FOURTEENTH LECTURE ALTERNATING CURRENT RAILWAY MOTOR. mN a direct current motor, whether a shunt or a series motor, the motor still revolves in the same direction, if the impressed e. m. f. be reversed, as field and arma- ture both reverse. Since a reversal of voltage does not change the operation of the motor, such a direct current motor there- fore can operate also on alternating current. With an alter- nating voltage supply, the field magnetism of the motor also alternates ; the motor field must therefore be laminated, to avoid excessive energy losses and heating by eddy currents (cur- rents produced in the field iron by the alternation of the mag- netism) just as in the direct current motor the armature must be laminated. In the shunt motor — in which the supply current divides between field and armature — when built for alternating voltage, arrangements must be made to have the current in the field (or rather the field magnetism) and the current in the armature, reverse simultaneously. In the series motor, in which the same current traverses field and armature, the field magnetism and the armature current are necessarily in phase with each other, or nearly so. Only the series or varying speed type of alternating current commutator motor has so far become of industrial importance. In the alternating current motor in addition to the voltage consumed by the resistance of the motor circuit and that con- sumed by the armature rotation, voltage is also consumed by self-induction; that is, by the alternation of the magnetism. The voltage consumed by the resistance represents loss of power, and heating, and is made as small as possible in any 178 GENERAL LECTURES motor. The voltage consumed by the rotation of the arma- ture, or "e. m. f . of rotation," is that doing the useful work of the motor, and so is an energy voltage, or voltage in phase with the current; just as the voltage consumed by the resist- ance is in phase with the current. The voltage consumed by self-induction, due to the alternation of the magnetism, or "e. m. f. of alternation", is in quadrature with the current, or wattless ; that is, it consumes no power, but causes the current to lag, and so lowers the power factor of the motor; that is, causes the motor to take more volt-amperes than corresponds to its output, and so is objectionable. The useful voltage, or e. m. f. of rotation of the motor, is proportional to the speed ; or rather the "frequency of rota- tion", No, is proportional to the field strength F, and to the number of armature turns m. The wattless voltage, or self- induction of the field, is proportional to the frequency N, to the field strengfth F, and the number of field turns n. The ratio of the useful voltage to the wattless voltage therefore is mNo -^ nN, and to make the useful voltage high and the wattless voltage low, therefore requires as high a frequency of rotation No and as low a frequency of supply N, as possible. Thus the commutator motors of more than 25 cycles give poor power factors; and for a given number of revolutions No, which is number of revolutions per second times number of pairs of poles, therefore is the higher, the more poles the motor has. Hence a greater number of poles are generally used in an alternating current than in a direct current motor. Good direct current motor design requires a strong field and weak armature, to get little field distortion and therefore good commutation ; that is high n and low m. But such pro- portions, even at low supply frequency N and high frequency of rotation No, would give a hopelessly bad power factor, and ALTERNATING CURRENT MOTOR 179 thus a commercially impractical motor. In the alternating cur- rent commutator motor, it is therefore essential to use as strong an armature and as weak a field (that is, as large a number of armature turns m and as low a number of field turns n) as pos- sible. Very soon, however, a limit is reached in this direction, even if the greater field distortion and the resultant bad com- mutation were not to be considered : the armature also has a self-induction ; that is, the alternating magnetism produced by the current in the armature turns consumes a wattless e. m. f. This magnetism is small in a direct current motor, but with many armature turns and few field turns it becomes quite con- siderable ; and so, while a further decrease of the field turns and increase of the armature turns reduces the self-induction of the field — which varies with the square of the field turns — it increases the self-induction of the armature — which varies with the square of the armature turns. There is thus a best proportion between armature turns and field turns, which gives the lowest total self-induction. This is about in this propor- tion : armature turns m to field turns n = 2 -4- i ; and at this proportion the power factor of the motor, especially at low and moderate speeds, is still very poor. In alternating current commutator motors it is therefore essential to apply means to neutralize the armature self-induc- tion and armature reaction, so as to be able to increase the proportion of armature turns to field turns sufficiently to get good power factors. This is done by surrounding the arma- ture with a stationary "compensating winding" closely adja- cent to the armature conductors, located in the field pole faces, and traversed by a current opposite in direction to the current in the armature, and of the same number of ampere turns ; so that the armature ampere turns and the ampere turns of the compensating winding neutralize each other, and the armature i8o GENERAL LECTURES reaction, that is, the magnetic flux produced by the armature current, and the self-induotion caused by it, disappear. This compensating winding for neutralizing the armature self-induction was introduced by R. Eickemeyer in the early days of the alternating current commutator motor, and since then all alternating current commutator motors have it ; so that the electric circuits of all alternating current commutator motors comprise an armature winding A, a field winding F, and a compensating winding C. Since the compensating winding cannot be identically at the same place as the armature winding (the one being located in slots in the pole faces, the other in slots in the armature face) there still exists a small magnetic flux produced by the armature winding : the "leakage flux", analogous to the leakage flux of the induction motor ; and the number of armature turns cannot be increased indefinitely, otherwise the armature self- induction, due to this leakage flux, would become appreciable, and the power factor would decrease again. The minimum total self-induction of the motor with compensating winding occurs at a number of armature turns equal to 3 to 5 times the field turns ; at this proportion, the power factor is already very good at low speeds, and the motor is industrially satisfactory in this regard. For best results, that is, complete compensation and there- fore zero magnetic field of armature reaction, it is, however, necessary notlonly to have the same number of ampere turns in the compensating winding as on the armature, but also to have these ampere turns distributed in the same manner around the circumference. With the usual armature winding this is not the case, but the armature conductors cover the whole circum- ference; while