which / is the frequency in cycles per second ; B is the maximum flux density in the iron core, in lines per square centimeter; V is the volume of the iron, in cubic centimeters; and y is a coefficient de- pending upon the magnetic quality of the iron. For plain electrical sheet steel used for transformer cores the value of ^ is about 0.0021. For silicon-steel, now much used in constructing the cores of trans- formers, the value of 7? may be taken as about 0.00093. The eddy current loss, in watts, is JF e = 6F/ 2 * 2 B 2 X10- 7 (36) in which t is the thickness of the laminations, in centimeters; and b is a constant depending upon the specific electrical resistance of the steel. For ordinary sheet steel the value of b is 1.65X10" 11 , and for silicon-steel 0.57 X 10" n . Insufficient insulation between laminations causes excessive eddy current loss, and results in a much higher loss than equation (36) indicates. The equation is derived on the condition of perfect insulation between laminations, a con- dition which is hardly ever realized in practice. Equations (35) and (36) may be used for calculating the ap- proximate hysteresis and eddy-current losses in any mass of laminated iron subjected to periodic magnetization, such as alternator armatures and the rotor and stator iron in an induction motor, but the losses thus calculated are usually smaller than the actual losses. It is preferable when possible to find by a wattmeter test the actual total core loss per pound of steel at different flux densities and ALTERNATING-CURRENT MACHINERY 263 TABLE VII Transformer Efficiencies, Losses, Etc. KV-A. WATTS Loss PER CENT EFFICIENCY PER CENT REGULATION * Exciting Current Iron Cop- per Full Load H Load Yz Load M Load TF % p % ??% P % * 15 13 94.7 94.4 93.2 88.7 2.62 3.21 3.28 3.16 8.0 1 20 24 95.8 95.7 95.1 92.0 2.42 3.03 3.12 3.04 5.5 1 2 L 25 35 96.0 96.0 95.5 92.7 2.36 2.96 3.07 3.00 4.0 2 30 42 96.5 96.5 96.2 93.8 2.12 2.76 2.88 2.86 3.6 2i 33 51 96.8 96.8 96.5 94.5 2.08 2.71 2.83 2.83 3.3 3 34 64 96.8 97.0 96.8 95.2 2.16 2.79 2.91 2.88 3.0 4 40 75 97.2 97.3 97.1 95.7 1.90 2.77 3.00 3.12 2.5 5 45 93 97.3 97.5 97.3 96.1 1.90 2.76 2.99 3.11 2.3 7i 62 125 97.6 97.7 97.6 96.4 1.70 2.60 2.84 3.00 2.2 10 80 148 97.8 97.9 97.7 96.5 1.51 2.42 2.68 2.89 1.9 15 105 212 97.9 98.0 97.9 97.0 1.44 2.36 2.63 2.85 1.6 20 25 131 268 98.0 98.1 98.0 97.1 1.39 2.51 2.87 3.21 1.5 147 319 98.2 98.3 98.2 97.4 1.33 2.45 2.82 3.17 1.3 30 163 374 98.2 98.4 98.3 97.6 1.32 2.45 2.82 3.16 1.2 37i 197 433 98.3 98.4 98.4 97.7 1.20 2.34 2.72 3.09 1.2 50 240 550 98.4 98.6 98.5 97.9 1.15 2.29 2.68 3.07 1.0 *In per cent of full load current. frequencies. Curves may then be plotted one for each fre- quency, using total core loss in watts per pound as abscissas, and B as ordinates. (b) Copper Losses. The copper losses, in watts, are W =R f I'*+R"I"\ (37) This loss is nearly zero when the transformer is not loaded; it in- creases with the square of the current; and becomes excessive when the transformer is greatly overloaded. Transformer Efficiency. The ratio power output -f- power intake is called the efficiency of a transformer. Table VII, shows the efficiencies, losses, and regulation of a recent series of combined core- and shell-type transformers designed and manufactured by a large American company. These transformers are designed for a frequency of 60 cycles per second and primary voltages of 1,100 or 2,200 volts, according to whether the two 264 ALTERNATING-CURRENT MACHINERY "'as*} T 95.X% $IOM !jtOM halves of the primary coil are connected in parallel or in series. The secondary voltages are 110 or 220 volts. Fig. 235 shows graphically the relation between the efficiency and the output for a 7.5-kilowatt, core-type transformer designed for primary voltages of 1,040 and 2,080, and secondary voltages of 104 and 208, and a frequency of 60 cycles per second. The core loss is 86.5 watts and the total copper loss at full load is 117.8 watts. The high efficiency throughout a wide range of load is worthy of note and is typical of all well-designed transformers. Fig. 236 shows graphically the various losses and the efficiency of a Westinghouse air-blast transformer rated at 550 kilowatts used to step up from 500 volts to 10,500 volts at a fre- quency of 25 cycles per second (3,000 alternations per minute). In Fig. 236, the curve representing the iron loss is plotted as a horizontal straight line because the iron loss for a given trans- former is practically con- stant for all loads. The curve representing the copper loss is a parabola. The efficiency of a given transformer is a maximum at that load for which the iron loss is equal to the copper loss. This load is evidently the abscissa of the point at which the iron-loss line intersects the cop- per-loss curve. As seen in Fig. 236, the maximum efficiency occurs at about 101 per cent of full load; at any other load (for the given transformer) the efficiency will be less than at 101 per cent of full load. The transformer output (non-inductive receiving circuit) is E"I". The internal loss is W h + W e + W c , so that the intake is E"I" + W h +W e +W e , and the efficiency is O tO XO 3O K/LOWATTJ OU7fUT X J * 5 e ?O 5O GO 70 SO 90 IOO // <. _ _ .0 3 ?; e ^ 50 75 PER CENT r LOAI> Fig. 236. EflSciency Curves for a Westinghouse Air-Blast Transformer low a value as possible. The ratio total energy given out by the trans- former -T- total energy received by the transformer during the day is- called the "all-day efficiency" of the transformer. In other words total watt-hours output it is the ratio - -r - during the day. The all-day total watt-hours input efficiency is given by the formula all-day efficiency = E'T'Xt E'T'X t+ (W h +W e ) X24 +W C X t (39) in which t is the number of hours during the day of 24 hours that the transformer is loaded, and 1" is the average current delivered by the secondary while the transformer is loaded. The other symbols have the same significance as on pages 262-264. 266 ALTERNATING-CURRENT MACHINERY Since the iron loss of a given transformer is continuous as long as the transformer is connected to the primary supply mains, it follows that to obtain a high all-day efficiency, it is necessary to use a transformer whose iron loss is as small as possible. In general, if a transformer is to be operated at light loads the greater part of the day, as is the case in electric lighting service, it is much more econom- ical to use a transformer designed for a small iron loss than for a small full-load copper loss. In very small transformers, the iron and the copper losses are made about equal, but for outputs of about 5 kilowatts and upwards, the iron loss is often made only about one-half as great as the full-load copper loss. This is illustrated by the data in the second and third columns in Table VII. On the other hand, in cases where transformers are to be used to supply a load of induction motors, and the conditions are such that the transformers are operated at or near full load during most of the day, it is more economical to use transformers designed to give equal iron and copper losses at full load. In general, a given transformer works at the maximum efficiency when it is operating at a load such that its iron loss is equal to its total copper loss. Transformer Regulation. The secondary terminal voltage of a transformer falls off in value with increasing load, and rises with decreasing load. The rise of secondary terminal voltage from rated non-inductive load to no load (at constant primary applied terminal voltage), expressed in per cent of the secondary terminal voltage at rated load, is called the regulation of a transformer. Example. A certain 5-kw. transformer gave a secondary terminal voltage of 200 volts at rated non-inductive load; when the load was reduced to zero, the voltage rose to 203 volts. In accordance with the above definition, the regulation of this transfer- meiiS (203-200) - X 100 = 1.5 per cent ^300 The regulation of the average distributing transformers on the market at present varies from about 1 per cent to 3.5 per cent, and when operated on power factors as low as 60 per cent, the regula- tion in some cases is as high as 4 per cent. In incandescent lighting it is especially important to have transformers with a low regulation, otherwise the voltage at the lamps will fluctuate excessively as the ALTERNATING-CURRENT MACHINERY 267 load changes. This means unsatisfactory illumination for the con- sumer, and more frequent lamp renewals for the lighting company. Thus, for lighting work it is important to specify a low regulation. The regulation of a transformer is lower when used for supply- ing a non-inductive load (such as incandescent lamps), than when supplying an inductive load (such as induction motors). For a given kind of load the regulation of large transformers is lower than for small sizes. These matters are clearly brought out by a careful study of the values of regulation given in Table VII. The method of calculating regulation is explained on page 313. Practical and Ultimate Limits of Output. When the secondary current of a transformer is increased, the secondary electromotive force drops off, and the power output increases with the current, reaching a maximum as in the case of the alternator. This maximum power output is the ultimate limit of output of the transformer. Practically, the output of a transformer is limited to a much smaller value than this maximum output, (a) because of the necessity of cool running; (b) because in most cases it is necessary that the secondary electromotive force be nearly constant; and (c) because the efficiency of a transformer is low at excessive outputs. Small transformers have relatively large radiating surfaces; and in such transformers the requirements of a small (good) regula- tion, as a rule, determine the allowable output. Large transformers, on the other hand, have relatively small radiating surfaces, and their allowable output is limited by the permissible rise in temperature. Some transformers ranging in rated output from 100 to 1,250 kilowatts are provided with air passages through which air is made to circulate by a fan. Transformers which are not cooled by an air blast up to about 500 kilowatts capacity are submerged in oil, which, by convection, carries heat from the trans- former to its enclosing case, where it is radiated. Very large trans- formers are not only submerged in oil, but are also water-cooled. Rating of Transformers. A transformer is rated according to the power it can deliver continuously to a non-inductive receiv- ing circuit without undue heating; and the ratio of transformation, together with a specification of the frequency and effective value of the primary electromotive force to which the transformer is adapted, are also given by the manufacturer. 268 ALTERNATING-CURRENT MACHINERY In order to secure uniformity in the rating of transformers by manufacturers, the American Institute of Electrical Engineers rec- ommends the following rules concerning the allowable rise of tem- perature: (1) In transformers for continuous service, the temperature rise should not exceed 50C. as measured by change of electrical resistance of primary and secondary coils and by thermometer in other parts. (2) In transformers intended for intermittent service, or not operating continuously at full load, but continuously in circuit, as in the ordinary case of lighting transformers, the temperature elevation above the surrounding air temperature should not exceed 50C. as measured by resistance in electric circuits, and by thermometer in other parts, after the period corresponding to the term of full load. In this instance, the test load should not be applied until the transformer has been in circuit for a sufficient time to attain the temperature elevation due to core loss. With transformers for commercial lighting, the duration of the full load test may be taken as three hours, unless otherwise specified. The over-load rating of a transformer is recommended to be 25 per cent for two hours; and it is based upon an allowable increase of temperature of 15 C. above that specified for full load, as given above. This extra rise of temperature is to be measured after the transformer has been operating for two hours on 25 per cent over- load, the transformer having previously acquired the temperature corresponding to full-load continuous operation. When, however, transformers are to be used with other apparatus for which an over-load capacity in excess of 25 per cent is guaranteed, the same guarantee should apply to the transformers. Abnormal Conditions of Operation. Transformers are fre- quently used under conditions departing more or less widely from the conditions specified by the manufacturer in regard to values of primary and secondary voltages, frequency, and current output. Thus, if a transformer is used with a primary applied voltage in excess of the rated value (frequency being unchanged), the core flux will be increased according to equation (34) and the core losses will be increased according to equations (35) and (36). If, on the other hand, a transformer is used with a frequency less than the rated frequency (the voltage applied to the primary being unchanged), the core flux will be increased according to equa- tion (34), and the hysteresis loss in the core will be increased according to equation (35); whereas, the eddy-current loss is un changed. ALTERNATING-CURRENT MACHINERY 269 The increase of core loss due to increase of primary applied voltage, or to decrease of frequency, or to both, may be compen- sated for by reducing the allowable current output, and thereby reducing the copper loss, in order that the total heating of the trans- former may not exceed the normal amount. Examples. A given transformer is rated at 5 kilowatts, and is designed to take current from 1,100-volt mains at a frequency of 60 cycles per second. Under these conditions hysteresis loss Wh, eddy current loss W e , and copper loss W c , will be called normal. (a) The transformer is loaded so that the output is 6 kilowatts at the rated electromotive force and frequency. Find W c in terms of normal. (b) The transformer is used at rated electromotive force, but at a frequency of 75 cycles per second. Find W h and W e each in terms of normal. (c) The transformer is used at rated frequency, but with primary electromotive force of 1,500 volts. Find Wh and W e each in terms of normal. (d) The transformer is used on primary electromotive force of 1,500 volts. Find / for which W^ is normal. (e) With primary electromotive force of 1,500 volts, what load would give normal W c t Solutions, (a) Increasing the output in the ratio 5 to 6, increases both primary current and secondary current in the same ratio and, therefore, in- creases I' 2 R f and I" 2 R" in the ratio of 5 2 to 6 2 . Therefore, the total copper Q A loss becomes = 1.44 times the normal copper loss. 2o (b) Increasing^ the frequency in the ratio 60 to 75 decreases the flux- density B in the same ratio, namely, 75 to 60. Hence, the hysteresis loss per cycle is decreased in the ratio 75 1 ' 6 to 60 U> , and the total hysteresis loss is 75 60 16 /60y 6 X 1<6 or (- J^ or o.87. Therefore, the total hysteresis loss at the in- creased frequency is 0.87 times the normal hysteresis loss. From equation (36), the eddy current loss is proportional to / 2 B 2 . In the case under consideration, / is increased and B is decreased in the same ratio, so that the product f^B? remains unchanged. Hence, the eddy-current loss in a given transformer is independent of the frequency with given primary applied voltage. (c) Increasing the primary voltage in the ratio 11 to 15 increases the flux-density B in the same ratio. Therefore, the hysteresis loss is increased /ISX 1 ' 6 in the ratio ( ) =1.64; and the eddy current loss is increased in the ratio / 15 \ 2 f ) = 1.86. That is, the hysteresis loss becomes 1.64 times its normal value; and the eddy current loss becomes 1.86 times its normal value. (d) The hysteresis loss is proportional to/B 1 ' 6 ; and B is proportional jftr x jjir * 1.6 ET/1.6 to - ; therefore, the hysteresis loss is proportional to / X ( ) or to v This ratio -^ must have the same value under the normal conditions as under 270 ALTERNATING-CURRENT MACHINERY the new conditions, if the hysteresis loss is to be the same. That is (1,100)V 6 (1,500) K6 (60) ( or (1,100) X z' 6 =(60)- 6 X (l,500r 6 or taking logarithms of both sides, we have (l.GXlog 1, 100)+ (0.6 X log z) = (0.6Xlog 60) + (1.6 X log 1,500); or, (0.6Xlog x) = (0.6 X log 60) + (1.6 X log 1,500) - (1.6 X log 1,100); from which we find x equal to 137 cycles per second. (e) The copper loss W c has normal value when the primary and sec- ondary currents have normal value. Therefore, when the primary applied voltage is increased in the ratio 11 to 15, the output increases in the same ratio, the current and W c being normal. Therefore, the output is X 5 kw., or 6.82 kilowatts, to give normal TF c with a primary applied voltage of 1,500 volts. COMMERCIAL TYPES OF TRANSFORMERS Transformers may be classified, according to the relative dis- position of the iron and copper, into core-type transformers and shell- type transformers. The distinc- tion between these two types of transformer maybe understood by referring to Figs. 237 and 267. In the core type, Fig. 237, it will be seen that the iron core is almost entirely surrounded by the copper windings, while in the shell type, Fig. 267, the coils are almost entirely surrounded by the sheet-iron laminations, form- ing a shell. There has been much discus- sion as to the relative merits of the shell and core types, some manufacturers going so far as to claim exclusive advantages for one or the other. The fact is that no gen- eral conclusion can be drawn as to which type is better, for each possesses inherent characteristics which specially adapt it to certain conditions. A brief comparison of these characteristics will aid in determining which type to use for specified conditions of service, size, voltage, and the like. Fig. 237. Core-Type of Transformer Assembled with Case Removed ALTERNATING-CURRENT MACHINERY 271 Fig. 238. Standard Form of Laminated Transformer Core The core type has relatively a lighter core of smaller sectional area but a greater length of magnetic circuit, while the copper is relatively heavier, containing more turns, although of shorter mean length. The core type is more easily wound as cyl- indrical formed coils may be used, and the coils are more accessible and expose more surface to radia- tion. The core type, with its relatively large wind- ing space, is better adapted for high voltages which require many turns and large space for insulation, smaller currents and, therefore, small wires, and higher frequencies with low magnetic flux densities. The shell type, on the other hand, is particu- larly suited for transformers of moderate voltage, requiring few turns and little insulation, large cur- rents, and low frequency with corresponding mag- netic flux. The net result is that manufacturers generally adopt the core type for transformers of small capacity and high voltage, and the shell type for large transformers, even up to 150,000 volts. Substantial advance- ment in transformer de- sign has been made in the past few years, notably in the combination of the advantages of both the core and the shell types in one transformer, in the use of silicon-steel for the laminations, thus greatly reducing the core loss, in the use of better insulat- ing materials and meth- ods, and in more effective methods of cooling. With these improved materials and methods it is even feasible to build reliable transformers with outputs up to 14,000 kilo volt amperes, and for voltages up to 150,000, or higher. Core Type. Fig. 237 shows a complete core-type transformer without its case; Fig. 238 shows the complete core built up of thin Fig. 239. Core with Upper Yoke Removed, Showing One Coil in Position 272 ALTERNATING-CURRENT MACHINERY sheet steel strips, or stampings. The two upright portions of the core upon which the wdre coils are placed, as shown in Figs. 237, 239, Fig. 240. Both Transformer Coils in Position, Showing Laminated Strips Which Form the Upper Yoke of Core and 240, are called the limbs of the core. The short horizontal parts of the core that do not have windings of wire upon them are called yokes. These yokes serve to complete the magnetic circuit, and they are made just long enough to give room for the coils, as shown in Fig. 241. Fig. 239 shows the core with its upper connecting yoke removed to permit of the slipping of the coils into position on the core; the left-hand limb of the core is shown wrapped with a thick layer of insulating material in order to prevent electrical contact between the wire of the coil and the iron of the COre. Fig. 240 shoWS the Coils in Fig. 241. Section Through Core- . Type of Transformer place, and a pile of loose sheet-iron stampings which are used to form the upper connecting yoke. Fig. 241 is a sectional view of the completed core and coils; and Fig. 242 is a sectional view of the complete transformer enclosed ALTERNATING-CURRENT MACHINERY 273 in a cast-iron case; which is usually filled with oil. Half of the primary coil and half of the secondary coil also is placed upon each limb of the core, as shown by the sec- tional view, Fig. 241. The terminals of each half of the primary (fine-wire) coil are connected to binding screws on a porcelain connection board mounted on top of the transformers inside the case, as shown at the right in Fig, 242. The terminals of each half of the secondary (coarse-wire) coil are passed through porcelain bushings to the outside of the case, as shown at the right in Fig. 242. The half coils of the primary may be connected in parallel or in series according to the value of the voltage applied to the primary; and the half coils of the secondary may be connected in parallel or in series according to the desired value of the secondary voltage, as explained on page 244. Fig. 242. Section Through Commercial Core-Type of Transformer Primaries in Series Primaries in Parallel Secondaries inSeriesorThree-wire Secondarfes in Parallel Fig. 243. Plan of Transformer Showing Strap Connections for Arranging Primaries in Series or Parallel Fig. 243 is a top view of the transformer with the cover of its case removed showing the terminals of the secondary coils, and the 274 ALTERNATING-CURRENT MACHINERY porcelain connection-board to which the terminals of the primary coil are connected. The change from series to parallel connection of the halves of the primary coils, is effected by means of the copper connecting straps S S. The style of core-type transformer illustrated in Figs. 237 to 242 is adopted by several manufacturers for transformers of small output. The style of core-type transformer adopted by the Gen- eral Electric Company for larger outputs up to 350 kilowatts, is illustrated in Figs. 244 to 248, which show a high voltage core-type transformer designed for 60 cycles. These transformers are submerged in tanks of oil, one of which is shown in Fig. 248, and which are made with a cast-iron base and sides of heavy steel with deep corrugations to fa- cilitate cooling of the tank by radiation. Fig. 246 shows the arrangement of the coils on the core; it also shows the pas- sages between the core and the inside coil, and between the two coils, for the circula- tion of the oil. Oil is a better heat-conducting medium than air; it carries heat from a transformer to the containing case much better than air, so that a transformer in oil will show a much lower temperature. The use of oil, Fig. 244. General Electric Core- . . Type Transformer Partly moreover, preserves the insulation, keeping Assembled . it soft and pliable, and preventing oxidation by air; consequently, its use is advantageous in producing proper conditions to maintain uniform core loss and a superior insula- tion. Furthermore, oil is itself a very good insulator having the valuable property common to all liquid insulators, that it is not .permanently damaged by a puncture caused by lightning; for example, in this case the resistance of the oil is only momen- tarily broken down, the oil immediately flowing into the break and sealing the insulation. Another variety of the core type is that adopted by the Crocker- Wheeler Company in their "remek-type" transformer. The core punchings are shown in Fig. 249. Each lamination consists of a ALTERNATING-CURRENT MACHINERY 275 core and yoke punching, which are punched simultaneously from the whole sheet by a compound die. In assembling the transformer, care is taken to get the core and yoke punchings back into the same Fig. 245. General Electric Core-Type Transformer with Tank Removed relative position as before punching, in order to secure the lowest possible magnetic reluctance of the joints, and hence a small mag- netizing current. The laminations when assembled are held to- 276 ALTERNATING-CURRENT MACHINERY gether by iron end plates bolted together and the core portions are securely clamped together and to the yokes, as shown in Fig. 250. The core, as seen, constitutes a double mag- netic circuit, the magnetic flux due to the windings on the central core dividing on pass- ing into the yokes, half on each side. The yoke laminations are made wider than one- half the width of the central core, in order to reduce the flux density in them, thereby re- ducing the core loss and the magnetizing cur- rent. The windings consist of former-wound coils which surround the core as illustrated in Fig. 25 1 . The high voltage winding is placed between the two halves of the low voltage winding in order to reduce magnetic leakage and thus improve regulation. Vertical ducts are provided between the windings, and between the core and windings, as may be seen in Fig. 251. This arrange- ment is to insure a free circulation of the oil in which the transformer is immersed and facilitates dissipation of the heat by radiation and convection. The assembled transformer core and coils ready to be placed in the oil-filled tank are shown in Fig. 252. The high voltage termi- Fig. 246. Coils and Core of General Electric Trans- former Showing Oil Ducts Fig. 247. Specimen Coils for General Electric Core-Type Transformer nals are shown connected to a terminal board in front, and the four secondary terminals, two for each coil, are shown in the rear. ALTERNATING-CURRENT MACHINERY 277 These "remek-type" transformers are wound for a primary voltage of 2,200, secondary voltages of 220 and 110, and are built in sixteen sizes ranging from 0.6 to 50 kilowatts. Fig. 253 shows a 10-kw. combination core- and shell-type Fig. 248. General Electric Core-Type Transformer Completely Assembled Showing Deep Corruga- tions to Facilitate Cooling transrormer removed from its tank. It is called the, distributed core type (type H form K) by its makers, the General Electric Com- pany, although it resembles the shell rather than the core type. 278 ALTERNATING-CURRENT MACHINERY The primary and the secondary coils are wound on formers of cylindrical shape, and placed on the center limb only. They are separated from each other and from the core by spacing blocks, thus forming ducts and passages for free circulation of the oil with which the containing tank is filled. Fig. 254 shows clearly the disposition of the coils, mica insulation, and oil ducts, with respect to the central limb. The core, as shown in Fig. 255, contains four magnetic circuits in parallel, each circuit consisting of a separate core similar in general outline to that used in the simple core type. One limb of each magnetic circuit is built up of two different widths of punchings, forming a cross-section such that when the four circuits are assem- Fig. 249. Core Punchings for Crocker-Wheeler "Remek-Type" Transformer bled together they interlock to form a common central limb upon which the primary and the secondary coils are placed. The four remaining legs consist of punchings of equal width. These occupy a position surrounding the coil at equal distances from the center, on the four sides, forming a channel between each leg and coil, thereby presenting large surfaces to the oil and allow- ing it free access to all parts of the winding. The punchings of eadi size of transformer are all of the same length, assembled alternately, and forming two lap joints equally distributed in the four corners of the core, thereby giving a mag- netic circuit of very low reluctance. This type of construction combines the best features of both shell and core types, namely a short mean length of turn in the windings, and a short length of magnetic circuit in the core. ALTERNATING-CURRENT MACHINERY 279 Fig. 250. Assembled Core Show- ing Method of Clamping Fig. 251. Method of Assembling Coils Showing Oil Ducts Fig. 252. Assembled "Remek" Core and Coils 280 ALTERNATING-CURRENT MACHINERY Fig. 253. General Electric Combination Core and Shell Type Transformer r 'M/CA SH/ELO M/CA SM&-0-A SECOMfijffy L Fig. 254. Details of Core and Coils Showing Oil Ducts ALTERNATING-CURRENT MACHINERY 281 MCA Stt/fLDS SECONDARY Pft/flARY 0/2. DUCT Fig. 255. Plan of Assembled Core and Coils Showing Four Magnetic Circuits in Parallel Fig. 256. Westinghouse Combination Shell and Core Type Fig. 257. Assembled Coils for Westinghouse Transformer 282 ALTERNATING-CURRENT MACHINERY Another example of the combined shell- and core-type of trans- former is the distributing transformer (types S and SA) of the West- Fig. 258. Section through Westing- house "Type S" Transformer Fig. 259. Part Section through Westinghouse Transformer Fig. 260. Pancake Coil for Shell-Type Transformer Fig. 261. Pancake Coil We found with Tape inghouse Electric Company. These transformers are almost iden- tical in form and design with the "type H form K" transformers just described. ALTERNATING-CURRENT MACHINERY 283 Fig. 256 shows the core and coils and terminals of the "type S" transformers removed from the case, and a general view of the case. Fig. 257 shows the appearance of the assembled coils and leads of 10- and 15-kw. transformers, with the ventilating ducts. The figure shows the high voltage winding mounted concentrically between two low voltage wind- ings, which arrangement reduces magnetic leak- age and thus improves "regulation." As illustrated in Fig. 256, separate high and low voltage porcelain terminal blocks are mounted upon extensions of the upper end frame, and are thus kept well apart to prevent mistakes in making electrical connection between the high and low voltage coils. Fig. 258 is a section through a J-kw. "type , Fig. 262. Pancake Coil S transformer, and shows the distribution of showing Use of insu- lated Flat Strips the windings in layers, the high voltage coils being placed between two sections of low voltage coils. Fig. 259 is a section through a 50-kw. transformer and shows the large oil ducts arranged between sections of the high voltage winding. The angle irons used to clamp the laminations together are shown at the top and bottom. These distributing "type S" transformers are built in sixteen sizes from \ to 50 kilowatts, and for the standard primary voltages of 2,200 and 1,100, and secondary voltages of 220 and 110. Standard frequencies are 25, 40, and 60 cycles. Shell Type. In shell-type transformers the coils, both primary and secondary, are usu- ally wound in pancake form on formers, as shown in Figs. 260 to 263. The coils in small transformers are wound with round wire, but in the larger sizes flat rectangular copper strip is used with one turn per layer, in many layers, Figs. 260 and 262. The insulation between turns consists of paper, mica, or varnished cambric, or all three together, according to the voltage Fig. 263. Method of Binding up Pancake Strip Coils 284 ALTERNATING-CURRENT MACHINERY and other conditions. The thin pancake coils are then treated with an insulating compound and wound with a number of layers of tape according to the voltage for which they are designed, each layer being given several coats of insulating varnish baked on in ovens. The coils are then assembled into groups of two or more sections, Fig. 263, and the groups into complete windings, the pri- mary and the secondary being intermixed or sandwiched in order to Fig. 264. Partially Assembled Air Blast Shell-Type of Transformer reduce magnetic leakage. Between the various groups suitable insulating barriers are interposed. In transformers for high voltages used for transmission of power, the insulation of a considerable length of the conductor nearest the terminal leads is heavily reinforced. This is a very important precaution, as the extra dielectric strength of the insu- lation of these end turns is a safeguard against break-downs which might otherwise occur due to the excessive voltages from light- ning discharges or other surges to which transmission lines are unfortunately subjected, ALTERNATING-CURRENT MACHINERY 285 The various groups of windings are encased in a box-like struc- ture which, while serving as an electrical and a mechanical protection, is so arranged that it does not obstruct the air or oil ducts which are provided between the various coil sections. The windings are then set up verti- cally in the bottom frame and the magnetic circuit is built up piece by piece around them in the form of rectangular sheets of steel, Fig. 264. In building up the core out of the stamp- ings care is taken to break joints in successive layers. The top frame is then put on and tightly clamped to the bottom, thus compressing and holding the core. After the addition of connection board, leads, in- sulating bushings, and the like, the transformer is ready for its casing or tank. Fig. 264 shows a large General Electric air-blast, shell-type transformer at that stage of construction where the core stampings are being built up around the coils, the coils being protected by thick strips of insulating material. In this air-blast transformer, air passages or ducts are provided between the layers of the coils, and at intervals, between the core stampings, as shown in the figure. The air for cooling the transformer is admitted at the base, and passes vertically through the ducts in the coils. Air is also admitted to the air ducts in the core through a damper on one side of the transformer, and es- capes through the perforations in the casing on the opposite side. Fig. 265. Sectional View of Fort Wayne Shell- Type Transformer Fig. 266. Sample Coils for Fort Wayne Transformer 286 ALTERNATING-CURRENT MACHINERY Figs. 265 to 268 give a general view and structural details of a 15-kw. shell-type transformer manufactured by the Fort Wayne Electric Works. The shape of the core stampings is shown in Fig. 265, which is a section through the transformer. The secondary winding, as seen in the figure, is subdivided into four sections, and the primary into two. The order of the sections as arranged in the "window" of the core is: One-fourth total secondary turns, one-half total primary turns, one-fourth secondary turns, one-fourth Fig. 267. Assembled Core and Coils for Fort Wayne Transformer secondary turns, one-half primary turns, and one-fourth secondary turns. This intermixing the various sections reduces greatly mag- netic leakage, and thus improves regulation. Fig. 266 shows a group of pancake coils insulated and taped ijeady to be assembled. Fig. 267 shows the core and coils assembled. The four leads on the left are from the secondary coils, and the six leads on the right are the four primary terminals to which are added two extra taps. Fig. 268 is a top view of the transformer with the cover re- ALTERNATING-CURRENT MACHINERY 287 moved and shows how the leads are brought up to the connection board and out of the case. Each of the two primary sections is provided with what is called a ten per cent tap. That is, while the normal voltage for which each primary section is wound is 1,100 volts, by using the ten per cent taps, it is possible to change the ratio of transformation, and obtain ratios of 20 : 1, 19 : 1, 18 : 1, 10:1, 9.5 : 1, 9:1, 5:1, and 4.5 : 1, according to the connections of the primary and the secondary sections, whether in series or in parallel. The advantage of these ten per cent taps is that it permits a transformer to be used near the station where the voltage is high or at the end of a feeder where the voltage may be five or ten per cent lower, by simply changing the electrical connections at the terminal board shown in Fig. 268. Fig. 268. Top View of Fort Wayne Transformer with Cover Removed Showing Arrangement of Lead Wires Figs. 269 to 271 show constructive details peculiar to very large shell-type transformers which are usually of the water-cooled type. Fig. 269 represents a General Electric transformer with its contain- ing tank removed to show the method of suspending the core and cooling coils from the cover. The cooling coils are of lap-welded wrought-iron pipe with electrically welded joints. Water is kept circulating through these coils in order that the oil filling the case and surrounding the transformer may be kept cool while the trans- former is in operation. The core and the coils are first assembled after which they are tightly clamped and suspended from the cover 288 ALTERNATING-CURRENT MACHINERY by means of heavy bolts, as shown in Fig. 269. The completed trans- former filled with oil may be easily lifted and moved about by means of the heavy lifting lugs on the upper side of the cover. Some of the advantages of this cover suspension construction (a) all coil terminals are brought out through the cover and, are: Fig. 269. General Electric Shell-Type Water-Cooled Transformer Tank Removed and Cooling Coila Shown in Section therefore, are not interfered with when the transformer is removed from its tank, (b) The terminal board at the top is accessible through openings in the cover and changes in electrical connection jan be made by simply raising the transformer a few feet out of the tank by a crane, without drawing off any oil. Inspection of all parts is also easily and quickly made, (c) There being but a few bolts to loosen, it is easy to remove the transformer from the tank, ALTERNATING-CURRENT MACHINERY 289 or to lift tank and all by means of the lugs on the cover, (d) With this construction there is no need of lowering crane hooks or chains into the tank, thereby avoiding possible danger to insulators and to coil insulation. The construction of water-cooled transformers is, in mechanical Fig. 270. General Electric Water-Cqoled Transformer Showing Coils in Position and electrical design, similar to the oil-cooled transformers, the only difference being in the tank. In the water-cooled type the tank is built up of heavy boiler plate iron riveted to a cast-iron base. All joints on the tank are heavily riveted and thoroughly caulked to make them oil tight. Three=Phase Transformers. During the past few years the size of transformer units has been steadily increasing in response 290 ALTERNATING-CURRENT MACHINERY to the demand from the power companies engaged in generating and transmitting electrical power on an enormous scale. Long- distance transmission of electrical power is most economically carried out by the three-phase system, which is the standard prac- Fig. 271. General Electric Water-Cooled Transformer Completely Assembled tice. These conditions have caused a demand for large three-phase transformers, which under certain conditions are to be preferred to three single-phase transformers of the same aggregate capacity, but no general rule can be given as to the relative value of the two types. The advantages of the three-phase transformer over a group of three single-phase transformers of the same total capa- city are: ALTERNATING-CURRENT MACHINERY 291 (1) Lower cost. (2) Higher efficiency (3) Requires less floor space. (4) Has less weight. (5) Connections and outside wiring very much simplified as only three primary and three secondary leads are usually brought out. (6) Lower transportation charges and cost of installation. (7) Presents a symmetrical and compact appearance. The disadvantages of the three-phase type are: (1) Greater cost of spare units. (2) Greater derangement of service in case of break-down. (3) Greater cost of repairs. (4) Reduced capacity obtainable in self-cooling units. (5) Greater difficulty in bringing out taps for a large number of voltages. In general single-phase transformers are preferable where only one transformer is installed and where the expense of a spare trans- former would not be warranted. In such installations the burnout of one phase of a three-phase unit would cause considerable incon- venience for the reason that the whole transformer would have to be disconnected from the circuit before repairs could be made. If, however, single-phase transformers are used, the damaged trans- former can be cut out with a minimum amount of trouble, and the other two transformers can be operated at normal temperature open A - (or V-) connected at 58 per cent of the normal capacity of the group of three transformers, until the third unit can be replaced. With a three-phase shell-type transformer, if both the primary and the secondary are A-connected, trouble in one phase will not prevent the use of the other two phases in open delta. By short- circuiting both primary and secondary of the defective phase and cutting it out of circuit, the magnetic flux in that section is entirely neutralized. This cannot be done, however, with any but A-con- nected transformers. Where a large number of three-phase trans- formers can be used, it is generally advisable to install three-phase units. Three-phase transformers are made both of the core and the shell types, according to circumstances. Thus, Fig. 272 shows a three-phase core-type transformer with its case removed, made by the General Electric Company. A three-legged core is used, each leg being wound with the primary and the secondary coils of one 292 ALTERNATING-CURRENT MACHINERY phase. The magnetic circuit, arranged as shown in Fig. 226, is explained on page 47. Since the weight of the core and the coils is greater than in the single-phase transformers, the core-clamps and other mechanical parts are made larger and stronger while two bolts on each side, instead of one, are used to support it from the cover. Three leads only are brought out of the cover on the primary and the secondary sides, all connections being made on the inside of the tank. The General Electric Company has built three-phase trans- formers of the shell type up to 10,000 kilowatts capacity, for 100,000 volts primary and 11,000 volts secondary, and a frequency of 60 cycles. They are now building them for an output of 14,000 kilo- watts. Fig. 273 shows a three-phase shell-type of transformer removed from its tank. It is rated at 1 ,800 kilovolt-amperes, 48,000 volts and 25 cycles. This transformer, which is built by the Westing- house Company, is designed to be placed in a tank filled with oil which is kept cool by water circu- lating in a coil of brass tubing sur- rounding the transformer and below the surface of the coil. Cooling of Transformers. Transformers of moderate size have large radiating surface compared with their losses. Such transformers, therefore, can radiate the heat due to core losses and copper losses without excessive rise of temperature. A transformer which is twice as large as a given transformer in length, in breadth, and in thickness, has eight times as much volume but only four times as much radiating surface as the latter. The large transformer having nearly eight times the losses and four times the radiating surface of the smaller transformer would rise to a much higher temperature than the smaller transformer in Fig. 272. General Electric Three-Phase Core-Type Transformer Case Removed ALTRENATING-CURRENT MACHINERY 293 order to radiate the heat due to its losses. Large transformers must, therefore, be provided with special means for cooling. It is much cheaper to provide special cooling devices than to attempt to make the transformer large enough to keep cool by natural radiation. Fig. 273. Westinghouse Three-Phase Shell-Type Transformer Tank Removed Various methods of cooling are adopted in practice, ac- cording to which the following classification of transformers may be made: Self-cooling dry transformers. Self-cooling oil-filled transformers. Transformers cooled by forced current of air. Transformers oil-filled cooled by forced current of water. Self-Cooling Dry Transformers. These transformers are usually of small output, and no special means of cooling is provided, the 294 ALTERNATING-CURRENT MACHINERY natural radiation being depended upon for cooling. Some larger ones up to 5- or 10-kilowatt capacity have been made in this way, but they are heavier and more expensive than if oil-cooled. Self-Cooling, Oil-Filled Transformers. Transformers of this type are very generally employed, the entire core and coils being immersed in oil. Transformers are practically always enclosed in a cast-iron or sheet-steel case, and this is simply filled with a special high-grade mineral oil. No increase in cooling surface is thereby secured, but the natural circulation of the oil tends to equalize the temperature of the various parts, and carries the heat to the case, from which it is radiated. In most self-cooling types, the case is made with external ribs or corrugations to increase its radiating surface. The large volume of oil also absorbs considerable heat, so that the temperature rises more slowly. Hence, for moderate periods of operation, up to 3 or 4 hours which is ordinarily sufficient in electric lighting the maximum temperature would not be reached. Another advantage gained by this arrangement is an improvement in insulation. This is due to the high insulating qualities of the oil itself, and to the fact that a disruptive discharge takes place through it much less readily than through the air that it displaces, distances being the same. This arrangement possesses, moreover, the power of self-repairing any break in the insulation. If ordinary materials, such as cloth or mica, become punctured, they lose their insulating properties, and the 'apparatus cannot be used until the fault is re- paired, which ordinarily involves considerable time and expense. On the other hand, if oil is punctured, it tends to close in and repair the break, unless the discharge lasts so long that a charring occurs, which may make a permanent conducting path. The chief objection to the use of oil is the danger of fire. If a short-circuit occurs inside the transformer, the oil may be thrown out and ignited at the same time; or a fire started in any other way might be made far more disastrous than it would otherwise be owing to the presence of a large quantity of oil. In this way, several power plants have been destroyed by fire with large loss of prop- erty. There is no special precaution that will entirely eliminate this risk; but care in locating such transformers, in avoiding over- heating, and in protecting the machines by effective lightning ar- resters., will reduce the hazard. ALTERNATING-CURRENT MACHINERY 295 Oil-cooled transformers can be built for any voltage and to almost any size, although the economical maximum limit in output is reached at about 500 kilowatts. ^^j^^^^^?