ELECTRIC MOTORS THEIR ACTION, CONTROL AND APPLICATION BY FRANCIS B. CROCKER, KM., PH.D. PROFESSOR OF ELECTRICAL ENGINEERING, COLUMBIA UNIVERSITY, PAST PRES. AM. INST. ELEC. ENG., MEM. BRITISH INST. ELEC. ENG. AND MORTON ARENDT, E.E. ASSISTANT PROFESSOR OF ELECTRICAL ENGINEERING, COLUMBIA UNIVERSITY FELLOW AM. INST. ELEC. ENG. SECOND EDITION REVISED AND ENLARGED NEW YORK D. VAN NOSTRAND COMPANY _ 25 PARK;. PLACE, 1914 COPYRIGHT, 1910, BY D. VAN NOSTRAND COMPANY COPYRIGHT, 1914, BY D. VAN NOSTRAND COMPANY THE SCIENTIFIC PRESS ROBERT ORUMMONO AND COMPANY BROOKLYN. N. Y. PREFACE TO FIRST EDITION. THE design and construction of electrical apparatus are well covered by existing literature, the books on such subjects being very numerous, and many of them are comprehensive as well as authoritative. On the other hand, the operation of electrical machinery has received comparatively little attention. This latter fact is anomalous when we consider that there are undoubtedly several hundred users for every designer or constructor of such apparatus, because each builder supplies a large number of customers. Hence the authors have endeavored to supply information that may be useful to those who operate or are interested in the operation of electric motors. Included among these are electrical engineers who install or run electric power plants, managers of manufacturing or other establishments in which electric- drive is employed, as well as students and others who desire to acquaint themselves with the working of various kinds of electric motors and their application to useful purposes. The subject is necessarily technical because it involves not only the mechanical factors speed and torque but also the electrical quantities voltage, current and flux. Moreover, any or all of these five fundamental quantities may, in fact usually do, vary and are affected by other factors or conditions. Hence the problems must be analyzed and solved with thoroughness to obtain results of real value and cannot be properly treated in a popular manner. Nevertheless care has been taken to introduce and explain each step or result as clearly as possible, and to illustrate each case, when feasible, by a specific numerical example based upon standard commercial motors. The general method herein adopted is an outgrowth of the course of lectures on electric motors and their applications given in Columbia University since 1889. It is based upon the consideration of counter e.m.f. and its relation to impressed e.m.f. as the important criterion of motor action. This point of view is, of course, not original, but it is claimed that the conception is more explicitly and iii 343*09 iv PREFACE. widely applied than heretofore. Furthermore, this idea brings together the motor and generator so that they may be regarded as identical except for slight differences easily seen, and our knowledge concerning one is applicable to the other. The plan of treatment also links voltage with speed, and current with torque, since in general they are respectively proportional. Thus we consider one pair of quantities at a time instead of four. The synchronous a.c. motor differs so radically from the d.c. type that the treatment must be modified, but even in this case a similar standpoint is adopted as nearly as possible. Throughout the book references are given to United States and foreign patents as well as articles and books in which may be found further descriptions of the various machines and methods considered. Those portions of the A. I. E. E. Standardization Rules relating to electric motors have been extracted verbatim and put together as Appendix A. The authors gratefully acknowledge their indebtedness to Messrs. J. H. Morecroft, A. G. Popcke, L. W. Rosenthal, A. H. Timmer- man, E. H. Waring and G. B. Werner for valuable information and to F. L. Mason for assistance in proof reading. They also take this opportunity to thank the Crocker- Wheeler, Electro-Dynamic, General Electric, Wagner and Westinghouse Companies for illustra- tions and data of apparatus manufactured by them and discussed in this book. January 5, 1910. PREFACE TO SECOND EDITION. THE present edition contains many amendments and addi- tions to make the subject matter clearer and more complete. These changes apply to the illustrations as well as to the text. Some sections of the book have been considerably revised or added to, as for example Starting Box Calculations for direct current shunt and alternating current induction motors, and an entire chapter on the power requirements of various tools, etc., has been introduced. The object of the authors is to set forth the action and opera- tion of the various types of electric motors with sufficient comprehen- siveness for most persons, who study or use these machines, even including students and practitioners who specialize in electrical engineering. At the same time particular care has been exercised in omitting matter that is only of theoretical or special interest. In other words, both the scope and volume have been kept strictly within the limits of a hand-book, and no attempt made to produce an encyclopedia of the whole subject or an exhaustive treatment of a particular branch. The authors gratefully acknowledge the assistance of Prof. Geo. F. Sever, Mr. J. Lebovici and Mr. Wm. Siebenmorgen in preparing some of the amended and new material in this revised edition. April, 1914. v TABLE OF CONTENTS PART I GENERAL CHAPTER I PAGE INTRODUCTION i CHAPTER II TYPES OF MOTORS AND ADVANTAGES OF ELECTRIC DRIVE 4 PART II DIRECT-CURRENT MOTORS CHAPTER III ACTION OF SHUNT MOTORS 10 CHAPTER IV SHUNT-MOTOR STARTING BOXES 33 CHAPTER V SHUNT-MOTOR SPEED CONTROL BY VARIATION OF RESISTANCE OF ARMATURE CIRCUIT 40 CHAPTER VI MULTIPLE-VOLTAGE SYSTEMS OF MOTOR SPEED CONTROL 51 CHAPTER VII SPEED CONTROL OF SHUNT MOTORS BY VARIATION OF FIELD CURRENT 70 CHAPTER VIII SPEED CONTROL OF MOTORS BY VARIATION OF FIELD RELUCTANCE 91 vii viii TABLE OF CONTENTS CHAPTER IX PAGE DIRECT-CURRENT SERIES MOTORS 99 CHAPTER X CONTROL OF DIRECT-CURRENT SERIES MOTORS 114 CHAPTER XI COMPOUND- WOUND MOTORS 1 24 PART III ALTERNATING-CURRENT MOTORS CHAPTER XII CLASSIFICATION AND HISTORY 1 29 CHAPTER XIII SYNCHRONOUS ALTERNATING-CURRENT MOTORS 137 CHAPTER XIV POLYPHASE INDUCTION MOTORS 173 CHAPTER XV STARTING OF POLYPHASE INDUCTION MOTORS . . .202 CHAPTER XVI SPEED CONTROL OF POLYPHASE INDUCTION MOTORS 214 CHAPTER XVII SINGLE-PHASE INDUCTION MOTORS 223 CHAPTER XVIII COMMUTATING ALTERNATING-CURRENT MOTORS 248 TABLE OF CONTENTS ix PART IV APPLICATIONS OF ELECTRIC MOTORS CHAPTER XIX PAGE SERVICE CONDITIONS. . . .281 CHAPTER XX POWER REQUIREMENTS OF VARIOUS TOOLS, ETC 288 INDEX 301 Electric Motors, their Action and Control. PART I. CHAPTER I. INTRODUCTION. An electric motor is a machine which converts electrical power into mechanical power. In function, therefore, it is the exact converse of the dynamo-electric generator. On the other hand, identically the same machine may be and often is employed to per- form either function, which fact is known as the reversibility of the dynamo-electric machine. In the earlier periods of their develop- ment, however, the two machines were usually regarded as quite different in character and were constructed on wholly different lines. Strange to say, the motor historically precedes either the magneto- or dynamo -electric generator. Barlow's wheel of 1823, the first electric motor, was similar in construction to Faraday's disc of 1831, which was the original magneto-electric generator. The Jacobi electric motor of 1838 was large enough to propel a boat carrying fourteen passengers at three miles per hour, and Page in 1851 constructed a car driven by a i6-horsepower electric motor at nineteen miles per hour, whereas the dynamo electric machine was not invented, by C. W. Siemens and Wheatstone, until 1867. These as well as other electric motors of those early times were far more powerful and were regarded as more practical or more promising than the contemporaneous magneto-electric generators. The Paci- notti ring of 1861, the prototype of modern armatures, was primarily intended to be used in a motor, although the inventor suggested that it could also be employed to generate electric currents. All of these early electric motors depended upon primary bat- teries for their supply of electrical energy, and it was found that the 1 2 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. cost of operation was excessive for any considerable power, especially with the low-efficiency motors and crude forms of battery then available. The result was that the motor had to stand aside while the generator was being developed to commercial success, which development began about 1880. Even then the electrical energy produced was used entirely for arc and incandescent lighting. In fact, it was not until about 1887 that central stations with their systems of distribution had become sufficiently large and well enough regulated so that the use of electric motors upon their circuits was encouraged or even permitted except in a few cases. The electric light having been practically introduced and more or less generally established, inventors, manufacturers, also those who produced electrical energy, turned some attention to electric power, which, from about 1888, has been a prominent part of electri- cal engineering, including railway as well as stationary motors. The former type, also the induction and synchronous alternating-current motors, began to be commercially introduced about that time or soon after. Since this comparatively recent epoch the progress of electric power in all its branches has been at an extraordinarily rapid rate and with most far-reaching results, unequaled by any other art or industry in anything like the same period of time. Relation between Generator and Motor. Either a dynamo- electric generator or a motor may be regarded as made up of a cer- tain number of centimeters of wire located in a magnetic field of given density in lines per square centimeter. The former machine will generate e.m.f. when the wire moves; the latter machine exerts torque if current flows in the wire, that being the essential distinction between the two. We may have, for example, a generator (sepa- rately excited) producing full e.m.f. with no current in its armature and we may have a motor exerting full torque with its armature prevented from turning, but we cannot have such a generator without motion or such a motor without current In practical operation, however, the generator has current flowing in the wire so that a certain torque, opposed to driving force, is also exerted; and in the motor an e.m.f., counter to energizing current, is set up by the motion of the wire. Hence either machine while working, develops both e.m.f. and torque, the only difference between them under these conditions being the fact that this e.m.f. is positive with respect to current in the generator and negative in the motor, while torque is negative with INTRODUCTION. 8 respect to motion in the generator and positive in the motor. Tt follows therefore that electrical power is positive in the generator and mechanical power is negative, whereas electrical power is negative in the motor and mechanical power positive. In fact the exact func- tion of these machines is expressed in the above statement, which means that they convert mechanical power into electrical power and vice versa. These distinctions in function or action do not, however, involve any necessary difference in the construction of generators and motors. As already stated, identically the same machine is equally operative for either purpose because the dynamo-electric machine is perfectly reversible. In practice, motors and generators are some- what different, but merely with respect to details of form or con- nections, so that they will be more convenient for the special uses to which they are applied. As a matter of fact motors differ among themselves, railway and stationary types, for example, fully as much as they differ from generators. While these differences in construction are for the most part mere matters of adaptability, the usual operation of generators is radically unlike that of motors. The former are almost universally driven at constant speed by steam engines, gas engines, turbines or other sources of mechanical power. Of course in practice the speed varies somewhat, but this is ordinarily undesirable and avoided as much as possible by most careful design as well as adjustment of governors. The few cases in which the speed variation is large, as, for example, the driving of a generator from the axle of a railway car, involve mechanical as well as electrical difficulties, special and often com- plicated auxiliary apparatus being employed. On the other hand, the speed of electric motors is very commonly variable or adjustable, the range in many cases being from zero to a maximum in either direction, as in railway or elevator service, and speed ratios of three or four to one or higher are common in factories, machine shops, etc. The means and methods used to accomplish such speed variation constitute an important branch of engineering, and it is the particular purpose of this book to discuss this subject of motor control. In those applications for which constant speed is desired, the motors may depart somewhat from this condition owing to their own action, which matter will also be given special attention, because it is often of practical importance to reduce or allow for even slight changes of speed. CHAPTER II. TYPES OP MOTORS AND ADVANTAGES OP ELECTRIC DRIVE. MANY kinds of electric motors are in use, each having its char- acteristics of design and operation. In general electric motors are divided into those of the direct-current and alternating-current groups, which in turn may be subdivided into particular types as follows: DIRECT-CURRENT MOTORS. Type Operative Characteristics. Shunt-wound motors Starting torque obtainable in actual practice is 50 to 100 per cent greater than rated running torque, and fairly constant speed over wide load ranges. Series-wound motors Most powerful starting torque of any electric motor, speed varying greatly (inversely) with load changes. Compound-wound motors . . . Compromise between shunt and series types. Differentially wound motors . Starting torque limited, for which reason these motors are rarely used practically. They are interesting scientifically because their speed can be made almost constant for reasonable load changes within rated capacity. ALTERNATING-CURRENT MOTORS. Synchronous motors Single-phase type not self -starting; polyphase type is self-starting with low torque, speed of both absolutely constant. Induction, motors Single-phase (per se) is not self-starting, poly- phase self-starting under load, substantially constant speed over wide load-range. Comrmitating motors Single-phase self -starting with powerful torque, speed may or may not vary with load changes, depending upon design. No attempt is made herein to describe the design or construction of electric motors except special features relating to speed control. The general subject of motor structure, mechanical and electrical, is treated in a number of standard works listed at the end of this 4 TYPES OF MOTORS AND ADVANTAGES OF ELECTRIC DRIVE. 5 chapter, and a reasonable knowledge of such matters is assumed. For example, the various parts of motors, their names, forms and relation, are supposed to be understood. The sole function of electric motors is to drive some other machine or device, but, as the number of such applications is practically infinite, their field of utility becomes almost universal. Some of the prominent uses include the driving of cars, pumps, fans, machine tools, looms, printing presses, hoisting apparatus, grinding and polishing machines, etc., etc. Before taking up the discussion of motor action and control it will be well to consider the advantages thereby secured. ADVANTAGES OF ELECTRIC DRIVE. i. Saving in Power. This is generally the first point to be considered, but it is by no means the most important, as the cost of power in manufacturing is rarely more than i to 3 per cent of the cost of the finished product, the expenditure for labor alone being usually many times greater. It is a fact, however, that, due to the absence of belting and shafting losses, which are usually 40 to 60 per cent of the total power required to drive the various machines, the saving is considerable. Furthermore, the complete cutting off of electric current whenever an individually operated machine is stopped, compared to the large practically constant loss with belt- ing and shafting, is much in favor of the former method. In fac- tories and similar industrial establishments the load factor or average power is only 20 to 60 per cent of the maximum or total amount required when all machines are working at full load. The losses with electric drive correspond to the usual or average conditions, while belting and shafting losses vary somewhat with load, but nearly correspond to possible or total capacity of the plant. This advantage is more or less offset by the losses involved in the double conversion of energy from mechanical to electrical and back to mechanical form, but in most cases and in the long run the former method does effect a real saving in power consumption. This is particularly true when the machines to be driven are scattered; on the other hand, if they are very compactly placed, with minimum distances between them, the saving in power by electric drive might be little or nothing, but some or all of the other advantages now to be stated would be secured. PLATE B. THE SAME SILK-WEAVING SHED WITH MOTOR DRIVE. TYPES OF MOTORS AND ADVANTAGES OF ELECTRIC DRIVE. 7 features must be sacrificed. A great advantage due to the flexibility of electrically driven machines is the use of portable equipments which are easily made up and operated, so that the tool is frequently brought to the work, as, for example, when a portable drill is brought to a heavy casting or to a large number of castings, or when a verti- cal slotter is applied to the outside of a large casting at the same time that the interior is being bored. 6. Clear Head Room. The elimination of overhead belting and shafting by the use of motors gives a clear head room, which enables overhead cranes to be used freely; a fact which results in great saving of time and labor in the bringing of the wo^'to the tools or removing finished pieces. The clear head room also gives better illumination and ventilation. In fact the saving in cost of proper illumination may be very considerable because, general instead of local lighting may be obtained, whether natural or, arti^ ficial. Comparing plate A, which shows the ' appearance of a silk-weaving shed operated by belting and shafting, and plate B, an illustration of the same mill in which the looms are operated electrically, the great advantages regarding head room- and illumi- nation are apparent. 7. Cleanliness. The dripping of oil from overhead bearings and shafting is a constant source of annoyance, and -the dirt thrown out from belting is an even worse enemy to cleanliness. The agitation of dust by belting and shafting keeps it in con- stant circulation, so that it penetrates everywhere and everything, This is an especially important matter in printing and textile work. 8. Health of Employees. On account of the better ventilation and illumination, and reduction of dust and dirt, it is shown by actual experience that the general health of those who work with electrically driven machinery is improved. In the Government printing office at Washington, it was found that the sick list was decreased as much as- 40 per cent after the electric drive was introduced. 9. Convenience for Detached Buildings. The electrical method enables power to be supplied easily and economically to detached buildings or sections, which is not possible with belt or steam trans- mission; therefore, the buildings, like the machinery within them, can be located for general convenience, and not with special regard 8 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. to supplying them with power. This subdivision of an industrial establishment into a series of detached buildings is an almost abso- lute safeguard against total destruction by fire; and is thus a practi- cal guarantee of continuous earning capacity. If electric power were not employed, it would be necessary to have very extended belting and shafting connections, involving great losses and extra heavy wall construction, or a number of small power plants with a larger force of men and considerably less economical in oper- ation than one central power house. 10. Freedom for Growth. For similar reasons, with electric drive it is a simple matter to extend a building, or add another in any direction, whereas shafting must be installed originally large enough to allow for extension; or else it must be replaced later; in which case the operation of the existing line shafts would be interfered with. 11. Reliability. Shut-downs or delays are less frequent and less serious, because an accident in an electrically driven plant usually has a local effect only, simply interrupting the service of one or a few machines, while with belting and shafting the breaking or slipping off of a belt, or the failure of a friction clutch, may require the shutting-down of a whole plant or a large section thereof. Fur- thermore the time required for repair is usually less with electric power. In a large establishment a delay of even a few minutes represents a considerable item in wages, and in addition the inter- ruption of the work is demoralizing. It might be argued that the central power plant may break down; this is, however, just as likely to happen with one form of power transmission as with another. In case of damage by fire or flood, the machinery can be moved, rearranged, reconnected and started again much more promptly with electric driving than with belting and shafting. 12. Speed Control. The variation of speed that is possible with the electric drive, and the convenience as well as the wide ranges of control, are great advantages which in many cases are sufficient in themselves to dictate the adoption of electric motors. The operator can drive the machine to its limit of capacity, and can, on the other hand, instantly relieve it of strain. With mechanical drive the methods of speed control are more limited and require more time to operate than with electric motors. The shifting of the belt on a cone pulley, or the throwing in and out of different sets of gears, TYPES OF MOTORS AND ADVANTAGES OF ELECTRIC DRIVE. 9 takes more effort than the simple turning of a controller handle, which can be placed in a much more convenient position than is usually possible with the mechanical device. The result is that the operator makes more frequent use of the former in order to gain even slightly in the efficiency or rapidity of his work. For example, the cutting speed in the case of the electric drive can be kept absolutely constant or at maximum value, whereas mechanical drive cannot be adjusted as quickly or as closely, the steps of speed variation being much greater. The saving in time thus obtained is consider- able and correspondingly reduces the shop cost of the article. It is the particular purpose of this book to discuss the matter of electric- motor speed control. 13. Increased Output. Owing to its many advantages, especially on account of clear head room for crane service and convenient speed control, the output of manufacturing establishments is in most cases materially increased or the running expenses decreased by the intro- duction of electric drive. An added output of 20 or 30 per cent is often obtained from the same plant, which in itself is sufficient to make the difference between profit and loss in a manufacturing business. 14. Overtime work, also work on holidays or during strikes, may be carried on conveniently and economically with a portion of the machinery or even with a single tool, because a small engine and generator may be run to supply the electric power. On the other hand, the main engine and the whole or a large part of the shafting and belting would have to be operated in order to supply the power by the ordinary mechanical transmission. 15. Noise. Rumbling of line shafts and slapping of belts are entirely done away with when electric drive is adopted. BIBLIOGRAPHY, ELECTRIC MACHINE DESIGN. CONTINUOUS CURRENT MACHINE DESIGN. Wm. Cramp. D.VanNostrandCo. 1908. DIE GLEICHSTROMMASCHINE, Vols. I. and II. E. Arnold. 1906 and 1907. DIE WECHSELSTROMTECHNIK, Vols. I-V. E. Arnold. 1904-1910. DYNAMO-ELECTRIC MACHINERY: Vols. I and II. S. P. Thompson. 1904. ELECTRIC MACHINE DESIGN. Parshall and Hobart. 1906. ELECTRIC MACHINE DESIGN. Alex. Gray. 1913. ELECTRIC MOTORS. H. M. Hobart. 1910. ELECTRICAL MACHINE DESIGN, Vols. I and II. J. W. Esterline. 1906. ELEKTRISCHE GLEICHSTROMMASCHINEN. J. Fischer-Hinnen. 1904. THE DYNAMO. Hawkins and Wallis. 1903. THE INDUCTION MOTOR. H. B. De La Tour. 1903. THE INDUCTION MOTOR. B. A. Behrend. 1901. PART II. DIRECT-CURRENT MOTORS. CHAPTER III. ACTION OF DIRECT-CURRENT SHUNT MOTORS. LET us now study the action of shunt-wound motors under various conditions of load, temperature, speed, etc. It is well to consider first what occurs due to the conditions existing or changing within the machine itself, by its own action, after which the effect of external or purposely introduced factors will be explained. To make the results as significant as possible, standard shunt-wound machines have been selected as examples. Three typical sizes are considered and compared, i.e., i, 10 and no horsepower. The exact data concerning these machines and calculations based thereon are given in Table i of the present chapter. It is to be remembered that the average size of motors is less than that of generators, several of the former being usually fed by one of the latter. Hence these sizes represent small, medium and fairly large machines. It is also a fact that the no-horsepower size is sufficiently large, so that still larger motors will correspond closely. For example, the efficiencies of the three sizes are about 81, 86 and 93 per cent, respectively, above which last figure the efficiency would increase only i or 2 per cent. Therefore the characteristic differences are found below no horse- power, and these machines may be taken to represent commercial practice with respect to shunt motors. A few simple tests determine the fundamental facts from which the action of these machines under almost any reasonable conditions may be readily calculated. Most of the tests are well known, but they are included here as a desirable part of the definition of these fundamental quantities, to avoid any uncertainty in regard to them. It is assumed that the construction of shunt motors is already understood by the reader, the present book being confined to action and control of this and other types. The literature of dynamo-elec- tric generators and motors is extensive in regard to their theory, design and construction, there being many works in which these matters are fully covered (see page 9), but their operation has not been given the attention that it deserves. 10 ACTION OF DIRECT-CURRENT SHUNT MOTORS. 11 i. The Voltage V, for which the motor is designed and at which it normally operates, is assumed to remain constant, being applied to the terminals of the armature and field circuits, which in the shunt type are in parallel (Fig. i). If V is not constant it should be main- tained so (for experimental investigation) by inserting a rheostat Armature FIG. I. SHUNT-MOTOR CONNECTIONS. which can be adjusted to correct any variations. This voltage should be that marked on the manufacturer's name plate and is gen- erally known as the rated voltage. It may be found later that some other voltage is preferable in order to obtain a different speed or other result, in which case a new series of tests should be made at the modified voltage. 2. The Total Current I taken by the motor at rated load is also marked on the name plate. This may be found later to differ from the current at which the rated horsepower is developed, or it may cause heating in excess of the limit specified in (4). In either case another series of observations should be made with the corrected current. For the present, however, it will be assumed that the rated voltage V and the rated current / are both correctly given on the maker's name plate. In the shunt-wound motor the total current / is the sum of the armature current I a and shunt-field cur- rent I sh . 3. The Room Temperature t is herein taken at 25 degrees C. as a usual average value.* If it differs from 25 degrees C. or a different standard adopted, allowance should be made. 4. The Temperature Rise 6 permissible in the armature or field * Standardization Rules, Anaer. Inst. Elec. Eng., 1911. 12 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. is 50 degrees C. as measured by increase in resistance of these respec- tive windings. This gives a working temperature of t + =75 degrees C. = T, at which the machine is said to be "hot" in contra- distinction to "cold" at the room temperature t. To determine whether the temperature rise is within the limit of 50 degrees C., the motor is supplied with the rated voltage V and operated with suf- ficient load to draw the rated armature current I a until a constant temperature T is reached, requiring from 6 to 18 hours, depending upon the size, speed and ventilation of the machine.* The resist- ance of the field and armature circuits is measured before, during and after the run, as explained in (5) and (6). This resistance at 25 degrees C. is R + (.0042^ X 25) = i.io$R , and at 75 degrees C. it is R + (.0042 R X 75) = 1.315 RQ, in which R is the value at o degree C. Hence the resistance at working temperature or "hot resistance" is as 1.315: 1.105 : : 1-19 : !> r I 9 P er cent greater than the "cold resistance." If the increase in resistance is found to be more than 19 per cent, the standard safety limit has been passed, but if found to be 1-ess than 19 per cent, so much the better, not only for safety, but also for constancy of speed, as shown later (see page 26). The capability of a motor to carry current and develop power depends upon temperature of the air and other variable conditions. Hence the rating of motors and other electrical apparatus is some- what arbitrary, but it is based upon the long experience and consen- sus of opinion of those who make and use them. The A. I. E. E. Standardization Rules are followed herein and are given so far as they relate to motors in Appendix A. 5. The Field Current I sh in the shunt motor is determined by con- necting the field terminals directly to the supply circuit, the voltage of which is V. This should be measured when the machine is " hot," that is, after the run specified in (4) to obtain working conditions. The field current should also be determined with the machine "cold," before the run, because speed variations are caused by the temperature changes, as explained later (Chap. Ill, p. 26). Further- more, with both values known, the increase in resistance and the temperature rise may be easily calculated. The shunt-field resist- ance "hot" R sh = V -f- I sh and the corresponding value cold is * Standardization Rules, Amer. Inst. Elec. Eng., 1911. ACTION OF DIRECT-CURRENT SHUNT MOTORS. 13 R' ah = V + r sh , from which R sh + R'^ = I' sh + I sh . With tem- peratures of 75 degrees and 25 degrees C., respectively, it was shown in (4) that R t + -f- R' t = 1.19, hence P sh =1.19 I sh . In any case, however, the temperature rise in degrees C. is: e - (238.1 + /,) (!* - i), in which / t and 7^ are the initial temperature and resistance, while R 2 is the final resistance. 6. The Armature Resistance R a , including resistance of brushes and brush leads, but not brush contacts, is also measured "hot." Potential difference or voltage "drop" due to the brush contacts, which depends upon the current density, should be measured at the rated current value and deducted from the total drop in the armature circuit, to get the true resistance of that circuit, or that quantity which, multiplied by the current, gives the IR drop. The nature and value of drop due to brush contacts is discussed later in the present chapter. The armature, before it has time to cool after the run specified in (4), is supplied with its rated current / , but is not allowed to rotate, under which condition suitable resistance must be inserted in series to compensate for the absence of counter e.m.f. The total drop V in volts across the armature terminals is then measured, also the drop D b due to the brush contacts, and we have R a = (V f - D b ) + I a . The armature circuit resistance "cold," if entirely of copper, is then R' a = R a -5- 1.19 = 84 R a , assuming "cold" and "hot" temperatures of 25 degrees and 75 degrees C., respectively. As a rule, however, the total resistance of the armature circuit includes that of the carbon brushes, which latter has a nega- tive temperature coefficient, so that the resultant increase between 25 degrees and 75 degrees C. is about 15 per cent, or R' a = R a + 1.15 = .87 ^ a . This variation in armature resistance is not very important, however, as it will be shown later that it has but little effect upon the efficiency or regulation of a motor. Not only does this statement apply to shunt motors, but it is true generally of the various types of electric motors and generators, as affected by varia- tions in armature resistance due to any reasonable temperature changes. This statement does not mean that armature resistance itself is insignificant in effect. 14 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. COUNTER E.M.F. OF SHUNT AND OTHER MOTORS. Before proceeding with the various problems to be considered in connection with electric motors, it is desirable first to study their counter electromotive force, as it plays an exceedingly important part in the action of such machines. The counter e.m.f. of a motor armature is the e.m.f. that it would develop as a generator when operated at the same speed with the same field flux. Hence the following well-known expression for the e.m.f. of a d. c. generator is equally applicable to both cases. Let ^ = flux entering or leaving the armature per pole, n = total number of inductors on the armature, N = revolutions per minute, p = number of pairs of poles, b = number of circuits in parallel in the armature winding, p then e = c.e.m.f. of motor armature = (2) 60 X io 8 X b By inspection of equation (2) it is readily seen that with 3>, n, p and b maintained constant, the c.e.m.f. varies directly with N the number of r.p.m., and conversely we may state that the speed of a motor varies as the c.e.m.f., other factors being constant. This is a very important fact in studying the action and speed control of electric motors, especially shunt motors, because the above-men- tioned quantities remain practically constant or do not change greatly in this type, unless purposely varied. In series or compound- wound motors the field flux usually varies considerably with the current and torque. In fact in a lightly loaded series motor it increases almost directly with the current. Even in the case of these machines their counter e.m.f. is used in Chapter IX as the criterion in determining their speed variation and control. This funda- mental and general significance of the counter e.m.f. of electric motors is the basis of the method of treatment set forth in the present book. The counter e.m.f. .of shunt and series motors or other direct-current types can be determined in several ways which will now be explained. i. Experimental Method of Determining the C.E.M.F. The armature shaft may be fitted with a heavy flywheel, so that the stored energy in the revolving parts is great. The motor is then ACTION OF DIRECT-CURRENT SHUNT MOTORS. 15 operated without load, but at rated speed (i.e., 'that corresponding to rated load) by introducing resistance in its armature circuit in order to reduce slightly the voltage applied to it, while the field is excited with the proper line voltage V. When the rated speed is attained, the armature circuit is suddenly opened, and the fly- wheel effect will cause the armature to maintain almost constant speed for a short time, during which the c.e.m.f. can be measured by a voltmeter connected to the armature terminals, since it then becomes the e.m.f. of the machine acting as a generator, the field current being kept constant. 2. Determination of E.M.F. from Torque of a Motor or Generator. - The shunt field circuit is connected to the supply conductors to allow rated field current I S h to pass through it. The armature is also connected to the supply, sufficient external resistance being inserted so that only the rated load current I a flows through its winding. This develops a torque, but the armature is not allowed to rotate, a metallic or wooden bar being clamped to the pulley or shaft of the machine. By means of known weights or a spring balance we measure in pounds, the pull 'plus the friction of bearings and brushes, also the pull minus friction, add these together and divide by two. The result multiplied by the length of brake arm in feet may be called the true torque (Tt) because it is the full amount developed by the interaction of the magnetic field and armature current. Of course the weight of the arm or lever should also be eliminated. The pull plus friction is easily found by forcing the armature to turn slightly against its tendency to rotate, the pull minus friction being measured by yielding slightly to the motor's torque. Then at any speed N in r.p.m., the gross power developed would necessarily be 2 xT t N foot-pounds per minute, which divided by 33,000 is the total mechanical horsepower evolved in the armature and corresponds to the indicated horsepower of a steam engine. This must equal the electrical horsepower supplied to the armature; hence T t N 33,000 I a 7.03 I where E is the motor c.e.m.f. or generated voltage at any speed N. The true torque or turning effort of a motor depends upon the armature current, the number of armature inductors and the flux 16 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. through the armature. It is independent of the speed, being equal to T t = KI a $> } where K is a constant, depending upon the number of poles, effective conductors, etc. This gross or true torque includes not only the effective torque developed by a motor at its pulley when running, but also the torque required to overcome friction, windage and core losses. In the case of a generator, the total torque is that necessary to revolve the armature and overcome friction, etc. Hence effective motor torque + (friction + windage + core loss torque) = true torque = generator torque (friction + windage + core loss torque). In the case of belt-driven machinery, the effective torque is equal to the difference in tension on the two sides of the belt multiplied by the radius of the pulley in feet plus one- third belt thickness. It might be thought proper to multiply the electrical power in equation (3) by the efficiency of the motor in order to equate it with mechanical power. We should remember, however, that field cur- rent is not considered, core loss and windage are absent when the armature does not rotate and friction is eliminated by the described method of measuring the true torque. Hence we are dealing here with ideal conditions, the usual practical losses being eliminated. 3. Calculation of C.E.M.F. The use of Equation (2) to cal- culate c.e.m.f. has already been explained. The quantities involved in that equation are determined by the designer of a motor or genera- tor, but all of them are not usually known to, or readily ascertainable by, the user of the machine. Hence the following method is given because it employs data easily obtained by the simple tests already indicated under the head of " The Armature Resistance" on page 13. These tests can readily be made after the machine is in practical service. In the armature circuit of any direct-current motor the applied voltage, F, overcomes three factors, namely, resistance drop, brush- contact drop and the c.e.m.f.; hence, V = I a R a + D b + c.e.m.f., or, rearranging, c.e.m.f. == V - (I a R a + D b ). (4) Brush-contact drop or fall of potential which occurs at the con- tacts of brushes and commutator being small has often been wholly ignored in tests and calculations concerning dynamo-electric ma- chinery. Nevertheless, it is a measurable quantity producing an appreciable effect upon the speed of a d. c. motor as well as upon the external voltage of a d. c. generator. As its value does not ACTION OF DIRECT-CURRENT SHUNT MOTORS. 17 ordinarily exceed i volt for each contact, it is common practice sim- ply to assume 2 volts for both brush contacts of a d. c. machine. This assumption is open to two criticisms: first, the fact that even the maximum brush drop may not be and usually is not quite as much as 2 volts, and second, in any case it varies somewhat with the current, so that it is certainly less than this amount at light loads. If brush drop increased directly with current it could be included in the armature resistance, giving a total value which multiplied by the current would be the total armature drop. This would be most convenient for both tests and calculations, but unfortunately does not accord with the physical facts. Brush drop appears to be the combination of a fall of potential which is fairly constant and a true resistance drop IR directly proportional to the current. Probably the approximately constant fall of potential is in the nature of a c.e.m.f., the phenomenon being analogous to that of the arc. In fact, such a contact, especially in the case of a carbon brush, may properly be regarded as an incipient arc. This is true even under favorable conditions, and with a poor contact due to dirt, vibra- tion, roughness of commutator, etc., the arc becomes actual and apparent. The curve in Fig. 2 is based upon the results of actual tests made with a number of brushes similar to those employed in the three sizes of motor specified above. The voltage is the total value, includ- ing the potential difference or drop at both positive and negative carbon brushes. It increases with the current density, being prac- tically a rectilinear function, as Fig. 2 shows, but is not directly pro- portional thereto, since the drop is i volt at i ampere per square centimeter and 2 volts at 6 amperes per square centimeter, these being the ordinary limits for small as well as large machines at rated load. The current densities for larger motors, also generators, are usually higher than for small machines, but are not adopted as desir- able, being practically necessary because in large machines many amperes must be carried, with a reasonable number and size of brushes, upon a commutator of moderate dimensions. For ordi- nary calculations not requiring very accurate results, the loss of potential at brush contacts may be included with that due to arma- ture resistance. This assumes that the former is a simple resistance effect like the latter, which, as already stated, is not the physical fact, but the percentage of error thus involved is usually small. Accord- 18 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. ing to this assumption the total lost voltage in the armature circuit is / a (R a -f- -K c ), in which I a is armature current, ^ a armature resist- ance and R c is a resistance equivalent to brush contacts. Taking these quantities at their rated values for the typical lo-horsepower shunt motor in Table I on page 20, we have 37 (.28 + R c ) = 10.4 + 1.4, from which R c = 1.4 -r- 37 .038 ohm. If now this resistance be multiplied by the current in the armature when it runs Total Volts Drop at Brush Contacts i_i ( i (-1 -* t- * io^bsccofo^osccC ^^1 x-" X X X ^ X ^ -'' ,X -" X" X 1 X X X" ^ X X x / / / \ _J \ I 2 3 45 ( Amperes per S*i. Cm. FIG. 2. TOTAL VOLTAGE DROP AT BOTH CONTACTS FOR CARBON BRUSHES. free, the brush drop thus calculated is only 2.3 X .038 = .09 volt. The actual brush drop found by test is .84 volt when the armature runs free, hence the calculated result is .84 .09 = .75 volt less than the experimental. This difference, which would be about the same for other normal motors, is only ' , or J of i per cent of the rated 230 terminal voltage, and would introduce a corresponding error in cal- culations of speed, etc. Ordinarily this error would be insignificant, especially as it disappears as rated load is approached. On the other hand, the percentage of error is twice as great for ii5-volt motors, and in the case of speed control by armature rheostat or mul- tiple voltage it would be too large to be neglected except for rough calculations. For example, the true speed of a motor running at Q \x H ^ one-eighth of rated voltage would be about '- , or 2.6 per cent 230 ACTION OF DIRECT-CURRENT SHUNT MOTORS. 19 less than that calculated by using the assumed brush-contact resist- ance of .038 ohm. This error can be almost entirely eliminated, however, by assuming that the voltage lost at brush contacts is made up of a constant c.e.m.f. independent of current, combined with a true resistance drop; that is, D b = b + I a R b . (5) Referring to Fig. 2, it is seen that the experimentally determined drop at brush contacts is represented by a straight line for all current densities between i and 6 amperes per square centimeter. Below i ampere per square centimeter the line curves downward, but if the straight portion were prolonged backward it would intersect the vertical axis at .8 volt. If the straight line thus completed be adopted as the basis of calculations, we have the simple numerical relation- ship that there is an initial loss of potential due to brush contacts of .8 volt at zero current and there is an increase of .2 volt for each ampere per square centimeter. For example, the drop at 3 amperes per square centimeter is .8 + (3 X .2) = 1.4 volts and so on. This assumption introduces no error whatever for currents above i ampere per square centimeter, and below that value the error is insignificant because the current never falls to zero, being a mini- mum of .19 ampere per square centimeter at no load. At this limit the distance between the straight and curved lines represents only .1 volt, which would be negligible in all practical cases. Substituting the above explained values in (5) we have for the typical io-horscT/ow^ motor, 1.4 = .8 + 37 R b , from which R b = (1.4 - .8) -s- 37.- .016. The general equation of the motor thus becomes V = e + b + I a (R a + R b ). (6) This form expresses the same facts as equation (4) but has the great advantage that the empirical quantity D b is eliminated, a rational quantity I a R b being substituted. The value of R b like that of b is constant for a given motor and does not vary much for motors of normal design, hence the drop in volts due to any value of ar- 20 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. mature current is completely represented by b + I a (R a + R b ), in which I a is the only variable and is easily measured. Shunt-Motor Problems. The following data of three standard sizes of shunt motors are given, so that the various features of these machines may be studied and the efficiency, effect of temperature, speed regulation, etc., may be calculated. These results are ob- tained by actual tests of the completed motors. The precise signifi- cance and in most cases the method of determining these data are stated in the preceding chapter, but in general the table speaks for itself and the figures given are found by simple measurements using voltmeter, ammeter and speed counter. It is also possible to assume or predetermine such data and use them as a basis for calculations similar to those which follow. TABLE I. TEST DATA OF TYPICAL SHUNT MOTORS. l-H.P. Machine. 10-H.P. Machine. 1 10-H.P. Machine. Rated Voltage, V Rated Current, / Arm. Current at Rated Load, Ia 230 volts 4 amps. 3.85 amps. 230 volts 38 amps. 37 amps. 230 volts 384 amps. 380.7 amps. Shunt Field Current, I s h. Arm. Resist " Hot," R a . . . Arm. Resist. "Cold."#V Field Resistance Hot,' R s h .15 amp. 3.1 ohms. 2.7 ohms. 1506 ohms. 1 amp. .28 ohm. .244 ohm 230 ohms. 3.3 amps. .0104 ohm. .009 ohm. 70 2 ohms. No-load armature cur- rent, l'a Speed at rated load Speed at no load Brush Contact Area, Ab Current Density of Brushes at rated load, Sb .4 amp. 1250 r.p.m. 1310 r.p.m. 3 sq. cm. 1.28 2.3 amps. 825 r.p.m. 865 r.p.m. 12 sq. cm. 3.08 14.2 amps. 585 r.p.m. 595 r.p.m. 72 sq. cm. 5.3 Current Density of Brushes at no load, Sb .126 .19 .20 Drop due to brush con- tacts at rated load, Db . Drop due to brush con- tacts at no load, D'b 1.05 volts .83 volt 1.4 volts .84 volt ( 1 . 86 volts .84 volt In the above table the potential difference or voltage drop due to the brush contacts has been taken from the curve in Fig. 2. Calculation of Speed of Shunt Motor Running Free (i -Horsepower Machine). It was shown that the speed of a motor is directly pro- portional to the c.e.m.f. at any instant, other quantities such as field ACTION OF DIRECT-CURRENT SHUNT MOTORS. 21 current and flux being constant, hence the ratio between rated load speed and no-load speed (i.e., free) is r.p.m.: r.p.m.y : : c.e.m.f. : c.e.m.f./. (7) From equation (4) c.e.m.f. == V - (I a R a + D b ), in which we substitute the values of F, I a R a and D b as given in the data sheet and obtain c.e.m.f. = 230 (3.85 X 3.08 + 1.05) = 217.1 volts at rated load, and c.e.m.f. == V - (I' a R' a + D'b) = 230 - [(.4 X 2.7) + .83] = 228.10 volts at no load. Substituting these values of c.e.m.f. and the rated motor speed in equation (7), we have 1250 : r.p.m : : 217.1 : 228.1. Therefore r.p.m.y = 1314 r.p.m., which is within .3 per cent of the test value (Table I) given as 1310 r.p.m. for the speed at no load. Speed Running Free (ic-Horsepower Motor). Rated load c.e.m.f. = V - (I a R a + D b ) = 230 - (37 X .28 + 1.4) - 218.2 volts. No-load c.e.m.f. = V - (I'R'a+ D' b ) = 230 - (2.3 X .244 + .84) = 228.6 volts. The rated speed is 825 r.p.m., and by substituting in equation (7) we have 825 : r.p.m.y : : 218.2 : 228.6; whence r.p.m./ = 863, which is within .3 per cent of the test value of 865 r.p.m., for the no-load speed. Speed Running Free (no-HorsepowerMotor). Rated load c.e.m.f. = V - (I a R a + D b ] = 230 -(380.7 X .0103 -f 1.86) - 224.2 volts. No-load c.e.m.f. = V - (I' a R' a + D' b } - 230- (14.20 X .009 + 84) = 229 volts, and from equation (7), 585 : r.p.m.y : : 224.2 : 229 or r.p.m.y = 597 r.p.m., being within .3 per cent of the test value of the no-load speed given in Table I as 595 r.p.m. These calculations assume armature flux the same at no load as at rated load, which is not usually the case because of armature reaction, as explained later in the present chapter. The close agree- ment between calculated and experimentally determined speeds is therefore partly accidental. The effect of armature reaction in reducing flux and increasing speed depends upon the position of the brushes but usually would be appreciable and in some cases actually causes the no-load speed to be less than that at rated load. Never- theless a rise in speed lends to occur, due to diminished drop in armature resistance and brush contacts when the load torque and armature current are reduced. This tendency is correctly repre- sented by the values calculated in the three examples above. In fact the speed will actually vary in accordance with the numerical 22 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. results obtained unless some other condition, for example that of armature reaction or of temperature, is also changed. It is better, however, to study and determine each influence separately and then combine them to ascertain their resultant effect. Such is the method of treatment adopted herein. It is obvious that any change whatever of armature current, whether from rated load to no load or otherwise, has a tendency to cause speed variation, the amount of which may be calculated by a similar use of equations (4) and (7), substituting the proper values for speed, current, etc. The above calculations of speed running free assume th-at arma- ture is "cold" (25 degrees C.). If 'the load were suddefily thrown off a motor which had been operating with full rated armature current for several hours, the armature would not cool immediately and its resistance would remain at practically rated value. In the case of the lo-h.p. machine, for example, the c.e;m.f. would then be 230 (2.3 X .28 + .84) = 228.5 instead of 228.6 volts, but -the cor- responding diminution of speed would be less than ^ of i per cent, which is inappreciable. If, on the other hand, rated load be suddenly applied to a "cold" motor, the speed will not diminish as much as if the armature were " hot " (75 degrees C.) . In this case the c.e.m.f. will be 230 (37 X .244 + 1.4) = 219.6 instead of 218.2 volts. Hence the speed will be .6 per cent higher, or 830 instead of 825 r.p.m., but this difference is practically insignificant. The effect of temperature upon speed is discussed further on p. 26. EFFICIENCY OF ELECTRIC MOTORS. Determination of Efficiency of Motor at Rated Speed and Load. The Standardization Rules of the American Institute of Electrical Engineers (paragraph 313) state: "All electrical apparatus should be provided with a name-plate giving the manufacturer's name, the voltage and the current in amperes for which it is designed. Where practicable, the kw-capacity, character of current, speed, frequency, type designation, and serial number should also be stated." From the data thus given the approximate or "name-plate efficiency" of a motor can be determined as follows: From Name-plate of i -Horsepower Motor. Input at rated load = 230 X 4 = 920 watts. Output at rated load = i h.p. = 746 watts. ACTION OF DIRECT-CURRENT SHUNT MOTORS. 23 Hence efficiency being the ratio between input and output, the name-plate efficiency of the i-h.p. motor = 746 -H 920 = 81 per cent. Calculation of Efficiency, Using Test Values (Table I). In de- termining the efficiency of a motor we take the motor input at rated load and then calculate the stray-power and other losses, using the values found by actual test and given in Table I. The difference between input and the total losses gives the output, hence the ratio of input minus losses to input gives the motor efficiency. The stray-power losses of the i-h.p. motor running free (i.e., at 1310 r.p.m.) are equal to the armature input at no load minus the armature no-load copper and brush losses; that is, the no- load stray power losses = VI' a - (I'a R 'a + I'aP'b) = 2 3X .4 - (.4 X 2.7 + -4 X .83) = 91.2 watts. The remaining losses at rated load are: Loss in Field Copper I sh V = .15 x 230 = 34.5 Watts Loss in Armature Copper I a 2 R a = 3.85* X 3.08 = 45.64 Watts Loss in Brush Contacts I a D b =3.85 X 1.05 = 4.04 Watts 84.18 Watts. If to this we add the no-load stray power of 91.2 watts, the total loss is 84.18 -f 91.2 or 175.4 watts. The motor output is equal to the input minus losses. The input by test (Table I) is 230 volts and 4 amperes, that is, 920 watts; hence the output is 920 175.4 = 744.6 watts, and the efficiency by definition equals or 81.0 per 920 cent; so that in this case the efficiency by calculation is exactly equal to that by name-plate determination. Ordinarily (as shown by the 10 and no h.p. examples which follow) there is a slight differ- ence because the former is based upon purely electrical data while the latter depends upon a brake or other test of actual mechanical power developed. The assumption that the stray power at no load is the same as at rated load is not absolutely correct, since it will be lower at rated speed, which is from 2 to 5 per cent less than with the motor running free, but the error introduced by this assumption is practically negligible, as will be proved in the following case of the lo-h.p. motor. 24 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Determination of Efficiency of lo-Horsepower Motor. . Output from name-plate 10 X 746 Name-plate efficiency = = - 1 - Input from name-plate 230 X 38 7460 - = 85.3 per cent. 8740 The calculation of efficiency of the lo-h.p. motor is similar to the foregoing example of the i -horsepower machine, but the stray-power losses will be corrected for speed. The stray-power losses of lo-h.p. motor running free (i.e., at 865 r.p.m.) equal the no-load armature input (230 volts and 2.3 amperes) minus the no-load armature copper and brush losses; that is: Stray power at no load = 230 X 2.3 - (2.3* X .244 + 2.3 X .84) = 526 watts. At rated load the motor is running at a slightly lower speed of 825 r.p.m., hence the stray power will be less because the eddy current constituent varies as the square of the speed, and the several losses due to hysteresis, windage and friction may be assumed to vary directly as the speed. In ordinary machines of this size the stray-power losses are usually divided as follows : 50 per cent due to windage and friction, 25 per cent due to hysteresis and 25 per cent due to eddy currents. Hence, 75 per cent of the stray-power losses vary as the speed, and 25 per cent vary as the square of the speed. The stray power corrected for change in speed from 865 to 825 r.p.m. or 4j per cent will be (.955 X .75 X 526) + (.Q55 2 X .25 X 526) = 495 watts. If the stray power had been assumed to have the same value at no-load speed as at rated-load speed, the error introduced would therefore be 526 495 =31 watts, or about .4 per cent of 8740 watts, the rated input. This difference is so small that it may generally be neglected in practical problems. Furthermore there is in most cases a rise in stray-power losses as the load increases. These are called "load losses," being partly due to larger mechani- cal forces and therefore friction, also to augmented hysteresis and eddy currents because of altered distribution of flux which is crowded into certain portions of the armature. The assumption of the higher figure for stray power would tend to cover load losses which are difficult to determine. ACTION OF DIRECT-CURRENT SHUNT MOTORS. 25 The losses at rated load in addition to stray power are as follows: Loss in Field Copper I sh V = i X 230 = 230. Watts Loss in Armature Copper P a R a = $f X .28 = 383.3 Watts Loss in Brush Contacts I a D b = 37 X 1.4 = 51.8 Watts 665.1 Watts This amount added to the corrected stray-power value gives a total loss of 495 + 665 = 1160 watts. Hence the output is equal to the input (230 X 38 = 8740 watts) minus this loss, that is, 8740 1 1 60 = 7580 watts. The efficiency is, therefore, 7580 -f- 8740 = 86.8 per cent. A comparison of calculated output (7580 watts) and rated output (10 X 746 = 7460) shows that the former is 136 watts greater; so that the manufacturer is on the safe side when the motor is rated to give 10 horsepower, and this is as it should be, overrating of ma- chinery being bad practice. In other words 10.18 h.p. are actually developed at rated input. Effect of Armature Resistance upon Speed of Shunt Motors. The principal and instantaneous cause of shunt motor speed varia- tion with changing loads is the varying armature current and conse- quent varying armature drop ( = I a R a ) ; hence the reason for making the resistance of the armature (R a ) as low as possible. This cause of speed change is shown by consideration of the typical zo-h.p. motor. Assume its armature to be "hot" (75 degrees C.) and let us determine the speed change due to variations of armature current alone. From Table I we have the following test values: R a = .28 ohm, brush drop at no-load .84 volts, at rated load 1.4 volts, and speed at rated load 825 r.p.m., with armature current I a of 37 amperes and terminal voltage V of 230. According to equation (4) the c.e.m.f. at rated load with arma- ture "hot " but running free is 230 - (.28 X 2.3 + .84) = 228.5 volts. Hence from equation (7), taking the c.e.m.f. at rated load from page 21, we have r.p.m. (free) = X 825 = 864; that is, 218.2 the speed changes from 825 to 864, amounting to 39 r.p.m., or 4^ per cent. Thus there is a speed rise of 4^ per cent solely on account of diminished armature drop (including brush contacts) when the rated load is removed from this lo-h.p. motor. The effect is the same as if the voltage supplied to the armature were raised 26 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. about 44 per cent. In fact the available voltage is actually increased to that extent. This calculation does not take into account the effect of armature reaction which tends to counteract more or less the speed variation determined above, as shown a little later. Effect of Temperature Changes upon Speed of Shunt Motors. - Heating of armature affects the speed only to a slight extent, and may be practically neglected, as already shown on page 22. For example, in the case of the lo-h.p. motor, the "cold" armature resistance is .244 ohm; while the "hot" armature resistance with a temperature change from 25 degrees to 75 degrees C. (i.e., 50 de- grees C. rise being permissible) is 15 per cent greater (see p. 13) or .28 ohm. The speed alteration at rated load due to this heating is determined as follows: C.e.m.f. with armature "cold" at rated load = 230 (37 X .244 -f 1.4) = 219.6 volts, the c.e.m.f. with armature "hot" and at rated load being 218.2 volts. Hence the speed at rated load and with armature "cold" is 219.6 -f- 218.2 X 825 = 830 r.p.m. instead of 825 r.p.m. when the armature is "hot," an increase of .6 per cent, which is not material in most practical cases, as the change due to varying load may be 4 or 5 per cent as shown above. As already noted on page 22, the load may be and often is sud- denly thrown off a motor when its armature is " hot," and conversely rated load is often applied to a " cold " armature. In each case the resistance of the armature winding depends simply upon its temperature at that particular time. The change in this resistance being only 15 per cent between its " cool " value (after continued stand still) and " hot " value (after continuous running at rated load), it produces little practical effect upon speed, as shown above, and usually this effect need not be considered. Change of Speed due to Heating of Field Circuit. - - The allowable temperature rise in the field winding is 50 degrees C., causing a 19 per cent increase in the resistance, as shown on page 12. Since the rated resistance is the working or " hot " value, being 230 ohms for the shunt field of the typical lo-h.p. machine, it follows that this resistance at ordinary temperature (assumed to be 25 degrees C.), herein called "cold" resistance, is R sh 230 = = 193.3 ohms. 1.19 1.19 Current in field (hot) I sh = = = i ampere. V 230 ACTION OF DIRECT-CURRENT SHUNT-MOTORS. 