ELECTRICAL EQUIPMENT McGraw-Hill BookContpany Electrical World The Engineering and Mining Journal Engineering Record Engineering News Railway A^e G azottx? American Machinist Signal knginoer American Engineer Electric Railway Journal Coal Age Metallurgical and Chemical Engineering Power ELECTRICAL EQUIPMENT ITS SELECTION AND ARRANGEMENT WITH SPECIAL REFERENCE TO FACTORIES, SHOPS AND INDUSTRIAL PLANTS BY HAROLD W. BROWN, B. S., M. M. E. DEPARTMENT OF ELECTRICAL ENGINEERING, CORNELL UNIVERSITY FIRST EDITION McGRAW-HILL BOOK COMPANY, INC. 239 WEST 39TH STREET. NEW YORK LONDON: HILL PUBLISHING CO., LTD. 6 & 8 BOUVERIE ST., E. C. 1917 COPYRIGHT, 1917, BY THE MCGRAW-HILL BOOK COMPANY, INC. *' -; *,- . ; ..,. 'OVZ* J ; *' THE MAPLE PRESS YORK PA INTRODUCTION A working knowledge of electrical engineering is becoming of greater importance every day as the applications of electrical apparatus increase. The usual course of instruction given to mechanical engineers includes a discussion of the theory of opera- tion of the various electrical machines but goes no further. It is often said in criticism of college instruction that the student learns more of engineering in the first three months of practice than he does during his whole college career. What he really does when he begins his practice is to work on a limited number of problems, to the solution of which he has to bring the sum total of his experience; and thus he gradually gains confidence in himself. The mechanical engineers at Cornell University cover the usual course on the principles of electrical engineering during their junior year, and it was considered desirable that the work of the senior year include one or two pretentious projects, consist- ing of the selection and arrangement of all the electrical equip- ment for an industrial plant, such as a machine shop or a cement plant problems that require for their solution a comprehensive rather than a detailed knowledge of electrical apparatus. It was soon found, however, that the student lacked the broad point of view that comes from practice. He knew for example that direct-current motors were good for speed adjustment and that alternating-current motors were essentially constant-speed ma- chines, but he was not able to reach the conclusion that there- fore direct-current supply was desirable for machine shops with many variable-speed tools. The set of notes prepared by Mr. Brown to guide the student in his work contained so much useful information not to be found in books that it was considered desirable to offer them to the engineering profession. There are innumerable books for wire- men and for the shop mechanic, but there was no book on the market written specially to guide the mechanical engineer in the selection of his electrical equipment. While it was considered not only advisable but necessary to give extensive references to the literature of the subject, it 349467 vi INTRODUCTION was recognized that the mechanical engineer would not have an extensive library of electrical texts, nor would he be familiar with the electrical periodicals. It was therefore decided that, except in rare cases, the references would not go beyond one of the standard texts in addition to the electrical handbooks. If the student can be trained to use the matter contained in the handbooks in an intelligent manner, and also to check up his theory from a reliable text, he will approach the dreaded electrical problems with great confidence. Our experience with this course at Cornell has given us great satisfaction. ALEXANDER GRAY, Head of Electrical Engineering Department, Cornell University. PREFACE A great mass of electrical data is now available in the various handbooks, and other technical literature. The engineer must not only have such data at hand he must know how to use them. The purpose of this book is to show how to apply the available data, and the principles laid down in textbooks, to the equipping of shops, factories and industrial plants. Numerical examples are worked out illustrating these applications; and in addition a progressive series of problems is placed at the end of the book. Both the text and the problems are drawn largely from the au- thor's experience as engineer with the Westinghouse Electric & Manufacturing Co., in the Detail and Switchboard Divisions. I acknowledge with thanks the privilege extended by the McGraw-Hill Book Co., Inc., publishers, to make use of the material in the Standard Handbook, 1 and a similar privilege extended by John Wiley & Sons, Inc., with reference to the American Handbook. 1 Only condensed data and brief state- ments of theory are included in the text; references are given throughout the book to fuller data in these two handbooks, and to fuller description and theory as given in Gray's "Principles and Practice of Electrical Engineering". 1 It gives me pleasure to express my thanks to Professor Alex- ander Gray, Head of the Department of Electrical Engineering, for suggesting the publication of this material, for painstaking reading of manuscript and proof, and for important suggestions, as to both details and general form of presentation. I am glad to acknowledge also my obligation for valuable suggestions and data, to members of the Engineering and Sales Departments of the Westinghouse Electric & Manufacturing Co. ; of the Engineer- ing Department of the National Lamp Works of the General Electric Co. ; of the Sales Department of the American Steel and Wire Co.; also to Mr. W. H. Kniskern, General Manager of the Cayuga Cement Corporation; and to Mr. R. A. Hunt, Power and Electrical Engineer at the Sayre Shops of the Lehigh Valley Railroad. H. W. B. CORNELL UNIVERSITY, January, 1916. ^ee footnote, page 1. vii CONTENTS PAQB INTRODUCTION v PREFACE vii CHAPTER I THE CIRCUITS OP POWER PLANTS AND DISTRIBUTION SYSTEMS ... 1 Diagrams of Electrical Connections. 3 Rules for Representing Wiring 4 Conventions for Representing Apparatus . ! . . . . . . \ . . 6 Notes and Labels . . . . .'. t 6 CHAPTER II THE REQUISITES OP POWER PLANTS AND DISTRIBUTING SYSTEMS ... 7 Safety to Operators and Equipment 8 Continuity of Service 9 First Cost, Fixed Charges, and Operating Cost 10 Voltage Variation 12 Adaptability to All Required Loads 13 General Appearance and Environment 14 CHAPTER III CHOICE OP SYSTEM 15 A.C. versus D.C. Systems 15 Number of Phases 16 Frequency 17 Voltage. ..... ,...' . . . . .' ........ ;',- . . . . 18 CHAPTER IV D.C. MOTORS , . . ; . . 22 Voltage .'.... 22 Locations Requiring an Enclosed Motor 22 Motor Rating and Allowable Overload 23 Speed Regulation ? 24 Speed Adjustment 25 CHAPTER V A.C. MOTORS 28 Types Available '. . ./'. .... .-.,.. ; . .*.... 28 Voltage, Frequency and Phases 28 Location , ..x .;.,' ^9 Operation at Various Loads . . . . 29 Starting Torque 31 ix CONTENTS PAGE Regulation V .. ^ --.".'..'. 32 Speed Adjustment ".'-." 33 Motor Applications < . . \. . . . . . 33 CHAPTER VI MOTOR-GENERATORS, CONVERTERS AND RECTIFIERS 34 Converting A.C. to D.C 35 Raising or Lowering D.C. Voltage 38 CHAPTER VII TRANSFORMERS AND AUTO-TRANSFORMERS 43 Applications and Operation of Transformers 43 Auto-transformers 46 Grouping of Transformers and Auto-transformers 48 CHAPTER VIII STORAGE BATTERIES. , 51 Comparison of Types of Storage Batteries 51 Cost 51 Space Occupied and Weight 52 Durability and Repairs 52 Current Discharging Rate 52 Current Charging Rate 53 Voltage 53 Efficiency 54 Applications to Stationary Service 55 In the Generating Station 55 In the Battery Sub-station 56 On Circuits that are Entirely Distinct 56 Applications to Portable Service 57 Automobile Lighting, Ignition and Starting 58 Electric Automobiles, Battery Trucks and Battery Locomotives . 58 Train Lighting 59 CHAPTER IX ILLUMINATION 61 The Essentials .-.,...... 61 Illumination Intensity . . 61 Glare V .' 67 Color 67 Shadows 67 Three Kinds of Illumination 68 Computations 68 CHAPTER X D.C. TRANSMISSION AND DISTRIBUTION SYSTEMS 71 Voltage Drop 71 Motor Circuits. 71 CONTENTS xi PAGE Lighting Circuits 71 Two-wire System 72 Ground or Rail Return V .' , .. ; . . i'V.. 73 Multiple Voltage Systems ...... ^ 74 Economical Size of Wire 74 Variable Current V . 78 Safe Size of Wire 79 Conclusions 79 Table of Data on Electrical Conductors 80 Notes on the Table. ...;'' 82 CHAPTER XI A. C. TRANSMISSION AND DISTRIBUTION. . . v.' '*-.; . . . ; , . 85 Voltage Drop . ;-. : . . . .... 85 Reactance in a Single-phase Circuit 85 Power Factor on Single Phase . 86 Polyphase Circuits 87 Economy and Safety . 90 Conclusions 90 CHAPTER XII D.C. GENERATORS 92 Characteristics 92 Regulation Curve. . . :. * . . 92 Efficiency 93 Load Rating 94 Parallel Operation 94 Cost and Available Sizes 96 Number and Size of Generators 96 Kilowatt Capacity of Plant 96 Allowance for Accidents and Repairs 97 Number of Generators 97 CHAPTER XIII A.C. GENERATORS f ~r 100 Various Classifications 100 Phases and Phase Connections 100 Frequency 101 Speed and Prime Mover 102 Voltage 102 Revolving Field and Revolving Armature 102 Characteristics 103 Regulation 103 Efficiency 103 Load Rating 104 Requisites for Plant Operation ^ . . . 104 Regulation of Prime Movers and Alternators. 104 xii CONTENTS PAGE Synchronizing , ; , 105 Connections, Switches and Meters , . .- 106 Excitation and Voltage Regulation . . . . . .. W ....... 107 Cost , , . . ' . . 108 CHAPTER XIV REGULATING TRANSFORMERS 109 Constant Current Regulating Transformer 109 Induction Voltage Regulator Ill CHAPTER XV INSTRUMENT TRANSFORMERS 114 Voltage Transformers 114 Current Transformers 116 Advantages of Using Instrument Transformers 120 CHAPTER XVI CONTROLLING AND REGULATING EQUIPMENT 121 Circuit Opening and Closing Equipment 121 Knife Switches 121 Oil Switches 123 Disconnecting Switches 123 Control Switches 123 Rheostats Controlling Motors and Generators 124 Automatic Regulating Equipment 126 Generator Voltage Regulator 126 Voltage Regulating Relay 127 Line-drop Compensator 128 CHAPTER XVII CIRCUIT-BREAKING EQUIPMENT 130 Fuses 130 Circuit-breakers 131 Rated Ampere Capacity 131 Ultimate Breaking Capacity 131 Oil Circuit-breakers 134 Carbon Circuit-breakers , '.. . ,, 135 Protective Relays 136 Time Limit .... ... .'.. . % . . . . ."..'.' - 137 Applications 139 CHAPTER XVIII LIGHTNING ARRESTER EQUIPMENT 142 Multigap Arrester, .. . . '. 142 Horn-gap Arrester 143 Magnetic Blowout Arrester 144 Condenser Arrester . .144 CONTENTS xiii PAGE Multipath Arrester *..... 144 Aluminum Arrester ..,.... 145 Relative Merits of Arresters ,.....; . 145 Ground Connections . . . ." , . . . r~. . 147 CHAPTER XIX MEASURING AND INDICATING APPARATUS 148 Meters and the Quantities Measured 148 Polyphase Wattmeters and Watthour Meters. 148 Power Factor Meters .' ; , j . ... .. . . .. v \ ''.; . .' 149 Synchronizing Apparatus . . . /:,.*/.... . . , 150 Frequency Meters 151 Ground Detecting Apparatus 151 Characteristics of Meters ..."..,... . . . 152 The Scale V. . ... . . . . . ... . 152 Causes of Errors . . . . . ' . . 154 Meter Switching Devices . . ... .... . 159 Voltmeter Plugs and Receptacles 159 Synchronizing Plugs and Receptacles 160 Ammeter Switches 160 Ground Detector Switches . 161 Meter Applications . ..-...'..' 162 D.C. Switchboards . , . . . . .*. . : 162 Three-phase Switchboards ....'. 162 CHAPTER XX MOTOR APPLICATIONS. . . , . 166 Table of Kinds of Motors . . 167 Table of Sizes of Motors. . ..!'. . . . . . .169 Notes on the Table. . .." . . . ......... . . . . . . 178 CHAPTER XXI COSTS .. ; . . . > ... 185 Generators and Motors * 185 Switchboard Meters V ... . 186 Instrument Transformers and Compensators . . . .191 Relays 193 Switches and Circuit-breakers 195 Transformers .^- r r 198 Lightning Arresters . . ... . . . . . . ... . 199 Current and Voltage Regulators . . . . . . . . . . ... . . . 200 Plug and Instrument Switches . . . . . .-. . . . 200 CHAPTER XXII PROBLEMS . . v . . 202 INDEX. . 221 ELECTRICAL EQUIPMENT ITS SELECTION AND ARRANGEMENT CHAPTER I THE CIRCUITS OF POWER PLANTS AND DISTRIBUTION SYSTEMS 1 2 This chapter is intended to give a general survey of the kinds of electric circuits in common use, and the customary ways of representing the circuits by diagrams. In plants of any con- siderable size, practically every circuit leads to or from a set of buses, as illustrated in Fig. 1. The generators furnish power to the buses, and feeders take it to its destination. Thus it is possible for the generators to operate in parallel, and to share in the fluctuation of the load on any feeder. The circuits most commonly found in practice, feeding to and from these various buses, are: D.C. generator circuits. D.C. power and lighting feeders. A.C. generator circuits. A.C. power feeders. A.C. constant-potential lighting feeders. Series lighting circuits. Exciter circuits. 1 Throughout the text the following abbreviations are used in references: G. = Principles and Practice of Electrical Engineering, by Alexander Gray. The numbers following G. refer to paragraphs. (First Edit ion, 1914, McGraw-Hill Book Co., Inc.) S. = Standard Handbook for Electrical Engineers. The numbers pre- ceding colons refer to sections; those following colons refer to paragraphs. In addition to the references cited, a bibliography is given at the end of nearly every section, and at the ends of 'many of the individual articles. (Fourth Edition, 1915. McGraw-Hill Book Co., Inc.) A. = American Handbook for Electrical Engineers. The numbers refer to pages. A bibliography is given at the end of nearly every article, and cross references to related subjects are given at the beginning. (First Edi- tion, 1914, John Wiley & Sons., Inc.) 2 G. 193-197, Parallel operation of D.C. generators. S. 10 : 765-768, 799-802, D.C. and A.C. switching connections. A. pp. 1474-1478, 1494, Switchboards and switching connections. 1 2 ELECTRICAL EQUIPMENT Each of these circuits should be controlled by a switch, and pro- tected if necessary by a fuse or circuit-breaker. In addition, it is necessary, for the intelligent control of the plant, to make D.C. Buses (usually 110 or 220 Volts) Carbon Circuit Breakers ( Sometimes Replaced by Fuses) nife Switches D.C. Power Feeders D.C. Generator Circuits FIG. 1. D.C. generator circuits, feeders and buses. Showing how D.C. generators deliver current to the buses, and the buses distribute it to the several feeders. 3 Phase Power Buses | Exciter Field Eheo. Exciters FIG. 2. A.C. Generator circuits, feeders and buses. If the exciters are to operate in parallel, an equalizer is to be added as in Fig. 1. measurements of current, voltage, power, energy, power factor and frequency, or some of them, on some if not all of these circuits. The economic considerations determining the kind of THE CIRCUITS OF POWER PLANTS 3 system to be installed are outlined in the next two chapters. In later chapters the equipment is taken up in detail. D.C. generator and feeder circuits are illustrated in Fig. 1, and A.C. circuits in Fig. 2. In large power plants, the connec- tions become much more complicated, but still the entire arrange- ment is based on that shown in these simple diagrams. The connections as indicated should be studied in detail. A complete diagram includes not only the power circuits shown here, but Each Bus 1 Strip 3"x X FIG. 3. Ammeters, ammeter switches and current transformers on three circuits. Illustrating relative widths of lines, spacing between lines, and between groups, and method of designating size of wire, if necessary. The significance ofithe various connec- tions will be better understood after a study of Chapter XIX. also the connections to meters, meter switching devices, and overload trip-coils of circuit-breakers. In many cases this equip- ment is not connected directly to the line, but to the secondaries of instrument transformers, whose primaries are connected to the line. In Fig. 3 are several current transformers connected to ammeters. The switching arrangement shown is discussed further in Chapter XIX. DIAGRAMS OF ELECTRICAL CONNECTIONS A good diagram of connections is important, from the begin- ning to the end of the development of a power system. Conven- tional forms and methods are adopted, which represent the apparatus and connections in one way or another. In some cases the apparatus should be represented more as it appears to the eye, and in others it is better to emphasize theoretical relation- ships. We consider three features of diagrams; namely: (a) 4 ELECTRICAL EQUIPMENT rules for representing wiring; (6) conventions for representing apparatus; and (c) notes and labels. (a) Rules for Representing Wiring. The following rules for drawing lines that represent wiring will be found conducive to clearness and compactness of the diagram, and ease of drawing: 1. Lines are to be drawn with no more turns (angles) than are necessary; they are to be drawn vertically and horizontally, ex- cept that short lines may be made slanting, where it is particularly advantageous, as in one of the diagrams of the three-phase gen- erator, Fig. 4c. 2. Lines must not cross other lines more than is necessary. 3. The width of each line is to indicate what is the general use of the conductor (see Table I). It is not always practicable to represent all the various sizes of wire by widths of lines, because only a few widths of lines can be distinguished clearly in the ordinary diagram. If the sizes of wire are to be given on the diagram, they may be put in a table, or marked alongside the line, as in Fig. 3. TABLE I. WIDTHS AND SPACING OF LINES. The dimensions given are suitable for drawings. They may be varied somewhat, depending on the nature of the diagram. For diagrams printed from plates, each dimension may be divided by 2. Lines representing Width of ink line (inches) Spacing, center to center, between lines Of the same group (inches) Of consecutive groups (inches) Small wiring for meters, instrument transformers, relays, trip-coils, D.C. generator and motor fields (usually about No. 10 B. & S. wire). Outlines of all apparatus (not used for wires) 0.005 0.01 0.02 0.035 0.05 He H He M y* M % y* Leads for exciter armatures, A.C. generator and motor fields; also buses for auxiliary circuits Leads for power circuits (i.e., gen- erator and motor armatures, trans- formers, feeders, etc.); also exciter buses ... . . . . Ordinary power buses THE CIRCUITS OF POWER PLANTS 5 4. The space from the middle of one line to the middle of the next should be from two to ten times the width of the line. The wider the line and the more accurate the drawing, the less rela- tive spacing is required. (a) (6) D.O. Generator or Motor (c) (d) 3-Phase Generator or Synchronous Motor Squirrel Wound Cage Rotor (a) (h) 3-Phase Induction Motors (/) 2 Phase Generator or Synchronous Motor I 1UU f rfrff ) (j) (k) Knife Switches ffl (o) (1) (m) (n) on switch Carbon Circuit- Breakers or Circuit Breaker FIG. 4. Conventional representations of several kinds of electrical equipment. (a) and (6) show two ways of representing D.C. machines, (a) shows the theoretical relationships and (6) is nearer the actual appearance, (c) and (e) correspond to (a), and (d) and (/) to (6), representing A.C. machines, (g) and (h) differ in that the squirrel-cage motor (g) has no rheostat connected to the rotor, such as the machine with a wound rotor has, as shown in (h). 5. Not more than three or four lines should be drawn in a group. (Usually a group consists of one circuit. See Fig. 3.) Spacing between groups should be at least twice the spacing between lines in a group. 6. A dot is used to indicate where one conductor is connected electrically to another, as in Fig. 3. If the dot is so used, 6 ELECTRICAL EQUIPMENT semicircular curves or " jumpers" should not be used to indicate that wires are not connected. (6) Conventions for Representing Apparatus. Several useful conventions are given in Fig. 4. Such conventional forms must sometimes be modified on account of changes in apparatus or in the purpose of the diagram. (c) Notes and labels should be added wherever they make the diagram easier to understand, or easier to draw. 1. Conventional forms should be labelled wherever there can be any doubt as to their meaning. Sometimes they are labelled by abbreviations, and a list of abbreviations is appended to the diagram. 2. It is well to label circuits e.g., "Lighting Feeder," " Power Feeder to Woodshop," "Feeder to XYZ Substation." 3. Notes can be added in many cases, to save drawing several duplications of circuits, as: "Total of 5 Generator Circuits Like This." "2 Such Feeders to Woodshop and 1 to Paint Shop." 4. Reference notes should be given, indicating where detail diagrams, and diagrams of related installations can be found, as : "See p. 47 for Details of Motor-starter Connections." "See Dwg. 2,732 for Diagram of Connections of Machine Shop." CHAPTER II THE REQUISITES OF POWER PLANTS AND DISTRIBUTION SYSTEMS 1 Whether a power plant is part of an industrial establishment or is a commercial plant furnishing power to customers, the requisites for a good stable investment are as indicated in the following outline, which is put in convenient form for ready ref- erence. The reference letters and Roman numerals refer to the fuller discussion, which begins on the next page. I. Safety to operators and equipment, which requires: (a) Voltage not unnecessarily high. (6) Adequate insulation of lines and equipment. (c) Protection against lightning and other excessive voltages. (d) Automatic protection against grounds, short-circuits, and overloads. II. Continuity of service, which requires: (a) All that is required for safety. (6) Duplication of all essential equipment. (c) Circuit-breakers that do not operate instantly, and some interlocking arrangement that keeps one breaker in when another goes out. III. Small first cost, fixed charges and operating cost, which require: (a) All that is required for safety. (6) Voltage neither too high nor too low. (c) Labor-saving apparatus without unnecessary complica- tions. (d) Installation of no unnecessary equipment. (e) Generating and other units neither too large nor too small. 1 S. Sections 10 to 13; Power Plants, Distribution and Wiring. A. pp. 1087, 1089, 1119, 1462, 1463; Power Stations and Substations. A. pp. 251, 352, 363, 1657, 1891, Distribution and Wiring. 7 8 ELECTRICAL EQUIPMENT (/) Units of high efficiency, operating at about their maxi- mum efficiency. (g) Equipment having little depreciation. (k) Equipment requiring little outlay for upkeep and repairs. ({) Conditions of low interest, insurance, and taxes. IV. Small per cent, voltage variation, which requires: (a), (6) High line voltage or large line wires for small per cent drop. (c), (d) Suitable compounding or voltage regulation of D.C. generators, voltage regulation of A.C. generators, or reg- ulation of feeder voltage, or any combination of these methods of regulation. (e) Power factor of the A.C. load as high as possible. V. Adaptability to all required loads, which requires: (a) Sufficient total capacity of generating units. (6) Capacities of some or all the units not much in excess of the minimum load. (c) Allowance for expansion. (d) Ammeters and wattmeters whose full-scale indications are sufficient for all ordinary overloads, and whose in- dications at customary loads are readable with fair accuracy. VI. Good appearance, which encourages keeping the plant in good condition, and requires: (a) Consideration of appearance in selecting equipment. (6) Orderly layout of switchboard and machines. (c) Good building. (d) Well-arranged natural and artificial lighting. I. SAFETY TO OPERATORS AND EQUIPMENT (a) High voltages are undesirable if the employees working around the circuits are not familiar with electricity. Usually there is no advantage that would warrant a voltage much above 550, in a plant employing non-electrical men (see Chapter III, p. 19, for customary voltages). (6) Insulation. All parts of the system should be insulated well enough so that there is no danger of breakdown to ground, nor from one wire to another. For all inside wiring, the insula- THE REQUISITES OF POWER PLANTS 9 tion should be in accordance with the rules of the National Electrical Code, 1 together with city ordinances, power company requirements and local insurance rulings, if there are such. (c) Lightning Arresters. When lightning strikes a line, the danger is on account of the excessively high voltage that may occur between the line and ground. If this voltage due to the lightning is high enough, the insulation breaks down at the weak- est point, and the lightning discharges to ground. A lightning arrester provides a direct and easy path to ground, thereby reduc- ing the strain on the insulation. This easy path is made opera- tive the instant the lightning strikes, but is made inoperative as soon as the strain due to the lightning is past (see Chapter XVIII) . Lightning arresters serve as a protection against other high voltages on the line, as well as against lightning. This is a mat- ter of considerable importance in case of some high-tension lines in which the voltage surges that are liable to occur in the opera- tion of the system become excessive. (d) Automatic Protection against Grounds, Short-circuits, and Overloads. If a system is already grounded at one point, the result of another ground, on another phase or polarity, is equivalent to a short-circuit. Such a ground or a short-circuit is very much like a heavy overload, and is to be treated accord- ingly. A circuit-breaker can be set to operate when the current in the line exceeds the maximum safe value, thereby protecting against all these troubles (see Chapter XVII). II. CONTINUITY OF SERVICE (a) Safety Requirements. Whatever is required for safety is also important for continuity of service, because a breakdown is likely to interrupt the service. (6) Duplicate Equipment. As far as practicable, the plant should be laid out so that if any one part is disabled it need not interrupt the entire plant. By providing at least one more of every piece of equipment than is required for normal operation, such danger of interruption is largely avoided. The emergency equipment should be available on an instant's notice, for use wherever required, if it is not actually in service. Consider two 1 Rules and Requirements of the National Board of Fire Underwriters for Electric Wiring and Apparatus; revised every 2 years. These rules can be obtained from any local insurance inspection office, or from the head- quarters in Philadelphia. 10 ELECTRICAL EQUIPMENT cases: (1) A spare generator takes its turn at lying idle, but it should be driven by an engine or other prime mover that can be started without delay (see Chapter XII, p. 97). (2) In case of transmission lines and distribution systems, two lines are kept in continuous service, in parallel. Either line may then drop out of service on account of some fault; the other line then carries the entire load (see Chapter XVII, p. 140) . For economic reasons this duplication of equipment cannot be carried to an extreme, and those responsible for laying out the plant must decide to what extent they are willing to sacrifice continuity of service in case of emergency, in order to reduce the first cost of the plant. (c) Restricted Operation of Circuit-breakers. Unnecessary opening of the circuit-breaker is to be avoided whenever pos- sible, so that the circuit-breaker should be set for as large a cur- rent as is safe; and usually it is better for the sake of continuity of service, to allow some time to elapse, so that if possible the trouble will clear itself without opening the breaker. This time element is introduced in many cases by means of a small relay which determines the time when the circuit-breaker is to open (see Chapter XVII, p. 137). In many cases it is necessary to lay out the wiring so that a single short-circuit tends to open two circuit-breakers. It is desirable that the breaker nearer the seat of trouble should open, but that the other should remain closed, because opening it usually affects some other machines. In such a case the circuit- breakers are made to operate "selectively;" that is, there is a mechanical or electrical interlock that prevents the one trouble from tripping more than one breaker. HI. FIRST COST, FIXED CHARGES, AND OPERATING COST The first cost is the cost of equipment, including transportation and installation. The building, steam plant, electric plant, and wiring naturally come under this head. Fixed charges are for annual depreciation, interest on the investment, insurance, and taxes. Operating cost includes labor, fuel, supplies, upkeep and repairs. (a) Safety. Whatever makes for safety reduces one element of the operating cost, by saving in repairs. (b) Voltage. The voltage best adapted to any particular circuit depends on the extent of the system. A high-voltage installation may cost more for protection and insulation than THE REQUISITES OF POWER PLANTS 11 one of low voltage, but on long lines the saving in copper by increasing the voltage is so great as to make the higher voltage more economical (see Chapter III). (c) Labor-saving Apparatus. Under this head is included whatever saves time or special attention, such as meters, instru- ment transformers, meter-switching devices, and indicating lamps (see Chapters XV and XIX). (d) Unnecessary Equipment. The dividing line between necessary and unnecessary equipment is not always easy to draw; but unless every $100 invested brings $10 or $15 return every year, in profit, labor-saving, or protection, it is not usually a very good investment. Sometimes meters can be omitted, and perhaps even more often they can be replaced by switching de- vices to shift an ammeter and voltmeter from one phase to another, and to shift a voltmeter from one circuit to another (see Chapter XIX). (e) Size of Generators and Other Units. Large generators and transformers cost less per kilowatt capacity than small ones; but the investment in spare machines becomes excessive if all the units are of too large capacity. Both the first cost of the entire plant and the operating cost must be considered in determining the most economical size of machines (see Chapters VII, XII and XIII). (/) High efficiency operation is desirable, not only because the cost of operation is less, but also because the machine runs cooler at high efficiency. (g) Depreciation. The total amount annually chargeable to depreciation is proportional to the investment, except that some equipment depreciates less rapidly than other. On account of the smaller depreciation, apparatus having the larger first cost is frequently the more economical. (h) Upkeep and Repairs. Some apparatus rarely requires any attention to keep it in perfect condition. Other apparatus is laid up for repairs an appreciable part of the time. The loss is twofold: The cost for making the repairs, and the loss of the use of the apparatus during the time of repairs. (i) Interest, Taxes, Insurance. Interest and taxes are essen- tially proportional to the total investment, except as better rates of interest are obtainable for a plant that is better pro- tected and more durable. Insurance depends on the quality of the building to such an extent that, considering the small cost of 12 ELECTRICAL EQUIPMENT the building, the saving in insurance usually warrants construc- tion that is at least approximately fireproof. IV. VOLTAGE VARIATION Except series lighting circuits, practically all power and light- ing apparatus operates from nominally constant-potential sys- tems. Only a small percentage variation from constant poten- tial is allowable in motor circuits, and on incandescent lighting circuits it should be still smaller. Excessive variation of voltage with load on account of line drop is avoided by adopting a suitable line voltage, and suitable size and spacing of conductors. (a) Line Voltage. If the amount of power transmitted, and the length of line remain constant, the higher the voltage the less is the per cent, voltage drop. The voltage must, therefore, be high enough so that a line can be designed whose voltage drop is not excessive, with reasonable sizes of wire (see Chapter III). In case of A.C. circuits it is good practice to use one voltage for transmission over considerable distances, and another voltage for local distribution. (6) Size of Conductors. After the voltage of transmission and distribution has been fixed, the conductors must be of such size and spacing that the drop does not exceed a prescribed maximum. On A.C. circuits, line reactance as well as resistance produces voltage drop, but of course on D.C. circuits only resistance is to be considered (see Chapters X and XI). (c) Compounding D.C. Generators. The voltage of small shunt generators decreases so much with load that if constant potential is at all important on D.C. circuits, small generators are compound-wound. If all the power is used so near the bus that there is no considerable line drop, the generator may be flat- compounded that is, compounded so that its voltage at full- load is the same as at no-load. But if all the power is trans- mitted to a distant point, such that the per cent, line drop is large, the machine may be overcompounded that is, com- pounded so that the generator voltage at full-load is higher than at no-load, thereby compensating for the line drop and main- taining constant voltage at the load (see Chapter XII). (d) Voltage Regulation. It is necessary, in many cases, to provide, external to the generator, some means of voltage regu- lation. This is of less importance on D.C. than on A.C. circuits, because A.C. machines are not generally self -regulating. If THE REQUISITES OF POWER PLANTS 13 all the feeders are short, the voltage of the entire system can be kept constant by a device called a voltage regulator, applied to the generators; but if some feeders are very long, each one may require additional individual regulation (see Chapters XIV and XVI). (e) Control of Power Factor. The greatest voltage drop due to line reactance occurs in case of a lagging current at low power factor. With current at 100 per cent, power factor, the reactance has no appreciable effect on the terminal voltage, and with a leading current the reactance tends to increase the terminal voltage (see Chapter XI, p 87). Where it is feasible, the load should be so arranged that the power factor is about 100 per cent., thereby eliminating reactance drop. V. ADAPTABILITY TO ALL REQUIRED LOADS (a) The total capacity of the generating units must be large enough to carry all ordinary loads with good efficiency, and with- out excessive heating, sparking, voltage drop, or other harmful effect of overload. The capacity should be such that if any one unit is disabled, the remaining machines can carry the load without seriously crippling the plant or endangering the remaining generators (see Chapters XII and XIII). (6) Units for Light Load. It should be possible to run the plant at good efficiency, at the lightest load that the plant is likely to carry for any considerable time. If the plant has a large number of machines, they should all be of the same size; in case of a small plant, having only two or three generators, and operating for long periods at a very light load, it may be advantageous to have one of the units of about one-half the size of the others. (c) Allowance for Expansion. If the plant is of such a nature that the demand for power is likely to increase, the plant should be large enough to anticipate the increase, or provision should be made for one or more additional generators and related equipment, to be installed later. (d) Meter Capacity. Ammeters and wattmeters should be of such capacity that they will indicate the largest load that the line is likely to carry for any appreciable time; but the meters should not have too large capacity, because in that case the deflection is very small under normal and light-load conditions, and it is not possible to take accurate readings. 