^^ Fig. 274. Bank of Air-Blast Transformers Showing Arrangement of Air Chamber Air-Blast Transformers. These transformers are now com- monly employed, and have advantages over those of the oil-cooled type, in that the danger of fire is avoided, and the cooling effect can be regulated in accordance with working conditions. They are so constructed that air can circulate through and around the core and coils, the ventilation being forced by a blower driven by a motor. A transformer of 100-kw. capacity requires about 450 cubic feet of air per minute at a pressure of 0.5 ounce per square inch, the power consumed by the blower set being less than one per cent of the full-load output of the transformer. The flow of air is controlled by dampers; and the proper amount of air can be determined from its tem- perature as it issues from the top; ordinarily this tempera- ture should not be more than 20C. above the temperature // AirCHamberY 7 \ Fig. -275. Section of Air-Blast Transformer Showing Circulating Currents of the room. Fig. 274 shows a bank of six large air-blast transformers sup- ported on ! beams over an air chamber supplied with air from a fan blower. The air passes in at the bottom of each transformer 296 ALTERNATING-CURRENT MACHINERY case, penetrates through ducts in the core and coils, and passes out at the top of the case, as shown in Fig. 275. The process of building up the core of a shell-type air-blast transformer is illustrated in Fig. 264. The air-blast type is used for moderate voltages where cooling water is expensive or not available, and is built for voltages up to 33,000 in sizes up to 5,000 kilowatts. The limiting voltage for this type is determined by the excessive thickness of the solid insulation needed and the consequent difficulty in radiating heat from the copper. For voltages above 33,000, the oil-insulated water-cooled type is, therefore, recommended. Water-Cooled Transformers. Transformers in which water is used for cooling are always immersed in oil. In an oil-insulated water-cooled transformer, all the heat generated in the iron and copper, except a small amount radiated from the surface of the case, is dissipated by means of water flowing through coils of pipe placed under the oil near the surface. As the copper and iron become heated, the heat is transferred to the oil coming into contact with their surfaces. As the oil is heated up, convection currents are pro- duced by the hot oil rising from the transformer and flowing over towards the sides of the case; here it comes into contact with the surface of the cooling coils, giving up its heat to them and then sinks along the sides of the case to the bottom where it is ready to be heated up again and repeat the cycle. This method of water cooling is so effective that very little heat is dissipated from the tank and there is nothing to be gained by corrugations. From this it is seen that the cooling coils form a very important part of the oil-insulated water-cooled transformer and that, if for any reason they should fail to perform their work of carrying away the heat or should develop a leak, the transformer may be seriously damaged or even destroyed. Water cooling is at present the most effective method for dissipating the heat generated in very large units and where- ever the voltages exceed 33,000 volts. It is very convenient for water-power plants, the supply of water being at hand; but where a natural flow is not available, and pumps or city water mains have to be utilized, the expense may be prohibitive. * *In any of these types of transformers depending upon forced circulation of air or water, it is vitally' important to avoid any stoppage of the flow, as this is likely to cause a burn- out of the transformer coils. ALTERNATING-CURRENT MACHINERY 297 Series or Current Transformers. The chief difference between the series (current) and the shunt (voltage) transformer is, as the name implies, in the manner of connecting it in the circuit. The series transformer has its primary coil in series with the line, while the shunt transformer has its primary shunted across the line wires. In the latter the primary voltage is determined by that of the main circuit, and the primary current is determined by the impedance of the transformer, varying according to the secondary load. In the series transformer, on the other hand, the primary current is determined by the current in the main circuit, and the primary vol- tage is merely the drop across the primary terminals due to the primary impedance. The total impedance of the secondary circuit being normally constant, the change in load is due to a simultaneous change in the primary current and voltage. In the shunt (voltage) transformer the act- ual ratio of the primary to the secondary current is of minor consequence, whereas a con- stant ratio of voltages (espe- cially in the so-called potential transformers used in connec- tion with voltmeters and watt- meters) is of the highest impor- tance. In the design of a series transformer, the questions of constant-voltage ratio and of high efficiency receive no attention, whereas the matter of securing a definite ratio of secondary to primary amperes receives the most care- ful attention. It was pointed out on page 249 that a transformer intended to give an accurately fixed ratio between primary and secondary cur- rents must be so designed as to have a very small magnetizing current. This is accomplished by using a very low magnetic flux density in the iron core, so as to reduce the watts of core loss to a minimum. The magnetizing current is also reduced by using iron- core plates of high permeability and with no breaks or joints in the magnetic circuit. Fig. 276. Portable Current Transformer Showing Primary Winding Terminals 298 ALTERNATING-CURRENT MACHINERY The series transformer is mostly employed for insulating an ammeter, a current relay, or series coil of a wattmeter or watt-hour meter from a high voltage circuit, or for reducing the line current to a value suited for these instruments. The secondary coils of such current transformers are usually wound for five amperes. Figs. 276 and 277 show two commercial types of portable cur- rent transformers made by the General Electric Company. Both types have cores built up of closed ring-shaped stampings of thin sheet steel. The primary winding of the transformer shown in Fig. 276 is divided into four coils, both ends of each coil being brought out at one end and connected to suitable terminal blocks. By con- necting- these coils in series, series-parallel, or in parallel, three different current ratios are obtained, the standard ratings being 200-100-50 amperes primary with 5 amperes secondary. The secondary is wound as one coil, the terminals of which are brought out at the top. The type shown in Fig. 277 has no regular primary winding, but the core is provided with an opening through which a cable carrying the current to be measured may be passed one or more times to make the primary winding. The ratio of transformation depends upon the number of times the cable is made to pass through this opening. This form of current transformer is designed to have 1,000 ampere turns at full-load rating. The standard transformer has a 5-ampere secondary winding giving a ratio with one primary turn passing through the center of the core of 100 : 5, or 200 : 1 . If the cable is passed through the open- ing twice, the ratio is 500 : 5, or 100 : 1, etc. Both types are rated at 40 watts, and may be used on circuits the voltage of which does not exceed 2,500 volts. The instrument, such as an alternating-current ammeter, to be used with these transformers, is connected directly in series with the secondary coil. If the ratio of transformation is 200 : 5 and the ammeter indicates 5 amperes, the actual value of the current passing Fig. 277. Portable Current Trans- former without Regular Primary Windings ALTERNATING-CURRENT MACHINERY 299 in the line and through the primary will be 200 amperes. In Fig. 217 the coil A, in series, with the secondary coil S*, may represent the switchboard ammeter. The question may arise as to why the switchboard ammeter is not connected as a shunt across the terminals of a low-resistance link inserted in the main circuit whose current is to be measured, exactly as in the case of switchboard ammeters for direct currents._ There are two reasons why this arrangement would not be permissible: (a) An alternating current does not divide between two branches of a circuit in inverse proportion to the resistances of the branches, when either branch has inductance. Therefore, the reading of a shunted alternating-current ammeter cannot be multiplied by a constant factor to give the total current in the main circuit. (b) It is objectionable to have the ammeter in electrical connection with high-voltage mains. The use of the series transformer is, therefore, prefer- able on the grounds of both accuracy and safety. Constant=Current Transf orm= ers. The constant-current trans- former is a transformer spe- cially designed to take a nearly constant current at varying angles of lag from constant-voltage mains, and to deliver a constant current from its secondary coil to a receiving circuit of variable resistance. The action of this trans- former is as follows: The primary coil P and the secondary coil S of the transformer surround a long, laminated-iron core, as shown in Fig. 278. This core, and the yokes at the top, bottom, and sides, form a double magnetic circuit. The magnetic flux , which at a given instant passes through the primary coil, flows partly through the secondary coil as the useful flux U, and partly leaks across be- tween the primary and secondary coils as the leakage flux L. The leakage flux L, in flowing across the air spaces from core to *The electrical connections for a series transformer have been described on page 249. Fig. 278. Magnetic Circuit in Constant- Current Transformer 300 ALTERNATING-CURRENT MACHINERY TABLE VIII Constant=Current Transformer Data* PRIMARY VOLTS NUMBER OP LAMPS VOLTS PER LAMP SECONDARY " VOLTS SECONDARY AMPERES 2,200 50 76.6 3,830 6.60 2,200 40 77.6 3,105 6.70 2,200 30 77.2 2,315 6.67 2,200 25 81.4 2,035 6.65 2,200 20 84.2 1,685 6.65 2,200 15 86.0 1,290 6.65 second. *Test of a 50-light, 6.6-ampere constant-current transformer, freqxiency 60 cycles per yokes, constitutes an intense magnetic field which pushes up on the secondary coil. The secondary coil is suspended, and partly counter- balanced by a weight, so that the upward push of the leakage flux just suffices to sustain the coil. If the resistance of the secondary receiving circuit is increased, the immediate result is to reduce the secondary current below its normal value, which lessens the upward push of the leakage flux on the secondary coil. The sec- ondary coil then, owing to the unbalanced action of the weight, moves down towards P; the leakage flux is lessened in amount and the useful flux U is increased in amount. This increase of useful flux increases the induced electromotive force in S; and the downward movement of S continues until the induced electromo- tive force in S is large enough to produce the normal value of the current through the increased secondary resistance. Similarly, a decrease of resistance of the secondary receiving circuit causes a momentary increase of secondary current which increases the upward push on the secondary coil. This coil moves upwards until the secondary current is reduced to the normal value. Table VIII shows the approximate constancy of secondary cur- rent in a constant-current transformer supplying current to a varying number of arc lamps connected in series to its secondary coil. With 50 lamps, the primary current is 14.83 amperes; arid with 20 lamps, is 14.78 amperes. In the latter case, the primary current lags 71 degrees behind the primary applied voltage; the power factor corresponding to this angle is 0.326; and the power received from the mains is 2,200 X 14.78 X 0.326=10,600 watts, or ALTERNATING-CURRENT MACHINERY 301 530 watts per lamp. With 40 lamps, the primary current lags 55 degrees behind the primary applied voltage; the power factor corre- sponding to this angle of lag is 0.574; and the power received from the mains is 2,200X14.81X0.574 which is equal to 18,800 watts, or 470 watts per lamp. The efficiency of the transformer with a 50-lamp load was 93.9 per cent, and with a 20-lamp load it was 85.7 per cent. The power factor of the system as a whole varies Fig. 279. Mechanism of General Electric Two-Coil Constant-Current Transformer from 72 per cent to 76 per cent at full load, decreasing considerably at light loads. Fig. 279 is a general view of the mechanism of the two-coil (one primary, and one secondary), constant-current transformer of the General Electric Company; and Fig. 280 shows the trans- former in its containing case. The mechanism of the transformer is surrounded by a corru- gated sheet iron casing designed primarily for the protection of the 302 ALTERNATING-CURRENT MACHINERY coils. The casing is enclosed by a cast-iron base and top which are provided with ample openings for the proper ventilation of the transformer. Within the working limits, the magnetic repulsion between the fixed and moving coils of the system for a given position is pro- portional to the current flowing in the coils, which makes the Fig. 280. General Electric Two-Coil Transformer Completely Assembled transformer capable of being adjusted, therefore, so as to main- tain any current, simply by changing the amount of counter- weight. In transformers up to and including 50-lamp capacity having but one movable coil the secondary, as in Fig. 279 the counter- weight is equal to the weight of the coil less the electrical repul- sion; and a reduction in the counterweight will produce an increase ALTERNATING-CURRENT MACHINERY 303 * in the current. A lever is supported by knife-edge bearings on hardened steel tables which are clamped to the top of the core. To one end of this lever are secured two fixed arcs to which are attached two cables which support the movable secondary coil. At the outer end of the lever an adjustable arc carries a counterweight suspended by a cable. In transformers designed to supply 75 and 100 lights, having two primary and two secondary coils, the movable secondary cotls are balanced one against the other by a system of double-rocker arms supported on knife edges. The weight necessary to balance the repulsion between the primary and the secondary coils is carried on a small auxiliary lever. In this case, a decrease in the counterweight is followed by a decrease in the current. The arc on the counterweight lever is made adjustable because the repulsion exerted by a given current flowing in the coils is not the same for all positions of the coils, being greater when the primaries and secondaries are close together and less when the primaries are separated. By means of the adjustable arc, the effective radius of the balancing weight is made to change as the coils move through their working range. When the primary and the secondary coils are separated by the maximum distance, the resultant force which tends to attract them to each other should be less than when they are close together. Regulation. When current flows in the primary and secondary coils, the mutual repelling forces separate the coils until equilib- rium is restored. The current corresponding to the position of equilibrium may be adjusted by changes in the counterweights, and the coils will then always take such a position as will maintain that current constant in the secondary coils, regardless of the external resistances to which the coils are connected. With any current less than normal, the repelling force diminishes, and the primary and secondary coils approach each other, thus restoring normal current. As soon as the secondary current exceeds normal, the resultant pull exerted by the counterweight and coils is overcome, and the sec- ondary coil moves away from the primary, again restoring normal current. Transformers of this design can be made to maintain constant current even more accurately than the constant-voltage transformer maintains uniform voltage. 304 ALTERNATING-CURRENT MACHINERY In Fig. 281 on the left is shown the diagram of electrical con- nections for a constant-current transformer for either 5, 35, or 50 lights, and on the right, the diagram for the 75- or 100-light trans- former. The 75- and 100-light transformers are furnished with two primary coils which may be connected in series for 2,200 volts or in parallel for 1,100 volts; for example, connecting B to C puts the two primary coils in series, and connecting B to D and C to A puts them in parallel for 1,100 volts. In these larger transformers there are two secondary coils also, and leads are arranged so that the / *v Contact next to Panel Contact farthest from Panel ightniny Arrester (When Current Transformer is supplied connect \ as shown. Otherwise conned from fti'.g \toAmmeter and from Ammeter to Lamps. Ammeter JacA^s Q)O/>e/? Circulating Plugr Jtyitcn Constant Current Transformer Wattmeter Current Transformer - Th/j dppardtus furnished only when sub-base. i<3 supplied Fuse ? Plug Switch ^IC Source A. & Sourc e Fig. 281. Connections for Constant-Current Transformer for 5, 35, 50, 75, and 100 Lights - tiooYolts connect BtoDandCtoA 2200 falts connect toC. lamps can be divided between two circuits by means of multi- circuit connections, as shown in Fig. 281 on the right. Open-circuiting plug switches are used to disconnect the line from the secondary of the transformer when testing for a ground or an open circuit. These are also used to disconnect one of the circuits of a multi-circuit transformer in order that it may be repaired without interrupting the other circuit. The ammeter jacks, which are provided, are used for the purpose of enabling one ammeter to measure the current in more than one circuit. By ALTERNATING-CURRENT MACHINERY 305 inserting the plug in any ammeter jack, that particular ammeter is connected in series with that circuit. The primary windings are provided with fuses which are made part of the primary plug switch and are mounted on the back of Fig. 282. Westinghouse Single-Pole Primary Fuse Block the panel. They are of the tube expulsion type, depending on the expulsive force exerted by the gases formed by melting of the fuse. This force is sufficient to blow out any arc which tends to form within the narrow tube in which the fuse is located. Transformer Fuse Blocks. The safety fuse links designed to protect a transformer from burn-out in case of short-circuit, are usually placed in circuit with both primary and secondary coils. The fuse links connected in circuit with the high-voltage coil are usually encased in a porcelain tube which encloses the arc that is formed when the fuse melts; and the expan- sion of the highly heated vapors in the tube extinguishes the arc by what is called "expul- sive" action. The tubes containing the fuses are usu- ally provided with brass terminals for the fuses, the terminals projecting as blades from one side of the tube. With fuse and termi- nals complete, the tube is pushed home in a receptacle containing metal spring clips that receive the fuse terminals somewhat after the manner of an ordinary knife-switch. Fig. 283. Fort Wayne Sin- gle-Pole Primary Fuse Block 306 ALTERNATING-CURRENT MACHINERY When the fuse melts or "blows," the tube carrying the fuse and terminals is withdrawn from the receptacle. A new fuse may Fig. 284. Method of Connecting Wires to Primary Fuse Block on Poles Before Current Passes to Transformer then be put in place without danger to the attendant, after which the tube and fuse may be replaced in the receptacle. In small transformers for moderate voltages, the fuse recep- tacles sometimes form part of the containing case; but in large transformers the fuse receptacles are, as a general rule, entirely ALTERNATING-CURRENT MACHINERY 307 separate from the transformer, and are mounted at any convenient point near the transformer, for example, on a cross-arm or on the pole where the transformer is placed. In the case of transformers for use out-of-doors, the fuse receptacles always consist of water- proof cases of cast iron or porcelain, usually the latter. Fig. 282 shows the type of single-pole primary fuse block fur- nished by the Westinghouse Electric Company. The block is made of porcelain, finished in black, and is weatherproof. The upper portion of the cut-out contains the stationary contacts which are deeply recessed in the porcelain and are well separated from each other. The contacts are so con- structed that the plug is held se- curely in place by giving it a par- tial turn after inserting it . When the plug is in position, the fuse is in sight, so that its condition can be easily noted without incurring the danger of opening the primary circuit by pulling out the fuse plug while the fuse is still intact and the transformer is under load. Fig. 283 shows a single-pole fuse-plug block made by the Fort Wayne Electric Works for pro- c tecting primary circuits. The block and plug are all porcelain, finished in black, and serve as -a ] Fig. 285. Sample Mounting for Out-Door Transformers both a primary switch and a cut- out. The block is mounted on the upper cross-arm of the pole and is shaped so as to be used as an insulator. The wires may be brought directly from the line to the cut-out, and thence to the primary winding of the transformer without other support, as shown in Fig. 284. The plug and cavity in the fuse block are elliptical in section. The plug, shown at the bottom of the block in Fig. 283, is inserted 308 ALTERNATING-CURRENT MACHINERY concentrically with the cavity and by a quarter turn the contact blade is forced between bronze spring terminal clips, and the major axis of the elliptical section of the plug brought into line with the minor axis of the cavity. The result is that the cavity is divided into two parts separated by the plug which tightly fits the cavity. The fuse lies in a groove in the plug with its ends held under screws on the plug blades. The groove is long enough to prevent arcing, and danger of shock is prevented by the insulated rim of the plug. The plug is firmly, held in place by the spring terminal clips. Mounting of Outdoor Transformers. Fig. 285 shows a trans- former in its water-tight containing case, mounted on a pole, with its primary coil connected through a double-pole fuse block to 1,100-volt mains, and its two-coil secondary connected to three- wire mains for supplying incandescent lamps in a near-by building. Two suspension hooks A, made of heavy strap iron, are attached to the back of the transformer case by slipping the bolt-heads B into the sockets C, after which the nuts are screwed tight. The transformer is hoisted to its position on the building or cross-arm by means of a rope or chain, which is slipped over the two hoisting lugs D. When the transformer is hoisted into posi- tion, the suspension hooks are slipped over the cross-arm, and screwed fast by means of lag screws. The lead wires from the primary coil are connected to the lower fuse-box leads, and the upper fuse-box leads are connected to the mains. The wires from the secondary coils are led into the building where the secondary fuses are placed. It is now more usual to install two single-pole fuse blocks as illustrated in Fig. 284, than one double-pole block as shown in Fig. 285. TRANSFORMER TESTS Heat Test. The simplest method of performing this test is to connect the primary co'il of the transformer to mains giving the rated voltage and frequency of the transformer, and to load the secondary with a bank of lamps or a water rheostat, adjusting the resistance so as to get rated full-load current from the transformer. The run should be continued until an approximately constant tem- perature is reached. The objection to loading the transformer in the way described ALTERNATING-CURRENT MACHINERY 309 is that it requires taking the full rated power from the transformer, which power, therefore, is usually wasted. If two transformers of the same voltage and rated capacity are available, the test may be made on the two simultaneously by what is known as the motor- generator method, as follows: t The two secondaries, Fig. 286, connected in parallel, are excited from a low-voltage circuit A at normal voltage and frequency; consequently normal voltage is induced in each primary winding. The two primaries are connected, in series, but in such a way as to oppose each other. The resultant voltage between the points a and b will then be zero, notwithstanding the fact that full voltage exists between the terminals of each transformer secondary. Therefore, if the points a and 6 be joined together no current will flow. If. however, instead of being joined, these terminals are connected to the ter- minals of the circuit C, any voltage impressed at C will produce a current in the circuit of the primary coils independent of the voltage existing in each of TransformerNo.z. Fig. 286. Connections for Transformer Heat Test the primary coils. Since each transformer is in effect short-circuited by the other, it follows that approximately twice the impedance voltage* of one transformer impressed at C will cause full-load current to flow through the primaries and secondaries of both. Under these conditions, the transformers will run at full load, while the total energy required for the test amounts to merely the losses in the two. The circuit A supplies the excitation current and core losses, the circuit C the full-load current and copper losses. The auxiliary electromotive force impressed at C may be derived from the same source as the electromotive force at A, by means of a transformer. *The impedance voltage of a transformer is the "electromotive force which must be applied to the primary coil to produce full-load current in both coils when the secondary coil is short-circuited. This voltage is from 2 per cent to 6 per cent of the rated full-load primary voltage. See page 312. 310 ALTERNATING-CURRENT MACHINERY A regulating resistance must be connected in series with it to allow adjust- ment of the electromotive force at C until the ammeter registers full-load current. The important temperatures to be observed are those of the coils, core, and room. The temperature of the case and of the oil may be observed as checks. The determination of the temperature of the coil may be made by thermometer or by measurement of resistance. If a transformer has remained in a room of constant temperature many hours, so that the temperature is approximately uniform throughout, thermometer measurement indicates quite accurately the temperature of the windings. If, however, the transformer is radiating heat, as during the heat run, the actual temperature of the copper coils will be much greater than the temperature of surface insulation. If we know the "cold" resistance, as measured under the first of the above conditions, and the temperature of the coil at the time of measurement, we have a means of finding the "hot" temperature of the coil by measuring its "hot" resistance. The rise in temperature above the temperature at which the "cold" resistance was measured, may be determined from the equation r = (238.1 in which R t is the "cold" resistance, Rt+ r is the "hot" resistance, i is the initial temperature, and r is the rise in the temperature expressed in degrees centigrade. The temperature coefficient for commercial copper wire is taken at 0.0042. If the room temperature differs from 25C. the observed rise in tempera- ture should be corrected by 0.5 per cent for each degree centigrade. Thus, with a room temperature of 35 C. the observed rise should be decreased by 5 per cent; and with a room temperature of 15 C., the observed rise should be increased by 5 per cent. Core=Loss and Exciting=Current Test. For this test, the trans- former is connected as shown in the diagram, Fig. 287, the primary being left on open circuit. Theoretically the test may be carried out with either coil connected to mains of the proper voltage and frequency. In practice, however, it is better, from the standpoint both of convenience and of safety, to connect the secondary coil. The electromotive force is adjusted by means of the variable resistance until the voltmeter indicates rated secondary voltage. The ammeter then indicates the exciting or no-load current; and the wattmeter indicates, very closely, the core loss, Resistance of Coils. The resistance of a coil of a transformer may be measured by the ordinary drop-of-voltage method. This method consists in passing through the coil a direct current, the value of which is noted by an ammeter, while the drop of voltage across the coil is measured by a voltmeter. Then the resistance is ALTERNATING-CURRENT MACHINERY 311 rr R= . Knowing the resistance of the coils, we can determine the drop in voltage due to resistance, under load. This loss of electro- Transformer 287. Wiring Diagram for Core Loss and Exciting Current Test motive force is usually expressed in per cent of the electromotive force supplied to the primary. Impedance. One of the important constants of a transformer is its impedance ratio, that is, the ratio of the voltage consumed by its total internal impedance at full-load current to its rated full- load voltage. The impedance of a transformer is measured by short- circuiting one of its windings, impressing an alternating electromotive force on the other winding and making simultaneous measurements of current and impressed voltage. E and I being thus observed, 7r OL nsformer Fig. 288. Diagram of Connections for Making" Transformer Impedance Test the impedance is found as Z= ohms. The value thus obtained includes the impedance of both primary and secondary windings. 312 ALTERNATING-CURRENT MACHINERY Impedance may be considered as constant at all loads. It is usually measured at full-load current, and the impressed voltage is then called the impedance volts, and when expressed in per cent of the rated voltage of the transformer, it is called the per cent im- pedance drop. It is evident that the ' Impedance ratio" as defined above is simply the per cent impedance drop divided by 100. The impedance of a transformer is an important factor in determining the regulation by calculation. To determine the im- pedance voltage, the transformer is connected as shown in Fig. 288. As the impedance voltage is not very large varying from 2 per cent to 6 per cent of rated primary voltage in standard transformers a much more accurately readable deflection of the voltmeter will be obtained if the primary coil is connected to the mains. As will be seen by referring to Fig. 288, the secondary coil is short-circuited. The primary coil is connected in series with an adjustable resistance to the low-voltage mains. The resistance is slowly cut out, until full-load current flows in the coil, as indicated by the ammeter. Then the voltmeter indicates the impedance voltage. This should be expressed in per cent of the normal voltage of the coil. From the equation the total impedance, inductance, and reactance can be computed, provided R and the frequency / are known. Example. In a test on a 7.5-kw. transformer with secondary short- circuited and primary connected to 2,080-volt mains, the impedance voltage was 61.1 volts at full-load current of 3.6 amperes in the primary, at a fre- quency of 60 cycles. The impedance drop being 61.1, the per cent impedance drop is Q =2.935 per cent, and the impedance is Z=-^- = 16.95 ohms. Since impedance = Z = Vfl 2 +Z 2 , X=^Z 2 -R 2 and the total reactive drop expressed in per cent is %XI = V(% impedance drop) 2 -(%fl/) 2 If % #7 = 1.57 from test, and % ZI =2.935 as calculated above, then %XI = \ 2.935 - 1.57 = 2.48 or 2.48 2080 ALTERNATING-CURRENT MACHINERY 313 Regulation. The definition of regulation given on page 266 will be repeated here, it is : the rise of secondary terminal voltage from rated non-inductive load to no load (at constant primary impressed terminal voltage) expressed in per cent of the secondary terminal voltage at rated load. The regulation of a transformer may be determined directly by exciting the transformer at rated frequency and with a primary voltage such that rated secondary voltage is obtained at full load, using lamps* or a water rheostat as load. The increase in secondary terminal voltage from full load to no load is then observed, the primary voltage and frequency being kept constant throughout the test. This increase in secondary voltage divided by the second- ary full load voltage is then the regulation expressed in per cent, This method is, however, unsatisfactory, because of the small differ- ence between the full load and no load values, and the liability of error in measuring either of them-. Much more reliance can be placed on results calculated from separate measurements of impedance drop and resistance than on actual measurement of regulation. A number of methods have been proposed for the calculation of transformer regulation, but the following formula will be found simple and practically correct. per cent regulation = pRI + qXI + ~^ - (40) . 200 in which RI is total resistance drop in the transformer due to load current expressed in per cent of rated voltage; XI is total reactive drop due to load current similarly expressed; p is power factor of the load on the secondary = cos 6; for a non-inductive load p=l; and q is reactive factor of the load = sin 0. Example. A 7.5-kw., 60-cycle transformer of 10 to 1 ratio with secondary voltage of 208 at full load has the following constants : Primary resistance, #1 = 5.65 ohms. 20 34 Primary #i/i = 5.65 X 3.6=20.34 volts, or %RJi = : X100 =0.978%. Secondary resistance, R 2 = 0.0334 ohm. Secondary R 2 Iz = 0.0334X36 = 1.24 volts, or %# 2 /2 = 0.596%. Total RI (reduced to primary) =20.34 + 10 X 1.24 =32.74 volts, or 1.57%. Reactive drop, XT' =2.48%. (a) Regulation on non-inductive load. Here cos 6 =l=p, and sin 0= = q. % regulation = 1 X 1.57 + X 2.48 + (* X 2.48 -1.57 X 0) 200 = 1.57 + 0.031 = 1.60 314 ALTERNATING-CURRENT MACHINERY (b) Regulation on inductive load. Assuming the power factor of the load to be 0.80 = p, gives q = yi -o. =0.6 % regulation = 0.8 X 1.57 + 0.6 X 2.48 + = 1.256 + 1.488 + 0.0054 = 2.75. (0.8 X 2.48 -0.6 X1.57) 2 200 The regulation for any other power factor p may be calculated in a similar manner. Efficiency Calculation. The efficiency of any piece of apparatus at a given load is equal to the output divided by the input. The input is equal to the output plus the losses. The efficiency may then be defined as the ratio of the output to the output plus the losses. In nearly every case the efficiency can be determined more accurately by measuring the losses, and then computing the efficiency accord- < Su00ly Mofn A 0000, B nnnn , row] <7|l/OW/AS-*|/> 0000 Fig. 289. Transformer Connected for Polarity Test Fig. 290. Wiring Diagram for Transformer Polarity Test . ing to the second definition, than by attempting to measure the total output and input, and then taking their ratio. Example. A given 5-kw. transformer is rated at 2,000 volts primary, and 200 volts secondary, at a frequency of 60 cycles per second. The coil resistances are found by measurement to be: Primary coil resistance 10.1 ohms Secondary coil resistance 0.067 ohms At full load, full-load currents are: Primary current 2.5 amperes Secondary current 25.0 amperes Core loss, as determined by test 70 watts Copper losses at full-load are: Primary loss =I' 2 R f = 10.1 X (2.5) 2 = 63 watts Secondary loss = I" 2 R" =0.067 X (25) 2 = 42 watts ALTERNATING-CURRENT MACHINERY 315 Total loss at full-load 175 watts Full-load output 5,000 watts Full-load intake 5,175 watts Full-load efficiency 5,000 -=-5,175 = 96.6 per cent At half load: , Total I 2 R loss 26 watts Core loss 70 watts Total loss 96 watts Half-load output 2,500 watts Half-load intake 2,596 watts Half-load efficiency, 2,500 -=-2,596 = 96.3 per cent The all-day efficiency of a transformer is the ratio of the output of work (watt-hours) during the day to the total input of work (watt-hours). The usual conditions of practice will be met if the calculation is based upon 5 hours at full load, and 19 hours at no load. See page 265. Output : 5 hours at full load = 5 hours X 5,000 watts = 25,000 watt-hours 19 hours at zero load = watt-hours Total output in 24 hours = 25,000 watt-hours Input : 5 hours at full load = 5hrs. X 5,175 watts = 25,875 watt-hours 19 hours at zero load =49hrs. X 70 watts = 1,330 watt-hours The zero load intake is but very little more than core loss, since I' 2 R' is negligible at zero load. Total intake in 24 hours 27,205 watt-hours All-day efficiency 25,000 -=-27,205 = 91.9 per cent Polarity Test. Transformers are generally designed so that the instantaneous direction of flow of the current in certain selected leads is the same in all transformers of the same type. For example, the transformer shown in Fig. 289 is designed so that the current at any instant flows into lead A and out of lead C. Such transformers run properly in parallel when similar primary and secondary leads on different transformers are connected together. The primaries of two transformers A and B are connected to supply mains, the con- nections of one primary being made without reference to the con- nections of the other. It is desired to determine first, how the sec- ondaries are to be connected in parallel to supply current to one and the same receiving circuit; and second, how the secondaries are to be connected in series to supply current to a receiving circuit. This test is made as follows : Connect a terminal, say a, Fig. 290, of the secondary of one transformer to one terminal c of the secondary of the other transformer. Then connect two 110-volt lamps in series (or a voltmeter) to the other two terminals 6 and d. If the lamps do not light (or if the voltmeter gives no deflection), then first, 316 ALTERNATING-CURRENT MACHINERY the terminals a and c are the proper ones to connect together to one service main; and the terminals b and d are to be connected together to the other service main ; and second, the terminals a and c are not the proper ones to be connected together, but 6 and c are properly connected together in order to connect the two coils in series, the other two terminals a and d being connected to the service mains. ALTERNATING-CURRENT MACHINERY PART V CONVERSION OF ALTERNATING INTO DIRECT CURRENT In spite of the increasing use of alternating currents, there is always a demand for devices of various kinds for converting the power received from an alternating-current circuit into power in the form of direct current. Thus, many electro-chemical processes, electrolytic refining of silver and copper, the charging of storage batteries, and direct-current motors all require direct current. Elec- tric power, as is well known, can be transmitted long distances most economically by the three-phase alternating-current system employ- ing high voltages. The demand for direct-current power arises when the power is to be utilized at the receiving end of the line. The conversion from alternating current to direct current is accomplished in practice in the following ways: (a) by the rectifying- commutator; (b) by the aluminum valve rectifier; (c) by the mercury- vapor rectifier; (d) by the rotary (or synchronous) converter; (e) by the motor-generator. Rectifying Commutator. The rectifying commutator is a com- mutator driven at a speed synchronous with the alternating current supplied to its brushes and it reverses the connections of the armature windings of an alternator as a whole. The rectifying action consists in reversing alternate (negative) half waves of the current delivered by the alternator so that the current in the receiving circuit is always in one direction. The application of this device to the compounding of alternators is explained on page 111. The rectifying commutator is limited to the rectifying of com- paratively small currents at moderate voltages on account of the prohibitive sparking which occurs at the brushes at high voltages and large outputs. Copyright 1912, by American School of Correspondence. 