27 V 23O Current in field (cold) /'*-7~ = ~ - = 1.19 amperes. A s* I 93-3 Hence the current in the coils cold is 19 per cent greater than when the latter are hot; and from magnetization curves of standard types of shunt motors a rise of 19 per cent in field m.m.f. causes an increase of about 4 or 5 per cent in the flux, or the field is this amount stronger " cold " than " hot." With the other conditions (e, b, n and p) constant, the speed (N) will vary inversely with the flux ($), as shown in the following transposed form of equation (2) : 2p e X io 8 X 60 X b e = - - or N = - - io 8 X 60 X b $n 2p With the flux 4 to 5 per cent stronger when the field winding is cold than with it hot, the speed is 4 to 5 per cent lower. This variation of speed with heating of the field winding is an objection- able characteristic of the ordinary shunt motor for work requiring almost perfectly constant speed, such as weaving. It can be overcome by employing a field so highly saturated that a moderate change in field current produces only slight flux variation; or a field winding composed of wire having a zero temperature coefficient would secure a like result. Both methods are costly, especially the latter, for which the only available materials are alloys with resistivity much higher than that of copper, demanding correspondingly greater cross section of wire. In some cases it may happen that the former plan employing a field approaching saturation is desirable for other reasons, such as improved commutation, so that the total advantage warrants the expenditure for additional ampere-turns in the field coils. It was explained under the preceding heading relating to heating of armature that rated load may be put upon a "cold" or a "hot" motor or may be thrown off either. In the field winding the full current flows whether the machine is loaded or not, so that the tem- perature of the former simply increases with the time of operation until maximum is reached. Hence the heating of shunt-field coils and the percentage of speed rise occasioned by it are practically the same whether the motor is running free or loaded. The same is approximately true of ar- mature heating by eddy currents and hysteresis in its core. On the 28 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. other hand, heating due to armature resistance increases as the square of the current (I 2 a Ra) and is therefore very small at light loads. At rated load it is about equal to the core heating in ordinary machines, in which case the temperature rise in the armature would be about one-half as great running free as at rated torque. It has already been shown (page 26) that speed change due to armature heating is small, the r.p.m. being .6 per cent higher when the armature is "cold" (25 degrees C.) than when "hot" (75 degrees C.). Hence the speed would be .3 per cent greater when the armature begins to run free (i.e., cold) than when it has been running unloaded for several hours. Effect of Voltage Variation upon Speed of Shunt Motors. The speed changes of shunt motors due to their own action have been discussed in all cases on the assumption that the voltage sup- plied to them was constant. Such constancy is the desirable con- dition to be maintained or approximated as closely as practicable. Nevertheless, appreciable variations of voltage do occur even on the best regulated circuits, and may often become very considerable, that is, 5 per cent or more, whether from central station or isolated plant. Fortunately the ordinary shunt motor is not very sensitive to these variations, the percentage of speed change being considerably less than that of the voltage change. In actual practice the former is usually from .6 to .8 of the latter; that is, a 5 per cent rise or fall in voltage will cause the speed to rise or fall 3 to 4 per cent. In this respect the shunt motor is far less susceptible than the incandescent lamp, the ordinary tungsten^filament type changing'its candle-power about 3.6 per cent when the voltage is altered only i per cent. A shunt motor in which the magnetic circuit is considerably below saturation runs at nearly constant speed even if the voltage varies widely. This is because the flux 4> varies directly with the voltage V, which in turn is very closely proportional to e, the c.e.m.f. in the e X 60 X io 8 X b expression r.p.m. = derived from equation (2), $n 2 p so that any change in the latter is cancelled by a corresponding change in the former, r.p.m. remaining constant. On the other hand, with a magnetic circuit completely saturated and therefore constant, the speed would vary directly with e which is nearly pro- portional to the supply voltage V in the normal shunt motor with an armature circuit of very low resistance. ACTION OF DIRECT-CURRENT SHUNT MOTORS. 29 In most practical cases the magnetic circuit is only partially sat- urated, and in order to determine the percentage of speed changes that will be produced by a certain percentage of voltage change v, it is necessary either to arrive at the result empirically by actual test or to calculate it from the magnetization curve of the machine, which is more or less individual for each design. It is convenient for this calculation to employ what is known as the percentage of saturation. This quantity according to the A. I. E. E. Standardization Rules, IV, par. 58) "may be deter- mined from the saturation curve of generated voltage as ordinates, against excitation as abscissas, by drawing a tangent to the curve at the ordinate corresponding to the assigned excitation, and extending the tangent to intercept the axis of ordinates drawn through the origin. The ratio of the intercept on this axis to the ordinate at the assigned excitation, when expressed in percentage, is the percentage i * of saturation." It may also be found from the relation p = i j, in which p is the percentage of saturation and / is the saturation factor, which is denned by the same Rules (par. 57) as "the ratio of a small percentage of increase in field excitation to the correspond- ing percentage increase in voltage thereby produced." It is not * Proof that p=i-i/f. Let oP, Fig. 2a, represent a typical saturation curve: and let Y represent any point upon said curve, the ordinate thereof having a value Z and the abscissa a value K. Through the point Y draw a tangent to the curve and con- tinue the tangent line until it inter- cepts the axis of ordinates, represent the distance of the point of intersection from the origin by the ordinate of value W. Let L represent the in- crease in voltage occasioned by a small increase M of the magnetizing force. Then by definition p the per cent saturation is equal to W-^-Z and similarly /, the saturation factor, is equal to M/K-s-L/Z or MZ/KL. From the relations existing between similar triangles M I K'.'.L I N wherein N=Z-W, whence MW = MZ-KL. If this- latter equation be divided through by MZ the following relation obtains: W/Z=i-KL/MZ wherein W/Z=p and #Z/MZ=i// and consequently p = i-i/f as above. 30 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. necessary, therefore, to determine the complete saturation curve of the machine. It is sufficient to ascertain the percentage rise or fall in the voltage developed by the machine running as a generator on open circuit at any constant speed when the shunt field is excited first by normal voltage V and then by the voltage V v, in which v is the percentage of variation of V in any particular case. For example, if the saturation curve or the test just mentioned shows that the voltage generated by a machine rises 2 per cent when the voltage V exciting the shunt field is increased 5 per cent, then its saturation factor/ = 5 -*- 2 = 2.5 and its percentage of saturation p = i =i =60 per cent. Ordinarily these quantities / 2.5 are referred to the rated or normal value of F,but may be based upon any other selected value. This percentage of saturation represents the extent to which the magnetization approaches saturation. If the armature core were wholly saturated this percentage would be 100, while it is practically zero for moderate flux densities. This same percentage of saturation represents the ratio between speed varia- tion of a shunt motor and change of voltage V supplied to its terminals. In the above example, therefore, the speed would rise .60 X 5 = 3 per cent when the voltage increased 5 per cent, the percentage of saturation being 60. At 100 per cent or complete saturation the speed rises or falls exactly the same percentage as the voltage. On the other hand, at zero saturation or with low flux densities practi- cally proportional to the excitation, the speed would be constant. In this discussion it is assumed that the resistance of the shunt- field circuit is constant, in which case the field current varies directly with the voltage V. This would be approximately true for a change of a few per cent in the value of V, which is all that usually occurs in practice. Of course any increase in V does tend to raise the tem- perature and therefore the resistance of the field winding, so that the shunt-field current would not increase or decrease quite as rapidly as the voltage V. This fact makes the percentage of speed varia- tion slightly greater than that stated above. This effect is similar to that due to the gradual heating of the field which occurs even when V is constant, as already explained on page 26. The definitions of percentage of saturation and saturation factor quoted above from the A. I. E. E. Standardization Rules refer either to one point on the saturation curve at which a tangent is ACTION OF DIRECT-CURRENT SHUNT MOTORS. 31 drawn or to "a small percentage of increase in field excitation." This limitation is imposed because a point of tangency or a very short distance on a curve may be regarded as a straight line. The voltage variations occurring in practice may be so small that this assumption is correct, but often they amount to 5 or 10 per cent or even more, and cases might arise in which the voltage may be acci- dentally or purposely varied 50 or 75 per cent. For the purposes of our problem, which relates to the effect of such variations upon the speed of shunt motors, it is sufficient to consider only the range of variation, the form of the curve between the limiting points being of no consequence. Hence the machine is driven as a generator at any constant speed, and d the difference in voltage developed is measured with field excited by voltages V and V v respectively. It is not necessary in this case to limit v to the "small percentage" stated in the definition. If d and v are expressed as percentages of the initial values, then v -*- d = /, the saturation factor; which signi- fies simply the fact that in order to raise the voltage generated by a certain percentage it is necessary to augment the magnetizing current a greater percentage. For example, a saturation factor of 3, which is an ordinary value, means that a 4 per cent rise in generated voltage demands a 3 X 4 = 12 per cent increase in magnetizing current or exciting voltage which are assumed to be proportional to each other in a shunt machine, because temperature and resulting resistance changes are gradual even when they occur. In making the test to determine d for a given value of v, only a voltmeter is connected to the armature; hence the driving power required is small, and the speed may have any reasonable value, provided it is constant. The above method of calculating the speed of a shunt motor at various values of terminal voltage V was applied in the case of a lo-horsepower ii5-volt General Electric shunt motor. The results thus obtained are given in Table II and are compared in Table III with the measured speeds noted during a speed-load test, at the same values of V as those employed in the calculations. A portion of the magnetization curve of the motor was carefully determined, and from this the following relations required for the calculations were deter- mined. V represents the voltage applied to the motor field winding; I 8h the shunt-field current = V -r- R sh \ e.m.f. represents the open- circuit voltage obtained when the motor was operated as a genera- tor at the specified speed, and is naturally proportional to the field 32 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. flux < developed by the corresponding value of I sh . The per cent saturation at rated excitation with 115 volts was determined directly from the magnetization curve. TABLE II. TESTS TO DETERMINE PERCENTAGE OF SATURATION OF A 10-H.P. SHUNT MOTOR. Terminal Voltage, V. Shunt Field Current , /.ft. E.m.f. at 970 r.p.m. co$. A, Change in /.ft. B, Change in e.m.f. or *. i-2, A per cent Saturation. 105 Volts. 1.5 Amps. 109 Volts. -8.5% - 3.6% 57.5 110 1.57 111.2 -4.3 - 1.7 60.0 115* 1.64 113.1 63.0 120 1.71 114.8 + 4.3 + 1.5 65.0 125 1.78 116.4 + 8.5 + 2.9 66.0 * Rated voltage and field current. The formula employed to calculate the various speeds of the motor at different values of V is in its simplest form as follows: r.p.m. at V = r.p.m. at rated V { i + (A - B) } , (8) wherein A and B must be given their proper signs. TABLE III. MEASURED AND CALCULATED SPEEDS OF A 10-H.P. SHUNT MOTOR WITH LINE VOLTAGE (F) VARIED. V. 105 Volts. 110 Volts. 115 V. 120 Volts. 125 Volts. /.ft 1.5 Amps. 1.57 Amps. 1.64 Amps. 1.71 Amps. 1.77 Amps. /a R.p.m. R.p.m. R.p.m. R.p.m. R.p.m. Amps. Test. Calc. Test. Calc. Test.* Test . Calc. Test. Calc. 10 906 920 938 946 970 992 996 1010 1024 20 894 910 926 934 958 976 984 996 . 1012 40 880 892 912 916 940 956 966 986 992 60 866 878 900 902 926 938 952 976 978 75 852 864 886 888 912 926 937 962 964 * Rated voltage and field current. The agreement between measured and calculated speeds is reason- ably close ; the differences being small enough to fall within ordinary errors of observation. CHAPTER IV. SHUNT-MOTOR STARTING BOXES. IN starting shunt and compound-wound motors, no trouble is likely to occur in connecting the shunt-field coils to the circuit because their resistance is high. The difficulty is with the armature winding, its resistance being very low in order to obtain high efficiency and good speed regulation, as already shown. If a low-resistance winding be directly connected across the line terminals, the current \ ,^- FIG. 3. ELEMENTARY STARTING BOX. would be so excessive that it would tend to injure or destroy it, When standing still an armature generates no c.e.m.f., so that the entire voltage of the supply circuit would have to be consumed by the fall of potential in the armature resistance and brush contacts Theoretically the armature current would rise in the typical 10- horsepower motor to a value I a = (V D b ) + R a = (230 1.4) -^-.28 = 816 amperes compared with rated current of only 37 amperes. This equation is obtained from equation (4) by making c.e.m.f. equal to zero. Practically the current would not reach such 33 34 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. an extreme value, because the fuses or circuit-breaker would act to prevent it; but a very excessive armature current would flow at least momentarily with injurious mechanical as well as electrical effects. To prevent injury and at the same time to obtain gradual accelera- tion, an adjustable rheostat, commonly called a "starting box," is inserted in series with the armature, the resistance of which is gradu- ally reduced as the speed increases. As a rule, starting boxes, unless otherwise specified, are designed to allow the motor to draw an initial current about 50 per cent greater than normal, so that the machine may develop ample torque to start under load.* The value of the starting current depends upon the working load, the equivalent moment of inertia of the various devices to be accelerated, and the rate of acceleration desired. The maximum starting current is thus a combination of the current required to accelerate and that required to overcome the working load torque. The mean current (amperes) producing acceleration is : v* herein co is the angular velocity, of motor armature at rated speed in radians per second, K is the equivalent moment of inertia of motor armature and other parts to be accelerated, V is the rated voltage of the motor, and t is the time in seconds allowed for starting. The maximum current at the moment of starting is : I st = I a + 2l ma . (9 5. Teaser systems. 6. Double-armature motors. 7. Variation of number of poles in motor. In the last two of the above cases, Nos. 6 and 7, the motor as a whole is supplied with constant voltage. Nevertheless the volt- age available for each armature is varied in case 6, and the group- ing of the armature conductors is altered in case 7 so as to change the c.e.m.f. developed; hence these two rather peculiar cases may be included in a general way under adjustable-voltage control. They also resemble the other five cases in the fact that field cur- rent and flux are usually maintained constant. The double-arma- ture motors of case 6 do not exert constant torque, as in the other six cases, but produce constant output in horsepower like the single voltage types. It is also to be noted that cases 6 and 7 in the above list apply to multi-speed motors as defined by the 50 MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 51 A. I. E. E. Standardization Rules * rather than to adjustable-speed machines. In the discussion of the relation between speed and c.e.m.f. it was shown that, with other conditions constant, the speed varies directly as the c.e.m.f. From the equation V = c.e.m.f . + I a Ra + D b it is evident if I a R a and D b are small with respect to the c.e.m.f. (as must be the case with an efficient motor) that increase in V will cause, at constant torque, a nearly proportional increase in c.e.m f., that is, a variation of impressed e.m.f. produces an almost cor- responding change in speed. This is the principle of the adjust- able-voltage or multi-voltage systems of control. Three-wire Multiple-voltage Systems. The simplest multiple voltage system is the ordinary three-wire circuit, with say 115 volts FIG. 7. SIMPLE THREE-WIRE MULTIPLE -VOLT AGE SYSTEM. between either outer and the neutral conductor, and 230 volts between the outer conductors as represented in Fig. 7. Thus a 230- volt shunt motor connected so that its field winding is supplied with 230 volts and its armature with 115 volts, as indicated by solid lines, will develop a certain speed. If the armature terminals are then connected to the 23o-volt supply, as indicated by broken line, the speed will be approximately twice as great. The two principal running points are nearly one -half and full speed, while those inter- mediate may be obtained by the introduction of armature rheostat or " field-weakening " control. Let us consider the lo-h.p. motor, the data of which were given on page 20. This machine has an armature resistance, hot, of .28 ohm; a brush drop of 1.4 volts and an armature current of 37 amperes at rated load. * Division i, Section E, Rules 1911. 52 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Then I a R a + D b = .28 X 37 + 1.4 = n.8 volts. With V = 115, the c.e.m.f. is 115 n.8 = 103.2. With V = 230, the c.e.m.f. is 230 - n.8 = 218.2. Hence the speed ratio at these voltages is as 103.2 : 218.2, or nearly a i to 2 speed change (i : 2.11). The objection to this method is that while three wires are neces- sary, only two running speeds are obtained. An additional speed can be secured from an unsymmetrical three-wire system, in which one of the sides has a voltage of x and the other side a voltage of 2 Xy but even then only three running speeds corresponding to x : 2 x : 3 x could be obtained with three wires. To gain a much wider speed range with only one-third greater number of wires, the four-wire systems were developed, and these will now be explained. Ward Leonard Multiple-voltage Control of Speed. The first of these four-wire methods, historically, is that of H. Ward Leonard,* who employed three generators of 62, 125 and 250 volts, respectively, and grouped them in series in the order named, as represented in Fig. 8. These voltages were supplied to the various motors by a four-wire system of distribution, connected to the three generators as shown. ^T^ 1 IS' 1 V. 1 \ t t V. 875 V. 250 V. 375 V. V. V. J 234 56 Running Points FIG. 8- WARD LEONARD MULTIPLE-VOLTAGE SYSTEM. The shunt-field windings of all the motors were supplied with a constant voltage, either the total amount obtained by connection with the outside wires A and D, or a smaller value, as, for example, that existing between the wires C and D. The armature terminals may be connected as desired to any two of the conductors A, B, C * U. S. Patent No. 478,344, July, 1892. MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 53 and D\ thus if applied to A and B, 62 volts would be obtained; across B and C, 125 volts; across A and C, 167 volts; across C and D t 250 volts; across 5 and D, 375 volts; and across ^4 and Z), 437 volts. Taking the speed at the highest voltage as the full or rated value, the various running points would give speeds of approximately -i-, T T> T> T> an d y> a sudden jump in the voltage increment occurring at the fifth point. The -f- speed value, or that corresponding to 312 volts, could not be obtained, because to get this voltage AB would have to be added directly to voltage CD, which would short-circuit the voltage EC. Crocker- Wheeler System. The next four-wire multiple-voltage system developed was that of the Crocker-Wheeler Company, employing voltages of 40, 1 20 and 80 in the order given. (Fig. 9.) By connecting the field terminals across the 24o-volt lines (AD) and shifting the armature terminals from AB to CD, to EC, to T V. ! v. L f 1 T 1 2 V. I ? 240V. 120 80 V. 40 V. 240 V. Running Points FIG. 9. CROCKER- WHEELER MULTIPLE -VOLT AGE SYSTEM. ^ AC, to BD, and finally to AD, six voltages and speeds are obtained as follows: AB gives 40 volts CD " 80 " BC " ..120 " AC gives 160 volts BD " 200 " AD " ..240 " These voltages correspond approximately to -J-, --, -|, -|, -J and f of the rated speed. Thus, with this system, the speeds increase in a straight line, or in an arithmetical progression, there being no jumps, but a uniform rise throughout. The actual speeds are from 106 to 862 r.p.m., giving a range of a little more than i to 8, as shown in the table of " Speeds and Efficiencies" on page 55. 54 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Bullock Multiple-voltage System. A third multiple-voltage method is that of the Bullock Company, Fig. 10, employing vol- tages which increase in geometrical progression ; that is, the voltages are in the following ratio: a : ar : ar 2 : ar 3 : ar 4 : ar 5 . As these values must all be obtained in practice from a single system consist- ing of only four conductors, it is necessary that ar 3 = a + ar, that ar 4 = ar + ar 2 and that ar 5 = ar 2 + ar 3 = a + ar + ar 2 . 450 Running Points FIG. 10. BULLOCK MULTIPLE-VOLTAGE SYSTEM. The only factor r which satisfies these conditions is 1.3247; that is, each voltage is 32 J per cent, or about one-third, higher than the preceding. The commercial system according to this plan employs 60, 80 and no volts in the order named. These are round numbers that are practically convenient, but are only approximately in the ratio stated, the theoretically correct values being 60, 79.5 and 105.3 volts. The armature voltages and speeds obtained by connecting the armature terminals of the typical lo-horsepower motor to the various conductors are given in the following table. The speed is proportional toe. e.m.f.; that is, r.p.m. = (c.e.m.f. -j- 218.2) X 825, the two latter being rated values. TABLE VII. SPEED CONTROL OF 10-H.P. MOTOR BY BULLOCK MULTIPLE-VOLTAGE METHOD. Terminal Volts. '..+*>. C.E.M.F. Speed in R.P.M. 60 11.8 48.2 182 80 11.8 68.2 258 110 11.8 98.2 374 140 11.8 128.2 485 190 11.8 178.2 673 250 11.8 238.2 900 MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 55 The Crocker-Wheeler method gives a speed range which is about i to 8, as stated above, while the Bullock arrangement gives a speed range of exactly i to 5, the total number of controller steps being the same for both. The range of voltage is 40 to 240 in the former and 60 to 250 in the latter. Hence the former starts at a lower speed of 106 r.p.m. instead of 182 r.p.m., and finally reaches about same maximum of 862 compared with 900 r.p.m. Multiple-voltage systems may be worked at any reasonable maxi- mum; they differ only in the ratio of voltages. It is desirable, how- ever, to have standard values for at least the maximum voltage and one of the sub voltages, in order that standard motors, arc and incandes- cent lamps, etc., may be fed from the same lines. The efficiency oj multiple-voltage speed control is much higher than that of the armature rheostat method for same torque and speed range. Let us consider the typical lo-h.p. motor, the data of which were given in the table on p. 20, and determine its efficiency at rated torque and the various speeds obtained by the Crocker- Wheeler multiple-voltage system. The several speeds at rated torque corresponding to impressed voltages of 40, 80, 120, 160, 200 and 240, respectively, are deter- mined as in the case of the Bullock system above. The input in watts in each case is found by multiplying the voltage input by the rated armature current (I a = 37 amperes) and adding 230 watts, which is the normal field input of this typical lo-h.p. motor. TABLE VIII. SPEEDS AND EFFICIENCIES OF 10-H. P. MOTOR WITH CROCKER- WHEELER MULTIPLE-VOLTAGE CONTROL. Volt- age Input. Input. Watts. Drop (I a R a + Db) Volts. C.e.m.f. Volts. R.p.m. Output 7580 X r.p.m.. Efficiency at Rated Torque. 825 Watts. 4] 1710 11.8 28.2 106. 971 56.6% 80 3190 11.8 68.2 258. 2370 74.2 120 4670 11.8 108.2 409. 3750 80.1 160 6150 11.8 148.2 560. 5150 83.5 200 7630 11.8 188.2 711. 6530 85.6 240 9110 11.8 228.2 862. 7920 86.7 Comparing the efficiency curve (Fig. u) of the multiple-voltage method of speed control with that of the armature rheostat method 56 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. at rated torque, Fig. 4, the much higher average efficiency of the former method is very marked. An even greater advantage of this method over the rheostatic control is its far better speed regulation under variable loads. The curves in Fig. 12 and values in the fol- lowing table show this superiority very clearly. The r.p.m. at rated torque are from table above, and r.p.m. at no load are 39 r.p.m. / Efficiency Rheostatic Control 85 50 75 Percent Rated Speed 100 FIG. II. COMPARATIVE EFFICIENCIES OF RHEOSTATIC AND MULTIPLE-VOLTAGE SYSTEMS. higher in each case for multiple voltage, because armature current is reduced from 37 to 2.3 amperes, which decreases armature drop 34.7 X .28 = 9.7 volts, and brush drop is .84 instead of 1.4 volts. The c.e.m.f. must rise therefore 9.7 + .6 = 10.3 volts, producing a speed increase of 10.3 -r- 218.2 X 825 = 39 r.p.m. The r.p.m. at no load for armature rheostat control are found as follows: The terminal pressure to give 106 r.p.m. at rated torque is 40 volts; hence 240 40 = 200 volts must be consumed in rheostat, the resistance of which is 200 -5- 37 = 5.4 ohms. At no load c.e.m.f. = 240 23 (.28 + 5.4) .84 = 226.1 volts. This corresponds to 226.1 -r- 218.2 X 285 = 854 r.p.m. as given in Table IX. MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. TABLE IX. SPEED REGULATION, 10-H. P. MOTOR. 57 Multi-Voltage Control, V max = 240 volts. Rheostat ic Control, V max = 240 volts. R.p.m. Rated Torque. R.p.m. No Load. Per Lt. Speed Change. R.p.m. Rated Torque. R.p.m. No Load. Per Ct. Speed Change. Curve A 106 145 33% Curve a 106 854 705 Curve B 258 296 11.5 Curve 6 258 865 235 Curve C 409 447 9.2 Curve c 409 875 114 Curve D 560 599 7.0 Curve d 560 883 48.4 Curve E 711 750 5.2 Curve e 711 892 25.2 Curve F 862 900 4.6 Curve F 862 900 4.6 1000 loa Percent Bated Load FIG. 12. SPEED REGULATION OF RHEOSTATIC AND MULTIPLE- VOLTAGE SYSTEMS. A multiple-voltage system may be supplied by three generators of 40, 80 and 120 volts respectively, each large enough to carry its corresponding fraction of the maximum load. This does not, how- ever, necessarily equal the combined watt capacity of the motors, as it is improbable that all machines will be simultaneously operating at full output. In fact the actual working load is not likely to exceed 30 to 50 per cent of the possible load. Instead of using the above combination of three generators, one generator of total voltage and 58 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. load capacity and a three-unit balancing set (Fig. 13) at 40, 80 and 120 volts can be and usually is employed. These balancers, it has \ 40 V 120 V, 80 V. FIG. 13. MULTIPLE-VOLTAGE SYSTEM WITH BALANCERS. been found by experience, need have a total capacity of only 5 to 10 per cent of the total load in ordinary cases. If, however, there is one extremely large motor, while the rest of the plant consists only of small motors, the balancer set should have a capacity equal to that of this large motor. The balancer arrangement is the one usually adopted, as it is advantageous in the following respects: (a) Lower cost of prime movers, only one instead of either three engines or a system of line shafts, belts, etc. (6) Lower cost of generator, a large one in place of three smaller ones of same aggregate power. (c) Lower cost of foundations. (d) Less steam piping. (e) Cheaper switchboard and electrical connections. The motors controlled by multiple voltage are ordinary standard machines, which is an important practical advantage. They are so connected that the field is permanently across the 24o-volt lines whenever the motor is in operation, and the six running speeds are obtained by shifting the armature terminals from sub-voltage to sub- MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 59 voltage by means of the controller drum as shown in Fig. 14. In FIG. 14. CONTROLLER AND MOTOR CONNECTIONS, MULTIPLE -VOLTAGE SYSTEM. some cases the changes from one running speed to another are made gradually by shifting to the next higher voltage with some resistance inserted in the armature circuit and then gradually reducing this resistance until that voltage is applied to the armature terminals and so on with the various sub-voltages until the maximum pressure is attained. These gradual changes with intermediate speeds are also obtainable by diminishing the field current until the next higher speed is reached, then connecting the armature to the corresponding voltage, at the same time reestablishing full field current. Since the speed steps differ by only 25 or 40 per cent, the ordinary shunt motor is capable of this range of field weakening, particularly as the speed is below normal which lowers the frequency of com- mutation and reduces the reactance voltage of the short-circuited coils. In some instances a combination of variable field current and armature rheostat control is employed in passing from one sub-volt- age to another, thus obtaining as many as 36 different speed points from minimum to maximum. 60 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The Motor-Generator and " Boost and Retard " Systems, both invented by H. Ward Leonard, are also multi- voltage or rather ad- justable voltage methods of speed control, but the speed changes being gradual, no intermediate steps are required. In the case of the motor- generator system* in addition to the working motor, a motor-dynamo is required for each machine so operated. The motor end (M) of the motor-dynamo is connected to the line or supply mains and controlled as any ordinary single-speed machine (Fig. 15). The generator terminals (D) are connected to the work- no. 15. WARD LEONARD MOTOR-GENERATOR SYSTEM OF CONTROL. ing motor's armature (WM). Adjustable voltages and speeds are obtained by changes in the field strength of the generator, the field of which, as well as that of the working motor, being connected to the supply circuit. Reversal of rotation in this case is by means of a reversing switch (5) and rheostat (R) in the generator field cir- cuit. By this method the reversal of voltage applied to the working motor is gradually accomplished, being first reduced to zero and then built up in the opposite direction. Furthermore the reversing switch S controls only a small field current instead of the armature current which would be 20 to 50 times greater, requiring large contact surfaces. While this system is extremely flexible, it is not extensively em- ployed on account of its first cost. The motor end of the motor- dynamo must be larger than the working motor by the amount of the losses in both the dynamo and working motor. For example, the lo-horsepower motor previously considered is of 86.6 per cent effi- ciency; hence to operate this machine at rated load, the input must be 10 -5- .866, or n.6 horsepower. The efficiency of the dynamo is also about the same, so the motor end of the motor-dynamo must be * U. S. Patent No. 463,802, November, 1891. MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 61 of 1 1.6 -f- .866, or 13.5 horsepower capacity. Thus three machines are required, each of a power equal to or somewhat greater than that needed for the actual work, the total rated capacity being 10 + 1 1.6 + 13.5 = 35.1 horsepower. An extensive use of this method of speed control was formerly the operation of turrets and gun platforms in modern war-ships, for which very fine adjustment and yet wide range in speed are neces- sary. It is now frequently used for driving large rolls in steel mills, where in combination with a heavy flywheel it is known as the Ilgner system. Both of the Ward Leonard methods and the " teaser" system, to be given later, involve motor-generator equipments. Their essen- tial advantage is forcibly shown by the following example: To obtain 55.5 amperes at 17 volts, sufficient to develop a torque to start the standard lo-horsepower motor from rest under load, assuming 50 per cent increase in armature current above the rated value of 37 amperes, would require a motor-generator of 80 per cent efficiency to draw 55.5 X 17 -=- .8 = 944 watts from the line, whereas to obtain the same starting torque directly from a 23o-volt line by means of armature rheostat control would require 55.5 X 230 or 12.77 kilo- watts, which is nearly 14 times as much power. The "Boost and Retard" System* of Ward Leonard is very similar in principle to the preceding method but reduces somewhat its high cost by the following scheme. A motor-generator is also em- FIG. 16. WARD LEONARD " BOOST AND RETARD" SYSTEM OF CONTROL. ployed, but the generator end is placed in series with the line, so that its e.m.f. may be added to or opposed to the line pressure. This e.m.f. is controlled by variation of field resistance, and its direction by rever- sal of field connections. The operation of this system may be under- stood by referring to Fig. 16. To obtain the same speed range with * U. S. Patent Xo. 572,903, December 8, 1896. 62 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. a 24o-volt motor as with an ordinary multiple-voltage system, the line potential is only 120 volts, while the generator end D also develops 120 volts. Thus if both line and generator pressures are in series, the voltage V at the motor terminals will be 120 + 120 or 240. Decrease in value of V from 240 volts to 120 volts is obtained by weakening the field of the generator to zero, while a reduction of V below 120 volts is obtained by reversing the generator voltage and thus subtracting it from the line pressure. This is accomplished by arranging the field rheostat F to reverse the field connections, the current in the same having been, however, first gradually reduced practically to zero, in which manner the high voltage and spark accompanying the opening of a field circuit are eliminated. Since the voltage of the generator end is only one-half of that required by the working motor WM at rated speed, the watt-capacity of the "boost and retard" equipment need be only a little more than one- half that of the working motor, and accordingly the parts of the motor generator MD are 50 per cent smaller than in the preceding system. The voltage and current relations existing between the units com- prising the "boost and retard" system are as shown in Table X, and it should be noted that when the generator end of the MD set is "crushing" or "retarding" the line voltage, it has reversed its function and is acting as a motor driving what was previously the motor end as a generator, which then pumps back into the sup- ply line, thus furnishing part of the current required by the work- ing motor. For example, to run the working motor at one-quarter speed or 206 r.p.m. requires n.8 + (218.2 4- 4) = 66.3 volts, that is, armature and brush drop plus one-quarter of rated c.e.m.f. Hence the machine D must generate 48.7, which, combined with 115, the line voltage, produces the required 66.3 volts for the working motor. The machine D, thus developing a c.e.m.f. of 48.7 volts, is therefore running as a motor, consuming 48.7 X 37 = 1802 watts. Assuming the combined efficiency of the machines D and M as 80 per cent, the latter will generate .80 X 1802 = 1442 watts at 115 volts, since it is connected to the supply lines. Hence it furnishes 1442 -r- 115 = 12.6 amperes and the supply circuit 24.4 amperes to make up the 37 amperes consumed by the working motor. The other values in Table X are calculated in a similar manner. For small currents the efficiency of D and M might be less than 80 per MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 63 cent, but the difference would be of little practical consequence. It is to be noted that this "boost and retard" method as well as the preceding "motor-generator" arrangement gives full rated torque with usual overload capacity at all speeds of the working motor, so that its horsepower output increases directly with its speed. TABLE X. BOOST AND RETARD EQUIPMENT VOLTAGE AND CURRENT RELATIONS. Working Motor. Motor-Dynamo. Supply Line. Dynamo End. Motor End. Speed Volts. Amp. Volts. Amp. Volts. Amp. Volts. Amp. 11.8 37 -103.2 + 37 115 26.5 115 10.5 206 66.3 37 -48.7 37 115 12.6 115 24.4 412 120.8 37 5.8 37 115 - 2.3 115 39.3 618 175.3 37 60.3 37 115 -23 115 60.0 825 230.0 37 115.0 37 115 -46.8 115 83.8 Examination of the table shows that this system of control is advantageous at speeds considerably below the rated value. For example, to start the working armature by supplying it with n.8 volts consumes only 115 volts and 10.5 amperes or 1208 watts from the supply lines. To produce the same effect with a rheostat in the armature circuit would demand 230 volts and 37 amperes or 8510 watts, which is seven times as large an input. On the other hand, with this system at or near rated speed, the efficiency falls from 86.7 7640 per cent for the individual motor to = 71.6 per cent for 115 X 83.8 the combination of the motor and motor-generator. Bullock " Teaser " System. This arrangement is designed espe- cially for printing-press operation when the "inching" or slight forward movement of the press must be effected very accurately for " making ready, " as it is called. While this could be accomplished by the two preceding multi-voltage methods, the cost of the equip- ment would be rather high, hence the development of this special method. The apparatus and connections of the electrical units are as shown in Fig. 17. The "teaser" or motor-generator MD is of compara- tively small capacity and its generator end generates a low voltage. 64 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The operation is as follows: The motor-generator acts as a current transformer, to supply currents of considerable value at low voltage to the main motor for starting large presses, inching them forward or even running them for long periods at low speeds. The speed of the working motor WM is gradually augmented by increas- Teaser Set Teaser Set Cut-out Working Motor connected directly to Line Pull Speed FIG. 17. BULLOCK "TEASER" SYSTEM. Working Motor Current Line Current with Rheos at Control Teaser Cut Line Current with Teaser Set 10 20 30 40 50 60 70 80 90 100 Percent Rated Speed FIG. 1 8. LINE CURRENTS, RHEOSTATIC AND "TEASER" CONTROL. ing the speed and voltage of D by decreasing the value of the series resistance R or by field weakening of the motor end until the main motor WM is rotating at such a rate that it can be operated with comparative economy from the main line through the resistance R m , at which instant the " teaser" is disconnected from the line and main motor. The great economy of the teaser system over armature MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 65 rheostat control is shown by the curves in Fig. 18, which represent the currents drawn from the line by the two methods when the working motor is performing the same duty. The dotted line com- pared with the solid line shows the reduction in line current with the teaser, the saving being more than 50 per cent up to about 30 per cent of rated speed. A little below half speed the teaser is cut out, above which point the armature speed and current are controlled by the rheostat R m in the usual way. The conditions while starting the typical lo-horsepower motor are represented in the upper diagram of Fig. 17. The armature current of the working motor WM is assumed to be 55.5 amperes, which is 50 per cent above rated value, in order to overcome inertia and initial friction. Of this current 48.8 amperes are generated by the generator end D of the teaser, and 6.7 amperes are supplied through the motor, as indicated. Merely to start the motor demands 17 volts and 48.8 amperes or 830 watts from the machine D. Assum- ing 80 per cent efficiency for the motor-generator, the input of the motor end M must be 830 +- .8 = 1036 watts. Hence the voltage consumed by it is 1036 H- 6.7 == 150 volts and the drop in the series resistance R is 230 (150 + 17) =63 volts, the amount of this resistance being 63 -7-6.7 = 9-4 ohms, which is gradually decreased to raise the speed of the teaser and working motor. At starting only 230 volts and 6.7 amperes are drawn from the supply lines, instead of 230 volts and 55 amperes, which is more than eight times the power in watts. When the teaser generator is of the simple shunt type, a sudden overload or sticking of the press rollers stalls the entire equipment, because the terminal volts of D fall too low to produce the current required. To overcome this difficulty the modification known as the Bullock Teaser Booster equipment has been developed. This is essentially like the preceding, but the generator end of the teaser is compound wound, so that any tendency to stall the working motor WM increases the current; thus the voltage of D and the motor torque are sufficiently augmented to carry it over the sticking point. With these teaser arrangements, the working motor may exert full torque at all speeds, so that its horsepower output increases with the latter as in the multiple- voltage or "boost and retard" systems. Holmes-Clatworthy System. This is similar to the teaser sys- tem in principle and is also applicable to the driving of printing presses, but the low speed for starting and inching purposes is 66 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. supplied from a special motor. The equipment comprises a main or working motor, a smaller auxiliary motor and a controller. In addition there is an electrically operated self-releasing clutch, situated between the two motors, by which the turning effort of the auxiliary motor is transmitted through worm gears to the press. The auxiliary motor is wound for such a speed that by means of the gearing it will drive the press for all purposes of starting up, inching, leading in, etc., and bring it up to a sufficient speed so that the main motor may take the load advantageously. As soon as the main motor overspeeds the auxiliary one, the releasing clutch operates automatically, and the latter machine is disconnected. Double-armature Method. This method of motor speed con- trol is placed under the general head of multi-voltage or ad- justable-voltage systems because even though the line voltage remains constant, adjustable speed is obtained by changing the voltage applied to a given armature winding, thus producing the same result as by altering line voltage. There are two gen- eral arrangements belonging to this class. The principle is the same for both, but with the first only two running speeds are obtained by connecting the armature windings either independently or in series, while in the second case four speeds can be secured by changes in the manner of connecting the two armature windings to the circuit. The first method (General Electric Company's,* and C. and C. Electric Company's systems) employs a motor with an ordinary field frame and winding which may be shunt or may be compound wound, but the armature core is provided with two windings and two com- mutators, which are alike in all respects. Thus if one armature winding be placed across the line a certain speed will be obtained. If both are placed across the line in series, the speed will be about one-half as great. This double-armature method is closely similar to the series-parallel control of railway motors. The successive steps in this method for the speed regulation of a compound-wound motor are as illustrated in Fig. 19. An extension of the same principle is exemplified in the motor developed by the Commercial Electric Company, which employs one common field frame and winding (shunt or compound) and two independent armature windings, but instead qf having these alike in * U. S. Patent No. 757,394, April, 1904. MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 67 number of inductors, one of them has 2 x inductors and the other 3 x inductors; i.e., one has 50 per cent more inductors in series than the other. Thus if the 2 x winding be opposed to the 3 x winding and connected in series to the line, only x inductors are effective in pro- ducing the c.e.m.f., hence the speed would be a maximum. If the Min. Speed Shunt Field L- Series Field Armature Windings Starting Resistance ToinroTflT Series Field Jumper Max Speed Slmnt Field inmnnnnra -B^t FIG. 19. GENERAL ELECTRIC, DOUBLE-ARMATURE MOTOR CONTROL. winding with 2 x inductors were connected by itself to the line, a speed of one-half the maximum would be obtained. If the winding with 3 x inductors were placed across the line, a speed of one-third the maximum would be obtained, while if both were placed in series across the line so that they generate e.m.f. in the same direction corresponding to 5 x inductors, a speed of only one-fifth the maximum would be the result. The general connections for these steps are shown in Fig. 20. If used in combination with field or with rheo- 68 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. static control this method would give an extremely wide range; for example, a 6 to i field range would give a 30 to i speed range. The series-parallel control, including ordinary railway motors, as well as the G. E. and C. and C. methods described above, gives full rated torque at all speeds unless the field is weakened, because both Min. Speed 2x FIG. 20. COMMERCIAL ELECTRIC COMPANY'S DOUBLE-ARMATURE METHOD OF MOTOR CONTROL. armatures may carry full current, if desired. The Commercial Com- pany's arrangement exerts only one-fifth torque at five times the speed, that is, constant power like the field weakening method. Speed Control by Variation of Number of Poles. The Bullock Company at one time manufactured a motor capable of giving various speeds by changing the number of poles. For example, consider a six-circuit armature with a six-pole field magnet. When the field coils are so connected that the ordinary relation of alternate north and south polarity exists, the number of armature circuits MULTIPLE-VOLTAGE CONTROL OF MOTOR SPEED. 69 (b) and the number of poles (2 p) are each 6, hence from equation (2), the speed would be eio 8 6o b e io 8 6o r.p.m. = = - . n<& 2 p n<& If, then, the connection of the field coils be changed so that three poles adjoining each other become S, and the other three AT", we have e io 8 6o with the same impressed voltage a speed in r.p.m. = - - or %x n$$ because the armature winding becomes a two-circuit one, the number of poles being two, while the flux per pole has increased to about 3 provided the yoke and armature have sufficient cross section to carry the increased flux. Hence the speed in the second case would be only one-third of what it was in the first instance. The cost of this design is so great, due to larger frame, complex windings and switches, that it has not been commercially successful. For example, it would be necessary even in the smallest motors to have six poles in order to obtain a 3 to i speed range. CHAPTER VII. SPEED CONTROL OP SHUNT MOTORS BY VARIATION OP FIELD CURRENT. IN the preceding chapters the speed regulation of shunt motors by the multiple voltage systems or equivalents was discussed and the ob- jections thereto were noted. This chapter is devoted to the discus- sion of a second method of speed adjustment not open to the same criticisms. QnN 2 p The equation e = 7 , shows that if e, the counter e.m.f., 60 X io 8 X b is kept constant, and the armature flux $ varied, the speed N varies inversely as the flux < because the other quantities do not change unless purposely made to do so by altered construction or arrangement of parts. This relation, therefore, indicates a method of speed varia- tion. Shunt motors are usually designed to have such high flux density in both field and armature that it is not practicable to increase it materially. Hence this method is confined to and commonly called field weakening. * In the case of ordinary shunt motors, the range of speed variation by means of field weakening is small. For instance, take the 10- horsepower motor previously considered and weaken its field by the introduction of extra resistance into its field circuit to produce 30 per cent increase in speed. Since the flux must be varied inversely as the speed, it must be weakened in the proportion 130 : 100 or 100 : 77, that is, 23 per cent, while to produce this change the field ampere-turns must be reduced by about 50 per cent. To develop rated torque with this diminished field strength the armature current must be increased 30 per cent, and the ratio between back ampere- turns and field armature-turns, instead of having its normal value of about .10, is raised to = .26, because the former is 30 per cent greater and the latter is reduced to one-half. This latter ratio is excessive. The corresponding increase in cross ampere-turns, acting collectively with the increased back ampere-turns, causes excessive 70 SPEED CONTROL OF SHUNT MOTORS. 71 sparking. Hence a 30 per cent increase of speed above rated value in the case of an ordinary standard shunt motor cannot be obtained by field weakening without objectionable sparking. Another difficulty arises from the fact that the increase of armature current necessary to maintain constant torque augments the I 2 a R a loss, which in the lo-horsepower motor armature rises from 37* X .28 to 48 2 X .28, an increase of 262 watts or 68 per cent, producing too much heat for the armature insulation to stand for any consider- able time. Adjustable speed motors of the flux-variation type are not constant- torque machines, but constant-horsepower or output motors; i.e., the torque falls to the same degree as the speed increases, or T X r.p.m. = a constant. In fact, unless the ratio of back ampere-turns to field ampere-turns is less than 10 per cent at minimum speed, an increase in speed of even 30 per cent with constant output is not practicable with the ordinary shunt motor because it demands a 50 per cent reduction in field m.m.f., as shown above. It is evident that a shunt motor, to have any considerable range of speed variation (i.e., increase of more than 20 or 30 per cent) by field control, requires some modification in design, because the field must be more powerful with respect to the armature than in the case of standard single-speed motors. Some special motors of this kind allow of speed variations of three or four to one, with constant- horsepower output, but not at rated torque. These increased speed ranges are obtained as follows: (a) Magnetic Circuit of Very Soft Steel. The magnetic properties of the material are such that even with high flux densities the bend of the curve is not reached, so that the change in m.m.f. to produce a large change in flux is not excessive; i.e., the rate of change of flux and m.i^.f. is almost in direct proportion. With these machines the field frame is large, the total flux being very great, while the armature winding consists of fewer turns per section and a larger number of sections, so that the self-induction per section is low. Thus, under normal or even exceptional conditions the ratio of field to back ampere- turns is kept low, so with low inductance per section sparking cannot become serious. However, a machine having a field of sufficient strength and enough armature sections to prevent spark- ing at high speeds must have a frame considerably larger than neces- sary for a single-speed motor of equal power and the commutator 72 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. must have a greater number of bars. In general, when considering this simple type of variable-speed motor, it can be stated that the percentage of speed increase of which a normally loaded motor is capable by means of field weakening, is a measure of its overload capacity with full field strength. In other words, a given range of speed variation demands a motor having a certain increased capacity, or special features of design. The following practical examples illustrate this point : Relative Sizes of Frames with Speed Ratio of 1:2: i5-horsepower frame for a lo-horsepower motor. Relative Sizes of Frames with Speed Ratio 0/1:3: 2o-horsepower frame for a lo-horsepower motor. (b) With the magnetic circuit specially designed so that the flux density is always great at the pole tips, the field distortion due to armature reaction is lessened and a sufficient flux is maintained in the commutation zone, giving sparkless operation within reasonable speed and load limits. Greater ranges of speed adjustment than a three to one ratio are frequently required, for example, five or even six to one; in such cases the preceding types are not economically available. With the (a) and (b) types it is difficult or costly to maintain the commutation fringe when the main field excitation is reduced sufficiently to obtain a speed range greater than three to one; thus some new feature in design to maintain the commutation flux becomes necessary. This feature, somewhat differently obtained, is present in two types of motors. (c) Historically the first of these is the Thompson-Ryan design of compensated motor, manufactured by the Ridgway Dynamo and Engine Works. This compensated form employs what is equiva- lent to a stationary armature built up in the polar faces, and traversed by the armature current or a portion thereof, which develops a m.m.f. opposed to the armature m.m.f., thus eliminating or even reversing armature reaction. It has been found, however, by M. E. Thompson that this compensating winding, by itself, is not sufficient to prevent sparking at the brushes, so he introduced commutating lugs, excited by special turns immediately around them as well as by the compensating winding, which establish a flux for reversal over SPEED CONTROL OF SHUNT MOTORS. 73 the coils undergoing commutation.* This motor thus possesses the two features of compensation and commutation, which are independent of the strength of the main field, and sparkless operation over wide speed changes is theoretically possible. (d) The second of these special forms is one wherein armature reaction and distortional effects are not overcome, but their presence is depended upon to obtain good speed regulation. The sparkless condition of operation is secured by the use of "interpoles" or auxiliary field poles, placed directly over the zone of commutation, the m.m.f. of these poles being opposed to that of the armature, and, as they are energized by coils carrying the armature current, their m.m.f. increases with and is designed to be superior to that of the armature. Thus the flux for reversal is locally maintained inde- pendently of the main field, and varies automatically, as required, with the result that sparkless commutation may be obtained. f The difference between types c and d is that the former embodies general magnetic compensation as well as local commutation flux, whereas the latter depends upon local flux for commutation alone, with no attempt to neutralize armature reaction. Machines of--Class (a) are built by many manufacturers, and while a number are in use they are larger than standard constant- speed motors, as already shown. An example of this class (a) is found in a 5-h.p. Bullock shunt motor, the data of which are as follows : Rated capacity, 5-h.p. Rated pressure, 220 volts. Speed, 350 to 1050 r.p.m. Armature current at rated load, 22.2 to 24.6 amperes (depending upon the speed). Field current, 1.3 to .23 amperes. No-load armature current, 2.1 to 3.6 amperes (depending upon field strength and speed). Armature resistance, hot, 1.12 ohms. Field resistance, hot, 195 ohms. Weight of motor complete, noo Ib. Tests were conducted upon this motor with the results shown in the following series of curves: * U. S. Patent No. 591,024, October 5, 1897. f U. S. Patent No. 775,310, November 22, 1904. 74 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. mm 175 150 126 |ioo 75 50 25 x^ ^ ,* // x Curve D Rated load R.p.m.l050Arm. Current 25Ampg| = .23 Amp, FIG. 23. FLUX DISTRIBUTION OF 5-H.P. BULLOCK MOTOR. the field. The currents, running free, rise with speed, owing to greater stray power losses. The flux-distribution diagram (Fig. 23) of this motor shows how much the field flux is reduced in value to obtain the highest speed, and how the armature distortional effects have forced the field mag- netism to the left, the crossing-point or zero flux value being no longer under the brush, which naturally causes the sparking noted above. The efficiency curves (Fig. -24) of this 5 -horsepower motor bring out the fact that, at corresponding loads, the efficiency of the machine is less the higher the speed. This is to be expected because frictional 76 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. losses increase more rapidly than the iron and excitation losses fall off, the same being true generally of all adjustable-speed motors un- less provided with ball or roller bearings. 234 Horse-Power Output FIG. 24. EFFICIENCY CURVES OF 5~H.P. BULLOCK MOTOR AT SPEEDS OF 350, 700 AND 1050 R.P.M. Machines of Class (b) were formerly manufactured by the Magneto Electric Company, and called Storey Motors, after the designer. These motors are of interest because they show how the concentra- tion and holding of the flux at the pole tips can be obtained by simply hollowing the field cores, as represented in Fig. 25. FIG. 25. FIELD FRAME CONSTRUCTION OF STOREY MOTOR. The frame of the motor is of soft steel and the flux density high, but not reaching the bend of the magnetization curve, as Fig. 26 shows; and the cores are relatively short. SPEED CONTROL OF SHUNT MOTORS. 77 The data of a 3-h.p., 3 : i adjustable-speed Storey motor exam- ined by the authors is as follows: Rated pressure, 115 volts. Armature current at rated load, 25 to 28 amperes, increasing with the speed. Field current, .5 min., 1.7 max. 1ft) 110 x s f 90 80 o70 I 60 50 O 40 30 20 10 ( /, y /, / /, / / / y 430 I .p.m. / 7 // ^ z / y ) .