14 ELECTRICAL EQUIPMENT VI. GENERAL APPEARANCE AND ENVIRONMENT This does not necessarily refer to ornamentation, but the plant should give the appearance of being adapted to its purpose. It should of course satisfy stockholders and directors and the general public, but it should also be so designed that it appeals to those who are operating the plant, as being worthy of the best of care. An operator takes pride in keeping a plant in perfect condition, if he believes it to be the best of its kind in the vicinity. (a) Consideration of Appearance in Selecting Equipment. The best manufacturers of electrical equipment take pride in their products, and are not willing to send them out poorly finished. It requires only a small additional expense to make a good piece of apparatus appear well, and to some extent the appearance serves as a guarantee of good workmanship and materials. For this reason, and because apparatus that is well- finished is easier to keep in order, it pays to consider appearance in selecting it. (6) Layout of Switchboard and Machines. Switchboard panels should be grouped, so that as far as possible all A.C. generator panels are in one group, all A.C. feeder panels in another, lighting feeders in another, etc. This plan is frequently carried so far as to leave blank panels for future additions, rather than to install the additions out of order. The machines should, if possible, be in essentially the same order as the switchboard panels. It is easier for the operator to act quickly in an emergency, and easier to give proper care at all times, if the entire layout is so rational that the operator has it perfectly in mind. (c) The Building. The cost of the building is small compared with the cost of equipment, but the convenience, safety and durability of the building play no small part in the safe and economical operation of the plant. (d) Natural and Artificial Lighting. The lighting of the power plant should be good : first, because every operation is performed more safely and easily with good light; and second, because if any part of the plant is in disorder good lighting reveals the fact. Good lighting, whether natural or artificial, includes (1) plenty of light, (2) uniform distribution, and (3) light so placed that there is no glare or blinding effect, due to strong direct or reflected light in the field of vision (see Chapter IX). CHAPTER III CHOICE OF SYSTEM The first things to be decided in arranging for an electrical installation are: (1) Whether it is to be an A.C. or a D.C. system; (2) If A.C., whether a one-, two-, or three-phase system; (3) Whether the frequency is to be 25 cycles, 60 cycles, or some frequency that is not standard; (4) What the line voltage or voltages are to be. 1. A.C. versus D.C. Systems. If a storage battery or any other electrolytic equipment is to be connected to the system, obviously a D.C. system must be employed. Otherwise, the decision must be based on the relative merits in the individual case, because either A.C. or D.C. can be used for both lighting and power. For incandescent lighting, either an A.C. or a D.C. system is satisfactory if the voltage can be maintained constant. For arc lighting, the D.C. has some advantages, but not enough to war- rant its use if there are many motors connected to the circuit which operate better on A.C. It is possible to use either an A.C. or a D.C. motor at will for every motor application ; but there are some cases, for example, in a powder mill, where D.C. is very undesirable; and others, for example, where there are cranes and many variable speed machine tools, where A.C. would be objectionable. In each plant where a choice is possible, the advantages of the two sys- tems must be balanced against each other. The chief advantages in using A.C. motors for industrial applications are: (a) The possibility of A.C. transmission and distribution at higher voltages, and stepping down to motor voltages. (6) The possibility of stepping down from the voltage of motor feeders to that of lighting feeders without a lower voltage generator or its equivalent. (c) The possibility of using induction motors which are rugged, and have no commutator to wear out or introduce fire hazards. 15 16 ELECTRICAL EQUIPMENT The chief advantages in B.C. motors are: (a) The possibility of automatic speed variation of series and compound motors, without excessive power lost in rheostats. (6) The possibility of speed adjustment in shunt motors by field rheostat, thereby obtaining at high efficiency any number of speeds that remain constant at the required values, at all loads. (c) The absence of reactance in line drop. (d) The possibility of connecting motors to the same circuit as storage batteries or other electrolytic apparatus. In Table XIII, p. 167 is a list of machine tools and other in- dustrial motor applications, indicating the motors that are usually preferable, and others that are sometimes satisfactory. 2. Number of Phases. The only systems in general use are single-phase, two-phase four-wire, two-phase three-wire, and AA/VWV1 (d) FIG. 5. Comparison of distributing systems. (a) Single-phase. (6) 2-phase 4-wire. The two voltages are equal, (c) 2-phase 3-wire. The CA voltage is \/2 times BA or CB. (d) 3-phase. The three voltages are equal. three-phase. These are illustrated in Fig. 5. The zigzag lines may be taken to represent transformers or any other source of power. Their angular positions indicate the phase relations of the several circuits. The principle advantage of a single-phase circuit is its simplic- ity. It requires only two wires for transmission, and the con- nections and equipment are sometimes a little simpler than for a polyphase circuit. The chief advantages of a polyphase system are that the size and cost of motors and generators are less, and that motor starting is easier. The advantages of the polyphase system far outweigh those of the single-phase, for ordinary indus- trial-motor applications. The only extensive single-phase appli- cation is to railway motors, which operate from only one trolley CHOICE OF SYSTEM 17 line and the ground return. There is but little choice between single-phase and polyphase systems for lighting circuits. For small currents, the use of only two wires is an advantage ; for large currents, the polyphase system offers a small saving in copper. After it has been decided to use a polyphase system, there is but little choice between two- and three-phase. The small ad- vantages of the several systems are as follows: The two-phase four-wire system has the advantage over the three-phase, in requiring only two transformers, and in having two phases that are insulated from each other and independent in their operation. The two-phase three-wire system has the advantage over the four-wire system, that the wiring is simplified, and there is a small saving in copper. The three-phase system has the advantage over the two-phase three-wire, that the three voltages between lines are equal, and all conductors are of the same size. Where a generating station is to be installed, the advantages of a three-phase system usually outweigh those of two-phase; but if there is a power company from which power can be obtained in an emergency, the system used by that company should be con- sidered. Transformers are used to step the power company's voltage down to that required by the industrial plant, and even in case the power company should have a two-phase system, the transformers could be Scott-connected (see Chapter VII, page 48), so that three-phase could still be used in the industrial plant. However, if two-phase emergency power could be obtained with- out stepping down the voltage, it might be well to introduce two-phase in the industrial plant, and save the transformers. It should be remembered that in transforming it is always possible to change not only from two-phase to three-phase, but also from three-phase to two-phase, and from two-phase three- wire to two-phase four-wire. 3. Frequency. In the choice of the frequency of an alternat- ing-current system, some considerations make a low frequency advantageous, and others a high frequency. Transformers, generators, synchronous motors, and induction motors can be made smaller, and at less cost, if they are to operate on a high than if on a low frequency. But very slow-speed induc- tion motors are more satisfactory on 25 than on 60 cycles, on account of the higher power factor that can be obtained on 25 18 ELECTRICAL EQUIPMENT cycles. The cost of a 25-cycle transformer or induction motor is usually from 25 to 50 per cent, higher than a corresponding 60-cycle equipment, and a 25-cycle generator or synchronous motor sometimes costs 20 per cent, more than a corresponding 60-cycle machine. Alternating-current commutating motors, used on electric rail- ways, operate at a higher power factor on low frequency. For this reason they are preferably designed for about 25 cycles. Arc and tungsten lamps flicker visibly on very low-frequency circuits. It is better not to use them on less than 50 or 60 cycles, on account of the resulting eye-strain. Transmission and Distribution Wiring. The voltage drop due to inductive reactance of the line is greater at high than at low frequency; it is entirely negligible at any ordinary frequency if the load current is at 100 per cent, power factor, but may be very large with load at low power factor. The desirable frequency to be adopted on any system depends on what equipment predominates. Alternating-current commu- tating motors are used on electric railways, but very little else- where; slow-speed induction motors are used only in steel rolling mills, and for other rather rare applications. For these motors a low frequency is better; but for all others the frequency should be higher. The reactance drop in the various machines, as well as in the line, would be excessive if the frequency were too high, but it is found in practice that at 25 to 60 cycles this drop need not be excessive, either in the machines or in the line. In the United States, 25 and 60 cycles are used in practice almost exclusively, for ordinary industrial purposes. Of the two, 60 cycles is far more common than 25, especially where slow-speed induction motors and alternating-current commutating motors are not employed. Besides these frequencies, 15, 30, 50, 133, and a few others are found occasionally. It is of advantage to use 25 or 60 cycles wherever possible, because standard equipment is made for these frequencies. If any other frequency is adopted, the delivery of equipment may be delayed, and the cost may be higher. 4. The voltage of a so-called "constant-potential" system, or of any circuit in such a system, depends, chiefly, on (1) the length of the system, (2) the kind of apparatus that it feeds, and (3) the danger to life or apparatus. All of these should be considered with reference to every circuit. CHOICE OF SYSTEM 19 Standard Voltages. The voltages in common use at the pres- ent time are 110, 220, 440, 550, 1,100, 2,200, 4,400, 6,600, 11,- 000, 13,200, 16,500, 22,000, 33,000, 44,000, 66,000, 88,000, and 110,000. All of these are derived from 110 volts, by multiplying by the factors 2, 3 and 5, as many times as necessary. Various other voltages, as high as 165,000 are in less common use. On account of line drop, the voltage cannot be standard along the entire length of the line. Either the beginning or the end of the line is usually kept at or near the standard voltage. Voltage of Distribution System. There is no simple, universal rule for the relation between voltage and length of line; but in practice, where transformers are necessary, the line voltage is nearly always between 500 and 2,000 volts per mile, and is ordi- narily about 1,000 volts per mile. Thus a line 10 miles long usually has a voltage of about 10,000. The choice would be between 6,600 and 11,000, which are standard voltages. It would be decided by considerations of cost, safety, and voltage regulation (see Chapter X). However, if some other line in this same system would dictate a higher voltage, sometimes it is of advantage to have the entire system at that higher voltage. If transformers are installed at the end of the line, it is usual practice to have a line voltage of at least 2,200, which is very common as a transmission and distribution voltage. Wherever it is possible to use 2,200 volts, it is better to adhere to this standard than to adopt the higher voltages which in- troduce added risks, especially in populous districts. Tungsten lamps connected in multiple are usually used on about 110 volts. (Lamps are also made for 220 volts, but in most cases, at present prices, the 220-volt lamps of a given wattage cost 20 per cent, more, and produce only 90 per cent, as much light.) In a plant of any considerable size, if only a 220-volt D.C. circuit is available, some convenient means can be provided, so as to have a 110- and 220-volt three- wire system. Various other arrangements can be employed at other voltages, to obtain a suitable lighting system. In case alternating current is avail- able, transformers may be employed to provide the desired voltage for the lighting circuit. Arc lamps connected in multiple on a D.C. circuit operate efficiently and satisfactorily on about 110 volts. (A little lower voltage could be used if there were a standard of about 70 volts, but the gain would not be very large.) When higher D.C. volt- 20 ELECTRICAL EQUIPMENT ages than 110 are employed, a relatively large series resistance is used to cut down the voltage across the arc, and the efficiency is very much reduced. For A.C. circuits, however, arc lamps are regularly made for 110, 220 and 440 volts. For voltages above 110, a transformer accompanies the lamp, and a good electrical efficiency is obtained, even on 440 volts. Special lamps can be obtained, also, to operate at still higher voltages. D.C. motors usually operate on 110 or 220 volts in industrial plants. Line drop is unnecessarily large in a large plant at 110 volts, but 220 volts is satisfactory if the motors are within 500 or 1000 feet of the power plant. A 550- volt system in an in- dustrial plant has two disadvantages: that it is approaching a dangerous condition if it is a plant in which non-electrical men are regularly at work; and that some provision must be made for lighting, other than the customary three-wire system. A.C. motors may operate at any desired voltage, without regard to the lighting circuit, because transformers can be employed for the lighting voltage. It is rarely desirable to operate the motors on less than 220 volts. A 440-volt system is more common in any plant requiring power several hundred feet from the generator; and even a 550-volt system is sometimes used. A motor-generator set has no electrical connections between the motor and the generator, so that the voltage of each machine may be whatever is required to adapt it to its circuit. The same fact holds for a dynamotor (see Chapter VI, p. 42). A synchronous converter, used to convert either from A.C. to D.C., or from D.C. to A.C., has only one winding for the A.C. and D.C. circuits, and there is a very nearly constant ratio between the A.C. and D.C. voltages. This ratio depends on the A.C. connections to the converter, and is as follows: _. ,. A.C. voltage A.C. connection to converter Ratio p Q vo it a Single-phase 0.707 Two-phase . 707 Three-phase 0.612 Six-phase, double-delta 0.612 Six-phase, diametrical . 707 The ratio given is for ordinary connections. Special connec- tions may be employed to obtain other ratios in case of the two- and six-phase connection. CHOICE OF SYSTEM 21 A storage battery, with or without a booster, has a voltage on discharge that is the same as the line voltage. The charging voltage depends on what method of charging is employed. If the storage battery is connected across only one side of a three- wire system, its discharge voltage is only the lamp-line voltage that is, one-half the motor voltage. CHAPTER IV D.C. MOTORS 1 In specifying the motor and its controlling apparatus for any given kind of service it is necessary to determine what kind of D.C. circuit is available, and what is required of the motor. We should know in particular: (1) What D.C. voltage is available, and whether it is a two- wire or a three- wire system; (2) Where the motor is located; (3) What is the maximum load on the motor, and how the load varies; (4) What automatic variation of speed with load is desired or allowable; and (5) What speed adjustment is required to be made by hand at the will of the operator, under various conditions. 1. The voltage of the motor and of the line must be adapted to each other. The best voltage for industrial plants is generally 220 volts; but if no motors are over a few hundred feet from the generator, 110 volts may be used. 2 2. Locations Requiring an Enclosed Motor. The motor should be enclosed, if it is in a place where dust would injure the com- mutator or interfere with commutation, where shavings might catch fire from the commutator, or where water might injure or short-circuit the machine. The motor may be semi-enclosed or entirely enclosed to keep flying objects from the moving parts of the machine. A semi-enclosed machine may also be used in some cases where there is slight trouble from dust, shavings or water, but not enough to require that the machine be entirely 1 G. Chapter XV, Characteristics; Chapter XVII, Applications; Chap- ter XVIII, Speed Control. S. 8: 157-180, 200-205, Characteristics, weights and costs; Section 15, Applications. A. pp. 957-971, Characteristics, weights and costs; pp. 892-896, 972-982 (also see references, p. 972), Applications. 2 See Chapter III, p. 20. 22 D.C. MOTORS 23 enclosed. Enclosing a motor increases its cost per horse-power, because it reduces the horsepower, that can be delivered con- tinuously without overheating. See Chapter XX, p. 183, as to the effect of enclosing on the motor rating. 3. Motor Rating and Allowable Overload. The motor rating given by manufacturers is usually based on the horsepower that the machine will deliver without causing excessive heating of any part 1 or excessive sparking at the commutator. Usually the rating is based on continuous duty (8 hr. or more), but some- times on 1-hr, duty, or even a shorter time. On the short-time basis the rating is much higher than on continuous duty (see page 183 for increase of rating). For example, the ordinary crane motor does not operate more than a few minutes at a time, and a M-hr- r & 1-hr- rating is sufficient; so that the motor size, weight, and cost can be much less than if it were driving a lathe for 8 hr. If the load is intermittent or variable, having maximum values of several times the average, it is well to specify that the motor is to deliver power according to a given time-load curve. The ratio of starting torque to full-load running torque should also be specified, especially if the motor is expected to exert a very great starting torque. After the full-load rating has been determined, in terms of either horsepower, or torque and speed, the full-load current can be found directly: 2irtf !T/33,000 = h.p. = El X efficiency/746 where T is the full-load torque in pound-feet and N is the speed in r.p.m. Table II gives reasonable figures for efficiency and rated speeds. If the motor is well-constructed mechanically, and commutation is good, the speed may be increased in each case by field control, to double the rated speed. If the torque is one-half as great at double the speed, the power delivered is the same as before, and the current input is practically the same. The ventilation is so much better at higher speeds that most motors running at double the normal speed will stand 120 per cent, of the rated armature current; so that the torque on continuous duty at double the rated speed may be as much as 60 per cent, of that at rated speed. 1 For A.I.E.E. standard of allowable temperature rise see footnote, p. 94. 24 ELECTRICAL EQUIPMENT TABLE II. D.C. MOTOR DATA FOR CONTINUOUS DUTY AT FULL-LOAD Rating in horsepower Efficiency, per cent. R.p.m. 1 2 80 1,000 5 83 800 10 85 600 20 88 500 . 40 89 400 100 90 300 300 93 200 1,000 95 100 1200 100 1000 26 50 75 100 125 150 Percent of Full Load FIG. 6. Typical efficiency and speed curves of D.C. motors. 4. Speed Regulation. Fig. 6 shows the comparative regula- tion (automatic variation of speed with load) of shunt, compound and series motors. The large majority of industrial applications 1 There are no definite limitations of rated speeds of D.C. as there are of A.C. motors. The speeds given in this table are in common use. The chief disadvantage of lower speeds is the increased cost of the motor. The chief disadvantages of higher speeds are commutation difficulties, and the extra gear reduction that is necessary. D.C. MOTORS 25 require that motor speeds remain approximately constant under all conditions of loading. A lathe is an example. See also the table of motor applications in Chapter XX, page 167. Shunt motors satisfy the requirement of constant speed so well that they are used in all such cases. (For extremely constant speed the motor may be differentially compounded, but at the present time the differential motor is almost obsolete.) A compound motor is adapted for operating such machines as shears, punches and crushers, which require a heavy torque intermittently. The kinetic energy of the rotating armature is utilized when the machine slows down at the instant of heavy torque. If a flywheel is on the motor shaft, the available kinetic energy is still greater. The compound motor is also adapted to certain hoists and pumps, and similar applications, in which the speed should be less at heavy than at light loads. A series motor has a still greater speed variation with load than a compound motor; it is in danger of running away at very light loads. Its speed regulation is adapted to hoisting and conveying, where it cannot run away; because it is always under the control of the operator, even if it is not always loaded. There are in use three methods of protection against overspeed of a series motor: (a) Gearing or other positive connection to a load that cannot be less than a safe minimum. (6) An operator who is present, controlling the speed whenever the motor is running. (c) Sufficient resistance in the motor circuit to keep down the back e.m.f., and therefore the speed of the machine. This makes the machine inefficient and the speed less, especially at heavy loads. It is only suited to very small motors. 5. Speed adjustment refers to changes made at the will of the operator, whereas speed regulation refers to the automatic change in speed due to change in torque. Speed Adjustment by Rheostats. The most common method of speed adjustment is by a rheostat in either the field or the armature circuit or one in each circuit. 1 The effects of these rheostats are shown very clearly by the expression for speed of a D.C. motor, r.p.m. = k (E a I a R a }/ 1 See Chapter XVI, for points to be considered in specifying rheostats. 26 ELECTRICAL EQUIPMENT Where E a is the applied voltage. I a is armature current in amperes. R a is resistance of armature circuit in ohms. is the flux in lines per pole. The armature rheostat is a part of the resistance R a ', when the resistance in the rheostat is increased, the speed decreases, until when R a l a E a the speed is zero. It should be noted that when R a is large, the regulation is poor; the variation of speed with load is excessive, because R a l a varies with 7 a , and therefore with the torque. The RP power loss is also large, so that this method of regulation is very inefficient. Compare with the foregoing the effect of the field rheostat. An increase of its resistance decreases the field current, and so the flux, <; and the speed variation is in inverse proportion to the flux. Since R a is now only the small armature resistance, its effect on the speed is negligible under all ordinary conditions. There is no large RP power loss, so that this is an efficient method of speed control. An excessive resistance weakens the field so much that commutation is bad, unless the motor has a commutating field. Motors can be built having speed adjust- ment by field rheostat, by which the speed can be increased from normal to twice the normal speed. The effect of an armature rheostat with any given current and resistance is calculated readily from the speed formula given above; but the effect of the field rheostat cannot be determined accurately without knowing how much the field is saturated. As an approximate guide, it may be assumed that for every 2 per cent, increase in field circuit resistance the speed is increased 1 per cent. Speed control by armature rheostat can be applied to shunt, compound and series motors. A field rheostat can be used on the shunt field of either a shunt or a compound motor. A similar field control of a series motor is accomplished by connecting a rheostat in parallel with the series field. Increasing the resistance of such a rheostat increases the field strength, by sending more of the current through the field winding. Multiple-voltage Speed Control. The difficulties of speed con- trol by armature rheostat are overcome by the multiple- voltage system. For slower than normal speed the armature is connected across a lower voltage, while the field remains un- D.C. MOTORS 27 changed. The motor runs at essentially the same reduced speed at light load as at full load, and has a reasonably good efficiency. The difficulty with this arrangement is that some form of motor- generator set must be provided to obtain the additional voltages, and additional wiring is necessary wherever the multiple voltage is required. The only multiple voltage in general use at the present time is the three-wire system having equal voltages on the two sides. For example, on a 110- and 220-volt three- wire system, if a motor armature is connected across 110 volts, the speed is one-half of the normal speed which the motor has when across 220 volts. Intermediate speeds are obtained by connect- ing the armature to 110 volts and weakening the field by a rheostat. Still higher speeds, as high as double the normal, are obtained by connecting the armature across 220 volts and weakening the field by the rheostat. CHAPTER V A.C. MOTORS 1 Types Available. Of all A.C. motors, the squirrel-cage and phase-wound 2 induction motors are the ones used most exten- sively for industrial purposes at the present time. Synchronous motors and various forms of commutating motors are used to a limited extent for power-factor adjustment, but the simplicity and ruggedness of the induction motors make them desirable wherever it is practicable to use them. The present discussion will be limited to the two kinds of induction motors. In pro- viding A.C. motors, the following should be considered: (1) The voltage, frequency and number of phases; (2) Where the motor is located; (3) What the maximum load is on the motor, and how the load varies; (4) What starting torque is required, compared with full-load torque; (5) Whether it is necessary that the speed at full-load be ap- preciably less than at no-load, and if so how much; and (6) Whether any speed adjustment is required to be made by the operator, and if so how much. 1. Voltage, Frequency and Phases. The motor should be designed for the same number of phases, the same voltage and the same frequency as the supply. If applied voltage >motor voltage 1 in)n logg Jg ^ s ^ If applied frequency < motor frequency J If applied voltage < motor voltage 1 maximum torque is If applied frequency > motor frequency j too low. Single-phase induction motors are of value where it is impor- tant to use only two wires for distribution of power; but poly- phase motors are to be preferred, particularly on account of 1 G. Chapter XXXVI, Theory and Characteristics; Chapter XXXVII, Applications; Chapter XXXVIII, Single-phase motors. S. 7 : 205, 215-221, 271-285, Characteristics; Section 15, Applications. A. pp. 983-988, 1005, 1007-1013, Characteristics, Costs and Weights; pp. 892-896, 972-982, also references on p. 972, Applications. 2 Otherwise known as "wound-rotor" or "slip-ring" motors. 28 A.C. MOTORS 29 special arrangements that are necessary for a good starting torque of the single-phase motors. There is no essential difference in operation between two- phase and three-phase motors. If it is necessary to operate a two-phase motor on three-phase, the Scott connection of trans- formers, or preferably auto-transformers, should be employed (see Fig. 23 h and i, p. 49 for the Scott connection, and Chapter VII, p. 46 for the advantage of using auto-transformers instead of transformers). 2. The location has less effect on the kind of induction motor than of a D.C. motor, because the induction motor has no com- mutator; and therefore no commutator trouble due to dust, and no fire hazard due to ignition from commutator sparking. The frame of the motor surrounds the winding to such an extent that injury from water or flying objects is almost impossible. If the motor is subjected to continued dampness or acid fumes, the coils should be treated with a special varnish. A phase- wound motor with external starting resistance requires slip-rings, which should be enclosed if the machine is in a very dusty location. The bearings of any induction motor in a dusty place should also be well protected from dust. 100 7. Charactistic curves of a 10-hp., 3-phase, 220-volt, 60-cycle induc- tion motor. 3. Operation at Various Loads. Increasing the load applied to an induction motor affects the motor operation, as to speed, power factor, efficiency and current. Referring to Fig. 7, it 30 ELECTRICAL EQUIPMENT will be seen that for the highest efficiency and power factor, the load should be neither very large nor very small. The speed decreases gradually with increase of load until a certain maximum horsepower output is reached. The motor would stop if a torque were applied much beyond the value for maximum horsepower. Usually the motor is made large enough so that the maximum possible torque is 100 per cent, more than the motor will be required to deliver. The full-load current of a three-phase induction motor is obtained directly from the expression, 27rA/T/33,000 = h.p. = ^%EI cos 6 X efficiency/746 (1) where N is the speed in r.p.m., T is the torque in pound-feet, cos 6 is the power factor, E is the voltage between lines and / the amperes per motor lead. Equation (1) holds for single-phase and two-phase motors, if the constant, -\/3, is replaced by 1 and 2, respectively. Table III gives reasonable values for full- load efficiency and power factor, no-load and full-load speeds and per cent, slip, for typical 60-cycle and 25-cycle motors. TABLE III. TYPICAL DATA AT FULL-LOAD AND NO-LOAD, ON SQUIRREL- CAGE AND PHASE-WOUND INDUCTION MOTORS 1 i 1 -* 60-cycle motor 25-cycle motor Horse- ^-d m ^1 power Rating |f sl3 2 \r 11 No-load or syn- chronous r.p.m. Full-load r.p.m. No-load or synchronous r.p.m. Full-load r.p.m. .PV3 PH * 1 82 78 5.5 1,800 1,700 (1,800-600) 5 85 86 5.5 1,800 1,700 (1,800-600) 10 87 88 5.0 1,200 1,140 750 712 (1,800-600) 25 89 89 4.0 1,200 (1,800-600) 1,150 750 720 50 89 89 3.5 900 870 750 725 (1,800-600) 100 90 90 3.5 600 580 500 485 (1,800-514) 500 91 91 3.0 600 582 375 365 1,000 92 92 2.5 450 440 250 244 1 The speeds given in parentheses are the highest and lowest for which the sizes given are ordinarily made. A.C. MOTORS 31 Both the usual load and the maximum must be considered, with reference to all the operating characteristics, in specifying a motor. Also the heating on short-time overloads must not be excessive (see Chapter XX, p. 183, for allowable short- time overloads). 4. Starting Torque. Induction motors are made with two kinds of rotating elements, or rotors. One of these, the wound rotor, has a winding in the rotor slots, with leads brought out to slip-rings. A rheostat con- nected to the slip-rings as in Fig. 8, is mounted outside the motor. It is used to insert Ante. transformer Primary or Stator Winding Connected Directly Through a Switch to the Line Slip Rings to which the Secondary or Eotor Connects FIG. 8. Induction motor with wound rotor connected to a rheo- stat. FIG. 9. Squirrel-cage induction motor with auto-transformers and switches for starting on low voltage. The diagram shows the theoretical con- nections. The circuit-breaker is opened by hand before the motor is started. Usually the switches and circuit-breaker are combined so that the change from starting to running position is made by a single switching operation. resistance in the rotor winding when the motor is being started. The effect of the resistance is to reduce the cur- rent, and to increase the power factor. Such an increase of power factor is important, because otherwise it is very low at starting, and the necessary starting current is very high. Where a rheostat in the motor secondary controls the starting current, the rela- tion between torque and current is the same as at full speed (see Fig. 7). The other type of rotor is called a squirrel-cage. It has heavy conductors, short-circuited on themselves. If the motor were started at full voltage, the current in the rotor (and also in the stator) would be excessive. The starting voltage is stepped down by auto- transformers as in Fig. 9 (see Chapter VII, page 32 ELECTRICAL EQUIPMENT 46; also G. 307, 332). The smaller the starting voltage, the smaller is the starting current. If the voltage is too low, the motor will not start. Table IV gives the starting voltages and the resulting currents that produce certain starting torques. The voltage should be sufficient for starting under the worst conditions; but if it is kept as small as practicable it prevents excessively large starting currents. For example, if in starting, a motor requires 1.1 times its full-load torque, then we find the starting voltage must be 80 per cent, of the full voltage, and the primary of the auto-transformer must take from the line 3.85 times the motor full-load current. In a motor with a wound rotor the current above full-load is nearly proportional to the torque, as shown by Fig. 7, so that it would have started with about 1.1 times full-load current. TABLE IV. CURRENT AND VOLTAGE REQUIRED FOR STARTING TYPICAL SQUIRREL-CAGE INDUCTION MOTORS AT VARIOUS STARTING TORQUES (These figures are subject to some variations, depending on the purpose for which the motor is designed) Per cent, of full- load torque Per cent, of full line voltage Per cent, of full-load current in motor Per cent, of full-load current in primary leads to auto-trans- former 27 40 240 100 60 60 360 220 110 80 480 385 170 100 600 600 5. Speed Regulation. Ordinary induction motors, whether squirrel-cage or phase-wound, correspond approximately to shunt motors in their speed regulation. This is in accordance with the usual requirements for industrial applications; but as applied to a punch press, for example, there is an advantage in decreasing the speed when the torque is very great. This is accomplished by permanently inserting resistance in the second- ary of either a squirrel-cage or a phase-wound motor, as men- tioned in a preceding paragraph. This produces a speed characteristic similar to that of a D.C. compound motor, but it is obtained at the expense of efficiency. The more resistance in the secondary, the greater the speed reduction and the less the efficiency. A.C. MOTORS 33 6. Speed adjustment requires either a special motor whose synchronous speed can be changed (A. p. 977, Multi-speed Induc- tion Motors), or a phase- wound motor with a rheostat to be in- serted in the secondary, just as is done in motor starting. The first of these is not used so extensively as the second, although by changing the synchronous speed a high efficiency is main- tained, and the speed is very nearly constant. Where the speed adjustment is by the phase- wound motor, the rheostat carries the secondary current continuously or for as long a time as the reduced speed is required. Such a rheostat must, therefore, have a heavier conductor than one used only for starting. If the rheostat is used for both starting and speed adjustment, the first few steps, which are necessary for a speed adjustment, may be heavy enough for continuous use; but more resistance is re- quired for starting, and the additional steps need only be heavy enough for short-time use. Motor Applications. Some of the customary applications of induction motors are listed in Chapter XX, page 167. Table V reviews the special advantage of each motor. TABLE V. THE KINDS OF SERVICE FOR WHICH THE SEVERAL A.C. MOTORS ARE PARTICULARLY ADAPTED Kind of motor Specially adapted for Example of application Phase-wound. . . Squirrel-cage... Synchronous Single-phase induction . . 25-cycle . 60-cycle. Large starting torque. Hand control of speed. Applications where phase- wound is not necessary. Control of power factor. Small starting torque. Very constant speed. Where only single-phase circuit is available. Very slow speed. Applications where 25 cycles is not necessary. Tube mill in cement plant. Elevator. Wood-working machinery that starts without load. Motor-generator set. Motor-generator set. Frequency-changing motor- generator set. Small fans. Steel rolling mill. Nearly everywhere. CHAPTER VI MOTOR-GENERATORS, CONVERTERS AND RECTIFIERS Electricity appearing in one form is converted to another in a variety of ways: 1. A. C. is converted from one voltage to another by an ordinary transformer (see Chapter VII). 2. A.C. is converted from constant potential to constant current by a constant-current regulating transformer (see Chapter XIV) . 3. A.C. is converted from one number of phases to another by a suitable combination of transformers, such as the Scott connec- tion (see Chapter VII) . 4. A.C. is converted from one frequency to another by a motor- generator set called a frequency changer, in which the generator has not the same number of poles as the synchronous or induction motor, so that the frequency of the generator is different from that of the motor circuit; for example, if a 25-cycle supply is available and a 60-cycle system is desired, a 25-cycle motor is used to drive a 60-cycle generator. 5. D.C.is changed to A.C. (a) by a shunt motor driving an A.C. generator, or (b) by an inverted synchronous converter. 6. A.C. is changed to D.C. (a) by a synchronous or induction motor driving a shunt or compound generator, (6) by a synchron- ous converter, (c) by a mercury rectifier, or (d) by a vibrating rectifier. 7. D.C. is changed from one voltage to another (a) by two machines comprising a balancer-set, connected in series across the higher voltage, with a neutral or low -voltage lead brought out from the connection between machines; (6) by a three- wire generator which is provided with one or two coils by which the neutral voltage of a three-wire system is established; (c) by a motor-generator set in which the motor and generator are de- signed for different voltages; or (d) by a dynamotor consisting of a machine having two distinct circuits one for the higher and one for the lower voltage, each circuit having its own complete winding and commutator. 34 CONVERTERS 35 This list includes only the conversions that are most frequently seen in industrial plants, and only the means most frequently employed for making them. The first three of the conversions listed are accomplished by transformers, and are discussed in Chapter VII. The fourth, frequency changing, 1 and the fifth, converting D.C. into A.C., 2 are not discussed further, because they are comparatively rare. "We shall consider the sixth and seventh in this chapter. 