318 ALTERNATING-CURRENT MACHINERY Aluminum Valve Rectifier. The aluminum valve rectifier is an electrolytic coil which depends on the property of aluminum electrodes in certain electrolytes to let electric current pass in one direction (when the aluminum electrode is cathode) but not in the other. The simplest arrangement of a single- cell valve rectifier is to connect the secondary of the transformer whose current is to be converted into direct current to the positive plate (iron) of the cell and the negative plate (aluminum) to the positive terminal of the storage battery to be charged. To complete the series circuit the negative terminal of the storage battery is connected to the other end of the transformer secondary. A common glass battery jar, containing an approximately neutral solution of pure ammonium phosphate in which an iron plate (the positive) and an aluminum /OAflF Fig. 291. Diagram of Connections for Aluminum Valve Rectifier plate (the negative) are immersed, constitutes the rectifier. With such a single cell only one-half of the complete alternating-current wave is utilized, the other half of the current wave of reversed sign being suppressed by the aluminum valve action. The reversed cur- rent which would tend to flow from the aluminum plate through the electrolyte to the iron plate is stopped because of the formation of a thin insulating film of aluminum oxide over the aluminum plate which resists the passage of current unless the voltage exceeds about 150 volts. A better plan of connecting a single cell so that both halves of the alternating-current wave will be rectified is given in Fig. 291. The choke coil C is an autotransformer, tapped at its middle point by a connection to the direct-current load B, such as a storage ALTERNATING-CURRENT MACHINERY 319 battery to be charged. The other terminal of the battery is connected to the iron plate in the cell. The coil C offers a high impedance to the alternating current, but very little impedance to the intermittent direct current flowing out from its middle tap, because an equal amount of current comes from each end and flows in opposite directions around the core of the choke coil. Assuming that at a given instant the terminal A of the secondary of the supply transformer is positive, the path of the current will be as shown by the arrowheads. Since current cannot enter the solution of the cell by way of an aluminum electrode, the 5 amperes have to pass through one-half of the choke- coil winding and out of the middle tap into the battery B, from which it enters the rectifier cell through the iron electrode and returns through the aluminum electrode at the right, back to the other ter- minal D of the transformer secondary. Since the tap point of the autotransformer C is at its middle point, the energy delivered to its lower half acting as a primary coil is transferred by transformer action to its upper half, so that the 10-ampere pulsating direct current is furnished by the combined currents of 5 amperes in each of the two halves of the coil C. It follows that at any instant there is twice as much current through the direct-current circuit B as through the alternating-current circuit, but at half the voltage. Thus, if both the alternating-current and the direct-current voltages are measured by alternating-current voltmeters, which give effective values, the alternating-current voltage would be, say, 100 volts, and the direct-current voltage would be 50 volts. In prac- tice the direct-current voltage would be less than 50 volts on ac- count of voltage drops and leakage in the cell. With each reversal of the alternating-current voltage, the direc- tion of the current in the alternating-current circuit, including the coil C, is reversed and each aluminum electrode alternates with the other in becoming an active negative terminal and then an inactive positive, but the direction of flow of the direct current through the battery B will remain unchanged. The aluminum valve rectifier is adapted for rectifying relatively small currents (up to about 25 amperes direct current), its capacity being limited by the heating of the cells. The efficiency is low, rang- ing from 50 to 60 per cent in practice. The power factor is never over 90 per cent, even on full load. The highest effective value of 320 ALTERNATING-CURRENT MACHINERY the alternating-current voltage that can be used with the cell at normal operating temperatures is about 175 volts, which means that about 55 volts will be obtained on the direct-current side. As electrolytes, ammonium phosphate or sodium phosphate are considered best. The cell should be artificially cooled to work satisfactorily. The voltage may be regulated by means of resistance in either the alternating-current or the direct-current circuits, but a better method is by adjusting the alternating-current voltage by a small autotransformer having a number of taps. The greatest advantage of the aluminum valve rectifier is its cheapness and sim- I A l v B Fig. 292. Mercury- Vapor Arc Rectifiers plicity. On account of its small direct-current output, its poor efficiency, and comparatively high cost of maintenance, it is not as yet of great commercial importance. Mercury=Vapor Arc Rectifier. The mercury-vapor arc rectifier, like the aluminum valve rectifier, operates by the action of electric valves. The rectifier bulb consists of a closed glass vessel provided with four electrodes; those marked A A, Fig. 292, called anodes (or positives) are of graphite, and the other two B and (7, called cathodes, are of mercury. The air is exhausted from the bulb which contains only mercury vapor. This like other metal vapors is an electrical ALTERNATING-CURRENT MACHINERY 321 i MAMMA/ i <4.C. SUPPLY conductor under some conditions, and the graphite or positive electrodes are immersed in this vapor. A pool of mercury in the bottom of the bulb forms the negative electrode or cathode B y and the small electrode C is used merely for starting the mercury arc between A and B. The rectifying action depends on the properties of mercury in the presence of mercury vapor, as follows: Current can readily^ pass from either of the solid graphite electrodes to the mercury vapor, but if it is attempted , to reverse the direction of the current, a very high apparent resistance is developed at the surface of the mercury elec- trode which prevents the flow of current from B to A. The diagram of electrical connec- tions for a mercury rectifier is given in Fig. 293. The alter- nating-current supply circuit is connected to the two posi- tive electrodes A and A' of the bulb and also to two re- actance coils F and E. On account of the check-valve action just described, the pul- sations of the alternating cur- rent pass alternately from A and A' into the mercury (neg- Fig. 293. ative) electrode B, from which they are delivered as an uni-directional current for charging the storage battery J. When the bulb starts to rectify there is a high resistance at the surface of the mercury which must be broken down before current can pass. This cathode resistance acts like an insulat- ing film over the surface of the mercury, but wheii a current is once started it will continue to flow, meeting with but small resistance as long as the current is uninterrupted. The briefest interruption, however, permits the cathode resistance to increase enormously, and thus stop the action of the bulb. Connections for Mercury- Vapor Arc Rectifier 322 ALTERNATING-CURRENT MACHINERY In order to break down the cathode resistance, on starting the rectifier, the bulb is tilted or shaken so that the small starting anode C, Fig. 293, is brought into contact with the cathode B by a mercury bridge. Current then passes between C and B and the little stream of mercury which bridges the space between the electrodes breaks with a spark "when the bulb is returned to its normal vertical posi- tion. This spark or initial arc breaks down the cathode resistance by forming mercury vapor which enables the graphite anodes to become active and the rectifying action to start. After the tube is in operation, the circuit through C is opened by a switch. The action of the rectifier may be followed in detail with the aid of Fig. 293. Let us assume an instant when the terminal // of the supply transformer is positive, then the path of the current will be shown by the small arrowheads, the electrode A will be positive, and the current is free to flow from A across the mercury vapor to By the mercury negative electrode (cathode). From B the current passes through the storage battery J, through the re- actance coil E, and back to the negative terminal G of the trans- former. A moment later when the impressed voltage falls below a value sufficient to maintain the arc against the counter-electromotive force of the arc and storage battery, the current would be interrupted at the end of the first half cycle before the current from anode A' could be established. In order to prevent this interruption two reactance (choke) coils F and E are connected in the circuit, as shown. The inductance of these coils delays the decreasing current from one anode, as A, until the voltage of the transformer has passed through zero, reverses, and builds up to such a value as to cause A' to start an arc between it and the mercury cathode B. The path of the current is now from G down to A' y across the mercury vapor to B, out through J, the coil F, and back to H, which is now the negative terminal. The new path is indicated by the arrows enclosed in circles. A moment later A again becomes active, and the path of the current is again indicated by the plain arrowheads as at first. Mercury-vapor rectifiers as made for charging storage batteries, operating arc lamps, small direct-current motors, etc., are furnished in four sizes, suitable for 10, 20, 30, and 40 amperes of direct current, respectively. They are designed for a frequency of 60 cycles, but can be adapted to com- ALTERNATING-CURRENT MACHINERY 323 mercial frequencies from 25 to 140 cycles per second. They are made for the standard secondary alternating voltages of 110 and 220 volts. The efficiency varies with the direct-current voltage delivered, and ranges from about 75 per cent from one quarter to full load when the direct-current voltage averaged 80 volts, up to over 80 per cent for direct-current voltages averaging 112 volts. Fig. 294. Front and Back View of Mercury- Vapor Arc Rectifier Equipment The rectifier tubes differ in size according to their ampere rating and in shape according to the direct-current voltage for which they are designed. The average life of a tube under normal operating conditions is about 600 hours, when a new tube should be substituted. Fig. 292 shows two different voltage types of the 30-ampere size of tube made by the General Electric Company. The tube on the left is for direct-current voltages from 25 to 45 volts; and the tube on the right is for 90 to 250 volts. A wide range of variation in the direct-current voltage is easily obtained by manipulating the dial switch mounted on the panel, shown in Figs. 294 and 295, 324 ALTERNATING-CURRENT MACHINERY Fig. 294 shows front and rear views of a complete mercury arc rectifier outfit with switchboard panel, as built by the General Electric Company. Fig. 295 shows the front, rear, and side views of such an outfit with standard dimensions. The outfit consists of a rectifier tube, reactance, regulating compensator, and panel. The panel is completely wired and simply requires connections to be made to the secondaries of the supply transformer and to the direct- current load circuit. The equipment of the panel is clearly shown in Fig. 295. The double-pole "line switch" to the left in Figs. 294 and 295 connects to the alternating supply circuit. The circuit breaker at the LO40 C/RCU/T B/tAHER AMMETER RECT/F/ER SHAKER ftODE SMTCH STAftT/MG -SW/TCH FUSE REGULAT/M6 COMFEMSA7VR ftEACTAMCE Fig. 295. Diagrammatic Views of Front, Back, and Side of Rectifier Equipment Showing Details top of the panel protects the direct-current circuit from excessive current. The "starting switch" is a single-pole double-throw spring switch which is used to start the tube on a resistance mounted on the back of the panel. The "anode switch" is an auxiliary spring switch mechanically operated by the "starting switch" and auto- matically opens the starting anode circuit as soon as the handle of the "starting switch" is released. The dial switch is provided with a double set of contact buttons, one set for rough and the other for fine regulation of the direct- current voltage. These contact buttons are connected to taps ALTERNATING-CURRENT MACHINERY 325 brought out from the regulating compensator mounted at the back of the panel. Another important application of the mercury-vapor rectifier is in connection with street and out-door lighting by series, constant, direct-current arc and incandescent lamps. The arc lamps commonly used for this service are known as "luminous arc" and "flame arc" lamps. The series incandescent lamps for this service have filaments - of tungsten, and are made for candle powers ranging from 25 to 80, and for currents of 4 and 6.6 amperes. This system of series, constant, direct-current lighting involves an alternator delivering alternating current at 2,200 volts to the primary of a constant-current transformer whose secondary is designed to give a constant alternating current of either 4 or 6.6 amperes at a voltage depending upon the number of arc lamps in series to be supplied. A mercury-vapor rectifier is then used to convert the constant alternating current into a constant direct current suitable for use in the arc lamps. The rectifiers constructed for this service differ from those described above only in certain details. The rectifier tubes are operated in oil-filled tanks, for cooling purposes, and the voltages of the rectified current range from 1,200 volts in the 12-light outfit up to 6,450 volts in the 75-light outfit. Rectifier tubes can be built for voltages up to 13,000. ROTARY OR SYNCHRONOUS CONVERTER The rotary or synchronous converter is a machine for convert- ing alternating current into direct current, or vice versa. The import- ance that such machine's have assumed in the electrical industry is due to several causes: (a) It is necessary, for economic reasons, to use alternating current at high voltages in long-distance transmission, as explained on page 4. Therefore, rotary converters are required for chang- ing the alternating current into direct current for use in electric railway motors, which must be supplied with direct current from the trolley wire at points at a distance from the power house. (b) Rotary converters are needed for charging storage bat- teries in places where the central station supplies alternating cur- rent, and inverted rotaries are necessary for factory driving with 326 ALTERNATING-CURRENT MACHINERY alternating-current motors in cases where direct current only is supplied by central stations. (c) Direct current is necessary in many of the chemical and electro-metallurgical industries such as the electrolytic reduction of aluminum from its ores, the electrolytic refining of copper, etc. If alternating current is generated and transmitted to these establish- ments, it must be converted into direct current before it can be utilized. The rotary converter is chiefly used to convert polyphase alter- nating currents into direct current on a large scale. Comparison with Direct=Current Dynamo. In general appear- ance and construction, the rotaiy converter resembles the direct- current generator very closely. The chief outward difference is the addition of a number of collector rings concentric with the shaft on one side of the armature, and the commutator is very much larger than in the ordinary direct-current generator. Another point of difference is in the relative dimensions of the magnetic circuit, including yoke and magnet cores, which are smaller than would be usual or desirable in ordinary direct-current generators. Under the usual condition of running, the armature is driven, as in a simple synchronous motor, by alternating current supplied to the collector rings from an external source. While so revolving direct current can be taken from brushes bearing upon the com- mutator. The current in the armature of a rotary converter may be thought of as the difference between the inflowing alternating cur- rents and the outflowing direct current. The average value of the current in a given armature conductor is, therefore, smaller in value than in the corresponding direct-current generator, and the heat- ing effect PR is correspondingly less'. Furthermore, the magnetizing action of the inflowing alternating current upon the armature is almost completely neutralized by the magnetizing action of the outflowing direct current. Therefore, a larger number of smaller conductors may be wound upon a given armature core if the arma- ture is to be used for a rotary converter, than would be permissible if the armature were to be used for a direct-current generator. That is, the allowable power output of a machine of given size is not limited to so small a value if the machine is to be used as ALTERNATING-CURRENT MACHINERY 327 TABLE IX Power Ratings of Rotary Converters in Kilowatts Continuous- Single- Three- Four- Six- Current Phase Phase Ring Phase Dynamo Converter Converter Converter Converter 100 85 132 162 192 / f+Brush a polyphase rotary converter, as it would be if the machine were to be used as a direct-current generator. Table IX gives the power ratings which a machine that would be rated at 100 kw. if used as a direct-current generator has, when it is used as a single-phase, three-phase, two-phase (four-ring), and six-phase converter, respectively. To Make a Direct=Current Dynamo into a Rotary Converter. Consider an ordinary bipolar* direct-current dynamo. Imagine two opposite commutator bars of the machine to be marked a and b, re- spectively, Fig. 296. Let the field mag- net of the machine be excited, and the armature be driven at a speed of n revolutions per second in a counter- clockwise direction, as shown by the arrow. At a given instant the marked bars will be midway between the direct-current brushes, as shown in the figure. Let us call the position of the armature at this instant the position A, Fig. 297, and let us consider the way in which the electromotive force between the given pair of commutator bars a and b changes as the armature rotates. (1) While the armature is making the first quarter of a revo- lution from the A position, the bars will move until bar a touches the + brush and bar b touches the brush; and the electromotive force between the bars will grow from zero to the full value E of the direct electromotive force between the brushes. 77- Fi; . 296. Commutator and Brush )iagram for D. C. Dynamo as a Rotary Converter *The following discussion applies to multipolar machines also, but the sta-tements are much simpler when limited to the bipolar machine. 328 ALTERNATING-CURRENT MACHINERY (2) While the armature is making the second quarter of a revolution from the A position, the bars will move until they are again midway beween the direct-current brushes; and the electro- motive force between the bars will drop from the value E to zero. (3) While the armature is making the third quarter of a revolution from the A position, the bars will move until bar a touches the brush, and bar b touches the + brush; and the electro- motive force between the bars, which must now be considered as negative, will grow from zero to the value E. (4) While the armature is making the fourth quarter of a revolution from the A position, the bars will move until they are again midway between the direct-current brushes; and the electro- motive force between the bars, which is still to be considered as negative, will drop from the value E to zero. first %ftevolufiort%fte\/ol: x "~ ~ lecting rings, four collecting rings, or y~7 six collecting rings, it is called a polyphase f / rotary converter. Fig . 29 8. Brush and Commu- Three=Ring Converter. Three equi- distant commutator bars a, b, and c, Fig. 298, of a direct-current dynamo are connected to three collector rings. It is shown later in this discussion that the electromotive force between bars a and b is, in phase, 120 degrees ahead of the electromotive force between bars b and c, and 240 degrees ahead of the electromotive force between bars c and a. Three electromotive forces related in this way are called three-phase electro- motive forces; and a direct-current dynamo provided with three slip-rings as specified, is called a three-phase, or three-ring, rotary converter. ,'' Four=Rmg Converter. Four equi- /' /' distant commutator bars a, b, a', b', !! ( ;, Fig. 299, of a direct-current dynamo are } connected to four collecting rings. Then \ '* ' the electromotive force between the rings a and a' is at its zero value when the electromotive force between rings b and b' is at its greatest value, or vice versa. Two Fig. 299. Brush and Conimu- electromotive forces related in this way are tator Diagram for Four- Ring Converter called two-phase electromotive forces; and a direct-current dynamo with four collecting rings, as specified, is called a two-phase, or more accurately, a four-ring rotary converter. 330 ALTERNATING-CURRENT MACHINERY Six=Ring Converter. Six equidistant commutator bars a, b, c, a', b', c', Fig. 300, of a direct-current dynamo, are connected to six slip-rings. Such a machine is called a six-phase or six-ring rotary converter. The electromotive force between the rings a and a' is, in phase, 120 degrees ahead of the electromotive force between rings b and b', and 240 degrees ahead of the electromotive force between rings c and c'. Three electromotive forces so related are called three- phase electromotive forces; and a six-ring rotary converter may, under certain conditions, be supplied with three-phase alternating currents. Multipolar Rotary Converters. The discussion already given on converters refers, for the sake of simplicity, to a bipolar machine. In case of a multipolar d. c. dynamo, having p field poles, the connections of the n rings of an n-ring converter are as follows: 7j Ring 1 is connected to the - - equidistant commutator bars 2i which, for a given position of the armature, are squarely opposite the centers of, say, the north poles of the field magnet. Let d be the distance between the commutator bars to which ring 1 is connected. Ring 2 is connected np to the equidistant commutator bars 2 1 which are - - of d ahead of the bars con- n nected to ring 1; ring 3 is connected to np the equidistant commutator bars which 2 Fig. 300. Brush and Commu- are - - of d ahead of the bars connected tator Diagram for Six- ^ Ring Converter to ring 1; and so on. For example, a 6-pole direct-current dynamo with 72 commu- tator bars, to be made into a three-ring converter, would have ring 1 connected to commutator bars 1, 25, and J$; ring 2 connected to commutator bars 9, 33, and 57; and ring 3 connected to commutator bars 17, 41, and 66. E. M. F. Relations for Rotary Converter. Let E be the value of the steady electromotive force between the direct-current brushes of a rotary converter; and let E 2 , E 9 , E*, E 6 , be the effective values ALTERNATING-CURRENT MACHINERY 331 of the alternating electromotive force between adjacent collecting rings of a two-ring, three-ring, four-ring, and six-ring rotary con- verter, respectively. It is desired to find the relationship between these various electromotive forces. Relationship between E and E 2 . The maximum value of the alternating electromotive force between the collecting rings of a two-ring converter occurs at the instant when the commutator bars to which the collecting rings are connected are in contact with the direct-current brushes, and this maximum value is, of course, equal to E. Therefore, the effective value E 2 of the alternating electromotive force between the slip-rings of a two-ring converter is E equal to / -. That is E (41) Relationship between E 2 and E 3 . The discussion of the relation- ship between E 2 and E s will be carried out for a very special case, namely, where the armature has 18 conductors; and this special discus- sion will lead to a very simple geo- metrical construction which will easily give the relationship between E 2 , E s , E*, and E 6 , and will also show the phase relations of the vari- ous electromotive forces of a three- ring, of a four-ring, or of a six-ring converter. Fig. 301 represents a direct- current ring-wound armature having 18 conductors, each conductor representing a turn of wire. Con- sider the conductor a. This conductor has an alternating electro- motive force induced in it as the armature rotates. Let this alter- nating electromotive force (effective value) be represented by the short line a in Fig. 302. Consider the next following conductor b. The alternating electro- motive force induced in this conductor has the same value as that Fig. 301. Diagram of a D. C. Ring- Wound Armature 332 ALTERNATING-CURRENT MACHINERY induced in conductor a, but is behind it in phase by the angle Fig. 302. E.M.F. Diagram for 360 18 20, where 18 is the total number of armature conductors. Let the electromotive force induced in conductor b be represented by the short line b in Fig. 302. Similarly, the short lines c, d, e, /, g, etc., in Fig. 302, represent the alternating electromotive forces (effective values) induced in the conductors c, d, e, /, g, etc. Consider first the two-ring converter. Suppose that its slip-ring 1 is connected to the commutator bar which is between conductors r and a, as shown in Fig. 301; then its other slip-ring will be connected to the bar which is between conductors i and j, and the alternating electromotive force E 2 between these two slip-rings will be the vector sum of the electromotive forces a, b, c, d, e, /, g, h, and i (Fig. 302), as shown in Fig. 303. That is, E 2 (effective value) is represented by the diameter of the polygon (circle) in Fig. 303. Consider now the three-ring converter. Suppose its three rings are connected as shown by the numbers 1, 2, and 3 in Fig. 301. Then the electromotive force E's, between rings 1 and 2 is the vector sum of the electromotive forces a, 6, c, d, e, and /, as shown in Fig. 303; the electromotive force E"a, between rings 2 and 3, is the vector sum of the electromotive forces g, h, i, j, k, and /, as shown in Fig. 303 and the electromotive force E r "a, between rings 3 and 1, is the vector sum of the electromotive forces m, n, o, p, q, and r, as shown in Fig. 303. Therefore, the effective value of the electromotive force E% between the two rings of a two-ring converter, being represented by the diameter of a circle, the effective value of the electromotive force Ea t between any two rings of a three-ring converter is repre- sented by a 120-degree chord of the same circle. Therefore Fig. 303. E.M.F. Relations Two- and Three-Ring Converter V 3 XE, ALTERNATING-CURRENT MACHINERY or, using the value of E 2 from equation (41), we have 333 (42) It is to be noted that in order to make a direct-current machine into a three-, four-, or six-ring converter, the number of armature conductors must be divisible by the number of rings. Therefore,' the armature shown in Fig. 301 is not suitable for a four-ring con- verter, although it is suitable for a three-ring converter. The foregoing discussion shows that if the effective value of E 2 is represented by the diameter of a circle, then the effective value of E s is represented by a 120-degree chord, the effective value of E* Fig. 304. E.M.F. Relations in a Four- Ring Converter Fig. 305. E.M.F. Relations in a Six- Ring Converter is represented by a 90-degree chord, and the effective value of E& is represented by a 60-degree chord of the same circle. This is shown in Figs. 303, 304, and 305. From Fig. 304, we have or, substituting the value of E 2 from equation (41), we have 1 E E (43) 334 ALTERNATING-CURRENT MACHINERY From Fig. 305 we have or, substituting the value of E 2 from equation (41), we have (44) 2 Summary of E. M. F. Relations for the Rotary Converter. Let E be the electromotive force between the direct-current brushes of a rotary converter; then, E 2 = 0.707 E ' E 3 = 0.612 E E, = 0.500 E E 6 = 0.354 E 45) in which E^ E s , E^, and E& are the effective values of the alternating electromotive force between adjacent collecting rings on a two- ring, three-ring, four- ring, and six-ring converter, respectively, and E is the steady value of the electromotive force between the direct- current brushes in each case. Examples. If a rotary converter is to deliver direct current at 500 volts : (a) It must be supplied with single-phase alternating current at 353.5 volts effective if it is a two-ring converter. (b) It must be supplied with three-phase currents at 306 volts effective between each pair of the three supply mains, if the converter is a three-ring converter. (c) It must be supplied with two-phase currents over four-wire supply mains with 250 volts effective between mains connected to adjacent collector rings, or 353.5 volts effective between mains connected to opposite collector rings, if the converter is a four-ring converter. (d) It must be supplied with six-phase currents over six-wire supply mains, with 177 volts effective between the mains connected to adjacent col- lector rings; or with 306 volts effective between the mains connected to rings 1 and 3; or with 353.5 volts effective between the mains connected to opposite collector rings. Modification of Theoretical Voltage Ratios in Actual Machines. The theoretical ratios of alternating to direct-current voltage are not always found to hold good in practice. This is owing to a variety of causes, chief among which are: the deviation of the generator voltage from a sine wave; the voltage drop in the armature wind- ALTERNATING-CURRENT MACHINERY 335 TABLE X Voltage Ratios of Rotary Converters PERCENTAGE POLE ARC 50 Per Cent 67 Per Cent 74 Per Cent 80 Per Cent 550-volt 67 63 62 61.5 Three-phase 250-volt 67.5 63.5 62.5 62 125-volt 68 63.8 63 62^ _ 550-volt 78 73.5 72.5 72 Two-phase 250-volt 79 74 73 72.5 125-volt 79.5 74.5 73.5 73 ings; the position of the direct-current brushes on the commutator; and the degree of field excitation. As the direct-current voltage at the commutator brushes, neglecting the resistance drop IR in the converter, is equal to the maximum instantaneous voltage between opposite collector rings, a flat-top wave gives a higher ratio, i. e., lower direct-current volt- age, and a peaked wave a lower ratio, i. e., higher direct-current voltage, for the same impressed alternating-current voltage. More- over, the shape of the electromotive force wave impressed by the generator upon the converter, is modified by the form of the counter- electromotive force wave of the converter. Hence, a short pole arc of the converter, producing a peaked wave of counter-electromotive force, tends to lower the direct-current voltage, and a long pole arc tends to raise the direct-current voltage, for the same impressed alternating voltage. A displacement of the brushes from the neutral point decreases the direct-current voltage for a given alternating-current voltage, the variation in extreme cases amounting to several per cent. Over-excitation of the field magnet may increase the direct- current voltage one or two per cent; while with under-excitation, i. e., with lagging current in the armature, the direct-current voltage may be decreased one or two per cent for a given alternating-current voltage. Under average conditions of full-load operation, the standard types of converters have ratios alternating-current voltage -^direct- current voltage, approximately as given in Table X. In the normal operation of the rotary converter, namely, when furnishing direct current, the drop in the armature reduces the direct-current voltage. When run as an inverted converter, i. e. t 336 ALTERNATING-CURRENT MACHINERY when delivering alternating current, direct current being fed to the brushes, the drop is on the alternating-current side; consequently the voltage ratio of a converter is lower when it is run inverted. For preliminary calculations where the data of operation is not known, the following voltage ratios may be used with most standard converters: No load Full load For single-phase 71.5 73 For two-phase 71.5 73 For three-phase 61 62.5 For six-phase 71.5 73 (measured on diameter) For six-phase 61 62.5 (measured on alternate rings) In operating rotary converters, it is customary to make allowance for the departure of the actual voltage ratio from the normal ratio. The amount of the allowance to be made cannot always be pre- determined; but any ordinary departure from the theoretical volt- age may be easily compensated for by using transformers provided with taps on the secondary windings which will permit a voltage change of about 5 per cent. Current Relations for Rotary Converter. The rotary converter, as ordinarily used to convert alternating current to direct current, behaves as a synchronous motor so far as its intake of alternating current or currents is concerned. In the case of the synchronous motor with a given belt load, the intake of alternating current varies with the degree of field excitation, the intake of current being a minimum for a certain field excitation, and the power factor nearly unity (see page 226). In the case of the rotary converter also, the intake of alternating current or currents is a minimum, and its power factor is unity for a certain field excitation, the direct-current output of power being given. Under these conditions there is a definite/ relation between the direct-current 7 delivered by the converter, and the effective value of the alternating current flowing in at each collecting ring. In fact 7 2 = 1.4147 7 3 = 0.9437 , 7 4 = 0.7077 > 7 6 = 0.471 7 ALTERNATING-CURRENT MACHINERY 337 in which 7 2 , 7 3 , I*> and 7 6 are the effective values of the alternating current entering at each collector ring of a two-ring, three-ring, four-ring, or six-ring converter, respectively; and 7 is the direct current delivered by the converter. These equations (46) are based on the assumption that the converter has unity power factor as above pointed out, and that the intake of power is equal to the output of power, i. e., the losses of power in the machine are ignored. Derivation of the Equations for 7 2 and 7 3 . The method of deriv- ing equations (46) will be sufficiently indicated by deriving the first two, namely, the equations for 7 2 and 7 3 . The direct-current output of power from the converter is El, E being the electromotive force between the direct-current brushes, and 7 being the direct current delivered. The intake of power is EzIzX power factor; but since the power factor is supposed to be unity, the intake of power is simply E 2 7 2 . Therefore, ignoring losses of power, we have E 2 7 2 = El But E 2 = 0.707 E, by the first of equations (45), so that E 2 7 2 = 0.707E X 7 2 = EL Hence 0.707 7 2 = 7 or, in final form, the equation for 7 2 is 7 2 = 1.4147 The direct-current output of power is El, and the intake of power is I 7 3 E 3 I S , the power factor being unity; that is, the power delivered by three-phase supply mains is equal to.V 7 3 times the voltage between mains times the current in each main, as explained on page 128. Therefore, ignoring power losses in the machine, we have 1/TE 3 7 3 = E7 But E 3 = 0.612 E, according to the second of equations (45), so that ^T E 3 I 3 = T/T x 0.612 X El 3 = EL Hence y Yx 0.612 7 3 =7 or, in final form, the equation for 7 3 is 7 3 = 0.943 7 Example. A rotary converter delivers 500 amperes of direct current, the field excitation being adjusted so that the intake of alternating current may be a minimum. 338 ALTERNATING-CURRENT MACHINERY (a) If this converter is a two-ring converter, it must be supplied with 707 amperes of alternating current from single-phase mains. (b) If this converter is a three-ring converter, it must be supplied with three-phase currents from three-wire mains, with 471.5 amperes effective in each main. (c) If this converter is a four-ring converter, it must be supplied with two-phase currents from four-wire supply mains, with 353.5 amperes effective in each main. (d) If this converter is a six-ring converter, it must be supplied with six-phase currents from six-wire mains, with 235.5 amperes effective in each main. ROTARY CONVERTERS IN PRACTICE Uses of the Rotary Converter. The uses of the rotary con- verter are as follows: (a) Direct-Current Generator or Motor. The rotary con- verter may be used as a direct-current generator or motor, in which cases the collector rings are not used, but the -machine must be provided with a pulley. (b) Alternating-Current Generator or Synchronous Motor. The machine may be driven by belt and used to deliver alternating currents from its collector rings as an ordinary alternator; or the machine may be supplied through its collector rings with alternat- ing currents, and may be driven as an ordinary synchronous motor delivering mechanical power to drive machinery. In either case the direct current for exciting the field magnet may be taken from the commutator of the machine, or from a separate direct-current dynamo (exciter). (c) Double-Current Generator. The rotary converter may be driven by belt, and used to deliver both direct current and alter- nating current from commutator and collector rings, respectively. When so used, the machine is called a double-current generator. (d) Regular Rotary Converter. The machine may be driven as a synchronous motor taking alternating current from supply mains, and delivering, not mechanical power, but electrical power in the form of direct current from its commutator. When so used, the machine is called a rotary converter. This is the most frequent use of the machine. The rotary converter does not require a pulley. Under these conditions, the machine exhibits all of the peculiarities of the synchronous motor. Thus, with under-excited field magnet, the rotary converter takes an unduly large amount of alternating ALTERNATING-CURRENT MACHINERY 339 current from the supply mains at a low power factor. As the field excitation is increased, the intake of alternating current decreases (for given output of direct-current power), and the power factor in- creases. For a certain degree of field excitation the power factor is nearly unity, and the alternating current or currents delivered to the machine are in phase with the alternating electromotive forces between the supply mains. When the field magnet is over-excited, the alternating currents supplied to the converter are ahead of the alternating electromotive forces in phase, and the power factor is less than unity. (e) Inverted Rotary Converter. The machine may be driven as a direct-current motor taking current from direct-current supply mains through its commutator, and delivering, not mechanical power, but electrical power in the form of alternating currents from its collector rings. When so used, the machine is called an inverted rotary converter; it does not require a pulley. Starting Rotary Converters. There are three methods in common use for starting rotary converters, namely, (a) from the alternating- current side, as an induction motor; (b) from the direct-current side, as a shunt motor; or (c) by means of a special direct-connected starting motor, usually of the squirrel-cage induction type. (a) As an Induction Motor. The first method is one of the most common and has been fully discussed on page 209. It consists in supplying polyphase currents at a suitable voltage to the collector rings. The resulting torque developed by the armature will bring the converter up to synchronous speed in from one to two minutes. The proper value of impressed voltage for starting is usually obtained from taps on the main transformers. This method of starting has the advantage of making synchronizing unnecessary, but on the other hand it requires a large starting current which, if the converter is relatively large as compared with the alternator supplying it, causes an excessive drop of voltage throughout the circuit. A difficulty sometimes met in this method of starting is, that a particular direct-current brush or set of brushes may be positive or negative according to the direction of the last pulse of alternating current just before the machine jumps into synchronism. There- fore, when several rotary converters are to supply direct current to common bus bars, special care must be taken to see that the polarity 340 ALTERNATING-CURRENT MACHINERY of a given machine is correct before it is connected to the bus bars. If the machine happens to come up to the speed with the wrong polarity, the field "break-up" switch, described on page 220, must be thrown from one position to the other, thus reversing the field. Self- starting converters are also generally equipped with a switch for disconnecting the shunt across the series coils from the series field winding, during starting, so as to prevent the circulation of a large induced alternating current in both the shunt and the series field winding. The induced current would not only cause excessive heat- ing, but would also hinder starting by its braking effect. (b) As a Shunt Motor. When a direct-current supply is available, the converter may be started as a direct-current shunt motor, by supplying direct current to the commutator side of the machine, the alternating-current main switch being open. \ This method is convenient and is used in many installations. In some cases the direct current is obtained from another converter already in operation, or from a small storage battery which may be at hand. Sometimes a small motor-generator set, consisting of an induction motor coupled to a direct-current generator, is installed to supply current for the starting of one or more converters in a station. In the direct-current method of starting, the fields should be fully excited by closing the field switch first, and there should be a resistance in series with the armature when the motor switch is closed. Failure to excite the field may cause the converter to increase its speed to a dangerous extent, just as in the case of a direct- current shunt motor with excessively weak field, for in starting the converter from the direct-current side, it is not running as a synchronous motor but as a simple shunt direct-current motor. If the converter is compound wound, the series field circuit must be opened, otherwise the current flowing through it will magnetize the field poles in opposition to the shunt field windings (differential com- pounding) and may even prevent the machine from starting. The operation of starting is as follows: (1) See that the alternating-current main switch is open. (2) Close the field switch. (3) Leave the starting resistance in circuit with the armature, and then close the direct-current main switch. (4) When normal speed is reached, cut out the starting rheostat, ALTERNATING-CURRENT MACHINERY 341 and vary the field strength until the synchronizing device shows that the converter is in synchronism with the generator. (5) Close the main alternating-current switch when the synchronizing device shows that the rotary converter is in step with the alternating-current supply. If the converter is furnished with a shunt winding only, adjust the field to give minimum alternating-current input. If the converter has a series winding also, the shunt field should be ad- justed to give at no load the direct-current no-load voltage at which it is rated. When a rotary converter is started as a direct-current motor, Fig. 306. Westinghouse Rotary Converter Equipped with Separate Starting Motor on Same Shaft it is easy to bring the machine into operation with a particular direct-current brush or set of brushes positive. (c) By Separate Motor. The third method of starting rotary converters, by means of a separate-starting motor, is often used. In such cases the starting motor is usually supported on the con- verter either on one end of the base plate or by the pillow block. The rotor is mounted on the armature shaft of the converter just outside of the bearing, as shown in Fig. 306. The starting motor is usually of the induction type with squirrel-cage rotor, and is 342 ALTERNATING-CURRENT MACHINERY furnished with a number of poles which is two less than the number of poles on the converter. This enables the motor to bring the con- verter up to and above the synchronous speed, and speed regulating devices are arranged for synchronizing the converter with the alter- nating-current supply mains. This method of starting has the great advantage of requiring a relatively small starting current, and is used especially when because of limited capacity of generators or transmission system it is essential to keep the starting current as low as possible. The disadvantages of this method are : that it requires time and skilled attendants to synchronize properly, and that the auxiliary motor adds to the cost and requires additional space. A modification of the last r method is sometimes employed in which the converter is started from rest by means of a separate starting motor and then thrown directly on to the alternating- current busses in series with a suitable reactance which limits the instantaneous rush of current to a safe value. This method com- bines the advantages of separate motor starting and self-starting from the alternating-current side, in that it obviates the necessity of synchronizing and insures the rotary coming in with the right polarity; on the other hand, it requires somewhat more time than the self -starting method and a heavier line current than with the induction starting motor alone. An example of this method is shown in Fig. 306, a 300-kilowatt three-phase rotary converter manufactured by the Westinghouse Company. It has 10 poles and when supplied with alternating three-phase, 60-cycle currents, at 367 volts per phase, delivers a direct current of 500 amperes at 600 volts from the commutator. This converter is built for electric railway substation service, and is provided with an induction motor for separate starting. At the right-hand end of the shaft may be seen the mechanical oscillator described below. Oscillators for Rotary Converters. The armature of a belted dynamo or motor is always caused by the belt to shift slowly to and fro endwise in its bearings, thus entirely obviating the uneven wear- ing away of the commutator in grooves where the brushes rub. The rotary converter, however, not being mechanically driven tends to run without end-play, and some special end-play device, or oscillator, is necessary. An effective device much used is an electro- ALTERNATING-CURRENT MACHINERY 343 magnet mounted opposite to the end of the rotary-converter shaft. This electromagnet is excited about ten times per minute, and at each time gives an endwise pull on the shaft, causing the desired endwise movement of the latter. A mechanical end-play device regularly used by the Westinghouse Company consists of a steel plate with a grooved ball race and ball backed by a spring, and mounted at one end of the shaft. As the grooved plate is not_ quite parallel to the end of the shaft, when the converter is prop- Fig. 307. General Electric 300-Kilowatt Three-phase Rotary Converter erly installed with the armature slightly inclined towards the oscil- lator, the hardened steel ball is caught at the lowest point be- tween the race and the end of the shaft. The ball is carried upward as the armature revolves and the spring is compressed. The reaction of the spring now forces the shaft away and the ball falls back to its normal position. Characteristic Types of Rotary Converters. Fig. 307 shows a 300-kilowatt, three-phase (three-ring) rotary converter built by the General Electric Company. It has a six-pole field magnet and six sets of direct-current brushes, each set having eight single brushes. 