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.S Field Amperes FIG. 26. MAGNETIZATION CURVE OF 3'H.P. STOREY 3 I I ADJUSTABLE-SPEED SHUNT MOTOR. No-load armature current, i.i to 4 amperes, increasing with the speed. Armature resistance, .31 ohm. Field resistance, 67.5 ohms. Speed, 430 to 1290 r.p.m. Weight, 800 pounds. The flux-distribution curves of this motor (Fig. 27) show a very uniform flux under the pole pieces at minimum speeds, also that the flux reversal line remains fixed independently of the load, thus main- taining a flux for commutation, which, however, is notably decreased in width as the main field is weakened, causing the ultimate develop- 78 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. ment of sparking as well as poorer speed regulation. This latter fact is also brought out by a study of the speed-load curves of this motor in Fig. 28. i 8 - I 6.0 Minimum Speed" ited Load ;oLoad Maximum Speed Rated Load No Load R.P.M.= 430 Ch=1.7 Amp. C = 25 Amp. C a ' =1.1 Amp. FIG. 27. FLUX-DISTRIBUTION CURVES OF 3~H.P. STOREY SHUNT MOTOR. 1600 1400 1200 1000 800 600 400 200 Field Curre Field Curre t.5Am t.7 Am Horse Power Output FIG. 28. SPEED-LOAD CURVES OF 3~H.P. STOREY SHUNT MOTOR. For example, the drop in speed from no load to rated load with the weakest field is 29 per cent, whereas the decrease in speed over the corresponding load range at the strongest field is only 16 per cent; thus the falling off in speed with weakest field excitation is 13 per cent greater. Only a small part of this is due to the greater I a R a drop (28 X .31 8.7 instead of 25 X .31 = 7.8); hence the extra falling off in speed occurring at weakest field must be primarily due to poorer contact and sparking at the brushes. SPEED CONTROL OF SHUNT MOTORS. 79 The efficiency curves of this Storey motor (Fig. 29) show that the second speed (860 r.p.m.) is probably the best for average service; while the highest-speed curve indicates large stray-power losses. Horse Power Output FIG. 2Q. EFFICIENCY CURVES OF 3-H.P. STOREY SHUNT MOTOR. It should be noted that the preceding types of adjustable-speed shunt motors (a and b) in all cases fall off considerably in speed as the load comes on, the greatest percentage of reduction occurring with weakest field, and, in all cases, the drop in speed is either equal to or greater than that caused by I a R a drop. Moreover, as the brushes of these machines are set back from the geometrical neutral ^one, they cannot run equally well in both directions of rotation, without brush shifting. The adjustable-speed motors (type S) of the Northern Electric Manufacturing Company represent a construction similar in prin- ciple to that of the above-described Storey motors and belong therefore to the same class (b). Their pole pieces are split in the direction of the flux, forming a field frame of clover-leaf form. This frame, built up of laminations, is similar to the construction shown in Fig. 31, but without the commutation lugs. Class (c) . The earlier form of Thompson-Ryan motor diagram- matically illustrated in Fig. 30 was originally brought out for 80 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. constant speed, but the demand for an adjustable-speed motor of wide range led to its adoption for this latter service as well. A machine of this type, however, is very expensive to build or to repair, FIG. 30. ORIGINAL FORM OF THOMPSON-RYAN MOTOR. and therefore for the smaller sizes commonly employed with ma- chine tools the modified design shown diagrammatically in Fig. 31 was developed in the spring of 1904. This modified type retains the compensating winding and commutation lugs of the earlier patented design, but discards the inner polar ring with its inherent cost, con- necting the commutation lugs directly to the field yoke and placing the compensating winding in slots formed in the main polar faces. The function of the compensation coils C, C (Fig. 30), in series with the armature winding, is primarily to prevent the distortion of the field flux and thus eliminate brush shifting with varying load. This, however, was not found effective to prevent sparking,* hence * Transactions A. I. E. E., March 20, 1895, Vol. XII. SPEED CONTROL OF SHUNT MOTORS. 81 the commutation lug was introduced to provide the necessary flux for reversal directly at the armature coils undergoing commutation, thus general compensation and local commutation phenomena are combined. FIG. 31. MODIFIED FIELD FRAME OF THOMPSON-RYAN ADJUSTABLE-SPEED MOTOR. The data of a 3-h.p. Thompson-Ryan motor of the modified type tested by the authors are as follows: Line voltage, 250 volts. Armature current at rated load, 11.4 to 12.2 amperes, rising with the speed. Field current, .28 to 1.15 amperes, increasing as speed falls. No-load armature current, i.o to 2.2 amperes, rising with the speed. Armature resistance, 2.1 ohms. Compensating and commutating coils' resist- ance, 1.17 ohms. Field resistance, 200 ohms. Speed, 350 to 1400 r.p.m., depending upon field strength. Weight complete, 650 pounds. The flux-distribution curves in Fig. 32 show that the armature reaction is reversed as load comes on, the leading corner being weak- ened and the trailing one strengthened, which is just the converse of the action occurring in other motors. As a result of this action, the leading corner is weakened more than the trailing corner is strength- 82 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. ened, and the net effect is a diminution of the field strength under load increase, with a slight improvement in speed regulation. The increased IR drop due to the compensating winding, however, does cause the speed to vary considerably with load. The reversal of armature reaction can readily be carried so far as to produce hunting and racing with large increase of load, especially Minimum Speed Field Current -1.J6 Maximum Speed Field Current = .28 FlG. 32. FLUX-DISTRIBUTION CURVES OF THOMPSON-RYAN 3'H.P. MOTOR. at the higher speeds. This scheme for obtaining very constant speed regulation is not, however, economically developed, since the weight of copper used in the compensating winding is approximately twice that used in the armature winding, which naturally means greater I a R c drop, P a R c losses and heating. Interesting features of this design are the extremely small air gap and the very high average potential difference of over 20 volts exist- ing between adjacent commutator bars. In fact this voltage is undoubtedly much more than that at some points, because, when the motor is operating at the higher speeds, the points of very high flux density can be approximately located by the lines of scintillation on the commutator, due to incipient sparking between neighboring bars. The speed-load curves of this motor (Fig. 33) represent both clock- and counter-clock-wise rotation and show just the reverse of the characteristic regulation of the ordinary simple field-frame shunt motors (types a and 6), in that the speed decrease under load is more pronounced at the low than at the higher speeds. This improve- ment in regulation is due to the field distortion and reduction caused by the action of the balancing windings. For example, at minimum speed the drop in speed from no load to rated load is 14.5 per cent, which is substantially that which occurs through I a R a drop. At the highest rate of rotation (field current = .28 amps.) the decrease in speed from no load to rated load is only 7.1 per cent, whereas it SPEED CONTROL OF SHUNT MOTORS. 1CSC 1400 83 1200 1000 800 600 400 200 IA=.28 Amp. =1.15 Amps. 1234 H. P. Load FlG. 33. SPEED -LOAD CURVES OF 3'H.P. THOMPSON-RYAN MOTOR. so f JC. A. A 1.1 B .! C .5 B. Amps. 01234 Hoise-Powtr Load FlG. 34. EFFICIENCY CURVE OF 3'H.P. THOMPSON-RYAN MOTOR. would be 15 per cent due to I a R a drop; so that speed regulation is improved 8 per cent by the effect of the balancing winding. The flux-distribution curves shown in Fig. 32 indicate not only very markedly the reversal of armature reaction, but also the net 84 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. decrease of main-field strength occurring at the highest speed. The falling off of flux to zero and even reversal, under the middle of the main poles, is due to the fact that the laminated construction employed has the main poles split from the pole face back through the yoke. This construction is necessary so that the removal of the various field coils for repair is feasible. The efficiency curves of this motor in Fig. 34 indicate nothing unexpected, because it is obvious from the construction that copper losses are great, and the flux-distribution curves show that the core losses are also large, on account of the crowding and numerous reversals of flux. This type of motor is extremely sensitive to change of brush posi- tion, a barely perceptible movement forward or backward producing quite different speed characteristics, so that the machine runs faster in the clockwise direction of rotation or more slowly in the opposite direction, acting in the one instance like a differential motor and in the other like a heavily over-compounded motor. The ordinary wear of the brushes or the formation of invisible sparks under the brushes, which is not unlikely to occur, alters the speed regula- tion considerably and frequently leads to more pronounced and objectionable sparking. Class (d). Inter pole* or commutation-pole motors (Fig. 35) con- stitute what is herein designated as Class (d) of adjustable-speed motors. In such machines auxiliary poles are introduced between the main-field poles. These interpoles are excited by coils connected in series with the armature, so that full or proportional part of the armature current flows through them. This type differs from Class (c) in that the compensation winding is discarded and armature reaction is therefore not eliminated or reversed, a local commutation flux alone being depended upon for sparkless operation throughout the range of speed. In fact, with this type armature reaction is actually exaggerated because the flux from the interpole strengthens the leading-pole corner and weakens the trailing-pole corner just as the armature m.m.f. does. This exaggeration of field distortion does no harm, but, on the contrary, it improves the speed regulation of the machine. The interpolar flux for reversal is independent of the main field ; being, however, directly dependent upon the armature * U. S. Patent No. 775,310, November 22, 1904. SPEED CONTROL OF SHUNT MOTORS. 85 current, it increases therewith and thus maintains the necessary commutating field. The connections of this type of motor are diagrammatically indi- cated in Fig. 36, N, S, N, S being the main poles, and n, s, n, s the interpoles. Each interpole is of the same polarity as that of FIG. 35. VIEW OF FIELD FRAME OF INTER-POLE MOTOR, SHOWING RELATIVE SIZES OF MAIN AND INTER-POLES. FIG. 36. CONNECTIONS OF INTERPOLAR MOTOR. the main pole immediately back of it, depending upon the direction of rotation; hence the illustration shows polarities for clockwise rotation. As represented, the interpoles are small with respect to the main-field poles, the arc of armature periphery subtended by 86 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. the former being about one-sixth that embraced by the latter. As already stated, the interpoles are provided with magnetizing coils connected in series with the armature and are placed midway between the main poles, directly over the armature coils under- going commutation. The commutator brushes are consequently SD set that they short-circuit coils in the geometrical neutral posi- tion. With this setting of brushes the motor will operate with substantially the same speed characteristics in either direction of rotation. V.R. is the variable resistance rheostat for adjusting the field strength in order to change the speed; and R.S. is the pole- changing switch for reversing the direction of rotation. Data of a 5-h.p., 6 to i variable-speed interpolar motor manu- factured by the Electro-Dynamic Company, and tested by the authors, are as follows: Rated voltage, 240. Armature current at 5-h.p. output, 22.2 to 24 amperes, increas- ing with speed. Armature current running free, .7 to 1.7 amperes, increasing with speed. Field current, adjusted between 1.27 and .16 amperes to obtain speeds from 210 to 1260 r.p.m. at 5-h.p. output. Resistance of armature winding, .9 ohm at 75 degrees C. Resistance of interpole winding, .2 ohm at 75 degrees C. Resistance of shunt-field winding, 176 ohms at 75 degrees C. Speed 205 to 1260 r.p.m. at 5-h.p. output, increasing with weaker field. Weight, 1200 pounds. The magnetization curve of this motor (Fig. 37) shows that, for the minimum speed, the flux density is carried well up above the bend; this is also quite apparent from the fact that a speed ratio of i : 6 is obtained with field currents at 8 : i. The speed-load curves of this motor (Fig. 38) indicate excellent speed regulation, with an actual increase of speed under load at the weakest field value. The regulating influence of armature and interpole reaction upon the main magnetic field and speed is brought out by the following examples: The no-load speed with field current of 1.27 amperes is 222 r.p.m. The speed diminution caused by IR drop is 22 r.p.m.; nevertheless, at rated load with field current of 1.27 amperes, the speed is 205 r.p.m.; hence the effect of armature and interpole reaction is to SPEED CONTROL OF SHUNT MOTORS. 87 raise the speed 5 r.p.m. compared with what it would otherwise be. At a speed of 740 r.p.m. the armature reaction exactly com- 200 180 160 140 > 100 80 60 40 20 n x ** / r // m. 210 // I / 9 / /f f // // t Field Current FIG. 37. MAGNETIZATION CURVE OF 5'H.P. INTERPOLAR SHUNT MOTOR. 1400 1200 1000 800 600 400 200 __ ~ - . Ish = .16 Am _ ^ ^ P- ! 1 It = .30 Air P- L) = 1.27 Ar np. , 12345678 Horse-Power Output FIG. 38. SPEED-LOAD CURVES OF 5~H.P. INTERPOLAR SHUNT MOTOR, SPEED RANGE 6 I I. 88 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. pensates for IR drop and the motor speed remains constant up to the rated output of 5 horsepower. With field current of .16 am- pere the motor speed rises from 1209 to 1260 r.p.m. when output increases from zero to 5 horsepower. Apparently it should fall from 1209 to 1084 r.p.m., which is the ratio between the c.e.m.f's (i.e., 238.1 : 213.6), but the reaction on the main field by the inter- poles and armature weakens the same sufficiently to raise the speed 176 r.p.m. Arm. Current .72 Amp. 22.2 1.27 " FIG. 390. FLUX DISTRIBUTION OF 5~H.P. ADJUSTABLE-SPEED INTERPOLAR SHUNT MOTOR, FIELD CURRENT 1.27 AMPS.; SPEED AT RATED LOAD 2IO R.P.M. It is thus evident that with this type of machine the speed regu- lation is the reverse of that obtained with adjustable-speed motors having the ordinary forms of field magnet (Classes a and b). The interpolar type of motor can be readily reversed in direction of rotation even while under load, on account of the great self- induction of the interpole and armature circuit and the production of the proper value of commutation flux. The flux-distribution curves in Fig. 390 indicate that at strong field excitation the interpoles do not produce a very great effect. The same fact was also shown in the speed-load examples on page 62. With small field flux, however (Fig. 396), the interpole m.m.f. and armature reaction produce a marked weakening and distor- tion of the main field, which phenomena are also apparent from the speed-load curves (Fig. 38). - If the brushes be displaced from [he geometrical neutral position, the motor speed is considerably changed. For example, with the brushes shifted backward (opposite to rotation direction), the speed SPEED CONTROL OF SHUNT MOTORS. 89 will rise under load, because then the interpolar flux develops in the armature an e.m.f. which decreases that produced by the main- field poles. If the brushes are advanced in the direction of rota- Arm. Current 1.7 Amp. " " 24.0 " I.* = ! FIG. 396. FLUX DISTRIBUTION OF 5~H.P. ADJUSTABLE-SPEED INTERPOLAR MOTOR, FIELD CURRENT .16 AMP.; SPEED AT RATED LOAD 1 260 R.P.M. tion the speed will fall under load, as in the case of a cumulative compound motor, because the interpolar flux generates an e.m.f. in the same direction as that due to the main poles. In conse- quence of this, the proper position for the brushes is obtained when H.P. A. 1.27 Amp . 03 Amp C. 0. 16 Amp 123456 78 FIG. 40. EFFICIENCY CURVES OF A 6- 1 ADJUSTABLE-SPEED "iNTERPOLE" MOTOR. the r.p.m. are the same in both directions for given load and field strength, and motors of this type should not be shipped by the manufacturer until such adjustment is secured at the highest speed. 90 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The efficiency curves of this motor (Fig. 40) indicate high values for light loads, which is due to the use of ball bearings. The rapid falling off in efficiency after rated load is reached is to be expected, on account of the additional PR losses caused by the interpole windings. The best running speed of this motor is apparently at a field strength of about .3 ampere, because at this value the general efficiency of the motor is considerably greater than at lower or higher speeds. Dunn Method. Another type of motor, the speed of which is ad- justable by varying field flux, was invented by Mr. Gano S. Dunn.* The armature is supplied with constant current and the field winding separately excited from a constant-potential circuit through a rheostat. The armature current being constant, the torque varies directly with field flux. By means of the field rheostat this flux may be regulated from a very low value up to full strength with corresponding increase of torque. This large range of control is obtained by regulating the field current, which is small, the heavy armature current being kept constant by an automatically regu- lated generator, as in constant-current arc lighting. The advan- tage is similar to that secured by the "field-weakening" method already described, but gives any torque or speed from zero to full value, while the latter is practically limited (unless special designs are employed) to a certain ratio of speeds, usually 2 or 3 to i. This method possesses an additional advantage over field-weak- ening control in having maximum field strength with maximum speed and torque. In these respects it would be adapted to adjustable-speed work in machine shops. On the other hand, the necessity for constant-current as well as constant-potential supply, and the high voltage required for any considerable power* are serious objections to this system. It is decidedly undesirable to operate motors below 20 horsepower with more than 100 amperes, at which current it would require about 1000 volts to supply 100 horsepower on one circuit a dangerous voltage in a shop. To multiply circuits is objectionable because each would demand its separate constant-current generator. Furthermore, the latter has not been developed commercially above 10 amperes. For these reasons, the field-weakening and multiple-voltage methods are preferred for machine shop or similar service. * U. S. Patents No. 549,061, October 29, 1895, and No. 591,345, October 5, 1897. CHAPTER VIII. SPEED CONTROL OF MOTORS BY VARIATION OP FIELD RELUCTANCE. THE preceding chapter dealt with the problem of shunt-motor ^peed adjustment by variation of the field-exciting current; this chapter is descriptive of those motors whose speed regulation de- pends upon the variation of the reluctance of the magnetic circuit. This method of control is based upon the fundamental fact that magnetomotive force reluctance flux = Thus, if the m.m.f. be maintained FIGS. 41 AND 42. METHODS OF VARYING RELUCTANCE OF THE MAGNETIC CIRCUIT. constant and the reluctance be varied, the field flux is changed in the inverse manner, and from the relation r.p.m. = e io 8 6o b <&n 2 p it is evident that the speed varies inversely as the field flux (<) or in the same ratio as the change in the reluctance. Among the earlier methods tried were those of T. A. Edison and the Diehl Company. The Edison Variable Reluctance Methods of Control. The re- luctance of the magnetic circuit in the machine was varied, as 91 92 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. shown in Fig. 41, by decreasing the amount of metal in the yoke of the field magnet, the wedge-shaped piece, A, being raised, thus decreasing the total flux. The range of speed adjustment is limited, however, as excessive sparking develops when the field is weakened because there is no feature of design to prevent flux distortion. This method was primarily intended for voltage regulation in connection with generators, and is of historical rather than commercial impor- tance. The Diehl Method of Control. In this type of machine flux re- duction was obtained by a lengthening of the air-gap. The field magnet was hinged so that the pole pieces could be moved away from the armature as indicated in Fig. 42. This construction was not very successful and was, like the preceding, originally intended as a means for regulating the voltage of generators. Two modern methods of speed control by variation of reluctance are those of the Stow Electric Company and the Lincoln Manufactur- ing Company. FIG. 43. STOW ADJUSTABLE-SPEED MOTOR. The Stow Adjustable-speed Motor. The speed increase of this machine also depends upon the removal of iron from its magnetic circuit, the pole cores being made hollow and provided with iron or steel plungers, the position of which is made adjustable through worm gears and pinions operated by the large hand-wheel, at the top, SPEED CONTROL OF MOTORS. 93 L dm hi J.710 Field Amperes FIG. 44. MAGNETIZATION CURVES OF 4-H.P. STOW MOTOR. MOO 300 2000 .1800 ^1600 1100 1200 1000 800 E ^600 400 >ger C 1 2 3 fft FIG. 45. SPEED-LOAD CURVES OF 4-H.P. STOW MOTOR. 94 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. as represented in Fig. 44.* When the plungers are withdrawn, the total flux decreases because of the lengthening and reduc- tion of area of the effective air-gap, also the decrease of effective metal in the field cores. The flux being now along the polar edges (in varying degree according to the position of the plungers), the field for commutation is maintained relatively strong, and the degree of sparking thereby considerably reduced. The concentra- tion of flux at the polar edges is well shown by the flux-distribution curves (Fig. 47) of this machine. On the other hand these curves also show that armature reaction forces the commutation fringe back, at the higher speeds and loads; consequently, to maintain sparkless commutation at the 3 to i range of speed, the brushes must be given a lag. This requires the brushes to be shifted if the direction of rotation is reversed, or if the speed range is greater than 3 to i. It is a fact, however, that the cross-section of the field core being diminished, the effect of armature reaction for a given current is less because the reluctance of the path of the armature flux is increased. The data of a 3 to i adjustable-speed 4-h.p. motor of this Stow type are as follows: Rated pressure, 220 volts. Armature current at rated output of 4 h. p., 16.75 to 17.0 amperes, increasing with speed. Field current constant at .64 ampere. No-load armature current, 1.3 to 2.2 amps, rising with speed. Field resistance, " hot," 344 ohms. Speed, 725 r.p.m. min. to 2175 r.p.m. max. Weight, 800 pounds. The operation of this machine under various loads and speeds is shown in the carves, Figs. 44, 45, 46, and 47, which represent, respec- tively, magnetization, speed-load relations, efficiency, and flux dis- tribution. A study of the speed-load curves indicates that the speed regulation at the higher rates of rotation is not as good as that with the stronger fields. This is due to the fact that invisible sparking at the brushes and poorer brush contact increase the I a R a drop, just as in the types (a) and (b) adjustable-speed motors which are controlled by variation of field current. The most efficient operating speed of this motor is at the second plunger adjustment, which gives 1090 r.p.m. * U. S. Patents Nos. 666,315 and 672,419, January and April, 1901. SPEED CONTROL OF MOTORS. 95 Horse-Power Output FIG. 46. EFFICIENCY CURVES OF 4-H.P. STOW MOTOR. A 710 r. p.m. at rated load. B 1090 r.p.m. at rated load. C 1490 r.p.m. at rated load. D 2130 r.p.m. at rated load. 10- Plungers in Field -Current .64 Amps. . Arm.-Ourreut 1.3 " II R.p.m 725. Hungers out Field - Current . Ann.-Current 2J2 . 17.0 y^. _ R.p.m. 2175. FIG. 47. FLUX DISTRIBUTION CURVES OF 4-H.P. STOW MOTOR. 96 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The Lincoln Adjustable-speed Motor. The variation in reluc- tance of the magnetic circuit of this type* is obtained both by lengthening the air-gap and by decreasing its effective area. The armature is formed as a truncated cone with corresponding polar FIG. 48. LINCOLN ADJUSTABLE-SPEED MOTOR. 1280 012345 6T 89 10 11 12 Horse Power Output FIG. 49. SPEED-LOAD CURVES OF IO-H.P. LINCOLN MOTOR. surfaces as represented in Fig. 48. The armature is movable in the direction of its axis, so that movement one way increases the length of the air-gap, thus decreasing the flux and raising the motor speed. The fact that the effective length of the armature inductors * U. S. Patent No. 829,974, September, 1906. SPEED CONTROL OF MOTORS. 97 within the magnetic field is decreased by this shifting of the arma- ture also increases the motor speed. The characteristic working curves of such a machine of 10 horse- power and 5 to i speed range are given in Figs. 49 and 50, being those of speed-load and efficiency at various loads, respectively. The principal objections to this construction are the extra space required for the armature and the large force required to move it. 012 3456 78 9 10 U Horse Power Output FIG. 50. EFFICIENCY CURVES OF IO-H.P. LINCOLN MOTOR The flux-distribution curves of the motors are similar to those of ordinary single-speed machines, the flux distortion at high speeds being limited on account of the increase of the air-gap lengths. However, to ensure sparkless operation at the high speeds, interpoles in series with the armature are employed in the more recent designs. It is to be noted that in this machine, as with types (c) and (d) motors having field-rheostat control, the speed regulation is better with weak than with the stronger magnetic fields, the variation in r.p.m. being 2.3 and 6 per cent respectively. Of the various means of motor speed control considered in the preceding chapters, the field rheostatic method is unquestionably the most used. It is simple, cheap, applicable to any system of D.C. supply, and enables any particular motor speed to be main- tained independently of load changes. The range of speed control obtainable is rather limited in ordinary shunt motors, but as already 98 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. shown the introduction ,of the interpole overcomes this limitation. The change of reluctance method is open to the objections: high cost of the machines, mechanically clumsy construction and lower speed ranges available. The multiple voltage systems though giving very wide speed ranges are expensive, because they require a com- plicated system of current generation as well as distribution and costly controllers. The Ward Leonard motor generator systems give the widest range of speed control obtainable, but their cost limits their use to those purposes for which first cost is a minor con- sideration. For further discussion of these various systems of shunt-motor control see the following publications: D. C. MOTOR SPEED REGULATION. J. W. Rogers. Prac. Eng., London, 1907. DIE GLEICHSTROMMASCHINE. E. Arnold. Vol. II, p. 616, 1908. ELECTRIC JOURNAL, Vol. I, p. 251; Vol. II, pp. n, 566; Vol. Ill, p. 348. ELECTRIC MOTORS. H. M. Hobart. 1910. ELECTRIC WORLD, Vol. XLIX, p. 947. ENGINEERING, September, 1905. LONDON ELECT., Jannary 27, 1905. MOTOR CONTROL. American Electrician, Vol. XVI, 1904, p. 391; Vol. XVII, 1905, p. 303. MULTIPLE-VOLTAGE CONTROL, Electric Power, 1904. PROCEEDINGS ENG. SOCIETY WESTERN PA., October, 1905. SPEED CHARACTERISTICS AND CONTROL OF ELECTRIC MOTORS. C. F. Scott. Eng. Mag., Vol. XXXI, p. 60, 1906. TRANSACTIONS A. I. E. E., Vol. XIII, p. 377, 1896; Vol. XX, pp. 111-197, 1902. Vol. XXIX, p. 621. ELECTRIC MOTORS IN MACHINE SHOP SERVICE. Chas. Day. Trans. Internat- Elect. Congress, Vol. I, 1904, p. 591. VARIABLE SPEED CONTROL. Eng. U. S. A., 1904. PUBLICATIONS of the various manufacturers of electric motors. CHAPTER IX. DIRECT-CURRENT SERIES MOTORS. As the name of this motor implies, the field and armature windings are in series, Fig. 51, hence the same current that flows through the armature also excites the field magnet. Arm FIG. 51. CONNECTIONS OF SERIES MOTOR. Series motors are of two general types, constant potential and ronstant current. Attention will be paid chiefly to the former, since the latter are no longer used commercially. The greatest of all applications of the electric motor is to electric traction, for which purpose series motors are almost universally used in this country. It is natural, therefore, to discuss the action and control of series motors from the railway standpoint, although their application to hoists, fans, pumps, etc., is also important. Speed-current Curve. The speed of a constant potential series motor rises when the current is diminished, the exact relation depend- ing upon the degree of magnetization of the magnetic circuit. At low current values, and therefore low flux densities, the speed is relatively high and the amount of its variation for a given change in torque is cor- respondingly great. The speed is much lower and more nearly constant when the field approaches saturation. The general formula for the speed of a motor as already given in equation (2) is r.p.m. = E io 8 6o b 2 p&n 99 100 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The flux $ is the only variable and depends upon the field current, which is the same as or proportional to the armature current in the case of a series motor. The flux at low densities increases almost directly with the current, and if there were no voltage drop due to resistance, tne speed current curve would take the form, r.p.m. X <3> = constant, which is an equilateral hyperbola asymptotic to both coor- dinate axes. However, as saturation of the magnetic circuit is ap- proached there is a gradual reduction in the rate of increase of flux with current, so that the speed does not fall as rapidly, thus raising the right hand portion of the curve. Moreover, the resist- ance drops of the armature and field windings increase with the current, tending also to raise the same part of the curve. This relation between speed and amperes input is brought out numeri- cally in the two following examples. The first assumes a series motor A with a field of relatively low flux density, and the sec- ond a series motor B of equal current capacity but having a field approaching saturation below rated load. The series motor A is assumed to run on a 5 50- volt constant- potential circuit, its armature resistance being 0.7 ohm and field resistance of the same value. This motor, operated as a generator (separately excited) at a constant speed of 200 r.p.m., gives, in terms of voltage generated, the magnetization curve A in Fig. 52 with field- current variations from o to 50 amperes. Armature reaction may be practically neglected, being relatively small in series machines since the brushes are in the neutral position and because field m.m.f. rises with armature current and m.m.f. Brush drop is also practically negligible in most series motors which run at 550 volts or more in railway service and usually at voltages of 220 or higher for stationary work. Moreover, the field winding being in series with the armature, drop due to resistance is about twice as great as in a shunt machine of the same voltage, making brush drop relatively small, and it will be considered as included with the armature drop. (See Chapter III, pp. 16-19.) The speed-current curve is calculated as follows: At five amperes input the voltage drop due to armature and field resistance is IR a + IR se = 5 (.7 + .7) = 7 volts; hence the c.e.m.f. generated with 550 volts applied =550-7 =543 volts. From Fig. 52, curve A, the field flux due to 5 amperes produces at 200 r.p.m. an e.m.f. of 56.5 volts; hence to develop a c.e.m.f. of 543 volts the DIRECT-CURRENT KS^ \ J / l\] \ /;101 500 450 400 350 2250 \ 150 .100 50 7 5 10 15 20 25 30 35 40 45 50 Magnetizing Current FIG. 52. MAGNETIZATION CURVES OF SERIES MOTORS, A AND B. r.p.m. = (543 ^ 56.5; X 200 = 1920. The speed at other torque conditions corresponding to 10, 20, 30, 40 and 50 amperes can be similarly calculated; the results being given in the following table. TABLE XI. CURRENT-SPEED DATA, SERIES MOTOR A, (LOW FLUX DENSITY). Amp. A B C C.e.m.f.= A-(B+C) Volts at 200 r.p.m. Curve A, Fig. 52. 200Xc.e.m.f. V. '*. IR M Volts at 200 r.p.m. 5 550 3.5 3.5 543 56.5 200 (543-*- 56. 5)= 1920 10 550 7.0 7.0 536 108.0 200 (536-5-108 )=1155 20 550 14.0 14.0 522 200.0 200 (522-f-200 )= 522 30 550 21.0 21.0 508 283.0 200 (508H-283 )= 359 40 550 28.0 28.0 494 350.0 200 (494-H350 )= 283 50 550 35.0 35.0 480 412.0 200 (480-^412 )= 233 Plotting these speed and current values as a curve, a, Fig. 53, and comparing them, it is seen that speed varies greatly with current, the range being from 1920 to 233 r.p.m. with currents from 5 to 50 amperes. , THEIR ACTION AND CONTROL. In the case of series motor B, the armature and field resistance are 0.4 and i.o ohm respectively, and the magnetization-voltage curve of this machine operating at 200 r.p.m. with 5 to 50 amperes field current is curve B, Fig. 52, showing much higher flux densities than curve A of the first machine. The rated load current is 50 amperes and line pressure 550 volts, as in the case of motor A. The relations existing between current and speed can be calculated as in the pre- ceding case, the results being given in Table XII. TABLE XII. CURRENT-SPEED DATA, SERIES MOTOR B, (HIGH DENSITY). Amp. A B C C.e.m.f.= A-(B+C) Volts at 200 r.p.m. Curve B, Fig. 2. 200 X c.e.m.f. V. IR* JR.. Volts at 200 r.p.m. 5 550 2 5 543 137 200 (543-5- 137) = 794 10 550 4 10 536 235 200 (536^- 235) = 455 20 550 8 20 522 360 200 (522- 360) = 290 30 550 12 30 508 420 200 (508-5- 420) = 242 40 550 16 40 494 450 200 (494- 450) = 220 50 550 20 50 480 462 200 (480^-462) = 207 The effects of low and high magnetic flux densities upon speed of series motors at various loads are shown by comparing the two speed- current curves in Fig. 53. For motor B with high flux density the speed range is 794 to 207 r.p.m., or 3.84 : i, while it is 1920 to 233 r.p.m., or 8.24 : i for motor A, which is more than twice as great as the variation in speed with the motor of high flux density, the current change being the same, that is, 5 to 50 amperes in both cases. The type of direct-current series motor commonly employed is that with the higher flux density because of the greater economy of material and better operation with variation in load and line voltage. Comparing the speed curves of series and shunt motors, Fig. 54, it is apparent that the speed changes in series motors are due not only to resistance drop / (R a + R se ) but more especially (up to heavier loads) to increase in field strength. The IR a drop is greater in series than in shunt motors of the same rating, because the former are usually designed for intermittent service, so that the current density in the field and armature windings can be made much higher than would be approved of in shunt machines, which are usually loaded more continuously. DIRECT -CURRENT SERIES MOTORS. 103 10 15 20 25 30 35 40 45 50 3 FIG. 53. ' SPEED-CURRENT CURVES OF SERIES MOTORS, A AND B. \ KSS /Torqa 10 20 30 40 30 W TO W 90 100 Percent Rated Current FIG. 54. COMPARATIVE SPEED AND TORQUE CURVES OF IOO-H.P. SERIES AND SHUNT MOTORS. The speed for any given current, as already shown, depends upon the c.e.m.f. generated or upon V I (R a +R se )', hence the speed at any line voltage V x is obtained from the equation r.p.m. x = V x -I(R a + R se ) r.p.m. -I(R a + RJ in which r.p.m. and r.p.m.,,. are the speeds at the standard and fractional voltages V and V x respectively while I (R a + R st ) 104 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. is the resistance drop in the motor windings. When V x is less than Vj the speed corresponding to it is lower than the value ' -, since the resistance drop is then a larger part of V x than it is of V. Similarly when V x is greater than V the reverse is true. When V x is equal to / (R a + R se ) the armature will stand still, exerting torque corresponding to /. For any increase in V x above this value the speed rises proportionately. 6000 60 ZOO 3OO 4OO 5OO FIG. 55. SPEED-CURRENT AND TRACTIVE EFFORT-CURRENT CURVES OF A SERIES MOTOR. G. E. 6pC Railway Motor. Gear Ratio, 1.885; Wheel Diam., 36 ins.; Resistance of windings, 0.14 ohms. By means of the preceding speed-current equations, the corre- sponding curves (Fig. 55) of the typical 2oo-horsepower series rail- way motor (G. E. Type 690, N.Y.C. "M. U." trains) at 150 and 300 volts have been calculated from the 6oo-volt curve given by the manufacturer. Torque-current Curve. The torque of any motor varies directly with the product of armature current and field flux. Hence in a series motor the torque at low flux densities varies directly as the square DIRECT-CURRENT SERIES MOTORS. 105 of the current; that is, torque = ~KP; but as the magnetization approaches saturation the torque becomes more nearly propor- tional to the first power of the current. The torque of a series motor is independent of the voltage except for variation in hysteresis, eddy-current, friction and windage losses resulting from the change in speed with altered voltage. At low voltages and corresponding speeds, these losses are reduced and the available torque per ampere is similarly increased; at higher voltages the reverse is true. Since the hysteresis loss varies with the first power, and eddy-current loss as the square of speed, the difference in torque for any given current is greater between the 150 and 300 volt curves than that between the 300 and 600 volt curves. The per- centage difference between torque values at any two voltages in- creases with the current on account of ratio of speeds at these voltages. Fig. 55 shows the torque-current curve for the typical 2oo-horsepower series motor at 600 volts, being approximately cor- rect for 150 and 300 volts also, because the variation in the losses is small compared with the total torque. The full-load value of motor current in the case of a railway equip- ment is generally employed as the starting current. A comparison of the speed-current and the torque-current curves of a series motor (Fig. 55) shows that the maximum torque exists at the minimum speed. This is the especially valuable feature of the series motor, as maximum torque can thus be obtained at starting, which gives rapid acceleration. Further study of these curves also brings out the bad feature of the series motor, namely, that very high, in fact dangerously high, speeds may be attained by the arma- ture if the load be totally removed. Series motors should, there- fore, be either geared or directly connected to their load to prevent any breaking of the mechanical connection which is likely to occur with a belt. The torque per ampere may be called the torque-efficiency of the motor. This varies with the current, but there is a gradual reduction in its rate of increase, because the magnetic circuit approaches saturation as the current becomes larger. The " torque per ampere- current" curve of a series motor, Fig. 56, is substantially the mag- T netization curve, because T = K&I. .'. = K<$. The torque per ampere curve in Fig. 56 gives the torque in terms 106 ELECTRIC MOTORS^ THEIR ACTION AND CONTROL. of pounds pull at i foot radius from motor shaft. This value is obtained by dividing the pounds tractive effort per ampere by the gear ratio and multiplying this quotient by the radius of the wheels in feet. For example, the torque per ampere at motor shaft with armature current of 300 amperes is 3120 X 1.5 -r- 300 X 1.885 = 8.27 pounds at a foot radius. 10 O /OO 200 30O 400 50O A/nperes FIG. 56. TORQUE PER AMPERE-CURRENT CURVE OF G. E. 690 RAILWAY MOTOR. A working value of the constant K can be obtained by determin- ing the torque T at any reasonable current / and dividing this by the product of that current and the electromotive force corresponding thereto when the motor is operated as a separately excited generator at any given speed. DIRECT-CURRENT SERIES MOTORS. 107 SL & ^ /OO 200 300 40O 300 A/nperes FIG. 57. BRAKE H.P.-CURRENT CURVES OP G. E. 6gC RAILWAY MOTOR Horsepower-current Curves. The output is readily calculated from the torque and speed, since torque is expressed as pounds pull at a one-foot radius and is obtained as follows : _ 27rPLr.p.m. _ torque (r.p.m.) 33,000 5250 (12) wherein P is the pull in pounds at rim of wheel and L is the radius of the wheel in feet. If the turning moment of the motor be expressed as tractive effort 108 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. or pounds pull at the rim of the car wheel (t.e.) and the speed in miles per hour (m.p.h.), then the output is (t.e.) (m.p.h.) * h.p. 375 d3) The horsepower output for a given current varies almost propor- tionally with the voltage; thus at 150 and 300 volts the horsepower output is practically one-quarter and one-half, respectively, of the value at 600 volts. More exactly, the output at sub-voltages is a little less than the same fractional part of the output at 600 volts, since the percentage increase of torque at the reduced voltage is less than the corresponding decrease of speed. Fig. 57 shows the curves of horsepower output and current at 600, 300 and 150 volts for the 2oo-h.p. motor previously considered. FIG. 58. EFFICIENCY-CURRENT CURVES OF G. E. 6gC RAILWAY MOTOR (GEAR LOSSES INCLUDED). Efficiency-current Curves. The efficiency curves of this typical 2oo-h.p. series motor at 150, 300 and 600 volts, Fig. 58, show that the efficiency of such a motor at rated voltage is nearly constant over a wide range of speed and load, but with very small or excess- r.p.m. = (m.p.h.) 5280 I2O7T L and torque = (t.e.) L. DIRECT-CURRENT SERIES MOTORS. 109 ively large currents it falls to low values. At voltages less than normal, trie efficiency is reduced, and this reduction is more marked as the voltage is lowered. The efficiency at any load may be readily determined by any of the following formulae: Efficiency = 746 (h.p. output) watts input torque (r.p.m.) 7 watts input 1.99 (t.e.) (m.p.h.) watts input (14) 1000 2000 Tractive Effort in Pounds FIG. 5Q. SPEED TRACTIVE EFFORT CURVES OF G. E. 6pC. RAILWAY MOTOR. Speed-Tractive Effort Curves. This is the most important charac- teristic of a railway series motor in the determination of its fitness for a specified service. The speed-t.e. curves are similar in form to the speed-current curves, due to the fact that the t.e.-current curve is almost a straight line. The equation for the speed-tractive effort curve is of the form t.e. = m.p.h. + B C. (15) 110 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The constants change with the voltage, resistance and degree of magnetization of the magnetic circuit. Fig. 59 gives the speed- tractive effort curves of the typical 2oo-h.p. series motor at 150, 300 and 600 volts, the equations of which are : 12,800 At 150 volts, t.e. = 970. m.p.h. 1.82 At 300 volts, t.e. = 2 5'3 1050. m.p.h. 5.78 At 600 volts, t.e. = - - - 740. m.p.h. 16.43 The insert points as shown in Fig. 59 are derived by calculation and agree very closely with the test values over a wide range of speed. Gears and Wheels. A railway motor is usually connected to the axle of the car drivers through a single pinion and gear, but in some cases the armature is directly mounted upon the axle of the car wheels. The speed and tractive effort at the rim of the driving wheels are respectively proportional and inversely proportional to the wheel diameter, while with gearing these two quantities are dependent upon the gear ratio, which is always designed to secure speed reduction. The proper ratio depends upon the type of service to be performed, but usually lies between 2 and 5, the lower value corresponding to high-speed service. The effect on tractive effort and speed of larger wheel diameter is exactly the reverse of that obtained by increasing the gear ratio. The speed and tractive effort for any gear ratio and wheel diameter may be found for any other known conditions from the following formulae, wherein m.p.h. and m.p.h. x are respectively the known and the unknown speeds, in miles per hour, for an existing gear ratio r and wheel diameter D. The gear ratio r x and wheel diameter D x correspond to the unknown speed (m.p.h. x ). T is the torque for the known gear ratio, and T x is the corresponding unknown value. m.p.h., - m.p.h. = - --- D Dr x (17) DIRECT -CURRENT SERIES MOTORS. Ill The characteristic curves of a standard Westinghouse railway motor with gear ratio of 22 : 62 are shown in Fig. 60. This has con- siderably smaller power than the typical General Electric machine to which the preceding curves relate, the maximum current being 200 amperes for the former and 500 for the latter. WESTINGHOUSE RAILWAY MOTOR 500 VOLTS GEAR RATIO. 22 TO 62-33"WHEELS CONTINUOUS CAPACITY, 30 AMPERES AT 300 VOLTS FIG. 60. CHARACTERISTIC CURVES OF A TYPICAL RAILWAY MOTOR. b. Motor Losses. The rated output of a motor is determined by its commutation and heating limits; hence even a small reduction of the losses within the motor is of considerable importance. For example, assume the efficiency of a motor to be increased from 90 to 91 per cent, a difference of but i per cent, in which case the total loss within the motor is reduced from 10 to 9 per cent, resulting in a decrease of 10 per cent in the heat to be radiated and a probable lowering of the temperature rise by nearly 10 per cent. Thus it is seen that the rated output of a motor depends largely upon its efficiency. The per- missible output is not, however, increased 10 per cent in the above example, because the heating effect is as the square of the current. For example, when current rises from i.oo to 1.05, the heating effect is augmented from i.oo to 1.1025. On the other hand, the PR heat 112 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. is only about one-half of the total (including core losses), so that armature current and output may be raised say 7 per cent. The increased output due to rise in field current would not be great, because the flux usually approaches saturation at maximum current and may be assumed to be i or 2 per cent additional. The distribution of the losses between field and armature is of wide variation. The following tables taken from a paper by W. B. Potter give values of the loss distribution of typical railway motors.* TABLE XIII. LOSSES AT RATED LOAD IN PER CENT OF OUTPUT. Armature. Commercial Field Motor Rating, h.p. I 2 R. Total. PR. Core. Total. 38 4.70 4.00 2.37 6.37 11.07 38 4.60 3.80 4.92 8.72 13.32 50 4.20 2.10 3.45 5.55 9.75 50 3.25 2.80 4.80 7.60 10.82 50 4.33 3.36 4.17 7.53 11.86 75 3.20 2.50 2.93 5.43 8.63 125 2.48 2.40 2.12 4.52 7.00 TABLE XIV. SEGREGATED LOSSES AT RATED LOAD IN PER CENT OF TOTAL LOSSES. Armature. Ratio Commercial Field Field Loss Rating, h.p. I*R. to I 2 R. Core. Total. Armature Loss. 38 42 36 22 58 .74 38 35 28 37 65 .53 50 43 22 35 57 .76 50 30 26 44 70 .43 50 37 28 35 63 .57 75 37 29 34 63 .59 125 36 34 30 64 .55 Rating.f There are two ratings by which railway motors are commercially classified. The nominal rating of the General Electric Company is the better known. This is defined as that output which * Trans. Amer. Inst. Elect. Eng., Vol. XIX (1902), p. 179. t Standardization Rules, A. I. E. E., 1911, Appendix B, p. 30. DIRECT-CURRENT SERIES MOTORS, 113 causes a temperature rise of 75 degrees C. from a room tempera- ture of 25 degrees C. after an hour's run upon test stand with motor covers open and 500 volts at motor terminals. This rating is much in excess of the continuous service capacity of the motor, but it gives a convenient means of classification and is a severe test upon the mechanical qualities of the machine. In view of the tendency to use higher voltages, a test at 550 volts in place of 500 volts would probably more nearly approach service conditions at the present time. The continuous rating used by the Westinghouse Company cor- responds to that current which supplied continuously to the motor on a test stand will produce a temperature rise of 60 degrees C. The voltage selected is an approximation of the average value throughout the period of starting as well as running. No attempt is made to reproduce the conditions of ventilation that obtain in actual service. The instantaneous capacity of a railway motor is limited by its commutation; nevertheless in well-designed car equipments the motor should be able to slip the wheels under normal track condi- tions before the sparking limit is reached. The rating of railway motors must necessarily be largely arbi- trary because of the intermittent character and very variable conditions of such service. The only conclusive test is actual operation under the practical conditions of each particular road, which a shop test can only approximate in a general way. It is well, however, to have some nominal rating, as denned above, in order to estimate and compare the performance of different railway motors. CHAPTER X. CONTROL OF DIRECT-CURRENT SERIES MOTOR. Function of Controller. For all series motors, with the possible exception of those operating small fans and pumps, a starting device is necessary to increase gradually the voltage applied to motor terminals. By this means the starting current and acceleration are regulated, while the heating and sparking of the motor are restricted within reasonable limits. Starting is accomplished by inserting proper values of resistance in series with the motor, usually supple- mented by change from series to parallel connection of two or more motors. Rheostat Control. The simplest form of control, Fig. 51 (page 99) , is by means of series rheostat or resistance (R) which is gradually re- duced in predetermined steps until the motor terminals are connected directly across the full line voltage. This method is practically the same as the rheostat control of shunt motors, except that only the armature current is affected in the latter machine. The same objections apply, however, in both cases. These include bulkiness of rheostat, widely varying speed with any considerable change in torque and very low efficiency. For example, the loss by this control with uniform acceleration is practically one-half the total energy supplied by the line during the period of starting and thus equals the energy consumed in the motor. Moreover, the only efficient running speed is the full value. Nevertheless this method is often used on account of its simplicity, particularly in the case of motor-driven fans and pumps. Series-parallel Control. Where two or more motors or two or more windings on the same armature are to be controlled simul- taneously, certain groupings may be obtained by means of which a single motor or winding receives but a fraction of the line voltage without external resistance in circuit. Thus, two motors operating together may be connected in series with each other and in series with starting resistance, which is gradually reduced until each motor receives one-half line voltage. The motors may then be thrown in parallel with each other and in series with resistance, which is 114 CONTROL OF DIRECT -CURRENT SERIES MOTOR. 115 again cut out by steps until each motor receives full voltage. Thus there are two points of the control at which no resistance is in cir- cuit. The total energy loss during the period from starting to full parallel operation is approximately one-third of the line supply or one-half of the motor consumption. The speed of the motors when operated in series is about one-half that with the parallel connection. This series-parallel control is the method generally adopted for single cars and for multiple unit trains CONTROLLER K 12 RES. MOTOR 1 MOTOR 2 TWO MOTORS. FOUR MOTORS. FIGS. 6l AND 62. SERIES-PARALLEL CONTROL FOR 2 AND 4 MOTOR EQUIPMENTS. whenever two or more motors are operated simultaneously. The various steps of this method of control are diagrammatically illus- trated in Figs. 6 1 and 62, which are respectively for two and four motor equipments. A modification of the series-parallel control is the so-called Bridge arrangement, in which the motor circuits are so arranged that they are not opened during transition from series to parallel connections. 116 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Series, Series-parallel, Parallel Control. When the equipment consists of four motors, it is a common arrangement, especially for electric locomotives, to employ three groupings of motors in start- ing them, as illustrated in Fig. 63. At first the four motors are all in series (A), then two groups, each consisting of two motors in series, are connected in parallel (B), while' finally the four motors are placed in multiple (C). During the first few steps of each combination, series resistance is gradually cut out of the circuit. This method secures higher efficiency than the others but necessitates more car wiring than either the simple rheostatic or series-parallel control. The energy loss with this method of control during the period of acceleration is about j\ of the total supply, or about f of the motor consumption. The three efficient running points are as shown in Fig. 63, and they correspond approximately to one-quarter, one-half and full speed. Jrwoforefietif 71 fcsisfance FIG. 63. RESISTANCE CONNECTIONS OF THE THREE RUNNING POINTS OF THE SERIES, SERIES-PARALLEL, PARALLEL METHOD OF CONTROL. Field Control. During the early development of electric traction it was customary to increase speeds after the efficient running points (without external resistance) had been reached by shunting some of the field winding or by arranging the field coils in parallel com- binations (field commutation). The resulting weakening of the field, however, led to objectionable sparking at the brushes, and this means of control for railway motors was discarded. The intro- duction of interpoles or commutating poles, which maintain a commutating flux independent of the condition of the main field, but proportional to the armature current, has revived the use of field control for high-speed railway motors. The performance curves of a 35-horsepower series motor con- CONTROL OF DIRECT -CURRENT SERIES MOTOR, 117 trolled in this manner are given in Fig. 64, while Fig. 65 illustrates the various steps of such a method of speed regulation.* The control is by series-parallel connection supplemented by the increase of speed obtainable with field weakening. After starting with resistance in the ordinary way, the machines are brought into the series running condition (No. 5) with full-speed strength. Higher speeds are attained by combining the four field coils of each motor in partial series-parallel (No. 6), then in series-parallel (No. 2000 1500 1000 40 70 90 100 50 60 Amperes FIG. 64. CURVES OF 35-H.P. E. D. CO. SERIES MOTOR WITH FIELD CONTROL. 7) and finally in parallel (No. 8), thus securing four field strengths for any given armature current. The speed rises as the field current is weakened but not in equal degree. Similar combinations of the field coils are employed with the parallel grouping of the motors. Thus by this method of control eight efficient running points are obtained with a two-motor equipment. The speed range at rated load, or even at any intermediate load, is not, however, 8 to i, because the field strength does not change directly with the m.m.f., the magnetic circuit being partially saturated. It is apparent from the characteristic curves of this equipment (Fig. 64) that the speed with armature and field windings of both motors all in series, at full field strength and rated load, is about 5.2 miles per hour, while with the two motors in parallel and all field coils in parallel (weakest field condition, No. 12) the speed is about * Article by G. H. Condict, Electrical World, Vol. XLVII, 1906, p. 1088. ELECTRIC MOTORS, THEIR ACTION AND CONTROL. POSITIONS START RUN 2 Ej~[ I I I Q-^MT^HVHAArVVV O-^^^^V-T^nM-v\V HUM O-'TffiSSMVHVW^ C^^J^AV-VVVT^ViAV 4 Lj | | | | O-^^^WrAVH^^ O-^^^^V-^n^ Armature Main Field -fTTTh Resistance FIG. 65. CONNECTIONS FOR FIELD CONTROL OF SERIES MOTORS. CONTROL OF DIRECT-CURRENT SERIES MOTOR. 119 26 miles per hour, a range of 5 to i. This is a large gain over the ordinary series-parallel arrangement, which gives about 2 to i range of speed. Drum and Master Controller. For the smaller series motors such as are used for ordinary cars, hoists, pumps, fans, etc., the power circuits are made and broken in the controller by means of stationary fingers and movable contacts mounted upon a drum or cylinder. Where large currents are required, the circuits are made and interrupted by separate switches, called contactors, which are controlled electrically from a master controller and operated by a pneumatic or solenoid device. This latter form of control is almost always used where several cars are to be operated together, and from any car irrespective of sequence. The control circuits are made continuous from the first to the last coach by means of a train line and "jumpers." Hand and Automatic Starting. The rate of acceleration in starting depends upon how rapidly the operator moves his con- troller handle from the first to the last notch. Some recent types either prevent the operator from passing to the next point until the current has decreased to a certain value, or cause the controller to move (" notch up") automatically at the proper rate to a point predetermined by the operator. The efficiency of control methods may be easily calculated, assum- ing that the current per motor is maintained constant by a gradual increase in voltage at the motor terminals. This is the ideal condi- tion because it secures uniform acceleration and is closely approxi- mated in practice with automatic starting. A comparison of the three methods of control described is set forth in Figs. 66, 67 and 68. In these and the formulae accompanying them / is the cur- rent per motor, R m is the resistance, E is line voltage and T the time from start to full speed (period of acceleration) . It should be noted that the assumption made in developing these diagrams is that the time during which the motors are in series bears the same relation to CE \ - IR m 1 bears to (E IR m ). The shaded portion of the diagrams represents the losses during the period of starting with uniform acceleration by each of the three methods. 120 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Ullrf (V ^ if + ^ >^ VH ..v I &3 ^ ^ ^ 7 ^ ^ N g \ ^ "* m 2 d S 6 I 1 h I & 3 = enoA ami \ ^ K\ + ^ 1 ^H i- ^ g >~H ^H ft- fa 1 - s^ rs 3 -H H - 1 + O 1 (^ < << a - " ; "^ CONTROL OF DIRECT-CURRENT SERIES MOTOR. 121 The following values are taken from a case in actual practice and he table below is calculated from them. Line pressure = E = 600 volts. Current per motor = / = 250 amps. Motor resistance, (R a + R se ) = R m = 0.14 ohm. Time occupied in starting = T = 30 sec. Number of motors = 4. An examination of the results set forth in table below brings out the great advantages of the series, series-parallel, parallel control compared with the other two- methods, not only as regards the greater speed range, but also higher efficiency of the equipment during the period of starting. COMPARISON OF DIFFERENT METHODS FOR STARTING SERIES MOTORS WITH UNIFORM ACCELERATION. Items. Rheostat ic Control. Series-parallel Control. Series, Series-parallel, Parallel Con- trol. Supply for 4 Motors, kw.-sec 18000 13800 12900 Motor Consumption, kw.-sec 9500 9500 9500 Total Controller Loss, kw.-sec 8480 4240 3340 Relative Controller Losses 1.00 0.50 0.39 Relative Input to whole Equipment. . Total Efficiency 1.00 0.53 0.77 0.69 0.72 0.74 Constant-current Series Motors. Although not used commer- cially at present, this type of machine is of such historical importance, and differs so radically from the constant-potential motors now universally adopted, that it deserves some attention. Electric motors were first used in considerable numbers about 1886 or 1887. Then, and for some time afterward, there were constant- current arc lighting circuits in many smaller towns and in portions of large towns where constant potential supply was not available. In* such cases motors were operated on these circuits in series with arc lamps. The connections are represented in Fig. 69, A being the armature and F the field circuit of such a motor, the windings of which were designed to carry the constant current (usually about 10 amperes) continuously. Hence, no starting box was required, the motor being introduced directly into the circuit by the cut-out switch S. 122 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. A plain series motor with constant current exerts constant torque (at full value), because T = K&I, in which field flux $ and armature FIG. 69. CONNECTIONS OF CONSTANT-CURRENT SERIES MOTOR. current / are both constant. Unless the load happens to equal this torque some means for adjusting the motor torque to the load must be provided, otherwise the motor will either stop or race. The usual method was to short-circuit more or less of the field winding, or to shunt it with variable resistance by means of a switch con- trolled by a centrifugal governor mounted on the motor shaft. A diminution in load below the motor torque would produce a rise in speed, and the governor would then reduce the field strength and torque to the proper value. The speed of these constant-current motors was also regulated by shifting the brushes or by moving the armature out of the field. The serious objection to the operation of motors on constant-current series circuits is the high e.m.f. of the latter, usually from 3000 to 6000 volts. Even the potential difference between the motor terminals is about 100 volts (1000 watts -r- 10 amperes) per horsepower, which would amount to 1000 volts for a lo-horsepower machine. All of this except 30 or 40 volts drop in the field winding (iR se ) would exist as a potential difference between the brushes, being dangerous to persons and high for com- mutation. To supply 50 horsepower by this system would involve CONTROL OF DIRECT-CURRENT SERIES MOTOR. 