6. Converting A.C. to D.C. is most often desirable where a relatively long line requires A.C. transmission, and a storage bat- tery or a number of variable-speed motors call for D.C. applica- tions. Each of the means of conversion has its advantage, and is the best to use in certain cases. (a) A motor-generator set may consist of either a self-starting synchronous or an induction motor, and either a shunt or a From A.C. Source and Starting Apparatus From A.C. Source and Starting Apparatus From D.C. Source and Field Switch FIG. 10. Motor-generator sets. (a) Synchronous motor-generator set, requiring connection to D.C. for field excitation. It may be started by an additional induction motor winding. (6) Induction motor-gener- ator set, having a squirrel-cage motor, which requires no connections to the rotor. compound generator. 3 The induction motor is more rugged and more easily started, and does not require synchronizing nor D.C. field excitation. The synchronous motor can be operated at unity power factor or with a leading current, and has no speed variation between no-load and full-load. Large motors of either kind, with suitable windings, can be connected to the line without transformers, if the line voltage is not over 13,200. For small motors the voltage must be lower. Self-starting synchronous motors are usually used in motor-generator sets of 500 kw. or more; induction motors are ordinarily used for 100 kw. or less on account of their simplicity; and between 100 and 500 1 S. 7 : 346-369; A. pp. 951, 952. 2 S. 9 : 95-102 A. p. 280. 3 Chapters V and XII; also G. Chapter XXXIX; S. 7 : 335-345; A. p. 950. 36 ELECTRICAL EQUIPMENT kw. either motor may be used, depending on conditions under which it is used. Diagrams of the connections of the two kinds of motor-generator sets are given in Fig. 10. (6) A synchronous converter has advantages over a motor-genera- tor set, in that the cost is less and the efficiency higher, especially if transformers are used in both cases. 1 It has the disadvantages of inflexibility: the ratio of D.C. to A.C. voltage is nearly con- stant (see Chapter III), and the power factor cannot be varied through any considerable range. 2 To overcome these difficulties, the synchronous booster-converter has been developed. 3 It con- D C. From "*7 3 Phase 0ut P ut Source Converter Booster W FIG. 11. Synchronous converter with and without a booster. (a) Three-phase synchronous converter. With this connection it can be started from the D.C. end. Special field connections are necessary for self-starting from the A.C. end. (6) Three-phase synchronous booster converter, showing A.C. connections from the three-phase source through the booster, to the converter. The rheostat can be manipulated so that the field current flows in either direction, and the boosting is either positive or negative. sists of a synchronous converter direct-connected to an A.C. generator which is used as a booster. This Combination costs somewhat more than the synchronous converter alone, but less than the motor-generator set. Its efficiency is nearly as high as that of the synchronous converter, and it has the voltage flexi- bility of the motor-generator set. The external connections of these machines and some of the internal connections of a syn- chronous booster-converter are shown in Fig. 11. 1 For data, applications and operation see G. Chapter XXXIX; S. 9 : 38-84; A. pp. 279, 280, 290, 291. 2 A. p. 950: Induction motor driving D.C. generator. p. 279: Synchronous converter versus motor-generator. S. 9: 55-61 Comparison of motor-generator sets and synchronous converters. 7: 332 Flexibility of motor-generator. 12: 63 Efficiencies, costs and floor space. 3 S. 9 : 20 The synchronous booster-converter. CONVERTERS 37 (c) A mercury vapor rectifier consists essentially of a receptacle containing mercury vapor, connected to a single-phase or poly- phase source, from which the mercury vapor causes the selection of a current flowing in only one direction. 1 Its best application at the present time is to circuits of 110 volts or more, and relatively small cur- rents. Rectifiers are made for use in connection with series lighting circuits, for as many as seventy-five 6.6 amp. arc lamps, but they are more often made to be connected to 110- or 220-volt A.C. circuits, for charging batteries at D.C. voltages from 2 to 120, and currents from 5 to 50 amp. A simple diagram of a mercury vapor rectifier is shown in Fig. 12. (d) A vibrating rectifier accomplishes mechan- ically what the mercury vapor does by other means it has a vibrating contact that closes, at such times that the current can flow in only one direction. 2 This rectifier is made for low voltage only, at which the efficiency of the mercury vapor rectifier is very low. It is connected to a 110- or 220-volt single-phase circuit, and de- livers as much as 8 amp. to three lead storage cells in series. The external connections of a vibrating rectifier may be represented as in Fig. 13. While there is some overlapping in the application of these converting devices, each has certain features peculiar to itself that adapt it to certain kinds of service: (1) If a transmission line has a large voltage drop, a motor-generator set or a synchronous booster- converter will probably be the best choice, because D the D.C. voltage can be maintained constant with D.O.Output to Battery FIG. 12. Mercury vapor rectifier. The A.C. source connects through an a u t o-transf ormer to the rectifier. The zig-zag line in the lower right- hand corner of the diagram represents a resistance leading to a starting ter- minal. to Battery FIG. 13. Vibrating rectifier. fluctuating A.C. terminal voltage. If the power factor of the line current is low, the motor field of the synchronous motor-generator set can be over-excited, to produce a leading current, and raise the power factor. (2) Where line drop is low and power factor high, the ordinary synchronous converter can be used to advantage, because it 1 G. 388; S. 6 : 269-281; A. pp. 1209, 1210. 2 S. 6:289-295; A. p. 1211. 38 ELECTRICAL EQUIPMENT is more efficient and a little less expensive, especially if it re- quires no additional transformers. (3) For smaller currents, the rectifiers have their field of usefulness, the mercury vapor rectifier being suited to all voltages above the 8 or 10 volts required for charging three lead storage cells. Table VI. shows the efficiency of the various means for converting, under the conditions for which they are adapted. TABLE. VI. COMPARISON OP EFFICIENCIES OF THE SEVERAL KINDS OF CONVERTING AND RECTIFYING APPARATUS. Apparatus Kw. Amperes in D.C. circuit D. C. voltage Approximate efficiency at full-load (per cent.) Motor generator 1 2 to 10 70 set / 15 to 300 80 Synchronous converter or synchronous booster con- 2 to 10 15 to 300 85 90 verter Mercury vapor rectifier, con- stant potential } 5 to 50 5 to 50 5 to 50 5 to 50 15 50 no 220 50 75 85 90 Mercury vapor rectifier For constant current 4 6.6 Up to 5500 Up to 4000 Up to 95 Up to 95 circuits . . . J Vibrating rectifier } 8 5 to 10 55 7. Raising or Lowering D.C. Voltage. There are several con- ditions requiring a change of D.C. voltage. Those most fre- quently found in practice, on constant potential circuits, and the means employed in producing the change are as follows: Three-wire System for Lighting Circuit. 1 The most satisfactory voltage for arc and tungsten lighting is 110 volts. This voltage is too low for satisfactory distribution in a large industrial plant, in which the motors are usually operated on 220 volts. The G. 368; S. 13:82, 83; A. p. 366. CONVERTERS 39 generators furnish power at the motor voltage, and some means is to be provided for furnishing the lower voltage to the lamps. If the lamps are adapted to one-half the motor voltage, they may be connected from one of the lines to neutral. A balancer-set may be installed as indicated in Fig. 14, to keep this voltage actually neutral that is, midway between positive and negative. 1 This set consists of two shunt or compound machines that are exactly alike, direct-connected mechanically, and connected electrically in series across the outside lines. The neutral line connects to a point between the two armatures. From Generators Balancer Set 2 & 3 Wire Li8htiug Feeders , To Motor To .Motor To Motor Power Feeder FIG. 14. Balancer set on D.C. 3-wire system. When the lamp load is exactly balanced that is, when the current flowing from the positive line is the same as that flowing to the negative there is no current flowing back in the neutral wire, and the two machines of the balancer-set float on the line as idle motors. But if more or larger lamps are on one side of the line than on the other, the joint resistance of lamps on that side is less. As a result, the voltage across that side would drop, and that across the other side would increase, except for the balancer-set, which keeps the neutral wire at or near actual neutral voltage, allowing the unbalanced portion of the current to return through the neutral and the balancer-set, instead of flowing through the lamps. With this unbalanced condition of the load, the machines are running, one as a generator furnishing the unbalanced current to the side requiring more; and the other as a motor driving the generator. If each of the machines has a series winding, the connections can be made as shown, so that the current of the neutral wire flows in such directions through the series fields as to decrease the voltage on one side and to increase 1 G. 347; S. 8: 224, 225; A. p. 375. 40 ELECTRICAL EQUIPMENT FIG. 15. Three-wire D.C. generator and balance coils. it on the other, compensating for RI drop in the neutral line and armature. Instead of the balancer-set, a three-wire generator may be employed, as in Fig. 15. x The generator itself is essentially a two- phase synchronous converter. Each balance coil is simply a coil of large reactance connected by slip-rings across one of the phases. On account of the high reactance, the alternating current flowing through the coil from one slip ring to the other is negligibly small. The middle points of the two coils are at neutral voltage; they are connected together, and to the neutral line. Since direct current is not affected by reactance, it can flow readily in the neutral line, either to or from the generator. Sometimes only a -single balance coil is used, and is connected to single-phase slip-rings. In Fig. 15 the slip-rings and the commutator are shown at opposite ends of the arma- ture. Frequently the two are at the same end, but the opera- tion is unchanged. Multiple Voltage for Motor-speed Adjustment. 2 A three- wire circuit such as has just been described for lighting purposes can be used for motor-speed adjustment. When the armature is connected from one line to neutral the speed is practically one- half that when it is across the outside lines. Further speed con- trol is obtained by a field rheostat, as stated in Chapter IV, page 25. Ward Leonard and Ilgner Systems for Motor-speed Adjustment. 3 The Ward Leonard system may be considered as a refinement of the multiple- voltage system. It is limited to the few cases where the fine adjustment obtained is worth the cost. The motor whose speed is to be adjusted has its field excited from a con- stant-potential source, and its armature is connected to the generator armature of a motor-generator set. Thus in Fig. 16, motor A, which is connected to the constant-potential source, drives generator B at constant speed. Rheostat R controls the voltage of the generator. Motor C, whose speed is to be adjusted, has a constant current in its field, and a variable 1 G. 348; S. 8: 190-199. 2 G. 129; S. 15: 448; A. p. 966. 3 G. 130; S. 7: 341-345; S. 15:99-107; A. p. 976. CONVERTERS 41 voltage across the armature, depending on how much of the resistance of rheostat R is in the circuit. The motor speed depends on its armature voltage and is thus controlled by the generator field rheostat. The direction of rotation of the motor is reversed by reversing the field connections to the generator. Sometimes motor A is an induction motor, but motor C and generator B are always D.C. machines. Generator Motor A B C FIG. 16. Ward Leonard system of motor speed control. Motor A is sometimes an A.C. motor. The Ilgner system is similar to this, having fly-wheel, as shown dotted, driven by motor A, which is usually an A.C. motor. The Ilgner system is similar to the Ward Leonard; in addition there is put on the shaft of the motor-generator set a flywheel that gives up its energy when the motor-generator set slows down. This system is used for hoisting, and is subject to short, heavy peak loads. As usually installed, motor -A is a slip-ring induction motor, and B and C are D.C. machines. The speed of motor A is controlled by an automatic device to utilize the energy in the flywheel when the peak loads come; and the speed of hoisting by motor C is controlled by the generator field rheostat. Boosters for Battery Control, and Line-drop Compensation. 1 A booster is merely a generator used to raise or lower the line or other voltage when it is necessary. It is used to raise or lower the voltage of a battery circuit, so as to make the battery charge or discharge more strongly than it would if it were floating on the line without the booster. A booster is also used to raise the voltage on a long feeder, in which the voltage drop would otherwise be excessive. i G. 204-209, 346; S. 8: 184; S. 12:86; S. 20:147-160, 165, 166; A. pp. 97-100. 42 ELECTRICAL EQUIPMENT The voltage of the booster, which is superposed on the battery or line voltage, may be regulated either by hand or automatically. If it is regulated by hand, the field circuit is connected through a field rheostat, across the line. The machine is then called a shunt booster. If the operation is to be automatic, at least one winding of the booster is in series with the circuit that controls the booster voltage. For example, for compensating for line drop the field coil of the booster is in series with the line ; and for maintaining approximately constant current in the generator, a booster field winding is in series with the generator. There are a number of ingenious arrangements used in connection with battery charging and discharging, that make the battery take a part or practically all the fluctuation of the load on the line, so that the load on the generator can be made to remain prac- tically constant. 110 or 220 Volt Source FIG. 17. Dynamotor for delivering very low voltage. Very Low D.C. Voltage. A dynamotor 1 is especially suited for supplying a heavy current at a very few volts; for example, for some electrolytic work. As shown in Fig. 17, a commutator at one end connects to the higher D.C. voltage usually 110 or 220 volts. This commutator and the corresponding winding serve as a motor element, driving the machine at constant speed. Another commutator at the other end, and its low-voltage wind- ing, serve as the generator element. The high- and low-voltage windings are entirely distinct, but are laid in the same slots and revolve in the same field. 1 S. 9: 126-133; A. p. 382. CHAPTER VII TRANSFORMERS 1 AND AUTO -TRANSFORMERS The original use of transformers was to step the A.C. voltage up at the beginning, and down at the end of a transmission line. Since the original applications, new applications of that type of transformer have been made, and various other kinds of trans- formers have been applied to new uses. We shall consider several of the more important applications, beginning with the original step-up and step-down transformers. Applications and Operation of Transformers. Transformers for stepping up the voltage are unnecessary if the generator voltage is high enough; but it pays to build small generators for relatively low voltage and to step the voltage up, rather than to build small generators for very high voltage. Voltage and Ratio. A transformer should not be put across a voltage much above its rated voltage, because if the amount of 00 W FIG. 18. Transformer with the secondary winding in two equal sections. In (a) the sections are in parallel, and in (Z>) in series. Sometimes the primary winding is also in sections, which may be either in parallel or in series. iron has not been generous, the resulting magnetizing current may be excessive. Too high voltage also endangers the insula- tion; but the magnetizing current, rather than the insulation, usually limits the voltage. If the line voltage is lower than the transformer rated voltage, no harm is done except that in ex- treme cases the voltage regulation and efficiency are poor, and the kva. capacity goes down with voltage. 1 G. Chapter XXXIV, Characteristics; XXXV, Connections. S. 6:85-126; 133-135; 137-145; 147-149; 155-161, Characteristics, data and connections. S. 10: 837-849, Applications. A. pp. 1606-1610, Classification and Theory. 1612-1617, Connections. 1632-1637, Applications, weights and costs. See Chapters XIV and XV, Regulating and Instrument Transformers. 43 44 ELECTRICAL EQUIPMENT To guard against applying to wrong voltages, it is well to ex- press the ratio in actual volts, as 2,200/110, rather than to make the numerator or the denominator unity, as 20/1 or 1/20. If a 2,200-volt transformer be put on a 2,500-volt circuit the operator will then know that the magnetizing current will be excessive. Some transformers are provided with voltage adjustment, which is in one or more of four ways: (a) The secondary is divided into two or more equal parts, which are to be connected either in parallel, as in Fig. 18a, or in series, as in Fig. 186, depending on the FIG. 19. required secondary voltage, (b) Several leads Transformer with , , , J f . , ' several secondary are brought out from points a few per cent. leads brought out f rom one end of the secondary winding, as in for adjustment of ,.,. _ ,. ,, A , , <. .-,. ., , voltage. Fig. 19. Of course the methods of Fig. 18 and Similar leads are Fig. 1Q cannot be Combined, UnleSS both Wind- sometimes brought . . out from the pri- mgs in Fig. 18 are provided with additional leads shown in Fig. 19, (c) The primary is divided into equal parts, as the secondary is divided in Fig. 18. (d) Primary leads are brought out as in Fig. 19. The application of transformers with extra leads, such as the foregoing, must be with due care. Unless there is definite infor- mation to the contrary, the transformer should not be put on a line of the highest rated primary voltage, unless the primaries are in series, and all or nearly all the end turns are in circuit; it should be assumed that primary end turns are intended for adjusting for low primary voltage. Capacity in Kva. The product of the voltage times the cur- rent in a single-phase circuit is called the apparent power, and is expressed in volt-amperes. 1 This is the same as the power in watts, if the power factor is at 100 per cent., but at lower power factors the number of volt-amperes is greater than the number of watts. This unit and the kilovolt-ampere (= 1,000 volt-amp.) are used to designate the capacity of A. C. apparatus to trans- form or deliver current at a given voltage. Transformers and A.C. generators are regularly rated in kilovolt-amperes (abbreviated to kva.), rather than in kilowatts, because a kilo- watt rating has no definite significance unless the power factor is given. If the current is balanced on a polyphase circuit, the 'G. 246; S. 24:27; A. p. 1298. TRANSFORMERS AND AUTO-TRANSFORMERS 45 same relation exists as on single-phase, between watts and volt-amperes. Thus, we have, On a single-phase circuit kva. = kw./P.F. = #7/1,000 On a two-phase circuit kva. = kw./P.F. = 2#//l,000 On a three-phase circuit kva. = kw./P.F. = \/3#//l,000 where E is the voltage between lines and I is the current per line leading to the transformer or group of transformers. It is at once evident that to get the maximum power from trans- formers and other similar equipment, they should deliver power at as high a power factor as possible. If a transformer is used only intermittently, it will deliver safely much more than the rated kva. output, for the short time (see Chapter XX, p. 183). 0.333 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Bated Amperes in High. Tension Winding FIG. 20. Curves showing full-load efficiency and regulation of typical transformers. Efficiency. Fig. 20 shows how transformer full-load efficiency depends on the current capacity of the high-tension winding. These curves are based on data on about 100 typical trans- formers of voltages from 2,200 to 110,000, and are correct in most cases within 0.1 per cent, or 0.2 per cent. It will be seen that the losses in transformers made for 25 cycles are nearly 1.5 times those for 60 cycles. The variation of efficiency with load in a typical 46 ELECTRICAL EQUIPMENT transformer is illustrated in Fig. 21. In some transformers full- load is a little nearer the point of maximum efficiency than is in- dicated on this curve. Regulation. The per cent, regulation is the per cent, ratio of the change in secondary voltage between no-load and full-load to the transformer rated secondary voltage. 1 Fig. 20 shows the regulation, as well as the efficiencies of typical transformers, when the load is at 100 per cent, power factor. At other power factors the regulation is poorer than at 100 per cent. In a transformer having a high-tension voltage of 13,200 or less, with 100 99.5 r 97.5 97 10 20 30 40 50 60 70 80 90 100 110 120 130 Percent of Full Load FIG. 21. Curve showing variation of efficiency with load in a typical power or lighting transformer. a lagging current, the regulation at 80 per cent, power factor is usually between 2 and 3 per cent. With a leading current at 80 per cent, power factor, the regulation is not far from zero. Frequency. The weight and cost of a transformer are greater for low than for high frequencies (see Chapter III, p. 17). A transformer designed for a given frequency can be used on a circuit of a higher, but not of a lower frequency, at the same voltage. An auto -transformer 2 is a transformer in which the primary and secondary are combined in a single circuit, as in Fig. 22a, where the primary is connected across AB and the secondary across AC, or vice versa. The winding from A to C is common !S. 24:560, 565; A. pp. 1327, 1328. 2 G. 307, 332, 333. 8.6:173-178, 180, 181. A. pp. 63-65. TRANSFORMERS AND AUTO-TRANSFORMERS 47 to both primary and secondary and that from C to B is in only the circuit of the higher voltage which we will call the primary. The advantage of an auto-transformer over a transformer is seen by comparing Fig. 22a with 6. Neglecting all losses and the reactance drop, the apparent watts input equal the output. If the avto -transformer is used to step down from 110 to 100 volts, and is to deliver 11 amp., the current taken from the primary line must be 10 amp. Winding CB must carry 10 amp., and winding CA must furnish the other 1 amp. Thus, winding CB is for 10 volts X 10 amp., or 100 volt-amp., and winding CA Secondary Primary Secondary Primary FIG. 22. Auto-transformer and transformer, with primary voltage higher than the secondary. If the contact at C is movable, and leads are brought out from suitable points in the winding, any desired secondary voltage can be obtained, between the primary voltage and zero. If the primary and secondary are interchanged, the secondary voltage is higher than the primary. Such a connection must be made with care, to avoid too high a voltage for the shorter winding. is for 100 volts X 1 amp. or 100 volt-amp. But each winding of a transformer, as in Fig. 226, must be for 1,100 volt-amp.; so that the required size of the auto-transformer is only one- eleventh that of the transformer to do the same work. One can readily see also that the efficiency of transformation is much higher in the auto-transformer than in the transformer, because the losses are very small in the case above, correspond- ing to the losses in a transformer of one-eleventh the size that would be required for ordinary transformation. The advantage in size and efficiency is less marked, when the ratio is farther from unity. In the case of Fig. 22, with the same primary voltage, if the output were at 11 volts, instead of 100, the volt-ampere capacity of the auto-transformer windings would be nine-tenths of that of the corresponding transformer. One of the most important applications of auto-transformers is to starting squirrel-cage induction motors. This is better than to use rheostats, because less power is wasted. It is better 48 ELECTRICAL EQUIPMENT than to use ordinary transformers, because the auto-transformers are smaller and less expensive. 3. Grouping of Transformers. The only groupings of trans- formers that we shall consider are some of those on three-phase circuits. Two-phase combinations are omitted because they are comparatively simple, and require no explanation except what is given in this chapter with reference to the Scott connec- tion. Six-phase connections are important for those who have to do with large synchronous-converter substations; but they are little used elsewhere. , Several of the more usual groupings of transformers are illus- trated in Fig. 23. Of these, the delta connection, Fig. 23a, is used more than any other, 1 for ordinary transformation. Perhaps the greatest advantage of having a delta connection in both primary and secondary is that in case of breakdown of one transformer, it can be removed, and the group can continue to operate with a V-connection. The two remaining transformers will be carrying a 73 per cent, overload, if they deliver the same power as was required of the three at full-load, but if the load is cut down to 58 per cent, of the original full-load, the two are only carrying normal load. This fact holds true, not only for a group of three single-phase transformers, but also for a three- phase transformer, if one phase is out of commission. In a three-phase core-type transformer care must be observed that there are no dangerous loose ends in the broken-down phase of the transformer; for there is full voltage in all phases, even though both primary and secondary of one phase are entirely disconnected from the system. The voltages are not as well balanced, and the efficiency of transformation is lower with the V than with the delta connection. The Scott connection is illustrated in Fig. 23/i. Two trans- formers are T-connected to a three-phase circuit; and the other windings of the transformers are connected to a two-phase circuit. This connection is suitable for transforming either from three- to two-phase or from two- to three-phase. To appreciate the importance of this somewhat familiar con- nection, imagine a machine shop in a city such as Philadelphia or New York, in which two-phase power has been obtainable for 1 Except on very high voltages where the Y-connection is used on the high-tension side; the low-tension side may even then be delta-connected. See Fig. 236 and d. TRANSFORMERS AND AUTO-TRANSFORMERS 49 a long time. Suppose that a change in management of the elec- tric power company results in changing from a two-phase to a three-phase system. It is only necessary to install a pair of Scott-connected transformers, to continue the two-phase system Delta to Delta Connection This connection is nearly always used for ordinary Power purposes except on very High Voltages ~7j ' r~ ^ A (bw Neutral Line or Ground on One or Both Sides if Required Y to Y Connection Ground Connection, if Required. Y to Delta or Delta to Y Connection C) V to V Connection Used in Emergencies when One Transformer in a Delta Connection Breaks Down. Used also with. Voltage Transformers for Meters. V Connection of Auto-trans- formers for Motor Starting This Transformer is Preferably Wound.for only 86.6 % of Line Voltage on each Side 50% Tap on each e of this Transformer 00 T to T Connection > 86.6 % Tap, or Winding for -* 86.6 % of Line Voltage T to 2 -Phase or Scott Connection FIG. 23. Transformer grouping. w Scott Connection of Auto-transformers in the shop; whereas without this connection or its equivalent, it would be necessary to take out every two-phase motor in the shop and replace it by a three-phase motor, costing from $100 to $1,000 for every motor changed. 50 ELECTRICAL EQUIPMENT The Scott connection may be employed to step the voltage up or down, or to transform with no change in voltage. If the two voltages are the same, or nearly the same, auto-transformers may be used; there must then be no other electrical connection between the two phases of the two-phase circuit. The auto- transformer connections for equal two-phase and three-phase voltages are shown in Fig. 232. CHAPTER VIII STORAGE BATTERIES 1 COMPARISON OF TYPES OF STORAGE BATTERIES The storage cells used in practice are of two kinds. One of these is the lead cell, whose electrodes are lead and lead peroxide, and whose electrolyte is dilute sulphuric acid. One form of lead cell has "pasted" plates, in which the active material is added in the form of a paste. Another has "Plante" plates which are formed electrochemically by putting them in the electrolyte and passing a current repeatedly first in one direction and then in the other. A third form of lead cell is the " Ironclad," whose posi- tive plates are made up of a series of hard-rubber slotted tubes containing the active material. Besides these lead cells are the alkaline cells. The Edison storage battery is of this type, and is the alkaline battery in most general use; it has caustic potash for the electrolyte, and nickel peroxide and iron for the electrodes. So much depends on the size and kind of installation, and the treatment the battery receives, that the following information must be taken only as a general guide, subject to large correc- tions in individual cases. Among the more important considera- tions are the following: Cost. Alkaline batteries cost about $80 per kw.-hr. capacity; portable lead batteries with pasted plates cost $30 to $55 per kw.-hr. capacity. The cost of stationary batteries with Plante plates is 1.5 to 2 times that for pasted plates, depending on size, liberality of construction and various other details. It is not enough higher to warrant the use of pasted plates where the more 1 G. Chapter XXIII, Characteristics; Chapter XXV, Applications; Chapter XXVI, Train Lighting. S. 20: 43-233, Characteristics and Applications; 10: 898, 900 Deprecia- tion; 12: 77-79 Stationary Installations; 17: 62-84; 22: 67-85 Vehicle Bat- teries and Charging; 22: 292-298 Train Lighting Systems. A. Lead Batteries pp. 103-119; Alkaline Batteries pp. 77-86; Applica- tions pp. 87-102. 51 52 ELECTRICAL EQUIPMENT durable Plante plates can be put in small enough compass, except for "stand-by" batteries, and others used infrequently. Space Occupied and Weight. The net space occupied by an Edison battery is about 0.7 cu. ft. per kw.-hr. of energy to be stored. That of a portable lead cell with pasted plates is from 0.5 to 0.7 cu. ft., and that of a lead cell with Plante plates about 1.5 cu. ft. per kw.-hr. The weight of the Edison battery is about 75 Ib. per kw.-hr., the lead battery with pasted plates 65 to 125 Ib. and with Plante plates about 200 Ib. The volume and weight of lead cells varies considerably depending on liberality of design, kilowatt-hour capacity, and kind of retainer, as well as on kind of plates. Durability and Repairs. The Edison battery is thoroughly guaranteed by the manufacturers, for a length of time depending on the conditions under which it is to operate. The time of this guarantee is usually 4 or 5 years. Lead cells with pasted plates 'for portable service are much less rugged: their life depends on how much their durability has been sacrificed to make them light, as well as on the kind of treatment they have. With rea- sonable care they can be fully charged and discharged from 300 to 500 times. Cells with Plante plates for stationary service, on account of more rugged construction, more liberal design, and more favorable conditions of operation, last with good care from 5 to 10 years. Pasted plates, under similar conditions, need not be very much shorter lived. Plante" plates are used in train lighting, which is between the portable and stationary in- stallations as to favorable conditions. Their life is about 40 per cent, of that of the stationary batteries. Annual cost for maintenance and repairs of portable batteries (not including cost of charging) is $10 to $20 per kw-hr. capacity for Edison batteries and about $25 for lead cells with pasted plates. Train-lighting batteries are so well constructed that maintenance and repairs need not be over 10 to 20 per cent, of that of other portable batteries. Stationary batteries, well- installed and properly cared for, require no allowance for main- tenance and repairs that cannot be included in ordinary attend- ance, together with the allowance for replacing them. The current-discharging rate of a Plante lead cell for stationary use is usually such that the battery discharges in 8 hr.; that is, a 600-amp.-hr. battery would normally deliver 75 amp. for 8 hr. If such a battery is called upon for more than 75 amp., STORAGE BATTERIES 53 the time is so much reduced that the total ampere-hour capacity is less. Table VII shows the maximum time of discharging, and the ampere-hour capacity, at various currents. TABLE VII. AMPERES AND AMPERE-HOURS OF PL ANTE LEAD STORAGE BATTERIES ON HEAVY DISCHARGE When the discharge rate is The battery will deliver that current for That is, at that current it is capable of delivering per cent, of rated ampere-hours 1 X normal amp. 8hr. 100 2 X normal amp. 3hr. 75 3 X normal amp. 1 hr. 35 min. 60 4 X normal amp. Ihr. 50 6 X normal amp. 30 min. 37 8 X normal amp. 20 min. 33 It should be noted that these percentages indicate the relative capacity of the battery at various currents. They are not the same as efficiency, although the efficiency does drop off very much with increase of current. For pasted lead cells with thin plates, suitable for vehicle use, the decrease in capacity on overload is not so great as with Plante plates. If they are discharged in 1 hr., their capacity is 60 per cent, of that in 8 hr. ; if discharged in 2 hr. the capacity is 75 per cent, of that in 8 hr. For other times of discharge the percentage is correspondingly higher than in the table. Pasted cells for portable use are not always rated on an 8-hr., but fre- quently on a 5- or 6-hr, basis. For Edison cells the ampere-hour capacity is practically the same at all rates of discharge. Their current rating is regularly based on discharging in 5 hr.; i.e., the ampere-hour capacity divided by 5. The Current-charging Rate. The most approved method of charging lead cells is to give the battery a " tapering charge" that is, starting the charging at a high rate and gradually reduc- ing the current. The charging rate starts at from 1 to 2 times the normal discharge rate, and finishes at J^ to % of the normal discharge rate. The best charging of Edison cells is at a steady rate, equal to the normal discharge rate. Voltage. The discharge voltage of a lead cell is about 2 volts, and that of an Edison cell about 1.2 volts, when the cells are dis- charged at their normal rates. Fig. 24 shows how the voltage 54 ELECTRICAL EQUIPMENT drops off during discharge at normal current, and also during discharge that is so rapid that the cell is discharged in 1 hr. It also shows the voltage required for charging at the normal rate. At higher rates of charging, the voltage is a little higher; but if the generator voltage is high enough for the final charge at the normal rate, it will be ample for the initial charge at a (a) Lead Cell Charging at Normal Discharge Rate (6) >t " Discharging at Normal Bate (c) " Discharging in 1 Hour at 4 Times Normal Eate (d) Edison Cell Charging at Normal Discharge Rate (e) " " Discharging at Normal Rate (/) > Discharging ia 1 Hoar at 6 Times Normal Eate 12845678 9 10 FIG. 24. Voltages of storage cells during charging and discharging. higher rate. The lead-cell voltages in Fig. 24 refer primarily to cells of the Plante type, but they apply rather closely to all kinds of lead cells, if the discharge rate is on the 8-hr, basis. Efficiency. The efficiency of a storage battery varies greatly with the rate of charging and discharging. In ordinary service, portable Edison batteries have a watt-hour efficiency of 40 to 50 per cent., and portable lead batteries 65 to 75 per cent. The STORAGE BATTERIES 55 ampere-hour efficiencies of the same batteries are 60 to 70 per cent, for Edison, and 80 to 90 per cent, for lead batteries. On very heavy loads, the watt-hour efficiency drops even lower than the lowest values given, and on very slow discharge rate it rises even higher than the high values. APPLICATIONS TO STATIONARY SERVICE Since space and weight are not of vital importance, it is not necessary to sacrifice ruggedness. The combined qualities of durability, high efficiency and relatively low cost of the Plante* - H"H FIG. 25. End-cell switch connected to storage battery. cell gives it first place in common use for stationary service, although pasted plates also prove satisfactory. However, if a battery is to be discharged infrequently, even at very high rates, it is more economical to install pasted plates, as their life com- pared with that of Plante plates is a matter of discharges, and not of time. Several of the more common applications of batteries may be mentioned: In the generating station a battery may be installed in parallel with the generators. Its main purpose is to relieve the generators at certain times, but it may also serve to maintain constant voltage. Such a battery may take the instantaneous fluctuations in the load current; or it may furnish a part of the steady power output of the plant when the station is overloaded, or when one 56 ELECTRICAL EQUIPMENT generator may thereby be shut down ; or it may furnish all the power in an emergency or when the load on the plant is light enough to shut down all the generators. 