344 ALTERNATING-CURRENT MACHINERY Its rated speed is 500 revolutions per minute, which, with a six-pole field, gives a frequency of 25 cycles per second. The three collector rings are mounted on the armature shaft on the end opposite to the commutator. It can be seen from the figure that the commutator is larger in comparison with the size of the machine than is usual in an ordinary direct-current generator. Each collector ring has three brushes bearing upon it in order to permit of the delivery to the machine "of the very large alternating currents from the supply Fig. 308. General Electric Three-Phase Rotary Converter for Electric Light Service mains. The use of three brushes on each collector ring is preferable to the use of one broad brush, inasmuch as it is desirable to make the rings narrow to save space. This machine is rated at 550 volts be- tween its direct-current brushes, so that the full-load direct-current output is 300,000 watts divided by 550 volts, or 546 amperes. There- fore, on the assumption of unity power factor and 100 per cent efficiency, as explained on page 336, the alternating current entering at each collector ring is 546 amperes X 0.943 = 515 amperes effective; and the effective voltage between collector rings is 550 volts X 0.612* = 336 volts. The alternating-current power supplied to the machine is VT X 336 volts X 515 amperes = 300,000 watts *See Equation (42), page 333. ALTERNATING-CURRENT MACHINERY 345 Fig. 308 shows a three-phase rotary converter manufactured by the General Electric Company for electric lighting service at 250 volts. The characteristic features which distinguish the rotary converter from the direct-current generator are here especially prominent, namely, the large commutator and great brush contact area, the comparatively large collector rings, and the relatively small magnetic system, including yoke and field-magnet cores. Hunting of Rotary Converter. A rotary converter (regular), being a synchronous motor in relation to its alternating-current supply, has a tendency to hunt, as explained on page 223. A rotary converter, however, having no pulley and not being mechanically connected to machinery, is much more sensitive in responding to the pulsations of an engine or to other causes of hunt- ing than is a synchronous motor delivering mechanical power. The hunting oscillations of a synchronous motor are always due to external disturbances, that is, to disturbances originating outside of the alternating-current generator, the line, and the synchronous motor. A sudden change of load on the synchronous motor, for ex- ample, is followed by a series of oscillations. Whether or not the oscillations due to a certain change of load give rise to trouble, depends largely upon the resistance and the reactance of the trans- mission line, and upon the frequency. The greater the resistance and the reactance of the transmission line, the greater the trouble from hunting; and at high frequencies the trouble from hunting is much greater than at low frequencies. Thus a 25-cycle rotary converter gives no serious trouble from hunting if the line resist- ance and reactance are not excessively high, whereas a 60-cycle rotary converter is more likely to give trouble unless special pro- vision is made to diminish hunting, as explained below. A periodic variation in the speed of the engine driving the alternator from which a rotary is supplied w r ith alternating current, produces very troublesome hunting when this variation of speed is in rhythm with the hunting oscillations. This class of hunting is obviated by increasing the fly-wheel capacity of the engine, or by changing the resistance or the reactance of the transmission lines. The latter method changes the rhythm of the hunting oscillations and thereby does away with the coincidence of rhythm, which is the chief cause of excessive hunting oscillations due to engine pulsations. 346 ALTERNATING-CURRENT MACHINERY The hunting oscillations, once started, usually reach their maximum under given conditions in a few minutes of time, so that serious trouble due to hunting, such as the dropping out of step of the rotary, or excessive sparking at the commutator, usually occurs soon after the hunting begins. Hunting which is approximately the same at all loads, from no- load to over-load, is more troublesome when the field of the rotary converter is over-excited so as to take leading currents, than when the excitation is such as to give either unity power factor or lagging currents. Hunting is also greater when several rotaries are supplied from an alternating-current generator, than when a single rotary con- verter is supplied, unless there are short lengths of alternating- current mains between them, or unless the converters supply direct current in parallel to the same direct-current mains. Dampers. Hunting, whether due to engine pulsations or to momentary outside disturbance, such as sudden change of load or momentary short-circuit, is greatly reduced by the use of massive copper bridges or frames Fig. 309. Diagram Showing Copper Dampers in Place in Field Poles these holes or channels heavy extending partly over the pole faces, as shown in Fig. ' 199. A more effective ar- rangement is shown in Figs. J 71 and 309. A number of holes or channels are pro- vided in each pole tip. In copper conductors A and B are placed, and these conductors are short-circuited by being connected at the ends by the bars C C. These copper frames or "dampers" diminish hunting oscillations because, when a machine is hunting, the magnetic flux from pole-face to armature core is shifted forwards and backwards over the pole-face, and this shift- ing flux induces electromotive forces in the copper frames, which produce currents tending to oppose the shifting of the flux, and thereby oppose the hunting oscillations. A rotary converter having solid cast-steel pole pieces has little or no tendency to hunt. The action of the solid poles is the same as the action of the massive copper conductors shown in Fig. 309. ALTERNATING-CURRENT MACHINERY 347 The use of solid poles, however, leads to excessive eddy-current losses; hence solid poles are not considered desirable. Inverted Rotaries. When a rotary converter is used to convert a direct current into an alternating current, taking direct current in at the commutator and delivering alternating current at the collector rings, it is called an inverted rotary converter. While the rotary converter is generally used to convert alternating current into direct current, it sometimes happens that inverted converters are desirable. For example, in a low-tension, direct-current system, a district remote from the central station may be supplied with current by converting direct current to alternating current at the station (by means of inverted rotaries); then, by step-up trans- formers, raising the voltage to a high value, and transmitting it as high-tension alternating current; and finally, at the distant point, reconverting it (using step-down transformers) to direct current. Again, in a station containing direct-current generators for short- distance supply, and alternators for long-distance supply, the con- verter may be used as the connecting link to shift the load from the direct to the alternating generators, or conversely. The ma- chine which is used for shifting the load in this way is caused to operate as a regular rotary or as an inverted rotary according to the demand for direct current or alternating current. The behavior of an inverted rotary converter is different in many respects from the performance of the same machine when used as a regular rotary converter. When converting from alter- (nating current to direct current, the speed of the converter is rigidly fixed by the frequency of the alternating current supplied to it, and cannot be varied by alternating its field excitation. Varying the field excitation would merely change the phase difference between the alternating electromotive force and current supplied to the machine, and hence the power factor, as in the case of the synchronous motor. When converting from direct current to alternating current, however, the speed of the converter, as in a direct-current motor, will be pro- portional to the applied direct-current voltage, and will also depend upon the field excitation. The effect of weakening the field is to in- crease the speed, and the effect of strengthening the field is to de- crease the speed. It is evident, therefore, that there should be little or no series field winding provided on an inverted rotary con- 348 ALTERNATING-CURRENT MACHINERY verier, as it will change in speed under load and deliver alternating currents at a variable frequency. If the field becomes greatly weakened, an inverted rotary converter may reach a dangerously high speed before the attendant has time to prevent it, and the armature of the machine may be torn to pieces by the excessively large centrifugal forces. Changing the field excitation of an inverted rotary converter will not change the voltage of the alternating current, because the ratio of transformation in a given converter is fixed; changing the field strength merely causes a change in the speed of the rotary. * The voltage of the alternating current may be changed by changing the voltage of the applied direct current, or it may be varied by using alternating-current .potential regulators, page 448. Speed-Limiting Devices. An inverted rotary being an alternating- current generator, its field strength depends upon the intensity and the phase relation of the alternating current supplied by its armature; thus a lagging current in the armature reduces the field strength, and hence increases the speed and the frequency; whereas, a leading current increases the field strength, and thus decreases the speed and the frequency. Again, if the alternating-current side of an inverted rotary converter delivers large lagging currents to induc- tive receiving circuits, such as induction motors, the demagnetizing action of the lagging armature currents on the field may result in a dangerously high speed. In operating inverted rotary converters, therefore, especially when they are liable to be overloaded on the alternating-current side, as in the starting of synchronous or induc- tion motors supplied from the inverted rotary, great care should be taken to see that the field excitation is always great enough to pre- vent excessive speeds. When used for the above purpose, special speed-limiting devices should be used. A method used by the Westinghouse Company to prevent this tendency of the inverted rotary converter to race, is as follows: The converter is separately excited by a small direct-current generator mechanically connected to, and driven by, it, The speed of the exciter will, therefore, change with every change in the speed of the inverted rotary. The magnetic circuit (magnet cores, yoke, etc.,) and magnet coils of the exciter are so designed that its armature can generate normal voltage when the machine is being worked at a point considerably below the "knee" of the saturation curve. Any increase in the speed of the exciter will, therefore, cause a great ALTERNATING-CURRENT MACHINERY 349 increase in its voltage. If then the speed of the inverted rotary converter increases, the voltage of the exciter immediately increases and strengthens the field of the converter, thus checking its tendency to race. The same result is attained by the General Electric Company, but in a different manner, as follows: A kind of centrifugal governor is attached to the shaft of the inverted rotary and revolves with it. If the speed of the rotary converter increase^ above a certain value, the governor acts, thus closing an electric circuit which automatically throws off the power supplied to the rotary. Control of Direct=Current Voltage. The relations between the alternating-current voltages supplied to a rotary converter and the direct-current voltage of the machine, as explained on page 331, and as expressed in equations (45), apply to the case in which the field excitation of the rotary converter is such that the alternating cur- rents delivered to the machine are in phase with the applied alter- nating-current voltages; that is, these relations apply to the case in which the power factor of the machine is unity. The direct- current voltage of a rotary converter may be slightly greater or less than the ideal value given in equations (45), but the fact remains that the ratio of the alternating-current voltage to the direct- current voltage of a given converter is nearly constant. The fixed ratio of these voltages is a serious handicap in electric railway and similar work where it is desirable to have the station voltage increase as the demand for current increases, in order that the voltage at a distant point of the line shall be kept up in spite of the increase in line drop. In order, therefore, to increase the voltage of the direct- current output of the rotary converter it is necessary to increase also the alternating-current voltage supplied to its collector rings. This increase of voltage may be secured by the following methods: (a) Variable ratio step-down transformers (b) Variable ratio low-voltage autotransformers (c) Voltage regulators (see page 448) (d) Synchronous regulators or boosters (e) Series field winding properly proportioned in connection with series inductive reactance (compounding) Methods (a), (b), (c), and (d) are non-automatic, and are used where the load is fairly constant over considerable periods. Method (e) is entirely automatic within a range of 10 to 15 per cent and is frequently used where the load is rapidly fluctuating, as in electric 350 ALTERNATING-CURRENT MACHINERY railway service. With the necessary auxiliary apparatus, methods (c) and (d) can be made to operate automatically. In using method (a) the transformers for rotary converter service .are designed for the normal secondary voltage which will give the required direct-current voltage. They are provided with taps on both primary and secondary windings which allow compensation to be made for line drop or for any small variation from the desired standard voltage. These taps, however, cannot be used for controlling the voltage while the apparatus is in service. Method (d) requires a synchronous booster which is merely an auxiliary alternating- current generator, with the same number of poles as the converter, and whose armature is mounted on the shaft of the converter. The armature winding of this generator is connected in series between the armature and the collector rings of the converter. The field Fig. 310. Rotor of Westinghouse Six-Phase Rotary Converter with Synchronous Regultaor magnets of the booster are separately excited so that its armature voltage may be varied from zero to a maximum. Thus, since the armature windings of both the converter and the booster are in series, the alternating-current voltage supplied to the collector rings may be varied at will, and the voltage on the direct-current side changed accordingly. If necessary the excitation and the polarity of the regulator may be reversed, so that the voltage derived from the regulator may be subtracted from the normal value of the impressed alternat- ing-current voltage instead of added to it, and a corresponding reduction of the direct-current voltage obtained. The regulator may be arranged for either automatic or manual control. With a synchronous regulator having 15 per cent of the rotary converter capacity, the direct-current voltage can be varied 30 per ALTERNATING-CURRENT MACHINERY 351 cent at will; or, if desired, a constant voltage can be maintained. Moreover, the regulator field can be so controlled that when operating in parallel with other rotary converters, or storage batteries, the con- verter will take its proper share of the total load. Fig. 310 shaws the rotor of a 770-kw., six-phase rotary converter provided with a synchronous regulator or booster, made by the West- inghouse Electric Company. Passing from left to right in the figurer there are : the commutator of the converter, the armature of the con- verter, the armature of the synchronous regulator, the six collector Fig. 311. Westinghouse Three-Phase Converter with Synchronous Regulator rings, and on the extreme right the squirrel-cage rotor of the auxiliary starting motor. Fig. 311 is a view of a Westinghouse three-phase 1,000-kw. converter with a synchronous regulator. The regulator frame, with its field poles and windings, is supported by brackets attached to the converter frame. The converter itself is the same as other standard converters, the main field windings being free from all regulating devices. Method (e) for voltage control depends on the action of the series field winding in connection with series reactance. As already 352 ALTERNATING-CURRENT MACHINERY shown on page 226, the field excitation of a synchronous motor may be varied through quite a range above or below that corresponding to unity power factor, the machine taking leading currents when its field is over-excited, and lagging currents when its field is under- excited. This is especially the case when the synchronous motor has considerable armature inductance, and when the transmission line also has considerable inductance. This remark applies to the rotary converter also; and, when the transmission line has considerable reactance, the alternating-current voltages between the collector rings of a rotary converter and the direct-current voltage between its direct-current brushes vary with the field excitation of the con- verter. Where there is both inductance and resistance drop in the feeders and a considerable variation in the alternating voltage supply, the converter, if provided with series field winding, can be made to regulate automatically for constant direct-current volt- age within reasonable limits, as explained below. As the ratio of the alternating current to the direct-current voltage of a converter is nearly constant, the impressed alternating voltage must be varied in order to vary the direct-current voltage. This can be done by taking advantage of the fact that an alternating current passing over an inductive circuit will decrease in voltage if lagging in phase behind its electromotive force, and will increase in voltage if leading. Just as in the case of a synchronous motor, a certain field excitation in any converter will give a minimum arma- ture current. If the excitation be decreased, the armature current will be increased but will be lagging. By providing, therefore, sufficient reactance in the alternating-current circuit connecting a converter with its source of power, the alternating-current voltage at the converter terminals may be varied by means of the field exci- tation of the converter, and without altering the generator voltage. When it is desired to control the direct-current voltage of a rotary converter independently of the voltage of the alternating- current generator that supplies the alternating currents, the trans- mission line is frequently given an artificial reactance by connecting inductance (reactance) coils in series with the alternating-current supply mains. Thus, the alternating currents delivered to the con- verter are caused to flow through these reactance coils. When, ALTERNATING-CURRENT MACHINERY 353 therefore, a rotary converter has a compound field winding (series and shunt), as described below, the use of reactance coils in the supply mains is necessary if the transmission lines do not of them- selves have sufficient reactance. Field Excitation. Various methods are employed for exciting the field magnet of rotary converters, as follows: (a) By Armature Reaction. WJien an alternating-current generator delivers leading current to a receiving circuit, the mag- netizing action of the armature currents tends to strengthen the field magnet poles, as explained on page 109. If an alternating-current generator were always used to deliver leading currents, it would be possible to depend upon the magnetizing action of the armature currents entirely for exciting the field magnet, without using any direct current whatever in the field windings; in fact, the field windings could be dispensed with altogether. When a synchronous motor, or rotary converter, takes lagging currents from alternating-current supply mains, the magnetizing action of these currents in the motor armature is to strengthen the field magnetism of the motor, or rotary converter. This action alone may be utilized for exciting the field magnet of a synchronous motor, or rotary converter, and rotary converters have been designed and commercially operated with this mode of field excitation. (b) Self -Excitation by Direct-Current Taken from the Commu- tator of the Machine. The usual method of exciting the field of a rotary converter is by means of direct current taken from the com- mutator of the machine itself. There are three schemes for carrying out this method of field excitation, exactly as in the case of ordinary direct-current generators, as follows: (1) Series excitation. This scheme, in which the entire direct-current output flows through the field winding (coarse wire), gives a field excitation which is zero when the direct-current output is zero, and which rises to full- rated excitation when full-load output of direct current is reached. This scheme of field excitation is not suitable for rotary converters, inasmuch as a rotary converter should have an approximately constant-field excitation, or a field excitation which changes through a comparatively narrow range only. (2) Shunt excitation. In this scheme the field winding is made of com- paratively fine wire. Its resistance, therefore, is comparatively high, and it is connected directly between the direct-current brushes with an adjustable field rheostat in its circuit, exactly . as in the ordinary shunt-wound direct- 354 ALTERNATING-CURRENT MACHINERY current dynamo. This scheme gives an approximately constant field excitation, and it is much used in rotary converters. The variation of field excitation for the purpose of controlling the power factor of the converter is accomplished by means of the adjustable field rheostat. (3) Compound excitation. The combination of series and shunt excita- tion is frequently used in rotary converters so as to provide for slightly increas- ing field excitation (by means of the series winding) with increasing direct- current output. This scheme of field excitation is, however, more limited when applied to a rotary converter than when applied to an ordinary direct- current dynamo, for the reason that too great an increase of field excitation in a rotary converter (as in the case of a synchronous motor) causes the con- verter to fall out of synchronism and stop, or "break down," as it is termed. Compound-wound rotary converters are used to advantage for supplying current which is constantly fluctuating (and where the generators supplying the converters do not greatly exceed the latter in kilowatt capacity) as in railway service, and in cases where it is necessary to maintain constant or increasing voltage with increasing load. More or less prominence can be given to shunt or series wind- ings as may be required. The regulation is made automatic by a series field winding on the converter, but the inductance of the transmission lines and gen- erator must frequently be increased by introducing reactive coils. The amount of raising ("boosting") or lowering of the volt- age is proportional to the reactance in circuit, for a given series field. Considering, however, that the maximum output of the converter and its stability are affected by too much reactance, the introduc- tion of reactance should not be carried too far. In compound-wound converters the shunt excitation is gen- erally adjusted to give a lagging current of from 20 per cent to 30 per cent of full-load current at no load by under-excitation, and the series field is adjusted to give a slightly leading current at full load. This arrangement lowers the impressed voltage at the converter at no load and raises it at full load enough (with constant voltage at the generator) to compensate for all the losses of voltage in the system, thus making possible the delivery of a constant direct-current voltage at all loads. It has been found in practice that the compound winding dis- tinctly diminishes the stability of running when the tendency to hunt is present to any extent. The series winding should be cut out ALTERNATING-CURRENT MACHINERY 355 when starting up from the direct-current side. This is conveniently accomplished by a double-throw switch which in one position con- nects the junction of the series winding and the negative brushes to the starting rheostat, and in the other position connects this junction with the equalizing bar. (c) Separate Excitation. The use of a small auxiliary direct- current dynamo to supply direct current for exciting the field of .a rotary converter has been mentioned on page 348, where it was pointed out that this method of field excitation is especially suited to inverted rotaries. Rotary Converter with Edison Three=Wire System. The Edison three-wire system must ordinarily be supplied with current from two direct-current generators connected in series between the outside mains, and with the neutral main connected to the junction of the two machines. To operate a three-wire system from a single direct-current generator would give rise to great differences of voltage on the two sides of the system when the number of lamps on one side differs greatly from the number of lamps on the other side; in fact, it would not be allowable to turn off a lamp on one side without turning off a lamp on the other side at the same time. To avoid this difficulty some arrangement which is equivalent to the use of two generators is necessary. A rotary converter may be used to deliver direct current to an Edison three-wire system giving every advantage ordinarily obtained by the use of two direct-current generators connected in series. For such service the third wire must be connected to the neutral point of the transformer group in such a manner that the current in the neutral will flow equally in both directions through the transformer windings and, therefore, will not change the effective magnetic flux in the core of the transformer. When three-phase distribution is used the direct current neutral is brought out from the common junction point of the inter-connected Y secondaries. With two-phase distribution the direct current neutral can be connected to the middle point of the secondary coils of the trans- former. The connections of a three-phase rotary converter for sup- plying direct current to an Edison three-wire system are shown in 356 ALTERNATING-CURRENT MACHINERY Fig. 312. Connections for a Three-Phase Rotary Converter on Edison Three-Wire System Fig. 312. The three primaries A, B, and C of the step-down trans- formers for supplying alternating currents to the rotary converter are either Y- or A-connected to the high voltage supply mains. The three secondaries a, b, and c are Y-connected to the collector rings r' , r", r'" y of the rotary converter. The two outside direct-current mains 1 and 3 are connected to the direct-current brushes of the converter as shown, and the middle main (neutral main) 2 is connected to the common junction N, or neutral point, of the Y-connected sec- ondaries a, b, and c. With these connections, the voltage be- tween mains 1 and 2 is a steady direct-current voltage, as is also the voltage between mains 2 and 3, and each of these voltages is equal to half the voltage between mains 1 and 3. When the number of lamps connected between mains 1 and 2 is different from the number connected between mains 2 and 3, or vice versd, the neutral main must carry a direct current equal to the difference of the direct currents in the mains 1 and 3. This direct current in the neutral main 2 is actually supplied through the secondary coils a, b, and c from the collector rings. The rotary converter may also be Fig. 313. Connections for D. C. Generator on Edison Three-Wire System with Two-Phase Rotary Converter Connected as a Balancer used to take the place of two machines, a generator and a motor, used in combination to form a "balancer" for keeping the voltages on the two sides of an Edison three-wire system the same. Fig. 313 shows an ordinary direct-current generator G, sup- ALTERNATING-CURRENT MACHINERY 357 plying current to the outside mains 1 and 3, of an Edison three-wire system, and a two-phase rotary converter R, connected as a "bal- ancer" to supply the necessary direct current to the middle main 2. The armature only of the rotary converter is shown in the figure, its direct-current brushes being connected to the outside mains 1 and 3. One pair of opposite collector rings of the converter R is connected to an inductance coil a a, wound on an iron core, and the other pair is connected to an inductance coil b b. The middle points of these two inductance coils are connected together and to the mid- dle or neutral main 2 of the three-wire system. Six=Phase Converter. The large rating of a six-phase converter as shown in Table X, together with other advantages enumerated below, make this machine the standard converter for large outputs. The advantages of the six-phase converter are as follows: (a) The high rating, namely, 1.92 times the rating of the same machine as a direct-current generator, or 1.45 times the rating of the same machine as a three-phase converter, means that the machine may be smaller and, there- fore, cheaper for a given" output. The high rating of the six-phase converter is due to the fact that its armature winding is tapped at six points (for a two- pole machine), and that alternating currents enter the armature winding at six points, so that the length and the resistance of the paths from collector rings to commutator are less, and the heating of the armature windings is less than it is in a three-phase converter, for example, for the same direct- current output. (b) The six-phase converter runs more stably than a converter having a smaller number of collector rings, and has less tendency to hunt. (c) The magnetizing actions of the alternating and direct currents in the armature are more nearly balanced in the six-phase converter than in a converter having a fewer number of collector -rings, and commutation is freer from sparking and flashing. In spite of the above advantages, the greater complication of the transformer and collector connections outweighs them in the smaller sizes. Six-phase converters, therefore, are rarely built in units of less than 500 kilo volt amperes. Above this output the six- phase is usually preferable to the three-phase machine. Transformer Connections for Rotary Converters. Figs. 314 to 321 show the connections which are commonly employed between transformers and rotary converters. The circular spiral winding at the bottom of each figure represents the armature winding of a bipolar converter, the collector rings being omitted to avoid con- fusing the diagram. Figs. 314, 316, 318, and 320 show the two-phase 358 ALTERNATING-CURRENT MACHINERY and three-phase connections, while figures 315, 317, 319, and 321 are for six-phase connection. I Phase A I I Phase B ( oooQOo Two-Phase D/ametr/cal Fig. 314. Transformer Connections'for Two-Phase Rotary Converter Six-f/jase Diatnetrical Fig. 315. Transformer Connections for Six- Phase Rotary Converter Each pair of figures, 314 and 315, 316 and 317, 318 and 319, 320 and 321 are closely related. Thus in Figs. 314 and 315, the two terminals of each transformer secondary coil are joined to the armature winding of the converter at points 180 degrees apart. Such connection is called the diametrical connection and it is possible when the rotary has an even number of rings. In any diametrical connec- tion of the secondaries of the step-down transformers (one trans- SGWTOTW Three -Phase A Con nee t/bn> Fig. 316. Transformer A Connections for Three-Phase Rotary Converter x-f-^hctse Double A Connect /on Fig. 317. Transformer Double A Connec- tions for Six-Phase Rotary Converter former for a two-ring converter, two transformers for a four-ring converter, and three transformers for a six-ring converter), the voltage ALTERNATING-CURRENT MACHINERY 359 in each secondary coil is the same in value, no matter how many rings the converter may have. >eA| \PhaseB\ |//>aseCi UmJ (smu Fig. 318. Transformer Y Connec- tions for Three-Phase Rotaryj Converter S/ x _/=57as Double ^Connection Fig. 319. Transformer Double Y Connec- tions for Six-Phase Rotary Converter The diametrical connection of the transformer secondaries to the converter rings is simpler than the connections shown in Figs. 317, 319, and 321, inasmuch as the diametrical connection requires only one secondary coil on each of the step-down transformers and, therefore, but two secondary leads are brought out from each trans- former; whereas the connections shown in Figs. 317, 319, and 321 require two secondaries on each trans- former and, therefore, four secondary leads from each transformer. The switch- ing arrangements for the diametrical connection are, therefore, simpler than they are for the connections shown in Figs. 317, 319, and 321. Fig. 316 shows three step-down transformers receiving three-phase cur- rents and delivering three-phase currents to a rotary converter, the secondaries being connected to the collector rings. Fig. 318 shows the same arrangement except that the secondaries are Y-con- nected to the collector rings. Fig. 320 shows a Scott* trans- Three-Pha.se ^Connect/on Fig. 320. Transformer T Connec- tions for Three-Phase Rotary Converter *The principle of phase transformation is explained in detail on page 257. 360 ALTERNATING-CURRENT MACHINERY former receiving currents from two-phase supply mains and deliver- ing three-phase currents to a three-ring converter. This Scott transformer arrangement is often called the T connection, and it may be adapted with slight modification to three-phase supply, that is, to transform three-phase alternating currents into two-phase alternating currents. For three-phase rotary converters, the transformers should preferably be connected in A, as this permits the system to be oper- ated with only two transformers, in case the third has to be cut out of the circuit temporarily for repairs. The T connection as shown in Figs. 320 and 321 requires only two transformers, and it can be used to change from either two-phase or three-phase to three-phase, as shown in Fig. 320, or from two-phase or three-phase to six-phase, as shown in Fig. 321. The Y or T arrange- ment of transformers is particularly advantageous where a rotary is to be used to supply direct current to an Edison three-wire system. Of course, two converters can be used, one on each branch of the three-wire system, and this is the preferable method where the branches are liable to be greatly unbalanced. With the Y or T connection of transformers a single converter can be connected across the outside wires of the three-wire system, the neutral wire can then be joined to the neutral point of the Y connection, or to a tap, in one of the transformer windings, which tap corresponds to the neutral point of the Y connection. TESTING ROTARY CONVERTERS Standard Tests. For a rotary converter, the saturation and sore loss curves are obtained in the same manner as in the case of an alternating-current generator, with the exception that the direct- current voltage is also recorded. The phase characteristic is deter- mined and the pulsation test is made in the same manner as for a synchronous motor, except that the direct-current voltage should be S/'x-&hctse Double T^Connectfon Fig. 321. Transformer Double T Con- nections for Six-Phase Rotary Converter ALTERNATING-CURRENT MACHINERY 361 recorded. The machine, of course, is run as a synchronous motor, being supplied with alternating current through its collecting rings. The pulsation test should be made with the converter self-excited. Heat Run. To make a heat run on a rotary converter it may be run either as a synchronous motor taking alternating currents through its collector rings, or as a direct-current motor supplied with direct current through its brushes and commutator. If driven as a synchronous alternating-current motor, the full load output is taken from the commutator end of its armature, or vice versa, and is delivered to a water rheostat or other receiver. During the run the following readings should be recorded every half hour: Volts D.C. Volts A. C. Amperes D. C. Amperes A. C. Amperes Field Volts Field Speed Room Temp. When the rotary converter has reached a constant temperature, after running continuously a number of hours under rated full-load conditions, the machine may be shut down, and thermometers quickly applied to measure the temperature of the following parts: Field spools Pole tip, leading Pole tip, trailing Frame Bearings Room Armature laminations Armature ventilating ducts Armature coils front end Armature coils back end Armature binding wires Commutator Collector rings The condition of constant temperature is indicated when the voltage applied to the terminals of the field winding in order to keep the current in the field winding constant, no longer increases. In other words, the resistance of the field winding increases for a time due to increasing temperature of the winding, and it takes an increasing voltage at the field terminals to maintain the field cur- rent constant. But when the temperature of the field windings becomes constant, the voltage required at the field terminals no longer increases. Motor-Generator Method. If two similar rotary converters are at hand, the heat test may be made by the "motor-generator" method, 362 ALTERNATING-CURRENT MACHINERY in a manner somewhat similar to the method described under trans- formers. This method applied to rotary converters involves running one machine as an inverted rotary taking power through its brushes and commutator from direct-current mains. The second machine is made to run as a regular rotary converter, taking its power from the collector rings (that is, alternating-current side) of machine No. 1. Machine No. 2 then delivers its power in the form of direct cur- rent from its commutator back to the direct-current supply mains, or to the commutator of machine No. 1. Machines Nos. 1 and 2 are now said to be "tied together in multiple on both the direct-current Fig. 322. Wiring Diagram for Motor-Generator Method of Testing Rotary Converters and the alternating-current sides." If the machines are similar in every respect, no appreciable current will flow between the two under these circumstances. Each will run from the direct-current side, the two together taking only enough power from the direct-current mains to supply the no-load losses in both machines. If, however, an auxiliary electromotive force is applied in the circuit between the machines on the alternating-current side, it will immediately cause current to flow between the machines precisely as in the case of the transformer test above referred to. Fig. 322 is a diagram of the electrical connections for this method of running. R 1 and R 2 are the two three-phase rotaries. They are shown connected together electrically on the alternating-current ALTERNATING-CURRENT MACHINERY 363 side (that is, the collector rings of Ri are connected to the corre- sponding collector rings of R 2 ) through an induction regulator R. This regulator supplies to the circuit of each phase the auxiliary electromotive force necessary to cause the current to flow between the two machines. The electromotive forces supplied by the regu- lator are adjustable. When the machines are first thrown together the regulator is in such a position that its electromotive forces are. zero. On the direct-current side (commutator ends) both machines are shown electrically connected to the direct-current supply mains. The method of procedure in starting the two machines is as follows: First, RI is started as a direct-current motor from the direct- current side (commutator). Its field current is adjusted until it runs at the right speed when the normal voltage is applied to its armature terminals; Ri is at this time entirely disconnected from R 2 . Next R% is started in the same manner. Then Ri and R% are synchronized and connected together, as shown in the diagram, by closing switches Si, S 2 , and S s on the alternating-current side.* The most convenient way to synchronize the machines for test- ing is to connect up a series of incandescent lamps across each of the switches Si, S 2 , and $ 3 . The number of incandescent lamps in each series should be such that they can stand twice the normal alternating voltage of each machine. The field current of R 2 is varied until it is almost in synchro- nism with Ri, as shown by the slow pulsations of the synchronizing lamps. When the lamps are dark, there is zero voltage across the terminals of the switches, and then they can be safely closed. If the lamps connected across the switches do not all go out at the same instant, the machines cannot be synchronized. In this case the direction of rotation of one of the machines must be reversed, or two of the leads on the alternating-current side of one of the machines must be interchanged. Up to this time the two machines have been running inde- pendently of each other. Each has taken sufficient current from the mains to supply the losses occurring in it while running un- loaded. When the two machines are connected together on the *The apparatus for determining when the two machines are in synchronism is not shown in the figure. 