123 a potential difference of 5000 volts between the two conductors, whether using one motor or several in series. The constant-current motor, however, possesses the great advantage that the armature may be stopped indefinitely with full current flowing and no injury results. In fact, its temperature is practically the same as at rated speed, because the absence of armature-core losses when standing still usually makes up for decreased ventilation. For further information concerning series motors see the following : AMERICAN ELECTRIC RAILWAY PRACTICE. Herrick and Boynton. 1907. DIE GLEICHSTROMMASCHINE. Vql II, p. 630. E. Arnold. 1904. ELECTRIC MOTORS FOR RAILWAY SERVICE. W. B. Potter, Trans. A. I. E. E., Vol. XIX, p. 170. ELECTRICAL TRACTION, Vol. I. Wilson and Lydall. 1907. ELECTRIC TRACTION FOR RAILWAY TRAINS. E. P. Burch, 1911. ELECTRIC JOURNAL, Vol. I, p. 479; Vol. Ill, pp. 14, 525; Vol. IV, p. 454. ELECTRIC RAILWAY ENGINEERING. Parshall and Hobart. 1907. ELECTRIC RAILWAY ENGINEERING. C. F. Harding. 1911. ELECTRIC TRACTION AND TRANSMISSION ENGINEERING. Sheldon and Hausmann, 1911. ELECTRIC RAILWAYS. McGraw Co. 1907. TRANS. A. I. E. E., Vol. XXIV, p. 65; Vol. XXVI, p. 1407. STANDARD ELECTRICAL HANDBOOK, pp. 456, 828, 841. McGraw. 1908. ELECTRICAL ENGS.' POCKET BOOK, H. A. Foster, pp. 614, 753, 760. D. Van Nostrand Co. 1908. CHAPTER XI. COMPOUND-WOUND MOTORS. THERE are two classes of compound-wound motors. In one case the ampere-turns of the series coil reinforce the shunt-field ampere- turns, producing the cumulative-compound motor; in the second case the series ampere-turns oppose those of the shunt winding, produc- ing the differential-compound motor. The former machine is com- mercially called the compound motor, the latter being known as the differential motor. The connections of a compound-wound motor are shown in Fig. 70. FIG. 70. CONNECTIONS OF COMPOUND-WOUND MOTOR As already noted, an ordinary shunt motor is usually operated with constant potential at the field-circuit terminals, so that the flux is practically constant at all loads (except for a slight effect of armature reaction which reduces this flux by a small percentage at rated arma- ture current and torque). Hence the torque increases practically in direct proportion to the armature current. In a cumulative-com- pound motor the conditions are somewhat different, since the field becomes stronger with increase in load, due to the magnetizing action of the series coil, so that the torque increases more rapidly than the armature current. That is, the torque is proportional to the arma- ture current and to the field flux (due to the sum of the series and V shunt excitations) , that is, torque = KI a (F -f F/ a ) . At the same R sh 124 COMPOUND-WOUND MOTORS. 125 time, since the field flux rises with increase in load, the speed decreases, due not only to I a R a effects but to the relation r.p.m. = . This decrease in speed becomes relatively less as the load n<& 2 p increases, because the field flux rises less rapidly as the magnetic density approaches saturation, further reduction in speed being caused solely by the armature and series field IR drop. Hence the compound motor combines the characteristics of the shunt and the series types, having a speed not extremely variable under load changes but developing a powerful starting torque. The stronger the shunt-field flux (no-load flux) the more nearly the action corre- sponds to that of the shunt motor; the weaker the shunt field the more closely does the machine resemble the series motor. In fact, two forms of cumulative-compound wound motors are commonly manufactured, one with a low ratio of series field m.m.f. to shunt field m.m.f. varying from 10 to 25 per cent at rated load; the other having a series m.m.f. equal to 50 or 75 per cent of the shunt field m.m.f. at rated load. The characteristic curves of a 4o-horsepower 22o-volt compound- wound motor having a field m.m.f. at rated load made up of 80 per cent shunt and 20 per cent series excitation, are shown in Fig. 71. The speed increase of this machine between rated load and running free is about 35 per cent. It is interesting to compare the curves of Fig. 71 with those given in Fig. 72 as the effect of weakening the shunt excitation and strengthening the series field becomes at once apparent. The curves in Fig. 72 are for a motor similar to that of Fig. 71, the ratio of excitations being now, however, 70 per cent series and 30 per cent shunt, there being just enough of the latter to prevent the motor from racing when load is entirely removed; never- theless the motor speed varies from 590 to 1010 r.p.m. The former machines are employed extensively in shop practice where a motor may be required to start under heavy load but must maintain an approximately constant speed after starting, or when the load is removed. The heavily compounded motor is employed where powerful starting torque and resulting rapid acceleration are needed with speed not too widely variable under load changes as in elevator, rolling-mill and similar service. In addition to the comparatively constant speed required after the running condition has been attained, elevator service has an additional requirement, namely, the motor 126 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. H.P. 90 B.H.P. Eir Torque E.P.M 800 500S Is i - g 200 100 25 50 75 100 125 150 175 200 Amperes FIG. 71. CHARACTERISTIC CURVES OF A 4O-H.P. 2O PER CENT COMPOUND WOUND MOTOR. To 600 -g a 400 25 50 75 100 125 150 175 200 Amperes FIG. 72. CHARACTERISTIC CURVES OF A 4O-H.P. 80 PER CENT COMPOUND WOUND MOTOR. COMPOUND-WOUND MOTORS. 127 must have the series field-winding cut out or short-circuited at full speed to avoid reversal of the machine should the elevator be heavily overbalanced. This would cause the motor to speed up, and a series winding relatively powerful compared with the shunt winding would reverse the excitation and result in a burn-out. Well-designed shunt motors have a speed regulation within 5 per cent ; series motors have a speed change of almost unlimited ratio from no load to full load or wee versa, while compound-wound motors have speed variation from 12 to 100 per cent above that corresponding to the rated torque, depending upon the ratio of shunt to series field m.m.f. as set forth above. The speed control employed with compound motors may be any of the various methods explained in connection with the shunt motor, though when used for elevator service the control is generally entirely rheostatic, with the final cutting out of the series winding after acceleration has ceased. The Differential Motor. In this class of compound -wound motors the m.m.f. of the series winding opposes that of the shunt winding and thus weakens the field with increase of load. The object is to compensate for I a R a drop and thus maintain the speed constant at all loads. These machines will operate satisfactorily if not overloaded, but with an overload the field flux becomes so much reduced that the torque is not sufficient to maintain rotation, hence the c.e.m.f. falls to zero and a burn-out of the armature winding would result, if the machine is not protected by fuses or a circuit-breaker. Even with this customary protection it is very troublesome to have the fuses blow or circuit-breaker open every time an overload happens to occur for a second or two. A "still further objection to this motor is its low starting torque due to weak- ening of the field with heavy currents. In fact, the series circuit being of considerably lower inductance than the shunt circuit, the motor if quickly started under load, would tend to rotate in the wrong direction. Hence the series winding should be automatically short- circuited while starting the machine. The constant speed secured by differential winding may be obtained from a specially designed shunt motor with exaggerated armature reaction, also by brush lag, by an excessive number of armature turns, or by proper use of interpoles, without the serious 128 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. weakness of the differential motor. For most practical purposes, however, the speed of a well-designed shunt motor .does not vary to an objectionable extent, so that this special regulation is not needed. The differential machine is, therefore, rarely used. Resume of Characteristics of Direct-current Motors. Shunt motors have a speed varying only slightly with load changes, and good starting torque. They are employed where the speed should be approximately constant and also where the speed may be adjusted by some of the methods described. Series Motors. These machines are employed when powerful starting torque and rapid acceleration are demanded, also when speed must be automatically adjusted to load, but they have no customary or even limit of speed with variable loads. Compound-wound motors have a speed decreasing considerably (12 to 50 per cent) with load, but are capable of exerting some of the powerful starting torque characteristic of series motors, and are employed for work requiring that capability and a speed not exces- sively variable under load changes. They may also be safely belted to their work, while series motors should never be, but must be positively connected to prevent racing. Differential motors may be designed to give an almost absolutely constant speed under all load changes within their rating, but beyond this the motor is too likely to be stalled ; the starting torque is also small. This motor is no longer used in practice, improvement in the design of shunt motors having secured nearly constant speed without the objectionable features of the differential type. PART III. ALTERNATING- CURRENT MOTORS. CHAPTER XII. CLASSIFICATION AND HISTORY. THE salient facts in the historical development of the electric motor were briefly stated in Chapter I. These related chiefly to direct-current types because the only source of electrical energy commercially available up to 1880 or thereabouts was the voltaic battery. Furthermore, the electrical generating plants employing dynamo-electric machinery in operation prior to 1890 were almost all designed to supply direct currents only. Hence the commercial progress and as a natural result the scientific advance of the a. c. motor were held back, while the d. c. machine received much more attention. Nevertheless, during all this time experiments were being made and ideas evolved which led up to the various a. c. types now known, these being quite numerous and differing widely from one another. On the other hand, d. c. motors are practically all of one species, which may be defined as embodying essentially a drum armature with commutator in a bipolar or multipolar field, and is the same as the d. c. generator except for mere differences in form to suit particular conditions, as, for example, those of electric railway service. While there is only one important kind of d. c. machine, we have the following distinct Types of Alternating- current Motors. 1. Synchronous Motor, being an ordinary a.c. generator reversed in function. 2. Induction Motor, having armature winding closed upon itself or through a local circuit not connected to source of supply and without commutator. 3. Repulsion Motor, having armature with commutator and brushes connected through local circuit wherein current is induced. 4. Similar to d. c. motor, having armature (with commutator) and field winding both connected to supply circuit, hence often 129 130 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. called conductive motor in contradistinction to the two foregoing types. 5. Shaded Pole or Creeping Field Motor , in which the phase of the flux is retarded in parts of the field. In addition to the above classification a further differentiation of a. c. motors is based upon the fact that either of the first two types may be operated by single, two, or three phase currents. The other three types are supplied with single-phase current, and even when fed from polyphase circuits each motor is connected to one phase only. It is to be noted that the conductive type, No. 4 of the above classification, is the only one capable of being operated either as a d. c. or as an a. c. motor. With the entire field as well as armature iron laminated and with certain features to limit sparking, as explained later, the same machine will run well with either direct or alternating current; in fact many electric locomotives are so operated on the New York, New Haven and Hartford Railroad. The other forms, types Nos. i, 2, 3, and 5 of the foregoing list, are incapable of operating with d. c. supply. It is true that type 3, the repulsion motor, has a commutator and is structurally similar to the d. c. machine; nevertheless it must be differently connected and current supplied to its armature by conduction instead of in- duction, in order that it may run as a d.c. motor. Hence it would no longer belong to the repulsion class. It is evident from these statements that we have only a single important type of d. c. motor or generator, while there are five types of a. c. motor, one of which is the same as the d. c. machine. The only other kind of dynamo that has been used for d. c. genera- tion is the unipolar or homopolar machine. This can also be operated as a d. c. motor and as an a. c. motor. Therefore the latter seems to include all cases of the former, in addition to which it has at least four distinct forms of its own. The complete history of a. c. motors would include, therefore, an account of the development of each of these five types, which for the most part owe their existence to different times and dif- ferent inventors It is sufficient at this point to indicate briefly the principal historical, facts, to be supplemented later by the dis- cussion of the individual types, which contain references to invent- ors, authors, patents, and articles. ALTERNATING-CURRENT MOTORS. 131 The history of the synchronous motor is essentially similar to that of the a. c. generator because the two machines are structur- ally identical; in fact the same machine may be used equally well for either purpose. This reversibility was not fully appreciated and applied by those who first brought out and experimented with electric generators and motors, so that they were designed quite differently and their development was to a large extent independ- ently carried forward. Nevertheless, any improvement in the generator was equally applicable to the motor, and vice versa. It is also a fact, as stated in Chapter I, that the reversibility of the electric generator began to be understood many years ago. For example, Pacinotti in 1860 invented his ring armature for use in motors as well as generators and later others described machines to perform both functions.* In 1868 Wilde while operating alter- nators in parallel observed the fact that the armature of one of them was caused to oscillate as a motor when fed with current generated from another. f Hopkinson in 1883 published the theory of this phenomenon and showed that continuous rotation of the motor could be maintained.! In conjunction with Prof. W. G. Adams he soon verified these conclusions experimentally with three De Meritens alternators of several horsepower each, at the South Foreland lighthouse, the results being given in a paper by Adams on "The Alternate Current Machine as a Motor." The polyphase generator or motor may be regarded as a com- bination of two single-phase machines. At the same time the poly- phase synchronous motor has the practical advantage that it is self- starting, while the corresponding single-phase motor requires some auxiliary means to bring it up to synchronous speed. This advan- tage is a fortunate incident, however, because polyphase systems owe their great importance not to this fact but to their capabilities of operating induction motors and their economy of material in trans- mission lines as well as in generators, etc. Early in 1887, Charles S. Bradley invented a machine to operate either as two-phase generator, * Siemens, Brit. Patent No. 3134 of 1878. Deprez, Brit. Patent No. 4128 of 1887. Dredges, Electric Illumination, London, 1882, Vol. I, p. 69. f Philosophical Magazine, January, 1869. t Lond. Inst. Ci^. Eng., 1883. Soc. Teleg. Eng. and Elec., November 13, 1884. 132 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. motor, or converter, but this invention is considered more fully under the following heading. The evolution of the induction motor is directly related to that of polyphase currents and the production of rotary magnetic fields by means of such currents. Many pieces of apparatus have been devised and physical facts noted which have contributed to the advance in this direction. Arago * in 1824 observed the retarding effect upon the swinging of a compass-needle produced by sur- rounding it with a copper ring. He also deflected a suspended mag- netized needle by motion of a copper disk immediately below it, and with more rapid motion he caused the needle to rotate continuously but at a lower speed than that of the disk. These phenomena are basic in relation to the induction motor, being due to the force set up between a magnet and a conducting body when they are moved with respect to one another so that electric currents are induced in the latter by cutting the magnetic lines of the former. It was not, however, until Faraday's discovery of magneto-electric induction in 1831 that the true explanation of these phenomena was forthcoming. Walter Bailey read a paper before the Physical Society of London on- June 28, 1879, entitled "A Mode of Producing Arago's Rota- tions," and exhibited a model in which a copper disk was caused to rotate by progressive shifting of magnetism among four fixed elec- tromagnets, by throwing on and off as well as reversing through a revolving commutator the current obtained from two primary batteries. Thus the Arago effect was produced without bodily moving the magnet, or in other words, it was what is now known as the rotary field. In 1880 Marcel Deprez presented a paper before the Societe Francaise de Physique describing a motor which operated by two-phase currents. It was, however, of the synchronous type, and is only interesting as a step of progress in this direction, but in 1883 he announcedf an important theorem on the production of a rotary field by the combination of two alternating magnetic field's differing in phase by one quarter of a period. Deprez was the first to appreciate that this phenomenon is analogous to the mechanical production of rotary motion by the combination of two forces (or cranks) acting at right angles, and he was also the first to work out the theory of the magnetic case. * Annales de Caimie et Physique, XXVII, 363; XXVIII, 325; XXXII, 213. t Comptes Rendus, Vol. II, p. 1193, 1883. ALTERNATING-CURRENT MOTORS. 133 A number of inventors had prior to that time constructed or published descriptions of generators for producing polyphase cur- rents; for example, Wheatstone, Gramme,* Cabanellas,f and others. The next important contribution was that of Professor Galileo Ferraris, who in 1885 built a two-phase motor having four poles, two of which were excited by one alternating current and the other two by another alternating current differing in phase, the rotary field thus produced causing the armature to revolve, without any electrical connection to the latter. This motor was not exhibited till 1888, on March i8th of which year Ferraris also read a paper before the Turin Academy in which he set forth the geometric theory of the rotary field and described experiments illustrating the same. He pointed out the fact that a motor armature in which the current is generated by induction must necessarily rotate less rapidly than the field, or in other words, there must be a slip. He employed armatures of iron, copper, as well as of mercury, and suggested that a. c. measuring instruments could be made in accordance with this principle. He also explained how two-phase currents could be obtained by dividing an a. c. circuit into two branches, one induc- tive and the other non-inductive, now known as the split-phase connection. Charles S. Bradley on May 8, 1887, filed an application for U. S. patent (^0.390,439) in which he clearly showed and described a two-pole generator having a Gramme armature tapped at four equi- distant points by connections to four collector rings. This machine generated two-phase currents, one of the objects stated being to obtain larger output by reason of the fact that one current is at a maximum when the other is zero and vice versa. This is one of the great advantages of polyphase apparatus of all kinds, which are usually capable of giving more power than single-phase apparatus of equal weight. Bradley also stated that his machine could be used as a motor if supplied with two-phase currents and that it would give out direct currents if fed with alternating currents, or conversely; that is to say, it was what is now called a rotary converter. In fact this was the basic and controlling patent on that machine in the United States. Bradley in another U. S. patent (No. 409,450) issued August 20, 1889, describes a similar machine with three * British Patent No. 953 of 1878. | British Patent No. 200 of 1881. 134 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. armature connections to generate three-phase currents or to operate as a motor when fed with such currents, which is nothing less than a three-phase system of power transmission, including also the rotary converter to supply d. c. railways, arc lamps, storage batteries, and other electrolytic apparatus. Nikola Tesla in October, November, and December, 1887, filed applications for U.S. patents which were issued in May, 1888, as Nos. 381,968, 381,969, and 382,279, setting forth a generator to produce two-phase currents, connected to a motor in which a rotary field was developed thereby and acted upon an armature of iron to cause it to revolve. It is interesting to consider wherein Tesla's work differed from the early contributions which have just been pointed out. Arago's disk and similar apparatus prior to that of Bailey were in no sense electric motors, because the motion of the disk was merely the result of bodily rotating the magnet by hand or by a belt. The device of Bailey was an induction motor, because currents supplied to its stationary magnets set up a rotary field which induced currents in and caused the revolution of its armature. These currents were not truly alternating, however, being merely direct currents from two batteries which were reversed by a commutator turned by hand. Furthermore, what Bailey exhibited in 1879 was on ty a small model with a disk about 2| inches in diameter, incapable of exerting any appreciable power, its design being wholly unadapted to practical use. On the contrary, Tesla particularly describes the use of a true " alternating current, each impulse of which involves a rise and fall of potential " in order that " the progression of the poles will be con- tinuous and not intermittent " as in the case of simple reversed currents. He also points out " the practical difficulty of interrupt- ing or reversing a current of any considerable strength." The draw- ings and specifications of these Tesla patents set forth machines which are evidently intended to be used as practical motors. The theory of the rotary field, published by Deprez in 1883, gave a definite mathematical basis for this important physical principle, but he did not embody it in a concrete motor, and could not therefore have obtained a patent for his results, original though they were, since a principle unapplied is not patentable. On the contrary, Ferraris did work out not only the theory involved but also constructed motors in accordance therewith. In this country, however, he labored under the legal disadvantage ALTERNATING-CURRENT MOTORS. 135 that he could get no benefit for what he did prior to his printed publications, while Tesla could go back to his earliest notes, experi- mental work, and private disclosures to others. This is the one respect in which a foreigner's rights are not equal to those of an American citizen in the eyes of the patent law. It is also true that Ferraris did not appreciate the great practical value of his inven- tion, but this is often the case even with the best ideas until they are applied and a demand created. In itself this would not invali- date his patent rights, especially as we have seen that he actually built working induction motors operated by polyphase currents and suggested the applicability of the same principle to a. c. meas- uring instruments. It would seem, therefore, that Ferraris had good moral claims to the credit of the invention, but in this country was barred legally from, using the earlier evidence in his favor. Bradley in his U. S. patent No. 390,439, already cited, which was applied for May 7, 1887, did not set forth or claim the induc- tion motor, but he clearly showed and described a machine to serve as a generator of two-phase currents, as a rotary converter, or as a two-phase synchronous motor, thus including all the im- portant elements of a polyphase system except the induction motor. It has already been stated also that Bradley's U. S. patent No. 409,450 fully describes a three-phase generator, converter, and synchronous motor. The earliest application for a patent by Tesla setting forth a three-phase generator and motor was filed Oct. 12, 1887. Even after the polyphase system and induction motor had been made known to the public during 1888 and 1889 by the patents and papers of Ferraris, Bradley, and Tesla, it required several years of experiment and design by many engineers, involving much labor and expense, before this type of motor became a really practical success. In fact, this cannot be said to have occurred until about 1894. Since that time still further improve- ment has been made in efficiency, higher power factor, economy of material, etc., as well as in auxiliary starting and regulating devices. No one is directly credited with having invented the alternating- current series motor. The first mention of the possibility of such a machine was apparently made by Alexander Siemens before the British Institution of Electrical Engineers in 1884. At the same 136 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. time he indicated the advisability of laminating the entire magnetic circuit.* It was not, however, until about 1890 that this idea was applied practically when the simple a. c. series type was manufactured quite extensively for use as small fan motors. During the early nineties, among others, Rudolph Eickemeyer and C. P. Steinmetz experimented with large-size a. c. series railway motors, but they did not meet with much success on account of the high frequency of the current usually employed at that time. Interest in this type of motor lapsed thereafter, on account of the development of the single -phase induc- tion motor, until G. B. Lamme of the Westinghouse Company pro- duced a more practical machine in 1902, upon which engineers again became much interested and many modifications were devised. The invention of the repulsion motor is generally credited to Prof. Elihu Thomson, who discovered the physical fact that any conducting body tends to be repelled by a magnet excited by alter- nating current. This phenomenon and various experiments illus- trating it are described by him in a paper read May 18, 1887, before the American Institute of Electrical Engineers on " Novel Phe- nomena of Alternating Currents."! Professor Thomson in this paper also showed how to apply the principle and thus obtain a new type of electric motor. In this machine only a portion of the armature coils are short-circuited at any given time, that is, those moving away from the poles. The form of motor now known as the repul- sion type embodies an armature having all of its coils short-circuited. This latter arrangement was first described by Professors Anthony, Jackson, and Ryan in their U. S. patent No. 389,352, issued Septem- ber n, 1888, the invention having been made in 1887. Neither of these forms of repulsion motor was considered to have much practical importance until 1902 or later, when the latter was experimentally tried and its use advocated for electric railway service by the General Electric Company and others. J * Journal British Institution of E. E., p. 527, Vol. XIII, 1884. t Transactions, Vol. IV, p. 160. U. S. Pat. No. 363,185 of 1887. t Papers and discussion by Slichter, Steinmetz, Blanck, and others in Trans. Amer. Inst. Elect. Eng., Vol. XXIII, pp. i-ioo, January, 1904. CHAPTER XIII. THE SYNCHRONOUS MOTOR. THE synchronous alternating-current motor is merely an inverted alternator; that is, the same machine may in general be used as a generator or motor. The simplest of this type is the single-phase machine, and a study of its characteristics will also explain the action of the corresponding polyphase motors. For example, a two-phase motor may be regarded as a combination of two single-phase machines. There is, however, the important practical fact that the single-phase synchronous motor is not at all self-starting (unless provided with special starting device), while the polyphase syn- chronous motor is self-starting without load, in which limitations both differ from the majority of other electric motors. It should also be noted that this type of motor whether single or polyphase requires a direct-current supply for field excitation. Not Self -starting for Following Reasons. The rotation of the armature of a direct-current motor is due to the fact that an uni- directional torque is exerted between the armature and field. Any increase in load tends to diminish its speed and counter e.m.f., which allows a greater armature current to flow, producing a correspond- ing increment in the driving effort of the motor. In the case of a synchronous motor we have the following con- ditions: a field excited by direct current and an armature supplied with alternating current. The first condition provides a field of fixed magnetic polarity, the second gives an armature the current in which is alternating, therefore the direction of rotation also tends to alternate. (Fig. 73.) If, however, the motor could be brought to such speed (by external means) that the half of the armature represented above in Fig. 73 would be below after the current reversed, a fixed polarity in space would result, because the top of the armature would always be of N polarity and the lower part of S polarity, producing a torque in one direction and therefore con- tinuous rotation. For this to occur, the armature must, however, revolve one-half a turn in the time occupied by one alternation of 137 138 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. current, and a full turn in the duration of one cycle of current. In other words, the armature _must generate a counter e.m.f. of the same frequency as the applied voltage. When this condition is ful- filled the motor is said to be operating in synchronism, hence the TIG. 73. VARIATION OF ARMATURE POLARITY. term synchronous motor. It is apparent that the speed of this motor must remain constant, since the torque must be unidirectional to maintain rotation, so that the speed of any synchronous motor is fixed by its number oj poles and the frequency of the applied voltage; i.e., periods per sec. X 60 speed in r.p.m. = : : (18) pairs of poles The polyphase synchronous motor is self-starting without load, because the polyphase currents set up a rotary magnetic .field in the surface of the armature, which reacts upon the field magnet to pro- duce mechanical rotation. The circuit of the field winding, however, must be open, because the rotary field would not be sufficiently power- ful in the presence of the field flux. After the rotary member Ts up to speed, the usual d. c. field excitation is established. To prevent the development of excessive voltage in the field coils, they are not connected together in series until synchronous speed is nearly attained. It is also desirable when thus starting synchronous motors to use less than their normal working voltage, which may be obtained from a transformer or autotransformer. Action of Synchronous Motor under Varying Loads. As shown above, the motor jmust operate at a definite speed on a circuit of given frequency, or not at all. The field strength being also con- stant, the effective counter e.m.f. of the motor is of constant value; therefore it would seem that with constant applied voltage, which is the practical condition, the armature current could not automat- SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 139 ically increase to enable the motor to exert more torque in order to carry additional load. The peculiar action which occurs and gives variable torque is explained as follows: Consider two ordinary single-phase alternators M and G driven by independent prime movers, but with their armatures electrically connected in series and jointly furnishing power to an external load Q (Fig. 74). The FIG. 74. ALTERNATORS CONNECTED IN SERIES. e.m.f. of the system (E r ) will under this condition be E m + E as shown by the wave and vector diagrams A and B, respectively (Fig. 75). Since there is inductance in the armatures and line, the or E FIG. 75, A AND B. ALTERNATORS IN SERIES WAVE AND VECTOR DIAGRAMS E.M.F.'S IN PHASE. current / will lag behind the resultant e.m.f. or E r by an angle 0. If the load should change or the steam pressure vary, the engine 140 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. governors would tend to maintain constant speed ; but one would naturally act before the other. This difference, however small, would cause one machine and its e.m.f. to fall behind, so that its phase angle with respect to the current would be less, its load there- fore greater, and the power required to drive it correspondingly increased. The converse is true of the other alternator, the conse- quence being that the former continues to drop back in phase until it is about 180 degrees behind the other machine. Referring to the vector diagram B in Fig. 76, let us consider exactly what occurs. KG. 76. PHASE RELATIONS BETWEEN ALTERNATORS IN SERIES, SWINGING OVER TO PARALLEL CONDITIONS. OEg represents the e.m.f. of the lagging machine and OE m that of the other, so that OE r is the new resultant e.m.f. and OI the new current position, which maintains the same phase relation with E r as before, because the resistance R and inductance L of the circuit have not changed. Let (f> m and fa represent the new phase displacement of E m and E g respectively with reference to the current. The load on each of the alternators is now EmI cos m and E g l cos (f> g respectively; the angle fa being less than (j) m , the load E g l cos fa on the engine that drives the machine G is greater than that on the engine driving the machine M , hence the latter tends to run faster while G will fall off in speed. The load on G increases more and more and the SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 141 angle between E m and I becomes greater until it passes through 90 degrees, after which cos $ m has a negative value, so that the work done by alternator M is negative, that is, it is operating as a motor, the phase relations being represented in Fig. 77. Hence the opera- tion of two or more alternators in series is a condition of unstable equi- librium, unless they are positively connected or driven from the same source of power so that any speed change is common to both; other- FI3, 77. SYNCHRONOUS MOTOR, PHASE RELATIONS BETWEEN LINE VOLTAGE CUR- RENT AND MOTOR E.M F. wise, at the least variation in load or speed, they will instantly fall out of step and tend to pass into the condition of opposition or 180 phase relation. On the other hand the parallel operation of two or more alternators is a stable one. If one machine tends to speed up, its voltage increases and thus it would supply a larger current, carry a heavier load and be compelled by the action of the engine governor to slow down, while the under-loaded machine would tend to speed up, thus equalizing conditions. This statement is a general one corroborated by the fact that a.c. generators are successfully run in parallel in thousands of commercial plants. On the other hand practical difficulties arise in some cases of parallel operation, because of slight differences in angular velocities due to " hunting " of governors or other causes, which may throw the machines out of synchronism. If, after the alternator M reaches the i8o-degree phase relation with respect 142 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. to the other machine, we disconnect its mechanical driving power, it would continue to operate as a motor, provided the current fed to its armature be sufficient to supply copper losses and stray power torque. If this is not enough, armature M will tend to lag a trifle, causing the resultant e.m.f. to increase so that more current flows. This increases its torque sufficiently to maintain rotation, unless the dropping back of M should be so great in duration or phase that the synchronous relation is broken. The preceding facts may be summed up as follows: The ability of a synchronous motor to carry a variable load is due to the^ phase shifting of its e.m.f., which action, taking the place of the speed changes of other types of motors, alters the resultant e.m.f. so that the armature current automatically adjusts itself to the load. Starting and Synchronizing of Synchronous Motors. The single- phase synchronous motor is not self-starting, as already explained, and requires some auxiliary motor to bring it up to synchronous speed and into proper phase relation before it can be properly con- nected to the supply circuit. Such auxiliary starting device may be a series, repulsion or induction (split-phase) motor, or if the direct- current field exciter is large enough, it may be used as the starting motor, being supplied with current from a storage battery, which at that time also furnishes the main-field exciting current. Small syn- chronous motors are often constructed with the starting device as an integral part as follows : The Armature core is ^provided with an additional winding and commutator, which, connected in series with an extra winding on the field cores, makes it possible to start the machine as a series motor. The commutated armature wind- ing is connected across the main field after synchronous speed is attained and the main armature is connected to the a. c. supply lines; thus the machine becomes self-exciting. Polyphase synchronous motors are self-starting, with about 10 to 15 per cent rated load torque, through the development of a rotary field by the currents in the armature windings, which, acting upon the polar faces or grids set in them, drags the rotor around. To start in this manner, the field circuit is opened, and the armature supplied with approximately rated load current at about one-half rated voltage through a transformer or compensator. This causes SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 143 the revolving member to rotate at a speed approximating that of synchronism. The operator then closes the field circuit, thus locking the rotor into step at synchronous speed, after which the armature is supplied with current at rated voltage and the machine is ready to carry its load. It is possible, if the pole faces are solid or provided with dampers, to start up with the field circuit closed, but this takes about twice as long and a somewhat larger current to synchronize. This method of starting may, however, especially if the motor be large with respect to the generators, cause serious voltage fluctuations, and it then becomes desirable to use some other starting devices, for example, an auxiliary machine as already stated in the case of the single-phase synchronous motor. A new method of starting synchronous motors has been proposed by Dr. Rosenberg, which also employs an auxiliary motor. It differs, however, from the earlier methods in the fact that the stator winding of the starting motor is connected in series with the stator winding of the main motor. This method may be designed to give almost any desired starting torque and automatically synchronizes the machine.* f This self -synchronizing method is particularly adapted to use in connection with rotary converters, as the rotary always " pulls in " with the correct polarity. It is impossible with ordinary mechanical speed-measuring instruments to determine the approach to synchronous speed as accurately as is necessary for the safe connecting of synchronous motors to the line. Furthermore, the phase relations of the line and motor voltages must also be correct. There are, however, simple electrical methods of determining definitely when agreement in frequency and proper phase relations exist, the simplest of these being the lamp methods, one of which follows : In Fig. 78 L and M represent respectively single-phase line terminals and a single- phase synchronous motor, which can be connected by the double pole switch S. Two synchronizing lamps / and /' are connected respectively, as shown, across the two gaps in the circuit, con- trolled by the switch S. By means of the auxiliary starting motor EF, the synchronous machine M is brought to approximately the * British Pat. No. 9644 of 1912. t Journal I. E. E., , London, Vol. 51, page 62, April, 1913. 144 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. proper speed. With the rotor member of the motor stationary, or when its frequency differs greatly from that of the circuit, the alternations of current and the corresponding flickering of the lamps /, l f are too rapid for the eye to detect, but when the motor frequency varies only slightly from that of the line, whether higher or lower, the synchronizing lamps will glow for one moment and be black the next. The smaller the difference in frequency the ' 511- j^ r^i * - L *" E ) F n 1 ' FIG. 78. CONNECTIONS FOR SYNCHRONIZING SINGLE-PHASE MOTOR. (LAMP FILAMENTS BLACK.) less rapid the flickering. At the instant that the voltages are opposite in phase, and of equal value, there will be no current through the lamps; but when the voltages are in phase their full sum is applied to the lamps, which then glow at their maximum brilliancy. When the flashing becomes very slow the motor may be connected to the line by closing the switch S at the instant that the lamps cease to glow. If the motor continues to operate properly, its field strength may be adjusted so that the line current will be small after the auxiliary motor or starting-up device is disconnected. It is better to connect the motor to the line as it ap- proaches exact synchronism rather than when it is departing from it; that is, the main switch S should be closed the instant the lamps cease to give light. Owing to the fact that incandescent lamps do not glow with less than 30 or 40 per cent of their rated voltage, it is impossible to determine exactly the minimum voltage difference which is the proper condition for connecting a synchronous machine, and thus the current passing between the machines and the line may be rather high upon closing of switch 5. To avoid the danger of SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 145 such rush of current, the lamps may be diagonally connected, that is, between G and F and between E and H (Fig. 79), in which FIG. 79. CONNECTIONS FOR SYNCHRONIZING SINGLE-PHASE MOTOR (MAXIMUM LIGHT). case they glow at full brilliancy when the phase relation is correct, so that this condition is much more definitely shown. The lamps may also be replaced by voltmeters, which if connected as in the first instance would indicate zero voltage and in the latter case show full voltage. The connections of the synchronizing lamps for a three-phase circuit are similar to the preceding, but three lamps are employed as shown in Fig. 80. If all three lamps simultaneously become FIG. 80. CONNECTIONS FOR SYNCHRONIZING THREE-PHASE MOTOR. bright or dark, the connections are correct, and the line switch may be closed at the instant of darkness. It may happen, how- ever, that the lamps do not glow at the same instant, but succes- sively. This indicates that the leads are not connected in their proper order. In this case the motor lines should be transposed 146 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. until the lamps brighten simultaneously. After the machines have been once properly connected their synchronizing can be accom- plished with a single lamp. If the voltage of the circuit is too high for the direct use of lamps, transformers should be inserted as indi- cated in Fig. 81, their secondary coils being in series with each Transformers FIG. 8l. CONNECTIONS FOR SYNCHRONIZING ON HIGH-VOLTAGE CIRCUIT. EXCITER USED AS STARTING MOTOR. other and with the lamp /. The latter will glow when the motor e.m.f. opposes that of the line, provided the connections of either the primary or secondary coils are reversed, the former case being represented in Fig. 81. It has become apparent with the continual increase in the size of units that better means of synchronizing than those afforded by the lamp methods just described were desirable because serious trouble may ensue if large machines with heavy moving parts are connected together when not exactly in step. Such a ^device is secured in the synchronizer or "synchroscope."* This instrument consists essentially of a small induction motor, of which the fixed winding or stator is excited from the line, and the revolving member or rotor is supplied with current (through a phase-splitting device) from the machine to be synchronized. A rotating magnetic field is thus set up in the synchronizer, and the rotor thereof will revolve at a speed that is governed by the difference between the line and motor * Electric Journal, Vol. I, 1904, p. 692; Vol. IV, 1907, p. 497. SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 147 frequency. The shaft of the rotor is provided with an arm, which, revolving with it, serves as an indicator. When the frequencies of the currents in the rotor and stator of the synchronizer are the same, the magnetic field due to both is no longer a rotary one, and the pointer remains stationary. This condition may, however, indicate only an equality of frequency, not necessarily one of correctness of phase relation. There is only one particular position of rest assumed by the rotor when frequency and phase agreement both exist; and its index is so set that it points vertically upward when these con- ditions are secured. Accordingly, while agreement in frequency is indicated by the fact that the index of the synchroscope is stationary, the angular difference between such direction and the vertically upward one shows the phase difference existing. Another feature of value in this instrument is the fact that it also shows whether the motor (or incoming machine) is running too fast or too slowly, because if running too fast the pointer will move forward in a clock- wise direction, while if revolving at too low a speed the pointer will move in a counter-clockwise direction. Thus the synchroscope accurately indicates frequency and phase relations, and its use is to be recommended in connection with large synchronous motors as well as generators. Phase Relations between Constant Line. Voltage and Motor Current. The synchronous motor in practice is supplied with current from a constant potential a. c. circuit, and load changes cause the machine to draw currents varying not only in value but also in phase relation with respect to the line voltage. Fig. 82, A, B and c, illustrates the changes in current value and phase angle which occur in the case of a 3O-kw. single-phase synchronous motor when the load is 5, 20, and 40 kw. respectively, motor e.m f. and line voltage being equal at 500 volts. These diagrams show that the angle between the current and line voltage becomes smaller and smaller as the load is increased, and would at about 45 -kw. load reach a zero value, after which upon further addition of load it becomes greater again, in the opposite direction, but shifting from a leading to a lagging angle. Change of load is not the only way to produce variations in the angular relation between line current and voltage; it may be caused by adjusting the excitation of the motor, and this is frequently done in practice to secure a leading current. 148 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The actions occurring in a synchronous motor with variable field excitation can readily be studied by means of vector or circle dia- grams. Assume for example any condition of motor load and let the line e.m.f. be represented by E gy the current by /, and the angle between them by g . The angle 6, existing between the resultant e.m.f. E r and the current / (which depends upon the values of fre- /I- 12 A A, Motor Load 5 K.W. B, Motor Load 20 K.W. I -92 A C, Motor Load 40 K.W. FIG. 82, A, B, AND C. CHANGES OF CURRENT VALUE AND PHASE WITH VARIATION IN SYNCHRONOUS MOTOR LOAD. quency /, inductance L, and resistance R of the circuit), is theoreti- cally constant, but in practice it varies slightly, since the permeability of the magnetic circuit is only approximately constant. Hence, know- ing the line frequency and voltage E g , the load amperes /, the watts input and the machine constants (L and R), we can readily draw vector diagrams representing the various phase relations of E g ,E m ,E r , and / at any input, or any mechanical output, if we recollect that this latter is equal to the input less the IaR a losses. Formulae can be derived from these vector diagrams by which any one of the various SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 149 components can be calculated if the others are known (pp. 158, 167). In Fig. 83, A, lay off OE g a,s the impressed or line voltage, OI as the current at an angle . The motor output, including core and friction losses, is E g l cos g 7 2 ^ a or E m l cos m , wherein E m is the motor e.m.f. and m angle between E m and /. With the line voltage E g maintained constant, the current / may have any value for a given input, depending upon the value of cos g . For instance, in Fig. 83, B, let the motor input have the same value as in Fig. 83, A, but let the current be in phase with the line e.m.f.; that is, Egl cos g = const, and g = o. The new position of E r (the resultant e.m.f.) is substantially the same angle ahead of the current /, since R and L have not changed materially, but E r is smaller than before because / is less for the same load. The new value of E m (the motor e.m.f.) is obtained by completing the paral- lelogram of which OE g is one side and E g Er another. This new position and value of E m are shown by the line OE m , which is longer than in the preceding case. This increase can be brought about only in one way, i.e., by increasing the strength of the motor field, because the speed is synchronous and therefore constant. In Fig. 83, c, let us assume a motor load of the same amount as in the two preceding instances, but with the current leading the line e.m.f. by an angle (/> g equal to the lag in the first case (Fig. 83, A). The angle between E r and / remains practically constant because R has not changed and L only slightly. Lay off OE r as the resultant e.m.f. an angle d ahead of OI. Completing the parallelogram we have OEm representing the phase and value of the motor e.m.f. that corresponds to these new conditions. An inspection of this diagram shows that the motor e.m.f. (E m ) is now of still larger value. In fact the field strength of the synchronous motor can be increased so much that the motor e.m.f. is considerably greater than the line 150 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. -100 A ^ Eg - 200 v. Eg I Cos s / - 17.5 K.W/ s - 29 lag. " 87.5 A Eg 200 Y. E- m -275V. Ir- 168V. FIG. 83. A, B, C AND D. VARIATION OF POWER FACTORS (COS g) OF CIRCUIT BY CHANGE OF SYNCHRONOUS MOTOR E.M.F. MAINTAINING CONSTANT INPUT. SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 151 e.m.f., the result being that the angle becomes a large leading one; which condition is shown in Fig. 83, D. Hence, if the motor field is gradually strengthened the line current can be made ultimately to lead the line e.m.f. This phenomenon of the synchronous motor is of value in the transmission of power, since a super-excited motor can be employed to raise the power-factor of the circuit, which usually tends to have a lagging current. Torque Conditions of Synchronous Motor depending upon Angle between Current and Motor E.M.F. The current flowing in the armature of a synchronous motor may have one of three general FIG. 84, A, B, AND C. VARIOUS PHASE RELATIONS OF SYNCHRONOUS MOTOR E.M.F. AND CURRENT WITH CONSTANT OUTPUT. phase relations with respect to the motor e.m.f. or E m . This phase angle may be 180 degrees (Fig. 84, A) because motor action is here considered, the same relation existing in a d. c. motor; it may 152 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. be less than 180 degrees (Fig. 84, B) or it may be more than 180 degrees (Fig. 84, c). The most efficient condition for motor output exists in the first case when the phase angle between E m and I a is 180 degrees, since then the current required to produce the desired torque is a mini- FIG. 85, A, B, AND C. POWER CURVES WITH (f> m = , <, AND > l8o. mum. This may be proven as follows: In Fig. 85, A, the wave diagrams of current and e.m.f. are shown with a phase displace- ment of 1 80 degrees; the resulting power curve P being negative at SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 153 every instant; thus fora given area (representing motor power) the value of / will be ? minimum. This condition is also shown by the equation E m l cos (j) m = motor power; because cos (f> m has its maximum negative value = i, when (f> m = 180 degrees; hence to produce a given power with E m constant, / will have minimum value. In Fig. 8c;, B, the wave diagram shows the current displaced less than 180 degrees with respect to motor e.m.f., in which case the resulting power wave has both negative and positive values; hence with a given current the motor power represented by the negative area is not only smaller than in the preceding case but is still further diminished by the positive area, so that the available motor power is considerably less than when the current and e.m.f. differ by 180 degrees. Therefore, to have the same power the current must be greater in the second case (Fig. 85, B). Evidently similar conclusions apply when the current leads the motor e.m.f. as shown in Fig. 85, c. There is also another effect when the cur- rent and motor e.m.f. differ less or more than 180 degrees in phase; namely a strengthening or weakening of the motor field. For example, if the phase displacement between current and motor e.m.f. is 1 80 degrees, the current reaches its maximum at the same instant as the e.m.f., which condition is represented by the position of the armature in Fig. 86. In this case the armature current FIG. 86. ANGLE RETWEEN E m AND LINE CURRENT IS l8o; DISTORTION OF MAIN FIELD. neither magnetizes nor demagnetizes the field, with the moderate flux densities usually adopted for a.c. machinery. The effect is merely to distort the field, since the N poles of the armature increase the flux at the S poles of the field and diminish it equally at the N 154 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. field poles a similar balanced effect being produced by the S arma- ture poles. If the armature current lags less than 180 degrees behind the motor e m.f. it reaches its maximum value at the instant indicated in Fig. 87. An inspection of this diagram shows that the field FIG. 87. ANGLE BETWEEN Em AND LINE CURRENT LESS THAN l8o; MOTOR FIELD STRENGTHENED. strength of the motor is increased because the flux direction of the armature favors that of the field. If the armature current leads the motor e.m.f., that is, the phase difference is more than 180 degrees, then this current attains its maximum before the armature reaches the position of maximum e.m.f. as shown in Fig. 88. The result with this phase relation is a FIG. 88. ANGLE BETWEEN E m AND LINE CURRENT MORE THAN l8o; MOTOR FIELD WEAKENED. weakening of the main field due to the opposition of the armature magnetization, like poles being contiguous. A summary of the preceding facts is as follows. i. For a given current a motor develops maximum torque when the phase angle between its e.m.f. (E m ) and the armature current (/) is 1 80 degrees. SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 155 2. When E m and I differ in phase by more or less than 180 degrees the torque for a given value of / is less than when the phase angle is 180 degrees. 3. When the phase angle (j) m is 180 degrees the armature reac- tion merely distorts the field, because one pole tip is strengthened as much as the other is weakened, with ordinary flux density. 4. When the angle (f> m is less than 180 degrees (line current lagging with respect to E m ) the armature reaction strengthens motor field. 5. When the angle (j) m is more than 180 degrees (7 leading E m ) the armature reaction weakens motor field. 6. The condition of lagging current with respect to motor e.m.f. ( m = o) and the machine stops, if it has not already done so before the go-degree limit is reached. Practically, the stopping would occur before (f> m is reduced to 90 degrees, because the driving power must be sufficient to overcome the core losses, windage, friction, etc., plus any external load. Investigation of the operative range curves (Fig. 89) of the syn- chronous motor indicates that it has two conditions of operation, namely, a stable and an unstable one. The unstable condition of operation exists when the phase angle between armature and line voltages is less than a certain value, ranging between 100 degrees and 120 degrees, depending upon the resistance and reactance of the motor armature. Thus, if the motor should be operating on the unstable portion of the curve, that is, between zero and about no degrees, any attempt to increase the load would be accompanied by retardation of the armature, which would not, however, aug- ment the driving power of the machine; with the result that the synchronous link is broken and rotation ceases. Conversely, while operating on this unstable portion of the curve, if the motor load be decreased, acceleration of the armature results, which augments the driving power of the machine, and acceleration is continued until the crest of the power curve is passed and the stable con- dition reached. The stable condition of operation for synchronous motors exists when addition of load causes a retardation of the armature with in- crease of driving power until load and driving power balance. If, however, it be attempted to overload the motor considerably, the resulting retardation of the armature causes the angle between the motor e.m.f. and line voltage to decrease so that it is less than SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 157 the value corresponding to that of maximum driving effort; therefore the motor passes into the unstable condition of opera- tion and stops. If the load on the motor be decreased while the 80 60 50 40 30 20 10 Condit on of Open i / Unstable tion Eg - 500 V. Ra - 0.8 w 2 TTfL- 2.5 f-60 E m -700V. 600V. -400V. 800V. \ Stable Condition of \ Operation \ Degrees 1.00 120 140 Angle between Line Volts and Motor E.M.F. 160 180 FIG. 89. OPERATIVE-RANGE CURVES OF A 3O-KW. 5OO-VOLT SINGLE-PHASE SYNCHRONOUS MOTOR. motor is operating on the stable portion of the curve, the arma- ture is accelerated and the driving power decreased until a balance is obtained. 