1 The internal drop in the battery is so great that it will not of its own accord furnish current for the instantaneous fluctuations, but an automatic booster in series with the battery may be excited by a field winding in series with the generator, in such a way that practically all the fluctuations in line current come from the battery. For taking a steady load, it is not necessary that the battery be provided with an automatic booster. A shunt booster may be used whose field rheostat is operated manually; or an end-cell switch such as is illustrated in Fig. 25 may be operated manually or by an automatic device, to make the battery carry the fluctua- tions of the load. In a battery substation, if the voltage drop on a feeder is excessive, a battery may be connected to the feeder near its end, to keep the voltage more nearly constant. A study of the battery characteristics at various charging and discharging currents will show how great voltage variation exists when the battery is doing its part to maintain a constant voltage. If closer regula- tion is required, a booster or an end-cell switch may be employed. The battery should be located far enough out on the feeder to keep up the voltage at the end; but the drop at intermediate points should not be excessive on account of having the battery too far out on the feeder. A study of the distribution of the load, and the characteristics of the battery, will indicate the most desirable location. On circuits that are entirely distinct, or connected through resistance, batteries are used for various purposes. If there are wide fluctuations of the voltage on the power circuit, a storage battery may be first charged and then used for lighting, or for laboratory or other purposes requiring a more constant voltage. This requires a separate period of charging, and perhaps some attention to voltage regulation. To avoid these difficulties the battery may be connected to the line through a rheostat. The higher the resistance of the rheostat, the less effect will fluctuations of line voltage have on battery voltage, but the resistance and the number of cells must be small enough to keep the battery charged. Besides use for obtaining very constant voltage, such a battery 1 For connections see footnote, p. 51. STORAGE BATTERIES 57 may be used to insure a voltage when there is no generator voltage. For example, a circuit-breaker should operate without fail when an accident reduces bus and line voltage to approximately zero, but this is just the condition under which it cannot operate if it has a trip-coil (see Chapter XVII, p. 135, Fig. 65), operating on generator voltage. Another important application is to lighting and other circuits that require power, 24 hr. per day, where the generator runs only during the daytime. Still another important application is to apparatus requiring small amounts of power at low voltage. Small motors, bells and other small equipment are sometimes better adapted to a few storage cells than to higher voltages. Even the three- wire system for lighting may be included in the same class. The neutral volt- age can be established by the middle point of a battery that con- nects across the entire line. APPLICATIONS TO PORTABLE SERVICE Batteries used for portable service should be as light and com- pact as possible, and at the same time they should be as efficient and durable as possible, and they should not be unnecessarily expensive. Obviously not all these conditions are obtainable to the fullest extent in any one kind of battery. Referring to the advantages of the various kinds of batteries; as already stated, we find that an Edison battery is light and durable, but is not quite so compact as the most compact lead cells, nor so efficient as lead cells, and it is more expensive. The several advantages and disadvantages must be weighed in each case, and those that are most important will usually dictate the battery to be used. Lead cells with Plante" plates are little used for portable service, because their only claim of great advantage over the pasted plates is in durability; durability must be sacrificed in portable batteries, in favor of less bulk and weight. The exception is in train-light- ing batteries, which are not subject to these restrictions to the same extent. The voltage of portable batteries is usually less than 110 volts and depends on the amount of power to be delivered. There are no standards of voltage that are now in universal use, but the following have been found satisfactory in a great many cases, and are gradually being adopted as standards: 58 ELECTRICAL EQUIPMENT Volts Automobile ignition, lighting and starting 6 Electric automobiles 60 to 85 Battery locomotives 80 to 240 Battery trucks 24 to 60 Car lighting (batteries fully charged, 32 and 64 volts) nominal voltage 30 and 60 The amount of power required for each purpose varies somewhat, but not greatly, from the following: Automobile lighting : Watts Headlights, two to be provided each, 15 Side lamps, two to be provided each, 3 Tail lamp, one to be provided each, 3 Other lighting, if desired, need not exceed 15 Gas engine ignition 3 Automobile electric starting, small automobile 500 Average automobile 600 to 700 Large automobile 1,000 Railway car lighting : Pullman sleeper, 16 section . 1,600 Coach 500 Mail car, 60-ft 650 Baggage car 300 Dining car. . . . ; 1,600 Automobile Lighting, Ignition and Starting Power. Auto- mobiles that are not self -starting require very small battery power for lighting and ignition. For self-starting, according to the above table, much more power is required, but even in that case the three-cell battery is sufficient. The large amount of power required for starting is used for only a very short time, so that it is allowable to overload the battery. It is accepted practice to furnish a battery of such size that it will deliver the required power for starting for a period of 20 min. A lead battery made for this purpose, with thin pasted plates, will deliver for 20 min. six or seven times the current that it will for 5 hr., at an average of 85 per cent, of the initial voltage that it would have at normal load. An Edison battery will deliver for 20 min. six times the current that it would for 5 hr., at about 70 per cent, of the initial voltage that it would have at normal load. Electric automobiles, battery trucks and battery locomotives are driven by motors operated from storage batteries. 1 For 1 See Proc. A. I. E. E., 1916, A. E. Kennelly and 0. R. Schurig, "Tractive Resistances to a Motor Delivery Wagon on Different Roads and at Different Speeds." STORAGE BATTERIES 59 battery trucks, less power is required than for automobiles and locomotives, and the customary voltage is less, as indicated above. The resistance to rolling on a floor or track depends on the kind of tire, the kind of floor or track, and the speed. As the speeds are not over 12 miles per hr., if we have wheels with solid or pneumatic tires, the total resistance, including chain and bearing friction, windage, and tire friction, is about 25 Ib. per ton weight of truck and load, on a good level floor. With wheels having steel tires, on good rails, the total resistance of a locomotive is about 20 Ib. per ton. If the floor or track is inclined in any part of the travel, the grade must be considered in addition to these figures. The weight of a battery truck or locomotive varies considerably, but values near enough for estimating the loads to be carried by the battery are the following : For battery trucks, up to 10 tons capacity, 1,000 Ib. + 30 per cent, of the weight of the load to be carried; and for locomotives with drawbar pull up to 2,400 Ib., 4,000 Ib. + 4 times the drawbar pull. The efficiency of the motor is about 80 per cent, at any ordinary load. This does not take into account losses due to speed adjustment by gear reduction or by controller series resistance; but it does per- mit speed variation with a ratio of about 3 : 4, by series and parallel connection of two sections of the series field. Train lighting is done in three different ways, each requiring the use of storage batteries: 1. The straight storage system. This requires a sufficient battery capacity on each car for all demands for light, to last until the car is brought again to a charging station. Usually it is not so satisfactory as one of the other systems. It is proposed to stan- dardize on a nominal voltage of 60 (32 lead cells) for this system. A number of roads are still using voltages ranging from 30 to 110, but new installations should be at 60 volts if possible, for the sake of uniformity. The size of cell most commonly used has a capacity of 300 amp.-hr. It is better to use this standard size if it is large enough, and not very much too large. 2. The head-end system has a generator, in the baggage car or on the locomotive, large enough to light the whole train. The battery need not be so large in this case as in the straight storage system, because it is only required to furnish lights during stops and for a time at the beginning and end of the run, for cleaning and other work about the cars. Some cars require light for longer times before and after the run than others. Thus, mail cars are 60 ELECTRICAL EQUIPMENT required to have sufficient battery capacity to light them for 12 hr. without recharging the batteries; diners require lights for laying in supplies, cleaning and other operations; Pullman cars require lights for cleaning, and for occupancy at the beginning of the run, before the car starts. One difficulty of the head-end system is that these several cars cannot be charged up in advance so readily as if each were handled independently. In some cases, but not all, the rule can be followed of making the total ampere- hour capacity of the battery one-half of what it would be if there were no generator on the train. This system has another dis- advantage where the train is broken up at junction points. If some cars are run on branch lines there must be some provision for lighting those that are so switched off. It is proposed to standardize the battery voltage for head-end systems on 60 volts. In a few cases 110-volt systems are in use at present, but new systems should conform to the standard. Cells of 300 amp.-hr. capacity should be used wherever practicable, but others may be used if necessary. 3. The axle-generator system has both a generator and a battery on each car, and is therefore the most flexible of all systems. The proposed standard for this system is 30 volts. At present dining cars and a few others commonly use 60 volts. Cells of 300 amp.- hr. capacity should be used if practicable. The same provision must be made as before indicated, for additional time of lighting mail cars, diners and Pullman sleepers. CHAPTER IX ILLUMINATION 1 THE ESSENTIALS It requires the application of only a few principles, in making the necessary computations for illumination. We shall consider enough of these principles to lay out the equipment for good industrial illumination. Illumination intensity refers to the strength of light on the object that is observed. If it comes from a single lamp, it varies as the candlepower and inversely as the square of the distance of the lamp from the surface (G. 398, 399). The candlepower is usually different in different directions, and the value used in computing should be found for the required direction, from a curve that shows the variation of candlepower with direction. In Fig. 28 are four such curves, showing the candlepower of a 100-watt lamp without a reflector, and with three different kinds of reflectors. Illumination intensity is expressed in foot-candles, and a surface is said to have one foot-candle of illumination when the light is from a one-candlepower lamp, one foot from the surface, if the beam of light is normal to the surface. The illumination intensity due to any lamp at any distance, with the light striking the surface at any angle, is / = C cos 6/D* where C is the candlepower in the particular direction, D the distance in feet between the lamp and the object, and 6 the angle between the light beam and the normal to the surface. If the i G. Chapter XLII. S. Section 14; also see list of references, paragraph 250. A. Theory, pp. 764-771; Interior Illumination, pp. 756-763; Street Illumination, pp. 772-778. Bulletins of Engineering Department, National Lamp Works of General Electric Co., Cleveland, O. In particular, Bulletin 20, Industrial Lighting. Chapters X and XI, D.C. and A.C. Lighting Circuits. Chapter VIII and references, p. 51, Train and Vehicle Lighting. 61 62 ELECTRICAL EQUIPMENT surface is lighted by several sources, the total intensity is the sum of the intensities from the several sources. The total amount of light, striking any surface, is the product of average intensity of illumination times the area. If the in- tensity is in foot-candles, and the area in square feet, the total light, or the light flux, is expressed in lumens. Consider a sphere, Fig. 26, of radius D, with a lamp at the center having a candle- power C, in all directions. The illumination intensity on the inside of the sphere is C/D 2 , and the total light flux in lumens on the inner surface of the sphere is 4irD 2 C/D* or 4irC. That is, the total light in lumens emanating from any lamp of uniform candlepower is 4?r times the candlepower. If the candlepower is not the same in all direc- tions, the total light is 4?r times the "mean spherical candlepower." Incandescent lamps are now rated in mean spherical candlepower. Up to the present time they have been rated in "horizontal candlepower," which is the . 26. Lamp of candlepower in a horizontal direction when C candle-power at the tip points straight down. This is about the center of a .,. , . , , ,-, . sphere having a ra- 1-25 times the spherical candlepower, but this dius of D feet. ratio differs in different lamps. At present, lamps are also rated in lumens or in watts or both. A 100-cp. (spherical) lamp produces 4?r X 100, or 1,257 lumens. The effect of reflectors is to change the direction of light, and to absorb a small amount of it. The light actually reaching the working plane is increased by the use of reflectors, and light that would be annoying, by shining in the eyes, is cut off. The dis- tribution of the light depends on the kind of reflector that is used. Reflectors are classified, with reference to distribution, as extensive, intensive, and focusing. An extensive reflector throws a large part of the light out toward the horizontal, and is suitable for use in lighting streets and large areas. An intensive reflector throws the light on an area that is less extended; it is suitable for ordinary industrial lighting, especially for purposes of general interior illumination. A focusing reflector concentrates the light on a relatively small spot, immediately below the lamp. It is not suitable for general lighting unless the lamps are placed close together, but it is especially good for spot lighting, where that is required. ILLUMINATION 63 The shapes of several of these reflectors are illustrated in Fig. 27, and the distribution of light is shown in Figs. 28 to 30. (a) Intensi ve ( 6) Extensi ve Bowl Shaped Dome Shaped (d) Angle ) Concentric Reflector and. Prismatic Glass Refractor FIG. 27. Typical reflectors. Co), (6) and (c) are made of steel, covered with porcelain or other reflecting material. They are suitable for in-door and industrial use. Extensive reflectors are usually dome- shaped. Intensive reflectors are either bowl- or dome-shaped; the dome-shaped are prefer- able on accout of high efficiency, and because they cast softer (less sharp) shadows. The chief disadvantage of dome reflectors is that they do not conceal the lamp filaments from view. See Fig. 28 for photometric curves of these reflectors, (d) is suitable for certain cases of special lighting. (See Fig. 29.) (e) is a combination reflector and refractor. It is suitable for out-door lighting. (See Fie. 30.) FIG. 28. Curves showing the distribution of light from a clear Mazda lamp, without a reflector, and with three kinds of steel reflectors. Radii are candle-power. This was a 100-watt lamp, operating at 9.1 lumens per watt, or 1.38 watts per spherical candle-power. The dotted line in Fig. 28 illustrates the fact that the candle- power of a Mazda lamp is relatively high in a horizontal direc- 64 ELECTRICAL EQUIPMENT FIG. 29. Curve of distribution of light from a 100-watt lamp (9.1 lumens per watt) with an angle-type reflector. FIG. 30. Curve of distribution from an 85 spherical candle-power series Mazda lamp. The power required varies somewhat, depending on the current for which the lamp is rated. It is about 0.8 watt per spherical candle. ILLUMINATION 65 tion, and that it is very low directly downward. The exten- sive reflector throws a stronger light than the intensive, at angles of more than 45 from the vertical, and the focusing reflector throws more than either of the others at less than 30. The first impression in comparing these curves would be that the total amount of light is several times as great with the focusing reflector as, with the others. This is not the case, because the solid angle is so small in which the light from the focusing reflector is very strong. As the light leaves the lamp and reflector, a large part of it is thrown directly on the working plane. 1 The rest falls on the walls, ceiling and other surfaces, and a part is reflected from there to the working plane. The utilization efficiency is the ratio of the light flux finally reaching the working plane to the total light produced. Thus, if 1,000 lumens are produced by the lamp, and the average illumination intensity is 3 foot-candles on an area of 100 sq. ft., the utilization efficiency for that area is 30 per cent. The efficiencies in Table IX are given by the National Lamp Works of the General Electric Co.; they show that in a large room having several rows of units in each direction, two or three times as much of the light reaches the working plane as in a very small room having only a single lamp. TABLE IX. UTILIZATION EFFICIENCIES OF ILLUMINATION Installation units spaced 1.5 to 1.6 times height above work Reflector Enameled steel dome, per cent. Enameled steel or pyro glass bowl, per cent. 1 unit 28 42 48 52 56 60 63 67 71 24 36 41 44 47 49 51 54 57 1 row of 5 units 2 rows of 2 units 2 rows of 3 units . . . 3 rows of 3 units 3 rows of 4 units 4 rows of 4 units 4 rows of 8 units 8 rows of 8 units . The intensity of illumination that is suitable for industrial purposes depends on considerations mentioned later particu- 1 That is, the horizontal plane at the average height of the work usually 30 to 40 in. above the floor. 66 ELECTRICAL EQUIPMENT larly on glare. It is sometimes easier to see in a room that has an illumination intensity of 1 or 1.5 foot-candles, produced by indirect lighting, than in a room having 3 foot-candles produced by direct lighting in which the lamp filaments are in the range of vision. If the filaments are concealed, and the lamps are placed as much as possible out of the field of vision, the intensities given in Table X should be sufficient, for ordinary cases; but special conditions may call for either higher or lower values. TABLE X. ILLUMINATION INTENSITIES Purpose of illumination Average values for well-placed lamps with suitable re- flectors, foot-candles Desk work 4-6 Fine machine work 5-10 Rough machine work 3-5 Storage 1 Passageways 1-2 The effect of dust and aging of lamps must be taken into account in providing for the illumination of a room or other space. The values of illumination intensity given in Table X refer to condi- tions that should be found in service, after dust has accumulated on the lamps and reflectors, and the lamps have become somewhat dimmed with age. The effect of dust depends on the frequency of cleaning the lamps, the location of lamps, general conditions as to dust, and the kind of reflectors used. An average dimming on account of dust, with the kind of reflectors commonly used in industrial plants, is 1 per cent, per week for the first 2 months, and a further dimming of 0.5 per cent, per week for the next 4 months. 1 The aging of a Mazda (tungsten) lamp, during a period of 1,000 hr. of use which is considered the normal length of life of the lamp, produces a decrease in lumens to 87 per cent. 2 of the initial value, at the end of the life of the lamp. The average during the life of the lamp is 94.5 per cent. 2 of the initial value. If we are willing to accept average illumination during the 1 These figures are for ordinary conditions. Under very bad conditions there may be a dimming of 50 per cent, in 1 month. 2 These figures show the decrease in useful, as well as in total light, if dome reflectors are used; but with bowl reflectors, the useful light drops in 1,000 hr. to 82 per cent, of the initial value, and the average during the life of the lamp is 92 per cent. ILLUMINATION 67 1,000 hr., we shall allow for the average effect of dust and aging; but if the minimum illumination intensity is not to fall below the specified value, it is necessary to allow for the accumulation of dust during the full period between cleanings, and the effect of aging in the full 1,000 hr. If the lamps are not spaced too far apart, and suitable re- flectors are used, the illumination is practically uniform over the entire area. The distances given in Table XI should not be exceeded, if uniform illumination is required. TABLE XI. MAXIMUM SPACING DISTANCES FOR UNIFORM ILLUMINATION (H is the height of the lamps above the working plane) Kind of reflector Distance between rows Extensive 2. OH Intensive ' 1 . 25H Focusing Q.75H Glare. The purpose of light is to make objects visible, and we should consider not only what intensity of light is produced, but also whether anything reduces the usefulness of the light. If the eye is accustomed to a very bright light, it is not in condi- tion to see very well on a moderately lighted surface. Any such interference with clear vision is called glare. There are several cases to consider: (1) Bright spot-lighting contracts the pupil of the eye, and makes it difficult to see objects in the vicin- ity, even if they are fairly well lighted. (2) A reflecting surface sometimes produces glare, if the light strikes it at such an angle as to be reflected to the eye. (3) The intensely bright filament of a tungsten lamp, or the arc of an arc lamp, if not covered, pro- duces glare when it is seen in looking less than about 20 above the horizontal. (4) A flickering light is similar to the other cases, and in addition the continuous changing tires the eye. Color of light has a considerable effect on its usefulness; for most industrial purposes the colors of tungsten and arc lighting are satisfactory. If the work requires careful color observations a test should be made before the installation is completed, to ascertain what type of lamp is most effective, and causes the least eye-strain. Shadows. If an object is so perfectly lighted that there are no shadows, the details of the object are not so plainly visible as if there are moderate shadows, showing by contrast where the depressions and projections are. On the other hand, if shadows 68 ELECTRICAL EQUIPMENT are too intense, the part in the shadow is entirely invisible. The best effect is obtained if the light comes from at least two or three directions. For drafting, and similar work on a plane surface, the less shadows there are, the better is the effect. THREE KINDS OF ILLUMINATION We have found that sufficient intensity, avoidance of glare, and moderate shadows are essential to good lighting. Each of these is obtained to a greater or less degree by each kind of illumination : Direct lighting has the advantage over indirect, that the light is used more efficiently. It has the disadvantage that the source of light is visible, and may produce glare. For the best results the lamps must be out of the field of vision, and close enough together so that the working plane is uniformly lighted and so that several lights from different directions show the form of every object by the shadows. Indirect lighting is more expensive on account of inefficiency. The surfaces lighted should be large enough to reflect sufficient light without producing glare. For even moderate efficiency, the lighted surfaces should be of a very light color. Semi-indirect lighting is produced by lamps, at least part of which are used for direct lighting, whereas some or all throw their light also on walls, ceilings or other surfaces, for indirect lighting. If the lamps are screened from the eye by an adequate diffusing medium, this kind of light may be very nearly as soft and free from glare, as indirect lighting, and it is less expensive. COMPUTATIONS There are two methods of finding the number, size and arrange- ment of lamps, to produce the required illumination. The point-by-point method is very tedious, and is as follows: a trial layout is made, of lamps and reflectors such as would be expected to give the necessary illumination intensity in all parts of the room or space. The intensity at a certain point is then computed by using the formula on page 61; it is the sum of all the inten- sities at that point from all the sources in the vicinity. There may be a dozen lamps whose effect at that point is to be computed. Then all these computations must be repeated at a large ILLUMINATION 69 number of points, so that the illumination in every part of the room is known. If a part or all the illumination is unsatisfac- tory, it must be changed, and new computations made. A much simpler, and quite satisfactory method is that of ob- taining the average illumination intensity. Reflectors have been so well developed that where proper spacing is not exceeded the illumination is practically uniform. If we assume a convenient height of lamps, the table of spacing distances on page 67 gives us the maximum allowable spacing between rows, and from that we know the area to be lighted by each lamp. The product of area and illumination intensity gives lumens per lamp. Or, if we assume a convenient size of lamp, we know the spacing and the minimum allowable height of lamps. The problem is one of finding consistent values of height, spacing and lumens of each lamp. If a satisfactory solution is not obtained directly, it may be desirable (1) to specify a spacing that is less than the maximum allowable, (2) to provide an illumination of higher intensity, or possibly slightly lower than was originally required, or (3) to provide general illumination that is considerably too weak, but is supplemented in certain localities by any method that is indi- cated below for special lighting. In any case computations are made for average illumination on the working plane, taking into account the effect of dust and aging of lamps and the utilization efficiency. In laying out the positions of the lamps, the scheme can some- times be modified to adapt it to the shape of the room and the lo- cation of machines, preventing dark corners and edges of the room, if good light is required in these places. The distance from the wall to the first row of lamps should not be more than one-half the distance between rows, unless good illumination is unneces- sary near the wall. Even if the distance between the first row and the wall is as little as one-half the spacing between rows, the illumination is appreciably less near the wall, unless the wall is of a very light color, and serves as a good reflector. The provision thus far is for general illumination. If this is not sufficient for all parts of the room, special lighting may be introduced to increase the illumination in certain sections (1) by increasing the size of lamps; (2) by reducing the spacing between lamps, (3) by using focusing reflectors, or (4) by providing addi- tional hand or stationary lamps. The use of stationary lamps probably gives the most satisfactory lighting for a small amount 70 ELECTRICAL EQUIPMENT of power used. 1 In a shop or factory, the lamp should then be placed where it lights the work to the best advantage. In general, a good rule is to place the lamp so that when the operator is at work, it is just to the right and in front of his right shoulder, and just above his head. The best height of the lamp is from 4 to 7 ft. above the floor, depending on whether the operator is seated or standing. It must be placed where no shadow is cast on the work. 1 Any such localized lighting is rarely necessary. It nearly always pro- duces glare in some form, and should be avoided if possible. CHAPTER X D.C. TRANSMISSION AND DISTRIBUTION SYSTEMS 1 This chapter applies very largely to A.C. as well as to D.C. systems ; some further points are brought out in the next chapter, with reference to A.C. circuits. It is assumed that the kind of system has been chosen in accordance with Chapter III ; we now proceed to find the size of wire, which must be large enough so that the current does not produce (1) excessive voltage drop, (2) excessive power loss in the line, nor (3) excessive heating of the conductor. It must also be large enough for mechanical strength, but not so large that the investment is unnecessarily large. VOLTAGE DROP Motor Circuits. When the voltage of a system drops, the motor current must increase, if the motor is to do the same work as at full voltage; also, the operating characteristics of some motors are impaired. A drop of 5 per cent, is satisfactory in motor circuits under practically all conditions, and usually 10 per cent, is not excessive. Still greater voltage drop may be necessary in extreme cases, but should not be allowed without careful consideration of the extra cost and the advantage of keeping it within 10 per cent. 2 Lighting Circuits. The voltage drop should be even smaller for lighting circuits than for motors, because every 1 per cent, decrease in voltage causes from 3 to 4 per cent, decrease in the 1 G. Chapter XLL S. Section 11; High-tension long-distance transmission. Section 12; Distribution systems and short transmission lines. Section 13; Interior wiring and local distribution. A. pp. 352-376, 1657-1707. 2 Overcompounded generators, voltage regulators, or boosters can be employed to maintain a steady voltage, but none of these will keep the vol- tage constant on all parts of the system without a large investment in equip- ment. If possible, the line drop, independent of automatic regulating de- vices, should not be excessive. 71 72 ELECTRICAL EQUIPMENT amount of light produced. For example, if the voltage drops 5 per cent., the candlepower of a tungsten lamp decreases about 16 per cent. For this reason, the voltage drop on a lighting circuit should not ordinarily exceed 3 per cent., and it is better not to exceed 2 per cent. There are three cases of voltage drop to be considered, for D.C. motor and lighting circuits: (1) The two-wire system, in which all the current flowing out on one wire necessarily returns on the other; (2) the ground- or rail-return system, in which the cur- rent flowing out on the copper wire returns through the ground, or over rails, or both; and (3) the three- wire and other multiple- voltage systems, in which at least one additional conductor at an intermediate voltage is provided for lighting, or for motor-speed adjustment. Two-wire System. If a feeder is very long and has branches taking considerable parts of the total current, the. conductor need not be so large at the end as at the beginning of the feeder. 1 Ordinarily, however, it does not pay to make the joints and to change sizes; a size is selected which is large enough for safety and economy, and which distributes the current to its various destinations without producing a drop at any destination, ex- ceeding the allowable maximum. The total RI drop in a D.C. line is computed by adding together the RI drops in the various parts that are in series. Example. A feeder of 500,000 circ. mil cable furnishes power to three motors, at distances along the feeder of 50, 100 and 200 ft. from the busbars. The motors take, respectively, 100, 200 and 250 amp. It is required to find the voltage drop. From Table XII, p. 80, the resistances of the three lengths are respectively 0.00216, 0.00216, and 0.00432 ohms. The total drop is 550 X 0.00216 + 450 X 0.00216 + 250 X 0.00432 or 3.24 volts. It is sometimes simpler to compute the drop due to the individual currents by multiplying each by the total resistance through which it flows. The total drop, computed by that method, is 0.00216 X 100 + 0.00432 X 200 + 0.00864 X 250, which agrees with the other computations. The voltage drop may be computed without the use- of the table, as equal to kLI/A, where k is the resistance of a circular- 1 In such a case it can be shown that the most economical distribution of copper for minimum line drop is obtained if the sectional area of each length of conductor is proportional to the square root of the current in that length. Thus, if the first 100 ft. carries nine times as much current as the second, and we consider minimum line drop and nothing else, the sectional area of the first 100 ft. should be three times as great as that of the second. D.C. TRANSMISSION AND DISTRIBUTION 73 mil-foot (10.6 ohms, for annealed copper at 25C.), L is the length of conductor in feet, I the current in amperes, and A the area in circular mils. It is simpler to use the table than to make this computation to find the voltage drop, but if the voltage drop is given, and the size of conductor is to be found, it may be simpler to use this formula than to try the various sizes of wire until the one is found that gives the right voltage drop. 1 Where it is specified that the voltage drop shall not exceed a certain maxi- mum, of course the full-load value of the current is to be sub- stituted in the expression for voltage drop, even though the average current is much smaller. However, this maximum drop does not necessarily refer to the period of heavy currents for motor starting, lasting for a fraction of a minute; for even if such currents cause an excessive drop for a very short time, they need not interfere with satisfactory operation of other machines. Example. If the total full-load current taken by all the motors on a feeder is 500 amp., the motors are 200 ft. from the power-station buses, and the maximum allowable drop is 15 volts, the required area of the wire is 10.6 X 2 X 200 X 500/15 or 141,000 circ. mils. From the table we find that No. 000 wire is the size to use. Ground or Rail Return. In a circuit having a rail or other return path, if the drop in the return circuit is appreciable, the resistance of the return circuit, and the current (if it is different from that of the wire) must be determined. The total drop is, then, the resistance of only one wire times its current, plus the resistance of the return circuit (if appreciable) times its current. If the return circuit is through a rail, the drop in the rail is usually small; the resistance of two 60-lb. rails in parallel is about 0.0083 ohm per 1,000 ft. The resistance of other weights of rail is very nearly inversely as the weight. 1 For rapid calculation of wire resistances without reference to tables, the following rules are convenient to memorize. At 20C. (68F.), for ordi- nary commercial copper wire of sizes from No. 0000 to 10, A.W.G., they are correct within 2 per cent. The errors are slightly larger for smaller wires. Rule 1. The resistance of No. 10 wire is 1 ohm per 1,000 ft.; adding 3 to the number of any wire doubles the resistance; and subtracting 3 from the number halves the resistance. That is, changing the number by 1 multiplies or divides the resistance by v^2 or 1.26. Rule 2. Adding 10 to the number of any wire multiplies the resistance by 10, and subtracting 10 divides it by 10. 