364 ALTERNATING-CURRENT MACHINERY alternating-current side by closing the switches Si, S 2 , and S 3 , they run in synchronism. If the regulator R is in its zero position, no appreciable current will flow between the two machines. As the voltage of R is increased, however, a current will circulate between the two machines. If the voltage of R is in such a direction that V 2 is greater than V\, then R 2 will run as a regular rotary converter, and Ei as an inverted rotary. The power will pass from RI to R 2 on the alternating-current side, and from R 2 to RI on the direct- current side. RI will run as an inverted rotary, driving R 2 on the alternating-current side, and ^2 will run as an ordinary rotary con- verter driving RI on the direct-current side. By sufficiently increasing the voltage of R we can cause full- load or even over-load conditions to obtain. The direct-current mains deliver merely the power to supply the losses in the entire system. Ammeter A indicates the current supplied to overcome the losses in the system. Under these circumstances, RI will have a load slightly greater than R 2 . Ammeter Ai should give a reading equal to the sum of the readings of the two ammeters A and A 2 . The chief advantage of this method . is that full-load condi- tions are obtained without actually supplying full-load current from the direct-current supply mains. The mains supply simply the power losses in both machines. Another great advantage is the ease with which the load can be controlled. By this method, more- over, two machines can be tested more easily than a single machine by the ordinary method. If it is desired to test one machine only, the load may be put on in the same way by connecting it up with a second machine. It is not necessary in this case that the second machine be of the same size as the first. It is only necessary that it be of the same voltage and frequency, and that it can carry with- out excessive heating the full-load current of the machine to be tested. The current, during the run, is adjusted to the normal full- load value of the machine to be tested. MOTOR=QENERATORS The last method of obtaining direct current from alternating- current circuits is by a motor-generator, which consists of an alter- nating-current motor driving one or more direct-current generators. ALTERNATING-CURRENT MACHINERY 365 The motor may be of the induction or of the synchronous type. In either case the motor is usually mounted on the same base with, and mechanically coupled to, the generator. The smaller sets up to about 150 kilowatts are usually driven by induction motors for 25, 40, or 60 cycles, and are customarily used for charging storage bat- teries or for furnishing the exciting current for the fields of alternators or synchronous motors. The larger sets, from 200 kilowatts upwards, are used for the supply of lighting, railway, or power systems, and are usually driven by synchronous motors, which have the advan- tage of permitting the control of the power factor of the circuit by varying the field excitation, but have the disadvantage of requiring a direct-current field excitation. Synchronous motors have an advantage over induction motors in that large sizes may be wound for 11,000 or 13,200 volts, whereas induction motors of the same size are limited to 2,080 or 6,000 volts. Comparison with the Rotary Converter. A motor-generator employing a synchronous motor does not seem to possess any essential advantages over the rotary converter, except in some cases where independent control of the direct-current voltage is desired. The use of the synchronous motor does not remove the objections to the rotary converter which are based on the fact that it is a synchronous machine. A motor-generator employing an induction motor has the ad- vantage of using induction instead of synchronous apparatus, thereby securing many of the advantages summarized later in the comparison between synchronous and induction motors, page 401. Circuits which are supplied by alternating-current generators the speed of which has rapid and periodic fluctuations, or in which for any other reason the conditions are such as to cause "hunting" in synchronous machines, may, however, satisfactorily supply an induction motor driving a generator. The various characteristics of the induction motor under emergency conditions, such as a sudden overload, momentary interruption, or lowering of the voltage of the supply circuit, may cause little or no inconvenience if the induction motor is used, whereas it might cause serious interruption to a rotary converter or a synchronous motor. The induction motor driving a generator is also to be preferred where the size of the units is quite small and the attendance is unskilled. It is also preferable for single- 366 ALTERNATING-CURRENT MACHINERY phase work. The armature for the induction motor should in general be of the squirrel-cage type as the required starting torque does not exceed 20 per cent of full-load torque, and the required starting torque can be obtained with about full-load current. The rotary converter, like the synchronous motor, is not suit- able for general distribution in small units. It has the advantage over the motor-generator in point of cost, there being but one ma- chine instead of two; in point of efficiency, there being the loss in one machine instead of two; and in its effect upon the voltage of the transmission system as a whole, as it may be compounded to overcome the drop which would otherwise occur in generator and transmission circuit. On the other hand, the electromotive force of the direct current delivered by the converter has a more or less fixed relation to the electromotive force of the alternating currents supplied; whereas the electromotive force of a motor-driven generator is independent of the electromotive force of the supply circuit, and it may be ad- justed or compounded as desired. In practice, however, it is found that the voltage delivered by a rotary converter can be satisfactorily adjusted and controlled by regulating devices or by compounding, so that usually the close relation between the electro- motive forces at the two ends of the converter is not disadvan- tageous, provided the electromotive force of the supply circuit is reasonably constant.- This statement applies to those cases where a practically constant direct-current voltage is desired. There are, of course, special cases, in which the voltage is to be adjusted over a wide range, or where, for other reasons, the motor-generator is to be preferred. In many cases the motor may be operated with- out requiring step-down transformers, whereas they would be neces- sary with rotary converters. Uses of Motor=Qenerators. Motor-generator sets are often used in central stations in connection, with arc-lighting service. In these cases alternating-current motors are used for driving arc- lighting generators, as in the large plant of the Buffalo General Electric Company. They are also used in some places for supplying three-wire direct-current lighting circuits from alternating-current mains. Motor-generators are often used in central stations for fur- ALTERNATING-CURRENT MACHINERY nishing direct current to the field-magnet windings of alternators, the direct-current generator part of the set acting as the exciter for one or more large alternators. The alternating current supplied to the (synchronous or induction) motor end of the motor-generator set is taken from tlie main station bus bars, and may or may not be stepped-down by means of reducing transformers. Examples of this use of motor-generator sets are to be ftyund in the immense power stations of the Interborough Rapid Transit Company, and the New York Edison Company, of New York City, and in many other places. Another use for motor-generators is as frequency changers (or converters). In such cases they are used for transforming an Fig. 323. General Electric Motor-Generator Set with D. C. Exciter alternating current of one frequency to another alternating current of higher or lower frequency, usually the former. The current may be transformed from three-phase to two-phase, or vice versd, at the same time. Thus it might be required to supply a two-phase, 60-cycle lighting system from a 25-cycle, three-phase transmission system, with or without change of voltage. Types. A motor-generator set built by the General Electric Company is shown in Fig. 323. It consists of a combination of three distinct machines, all mechanically coupled to" the same shaft, viz, a three-phase synchronous motor of the revolving-field type; a three-phase alternator also of the revolving-field type; and a direct- 368 ALTERNATING-CURRENT MACHINERY current generator acting as an exciter for the fields of both the alter- nator and the synchronous motor. Summary of Data for Fig. 323 Synchronous Motor Alternator 8 12 275 250 570 570 38 57 Exciter 10 10 570 No. of poles Rated output (kw.) Speed.. Frequency Volts.... 60 This particular motor-generator set is designed to be used as a frequency-changer to change from 38 to 57 cycles per second. Fig. 324. General Electric Motor-Generator Set Induction Motor Driving Compound-Wound D. C. Generator The three-phase alternating currents having a frequency of 38 cycles per second are led into the stationary armature terminals of the synchronous motor (shown next to the pulley in Fig. 323). The motor having eight field poles, its synchronous speed is 570 r. p. m.; and since both the alternator and the exciter are coupled to the same shaft, all the machines run at 570 r. p. m. The alternator having 12 field poles, the frequency of its in- duced electromotive forces is 57 cycles per second. Hence, the ALTERNATING-CURRENT MACHINERY 369 three-phase alternating electromotive forces induced in the station- ary armature of the alternator have a frequency 50 per cent higher than those of the alternating currents which are supplied to the driving synchronous motor. 370 ALTERNATING-CURRENT MACHINERY Fig. 324 is a view of a motor-generator set built by the General Electric Company. It consists of a compound-wound direct-current generator driven by an induction motor, both machines being mounted on the same bed-plate and having a common shaft. Fig. 325 shows two motor-generator sets, each consisting of two Brush arc-light dynamos directly coupled to an induction motor. In some of the stations of the Brooklyn Edison Company Brush arc-light dynamos are driven in pairs by being directly coupled to three-phase, 25-cycle synchronous motors. The constancy of the speed of synchronous motors makes them especially well suited for driving electric generators. ALTERNATING-CURRENT MACHINERY PART VI INDUCTION MOTOR Constructive Elements. It has already been pointed out that the successful use of alternating current for power purposes depends largely upon the use of the induction motor supplied with polyphase currents. This machine consists of a primary member and a second- ary member, each with a winding of bars or wire. The primary Fig. 326. Diagram of Squirrel-Cage Type of Rotor Fig. 327. Diagram Showing Slots in Stator of Induction Motor member is usually stationary, and is often called the stator. The secondary member is usually the rotating member, and is often called the rotor. Fig. 326 shows a rotor of the squirrel-cage type. It con- sists of a drum A built up of thin circular sheet-iron disks. Near the periphery of this drum are a number of holes parallel to the axis of the drum. In these holes heavy copper rods b are placed, and the projecting ends of these rods are screwed and soldered to massive copper rings r, one at each end of the drum. The stator is a laminated iron ring FF, Fig. 327, closely sur- rounding the rotor. This ring is slotted on its inner face, as shown; windings are arranged in these slots, and these windings receive Copyright, 1912, by American School of Correspondence. 372 ALTERNATING-CURRENT MACHINERY current from polyphase supply mains. These polyphase currents produce in the stator iron a rotating state of magnetism, the action Fig. 328. Squirrel-Cage Rotor Mounted in Ordinary Four-Pole Field Fig. 329. Diagram of Four-Pole Two- Phase Induction Motor of which on the rotor is the same as the action of an ordinary field magnet mechanically revolved. Thus Fig. 328 shows a squirrel- cage rotor A surrounded by an ordinary field magnet revolving in the direction of the arrows. This motion of the field magnet induces currents in the short-circuited copper rods of the rotor; the field magnet exerts a dragging force on these currents, and causes the rotor to revolve. No electrical con- nections of any kind are made to the rotor. Stator Windings and Their Action. The stator windings are arranged in the slots s, Fig. 327, in a manner exactly similar to the arrangement of the windings of the two-phase or three-phase alternator arma- ture according as the motor is to be supplied with two-phase or three-phase currents. Two-Phase. Fig. 329 shows an end view of a four-pole two- phase induction motor. In this figure, the outline only of the rotor (or secondary) is shown. The stator conductors are represented in section by the small circles, the slots being omitted for the sake of clearness, and the end con- Fig. 330. Diagram of One-Half the Stator Conductors of Fig. 329 ALTERNATING-CURRENT MACHINERY 373 nections of half the stator conductors are shown in Fig. 330. In this figure the straight radial lines represent the conductors which lie in the slots of the stator, and the curved lines represent the end connections. The stator conductors are arranged in two distinct Iron Iron O Q Q O O ffon Up f~bv*iny Cur fen ts Fig. 331. Magnetic Action of Conductors Carrying Currents between Iron Poles Iron Oo wn Flo wing Cure en ts Fig. 332. Magnetic Action of Conductors Carrying Currents between Iron Poles circuits. One of these circuits includes all of the conductors marked A, and this circuit receives current from one phase of a two-phase system. The other circuit includes all of the conductors marked B and this circuit receives current from the other phase of the two- phase system. The terminals of the B circuit are shown at t t f , Fig. 330. The conductors which constitute one circuit are so connected that the current flows in opposite directions in adjacent groups of conductors as indicated by the arrows in Fig. 330. The action of a band of conductors between the two masses of iron is shown in Figs. 331 and 332. The small circles represent the conductors in section; conductors carrying down-flowing currents Fig. 333. Instantaneous Effect of Alternating Fig. 334. Magnetic Effect of Alternating Cur- Current for a Given Position of Rotor rent i of a Cycle Later than in Fig. 333 are marked with crosses and those carrying up-flowing currents with dots. The action of the currents in these bands of conductors is to 374 ALTERNATING-CURRENT MACHINERY Fig. 335. Magnetic Effect of Alternating Current J of a Cycle Later Than in Fig. 333 produce magnetic flux along the dotted lines in the direction of the arrows. The lines A' and B' in the clock diagrams of Figs. 333, 334, and 335 are supposed to rotate, and their projections on the fixed line ef represent the instantane- ous values of the alternating cur- rents in the A and B conductors, respectively. These conductors are represented in section by the small circles. These small cir- cles are marked with crosses when they carry down-flowing currents, with dots when they carry up-flowing currents, and they are left blank when they carry no current. Fig. 333 shows the state of affairs when the current in conductors A is a maximum and the current in conductors B is zero, and the dotted lines indicate the paths of the magnetic flux. This flux enters the rotor from the stator at the points marked N and leaves the rotor at the points marked S. Fig. 334 shows the state of affairs, one-eighth of a cycle later, when the current in the B conductors has increased and the current in the A conductors has de- creased to the same value, so that equal currents flow in the A and in the B conductors. The points N and S have moved over one-sixteenth of the circumfer- ence of the stator ring, from the positions they occupied in Fig. 333. Fig. 335 shows the state of affairs, after another eighth of a cycle, when the current in the B conductors has reached its maximum value, and the current in the A conductors has dropped to zero. The points N and S have moved again over one-sixteenth A Fig. 336. Complete Connections for Four Poles of One Circuit of Three-Phase Induction Motor ALTERNATING-CURRENT MACHINERY 375 of the circumference of the stator ring. This motion of the points N and S is continuous, and these points make one complete revolu- tion (in a four-pole motor) during two complete revolutions of the vectors A' and B', that is, while the alternating currents supplied to the stator windings are passing through two cycles. In general: n = (47) in which n is the revolutions per second of the stator-magnetism, p is the number of poles, N and S, and / is the frequency of the alternating currents supplied to the stator windings. Three-Phase. When an induction motor is driven by three- phase currents, the stator conductors are arranged in three distinct circuits A, B, and C, which are either A-connected or Y-connected to the supply mains. Fig. 336 shows the complete connections, for four poles, of the A circuit with its terminals t 1'. The B and C circuits are similarly connected. In general, the g-phase stator winding for p poles has p q equi- distant bands of conductors. The first, (g-f-l)th, (2g+l)th, etc., bands are connected in one circuit, so that currents flow oppositely in adjacent bands, and this circuit takes current from one phase of the g-phase system. The second, (g+2)th, (2g+2)th, etc., bands are similarly connected in another circuit and take current from the second phase of the g-phase system. The third, (g-f-3)th, (2g +3)th, etc., bands are similarly connected in another circuit and take current from the third phase of the g-phase system; and so on. ACTION OF INDUCTION MOTOR Many important details of the action of the induction motor are most easily explained by looking upon the induction motor as a rotor influenced by an ordinary field magnet, mechanically revolved. The complete theory of the action of the induction motor is, how- ever, similar to the theory of the alternating-current transformer. Torque and Speed. Let n be the number of revolutions per second of the field, and n' the revolutions per second of the rotor. When nn', the rotor and field revolve at the same speed, so that their relative motion is zero; no electromotive force is then induced in the rotor conductors and no current flows and, therefore, the revolving field exerts no torque upon the rotor. As the speed of the 376 ALTERNATING-CURRENT MACHINERY rotor decreases, the difference of the speeds of rotor and field (n n'), increases and, therefore, the electromotive force induced in the rotor conductors, the currents in the conductors, and the torque with which the field drags the rotor, all increase. If the whole of the field flux were to pass into the rotor and out again in spite of the demag- netizing action of the current in the rotor conductors, then the torque would increase in strict proportion to (n n'). As a matter of fact, because of the demagnetizing action of the rotor currents, a larger and larger portion of the field flux passes through the space between the stator and rotor conductors as the speed of the rotor decreases, and this magnetic leakage causes the torque to increase more and more slowly as (nn f ) increases. The torque usually reaches a maximum value, and then decreases with further increase of (nn'). Fig. 337 shows the typical relation between torque and speed of an induction motor. Ordinates of the curve represent torque, and abscissas measured from represent rotor speeds. The rotor is said to run above synchronism when it is driven so that n f is greater than n. The rotor never actually reaches syn- chronous speed, but ap- proaches it very nearly when the induction motor is running unloaded. In order to cause the rotor to run above synchronism, that is, to make n' greater than n, or in order to cause the rotor to run backwards, that is, in a direction opposite to that of the revolving magnetism in the stator iron, the rotor must be driven mechanically from an outside source of power. Starting Resistance in the Rotor Windings. The speed of the rotor for which the maximum torque occurs, depends upon the resistance of the rotor windings, and it is advantageous under cer- tain conditions of operation to provide at starting such resistance in these windings as to at once produce the maximum torque, this resistance being cut out as the motor approaches full speed. Efficiency and Speed. For the sake of simplicity, let us assume that the only opposition to motion of the revolving field magnet Fig. 337. Graphical Relation of Torque and Speed of Induction Motor ALTERNATING-CURRENT MACHINERY 377 is the reaction of the torque which it exerts on the rotor. Let this torque be represented by T. Then the power expended in driving the field magnet is 2nnT, and the mechanical power delivered to the rotor is 2nn'T, and this power Znn'T is available at the pulley of the motor, except for slight losses due to friction in the bearings and to air friction. Therefore, ignoring friction losses, 2nnT is the input of power in driving the revolving field, and 2nn' T is the output of power, so that the efficiency of the induction motor is . n This expression for efficiency ignores all the losses of power in the revolving field magnet* and the friction and windage losses in the rotor, and shows that the efficiency of an induction motor is zero when the rotor stands still, that it increases as the rotor speeds up, and approaches 100 per cent, ignoring field losses and friction, as the rotor speed approaches the speed of the revolving field. The n' ratio - ranges from 0.85 to 0.95 or more, in commercial induction iL motors under full load, but the actual full load efficiencies of induc- tion motors range from 75 per cent, or even less for small motors, to about 95 per cent for very large motors. Ratio of Mechanical to Electrical Energy in Rotor. The total power delivered to the rotor is equal to 2nnT where n and T have the meanings above specified. That is, all of the power used to drive the field magnet (ignoring losses in the field) is delivered to the rotor. Now, the mechanical power delivered to the rotor is equal to 2nn'T, as already explained; therefore, the difference 2nnT f l7tn'T\ is electrical power used to force the rotor currents through the rotor windings. Therefore, when the field speed is n and the rotor speed is n f , the total power delivered to the rotor, the mechanical power developed in turning the rotor, and the electrical power developed in the rotor windings are to each other as, n, n', and (nn'), re- spectively. *In the actual induction motor, it ignores the loss of power due to the heating of the stator windings by^ the supplied alternating currents, and the loss of power due to core losses in the stator iron. fWhen torque is expressed as pounds weight on a lever arm of one foot in length the torque is said to be expressed in pound-feet, and power in watts is equal to ~T^ 8.52nT watts, where n is the speed in revolutions per second. 378 ALTERNATING-CURRENT MACHINERY Ratio of Rotor Voltages to Stator Voltages. When the rotor is wound with the same number of conductors as the stator, then when the rotor is standing still, the rotating stator magnetism in- duces in the rotor windings electromotive forces of the same value and of the same frequency as the electromotive forces induced in the stator windings by this rotating stator magnetism (neglecting magnetic leakage). Moreover, the electromotive forces induced in the stator windings are very nearly equal and opposite to the volt- ages applied to the stator windings. When the difference of the speeds of the rotor and the stator magnetism is (nn f ), the electro- motive forces induced in the rotor windings are the fractional part, I, of the voltages applied to the stator windings, and the frequency of the electromotive forces induced in the rotor windings [H n f ~i I of the frequency of the voltages // ~J applied to the stator windings. Example. Let a certain three-phase induction motor having a stator wound for 6 poles, and taking three-phase alternating currents at a frequency of 60 cycles and a voltage of 220 between any two of the three supply mains, have a rotor furnished with the same number of conductors as the stator. Further, let the no-load speed of the rotor be 1194 r.p.m., and its full-load speed be 1,143 r. p. m. Assuming that the magnetic leakage is negligible, it is required to find: (a) The synchronous speed. (b) The electromotive forces (three-phase) induced in the rotor wind- ings at no-load and at full-load. (c) The frequency of the electromotive forces induced in the rotor windings at no-load and at full-load. Solution: (a) The synchronous speed of the rotating magnetism in the stator is, according to equation (47), 60 n = = 20 revolutions per second, or 1,200 r.p.m. IF (b) The electromotive forces (three-phase) induced in the rotor wind- ings at no-load are - X voltage applied to stator or / 1200 - 1194 \ 1200 )x220 = 1.10 volts ALTERNATING-CURRENT MACHINERY 379 The voltages induced in the rotor windings at full-load are / 1200-1143 1200 X 220 = 10.45 volts It is interesting to note that if the slip of the rotor at no load were zero, or in other words, if n' were equal to n there would be zero electromotive forces induced in. the rotor windings at no-load. (c) The frequency of the electromotive forces induced in the rotor windings at no-load is - X frequency of stator voltage or 1200 -1 194 \ ) X 60 = 0.3 cycles per second V 1200 The frequency of the electromotive forces induced in the rotor wind- ings at full-load is 1200 -1 143 \ 1200 / X 60 = 2.85 cycles per second When the rotor is at rest, the frequency of the electromotive forces induced in the rotor windings is the same as the frequency of the stator volt- age, namely, 60 cycles per second. Efficiency and Rotor Resistance. For a given difference (nn f ) between field speed and rotor speed, a definite electromotive force is induced in the rotor conductors, and the less the rotor resistance, the greater the current produced by this electromotive force, and the greater the torque. Therefore, a given induc- tion motor will develop its full load torque for a small n' value (nn f ) or for a large value of (efficiency) if its rotor resist- n ance is small. High efficiency depends, therefore, upon low rotor re- sistance. The necessity of high rotor resistance to give large torque at starting has nothing to do with the necessity of making the rotor Fig. 338. Iron Stator Frame for Westinghouse Induction Motor 380 ALTERNATING-CURRENT MACHINERY resistance small in order to secure full load torque at as nearly synchronous speed as possible. These two conflicting conditions may be realized in one motor by an arrangement whereby a resist- ance which is in circuit with the rotor conductors at starting, may be short-circuited when the motor nearly reaches its rated speed. Structural Details of a Typical Induction Motor. Figs. 338 to 341 show the structural details of a typical induction motor manu- factured by the Westinghouse Electric and Manufacturing Company. It has a stationary primary member (often called the stator or field) and a rotating secondary member (often called the rotor or armature). The primary member is mounted in a hollow cylindrical frame of cast iron shown in Fig. 338. This frame forms a base for the machine, and also supports the two end- brackets which carry the self -oiling bearings. In- side the frame are sev- eral lugs that support the stator core lamina- tions far enough from the frame to leave space between the frame and the core for ventilation. The iron core of the pri- mary member consists of a ring built up of sheet- steel stampings slotted on the inside to receive the primary con- ductors as ; shown in Fig. 339. The laminations are assembled, clamped, and keyed between stiff end rings inside the lugs on the frame. Steel keys in one or more of the lugs prevent circular move- ment of the laminations. Fig. 339 shows the primary member com- pletely wound. The primary conductors are usually grouped in former-wound coils of wire which are thoroughly taped and insulated before being slipped into place in the slots in the stator core. In larger motors Fig. 339. Primary Member of Westinghouse Induc- tion Motor Completely Wound ALTERNATING-CURRENT MACHINERY 381 Iron Rotor Core for Westinghouse Induc- tion Motor copper strap bent into the proper form is used instead of wire for forming the coils. The terminals of the primary winding are brought out, usually at one side of the motor, and are clamped in insulated cleats or bush- ings. The leads which sup- ply alternating currents to the motor are attached to these terminals through Fig. 340 suitable connectors. The iron core of the secondary member (rotor), shown in Fig. 340, is also built up of ring-shaped stampings of sheet-steel assembled, clamped, and keyed between stiff end plates on the arms of the rotor spider. The spider is pressed on the shaft and keyepl.^ Ventilating plates on the rotor cores of the larger motors act like the blades of a fan and force strong currents of ,air between the rotor end rings and the core and through all the openings in and around the stator wind- ings and core, thus keeping all parts cool. The secondary conductors consist of rectangular cop- per bars placed in nearly closed slots, around the periphery of the core. These bars project beyond the laminated core, and they are screwed and soldered at each end to massive rings of copper, thus forming a short-circuited secondary winding, as shown in Fig. 340. This type of secondary member is called a squirrel- cage rotor. The complete motor is shown in Fig. 341 . Types of Rotors for Constant and Variable Speed. There are three types of secondary members used in commercial induction motors, according to the conditions of service to be met. The start- Fig. 341. Westinghouse Induction Motor Com- pletely Assembled 382 ALTERNATING-CURRENT MACHINERY ing and running conditions determine which type to adopt in any given case. These types are shown in Figs, 342, 343, and 344. Fig. 342. Squirrel-Cage Type of Rqtor /Fig. 342 is a squirrel-cage rotor. Fig. 343 is a rotor wound with insulated wire, forming what is called a definite*, or polar, winding. The terminals of this winding are connected to a start- ing resistance mounted inside of the rotor. A switch is arranged to short-circuit this starting resistance, and is operated while the motor is running by means of a rod which lies inside of the hollow shaft of Fig. 343. Rotor with Definite or Polar Winding with Starting Resistance Inside Core the rotor. This rod terminates in a small handle or knob at the end of the rotor shaft as shown in Fig. 343. Fig. 354 shows a complete three-phase induction motor with the knob and rod for operating the internal starting resistance. *This winding is identical with the stator winding as described on page 374. ALTERNATING-CURRENT MACHINERY 383 Fig. 344 is a rotor with a winding similar to the winding of Fig. 343, but instead of connecting the terminals of the rotor winding to an internal starting resistance, these terminals are brought out to collector rings on the end of the shaft as shown in Fig. 344. The circuits of the rotor windings are completed through adjustable external resistances which are connected to the rotor windings by means of brushes rubbing on the collector rings. These adjustable- external resistances are regulated by a cylindrical switch or controller similar in general to the ordinary electric street-car controller. Fig. 345 shows a Westinghouse controller for induction motors used for cranes, hoists and similar apparatus. The cylinder has a set of contacts for making, breaking, and reversing the primary circuit, and another set of contacts for control- rig. 344. Rotor with Polar Winding and* Collector Rings ling the speedcof the motor by varying the resistance in the secondary circuits. The two sets of contacts on two drums are mounted on the same shaft and all operations are performed by moving one controller handle. The number of speed steps in each direction of rotation may be 6, 9, 12, or 15, according to the capacity of the controller. An induction motor provided with a squirrel-cage rotor takes excessive current from the alternating-current supply mains at starting. The squirrel-cage rotor requires from three to four times full-load current to produce at starting a torque equal to the torque developed when running at full-load. Hence, when the motor has to start under a heavy load, or where the taking of excessive currents from the supply mains will interfere with other apparatus supplied 384 ALTERNATING-CURRENT MACHINERY from the same mains by causing excessive drop of voltage, the squirrel- cage type of rotor is objectionable, especially in large size motors. On the other hand, the extreme simplicity of the squirrel-cage rotor and its ability to carry enormous currents without injury, largely compensate for the above mentioned disadvantages. Its speed is practically constant, varying only a few per cent from full load to no load. The operating characteristics of the squirrel-cage type of induction motor are such as adapt them to a wide variety of pur- poses and make them especially suitable for continuous constant- speed service. An induction motor provided with a rotor like that shown in Fig. 343, with an internal starting resistance, takes at starting only about one and one-half full-load rated current from the supply mains, giving a starting torque of about one and one- half full-load torque. Such an induction motor, therefore, is used only where a starting torque not greatly in excess of full-load torque is required. The advan- tage of this type of rotor is that it does not take excessive currents at start- ing, and it will start, therefore, with- out producing excessive drop of electro- motive force in the alternating-current system from which the motor receives its power. The starting resistance in motors up to about 50 horse-power consists of cast-iron grids enclosed in a triangular frame which is bolted to the end plates holding the rotor laminations together. Fig. 345. Westinghouse Controller rrn r i , -, . . , . . for Induction Motors 1 ne Wnole OI thlS resistance IS in SCRCS with the secondary winding at starting. As the motor increases in speed, the resistance is short-circuited by sliding spring metal brushes along the inside surface of the grids. The brushes are supported by a metal sleeve sliding on the shaft which is operated by a rod passing through the end of the shaft. An induction motor having a rotor provided with collector ALTERNATING-CURRENT MACHINERY 385 rings is generally used for cranes, hoists, elevators, and other work where variable speed is required. The starting resistance used in the type of rotor shown in Fig. 343 is designed to carry the rotor current for a short time only, that is, during starting; if kept con- tinuously in circuit for the purpose of speed control, this starting resistance would become excessively hot. For speed control, there- fore, an external resistance must be used. The range of speed control possible in the case of an induction motor provided with a rotor having collector rings connected to external adjustable resistances is about the same as the range of speed control obtainable with a shunt-wound direct-current motor having a regulating rheostat in its armature circuit. Behavior at Starting and in Operation. When an induction motor is running without load, its speed is nearly equal to the speed of the rotating magnetic field, namely, synchronous speed. Under these conditions the stator takes only sufficient current to force the magnetic flux through the reluctance of the magnetic circuit, and to supply the PR losses of the stator windings, the core loss, and the friction and windage loss of the rotor. When the motor is loaded, its speed decreases in nearly direct proportion to the load, from nearly synchronous speed at no-load to about 98 per cent of synchronous speed in the case of large motors, and to about 92 per cent of synchronous speed in small motors at full-load. Therefore, the induction motor is practically a constant-speed motor. The decrease in speed expressed as a per- centage of synchronous speed is called the slip of the motor. The slip of large motors is thus about 2 per cent at full-load, and that of small motors is about 8 per cent at full-load. When an induction motor is overloaded, it takes excessive cur- rent from the supply mains, and its torque increases up to a cer- tain value of the slip (a definite value for a given motor). When loaded up to this point the machine is unstable, and the least ad- ditional loading causes the machine to "break down" or stop. This maximum output which a given motor can deliver is usually about one and one-half to two and one-half times as great as its rated full-load output. This maximum output is proportional to the square of the electromotive force of the alternating currents supplied to the motor. Thus a certain induction motor rated at 220 386 ALTERNATING-CURRENT MACHINERY volts has a maximum power output of 1 .8 times its rated output. The same motor supplied with currents at 200 volts would have a maxi- mum output off- -)* X 1.8 = 1.49 of its rated output. When an induction motor is operated at slightly less than its rated frequency but with full-rated voltage, the speed of the motor will be decreased in proportion to the frequency, but its power output will not be greatly affected. The efficiency of the motor, and its rise of temperature under full-load, will be approximately Fig. 346. Three-Phase Starting- Compensator Complete/'' Fig. 347. Three-Phase Starting Com- pensator with Cover Removed unchanged, and the maximum power output will be slightly increased. An induction motor having a squirrel-cage rotor will develop sufficient torque to start satisfactorily with from 40 per cent to 80 per cent of the rated voltage applied to the primary member. There- fore, the current required at starting may be greatly reduced by supplying the primary member, at starting, with current through a step-down transformer which is designed to reduce the supply voltage to 40, 50, 60, or 80 per cent of the rated voltage of the motor, and to multiply the delivered current in the same ratio. This ALTERNATING-CURRENT MACHINERY 387 step-down transformer is usually an autotransformer. An auto- transformer, with its special switching device for changing the motor connections quickly from the low-starting voltage to the full- running voltage, is called an autostarter, or a compensator. The autostarter may be located at any convenient point, either near the motor or at a distance from it. Figs. 346 and 347 are general views of a three-phase hand- operated starting compensator of the wall type, as manufactured by the General Electric Company. The compensator consists of three core-type autotransformers, a cable clamp, and a special switch assembled in a metal case with external handle and release lever. In the wall-suspension type the switch is located at the bot- tom, as seen in Fig. 347, and is enclosed by an oil-filled tank, Fig. r~7 j r | - j / i/$$ < i / VUS NO VOL TAGC r r / r^L 5 o-l > i BACK S/DL~ F/A&fff BLOCK ^\\ O/L SW/TCH L 1 L J L I CMMXR s. te r0f? START/NO 3 1 -I t \ r/MGfff BLOCK \^_ } ] TAPS' Fig. 348. Electrical Connections for Three-Phase Starting Compensator 348 is a diagram of the electrical connections. The three coils of the three-phase winding are connected in Y, the line to the three free ends of the coil, and the starting connections of the motor to the taps. For motors from 5 to 18 horse-power, these compensators are provided with taps for starting the motor at 50, 65, and 80 per cent of the line voltage, any one of which may be selected after trial for permanent connection to the switch, for starting according to the requirements of any individual case. The shaft of the switch, as seen in Fig. 347, extends through the sides of the compensator case, and is operated by a lever at the right, being held in the running position by a lever at the left. The switch, provided with heavy wiping contacts, is immersed in oil 388 ALTERNATING-CURRENT MACHINERY and is intended to be used as a line switch as well as for starting the motor. The lever has three positions: "off", "starting", and "running". To start the motor, the switch is thrown to the "starting posi- tion", and is left there until the motor reaches nearly full speed and then the swkch is quickly thrown over to the "running position". The time required for bringing an induction motor from rest up to rated speed is about one minute, the time in any given case depend- ing upon the value of the starting voltage used, and the amount of load on the motor at starting. In the "off position" both compen- sator and motor windings are disconnected from the line. In the Fig. 349. Diagram of Electrical Connections for a Two-Phase Compensator for Starting Two-Phase Induction Motors "starting position", Fig. 348, the switch connects the line to the terminals, and the motor to the taps, of the compensator winding, without circuit breakers or fuses in circuit. In the "running position" the compensator winding is cut out and the motor is connected to the line through suitable fuses or overload relays mounted directly above the compensator. For instance, Fig. 347 shows three "cartridge" fuses, one in each of the three line wires. To prevent the attendant from carelessly throwing the motor directly on the line at starting, an automatic latch is provided, so arranged that the lever at the "off position" can be thrown only into the "starting position" ALTERNATING-CURRENT MACHINERY 389 (backward), and can be thrown into the "running position" (for- ward) only by a quick throw of the lever. The "no- voltage release", shown in Fig. 348, is an electromagnet whose laminated plunger holds a tripping lever which engages with the lever mounted on the switch shaft. If for any reason the supply voltage is cut off from the motor, the no- voltage release acts promptly to release the tripping lever which in turn, through the action of a- strong spring, throws the operating lever to its "off position", where it remains until the supply voltage is again restored, and the motor is to be started. Fig. 349 shows a diagram of connections for a two- phase com- pensator for starting two-phase in- duction motors. In this case the line is connected to the ends of each coil, and the starting connec- tions of the motor to one of these ends and the taps. The arrange- ment and wiring is essentially simi- lar to the three-phase compensator. For starting constant - speed polyphase motors above 5 horse- power, a starting compensator is generally used, but below 5 horse- power it is customary to connect polyphase motors directly to the F i g . 