158 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The range of driving power, and thus the capacity of a given synchronous motor, depend upon its field excitation and armature constants (resistance and reactance of its winding). The driving power of such a machine is generally greater the stronger its field, within the limits attainable in practice. For different machines, other things being equal, that one having the larger impedance angle (tan" 1 2 TtfL -T- R) has the higher overload capacity or greater stability. The approximate operative range of any synchronous motor for various conditions of excitation can be predetermined if the line FIG. 90. GENERAL VECTOR RELATIONS OF LINE VOLTAGE, CURRENT AND MOTOR E.M.F. voltage, armature e.m.f., resistance, inductance and frequency are known. The equations for this calculation are obtained from the ordinary diagram showing the vector or space relations between the line voltage, current and armature e.m.f. Inspection of Fig. 90 shows that the resultant voltage E r = VEm 2 + Eg 2 + 2 E m Eg COS d, (19) where d is the angular relation between motor and line voltages. From this value the armature current can be obtained by dividing the resultant voltage by the armature impedance, thus: v ^m + Eg + 2 E m Eg COS d rL r -r- Z = (2O) ~+ (27T/X)" The driving power of the motor, or that portion of the input con- verted into mechanical power, is W m = E m l COS m= .368. From eq. (22), Wm or motor output = 500 X 291 X 368 = 53.5 kw. =m- d = in 30' - 80 = 31 30'; cos fa =+ .852. W * 500 X 291 X .852 = 124 kw. The same steps could be followed out in deriving corresponding values of E r , I, W m , etc., when d is varied. Collecting the results of such calculations and tabulating them we have: 9. Er, volts. I- Er 7 ~z' m. W m> kw. 0- COS <0. Wg, kw. amps. 80 766 291 ill 30' -.368 -53-5 3i 30' .852 I24.O 100 643 245 122 -53 -66.0 22 927 "5-3 120 500 , 190 132 -.669 -63-5 !2 .978 93-o l 4 350 128 142 -.783 -S2-5 2 999 64.4 160 175 66 152 -.883 29.6 -8 .990 32.9 1 80 o o o A curve plotted between the listed values of d and W m as abscissae and ordinates respectively gives the 5oo-volt load-range curve shown in Fig. 89. The corresponding curves for 300, 400, 600, and 700 volts indicated in the same figure could be obtained in the same manner, though for convenience the authors employed the circle diagrams shown later, on pages 163, 164. It is to be noted that the possible load capacity of this machine is greatest at the highest field excitation considered; but at these heavier loads the armature current is excessive, as shown in Fig. 95. When excited so that its armature produces 500 volts counter e.m.f. the motor has about 80 per cent overload capacity before instability is reached. At greater excitations, for example at 600 volts, the crest of the power curve occurs at 140 per cent overload. The im- portant fact follows that with a synchronous motor liable to be subjected to widely variable loads, greater stability is obtained by ad- SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 161 justing its field excitation so that the armature e.m.f. is equal to or somewhat greater than the line voltage. Increase of armature volt- age should not, however, be carried very high, since the current 80 100 120 140 160 180 Angle between Line Volts and Motor E.M.F. FIG. 91. "CURRENT-PHASE SWING ANGLE" CURVES OF 3o-xw. SINGLE-PHASE SYNCHRONOUS MOTOR. for a given load thereby becomes too great, causing excessive heating of the machine. For example, the armature current for 30 kw. at 500 volts is 70 amperes, at 600 volts, 82 amperes; and at 700 volts, 162 ELECTRIC MOTORS, TffEIR ACTION AND CONTROL. 120 amperes. The curves between current and phase angle given in Fig. 91 are derived from the values of d and / in the table just preceding. These show that as the angle between line e.m.f. and motor e.m.f. becomes smaller the current increases rapidly. Circle Diagrams of Synchronous Motor. If we consider a given line voltage and armature e.m.f. of a synchronous motor, and plot the vector positions and relative values of line voltage, armature e.m.f., resultant voltage and current through a phase swing of 180 degrees, it is found that while naturally the locus of the motor volt- age is a circle, the loci of the resultant voltage and armature current are also circles. The centers of these circles are at different points. These circle diagrams of the synchronous motor are useful, and by their application the values of E r , I, and (j> m can be directly deter- mined and thus the power or operative load range curves of the motor obtained without the lengthy calculations based upon the preced- ing equations. One set of circle diagrams is required for each motor excitation. Their construction and application are as follows : Lay off in a horizontal direction and to scale the line OE g (Fig. 92) representing the line voltage, then add to it, in the same direction, the line E g E m f which corresponds with and is proportional to the motor e.m.f.; also, lay off to the left of E g a distance proportional to and representing the motor voltage as above. Then with E g as a center and E g E m ' as a radius describe a circle. This is the locus of the resultant voltage E r . Its maximum value is propor- tional to the distance from O, through E g to E m , while its minimum value is the distance along the same diameter from O to the point at which this diameter cuts the left-hand side of the circumference; that is, the length OQ. With the point O as a center describe a circle having a radius representing and proportional to E m . This is the locus of the motor voltage. Through the point O draw the line OK so that it makes an angle (the impedance angle = tan" 1 - \ with the horizontal line OE . The location of the point K is ob- tained by using the point O as a center and a radius equal to OEf m ] that is, OK is equal to OE m '. Then from the point K along the line OK towards O lay off a distance equal to E m and with this new point P as a center and PK as a radius describe a circle. This repre- sents the locus of the current vector and is proportional thereto, SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 163 being the impedance drop, which is numerically, for any vector posi- tion, equal to the amperes multiplied by the impedance Z. The maxi- mum current and its position are represented by the line OK, which are numerically equal to OK -=- Z, the impedance, being obtained when the motor voltage and line voltage are in phase (i.e., 360 degrees dis- placement existing between them) . The minimum value of the current is equal to the distance OL divided by the impedance, and is obtained when the motor voltage is 180 degrees from the line voltage Three sets of circle diagrams are given. The first of these (Fig. 92) shows the FIG. 92. CIRCLE DIAGRAM OF SYNCHRONOUS MOTOR; Eg = 500 VOLTS, Em =300 VOLTS, = 60. loci of resultant voltage, motor voltage and current, when the motor voltage is 300 and the line voltage 500 volts. The second circle dia- gram (Fig. 93) illustrates the corresponding loci when motor and line e.m.f. are each 500 volts; and the third diagram (Fig. 94) sets forth the loci when the motor voltage is 700 and the line voltage 500 volts. Con- sider the first diagram, and let it be desired to obtain the resultant voltage, motor current, value of m , and driving power of motor when the angle between the motor voltage and the line voltage is 130 degrees. The procedure is as follows: From the point O draw the line OE m so that it makes an angle of 130 degrees with OEg. This represents the angular position of the motor voltage with respect to the line voltage. Then with the point E m as a center and a radius equal to OE g describe an arc which cuts the resultant voltage vector locus at the point E r . OE r represents the scale value (= 375 v.) of the resultant voltage and its correct angular position. With O as a center and with the distance OE r as a radius describe an arc inter- 164 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. FIG. 93. CIRCLE DIAGRAM OF SYNCHRONOUS MOTOR; Eg = 500 VOLTS, E m = SCO VOLTS, 9 = 6o FIG. 94. CIRCLE DIAGRAM OF SYNCHRONOUS MOTOR J Eg E m = 700 VOLTS, 6 = 60. 500 VOLTS, seating the current vector locus at the point /. The line OI repre- sents the true vectorial position of the armature current, and if its scale value be divided by the impedance, the correct value of this current is given in amperes (||f = 146). The angle contained between E m OI is the angle (j> m) and its value (166 degrees) can be measured on the motor voltage circle. The product then of E m l SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 165 cos (j> m gives the motor driving power, or 300 X 146 X 0.97 = 42.5 kw. Various angular positions between E g and E m can similarly be assumed, the motor current and driving power determined as 300 280 10 20 30 40 50 60 70 80 90 100 110 120 130 FIG. 95. CURRENT-POWER CURVES, 3O-KW. SYNCHRONOUS MOTOR. shown, and these different values plotted as ordinates with the phase angle between E and E m as abscissae; the result being the motor "load-range curves" and "current curves" shown in Figs. 89, 90 and 95, respectively, of which the 5oo-volt series were obtained by calculation, using equations (19) to (22), It can very readily 166 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. be seen by means of the operative range curves what angle between motor voltage and line voltage represents the pulling out or stop- ping condition. This occurs with the typical 4o-h.p. motor ex- amined when the angle E m OI is a little less than no degrees. V Curves of the Synchronous Motor. All the working charac- teristics of a synchronous motor can be determined from its opera- tive load range curves, which were derived and discussed in the preceding pages. For example, the current-power curves are obtained by plotting curves, having corresponding current values as ordinates and kilowatts as abscissae. Two sets of current- power curves exist (Fig. 95); one group showing the relation between current and output or driving power, and the other between current and armature input. The armature watts input are determined by adding the corresponding PR a losses and watts output. The characteristic V curves of the synchronous motor are readily determined from the current-power input curves, by proceeding as follows: Assume any constant input, say 3o-kw., then draw the straight line AB vertically through the 30-kw. abscissa point of the current-power curve (see Fig. 95), and from the intersection of the line AB with the different input curves we can determine the various armature currents. For example, at 300 volts this current is 93 amps., at 400 volts it is 68 amps., at 500 volts 60 amps., etc. The relation existing between the motor e.m.f.'s and these current values used as abscissae and ordinates, respectively, gives the 3O-kw. current V curve for the motor of the text. The complete series of these V curves given in Fig. 96 were obtained by following a like procedure for the different values of input employed. A study of these curves indicates that at any given input, as the motor voltage is increased the armature current decreases, until a minimum value is reached, after which, upon further strengthen- ing of the motor field, the current begins to increase. The varia- tion in the armature current for a given range of field excitation is greater at the lighter loads, and the curve obtained when the motor is running practically without load is of substantially a V shape, hence the name; while at the higher loads the curve flattens out considerably, appearing like an arc of a large circle at rated load input and beyond. These same V curves can be determined by calculation, employing a formula which is derived from the simple vector relations existing between line volts, motor volts, SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 167 100 300 400 600 700 500 Motor E.M.F. FIG. 96. PHASE CHARACTERISTICS (7 CURVES) OF 3O-KW. SYNCHRONOUS MOTOR. and current in the case of a synchronous motor, this equation being E m = V(E cos <0 - IRaY+ (E g sin - IX) 2 * (23) but its use is rather tedious. * From vector diagram, Fig. 90, it is apparent that Em 2 = Eg 2 + E r 2 - 2 EgE r cos (0 - fa\ (A) wherein Eg 2 = 2 (cos 2 00 + sin 2 00). E r 2 = IZ? = P (R 2 + X 2 ). = cos- 1 1 = sin" 1 - R X cos (Q 00) = cos 6 cos 00 + sin 6 sin 00 = cos 00 H sin 00. Z, Z Substituting the above for their equivalent terms in equation (A) we have E m a = Eg 2 cos 3 00-2 IREg cos 00 + PR 2 + Eg 2 sin 2 00-2 IXEg sin 00 + PX* = (Eg cos 00 - IR) 2 + (Eg sin 00 - IX) 2 or E m = \/(Ey cos 00 - IR) 2 + (Eg sin 00 - IX) 2 , (B) which equation expresses the value of the motor e.m.f. in terms of the line voltage, current, phase angle, and armature constants. 168 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. A second set of these V curves is obtainable from the current- power curve, namely, those showing the relation between power factor and field excitation. These are determined as follows: Assume any condition of constant input and let this, as before, be 30 kw.; then refer to the current-power curves (Fig. 95) and mul- tiply the current at any motor e.m.f. by the line voltage. This product is the volt-amperes input. A division of the correspond- ing abscissa value (which was 30 kw. input in this instance) by the volt-amperes gives the power factor at the selected field excitation and input. The curves obtained by using the power factor and the corresponding motor e.m.f.'s as ordinates and abscissae, respectively, are those represented by the broken lines in Fig. 96. It is to be noted that these are the converse in shape of the current V curves. The power factor rises with field excita- tion up to about unity at 500 volts, but as the field strength of the motor is still further augmented the power factor decreases, even more rapidly than it rose. It is also apparent that the power factor of the machine is improved by addition of load to the motor. Capacity Effect of Synchronous Motor. Reference to the curves of Fig. 96 and to the formula for motor e.m.f. (Em) shows that the smaller values of motor e.m.f. exist when (j> ff is a lagging angle, because sin (j) g is then plus. The larger values of the motor e.m.f. exist when (> g is a leading angle, sin (f> then being minus, which increases the second term under the radical and consequently E m is greater. Hence, variation of the motor excitation produces a change in the angle between line voltage and motor current. Low field excitations cause a lagging current to flow and high excitations draw a leading current. Thus a synchronous motor whose e.m.f. is greater than that of the line (field super-excited) draws a leading current and acts as if it possessed electrostatic capacity. The above property of the synchronous motor, used as such or as a rotary converter, is frequently utilized to improve the power factor of transmission systems, usually because induction motors or lightly loaded transformers are supplied thereby, which operate at a low power factor (p. 200), necessitating lagging current to be transmitted. This condition not only increases the line drop but also interferes seriously with alternator regulation. The installa- tion of a super-excited synchronous motor at the receiving end of a line reduces this angle of lag by drawing a leading current. SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 169 In fact, the motor field can be so adjusted that the phase displace- ment between line voltage and current becomes small or even nil. Balancing Action of Synchronous Motor. The fact that a syn- chronous motor draws a leading current when super-excited, and that the extent of lead is increased with the degree of super-excita- tion, gives to this type of polyphase machine the capability of restoring a balance to an unbalanced polyphase circuit. When a slightly super-excited motor is connected to the terminals of a balanced or an unbalanced polyphase system, all phases of the motor armature draw a leading current. That phase winding, however, which is connected across the line terminals of lower voltage draws a current of greater lead than the other windings because its super-excitation is relatively higher; hence, the com- pounding tendency of the leading current is more marked in this phase than in the others and the voltage thereof is increased. This action tends to balance the circuit, not only in voltage but in cur- rent as well, because combinations of leading and lagging currents give reduced resultant currents. Hunting of Synchronous Motor. A trouble which sometimes arises in connection with synchronous motors is that of hunting, pumping, or phase swinging. These terms signify the periodic fluctuations in speed and armatunTcUritent occurring under certain conditions. Such surgings may be produced by several causes. Let us suppose the typical motor to be running at a constant load up to a certain instant, and that then the load is suddenly varied, say increased. A momentary retardation of the motor armature natu- rally results, and we see from power curves (Fig. 89) that the angle between E m and E g must decrease to allow of increase in the driving power of the motor. The proper value of armature current is not obtained at the instant that the correct phase angle between E m and Eg exists, but somewhat later, owing to time lag caused by inertia and the inductance of the armature winding. Thus, while the armature may momentarily pass through the right angular relation with respect to the line e.m.f., it will ultimately draw more current than its load requires. This causes the armature to be accelerated and the angle (d) between E m and E g is increased beyond its proper value, until the driving torque is so much reduced as to be insuffi- cient for the motor load, whereupon lagging or retardation of the armature again ensues, and so on. This phase swinging of the arma- 170 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. tiire and current variation accompanying are included under the term hunting. The amplitude of this swinging or pendular action usually dies down, due to frictional, eddy current, and hysteretic effects, so that the armature finally finds its correct load position. The irregular rotation above considered may be regarded as con- sisting of a uniform motion of rotation at synchronous speed with a to-and-fro or pendular motion superposed upon it. We see, therefore, that a change of load upon a synchronous motor will produce oscillations in its angular velocity and armature current. There are, however, other actions by which such fluctuations may be started. Assume the motor load to remain quite constant but the speed of the generator to undergo a sudden rise. This corresponds to an advance^of the angular position of the line voltage vector, which naturally decreases the angle between E m and E g , thus increas- ing the driving torque of the synchronous motor (see operative- range curves, Fig. 89), and producing an acceleration, which will, as already shown, develop the hunting phenomenon. Similarly a sudden change in excitation of the motor or in the line voltage will produce a variation in the motor torque and thus bring about the phase surging action. This fluctuation is very slow compared with the line frequency, and its period can be readily determined by the violent swinging of the needle of an ammeter connected in the supply line. As already stated, the hunting started by any one sudden dis- turbance will gradually subside. If, however, before the oscilla- tions due to one cause have been damped out, another disturbance should occur of such nature as to reenforce the already existing fluctuations, their combined amplitude may become so great as to cause the motor armature to swing beyond the range of stability, with the result that it falls out of step and stops. This action is w likely to occur, because sudden load changes are usually accom- panied by marked variations in line voltage, both acting to produce a like effect. The surges of the armature current thus developed may affect the alternator speed, and then all three dis- turbing factors act in unison. If the phase swinging is not violent enough to cause the motor to pull out of step, the variation thus produced in the line voltage may be, nevertheless, of such low periodicity that it becomes apparent in the flickering of lamps con- nected to the circuit, and it may even develop hunting in other SYNCHRONOUS ALTERNATING-CURRENT MOTOR. 171 synchronous motors fed thereby. The fact that hunting of a syn- chronous motor not only interferes with its own stability but may react upon other synchronous units connected to the line is very objectionable, and the prevention of marked hunting becomes a practical necessity. Prevention of Hunting. Inspection of the power or operative range curves of synchronous motors (Fig. 89) shows that a given FIG. 97. THE HUTIN AND LE BLANC AMORTISSEUR OR DAMPER. Damping Grids FIG. 98. ANTI-HUNTING DEVICE. change in torque will be obtained with a smaller phase swing when the field is strong than when it is weak, and naturally the smaller the initial phase shift the less the resulting oscillation; hence, use of strong fields is favorable in checking hunting, but this by itself will not suffice. It has already been indicated that the production of eddy currents by the surging of the armature current tends towards the reduction of the duration of the surges, and theoretically any device wherein currents are generated by the pendular motion of the armature will act by Lenz's law to stop the motion producing them. Hence it is 172 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. necessary to design a device wherein heavy eddy currents are developed by even slight hunting tendencies, in order to arrest the surging in its very development. The devices producing this checking effect are known as damping coils or dampers. One of the earliest of these dampers was proposed by Hutin and Le Blanc* (Fig. 97), and consists of a series of thifck copper bars embedded in each pole 'piece, parallel to the armature 'axis, and connected in parallel by heavy copper rings concentric with the armature. A more modern and probably more economical arrangement (Fig. 98) consists of a copper grid placed in corresponding slots cut in the polar face,, the outer rim of the grid forming a closed band around the pole piece. f * U. S. Patent No. 529,272, November 13, 1894. f U. S. Patent No. 575,116, January 12, 1897. For further information concerning synchronous motors see : ALTERNATING CURRENTS, D. C. and J. P. Jackson, p. 571. ALTERNATING CURRENT PHENOMENA. C. P. Steinmetz. 1908. DIE WECHSELSTROMTECHNIK, Vol. IV. E. Arnold. 1904. SYNCHRONOUS MOTORS AND CONVERTERS. Blondel-Mailloux. 1913. ALTERNATING CURRENT MOTORS. A. S. McAllister. 1909. ELECT. ENG. POCKET-BOOK, Van Nostrand, N. Y. 1910. p. 340. STANDARD HAND-BOOK, p. 341. McGraw. 1908. SYNCHRONOUS MOTORS. F. G. Baum. Electric World, March, 1902. SYNCHRONOUS MOTORS. Prof. C. A. Adams. Harvard Eng. Journal, 1907. LONDON INST. Civ. ENGS. J. Hopkinson. 1883. Trans. A. I. E. E., Vol. XIX, 1902, p. 718. C. P. Steinmetz. Trans. A. I. E. E.., Vol. XXIII, 1904, p. 481. G. B. Lamme. Trans. A. I. E. E., Vol. XXVI, 1907, p. 1027. M. Brooks. Trans. A. I. E. E., Vol. XXXI, April, 1912, p. 305. C. J. Fechheimer. Jour. I. E. E., London, Vol. XLII, April, 1913, p. 62. E. Rosenberg, CHAPTER XIV. POLYPHASE INDUCTION MOTORS. ) THE polyphase induction motor as developed through the inven- tions of Ferraris, Tesla, and. others is undoubtedly the most impor- tant of alternating-current motors.* Two-phase or three-phase machines are employed, depending upon the system by which the current is supplied. The operation of the induction motor is very different from that of the preceding types because there is no elec- trical connection between the armature (usually called secondary .or rotor) and the source of current supply. The motion of the arma- ture is produced by a rotating magnetic field, and it is this peculiar field which is the characteristic of induction motors. Production of Rotary Field. A laminated iron ring, wound with insulated wire, as represented in Fig. 99, is supplied with two- phase or quarter-phase currents at four equidistant points A, B, C and D. Two conductors of one phase are connected at A and B, and those of the other phase across C and D respectively. The direction of the winding is such that a current entering at A will produce a south pole at this par- ticular point and a north pole at B, therefore if a compass needle were placed inside of the ring, it would tend to point vertically up- ward as indicated by the dotted arrow. This condition is represented at i in Fig. 100, the current of phase AB having its maximum positive value, and that of phase CD zero value, in accordance with the usual phase difference of 90 or one-quarter period existing between two-phase currents. A moment later, i.e., one-eighth of a period, the current in AB * See pages 131-134. 173 FIG. 99. RING SUPPLIED WITH TWO-PHASE CURRENTS. 174 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. has decreased somewhat, and the other has increased, so that they are now equal. In this case, each current will tend to produce a south pole where it enters the winding at A and D respectively, so that a resultant polarity is developed midway between, as shown at point 2, the arrow being inclined at an angle of 45. The next instant, the current of phase AB has fallen to zero, and that of CD has reached its maximum, so that the needle takes the hor- izontal position as represented at 3 in Fig. 100. Again at 135, the current AB has reversed, tending to make a south pole at B, the needle being inclined downward at an angle of 45 as shown at point 4. By following the successive conditions, the needle will FIG. 100. MAGNETIC RESULTANTS DUE TO TWO-PHASE CURRENTS. be found to take the various positions represented at points 5, 6, 7, 8, and finally at 9 it assumes its original vertical direction, the current having then completed one cycle of its changes, having passed through two alternations. Thus, the compass needle tends to be rotated on its support continuously by the shifting result- ant field, as long as the winding is supplied with two-phase or quarter-phase currents. If either one of the connections AB or CD (Fig. 99) were reversed, the direction of rotation of the needle would then be counter-clockwise, instead of clockwise. Hence, to reverse the direction of rotation of such a field, it is necessary to interchange the terminals of one of the two phases. The Action of Three-Phase Currents in producing a rotary field is quite similar to that explained for two-phase currents. The POLYPHASE INDUCTION MOTORS. 175 laminated ring of Fig. 101 is wound as before, but the current is led in at the three equidistant points X, Y and Z, instead of at four points, as was indicated for two-phase currents. Taking the instant when the current flowing in at X is a maximum, then cur- rents flowing out at Y and Z each have one-half the value of that entering at X. This tends to produce a south pole at X, and two north poles at Z and Y respectively. The resultant due to the latter is a north pole at T, midway between Y and Z; conse- quently a magnetic needle placed within the ring would assume the position indicated by the dotted arrow at i in Fig. 102. One-sixth of a period later,, currents enter at both X and Z, and a maximum current flows out at F, FIG. 101. RING SUPPLIED WITH THREE-PHASE CURRENTS. FIG. 102. MAGNETIC RESULTANTS DUE TO THREE-PHASE CURRENTS. hence the needle would point towards V. At the end of another one-sixth of a period, the maximum current would enter at Z, and the needle would turn to that point as shown at 3 in Fig. 102, and so on until it had made a complete revolution in one period of the alternating current. If any of the two connections shown 176 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. in Fig. 101 be transposed, the direction of rotation of the corre- sponding magnetic field will be reversed. Variation of Flux with Two-phase and Three-Phase Stator Wind- ings. To determine the magnitude of flux of the rotating field at any moment when set up by 2 -phase 'Currents, consider two similar coils at right angles to each other as in Fig. 102 A. At any point of space each coil will produce when carrying current, a field propor- tional to such current. With balanced two-phase currents, = 7 sin = 7 cos The magnetic field at the common centre O of the two coils is x = M sin 6, along X axis, and y = M cos 0, along Y axis. Now since the fields are at right angles to each other, the resultant field OR is given by the square root of the sum of their squares or: x* + y 2 = M. In other words the magnitude of the resulting rotating magnetic field is constant and equal to the maximum field set up by any one of the two balanced phases. FIG. IC2 A. FIG. 102 B. To determine the condition of the resulting field with three-phase currents, consider three coils, a, b and c, placed at 120 to each other, as in Fig. 102 B, conveying three currents of equal amplitude: i a = I sin 0; 27: i b = 1 sin ( 0- = - o7 sin + .8667 cos 0; i e = I sin -. j = - .57 sin - .866 7 'cos 0. \ o / The directions of the magnetic fields produced by the three coils at POLYPHASE INDUCTION MOTORS. 177 their common center O are indicated by the vectors x, y and z, respec- tively. The relative values of these fields at any instant are : x = M sin 6; y = - .$M sin 6 + .S66M cos 0; z = - .$M sin - .866M cos 0. The total horizontal component of the above fields at any moment is : X = x - (y +2) cos 60 = M (sin + | sin 0) = |- M sin 0. The corresponding total vertical component is: F = (y -z) sin 60 = -f M cos 0. So the magnitude of the resulting field (at any moment) is OR = X 2 + F 2 - |M. In other words, the resulting rotating magnetic field produced by balanced three-phase currents is of constant magnitude, and 50 per cent greater than any one of the alternating fields producing it. The particular advantage of the three-phase motor with respect to the two-phase machine is that it is more economical as regards copper for its stator winding, since the smaller current per phase of the former would produce an equivalent induction. The three- phase winding also lends itself better to the use of a simple starting device, being connected in "Y" at starting and in delta for running. In practice three-phase machines are more gener- ally employed, because the corresponding generators, transformers, transmission lines, etc., are more economical of material. The ring with the magnetic needle as described, illustrates the synchronous polyphase motor, since the armature revolves in syn- chronism with the angular velocity of the currents. If the needle is replaced by a laminated cylinder of iron wound with inductors like an ordinary armature except that they are short-circuited, it is found that this will also revolve, but in this case the speed is a little less than that of the synchronous field. The difference in speed (angular velocity) between the rotary field and the armature divided by that of the former is called the slip; or denoting the slip by s, the angular velocity of the rotary field by coi and that of the arma- ture by co2, we have 5 = (on twm* This slip represents a relative motion of the rotating field, with respect to the armature inductors; consequently the latter are cut by lines of force and therefore currents are induced in them. Since 178 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. it is the action of the field upon these induced currents which causes the armature to revolve, this type of machine is called the induction motor. It is to be noted that no current is supplied to the moving part, hence it need have no electrical connections made to it, except (as will be shown later) for purposes of starting and speed regula- tion, in which case electrical connection is necessary. The stationary part of the usual induction motor is connected to the source of current, and is termed the stator or primary. The moving part called the rotor forms a secondary to the stator. The terms field and armature could without error be retained, be- cause the primary forms the inducing member or field, while the sec- ondary or rotor is that part acted upon inductively, or the armature. Typical Induction Motor. The type of winding illustrated in the development of the rotary field does not lend itself to the pro- duction of a commercial machine on account of the waste of copper and its high leakage reactance.* The rotor winding and core must also be modified to suit practical conditions. The typical stator core consists of an assemblage of thin iron or mild steel rings of about .014 to .025 inches in thickness, with teeth and slots upon the inner circumference. These slots contain a distributed drum winding of substantially the same character as the armature winding of polyphase alternators. The magnetic poles are therefore not produced by windings concentrated at certain points of the gap periphery on salient or separately projecting masses of iron as in d. c. machines. Nevertheless, magnetic poles are formed by properly connecting the groups of coils. Although a diagram as in Fig. 99 may be used to represent the stator winding for theoretical discussion, it does not portray the actual commercial machine. The windings are seldom closed-coils, the three-phase stator is usually Y connected, although certain manufacturers employ this grouping simply for starting, changing to delta con- nection when running. The winding is divided into a number of groups, equal to the product of the number of phases and the number of poles. Fig. 103 represents the diagram of an 8-pole two-phase winding. Consider the instant when the currents in the two phases are in the same direction (that is between o and 90 or 180 and 270, Fig. 100), * Leakage reactance is that component of the inductive reactance, due to such lines of force (stray) as are not effective in the production of torque. POLYPHASE INDUCTION MOTORS. 179 then by tracing out the connections, it will be found that the currents circulate in the same direction in two adjacent groups. Thus a pole is formed by two groups, both phases being represented in each pole. When the current in each phase reverses (after a half cycle) the pole shifts the angular distance covered by two groups, so that the field completes one revolution in eight alternations of current. Thus if the current supplied had a frequency of 60 cycles per second, the field would make 15 revolutions per second, or 900 per minute. To minimize the length of cross-connecting wire, it will be seen that every fourth group is connected in the same direction in each phase. A coiled arc such as A represents a group comprising a FIG. 103. EIGHT-POLE TWO-PHASE STATOR WINDING. certain number of coils in series, each coil located in a separate pair of slots and the end of one being connected to the beginning of the next. A six-pole three-phase winding of 18 groups is indicated in Fig. 104. The phases are represented in counter-clockwise direction in the order A, B, C, A, B, C, analogous to the two-phase winding. 180 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The phases are thus only 60 degrees apart. To get the star or Y y which is a i2o-degree relation, the middle phase is reversed, as in Fig. 105, so that a pole will be formed by the three consecutive phases when the current is in the same direction in A and C, and opposite in B. The beginning of the middle coil (C), and not the end, as with the other two, is connected to the common point O. B C FIG. 104. SIX-POLE THREE-PHASE STATOR WINDING. C * FIG. 105. REVERSAL OF MIDDLE COIL WITH 60 SPACING TO OBTAIN 120 STAR ARRANGEMENT. POLYPHASE INDUCTION MOTORS. 181 In this case the pole shifts the distance of three groups for each alter- nation, so that one revolution of the field is completed in three periods, making 20 r.p.s. or 1200 r.p.m. with 60 cycle current. The speed or number of revolutions made by the rotating field accordingly depends upon the frequency as well as upon the num- ber of poles, being directly as the former and inversely as the latter, or r.p.m. = 60 X frequency -J- pairs of poles. (25) Since speeds of more than 1800 r.p.m. are higher than can con- veniently be employed, the majority of induction motors have four, or a still greater even number of poles. For example, a group of commercial 60 cycle machines has two pairs of poles up to 5 horse- power capacity, three pairs from 7.5 to 30 horsepower, four pairs from 30 to 50 horsepower and five or six pairs for sizes between 50 and 200 horsepower. The rotor core consists of a laminated iron cylinder, with the winding either of copper bars or of wires embedded in it. The simplest form of rotor construction employs what is known as the squirrel-cage winding devised by Dobrowolsky. It con- sists of a number of lightly insulated copper rods or bars arranged in holes or slots around the rotor periphery, and connected at each end by brass or copper rings of ample cross-section. There must be no common factor between the number of rotor and stator slots, otherwise the latter may tend to " lock," or fail to start when current is supplied to the stator winding. The end rings may be solid or laminated copper punchings, and connection to the bars can be made by means of rivets, screws, solder or welding. Riveting is expensive in labor, and if not done well gives poor contact, which results in heating and large slip. Screws and bolts are also expensive and poor contacts are likely to exist. Soldering by itself secures good contact but at heavy overloads or slow starting it is. likely to melt. Hence a combination of two of such means of connection is usually employed, though welded connections would be very satis- factory if uniformity of material at the joints could be assured. In the squirrel-cage winding which Fig. 106 illustrates, the rotor has a number of equidistant rectangular holes near its periphery, and through these holes pass copper rods, the projecting ends of which are bolted and soldered to two cast metal end rings. A type of rotor winding frequently adopted is similar in form to the three-phase 182 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Y stator winding already described, but the three free ends of the winding are led to three slip rings upon which brushes bear FIG. I06. SQUIRREL-CAGE ROTOR. (Fig. 107). These are connected to the three terminals of a Y-arranged variable resistance, the function of which will be considered later. FIG. 107. SLIP-RING (WOUND) ROTOR. Fundamental Equation of the Induction Motor. The funda- mental equation of the induction motor is the same as that of the transformer, with the exception of the winding constant K r m jo' 8 , (26) where E = the c.e.m.f. in volts, N the turns in series per phase, / = cycles per second and < m the total maximum flux per pole. K 1 is a constant required to correct for the departure of the flux distribution from the true sine wave form, and its value varies between .63 and i.oo, depending upon the number of phases, slots per pole and pitch of end connections. POLYPHASE INDUCTION MOTORS. 133 jM Since ro = 3> a , the total average induction around the air gap is 7T P = = - - where P = number of poles. Putting P&a = $ and = - we have, 120 _ 4.25 io 9 ^ 2 \/ 2 l^AT r.p.m. #1^ r.p.m. Formulae 26 and 27 show that for a given induction or flux the turns of winding are directly proportional to the voltage and inversely as the speed. With a given motor winding the flux varies directly as the volts and inversely as the frequency. An equation for the magnetizing current of the induction motor may be developed as follows: To produce a certain flux density in the gap, a number of ampere turns / N a are required for the path through air, and a number / Ni are necessary for the path through the iron. Then the magnetization factor, or the total m.m.f. in terms of that required to produce the necessary flux in the air path, is This quantity varies in actual design from i.i to 1.5. That is, the ampere-turns required for the gap are the controlling factor. The / NI may be calculated from magnetization curves of the punchings employed, and the / N a may be obtained from the relation I-N a = . 3 i 33 m L g + S. (29) Where S the total surface of air gap is the length of core X cir- cumference, L g is the effective length of mechanical gap X 2. All values of the above are expressed in inches. The magnetizing current 7 mag corresponding to the air gap should be as low as possi- ble because it is entirely wattless and thus reduces the power factor of the motor. It can readily be obtained with the magnetization factor evaluated, being /mag = la X M.F. ( 3 o) The value of / mag varies from 15-30 per cent of the rated load 184 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. current, depending upon the size of the motor, the larger per cent being for the smaller sizes. The value of I a is given by the equation .31.33 m L a P ' VzNS The magnetization volt-amperes will therefore be / mag - ' 3I3 ^y >P M - F - (32) Combining this with equation 28, we get for a given motor the relation -pi p2 *-F. (33) That is, the magnetizing volt-amperes for a certain magnetic circuit are proportional to the square of the voltage and to the square of the number of poles, while inversely proportional to the square of the number of turns and to the frequency. It is evident from this that to keep the same percentage of magnetizing current, the turns and volts must be proportional if one or the other change. Similarly with a change in the frequency, the volts should vary as the square root of the frequency. The leakage reactance, or those portions of primary and second- ary reactances which are due to leakage of flux, is difficult to determine accurately without tests. It may be predetermined within about ten per cent by means of such a formula as given by Professor C. A. Adams (A. I. E. E. Transactions, June, 1905). It may be expressed in reactance ohms or inductive volts per ampere, or per cent of total flux. There are four components comprising the total leakage, namely, primary, secondary, zig-zag, and end- leakage. Each of these four factors is proportional to the ampere turns per slot. The slot leakage, primary and secondary, varies inversely as the slot-width, and directly as the slot depth, the exact functions being quite complex. The zig-zag leakage, threading from primary to secondary slots, varies inversely as the air gap length. The end leakage varies roughly with the throw or circular span of the coils, or inversely as the number of poles. A certain number of corollaries follow from the above relations. POLYPHASE INDUCTION MOTORS. 185 (a) Either of the quantities which determine a low-speed motor, i.e., low frequency or large number of poles, increases the per cent of leakage for a given total induction by decreasing the flux per pole. (b) The per cent leakage varies inversely as the square of the voltage, since for a given apparent watts input, the current and the flux per pole respectively vary inversely and directly as the voltage. (c) The effect of the slot openings is to cut down the slot leakage flux. Hence the use of open secondary slots, even where the con- ductors are placed in slots from the ends and not from above as in the primary. The leakage current is that additional magnetizing current re- quired to maintain the primary flux against the secondary reactions. It may be determined from tests, very easily and with considerable accuracy; either from pull out (or maximum torque), or from the locked current (which is the current drawn by the motor when rated voltage is applied to the stator with the rotor held stationary). The following empirical relation between pull out torque and per cent leakage has been found to hold : T. ..11 40 Rated load torque , N Per cent leakage = ^ (34) Pull out torque If readings of voltage, amperes and watts are taken with the rotor locked, the leakage ohms (a>i) are : El sin "'= -J 2 ' (35) The conditions being even more exaggerated than they are in a transformer with its secondary short-circuited, the mutual flux is reduced to the very small value required to maintain the current through such an exceedingly low resistance, and the magnetizing current is also very low. Under these conditions we have Per cent leakage = COS - (36) E - Ir l Where I equals rated load current, cos equals full load power factor, E equals rated voltage, and Ir l equals rated load primary drop. The denominator only approximates the useful or c.e.m.f., since it does not take into account the Ix drop; but will be found 186 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. satisfactory as far as practical results go, although not rigorously exact. The power factor of an induction motor may be determined for any load, from the per cent value of the two wattless components of the current input, by means of the relation : cos = Vioo 2 (per cent magnetization + per cent leakage) 2 = Vioo 2 - (M + L? . (37) wherein M and L are the respective values of the magnetization and leakage currents in per cent of the total current. The magnetization current is substantially the same at all loads, hence its percentage varies inversely with the load. The leakage current, however, is a direct function of the load, being substantially zero at no load. To show the effect of various relative values of percentage leakage and magnetization currents the following example is given, the selected motors having the same value of M + L at rated load. EFFECT OF LEAKAGE AND MAGNETIZATION CURRENTS UPON MOTOR POWER FACTOR. Load. Motor No. 1. Motor No. 2. Motor No. 3. 50% Rated Per cent L Per cent M P.F. 5 60 76 10 40 86.6 15 20 93.5 Rated Per cent L Per cent M P.F. Pull Out Torque* 10 30 91.7 4 20 20 91.7 2 30 10 91.7 1.33 125% Rated Per cent L Per cent M P.F. 12.5 24.0 93 25 16 91.3 37.5 8 89.2 150% Rated Per cent L Per cent M P.F. 15.0 20.0 93.6 30 13.3 90.1 45.0 6.6 85.6 175% Rated Per cent L Per cent M P.F. 17.5 17.2 93.8 35.0 11.4 88.5 52.5 5.7 81.3 200% Rated Per cent L Per cent M P.F. 20 153.6 40.0 86.5 Pull Out. * In terms of rated load torque. POLYPHASE INDUCTION MOTORS. 1S7 Examination of this table indicates that motor No. i is best suited to heavy overloads on account of the small percentage of its leakage current. Motor No. 3 is best suited to light loads by reason of the small percentage of its magnetization current. The curves given in Fig. 108 are drawn from the data of the above table, per cent load and per cent power-factor being employed as abscissae and ordinates respectively. IOC I 90 8C 70 25 75 100 125 Percent Rated Load 150 175 200 FIG. I08. EFFECT OF LEAKAGE AND MAGNETIZATION CURRENTS UPON PGWZX FACTOR CF DIFFERENT MOTORS. Torque and Speed. It was shown in the development of the elementary induction motor that the phenomenon which caused the secondary of the motor to revolve was the mutual action of the rotary field and the secondary currents. That is, the rotating magnetic field induces currents in the secondary, and these currents acting according to Lenz's law tend to stop the motion producing them. The rotation of the rotary field cannot, however, be halted by the secondary currents, but its speed can be relatively reduced; that is, the rotor can follow the field. At no load the e.m.f. induced in the rotor need only be extremely small, hence the rate of relative motion between field and secondary is very low, and the rotor revolves at approximately synchronous speed. As the load gradually increases, the current required in the secondary becomes larger; at the same time the frequency of the secondary e.m.f. is higher. The current does not increase at the 188 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. same rate as the e.m.f., since the reactance of the circuit is greater, hence the speed must fall off more rapidly than the torque growth would indicate. In addition to this, the decrease in speed with increase in torque is still further accentuated because the second- ary current lag becomes greater, consequently a proportionately larger current is required to produce the corresponding torque. Finally magnetic leakage becomes pronounced, the effective flux reduced and the speed must drop off an extra amount to compen- sate for this condition. Ultimately the required rate of flux cutting can no longer be maintained, and the motor stops or becomes stalled. The exact form of the speed torque curve depends upon the relation existing between the resistance and reactance of the secondary winding, and upon the leakage factor. The general form of the speed-torque or more correctly the torque- slip curve of an induction motor is indicated by the following equa- tion : T = N 2 e 2 r 2 s + ^ (r 2 + s 2 x 2 ) (38) * wherein N 2 is the number of turns per secondary circuit. e = induced volts per turn at standstill. r 2 = resistance per secondary circuit. oc 2 = reactance per secondary circuit at standstill. s = rotor slip. Wj = angular velocity of the rotary field. * The derivation of equa. 38 is based upon relations existing between the corre- sponding quantities in a transformer as follows: LetEi be the line voltage per primary circuit, and with the usual' low resistance of the stator winding, it may be placed equal to the voltage induced per primary circuit by the rotary field, or if e is the voltage induced per turn, N,e = E r Similarly at standstill the secondary induced voltage per circuit may be written , = N 2 e, and at any slip s, this secondary voltage becomes sNtf. This voltage has two components, its resistance and reactance drops, or sN 2 e = It r from which, ' The energy component of the secondary current is consequently This energy current, in terms of the primary current, when multiplied by the pri- mary voltage corresponds to that part of the motor input which represents the power of POLYPHASE INDUCTION MOTORS. 189 An examination of this torque formula indicates many of the characteristics of the induction motor, for example: i. The torque becomes a maximum when r 2 = 3x2', this follows directly by differentiating equ. 38 with respect to r: d I r 2 sN 2 2 e 2 \ MI (r 2 2 + s 2 x 2 2 )sN 2 2 e 2 - wi zr 2 (r 2 sN 2 2 e 2 ) which is placed equal to zero and simplified, giving: r- 2 2 + S 2 2 X 2 2 - 2r 2 2 = O, or 8X2 = r 2 for maximum torque as above stated. 2. The torque of an induction motor at standstill is (39) which is evidently greater the less the resistance of the motor winding, and the lower the angular velocity of the rotary field. 3. The maximum torque of a motor occurring when r 2 = sx 2 shows that maximum torque is exerted at standstill when r 2 = x 2 because 5 is then unity, or, AT" V 2 rr> -t o & / \ r <' ma * = ^7 2 (40 ' which varies inversely as the resistance and consequently to produce a great starting torque not only should r 2 and x 2 be equal but they should both be as small as possible. the rotor. The relation between these two currents is, however, expressed by the inverse ratio of turns, or this energy component of the primary current is: ***** (d) Ni(r* + S 2 *2 2 ) and when multiplied by the primary voltage E l = N t e it gives the watts input repre- senting the power of the rotor, or Rotor power = ** ' () This quantity, however, includes the copper losses occurring in the rotor, and these from equation (&) of the rotor becomes: are from equation (&) expressed by the term I 2 2 R 2 = ' * 2 ; thus the available power r 2 -f rx 2 r*+s*x* r* + s*x* . r* + LJ. v (3) That due to the mutual flux existing between primary and secondary. To determine the value of this we must consider the secondary current. Let its instantaneous value be i 2 = 7 2W sin cut. The flux through the primary due to this current is Mi v wherein M is the coefficient of mutual induction between primary and second- ary. The voltage thereby induced in the primary, assuming a one to one ratio of transformation, is a)MI 2m cos cut. To balance this the impressed voltage must have an opposite component or + cuMI 2m cos a)t, the effective value of which is ooMI 2 in quadrature with 7 2 . The total secondary e.m.f. is that due to the primary current, its value being ajM^ lagging in quadrature with respect to the current 7 r It is made up of two components, one in phase with the secondary current, namely, the 7/ 2 drop, and one the leakage reac- tion, wL 2 I 2 in quadrature with 7 2 . These various voltages are shown vectorially in Fig. no, wherein the primary current O7 X is taken as the horizontal axis of reference. The re- sistance drop of the primary is repre- sented by OA in phase with O7 X ; the leakage reaction of the primary is ALi, 90 behind Oh. OP is the induced e.m.f. in the secondary, due to the mutual flux. Its two components are PR 2 and OR 2 , corresponding to the secondary leakage reaction and resist- ance drop respectively. The compo- nent of the primary applied voltage, FIG. no. VECTOR DIAGRAM OF d ue to the mutual inductive reaction, VOLTAGES PER PHASE OF IN- is L c p er p en dicular to OR y The DUCTION MOTOR. , . ^ ,, impressed primary voltage is then the vector resultant of OA, AL l and Lf, or it is represented by the vector OC at an angle CO A or ahead of the primary current, the cosine of which represents the power factor of the motor. The POLYPHASE INDUCTION MOTORS. 193 angle POR 2 corresponding to < 2 is the phase angle between the secondary voltage and current. From C draw a line parallel to OR 2 , let this intersect AL l at N. This construction gives us two similar triangles, namely, OPR2 and NLiC, wherein L\C and PR? are pro- portional. Now divide each vector by I lt thus fixing the points A, L l and P in position, because now the corresponding vectors represent R v a>L v and cuM which are of constant value. Hence as the secondary current varies, the angle OR 2 P being a right angle, the point R 2 must describe a semicircle about OP as a diameter. The triangle NLf, however, is similar to OPR 2 , so that any change in the latter must be accompanied by a corresponding change in the former, or the point C must describe a semicircle on L^N as a diameter. The point O, being the origin of axes, is fixed in position; accord- ingly it follows that OV X OC is constant for all values of C*, which may be expressed as OV X OC = K or OV = K + OC. Tf jr However, by construction OC = ; consequently O V = 7 r The voltage E is constant, therefore OF is directly proportional to the primary current, and we have the important fact that the extremity of the vector representing the primary current moves along an arc of a circle as the load of the motor changes. With this rule estab- lished, we can construct particular circle diagrams adaptable to practical use. We shall employ the circle diagram proposed by A. S. McAllister in the " Electrical World " of April and May, 1906.! For the construction of this diagram the following readings must be determined, namely, voltage, current and watts with motor running without load, and voltage, current and watts with its rotor locked, also the resistance of each primary phase winding. The equivalent single-phase current is obtainable from the no load ammeter reading, and the equivalent single-phase locked current is derived from the locked conditions. The equivalent single-phase resistance of the stator can be calculated if resistance per phase winding is known. The reason for using single-phase equivalents is that the circle diagram when thus constructed gives directly the true motor input, torque and output. * The area of the rectangle constructed upon any total secant and its external part is equal to the square of the corresponding tangent. f Alternating Current Motors, A. S. McAllister, p. 109, 1909. 194 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The equivalent single-phase current in the case of two-phase circuits is the sum of the current in both phases, while in the three- phase system the equivalent current is V^ 7, where 7 is the average of the currents in each line. The equivalent single-phase resistance for any two-phase or three-phase system, when considering the like currents, is one-half that measured between phase lines by direct currents.* The watts input and current for the locked condition cannot be obtained safely with rated line voltage because of the danger of damaging the motor by the large current which then flows. In practice a locked saturation curve is obtained by plotting a series of four or five readings of current, power and torque with the test voltage at rated frequency, and varied between one-fifth and about three-fifths of the operating pressure employed as abscissa. The various curves are then continued beyond the test points by exter- polation. A rough approximation of the locked current and watts can be made by testing at one-half rated voltage, and then multiplying the current by two and the watts by four, but possible change in saturation is likely to introduce an error of large value, especially in the power-factor.f The above curve method is therefore preferable, although it is evidently open to some question. A series of locked saturation curves of a three- phase 8-pole, 60 -cycle, 2i5-volt 20-h.p. induction motor is illus- trated in Fig. in. Construction of Diagram. Let the vertical line OE, Fig. 112, represent the line voltage vector. Draw at their proper phase positions, to scale, the equivalent single-phase no load current OM and locked current OF, using the power factor quadrant to obtain the proper phase angles. Through M draw a line HM perpendicular to OE, join M and F\ draw, also, a line per- pendicular to the middle of ME, intersecting MH at X. With X as center and either XM or XF as a radius, describe the arc MCF; this is the locus of the primary current. The distance HG represents the added primary or stator loss existing with rotor locked, its length = (added primary copper loss -H total locked watts) X IF. Draw the line GM. With this construction com- pleted, the performance of the machine may be determined directly * A. S. McAllister, Alternating Current Motors, pages 13, 14, 15. f The rotor should be allowed to rotate very slowly during this test, or the position of the rotor should be varied and the results ave r aged. POLYPHASE INDUCTION MOTORS. 195 J 200 ISO ICO I 400 30 20 200 -100 50 100 Volts 150 200 FIG. III. LOCKED SATURATION CURVE, 2O-H.P., 2I5-VOLT, INDUCTION MOTOR. by inspection. For example, the factors indicating the operation of the motor with a current P are as follows: OP to scale represents the equivalent single-phase primary current. Cos. angle POE equals power factor of motor. MP equals secondary current in primary equivalents. PT equals primary input in watts. TS equals no load losses in watts. RT equals total primary loss in watts. PR equals total secondary input in watts. RS equals the added primary copper loss. QR equals secondary copper loss. QP equals motor output in watts. 196 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. ^ pT equals motor efficiency. ~QR -r- PR equals the per cent rotor slip. Torque . 746 r.p.m. r.p.m. OM' = magnetizing current. U'T = leakage current. POLYPHASE INDUCTION MOTORS. 197 OM ' -*- OP = per cent magnetizing current. M 'T -j- OP = per cent leakage current. Maximum torque is CG', the point C is the extremity of a radius perpendicular to MG. Maximum output is BJ, the point B is the extremity of a radius perpendicular to MF. Maximum power factor exists when primary current vector is a tangent to the arc, corresponding to point P in the diagram. 100 10 50 300 100 150 200 250 Torque in Lbs. at Ft. Radius FIG. 113. CHARACTERISTIC CURVES OF A 6o-CYCLE, 2IS-VOLT, THREE PHASE, 20-H.F. INDUCTION MOTOR. The characteristic curves in Fig. 113 are those of a three-phase, 6o-cycle, 8-pole, 2i5~volt induction motor of 20 horsepower capacity, the values for the construction of these curves being obtained from the circle diagram just given. The fundamental data employed were derived from test and are as follows: No Load Values. Locked Values, (See Fig. 112.) Volts 215 215 Eouivalent single-phase amperes 33.5 430 Total watts 930 43 X 10 3 Power factor 12.9% 46.5% Hot resistances of stator: Ph (1 - 3) = .107 w; Ph (3 - 2) = .109 &; Ph(2- 1) = .106 w 1 08 ' Equivalent single-phase resistance = '- = 054 w 198 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Construction. Draw the power factor quadrant Fig. 112 with radius of 2.5 in. (.25 in. = 10 per cent p.f.)- Lay off p.f. = 12.9 per cent and draw the no-load current vector OM = .42 in. (i in. = 80 amperes). Lay. off p.f. = 46.5 per cent and draw locked current vector MF = 5-375 in - Join F and M, draw MH perpendicular to voltage line OE. Bisect MF and erect a perpendicular at this point. The inter- section of this perpendicular with MH at point X is the center of current locus. Draw an arc through M and F with X as center. Determination of motor performance at load corresponding to current OP : OP = 1.57 in. = 126 amperes; continue OP until it intersects power factor circle, then project intersection to power factor ordi- nate = 2.20 in., or 2.20 -H 2.5 = 88.0 per cent. Line M G is drawn as follows : PI = total power input at starting motor with rated voltage = 43 kw. (to scale 2.46"). HI = MN = power input at no load = 930 watts. HF = secondary copper loss at starting + increase of primary copper loss for the starting current OF of 430 amperes. Primary copper loss in watts at starting with rated line voltage = 430 2 X - - = 10 kw. 2 .I08 No load primary copper loss = 33. 5 2 X = .06 kw. Increase in primary copper loss at starting = 9.94 kw. The line FI = 43 kw. = 2.46 in. or 17.5 kw. = i in.; thus dis- tance HG or added primary loss = - = .57 in., which determines position of G, and from it MG is drawn. The slip at load corresponding to current P (i.e., 126 amperes) is QR -7- RP = - = 10.9 per cent. Synchronous speed = = = 900 .*. speed = 900 (.100 .109) = 802 r.p.m. Motor input = TP = 1.4 in. or 1.4 X 17.5 = 24.5 kw. Motor output = PQ = 1.15 in. or 1.15 X 17.5 = 20.2 kw. = 27h.p. Motor efficiency = PQ -i- P T = 20.2 -r- 24.5 = 82.5 per cent. POLYPHASE INDUCTION MOTORS. 