74 ELECTRICAL EQUIPMENT Thus the drop in the rail return 1 mile long, consisting of two 40-lb. rails in parallel, when carrying 150 amp., is 150 X 0.0083 X 5.28X40X60 or 4.4 volts. If only one rail is used as the return circuit, of course the drop is twice as great. Multiple-voltage Systems. A direct-current three-wire circuit should be treated the same as a two-wire circuit, if the circuit is balanced. The current in one line and the voltage between outside lines should be used in computing per cent, voltage drop. Example. If 100 lamps each taking 1 amp. at 110 volts are balanced on a 110- and 220-volt three-wire circuit, there are 50 lamps on each side, and the per cent, line drop is the same as for 50 amp. on a 220-volt circuit. If a three-wire circuit is unbalanced, the voltage on either side of the system may be .either too high or too low. (See the three-wire feeder in Fig. 14.) If there is a larger current on the positive than on the negative side, a part of the current returns through the neutral. Designating the positive, neutral and negative currents by I +} I n , and /_, and the resistances of the outside and neutral wires by R and R n , the drop in the posi- tive line is R I+ + R n l n - I n is usually a small fraction of /+, but R n may be larger than R . A numerical example will illustrate : Assume that on a 110- and 220-volt system the maximum current that will flow in an outside line is 50 amp. and at least 80 per cent, of this current is balanced by a current returning in the other outside line. If the resistance of the outside line is 0.04 ohm, and the neutral 0.08 ohm, the maximum vol- tage drop with balanced load is 0.04 X 50, which amounts to 2 volts on each side, or 4 volts on both sides. This is a drop of 1.8 per cent. The maxi- mum drop with unbalanced load is 0.04 X 50 + 0.08 X 10 or 2.8 volts on one side. This is a drop of 2.5 per cent. Note that if / is reversed, the second term of the voltage drop is negative. ECONOMICAL SIZE OF WIRE Even if a large conductor is not required for any other reason, it may be required for economy. Evidently a very small con- ductor is not economical, because the annual power lost is pro- portional to the resistance, or inversely proportional to the sectional area of the wire. On the other hand, a very large con- ductor costs so much that there is an excessive annual outlay for interest and other fixed charges that is, for charges that exist whether the conductors are carrying current or not. There is an intermediate size of wire that is most economical, whose D.C. TRANSMISSION AND DISTRIBUTION 75 exact size would be dependent on the cost per kilowatt-hour for energy, and on the necessary allowance for fixed charges, which include interest, taxes, insurance and depreciation. Some of the items of cost in installing a transmission or dis- tribution system are the same for any ordinary size of wire. Other items are about proportional to the weight, and therefore to the sectional area of the wire. Since the fixed charges are a certain per cent, of the first cost, some fixed charges are constant, whereas others are proportional to area of conductor. Thus the total annual outlay on account of the line includes the fixed charges, in two parts, and the cost of energy. It may be ex- pressed as C = K, + K 2 A + K S /A where A is the area, KI the invariable fixed charges, K Z A the annual fixed charges proportional to the area, and K 3 /A the cost of energy lost on the line per year. Differentiating the annual outlay, with respect to area, and setting the first derivative equal to zero, to find the area for minimum cost, we have dC/dA = K 2 - K S /A 2 = from which we have K$A = K Z /A. That is, the annual fixed charges that are proportional to the area should equal the cost of energy lost on the line per year. The cost of energy lost is RIHC e / 1,000, where t is the number of hours per year that the current flows, and C e is the cost of en- ergy in cents per kilowatt-hour. If the line is of annealed cop- per, the resistance is 10.6L/A where L is the length in feet and A the area in circular mils; and the cost of energy lost is 10.6LIW./1,OOQA. The fixed charges are 3.03 X 10~ 6 LACJF, where 3.03 X 10~ 6 is the weight of a circular-mil-foot of copper, C c the cost of copper, installed, in cents per pound, and F the fraction to be allowed annually for fixed charges. Equating the fixed charges to the cost of energy, and solving for area, if t is 365 X 24 hr., (2) When, as is usually the case, the value of A is not a commercial size of wire, the nearest size should be selected not necessarily 1 See S. 13: 75, 76 for a similar statement, based as this is on Kelvin's law. 76 ELECTRICAL EQUIPMENT the next larger size. If the time of operation per year is not 365 days of 24 hr., the area is proportional to the square root of the time. Thus, if a line is in service only 8 hr. per day, 300 days per year, expression (2) becomes A = 5,500 X V8 X 300/(24 X 365) X VC e /(CtF). The cost of energy per kilowatt-hour, C e , is usually between 1 and 10 cts. if purchased from a power company. If not pur- chased, but generated in a plant of 2,000-kw. capacity or more, it should ordinarily cost from 0.5 to 1 ct. per kw.-hr., depending largely on the size of the plant and the cost of coal. The cost of copper per pound, installed, C c , including insulated wire, supplies and the labor of wiring, depends on the prevailing base on which wire costs are computed, size of wire, the kind of wire insulation, discounts obtainable, cost of labor, and kind of wiring system that is, whether an out-of-doors pole line, an in-doors conduit system, or some other kind of installation. This total cost may be as low as 25 cts. per Ib. of copper installed, or as high as 75 cts., for such sizes as are used for power purposes. To find this total cost, proceed as follows : 1. Knowing from market quotations the base on which the re- quired wire is sold, find from the price list in Table XII the list price of the wire per 1,000 ft., and take off whatever discount is allowed. 2. Add to this the cost per 1,000 ft. for conduits or other sup- plies, and labor. 3. Divide by the weight of bare wire, in pounds per 1,000 ft. It is required to find the total cost of a pole line, not including poles, per pound of copper. Market quotations for the required wire are on the 15-ct. base, and a discount of 45 per cent, is obtainable. The line is to be of No. 4 stranded conductor. List price of wire per 1,000 ft. is $122.00 Taking off the discount, 122 X 0.55 is 67 . 10 Labor and supplies cost, per 1000 ft. 25.00 Total cost per 1000 ft. $92 . 10 Dividing by the weight in pounds, the - cost per pound of copper is 0.71 The rate of interest is usually about 5 or 6 per cent., depending on financial condition. D.C. TRANSMISSION AND DISTRIBUTION 77 Taxes, of course, depend on the locality. An allowance of 1.5 per cent, is reasonable. Fire insurance is placed on buildings and their contents, and other equipment that may be destroyed by fire; but it is not customary to insure transmission and distribution lines that are outside of buildings. Insurance on power stations and other equipment ranges from practically zero to 1.5 per cent. In a well-constructed building it is not far from 0.5 per cent. Depreciation is an allowance for a decrease in value, due to ordinary wear and tear, effect of the weather, and being displaced by equipment better adapted to the requirements. (Scrap value of old equipment reduces the necessary allowance for deprecia- tion.) It is not customary to charge maintenance and repairs to depreciation, but these charges should be included here if they are not elsewhere. Allowance for depreciation ranges in most cases from 5 to 10 per cent., but in a few cases it goes much higher or lower. For ordinary wires and wiring equipment (not includ- ing trolley wires) it is about 7 per cent. This brings the total fixed charges for an out-of-doors circuit to about 14 per cent. (It usually ranges between 12 and 16 per cent.) This percent- age allowed for fixed charges is to be substituted for F in equa- tion (2). It is to be written as a decimal fraction, e.g., 0.14, not as a whole number, as 14 per cent. A good check on the results of equation (2) may be made by comparing the total annual cost for the chosen size with that for the next sizes above and below. Example. Let us apply the formula and then check it, to find the most economical size of triple-braid weatherproof wire to carry 300 amp., 9 hr. per day, 295 days per year, over an out-of-door pole line, if this wire is sold on the 20 ct. base, there is a discount of 53 per cent., fixed charges are 14 per cent, and energy costs 1 ct. per kw.-hr. Usually results come out at about 1,000 to 2,000 circ. mils per amp., so we shall look for a conductor of 300,000 to 600,000 circ. mils. Let us work the problem, using the data for 500,000 circ. mils. From Table XII and Note 9 of that table, we find that: Cost of the conductor per 1,000 ft. is $604 Cost of labor and supplies per 1,000 ft. is $28 Total cost per 1,.000 ft. is $632 Weight in pounds per 1,000 ft. is 1,540 Cost per pound is $0.41 78 ELECTRICAL EQUIPMENT Substituting this value for C c , in equation (2), we have VI Q V 2Q^ 40<"al4 X 24 X 365 = 379 ' 000 circ ' mils ' The best commercial size is 400,000 circ. mils. This solution was obtained, by finding the value of C c for a 500,000-circ. mil conductor. This would be so nearly the same for 400,000 circ. mils that usually no further computations are necessary. As an extra precaution, the solution can be repeated, after finding C c for 400,000 circ. mils. Checking the foregoing by comparing costs for 400,000 circ. mils with those for 350,000 and 450,000, we find: Area in circular mils 350 000 400 000 450 000 Cost of insulated wire $442 00 $496 00 $549 00 Labor and supplies 23.80 25.50 27.00 Total first cost $465 80 $521 50 $576 00 14 per cent, fixed charges $65 20 $73 10 $80 60 Cost of energy lost at 21C 72.40 63.40 56.30 Total annual outlay $137.60 $136.50 $136.90 These results agree with those obtained by the formula, in indicating that 400,000 circ. mils is the most economical size. There is a small theoretical error in using the formula, because in deriving it we assumed that the cost of copper per pound is constant, whereas it is slightly less for large sizes than for small sizes of wire. For this reason, results obtained by the formula are usually about 3 per cent, too low. Variable Current. If the current is not steady, but has values Ii t It, 7 3 , . . . respectively, for h, t z , h, . . . hr. per year, the value of / to use in equation (2) is equal to \/A + 1 V*2 + I*% + - . /24 X 365. Center of Distribution for Branched Circuits. If the circuit branches near the outer end, so that the current in the main feeder is smaller at the end than at the beginning, it is allowable to make computations as if the feeder carried all the current to a center of distribution, whose distance from the power buses is less than that of the farthest load, but more than that of the nearest load. A large error may be introduced, however, if the current branches at a point near the power station. D.C. TRANSMISSION AND DISTRIBUTION 79 SAFE SIZE OF WIRE The wire must be safeguarded against mechanical strain and electrical heating. The National Electrical Code lists the carry- ing capacities allowed by the National Board of Fire Under- writers. This list (see Table XII, page 80) is generally accepted as in accordance with good practice. Smaller than No. 14 wire is not permitted for any power or lighting current, except in special cases. The carrying capacity of a triple-braid covered or other wire without rubber is greater than that of a rubber- covered wire, on account of deterioration of the rubber with heat. However, rubber-covered wire is required by the Under- writers in certain cases. Overhead wiring out of doors should be strong enough to with- stand wind and sleet, and good practice calls for a No. 6 wire as the smallest to be used as a pole line. Larger sizes should be used for larger currents, in at least approximate conformity to the ratings of the National Code, even though it be where the Fire Underwriters have no jurisdiction. Underground circuits, in cables and conduits, should usually have not smaller than No. 8 wire, and should conform at least approximately to the National Code. CONCLUSIONS After finding the size of wire required for allowable line drop, economy and safety, the largest of the three is to be selected for obviously we cannot exceed the maximum voltage drop, just because it is safe and economical; nor can the other limitations be disregarded. If the requirements for voltage drop or safety call for an excessively large wire, it may be possible to obtain special concessions from the proper authorities. If the size required for economy is very large, it may not be possible to tie up extra capital, even if it is economical to do so. In any of these cases it is well to consider whether a higher line voltage can be used, thereby reducing the line current. 80 ELECTRICAL EQUIPMENT TABLE XII. DATA ON WIRES Size of wire or cable Weight of copper* w i re or cable in pounds per 1,000ft. Safe carrying capac- ity of copper* wire or cable in amperes (National Elec. Code) B. & S. or A. W. G. No. Area in circular mils 1 Outside diameter in mils 1 Bare Triple braid* Bare Triple braid weather- proof 4 Rubber insula- tion Other insula- tions Stranded conductors 1,000,000 950,000 900,000 850,000 800,000 750,000 1,152 1,123 1,093 1,062 1,031 998 1 451 1 300 3,090. 2,930. 2,780. 2,620. 2,470. 2,320. 3,478 2,6i5 650 600 550 1,000 920 840 700,000 650,000 600,000 550,000 500,000 450,000 964 929 893 855 814 772 1 i --Trip-con opened by a single breaker. Sometimes the three poles of the breaker are in separate oil L 6 3 _ o . , tanks, but the operating mechanism is all in circuit-breaker, one system, so that there is essentially only operated by two one operation in opening all the poles. Two- tnp-coils. r phase tour-wire and three-wire circuits re- quire respectively four-pole and three-pole breakers. An oil circuit-breaker is tripped either by a trip-coil in series with the line, or by a coil connected to some other circuit. Fig. 63 is a diagram representing a three-pole oil breaker that is tripped by coils in series with two of the conductors of the three- phase line. As an overload or short-circuit cannot occur under ordinary conditions in the third conductor, without flowing also in one or both of the other two, it is frequently considered ade- quate to provide only the two-pole protection. But there is a possibility of circumstances in which both the generator and the third conductor become grounded, as in Fig. 64. The current then flows from the generator through the ground to the grounded wire and back, without flowing through either trip-coil. For CIRCUIT-BREAKING EQUIPMENT 135 this reason, as an added precaution, sometimes the third pole of the breaker also has overload protection. Trip-coils in series with the main conductors are satisfactory, if there are no restrictions as to the time and circumstances of operation of the breaker; but the difficulties already men- tioned, and others make it im- portant in many cases that the breaker exercise a high degree of discretion in its operation. The breaker may then be tripped from an auxiliary circuit, as in Fig. 65, by a trip-coil that oper- ates when a contact, R, is closed by a relay. 1 If the breaker is FIG. 64. A possible, but improba- at a distance from the switch- J>le condition of grounding, in which . . two trip-coils are inadequate for board where it is controlled, or protecting the circuit. if it is too heavy to be con- trolled by hand, a coil is provided for closing, as well as for opening the breaker. Such a closing coil is shown in Fig. 65, which is operated by closing contact C by hand. For tripping (opening) the breaker by hand, contact T is employed. Carbon circuit-breakers are ordinarily limited to A.C. and D.C. circuits of 750 volts or less. The operation of the breaker in its simplest form is as follows: when an overload occurs, an electromagnetic device FlQ 65 Elec- re l eases the arm of the breaker, which flies trically-operated cir- open. As it starts to open, it separates the C -c7i!tabt r ' closed mam c PP er contacts, and the current is de- automaticaiiy on over- fleeted to flow through a carbon contact. As load, to trip the breaker. & r= contact closed the breaker opens still further, the carbon manually to trip the r breaker. contacts open; by this time the copper con- C = Contact closed J manually to open the tacts have moved away, so that there is no breaker. arcing of the copper, but it is restricted to the carbon. The non-arcing tendency of the carbon allows the breaker to open without excessive arcing. In some breakers there is an intermediate contact of phosphor-bronze, that breaks after the copper, but before the carbon contacts. This serves to 1 See Relays, pp. 136-141. 136 ELECTRICAL EQUIPMENT protect the copper in case the carbon contacts are burned away, or for any reason fail to operate. Some two-pole carbon breakers are arranged so that one pole may be closed at a time, but both poles are opened automatic- ally by an overload. The advantage of this arrangement is that it is not necessary to provide a knife switch to use along with the breaker. The electromagnetic device that trips the breaker is usually energized by the main current of the breaker, but instead of this or in addition to it the breaker may be actuated by a device operating on underload (when the current becomes less than a prescribed amount), or undervoltage. An example of underload operation is in case of charging a storage battery. When the battery becomes charged, the current drops off and the breaker is opened. 1 An example of undervoltage operation is the discon- necting of a motor, if for any reason the line is temporarily dead. When the power comes on the line again, if the motor were not disconnected, it might be damaged by the excessive current that would flow. Sometimes the breaker is provided with a trip-coil that is operated by some outside appa- ratus, as in Fig. 66. The coil has no current in it, until the contact, R, is closed. The contact, R, may be closed by hand, or in case of any ab- R - normal condition of the circuit it may be closed Carbon circuit automatically. 2 breaker, tripped Carbon breakers can be obtained with one, is 7 energized by two > three or four poles as required. As they closing a con- are used more often on D.C. than on A.C. cir- cuits, the breakers are in most cases single-pole or two-pole. The size of breaker must be sufficient to carry the rated current and to open any possible overload or short-circuit, as already explained. PROTECTIVE RELAYS Relays are either protective or regulating. Examples of regulating relays were explained in Chapter XVI. In this chap- ter only protective relays are considered. 1 However, this is not the best criterion on which to limit the battery charge. 8 See Relays, p. 139, Fig. 70. Circuit- Breaker CIRCUIT-BREAKING EQUIPMENT 137 12 10 A protective relay is used to operate a circuit-breaker, where it is necessary to exercise much discretion in opening the circuit. If instantaneous operation is permissible on overload, the breaker can be set to operate satisfactorily without a relay; and it is even possible to make the breaker delay its operation, thereby introduc- ing a time element. But the most accurate control of the time element, and various other desirable features are obtained only by means of relays. A protective relay is an electromagnetic device that opens or closes a contact and thereby operates a circuit-breaker, when certain abnormal conditions exist on the line that is being pro- tected. Thus relays are employed to open the breaker in case of overload, underload, overvoltage, undervoltage, overspeed, reversal of flow of power, and various other abnormal conditions. Of these, overload relays are in more general use than any of the others. Relays are somewhat like meters in their operation, except that they are more rugged; and whatever can be meas- ured by any electric meter can be used to operate a relay, and thereby a cicuit-breaker. Time Limit. Just as the motion of a meter can be damped, so the operation of a relay may be made slow. In some relays the time of operation is the same under all con- ditions of overload, and in others it is less in case of a heavy overload than when the overload is slight. The first is known as a definite time- limit, and the second as an inverse time-limit relay. The name does not mean that the time is strictly in inverse proportion to the load, but that in general at larger overloads the time is less. This inverse time-limit is of great importance, because the cir- cuit is not opened without giving an opportunity for the overload to stop; and the less the overload the longer is it safe to leave the circuit closed. In Fig. 67 are curves showing the operation of an inverse time- ice 300 500 Percent of Maximum Continuous Load FIG. 67. Time-load curves of an inverse time-limit relay. 138 ELECTRICAL EQUIPMENT limit relay. The lower curve, representing the operating charac- teristics with setting A, shows that if the current is 125 per cent, of the maximum continuous load (25 per cent, overload), the relay operates in about 3 sec.; but at 400 per cent., it operates in 0.3 sec. With settings B and C the time is longer in each case; but with any setting the time is relatively long at small over- loads, and very short on heavy overloads. If the load is heavy enough, the time with any setting is less than 2 sec. It will be remembered, however, that the circuit-breakers have less work to perform if there is a time element of at least 2 sec. The operation of a definite time-limit relay is very different from Fig. 67. The relay can be set to operate in the required number of seconds, and it will take that length of time, whatever the over- load. This type of relay obviously has the advantage of not tripping the circuit-breaker too quickly on heavy overloads. Fig. 68 shows the operation of a relay that combines the definite and inverse time limits. The time is long on a slight overload, CHARACTERISTIC TIME CURVE N'lOTIMESniWG Time is proportional to lever setting % OF AMPERES NECESSARY TO CLOSE CONTACTS 2000 FIG. 68. Time-load curve of an inverse time-limit relay having a definite minimum time. and shorter on a heavy overload, but it is never appreciably less than a certain minimum which can be made large or small, as desired. With No. 10 setting, for example, at 200 per cent, of maximum continuous load (100 per cent, overload), the relay operates in 5 sec.; at 1,000 per cent, it operates in only 2 sec.; but at heavier overloads there is hardly any appreciable decrease in time. This operation is not so severe on the circuit-breaker as the ordinary inverse time limit; and the system is not as likely to be tied up unnecessarily by a slight overload, as if the relays had a definite time limit. There is another important use of this relay, in case two breakers are in series. Let us consider breaker A , Fig. 69, which is on a feeder circuit, and is located at the power CIRCUIT-BREAKING EQUIPMENT 139 station; and breaker B, which is on a motor circuit branching from the feeder. If the relay at the motor has a 1-sec. setting, and the one at the power station a 2-sec. setting, as in Fig. 68, there is no possibility that motor trouble will tie up the entire feeder, because the motor breaker will open ahead of the feeder breaker, and restrict the trouble to the motor circuit. But if the relay of Fig. 67 is used on both breakers, the difference in time between settings A and B is so little that both breakers may be opened, and the entire feeder will be temporarily tied up. To Motor FIG. 69. Circuit-breakers which should preferably be operated by re- lays having inverse time limit with definite minimum. Auxiliary Circuit FIG. 70. Two overload relays and two current transformers pro- tecting a three-phase circuit. Applications. Relays used on A.C. circuits are not connected in series with the line, but are used with current transformers. 1 The connection of two relays to two current transformers on a three- phase circuit is illustrated in Fig. 70. If there is an overload on either outside conductor, at least one of the relays closes its contact, and the breaker is opened by a current flowing from the auxiliary circuit, through the contact of that relay, and through the trip-coil. The circuit is fully protected by this arrangement except in a special case of grounding, such as is illustrated in Fig. 64, page 135. Fig. 71 is the same as Fig. 70, except that there are three instead of two current transformers, offering the full protection on three conductors which is not afforded by Fig. 70. There are only two relays, but each takes the resultant of the secondary 1 See Chapter XV. 140 ELECTRICAL EQUIPMENT currents from two of the current transformers. If it is remem- bered that no current can flow in the transformer secondary, without a corresponding primary current, it can be shown that any possible overload, short-circuit or ground will operate one or both of the relays, and so the circuit-breaker. Three relays are sometimes used in this case instead of two, but the added relay is superfluous. This combination of three current transformers and two relays is known as the Z-connection. By an ingenious arrangement of connections, it is possible to utilize the current from the transformers, to trip the breaker * \ Current Transformers J crrl I 111' Overload Belays Auxiliary Circuit FIG. 71. Two overload relays and three "Z-connected" current trans- formers protecting a three-phase circuit. without the necessity of an auxiliary circuit, but the standard connections of Figs. 70 and 71 are sufficient to illustrate the applications. Another application of relays is on parallel feeders. In Fig. 72, there are two three-phase lines feeding from the generating station to the substation. This is done, so that if there is a break or short-circuit in one line the other will carry the power. But if a short-circuit occurs at S, the current on short-circuit may feed into it from both directions, and breakers will be opened on both lines. In this case, reverse-load relays that are instan- taneous in their action may be installed in the substation. These relays open as soon as the power begins to feed back, and it is then impossible for that fault to open the other feeder. This serves as an outline of an application of reverse-load relays; some difficulties have to be met, which need not be considered in this discussion. CIRCUIT-BREAKING EQUIPMENT 141 A different application of relays is utilized in the Mertz- Price system. Two current transformers, a and 6, Fig. 73, are at opposite ends of a transmission line, and their secondaries are connected together by two small wires running the entire length of the line, with a relay, R, in their circuit. The connec- tions are such that in normal operation the two transformers Circuit-breakers Operated by Overload Relays Circuit-breakers Operated by Reverse Load Relays Buses at End of Line Buses at End of Line FIG. 72. Parallel feeders protected by overload and reverse-load relays. FIG. 73. Mertz-Price system for protecting against short-circuits and grounds. oppose each other. If the current at a is the same as that at 6, the effects of the two transformers are equal, and neutralize each other. But if there is a ground or short-circuit at S, disturb- ing the balance, or reversing the current at one end, a current flows through the relay, which closes a contact and trips the circuit-breaker. CHAPTER XVIII Buses LIGHTNING-ARRESTER EQUIPMENT 1 . When lightning strikes a line, it passes on to ground by the easiest path. If there is no easier one, it punctures the insulation of a machine or transformer, or passes through the thin flanges of a line insulator. The excessive voltage comes with such extreme suddenness as to make it impracticable to operate a mechanism, connecting the line by an easy path to ground. The lightning discharge is equivalent to a high-frequency current, and on this account a choke coil located where the line enters a power station, and connected in series with the line, prevents most of the light- ning discharge from entering the building. But this alone is not suffi- cient ; an easy path to ground must be established. This path cannot exist under normal operating conditions, because the line would then be grounded; but it must be established instantly when the lightning strikes. Two satisfactory media have been found for this purpose: the air gap and the aluminum cell. Either of these, when connected between line and ground and properly adjusted, breaks down the instant there is an excessive voltage stress. We shall consider several arrangements of air gaps, and some further facts about aluminum cells. A multigap arrester, Fig. 74, consists of a series of metal cylinders, placed close together but not touching. These little cylinders are arranged so that each line of the three-phase or 1 G. 372, 374. S. 10:850-868; 11:69-80, 220; 12:146-154; 24:733-735. A. pp. 869-872; 360-361. 142 ^=r Ground Outgoing or Incoming Feeder FIG. 74. Multigap lightning arrester. Disconnecting switches, if used to disconnect the lightning arrester, are inserted at D, D, D. For sim- plicity, circuit-breakers and discon- necting switches are omitted from succeeding diagrams of lightning arresters. They are connected as shown here. LIGHTNING-ARRESTER EQUIPMENT 143 other circuit is connected to ground through a series of small air gaps. The advantage of several small gaps in series, instead of one large gap, is that after the lightning has passed, the spark across the little gaps does not develop into an arc, which might continue grounding and short-circuiting the line; whereas with a single long gap the vapor of the metal might produce a serious persistent short-circuit. The cylinders of the arrester are made of a " non-arcing" alloy that is, one that does not tend to continue the arc. This also tends to quench the spark as soon as the light- ning has passed. Every arrester must have some such means of stopping the current from flowing to ground after the lightning Outgoing or Incoming Feeder FIG. 75. Multigap arrester with resistance elements in parallel. Connections to the other phases are the same as the one shown. orn-gaps D Other Arrester FIG. 76. Horn-gaps used in con- junction with some other lightning arrester. discharge. This type of arrester is suitable for A.C., but not D.C., and is made for use in conjunction with various arrange- ments of resistances for example, such as that in Fig. 75 for voltages ranging as high as 50,000. A horn-gap arrester, Fig. 76, consists of a gap such as would be formed between the two sides of a V if the bottom of the V were removed. A three-phase arrester consists essentially of three such horn-gaps, each connecting from one line to ground; a resistance rod, an aluminum arrester, or some other additional element is put in series with each horn-gap, preventing a bad short-circuit which would otherwise occur. When the lightning discharge has passed, the heat of the arc and the magnetic effect of the current tend to carry it upward until it is stretched out so long that it breaks, 144 ELECTRICAL EQUIPMENT A magnetic blowout arrester, Fig. 77, is similar to a horn- gap, in that there is a gap in series with another element, and there is a means of blowing out the arc. In this case the series element is a resistance rod made of carborundum, and the blowout is an electromagnet connected in parallel with a part of the resistance. This arrester is made for all D.C. circuits up to 1,500 volts. FIG. 77. Magnetic blowout lightning arrester. FIG. 78. Condenser lightning arrester. A condenser arrester, Fig. 78, consists of a condenser in parallel with a resistance rod, and these two in series with a very small spark gap. The high-frequency A.C. charges and discharges the condenser, with a very low voltage to ground. If there is also a continuous charge (not alternating), it flows to ground through the resistance. This is intended for line and car use on D.C. circuits up to 1,500 volts. Horn-gaps Aluminum Cell FIG. 79. Aluminum cell lightning arrester, with horn-gaps in series. A multipath arrester is one in which a special composition acts as a partial conductor, filled with minute spark gaps. The relatively large mass of the substance absorbs the heat of the minute sparks; and as soon as the lightning discharge stops, the LIGHTNING-ARRESTER EQUIPMENT 145 path to ground ceases to conduct the current. This arrester is intended for use on 400- to 750-volt D.C. and A.C. circuits. An aluminum arrester, Fig. 79, is made up of a series of alumi- num pans, each inside of the one below, and each filled with an electrolytic solution, and immersed in oil. The surfaces of the aluminum pans are covered with a film of aluminum hydroxide, which acts almost as an insulator until the voltage reaches a cer- tain value. But when the voltage becomes excessive, the film breaks down, and the whole arrester becomes a good conductor. And as soon as the voltage drops again, the insulating film is again restored. This type of arrester is commonly used in con- junction with a horn-gap, which has the effect of insulating the arrester from the line, except during the lightning discharge. This arrester is adapted for all voltages from 2,000 up, and for lower voltages on D.C. Relative Merits of Arresters. In selecting a lightning arrester, the line voltage of the system is to be considered first, then the severity of the lightning and the total current capacity of the generators in the vicinity; because large generators may be capa- ble of pouring a large current through the arrester to ground, after the lightning strikes, before normal conditions are again established. The ideal arrester acts on an electric circuit as a good safety valve does on a steam boiler. When there is a slight excess in voltage, the current flows freely to ground, through the equivalent of a low resistance; but as soon as the voltage becomes normal, the current to ground is cut off by an increase in the equivalent resistance. The aluminum arrester performs its function much better than any other does on very high voltages; and it is recommended as the preferable arrester to apply on voltages even as low as 2,000. For D.C. circuits of even lower voltage it is of advantage, where the lightning is very severe. The reason for the success of the aluminum arrester is that the resistance is extremely low to the high voltage of the lightning, but it is extremely high as soon as the lightning has passed. The disadvantage of the aluminum arrester is that it requires a little attention. It should be " charged" every day that is, connected across the line without an air gap in series to keep the insulating film in good condi- tion. The cost is higher for aluminum arresters than for suitable types of spark-gap arresters, especially for low-voltage circuits. Choke coils are put in series with the line, as it enters or leaves 10 146 ELECTRICAL EQUIPMENT a building, to keep the lightning out. They are coils, wound without iron, and having a quite low inductance. Being in series with the line, they introduce a reactance, 2irfL, where / is the line frequency and L the inductance of the coil. The inductance is so low that the drop is almost negligible in normal operation; but the lightning is equivalent to an extremely high- frequency current, and the opposition offered to the lightning is correspondingly great. Choke coils differ, first, as to current-carrying capacity, second, as to the voltage for which they are insulated from ground, and third, as to their inductance. An increase in any of these increases the cost of the choke coil. The current and voltage capacities must be those of the line. The larger the inductance, the more effective is the choking in keeping the lightning out of the build- ing. On short lines of low voltage, the importance of choke coils is relatively small in fact, sometimes both choke coils and ar- resters may be omitted but on voltages from 2,200 up on long lines, subject to severe lightning disturbances, choke coils and arresters are important. A choke coil may be considered as equi- valent to a multiplier, increasing the effectiveness of the arrester. It can be omitted where the arrester is adequate without it. -Ground Wire Outgoing or Incoming Feeder FIG. 80. Relative connections of lighting arrester, choke coils and circuit-breaker. Disconnecting switches at D, D, D, if used. FIG. 81. Ground wire, above the line wires L, L, L. The larger the inductance of a choke coil, the more must one end of the coil be insulated from the other; for when the lightning strikes, the pressure exerted may be about proportional to the inductance. Sometimes choke coils of heavy inductance, in- tended for use on high-tension lines, are immersed in oil for better insulation. LIGHTNING-ARRESTER EQUIPMENT 147 Fig. 80 shows a suitable arrangement of choke coils and ar- resters, connected to a line entering a power station. Ground Connections. The path to ground must be good, to insure proper operation of the arrester. It should have as few turns, and as little horizontal length, as possible. It should go down to moist earth, and should have exposed there several square feet of surface, consisting of sheet copper, iron pipes or rods, or other adequate grounding surface. If the ground wire is continued along the transmission line, over the other wires, as in Fig. 81, it helps to shield the other wires and the arrester from lightning disturbances. This would be an unnecessary expense in some cases, but in others it is of material value. CHAPTER XIX MEASURING AND INDICATING APPARATUS This chapter is treated under four heads: 1. Meters and the quantities measured. 2. Characteristics of meters. 3. Meter switching devices. 4. Meter applications. METERS AND THE QUANTITIES MEASURED The instruments that we consider are those in common use in commercial and industrial plants, for indicating current, voltage, frequency, grounds, and single-phase and polyphase power, energy and power factor. Some of them are so well known as to require little or no description. FIG. 82. Diagram of polyphase wattmeter. Current circuit Ii and voltage circuit Ei comprise one meter element; circuits 1 2 and Ez comprise the other element. In induction types of meters both the current and the voltage circuits are stationary. The currents in these stationary circuits induce eddy currents in rotable disks or drums, and these eddy currents react with the magnetic field, rotating the moving element. A polyphase wattmeter consists of two single-phase meter elements that are electrically complete and distinct; they act on a single pointer, tending to deflect it, and the deflection is opposed by a spring. The connections are illustrated in Fig. 82. The scale indication is equivalent to that of two single- phase wattmeters. It is at once evident that two-phase power can be measured with this meter, by connecting one phase to 148 MEASURING AND INDICATING APPARATUS 149 one wattmeter element and the other phase to the other element. It has been shown in a variety of ways that three-phase power can be similarly measured. 