350. Snyder Drill Press ! . Driven by Induction Motor supply mains. The direction of rotation of a two-phase motor is reversed by reversing the connections of one of the phases, that is, by reversing the connections of the two wires belonging to one-phase supplying current to the stator windings of the motor. The direction of rotation of a three-phase (three-wire) motor is reversed by interchanging the connections of any two of the three wires used to lead the three-phase currents to the stator windings of the motor. Typical Induction Motor Installations. The application of induction motors to the driving of machine tools has been rapidly 390 ALTERNATING-CURRENT MACHINERY growing in favor. Among the advantages of electric driving over mechanical driving through shafting and belting are: increased shop production, economy of power, ease of control, better and more convenient arrangement of machines, and saving of floor space. Fig. 350 is a view of a Snyder 21 -inch drill press driven by a constant- speed two horse-power induction motor. Fig. 351 shows a 24-inch Chandler planer driven by a 7| horse-power induction motor running at 1,200 revolutions per minute. The individual motor drive as Fig. 351. Chandler Planer Driven by Induction Motor applied to planers has many advantages, among them being the use of independent motors, one for planing and one for raising or lower- ing the cross-head. A typical application of constant-speed induction motors to the driving of spindles in a textile mill is illustrated in Fig. 352. A large number of Crocker-Wheeler 5-h.p. motors are directly coupled to the shafts which drive the spindles above, thus eliminating all over- head shafting, hangers, and belts. An application of variable-speed induction motors having phase-wound rotors with slip rings is given in Fig. 353, which shows several Westinghouse 500-h.p. "type HF" induction motors operating Worthington centrifugal pumps in a municipal pumping plant. ALTERNATING-CURRENT MACHINERY 391 Fig. 352. Constant Speed Induction Motors Used for Driving Spindles in a Textile Mill Fig. 353. Westinghouse 500-H. P. Induction Motors Operating Centrifugal Pumps in a Municipal Pumping Plant 392 ALTERNATING-CURRENT MACHINERY The laminations of both the stator and the rotor of large induc- tion motors are provided with ventilating ducts through which air is driven by centrifugal action. Fig. 353 shows a number of holes in the cast-iron casing through which the air flows after passing through the ventilating ducts between the laminations of the rotor and the stator. Fig. 354 shows an induction motor driving a triplex pump. This induction motor has a starting resistance inside of the rotor, Fig. 354. Induction Motor Driving Triplex Pump and the switch rod and knob are shown projecting from the rotor shaft at the left. . Fig. 355 illustrates a very common method of installing in- duction motors when used for driving line shafting in shops or factories. The motor is shown bolted in an inverted position to the ceiling by means of lag screws, and is furnished with two pulleys, each belted to a different line shaft. Since induction motors require no adjustment and practically no attention, they may be installed where direct-current motors are ALTERNATING-CURRENT MACHINERY 393 not suitable, with the consequent advantage of saving valuable floor space. Single=Phase Induction Motor. When an induction motor (two-phase or three-phase) is once started, and is running at full speed, all the phases but one may be disconnected from the primary member and the machine will continue to operate and to carry approximately 70 per cent as much load with the same slip aad temperature rise as when all the phases are connected to the supply mains. An induction motor, however, will not start when one phase only of its primary member is connected to single-phase supply Fig. 355. Induction Motor Suspended from the Ceiling, Driving Shafting by a Belt mains. Therefore, when it is to be operated from single-phase mains, special provision for starting must be made. An induction motor designed to operate from single-phase mains and provided with special arrangements for starting, is called a single-phase induction motor. Three methods of starting single-phase induction motors are in general use, as follows: (a) by hand; (b) by phase splitting; (c) by repulsive action of field on armature obtained by temporary con- nections during starting. (a) Hand Starting. Very small induction motors may be 394 ALTERNATING-CURRENT MACHINERY started by giving a vigorous pull on the belt which connects the motor to the machinery which it drives. (b) Split-Phase Starting. When an alternating current divides between two branches of a circuit, there is a phase difference between the currents in the two branches, if the ratio of resistance to reactance is different in the two branches. This is especially true if one branch contains a condenser. A two-phase motor will start when the cur- Fig. 356. Stator of Single-Phase Induction Motor Holtzer-Cabot Company rents in the two stator circuits are less than 90 degrees apart in phase, although the starting torque grows less and less as the phase differ- ence of the two currents decreases. The dephasing of the two parts of a single alternating current in two dissimilar branches of a given circuit is called phase-splitting, and a single-phase induction motor may be arranged to start as a two-phase motor, by splitting a single- phase current and using the two parts of the split current exactly as one would use two genuine two-phase currents. Fig. 356 shows the stator of a 2-h.p. single-phase induction motor of the Holtzer-Cabot Electric Company. One set of stator ALTERNATING-CURRENT MACHINERY 395 coils, the "working coils", consist of many turns of coarse wire occupying three-fourths of all the stator slots, and the other set of stator coils, the "starting coils", consist of fewer turns of fine wire occupying one-fourth of all the slots. At starting, both sets of coils are connected to the single-phase supply mains, and the difference in the resistance and the reactance in the two sets of coils splits the single-phase current supplied, suffi- ciently to give a slight starting torque. This type of split-phase induction motor cannot start with any considerable load, hence the load, if it is difficult to start, should be thrown on to the motor Fig. 357. Wagner Single-Phase Induction Motor by means of a friction clutch after the motor is running at full speed. The rotor used in the Holtzer-Cabot motor is of the squirrel-cage type. A single-phase motor will run in either direction equally well, depending only upon the direction in which it is started. There- fore, the hand-started motor may be started in either direction. The direction of starting of the split-phase motor may be reversed by reversing the connections of the starting winding. (c) Repulsion Motor Starting. If an ordinary direct-current dynamo were provided with a laminated field magnet, and if its field magnet were excited by an alternating current, currents would 396 ALTERNATING-CURRENT MACHINERY be induced in the armature windings by the alternating field, pro- vided the brushes of the direct-current machine were set at an angle of about 45 (for a two-pole machine) from their proper position for collecting a direct current. These currents induced in the arma- ture would be acted upon by the alternating field so as to pro- duce a torque which would cause the armature to rotate. A self- starting, single-phase, alternating-current motor constructed on this principle is called a repulsion motor. It is not entirely satisfac- tory in operation, but the repulsion-motor principle furnishes the Fig. 358. Part Section of Wagner Single-Phase Induction Motor best means for making a self -starting single-phase induction motor, that is, a motor which is arranged so that it can act as a repulsion motor while starting, and which by changing certain inside con- nections can be altered into an induction motor when it reaches full speed. Fig. 357 is a general view of a single-phase induction motor arranged to start as a repulsion motor, and built by the Wagner Electric Manufacturing Company. The motor shown has a four- pole stator winding, the iron stator core being made very much like the core of an ordinary induction motor, namely, in the form of a laminated ring closely surrounding the armature, and slotted on its inner face. ALTERNATING-CURRENT MACHINERY 397 The armature is of the ordinary direct-current drum type provided with a disk commutator with radial commutator bars. Four (for a four-pole machine) short-circuited brushes are pressed against the face of the disk-shaped commutator, as shown in Fig. 357. At starting, the stator winding is connected to the single- phase supply mains, and the machine starts as a repulsion motor. Inside of the armature are two governor weights V, Fig. 358, which" are thrown outwards by the centrifugal force when the machine reaches full speed, thus pushing the solid copper ring K into con- tact with the inner ends of the commutator bars L, and thus com- pletely short-circuiting the armature winding. At the same time barrel I, which is pushed endwise by the governor weights and which carries the short-circuiting copper ring K, pushes the brush holder or rocker arm endwise, and lifts the brushes off the commutator. In starting the Wagner single-phase motor, the supply volt- age is usually reduced to a fractional part of the full running voltage. This is accomplished by the use of a small step-down transformer, usually an autotransformer, in much the same way as has been ex- plained in connection with the autostarter or starting compensator for two-phase and three-phase induction motors. Induction Generator. An induction motor runs as a motor at a speed less than the speed of the rotating magnetism in the stator iron (synchronous speed). When the motor load is decreased, its speed approaches synchronous speed, and the intake of power from the alternating-current mains falls off more and more. If the rotor is driven by an external source of mechanical power, it may be speeded up to synchronism, in which case the intake of power becomes zero, except for core loss in the stator iron. If now the rotor is speeded above synchronism by the external source of power, the stator windings deliver power to the alternating-current mains, provided the alternating-current generator remains connected to the mains to fix the frequency. When an induction motor is so used, it is called an induction generator. The use of the induction motor as an induction generator is not of much commercial importance. Frequency Changer. An induction motor provided with a rotor having a definite winding with terminals brought out to collect rings, see Fig. 344, may be used as a so-called frequency changer. When the rotor stands still, the rotating stator magnetism induces electro- 398 ALTERNATING-CURRENT MACHINERY motive forces at full frequency, that is, of the same frequency as the alternating currents supplied to the stator. If the rotor runs at one-fourth speed, let us say, the relative speed of the rotor and the stator magnetism is three-fourths of the speed of the latter, and hence electromotive forces of three-fourths full frequency are induced in the rotor windings. If the rotor is run backwards at, let us say, one-half of the speed of the stator magnetism, then the relative speed of the rotor and the stator magnetism is one and one-half times the speed of the stator magnetism, and electromotive forces of one and one-half times full frequency are induced in the rotor windings. Example. A certain induction motor runs at one-third synchronous speed (n' = $n, page 377), then, ignoring stator losses, all of the power de- livered to the stator is transmitted to the rotor, and of this total power one- third appears as mechanical power driving the rotor, and two-thirds appears as electrical power developed in the rotor windings. This electrical power, ignoring the resistance loss in the rotor windings, is delivered to the rotor collecting rings. Furthermore, if the rotor has the same number of conductors as the stator, then the electromotive forces between collector rings are two-thirds as great as the voltages applied to the stator windings, and their frequency is two-thirds as great. If the rotor of an induction motor is driven backwards by an external source of power at one-half synchronous speed (n f = $n), then all of the elec- trical power delivered to the stator together with the mechanical power used for driving the rotor, appears as electrical power in the rotor windings, and the rotor voltages are one and one-half times as great in value, and one and one-half times as great in frequency as the voltages applied to the stator. The stator current in an induction motor, or a frequency changer, is sufficient at no-load to magnetize the stator. This stator current is called the no-load current of the machine. When current is taken from the rotor, an equal (and opposite) additional current is taken from the supply mains by the stator windings, exactly as in the case of the transformer. The above statements are based on the assumption that the rotor windings are exactly like the stator windings, both as to the number of conductors, and as to the grouping of the conductors into separate circuits or phases. If the rotor has half as many conductors as the stator, the rotor voltages are halved and the rotor currents are doubled, other things being equal. In alternating-current plants, designed primarily for the trans- mission of power, and hence using a low frequency (e. g., 25 to 40 cycles per second), there is sometimes a need for a limited amount ALTERNATING-CURRENT MACHINERY 399 of current of a higher frequency. To meet such conditions, a fre- quency suitable for lighting purposes, 60 cycles or more, may be cheaply and easily obtained by means of the frequency-changer. This is essentially an induction motor as explained above, the rotor of which is driven mechanically by an auxiliary synchronous motor in a direction, usually opposite to its natural rotation. The current of lower frequency is fed to the stator windings and the curremTof higher frequency is taken out of the rotor windings by means of collector rings. The frequency of the motor current will depend on the speed at which the rotor is driven. Thus, if the rotor is driven at its rated speed but in a direction opposite to its natural rotation, the frequency of the current delivered by it to the collector rings will be twice the normal, or if run at half the normal speed in its natural direction, the frequency will be one-half the normal. To change a current with a frequency of 40 cycles into one of 60, the motor would be run at one-half speed in an opposite direction, while to obtain 60 cycles from a 25-cycle current, the rotor would run nearly one and one-half times the rated speed in an opposite direction. The total power delivered to a frequency changer is partly electrical power delivered directly from the low frequency supply mains to the stator of the frequency changer, and partly mechan- ical power delivered by belt to the rotor of the frequency changer from the auxiliary driving motor. The power output of the fre- quency changer is wholly electrical and in the form of increased- frequency alternating currents from the rotor. The electrical power delivered to the stator of the frequency changer is and the mechanical power delivered by belt to the rotor of the frequency changer is p*= where P is the total power delivered to the machine, being a little greater than the total power delivered by the machine at the in- creased frequency ; / is the low frequency of the alternating currents supplied to the stator of the machine ; and /' is the higher frequency 400 ALTERNATING-CURRENT MACHINERY of the alternating currents delivered by the rotor of the machine. Therefore, the rotor of the frequency changer must be designed for the total output of power at the higher frequency, and the stator of the frequency changer must be designed for the intake of the amount of power P e , which is supplied to it electrically. For example, a frequency changer rated at 100 kw. to change a 40-cycle current to one having a frequency of 60 cycles per second would be made up as follows: An auxiliary driving synchronous motor rated at 33.3 kw. designed to take current from 40-cycle mains at a speed of, say 600 r. p. m. ; it would, therefore, have eight poles. Its armature would be direct connected to the rotor of the frequency- changer which would be rated at 100 kw. The stator of the frequency- changer would be rated at 66.7 kw. and would be supplied with current having a frequency of 40 cycles per second. If wound for four poles, the synchronous speed (as an induction motor) of the rotor of the frequency-changer would be normally 1,200 r. p. m. But by driving the rotor (by the auxiliary synchronous motor) at a speed of 600 r. p. m. in a direction opposite to its natural rotation, the frequency of the rotor currents would be that due to an equiva- lent speed of 1,200+600= 1,800 r. p. m.; corresponding thus to a frequency of 60 cycles per second. For the sake of a simple illustration, the ratings as given above are based on the assumption of a 100 per cent efficiency, which, of course, on account of the unavoidable power losses, is never realized in practice. It is evident that the frequency-changer can at the same time be designed to change the electromotive force by using a suitable number of turns in the stator and rotor windings. It can also be used to change the number of phases of the system by providing a rotor wound for a number of phases different from that of the stator. For instance, it may be designed to convert from three-phase, 6,000 volts, and 25 cycles, to two-phase, 2,500 volts, and 62.5 cycles. On account of this flexibility the frequency changer is sometimes called a "general alternating-current transformer." A number of these frequency changers are in present use, but on account of excessive magnetic leakage, they are not as satisfactory in operation as motor- generators. One of the large manufacturing companies is now recommending as a frequency changer a motor-generator consist- ALTERNATING-CURRENT MACHINERY 401 ing of a polyphase induction motor of one frequency driving mechan- ically an alternator designed for the frequency desired. COMPARISON OP SYNCHRONOUS MOTOR AND INDUCTION MOTOR To summarize the characteristic behavior in service of synchro- nous and induction motors, and to simplify the comparison between them, the following tabular statement in parallel columns, prepared by C. F. Scott, is given. The induction motor chosen for comparison with the synchronous motor is of the so-called ' "squirrel-cage" type, started by applying a low electromotive force to the primary winding. The description following will, of course, require modification in some particulars, if the secondary is furnished with adjustable resistance, but these modifications are of minor importance and do not affect the general comparison. SYNCHRONOUS MOTOR INDUCTION MOTOR Auxiliary Apparatus Required 1. A starting motor; or, if self- 1. A two-way main switch with starting, some form of resistance or autotransformers giving a low e.m.f. transformer for reducing the volt- for starting. This may be at any age. distance from the motor. 2. An exciter, driven by the mo- 2. No exciter is required, tor or otherwise, with circuits to switchboard and motor. 3. Rheostats for exciter and mo- 3. No field rheostats are required, tor. 4. Instruments for indicating when 4. No instruments are required, field current is properly adjusted. 5. Main switch and exciter 5. No exciter switches are re- switches, quired. 6. A friction clutch is required 6. No friction clutch is required, in many cases. as the motor starts its load Construction 1. Armature winding. 1. Primary winding. 2. Field winding with many turns. 2. Secondary, short-circuited. Liable to accident from "field dis- charge" if exciting current is suddenly broken; or from high e.m.f. by induc- tion from the armature if the field cir- cuit is open. 3. Collector rings and brushes. 3. No moving contacts on "squir- rel cage" secondary. 402 ALTERNATING-CURRENT MACHINERY Starting Normal 1. Motor is brought up to speed without load; if starting motor is used, the main motor must be brought to proper speed and "synchronized"; if self-starting, the starting devices must be cut out of circuit at the proper time. 2. Exciter is made ready for de- livering proper current and the motor field must be excited, adjustments being made by rheostats until instru- ments give proper indication. 3. Load is thrown on by friction clutch or other means. 1. Throw switch to starting and then to running position. 2. There is no exciter. (The mo- tor is magnetized by lagging current from the generator.) 3. The motor starts its own load. Starting Abnormal 1. If the several operations in starting be performed improperly or in wrong order, injury may result. If a starting motor is used, the syn- chronizing may be attempted at an improper speed or phase; if the motor is self-starting and it is connected to the circuit without the starting de- vices, a large current will flow which may induce a high e.m.f. in the field circuit; if the field circuit be open, a high e.m.f. may be induced in it at other times also. 2. If a load having inertia be ap- plied by closing the friction clutch too quickly the motor may be over- loaded and stopped. 3. If motor stops owing to failure of current supply, it is not self-start- ing when the current returns. An attendant is always required for starting. 1. The only possible error is in starting with the switch in the run- ning or full voltage position, which simply causes the motor to exert a greater torque and consume a greater current than is necessary. 2. The motor starts its own load and requires no friction clutch. 3. The motor will stop if the cur- rent is cut off at the power house and then start again when the cur- rent is supplied to the circuit. Starting and Maximum Running Torque 1. The starting torque of the self- starting motor is very small and an excessive current is required for de- veloping it. The motor starts as an induction motor, but inefficiently, as the design which is best for synchro- nous running is not good for starting. 1. The starting torque is adjusta- ble and may be several times full load torque. ALTERNATING-CURRENT MACHINERY 403 2. The maximum torque is sev- eral times the full load torque, and occurs at synchronous speed; below this speed the torque is very small; any condition which momentarily lowers the speed causes the motor to stop. Speed 2. The maximum torque is usu- ally greater than that of the syn- chronous motor, but it occurs at a reduced speed and there is a large torque at lower speeds. 1. The motor has a single definite speed; at other speeds its torque is very small, and the current is very large. 1. The motor may be designed for a practically constant speed, with large torque at lower speeds; or for several definite speeds by changing the number of poles; or for variable speed, for cranes, elevators, hoists, and the like. Current 1 . If there is useful starting torque, the current required for producing it is very great. 2. The running current depends upon the wave form. If the wave form of the motor and of the circuit differ, a corrective current will fol- low, which cannot be eliminated by adjustment of field excitation. 3. The running current depends upon uniformity of alternations of the current, i.e., upon the uniformity of the speed of the generator and other synchronous motors. The motors at- tempt to follow the generator speed exactly. If the latter pulsates, the motors pulsate also; they vibrate about a mean position, "hunting" or "pumping." One motor pumping incites others. The current is in- creased even though the conditions may still be operative. 4. The running current depends upon the relation between the field current (which is adjusted by the at- tendant) and the e.m.f. of the circuit. The main current may be made lead- ing or lagging or theoretically it may be neither. The e.m.f. of the circuit is an element which is under the partial control of the attendants at every mo- tor, as well as at the generator station. 1. The starting current may be made proportional to the torque, and is 1 \ to 2 1 times that required for the same torque at high speed. 2. The running current is prac- tically independent of the difference in wave form, as it has no wave form of its own. 3. The current is practically in- dependent of fluctuations in gener- ator speed, as there is a slip between the synchronous and the actual speed of the motor. 4. The current is not subject to any adjustments which the motor at- tendant can make, nor is the e.m.f. of the circuit in any way under his control. 404 ALTERNATING-CURRENT MACHINERY Power Factor 1. As the power factor is the re- lation between actual current and energy current, it is dependent upon wave form, hunting, and field cur- rent. Under favorable conditions, the motor may have a high power factor; under many actual conditions it may not; under some conditions the highest attainable power factor is less than that of the induction motor. 2. The current may be lagging or leading, depending upon the motor field strength. 1. The power factor varies with load, but is definite and is practi- cally independent of wave form and hunting. 2. The current to the motor is always a lagging current. Reaction Upon Generator and Circuit 1. The motor impresses its own wave form on the circuit. 2. A motor may augment the fluc- tuations in generator speed by the os- cillation of its own armature. One motor may increase the disturbance in the circuit so as to interfere with other motors not otherwise seriously affected. 3. As the current may be either lagging or leading, the drop in e.m.f. in the generator, and between gener- ator and motor may be either more or less than that which could be caused by a non-inductive load or by an in- duction motor. 4. If a short-circuit occurs in the transmission system, the motor acts as a generator, which thereby greatly increases the current and the intensity of the short-circuit. 5. If the circuit is opened, either by a switch, a circuit breaker, a fuse, or the breaking of the line, the mo- tor speed falls, its e.m.f. is no longer in phase with that of the circuit; the two are thereby added, thus doubling the normal e.m.f. and bringing in- creased strains on the insulation and the opening devices. 1. The motor has no wave form to impress upon the circuit; its tend- ency is to smooth out irregularities in a wave not a sine. 2. The motor has a damping ac- tion upon fluctuations in frequency; in some cases a synchronous motor which hunts may run smoothly when an induction motor is connected to the same circuit. 3. The drop in e.m.f. is always greater than would be caused by non- inductive load. 4. The motor does not generate current when there is a short-circuit. 5. The motor does not generate e.m.f. when it is disconnected from the circuit. ALTERNATING-CURRENT MACHINERY 405 Causes which May Accidentally Stop a Motor 1. Momentary lowering of e.m.f. caused by short-circuit on the line, or by accident to another motor, or by error in synchronizing a generator, or by the "switching over" of the mo- tor from one circuit to another, is apt to cause the motor, particularly if carrying load, to fall from synchron- ism and stop. 2. A heavy load, even momentary, may exceed the limiting torque and cause the motor to drop from syn- chronism, even though the load be removed immediately. The connec- tion between generator and motor is rigid. 3. If the generator speed suddenly increases, a motor carrying a load having inertia may be unable to in- crease its speed quickly without ex- ceeding the limiting torque, which will cause the motor to stop. Summary 1. Momentary lowering of e.m.f. causes momentary decrease in speed. 2. An excessive load receives the stored energy of the motor and of the load itself as the motor speed falls; when the excess load is removed the motor speed increases again. The connection between generator and motor is elastic. 3. The motor readily follows changes in generator speed. 1. The motor is an active element in the system; it acts as a generator in impressing its own wave form, its e.m.f. and its fluctuations upon the circuit. These fluctuations may be caused by an intermittent load. 2. The m^tor is a sensitive ele- ment in the system. Its successful operation is dependent upon a proper relation between the design of the mo- tor itself and of other machines in the system. Its successful operation also depends upon the proper adjustment and freedom from speed fluctuation in generators and other motors. It is liable to momentary variations from normal conditions, such as a sudden over-load and sudden increase of gen- erator speed or a momentary fall in e.m.f. 3. The motor requires skill and care on the part of the attendant for starting, for readjusting and for keep- ing the various brushes and auxiliary apparatus in condition. 1. The motor is a passive element in the system. Each motor attends to its own work and does not try to run the system. 2. The motor is not sensitive to differences in the design of other ap- paratus operating on the same system. 3. No experience and electrical skill are required of the attendant and there is little or nothing to get out of order either through carelessness or 406 ALTERNATING-CURRENT MACHINERY 4. The power factor is under the 4. The motor has a definite power control of the operator and the cur- factor, depending upon the load; the rent may be made leading or lagging. out-of-phase current does not vary Instruments are necessary in order greatly at different loads. Thechang- that proper adjustments may be made ing load, therefore, has comparatively by the attendant. little effect upon the drop in voltage and in regular service there is little liability that the motor will disturb the e.m.f. of the circuit. 5. The motor and its operation 5. The motor and its operation are complex and involve many pos- are simple and reliable. sibilities of accident. The synchronous motor is obviously not suitable for general distribution of power, owing particularly to its lack of starting torque, the skill required in attendance, and the liability of the motor to stop if the conditions become abnormal. These objectionable features, however, are of much less importance when motors are installed in substations or are of sufficiently large size to justify an attendant. The characteristic of the synchronous motor which may be particularly advantageous is the fact that the power factor of the current can be varied and that the current may be made leading. INDUCTION MOTOR TESTS Heat Test. The heat test on induction motors is usually carried out by connecting the terminals of the stator windings to mains of the proper voltage and frequency, and taking rated full-load mechanical power from the rotor until a constant temperature has been reached. Then the motor is shut down, and the temperatures of the following parts are taken: Armature laminations Armature conductors Field conductors Field laminations Frame Bearings Room If the motor is small, the mechanical power output may be absorbed by a brake. For large power motors, however, a brake becomes troublesome. In any case, the most convenient way to ALTERNATING-CURRENT MACHINERY 407 measure the power is to belt to the motor a direct-current sepa- rately-excited generator, whose losses can be easily determined, and measure the ouput of this generator. The output of the motor is equal to the output of the generator plus the losses in the gener- ator. The losses in the generator are: the I 2 R loss in the armature, brush and bearing friction, and windage loss, and the core loss due to hysteresis and eddy currents. The field being separately excited, the field loss need not be considered. The field current must, however, be kept constant. During the heat run, the following observations are regularly recorded : MOTOR GENERATOR Volts Amperes Speed Volts Amperes Armature Amperes Field Speed The PR loss in the generator armature can be calculated from the current flowing, and the resistance of the armature. To determine the stray-power losses (equal to all the losses except PR) proceed as follows: Disconnect the motor from the alternating-current mains, and run the combination of motor and generator from the direct-current end, at the same speed as that recorded during the run, the direct-current machine being now used as a motor. Its field is excited to the same value that it had during the run. The speed is adjusted to the right value by varying the voltage supplied to the direct-current machine used as a motor. When the speed is correct, record the amperes and volts taken by the direct-current machine. Next the belt is thrown off, and the voltage supplied to the armature termi- nals of the direct-current motor is adjusted until proper speed is attained, the field current remaining as before, and the input of power is again recorded. Let Wi = the watts input to the direct-current motor with the belt off; Wz watts at same speed to drive both machines with belt on; C = core loss of direct-current machine; F = friction of direct-current machine without belt; FI= friction of induction motor without belt; / = increase of bearing friction of direct-current machine due to belt tension; and /i= increase of friction of bearings of induction motor due to belt tension. Then Wi = C + F (t) and W 2 =C + F + F 1 +f 1 + (ii) Subtracting (i) from (ii) we get or 408 ALTERNATING-CURRENT MACHINERY Now, since the two machines are of about the same size, we may assume that the increase in friction of each, due to belt tension, is the same, so that TF 2 - Wi - F! But Fi, the friction of the induction motor, can be determined as de- scribed under core-loss test. Hence, we have the following expression for the stray-power loss of the generator: TF, -TF 2 -Fi stray-power loss = W \ -\ - total output of motor = stray-power loss + El + I 2 R This expression gives an exact method of determining the output of any kind of motor, by using a direct-current generator as a load. If Fi is unknown, we may neglect the increase of bearing friction due to belt tension, giving results sufficiently accurate for a heat run. Breakdown Test. The breakdown test, as in the case of a syn- chronous motor, is to determine the maximum output of the motor, that is, the load which will cause the motor to "break down", and stop. To make this test the load on the motor is increased, until the motor breaks down, the maximum output being noted. It is essential that the alternating currents be supplied to the motor at normal voltage. As the torque and, therefore, the maximum output, varies as the square of the voltage, the results obtained by this test will not be accurate unless the voltages applied to the stator windings are kept constantly at normal value. If for any reason it is impos- sible to load the motor to the breakdown point, the voltage may be reduced, and the maximum load at normal voltage may be calcu- lated from the value obtained at the reduced voltage by multiplying . . . / normal voltage \ 2 the observed maximum load by I - I . V reduced voltage / If, for example, an induction motor rated at 50 horse-power gives a maximum output of 25 horse-power at one-half its rated voltage, its maximum output at normal voltage would be approxi- mately 100 horse-power. The most convenient way to load the motor in the above test is to belt it to a direct-current generator, as previously explained. To determine the output of the induction motor, from observations on the direct-current generator, proceed in the same manner as described under heat run. ALTERNATING-CURRENT MACHINERY 409 In commercial work it is not customary to run this test to the breakdown point. The usual method is to find out if the motor will stand 50 per cent overload without breaking down. When, how- ever, it is desired to obtain a full set of data on a machine, the test is carried to the breakdown point. Starting Torque Test. The stationary or starting torque de- veloped by a motor determines the amount of load under which it will start. To perform this test a brake is clamped to the pulley as shown in Fig. 359. A is the center of the pulley; B is the point of suspension of the brake arm from the spring dynamometer S; C is a reference pointer, carried on a standard, on a horizontal line through the center of the pulley. The spring S will measure the Fig. 359. Prony Brake Arrangement for Testing Starting Torque tangential force exerted by the motor at a radius AB when B has moved down to the point C, and the spring dynamometer S is sus- pended vertically. The reading of S will be affected by the friction in the motor bearings. The following procedure will eliminate this error: Fasten to the brake arm a weight W, sufficient to overcome the friction of the bearings, so that the arm, if left unsupported, will always be carried downward by its weight. By slowly raising and lowering the brake arm by means of the cord which supports the spring dyna- mometer S (no current passing through the motor), we get two different readings of S when B passes the point C. While the arm is being raised the friction in the bearings acts in the same direction 410 ALTERNATING-CURRENT MACHINERY as the weight W. Second, as B passes (7, while the arm is being lowered, the friction of the bearing acts against the direction of the pull due to W. L e t W pull, exerted by the weight of the brake and arm weight; F= friction of the bearings; a = scale reading when arm is being raised; and 6= scale reading when arm is being lowered. Then W+F=a and W-F=b Let T be the force exerted by allowing the current to act on the motor, this force being in the same direction as W. Then, with current flowing in the motor, let c= scale reading while arm is being raised; and d= scale reading while arm is being lowered. Then T+W+F=c and T+W-F=d or But from above we have w- a+b 2 substituting this value of W, we have T = C ~^~ d a By this method both the weight of the brake arm and the friction of rest in the bearings are eliminated. To carry out this test, this set of observations is repeated for as many values of the current in the motor as desired. The current should range at least from one-half to twice the normal full-load current. To obtain the desired current, the voltage across the motor terminals must be adjusted. Core Loss Test. The core loss of an induction motor cannot ALTERNATING-CURRENT MACHINERY 411 be measured by the method used in the case of a synchronous motor or alternating-current generator, namely, by driving it by a direct- current motor and observing the motor input for various voltages generated, since an induction machine will not generate electro- motive force unless it is connected to the mains. If it is connected to the mains, the mains may supply power to it, and hence the power supplied by the direct-current motor is not the total power- delivered to the machine. The core loss is measured in the same manner as in the case of a transformer, namely, by connecting it to the alternating-current 200 Q / 160 / / 'o / 120 100 80 I / & / / * c X / 60 40 20 0< x / ^ ^ > - ,_- *~*~~~~ O 100 200 300 400 500 600 700 l/o/fs Fig. 360. Curve Showing Watts Input to a 1-H. P. Motor at Different Voltages mains at normal voltage and frequency, and measuring the watts input at no-load. There is this difference, however, between the case of the transformer and that of an induction motor, that, whereas in the former the power input is practically all needed to supply the core loss, in the latter, a part of the power input goes to supply the friction and windage losses of the motor, and the appreciable PR loss in the stator windings. When an induction motor is run at no-load, the speed remains practically constant as the voltage is reduced, until the motor "breaks down" and stops. The power input to the motor (at no-load) at any voltage consists of the core loss at that voltage plus the friction 412 ALTERNATING-CURRENT MACHINERY and windage loss plus the I 2 R losses in the stator and rotor wind- ings. But the friction and windage loss is nearly constant at con- stant speed, therefore the watts input to the motor at various voltages consists in part of the constant friction and windage loss, and in part of the variable core loss. Such a series of observations on a 1-h. p., 550-volt, 3-phase motor is shown plotted in Fig. 360. The voltage can be reduced to the point at which the motor breaks down; beyond this we cannot go. As the ordinates on this curve are equal to the sum of a constant and a variable part, and since at zero voltage the variable part becomes zero, it follows that if we prolong the curve as shown in the dotted portion until it crosses the axis of watts, that is for zero volts, the value intercepted on the axis of watts may be con- sidered, without much error, to be the constant part of the watts, namely, the friction and windage loss. Thus in Fig. 360, 26 watts represents the power lost due to friction and windage in the case of the induction motor above mentioned. The sum of the core loss, friction losses, and PR loss at normal voltage, viz, 550 volts, is found from the curve to be 109 watts. For each observed value of the volts and the watts, the current per phase to the stator wind- ings must be recorded. Then, the resistance of the stator wind- ings having been measured, the PR loss corresponding to any given voltage can be calculated. To obtain the core loss corresponding to any voltage, we must, therefore, subtract the constant friction losses plus the PR loss in the stator windings from the total ob- served input to the motor (when running unloaded). For example, in the case of the 1-h. p. induction motor under consideration, the current input per phase at no-load was measured and found to be 0.655 amperes, when the voltage between supply mains was 550 volts. The resistance per phase was also measured and found to be 15.5 ohms. The total PR loss at no-load was, therefore, 3 X O65? X 15.5 = 20 watts. Therefore, at normal voltage core loss = 109 - 26 - 20 = 63 watts Impedance Test. This test is carried out in the same manner as the core-loss test. The motor is connected to the alternating- current supply mains and the amperes flowing, the watts input, and the volts at the terminals of the motor are measured, the watts, ALTERNATING-CURRENT MACHINERY 413 of course, being measured by wattmeters. In this test, however, the motor is not allowed to run free, but its armature is blocked to prevent it from turning. Instead of supplying normal voltage to the motor, the voltage supplied is cut down to only five or ten per cent of the normal value and is then raised carefully until the ammeters show about one-third to one-half of the full-load current. The motor must be supplied with current at normal frequency. The following observations should be recorded: Amperes in each line Volts for each phase Total watts The observations are repeated with increasing value of the voltages, until the current has reached from one and one-half to over twice the full-load value. Fig. 361 shows curves of observa- tions taken in this way for the 1-h. p., 550-volt, three-phase mo- tor referred to above. This motor takes one ampere of current per phase at full-load with normal voltage of 550 volts. The two curves are plotted with current and volts, and with watts and volts as ordinates and abscissas, respectively. The former curve is the straight line. Since for full-load current with the armature standing still, between Terminate Fig. 361. Curve for Impedance Test of Induction Motor the voltage applied to the termi- nals of the motor is very low, the number of watts supplied to overcome core loss is very low. In fact, practically all the watts supplied are used up in heating the con- ductors of the stator and rotor. The watts input, therefore, for normal rated full-load current but with rotor blocked, may be taken as a measure of the total PR losses (primary and secondary) of the entire machine at full-load. For the motor for which the curves are shown, the PR losses at full-load current are equal to 100 watts. Efficiency Test. As in the case of the machines previously 414 ALTERNATING-CURRENT MACHINERY considered, the efficiency of an induction motor, at a given load is equal to the output, divided by the output plus the losses, at that load. All the losses can be determined from the core-loss test and the impedance test; the core loss and friction and windage losses being determined from the former, and the copper losses from the latter. For the three-phase, 1-h. p. motor above mentioned, the losses at full load are as follows : Friction and windage loss = 26 watts Core loss = 63 watts Copper loss (7 2 /2) = 100 watts Total losses = 189 watts therefore, 746 efficiency at 1-h. p. output = = 79.8% /rrO ~| The efficiency of large machines is generally calculated in this way. For smaller machines it is more usual to determine the effi- ciency by actually measuring input and output. The reason for this is that the actual losses occurring in the motor when it is loaded, are different from the values as calculated from results of tests at no-load. The differences between these calculated and actual losses are comparatively large in a small machine. To test the efficiency by measuring the total input and out- put, the most convenient method is to belt the motor to a direct- current generator, measuring the output of the generator, and the input to the motor. The output of the motor is, of course, equal to the output of the generator plus the generator losses. The losses of the generator may be calculated as described under the heat test. The output and input of the motor are measured for various loads successively from a very small load to perhaps 50 per cent overload, so that a curve may be constructed showing the relations between efficiency and load. The losses of the generator must be determined at each speed occurring in the series of observations, because the generator losses vary with the speed. Slip Test. The slip at a given load on the motor is the ratio where n is the synchronous speed of the motor, and n f is the speed ALTERNATING-CURRENT MACHINERY 415 under the given load. Slip is usually expressed in per cent, in which case per cent slip = - - X 100 n The synchronous speed in revolutions per second is equal to 2f , where / is the frequency of the alternating currents supplied P to the stator, and p is the number of "poles" of stator magnetism, NS, NS in Figs. 333, 334, and 335. The slip is independent of the number of phases. The speed of the motor at zero-load is very nearly equal to the synchronous speed, and where / and p are not known, the zero-load speed may be used for n without great error. The determination of the slip of an induction motor at full- load with full-rated voltage applied to the stator windings, is an important test. This slip may be determined by observing, by means of a speed counter or tachometer, the speed n' of the motor under full load and with full rated voltage. If p and / are known, the slip may be calculated as explained above. Various methods have been proposed for measuring the slip of an induction motor directly. A simple piece of apparatus for this purpose consists of a black disk with p white radial lines or sectors painted upon it, where p is the number of poles of the motor. This disk is attached to the motor pulley. If, when the motor is running, the disk is illuminated by an alternating-current arc lamp supplied with current from the same mains as the motor, the white lines on the disk would appear stationary if the motor were running in synchronism. If the motor were below synchronism, the disk would appear to rotate with a speed equal to the difference between the synchronous and the actual speed. Thus the slip can be measured directly by counting the apparent revolutions per minute of the disk. For example, the synchronous speed of the 1-h. p. motor con- sidered above, is 1,200 r. p. m. In applying the above test the disk made 70 apparent revolutions per minute when the motor was running at full load and, therefore, the slip at full-load was ' or 5 - 83% 416 ALTERNATING-CURRENT MACHINERY Performance Curves of an Induction Motor. Fig. 362 shows the performance curves of a 175-h.p., three-phase, twelve-pole induction motor having a synchronous speed of 600 r. p. m., when supplied with three-phase currents at 550 volts and 60 cycles per second. The abscissas of all the curves are horse-power output. The dropping of all the curves in the region between 340 and 360 horse-power shows that the maximum power which the machine can develop is about 350 horse-power. There are three scales of ordinates, namely, amperes input, per cent, and pounds torque. The ordinates of the curve marked "synchronism" are measured on the per cent scale, and they represent the speeds in per cent of the synchronous speed at the various loads. The ordinates of the curve marked "amperes input" give the amperes input ner phase at the various loads. 