199 watts output X 7.05 20200 X 7.0=; Motor torque = ^ ^ = i761b. r.p.m. r.p.m. at a ft. radius. Since the torque corresponding to current OP is by calculation 176 Ibs. at a ft. radius, and the vector RP correspond- ing thereto is 1.38 in. long, we can state that i in. on the torque lines of Fig. 112 is equivalent to an effort of 137 Ibs. at a foot radius. Per cent magnetization current = ON + OP = ~ cent. 26 per Per cent leakage current = NT -^ OP = - = 24.6 per cent. DATA FOR CHARACTERISTIC CURVES OF A 20-H.F. INDUCTION MOTOR DERIVED FROM CIRCLE DIAGRAM. FIG. 112. Point. Equi. Primary Amps. Per Cent Slip Q ^ino Pfl 10( R.P.M. Altsd -s) Per Cent Efficiency f xioo H.P. Output PQ" X23.5 Torque Lbs. at Ft. Radius. PR" X137 Per Cent P.F. poles M 33.5 900 13 1 52 4.7 858 82.0 9.1 59 74 2 84 6.2 844 86.2 17.6 110 86 P 126 10.9 802 82.5 27.0 176 88 4 170 16.0 756 79.0 33.6 233 87 5 218 21.7 704 70.7 38.0 284 83 B 254 27.0 657 66.0 39.5 315 80 7 292 33.8 596 58.5 38.0 336 76 C 324 40.0 540 51.2 34.5 343 70 9 364 53.4 419 39.0 26.5 333 64 10 397 79.0 189 23.8 15.5 307 56 F 430 100 255 46.5 Maximum motor torque CG' = 2.5 X 137 == 343 ft. Ibs. Maximum motor output = 1000 (BJ X 17.5) -* 746 = 29.6 -f- .746 = 39.5 horsepower. The performances for different current values corresponding to a series of points indicated on the circle diagram were simi- larly obtained, and for convenience of reference are arranged in the preceding table, to which the curves in Fig. 113 cor- respond. The speed regulation of this particular motor is fairly good up to 150 per cent of rated torque, beyond which limit the drop in speed becomes pronounced, and at a torque of 343 ft. Ibs. (2.7 times rated value) the motor reaches its "pull out torque." The "pull put" 200 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. limit (i.e., maximum torque developed) of an induction motor is that point upon its speed-torque curve at which any attempt to further increase the torque causes the motor to fall rapidly in speed and stop. This characteristic is very pronounced in induction motors, the exact location of this point depending largely upon the flux leakage occurring. It usually varies between two and three times the rated torque, depending upon the size of the motor, and may be obtained at starting if the rotor resistance and "standstill" reactance are made equal (p. 189). The maximum horsepower output of this induction motor is obtainable at a speed greater than that existing at its maximum torque, and this is usually the case with electric motors. The power factor curve indicates one of the diffi- culties caused by induction motors on a circuit, namely, the produc- tion of a wattless current, which is particularly pronounced at light loads. The power factor of an induction motor increases with the load to nearly the " pulling out " point, after which it decreases, and unless a special method be employed to secure maximum torque at starting, the power factor at standstill is usually much lower than when running at or near rated load. The following table gives the characteristics of operation attained DATA OF CROCKER-WHEELER POLYPHASE INDUCTION MOTORS. H.P. RPM. Poles. Slip Per Cent. Start Amps. Per Cent. Start Torque Per Cent. Pull Out Torque Per Cent. Per Cent Power Factor. Per Cent Eff. i f 1 f } f f 1 * 1800 4 5.0 350 170 250 50 62 69 74 70 74 75 74 1800 4 5.0 400 150 240 62 72.5 78 81 74 78 79 79 2 1800 4 5.0 550 200 320 64 75 83 85 77 81 82 82 3 1800 ,4 4.4 650 220 350 74 83 88 90 79 83 84 83 5 1800 4 4.4 625 230 350 76 85 89 90 82 84 85 84 7.5 1200 6 5.0 625 250 300 76 84 88 89 84 85 86 85 10 1200 6 5.8 500 200 275 78 86 89 90 84 85 85 83 15 1200 6 5.0 625 200 300 78 86 89 91 85 87 87 86 20 1200 6 5.0 625 250 300 76 85 89 90 85 87 87 86 20 900 8 6.5 550 160 250 72 82 86 88 86 87 87 86 25 1200 6 4.2 650 250 325 79 87 90 92 86 88 88 87 30 900 8 5.5 625 200 300 78 86 90 91 87 89 88 87 40 900 8 4.4 600 200 300 79 86 89 90 86 88 89 88 50 900 8 3.9 650 225 325 74 84 89 90 87 89 90 90 75 720 10 4.1 600 200 300 78 86 89 90 88 90 90 89 100 720 10 4.1 650 210 325 76 85 89 90 88 90 90 90 150 720 10 3.5 725 200 360 81 88 91 92 88 90 91 90 POLYPHASE INDUCTION MOTORS. 201 by standard machines, and it should be noted that the power factor increases somewhat with the size of the motor. Starting and pull out torques are in terms of rated load torque. Starting amperes equal amperes to start with rated load torque at line voltage, in terms of rated load current. For further information upon theory, construction and control of induction motors, the reader is referred to the following standard works: ALTERNATING CURRENT MOTORS. A. S. McAllister. New York, 1909. ALTERNATING CURRENT PHENOMENA. C. P. Steinmetz. New York, 1908. COURANTS ALTERNATIFS, Vol. II. G. Sartori. Paris, 1905. DYNAMO-ELECTRIC MACHINERY, Vol. II. S. P. Thompson. London, 1905. ELECTRIC MACHINE DESIGN. Alex. Gray. 1913. ELECTRIC MOTORS. H. M. Hobart. London, 1910. ELECTRIC TRANSMISSION OF ENERGY. Gisbert Kapp. THE INDUCTION MOTOR. Behrend. New York, 1903. THE INDUCTION MOTOR. De la Tour-Mailloux. New York, 1904. WECHSELSTROM-TECHNIK, Vol. V. E. Arnold. Berlin, 1909. CHAPTER XV. STARTING OF INDUCTION MOTORS. THE fact that an induction motor is substantially a transformer with a short-circuited secondary causes difficulty in starting, especially when its terminals are directly connected to full line pressure. For example: The locked saturation curves of an induction motor, as shown in Fig. in (p. 195), 'indicate that direct application of the full line pressure to the stator terminals, with the rotor short-circuited and standing still, produces an inrush primary current which is nearly five times rated value. Such excessive cur- rent is likely to injure the insulation of the windings and should be avoided. In addition to this, the power factor of this current is very low., being about thirty to forty per cent. It also affects the line regulation, causing voltage fluctuation. Consequently, when the motor to be started is of even moderate size (over i h.p.) some means should be employed to limit the inrush current to reasonable values. Two general forms of rotor windings are employed in practice as already stated on pp. 181-2, and as a result two methods of starting have been developed which depend respectively upon: (a) Reduction of Line Voltage. (b) Resistance Control. Starting by means of reduced line voltage is adopted when squirrel-cage rotors are employed, and it is generally accomplished through the introduction of an auto -transformer or compensator into the primary circuit. The underlying principle of this type of starter will be understood by referring to Fig. 114. The device is equivalent to a single-coil step-down transformer, the ratio of trans- formation being that existing between the total number of turns across which the primary terminals are connected and those between which the load is placed. In the specific instance illustrated in Fig. 114, the primary potential is 440 volts, the secondary voltage is 176, secondary current 200 amperes, and primary current 80 amperes. The voltage across the stator terminals is only a frac- 202 STARTING OF INDUCTION MOTORS. 203 tion of the line potential, when the switch is placed in the starting position, but after the motor has approximately reached its rated speed, the switch is thrown over rapidly into the running position, the stator winding being then directly connected to the supply voltage. 200 Amps. 80 Amps. 1 1 FIG. 114. SIMPLE AUTO-TRANSFORMER CONNECTIONS. The compensator windings for a three-phase motor consist of three coils, one for each phase, each coil being placed upon a separate leg of a laminated iron core. Each coil is provided with three or more taps, so that a number of sub-voltages may be obtained, any one of which may be selected for permanent connection to the throw-over (5) Running a. Starting FIG. 115. CONNECTIONS OF STARTING COMPENSATOR FOR THREE-PHASE INDUCTION MOTOR. switch, according to service conditions. The three coils of the com- pensator are Y-connected, the supply line to the three free ends and the starting connections of the motor to the taps being as shown in Fig. 115. To meet various requirements, compensators are gen- erally provided with taps giving potentials approximately equal to 204 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. 40, 58, 70 and 80 per cent of the line voltage, though the 70 per cent value meets most of the commercial requirements, as it gives prac- tically full load torque for starting. The line currents with the above taps are respectively 16, 34, 50 and 64 per cent of that which would be drawn by the motor if no compensator were employed. The chief objection to the compensator is its cost, being about 25 per cent of that of the motor. It has been suggested that this expense could be reduced by using one compensator for starting a number of motors, the method recommended being as follows:* A throw-over switch is provided for each motor to be started, and a three-pole compensator supply switch. Only one motor can be started at a time, thus avoiding the line disturbance caused by simultaneous starting of two or more motors, each motor switch being thrown into the running position as soon as the machine Supply Bus Bars t I ^ f I ( I c t r t r C.B.C I ft ] C H ] C ^ ?! I i i \ rl 1 [ f P I r r 1 ~T~I 1 ~T~1 C.B. Q Q Q ODD Running Side r; n i: 1 x 1 JC To Motor No. 3 To Motor To Motor in No - * D n n No ' 2 n n n Starting Side t i c r 5 r c t U i Compensator Bus Bars Compensator 1 i i FIG. 1 1 6. CONNECTIONS FOR STARTING SEVERAL MOTORS BY MEANS OF ONE COMPENSATOR. approximates normal speed. When all motors have been started, the compensator supply switch should be opened. The diagram (Fig. 1 1 6) shows the method of connecting three motors to one compensator. * G. Stevenson, Journal Institution of Electrical Engineers, Vol. XLI, 1908, p. 685. STARTING OF INDUCTION MOTORS. 205 Star-Delta Method. Three-phase motors maybe started without a compensator, by F-connecting the stator windings at starting, and employing delta connections for running, the change being rapidly made by means of a special throw-over or double-throw four point switch. The connections for such a starting scheme are illustrated in Fig. 117. By this method the voltage per phase at starting is only i -7- \/3 or 58 per cent of the line voltage. It follows, then, that the starting current and torque are also reduced. For example, con- Motor Windings A A' B B C AAAAAA A/WWV\ Starting c [[ LJ fill Li All Supply Lines L ] [ ] [ ] .{ [ r [ r t r Running BT r ] A[ lL [ ]c-.-- J FIG. Iiy. CONNECTIONS FOR STARTING THREE-PHASE INDUCTION MOTOR. STATOR Y-CONNECTED FOR STARTING. sider the 2o-h.p. motor already referred to; the starting current with F-connection would be only one-third of that taken if the motor were thrown directly on the line with delta-connected stator, or it would be (470 -r- 3) -r- 97 = 1.62 times full load current.* The starting torque being proportional to the square of the potential difference employed, would give a value of torque equal to one-third of the value obtained with full line voltage. Boucherot Method. An excellent method for starting induction motors provided with squirrel-cage rotors is that devised by M.P. Boucherot. | The general scheme is to employ the ordinary form of stator as the primary, and to provide a rotor with several squirrel- cage windings of graded resistance and reactance varying from high resistance with low inductance to low resistance with high * Rated load current equals 97 amperes. t Bulletin de la Societe Internationale des Electriciens, February, 1898, and Electric Motors, H. M. Hobart, pp. 325~337> London, 1910. 206 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. inductance. The high resistance circuits are the seats of large induced currents at starting, while those of high inductance have only small currents, because at standstill their reactance is high. The starting is due to the high resistance windings. As the rotor speeds up from standstill, the frequency of the secondary e.m.f. decreases; consequently the reactance of the windings diminishes, and all circuits carry current, that of the highly inductive circuits becoming relatively larger, because their resistance is extremely low. Thus the advantages of a high resistance rotor for starting are secured, while the poor speed regulation and low efficiency of such a winding under varying load are avoided by the fact that the low resistance (high reactance) windings are the working ones. A double squirrel-cage winding is usually found to be sufficient to meet practical requirements, Fig. 118 showing a rotor punching 12 13 FIG. Il8. ROTOR LAMINATIONS OF A BOUCHEROT MOTOR. of such a motor. The radial openings joining the upper and lower slots are designed to prevent the occurrence of excessive magnetic leakage with respect to the inner winding. Copper bars are placed in the outer series of holes, and these are connected by means of high resistance end rings formed of German silver or other resistance alloy. Copper bars of larger cross section than those of the outer STARTING OF INDUCTION MOTORS. 207 group are placed in the inner series of slots, and these are connected by low resistance end rings. The speed-torque curves of such a motor are illustrated in Fig. 119;* of these, curve A represents the action when the motor is op- 100 A 20 < FIG. IIQ. SPEED-TORQUE CURVES OF A BOUCHEROT INDUCTION MOTOR. 60 80 100 120 140 160 180 200 Percent Rated Torque erated with only the outer or high resistance winding active. In this case the starting torque available is nearly twice that at rated load, and the slip at rated load is about 25 per cent. Curve B indicates the speed-torque relations when the inner or highly reactive winding only is used. Under this condition the motor has practically no starting torque, while the maximum available torque when running is only 60 per cent of the rated value, and the corresponding slip is 6 per cent. The speed torque characteristic of the motor with both windings active is shown in curve C. The starting torque then obtained is substantially twice that existing at rated load. The speed regulation is excellent, a slip of but 6 per cent occurring at rated load. It is surprising that this method of control is not more widely employed, since the efficiency of the motor thus designed is high, the starting torque good, and the control extremely simple, all that is necessary to start the motor being the closing of an ordinary supply switch. Resistance Control. It was shown in the discussion of the torque equation of the induction motor (p. 181) that the starting torque of this type of machine may be varied by changing the resist- ance of its secondary winding. With this method of control the starting torque can be made to have any value, up to the maximum ; * Electric Motors, H. M. Hobart, p. 330, London, 1910. 208 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. that is, two or three times the rated load torque. In the case of small machines (3 to 5 horsepower), in which no speed regulation is required, provision may be made to locate the special resistance grids in the annular space between rotor core and shaft, employ- ing for this purpose an overhung core. For example, the three free ends of the rotor winding are connected to three resistance grids placed within the rotor spider. This resistance is subsequently cut out, by operating a lever which engages a collar free to slip longitudi- nally upon the shaft. This collar moves over the resistance grids, gradually reducing their value, until they are completely short-cir- cuited. This method, while applicable to small machines, is not advisable for large ones on account of excessive PR loss in the resist- ances, which if confined within the rotor would produce extreme heating and perhaps ultimately injure the motor. Consequently, in large machines, or in the case of those whose speed is to be ad- justed, the regulating resistances are placed external to the motor, connections being made to the free ends of the Y-rotor winding by means of three slip-rings and brushes, Fig. 120. This type of resistance control, owing to the presence of the slip-rings, is commer- cially known as the slip-ring method. b c FIG. 120. CONNECTIONS OF SLIP-RING STARTING DEVICE. The slip of an induction motor at a given torque varies directly as the secondary copper losses (p. 190); hence if the rotor resist- ance per phase winding be doubled, the slip for any given torque will be increased 100 per cent; if the resistance be increased to three times its initial value, the slip will be thrice its former amount, etc. The curves shown in Fig. 121 are obtained from the speed-torque curve of Fig. 113, and they correspond to secondary rotor resistances of one, one and one-half, two, four, five and eight times that existing with the rotor short-circuited. These externally added resistances STARTING OF INDUCTION MOTORS. 209 are Y-connected and the movable contact arms cut out resistance equally in each of the branches, as shown in Fig. 120. The amount of external resistance needed to obtain any given starting torque within the range of the motor's capabilities can be readily determined from the speed-torque curve obtained when the rotor is operated with its windings short-circuited. For example, it is desired to have the typical motor operate so that it will give, as a maximum, approximately rated torque when starting; and Fig. 113 shows that rated torque exists when the slip is eight per cent. 1000 500 900 800 M 400 700 400 200 I g 300 g 200 ,| 100 100 \ \ 50 100 150 200 Ft. Lbs. Torque 250 300 350 FIG. 121. SPEED-TORQUE CURVES OF A 2O-H.P. INDUCTION MOTOR, WITH VARIOUS VALUES OF ROTOR RESISTANCE. Hence to have this torque developed at standstill, the desired resistance of the rotor circuit must be such as to increase the slip about twelvefold. However, since the resistance per phase winding of the rotor is .044 ohm, approximately .5 ohm additional must be placed in each branch. Similarly, if it be desired that the motor exert the maximum torque available at starting, the necessary ex- ternal resistance can be also determined directly from the speed- torque curve of Fig. 113. The slip at maximum torque is 40 per cent, therefore to have 100 per cent slip and same torque, the rotor resistance must be increased to about 2.5 times its initial value, that is, a total of .044 X 2.5 = ,110 ohm must be placed in each phase circuit of the rotor. The advantage of employing an adjustable resistance in the rotor 210 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. circuit for starting a motor is clearly indicated by the curves in Fig. 122. Of these, curve A shows the starting current drawn by the typical induction motor, when connected directly to the line without starting resistance in the rotor circuit. Curve B shows the 500 8 10 12 14 16 18 20 22 Time in Seconds FIG. 122. STARTING OF 2O-H.P. INDUCTION MOTOR, WITH VALUES OF ROTOR RESISTANCE. r 2 ro 3 r 4ro 5ro 6ro 7ro 8ro Rotor Resistance in Terms of-the Short Circuit Value FIG. 123. EFFECT OF ROTOR RESISTANCE UPON THE ACTION OF A 20-H.P. INDUCTION MOTOR starting current existing when three times the initial rotor resist- ance (.132 ohm per circuit) is employed and the increase of current occurring when this resistance is short-circuited on the second step after twelve seconds acceleration. Curve C shows what results when the added external resistance is four times the rotor STARTING OF INDUCTION MOTORS. 211 resistance (total per phase .22 ohm), and is gradually reduced in five steps to its short-circuit value. This method causes a very marked reduction in the average starting current, and is used where the supply circuit must not be disturbed by voltage fluctuations. The effects of adjustable resistance in the rotor circuit upon power factor, torque and primary current at starting, as well as upon the speeds attained at rated torque, are indicated in the curves of Fig. 123, which refer to the 2o-h.p. motor previously considered. These curves show that addition to rotor resistance at starting improves the power factor, reduces the starting current, while it also in- creases the starting torque until the rotor resistance equals rotor reactance, beyond which the torque falls off. Calculation of Slip-Ring Control. The determination of the resist- ance steps for the starter of a slip-ring induction motor is a simple matter, and it depends entirely upon the fact that the slip at a given torque varies directly as the resistance of the rotor circuit. For example, let it be supposed that the typical 20 h.p. induction motor is to be provided with a starting controller such that the following specified conditions will be met: Starting torque to be twice rated value. Inrush current at starting not to exceed 200 amperes (single phase equivalent). Current not to exceed above value upon change in setting of con- troller arm. Power factor during period of acceleration not less than 85 per cent. In the following procedure it is assumed that the motor is to accelerate against rated torque, and that the controller arm is to be held upon any given notch until acceleration for such setting has ceased. If this is not the case, it is well to double the number of resistance steps, making these added steps one-third less resistance than those originally determined. Draw the torque-per cent slip, torque-current and torque-power factor curves of the motor as in Fig. 1230. Reference to this figure shows that twice rated torque is 250 Ibs. at one foot radius, and that it occurs under normal operat- ing conditions with a current of 180 amperes (s.p. equiv.) and at a power factor of 85 per cent, thus conditions specified are obtainable. The slip at 250 Ibs. torque (rotor windings short circuited) is 17 per cent; hence to obtain a like torque at starting the rotor circuit 212 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. resistance must be increased from normal value to substantially six times the same or from .044 to .27 ohm. With such resistance per phase in the rotor, the motor will develop a starting torque of 250 ft. poundals and accelerate the rotor along slip curve CD against rated torque until the slip is six times the rated value, or 48 per cent (468 r.p.m.). To produce further acceleration, the rotor resistance must be again reduced. To have a slip of 48 per cent and a torque of 250 ft. poundals, the rotor resistance must be decreased from 6 times to 2.83 times normal or from .27 to .127 ohm because with normal conditions of operation the slip at above torque is 17 per cent. If 100 50 100 150 200 250 Torque in Lbs. at Ft. Radius 300 850 FIG. 123 A. DETERMINATION OF NUMBER OF SETS IN SLIP RING CONTROL. such a change is made in the controller setting, the motor again develops a torque in excess of load requirements and the rotor accel- erates along line EF until rated torque is attained, which occurs at a slip of 23 per cent or 680 r.p.m. Further acceleration being still required, rotor resistance must be again reduced. Reference to the torque-slip curve shows that 23 per cent slip at a torque of 250 ft. poundals is one and one-third times normal slip, hence to produce an acceleration beyond 680 r.p.m. the rotor resistance must be reduced from .127 to .06 ohm causing the rotor to speed up as shown by line GH until rated torque at if times rated slip is attained, which is 10.7 per cent slip or 804 r.p.m. Further reference to the slip-torque curve shows that a torque of 250 ft. poundals is not STARTING OF INDUCTION MOTORS. 213 obtainable with a 10.7 per cent slip, neither can the current rise above 130 amperes nor the power factor fall below 88 per cent if the rotor resistance is short circuited. Hence after acceleration has ceased on the third point, the control arm may be moved with safety over to the full speed position. Collecting the above results in tabular form we have : Control Notch. Rotor Circuit Resistance per Phase. Controller Resistance per Phase. Upon Movement of Controller Arm. After Acceleration has Ceased. Amps. P.F. R.P.M. Amps. P.F. R.P.M. Starting 2nd 3rd 4th .27 ohm .127 .06 .044 .226 ohm .123 .16 180 180 180 125 85 85 85 88 468 680 804 95 95 95 95 88 88 88 88 468 680 804 830 Excellent discussions concerning the various methods employed for starting poly- phase induction motors are given in the following : ALTERNATING CURRENTS. A. Hay. London, 1906. ELECTRIC MOTORS. H. M. Hobart. London, 1910. ELECTRIC MOTORS. N. G. Meade. 1908. HANDBUCH DER ELECTROTECHNIK, Vol. IX. Leipzig, 1901. WECHSELSTROMTECHNIK. E. Arnold. Vol. V, Berlin. 1909. POLYPHASE MOTOR. B. G. Lamme. Electric Journal, Vol. I, 1904. THE INDUCTION MOTOR, CHOICE OF TYPE. G. Stevenson. Journal Inst. E. E., Vol. XLI, 1908. CHAPTER XVI. SPEED CONTROL OF POLYPHASE INDUCTION MOTORS. THE induction motor, as already shown, is substantially a constant speed machine. Its change in speed beween rated load and no load is from 4 to 8 per cent, depending upon the capacity of the machine, the larger sizes usually having the better speed regulation. However, for many practical applications, such as hoisting, machine tool and traction work, it is desirable and often essential to vary the speed of a motor. It is the object of this chapter to examine the various methods of controlling the speed of induction motors, which is even more difficult than it is for direct-current motors. These methods are: Variation of the frequency of the supply voltage. Variation of the number of motor poles. Variation of the rotor resistance. Cascade cr concatenated connection. Variation of applied potential. Combinations of these methods are often employed to obtain wider speed ranges, better regulation or more gradual steps of adjustment than those economically possible with any single control. Variation of Supply Frequency. The speed in r.p.m. of the rotary field of an induction motor being, as already shown, equal to 60 frequency -.-pairs of poles] any change in the periodicity of the applied voltage would be reproduced in exact proportion in the speed of the rotary field. Hence, variation of frequency is theoreti- cally the ideal means of speed control; unfortunately, however, the obtaining of such a source of power supply is not commercially feasible at present. In case only a single motor is operated, the generator speed could be altered, and thus the frequency of the current. The voltage should be varied in proportion to the fre- quency with this method of control, otherwise the no load current 214 SPEED CONTROL OF POLYPHASE INDUCTION MOTORS. 215 would either be excessive or too small, according as the frequency is low or high, thus considerably changing the power-factor of the machine. cated, is substantially a constant torque machine, in the sense employed in this book (p. 71). The speed-torque, current-torque, and power factor-torque curves of a 2o-h.p. motor, when oper- ated with currents having frequencies of 20, 40 and 60 cycles, are respectively as shown in Fig. 124. The supply voltage is 100 150 Ft. Lbs. Torque FIG. 124. CHARACTERISTIC CURVES OF AN INDUCTION MOTOR WITH FREQUENCY CONTROL. altered with the frequency; that is, pressures of 72.5, 143 and 215 volts are employed. This group of curves shows that the current and power factor for any given torque equal to, or less than, the rated value are practically constant, and independent of the fre- quency employed. The speeds attained at a given torque vary in a greater ratio than the frequency, being 836 r.p.m. at rated torque and 60 cycles, 530 r.p.m. at 40 cycles; and 230 r.p.m. at 20 cycles, practically a 4 to i range. This difference is due to the departure of the machine from ideal conditions, since resistance and leakage 216 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. are present. The regulation becomes poorer as the periodicity is lowered. Speed Control by Changing Number of Poles. The synchronous speed in r.p.m. of an induction motor being given by the expression 60 X / -v- p (p. 181), it is evident that the speed varies inversely as the number of poles. Thus a motor wound for six pairs of poles and operating normally at 600 r.p.m. will rotate at a speed of about 1 200 r.p.m. if its stator winding be rearranged so as to have 3 pairs of poles. The simplest method of applying this control is to employ a stator having two or more separate windings, correspond- ing to different numbers of poles. One winding may also be used, the different speeds being obtained by means of a commutator switch, which alters the grouping of the coils and thus the number of poles. A rotor of the squirrel-cage type is the only practical one, because it is short-circuited upon itself and is therefore adapted to any number of poles. A grouped or polar rotor winding requires a rearrangement of its coils in the same order as those of the stator, though two or more independent rotor windings could be used. The connections of a multi-speed motor of this type are relatively simple, especially if only two to one ratio in speed is required and the rotor is of the cage type. In this case, only six leads are brought out from the machine for three-phase circuits, and eight for two- phase lines. In case, however, a polar rotor winding is used (to allow for slip-ring control), twelve leads must be brought out from the machine for three-phase connection, six of these terminals being for the stator winding and the remainder for the rotor. Similarly if a three to one speed adjustment (in three steps) were wanted, three-pole groupings would be required, with eighteen leads brought out from the motor, nine for the stator and rotor respectively. Con- sequently this method of control is objectionable in the complication of connections when more than a two to one speed is desired, espe- cially for machines having wound rotors. A further criticism is that the speed changes can be made only by opening and closing the connections to the supply lines, which as already shown (p. 202) is very likely to cause wide variations in the primary current and fluctuations in the line voltage. The power factor of this type of multi-speed machine is not greatly affected by change in the number of poles, though it is somewhat SPEED CONTROL OF POLYPHASE INDUCTION MOTORS. 217 higher with the smaller number. The efficiency and speed regu- lation are better with the greater number of poles. Variation of Resistance of Rotor Winding. The third method of adjusting the speed of an induction motor is by varying the resist- ance of the rotor winding. This arrangement has already been considered under the heading of slip-ring control, and curves show- ing the effect of resistance in the rotor are given on pp. 209, 210. It does not give a constant speed over the torque range, in fact, the speed changes occurring upon variation of torque are very marked, and depend upon the value of the resistance employed, as shown in the curves of Fig. 121. The speed regulation is comparable to that of a d. c. shunt motor having an external resistance in series with the armature, and the other objections of low efficiency and con- siderable space occupied by the controller also obtain. Consequently, this method should be employed only when the periods of speed _^ adjustment are of relatively short duration, the motor being operated most of the time at rated speed. It is, however, used considerably in connection with the other methods of speed control, for transition from one running speed to another. Speed Control by Cascade Connection. The fourth system of induction motor speed adjustment is variously known as the cascade, concatenation or tandem control.* The application of this method necessitates the use of at least two motors, the revolving members of which are coupled together, either directly upon the same shaft or indirectly by the load, as in the case of an electric locomotive. The first of the motors (i.e., that normally connected to the line) has its rotor provided with a polar winding, arranged so as to deliver at standstill a voltage of the same pressure and number of phases as those of the power circuit. This secondary is connected to the stator winding (primary) of the second motor. The rotor of this latter machine may be of the squirrel-cage or slip-ring type. In case the slip-ring rotor is employed, resistance control may be utilized for transitional steps. * C. P. Steinmetz, Electrotechnische Zeitschrift, 1899, Vol. XIX, p. 884. Speed Control of Induction Motors, H. C. Specht, 1909, Elect. Journal, Vol. VI, Nos. 7 and 8. Multi-speed Induction Motors, H. Reist and H. Maxwell, Trans. A. I. E. E., Vol. XXVIII, 1909, p. 971. .Wechselstrom-technik, E. Arnold, Vol. V, pp. 485-519, 1909. 218 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The cascade connection of two three-phase induction motors is shown diagrammatically in Fig. 125. Motor A receives the line voltage at rated frequency upon closing the supply switch S. Its secondary delivers three-phase currents of the same frequency and voltage to the stator of machine B when switch S f is connected to its lower set of terminals; consequently both motors will accelerate. As motor A speeds up, however, the frequency of its rotor currents will decrease, and at fifty per cent rated speed this latter current will have a frequency of one-half of that of the line. Motor B receiving a current of one-half line frequency will also run at one-half A FIG 125. TWO THREE-PHASE MOTORS ARRANGED FOR CASCADE OR INDEPENDENT OPERATION. (Switch 5 up and S' down for cascade connection.) speed. Therefore, if both motors are coupled as shown, this half speed is the point at which the machines tend to operate together. Rated speed is obtained when one machine only is employed, the second being cut out entirely by short-circuiting the slip-rings of rotor A by means of the switch S'\ consequently this system gives a two to one speed adjustment. The above explanation applies to the use of two motors having an equal number of poles. If, however, the machines connected in cas- cade have a different number of poles, they will operate at speeds other than half normal. For example, referring to Fig. 125, when either motor A or B is operated singly, the synchronous speed in r.p.m. = frequency X 60 -r- pairs of poles, or the cascade set could be employed to give the speed of either motor, depending upon which one was connected to the line. The next step would be to connect the secondary of machine A to that of the primary of motor SPEED CONTROL OF POLYPHASE INDUCTION MOTORS. 219 B, short-circuiting the rotor of the latter. This connection also gives one of two speeds, depending upon the employment of direct or differential concatenation. If the former is used, both motors tend to rotate in the same direction and the synchronous speed of such a combination is given by the expression: r.p.m. =/x 60 -* (p A + p B ) (41) wherein /is the frequency of the supply circuit in cycles per second, while pA and pB are the pairs of poles of motors A and B respec- tively. Inverse or differential concatenation is obtained when the machines are so connected that they tend to start up in opposite directions; in such case the synchronous speed is: r.p.m. = / X 60 -^ (p A - PB). (42) Cascade connection of two motors having a different number of poles consequently provides a method of obtaining a four speed outfit, the speed range depending upon the number of poles of the respective machines. For example, if motor A has 6 pairs of poles and B has 2 pairs, while the line has a frequency of 60 cycles per second, the following synchronous speeds could be obtained : 1. Motor B operating alone, r.p.m. = / X 60 -j- p 3 = 60 X 60 -v- 2 = 1800. 2. Motors A and B connected in differential concatenation, r.p.m. = / X 60 -^ (pA ~ PB) = 60 X 60 -*- (6 - 2) = 900. 3. Motor A operating alone, r.p.m. = / X 60 -r- pA = 60 X 60 -r- 6 = 600. 4. Motors A and B connected in direct concatenation, r.p.m. = / X 60 ~ (p A + p B ) = 60 X 60 - (6 + 2) = 450. Hence a speed range of four to one is attained. The torque developed by a group of motors in cascade depends upon whether they are connected in direct or differential order, and it may be determined by the following equation : Torque in Ibs. at i ft. radius = .117 (Wi -W) PA ^ PB (43) wherein Wi represent motor watts input, W t watts lost in primary 220 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. of the motor, p A and p B number of pairs of poles of machines A and B respectively, and / the frequency of the supply current in cycles per second. The plus sign is employed in case of direct concatenation and the minus sign for differential connection. The latter gives the lowest starting torque, and the set will not start up if the motor having the larger number of poles is connected to the line. The method of starting in this case is to speed up the set by using the motor with the smaller number of poles singly, and when the synchronous speed for differential connection has been slightly exceeded, the switches are thrown over so that the desired differential arrangement is secured, after which the equipment will continue to work properly. It is possible to operate two motors in cascade, having the motor with the smaller number of poles normally con- nected to the line, which condition gives a self-starting differential arrangement, but this order of connection is not particularly desirable, because the iron losses of the set would be greatly exag- gerated owing to the high frequency of the current in the secondary circuit. The characteristic curves of a group of induction motors con- nected in cascade can be determined by means of any of the circle diagram methods, the test data necessary for the construction being determined in substantially the same manner as for a single machine. The power factor and efficiency of a cascade group of given capacity at any torque and speed will be lower than that of a single machine having the same rating on account of the combined wattless com- ponents and losses. A great advantage of the two-motor equipment is that two efficient running speeds can be obtained without opening the supply switch, hence the line disturbances that occur with the other form of multi- speed induction motors, are eliminated. It is possible by an extension of the cascade connection to three motors to obtain a very wide speed range of many steps. On a 6o-cycle circuit with a set of three motors having 14, 8 and 2 pairs of poles respectively, a speed range from 138 to 1800 r.p.m. is secured, which is a ratio of i to 13. The cost of such a system is extreme, however, and the usual demands of practice are more economically met by employing a two-motor set, utilizing gearing to secure the wider speed ranges. SPEED CONTROL OF POLYPHASE INDUCTION MOTORS. 221 Speed Control by Variation of Applied Potential. The slip of an induction motor, at a given torque, varies approximately inversely as the square of the primary voltage (p. 182), and this is the prin- ciple of the potential method of speed control. The usual means of securing this adjustable voltage is a compensator, with several taps, which is introduced into the primary circuit. The connec- tions for this method are substantially the same as those of the com- pensator starting device shown in Fig. 115 (p. 203), excepting that the contactors slide over the taps, instead of being fixed in position. The speeds obtained at different values of potential with various 700 600 a" 6 500 I: 1WO Rated Torqu 75 Rated Torque 125 L )s. Ft. 50$ Rated T< rque ' Rated Torque 60 70 80 90 Percent Rated Volts 100 110 120 130 140 J50 HO. 126. SPEED- VOLTAGE CURVES OF A THREE-PHASE 2O-H .P. INDUCTION MOTOR. values of torque are shown in the curves of Fig. 126, and from these the speed regulation, for any selected voltage, with change in torque is readily obtained. For example, the speeds for different torques at 50 per cent rated potential are determined by drawing a vertical line through this voltage abscissa. The intersections of this line with the curves gives the speed developed at the corresponding torques. The speed regulation of a motor con- trolled by this method is very poor, while the power-factor and efficiency decrease with the speed. The regulation and efficiency are even less satisfactory when adjustable resistance in the primary is employed in place of the compensator. In fact, the potential method of induction motor speed ELECTRIC MOTORS, THEIR ACTION AND CONTROL. control is unsatisfactory. It should not be used except for special service, as in the case of traveling cranes, for which the more desira- ble methods already considered introduce too great a complica- tion in wiring and trolley connections. For further information concerning induction motor speed control see the following: ALTERNATING CURRENTS. A. Hay. London, 1906. ELECTRIC MOTORS. H. M. Hobart. London. 1910. WECHSELSTROMTECHNIK. Vol. V. E. Arnold. Berlin. 1909. DIE REGULIRUNG VON DREHSTROM-MOTERN. W. Burkard. E, T.Z., August, 1903. DlE TOURENREGULIRUNG VON INDUKTIONS MOTERN. M. OsnoS. E.T.'Z .,igO2, p. 1075. SPEED CONTROL BY FREQUENCY CHANGERS. H. C. Specht. Elect. Journal, Vol. VI, 1909. SPEED CONTROL: POLYPHASE MOTOR. B. G. Lamme. Electric Journal, Vol. I, 1904; Vol. VI, 1909. THREE-PHASE MOTORS WIDE SPEED RANGE. Dr. H. B. Eschenburg. Electri- cian, London, 1903. TOURENREGULIRUNG VON INDUKTIONS MOTERN. J. K. Sumac. Z. E. T., 1904. METHODS or VARYING SPEED OF A. C. MOTORS. G. A. Maier. Trans. A. I. E. E., Vol. XXX, Part III, 1911. p. 2455. CHAPTER XVII. THE SINGLE-PHASE INDUCTION MOTOR.* THE simplicity of single-phase systems in comparison with poly- phase ones makes them more desirable for small alternating-current plants. The constant-speed motor most extensively used in con- nection with such service is of the single-phase induction type and structurally it is very similar to the corresponding polyphase machine. f In fact any polyphase induction motor will operate as a single-phase machine of somewhat smaller capacity and lower power factor, if it is first caused to rotate at nearly synchronous speed by some starting device. The necessity for some such auxiliary device arises from the fact that the single-phase motor, per se, has no starting torque. Absence of Starting Torque. - Consider a bipolar single-phase motor, provided with a squirrel-cage rotor. The distribution of current in the secondary at standstill is as indicated in Fig. 127. The current in bars aa! is zero, because these are equivalent to a closed loop the plane of which is parallel to the flux. The maximum current is set up in bars W. However, this equivalent loop, if it moves at all, must move parallel to the direction of the lines of force, hence it exerts no turning effort. The bar m } carry- ing current as indicated, will exert a torque upon the rotor, as shown by the arrow alongside it. However, owing to the symmetry of the secondary winding, for every bar m there is another m' hav- ing a current of equal amplitude but of opposite sign. This latter bar being in a field of the same strength and direction as that in which m is located, will exert a torque equal to that developed by m, but in the reverse direction, as indicated by the corresponding arrow. In the same way the effort exerted due to the current in any bar of the winding will be neutralized by that of another sym- * The Single-phase Induction Motor, M. Arendt and J. H. Morecroft, G. E. Review, Vol. XIII, No. 5, 1910. f The first successful motor of this type was built by C. E. L. Brown, see Lon- don Electrician, Vol. XXX, p. 358, 1893. 223 224 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. metrically located with respect to the axis of the primary field. Consequently at standstill no turning effort is developed and the rotor fails to accelerate in a single-phase induction motor which has an oscillating in contradistinction to a rotary field. b FIG. 127. DISTRIBUTION OF CURRENT IN STATIONARY ROTOR OF SINGLE-PHASE INDUCTION MOTOR. The above fact may be proved as follows: Assume the rotor winding as composed of symmetrically placed short-circuited coils, and consider one having its plane at any angle a to the axis of the FIG. 128. SHORT-CIRCUITED COIL INCLINED TO AXIS OF MAGNETIC FIELD. field NS, as illustrated in Fig. 128. Further suppose the flux distribution to be a cosine function of a; this is approximately the case with actual motors provided with distributed stator windings, THE SINGLE-PHASE INDUCTION MOTOR. 225 and then let B represent the maximum flux density at a, = o, B cos pt represents the instantaneous flux density at a, = o, B cos pt cos a represents the corresponding value at the inductors selected, and with A as the area of the coil the flux passing through it becomes < = / AB cos pt cos ada = BA cos pt sin a. (44) "9 The e.m.f. induced in the selected coil is d$ e = = BAp sin pt sin a. (45) The instantaneous value of the corresponding current is i = BA p sin (pt 6) sin a -f- Z'. (46) Naturally in the case of a single coil this current will react upon the stator field and produce flux distortion; if, however, we sum up the effects of all of the rotor coils, the individual reactions balance, and the field distortion becomes negligible. It is to be noted that the impedance of a coil will be modified by the action of the neighboring coils, consequently Z' in equa. 46 repre- sents the effective impedance. The angle 6 = cos" 1 ( r' -*- Z'), where- in r' is the effective resistance of the coil and Z' the impedance as above defined. If there are n coils on the rotor equally spaced from one another, the torque of the Kth coil will be t k = lB*Ap [sin (2 pt - 6} + sin 0] sin n + 2 Z', (47) wherein / is the length of one coil. The instantaneous torque exerted by the whole rotor is T= 2/ = lB 2 Ap[sm (2 pt - 6)+ sin flJS^sin TT - 2 Z'=o* (48) Development of Revolving Field. We have just shown that when we have an oscillating magnetic field the rotor placed therein fails to exert any starting torque. Therefore, if a single-phase induction motor does develop a turning effort after it is caused to revolve, it must be because it has, by some reactions of the rotor * This same result is obtained from analysis of equation 60, p. 235. 226 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. currents upon the stator flux, set up for itself a rotating magnetic field. That such is the case may be shown non-mathematically. Assume a two-pole motor (Fig. 129) the stator winding of which is supplied with a single-phase alternating current, producing an oscillating field between the poles A A'. The rotor currents pro- FIG. 129. MAIN AND QUADRATURE FIELDS, SINGLE-PHASE INDUCTION MOTOR. duce a field at right angles to the main field, and for convenience we will assume this to be represented by the poles BB'. In com- mercial machines no such empty pole spaces exist, as practically all of the stator is covered with coils. The inductors of the revolving rotor have e.m.f's induced in them due to two actions, namely by motion through the field and by the time rate of change of the flux threading the coils. The first we shall designate as a rotational e.m.f. and the second as a transformer e.m.f. The inductors aa f will always have a rotational e.m.f. set up in them except when the stator field passes through zero value. The amplitude of this e.m.f. for any given speed will be proportional to the instantaneous value of the stator flux.- Conductors aa f may be considered equivalent to closed coils, and the current flowing in them will produce a field in direction BE'. Neglecting tem- porarily the IR drop in the rotor, the e.m.f. induced in aa f may be placed equal to -~ , where 4> r denotes the cross field developed by THE SINGLE-PHASE INDUCTION MOTOR. 227 the currents due to the motion of the rotor in the main field. The rotational e.m.f. is in time phase with the main field, hence the cross field 3>r will be in time quadrature with it. The direction of the main field and the motion of the rotor inductors are such that the e.m.f. generated in aa r is positive.* The rotor currents are in such direction that when pole A is of north polarity and decreasing, pole B will be of like sign but increasing, reaching its maximum strength one-quarter of a period later. The stiength of pole B decreases after a similar lapse of time, the main field reverses and a north pole begins to build up at A'. That is, the main field and quadrature field so combine that a north pole travels around the stator in the direction ABA'B'' at synchronous speed. Hence there exists a rotating field produced by the combined action of stator and rotor currents. This simple explanation gives an idea of the pro- duction of the rotating field in the single-phase induction motor, but it does not consider all the reactions which occur. The inductors bb' moving in the quadrature field have a rota- tional e.m.f. induced in them, in the same manner as those passing through the main field, and this is of maximum positive value when the north pole at B attains its highest value. In addition to these two rotational e.m.f.'s, the varying fields AA' and BE' set up transformer e.m.f 's, in coil groups W and aa r respectively. Conse- quently, there are four e.m.f.'s, to be considered before the actual rotor currents which produce the quadrature field can be determined. The rotational e.m.f. induced in inductors aa' is of maximum positive value when the pole A is at its greatest north polarity, but the transformer e.m.f. set up in these bars by the quadrature field is at the same moment of maximum negative value. Hence the actual e.m.f. (E a ) existing in A A ' is the algebraic sum of these two voltages. The rotational e.m.f. due to the main field must be greater than the transformer e.m.f. of the quadrature field; in fact the latter is of such strength that the actual e.m.f. (E a ) will be just enough to establish the current which produces the field BB'. Since this quadrature field is at right angles to the main field, its m.m.f. cannot be set up directly by the stator magnetizing cur- rent, so we must investigate further to see how it is taken, as it must be, from the line. It must be remembered that the imped- * Currents flowing away from the reader into the plane of the paper are called positive. 228 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. ance of the rotor coils is here assumed to be such that the IZ drop is negligible; if this is not the case, the rotational and transformer e.m.f.'s will not be in time opposition and their vector sum, instead of algebraic sum, must be considered. The main field, by transformer action, induces an e.m.f. in bars W, and this is opposed to the e.m.f. developed in the same inductors by their motion through the quadrature field. The resultant e.m.f. (E b ] in these conductors sets up a current affecting the main field and consequently the current drawn from the line. The current flowing in inductors W due to E b is equal to that existing in bars aa f , which latter is that producing the cross m.m.f. Moreover, the current bb' is in such direction that it increases the magnetizing current taken from the line, the increment being that which would be necessary to directly magnetize the quadrature field. The reluctance of the cross field's magnetic circuit is substantially the same as that of the main field, consequently the m.m.f. required for both will be the same, and obviously, therefore, a two-phase motor run on one phase will draw twice its nor- mal magnetizing current. This conclusion is borne out by actual practice, tests showing that the magnetizing current of a single - phase motor is double that taken per phase by a two-phase and three times that required by a three-phase machine, the potential difference, fre- quency and turns per phase winding being the same. At synchronous speeds the two component fields are of equal strength; accordingly they com- bine to give a circularly rotating field. Below synchronous speed the rotating e.m.f. in the bars aa' is reduced in inverse proportion to the slip, and thus the FIG. 130. FORMS OF ROTATING FIELD AT VARIOUS ROTOR SPEEDS. THE SINGLE-PHASE INDUCTION MOTOR. 229 quadrature field diminishes, while the main field remains constant. Consequently the strength of the rotating field developed below synchronous speed is represented by an ellipse, the shorter axis being in the direction of the quadrature field BB' '. When driven above synchronous speed the field is also elliptical, the major axis, however, being in the direction of the cross field. ' The fields for different speeds are as illustrated in Fig. 130, a, b, c, respectively, correspond- ing to synchronous, sub-synchronous and super-synchronous speeds. The maximum torque which a motor is capable of exerting, other things being equal, depends upon the average value of the magnetic field in which the rotor moves. This mean value, neglecting IR drop and leakage, is in the polyphase induction motor independent of the slip, while for the corresponding single-phase machine the average value of the field decreases as the slip increases; thus the pull-out torque of a polyphase machine connected single phase will be less than when normally operated. Many interesting facts concerning the rotor currents as well as the development of the rotating field may be brought out by FIG. 131. COILS INCLINED TO AXIS OF OSCILLATING FIELD. simple mathematical analysis. Let us consider the elementary bi- polar single-phase induction motor represented in Fig. 131 with a coil at an angle a to the main polar axis. Assume as before (p, 224) that the flux distribution is a cosine function of time, and adopt the following notation: 230 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. A = area of coil. oj = angular velocity of the coil, or a = cot. A sin a = A sin wt = projected area of coil on plane CC' perpendicular to the flux NS. B = maximum flux density, its instantaneous value being B cos pt. Instantaneous flux interlinking coil a is < = AB cos pt sin cot = .5 AB [sin (p + w) t sin (p - co) /]; (49) the e.m.f. induced in coil a is e = = .$AB {(p-a>)cos(p-a>)t-(p+a>) cos(p + co) /}. (5o) Let r 1 and Z t represent respectively the effective resistance and inductance of the coils; the values of these constants are based not only upon the character of an individual coil but also to some extent upon the action of neighboring coils. With this notation the cur- rent in any secondary coil can be considered as resulting from the e.m.f. of equa. 50, or wherein The fluxes produced by one rotor coil and the main field will so react upon each other that the value of the secondary current, if but a single coil be considered, can be expressed only by an infinite series. It has been experimentally shown, however, that the flux-distorting reactions between primary and secondary do not exist with a rotor winding composed of a number of coils divisible into pairs, the members of which are placed at 90 degrees (electrical) apart. The rotor winding of a commer- cial machine substantially satisfies this condition; consequently the THE SINGLE-PHASE INDUCTION MOTOR. 231 higher harmonics of the rotor current disappear and the same is correctly represented byequa. 51 given above. This equation indi- cates that the rotor current consists of two parts having different fre- quencies and amplitudes. At standstill any coil spaced an angle y from the axis of the mag- netic field will have a current of the following form: T 4 A *-n ABpcos (pt + y d ss ) * standstm = ~ (r.'+jz.') ' (52) which shows that the secondary current at standstill is of line frequency. The current component with frequency (p w) de- creases in value as the rotor speed rises toward synchronism, being zero at that limit, and the secondary current then becomes j ABpcos (2 pt + y 0syn) /syn= -- ' which is of double-line frequency. These variations of rotor current frequencies as well as the pres- ence of the differential (p aj) and additive (p + a>) compo- nents may be conveniently observed by the application of a reed frequency meter. Connect such an instrument across the slip rings of the wound rotor of a polyphase motor, excite the stator with single-phase current and then start the machine. As the speed of the rotor increases, the frequency meter will indicate the presence of two currents, one increasing and the other diminishing from the line frequency. Let us now select a coil on the rotor displaced any angle ft from the loop a we have just considered, Fig. 131. The flux through this new coil at synchronous speed (a = ut = pt) will be, from equa. (49), <> = AB cos pt sin (pt + ft), e.m.f. coil ft = e = - = -ABp (cos (2 pt + /?), (55) current coil ft = i = -- ^ cos (2pt + ft 6), (56) Vr 2 + 2pL* = ^005(2 pt + ft - 6). (57) 232 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The total magneto-motive force of all the coils on the rotor may be expressed as K^i. The maximum m.m.f. exists in the plane of the coil in which the current is equal to zero, and hence the poles of the rotor will be in the same plane. Let /?' be the angle of that particular coil; then * = ^COS (2pt + /?' 0). But since i is equal to zero, ^cos (2 pi + /?'- 0)= o, whence and /7T \ - 2 pi. This means that the angle between the reference coil and the magnetic pole of the rotor changes at the rate of 2pt. It also indicates that the pole rotates backwards on the rotor. The latter, however, is turning forward at a rate pt, consequently the rotor poles revolve backward in space at a rate pt, and the equation of this pole in space is If the equation for the current in the general coil is referred to the magnetic axis instead of to the reference coil, we have * = t cos j (2 #* + /? That is, referred to the magnetic axis of the rotor the current distribution is constant, hence the m.m.f. of these currents is constant and rotates backward at synchronous speed, as above proved. The relative value of the stator and rotor m.m.f 's may be derived as follows: Assume the rotor stationary; this corresponds to considering it the same as the short-circuited secondary of a trans- former. Thus the relations existing between primary and secondary m.m.f 's of a transformer apply or, neglecting resistance and leakage, the secondary m.m.f. is equal and opposite to that of the THE SINGLE-PHASE INDUCTION MOTOR. 233 primary. The current distribution in the bars on the rotor on the basis of the above assumption is expressed by equa. 46 as sin (pt-d)sinp, which upon neglecting r makes 6 = - and reduces to i = ^cos^sin/3; pL this if t = o becomes * = -^sin/?. (58) Jt is to be noticed that when / = o, the equation of the rotor cur- rents at synchronous speed (equation 56) reduces to i r = -- cos (/? - 6) , v r *+2pL* which can be still further simplified, if r is negligibly small with respect to pL, to the following form, *V=- sin. (59) Comparing these values of i and ir we see that these currents have the same distribution in the rotor, but the amplitude of the latter is only one-half that of the former. Consequently, the m.m.fs of the stationary rotor and of the stator being equal, the m.m.f. of the synchronously revolving rotor is one-half that of the stator winding. The magneto-motive force effective in developing the flux B cos pt when the two fields coincide may be expressed as Y X, wherein F represents the maximum m.m.f. developed by the stator and X that due to the rotor. But, as above shown, X = Y -=- 2, hence the excitation necessary to produce the flux B cos pt throughout the Y magnetic circuit of the machine is or X. 2 The two magneto-motive forces acting at any instant in this type of machine are: F cos pt, stationary in space. X, constant in value, but rotating backward at synchronous 234 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. speed. Since X rotates backwards it may be written X = X cos pt X sin pt, and consequently Y-X, the total magneto-motive force acting at any instant, becomes Y cos pt - X cos pt + X sin pt = X cos pt + X sin pt. This means that the total m.m.f. acting at any instant is of constant value and rotates forward at synchronous speed. The magnetic reluctance of commercial single-phase motors, due to the use of uniformly distributed windings, is practically the same, whatever the axis of the field, consequently the reactions existing between stator and rotor currents produce at or near synchronous speed a circular rotating field, and the formulae which apply to polyphase motors may be utilized. The effect of leakage and rotor resistance will modify this rotating field somewhat, changing it from circular to elliptical form. Torque Equations. It has been indicated on p. 230 that when the secondary of a single-phase induction motor is caused to rotate at any rate w, its current may be expressed as Inspection of this equation shows that the rotor current is composed of two parts, one of a lower and the other of a higher frequency than the rotating field. We may consequently consider that this current is set up through the action of two synchronously rotating fields, one revolving in the same direction as the rotor and the other oppositely.* The frequency of the rotor current component due to the suppositional field revolving in the same direction as the rotor is naturally less (by the velocity of the rotor) than synchronous value or it is (p a>). The component due to the oppositely rotating field has a frequency higher than that of the line, its value being (p + (*>). The per cent slip of the rotor with respect to the first field is 100. (- ) 100, and referred to the second field it is I- ) p.l \ P I * G. Ferraris, Mem. Reale Accad. di Scienze Torino, Series II, Vol. xliv, December 1893. Electrician, Vol. 33, pp. no, 129, 152, 184. London, 1894. THE SINGLE-PHASE INDUCTION MOTOR. 