1 A watt-hour meter (formerly called an integrating wattmeter or a recording wattmeter) makes a continuous record, summing up the total energy that has flowed since any given time, so that by reading dial indications every day, month, or other period, the energy used in each period is obtained directly. Watt-hour meters are either single-phase or polyphase. The polyphase +x FIG. 83. Diagram showing the principle of operation of a single-phase power-factor meter. Winding Z is a current winding; it is connected either in series with the line or to the secondary of a current transformer whose primary is in series with the line. Windings + R and R are voltage windings with resistance in series. Windings -f- X and X are voltage windings with reactance in series. These windings are connected either across the main circuit or to the secondary of a voltage transformer whose primary is across the circuit. In polyphase meters, sometimes winding I is a voltage winding, connected across the circuit, and windings + R, R, + X, X, or similar windings are current windings, connected in series with the several phases. The resistance and reactance are then omitted. In one important type of power factor meter, winding J is stationary, but the form of the coil and the iron core are such that the core can rotate as required to indicate the power factor. watt-hour meter consists of two single-phase elements, arranged as in wattmeters. Power-factor meters, or power-factor indicators, usually have two fields, one produced by the line current and the other by the line voltage. Either the current or the voltage field is made rotating, the other being simply an alternating field. The mov- ing element of the meter is made so that it is deflected to a posi- 1 Compare G. 267, Fig. 271, which applies to one polyphase meter, as well as to two single-phase. S. 3: 171-177. A. p. 1825. 150 ELECTRICAL EQUIPMENT tion that is determined by the relation of the current and voltage fields. These meters can be obtained for single-phase, two- phase and three-phase circuits. The principle of operation (but not the form of the instru- ment) of a single-phase power-factor meter is illustrated in Fig. 83. The four stationary poles, +R, -\-X, R, X, are excited by windings connected across the voltage, with reactance and resistance respectively in circuit, as indicated. The combined effect of these four windings is to produce a rotating field. The central rotating electromagnet is excited by a winding, 7, which is connected in series with the line, and carries the line current. It is attracted to the position in which the rotating field of -\-R, -\-X, R, X is in phase with the current in 7. The angular position of the moving element is shown by the pointer, and with suitable calibration it indicates the power factor. With some variation, this discussion applies to all types of power-factor meters, for single-phase and polyphase circuits. Synchronism indicators, or synchronoscopes, are made on the same principle as power-factor meters, except that windings -\-R, -\-X, R, X are voltage windings connected across the "running" machine, and winding 7 is a voltage winding connected across the "starting" machine. 1 The pointer takes a position depending on the relation of these two fields. If one is gaining on the other, or losing, the fact is indicated by a rotation of the pointer, to the right or left. This rotation is very slow when the starting machine has about the right speed. When the pointer is stationary, or nearly so, pointing vertically upward, the machines are exactly or approximately in synchronism, and the switch of the incoming machine may be closed. Lamps for Synchronizing. Lamps may be connected in series between the two machines, so as to show by their brilliancy when the machines are in phase. With the customary arrange- ment of connections, when the lamps go out the machines are in synchronism. The most approved method of synchronizing is by using both the lamps and the synchroscope. 2 x The terms "running" machine and "starting" machine refer to the machine already connected to the buses and the one to be synchronized. Sometimes the starting machine is referred to as "incoming." 2 HAKOLD W. BROWN, "Apparatus for Synchronizing," The Electric Journal, vol. v, p. 530, September, 1908. HAROLD W. BROWN and S. S. NEU, "Phasing out for Synchronizing Polyphase Circuits," The Electric Journal, vol. ix, p. 427, May, 1912. MEASURING AND INDICATING APPARATUS 151 Frequency meters are of two distinct kinds. One is similar to a differential voltmeter, in that it has two voltmeter elements opposed to each other, tending to deflect the pointer in opposite directions. As in Fig. 84, both of the voltage windings are connected across the same circuit, so that if each had the same impedance in series with it the meter would always indicate zero. One winding has a large resistance and no reactance in series with it, so that its current at a given voltage is the same at all frequencies; the other winding has a large reactance and small resistance, and of course the reactance varies directly with the frequency. Thus the element having the large reactance is weak at high frequencies, but strong at low frequencies; so FIG. 84. Connections of re- sistance- and-reactance fre- quency meter. The resistance element exerts a torque that is independent of fre- quency; the reactance element exerts a torque that decreases as the frequency is increased. FIG. 85. Electrostatic ground detector. that with the right calibration the pointer indicates the different frequencies. The other kind of frequency meter has a series of vibrating reeds each tuned for a different frequency. A coil is placed in such a position that it tends to make all the reeds vibrate, and the frequency is indicated by the one having the greatest vibra- tion. A ground detector is sometimes made in the form of a differen- tial electrostatic voltmeter that is, an electrostatic voltmeter which shows by its deflection if the voltage from one line to ground is greater than from another to ground. One form is illustrated in Fig. 85. - 152 ELECTRICAL EQUIPMENT A ground detecting lamp may be connected from each line to ground. If one lamp goes out or burns dim, it indicates that the corresponding line is grounded. This is illustrated in Fig. 86 (see also Fig. 99, p. 161). D.dor Single Phase 3 Phase 2 Phase FIG. 86. Arrangement of ground detecting lamps on D.C., single-phase and polyphase circuits. CHARACTERISTICS OF METERS There is a great variation in the characteristics of the various kinds of meters; the accuracy depends on the calibration and construction of the meter, and in part on whether the meter is used under exactly the conditions for which it was made. Some meters can be used indiscriminately under widely varying con- ditions; others are subject to considerable errors 1 due to excess- ively high or low temperature, mechanical balance, friction, aging, distorted wave form of current or voltage, thermo-elec- tromotive forces, and perhaps a few other causes, which are more or less beyond the control of the user, but should be specified in purchasing meters. In addition to these are several features and conditions causing errors, as mentioned below, which should be considered in both purchasing and using meters: The scale of a meter may have equally spaced divisions, such as are in most of the permanent magnet types of voltmeters and ammeters, or the spaces may be wide near the middle of the scale and narrow at each end, or they may increase gradually, so that they are widest at the end of the scale. These are illustrated in Fig. 87. Where it is possible, the best meter for all-round use has a uniform scale, but if readings are nearly always taken in a certain part of the scale, there is a possible advantage in having the divisions wider in that part. Also, in general, the longer the scale the smaller will be the reading error. 1 CYRIL JANSKY, "Electric Meters," p. 345 (New York: McGraw-Hill Book Co., Inc.), First Edition, 1913. MEASURING AND INDICATING APPARATUS 153 A meter should be selected, preferably, of such a full-scale in- dication that in ordinary use the indications are beyond the mid- 70 so FIG. 87. Typical voltmeter or ammeter scales. (a) Uniform scale divisions. (6) Wide divisions near middle of scale, (c) Wide divi- sions near end of scale. die of the scale. Accuracy is sacrificed in taking readings at much less than one-half of full scale. For example, a current of 50 amp. cannot be measured on a 200-amp. ammeter with 154 ELECTRICAL EQUIPMENT the same per cent, accuracy with which 150 amp. can be measured. Any meter may have more than one set of terminals or con- nections, by which the meter can be made to indicate either large or small quantities, as illustrated in Fig. 87a. It is then convenient to have the numbers on the two scales in different colors, to agree with markings on the corresponding meter ter- minals. If two sets of numbers are shown on the scale, such a meter is called a " double-scale " meter. In some cases meters are required to read both positive and negative quantities e.g., incoming and outgoing kilowatts. They are then provided with a scale extending both to t'~e right FIG. 88. Scales with Shifted zero. (a) Zero-center scale, (b) Suppressed-zero scale. and to the left of zero, as in Fig. 88a. Such a meter is called a " double-reading " meter, or the scale a " zero-center " scale. In certain meters a zero reading is never required, and relatively wide scale divisions are desirable. The zero may then be " sup- pressed, " as in Fig. 886. This has the disadvantage, however, that it is less convenient to adjust the pointer if an error on zero is introduced by a bent pointer or in any other way. Frequency affects some kinds of meters, and not others. In general, it has little effect on meters of the dynamometer and Kelvin balance type. The effect of frequency on some induction- type meters is greater than on others. The fact that meters are nearly always used on either 25- or 60-cycle circuits, and that the variation from the normal frequency is very slight, makes MEASURING AND INDICATING APPARATUS 155 the disturbance due to frequency rather insignificant in most cases. Voltage. A change of the magnetic condition of the iron with voltage may effect the accuracy of a wattmeter, or a watt-hour meter. It may also effect an ammeter whose field is produced by an electromagnet. Voltage variation may effect the resist- ance-and-reactance type of frequency meter, because both of the opposing elements are weaker at low voltage. Usually all these effects are negligible when the meter is used within 10 per cent, of the rated voltage. Low power factor has no effect on the accuracy of any meters except wattmeters and watt-hour meters. If even these meters are properly adjusted for power factor it should have no effect on them. Unbalancing of phases of a polyphase wattmeter or watt-hour meter should have no effect if the two elements of the meter are independently correct at high and low power factors and there is no stray-field effect of one element of the meter on the other. A stray-field may affect the accuracy of a meter if it has the same frequency as the quantity that the meter indicates, or both the field and the quantity measured are from a D.C. source. A D.C. ammeter or voltmeter should not be too near a D.C. conductor carrying a heavy current. Strong A.C. fields are not so likely to be near the A.C. meters, but they also should be avoided unless they are known to have a negligible effect. Some switchboard and other meters have iron cases, which shield them very largely against such magnetic disturbances. Instrument transformers, including both current and voltage transformers, have negligible errors if they are not furnishing power to too many instruments; but if the number of instruments is too great, considerable errors are introduced, both in ratio and in " phase displacement" (i.e., phase error). Ammeters, voltmeters and other apparatus operating on current or voltage 'are affected by ratio errors; wattmeters and watt-hour meters are affected by both ratio and phase displacement. 1 The effect of current transformers on the accuracy of watt-hour meter indications is illustrated in Figs. 89 to 91, in which typical instru- ments are connected to current transformers that are compen- sated for a secondary load (i.e., load due to the meter winding) J See Chapter XV, "Current Transformers." 156 ELECTRICAL EQUIPMENT of 25 volt-amp. 1 In each of these figures, six curves are drawn. Three of them show the error in the watt-hour meter itself, when fl. li Accuracy Curves Current Transformer win S ingle - Pnose Watt hour Meter Only. 1007. Loaa*5 Amperes. 60 Cycles Primary I ^WW 1 130 140 FIG. 89. Typical accuracy curves, showing the errors at various loads and power factors, when a single phase watt-hour meter is connected directly and through a current transformer to a single-phase 60-cycle circuit. Accuracy Curves Current Transformer with Poly- Phase Wat I hour Meter Only 100% Load -5 Amperes. 60 Cycles FIG. 90. Same as Fig. 89, except that a polyphase watt-hour meter is con- nected to a /wee-phase circuit. the load is respectively at 50, 75, and 100 per cent, power factor. 1 The meaning of "a secondary load of 25 volt-amp." is somewhat arbitrary, referring to the secondary volt-ampere output of the current transformer at rated full-load current. A watt-hour meter is a load of about 2 volt-amp. An ammeter is a load of about 5 volt-amp. A trip-coil is a load of about 50 volt-amp. MEASURING AND INDICATING APPARATUS 157 The other three curves in each figure show the combined error of the wattmeter and the current transformer. The difference between the two curves shows the transformer error. 1 Fig. 89 shows that on a single-phase system, if nothing but the watt- hour meter is connected to the transformer, the transformer error is practically zero at light load, rising to about 0.5 per cent. Accuracy Curves Current Transformer with Poly - Phase Watt hour Meter, Ammeters. and Trip Coils. 100% Load '5 Amperes 60 Cycl Cycles. J ppy (0 10 30 40 50 110 IZO 130 140 FIG. 91. Same as Fig. 90, except that ammeters and circuit-breaker trip-coils are connected in series with the current transformers. When the trip-coils are inserted, note the relatively large errors with light load at all power factors, and with full load at 50 per cent, power factor. This is in spite of the very high quality of design and construction of this type of transformer. at 30 per cent, overload, depending a trifle on the power factor. Fig. 90 shows that on a three-phase system, with a polyphase watt-hour meter, the errors are of about the same order, except that there is a negative error of about 1 per cent, at light loads. Fig. 91 shows that if trip-coils and ammeters are connected in series with the watt-hour meter, this negative error at light load is considerably increased, but there is a larger positive error, *The curves were furnished by courtesy of the Westinghouse Electric and Manufacturing Co. and represent tests on Westinghouse equipment. 158 ELECTRICAL EQUIPMENT at large loads and low power factor. (This error is chiefly due to the trip-coils not to the ammeters.) The conclusion to be reached from these curves is that where considerable accuracy is required 1 at light load or low power factor, the watt-hour meter should not be put on current transformers that operate trip-coils. Typical Ratio Phase Displacement Curves. Type "A' 'Current Transformer. Secondary Load at 5 Amps. 2*4 Volt-Amps, at JJ7. P. fl- 60 Cycles. 2-10. - " 90% " - FIG. 92. FIG. 93. (Curves are for 60-cycles.) (Curves are for 25 and 60 cycles.) The advantage of this transformer is in The advantage of this transformer is its small errors. that its cost is only two-thirds the cost of the transformer of Fig. 92. FIG. 92. Transformer of 50 volt-amperes capacity, compensated for 25 volt-amperes. Fig. 93. Transformer of 10 volt-amperes capacity, compensated for 10 volt-amperes. FIGS. 92 and 93. Ratio and phase displacement curves of current trans- formers. The meaning of 101 per cent, ratio is that primary current/secondary current is 101 per cent, of the correct (rated) ratio. The meaning of 60 minutes phase displacement is that the secondary current is 1 degree (1/360 of a cycle) ahead of the primary. If the primary has a lagging current, this dis- placement tends to make a wattmeter or watt-hour meter reading too high, whereas the ratio error tends to make it top low; so that the two errors tend to neutralize each other. The effect of phase displacement on wattmeter and watt-hour meter indications is as follows: 1.00 f 0.02 of 1 0.90 The error per cent, introduced per \ 0.85 of 1 0.80 degree of phase displacement is } 1.3 0.70 1.7 With a lagging current having a power factor of Fig. 92 shows ratio and phase displacement curves of a trans- former such as was used in Figs. 89 to 91, under various condi- tions of loading, with various instruments connected to the transformer secondary. Fig. 93 is similar to Fig. 92, but refers to a transformer of 10 volt-amp, secondary capacity. 1 For example, where charges for electric energy are made from the meter readings. MEASURING AND IN DIG A TING APPARA TUS 159 Beceptacles METER SWITCHING DEVICES The number of meters required in a plant is very much reduced, if suitable plugging or other switching devices are employed for shifting some of the meters from circuit to circuit, or from phase to phase of a circuit. Follow- ing are a few of these devices: Four-point Voltmeter Plugs and Receptacles. The recepta- cle is merely four terminals, to each of which a permanent and a plug connection can be made. Fig. 94 shows a suitable ar- rangement of connections. Of Frora Generators the three receptacles, one con- for m easur i ng generator andfbus volt- nects to the buses and two to age on a two-wire system, generators. All the receptacles connect to the one voltmeter. The plug, when inserted in any receptacle, puts the voltage of the buses or of one of the gene- rators on the voltmeter. It is essential that only one plug be . 1 lAAAA/v\A /vw\jwv\ JM rr r LO c~ 1 ok> (6) FIG. 95. Eight-point and six-point receptacles. (o) Eight-point receptacle for measuring voltage on three-phase circuits; may also be used on D.C. or single-phase, three-wire circuits, (b) Same as (o) where voltage trans- formers are used, (c) Six-point receptacle for measuring voltage on two-phase circuit. (d~) Same as (c) where voltage transformers are used. provided for the entire plant ; because if there were two, inserted by mistake in different receptacles at the same time, they might introduce a short-circuit between machines. 160 ELECTRICAL EQUIPMENT ^Erom Generators'* macl Eight-point Receptacles. Fig. 95, a and 6, shows how an eight-point receptacle may be used on a three-phase circuit just as the four-point is used in Fig. 94, on B.C. or single- phase. A four-point plug is used as before; there are three possible positions for the plug in each receptacle, for measuring the voltage of the three phases. In Fig. 95 FIG. 96. Synchronizing plugs and c and d, a six-point receptacle -ironizing "between ig uged Qn ft two . p h a se cir- cuit. Synchronizing Plugs and Receptacles. Various plugs and receptacles, such as those just described, are used for connecting the machines to the syn- chronism indicator. One arrangement is shown in Fig. 96, in which there are two kinds of plugs one for the starting and one for the running machine. The difference between the plugs is such that the starting machine connects to the top of the synchronism indicator, and the run- ning machine to the bottom. The diagram shows one of several synchronism indicators that are on the market. The order of leads is different in different types, but the method of switch. synchronizing is essentially the same. Some engineers prefer to connect the lower, or running leads of the synchronism indicator to the buses in all cases, instead of connecting to one partic- ular machine. The arrangement of wiring is then somewhat different, but the results are essentially the same. This is illustrated in Fig. 103. In the one case they synchro- nize " between machines" and in the other to the buses. Rotating Ammeter Switch. Switching devices for use with current transformers and ammeters should be constructed so that the ammeter can be inserted in any phase, without opening the circuit of any current transformer. Such devices are made FIG. 98. Ammeter plug and receptacles. MEASURING AND INDICATING APPARATUS 161 on two different plans, illustrated in Figs. 97 and 98. A drum switch is made in several forms similar to Fig. 97. With the switch in the position shown, the currents from current trans- formers A and C, entering the switch at terminals 1 and 3, must flow through the ammeter before they can return. Since the resultant of the A and C currents is the B current (see Chapter XV, p. 119), the ammeter must now be indicating that current; but if the switch is rotated so that the small segment of the drum bridges from 1 to 2, only the C current flows through the ammeter; and by rotating the switch in the other direction the A current is indicated. Ammeter Plugs and Receptacles. The other device, which is represented in Fig. 98, consists of one plug and as many recep- tacles as there are lines in which the current is to be measured. Push Indicating Lamp (or Voltmeter) FIG. 99. Ground detecting switches. The plug is inserted across 1-4, 2-4, or 3-4, connecting one end of the voltage transformer primary to any one of the three line wires. If the ground detecting push is pressed, it makes contact at g and connects the other end of the primary to ground. The indicating lamp in the transformer secondary will be dark, or at most only dimly lighted, if connection is made to a line wire that is grounded. When the push is not depressed, a spring presses the movable contact against a, con- necting to line 1 . If the plug is in position 2-4, or 3-4, the lamp then indicates full-line volt- age by its brilliancy. In case a little better indication is required in measuring ground voltages, the indicating lamp is replaced by a voltmeter. The construction of plugs and receptacles is not exactly according to the diagram, which is intended to show the principle, rather than the form of the apparatus. As the plug is inserted, it first connects its two contact surfaces to the ammeter, and then to the transformer; and after that the lower path of the current is opened, forcing the current to flow up through the ammeter. This device is sometimes preferred to the rotating switch, as by use of a special plug it permits connecting portable meters in the circuit to check the switchboard instruments. Ground detector switches are made in a variety of forms. One arrangement of switches and a transformer is illustrated in Fig. 99, by which the voltage from any phase to ground is indi- cated roughly by a lamp or a little better by a voltmeter. Only 11 162 ELECTRICAL EQUIPMENT a 110-volt lamp or voltmeter is required, if it is connected to the line through a voltage transformer, as shown. METER APPLICATIONS Meter Equipment for D.C. Switchboards. Only one volt- meter is necessary for the entire board. One four-point plug should be furnished and there should be a receptacle for each generator. One receptacle in addition may be provided to measure bus voltage, if desired. This is illustrated in Fig. 94. One ammeter is necessary for each D.C. generator. It is not customary to switch D.C. ammeters from one ammeter shunt to another, first, on account of the possible error due to contact From Generators FIG. 100. Voltmeter receptacles for measuring voltages on all phases of the buses and on one phase of each generator. On high voltage circuits two voltage transformers are inserted at Ti, and one each at Tz and T t . resistance, and second, because each generator needs its own ammeter continuously on the circuit. Ammeters are used on the more important outgoing feeders, but may be omitted from small feeders whose current is not likely to be excessively high. Wattmeters are not ordinarily used on D.C. circuits, but watt- hour meters may be used on any generator or feeder circuit whose total energy is being observed closely. Meter Equipment for Three-phase Switchboards. There is usually only one voltmeter for the entire board, with the neces- sary plug and receptacles. An arrangement very frequently used is illustrated in Fig. 100, in which an eight-point receptacle is used to measure voltages on all phases of the buses, and in addition one four-point receptacle connects to each generator circuit, to measure the generator voltage before synchronizing. Sometimes an eight-point receptacle is connected to each genera- MEASURING AND INDICATING APPARATUS 163 tor, to measure voltage on all phases as in Fig. 101. The bus receptacles are then omitted. In still other cases two voltmeters are used as in Fig. 102; one for making measurements on all phases of the buses, and the other for one phase of each generator. A synchronism indicator with all necessary plugs and recepta- cles, should preferably be installed in every large plant. Fig. /vv From Generators FIG. 101. Voltmeter receptacles for measuring voltages on all phases of the generators. Voltage transformers are added on high voltage circuits. 96 shows an arrangement for synchronizing between two genera- tors, and Fig. 103 for synchronizing between a generator and the buses. Both arrangements are in common use. One ammeter is provided for each generator, usually with a switching device for measuring the current in any particular From Generators FIG. 102. Voltmeter receptacles used with two voltmeters one indicating bus voltages and one, generator voltages. Voltage transformers are added on high-voltage circuits. conductor. Ammeters may be installed also on the more im- portant feeder circuits. The connections may be as in Fig. 97 or 98. Wattmeters, watt-hour meters and power-factor meters are important in many cases; they may be used in any combination. 164 ELECTRICAL EQUIPMENT on generator and feeder circuits, if there is special need of them. Fig. 104 shows a suitable arrangement where all these instru- ments are on a three-phase generator circuit. A constant-current lighting circuit requires no meters except an ammeter in the constant-current side of the transformer. It may be connected as in Fig. 105, or the ammeter transformer Plug FIG. 103. An arrangement of synchronism indicator and lamps for syn- chronizing a generator or motor "to the buses." (shown dotted) may be omitted, in which case the ammeter is put directly in the lighting circuit. Recording Meters. There are three kinds of records made by meters: (1) a graphic record made by a pen, showing the fluctua- tion of current, voltage, power, power factor, frequency, or any other quantity to be measured; this is made by a "graphic" or Current Transformers FIG. 104. Watt-hour meter, wattmeter and power-factor meter on a three-phase circuit. If ammeters are also used, they may be inserted at the three dots at A. " curve-drawing" meter. (2) An integrated record, showing the total energy in watt-hours or the total ampere-hours that have been delivered in a prescribed time; this record is made by a watt- hour or ampere-hour meter. (3) A record of the maximum power taken during any one minute, or other interval of time; it is made by a "maximum-demand" meter. The graphic meters may be an unnecessary luxury on ordinary circuits, on MEASURING AND INDICATING APPARATUS 165 account of the first cost and paper, but in some cases the commer- cial and industrial advantages gained would warrant even a greater outlay. Meters of the integrating type are less expen- sive, and are in common use on all kinds of circuits. Comparing FIG. 105. Connections for a series-lighting circuit. The connections for circuits B and C are the same as for A. The ammeter transformer is sometimes omitted, and the ammeter is connected directly in the lamp circuit. them with the graphic meters, the graphic record has the advan- tage of furnishing information not only at fixed times, but con- tinuously. A maximum demand meter is used where a large user obtains a low rate for power, but is penalized for peak loads. L CHAPTER XX 3 MOTOR APPLICATIONS 1 In selecting a motor for any particular service, the following are to be considered: 1. The kind of motor that is best suited to that service: whether D.C. shunt, series or compound; or A.C. squirrel-cage induc- tion, phase- wound induction, or synchronous; also whether any special features (such as a flywheel, or series resistance) are desirable on account of special conditions of loading, or speed requirements (see Table XIII, p. .167, also Chapter IV, "D.C. Motors," p. 22, and Chapter V, "A.C. Motors," p. 28). 2. The kind of system best suited to that service: whether 110, 220, 440, 550 volts, or a higher voltage; and if A.C. whether 25 or 60 cycles; one- two- or three-phase (see Chapter III, p. 16). 3. The size of motor required for continuous duty (see Table XIV, p. 170, and the notes following the table). 4. The best available speed of motor. See Table II, p. 24, for D.C. motors, and Table III, p. 30, for induction motors. The best speed for a synchronous motor is usually the same as the no-load speed of an induction motor of the same size. 5. Changes in motor rating, on account of (a) inclosing the motor, (6) intermittent or variable loading of the motor, or (c) effect of change of speed on motor rating (see p. 183). 6. Available sizes of motors. Usually a motor can be obtained within 25 or 50 per cent, of any desired size. The exact sizes that are available are different in different lines of machines; the following sizes (horsepower) of D.C. and A.C. motors are usually available: 1, 2, 3, 5, 7^, 10, 12^, 15, 20, 25, 35, 50, 60, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 750, 1,000, 1,250, 1,500, 2,000, 2,500. If the power required for any motor appli- cation exceeds a standard size by 5 per cent, or less, it is usually safe to use that standard size. 7. The total load in case of group drive is less than the sum of the loads on the several machines, unless all the machines in the group are operating at full-load at the same time. Usually the horsepower of the motor driving a group of machines should be 40 to 80 per cent, of the sum total of the full-load horsepower required by the several machines in the group. ^ee foot note, p. 167. 166 MOTOR APPLICATIONS 167 TABLE XIII. KINDS OF MOTORS FOR VARIOUS INDUSTRIAL MOTOR APPLICATIONS l Motor application Motors usually preferred (See list of abbreviations, p. 168.) Motors sometimes satisfactory Machine shops: Bending and forming machines Bolt and rivet headers Com. Shu., VS. SC. or WR., Sip. SC. or WR. Boring mills Com. SC. or WR., Sip. Drill presses Shu., VS. SC. or WR. Drills, radial Shu., VS. SC. or WR. Emery wheels GD. SC. GD SC. Grind stones GD SC. Hammers .... Com SC. or WR., Sip. Lathes, axle Shu , VS. SC. or WR. Lathes, engine Shu., VS. SC. or WR. Lathes, wheel Shu., VS. SC. or WR. Milling machines Shu., VS. SC. or WR. Pipe threading and cutting-off machines Planers Shu., VS. SC. or WR. SC. Polishing and buffing Presses, hydrostatic Shu. or SC. Shu. or SC. Punch presses Com., FW. SC. or WR., Sip., Rolls, bending . . . Com FW. WR. Saws . . . Shu , VS SC. or WR. Screw machines, automatic: Large Shu , VS. SC. or WR. Small GD. Shapers (if not GD ) . Shu. SC. or WR. Shears Com , FW. SC. or WR., Sip., Slotters and key-seaters (if not GD.) . . Shu., VS. FW. SC. or WR. Wood shops: Wood-working machinery: Small starting torque SC. Shu. enclosed. Large starting torque WR. Shu. enclosed. Various industrial applications: Air compressors: Reciprocating Syn ,FW, or Com SC or others. Centrifugal WR or Shu SC. Blowers .... Shu , SC or WR Cement mills: Applications requiring large starting torque WR. Applications requiring variable speed. All others WR. SC. Coal and ore handling Ser. WR. Coal crushers Com. enclosed or WR. SC. Cranes Ser. WR. Elevators Spe., Com. or WR. RI. or SC. 1 See also the following, regarding industrial motor applications : G. Chapter XVII, XVIII, XXXVII, XL, paragraph 364. S. Section 15. A. pp. 892, 972. 168 ELECTRICAL EQUIPMENT TABLE XIII. KINDS OF MOTORS FOR VARIOUS INDUSTRIAL MOTOR APPLICATIONS. Concluded Motor application Motors usually preferred (See list of abbreviations below.) Motors sometimes satisfactory Fans: Centrifugal Shu SC or WR Propeller Ser SC or WR Shu Hoists WR Ser or Com orllg Locomotives Ser WR if polyphase Paper and pulp mills: Small units Large units Low starting torque Powder mills Pumps: Centrifugal Reciprocating Refrigerating: Ammonia compressors SC. WR. Syn. SC. Shu., SC., WR. (Motor depends on conditions) Com., Shu., SC., WR., or Syn. WR MS Syn. Shu. Steel rolling mills WR., SS., FW. Com., FW. Telpherage Ser WR. Textile mills SC or Syn Shu. or Dif. Turn tables and transfer tables Ser. WR. Abbreviations (arranged alphabetically} Com. = D.C. compound motor. Dif. = D.C. differential compound motor. FW. indicates that a flywheel should preferably be mounted on the motor shaft or geared to the motor, to relieve the motor of short-time overloads. GD. Group drive by any approximately constant-speed motor. Ilg. = Ilgner system, consisting of an induction-motor-generator set with flywheel, driving a shunt motor. See p. 40. MS. = Multi-speed induction motor, which has two or three synchronous speeds. See A. p. 977, S. 7 : 276; 15 : 304. Rev. = D.C. motor specially adapted to reversing. RI. = Combination single-phase repulsion-induction motor. SC. = Squirrel-cage induction motor. Ser. = D.C. series motor. Shu. = D.C. shunt motor. Sip. indicates that squirrel-cage and phase-wound induction motors are to have sufficient resistance in the rotor circuit to introduce about 10 per cent, slip at full-load. Spe. indicates that the motor is to be of special construction. SS. indicates that a 25-cycle motor is desirable, for slow speed. Syn. = Synchronous motor with self-starting winding. VS. indicates that the motor specified is especially preferable only where variable speed by hand regulation is required. WR. = Induction motor with wound rotor and slip-rings connecting to an external rheostat. MOTOR APPLICATIONS 169 SIZES OF MOTORS In the following table some of the formulas for motor horsepower have a purely theoretical basis and others are empirical, being based on experi- mental data. 1 The power required depends so much on the nature of the work to be done that in some cases the formulas cannot be more than a first approximation . Where the power can be expressed in terms of a single variable, that variable is written as A in the formula in the second column; the third column states what is meant by A, or by the several variables; and the fourth column gives the range through which the data indicate that the formula is correct. In most cases the range may be continued both upward and downward, without excessive errors. Where this column is left blank, the formula holds for all ordinary ranges. 1 Most of the formulas are revised from data appearing in Section 15 of the "Standard Handbook for Electrical Engineers," Fourth Edition, 1915 (New York, McGraw-Hill Book Co., Inc.); and from Leaflets 3,516A and 3, 554 A on "Machine Tool Applications," issued by the Westinghouse Electric and Manufacturing Co. 170 ELECTRICAL EQUIPMENT M i~ > ^? [fl m * /-*\ o c a S S o o * ij Jj]j fll Us | O O sfli co - ^ > *O CO 1 1 * *f a 1 * 1 a| | d ill 5 li M a ^ 2.S 1 ill 9 ^j rC "3 - ^ s ** fl fl oo "a! g 2^3 _W 'rt 1 1 JO X. "^ M *-* -^ ^J g 2 : : ^ : * : s : 1 : g i- I- ^ ^ 1 3 a f'.^ ':' :. 4 : 1 : f f '"T *d (J3 i 1 ! 1 1 1 S 1 1 J 1 '* If 1 : 9 : -g I 51 liiil!!:!;!! 1 fl Illil 1 1 H|1 II flffijiiffiiit] 15 fl S 'C 'S fe 'C te 'C fe 3 -g S 3 ,2 oo B ooo^o^t< S W MM MM M M Q Q i B 1 I I I 1 g o5 -g 3 2 S s|a I || I iiiilil 2 i.ii >1 '1*5 i Q Q Q Q Q O MOTOR APPLICATIONS 171 IJIf 0.2 * E cj fr>_S 00 00 1-4 C$ ^ O fi ^ 22 22 22 f 2222 CO O O iO TH (N M >vi ^1 ^2 * $ SMI **! |j K 1 .2 1 ^liii i ^ liMS^ j 42 sfl^lS .1 illM- ,nd other letters indicate (in stated otherwise) i wheels. f wheeJ. lammer head in pounds. lammer head in pounds. of rotation in feet per minute, ifference between radius before L inches, distance along axis) in inches ; on the material being machined With a round nosed tool, for ; for wrought iron or machin d steel (0.5 per cent, carbon an for brass and similar alloys, K ; ' - 1 i ^ 2 1 S3 & 1:r || f f 3 Q f | co S5fi*Js 5 fl o o t^ S-2 a2 ^| 2 a i-S-sS^a &, fcJ 5 "s | a MM "3 ^ 5 " "g .go ^ ^ .M 3 . II * ?WH! 0) i i2 ^ % o CO <* ioo o ^^ 2 ^ 1 $$ ^j S*8g 1 ' 'o v. J. ** " i 7 ^ o 10 oo *> 1-1 j*< J* |rf ^ *s^3 33.^s-;?||i 2 ^ ^ ^ 1 o g I S 1 I 3 S > 3 ' a -a -a : rt rl 1 g a : ft a a "i ^ 'S *3 a3 2 : I! | : HJ fl fl rt > x TJ -a : 2 a 15 ^ S 03 S 'o ! stii II Ullr I! elisf4ii^ 1^l2 : l2 r a II ^lil lii jiil flO . ."S'cB'a5J322lS .rt'rtqjgjaJOOjWOOO g iilfl 1 ! a I a a 8" 8" 8" 8" S 8" 8" a a a a 4 S 4 a ^ 4 4 3 a 3 a c *c 'C S * -g O O O B B J JS^^J^^iiJiiii 172 ELECTRICAL EQUIPMENT ange of da he formu (in inche stated ot o o 1 <* ^iX o S 22 a os x r;g?38x r w - 3 3S;|" X X 333 . x x 3 u 6 .1 *#> r n A . >-" CO II a!|5 5 ^ CO ft} CQ -3 fQ tti ^J7 7 3 I >n Sc^ ^ d d MOTOR APPLICATIONS 173 gaj? * i Ifii sl-S Jsa< s-s *l II ii D II o o ij 1. tl T3 ^g ?9ff 5 - 11.1 a: es ti 174 ELECTRICAL EQUIPMENT X IO y*i S X 2t 5*1 l^s^ rt 3 J O CO CO 3 oo o a ^ CO O 3 o 2 ^ -* oo o* X ; ?|| 00 N CO O ?|-i-8 al (N H * OS ^ N 1 1 is* s 8 g ?0 Hi- 6 2 S J .2 a 5 . s a 3 'H O o! o f J .s al 5 - ij || .1 11 pi 'w .-*; i 1 | s^ S'v * 1 "3 es I 1 "o . llf S ^ 4J a a ^ 3 .u 'O ** s a a s a 4A - -g oj 3 p a a 'a | | 'QJ 'S-i 0) 1 Diameter. Si I ! 9 l_ 1 1 1 i 1 a a 1 -3 -s -Q S o oo . M ^ O . 11 a ; ill : a s ; a ; a ; 4 ,?l M 1 S ' S a ai . belt sanders idle indie ' ) ^ ^ "o Circular saws, rip, Circular saws, resi per min Planers, matchers a (feed 40 to 80 ft Timber sizers, 4 h min., average.. . 5 1 B 1 1 1 i 1 -* : --S- .S .9 ff b 1 1 ? 1 1 -S 1 1 1 1 1 * 1 a a 'S **. s 2 -s i s |ilfjli|ll*j a J -F* J J * 1 *. a 1 H HO SM w m-S Sanding machines, Shapers, single-spi] Shapers, double-sp Lathes, speed Lathes, pattern. . . Cranes, Hoists anc Hoisting, continue MOTOR APPLICATIONS 175 'd o 5^ 1 g g s 11 ll w I I i-S* 8 lafai* S * .a & a 5 1 J 4S f I 1 ii s .2 g - g - *o&*-sg ' 1 t5 1 2 8 - i EI i .a -s + ^ TJ 13 'C oS ,0 II S.2 2 s 5 a S S Eniciency about 80 * ^^.2.2 i^ s? a is ^ " 5 -s a ^ 1 1 in I H IN JB S 9 P II il II 8 II a H t ^3 o . -S .2 1! 