20 40 60 80 100 120 140 160 180 300 220 240 260 280 300 320 340 Horse F'oi^er Output Fig. 362. Performance Curves of a 175-H. P. Three-Phase Induction Motor The ordinates of the curve marked "torque" are measured on the pounds torque scale, and they represent the torque developed by the motor at various loads, these torques being expressed in pounds of force acting at one foot radius, that is, in pound-feet. The ordinates of the curves marked "efficiency", "apparent efficiency", and "power factor", are measured on the per cent scale, and they represent these quantities at various loads. The efficiency is, of course, the ratio, The power factor is the factor by which the product of volts times amperes must be multiplied to give the true input of power per phase. ALTERNATING-CURRENT MACHINERY 417 The product volts times amperes is called the apparent power delivered by an alternating-current generator, and the apparent apparent power input efficiency of an induction motor is the ratio, - . output The apparent efficiency is thus equal to the true efficiency multiplied by the power factor. The relation between the three factors, efficiency, power factor, and apparent efficiency, will be made clear by the following equa- tions, each based on definition: useful output efficiency = power factor = apparent efficiency = actual input actual input apparent input useful output apparent input From the above three equations, it is evident that apparent efficiency = efficiency X power factor When the output of a motor under a given load is measured mechanically by observing the useful torque developed at the arma- ture shaft, and the speed of the armature, we have 27m' T 7 , useful output = 00 horse-power = 0.142 n'T watts where n' is the speed of the armature in revolutions per minute; and T is the useful torque in pound-feet, developed by the armature. The actual input to a three-phase alternating-current motor of the induction type may be expressed as follows: actual input = V 7 3 EIP watts where E is the electromotive force (effective value) applied between any two of the stator terminals; / is the current (effective value) in any one of the leads supplying the stator windings; and P is the power factor of the motor. The apparent input to a three-phase induction motor may be expressed as follows: apparent input = V 3 El watts 418 ALTERNATING-CURRENT MACHINERY From the above equations we may easily write the expressions for the efficiency, power factor, and apparent efficiency, as follows : 0.142 n'T v^TEIP power factor = P - - / 0.142wT apparent efficiency = , ,. 1 o J-J*- Exartiple. A certain 12-pole induction motor whose performance curves are given in Fig. 362, is rated as follows: Number of phases, 3 Output rated at full load, 175 horse-power Voltage supplied (between mains) 550 volts Speed, at full load, 585 r.p.m. Frequency, cycles per second, 60 A brake test on the motor when running at full load gave the following data (see curves in Fig. 362). Useful torque on rotor shaft, pound-feet. 1,560 Speed, in revolutions per minute 585 Amperes input per phase 170 Power factor, in per cent 88 It is required to calculate the following quantities in order to check the accuracy of the curves plotted in Fig. 362. (a) The synchronous speed in r.p.m. (b) The slip at full load in per cent. (c) The useful output in watts and in horse-power (d) The actual input in watts (e) The efficiency (f) The apparent input in watts (g) The apparent efficiency Solution. (a) The synchronous speed, according to equation (47) is 2/ 2 X 60 = 10 revolutions per second or 10X60 = 600 r.p.m. (b) The slip as referred to on page 415, is 100 = (- ^^-} 100 = 2.5 per cent ALTERNATING-CURRENT MACHINERY 419 TABLE XI Capacities of Standard Transformers H. P. CAPACITY MOTOR THREE-PHASE I TWO-PHASE 2 TRANSFORMERS 2 TRANSFORMERS 3 TRANSFORMERS 1 .6kw. .5kw. .6kw. 2 1.5 kw. 1.0 kw. 1.0 kw. 3 2.0 kw. 1.5 kw. 1.5 kw. 5 3.0 kw. 2.0 kw. 3.0 kw. Ti 4.0 kw. 2.5 kw. 4.0 kw. 10 5.0 kw. 3.5 kw. 5.0 kw. 15 7.5 kw. 5.0 kw. 7.5 kw. 20 10.0 kw. 7.5 kw. 10.0 kw. 30 15.0 kw. 10.0 kw. 15.0 kw. 50 25.0 kw. 15.0 kw. 25.0 kw. 75 25.0 kw. 35.0 kw. 100 30.0 kw. 45.0 kw. (c) The useful output from the above equations is 0.142 n' T watts. Substituting the given values of n r and T, we have 0.142 X 585 X 1560 = 129,600 watts or 129,600 746 = 173 horse-power (d) The actual input in watts is V 3 EIP, whence substituting the given values of E, I, and P gives actual output = 1.732 X 550 X 170 X 0.88 = 142,500 watts (e) The efficiency (real) is useful output 129,600 - = - = 90.9 per cent actual input 142,500 (f) The apparent input in watts is V 3 El, or 1.732 X 550 X 170= 162,000 watts (g) The apparent efficiency is useful output 129,600 = - = 80 per cent apparent input 162,000 A comparison of the above results with the corresponding values obtained from the curves in Fig. 362, shows on the whole a very satisfactory agreement. The power factors of standard commercial induction motors of American manufacture vary at full load from 0.75 to 0.92, de- 420 ALTERNATING-CURRENT MACHINERY pending upon the size and the frequency of the motor. The effi- ciencies range from 0.80 to 0.95. The apparent efficiencies in motors above 5 h. p. output will be found, as a rule, not less than 0.75. This means that the transformers supplying current to induction motors of average sizes, must have an aggregate capacity of about one kilowatt for every horse-power of rated output of the motors. Table XI gives approximate capacities of standard trans- formers that should be used with two-phase and three-phase induc- tion motors. SWITCHBOARD AND STATION APPLIANCES The switchboard is a supporting frame upon which are mounted most of the measuring instruments and safety and controlling devices used in a generating or receiving station. Lightning arresters are usually mounted on the wall of the station building at the points where the lines enter the building. Large apparatus such as rheostats, oil- switches, circuit breakers, and voltage regu- lators are frequently installed at a distance from the switchboard, and are controlled from the switchboard by systems of levers, by compressed air, or by electrical relays which are in turn controlled by contact switches on the switchboard. Thus Fig. 363 shows a large rheostat mounted underneath the floor upon which the switchboard stands. The rheostat is operated by means of sprocket-wheels and chain which are moved by a hand-wheel on the switchboard. SWITCHBOARDS Alternating-current switchboards differ from direct-current switchboards as follows : (a) On account of the many special devices such as frequency meters, switchboard transformers, power-factor indicators, and synchronizing devices, which are used in alternating-current work, and are not used in direct-current work. Fig. 363. Diagram Showing Switchboard Control for Large Rheostats ALTERNATING-CURRENT MACHINERY 421 (b) Because of complications associated with the use of an auxiliary direct-current generator for exciting the field magnets of alternators. (c) Because of the frequent use of extremely high voltages, as high as 100,000 volts or more, in alternating-current systems. Switchboards are now usually built up of standard panels^ uniform in size, the style varying with the service required. Large switchboards for handling many generators or many feeder circuits, are built up by placing a number of these standard panels side by side. This method of building large switchboards has the following ad- vantages : (1) It reduces the necessity of special work to a minimum, and permits the use of standard apparatus, thus reducing cost. (2) It provides for interchangeability of panels, thus making rearrangement of feeder, generator, and exciter panels easy and convenient. (3) The use of standardpanels uniformly equipped with standard apparatus makes it easy and cheap to renew damaged parts. (4) It enables extensions to be made easily and systematically. In some large stations as many as ten or more generator panels and fifteen or more feeder panels are erected side by side. The panels of a switchboard are usually erected and wired completely at the factory, and all the instruments are attached. After thorough inspection and testing, they are shipped to their desti- nation, the instruments being detached, Fig 3 ~ 64 Standard switchboard and shipped separately. ''Ind^S^M?' Switchboards are usually constructed of a skeleton frame of angle iron, to which panels of marble or slate are fastened by bolts and nuts. The various instruments are 422 ALTERNATING-CURRENT MACHINERY attached to the marble or slate panels/ In many cases the appara- tus itself is located behind the board, the hand-wheel or operating lever only, being placed on the front of the board. Slate, when entirely free from metallic veins, is a fair insulator, but the frequent occurrence of such veins, and the tendency of slate to absorb moisture from the air, render it unreliable, especially for switchboards on which high-voltage apparatus is to be installed. 0.PS.T.Q.C.JMTCH J- CA/W RECE/YER /1AME L - M-SM4LL. rW SLOCKS Fig. 365. Front Elevation and Connections for Board Shown in Fig. 364 Marble is the standard material for switchboard panels, and it only is used for high-voltage panels. Typical Single=Phase Switchboard. Fig. 364 shows a front view of a standard switchboard for one single-phase generator and two feeder circuits. The front elevation of this board and the com- plete diagram of electrical connections are shown in Fig. 365. These switchboards are manufactured by the Fort Wayne Electric Works, for electric lighting plants of moderate size. The standard voltages for this type of panel are 1,150 and 2,300 volts, at frequencies of from 25 to 140 cycles per second. ALTERNATING-CURRENT MACHINERY 423 The kilowatt capacity or rating of switchboard panels depends on the current carrying capacity of its equipment. Thus, the rating of these panels ranges from 15 to 300 kilowatts at 2,300 volts and from 1\ to 150 kilowatts at 1,150 volts. The ammeter and voltmeter used on these boards, indeed nearly all switchboard amme- ters and voltmeters, are of the electromagnetic type, that is, they con- sist of a coil of wire ac- tuating a movable piece of soft iron to which the pointer is at- tached. The electromagnetic type is described on page 67. The essential features of the electrostatic ground detector are also described on page 67. Fig. 366. Details of Switchboard Fuse Holder Generator Sw. S uses _Fje1dAmmete Voltmeter- A.C.Ann metei A -Synchronizinq Lamp L Exciter Rhe -Held Switch Generator Rhe. Synch ronizincj Plua } Discharge Resistance" Generator Fig. 367. Front and Side Views and Complete Diagram of Connections for Standard Panel for Two or More Single-Phase Alternators in Parallel The two double-pole quick air-break generator switches with marble barriers are shown in Fig. 364. They are used for connect- 424 ALTERNATING-CURRENT MACHINERY ing the generator terminals to either or both of the feeder circuits at the top of the panel, as may be seen in the diagram of connections shown in Fig. 365. When either switch is opened, the arc which is produced is prevented by the marble barrier from flashing across from blade to blade of the switch, and thus short-circuiting the generator. Instead of these air-break switches, panels may be equipped with non-automatic oil switches, mounted at the back of the board. The type of switch illustrated in Fig. 364, and which breaks the circuit in air is suitable only for moderate voltages up to perhaps 2,300 volts, on account of the danger of arcing. One of the fuse holders, shown at A in Fig. 365, is illustrated in Fig. 366. The body of the fuse holder consists of an insulated metallic chamber, into which is screwed a fiber tube. That portion of the fuse located in the lower chamber has a smaller cross-section than the portion in the upper tube, which insures that the fuse will melt inside the chamber. When the fuse melts and breaks, the arc is extinguished by the explosive action which accompanies the sudden heating of the air enclosed in the fuse chamber. New fuses can be easily inserted in the holder by removing the screw plug shown at the bottom of the bulb. The holder is connected by means of blades fastened to each end which fit into clips mounted at the top of the panel, as shown in Figs. 364 and 365. The potential trans- former has its secondary connected through a high resistance to the voltmeter. If the ratio of transformation of the potential trans- former is 20 to 1, and the voltmeter reads 110 volts, the voltage be- tween the generator terminals is 20 X 110 =2,200 volts. Typical Switchboard for Operating Two or More Single-Phase Alternators in Parallel. Fig. 367 shows front and side elevations and a complete diagram of electrical connections of one of the General Electric Company's standard panels for one of several single-phase generators to be operated in parallel. The equipment of this panel is as follows : 1 double-pole generator switch, mounted on the back of the board, and operated by a lever on the front of the board. 2 expulsion fuse blocks complete. 1 generator ammeter. 1 field ammeter. 1 voltmeter and one potential transformer. 1 field switch. ALTERNATING-CURRENT MACHINERY 425 1 synchronizing device, complete. Tripod and front plate for "generator rheostat" (in the field circuit of the generator) with shaft and hand-wheel. Tripod and front plate for "exciter rheostat" (in the field circuit of the exciter.) All necessary framework and connections. The location of apparatus for this panel, as designated by letter, is as follows: R= rheostat in the field circuit of the generator, "generator rheostat." a = ammeter in the field circuit of the generator "field ammeter." V = voltmeter between the generator terminals (through potential trans- former). A = ammeter for the main alternating current. L= synchronizing lamp. S = ' 'generator switch" in the main circuit. r = rheostat in the exciter field circuit, "exciter rheostat." This apparatus is essentially the same as the apparatus already described on page 424 with the exception of the ground. detector, the field switch with discharge resistance, and the synchronizing device. One ground detector is sufficient for a number of machines operated in parallel, and it is usually mounted on a bracket attached to one of the generator panels. The field switch is arranged to short-circuit the field winding of the alternator at, or just before, the instant of disconnecting the exciter from the field windings. This allows the current in the field winding to die away slowly. The opening of a field switch which is not provided with a resistance produces an excessively high electromotive force between the field terminals, which is likely to cause puncture of the insulation of the field windings. The synchronizing device consists of the synchronizing bus bars connecting the various generator panels; the synchronizing lamps; a small transformer (the same one being used for the volt- meter) for stepping down the voltage to a suitable value for the synchronizing lamps; and connection plugs for connecting the sec- ondary of the potential transformer through the synchronizing lamps to the synchronizing busses. Two types of connecting plugs are used, one of which reverses the connections made by the other. The complete connections of the synchronizing device for two single-phase machines is shown in Fig. 368. Polyphase Switchboard. In case of polyphase machines, one phase only is connected to the synchronizing device, which is, there- 426 ALTERNATING-CURRENT MACHINERY fore, the same for single-phase and for polyphase alternators. The operation of alternators in parallel is very common in modern cen- tral stations.* The voltmeters and synchronizing device are always connected back of the main generator switch, that is, between the generator and the switch, for the reason that the voltage of the machine must be synchronized before the main switch is closed. The connections of the two types of synchronizer plugs are shown in Fig. 368. Neither plug is used when one alternator only is operated. When another machine is to be put into operation, Synchronizing Buses i ToBu&es Mafrt Switch Q To Buses Potential Transformer Mo/n Switch ?i?k A -:-h tift Potential Transformer) two Types of P/ugs n, General -*"" *>"*"""*'''*> To S e induCl> ance. The armature of the small machine has attached to - it a pointer which moves over a divided circle. When machines A and B have exactly the same frequency, this pointer is stationary and its reading on the circle indicates the phase difference between the two machines. When machine A is Fig. 376.- Lincoln Synchroscope &/S BAffS WWW VWW\ 1 VOL TAG TffAMSfOffrtL-ff VOL TffA A \ \ WA/MSW /TCtf vo 7^ A 1 WA <.TAG c \ \ //YSW, \ e vj f d)\ \-SCOPE} WWW WWW. ^T WWV\r ' WWWl ,7"^ GfM/y 2/ TO GEN.NL e Fig. 377. Wiring Diagram for a Synchrosc9pe for Single-Phase or One Phase of Poly- phase Circuit running at a slightly greater frequency than B, the pointer rotates in a certain direction indicating at each instant the changing phase- difference between A and B. When machine A is running at a ALTERNATING-CURRENT MACHINERY 435 slightly lower frequency than B, the pointer rotates in the oppo- site direction, indicating at each instant the changing phase-differ- ence between A and B. The speed of the pointer in revolutions per second is in each case equal to the difference in frequency of machines A and B } in cycles per second. Fig. 376 is a view of the exterior of a Lincoln synchroscope, "type A," as made by the Westinghouse Electric Company. It has a 9-inch dial and is intended to be mounted on the switchboard. Its coils are wound for 100 volts so that a voltage transformer is neces- sary for connecting it to the bus bars, and one voltage transformer for each machine to be synchronized. Fig. 377 is a diagram of connections for the "type A" synchro- scope for single-phase or one phase of polyphase circuits for voltages of 500 and over. CIRCUIT=INTERRUPTINQ DEVICES Devices whose sole function is the opening of an electric circuit may be classified into: fuses, switches, and circuit breakers. In the design of any circuit-interrupting device four general require- ments are involved: (a) To provide means for carrying the rated current of the device without excessive drop of voltage or heating, and also such overloads as will occur in practice. (b) To insulate all live parts for the maximum voltage. (c) To provide mechanical means for opening the circuit. (d) To prevent or make harmless any arc that may form. Circuit-interrupting devices may be automatic or non-auto- matic. In general, fuses are always automatic, switches always non-automatic, and circuit breakers either one or the other. In general, circuit breakers are so arranged that a spring or gravity tends to open them, and they are held closed by latches or toggle mechanisms It is this feature which makes the difference between a switch and a non-automatic circuit breaker. Fuses. A fuse is the simplest and cheapest circuit-breaking device. It consists of a wire or link of fusible metal (usually of lead alloyed with zinc and tin) connected electrically in series with the circuit to be protected. When more than the predetermined current passes through it, the heat generated in it due to its resistance is sufficient to melt it, and the fuse is said to "blow", thus opening the circuit. The open or exposed type of wire- or link-fuse is rarely used 436 ALTERNATING-CURRENT MACHINERY on account of its tendency to scatter molten metal and increase the fire hazard. Moreover, open link fuses cannot be accurately rated on account of the exposure and variable length of the fusible strip. It was to overcome these objections that the enclosed type of fuse was developed. The latter type of fuse* consists of a fusible wire or link enclosed within a fiber tube fitted with a fireproof material to exclude the air and suppress the arc formed when the fuse em- bedded in this material melts and opens the circuit. Suitable ter- minals are provided so that the fuse may be mounted in a fuse block. Enclosed fuses have been standardized into classes according to the voltage and ampere capacity, and are rated so that they will carry ten per cent overload indefinitely, and will open at twenty-five pfcr cent overload. A very important difference between the fuse and the circuit breaker is the matter of time element. Thus the over-load circuit breaker depends for its operation on the value of the current only, whereas the blowing of a fuse depends not only on the current but also on the time it flows. That is, the breaker will open immediately atTany load in excess of that for which it is set, and will not operate at any smaller current, no matter how long it may continue. Stand- ard fuses, on the other hand, will in time operate at an overload of only 25 per cent, and will open in a proportionately shorter time with greater over loads. Certain conditions, therefore, require the use of fuses, while others are better met by breakers, or by a combination of the two. - . - Air=Break Switches. Special styles of switches are required in many cases in alternating-current work on account of the excessively high voltages, especially when the switch has to handle large currents at high voltage. Mention has been made on page 421 of the use of a marble slab between the blades and contact points of a double-pole switch to prevent the arcs, formed when the switch is opened, from flashing across and short-circuiting the points of the switch which are con- nected to the generator terminals. Such a switch with marble barrier is shown in Fig. 364. Arcing is reduced to a minimum in every case by opening the switch very quickly, and all of the'special alternating-current switches Evatem^if/^K 8 ^ 11 type f on U8e '- much used for medi u m voltages in alternating-current systems, is described on page 305, m connection with transformer fuse blocks. ALTERNATING-CURRENT MACHINERY 437 described below, are arranged to open with a snap,, even though the controlling lever is moved slowly. Fig. 378 shows a common form of knife switch specially con- structed to give a quick break. The switch blade is in two parts a and b, to one of which, a, is attached the operating handle. The two parts a and b of the blade are connected by strong helical springs Fig. 378. "Quick Break" Knife Switches s s, one on each side of the blade. When the handle is pulled forward to open the switch, blade a leaves clip c, while the springs are put in tension more and more until the switch blade b is pulled from the clip c, with a snap, thus causing a sudden breaking of the circuit. Fig. 378 shows two single-pole quick-break switches. The one on the left, designed for 600 amperes, is provided with two massive tubular terminals 1 1, into which the terminals of the main circuit are inserted and soldered. The switch on the right, designed to carry 3,600 amperes, is furnished with massive studs d d, which project through holes drilled in the switchboard to the back of the board. 438 ALTERNATING-CURRENT MACHINERY Connection to these studs is made by clamping the terminals of the main circuit to them by means of the threaded nuts n n. On account of the large current^to be carried, extra large contact surfaces are provided between switch blades and clips, as seen in the figure. Damage to switch-blades and clips, due to arcing, is reduced to a minimum by providing carbon blocks which make a connection in parallel with the switch connection proper. When the switch is opened, the metal connection is broken first, and the carbon connec- tions afterwards, so that the arcing always takes place between the carbon blocks. Fig. 379. Half Section of Over-Load Type of Circuit Breaker Closed Circuit Breakers. In Fig. 379 there is shown in elevation and half section a plain overload type of circuit breaker for either direct- .or alternating-current circuits up to 600 volts. Fig. 380 is a general view of the same single pole breaker as manufactured by the Rclkr- Smith Company. Fig. 379 shows the breaker closed and Fig. 380, open. Referring to Fig. 379, A is a rectangular magnet core journaled on the cylindrical shaft shown, and supported by it between two non-magnetic supporting frames, only one of which, B, is shown in the sectional view. To A is secured one of the terminals of the main ALTERNATING-CURRENT MACHINERY 439 coil C, which is made up of a number of hard-rolled copper strips; and also the arm D. The other terminal of the laminated winding is secured to the lower stud P. Current entering through the stud P passes through the main windings C into the arm D, through the contact plate E into the stationary copper brush F, and finally out through the upper stud Q. When the current through the breakei exceeds the predetermined value for which it is adjusted, the attrac- tion exerted by the core A on the ends K of the pivoted armature causes it to rise with increasing speed until the finger M, a part of the armature, strikes R. The roller H is thus forced downward past the roller G, thus permitting the strong outward pressure of the brush F and that of the coil C to throw the arm outward and thus break the circuit, first be- tween the brush fingers and the contact plate E, and finally be- tween the carbons S and F. It will be noted that the carbon block F is mounted on a spring support and thus maintains its contact with S after the main contacts between the copper brush F and plate E have been broken. This action effectually prevents arcing and pitting of the main contacts which would soon wear out the breaker. To reset the breaker, the handle which the act of opening has raised, is pulled down, thus forcing the roller H up past the roller G, which compresses the brush F and the coil C, by forcing the arm back into its initial position. \^_ These breakers are also made with under-load, no-voltage, and shunt-trip attachments, and may be operated from any distant point if desired. The same methods are available for suppressing the arc be- tween the break points of a switch as for suppressing the arc which tends to maintain itself between the terminals of a fuse link. when Fig. 380. General View of Roller-Smith' Single-Pole Breaker Open 440 ALTERNATING-CURRENT MACHINERY the link fuses. Thus some of the older switches were designed to break the circuit between the end of a movable copper rod and a sta- tionary copper socket, both of which are surrounded by a porcelain tube, thus utilizing the expulsion principle for suppressing the arc. The most effective method, however, for suppressing the arc* on a high-voltage switch is to design the switch so as to open quickly under oil, and for very high voltages to provide for several simul- taneous breaks in series. Fig. 381. General Electric Oil Break Switch Mounted on Panel Fig. 382. General Electric Oil Break Switch Rear View Switch Open Oil=Break Switches. Air-break switches are not now considered reliable or safe for alternating-current circuits above about 600 volts, hence the adoption of oil-break switches for practically all alter- nating-current circuits of 440 volts and over is today almost universal. Figs. 381 and 382 are views of a typical General Electric oil- break switch, mounted on a panel. The six insulators which support ALTERNATING-CURRENT MACHINERY 441 the stationary contacts and studs each consist of a single piece of porcelain, and can be easily detached from the frame when necessary. The contact fingers are flared, are of copper, and are fastened to the studs by heavy springs which insure good contact with the moving blades. The oil tanks are of sheet metal lined with maple, and are de- signed to be easily removable for inspection of switch contacts and oil. These oil switches are designed with operating mechanism mounted directly on switchboard panel and the oil switch proper mounted : Fig. 383. Westinghouse Three-Pole Oil Circuit Breaker "Remote Control" (a) directly on panel; (b) on pipe framework directly back of panel; (c) on pipe framework remote from panel; or (d) on flat surfaces or in cells remote from panel. These switches are made single-throw with one, two, three, or four poles; and for voltages of 2,500 up to 15,000, and with current capacities ranging from 1,000 to 300 amperes, respectively. Any of these oil switches may have operating mechanisms which are either non-automatic, or automatic with one, two, or three overload-coil attachments. Fig. 381 shows front view of the switch closed, and Fig. 382 back view of the switch open. 442 ALTERNATING-CURRENT MACHINERY Great improvement has been made in recent years in the design of large circuit breakers for protecting alternating-current circuits up to 110,000 volts and over. Fig. 383 shows a Westinghouse three- pole oil circuit-breaker for 44,000-volt circuits. Its capacity is 300 amperes. It is hand-operated and illustrates what is called "remote- control". This type of breaker may also be electrically operated Fig. 384. "Type GA" Westinghouse Oil Circuit Breaker Removed from Tank Showing Open and Closed Positions by means of suitable solenoids or coils energized by direct current under the control of the operator. They may also be made automatic in action by the addition of suitable relay attachments. Fig. 384 shows a single-pole of the "type GA" Westinghouse oil circuit breaker removed from its tank, in both the open and closed positions. The upper or fixed contacts are firmly secured to the lower end of the leads, which are clamped by iron collars, which in ALTERNATING-CURRENT MACHINERY 443 turn are bolted to the tank cover through which the leads pass. This upper contact consists of a circular piece of brass of greater area than the moving contact, which insures the entire surface of the latter bearing upon the stationary contact, the object being to eliminate the necessity of accurately centering the contacts one upon the other. The lower or movable contacts are carried by a heavy metallic cross bar and consist of pieces of cylindrical brass rod backed by compression springs of sufficient strength to insure good contact and which also render the contacts self-aligning. Copper braid shunts are used to carry the current around the springs in order to pre- vent any deterioration by the pas- sage of current through them. It will thus be seen that the contact is of the simple "butt" type between two circular plane surfaces. The high voltage necessitates the unusually long, widely separated, and specially irrsulated "condenser" type of terminals shown in Fig. 384. The general type of oil switch illustrated in Fig. 383 is today the standard for high-voltage circuits carrying large alternating currents, and is used extensively in many of the most recent high-voltage gen- erating stations. . Fig. 385 is a general view of a General Electric three-pole oil switch, or rather three single-pole switches operated by an electric motor, which is controlled from the switchboard. Each single- pole switch in Fig. 385 is mounted in a separate brick compart- ment. The oil is contained in long metal cylinders in order to reduce the amount of oil to a minimum. The long brass connecting rods pass through holes in the porcelain bushings in the tops of the cylinders and the ends of these rods carry conical lugs which fit into spring sockets. These spring sockets are connected to rods which pass Fig. 385. General Electric Three-Pole Oil Switch Operated by Electric Motor 444 ALTERNATING-CURRENT MACHINERY out through porcelain bushings in the bottoms of the cylinders, and these lower rods are connected together in each compartment, thus making a double-break single-pole switch. The metal cylinders in Fig. 385 are entirely insulated from the switch points proper. Feeder or Voltage Regulators. When several feeder circuits are supplied from the same bus bars, and when it is de- sired to control the voltage on each feeder circuit independently of the others, feeder regulators are required. Single-phase feeder regulators are autotransformers with their primary coils connected across (that is, as a shunt to) the bus bars, and their secondaries connected in series with the feeder cir- cuit. They are of three types, as follows: Fig. 386. Stillwell Single-Phase VoltageRegulator Complete (a) A type in which the secondary coil has many leads brought out to points on a dial switch, so that the number of active turns on the secondary coil may be changed at will, thus permitting the adjustment of the feeder voltage to any desired value, (b) A type in which the primary and secondary coils are wound at right-angles to each other on the inner face of a laminated iron ring very much like the stator ring of an induction motor. The magnetic flux, due to the primary coil, is made to pass in whole or in part through the secon- dary coil by turning a laminated core as explained below. (c) Polyphase feeder-regulators usually consist of several single-phase regulators, one for each phase. The advantage of this arrangement is that the voltage of each phase may be controlled separately. A com- bined poly-phase feeder regulator is, however, sometimes used. (a) Stillwell Regulator. Fig. Fig. 387. Dial of Stillwell Single-Phase oo ~ . l i> o^ll Voltage Regulator 386 is a general vie.w or a btill- well single-phase voltage regula- The dial switch alone is shown in tor with its dial switch complete. ALTERNATING-CURRENT MACHINERY 445 Fig. 387. The complete internal connections are shown in Fig. 388. The primary coil of the regulator is permanently connected across the bus bars (or generator terminals). One feeder wire passes out directly from the generator and the other passes through few, or many, turns of the secondary coil of the regulator, and thence to the line. The re- versing switch A\ serves to connect the feeder / to one or the other ter- minal of the secondary coil, and the arm of the dial switch connects the line wire to any one of the taps which are brought out from the secondary coil. For one position of the reversing switch, the induced voltage in the secondary turns, which are connected in series with ' the feeder circuit, is added to the gen- erator voltage, thus raising the feeder voltage. For the other position of the reversing switch, the induced voltage in the secondary turns, which are in series with the feeder circuit, is subtracted from the gen- erator voltage, thus reducing the feeder voltage. When the arm of the dial switch touches two adjacent contact points (and it must be arranged to always touch one point before it leaves the other), the intervening turn (or turns) of the secondary coil of the regulator is short-circuited. To overcome this difficulty, the arm is made double, that is, two arms A and B move together side by side as shown in Fig. 389. These arms are shown connected to two contact points, say Id and 16. C represents a special form of choke coil consisting of two windings on one iron core. These two windings are arranged so that equal currents flowing out from A and B circulate around the core in opposite directions, so that the Fig. 388. Complete Internal Connec- tions for Stillwell Voltage Regulator Fig. 389. Diagram of Special Form of Dial Switch for Stillwell Regulator 446 ALTERNATING-CURRENT MACHINERY .Fig. 390. Diagram of Magnetic Voltage Regulator core is not magnetized, and the windings of C have no choking action. When, however, the two fingers A and B touch adjacent points of the dial switch, the turns of wire S on the reg- ulator secondary tend to send a very large current out on A, say, and back on B. These oppositely flowing currents circu- late around the core of C in the same di- rection. These currents, therefore, mag- netize the core, and the windings have in consequence a very considerable chok- ing action, the effect being to choke down oppositely flowing currents in the fingers A and B, and to allow currents in the same direction to flow freely through it. (b) Magnetic Voltage Regulator. The type of voltage regulator mentioned under (b) is sometimes called the magnetic voltage regulator. A laminated iron ring RRR.R, Fig. 390, has four large deep slots on its inner face in which the primary coil P P and the secondary coil S S are placed. A laminated core C C mounted on a spindle is arranged to be turned into any desired position by means of a hand wheel. In the posi- tion indicated by full lines, the core carries the magnetic flux due to the primary coil in one direction through the second- ary coil, and in the position indicated by dotted lines, the core carries the magnetic flux due to the primary coil in the other direction through the secondary coil. Therefore, when the core is moved slowly from position 1 to position 2, in the direction indicated by the arrows, the voltage in- duced by the secondary coil changes gradually from a full positive value to an equal negative value in its relation to the primary volt- age. That is, when the core is in position 1, the induced voltage in the Fig. 391. Magnetic Voltage Regulator in Case with_Cover Removed ALTERNATING-CURRENT MACHINERY 447 secondary coil has its greatest value of say, 100 volts which, if the coils are properly connected, is added to the bus-bar voltage E, giving a feeder voltage of E-f- 100. When the core is midway between the two positions 1 and 2, the induced voltage in the secondary coil is zero, and the feeder voltage is then equal to the bus-bar voltage E. When the core is in position 2, the induced voltage in the secondary coiLis Fig. 392. General Electric Six-Phase Induction^ Regulator Complete in Case again at a maximum of say, 100 volts, but in such a direction as to oppose the bus-bar voltage, so that the feeder voltage is E 100 volts. Fig. 391 is a view of a magnetic voltage regulator with the covering of its containing case removed. The two coils, primary and secondary, at right angles to each other, are clearly shown with their leads passing out to the connection board which occupies the compartment on the back of the case in the figure. The hand wheel 448 ALTERNATING-CURRENT MACHINERY for turning*the;iron core is also shown, A valuable feature of ! this type of regulator is that it produces a continuous variation of vol- tage, whereas the Stillwell regulator produces a step-by-step variation, (c) Induction Regulator. The combination polyphase induc- tion regulator is called the induction regulator from its similarity to the induction motor. The action of the induction regulator stated in simplest terms is as follows: A regular induction motor stator has its polyphase windings connected across the polyphase bus bars. This produces a rotating magnetism in the stator iron as explained on page 372. This rotating magnetism rotates in synchron- ism with the generator or generators which are supplying current to the bus bars. Inside of this induction motor stator (or primary) is placed a polyphase armature which does not revolve, but it is mounted Fig. 393. Laminated Core of the Stationary Member ^of an Induction Regulator on a spindle so that it may be turned through an angle of 60 or 90 by means of a hand wheel and worm gear. The rotating stator mag- netism induces polyphase electromotive forces in the windings of this polyphase armature; these polyphase electromotive forces are in synchronism with the electromotive forces between the bus bars, and the two (or more) windings of the polyphase armature are con- nected in series with the two (or more) feeder circuits (constituting of course one set of polyphase feeders) which are to be regulated. The electromotive forces in the stationary armature windings may be in phase with the bus-bar electromotive forces, in which case the regulator raises the voltage by the greatest amount of which it is ALTERNATING-CURRENT MACHINERY 449 capable. By turning the stationary armature by means of the hand wheel and worm gear, the phase difference between the bus-bar voltages and the voltages induced in the stationary armature windings may be gradually changed from coincidence of phase to opposition of phase, during which time the boosting effect of the regulator will gradually drop to zero, become negative, and reach its greatest negative value when opposition of phase is reached. Thus, if the electromotive force induced in each armature winding of the regulator is 100 volts, and if the bus-bar voltage is 1,000 (each phase), then the voltage between the feeders can be varied from 900 volts to 1,100 volts by means of the regulator. ' Fig. 392 is a general view of a six-phase induction regulator in its containing case, manufactured by the General Electric Com- pany. This particular machine has the primary windings on the movable member, whereas the armature windings are on the stationary member. It is used for control- ling the alternating-current voltages applied to the collector rings of a six-phase rotary converter. The figure shows a small direct- current motor mounted on the regulator case for turning the movable member of the regulator, this motor being controlled by the hand wheel shown in the figure. Fig. 393 shows the laminated iron core of the stationary mem- ber of an induction regulator, the stampings of which are clamped in a cast-iron shell. Fig. 394 shows the movable core. Ratings of Voltage Regulators. It was shown, page, 253 that the total amount of power delivered to service mains at a slightly in- creased voltage produced by an autotransformer is very much greater than the power actually transformed from the primary to the secondary of the transformer. In fact, the power actually transformed is equal to the increase (or decrease) of voltage mul- tiplied by the total current delivered, and the rating of the auto- transformer (which determines its size) is based upon the power transformed. Fig. 394. Movable Core of the Primary Member of an Induction Regulator 450 ALTERNATING-CURRENT MACHINERY For example, a voltage regulator is to be used for raising the voltage of 2,000- volt bus bars to a maximum of 2,100 volts, and the XR Fig. 395. Diagram of Connections for Voltmeter Compensator maximum current to be handled is 100 amperes. In this case the transformer rating of the regulator is 100 amperes at 100 volts (or 10 kilowatts), whereas the total power to be delivered to the feeders is, at its maximum, 2,100 voltsXIOO amperes or 210 kilowatts. Since a voltage regulator transforms only a small fraction of the power delivered to the feeders which it controls, the losses of power in the regulator are very small indeed. Thus the 10-kilowatt regulator might have a total loss of 300 watts, which is 3 per cent of the power actually transformed by the regulator, and only one- seventh of one per cent of the total power delivered to the feeders. Voltmeter Compensator. The voltmeter compensator is a device by means of which the voltmeter on the switchboard in a station is made to indicate the voltage between the transmission lines at some remote feeding center or receiving station. The essential principles of this instrument are made clear from Fig. 395. An alternating- current generator G delivers alternating current at voltage EQ be- Figs. 396 and 397. Vector Diagrams Showing E.M.F. Relations in the Line and in the Compensator tween its terminals. This current is transmitted over a line of which the resistance is R (including r) and the reactance is X (including ALTERNATING-CURRENT MACHINERY 451 x), and the voltage at the receiver is E. The line loss of electro- motive force is XI due to reactance, and IR due to resistance, as XR Fig. 398. Wiring Diagram for Voltmeter Compensator as Used in Practice in High Voltage Circuits explained on page 58. The relation between E , E, IR, and XI \s shown in Fig. 396. T, Fig. 395, is a transformer which supplies to the voltmeter V a voltage exactly in phase with E , and, say, one- Fig. 399. Diagram of Circuits and Apparatus in Connection with Westinghouse Compen- sated Voltmeter tenth as great. Let x be a reactance one-tenth as great as X, and r a resistance one-tenth as great as R. Then the voltage between 452 ALTERNATING-CURRENT MACHINERY the points a and b consists of two parts, rl and xl, which are in phase with, and one-tenth as great as, IR and X /, respectively. There- fore rl and xl subtracted from one-tenth E give a voltage which is exactly equal to, and in phase with, one-tenth E, as shown in Fig. 397, so that the voltmeter being acted upon by (one-tenth E-^ rl xl) gives a reading which multiplied by 10 is equal to E. In practice, it is not desirable to connect the voltmeter wires to the high-voltage mains, and the essential features of the arrange- ment shown in Fig. 395 are realized by introducing r and x in series with the secondary of a current transformer T 7 ', as shown in Fig. 398. More or less of the resistance r and of the reactance x may be included in the voltmeter circuit by means of two dial switches of which the arms are represented by d d, Fig. 398. The object of these dial switches is to enable a given compensator to be adjusted to any given transmission line. Fig. 399 shows all of the apparatus used in connection with the Westinghouse compensated voltmeter; the lines II are shown at the left, T is the step-down potential transformer, T' is the series transformer, dd is the case containing the resistance r and the reactance x, and upon which the dial switches are mounted, V is the voltmeter, -and VR is the resistance in series with the voltmeter. LIGHTNING ARRESTERS Effects of Lightning. When a lightning stroke occurs in the neighborhood of a transmission line, a sudden rush of electric cur- rent takes place over the line due to one or more of the following causes : (a) Electric charge accumulated on the line is suddenly released and tends to flow to earth. (b) The magnetic action of the lightning discharge induces a sudden rush of current in the line. (c) When the lightning discharge actually strikes the line, an enor- mous rush of current takes place over the line and to earth. When the sudden rush of current which. accompanies a light- ning discharge encounters a portion of the circuit which has con- siderable inductance, very great, electromotive forces are created, in the same way that enormous mechanical forces are created when a moving body strikes a heavy obstacle, and the insulation of the circuit, be it air or solid insulation, is likely to be broken down, or ALTERNATING-CURRENT MACHINERY 453 punctured, giving a short path to the earth for the rush of current. Thus, a sudden rush of current coming into a station over a trans- mission line, encounters the highly inductive windings of wire of a transformer, dynamo, or other apparatus. The electrical inertia (inductance) of the windings dams up the rush of current, as it were, and the current rush is almost sure to break through the insulation at the very entrance to the winding, passing from the copper wire over to any metal which is connected more or less thoroughly to earth, resulting in the coils or apparatus being burned out or badly damaged. A lightning arrester is a device for shielding a transformer, dynamo, or other piece of apparatus from the rushes of current which come into a station on an overhead line during a thunder- storm and from disturbances due to the static unbalancing of the electrical circuits. The lightning arrester consists of: (a) An inductance coil, or choke coil as it is called, for damming up more or less completely the rush of current before it reaches the dynamo or other appa- ratus. (b) A weak place, specially arranged in the in- sulation of the line, that is, a spark gap, just in front of the choke coil through which the dammed up rush of cur- rent may break, and flow harmlessly to earth. (c) A conducting path to earth as straight and direct as possible, and a good earth connection. Turns and bends in the conducting wire connected to earth are to be avoided, inasmuch as they introduce inductance which tends to check the quick flow of current to earth. (d) A device for extinguishing the electric arc which may be main- tained across the spark gap by the regular line current itself, after the rush of current from the lightning discharge has passed and gone. The choke coil of a lightning arrester must be very highly in- sulated. Two types of choke coil are commonly used. Fig. 400 shows a cylindrical choke coil mounted on a marble base, and con- sisting of three layers of large and very highly insulated wire. The Fig. 400. Choke Coil Mounted on Marble Base 454 ALTERNATING-CURRENT MACHINERY terminals of the coil are at its two ends. Fig. 401 shows a choke coil consisting of an insulated copper strip wound like a roll of tape. One terminal of this choke coil is on the outer edge and the other is at the center. The figure shows the method of supporting the coil on a special insulator carried on a bracket. Iron cores do not add to the damming action of a choke coil in the case of the excessively quick rushes of current which accompany lightning discharges, for the reason that the iron does not have time to become magnetized. Therefore, lightning arrester choke coils are always made without iron cores. Multi=Gap Non=Arcing Arrester. The spark gap of alternating-current lightning arresters is usually made between blocks of metal containing zinc or cadmium. A. J. Wurtz made the discovery that it takes a very high voltage to maintain an alter- nating-current arc between metal elec- trodes which contain zinc or cadmium. The behavior of these alloys is somewhat analogous to that of the mercury-vapor rectifier described on page 321 in that reversal of the cur- rent flow which makes the mercury an anode requires a greatly increased volt- age. If this is not supplied the flow could not be maintained. Thus an al- ternating electromotive force of a fre- quency of 60 cycles per second, and 500 volts (effective value), cannot main- tain an arc across a &-inch air gap between massive blocks of brass. This non^arcing property of certain alloys is made use of in the multi-gap arrester of Fig. 402. westinghouse Multi-Gap the Westinghouse Electric Company. Lightning Arrester . . shown in Fig. 402. It consists of seven independent knurled brass cylinders supported by two overhanging Fig. 401. Choke Coil made of In sulated Copper Strip ALERNATING-CURRENT MACHINERY 455 porcelain blocks forming a unit which is mounted in a weatherproof cast-iron case. The two end cylinders are connected, respectively, to the two line wires and the center cylinder is connected to ground. This arrester may also be used single pole, one for each side of a two-wire cir- cuit. Connections for single-phase and for three-phase circuits are given in Fig. 403. For two-phase circuits each phase is to be con- nected as in the single-phase diagram. This type of arrester is suited for use within a radius of three miles from the source of power on systems of 200-kw. capacity or less, and up to 400 kilowatts when connected more than three miles away from the power source. 