235 The effective turning effort of the motor is the resultant of the interaction between the rotor current and two oppositely rotating fields. But, since the rotor and one field turn in the same direction, the torque due to this latter field must be greater than that set up by the other. We have seen from equa. 38 (p. 188) that the torque developed by a polyphase induction motor is expressed by the following equation: 7" 1 _ 2 e sr i wherein s is the per cent slip between rotating field and rotor core, while (Jt) = p is the angular velocity of the revolving field. We may accordingly write the two component torques existing in the single- phase motor as N 2 e 2 * r T = 2 12 1 '''' r,-- p w w. oj wherein s t = - - = 1 and s P <*>i The total effective torque is T = T 4- T = ^^a ^ wherein s 2 s v is positive for speeds below synchronism, while s^ is variable but never greater than unity. Analysis of this equation brings out the following facts: 1. That the torque of the single-phase machine varies as the square of the impressed voltage, this being the same relation as obtains in polyphase induction motors. 2. That the motor exerts no torque at standstill because s 2 5, then equals zero, which makes the numerator of the same value.* 3. The motor cannot operate at synchronous speed, because this makes s t zero, in which case the torque developed is of negative value, 5j5 2 X 2 2 r 2 2 reducing to-r 2 2 , and the machine tends to act as a generator. Consequently the single-phase induction motor must rotate at less than synchronous speed. * (See equations (47) and (48), p. 225.) 236 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. 4. The maximum value of s^s^ being unity indicates that the single-phase induction motor cannot operate unless the reactance of its rotor winding at standstill is greater than its resistance. Un- less such is the case s^Xf r 2 2 will have a negative value, which means that the machine would tend to develop a negative 75 50 25 -f- 25 50 75 Percent Rated Torque 100 125 150 FIG. 132. SPEED-TORQUE CURVES WITH ROTOR RESISTANCE VARIED. torque or act as a generator. Fig. 132 indicates how the speed- torque curves of a single-phase induction motor are affected by change in the value of rotor resistances. Curves A and B may be considered as representative of standard machines. Curves C and D indicate the effects produced by inserting relatively large resist- ances into the rotor winding. It is apparent from these curves that the introduction of resistance into the rotor circuit for purposes of speed regulation is attended by a marked reduction of the over- load capacity of the motor, and cannot be used as advantageously as with polyphase motors (p. 209). It is, however, employed to limit the starting current (pp. 207 and 249). 5. The torque developed by a polyphase motor operated as a single-phase machine is less than that produced when normally connected, because of the presence of the counter torque T 2 . 6. If we take the first differential coefficient of equa. 60 with respect to r 2 and place it equal to zero, we find that the maximum torque developed for any rotor speed u exists when r 2 =X^+(7^ + 2), THE SINGLE-PHASE INDUCTION MOTOR. 237 and that the maximum torque T mai = AT 2 Vv 2 (,, - S| ) i- ai r r (61 ) This equation shows that the torque at any selected speed is greater the less the value of r r Characteristic Curves. The preceding torque equation, while valuable in that it indicates the general characteristics of single- phase induction motors, is not readily applied to the detail study of any specific machine. The working curves are most accurately determined by actual test. They may, however, be derived with moderate accuracy by means of a circle diagram somewhat similar to that utilized for the study of the polyphase motor. The particular diagram described herein (Fig. 133) is substantially that developed by A. S. McAllister, its construction being as follows :* Let the vertical line OE (Fig. 133) represent the line voltage. Draw at their proper phase positions and scale values the no-load as well as the locked currents OM and OF respectively. The value of OF is determined as already explained in connection with the polyphase induction motor (p. 194). MN and IF represent the energy components of the corre- sponding currents, and are therefore directly proportional to the respective inputs. Through M draw a line MK perpendicular to OE, join M and F; draw also a line perpendicular to the middle of MF intersecting MK at X. With X as a center and either XM or XF as a radius, describe the circular arc MPF, this being the locus of the primary current. The distance IG represents the added primary or stator loss existing with the rotor locked, its length = (added primary copper loss -=- total locked watts) X HF. Draw the line GM. With this construction completed the perform- ance of the motor may be determined by inspection. For example, the factors determining the performance of the motor with a current P are as follows: OP to scale represents the primary current, cos POE is the power factor of current OP. PT represents the watts input at current OP. MN represents the watts input at no load. TQ represents the watts loss at current OP. * Alternating Current Motors, A. S. McAllister, pp. 115-119. McGraw Co., New York 1909. 238 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Percent Power Factor 10 20 30 40 50 60 70 80 90 100 \ Scale :- \ 1 cm. = 10 Amperes. N lcm.=2.2K.W. 10 FIG. 133. MCALLISTER CIRCLE DIAGRAM FOR A IO-H.P. SINGLE-PHASE INDUCTION MOTOR. THE SINGLE-PHASE INDUCTION MOTOR. 239 TR represents the total primary loss at current OP. QR represents the added secondary copper loss. QP represents the watts output at current OP. QP -T- PT represents the efficiency of the motor at current OP. loo (PQ -*- PR)* represents the per cent slip. 7.05 QP -r- r.p.m. represents the torque at current OP. The field set up by the motion of the rotor varies as the speed (ai), consequently the torque (T) for a given rotor input (W) is proportional to the product of wW, or T = K^W. (62) The torque, however, is also proportional to the secondary output (W") divided by the speed (o>), or T = K, (W" whence 1800 (6.3) 5 10 40 45 50 15 20 25 30 35 Ft. Lbs. Torque FIG. 134. CHAJtACTERISTIC CURVES OF A 22O-VOLT, 6o-CYCLE, 4-POLE, IO-H.P. SINGLE -PHASE INDUCTION MOTOR. The secondary input, from the circle diagram, is proportional to PR', the output is similarly represented by PQ; consequently (PQ -*- PR)* corresponds to the rotor speed as above stated. The diagram shown in Fig. 133 has been applied to the deter- mination of the characteristic curves of a standard 22o-volt, 60- cycle, 4-pole, lo-horsepower single-phase induction motor. The fundamental data employed in the construction of this diagram 240 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. were derived by test, and are as follows: stator resistance, .304 ohm; current with motor running free, 19 amperes; corresponding input, i k.w.; and power factor 24 per cent. The current with rotor at standstill is 170 amperes; input, 13.4 k.w.; power factor, 36 per cent. Line potential in both instances is 220 volts. The values derived from the diagram are given in the following table and presented in the form of curves in Fig. 134. CHARACTERISTICS OF A 220-VOLT, 60-CYCLE, 10-HORSE- POWER, SINGLE-PHASE INDUCTION MOTOR. Point. Amp. % P-F. K.W. Input. H.P. Output. % Eff. R.P.M. Ft.-lbs. Torque. M 19 24 1 1800 1 25 65 3.56 3.4 70 1782 10 2 30 75 4.95 5.03 76 1772 15 3 40 81 7.15 7.65 80 1760 23 P 50 83 9.24 10.0 81 1745 30 5 60 85 11.2 12.0 80 1738 36 6 80 81 14.2 14.75 78 1715 45 7 100 78 17.2 16.5 72 1690 51 8 119 70 18.7 16.0 64 1640 51 9 140 61 18.8 13.1 52 1550 44.5 10 155 51 17.6 8.8 37.5 1400 33.0 F 170 36 13.4 Comparison of these characteristic curves of the single-phase induction motor with those of the standard polyphase induction motor brings out the fact that the former has zero torque not only at synchronous speed but also at standstill, whereas the latter usually has a starting torque in excess of that developed at rated load. The following table gives operating characteristics of standard single-phase induction motors. DATA OF STANDARD SINGLE-PHASE INDUCTION MOTORS. 110 TO 440 VOLTS. Per cent Power Per cent. H.P. Poles. Per cent Slip. Pull-out Torque.* Factor Load. Efficiency Load. i i Rtd. H i 1 Rtd. li 1 4 6 .5 46 58 66 68 53 60 63 60 1 4 4 .6 55 59 73 75 60 63 68 62 2 4 2.5 .8 56 65 77 76 71 75 78 77 5 4 2.5 .8 78 83 86 86 71 76 77 76 10 4 2.5 .8 75 81 84 83 75 79 80 79 20 6 2 .9 78 80 86 87 85 88 86 85 30 8 2 .9 68 80 85 84 77 81 83 82 50 4 2.3 2.0 91 94 93 91 82 84 86 86 * Pull-out torque in terms of rated load torque. THE SINGLE-PHASE INDUCTION MOTOR. 241 Comparison of the values of this table with the corresponding one on p. 200 shows that in general the power factor, efficiency and pull-out torque are higher for polyphase than for single-phase motors, while the speed regulation of the single-phase machine is better. This latter feature of the single-phase induction motor is accounted for by Dr. C. P. Steinmetz as follows:* " Since in the single-phase motor one primary circuit and a multiplicity of second- ary circuits exist, all secondary circuits are to be considered as corresponding to the same primary. Thus the joint impedance of all secondary circuits must be used as the secondary impedance, at least at or near synchronism. Thus, if the armature has a quarter-phase winding of impedance Z t per circuit, the resultant secondary impedance is ; if it contains a three-phase winding of impedance Z l per circuit, the resulting secondary impedance is . In consequence thereof, the resulting secondary im- o pedance of a single-phase motor is less in comparison with the primary impedance than in the polyphase motor. Since the drop in speed under load depends upon the secondary resistance, that occurring in the single-phase induction motor is generally less than with the polyphase motor." Methods of Starting. As already shown (pp. 225 et seq.), the simple single-phase induction motor cannot exert any starting torque. In practice, however, except in the smallest sizes which may be started by hand, the conditions of service which this motor is to meet require a starting torque as high as 150 per cent of the rated value, consequently some device producing this feature must be connected with or incorporated into the machine. The methods of accomplishing this result may be grouped into two general classes. The first is technically known as phase-splitting and the second as the repulsion-motor method. Split-phase Starting. Two-phase currents may be obtained on a single-phase circuit by dividing it into two branches one of which is inductive and the other non-inductive. If supplied with two- phase currents, even though these be less than 90 degrees apart, an induction motor is self-starting; and when synchronous speed is approximated the phase-splitting device may be cut out and the ma- chine will then run as a single-phase motor. There are many ways to obtain such split currents. The two parts of the circuit may be in series, one being shunted by inductance or capacity (Fig. .135). They may also be put into inductive relation to each other to pro- duce a phase difference.! * Elements of Electrical Engineering, 1902, p. 284. t U. S. Patent No. 401,520, April 16, 1889, to Nicola Tesla. 242 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Motors employing the above starting methods are provided with two stator windings, a working winding and a starting winding. The two windings are displaced from each other by about ninety magnetic degrees, just as in the ordinary two-phase motor. The FIG. 135. SLIP PHASE CIRCUIT, USING INDUCTANCE AND RESISTANCE. working winding, however, is of more turns, being spread over a larger surface, and of heavier wire than the starting winding, because it remains in circuit as long as the motor operates, whereas the starting coils are only in use momentarily. * FlG. 136. CONISTECTIONS FOR START- FIG. 137. PHASE-SPLITTING METHOD ING SMALL SINGLE-PHASE INDUCTION DEVISED BY BROWN, BOVERI FOR MOTORS. USE WITH LARGE MOTORS. The method illustrated in Fig. 136 has been developed by Brown, Boveri and Co. of Baden, Switzerland. At starting the two windings are placed in series across the supply lines, the starting winding S being shunted by the condenser. The current THE SINGLE-PHASE INDUCTION MOTOR. 243 consequently lags more in that winding, the difference in phase between the currents in R and S being sufficient to set up a so-called elliptical rotating field, that is one having greater strength in one direc- tion than in that at right angles thereto. The starting winding and its condenser are cut out, and the working winding is connected across FIG. 138. CONNECTIONS OF GENERAL ELECTRIC COMPANY CONDENSER COM- PENSATOR FOR PHASE-SPLITTING AND STARTING SINGLE -PHASE INDUCTION MOTORS. the line by means of the double-throw switch T when the motor has approximately attained synchronous speed. This method is slightly modified when machines of over 5-h.p. capacity are to be started. The two windings in such instances are placed in parallel, as shown in Fig. 137. By this means the working coil circuit is not broken and the flash occurring upon cutting out the auxiliary winding is eliminated. An excellent method for starting single-phase motors has been developed by the General Electric Company under patents granted 244 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. to Dr. C. P. Stein me tz, the connections for which are substan- tially as shown in Fig. 138.* Two terminals of the stator winding, which is substantially of standard three-phase construction, are connected directly to the supply lines. The third terminal is also connected to either one of the mains through an auto-transformer (p. 202), the order depending upon the direction of rotation desired. The ends of this compensator are placed across a con- denser. This combination is technically known as a condenser-com- pensator, and is employed because a condenser of given volt-ampere FIG. 139. ARRANGEMENT OF WORKING AND AUXILIARY STATOR COILS, HEY- LAND SELF-STARTING SINGLE-PHASE INDUCTION MOTOR. capacity is more economically constructed for high than for low voltage. The starting winding can be cut out by opening the switch at S after the motor is up to speed. It may, however, be advan- tageous to keep the starting coil in circuit, if of sufficient current capacity for continuous service, because the increased power factor at light loads thus obtained more than compensates for the losses occurring in the transformer. The use of external phase-splitting apparatus may, however, be dispensed with if the two stator windings are arranged to have different time constants. This is accomplished by having the auxiliary winding of larger self -inductance than the main coil. * U. S. Patent Nos. 602,920 and 602,921, April 26, 1898. THE SINGLE-PHASE INDUCTION MOTOR. 245 Heyland devised a very successful motor of this type, utilizing the scheme suggested in the Tesla patent cited above (page 241). The working winding P is distributed in a series of semi- closed slots. The starting coils S are short-circuited upon themselves and placed in closed ducts, the result being a highly inductive secondary circuit, the general arrangement being as illus- trated in Fig. 139. The current induced in the secondary winding lags almost 90 degrees with respect to the primary current, pro- ducing a field component similar to that caused by the second phase of a two-phase current. The starting torque thus produced is large, though the power factor of the machine is necessarily low, and therefore the starting coil should be cut out as soon as the machine has come up to speed.* The rotor windings employed in connection with any or all of the preceding methods for starting may be of the standard squirrel- cage or slip-ring type. Repulsion Motor Starting. A very interesting type of self- starting single-phase induction motor is manufactured by the Wagner Electric Mfg. Co. of St. Louis, f This motor is provided with an armature of the ordinary direct-current drum type, having a disk commutator with radial bars. The brushes bear- ing upon the commutator are displaced about 45 degrees from the corresponding neutral zones and short-circuited upon each other. The stator winding is connected to the supply lines, and at starting the machine speeds up as a repulsion motor (p. 266). In the annular space between the armature core and the shaft are two governor weights w (Fig. 140), which are forced outward, further and further, by centrifugal force as the machine accelerates. When synchronous speed is nearly attained the force acting upon these weights is sufficient to push the heavy copper ring R, against the action of spring S, into contact with the inner cylindrical surface of the commutator bars G, thus completely short-circuiting the armature winding. Simultaneously with this action the sleeve P is forced to the left sufficiently to lift the brushes B from the com- mutator. This series of automatic actions transforms the machine from a repulsion to a single-phase induction motor, having in the latter form what is substantially a squirrel-cage armature winding. * Electrical Engineer, Vol. XXXVI, p. 306. London, 1896. f U. S. Patent No. 543,836, December 4, 1894. 246 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. FIG. 140. GENERAL ARRANGEMENT OF WAGNER MOTOR SHOWING AUTOMATIC SHORT-CIRCUITING DEVICES. The starting torque thus obtained may be readily adjusted to about twice the normal value, without an excessive current being required. A very interesting feature of the Wagner motor, and of repul- sion motors in general, is the relation between torque and thickness of rotor brushes. The series of curves shown in Fig. 141 were determined from tests of a 5-h.p., 220-volt Wagner motor. Curve A shows the speed-torque relation on accelerating with normal brush thickness, this being substantially that of one commutator bar. Curve B represents the relations existing with a brush of twice normal thickness, etc. It is apparent from these curves that the normal thickness of brush gives the highest starting and synchronous speed torques. Further study of Fig. 141 indicates that use of a brush thinner than normal might tend to produce start- ing and synchronous speed torques of greater value than occur with normal brush thickness. Practical questions, however, such as mechanical strength limit the reduction of brush thickness. Single-phase induction motors, in addition to being provided with one or another of the preceding means for developing starting torque, require, when over moderate size (3 or 5 h.p.) THE SINGLE-PHASE INDUCTION MOTOR. 247 70 60 50 Normal Thidkness Doubi Triple Quadruple 40 V T 30 20 10 Railed Lot d Tormie ^S^ 200 400 600 800 1000 1200 1400 1600 1800 R.P.M. FIG. 141. SPEED-TORQUE CURVES WITH VARIOUS BRUSH THICKNESSES. starting compensators (p. 203), or wound rotors with slip-ring con- trol (p. 207) corresponding to those needed by polyphase induction motors. This precaution is necessary, as the inrush current other- wise occurring would be considerable and likely to react upon the line, producing voltage fluctuations. For further information on single-phase induction motors, the reader is referred to the following: ALTERNATING-CURRENT MOTORS. A. S. McAllister. New York, 1909. SINGLE-PHASE INDUCTION MOTOR. A. Still. Elect. World, New York, 1906. Vol. XLIII, pp. 1108, 1152, 1182, 1202. SINGLE-PHASE INDUCTION MOTORS. Dr. C. P. Steinmetz. Trans. A. I.E.E., Vol. XV, 1898, pp. 35-110. SINGLE-PHASE INDUCTION MOTORS. W. S. Franklin. Trans. A.I. E.E., Vol. XXIII, 1904, p. 429. THEORY OF SINGLE-PHASE INDUCTION MOTOR. V. A. Finn. Elect. Rev., London, February, 1906. WECHSELSTROMTECHNIK. Vol. V, by Arnold and La Cour, pp. 112 and 275. Ber- lin, 1909. CHAPTER XVIII. COMMUTATING ALTERNATING-CURRENT MOTORS. A COMMUTATING alternating-current motor has a closed-coil arma- ture provided with a commutator, being similar to a d.c. motor in general construction. Fundamentally there are three such types, namely, series, repulsion shunt, and shunt-induction motors, but many modifications and combinations have been devised, the more important of which will be considered. The general historical facts concerning these and other a.c. motors have already been given in Chapter XII. The Alternating-current Series Motor. The powerful starting torque and adaptation of speed to load which are characteristic of the d.c. series motor make it particularly suitable for traction and many other uses requiring such qualities. The limitations of direct current generation and transmission, as well as special advantages of a.c. voltage control, have made the development of a correspond- ing a.c. motor very desirable, a fact appreciated for many years.* Synchronous motors, whether single or polyphase, and single- phase induction motors, all having little or no starting torque, are obviously unsuited to railway and many other purposes. Poly- phase induction motors require at least three supply conductors and are for that reason less desirable than single-phase apparatus, especially for traction. Hence a single-phase motor with a power- ful starting torque has an enormous field of usefulness. Up to the present time the series and repulsion motors both with commu- tators are the only a.c. types fulfilling this condition. The operation of series motors on the high frequency circuits formerly employed (100 to 133 p.p.s.) was attempted at various times, but not with success, except in the case of very small machines. This failure was due to excessive transformer action in the coils of * Alex. Siemans, Journal British Inst. of Elect. Engs., p. 527, Vol. XIII, 1884. 248 COMMUTATING ALTERNATING CURRENT MOTORS. 249 the armature winding, short-circuited during commutation, as well as to the low power factor caused by the large reactance of the field windings. The introduction and use of low frequency (25 p.p.s.) systems for power transmission is the basis for the later commercial develop- ment of the a.c. series motor. In 1902, G. B. Lamme, of the Westinghouse Elect. & Mfg. Co., called attention to an a.c. series motor which operated on circuits having a frequency of i6f p.p.s.* This motor had a powerful starting torque, high power factor, and was of relatively high efficiency, but the low frequency necessary for its proper operation unsuited it for service on circuits of stand- ard frequency. Furthermore, illumination by arc or incandescent lamps at i6 cycles is not satisfactory. The design has since then been modified to adapt this type of motor to the standard frequency of 25 p.p.s. The same current flows through both field and armature windings of an a.c. as well as d.c. series motor, hence there can be no phase difference between field and armature currents. In this respect it differs from the a.c. shunt motor whose torque is proportional not only to field and armature currents but also to the cosine of the phase angle between them. There are, however, other limitations of the a.c. series motor and special features of design are found necessary by reason of the following phenomena, peculiar to a.c. commutator machines : 1. Iron losses throughout the magnetic circuit, due to alterna- tions of the flux. 2. An e.m.f. generated in the armature windings by the alter- nating magnetic field, and defined as a transformer e.m.f. in con- tradistinction to the voltage developed by armature rotation. 3. A local current circulating in those coils short-circuited by the brushes. This current is due to the transformer e.m.f. of No. 2. 4. An e.m.f. of self-induction in the field and armature windings. 5. Power factor less than unity, due to inductance of the wind- ings. i. Iron Losses. The total iron losses occurring in the a.c. series motor may be divided into two parts: that taking place in the arma- * Transactions A. I. E. E., pp. 10-49, Vol. XX, 1902. 250 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. ture coil and polar faces due to the rotation of the former, and that occurring in the entire magnetic circuit, due to the alternations of the magnetic flux. The losses arising from rotation of the armature coil, being common to both a.c. and d.c. motors, are often called "d.c. iron losses." These are supplied mechanically and act like the resisting torque of friction. The losses caused by flux alter- nation are supplied electrically and are due to eddy currents and hysteresis, the former being reduced to a reasonable amount by employing laminated field magnets as well as a laminated armature coil. A reduction in hysteresis loss is possible by operating at low flux densities. Hence in the case of a series motor designed for a.c. service a wholly laminated magnetic circuit is necessary. This feature, combined with the limitation of flux densities, results in a total weight 30 to 50 per cent greater than that of a corresponding d.c. machine. The overall dimensions are also correspondingly larger. 2. Transformer e.m.f. In addition to the c.e.m.f. of rotation a second e.m.f. is generated in the armature winding, which does not, however, appear at the brushes, except locally as a cause of sparking, explained in 3. The rate of cutting lines of force due to armature TIG. 143. ARMATURE OF SERIES MOTOR SHOWING DIRECTION OF C.E.M.F. rotation is a maximum at the position of coils A and B, Fig. 143, while the minimum rate of cutting occurs while the coils are in the COMMUTATING ALTERNATING CURRENT MOTORS. 251 so-called " neutral" position, CD. The e.m.f. generated by the armature rotation tends to cause current flow from coil D upwards through each half of the armature winding to the coil C, producing poles at C and D. The maximum c.e.m.f. exists between the brushes as in any direct current machine. The second or transformer e.m.f. is produced by the alternations of flux passing through the armature coil. For example, in the case of a ring wound armature placed in a bipolar field, one-half of the total flux passes through the sections at FIG. 144. DIRECTION OF TRANSFORMER E.M.F. INDUCED IN ARMATURE WINDING BY AN A.C. FIELD. C and D respectively, hence maximum flux variation occurs there On the contrary, no lines of force pass through coils A and B, so that no transformer action occurs and no e.m.f. is induced in them. This transformer e.m.f. produces no difference of potential at the brushes placed vertically in their usual position as indicated, because the portions of the armature winding in connection with them are of equal potential, the transformer e.m.f. being shown by heavy arrows (Fig. 144). The maximum transformer e.m.f. exists between coils A and B. It is evident that this e.m.f. does not act against the flow of current from the supply lines and produces no effect, except that the particular coils short-circuited by the brushes experience maximum transformer induction, which tends to set 252 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. up large currents in them, as already explained under heading 3. p. 249. The value of this transformer e.m.f. in the armature is -i (64) J.O this being the well-known expression for transformer e.m.f. in which / is the frequency, <& m the maximum armature flux due to the field m.m.f., and S the equivalent number of armature turns. If the total number of armature conductors, counting all around the per- iphery, is S a , the turns in series are Sa * 2 on the bipolar ring arma- ture in Fig. 144, which is simpler than a drum winding to represent and study. As already stated, the field flux linked with each turn of armature winding depends upon its position, being proportional to the cosine of its angular displacement from the line CD. The average value of the cosine in each quadrant is - , and each turn in a ring armature carries only one-half of the flux, hence the effect, of S a total conductors is equivalent to _ S a m 2 Sa ^ 2 271 271 Substituting this value of S in equa. 64, the transformer e.m.f. induced in the armature winding by the alternating field flux, and lagging one-quarter period or 90 degrees with respect to it, is = ^-p (65) This same formula applies equally well to a drum armature, in which case the turns are only one-half as many as the total conductors, but the flux is not divided between two turns as in a ring winding. Hence these two factors would cancel out if introduced in equation. 3. Local Armature Current. Since those coils undergoing com- mutation are the seat of maximum transformer action, a large cur- rent will flow in them as in any short-circuited secondary. The flux due to this secondary current being in opposition to the primary flux tends to weaken the field just when and where its greatest strength is required for commutation. This local current may be greatly in excess (5 to 15 times) of the normal armature current, COM MUTATING ALTERNATING CURRENT MOTORS. 253 thus producing local heating as well as an additional current and PR loss in the primary (i.e., field) winding.* The sudden inter- ruption of the heavy current in the short-circuited coil also draws a large spark at the brushes and causes commutator troubles. Since the transformer e.m.f. and current depend upon the frequency of flux alternations, the number of turns in, and resistance of, each short- circuited coil, they can be reduced by proper modification of these three factors. That is, frequency of the supply circuit should be as low as possible, consistent with standard practice, and the num- ber of turns of wire or inductors in series between consecutive commutator bars should be small, the transformer e.m.f. being directly proportional to these two factors. This latter condition increases the number of armature sections as well as commutator bars, and therefore cost of construction, but greatly diminishes the tendency to spark, self-induction being proportional to the square of the number of turns of wire. The e.m.f. is reduced, as just shown, while the resistance of the coil circuit is increased by employ- ing brushes of higher contact resistance than is usual with d.c. machines, or by inserting high resistance connectors or preventive leads (P) of German silver between the armature sections and the FIG. 145. PREVENTIVE LEADS. commutator bars (Fig. 145). f The high resistance of the special brush contacts or of the preventive leads is only a small factor, of the resistance of the whole armature winding, consequently it does not lower the efficiency of the machine to any marked * Electric Journal, p. 7, Vol. VI, 1909. fThis was done as early as 1891 in the small Hochhausen-Excelsior A. C. Fan Motors. 254 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. extent. The additional resistance of the preventive leads is, however, a considerable factor of the local circuit, hence it is very effective in cutting down the value of the local or short- circuit current. 4. E.M.F. of Self -induction occurring in both field and armature windings. This e.m.f. is due to the alternating flux, and introduces a factor not existing in the d.c. motor, that is, in addition to the c.e.m.f. of rotation, the impressed voltage must overcome an e.m.f. of self-induction equal to wLI. To cause an equal current to flow, with other conditions the same, the applied e.m.f. must there- fore be greater than in the d.c. machine. As a matter of fact, the actual voltage supplied to the terminals of an a.c. series railway motor is usually lower than for the d.c., being about 250 compared with about 550 volts, hence the e.m.f. of rotation must be relatively still lower in the former machine. In other words, the a.c. machine is designed for a lower voltage. Another difficulty due to this inductive e.m.f. arises when one or more field turns become short- circuited, forming a closed secondary circuit and drawing excessive current, very likely to result in a burned out field winding. The transformer e.m.f. set up in the armature of a bipolar machine, as given in equa. 65, is equal to the full c.e.m.f. developed by rotation when the line frequency equals motor r.p.m. -* 60, which relation corresponds to synchronous speed. This is evident because the armature turns cut the flux at the same rate in both cases. Hence the effect of a short circuit in the armature is aggravated in the series a.c. motor. 5. Power Factor. The field winding being highly inductive, and the armature winding having considerable inductance, the current flowing through them is not in phase with the line e.m.f. This condition does not affect the torque of the motor, but the result- ing low power factor impairs the regulation of the alternators, trans- formers and line, at the same time lowering the efficiency of the entire system. This e.m.f. of self-induction in the field winding is directly proportional to the frequency of the supply voltage, to the field flux and to the square of the number of its turns, hence in any attempt to improve the power factor of the motor, these various factors must all be considered. a. The frequency cannot be lowered indefinitely, because the motor COMMOTATING ALTERNATING-CURRENT MOTORS. 255 must operate on existing circuits, few of which have a frequency of less than 25 p.p.s. The use of i5-cycle circuits for series motors has been proposed in order to improve the power factor, and while that result undoubtedly would be thus obtained, the greater cost of the alternators and transformers might readily off- set this gain. b. Total Flux. By increasing the number of inductors and at the same time the number of sections on the armature, the field flux can be reduced and the torque maintained constant. That is, the armature is made strong with respect to the field, so that the field inductance may be decreased. Hence in practice the total flux of the a.c. series motor at corresponding current values is not as high as in the d.c. machine. The increase in armature inductance is eliminated by " compensation," as explained later. c. Turns in the Field Coil. These can be kept down, first, by having steel of high permeability, and secondly by having a some- what shorter air gap, thus obtaining the necessary flux with a smaller number of ampere-turns. In fact, the lower value of flux indicated above (b) tends to reduce the number of field turns required. It has been suggested that the power factor of the series motor could be improved by increasing its resistance in order to decrease the ratio uL+R. This short-sighted improvement in power factor would merely result in an increased I 2 R loss with lowered efficiency and horsepower capacity. The electrical action in a plain a.c. series motor can be readily shown by vector diagram, as in Fig. 146. In the field we have a large inductance reaction OY and a small resistance reaction YR y the resulting voltage across the field terminals being OR. The inductance drop RJ in the armature is relatively small, and not very much greater than its resistance drop JD. The vector RD represents the voltage required to overcome the impedance reaction of the armature winding, and OD measures the voltage balancing the joint impedance reaction of armature and field windings. This is also the starting voltage of the motor, the reaction represented by OD being all that exists with the motor at rest. A c.e.m.f. is developed in the armature winding, when it ro- tates in the magnetic field. This c.e.m.f. or rotative e.m.f. is 256 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. vectorially represented as an energy component, since it is propor- tional to the power required to overcome the rotative iron losses Field Armature 27TfL f l ^ Current M FIG. 146. VOLTAGE DIAGRAM OF SERIES MOTOR. and counter torque of the load. If, as before with a bipolar model, S a be the total number of inductors around the armature core, and s lay off to scale the short-circuit current OS. This short-circuit current is taken at full line voltage, and its phase angle 1 watt meter reading , XT , , cos = 9. Next apply a brake to motor volts X amperes shaft or driving wheel and allow the armature to rotate, adjust the brake so that the machine draws a certain current, say about one- third of the short-circuit value, note volts, amperes, watts input, speed and torque, calculate speed in miles per hour, tractive effort, horsepower output, and efficiency at this selected load. Lay off this last current OI to scale and in its determined phase position with respect to line voltage. The locus of the current drawn by the motor with different loads at a fixed voltage and frequency is then the arc of a circle drawn through the points O, 7, and S. The power factor of any current can be determined by project- ing the intersection of its vector with the circle KNC across to the power factor scale upon OX. The speed corresponding to any load current OI, for example, is also determinable from the circle diagram, as follows: Draw a line through S parallel to OX, then continue the current vector OI until it intersects this line at P; the distance between points S and P is proportional to the motor speed existing when the current is OI at voltage OX. This rela- tion is true by construction, because OI is at constant impedance proportional to the impedance drop and OS to the line voltage; consequently IS is proportional to the c.e.m.f. due to rotation and therefore to speed of rotation itself. Comparing the triangles OIS and OSP, it is seen that they are similar because the three included angles of one are equal to those of the other; accordingly SP is pro- portional to IS, or to the motor speed as above stated. If line SP be divided to scale, the speed at any assumed current can be read off directly by continuing its vector to intersect SP or its prolon- gation. The torque of the motor can be calculated with an error not exceeding a few per cent, by the relation T = k$I. Relative values of the field flux $ existing at different current values are deter- mined from the e.m.f. of rotation, which in the circle diagram is COMMUTATING ALTERNATING-CURRENT MOTORS. 259 FIG. 147. CIRCLE DIAGRAM FOR A.C. SERIES MOTOR. 260 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. 6000 100 Diam. Wheels 38 Gear Ratio 3.2i 100 200 700 900 1000 300 400 500 60 Amperes FIG. 148. CHARACTERISTIC CURVES 25~CYCLE, 25O-VOLT, I50-H.P. SERIES MOTOR. proportional to the line 75 for the current OI, and for any other current proportional to the corresponding line. Naturally, to obtain the true ratio existing between the different rotational e.m.f.'s and therefore between corresponding values of field flux, these e.m.f.'s must be reduced to a common speed basis. The torque T l with any current 7 X is then determined from that exerted with the test current / flowing, by the relation: (68) The results obtained from the circle diagram in Fig. 147 are compared in the following table, with values taken from test curves of Fig. 148. These characteristic curves are fora 1 5o-horsepower, 250- volt, 2 5 -cycle compensated-series motor. COMMUTATING ALTERNATING-CURRENT MOTORS. 261 COMPARISON OF RESULTS OBTAINED BY TEST AND FROM CIRCLE DIAGRAM. 150-h.p., 25-cycle, 250-volt Compensated Series Motor. Power Factor. Speed in M.p.h. K.V.A. H.P. Input. Amperes. Test. Diagram. Test. Diagram. Input. Test. Diagram. 400 .90 .90 33.5 32.7 100 121 121 500 .86 .86 27.5 27.5 125 144 144 600 .83 .83 23.2 23.4 150 167 167 700 .79 .80 20.0 20.0 175 188 188 800 .75 .75 17.5 17.0 200 201 201 900 .71 .70 15.2 14.5 225 214 212. 1000 .67 .66 13.2 12.2 250 224 221. Amps. Rotational C.E.M.F. Torque Ratio *!/! -*-*/ Pounds, Tractive Effort. Horsepower Output. Per cent Efficiency. Diag. At Common Speed. Test. Diag. Test. Diag. Test. Diag 400 500 600 700 800 900 1000 Volts. 216 207 195 182 167 153 138 Volts. 181 207 227 250 273 290 313 .70 1.00 1.32 1.71 2.14 2.52 3.02 1175 1750 2300 2975 3650 4300 5050 1210 1750 2300 3000 3740 4410 5300 108 128 142 150 170 175 178 105 128 143 158 170 172 174 89.4 89 85 84.5 84.5 81.6 78 89 89 85 85 84.5 82 78 The agreement between test and calculated results is reasonably close. The differences existing may be charged to two facts, namely, the basic assumption in the construction of the diagram that the winding impedance remains constant, which is not strictly true, and again to the difficulty of making very close linear measurements in a small diagram. An examination of the working curves (Fig. 148) of the a.c. series motor indicates that the speed- and torque-current characteristics are very similar to those of the corresponding d.c. machine, but owing to the lower flux densities of the former, the torque increases more nearly as the square of the current throughout the load range. For the same reason its speed does not tend to become so nearly 262 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. constant at heavier loads. The motor speed at fractional voltages can be determined from the circle diagram, or by the relation exist- ing between the rotational e.m.f's. 2 rot= 2 line + /Z 2 - 2 line /Z COS (& - 0). (69) In this expression 7 is the armature current at which the value of E TOt is desired; Z the impedance of the windings, the phase angle of the short-circuit current, the phase angle of the current /, and Eiine is the value of the line voltage at which the speed is to be calculated. The losses in an a.c. series motor may be divided into two classes, namely, those which also occur in d.c. machines, and those peculiar to machines having magnetic fields set up by alternating currents. Since these latter motors operate at a lower voltage than the corre- sponding d.c. machines, the current required for a given power is greater, hence the copper losses will be more, or the amount of copper required in the windings will be larger. As a matter of fact, the total losses in an a.c. series motor are about twice those occurring in a corresponding d.c. motor. The various losses of the a.c. motor may be conveniently arranged as shown in the fol- lowing chart.* Total Useful Mechanical Power Mechanical Mechanically ( D.C. Iron Losses ' Losses com- Total Electrical Power Power Evolved Supplied j Friction Losses Losses i. Winding Losses > - mon to A.C. and D.C. Total Supplied Electrically f Ordinary PR Losses Losses. Supplied j Transformer I 2 R Losses | Losses i A.C. Iron Losses / Special A> C. Losses Compensation of Armature Reaction and Inductance. It has already been shown (p. 256) that to improve the power factor of an a.c. series motor, the practice is to weaken the field and strengthen the armature by decreasing the turns of the former and increasing those of the latter. This change in design tends, however, to exag- gerate armature reaction as well as commutation difficulties. These two troubles can to a very marked extent be reduced or even elim- inated by the introduction of a compensating m.m.f. in substantially * Electric Club Journal. Vol. I, 1904, p. 16. COM MUTATING ALTERNATING-CURRENT MOTORS. 263 the same manner as employed in the case of d.c. adjustable speed shunt motors of the Thompson-Ryan design pp. 73, 79. This method of preventing the field distortion by the armature m.m.f. and reduc- ing the armature inductance is to surround the revolving armature with a fixed winding placed in slots cut in the polar faces if salient TF C FIG. 149. SERIES MOTOR WITH COMPENSATING WINDING (CONDUCTIVE COMPENSATION). poles are employed, or if an induction motor stator frame is used the compensating winding is displaced 90 magnetic degrees or half a pole pitch from the field winding. The compensating coils carry a current equal in m.m.f. and opposite in phase to the current in the armature, and this current may be obtained either conductively by connecting the balancing winding directly in series with the field FIG. 150. SERIES MOTOR WITH COMPENSATING TRANSFORMER (INDUCTIVE COMPENSATION) . and armature windings, as in Fig. 149, or inductively by using the stationary winding as the short-circuited secondary of a trans- former, of which the armature is the primary, as in Fig. 150. It is found that the best effects are produced when the balancing of the armature reaction is complete. The conductive method is the more desirable when the motor is to be operated on mixed service, that is, partly on a.c. circuits and partly on d.c. circuits. Methods of Control of A.C. Series Motors. The series motor may be controlled by means of a rheostat in series with it, an auto- transformer or an induction regulator. With external resistance 264 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. the efficiency of the system is low, as in the case of rheostatic control with d.c. machines. When the auto-transformer is em- ployed, the line is bridged by a single coil transformer provided with taps, so that various voltages can be applied to the motor circuit, and low voltages for starting can be obtained without the FIG. 151. CONNECTIONS FOR OPERATING SERIES MOTOR ON MIXED SERVICE. large losses involved in resistance control. The speeding up of the motor is accomplished by including more and more sections of the auto-transformer between the motor terminals. The trolley voltage usually employed in a.c. traction work is 11,000 volts or thereabouts, while the motors are designed to oper- ate at 250 volts or less. The line from the trolley to the ground passes through an auto-transformer designed with taps so as to give an adjustable secondary pressure up to 500 volts, sufficient to COMMUTATING ALTERNATING-CURRENT MOTORS. 265 operate two motors in series. The general scheme of connecting a.c. series motors, with auto-transformer control, for traction service is shown in Fig. 151. Since these motors are rated at 250 volts each, they are connected in series-parallel groups, two motors being per- manently connected in series so as to fit them also for d.c. operation. The switch 5, automatically operated, cuts out the auto-transformer when the alternating current fails, and inserts the rheostatic or series- parallel control of the two groups necessary for d. c. service. Switches aa and cc are open, while b and d are closed for series connection of motors the converse is the order for parallel service. The switch RS reverses the current in the field coils and thus the direction of rotation. The auto-transformer method of controlling the speed of a.c. series motors corresponds to the multiple voltage (p. 50) and motor-generator systems for d.c. motors, because they all supply an adjustable voltage corresponding to the speed desired. The a.c. means are much simpler, however; in fact the facility of transforming voltage is the great advantage of the a. c. control. The Repulsion Motor. As stated in Chapter XII, the physical phenomenon upon which the operation of this motor largely depends FIG. 152. REPULSION OF SINGLE COIL. was discovered by Prof. Elihu Thomson in 1887, and he applied it the same year to the development of an experimental motor.* The production of the repulsion phenomenon is as follows: If a closed coil of wire be suspended or pivoted near the pole of an alternating current magnet in such a manner that lines of force from the later pass through the former as represented in Fig. 152, an alternating e.m.f. will be induced in the coil. This secondary e.m.f. will be 90 degrees later in phase than the inducing flux, and * Transactions A. I. E. E., Vol. IV, 1887, p. 160. * U. S. Patent No. 363,185 of 1887. 266 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. if the coil contains no inductance, the secondary current will also be in quadrature with the primary flux. Under such conditions there will be no development of torque by the mutual action of the magnetic field and current as explained on page 250. Practically, however, every coil of w.ire contains some inductance, so that the secondary current lags more or less with respect to the secondary e.m.f., being therefore more than 90 degrees later than the primary flux, with the result that the cosine of this phase relation becomes a negative quantity. This means that the coil is repelled by the field. The maximum repulsion occurs theoretically if primary flux and secondary current differ in phase by 180 degrees, but even to ap- proximate this value requires the coil reactance (coL) to be very large with respect to its resistance. This condition implies an extremely small current, so practically the maximum repulsion occurs when the coil has such impedance that the secondary current lags about 45 degrees with respect to the e.m.f. If the movable coil above considered be pivoted in the magnetic field, the only way in which the negative torque or repulsion can act is by turning this coil on its axis, until such position is reached that no lines of force pass through it, or in other words, it will turn until it assumes a position parallel to the lines of force. A coil perpen- dicular to the flux may rotate in either direction, hence it must be placed obliquely with respect to the flux, to compel rotation in a definite direction, and if the inertia of the coil be sufficient to carry it beyond the dead center, continuous motion will be developed. The elementary repulsion motor devised by Professor Thomson is diagrammatically illustrated in Fig. 153. The magnetic circuit was completely laminated and the armature winding was of the open coil type, the terminals of each coil being connected to diametrically opposite commutator bars. The field winding was connected directly across the line, and the armature short-circuited by means of diametrically opposite brushes connected by a copper lead. With these brushes placed so that they short-circuit the armature coils at an oblique angle to the flux direction, torque and rota- tion are set up as for the single coil already considered and are con- tinued through the successive action of the different coils. The limitation of this early type of repulsion motor is the fact that the effective torque developed at any moment is due only to a single armature coil, since no current exists in the others whose circuits COMMUTATING ALTERNATING-CURRENT MOTORS. 267 are open. Hence to develop any considerable power, the current in the short-circuited coil must necessarily be high, and the opening of this circuit as the corresponding commutator bars pass out of contact with the brushes causes excessive sparking. Professors FIG. 153. EARLY THOMSON REPULSION MOTOR. Anthony, Ryan and Jackson appreciated the seriousness of this defect, and in 1888 suggested the use of a closed coil armature winding in place of the open coil type.* This resulted in a greatly increased power for a given weight, because the effective turns on the armature were augmented and a given current pro- duced more torque, or a smaller current produced the same torque without as much sparking. On the other hand, sparking with this type of armature is due not only to reversal of current in the coil short-circuited by the brush, as in d.c. machines, but also to trans- former action, as already explained with reference to the series a.c. motor (p. 251). Sparking in the brushes in the more modern designs is reduced by compensation, as in the series motor; by use of a distributed field winding; high brush contact resistance; prevention leads, etc. With the simple form of repulsion motor indicated in Fig. 154, the field winding is directly across the line and there is no electrical connection between the armature and field or supply circuit, re- sembling in this respect the transformer with a leaky magnetic circuit and movable secondary winding. * Trans. A. I. E. E., Vol. XXIII, 1904, p. 77. * U. S. Patent 389,352, September, 1888. 268 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The flux impressed on the armature core by the field, and repre- sented in Fig. 155 by the vector OR, may be resolved into two components, the first being OB along the line of commutation of the armature winding and the second OA perpendicular thereto. FIG. 154. CONNECTIONS OF SIMPLE REPULSION MOTOR. Currents are developed in the armature winding by two independ- ent actions, namely, transformer and rotational induction. The component OB is that which produces current in the armature winding by transformer effects, while OA is that producing current FIG. 155. COMPONENTS OF REPULSION MOTOR FIELD FLUX. in the secondary winding by rotation. These two independently produced armature currents are combined to give the total armature current actually existing. The line voltage applied across the field terminal may be regarded as being made up of three components, namely: First, the com- ponent required to overcome the resistance drop in the field winding; COMMUTATING ALTERNATING-CURRENT MOTORS. 269 second, that required to overcome the equivalent reactance of the primary winding; and third that needed to overcome the e.m.f. induced in the field winding by the rotation of the armature. By considering these three components, a circle diagram some- what similar to that of the series motor can be developed. The torque of the repulsion motor cannot be assumed to vary as the square of the current, since the phase relation between stator and rotor currents, as well as the brush position, must be considered. An excellent performance diagram of the repulsion motor was given by Osnos in the " Electrotechnische Zeitschrift " for Oct. 29. 1903, p. 905. This diagram is shown in Fig. 156, and its construc- B D r v c- KG. 156. THE OSNOS CIRCLE DIAGRAM OF THE REPULSION MOTOR. lion is as follows: OE represents the direction of the impressed voltage, OI t the primary current with the rotor locked, drawn at an angle O/ t corresponding to its phase displacement. Olf is the current with the rotor revolving without load and EOIf is its corresponding phase angle. Draw an arc of a circle through Olfl lf the point C being the center of the circle. This is the circle of current input, and the angle EOI corresponds to the phase angle of any particular primary current. The ordinate IP is the working or energy component of the current / and is proportional to the input. Describe the circle B^D, which has its center on OB perpendicular to OE. Then on OB as a diameter describe .a second semicircle OlfD. The first of these semicircles or BIiD is the circle of speed and the second circle OI f D is the torque circle. Thus for a load requiring a current /, the speed is represented by the ratio IP/PD, and the torque by the product OI X IN. The line ID represents the secondary current in phase and magnitude 270 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. (reduced to primary equivalents). The angle d is the difference in phase between corresponding primary and secondary currents. This diagram takes into account the copper losses as well as leakage effects, but does not include the windage, friction or iron losses, which must be allowed for, either by addition to the input or sub- traction from the output. The speed regulation of the repulsion motor may be altered by simply shifting the brushes. This type of motor, however, is very sensitive to comparatively slight change of the brush position, hence extreme care should be taken in attempting thus to vary the speed characteristics. The direction of rotation of the repulsion motor may be reversed, either by shifting the brushes over to the other side of the neutral line of the field flux, that is from AB to CD in Fig. 154, or by shifting the primary connections by 90 degrees magnetically when a distributed closed coil stator winding is employed. The Compensated Repulsion Motor * is a development of the preceding motor and was designed with the object of overcoming field distortion, and increasing the power factor of the machine. FIG. 157. CONNECTIONS OF COMPENSATED REPULSION MOTOR. The diagrammatic connections of the simplest form of this modification are shown in Fig. 157. At first sight, this motor does not differ much from the ordinary series machine, but the presence of brushes B and B considerably modifies its action. One effect is largely to neutralize the self-inductance of the field winding, since the current flowing in the armature across these brushes acts as the current of a short-circuited secondary, of which the field wind- ing is the primary. The field winding, therefore, acts as a trans- former coil; on the other hand, it does not supply the entire magnetic * Transactions, 1904, International Elect. Congress, Vol. Ill, pp. 129-185. COMMUTATING ALTERNATING-CURRENT MOTORS. 271 field necessary for the production of the turning effort. This latter field is mainly supplied by that component cf the. current which passes through the armature at brushes bb. The current flowing between bb is variously known as the exciting or compensating current, while that developed between brushes BB is called the short-circuit current. This type of motor is characterized by high power factor at speeds above synchronism, but at low speed its power factor is less than with the a.c. series motor, while at all speed points its torque per ampere is not as high. A further criticism of this construction is the fact that the greatest advantage of the repulsion motor its connection directly to high tension lines is no longer practicable, because the revolving member is also in the main circuit so that the necessary insulation is difficult. This bad feature of the compensated motor is avoided by the Winter-Eichberg modification shown in Fig. 158. In this FIG. 158. CONNECTIONS OF WINTER-EICHBERG COMPENSATED REPULSION MOTOR. design the armature exciting current, flowing between brushes bb, instead of being supplied directly to the armature from the high tension lines, is obtained from the secondary of a transformer whose primary is in series with the stator and high tension circuit. Vari- able speed is obtainable by variation of the voltage supplied by this secondary, which is provided with taps, as shown. Even though the repulsion motor, in its various forms, possesses the majority of the desirable features of the a.c. series motor, with the possibility of even better power factor at higher frequencies, combined with high voltage supply to which the armature is not subjected, nevertheless, it has not been as favorably received in 272 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. this country as the series machine, on account of the following features : 1. Noisy in starting up. 2. Lower starting torque per ampere, on account of the "blow- ing out" of the primary flux, due to secondary flux of the armature, as well as the difference in phase. 3. Greater tendency to spark at the brushes, due to excessive transformer currents, upon which its action largely depends. 4. With compensated motors, though sparking is much less, the motor is provided with twice as many sets of brushes, which are always a source of weakness and trouble, especially in traction service. 5. Reversal of direction of rotation not as convenient as with the series motor. 6. Shifting from a.c. to d.c. and back again not as easy as with series motor, because of the extra short-circuiting brushes, commutation devices, etc. Alternating Current Shunt Motor. Owing to the fact that the direction of rotation of any d.c. motor is the same irrespective of the direction of current supply, early attempts were made to adapt such machines for service on alternating-current systems. Of course these motors must be provided with laminated field frames to reduce the great losses due to eddy currents that would otherwise occur. The simple shunt motor of this type is not, however, of any com- mercial value, for the following reasons: 1. Low power factor, due to the many turns of the field winding, the inductance of which is extremely large. 2. Severe sparking at the brushes, due to the fact that as each coil passes under a brush it becomes a short-circuited secondary of a transformer, and thus a seat of heavy currents. 3. Low weight efficiency. This trouble is caused by the phase difference between armature and field currents and corresponding relation between their respective fluxes, resulting in greatly reduced torque. The inductance of the armature is small with respect to that of the shunt-field circuit, hence, the two currents differ considerably in phase. Let the vector OB (Fig. 159) represent the current in the armature circuit and d 1 its small angle of lag with respect to the line e.m.f. OA, while AC represents the field current and 2 its large angle of lag; then < = 6 2 6 1 is the angular difference between OC COMMUTATING ALTERNATING-CURRENT MOTORS. 273 and AB, the field and armature currents (also fluxes), respectively. The relation between these currents is also shown in the wave diagram of Fig. 159. The torque at any moment is t = KI a sin OJf sin 2 , which by reduction gives T = Kljf cos < as the effective value. In other words, the torque developed by an a.c. shunt motor is / c FIG. 159. VECTOR AND WAVE DIAGRAMS OF E.M.F. AND CURRENT OF A.C. SHUNT MOTOR. dependent not only upon the field and armature currents, but also upon the cosine of the angle between them. If the currents were in phase cos (f> would be unity and the torque of the a.c. shunt motor would be equivalent to that of the d.c. machine. Economy of design and efficiency of operation demand a shunt-field winding of many turns and an armature wind- ing of comparatively few turns, so that <, which is the phase differ- ence between field and armature currents, will always be large. Hence, as already stated, the torque and power per unit of weight of such an a.c. shunt motor are necessarily small. The a.c. shunt motor may be used with greatly increased weight efficiency on two-phase circuits, by supplying the field winding from the leading phase, and the armature from the lagging phase. The lag of field current would bring it very closely in phase with the armature current; thus would be small and the torque corre- spondingly increased with the same values of I a and //. Another advantage of this scheme is that adjustable voltage speed control becomes available through the simple introducing of a variable ratio auto-transformer in the armature circuit. This modification of the shunt motor, however, has not been adopted to any extent in practice, because induction motors with their higher power factor and balanced condition would naturally be used when two-phase currents are available. 