8 .2 ao + i T i ' -i ^ in ii iii g 3 a 5 I!! * T3 roqoot> tl ,3 a Z S - .S -a 1|| Sg be g.y 1 i ii 12 178 ELECTRICAL EQUIPMENT Notes on Table XIV The following examples will illustrate the use of the formulas: Bending and Forming Machines. A machine taking work 34 in. wide requires 0.4 (34 15) or 7.6 hp. From the list of available sizes on page 166, a 7^-hp. motor would be selected. Drilling Machines. There are seven different formulas for the horse- power required for a drilling machine. For a machine that is required to perform well-defined operations the first formula may be used. If it has two spindles, and is to drill holes ^ in. in diameter in brass, with a feed of 1 in. per min., the power required is 2 X 0.3 (or 0.5) X 0.5 2 X 1 = 0.167 hp. Probably a 1-hp. motor would be used unless this is one of a group of machines driven by a single motor (see page 166, paragraph 7). If the same drilling machine is for general use, the power required per spindle may be obtained from the formula A/8, where A is the diameter of the largest drill that will be used. The total power, then, is 2 X K/8 or 0.125 hp. If the diameter of the spindle, or the diameter of the table is given, or if the length of arm of a radial drilling machine is given, ordinary values of power required for such machines are similarly obtained. Where data are obtainable for applying more than one formula, the values of horsepower may be computed by the several formulas, and averaged. Edgers (for Wood-working). If an edger has four saws, and is used on 6-in. stock, the motor should be of 8 X 4 X 6 or 192 hp. A 200-hp. motor would be selected. , Ripping and resawing by band saws. The power required for this and other wood-working depends on the dryness and kind of wood. For ripping 12-in. (1-ft.) wet oak with a feed of 30 ft. per min., the power required is 1.2 X 1 X 30 = 36 hp. A 35-hp. motor would be used for this purpose. Cranes ASSUME THE FOLLOWING FULL-LOAD DATA: Capacity of crane (load) 60 tons (Weight of hook is negligible) Weight of trolley, including motor 25 tons Weight of bridge 50 tons Hoisting and lowering speed 20 ft. per min. Maximum trolley-travel speed 100 ft. per min. Maximum bridge-travel speed 250 ft. per min. Average trolley acceleration and retardation 3 ft. per sec. per sec. Average bridge acceleration 0.5 ft. per sec. per sec. Bridge retardation must be at least as much as its average acceleration. Wind velocity 20 miles per hr. Area facing the wind 280 sq. ft. Diameter of trolley-motor armature 18 in. (radius of gyration = 0.7 X 18 in.) MOTOR APPLICATIONS 179 Diameter of trolley track wheel 12 in. motor r.p.m. Trolley gear ratio, - > 2:1 track wheel r.p.m. Weight of motor armature and gear 1,000 Ib. Motor efficiency for each motor 0.90 Maximum hoisting distance 20 ft. Maximum distance of trolley travel in each direction , 40 ft. Maximum distance of bridge travel in each direction 400 ft. One-half of the hoisting and lowering may be performed during bridge and trolley travel. Other data are found in the table. DYNAMIC BRAKING. As is customary, the motors are provided with "dynamic braking" that is, in place of a friction brake, each motor is connected to operate as a generator sending power back into the line. Let P m = mechanical power developed, which is delivered either from the motor to the crane, or from the crane to the motor; Pi = electric power input when the machine is running as a motor ; P = electric power output when the machine is running as a generator; e = efficiency of conversion in either direction. It takes account of all or nearly all losses in the motor; it also takes account of losses in gears and bearings of the crane, if they have not been included elsewhere. The relations between mechanical and electrical power are: Pi = P m /e; Po = P m e so that if the same mechanical power is developed in the two directions, Po = Pie*. That is, the electric power output in lowering a hoist or retarding a trolley or bridge is e z times the electric power input to produce the same mechanical power in the reverse direction. Heating of the armature depends on the electric, not the mechanical power developed, so that the ability to receive is greater than that to deliver mechanical power. For example, if a motor has an efficiency of 0.90, and is receiving 123 mechanical hp., the heating is the same 1 as if it were delivering 123 X0.90 2 or 100 hp. Stated differently if a machine is operating as a generator, receiving a certain amount of mechanical power, the equivalent mechanical power output (in its effect in heating the motor) is the mechanical power received multiplied by the square of the efficiency; P n - P m e\ This is an important point that is not always understood clearly. 1 Subject to slight variations on account of different field excitation during braking. 180 ELECTRICAL EQUIPMENT COMPUTATIONS. Hoisting motor: 20 b^ fiO ^ 2 000 Power for hoisting = 33 QQQ x 0*80 = 91 hp * where 0<8 is tne efficiency of gears and crane bearings. Mechanical power developed in lowering is the same as in hoisting. Overall efficiency is 0.80 X 0.90 or 0.72. The equivalent power, in its effect on the motor during dynamic braking = 91 X 0.72 2 = 47 hp. Trolley motor: . 100 X (60 + 25) X 2,000 X 0.015 Power for steady travel = - 33 0ftQ - = 7.7 hp. Average power during acceleration 1 = 100 X (60 + 25) X 2,000 / 3\ , -GpOQ- - i ' 015 + 32^1 = 28 hp ' Average mechanical power during retardation is due to the difference, instead of the sum of the effects of acceleration and friction: 100 X (60 + 25) X 2,000 / [3 66,000 -\32 The equivalent power, in its effect on the motor during dynamic breaking, is 20.2/e 2 . The efficiency, e, takes account of only motor losses, since other losses are included in the allowance for friction. The equivalent power = 20.2 X 90 2 = 16.3 hp. Bridge motor: In computing wind pressure, the velocity of the bridge in miles per , 250 X 60 _ .. , hour is required : g ^Q = *> mile s per hr. Wind pressure per square foot, w = 0.004 (3 + 20) 2 = 2.1 Ib. Total power for steady travel against the wind is 250 (60 + 25 + 50) X 2,000 X 0.02 + 2.1 X 280 33,000 45.5 hp. Average power for acceleration and retardation is computed the same as for the trolley motor. During retardation, friction exerts a retarding force of WF Ib. on a mass W, thereby producing a retarda- tion of WFg/W, or Fg. The retardation due to friction is, there- fore, Fg = 0.02 X 32.2 = 0.644 ft. per sec. per sec. Since this is greater than the required retardation (0.5), dynamic braking is unnecessary except for emergencies. Time computations: Time for hoisting = 20/20 1 min. Time for lowering 1 min. 1 f\f\ Time for accelerating trolley = V/A = 6Q X3 0.56 sec. 1 The effect of acceleration of rotation of the motor armature is neglected in this computation. If it is taken into account, the following must be added to the mechanical horsepower during acceleration and retardation: 100 X 1,000 X 3 / 18 66,000 X 32.2 \ X ' 7 X MOTOR APPLICATIONS 181 Distance covered during acceleration Distance covered during retardation Remaining distance of trolley travel Time for steady travel Time for accelerating bridge Space for accelerating Time for retarding Space for retarding Space for steady travel Time for steady travel Total time for crane travel 100 2X60 X0.56 = 40 - 2 X 0.46 = 39/100 250 ~ 60 X 0.5 _ 250 X 8.3 2 X60 250 " 60 X 0.644 _ 250 X 6.5 2X60 = 400 - 30 370 ~ 250 8.3 + 6.5 = 1.48 + 60 = 0.46 ft. = 0.46ft. = 39ft. = 0.39 min. = 8.3 sec. = 17ft. = 6.5 sec. = 13.5 ft. = 370 ft. = 1.48 min. = 1 min. Since one-half of the lowering and hoisting can be during bridge travel, and trolley travel is during bridge travel, the total time of one trip = s H 1M = 2% min., approx. If the crane carries no load on the return trip, the time for raising and lowering the hook may usually be neglected; and we may assume that the bridge and trolley speed is the same at light-load as at full-load. The time for the round trip then = 1% + 2% = 4^ min. Root-mean-square computations: Root-mean-square of power in hoisting 1 = 2 x4? + 47 ' x lk 48 h *- That is, the motor should be able to carry 48 hp. continuously, and 91 hp. for a short time. The root-mean-square horsepower of the other motors is found in the same way. Motor speeds and gear ratios must be determined after referring to speed characteristics of available motors. Cement Plants Ball and Tube Mills. These mills are cylindrical in form, as in Fig. 106. They are filled about one-half full of cast-iron balls, pebbles, or other means for pulverizing the cement, and they are filled to the same depth with the material to be pulverized. A motor is connected to each mill, rotating it and causing the balls or pebbles to fall on the material, pulverizing it. The center of gravity of the contents is at a dis- tance OG from the center of rotation; this distance can be obtained from Table XV, after finding the area of cross-section of the material. The slope of the surface at which the balls or pebbles begin to fall is about 45 from the 1 Theoretically, the values of PI and P 2 should themselves be the root- mean-square values, for times t\ and h respectively. Practically, the average value is easier to find, and in all ordinary cases the error is negligible in assuming that the average values are the same as root-mean-square values for the respective times. 182 ELECTRICAL EQUIPMENT horizontal. The power in foot-pounds per minute, required to rotate the mass is 2?r X torque in foot-pounds X r.p.m. From these data, having given the weight of material and r.p.m., the power expended in the tube can be computed. The formula for horsepower given in Table XIV is based on the assumption that the angle is 45, and that about 25 per cent, is added to the power expended in the tube for gear and bearing friction. These are usually good assumptions to make. ASSUME THE FOLLOWING DATA: The mill contains 60,000 Ib. of cast-iron balls, and the interstices between the balls are filled with material to be pulverized. Cast iron (solid) weighs 450 Ib. per cu. ft. Cement material if balls were omitted would weigh 85 Ib. per cu. ft. Of the total space filled with the mixture, 60 per cent, is occupied by iron and 40 per cent, by cement material. ast Iron Balls r other means for Pulverizing, and Cement Material that is to be Pulverized FIG. 106. Position of load in a tube mill during rotation. The surface inclined about 45 degrees from the horizontal. O = Center of rotation. G = Center of gravity of load. The inside diameter of the tube is The mill makes 22 r.p.m. ft.), and the length 21 ft. COMPUTATIONS: Cubic feet of mixture = 60,000 0.60 X 450 222 cu. ft. TABLE XV. CENTER OF GRAVITY OF SEGMENT OF A CIRCLE (Used in Computing Horsepower of Tube-mill and Ball-mill Motors) Columns headed "Area" give the area of the, segment in terms of the square of the diameter, D 2 . Columns headed "Distance" give the distance from the center of gravity to the center of rotation in terms of the diameter, D. Area Distance Area Distance Area Distance 0.05D 2 0.431D 0.3927D 2 0.212D 0.6D 2 0.103D 0.1D 2 0.391D (L of com plete circle) 0.65D 2 0.076D 0.15Z) 2 0.355D 0.7Z> 2 0.049D 0.2D 2 0.322D 0.4D 2 0.208D 0.75D 2 0.021D 0.25D 2 0.292D 0.45D 2 0.181D 0.7854D 2 0.3D 2 0.263D 0.5D 2 0.155D (complete circle) 0.35D 2 0.235D 0.55D 2 0.129D MOTOR APPLICATIONS 183 222 Sectional area = -^- = 10.6 sq. ft. In terms of square of diameter, area = '-- 2 = 0.32D 2 From Table XV, distance to center of gravity = 0.252D = 1.44 ft. Weight of cement material = 4~^ X 60,000 = 7,600 Ib. Total weight inside the tube = 60,000 + 7,600 = 67,600 Ib. Power required to operate 22 X 1.44 X 67,600/6,000 = 280 hp. CHANGES OF MACHINE RATINGS (a) Enclosed Motors. A motor designed to be run open (with windings exposed to the outside air) may usually be run enclosed (with lids closed, decreasing the cooling effect of the outside air) and carry continuously two-thirds of the rated full-load. (6) Intermittent and Variable Loads. Electrical machines, transformers, and other apparatus having heavy windings, which require 6 or 8 hr. to approach their maximum temperature, do not usually heat the windings excessively when they carry: 150 per cent, of continuous full-load rating for 1 hr. 200 per cent, of continuous full-load rating for }/% hr. This logically implies that the machine is not already heated when the overload occurs; if it is overloaded, following a long full-load period, the size of the machine should be increased a little on that account. If the motor is loaded only a few minutes at a time, or if the load fluctuates, the average heating of the armature may be obtained as in the case of crane motors. 1 Change of Speed. A motor is sometimes run at a higher speed than that for which it was originally designed. For this change it is necessary (1) that the principal field, or a special commutat- ing field be strong enough for commutation at the higher speed; (2) that the mechanical construction of the armature be strong enough to withstand the increased centrifugal force; and (3) that the armature be well-balanced, and the bearings ample, to prevent excessive heating and wear of bearings. Under these conditions, the speed of a D.C. motor can frequently be increased to twice its original speed, by weakening the field. The increased speed increases the ventilation, and thereby reduces the heating of the armature, so that at double the speed the motor will usu- ^ee " Hoisting, Trolley or Bridge Intermittent Duty and Variable Duty," Table XIV, p. 175, and the footnote referring to this application. 184 ELECTRICAL EQUIPMENT ally deliver 20 per cent, more power than at the original speed (see p. 23 for allowable torque at higher speed). If the commutator and winding will stand a higher voltage, the speed may be increased by increased armature voltage, instead of by field control. In that case the armature will not heat excess- ively, even when the power delivered exceeds 200 per cent, of full-load; usually commutation, rather than heating of the arma- ture, limits the maximum possible load under these conditions. When lower than rated speeds are obtained by inserting resist- ance in the armature circuit, the maximum allowable horsepower is reduced about in proportion to the speed, since the armature current is the same at the reduced horsepower as at full horse- power at full-speed. CHAPTER XXI COSTS Even under the most favorable conditions it is not possible to give a simple general expression for costs, that can be applied to all kinds and grades of apparatus. This is especially true under the present fluctuating industrial and commercial conditions. The costs given in the following table should therefore be checked with quotations from manufacturers, before they are used as a basis for installing equipment. For cost of copper wire, see Table XII, and accompanying notes, pp. 81 and 84. For cost of batteries see p. 51. Costs of engines, generators and motors depend so much on speed and other features of design, that it is impracticable to express even a close approximation to costs in any simple way. The following expressions are not put in the table, because they are only rough approximations; they will be found to be too high for average conditions: For 1 to 25 kw., or 1 to 35 hp. D.C. generators or motors running at 1,200 to 400 r.p.m., $75 + $21 per kw., or $75 + $16 per hp. For 50 to 2,000 kw., or 60 to 2,400 hp. D.C. generators or motors running at 400 to 200 r.p.m., $300 + $8 per kw., or $300 + $6 per hp. For 60-cycle, 100 to 800 kva. alternators or synchronous motors, running at 300 to 100 r.p.m., $1,000 + $14 per kva. For 25-cycle, 1,000 to 2,000 kva. alternators or synchronous motors running at 100 r.p.m., $7,000 + $7 per kva. For 25-cycle alternators or synchronous motors, add 10 per cent, to the cost for 60 cycles. For 60-cycle induction motors, 25 to 300 hp., $250 + $5 per hp. For 25-cycle induction motors, 25 to 300 hp., $200 + $9 per hp. For motor generator sets, sum of motor and generator cost. For 300 to 1,000 kw. synchronous converters, $4,000 + $15 per kw. For A.C. or D.C. turbo-generator sets (including turbine), up to 300 kw., $300 + $30 per kw. For 60-cycle, 500 to 5,000 kw. turbo-generator sets, $5,000 + $10 per kw. For 60-cycle turbo-generator sets, above 5,000 kw., $30,000 + $6 per kw. For reciprocating compound steam engines, $500 + $20 per kw. capacity of the generator. 185 186 ELECTRICAL EQUIPMENT oo^ag fe*5 P.C8 m 3*J s!s| a a S S c3 cS ^ 1C O fl 4! 8 !! II 3 > > o || 33 t O o B. 2 IIS 3 | 3 Us- 414 Ij It! 3 3 2 E 1 2 * I I a *o * a \ 3 * s II 1-B il 1 a Si 15 !!?!!! !?S 1 COSTS 187 3 f ftft'o ^ 'C 1 03 8-og5 .0 d 1-1 ^ d a a TT d hMli 1 ^ ^ {^ ft ^ 2 tj CN es "1 . lg . ** . P* 10 f^ g (N *O CO M O ^ 73 T3 | 1 d S CO CO CO (N d 3 d CQ "o ^-> f' a 2 2 2 2 2 2 2 .2 I fsf ?S4 1 9 S 88 S 8 8 ll ^ d <1 ^ e 1 ijf |i|g|i||i|i|i| ss^ ft 03 M Ammeters Voltmeters Ammeter shunts Ammeter 2 d 3 ,d Ammeters > <>-cpLt-^>'o3cw strument transfo ransformers. Cc %. 1 ;r d ** 73 3 l-g|l X <0 1 1 III" IN Ci' 5 ^ 1 ll J2$ T V 3 -g *d i 1 3 Illuminated dial, for large switchboard panels, orswing- ing brackets. Negligible temperature coeffi- cient; negligible thermo-elec- tromotive force; thus avoiding errors due to heating. Low price; only moderate ac- d *o 0) o d p _S OQ * =5 A. C. Switchboard Meters. Low-price, simple construction, only moderate accuracy; am- 11 p 8? | * O ^ '13 30 ,>> "8 T}< 73 3 ^ ^ Ui 02 ft PL| i XX CO CO X X ^H i-H i-K >-\ iO *O s li 5 .2 ill 1!i 113 ! I ? o co o 2 ^ "S 190 ELECTRICAL EQUIPMENT 1 S lll s o 4- 8 8 8 D, O s 8 o 58 88 .2 -2 S"S- S" o fl o fl o fi II o * li^iiiii I ! fr, -si |i M I! i!j t S3 >-, d O. 113"^ 11 XQ ; lit 2 * _ '3 c i-H'l T3 >>4J J? & S COSTS 191 H"* 88*51 is'i S" 1-2 Ifllp 8 ^^ ^ j (N o ^ fc* "*5 "1 "3 ^ 3 d w M d N -a a g OS OS OS OS -< * 1-1 CO I-l 81 ^ft C4 1 II ft ft a s V f C8 * 1 f | j* o" W rH o a ft ft ft o 6 * o 2 a fl %ii OJ O CO S |- o t- a 111 i i ' 1 ft S n |1 1 " jljjjl a ** 5 xo O O T* O O ' O " O O b Q ft O 3 oo J ^ J * 2 ** w S 03 CO* IQ a 1 A i 1 III 03 I 2 2 2 222 i| o 13 M a 1 a S ii s It ill ^J Q, ft ft a 2 a S N 8 M 9 M M i If O *" 1| 5 ^ J5 ^ t/S'S'g 1 3 c I E g M 13 1 cc For above meters; 248 ft. per roll, for feeding at 2, 4 or 8 in. per hr. INSTRUMENT TRANSFORMERS Through-type indoor, second- ary capacity, 25 to 50 volt- amp., compensated for 10 or 25 volt-amp., for slipping over a cable stud or busbar; nominal secondary current, 5 amp. Dry-type indoor, compact, low priced; capacity 10 volt-amp.; compensated for 10 volt-amp. ; nominal secondary current, 5 amp. Dry-type, indoor, capacity 50 volt-amp.; compensated for 25 volt-amp.; nominal second- ary current 5 amp. Same as the foregoing, except insulation. Same as the foregoing, except insulation. Dry-type, outdoor, capacity 50 volt-amp. ; compensated for 25 volt-amp.; nominal sec- ondary current 5 amp. 192 ELECTRICAL EQUIPMENT Ml S i ""I . 10 fc>- S -4- -. o + + o o d + i Is 'oo -|- "J t. d s - 2 g S fe $$ d 1 i 'o Accuracy; maximum error in per cent, of full- scale reading h-g d la S S - H 9 83-OW-b^t- OS * M :l! 0>0>go3 ^ si l|l|li s >> %> d fn d iOi-niO(NW5O iO O ^ ^ , ^22 22 6 S S 5* ill i i 2 g 8 8 2 ^ 43 43 2 *H 2 t. 2 *1 |!|Sf i |}| a S 1 3 5 | -S 1 S d * Sail a* S|g|o 1 If 1 o^o^o^ g^^"" 30 ^ > *** > s, | &! a ."a 111" m Q, ft *! S 8 ^ 11 1l 4 is & 1 1 02 i I Illfi I! M ,flSO g .g .g c- a . * a g o 1 1 | 8 t*S S? I ; l^!fi a; a T3 g "3 W *3 -S g^gjr^ :-i !JUP 8 !hlJ lljlgslljlll 200 volt-amp., compensa for 15 volt-amp.; nomi secondary voltage, 100. Oil-insulated, especially sui for voltages that are too h for dry-type transforme capacity, compensation s secondary voltage same above. To correct the voltmeter in cation for ohmic and indu ive line-drop, on three-phi 60-cycle circuit. Same as the foregoing exc for single-phase or two-phs For 25-cycle circuit. COSTS 193 s-S 1 " 3 ' 1-olS. S fl OtJ M jj T3 -^ 2 03 2 ,>8a co -13, * CO 1 'i "o + 8 8 TJ CO CO CO 1C fl G 1 'R a 2 a a S ft . - Q 3 g I I % s| c 2 D - o Q 1 a 6 M Jr 1 I 1 i 1 | O i QQ FH CO ^-N ,,^ ^ c8 ^ fl Q? a J3 2*" 1 ^ CO hH *O tj > 1 s 5 1 13 K*^ 'a * 1 1 * a M 1 1 a O P^ > Q H !& <0 1 f X sH^" X S|a CO iisi|ll*Jf 8 -ll|I bO _fl o ,d c i 5 4S fc - S S 1 .2 d Special features and use J i c^o^ft- a '?-3.3 g i * o 2^ S > '3 a- 1 * >> * '1 fe 8 If I s H f^lil &lf ^1 U II 'S d 5 1 ^ 2 3 33 high or low voltage. Auxiliary relays. Used in combination with overload or reverse po relay to introduce a defii time element in the operat: For ringing an alarm when ; particular abnormal condil occurs in any circuit. COSTS 195 lllfi ilin ^ 3 CO . * ft o a|ag . 8, * .s , >> " Full scale or ormal curren or voltage ^rf ~- |- | >S.3.o5.o3. oftoftofto ft .i-H S -CO oSCOCO SCOCO CO CD i-H CO Is +3 0) c ."a 02^ -2 a. X it 196 ELECTRICAL EQUIPMENT aao !i|ll nil! {^ g ^J ^^^^^ _,?_<_, ^*^'^ T 'Jo o ^.^^i>ot-*' o o M^OOO r-i^(NO "5 TJ + + + ++ + + + + * M ' "5 d b-^HOOO OOO>OO i-ICO>O & > * 'gogS'MgSs o o o f "S a HB.HHJ 3 N S M S i s A I T| "* t II :J! BO g ijg , 3 , 3 a) 8 ' B 1 "S ^ i 1 1 5 |J 'i J B | s -I l!o|^?ll 1 1 1 2 1-1 " '^ 8 " 1 - 2 1 lon-auto- xatic circuit reakers) 3 M u* $* ^ g-s.ng-a.og C ^5 H ft, P r! ""* OS'S +* S| .2 8 g^ -SS>o -S^c N fl iltjll ll 1 1 8 is H|fl"^ Hi -2 -g,i'-S t! oa ^*^ 3 o ^2tel)" S"o 1 | 4 q S s ||g|| "=3^ **" sl H*. If! 03 1 |Is llfilll Jlli 197 a i, o ^ c8 lilii o ^4.^-5 1 ft ft 5J || >> >l >> ! g-o "g, g S ?! 3 ,-| g a gra S S S rf a n ^o^O 5i5^'l^^3^5 w 3'S ^^^^^^ a^ a^ a ^| a lSSjo S 1 *"3 o ^ t/g*rt OB g m rl O |=i! O i ^ c S 3 J fa fa '8 ft o 1 " i 1 ^3'i2 S > r S s s Si 1 "3 "g ^2 _ g M Q o 3 M >< "fl o -3 i 3 '3 If 2 ~- ^ Sow g ^ c*i 1 1 i i s |- 1 2 "3 5 2 9" S 4< o o2-s^| ~ s - -rt o "2 . o oT * "lai lift ^3 S S g g | 72 if li ss i g ft- g * ^ * a t* * - 5 ^ "? a| &s ^It-a v v S S3 fl p, -+- *jj bo gg|sa Bill OQ ^ w ^ n w fe e^ft SB ffi - a 198 ELECTRICAL EQUIPMENT fa-3ft' CO ^Jl lO 1> odd d ^ TO * t-H T* K o o . o ^ * ft i n o CO O i-l C3 i! r ft co -s| l a a f a f Ml i' S O c3 i 1 ^ a ts -s 1 COSTS 199 ftftO Tt< ""* o3 tf> < 5-3 !l 2 c.2g M 03 s ss s * 01 i. 5; i 2 i-< CD 00 O O "3 rH ;j 2 i 4- 4. S- o o o o s S + + .2 So o o o o o co co co co 10 o CO 00 O CO CN 4j i * CN O O 1 "rt 6 I o o o o o o o *O (N >O r-l O i-H *O O O O O O ^1 ;1 1 -2 ft ? i -S -2 ft B. a 2 M < rH IO TH O O O O s. 1 1 S p 1 p.rt'^ O (H 3=1 O O > > > 1 1 1 1 o o B "** -2 o If ^ js ^ j co o 1 l !*l*|* & S 2 2 2 s - s 4 S S k jn "-I-2 II II 1 ft ft I* i ^O O O O 98 a 03 c4 o3 C8 H H H H *2 bD ^* tC *-< d 2 '& S 3 S b S S l^w 2 l a i i | |i| j| J " a 1 | r fl rS 03 C3 o o* ? Q, O Sfi 'C * ^ " . ^ a cS I ^ a * I - 1 s 1 '^ 1 1 2ll 5 ljM ^ S g OnT^^C^ v -2T5T5 --H^aajS ^ _|00 cS^r^G 3 llll 1 "S 1 !: 4 3 . g 1 a 1 * * f '' 3 ^'sJa;^ -sI'lS 10 Iili || ft CG 8 &&S rf B^ S^ a 10 ^3 . . .c a a o cJcsO fto'CO'S o3 cJi t 02 CO 55 Ofl 200 ELECTRICAL EQUIPMENT fifl o^ a*> S fl.2 2 S OO CO i-H lO * O5 i-H O CO lO S S J9 1 ^ ^ o 8 o c o -. Hi l!|i K &i S & w- CO CO" CO" ^ (2 u || ** O 33 w ^ 0} Full scale or normal current or voltage 4 & iliS* 10 a ^ ^g 1 " ^^ ^O >J. >>IOC -^ s^ mill i t Q " ^-^ CO o "o a a 03 i-H 1 &. 1| | If 2 A rd "I gi 1! Sllll i 5 1 1 1 1 c a ft3 ^ 2 -M d W to M M M M _ lj!j 1 S S S o"--S S a 1 1 1 I g s s I 1 ' ' : 1 J S id l - 1 .'J| fgs iC o /K o -o ~ ^gQ doo | - | f; t Q^!, ts-tJ^n 110 a <- r i ? !'i 2 ^ i| s -s a J 1 |8|& | For series arc lamp circuits. For ground detector circuits, multiple-point form, 1, 2, 3, and 4 point. For ground detector, bayonet form. For synchronizing. COSTS 201 Hill 1 *o o *o o ^o *o o fl o rH ^ rQ '0 ,13 ,3 S ^ | -t Sf^-a.f-s 1 ' tJgw>g'5ofcg ^S^ajj-'S^g^jS PnrtpHtfH H H^ o . ^.s I tlflll KATUS 11 111 o> o S 4 > ^ a S g 8 g J 1 S a CO O S 03 g -s -g > "o g "E '1 '? X e p, bjo bo r --^ 3 ^ 3 ^ PH PH Q q M^ a H 3^"" -H^ ro 3 Ji 2^"* a 9 1 S ^ QO ^^ i 1 ft page 49, for suitable arrangements of auto-transformers and con- nections, to obtain low voltage for starting squirrel-cage induction motors.) On Feeder No. 2. Ten 100-hp. synchronous motors driving recipro- cating pumps. The motors are provided with squirrel-cage induction motor windings (which makes them self -starting), in addition to the regular synchronous motor windings. The external A.C. connections are the same as for squirrel-cage motors. In addition D.C. connections are necessary to excite the fields. Power is obtained from the exciter buses for exciting the fields of the motors as well as of the generators. On Feeder No. 3. Four 75-hp. induction motors with wound rotors, for conveyors. Lighting Feeders. Besides these power feeders, there are nine 110-volt 60-amp. single-phase lighting feeders, obtaining power from three lighting transformers. Power is furnished by three-phase 440-volt generators, excited from exciter buses by two exciters which are connected to operate either singly or in parallel. For solving this problem, assume the following : (a) The combined kilo- watt capacity of all the generators is to be equal to the total kilowatt input of all the motors at rated full-load, assuming a motor efficiency of 80 per cent.; plus the kilowatts required for lighting. Power factor is ordinarily taken into account in determining the size of A.C. generators; this and several other points are neglected in the present problem, but are considered in later problems. (b) At least two generators are to be installed. There may be more than two if the total power required is so great that each generator has a capacity of 1,000 kw. (c) The combined capacity of all transformers is to be equal to the rated capacity of all equipment connected to their secondaries. (d) Three transformers are to be installed, unless special conditions make some multiple of three desirable. All these assumptions are not far from standard practice. They are taken up more fully in connection with later problems. REQUIRED. (a) Determine the number of generators and the size of each, and the size of each transformer. 204 ELECTRICAL EQUIPMENT (6) Draw a diagram showing all connections, fully labelled. Show only one ammeter connection and one voltmeter connection in each circuit in which you consider them desirable for satisfactory operation of the plant. (In practice sometimes there is provided an ammeter connection in each line, and a voltmeter connection across each pair of lines.) See Fig. 1076, for meter connections. PROBLEM ON CHAPTER II 3. Safe Equipment. This problem is to be based not only on the text and foregoing problems, but also, as far as possible, on the student's previous observation. DATA. A small shop requires power for motors and lighting. This is to be purchased from an electric power company whose 2,200-volt, three-phase, three-wire feeders pass the building. Induction motors are to be used throughout the shop. REQUIRED. What voltage would you specify for power? For lighting? What provision would you make for safety to equipment and operators? Draw a diagram of connections from the 2,200 volt line to one or two motors and lamps. Where there is any other possible arrangement of equipment, give reasons for your choice. PROBLEMS ON CHAPTER III 4. Cement Plant. DATA: A cement plant uses a total of 3,000 kw. for power and lighting. About 80 per cent, of the power is used at the main plant, which adjoins the power house. The remainder of the power is used at the quarry, which is 1 mile from the power house. REQUIRED. Answer the following questions and give reasons for your choice. (a) Should an A.C. or a D.C. system be installed? (If you consider both A. C. and D.C. desirable, for what purposes should each be used?) (6) If an A.C. system is selected, what should be the number of phases, and the frequency? (c) What should be the voltage of the system? (Or, if there is more than one voltage, what should the several voltages be?) 6. Machine Shop. DATA: Power for a small machine shop is to be purchased from an electric power company, which furnishes it at 2,200 volts, on a three-phase, three-wire system. The power is used for lighting, and for direct-connected motors driving the following machines : 6 engine lathes, 2 planers, 2 drill presses, 1 saw, 1 milling machine, 1 punch, 1 shear, 2 emery wheels. REQUIRED. Answer the same questions as in Problem 4. (d) If you select a D.C. system, how would you obtain it? (e) If you select A.C., how would you vary the speed of the lathes and drill presses? 6. Machine Shop. If a generator is to be installed to furnish power to the shop of Problem 5, how would the questions be answered, assum- PROBLEMS 205 ing that the generator may be either A.C. or D.C., and of any desired voltage? 7. Machine Shop. DATA: A railroad repair shop requires power for driving a large number of engine lathes, boring mills, planers, and drill presses, several wheel and axle-lathes, rivet headers, a turn table, cranes, and a large wood shop. The wood shop is about 2,000 ft. from the power station. The turn table and yards are to be lighted, and all the buildings are to be lighted inside. REQUIRED. Answer the same questions as in Problems 4 and 5. PROBLEMS ON CHAPTER IV 8. Pump. DATA : A motor is to drive a reciprocating pump, pump- ing water to a reservoir at a height of 200 ft. The pump has an effi- ciency of 75 per cent., and is required to deliver 100 cu. ft. per min. continuously. D.C. power is available for the motor. (Weight of water is 62.5 Ib. per cu. ft.) REQUIRED. What kind and size of motor would you use? 9. Hoist. DATA: A motor is used to drive a hoist that lifts a load of 10 tons at the rate of 100 ft. per min. The efficiency of the hoist is 75 per cent. The motor is used for 1 hr. at a time. REQUIRED. What kind of a motor would you use? How much power must it deliver for 1 hr.? How much would it deliver continuously? 10. Variable Speed Dust. DATA: A motor is to be installed in a very dusty place. It must be large enough to deliver any power up to 25-hp. continuously, to a machine that runs at any speed from 25 to 100 r.p.m. A 220-volt D.C. system is available for power. When the machine is adjusted for a certain load, it should maintain a speed that is approximately constant. Assume that the efficiency of gear reduction per pair of gears is 95 per cent., taking into account bearing friction. The gear reduction must not exceed 1:7 per pair of gears. 1 REQUIRED. (a) Suggest in detail a suitable motor-driving mechan- ism (by gear or other mechanical means) . (6) Give as full information as you can, by which the motor can be ordered, (c) What current will the motor take? (d) What horsepower can it deliver when the en- closing lids are removed? 11. Variable Torque and Speed. DATA: A shunt motor drives a shaft at various speeds as required and is to exert a torque at each speed as follows : 50 to 150 r.p.m. 4,000 Ib.-ft. torque. 151 to 300 r.p.m. 3,000 Ib.-ft. torque. 301 to 400 r.p.m. 2,000 Ib.-ft. torque. 1 See paragraph on p. 166, entitled The Best Available Speed of Motor. S. 15: 221; 23: 28-34. A. pp. 612-615. 206 ELECTRICAL EQUIPMENT The motor must be large enough to exert this torque continuously. The field current at rated speed (without field rheostat) is 2 per cent, of the armature full-load current. The motor is connected to a 220- volt B.C. circuit. REQUIRED. (a) Suggest a satisfactory arrangement for obtaining these results; (6) Specify the motor horsepower and the maximum resistance required in any rheostats that are to be provided. PROBLEMS ON CHAPTER V 12. Small Starting Torque. DATA: A machine runs at 200 r.p.m., and requires a torque of 800 Ib.-ft. It is located where men are working who are not familiar with electric circuits. A 2,200-volt, three-phase, 60-cycle circuit is available for power. No speed control is required, and there is only a small starting torque. The speed reduction per pair of gears must not exceed 1:6. REQUIRED. Give as full information as you can for purchasing the motor that you would install. Give reasons for your choice. 13. Large Starting Torque. DATA : A 220-volt, three-phase, 60-cycle, 25-hp. squirrel-cage induction motor runs a machine having large start- ing friction; so that the motor must exert a starting torque that is 150 per cent, of the rated running torque. REQUIRED. (a) What voltage must be provided for starting? If this voltage is provided by an auto-transformer, what is the starting current in the motor, and in the line? (6) What would these currents be if the motor started at 27 per cent, of full-load torque? Note the advantage of starting light if possible. 14. Large Starting Torque. DATA: The same as in Problem 13, except that a phase-wound motor is installed. Assume, in accordance with the current curve in Fig. 7, that above full load the primary current is nearly proportional to the torque. REQUIRED. Find the starting current taken from the line at 150 per cent, of rated running torque, and compare with that in Problem 13a. 15. Pump. DATA: A reciprocating pump is to be operated by an A.C. motor, and a 440- volt, three-phase, 60-cycle circuit is available. Four plans are under consideration with reference to the pump and motor installation: (a) According to the first plan the pump is to be connected directly to the piping system pumping into the reservoir, so that in starting it must overcome its own starting friction and the inertia of the column of water, and in addition it must work against the water pressure. (6) The second plan is that the pump be installed in or near the generating station, pumping into the reservoir as before, but that it be provided with a by-pass for the water making it possible to start with but little torque on the motor. PROBLEMS 207 (c) The third plan is the same as the second, except that the motor is to be on a rather long, heavily loaded transmission line carrying other loads at low lagging power factor, and it is proposed to use a motor that will improve the power factor more than an induction motor will do. (d) The fourth plan is the same as the first, except that the reservoir will be omitted, so that fluctuations in the amount of water used must vary the speed of the motor. The pressure in the water mains is 60 Ib. per sq. in. The average demand for water is 1000 cu. ft. per min., and the reservoir is large enough to take care of all fluctuations. The maximum demand, which will last for an hour at a time, is 4000 cu. ft. per min. The pump efficiency is 70 per cent. REQUIRED. Specify the kind and size of motor to be provided accord- ing to each plan. PROBLEMS ON CHAPTER VI 16. Machine Shop. DATA the same as in Problem 7. REQUIRED. Work out a different system, based on this chapter, that would meet all requirements. Is this preferable to the arrange- ment worked out in Problem 7? Give reasons. 17. Battery Charging from A.C. Power. DATA: A factory pur- chases electrical energy at 2 cts. per kw.-hr. A 110- volt, three-phase 60-cycle circuit is available in a garage of the factory, and is to be used for charging automobile batteries. This will require 9.5 amp. at 10 volts, for each of fifteen automobiles, 3 hr. per day, 6 days in the week, throughout the year. REQUIRED. Work out an arrangement for charging the batteries, draw a diagram of connections, and estimate the cost per year for electrical energy. PROBLEMS ON CHAPTER VII 18. A.C. Transmission. DATA : A factory has a 440- volt three-phase 60-cycle power plant. A branch of the factory 1 mile distant is to use 100 kilowatts at 90 per cent, power factor. Assume that the line- drop will be 10 per cent., line reactance being negligible. REQUIRED. What would you specify for voltage of the transmission Jine? (See Chapter III.) Specify the voltage ratio, size, and number of transformers to be installed at each end of the line, and draw a diagram showing all connections from the buses at the main power house to the substation buses at the branch factory. 19. Machine Shop. DATA: The machines in a machine shop using a total of 200 kw. at 85 per cent, power factor are to be driven by A.C. motors. A 2,200-volt two-phase 60-cycle system is available as a source of power. It is to be expected that this will later be changed to a three-phase system. 208 ELECTRICAL EQUIPMENT REQUIRED. What system would you adopt for the motors? Give full information for purchasing the transformers. 20. Motor Starter. DATA: One of the machines in Problem 19 requires a 50-hp. squirrel-cage induction motor, which starts with full- load torque. REQUIRED. Specify the number of auto-transformers to be provided for starting, and the voltage and short-time current capacity that is required in each winding of each auto-transformer. Draw a complete diagram of connections, indicating on it the current capacities of the several parts of the equipment. PROBLEMS ON CHAPTER VIII 21. Storage Battery for Off-peak Load. DATA: An office building is provided with a power plant, in which the generators are running during the day. A storage battery is to be installed to furnish power for lighting when the generators are shut down. The battery capacity must be sufficient to light 100 60-watt lamps for 6 hr. An end-cell switch is provided for regulation of the voltage on discharge. The lighting of the building is from a 110- and 220- volt three- wire system. It is permissible if desired to arrange switches by which the circuits are changed to a 110- volt two-wire system when they are fed from the storage battery. REQUIRED. (a) Specify the kind of battery that is required, the total number of cells, the number of end-cells, and the ampere-hour capacity of each cell. (6) Draw a diagram showing connections to the battery, and all necessary electrical equipment for furnishing power, and for controlling and protecting the battery. 