5/rtGLE PHASE CESS THAM 12 50 -VOLTS S/MGL E PHASE 1Z5O TO 25OO. VOLTS THREE PHASE THAM /25O VOLTS THREE PHASE 1250 TO 2500 VOLTS Fig. 403. Lightning Arrester Connections for Single-Phase and Three- Phase Circuits This type of arrester is not recommended for protecting circuits liable to low power factors, nor for 25-cycle circuits, for experience has shown that under these conditions the arcing cannot be suppressed. Multi=Path Arrester. The multi-path arrester of, .the Westing- house Company is single pole and adapted to either alternating- or direct-current circuits of any voltage up to 1,000. It can be mounted upon a pole. It consists of a spark gap in series with a high resist- ance block of carborundum. This block offers a very high resistance under normal conditions and moderate voltages, but its resistance to a static discharge is very low, and it readily conducts a lightning 456 ALTERNATING-CURRENT MACHINERY discharge to ground. After the discharge has passed, it resumes its normal high resistance, thus preventing the line voltage from main- taining an arc. Fig. 404 is a view of the arrester in the lower half of its iron casing, the upper half being shown removed. Electrolytic=Cell Lightning Arrester. This type of arrester, which has proved highly effective, is widely used. A single cell con- sists of two aluminum plates and an electrolyte, such as ammonium phosphate, which form a condenser that will stand about 340 volts alternating before breaking down. Up to this critical value of the voltage, which varies somewhat with the electrolyte used, a very small current passes through the cell, but as soon as the critical voltage Fig. 404. Westinghouse Multi-Path Lightning Arrester Upper Half of Case Removed is exceeded, the current increases very rapidly with even a slight increase in the voltage. As soon, however, as the voltage drops below the critical value, the resistance of the cell increases greatly. In applying this interesting property of the aluminum cell to the lightning arrester, a large number of cells are made up in series and mounted in standard size units to be connected in series, the number of units chosen depending upon the voltage of the lines to be protected, allowing on the average about 300 volts per cell. Fig. 405 shows a cross-section of an aluminum cell lightning arrester as made by the General Electric Company. It consists of ALTERNATING-CURRENT MACHINERY 457 TO HORN CAP a series of concentric inverted cones of aluminum placed one above the other with a vertical spacing of about 0.3 inch. The electrolyte is poured into the cones and partly fills the space between adjacent ones. The pile of cones with the electrolyte between them is then immersed in a tank of oil, which helps to insulate cones from each other except for the electrolyte, and also prevents evaporation of the solution. A cylinder of insulating fiber concentric with the stack of cones is placed between the latter and the steel tank. This arrange- ment assists the free circulation of the oil and increases the insulation between tank and aluminum cones. A stack of cones is used for each phase, and up to 7,250 volts all the stacks are placed in a single tank. Where the line voltage exceeds 7,250, each stack of cones is placed in a separate tank. The connection of the stacks to the line wires through spark gaps and to the ground is different, depending upon whether the circuits have their neutral point purposely grounded or not, and upon the line volt- age. The top cone of each stack is not directly connected between a line wire and the ground, because at normal line voltage some current would flow contin- Fig. 405. Cross-Section of Aiumi- . . . . . . ,. , num Cell Lightning Arrester uously through the aluminum cells, which would heat them and evaporate the electrolyte. To prevent this flow of current, the aluminum stacks are in practice connected to the line wires through spark gaps adjusted for a sparking voltage just above that of the normal line voltage. Under normal conditions no current passes from the line wires through the aluminum cells, but a lightning discharge with its accompanying high voltage easily jumps the spark gap and passes through the aluminum cells to ground. As soon, however, as the discharge is over, the resistance of the spark gap in series with the aluminum plates is too large to permit the line voltage to maintain an arc. Fig. 406 is a view showing a typical installation of an aluminum lightning arrester for a 6,600-volt, three-phase, non-grounded neutral 458 ALTERNATING-CURRENT MACHINERY circuit. As shown the three stacks of cones, one for each phase, are installed in one tank which is placed on a rack insulated from the ground. Two of the stacks are connected directly to two of the Fig. 406. Typical Installation for Aluminum Lightning Arrester on Three-Phase Non-Grounded Neutral Circuit three line wires, each through a "horn-gap"; the third is connected to the remaining line wire through a "transfer device" or rotating switch, and a horn gap. The fourth stack is connected to the ground through the transfer device, which may be explained as follows : When the stacks of aluminum cones have been disconnected from the line wires for a certain length of time, the insulating film ALTERNATING-CURRENT MACHINERY 459 on the cones deteriorates, and experience has shown that in order to keep the aluminum cells in good working order, they must be connected to the line wires and charged at certain definite intervals. In some cases this type of arrestor must be charged daily, and in others weekly. The charging process consists simply of short-circuit- ing the spark gaps between the stacks and the line wires for a few moments. The transfer device provides a means of interchanging the ground stack with one of the line stacks during the charging operation so that tne insulating films of all the cells may be reformed to an equal extent. For arresters up to 27,000 volts, the device is mounted with three insulators on the pipe frame work and is operated by a hand- wheel, as shown in Fig. 406. Combination of a Condenser with a Choke Coil. The choking action of a coil of wire is due to its electrical inertia or inductance, CHOKE COIL TO LINE b ///J^V?\\\ b TO APPARATUS GAPS III GROUND GROUND Fig. 407. Combination of Condenser and Choke Coil as Lightning Arrester and the shielding of the apparatus behind a choke coil from severe electrical strains is precisely analogous to the following mechanical arrangement : A wall is shielded from the severe strains due to a hammer blow by allowing the hammer to strike against a very heavy ball of iron which rests against the wall. The completeness with which this heavy iron ball shields the wall depends upon the interposing of an elastic substance between the ball and the wall. The ball alone cannot shield the wall unless the wall itself is slightly elastic. In the same way the completeness with which a choke coil shields the dynamo and other apparatus depends upon the presence of some electro-elasticity*, or capacity in the circuit behind the choke coil. In some cases the circuit behind the choke coil has suf- *See 460 ALTERNATING-CURRENT MACHINERY ficient capacity for this purpose; it is always best, however, to con- nect a condenser behind the choke coil as shown in Fig. 407. The choke coil and condenser are immersed in an oil tank when installed. From Fig. 407 it is seen that three wires bbb con- nect to choke coil and condenser. This combination of choke coil and condenser is called a static interrupter by the Westinghouse Company. INSTRUCTIONS FOR INSTALLING LIGHTNING ARRESTERS Location. As regards the location of lightning arresters, electric plants may be divided into two groups: (a) Plants in which the individual pieces of apparatus such as trans- formers, motors, arc lights, etc., are many in number and widely scattered. In these cases lightning arresters should be ocated for the purpose of pro- tecting the whole line. They should be located at a number of points, more numerous on the parts of the line particularly exposed, and fewer in number on the parts that are naturally protected, especially those parts shielded by tall buildings or numerous trees. No definite statement can be made as to the number of arresters needed per mile, as the requirements of different cases vary widely. Under average conditions no point of the circuit should be more than 1,000 feet from an arrester. It is not usual to find distributed apparatus on circuits of over 2,500 volts, but when such cases occur, a light- ning arrester should be placed as near as possible to each piece of apparatus. (b) Plants in which the apparatus is located at a few definite points in the system, as in a high-voltage transmission line. In such cases the ar- resters should in general be lecated to protect especially those points where apparatus is situated, that is, should be placed with the object of protecting the apparatus rather than the line as a whole. The lightning arrester should always be so connected that in passing from the line to the apparatus the arrester is reached first, the choke coil second, and the condenser, if one is used, third. Insulation. A lightning arrester is naturally exposed to severe volt- age strains and, therefore, all active parts must be well insulated. On ar- resters for low voltages it is not a difficult matter to secure proper insulation, as the construction of the arrester itself affords protection. On high-voltage arresters, however, proper insulation is a more difficult matter. All arresters are marble or porcelain mounted, the marble or porcelain serving as an insu- lating support for the arrester. On circuits exceeding 6,000 volts, to obtain further insulation, the marble or porcelain bases should be mounted on wooden supports, well dried and shellaced. For 12,000 volts and above, the bases or panels should receive additional insulation in the form of porcelain or glass insulators used as supports. Two high-voltage arresters attached to different line wires should not be placed side by side without either a barrier or a considerable space between them. Grounds. Too much importance cannot be attached to the making of proper ground connections which should be as short and straight as pos- ALTERNATING-CURRENT MACHINERY 461 sible. A poor ground connection will render ineffective every effort made with choke coils and lightning arresters to divert static electricity into the earth. It is important, therefore, to not only construct a good ground con- nection, but to maintain it so. A good ground connection for a bank of station arresters may be made as recommended by the Westinghouse Company in the following manner: First, dig a hole 4 feet square as near the arrester as possible until perma- nently damp earth has been reached; second, cover the bottom of this hole with crushed charcoal (about pea size); third, over this lay 10 square feet of tinned copper plate; fourth, solder the ground wire, preferably No. copper, securely across the entire surface of the ground plate; fifth, cover the ground plate with crushed charcoal; and sixth, fill the hole with earth, using running water to settle. The above method of making a ground connection is simple, and has been found to give excellent results, and yet, if not made in proper soil, it will prove of little value. Where a mountain stream is conveniently near it is not uncommon to throw the ground plate into the bed of the stream. This, however, makes a poor ground connection, owing to the high resistance of the pure water and the rocky bottom of the stream. Clay, even when wet, rock, sand, gravel, dry earth, and pure water are not suitable materials in which to bury the ground plate of a bank of lightning arresters. Rich soil is the best. It is, therefore, advisable, before installing a bank of choke coils and lightning arresters, to select the best possible site for the lightning arrester installation with reference to a good ground connection. This may often be at some little distance from the station, in which case it is, of course, necessary to construct a lightning-arrester house. Where permanent damp- ness cannot be reached it is recommended that water be supplied to the ground through a pipe from some convenient source. Where possible, a direct connection to an underground pipe system, especially of a town or city water main, furnishes an exce]Jent ground on ac- count of the great surface of pipe in contact with the earth and the numer- ous alternative paths for the discharge. In a water-power plant the ground should always include a connection to the pipe line or penstock and to the case or frame of the apparatus to be protected. An effective and easily made ground may be effected by using a large, old iron casting, like an old car wheel, fitted with a riveted copper strap and buried in damp earth. A few pounds of common salt thrown around any ground terminal before covering helps to maintain dampness. Inspection. As the effectiveness of every arrester is of great impor- tance, they should be inspected from time to time and the resistances and earth connection tested for open circuit. 462 ALTERNATING-CURRENT MACHINERY APPENDIX PARALLEL OPERATING OF ALTERNATORS Necessary Conditions. In the parallel or multiple operation of alternators, it is necessary that they be similar in three respects in order to Insure their working together properly when connected in parallel,* viz, frequency, phase, and voltage. (a) Frequency. Two generators are of the same frequency when the numbers of alternations or reversals of their electro- motive forces in a given time are equal. This requirement is fulfilled when the product of the number of poles by the revolutions per minute is the same for each machine. The frequency of a generator, being dependent upon its speed, may be controlled by the regulation of the speed of its prime mover. (b) Phase. Two generators are in phase when the positions of their armatures with respect to their field poles are the same, i.e., when similar armature coils are opposite positive field poles at the same instants. When this condition exists the electromo- tive forces of the machines are both positive at the same time, and their maximum values occur at the same instant; the electromotive forces are said to be coincident in phase. (c) Voltage. Two generators are of the same voltage when the pressure me*asured across the armature terminals is the same for each machine. The voltage for a given speed being dependent upon the field strength may be controlled by means of the rheostat in the field circuit. Determination of Relative Frequency and Phase Coincidence. If two similar generators are running at exactly the same speed their difference in phase remains constant. This condition, however, does not exist in practice unless the armatures are rigidly connected, as the inevitable fluctuation in engine and water-wheel speeds and in belt slippage causes the position of the armatures with refer- ence to their field poles to be continually changing, and consequently the difference between the phases to be likewise changing. As generators should not be thrown in parallel excepting when their frequencies are practically the same, and at the time their phases , *Furthermore, two alternators, to operate satisfactorily in parallel, must have electro- motive force waves of the same shape. ALTERNATING-CURRENT MACHINERY 463 are in exact coincidence, or nearly so, it is essential to have an accurate means of determining when these conditions exist. The principle of the most common method of determining when generators are of the same frequency and are coincident in phase, is illustrated in Fig. 408. A and B represent two single- phase generators, the leads of which are connected at the switch^C,- through two series of incandescent lamps D and E. It is evident that as the relative positions of the phases of the electromotive forces change from that of exact coincidence to that of exact oppo- sition, the flow of current through the lamps varies from a mini- mum when the machines oppose each other in forcing current through the lamps as shown by the arrows in Fig. 408, to a maximum when the machines help each other in forcing current through the lamps.* If the electromotive forces of the two machines are exactly equal and in phase, the current through the lamps will be zero, and as the difference in phase increases, the lamps will light up and will in- crease in brilliancy until the maximum is reached; when the phases are in exact opposition. From this condition they will decrease in brilliancy until completely dark, indicating that the machines are again in phase. The rate of pulsations of the lamps depends upon the difference in frequency, i.e., upon the difference in the speeds of the machines, and by ad- justment of the governors of the engine or water-wheel, or the tension of the belt, the rate can generally be reduced to as low as one pulsation in ten seconds, which affords ample time for throwing the switch connecting the gen- erators in parallel. Synchronizer. When the phases of two generators coincide, the machines are said to be "in phase," "in step," or "in synchron- ism". The apparatus used for determining when generators are in phase is called a synchronizer. In Fig. 408 the lamps constitute the synchronizer. While a series of lamps, alone, may be used for syn- Fig. 408. Wiring Diagram Showing Method of Determining Coincidence in Phase and Frequency of Two Generators *When the two machines oppose each other in forcing current through the lamps they would help each other, or be in phase with each other, in producing current in the receiving circuit to which the machines are to be connected in parallel. 464 ALTERNATING-CURRENT MACHINERY chronizing machines of very low voltage, it is not safe or practical to use this method for machines of high voltage. The most common arrangement for synchronizing alternators, in general, is illustrated in the diagram, Fig. 409. A and B represent two single-phase generators with switches in the main leads. There are two trans- formers, the primaries of which are connected across the main leads of A and E, respectively, the secondaries being connected in series through the lamps E. Now, if the transformers are connected similarly in the two circuits, as shown in the diagram, then when the generators A and B are in phase, the electromotive forces in the secondaries will be in phase and no current will flow through the lamps, but when the generators are out of phase, the electro- motive forces in the secondary circuits will be out of phase also, and current will flow through the lamps. The amount of this cur- rent and the resultant brilliancy of the lamps depends on the dif- ference in phase. If the connections of either the primary or the secondary of either transformer i be now reversed from those -o-o- Trans. A 7>/wi$3QQJ B shown in Fig. 409, the indications of the lamps will be reversed; that is, when the generators are in phase the lamps will burn at maximum brilliancy, and vice versa. It is the common prac- ^ ce ^ the Westinghouse Com- pany to arrange the transformer connections so that the lamps shall be dark when the generators are in phase. In order to determine wheth- er the synchronizer lamps will be bri S ht or d ark for a given con- nection of transformers when the generators are coincident in phase, remove the main fuses or raise collector brushes from one machine, and throw in the main switches with the other generator at full voltage. Since both primaries are low connected through the switches to one machine, the lamps will be in the same condition as when the main or paralleling switches bT th d ALTERNATING-CURRENT MACHINERY 465 are open and both generators are coincident in phase. If the lamps burn brightly and it is desired that they be dark for an indication of phase coincidence, the connections of one of the primaries or of one of the secondaries of the transformers should be reversed. The lamps which are used with the synchronizer should be adapted for the highest voltage which they will receive. Thus r if they are placed upon the secondaries of two 100-volt transformers, there should be two 100-volt lamps or four 50- volt lamps in series. If two 200-volt machines have the lamps applied directly without transformers it will be necessary to use four 100-volt lamps, or their equivalent. Rate of Pulsation and Size of Pulley. The difference in speed between two machines may be determined by the rate of pulsation of the synchronizing lamps. It is sometimes convenient to know this difference in speed, especially when two generators are belt- driven from the same shaft. If the speeds are not equal, it may be necessary to turn off one of the pulleys in order to make them equal. One pulsation of the lamps, i. e., the interval between two consecutive occurrences of maximum brilliancy, indicates a gain of one cycle or two alternations of one machine over the other. Thus, if there is one pulsation of brightness per minute, and the number of alterations is 7,200 per minute, then one machine gives 7,202 alternations, while the other gives 7,200. If the number of pulsations of bright- ness is 36 per minute, then one machine gives 7,272 alternations, while the other gives 7,200 alternations, and the first machine is, therefore, running 1 per cent faster than the second machine. In order to determine which machine is running the faster, the load may be thrown upon one machine, or its belt may be slackened so as to decrease its speed. If this be done to the machine which at- tempts to run too fast, the pulsations will become less rapid; while if it be done to the machine which is running slower, the pulsations will become more rapid. If one machine is running 1 per cent faster than the other, it will be necessary to reduce the diameter of the pulley of the other (slower) machine by 1 per cent. The thickness of the belt, the tightness of the belt, the slip- page (dependent upon the kind qf belt, the condition of the sur- faces of the pulley and belt, and the load) are all factors which affect the speed and must all be kept in mind. 466 , ALTERNATING-CURRENT MACHINERY Directions for Connecting One Alternator in Parallel with Another Alternator. (1) Frequency. The speed of the new machine which is to be connected in parallel must be made such as to give the same frequency as the one already running. If the latter is carrying a load, it may be necessary to reduce the speed of the unloaded ma- chine below that at which it tends to run with no load, by adjusting the engine valve, or the water wheel gate or nozzle, or the belt slip- page, in order to secure the proper speed. The adjustment of the engine should preferably not be by throttling, as the governor is liable to "hunt" when the throttle is opened. It is desirable to be able to adjust the governor for chang- ing the speed while the engine is running. (2) Voltage. The field excitation of the new machine should be adjusted so that its voltage is the same as that on the bus bars, the measurement being made by a voltmeter. (3) Phase Coincidence. Synchronizer lamps indicate by their slow rate of pulsation that the machines are of practically equal frequency. When the synchronizer lamps indicate the proper phase relation, i.e., phase coincidence (preferably when the lamps are dark), all is ready for closing the switch. (4) Main Switch. Close the main switch a little too soon (when the machines are approaching the proper position) rather than too late (when they are receding from it). If the switch is operated by compressed air or for any other reason does not close the instant the handle is operated, due allowance must be made for the interval. (5) Equalizing Switch. If the generators are composite wound, close the equalizer switch. (6) Adjustment of Load. Adjustment may now be made by means of the governor or otherwise so that each machine receives its proper share of load. (7) Adjustment of Field Currents. The field currents of the several generators should be properly adjusted to eliminate cross currents between the armatures and maintain the proper voltage on the bus bars. Directions for Cutting Out An Alternator Which is Running in Parallel with One or More Alternators. (1) Preferably cut down the driving power until it is just ALTERNATING-CURRENT MACHINERY 467 about sufficient to run the generator at zero load. This will auto- matically reduce the load on the generator. (2) Adjust the resistance in the field winding of the machine which is to be cut out until its armature current is a minimum. (3) Open the main switch, then the equalizer switch. It is usually sufficient, however, to simply disconnect a machine from the bus bars, thereby throwing all the load suddenly on the remaining machines, without having made any special adjustments of the load or the field current. The objection to this method is that it may cause serious hunting of the remaining machines. The field circuit of the generator to be disconnected from the bus bars must not be opened before the main switch has been opened; for if the field were opened first, a heavy current would flow between the armatures. INDEX A Page A. c. circuit, fundamental equation of 32 Air-brake switches 436 All-day efficiency 265 Alternating currents, advantages and disadvantages of 4 a. c. machines, miscellaneous 6 a. c. machines, simple, construc- tion of 4 high e.m.f s, transformation of 5 Alternating-current machinery 1, 467 appendix 462 alternating into direct current, conversion of 317 alternating electromotive force and currents 1 alternators 97 armature windings 135 commercial types of machines 145 induction motor 371 measurement of power 128 measuring instruments 60 motor-generators 364 rotary converter 325 switchboard and station appli- ances 420 synchronous motors 216 transformer 233 Alternating electromotive forces and currents 1 a. c. circuit, fundamental equa- tions of 32 alternating currents, advan- tages and disadvan- tages 4 alternating e. m. f's and cur- rents 12 d. c. and a. c. calculations 10 d. c. and a. c. problems, com- parison of 7 capacity 30 condenser, capacity of 31 Page Alternating electromotive forces and currents condenser as compensator for lagging current 47 circuits in parallel 51 circuits in series 50 electrical resonance 41 electromotive force, average and effective values of 14 electromotive force losses in alternator 56 electromotive force losses in transmission lines 56 electromotive force, variations of 3 graphical representations of 12 harmonic electromotive forces . and currents 16 inductance 27 maximum and effective values, relation between 24 power, instantaneous and aver- age 15 simple alternator 1 speed and frequency, relations between 3 Alternating into direct current, con- version of 317 aluminum valve rectifier 318 mercury-vapor arc rectifier 320 motor-generators 364 rectifying commutator 317 rotary or synchronous converter 325 Alternator testing 188 alternating-current testing in general 188 armature resistance 198 characteristic curves 193 saturation 193 synchronous impedance 196 INDEX Page Alternator testing core loss and friction test 209 efficiency, calculation of 214 heat test 205 insulation testing 188 break-down 191 dielectric strength 189 resistance 189 regulation 200 American Institute rules 203 curve 202 Alternators 97 armature inductance 106 armature reaction 104 armature windings 135 commercial types 145 economy factors 179 electromotive force lost in arm- ature drop 107 field excitation 109 fundamental equation of 97 measurement of power 128 parallel operating of 462 polyphase alternators 118 single-phase system 118 three-phase system 121 two-phase system 118 regulation of 108 Aluminum valve rectifier 318 American Institute rules 184 limiting temperature rise 185 overload capacities 187 rating 184 Appendix 462 Armature drop, e. m. f. lost in 107 Armature inductance 106 Armature reaction 104 Armature windings 135 classification 135 according to construction of core 136 according to disposition of coils 139 according to form of con- ductor 141 according to progression of winding 139 according to shape of core 135 Armature windings single-phase three-phase two-phase Auto transformer B Bar winding Circuit breakers Circuit interrupting devices air-break switches circuit breakers feeder or voltage regulators fuses lightning arresters oil-break switches voltmeter compensator Commercial types of machines Page 142 143 143 251 142 438 435 436 438 444 435 452 440 450 145 revolving-armature alternators 147 revolving field alternators 154 Compensated wattmeter 79 Condensers, capacity of 31 Constant-current transformer 293 Cooling of transformer 292 self-cooling dry transformers 293 self-cooling, oil-filled trans- formers 294 water-cooled transformers 296 Core-loss and exciting-current test 310 Core type transformer 271 Current relations for rotary con- verter 336 Curtis turbo-alternator 175 Customer's bills, calculations of 95 D Dielectric, inductivity of 31 Direct- and alternating-current problems, comparison of 8 alternating current 8 direct current 7 Direct-current voltage, control of 349 E Economy factors in alternators 179 alternator testing 188 INDEX Page Economy factors in alternators conditions affecting cost 179 frequency 180 regulation 180 speed 179 voltage 179 efficiency 181 power losses 181 rating and overload capacities 183 Efficiency of alternator 181 influence of power factor upon output 183 practical and ultimate limits of output 182 Efficiency calculation 314 Electrical resonance 41 multiplication of current by 46 multiplication of e. m. f. by 44 Electrodynamometer 72 used as an ammeter 73 used as a voltmeter 74 Electrolytic-cell lightning arrester 456 Electromagnetic ammeters and volt- meters 67 Roller-Smith repulsion ammeter 70 Thompson inclined-coil meter 70 Electromotive force, average and effective values of 14 Electromotive force relations for rotary converter 336 Electromotive force, variations of cycle 3 frequency 3 period 3 Electrostatic ground detector 67 Electrostatic voltmeter 64 Feeder panels 430 Feeder or voltage regulators 444 induction 448 magnetic 446 ratings of 449 Stillwell 444 Field excitation 109 Field excitation and power factor 226 Field excitation of rotary converter 353 Form factor 15 Fort Wayne single-phase alternator 147 Four-ring rotary converter 328 Frequency changes 397 Fuse blocks transformer 305 Fuses 435 General Electric rotary converter 343 General Electric three-phase alter- nator 152 H Harmonic electromotive forces and currents 16 addition of 21 algebraic representation 19 clock diagram representation 17 graphical representation 18 phase difference 19 subtraction of 24 synchronism 19 Heat run 361 Heat test 308 Hot-wire ammeter and voltmeter 60 Hunting action 223 Hunting of rotary converter 345 I Impedance 311 Indicating instruments 60 elect rodynamometers 72 electromagnetic ammeters and voltmeters 67 electrostatic voltmeter 64 hot-wire ammeter and voltmeter 60 induction instruments 75 wattmeter 77 Inductance 27 formulas for 29 series and parallel 30 Inductance generator 397 Induction instruments 75 Induction motor 371 action of 375 behavior at starting and in operation 385 efficiency and rotor-resistance 379 efficiency and speed 376 INDEX Page Induction motor action of mechanical to electrical en- ergy in rotor, ratio of 377 rotor for constant and vari- able speed 380 rotor voltages to stator volt- ages, ratio of 378 rotor windings, starting resis- tance in 376 torque and speed 375 typical induction motor, struc- tural details of 380 constructive elements 371 frequency changer 397 induction generator 397 installations 389 single phase 393 hand starting 393 repulsion motor starting 395 split-phase starting 394 stator windings and their actions 372 tests 406 break-down 408 core loss 410 efficiency 413 heat 406 impedance 412 performance curves of 416 slip 414 starting torque 409 Induction watt-hour meter 85 meter friction, compensation for 91 moving element 89 polyphase 93 power factor adjustment 89 single-phase 86 Inductivity of dielectric 31 Integrating instruments 82 customer's bill, calculation of 95 induction watt-hour meter 85 sparks gauge 95 Thomson watt-hour meter 82 watt-hour meter dials 93 Inverted rotaries 347 Iron losses 262 J Joule's law 7 K Kirchoff s law Page 7 Lightning arresters 452 condenser with choke coil, com- bination of 459 electrolytic-cell 456 installing, instructions for 460 ground 460 inspection 461 insulation 460 location 460 lightning, effects of 452 multi-gap, non-arcing 454 multi-path 455 Lincoln synchronizer . 433 M Measuring instruments 60 indicating 60 integrating 82 recording 60 Mercury-vapor arc rectifier 320 Motor generators 364 comparisons of with rotary con- verter 365 types 367 uses of 366 Motor testing 229 break-down test 231 phase characteristic 229 pulsation test 230 self -starting test 231 Multi-gap, non-arcing lightning arrester 454 Multi-path lightning arrester 455 Multipolar rotary converter 346 Ohm's law 7 Oil-break switches 440 Oscillators for rotary converters 342 Outdoor transformers, mounting of 308 Parallel operating of alternators 462 connecting one alternator in par- allel with another 466 equalizing switch 466 INDEX Page Parallel operating of alternators connecting one alternator in par- allel with another field currents, adjustment of 466 frequency 466 load, adjustment of 466 main switch 466 phase coincidence 466 voltage 466 cutting out alternator 466 necessary conditions 462 frequency 462 phase 462 voltage 462 relative frequency and ohase coincidence 462 pulsation and size of pulley 465 synchronizer 463 Phase difference 19 Phase transformer 257 Polarity test 315 Portable torsion wattmeter 80 Power, expression for 26 Power, measurement of 128 balanced systems 129 three- wire three-phase 131 three-wire two-phase 129 unbalanced systems 131 four- wire two-phase 131 six- wire three-phase 132 three-wire three-phase 133 three-wire two-phase 132 Power factor, influence of upon out- put 183 Power law 7 Power losses 181 R Rating and overload capacities 183 American Institute rules 184 limiting temperature rise 185 overload capacities 187 rating 184 Rating of transformers 267 Rectifying commutator 317 Regulation 312 Regulation of alternator 108 Resistance of coils 309 Page Revolving-armature alternators 147 field structure for Westinghouse 180-kw. 152 Fort Wayne single-phase 147 General Electric three-phase 152 Westinghouse armature with dis- tributed winding 151 Westinghouse uni-coil armature 150 Revolving-field alternators 154 construction 151 armature 155 armature coil 157 bed plate and bearing pedestal 1 56 excitation 163 field coils 163 frame 1 54 rotating field 159 supporting ring 158 terminals 159 steam-turbine-driver 169 water-wheel-driver Ifi5 Roller-Smith repulsion ammeter 70 Rotary converters in practice 3118 characteristic types of 343 direct-current voltage, control of 349 with Edison three-wire system 355 field excitation 353 hunting of 345 inverted 347 oscillators for 342 six-phase 357 starting of 339 transformer connections for uses of 338 Rotary or synchronous converter 325 comparison with direct-current dynamo 326 current relations for 336 direct-current dynamo into a rotary converter 327 e. m. f. relations for 330 four-ring 329 multipolar 330 in practice 338 six-ring 330 testing of 361 heat run 361 standard 300 INDEX Page Rotary or synchronous converter three-ring 329 Saturation curve ' 193 Scott transformer 259 Series or current transformer 297 Shell type transformer 283 Simple alternator 1 Single-phase alternator 118 Single-phase windings 142 Six-phase rotary converter 357 Six-ring rotary converter 330 Spark gauge 95 Speed and frequency, relations be- tween 3 Speed-limiting devices 348 Steam-turbine-driven alternators 169 advantages 170 Curtis turbo-alternator 175 rotor 171 stator 171 Strap winding 142 Synchronism 19 Synchronous impedance curve 196 Synchronous motor and induction motor, comparison of 401 Synchronous motors 216 advantages 218 compared with d. c. motors 219 disadvantages 219 field excitation and power factor 226 hunting action 223 motor testing 229 starting motor 220 exciter 220 polyphase 220 polyphase type, self-starting of 221 separate starting 221 single-phase circuit 220 torque and power output 225 use as a condenser 227 Switchboard apparatus 433 circuit interrupting devices 435 lightning arresters 452 Lincoln synchronizer 433 Switchboards 420 Switchboards feeder panels high voltage panels polyphase typical single-phase Page 430 433 425 422 Tables capacities of standard transform- ers 419 constant-current transformer data 300 A and Y connection data in mains 127 A and Y connection data in re- ceiving circuits 128 inductivities of dielectrics 31 power ratings of rotary conver- ters in kilowatts 327 size and cost of copper wire two-wire system 5 sparking distances for various voltages 192 transformer efficiencies, losses, etc. 263 turbine speeds for alternators 170 voltage ratios of rotary conver- ters 335 Thomson inclined coil meter 70 Thomson watt-hour meter 82 Three-phase alternator 321 A-connected armatures 125 current relations 125 electromotive force relations 125 receiving circuits 126 dissimilar circuits (unbal- anced system) 126 similar circuits (balanced sys- tem) 126 Y-connected armatures 124 current relations 125 electromotive force relations 124 Three-phase transformers 289 Three-phase windings 143 Three-ring rotary converter 329 Torque and power output 225 Transformer 235 automatic action of 238 INDEX Page Transformer coil resistance and magnetic leakage 242 commercial types of 270 constant current 299 fuse blocks 305 cooling of 292 core 271 series or current 297 shell 283 three-phase 280 connections 242 auto-transformer 251 auto step-down trans- formation 252 auto step-up transforma- tion 252 Current relations 253 parallel - constant - voltage transformers 242 banking of 247 Edison three-wire system single-phase 246 multi-coil type 243 series-current transformers 249 description 233 ideal action graphically repre- sented 239 ideal and practical 206 maximum core flux 237 physical action 234 current relations 235 electromotive force relations 234 with load 234 without load 234 polyphase systems 254 with compound magnetic circuits 257 Page Transformer polyphase systems phase transformer 257 three-phase 255 two-phase 255 practical consideration 262 efficiency 263 rating of 267 regulation 266 transformer losses 262 regulation 241 with highly inductive load 241 with non-inductive load 241 tests 308 core-loss and exciting-current 310 efficiency calculations 314 heat 308 impedance 311 polarity 315 regulation 313 resistance of coils 310 Transformer connections for rotary converters 357 Two-phase alternator 118 Two-phase windings 143 Voltmeter compensator W 450 Water-wheel-driven alternators 165 Watt-hour meter dials 93 Wattmeter 77 Westinghouse armature with dis- tributed winding 151 Westinghouse uni-coil armature 150 Wire winding 141 " THE NOV D UNIVERSITY OF CALIFORNIA LIBRARY