274 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Shunt-Induction Motors. The ordinary single-phase shunt motor as above explained has a very low torque efficiency, owing to the considerable phase displacement existing between armature current and field flux. To avoid this displacement it is necessary to excite the field by an e.m.f. leading the armature e.m.f. by 90. The method of accomplishing this without the introduction of a second phase was invented by L. B. Atkinson and the design is embodied in lines of commercial machines variously known as shunt-induction or commutator -induction motors. Assume an armature of the d.c. type provided with a pair of diamet- rically placed short-circuited brushes as in Fig. 160. This armature FIG. 160. ATKINSON'S METHOD OF OBTAINING SHUNT EXCITATION. FIG. l6l.- 5IMPLE SHUNT-INDUCTION MOTOR. is caused to rotate in an a.c. field whose axis YY is 90 from the brush axis XX. If an e.m.f. is applied to the stator coils, the flux produced thereby will lag 90 behind it. Now as the rotor revolves in this field an e.m.f. of rotation will be induced in its coils, as in the case of a d.c. machine, in phase with the field flux and thus 90 behind the applied voltage. This e.m.f. of rotation in turn pro- duces a flux 90 degrees behind it in phase and in space quadrature with the stator flux. This rotor flux is therefore in phase with the line voltage, and constitutes the " in phase flux " necessary for the efficient production of torque. The flux due to rotation is not of constant magnitude, but is directly dependent upon the rotor speed, COMMUTATING ALTERNATING-CURRENT MOTORS. 275 thus this type of single-phase machine has no starting torque and it must be started as a series or repulsion motor. It differs from these machines, however, by the fact that the flux is independent of the load and hence it has the constant speed characteristics of the d.c. shunt motor. The stator core of the Atkinson type of motor is made up of slotted laminations similar to the corresponding core of a polyphase induction motor. The stator winding is of the drum type distributed over a large portion of the stator surface. The rotor is similar to the direct current armature and the commutator has brushes bearing upon it. There are two sets of brushes in the bipolar representation of Fig. 161, and generally two sets of brushes for every pair of poles. FIGS. 162, 163, AND 164. METHODS OF COMPENSATION EMPLOYED WITH SHUNT INDUCTION MOTORS. If, however, the armature winding is of the two-circuit type, the number of brushes may be reduced to two sets regardless of the number of poles. The brushes commutating coils in line with the field axis YY are called the energy or working brushes, as the torque-producing current occurs in the circuit made up by them. The brushes XX displaced 90 from the stator axis are called the exciting brushes as above explained. This machine has the same characteristics as the single- phase induction motor, namely, relatively constant speed under load changes, low power factor and absence of starting torque. Its efficiency is, however, lower due to the introduction of the brush and commuta- tor losses. It is possible, however, to improve considerably the power factor by compensation as suggested by Heyland, Fynn, Punga, 276 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. and others. This may be accomplished by employing a transformer T to supply a small voltage in phase with the line e.m.f to the excit- ing or compensating brush circuit as shown in Fig. 162. The trans- former T can be omitted and the stator winding itself may be used as the primary of a step-down transformer, independent secondary coils being wound concentricly with the stator winding and connected to the exciting brush circuit as shown in Fig. 163. Or the separate secondary stator coils may be omitted and the main stator winding be used as an auto-transformer to step down the line potential as shown in Fig. 164. The self-starting characteristic is obtained by shifting the brushes from the positions shown in the preceding diagrams by a small angle, FIG. 165. SHIFTING OF BRUSHES TO OBTAIN SHUNT INDUCTION MOTOR WITH HIGH STARTING TORQUE. FIG. l66. CONNECTIONS OF REPULSION- INDUCTION MOTOR. usually 15 degrees, and opening the short circuit between the excit- ing brushes by means of the switch S w as in Fig. 165. The machine will then start on the repulsion motor principle as explained in con- nection with the Wagner motor, p. 245, and as synchronous speed is approached, the switch may be closed either manually or auto- matically by means of some centrifugal device. After the exciting brush circuit is thus closed the machine will operate at a substantially constant speed and at a high power factor. This type of machine is commercially known as the compensated shunt induction motor. If the switch S w is not opened during the starting period, the flux COMMUTATING ALTERNATING-CURRENT MOTORS. 277 necessary to produce acceleration is materially reduced and the starting torque for a given line current is consequently much less. Such a machine is shown in Fig. 166 and is commercially known as the repulsion-induction motor. The characteristic curves of a typical motor of this class are shown in Fig. 167. The decrease in speed with increase in load, from no load to full load, is about 8 per cent, about the same as that of ordinary a.c. shunt motors of equal capacity. The compensation is very effective, as is indicated by the high power factor which is maintained over a wide load range. 100 g 60 300 I 40 200 "3 * 20 100 2000 1800 1600 1400 25 75 100 Percent Rated Load 125 150 175 FIG. 167. CHARACTERISTIC CURVES OF A 5 H.P. REPULSION-INDUCTION MOTOR. Shunt induction motors may be made to operate as constant output machines of a moderate speed range (about 2 to i) by several methods. The simplest way is to strengthen or weaken the field along the XX axis by the insertion of capacity or inductance in the excitation brush circuit, as shown in Figs. 168 and 169. The insertion of capacity increases the excitation current and strengthens the field, thus reducing the motor speed, whereas, the use of inductance weakens the field and raises the speed. Another method is that used in connection with the repulsion-induction motor. This com- prises a transformer the primary of which is connected across the supply lines and the secondary thereof is composed of two coils; 278 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. one, the " regulating coil," is placed in the energy brush circuit, and the second coil is connected into the exciting brush circuit. As stated, however, the speed range is limited, because commutation L I FIGS. 168, 169. METHODS OF CONTROLLING SPEED OF SHUNT INDUCTION MOTORS. becomes less and less satisfactory as the motor speed is caused to depart from synchronism. COMPARISON OF D.C. AND A.C. COMMUTATOR MOTORS. With d.c. machines, each circuit or portion thereof, whether in series or shunt, must obtain its current by electrical connection to the supply conductors. On the other hand, a.c. circuits may be supplied inductively as well as conductively. For example, any d.c. motor must have its field and armature windings, also its com- pensating winding to neutralize armature reaction (if provided therewith), all connected to the supply circuit. The windings of an a.c. motor, however, may be energized in four different ways: first, by electrical connection to and conduction from the supply; second, by induction through a transformer; third, by induction in the winding short-circuited upon itself; and fourth, by induction to another one of the three circuits to which the given winding is connected as a tertiary circuit. The Winter-Eichberg motor in Fig. 158 has its field winding directly connected to the supply circuit; the armature receives COMMUTATING ALTERNATING-CURRENT MOTORS. 279 current by the brushes b and b from the secondary of the trans- former PS and at the same time the brushes B and B are connected by very low resistance so as to short-circuit the armature. Thus tne first three of the above arrangements are present in this one machine. In most a.c. commutator motors the field winding is con- nected directly to the supply conductors because it is more easily insulated for the high voltage which they usually carry. The arma- ture, on account of its construction and motion, is more difficult to insulate, and for that reason is often connected as a secondary cir- cuit, the voltage of which may be made as low as desired. This arrangement is characteristic of the repulsion motor and constitutes one of its most prominent advantages. The ftossibility of ener- gizing any one of the three windings of an a.c. commutator motor in any one of the ways specified above affords opportunity for making many different combinations, but this does not mean that all of them are practically advantageous. Any a.c. machine may be protected from high-voltage by supply- ing it through a transformer. Such protection is not absolute in the case of an auto-transformer, but the potential at the motor may be made as low as desired. This arrangement at the same time enables variable voltage to be easily obtained for speed control, as explained in connection with Figs. 151 and 158. The facility of transformation and variation of voltage constitutes the advantage of a.c. compared with d.c. commutator machines. On the other hand, the low flux densities and power factor, also sparking diffi- culties of the former, render them less powerful and more trouble- some than the latter. In other words, they cost more and develop less power pound for pound, and at the same time are less satis- factory in operation. For further information on commutating a.c. motors, the reader is referred to: SINGLE-PHASE COMMUTATOR MOTORS. F. Punga, R. F. Looser. 1906. ELECT. TRACTION. Wilson and Lydall. Vol. II. London, 1907. ALTERNATING-CURRENT MOTORS. A. S. McAllister. New York, 1909. ELECTRIC MOTORS. Hobart. London, 1910. DIE WECHSELSTROMTECHNIK. E. Arnold. Vol. V, Book 2. Berlin. 1912. ALTERNATING-CURRENT ELECT. RAILWAY. G. B. Lamme. Trans. A. I. E. E., Vol. XX, 1902. p. 15. ALTERNATING-CURRENT RAILWAY MOTORS. W. I. Slichter, Dr. C. P. Steinmetz and W. A. Blank. Trans. A. I. E. E., Vol. XXIII, 1904, pp. i, 9 and 83. 280 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. HISTORY AND DEVELOPMENT OF SINGLE-PHASE COMMUTATOR MOTORS. Feldmann, Haga, and Noome. Der Ingenieur, March, 1909. SINGLE-PHASE COMMUTATOR MOTORS. F. Greedy, D. Van Nostrand Co., N. Y. SINGLE-PHASE COMMUTATOR MOTORS. J. Fischer-Hinnen. Elect., London, Vol. 63, 1909. SINGLE-PHASE COMMUTATOR MOTORS. M. Deri, M. Latour, O. Bragstad, E. Dan- ielson. Inter. Elect. Cong., St. Louis, 1904. Vol. Ill, pp. 129-184. SINGLE-PHASE R. R. MOTORS AND CONTROL. T. H. Schoepf. Jour. Inst. of E. E., London, Vol. 36, 1906. UEBER WECHSELSTROMME-KOMMUTATOR MOTOREN. M. Osnos, also F. Eichberg, Ekctrotechnische Zeitschrift, Vol. 25, Vol. 27, Vol. 29 (1904-1908). PART IV. APPLICATIONS OF ELECTRIC MOTORS. CHAPTER XIX. SERVICE CONDITIONS. ELECTRIC motors employed as a source of driving power must be adapted in speed and torque to the particular purpose to which they are applied. For example, if the speed of the driven machine is required to be practically constant, of course the motor should be suitable for constant speed operation. It is not necessary in such a case for the motor and the machine driven by it to have the same speed, the conditions being often fulfilled more conveniently by a fixed ratio of speeds obtained through gearing or belting instead of by direct connection. To secure very low or very high speeds this arrangement with large speed ratios usually becomes practically necessary, as in the case of a triplex plunger pump running at about 50 r.p.m. connected to a 5-h.p. motor whose normal speed is about 800 r.p.m. Another example is afforded by the " buzz " wood planer, the cutting cylinder of which rotates at about 4000 r.p.m. driven by a 3-h.p. motor at say 1000 r.p.m. If the driven machine or street car, for example, is to run at variable -velocity, then the speed of the motor must be varied in the same proportion with either direct connection or fixed ratio of speeds. It is possible to make use of mechanical connections, such as two or more different sets of gearing as in gasoline automobiles, to obtain 'variable speed ratios. These may be employed either in place of or in combination with the speed alteration of the electric motors, depending upon circumstances. In many cases, however, it is preferable to change the speed of the motor electrically rather than to introduce two or more sets of gears or other mechanical means of speed control. For instance, a radial drill costs less and is more 281 282 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. convenient with a 3 : i adjustable-speed motor than with change gear box and constant -speed motor. There are some conditions, especially with large machines running at one speed for several hours, and only infrequently at a different speed, under which it would probably be as well to use a practically constant-speed motor and adjust the speed ratios mechanically. The time required for gear changing is unimportant in such cases, because it occurs but occasionally. In lathes, milling machines and similar tools it is practically necessary to introduce gearing for the very low speeds. The Speed Classification of Motors recommended in the Standardi- zation Rules of the A. I. E. E., Transactions, Vol. 26, p. 1800, June, 1907, is as follows: 1. Constant-speed motors, in which the speed is either constant or does not materially vary, such as synchronous motors, induc- tion motors with small slip and ordinary direct-current shunt motors. 2. Multispeed motors (two-speed, three-speed, etc.), which can be operated at any one of several distinct speeds, these speeds being practically independent of the load, such as motors with two arma- ture windings. 3. Adjustable-speed motors, in which the speed can be varied gradually over a considerable range, but when once adjusted remains practically unaffected by the load, such as shunt motors designed for a considerable range of field variation. 4. Varying-speed motors, or motors in which the speed varies with the load, decreasing when the load increases, such as series motors. Classes of Service. In order properly to understand the action and speed control of electric motors it -is important to consider it least in a general way their application to the many practical uses for which they are now employed. The operating conditions of almost all kinds of machinery with respect to speed, torque and power may be divided into six general cases, as follows: (a) Service requiring practically constant speed, regardless of changes in torque, sudden as well as gradual. (b) Service in which the torque is steady or varies as some func- tion of the speed should the latter change. SERVICE CONDITIONS. 283 (c) Service which involves frequent starting and stopping or wide variations in speed with rapid acceleration. (d) Service which involves frequent starting and stopping or wide variations in speed with gradual acceleration, including very slow operation or " inching." (e) Service in which the torque varies regardless of the speed, or for which speed variations may be desired irrespective of torque. (/) Service of a cyclic nature, for which energy is stored in a fly- wheel during part of each cycle and is given out during another part. The first case (d) includes the driving of one or more machines by a single electric motor running at practically constant speed. A wood-working shop with circular saws, band saws, planers, etc., is a common and typical example. The direct-current shunt- wound motor and the alternating-current induction or synchronous motors are applicable to this service. Direct-current compound- wound motors are particularly suitable if heavy machinery must be started from rest or where heavy overloads, even momentary, are likely to occur. On the other hand the speed of compound motors with variable torque is not so closely constant as in the case of shunt- machines, but in many practical instances the difference would not be objectionable. The torque-exerting capabilities of compound motors and their speed characteristics have been discussed in Chapter XL A single machine driven by a motor may be directly connected by coupling their shafts or by employing the same shaft for both, provided they are adpated to run or can properly be made to run at the same speed. Buffing wheels, emery wheels or tool grinders mounted upon or directly coupled to the motor shaft are prominent and characteristic examples. If their speeds are different they may be connected by belting for moderate speed ratios, and these should not ordinarily exceed 4 to i. It is customary to use gear- ing or chain belt for reducing speed in higher ratios, especially for positive driving. About 8 to i speed ratio is the practical limit of a satisfactory chain drive. On the other hand, with sufficient distance between centers, spur gearing will give almost any speed reduction with high efficiency. The driven machine may require constant torque as well as constant speed and therefore constant power as in an ordinary pumping installation. Thus there would be nothing to cause speed 284 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. variation, but even with these simple conditions d.c. shunt motors or a.c. induction motors should be used because their speed is definite with given voltage or frequency, a certain speed being usually desired, and the danger of running away which exists with a series motor is avoided. In most power applications, however, included in this first case (a) the torque and power demanded of the motor vary even with practically constant speed. Very often, indeed, the torque and power may be almost nil at one moment and at rated value or even overload a second or two later. Such extreme changes occur very frequently with circular saws, grind- stones, drills, punching presses, shears, buffing wheels and many other kinds of machinery. For extremely or even moderately variable torque with constant speed the d.c. shunt motor or a.c. induction or synchronous motors are especially applicable, also the d.c. compound motor for strong starting torque or temporary over- loads as noted above. A number of machines operated by one motor are usually driven through a line shaft by pulleys and belting. This arrangement dis- tributes the power conveniently, enables the various machines to be run at different speeds by using various ratios of pulley diameters and readily permits the starting and stopping of individual machines by clutches or shifting belts. A group of wood-working or many other kinds of machines are often driven by a single motor in this way, as already stated. Usually the number in use and the torque de- manded by each are varying greatly, at the same time approximately constant speed is desired, hence the direct-current shunt or the alternating-current induction or synchronous motor is employed. A refinement of this problem is encountered in the driving of textile machinery, especially silk looms, with which even a slight speed variation might affect the appearance of the finished product. In such instances the alternating-current induction or synchronous motors are generally employed because the speed of direct-current motors varies considerably with voltage changes and with the variation in temperature which occurs after several hours of opera- tion, as explained in Chapter III, whereas the speed of the alter- nating-current motors, unless the voltage varies greatly, is dependent upon the frequency of the supplied current. The second case (b) covers service in which the torque is fairly steady or varies with, but usually more rapidly than, the speed if SERVICE CONDITIONS. 285 the latter changes. This case includes the operation of pumps, fans, blowers, etc., and its requirements are satisfied by the series motor, whose speed adjusts itself to the work and at starting the rush of current does not tend to be so great as in a shunt motor. It must be, however, either geared or directly connected to the apparatus, because the breaking of the belt or the sudden removal of the load would cause a series motor to race and become injured (p. 105). To avoid these dangers or to permit the use of a clutch which might allow a series motor to run away, also because very widely variable speeds are undesirable, it is common practice to employ heavily compound-wound motors for driving reciprocating pumps or positive blowers. If a break should occur in the suction pipe of a pump a series motor is likely to race, while a compound motor would not rise in speed above the danger limit. The operation of pumps by electric motors is usually effected by gearing, since ordinary plunger pumps do not operate efficiently if driven in excess of fifty strokes per minute, and to accomplish this by direct connection would demand a very low speed and costly motor. Centrifugal pumps or blowers operating at high speed may be direct driven. The third case (c) includes electric traction and crane service, in which the motor is frequently started and stopped and rapidly accelerated at starting, adjusting itself automatically to the load, slowing down when heavily loaded as when a car is climbing a steep grade. These conditions are satisfied by series motors of either the direct- or alternating-current types, depending upon the current available. Elevator service is of this character, as regards frequent starting and stopping, but after rapid acceleration it calls for a speed independent of the load. Hence, to fulfill both these requirements elevator motors when of the direct-current type are heavily over-compounded to give the series characteristic at starting; then, when the motor is up to speed, its series field winding is short-circuited and it operates as a shunt machine. Recently, however, two-speed shunt motors have been employed for this service, the field being of maximum strength for starting, and sparking prevented by use of interpoles. If only alternating cur- rent is available, the polyphase induction motor should be employed, but for powerful starting torque slip-ring control would be necessary in order to avoid very excessive currents and low power-factors Jn starting up or at low speed. 286 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. The fourth case (d) requires the motor to be started and stopped frequently and not rapidly accelerated, but on the contrary slightly moved or " inched " forward at the start, as in the operation of printing presses, gun turrets, etc. These conditions of service are satisfied by direct-current compound-wound motors provided with double armature and series-parallel control. This character of work is also well performed by having a double or variable potential source of current supply for the working motor, low voltage being used for starting and " inching " and higher voltages for running. These features are found in the Bullock " teazer " system, the Holmes-Clatworthy two-motor method, or the Ward-Leonard motor-generator equipment. The last named, however, being somewhat expensive, is employed for the operation of gun turrets, steel-rolls and such special service, in which cost is a secondary consideration. " Inching " or very slow operation can also be accomplished by the use of a multiple disk oil clutch, which can be very gradually and smoothly applied. The fifth case (e) includes individual machine-tool service, for which the maximum allowable cutting or turning speed requires the number of revolutions of the work or tool to vary inversely as the diameter of the cut, maintaining the load at a constant value. This condition is satisfied best by direct-current shunt motors, as these are readily controlled in speed, as described in Chaps. V-VII. It is to be noted that in cases (a) and (6), the motor usually regulates automatically to maintain practically constant speed. In remaining cases (c), (d) and (e), on the contrary, the motor is con- trolled by hand to give variable speeds. Furthermore, in case (c) the motor is under control of the hand at all times, while in cases (d) and (e) the motor or machine driven by it, after being started, is set to operate at a desired speed for some time and regulates automatically when so adjusted to maintain that speed. The sixth case (/) is represented in the operation of shears, presses, punching machines and the like, which run without load for the greater part of the cycle, but the shock or effort required for short periods is very great. If this intermittent but large demand for power were met directly by an increased supply of current from the line, it would call for an excessive value of current, sufficient to produce marked fluctuations in line voltage. To avoid this disturbing result, the tool or machine is provided with a fly wheel, SERVICE CONDITIONS. 287 and as the motor tends to slow down because of the increased turn- ing effort required the fly wheel gives up some of its stored energy, thus supplementing temporarily the power supply from the line. The type of motor particularly well suited to this kind of work is a compound-wound d.c. motor or an induction-motor with a high resistance motor. For further information on the application of electric motors, the reader is referred to the following: ELECTRIC DRIVEN MACHINERY. Dr. S. S. Wheeler. Elec., N. Y., May, 1898. ELECTRIC POWER IN ENGINEERING WORKS. Dr. Louis Bell. Eng. Mag., October, 1899, January, 1900. ELECTRIC DISTRIBUTION OF POWER IN WORK SHOPS. F. B. Crocker. Franklin Inst., January, 1901. THE CASE FOR ELECTRIC POWER DISTRIBUTION. W. B. Esson. Elect. Eng., London, January u, 1901. ELECTRIC POWER IN MFG. PLANTS. D. C. and W. B. Jackson. Cassier's Mag., Vol. 26, 1904, p. 151. ELECTRIC MOTORS AND THEIR APPLICATIONS. W. E. Reed. Proc. Eng. Soc., W. Penn., October, 1905. INDUSTRIAL ENGINEERING. H. W. Peck. Electric Journal, Vol. VI, 1909, p. 83. APPLICATION OF MOTORS TO MACHINE TOOLS. J. M. Barr. Electric Journal, Vol. II, 1905, p. ii. POWER REQUIRED BY MACHINE TOOLS. G. M. Campbell. Proc. Eng. Soc., West Pa., Vol. 22, 1906, p. 10. APPLICATIONS OF MOTORS. Electrical Record, June, 1909. COST OF OPERATING MACHINE TOOLS. A. G. Popcke. Electric Journal, Vol. VI, 1909, pp. 674, 757. ECONOMIC FEATURES OF ELECTRIC DRIVE. Chas. Robbins. Trans. A. S. M. E., April, 1910. GROUP AND INDIVIDUAL DRIVE. C. W. Drake. Proc. Eng. Soc., W. Pa., Oct., 1911. GROUP VERSUS INDIVIDUAL DRIVE. A. S. Popcke, Elect. Journal, Vol. VTII, 1911, p. 999. MOTOR DRIVE IN MACHINE SHOPS. G. H. Hall. Machinery, June, 1912, p. 780. TOOLS AND MACHINES TO FIT THEM. H. I. Brachenburg. Trans. A. S. M. E., Vol. 32, p. 727. MOTOR DRIVE FOR SHOPS. A. G. Popcke. Am. Mach., Vol. 38, pp. 17, 351, 467. CHAPTER XX. POWER REQUIREMENTS FOR DIFFERENT PURPOSES. THE previous chapter sets forth the various service conditions, as regards speed and torque characteristics, which must be met by any driving unit. This chapter deals more specifically with the power requirements of particular devices. It is impossible in the limited space here available to consider the many cases that occur. The problems of usual occurrence are, however, discussed. Fans and Blowers. The series d.c. or a.c. motor is the most satisfactory type for driving a fan or blower, and is preferably direct connected. The speed control should be by resistance in the armature circuit for d.c. and by autotransformers in the case of large a.c. motors. As this service is usually continuous for long periods of time, the motor should have a corresponding rating. The volume of air moved by a fan is directly proportional to the speed. The pressure developed varies as the square of the speed, hence the h.p. required varies as the cube of the speed. In general, the power required to drive a fan or blower may be approximated by the following formula: KQP h.p. = - , (70) 19,000 wherein K is a constant depending upon the type of fan or blower; Q is the quantity of air to be moved expressed in cubic feet per minute; P is the pressure in ounces above atmosphere, against which the air is to be moved. The values of K given in the following table are based upon a combined efficiency of 60% for drive and fan. VALUES OF FAN CONSTANTS Axial or disk fans 10 to 12 Centrifugal fans 8 to 9 Steel plate fans 12 to 14 Positive pressure blowers 6 to 7 288 POWER REQUIREMENTS FOR DIFFERENT PURPOSES. 289 Pumping Machinery. In general, either d.c. shunt or squirrel- cage induction motors are satisfactory for the operation of pumps. In case, however, a pump must be started against the back pressure of a full discharge or stand pipe, demanding strong initial torque, a compound-wound d.c. or slip-ring control induction motor should be used. The speed at which piston pumps can be worked is limited, owing to the fact that water is incompressible. A usual piston speed is 50 to 135 feet per minute. This low speed necessitates a high ratio of speed reduction between motor shaft and pump crank shaft. Centrifugal pumps, however, may be operated at high speeds, so that direct coupling between armature and pump shaft is very convenient and operates satisfactorily for moderate heads. This form of pump, however, is not positive in action like the piston pump and the slip increases markedly with height of lift, hence it is not much used for heads over 50 feet, though several may be coupled in tandem to produce 2 or 3 stage lifts. The power required to drive a pump can be approximated by the following expression: Q X (H + F) . h 'P- = 530 XE > wherein Q represents the volume of water in cubic feet per minute; H represents the vertical lift in feet ; F represents the friction head; E represents the efficiency of the pump which varies between 50% and 75%, depending upon the tightness of the fittings, etc. The friction head F is = .4 LV 2 D -i wherein L is the length of pipe in feet; D is the diameter of the pipe in inches; V is the velocity of water flow in feet per second. Power Requirements of Machine Tools.*ft The power required to remove or cut metals depends upon the material to be worked, * Electric Motor Applications, by Chas. Robbins. Trans. A. S. M. E. Vol. 32, 1910, pp. 199-215. t Power Required by Machine Tools, G. M. Campbell. Proc. Eng. Soc. West Pa., Vol. 22, 1906, pp. 11-14. t Power Required in R. R. Shop Tools. R. L. Pomeroy. Proc. Cen. R.R. Club, Vol. 14, 1908, pp. 140-156. 290 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. on the size of the cut, on the cutting speed and upon the condition of the tool. In general with proper edge on the working tools, it may be expressed as follows : wherein F is the feed of tool in inches per revolution; D is the depth of cut in inches ; S is the cutting speed in inches per minute ; K is a constant or the horsepower required to remove i cu.in. of material per minute, with tool in good working condition. Values for K : Material. K Cast iron ................................. 0.4 -0.5 Wrought iron ............................. 0.5 -0.7 Mild steel (low carbon) ..................... 0.45-0.6 Hard steel (high carbon) ................... i. -1.2 Brass, etc ................................. 0.25-0.3 The values of K should be doubled when used for drills and milling cutters, because of the packing action of the metallic chip. Owing to the losses in the driving connections between motor and tool, the power determined by the above formula should be increased by about 30 to 40%. Example. Determine the power required to drive a lathe performing the fol- lowing work: Diameter of work ...................................... 12 inches Material .............................................. mild steel Spindle speed . . . . ..................................... 50 r.p.m. Depth of cut ........................................... i inch Feed per revolution ..................................... 4 inch Assuming an efficiency of 70% it follows that the motor to drive the lathe would have to be of 5 h.p. capacity, intermittent service rating. POWER REQUIREMENTS FOR DIFFERENT PURPOSES. 291 Lathes. The motor best suited for lathe drive is a semi-enclosed shunt-wound interpole machine designed for speed control by field weakening. The speed ranges vary between i : 2 and i : 6, usually however, a 1:3 or 1:4 ratio will suffice. The controller is generally of the combined starter and field rheostat type located so that the operator can move the handle without leaving the tool carriage. The motor is usually connected by gearing or silent chain drive to the spindle and is located near the head stock of the lathe. The following table gives approximately the power required by various sizes of lathes : SIZES AND SPEED RANGE OF MOTORS ON LATHES Swing. Inches. Light Duty, h.p. Medium Duty, h.p. Heavy Duty. h.p. Speed Range. 14 I* 2* 4 i : 3 or 1:4 16 2 3 5 18 2 3 5-7 2O 2| 3 1\ 24 3 5 10 28 5 72 12* 30 7* 10 15 36 7i 10 I5.2O 38 10 15 20 42 15 20 48 20 2O 54 20 25 72 25 30 84-90 3 40 Planers. Mechanically reversible planers (old style) are usually driven by 25% to 30% over-compounded d.c. motors or polyphase induction motors having wound rotors. The motors may be of adjustable speed but they need not exceed a i : 2 ratio. The motor is usually direct connected or geared to the driving shaft which should in all cases carry a fly wheel. Planers with reversible motor drive employ interpole shunt-wound motors with a 4 : i speed ratio through field weakening. Power requirements for planers are given in the following table: 292 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. POWER REQUIREMENTS OF PLANERS MECHANICALLY REVERSIBLE. Length of Planer Bed. Inches. Light Service, h.p. Medium Service, h.p. Reversible Motor Drive. Heavy Service, h.p. Very Heavy Ser- vice, h.p. 22 I -S 3 5 7-5 30 3 6 7-5 10 36 4 8 10 IS 42 5 10 20 25 48 6 12 25 30 54 7 15 30 35 60 8 17 30 40 72 10 22 35 50 86 13 26 5 60 96 15 30 5o 60 1 20 20 40 75 85 Drills. The motor employed is usually of the shunt interpole type, designed with field control. The speed ratio is 1:3 or 1:4. The connection between motor and tool is usually by means of belt or gearing, when the motor may be mounted on top of the column or on an extended base. Silent chain drive between motor and gear is also frequently used. The sizes of the motors required to operate drills for working conditions encountered in practice are as in the following tables : SIZES AND SPEED OF MOTORS ON DRILLS Radial Drills Size in Ft. h. P . Adjustable Speed Rati 4 3 i : 3 or i : 4 5 5 6 5 10 7i Upright Drills. Size in Ft. H.p. Adjustable Speed Ratio . I \ i : 3 or i : 4 2 i 3 2 4 3 POWER REQUIREMENTS FOR DIFFERENT PURPOSES. 293 Multiple Spindle Drills No. Size. h.p. Adjustable Speed Ratio. CR 4 2" 7^ i : 3 or i : 4 6 2" 10 8 2" 10 Boring Mills. The types of motor employed are similar to those suitable for driving drills or lathes and may be connected either through gearing or chains. The power of motors employed in practice are indicated in the following table : HORSEPOWER REQUIRED FOR VERTICAL BORING MILLS. Size in Ft. h.p. Adjustable Speed Ratio. 2 5 i : 3 or i : 4 3^ 7* 5-7 10 9 IS 10 20 12 2O 14 25 16 30 Shearing and Punching Machines. The type of motor employed to drive these tools may be either a 25% over-compounded d.c. machine or a slip-ring induction motor having a high resistance rotor. The connection between the motor and tool is usually by gearing and a fly wheel must always be a part of the combination. The fly wheel serves to steady the load, giving up some of its stored energy during the shearing process. The power required is approx- imately as follows : POWER REQUIRED FOR SHEARING MACHINES Thickness of Metal Cut. Length of Cutter. Inches. h.p. by Test. Material Cut. i 6 sheet 33 2.25 iron A " 5 .46 to 2.5 steel A " 3 steel A " 61 5 I " 6 to 19 steel 1 " 5 3. 8 10 3 . it 4 ii i3-3 I " 14 20 / i j round 28 7.12 i* " 2 . 28 (max.) 294 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Grinders. Motors to operate emery grinders are usually of the high speed totally enclosed shunt or induction types and are generally belted to their loads. BELT POWER EMERY GRINDERS (DOUBLE) Sizeof wheels 3f"Xf' 4i"Xi" 6"xf" o"Xi" i2 f 'Xi' Speed 6000 4250 3400 2500 1750 h.p. for 2 wheels 15 .3 .6 1.25 2 Size of wheels i8"X2" 2 4 "X3" Speed 1 1 70 876 h.p. for 2 wheels 3.5 5 PRINTING PRESSES. Large Rotary Newspaper and Magazine Presses. A low speed of about 10 r.p.m. for the cylinders of such presses is required for mak- ing ready, threading in papers, beating in blankets, etc. For color work a speed of about 100 r.p.m. of press cylinders is usual. Ordinary black ink printing is done at 200 to 300 r.p.m. of cylinders. The latter speed refers to the high speed presses lately put on the market. Four methods of driving these presses are in use at the present time : The first is a single motor drive with either a mechanical attach- ment for getting extreme low speeds, or the armature rheostatic method of speed control, using shunt motor having a speed range of from 2:1. This method is not very satisfactory as it does not give uniform low speed or uniform acceleration. The second method is the two-motor drive. A large motor drives the press at the higher speeds and a small motor is used to obtain low speeds by means of a worm or a train of gears. A release mechanism is interposed between the small motor and the large motor, so that when the control is transferred from one to the other, the former is released from the drive. The small motor, as a rule, has about 20% of the capacity of the large motor and of standard speed. This system has been well tried out and gives good satisfaction. The third method employs a single motor of the double armature type to obtain the full variation of speed from 10 r.p.m. to 300 r.p.m. on press cylinders by field control. The fourth method comprises the "Teaser" system described, p. 49. In all cases the working motors are either geared or direct connected to press shaft. Belts, etc., are not very serviceable on account of the extreme variations in power, and the heavy starting torque necessary for this work. POWER REQUIREMENTS FOR DIFFERENT PURPOSES. 295 Name of Press. R.p.m. of Press Cylinders. Capacity in 8-page Papers per Hour. Approx. h.p. at Maximum Speed. i quadruple. 2OO 24,000 2O Slow speed quadruple 2OO 48,000 7C High speed quadruple 3OO 72 ooo to Slow speed sextuple High speed sextuple .... 2OO 3OO 72,000 108,000 50 7"? Slow speed octuple High speed octuple 200 ?oo 96,000 144. OOO 75 IOO Double quadruple, double sextuple and double octuple presses are in use which take double the power shown in above table, and are usually driven by two motors one for each half, so arranged that they can be coupled electrically and mechanically, if desired. A.C. drive has not been very successful with the possible exception of the two-motor drive, and even then the severe requirements of torque and acceleration make the d.c. drive more desirable. In general it may be stated that large rotary presses use about 600 watts at full load for each 1000 newspapers, 8 pages each, printed per hour. Cylinder Printing Presses. The type of motor employed to drive all cylinder presses is usually 25% compounded, slow speed, rated for continuous service and capable of 25% increase of speed by field control. The motor equipment is provided with an armature resistance whereby the speed of the machine may be reduced 50% below normal so that a 3 : i speed range is obtained. The motor is sometimes geared but more frequently belted to the press, which should be provided with a fly wheel of ample capacity. The power required varies somewhat with the make of press and class of work, the following table giving good average figures : Size of Press Bed in Inches. 17X22 26X34 29X41 33X46 35X50 39X53 43X56 46X62 46X65 h .p. of Motor. I i-5 2-5 3- 3-5 4- 4-5 5- 5^5 to 7. 5 296 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. Bed and Platen Printers. The type of motor employed to drive these presses is about 50% compounded of moderate speed non- reversible. The controller is designed to give 50% reduction in speed by armature resistance and 25% increase in speed by field resistance. The motor is usually belted and an idler attachment for tightening the belt is employed. The power required for these presses is given in the following table: BED AND PLATEN OR " GORDON " PRINTING PRESSES. Size of bed in inches 7X11 8X12 10X15 12X18 14X22 Horse-power of motor -J J-J J f J-i J-i Paper Cutting Machines. The type of motor employed to drive paper cutting machines is from 25 to 50% compound low speed and non-reversible. The mechanical connection is either gearing or belting. The power required to drive paper cutting machines is approximately as follows : PAPER CUTTING MACHINES. Size of cutter in inches 25 30 36 44 50 57 Horsepower of motor i 1.5 2 2.5 3 4 Sewing Machines. The best type of motor for sewing machine drive is either the d.c. shunt or the polyphase induction motor, though single phase induction or " repulsion induction " motors give satisfactory service. Usually sewing machines are driven " in group " by line shafting, but individual drive is sometimes employed. The power required varies with the kind of work about as follows : Light cloth sewing 1 8 to 20 machines per h.p. Heavy cloth sewing 12 to 14 machines per h.p. Leather sewing 10 to 12 machines per h.p. Woodworking Machinery. The motors recommended for driv- ing most wood-working machines are either the d.c. shunt-wound or a.c. squirrel-cage induction motors, depending upon the available current supply. If both are obtainable induction motors are usually preferable because they involve less fire hazard. The connection between motor and tool depends upon the type of drive and with POWER REQUIREMENTS FOR DIFFERENT PURPOSES. 297 group drive the connection is by means of belting to counter shafts. In case of individual drive, it is found that usually band saws, shapers, surfacers, tenonizing machines, etc., are direct connected, while sanders, circular saws, spindle borers and even lathes are belted unless very wide speed ranges are desire for the lathe. The power requirements vary considerably not only with the kind and quality of lumber encountered but also with the depth of cut, rate of feed and condition of tool edge. The following tables give average power demands for different kinds of tools.* FLOORING MACHINES Capacity of Machine. Average h.p. 10-15 10" wide X 4" thick 10" " X6" " 12" " X6" " 15" " X6' "' I2-2O 15-25 25-35 TENONIZING MACHINES Kind. Length of Tenon. Inches. Timber Length. Feet. Style. Average h.p. Double end 7-5 6-5 No copes or saws 6-8 Double end 7-5 6-5 Copes but no saws 8-10 Double end 7-5 6-5 Copes and saws 9-i5 Single end 3-75 . . . Single heads 1-2 Single end 6-5 Double head 2-4 OUTSIDE MOULDERS Size. 4 Sides. 3 Sides. Average h.p. 2 Sides. i Side. 3"X 4 " 3-5 2-3 1-5-3 1-2 4"X4" 3-6 2-4 2-3 1-3 6"X6" 5-8 3-6 2-4 1-5-3 8"X4" 5-8 3-6 2-4 2-3 8"X8" 6-12 5-9 3-6 2-4 14" X 5" 7-14 6-10 4-7 3-S 9" extra heavy 8-14 6-10 4-8 3-6 10" " " 9-iS 7-12 12" " " 10-18 * From data furnished by Wm. Siebenmorgen of the C and C. Electric Company. 298 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. SURFACES OR PLANERS Type. Size. Average h.p. Cabinet single surfacer I2"X 7" 3 2 . C 24" X 7" C-7 C < 30" X 7" 7 ^ IO 7C"X 7" Q 12 Pony single surfacer 20" X 8" 2 A < 26" X 8" 24 Eight roll single surfacer < 24" X 8" 30" X 8" 5-7-5 6 10 < 36" X &" 7 ^-I 1 ? ' l double surfacer 24" X 8" ^-10 < < 30" X 8" 8-iS 36" X 8" 10-18 Four 'sided Q"X o" 8-12 < I2"Xl2" 10-14 I2"X 3 0" 35-40 WOOD TURNING LATHES Swing of Lathe. h.p. Motor Speed Range. IO i-3 i : 3 or i : 4 1(5 i-S-3 24 2-5 30 2-5-7-5 SAWS Type. Diameter of Wheel or Disc in Inches. h.p. Jitind saws 3O i c 3 Re-saw machines 34 36 40 40 at 600 r.p m. 2-3-5 2-S 2-5-5-5 t 7 r Single cut-off . 48 at 600 r.p.m. 54 at 600 r.p.m. 60 at 500 r.p.m. 8 1 2-1 8 I4-2O 16-25 12 1 Rio. . . 12 18 36 8 2-4 2.5-10 4-15 2-4 18 36 5-io 10-15 POWER REQUIREMENTS FOR DIFFERENT PURPOSES. HORIZONTAL BORING 299 No. of Spindles. Size of Holes. h.p. I \" to i" 5-1 2 \" to i" 1-2 3 I" to i" 1-5-3 4 |" to i" 2-4 DRUM SANDERS Drum Diameter. h.p. 16" 3-5 42" IO-I2 60" 15-25 80" 25-35 102" 30-40 Hoisting. The power required to operate elevators, hoists or cranes depends upon the acceleration, height of lift, etc. Ordinary cranes as a rule consume one horsepower per ton lifted at the rate of 10 feet per minute. This assumes an efficiency of about 60%. The motor employed with d.c. service for operating elevators is usually a 20 to 25% compound-wound machine or an interpole motor, with a 2 : i field control. With a.c. supply, induction motors having a relatively high rotor resistance are suitable, the rotor being either of the high resistance squirrel-cage type or of the slip ring form with external resistance. The connection between motor and hoist drum is usually by a worm gear giving the desired speed ratio. For elevators counter-weights are used to offset to some extent the weight of the car and the load to be lifted. An ordinary arrangement- is to employ car counter- weights of about 400 to 500 Ibs. less than the weight of the car and a variable counter- weight of about one-half of the rated lifting capacity of the elevator. The power required to operate an elevator is h.p. x v 33000 X E 300 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. wherein W\ the combined weight in pounds of the load and car to be lifted; W2 = the combined weight of counter- weights for both car and load ; V = speed of lift in feet per minute; E = efficiency of the hoisting mechanism, which is some- what variable depending upon the condition of cable, maintenance of runways, etc. A safe value, however, is from 60 to 70%. For further information concerning power requirements for various industrial purposes the reader is referred to the following articles and publications in addition to that given on pages 287 and 289: APPLICATION OF MOTORS TO MACHINE TOOLS. J. M. Barr, Elect. Journal, Vol. 2, 1905, p. ii. MOTORS FOR METAL AND WOODWORKING MACHINERY. L. R. Pomeroy. G. E. Rev., 1907. SELECTION OF MOTOR OF PROPER CAPACITY. A. G. Popcke. Am. Mack., Vol. 37, 1912, pp. 510 and 550. SPEED AND POWER OF MACHINE TOOLS. J. C. Clifton. Practical Eng., June 9-16, 1911, pp. 712, 739. POWER REQUIREMENTS, FANS, BLOWERS, ETC. C. W. Drake. Elect. Journal, Vol. X, 1913, p. 245. MOTOR DRIVEN CENTRIFUGAL PUMPS. E. C. Wayne. Elect. Journal, Vol. X,p. 228. PUNCHING AND SHEARING MACHINERY. G. A. Anthony, Am. Mach., May, 12, POWER REQUIREMENTS FOR DRILLING. N. T. Sears. Am. Mack., Vol. 35, p. 209, and Sept., 1912. POWER REQUIREMENTS FOR DRILLING. A. L. De Leuw. Trans. A. S. M. E. Vol. 30, p. 837, Vol. 33, p. 245. HANDBOOK FOR MACHINE DESIGNERS AND DRAUGHTSMEN. F. A. Halsey. McCraw Co. 1913, p. 270, et seq. HOISTING AND CONVEYING MACHINERY. G. E. Titcomb. Trans. A. S. M. E., Vol. 30, p. 107. THE ELECTRIC CRANE. C. W. Hill. Lippincott & Co., 1911. PRINTING PRESS, POWER REQUIREMENTS. Elec. Rev., Aug., 1909. Elec. Rev. and W. Elect., May 18, 1912, May 24, 1913. LAUNDRY POWER DRIVE. Elec. Rev. and W. Elect., Jan. 6, 1912. ELECT. ENGS. POCKET BOOK. D. Van Nostrand Co. 1910 Edit., pp. 1515-1529. INDEX. Adams, Prof. C. A 184 Adams, W. G 131 Adjustable-speed Motors 283 Advantages of Electric Power 4 Alternating-current Motors History of 129 Polyphase Induction 131, 173 Repulsion 136, 265 Series 135, 248 Shunt 272 Shunt-induction. 274 Single-phase Induction 223 Synchronous 131, 137 Alternators Parallel Connection 131, 142 Series Connection 139 Anthony, Prof. W. A , 136 Application of Motors 281 Arago, F. J 132 Armature Heating 13, 26 Resistance 13 Resistance Control 40 Armature Reaction 22 Balancing Winding 82, 92 Compensation of 80, 90 Effect on Speed 25 L. B. Atkinson 274 Auto-transformer 202 Auxiliary Poles 81, 92 B Bailey, W 132 Balancer Sets, Multiple Voltage 57 Balancing Actions of Synchronous Mo- tors 169 Balancing Windings, Ryan, H. J 82, 92 Barlow's Wheel 1 Blowers Motor for 284 Power for 288 Boost and Retard System 61 Boring Mills Motor for 293 Power for 293, 299 PAGE Boucherat, P 205 Bradley, C. S 130, 133 Brush-contact Resistance 16, 19 Brush Drop 18 Brush Shifting 48 Bullock Elec. Mfg. Co. Adjustable-speed Shunt Motors. ... 73 Multiple-voltage System 54 Teaser System 63 Cascade Control 217 Chain Drive 284 Characteristic Curves of Compound Motors 126 Induction Motors 197,215,239 Series Motors Ill, 260 Shunt Motors 74, 97 Shunt-Induction 274 Synchronous Motors 157, 161, 167 Circle Diagram of A. C. Series Motor 259 Heyland, A 190 McAllister, A. S 194, 237 Polyphase-Induction Motors. . 190, 194 Repulsion Motor. 269 Single-phase Induction Motor 237 Synchronous Motor 160 Classification of Motors 4, 129/283 Commercial Electric Co. Double Armature Control 66, 68 Commutating Fringe 73 Commutation Lug. . . . : 72 Commutation Poles 73, 84 Compensation Armature Reaction 73 Thompson-Ryan 72 Windings 73, 262, 268, 271 Compensator Starter 202 Compound-wound Motors. . 124 Condenser-Compensator 243 Condict, G. H 117 Constant Current Motor 121 H. P. Motors 50, 71 Torque Motors 50, 71 301 302 INDEX. PAGE Controller Drum and Master. .. 119 G. E. Type K 115 Resistance 41, 114 Counter E. M. F 14, 16, 70 Crocker- Wheeler Co. Multiple- Voltage System 53 Current Equivalent Single Phase 194 Damping Grids 170 Data of Induction Motors 200, 240 Shunt Motors 20 Deprez, M 132 Design Features of Series A. C. Motors.. 253 Design Features Shunt Motors 71 Design of Resistance Controller 42 Diehl Co. Field Control 92 Differentially Wound Motors 124, 127 Direct Current Motors 4 Double-armature Control Methods 66 Double-frequency Currents 232 Drills Motor for 292 Power for 292 Drum and Master Controller 119 Dunn, G. S 90 E 91 Edison, Reluctance Control Efficiency Calculation of 22 Field-controlled Motors, 76, 79,83, 89, 97 Induction Motors 200, 241 Multiple- voltage Control 55 Name Plate 22 Resistance Control 44 Series Motor Control 108, 117 Eichmeyer, R 136 Electric Drive Advantages of 5 Applications of 282 Reliability of 8 Speed Control 8 Elevators Motor for 285 Power for 299 Electro-Dynamic Co 86 Equivalent Single-phase Current 194 Resistance 194 Equivalent Transformer 191 Fans Motor for. Power for . 284 289 PAGE Faraday's Disk 1 Ferraris, Prof. G 133, 173, 234 Field Heating of Windings 12, 26 Reluctance Variation 91 Rotating 132. 173, 225 Speed Control 70, 110 Flooring Machines Motor for 297 Power for 297 Flux Distribution 75, 77, 81, 88 G Gears 110, 282 General Electric Co. Double-armature Control 66 Induction Motor 243 Series Motor Type 69 110 Type K Controller 115 Grinders Motor for 293 Power for . . 293 H Heating Armature Field Limits Heyland, A Heyland Circle Diagram Heyland Induction Motors. . . . History of Motors Holmes-Clatworthy System. . . Hopkinson, J Horse-power Current Curve. . . Hunting of Motors 13, 26 12, 26 12, 111, 114 . . . 191, 246 191 245 130 65 131 107 . 169 Induction Motor, Polyphase 133, 173 Advantages of Three-phase 177 Cascade Connection 217 Characteristic Curves 197 Circle Diagram 190, 194 Compensator for 202 Construction of 173 Development of 133 Efficiency of 200 Equivalent Single-phase 193 Equivalent Transformer 194 Field- Magnetic 173 Flux Equation 182 Leakage Component 184 Current 186 Reactance 184 Locked Current 195 Saturation Curve 195 Magnetizing Current 183 Factor 183 Poles '...'.. 181 Power Factor 186, 200 Pull-out Torque 185, 189, 199 INDEX. 303 PAGE Induction Motor, Polyphase Continued. Resistance Control 181. 217 Rotors for 173, 181, 205 Slip 177, 200 Slip-ring Control 181 Speed Control 214 Starting of 202 Starting Torque 189, 210 Statorfor 173, 178 Torque and Resistance 188, 209 Torque-Voltage 190, 221 Induction Motor, Single-phase 223 Absence of Starting Torque 223 Brown, Boveri & Co. Types 242 Characteristic Curves 239 Condenser Compensator 244 Form of Field 229 G. E. Starter 244 . Methods of Starting 241 Rotating Field 226 Rotor Currents 232 Slip 241 Slip-rotor Resistance 237 Split-phase 242 Starting of 242 Steinmetz, C. P 242, 244 Tesla Patent 241 Wagner Electric Co 245 Interpoles 73 Interpole Motor 73, 84, 116 Iron Loss. Series Motor 249 Jackson, Prof. D. C 136 Jacobi Motor 1 Lamme, G. B 136, 249 Lathes Motor for 286 Power for 291 Leakage Current 186 Flux 184 Primary 184 Reactance 178, 184 Reduction of Power Factor by 186 Secondary 184 Lincoln Motor 96 Load Factor 5 Locked Current 195 Locked Saturation Curve 195 Losses, Division of 24 M McAllister, A. S 193, 240 Machine Tools 289 Magneto Electric Co 76 Master Control 119 PAGE Motors Advantages of 5 Application of 282 Classes of Service 282 Compensated 72, 79, 236, 271 Definition 1 Compound 124 Induction 133. 173, 223 Interpole 73, 84. 116 Series 99, 248 Shunt 10, 272 Synchronous 131, 137 Repulsion 136, 266 Types of 4, 129 Motor-generator System 59 Moulding Machines Motor for 297 Power for 297 Multiple- voltage System Balancer Sets for 58 Bullock Electric Co 54 Control of 59 Crocker- Wheeler Co 53 Efficiency of 55 Three- wire 51 Ward-Leonard, H. . ,52 N Name Plate Data 22 Efficiency 22 Northern Elec. Mfg. Co 79 Osnos, M 269 Osnos Circle Diagram 269 Pacinotti 1, 131 Page Motor 1 Paper Cutters Motor for 296 Power for 296 Pfatischer, M 73, 84 Phase Splitting 242 Swinging 169 Planers Motor for 291 Power for 291, 298 Potter, W. B 112 Power Factor of Induction Motors.. 187, 202, 211, 241 of Repulsion Motors 271 of Series Motors 250, 256, 236 of Synchronous Motors. , 148, 168 Press Control 63, 286 Printing Presses Motor for , 294 Power for . 295 304 INDEX. PAGE Preventive Leads 255 Pumps Motor for 284 Power f or . . . 289 Quarter Phase 173, 176 Rated Current 11, 12 Voltage 11 Rating Series Motors 112 Repulsion Motors. Anthony-Jackson-Ryan 136 Circle Diagram 269 Compensated 268, 271 History 136 Speed Regulation 271 Thomson, Prof. E 136, 266 Winter-Eichberg 271 Resistance Control Induction Motors 207, 217, 221 Series Motors 114 Shunt Motors 33, 41 Starting 33, 114, 207 Ridgway Motor 72, 79 Rotary Field 132, 173, 226, 229 Rotor Windings 173, 181, 205 Ryan, Prof. H. J 72, 136 Saturation Factor 29 Saws Motor 298 Power 298 Series Motor, AC 129, 135, 248 Characteristic Curves 260 Circle Diagrams 259 Compensated 263 Construction 251 Control 263 Losses in 249, 263 Power Factor 250, 256 Preventive Leads 253 Siemens, Alex 135 Sparking 253, 263 Transformer Action 250, 252 Vector Diagram 255 Voltages occurring in 250 Series Motor, D.C 4, 99 Constant Current 99, 121 Constant Potential 99 et seq. Characteristic Curves 99 Connections 99, 105, 110 Control of 114 Drum and Master Control 119 Field Control 116 Gears for 110 G. E. Type 69C 104 PAGE Series Motor, D.C. Continued. Racing of 105 Rating of 112 Resistance Control 114 Series-parallel Control 114 Speed-current 99 Speed-tractive Effort 109 Torque-current 104 Torque per ampere 105 Westinghouse Motor Ill Shunt Motor 10, 272 Adjustable-speed Types 70, 91 Armature Resistance-Speed 25, 41 Auxiliary Pole ' 73, 84 Brush Shifting and Speed 48 Bullock Co 73 Commutation Pole 73, 84 Compensated . 72, 79 Data, Constant-speed Types 20 Double Armature 66 Efficiency 22, 76, 79, 83, 89, 95 Electro-Dynamic Co 73, 84 Field Control 70, 91 Flux Distribution 75, 78, 82, 89, 95 Heating 26 Interpolar 85, 87 Lincoln Mfg. Co 96 Magneto Electric Co 76 Methods of Speed Control.. 41, 50, 70, 91 Multiple-voltage Control 50 Resistance Control 41 Speed-load Curves. .74, 78, 83, 87, 93, 96 Starting Box 33 Storey Motor 76 Stow Mfg. Co 92 Voltage Variation-Speed 28 Ward-Leonard, H. Controls 52 Shunt Induction Motor Atkinson's 274 Compensation for . . .- 275 Speed Control 278 Shearing and Punching Machines Motor for 286 Power for 293 Siemens, Alex 135 Single-phase Induction Motor (see In- duction Motor).. Slip 133, 177, 202, 241 Slip-ring Control 207 Slip-ring Rotors 182 Slip-ring Resistance 190, 209, 237 Speed Control of Induction Motors 214 Repulsion Motors 270 Series Motors 114, 263 Shunt Motors 41, 50, 70, 91 Split Phase 133, 242 Squirrel-cage Rotor 181 Starting Box 33 Compound Motors 124 Induction Motors 202, 245 INDEX. 305 PAGE Starting Continued. Repulsion Motors 270 Series Motors. . 114, 264 Shunt Motors 33 Synchronous Motors 142 Stator Windings 177 Steinmetz, Dr. C. P 136, 243, 244 Storey Motor 76 Stow Motor 92 Synchronous Motor 137 Action 138, 155 Balancing Action 169 Circle Diagram 162 Current-phase Angle Curves 161 Current-power Curves 165 Damping Coils 172 Driving Power 159 History . . 131 Hunting 169, 171 Limit of Stability 155, 156 Maximum Output 155 Phase Relations 147 Power-factor, Improvement 150 Speed of 138 Starting of 142 Starting Torque 142 Super-Excitation 151 Synchronism 138 Synchronizing 142 Torque 151 V Curves 167 Variation of field 150 of Power Factor 151 Synchroscope 146 Tandem Control 217 Teaser Control 61 Temperature Rise 11 Tesla, N 134, 173, 241 Test Voltage 11 PAGE Tenonizing Machines Motor for 292 Power for 297 Thompson, M. E 72 Thompson-Ryan Motor 73,79 Thomson Prof. E 136, 266 Torque 15 Tractive Effort 109 Train Lines 119 Types of Motors 4, 129 Types of Service 283 V Curves 166 Vector Diagrams Induction Motor 192 Repulsion Motor 269 Series Motor 256 Shunt Motor 273 Synchronous Motor 151, 158 Voltage-speed Induction Motor 190, 221 Repulsion Motor 271 Series Motor 104, 262 Shunt Motor 28, 50 Voltage Test 11 Voltage-Torque 190, 282 W Wagner Motor 245 Ward-Leonard, H 52, 60 Westinghouse Series Motor. Ill Wheels 110 Wilde, Prof 131 Windings Balancing 72, 82 Compensation 72, 263, 268, 271 Double-armature 66 Rotor 173, 181, 205 Starting 242 Stator 177 Wood Working Machines 296 D. VAN NOSTRAND COMPANY 25 PARK PLACE New York SHORT=TITLE CATALOG OF OF SCIENTIFIC AND ENGINEERING BOOKS This list includes the technical publications of the following English publishers: SCOTTi GREENWOOD & CO. JAMES MUNRO & CO., Ltd. CONSTABLE & COMPANY, Ltd. TECHNICAL PUBLISHING CO. ELECTRICIAN PRINTING & PUBLISHING CO. for whom D. Van Nostrand Company are American agents. JUNE, 1915. ~~ SHORT-TITLE CATALOG OF THE Publications and Importations OP D. VAN NOSTRAND COMPANY 25 PARK PLACE, N. Y. Trices marked bvith an asterisk (*) are bindings are in cloth unless dhertuise noted. Abbott, A. V. The Electrical Transmission of Energy. 8vo, *$5 oo A Treatise on Fuel. (Science Series No. 9.) i6mo, o 53 Testing Machines. (Science Series No. 74.) i6mo, o 50 Adam, P. Practical Bookbinding. Trans, by T. E. Maw i2mo, *2 50 Adams, H. Theory and Practice in Designing 8vo, *2 50 Adams, H. C. Sewage of Sea Coast Towns 8vo, *2 OD Adams, J. W. Sewers and Drains for Populous Districts 8vo, 2 50 Addyman, F. T. Practical X-Ray Work 8vo, *4 oo Adler, A. A. Theory of Engineering Drawing 8vo, *2 oo Principles of Parallel Projecting-line Drawing 8vo, *i oo Aikman, C. M. Manures and the Principles of Manuring 8vo, 2 50 Aitken, W. Manual of the Telephone 8vo, *8 OD d'Albe, E. E. F., Contemporary Chemistry i2mo, *i 25 Alexander, J. H. Elementary Electrical Engineering 12 mo, 203 Allan, W. Strength of Beams Under Transverse Loads. (Science Series No. 19.) i6mo, o 50 Theory of Arches. (Science Series No. 1 1.) i6mo, Allen, H. Modern Power Gas Producer Practice and Applications, i^mo, *2 50 Gas and Oil Engines 8 vo, *4 50 Anderson, F. A. Boiler Feed Water 8vo, *2 50 Anderson, Capt. G. L. Handbook for the Use of Electricians 8vo, 3 oo Anderson, J. W. Prospector's Handbook i2mo, i 50 Ande*s, L. Vegetable Fats and Oils 8vo, *4 oo Animal Fats and Oils. Trans, by C. Salter 8vo, *4 oo Drying Oils, Boiled Oil, and Solid and Liquid Driers 8vo, *5 oo Iron Corrosion, Anti-fouling and Anti-corrosive Paints. Trans, by C. Salter 8vo, *.| oo Ande*s, L. Oil Colors, and Printers' Ink. Trans, by A. Morris and H. Robson 8vo, *2 50 Andes, L. Treatment of Paper for Special Purposes. Trans, by C. Salter. i2mo. *2 50 D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 3, Andrews, E. S. Reinforced Concrete Construction nmo, *i 25 Theory and Design of Structures 8vo, *3 50 Further Problems in the Theory and Design of Structures. .. .8vo, *2 50 Andrews, E. S., and Hey wood, H. B. The Calculus for Engineers, izmo, *x 25 Annual Reports on the Progress of Chemistry. Nine Volumes now ready. Vol. I. 1904, Vol. IX, 1912 8vo, each, 2 oo Argand, M. Imaginary Quantities. Translated from the French by A. S. Hardy. (Science Series No. 52.) i6mo, o 50 Armstrong, R., and Idell, F. E. Chimneys for Furnaces and Steam Boilers. (Science Series No. i.) i6mo, o SD Arnold, E. Armature Windings of Direct-Current Dynamos. Trans, by F. B. DeGress 8vo, *2 oo Asch, W., and Asch, D. The Silicates in Chemistry and Commerce . 8vo, *6 oo Ashe, S. W., and Keiley, J. D. Electric Railways. Theoretically and Practically Treated. Vol. I. Rolling Stock i2mo, *2 50 Ashe, S. W. Electric Railways. Vol. II. 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