22. Storage Battery Truck. DATA: A battery truck is to be used for carrying material from section to section of a factory. The truck must make four trips daily, over a distance of 2 miles. The truck must be large enough to carry 5 tons, but the average load carried will not exceed 2 tons. A three-phase, 60-cycle, 220-volt circuit is available for obtain- ing power to charge the storage battery. REQUIRED. (a) Specify the kind of battery, the number of cells, and the ampere-hour capacity of each cell. (6) Specify the kind of apparatus that you would use for charging, and the A.C. and D.C. voltage and current capacity. (c) Draw a complete diagram of connections for charging. PROBLEMS ON CHAPTER IX 23. Lamp Economy. DATA: Tables I and II, in Supplement to Bulletin 20 of National Lamp Works. REQUIRED. (a) Compute the amount of light in lumens per dollar invested in lamps, for 25-, 40-, 60- and 100- watt, 110- volt lamps, and PROBLEMS 209 plot a curve with sizes of lamps (in watts) as abscissse and lumens per dollar invested in lamps, as ordinates. (6) Plot a similar curve on the same sheet, for 220-volt lamps. (c) Plot on the same sheet, curves between watts and lumens per watt for 110- and 220-volt lamps. (Make these curves dotted, or other- wise distinguished from (a) and (6). (d) Conclusions. Of these eight kinds of lamps, which is the most economical to use, considering only the cost of lamps? Which is the most economical, considering only the cost of energy? (Obviously other considerations sometimes make it economical to use neither of these lamps.) 24. Shop Lighting. DATA : A shop 50 ft. wide by 90 ft. long has a lighting system consisting of four rows of 110- volt, 100- watt Mazda lamps, with eight lamps in a row. The rows are uniformly spaced, and the lamps are uniformly spaced in each row. Dome reflectors are pro- vided for direct lighting, making the illumination practically uniform. Average conditions exist as to the effect of dust on the reflectors. REQUIRED. Find the illumination on the working plane, (a) When the lamps are new and reflectors clean. (6) After 1,000 hr. of use when reflectors are clean. (c) When lamps are new, but reflectors have not been cleaned for 6 weeks, and (d) After 1,000 hr. of use, when reflectors have not been cleaned for 6 weeks. PROBLEMS ON CHAPTER X 25. Size of D.C. Conductor. DATA: A D.C. 220-volt, 250-hp. motor is 300 ft. from the buses. The line leading to the motor is a rubber-covered copper cable. Market quotations for this kind of wire are on the 20-ct. base, and there is a discount of 50 per cent, on the wire. Net cost of labor and supplies is assumed to be the same for any size of wire that will be used. The motor is running 24 hr. per day, 365 days per year. Energy costs 1 ct. per kw.-hr. REQUIRED. (a) What size of cable is required for safety? (b) What size of cable must be used, in order that the line drop at full-load shall not exceed 10 per cent.? (c) What is the most economical size of wire? (d) Which of these three sizes should be provided? 26. Size of D.C. Conductor. DATA the same as in Problem 25, except that the motor is in operation only 8 hr. per day, 6 days in the week, throughout the year. REQUIRED. Find the sizes of wire for safety, voltage drop, and economy, as in Problem 25. 14 210 ELECTRICAL EQUIPMENT 27. Size of B.C. Conductor. DATA the same as in Problem 26, except that the motor carries full-load, 2 hr. per day; one-half load 3 hr.; and one-quarter of full-load 3 hr. per day. REQUIRED. Obtain results as in Problem 26. PROBLEMS ON CHAPTER XI 28. A.C. Line Voltage Regulation. DATA: A single-phase line has a resistance of 1 ohm and reactance of 1 ohm. REQUIRED. What is the decrease in voltage caused by 100 amp. flowing in the line, if the power factor of the load is 100 per cent.? 80 per cent.? 60 per cent.? (Use the approximate solution.) 29. Size of A.C. Conductor. DATA the same as in Problem 25, except that the motor is a three-phase, 60-cycle induction motor. REQUIRED. What size of wire should be provided, in order to be large enough for all requirements? [Suggestion: Reactance is not inversely proportional to area, so that the solution for size of wire for allowable line drop is somewhat complicated. A good method of pro- cedure is to find first the size required for safety and economy, and then to find by trial whether the voltage drop is excessive. If it is excessive, find by trial a larger size that does not have an excessive drop. Use the approximate solution in computing voltage drop.] PROBLEMS ON CHAPTER Xll 30. Generator Compounding. DATA: A 550- volt D.C. generator furnishes power to a feeder whose center of distribution is 2,000 ft. from the generator. The feeder delivers 300 kw. The size of the feeder is 500,000 cir. mils. REQUIRED. What must be the per cent, overcompounding of the generator, to maintain constant voltage at the center of distribution? 31. Maximum Demand. DATA: A factory has in it 20 200-hp. shunt motors, 30 100-hp., 50 25-hp., 100 10-hp., and 100 5-hp.; also 400 60- watt lamps. The demand factor during the daytime is 55 per cent, for motor loads, and 40 per cent, for lighting. REQUIRED. How much power is demanded of the generating station? 32. Number and Size of Generators. DATA the same as in the numerical example on page 98, except as follows: Interest is at 5^ per cent., depreciation 5 per cent., and insurance and taxes each 1 per cent.; other costs for switchboard, wiring, and equipment are $6 per kw. of total generator capacity; power is required 8 hr. per day, 6 days per week. REQUIRED. Find the best number and size of generators, and the cost per kilowatt for power, including fixed charges. PROBLEMS 211 PROBLEMS ON CHAPTER XIII 33. Size of Alternator and Engine. DATA: A three-phase engine type generator is required to furnish steady power to 200 1-hp. in- duction motors, running at full-load. REQUIRED. The kilovolt-ampere capacity of the generator, and the horsepower of the engine driving it. 34. Combined Load at Several Power Factors. DATA: The generator of Problem 33 is also required to furnish power to four 100- hp. induction motors, running at full-load, and fifty 100-watt lamps. REQUIRED. The kilovolt-ampere capacity of the generator. [Sug- gestion: Find for each part of the load, the power component of the kilo- volt-amperes (= kva. X power factor), and the "reactive" component (= kva. X sin 6, if cos 8 is the power factor). 1 Add together the power components to find the total power component, and the reactive components to find the total reactive component. The total kilovolt- ampere capacity must be the square root of the sum of the squares of the two components.] 35. Combined Synchronous and Induction Motor Load. DATA: The motor and lighting loads are the same as in Problem 34, except that some of the induction motors are replaced by synchronous motors, whose fields are adjusted so that the power factor of the generator load is 100 per cent. REQUIRED. Find the necessary kilovolt-ampere capacity of the gen- erator. PROBLEMS ON CHAPTER XIV 36. Constant -current Regulating Transformer. DATA: A constant- current regulating transformer operating from a 2,200 volt circuit is to furnish power for 100 60-watt 6.6 amp. series lamps. The line drop in the circuit is 25 volts. The power factor of the load is practically 100 per cent. At full-load, the transformer efficiency is 93 per cent., and the power factor of the primary 84 per cent. REQUIRED. The kilovolt-ampere capacity, and the primary and secondary current and voltage capacities. 37. Feeder Voltage Regulator. DATA: A three-phase feeder has a voltage of 2,300 at the buses, a line resistance of 0.1 ohm, and a line reactance of 0.25 ohm per line. The feeder is required to deliver power from zero to 8 kw., at any power factor from 60 per cent, (lagging) to 100 per cent. An induction regulator is to be used to maintain constant voltage at the end of the line. REQUIRED. The current capacity, the boosting voltage, the per cent, boosting, and the total three-phase kilovolt-ampere capacity of the regulator. 1 See brief table of sines and cosines on page 90. 212 ELECTRICAL EQUIPMENT PROBLEMS ON CHAPTER XV 38. Instrument Transformers. DATA as in Problem 37: A watt- hour meter is used to record the energy delivered by the feeder. The meter has 100-volt potential windings and 5-amp. current windings. REQUIRED. What should be the theoretical transformer ratio of the current and voltage transformers, according to these data? 39. Instrument Transformers. DATA: If voltage and current transformers cannot be obtained that produce exactly 100 volts and 5 amp. respectively, in their secondaries at rated voltage and full- load, the next higher or lower rating is selected, and the meter is cali- brated to suit (see list of transformers on pp. 116, 120). For the most accurate meter indications, the voltage elements of the meter in Problem 38 should have between 90 and 125 per cent, of the rated voltage, and at full-load the current elements should have between 75 and 150 per cent, of the rated current. REQUIRED. Select current and voltage transformers of commercial sizes, suitable for this service. PROBLEMS ON CHAPTER XVI 40. Switches. DATA: 'A 220- volt D.C. generating plant contains four 1,000 kw. -generators, and the following power feeders: Four 500 kw. feeders. Four 300 kw. feeders. Four 200 kw. feeders. REQUIRED. Give as full information as possible, as to the switches to be installed for the control of these circuits. 41. Rheostats. DATA: A 25-hp. motor on a feeder in Problem 40 is to run normally at 600 r.p.m. Field and armature rheostats are to be provided for varying the full-load speed. There are to be enough steps in each rheostat to vary the speed, by steps of 100 r.p.m., from 100 to 1,200 r.p.m. REQUIRED. Give all possible specifications for the rheostats. 42. Balancer-set Outfit. DATA: In the plant of Problem 40 are a balancer set and eight, three-wire 15-amp. lighting feeders. The lighting system is so arranged that the unbalancing of the current will not exceed 15 per cent, of the maximum possible lighting current. The armature current of the balancer set is 8 per cent., and the field current 2 per cent, of the machine full-load current, at no load. REQUIRED. (a) Give full information as to the switches to be pro- vided in addition to those of Problem 40. (6) Specify the kilowatts and voltage of each machine of the bal- ancer set. (c) Give full information regarding the motor starting rheostat. PROBLEMS 213 (d) Draw a complete diagram of connections of the balancer set. Note that if the neutral line is closed before the resistance is all cut out of the armature rheostat, the voltage is unbalanced, and all the lamps on one side of the circuit may be destroyed. Provide if possible some device making this condition impossible. 43. Line-drop Compensator. DATA as in Problem 37. In addition, a voltmeter and a voltage regulating relay are to be connected to current and voltage transformers and to a line-drop compensator, in the gener- ating station, to indicate and regulate the voltage at the end of the feeder. REQUIRED. (a) Specify the per cent, compensation of the com- pensator, and all transformer ratios. (6) Draw a complete diagram of connections of instrument trans- formers, compensator, voltage regulating relay, and induction voltage regulator. [Suggestion: The diagram may be simplified by notes referring to diagram of Problem 37.] PROBLEMS ON CHAPTER XVII 44. Feeder Protection and Voltage Regulation. DATA: The feeder of Problem 37 is to be protected by a circuit -breaker that carries the rated full-load without excessive heating. The circuit-breaker is tripped by a relay that operates in case an overload occurs, exceeding double the normal current. The relay receives its current from cur- rent transformers. It can be set to operate on any current from 3 to 6 amp. Required : (a) Specify the current and voltage rating of the circuit- breaker, and the current transformer ratio, selecting transformers of customary rating. (6) Draw a diagram of connections of the voltage regulator, circuit- breaker, relay, and current transformers operating the relay. 45. Overload Protection. DATA as in Problems 40 to 42. REQUIRED. Specify fully the equipment for overload protection. PROBLEMS ON CHAPTER XVIII 46. Lightning Protection. DATA as in Problem 37. In addition, the feeder is to be protected against lightning. The line is in a mountainous district where lightning discharges are very severe. It is proposed to put arresters at each end of the line, and every 1,500 ft. along the line. REQUIRED. Select suitable lightning arrester equipment, and draw a diagram showing the complete connections. 47. Lightning Protection. DATA : A 2,200-volt, three-phase, 60- cycle power plant furnishes power for coal mining. Power is used 214 ELECTRICAL EQUIPMENT outside each mine, for machine work, pumping, ventilating, and hauling, and inside the mine for mining, hoisting, and hauling. Motor generator sets are located in the mines and elsewhere as required, to obtain power at 275 volts, D.C., for hauling and other purposes. REQUIRED. Select suitable lightning arrester equipment, and draw a diagram showing connections. PROBLEMS ON CHAPTER XIX 48. Meters. DATA as in Problems 40 and 42. REQUIRED. (a) Draw a diagram showing the circuits, including all meters and ground detecting and meter switching apparatus, and specify the full scale indication of each meter. 49. Meters. DATA as in Problem 47. There are three generators in the power plant. REQUIRED. Lay out suitable feeders, and show all necessary meters, meter switches, ground detecting and synchronizing apparatus. PROBLEMS ON CHAPTER XX 50. Machine Shop Motors. DATA: A machine shop is to be equipped with motors for individual drive for the following machines: Engine lathe, 16-in. swing, for heavy duty. Engine lathe, 12-in. swing, for average duty. Planer, 6-ft. bed, 42 in. wide between housings. Boring mill, 6 ft. diameter of table, for average duty. Pair of emery wheels, 16 in. diameter, for heavy duty. Vertical drill press (upright drilling machine), 32-in. table. Punch press for punching a 1,^-in. hole in J^-in. soft steel. Lever shear for cutting y in. by 48-in. stock. REQUIRED. Specify the horsepower of motor for each application. 51. Crane Hoisting Motor. DATA: A 50-ton travelling crane is required to hoist full-load at 15 ft. per min., and to lower it at the same speed, with dynamic braking. The motor is hoisting one-third of the time, lowering one-third, and idle one-third. REQUIRED. (a) What power is the motor required to deliver during hoisting? (6) What is the equivalent power during lowering, in its effect in heating the motor? (c) What size of motor is required? 52. Crane Trolley Motor. DATA as in Problem 51. In addition, the weight of the trolley is 20 tons; maximum trolley speed is 120 ft. per min.; average acceleration and retardation is 4 ft. per sec. per sec. The trolley is accelerating during one-third of the time, retarding during PROBLEMS 215 one-third, travelling uniformly during one-sixth, and either stationary of drifting 1 (without power) during one-sixth. REQUIRED. (a) The power required during steady travel. (6) The power required during acceleration. (c) The equivalent of the power during lowering, in its effect in heating the motor. (d) The size of trolley motor. 53. Crane Bridge Motor. DATA as in Problems 51 and 52. In addition, the weight of the bridge is 40 tons; maximum bridge velocity is 300 ft. per min. No power is used during retardation, and accelera- tion is small enough so that power during acceleration is practically the same as during steady travel. Period of acceleration and steady travel, 30 sec. Period of retardation and rest, 45 sec. REQUIRED. (a) The power required during steady travel. (b) The size of the motor. [Suggestion: No variation of power is considered, except as it changes from zero to the maximum. Since there is only one value, the expression for intermittent, instead of variable power can be used, if desired.] 54. Acceleration of Motor Rotation. DATA as in Problem 52. In addition, the weight of the gear and motor armature is 1,000 lb.; outside diameter of the armature is 16 in.; diameter of trolley track wheels is 10 in.; and gear ratio from motor to track wheel is 2: 1. REQUIRED. How much additional power is required during accelera- tion, on account of accelerating the rotation of the armature and gear? GENERAL PROBLEMS 55. Railroad Repair Shop. DATA: A small railroad repair shop (see Fig. 108) is to be provided with D.C. power for motors, lighting and battery charging, for the following equipment: Motor drive for Four 26-in. lathes for heavy duty. Three 504n. lathes for average duty. One lathe used on machinery steel cutting speed, 60 ft. per min., %-in. cut; ^-in. feed. One lathe used on soft cast iron cutting speed, 40 ft. per min.; ^-in. cut; Y^-m. feed. One wheel lathe for heavy duty on 84-in. wheels, with separate motor for tail stock. One 12-ft. boring mill. One 6-ft. boring mill. 1 Strictly, if it is drifting without power, it is losing velocity, so that the power during retardation will be less. Practically the error is small in assuming that there is no change of velocity during a small amount of drifting. 216 ELECTRICAL EQUIPMENT One 36 in. X 8-ft. planer for average duty. One 42 in. X 16-ft. planer for average duty. One 60 in. X 20-ft. planer for average duty. One 14-in. slotter. One % in. X 96-in. shear. 2 1 I One multiple-spindle drilling machine, for 10 1-in. drills. One radial drill for heavy duty, 10-ft. arm. Two upright drills, 22-in. table. One 16-in. shaper. One 2>-in. bolt cutter. PROBLEMS 217 One 2-in. stucTcutter (same power required as for the same size of bolt cutter). One 6-in. pipe cutter. One 16-in. emery wheel. One 60-ton hydrostatic wheel press. One small hydrostatic press requiring 5-hp. motor. One 80-ton crane hoisting and lowering 5 ft. per min.; trolley travel 30 ft. per min.; bridge travel 100 ft. per min.; acceleration negligible; weight of trolley, 25 tons; weight of bridge, 70 tons. Each motor is working only a few minutes out of an hour. One 100-ft. turn table requiring 35-hp. motor. Lighting. General illumination also special illumination as required, in pits, inside locomotives, etc. (a) In the main shop. (b) In the round house. (c) On the turn table. (d) In the yards. Battery charging for train lighting, for two trains each composed of One baggage car. One 60-ft. mail car. Two 16-section Pullman sleepers. Four coaches. In addition to other demands, the batteries on the Pullman cars are to be adequate for lighting 2 hr. before the train is made up, and 1 hr. at the end of the trip. The total time of the trip is 8 hr. in each direction. Each train makes the round trip every day. If the axle-generator system is adopted, it is still necessary, on account of frequent and long stops, to give the batteries a charge at the end of each round trip, sufficient for 2 hr. service after the train starts. Generators, Switchboard Equipment and Wiring. For corresponding sizes of generators, the cost of generators and other equipment per cent, fixed charges, and cost of energy are as in Problem 32. The demand factor of the total load is 55 per cent. The plant is in operation 24 hr. per day, 300 days per year. REQUIRED. Draw a complete diagram of connections (condensed by notes); select the voltage or voltages of the system; specify the kind of motor for each application and horsepower of each; the number, size and kind of generators; the size, kind and voltage of each machine of each motor-generator set; and the size and location of all lamps. Give all possible information regarding meters, switches, circuit-breakers, rheostats, and other electrical equipment that should be provided in the power plant and shop, and the size and number of cells in each battery. 66. Cement Making Plant. DATA: The mechanical process of cement making is, in brief, as follows: Limestone and shale are 218 ELECTRICAL EQUIPMENT quarried, crushed, dried and stored ready for use. They are then mixed in suitable proportions, and reduced by several additional processes to a fine powder. This powder is passed through a kiln and melted to a clinker formation, and certain chemical changes take place. The clinker is then sometimes exposed to the weather for a few weeks, but this weathering may be eliminated. A small amount of gypsum is then added, and the material is again pulverized. It is then in its final state and is stored ready for shipment. Following is a list of machines and operations requiring motors in a typical cement-making plant; 1 the list is in the order of handling the cement (see Fig. 109). Wherever 5 hp. or less is required, a 5-hp. motor is installed for simplicity of layout, and to reduce the number of spare motors to be kept on hand. Where the horsepower is not given in the list, it is to be worked out as a part of the problem. In the quarry, 1, 3 a , 85, crushers (150, 75 and 30 hp.; 2, 4, belt elevators (capacity 100 tons per hour, lift 60 and 40 ft. respectively); and 5, belt conveyor (5 hp.). In the dryer department, 6, tram conveyor (10 hp.); 7, rotating the cylindrical dryer (25 hp.); 8, belt elevator, (10 hp.); and 9, belt conveyor (5 hp.) to separate storage tanks for shale and for limestone. In the mix department, 10 a and 10 6 , belt conveyors for shale and lime- stone, respectively (5 hp. each) to the mixing bin; 11, belt elevator (5 hp.) to the ball mills. In the raw department, 12 , 12 b , ball mills (75 hp. each); 13, belt conveyor and belt elevator (5 hp.); 14 a , 14 6 , 14 C , Kent-Maxecon mills (50 hp. each); 15, screw conveyors (5hp.) 16 ,&, belt elevators (5 hp.); 17 ,h, tube mills for raw material; 2 18, belt elevator (5 hp.); 19, 20, belt conveyors (10 and 5 hp. respectively) to the kilns. In the kiln department, 21 a to 21 e , rotating the kilns (20 hp. each); 22, Peck carrier (10 hp.); 23, belt conveyor (5 hp.) to clinker storage department. In clinker storage department, 24, 25, belt conveyors (5 hp. each); 26, belt elevator (5 hp.); 27, belt conveyor (5 hp.); 28, rolls (10 hp.); 29, cable (75 hp.). In the clinker department the processes are the same as in the raw department, beginning with the Kent-Maxecon mills Nos. 14 to 20. In the coal department are one 60-hp. and one 25-hp., two 10-hp. and three 5-hp. squirrel-cage motors. In the machine shop are located one 15-hp. motor for line shaft and one 25-hp. motor for air compressors. In the stock house are five 15-hp. and two 5-hp. squirrel-cage motors. 1 In nearly all details the motors listed are as in the plant of The Cayuga Cement Corporation, Portland Point, N. Y.; information was furnished by courtesy of Mr. W. H. Kniskern, General Manager. A few small motors have been omitted from the list, but none that would affect the layout of the system. 2 Assume data as on p. 182, unless otherwise specified. PROBLEMS 219 - Mr - v .:.. -- 7 : A.C- 222 INDEX Compressors: ammonia, motors for, 168, 177 air, motors for, 167, 177 Condenser lightning arrester, 144 Conductors, see Wires. Connection diagrams, see Diagrams of Connections. Constant-current lighting circuit, 164 Constant-current regulating trans- formers, 109 Contact-making voltmeter, 127 Continuity of service, 9 Controlling and regulating equip- ment, 121-129 Control switches or controllers: 123 applications, 124 Conventional representation of equipment, 5 Converters, see Synchronous Con- verters. Conveyors, power for, 176 Copper wires, data on, 80-84 Costs: 185-201 choke coils, 199 circuit-breakers, 196-198 copper wires, 81, 84 energy, affecting size of wires, 76 generators, 185 graphic meters, 190 integrating meters, 189 lightning arresters, 199 meters, 186-191 motors, 185 plug and instrument switches, 200 relays: bell-ringing, 194 protective, 193 steam engines, 185 switches, 195 transformers, constant current regulating, 200 instrument, 191 power and lighting, 198 turbo-generators, 185 voltage regulators, 200 wiring, 84 Cranes, motors for: 167, 174, 178 Problems 51-55; 214, 217 Current carrying capacities of con- ductors for safety, 79. 80 Current transformers, see Trans- formers, Instrument. Curve-drawing meters, 164 D.C. versus A.C. systems, 15 Delta connection of transformers, 49 Demand factor: 96 Problem 31; 210 Demand meters, 164 Detectors, ground, 151 Diagrams of connections: balancer sets, D.C., 39 conventional representations, 5 dynamotors, 42 end-cell switches, 55 exciters, 2, 127 generators: A.C., 2, 5, 101, 108, 110, 112 D.C.: 2, 5 three-wire, 40 instrument switches, 3, 159-164 lightning arresters, 142-146 meters, 148, 149, 151, 159-165, 202 motor-generators, 35, 39, 41 motors: A.C., 5, 31, 35 D.C., 5, 41 Problems 1, 2; 202, 203 rectifiers, 37 relays, 127, 129, 139-141 rheostats and D.C. motor start- ers, 2, 5, 35, 36, 39-42, 108, 127 rules for making, 3-6 starters, induction motor, 31 switches and circuit-breakers, 2, 5, 122-124, 134-136, 139-141 synchronous converters, 36 transformers and auto-trans- formers, 47, 49, 110, 115, 116, 118, 119, 124 typical A.C. and D.C. circuits, 2 voltage regulators, 112 Ward Leonard system, 41 INDEX 223 Diameters of wires, 80, 82 Direct lighting, 68 Disconnecting switches: 123 applications, 124 costs, 195 Distribution of candlepower, 63, 64 Distribution systems: A.C., 85-91 circuits of, 1 D.C., 71-84 requisites of, 7 Duplicate equipment, 9 Dust, effect of, on illumination, 66 Dynamic braking, 179 Dynamos, see Generators. Dynamotors, 42 E Economical size of conductor, 74-78, 91 Efficiency: auto-transformers, 47 generators, A.C., 103 D.C., 93 motor-generators, 38 motors: D.C., 24 induction, 30 rectifiers, 38 synchronous converters, 38 transformers, 45, 46 utilization of illumination, 65 Electrical conductors, see Wires. Electrolytic lightning arrester, 145 Enclosed motors, see Motors, D.C., Enclosed. End-cell switch, 55, 56 Engines, steam, costs, 185 Exciters: rheostats for, 108 and exciter circuits, representa- tion of, 2, 4 steam- and motor-driven, 107 Expansion, allowance for, 13 Feeders: parallel, protection of, 140 representation of, 2, 4 voltage regulators for, 111 Field: discharge resistance, 107 rheostats, 26, 124-126 switches: 107 plug, cost, 200 Fixed charges, 10, 11 Foot-candle, illumination, 61 Frequency: choice of, 17 meters, 151 variation affecting meter accur- acy, 154 Fuses, 130 Gas engines, ignition, 58 Generators, A.C.: 100-108 characteristics, 103 classifications, 100-103 connections, 2, 106 efficiency, 103 equipment of circuits, 106 excitation, 107 frequency, 101 number and size required, Problem 2; 203. See also Generators, D.C. phases, 100 Problems 2, 33-35; 203, 211 rating, 104 regulation: 103, 104 of prime movers, 104 requisites for plant operation, 104 revolving field or armature, 102 rheostats, 108 speed and prime mover, 102 synchronizing, 105 voltage: 102 and power-factor adjust- ment, 108 Generators, A.C. and D.C.: costs, 185 representation of, 5 voltage regulators for, 107, 126 Generators, D.C.: 92-99 adjustment of compounding, 94 available sizes, 96 characteristics, 92-94 circuits and equipment, 2, 95, 96 connections, 95 efficiency, 93 224 INDEX Generators D.C.: number and size required, 96-99 parallel operation, 94 Problems 30-32; 210 rating, 94 regulation, 94 temperature rise, 94 three-wire, 40 Generator circuits, representation of, 2, 4 Glare, 67 Graphic meters: 164 costs, 190 Ground detecting lamps, 152 Ground detectors: 151 switches, 161 Ground wire for lightning arresters, 147 Group drive, 166 H Hoists, motors for: 167, 175, 178 Problem 9; 205 Horn-gap lightning arrester, 143 Ignition, gas engine, 58 Ilgner system for motor speed ad- justment, 40 Illumination: 61-70 computations, 68 direct and indirect, 68 intensity, 61, 66 of power plant, 14 Problems 23, 24, 55; 209, 215 special, 69 train lighting, 58, 59 utilization efficiency, 65 Impedance drop, see Line Drop. Indicators, see Meters, Lamps. Indirect lighting, 68 Induction voltage regulator, 111 Industrial applications of motors: 167, 170 Problems 50-56; 214-220 Instrument switches, see Switches. Insrument transformers, see Trans- formers, Instrument. Insulated wire, diameters and weights, 80, 82 Integrating meters: 164 costs, 189 Intensity of illumination, 61, 66 Intermittent loading of motors, 183 K Kelvin's Law, 75 Knife switches, see Switches. Labels on diagrams, 6 Lamps: for synchronizing, 150 ground detecting, 152 Layout of power station, 14 Light flux, 62 Lighting: 61-70 automobile, 58 circuits: D.C., line drop, 71 frequency for, 18 voltage for, 19 of power plant, 14 Lightning arresters: 142-147 choke coils for, 145 costs, 199 ground wire and ground con- nections, 147 Problems 46, 47; 213 relative merits, 145 Line drop : see also Resistance of Con- ductors and Reactance of Transmission Lines. A.C.: 85-90 affected by power factor, 86 single-phase, 85 three-phase, 87 two-phase, 90 vector diagrams, 85, 86, 89 compensator: 128 Problem 43; 213 D.C.: 71-74 ground or rail return, 73 multiple voltage, 74 two-wire, 72 reactance, 81, 83 voltage regulation, 90 Locomotives, battery, 58 Lumen, 62 INDEX 225 M Machine tools, motors for: 167, 170, 178 Problems 50, 55; 214, 215 Magnetic blowout lightning arrester, 144 Maximum demand meters, 164 Measuring and indicating apparatus, see Meters, Lamps, Switches. Mechanical rectifiers, see Rectifiers. Mercury rectifiers, see Rectifiers. Mertz-Price system of relays, 141 Meters: 148-165 accuracy with instrument trans- formers, 155 applications, 162-165 characteristics, 152 constant-current lighting cir- cuit, 164 costs, 186-191 effect of : low power factor, 155 stray field, 155 varying frequency, 154 varying voltage, 155 frequency, 151 graphic or curve-drawing, 164 ground detectors, 151 integrating, 164 maximum demand, 164 power factor, 149 Problems 48, 49, 55, 56; 214- 220 scales, 152 synchronism indicators or syn- chronoscopes, 150 watt-hour, 149, 164 watt meters, 148 switches, see Switches. Motor circuits: frequency for, 17, 18 line drop, 71 voltage for, 20 Motor-generators : 34-42 efficiency, 38 Problems 16, 17; 207 induction vs. synchronous mo- tors, 35 vs. synchronous converters, 36 15 Motors, A.C.: 28-33 suitable applications, 33 types available, 28 induction : adapted to location, 29 connections for starting, 31, 49 data at starting: 31, 32 Problems 12-15; 206 efficiency, 30 operation at various loads, 29 power factor, 30 slip, 30 speed adjustment, 33 speed regulation, 32 speeds, usual, 30 voltage, frequency and phases, 28 Motors, A.C. and D.C.: applications: 166-184 Problems 3-16, 50-56; 204- 207, 214-220 available sizes, 166 costs, 185 for group drive, 166 kinds for various applications, 167 representation of, 5 speed affecting rating, 183 variable and intermittent load- ing, 183 sizes for various applications, 170-184 Motors, D.C.: 22-27 efficiency, 24 intermittent and variable load- ing, 23 loading at high speeds, 23 enclosed : change of rating, 183 Problem 10; 205 required in some cases, 22 overloading, 23 rating, 23 speed adjustment: 25 by multiple voltage, 40 by Ward Leonard and Ilgner systems, 40 Problems 10, 11; 205 regulation, 24 226 INDEX Motors, D.C.: speeds, usual, 24 voltage, 22 Multigap lightning arrester, 142 Multipath lightning arrester, 144 Multiple voltage for D.C. motor- speed adjustment, 26, 40 N National Electrical Code: (foot- note), 9 sizes of conductors, 79, 80 Notes and labels on diagrams, 6 Number of -phases, 16 Oil circuit-breakers, costs, 196-198 Oil switches, see Circuit Breakers, Oil, Non-automatic. Operating cost, 10, 11 Overload relays, see Relays, Protective. Parallel feeders, protection of, 140 Phases, number of, 16 Plug switches, see Switches. Point-by-point computation, 68 Potential regulators, see Voltage Regulators. Potential transformers, see Trans- formers, Instrument. Power factor: affecting line drop, 86 affecting meter accuracy, 155 brief table of sines and cosines, 90 meters, 149 Power plants: circuits of, 1 requisites of, 7 Protective equipment, 130-147 Protective relays, see Relays, Pro- tective. Pumps, motors for: 168, 177 Problems 8, 15; 205, 206 R Reactance drop, see Line Drop. Reactance of transmission lines, 81, 83 Receptacles, voltmeter and am- meter, see Switches. Rectifiers: 34-38 connections, 37 efficiency, 38 Rectifiers: Problems 16, 17; 207 suitable applications, 37, 38 Reflectors: distribution curves, 63-65 effect of dust, 66 effect of reflectors, 62 types, 62, 63 Refrigerating plants, motors for, 168, 177 Regenerative control, or dynamic braking, 179 Regulating transformers, 109-113, see also Voltage Regulators. Regulation: line, see Line Drop and Voltage Regulators, also Con- stant-current Circuits and Transformers. generator: A.C., 103, 104 D.C., 92 motor: A.C., 32 D.C., 24 Regulators, voltage, seeVoltage Regu- lator. Relay, voltage regulating, 127 switches, 128 Relays : auxiliary: relay switch, 128 bell-ringing, costs, 194 time-limit, costs, 194 protective: 136-141 applications, 139 costs, 193 overload, 137 time-limit, 137 Z-connection, 140 voltage regulating, 127 Representation of equipment, 5 Resistance: drop, see Line Drop. of aluminum conductors, 81, 83 of copper conductors, 81, 83 Rheostats: 124-126 applications and specifications, 124-126 alternator and exciter field, 108 D.C. generator field, 96 D.C. motor, for speed control, 25 field, representation of, 5 Problems 11, 41, 42, 55, 56; 206, 212, 217-220 Rolling mills, motors for, 168, 177 INDEX 227 Rotary converters, rotary trans- formers, see Synchronous Converters. Rubber-covered wires, current carry- ing capacities, 80 Safe carrying capacities of con- ductors, 79, 80, 91 Safety to operators and equipment: 8 Problem 3; 204 Scott connection of transformers and auto-transformers, 48-50 Screw conveyors, power for, 176 Segment of circle, center of gravity, 182 Semi-indirect lighting, 68 Series lighting circuit, 110 Series transformers, see Transform- ers, Instrument. Service, continuity of, 9 Shadows, affecting illumination, 67 Shunt transformers, see Trans- formers, Instrument. Size of conductor: based on allow- able line drop, 71, 85 based on economy, 74 for safety, 79, 80, 82 with variable current, 78 Solid conductors, see Wires. Spacing of lamps, 67 Speed : adjustment of D.C. motors, 25 variation of motors, affecting rating, 183 Station layout, 14 Steam engines, costs, 185 Steel rolling mills, motors for, 168, 177 Storage batteries : 51-60 capacity on heavy discharge, 53 charge and discharge vol- tages, 54 comparison of various kinds, 51-55 construction, 51 cost, 51 current charging rate, 53 current discharging rate, 52 Storage batteries: durability and repairs, 52 efficiency, 54 end-cell switch, 55, 56 Problems 17, 21, 22, 55; 207, 208, 217 space occupied and weight, 52 types, 51 Storage battery applications: auto- mobiles, trucks, battery locomotives, 58 battery substation, 56 generating station, 55 on separate circuits, 56, 57 portable service, 57-60 stationary service, 55-57 train lighting, 58-60 Stranded conductors, see Wires. Stray field affecting meter accuracy, 155 Speed control by rheostats, see Rheostats. Switchboards: A.C., 2, 3, 162 D.C., 2, 162 layout, 14 Switches: 121-123 costs, 195 plug and instrument : 159-162 ammeter, 160, 161 costs, 200 ground detector, 161 synchronizing, 160 voltmeter, 159 Problems 40, 42, 55, 56; 212-220 representation of, 5 Synchronizing A.C. generators, 105 Synchronous booster converters, 36- 38 Synchronous converters: 34-38 efficiency, 38 Problems 16, 17; 207 vs. motor-generators, 36 voltage ratios, 20 Synchronism' indicators or syn- chronoscopes, 150 Synchronizing: lamps, 150 plugs and receptacles, 160 System, choice of, see Choice of System. 228 INDEX !F-connection of transformers and auto-transformers, 49 Temperature coefficient of resist- ance, 81, 83 Three-wire generators, D.C., 40 Three-wire lighting system, 38 Time-limit relays, see Relays, Pro- tective. Train lighting, 58, 59 Transf ormes : constant current regulating: 109 costs, 200 Problem 36; 211 current, voltage, see Trans- formers, Instrument. instrument: 114^120 advantages of using, 120 affecting meter accuracy, 155 costs, 191 Problems 38, 39, 44; 212, 213 current (series or ammeter): 116-120 ratings, 120 representation, 116 on series lighting circuit, 164, 165 Z-connected, 140 voltage (shunt, potential or voltmeter): 114-116 connection, 115 ratings, 116 power and lighting : 43-50 costs, 198 efficiency, 45, 46 frequency, 46 grouping, 48-50 kva. capacity, 44 overloading, 45 Problems 2, 18, 19; 203, 207 ratio, 43 voltage adjustment, 44 voltage regulation, 45, 46 regulating, 109-113 voltage regulating, see Voltage Regulators. Transmission and distribution: A.C.: 85-91 Problems 18, 28, 29, 36, 37, 43, 49, 56; 207-220 B.C., 71-84 Problems 25-27, 55; 209, 210, 215 Trucks, battery, 58 Tube mill: distance to center of gravity, 182 Problem, 56; 217 Turbo-generators, costs, 185 U Utilization efficiency of illumination 65 Variable loading of motors, 183 ^-connection, power transformers and auto-transformers, 48, 49 Vector diagrams of line drop, 85, 86, 89 Vibrating rectifiers, see Rectifiers. Voltage: choice of, 18 control by rheostats, see Rheo- stats. drop, see Line Drop. compensator, see Line Drop Compensator. regulating relay, 127 regulators: costs, 200 generator, 126 induction, 111 Problems 37, 44; 211, 213 transformers, see Transformers, Instrument. variation: to be avoided, 12 affecting meter accuracy, 155 Voltmeter, contact-making, 127 Voltmeter plugs and receptacles, see Switches. W Ward Leonard system for motor- speed adjustment, 40 INDEX 229 Watt-hour meters: 149, 164 Wiring: costs, 84 costs, 189 diagrams, see Diagrams of Con- Watt meters, 148 motions. Weatherproof wires, diameters, rules for representing, 4 weights, and carrying ca- Underwriters' rules (footnote), 9 pacities, 80 Wood working, motors for, 167, 173, Weights : of aluminum wires, 80, 82 178 of copper wires, 80 Y Wires : areas, diameters and weights, y. connection . auto-transformers, 31, 80 49 costs > 81 > 84 transformers, 49 current carrying capacities, 80, 82 Z data on electrical conductors, Z-connection of current trans- 80-84 formers, 140 1*49167' YC 19599 UNIVERSITY OF CALIFORNIA LIBRARY