the compensating coil conductors cover only the pole arc, as the space between the poles is taken up by the CM •00 1 82 GENERAL LECTURES field winding. That is, the magnetic distribution around the armature circumference is as shown developed in Fig. 42 : the field gives a flat topped distribution, the armature a peaked, and the compensating winding has a small flat top and with the total ampere turns of the compensating winding equal to those of the armature, the compensating winding preponder- ates in front of the field poles, the armature between the field poles, or at the brushes, and there is thus a small magnetic field of armature reaction remaining at the brushes, just where it is objectionable for commutation. As it is not feasible to distribute the compensating wind- ing over the whole circumference of the stator, the armature winding is arranged so that its ampere turns cover only the pole arcs. This is done by using fractional pitch in the arma- ture; that is, the spread of the armature coil or the space between its two conductors, is made, not equal to the pitch of the pole, as shown in Fig. 43, but only to the pitch of the pole arcs, as shown in Fig. 44. With such fractional pitch winding, the currents in the upper and the lower layer of the armature conductors, in the space between the poles, flow in opposite dir- ections, and so neutralize, leaving only that part of the armature winding in front of the pole arcs as magnetizing. Hereby the distribution of the armature ampere turns is made the same as that of the compensating winding, and so complete compensa- tion is realized. The compensating winding may be energized by the main current, and so connected in series with the field and armature ; or the compensating winding may be short circuited upon itself, and so energized by an induced current acting as a secondary of a transformer to the armature as primary ; and as in a transformer, primary and secondary current have the same number of ampere turns (practically) and flow in opposite ALTERNATING CURRENT MOTOR 183 directions, such "inductive compensation" is just as complete compensation as the "conductive compensation" produced by passing the main current through the compensating winding. Fife. 43 Vice versa, the armature may be short circuited and so used as secondary of a transformer, with the compensating winding acting as primary. In either of these motor types, Fife. 44 which comprise primary and secondary circuits, that is, in which armature and compensating winding are not connected directly in series, but inductively, the field may be energized i84 GENERAL LECTURES by the primary or supply current, or by the secondary or induced current. In such a motor embodying a transformer feature, instead of impressing the supply voltage upon one circuit as primary, while the other is closed upon itself as secondary, the supply voltage may be divided in any propor- tion between primary and secondary. As primary and secondary current of a transformer are proportional to each other, it is immaterial, regarding the varia- tion of the current in the different circuits with the load and speed, whether the circuits are directly in series, or by trans- formation; that is, all these motors have the same speed — ^torque — current characteristics, as discussed in the preceding lecture, and differ only in secondary effects, mainly regarding commutation. The use of the transformer feature also permits, without change of supply voltage, to get the effect of a changed supply voltage, or a changed number of field turns, by shifting a cir- cuit over from primary to secondary or vice versa. For in- stance, if the armature is wound with half as many turns, that is, for half the voltage and twice the current, as the compen- sating winding, by changing the field from series connection with the compensating winding to series connection with the armature, the current in the field and thus the field strength, is doubled; that is, the same effect is produced as would be by doubling the number of field turns. According to the relative connection of the three circuits, armature A, compensating circuit C, and field F, alternating current commutator motors of the series t)rpe can be divided into the classes shown diagramatically in Fig. 45 : /7\ v^ y VsC-^ /^\ kczzrsj !0I ^ '*.<=>^ in k^3 : FACUury or AffuiEO science. 86 GENERAL LECTURES Primary : Secondary : A + C + F Conductively Compensated Series Motor. (2). A + F C Inductively Compensating Series Motor. (3). A C + F Inductively Compensating Series Motor with Second- ary Excitation, or In- verted Repulsion Motor. (4). C + F A Repulsion Motor. (5). C A + F Repulsion Motor with Secondary Excitation (6). C & A + F Series Repulsion Motor A. (7). C + F & A Series Repulsion Motor B. (8). The main difference between these tjrpes of motors is found in their commutation. In a direct current motor, with the brushes set at the neutral; that is, midway between the field poles (as is custom- ary in a reversible motor like the series motor), the armature turn, which is short circuited under the brush during the commutation, encloses all the lines of magnetic force of the field; so during this moment it does not cut any lines of force by its rotation, and thus no e. m. f . is induced in this turn ; that is, no current is produced, if the armature reaction is com- pensated for, or is otherwise negligible. If the motor has a considerable armature reaction, and thus a magnetic field at the brushes, this magnetic field of armature reaction induces an e. m. f. in the short circuited turn under the brush, and so ALTERNATING CURRENT MOTOR 187 causes sparking. Hence high armature reaction impairs the commutation of the motor. In an alternating current series motor the armature reac- tion is neutralized by the compensating winding, and therefore no magnetic field of armature reaction exists ; hence no e. m. f . is induced in the turn short circuited under the brush by its rotation through the magnetic field. As this field, however, is alternating, an e. m. f. is induced in the short circuited turn by the alternations of the lines of magnetic force enclosed by it, and causes a short circuit current and in that way, sparking. This e. m. f., being due to the alternation of the enclosed field flux, is independent of the speed of rotation ; it also exists with the motor at a standstill, and is a maximum in the armature turn under the brush, as this encloses the total field flux. The position of the armature turn during commutation, which in a direct current motor is the position of zero induced e. m. f., is therefore in an alternating current motor, the position of maximum induced e. m. f., but induced not by the rotation of the turn, but by the alternation of the magnetism. That is, this turn is in the position of a short circuited secondary to the field coil of the motor as primary of a transformer ; and as primary and secondary ampere turns in a transformer are approximately equal, the current in -the armature turn during commutation is very large; if not limited by the resistance or reactance of the coil, it is as many times greater than the full load current, as the field coil has turns. This causes serious sparking, if not taken care of. One way of mitigating the effect of this short circuit cur- rent is to reduce it by interposing resistance or reactance ; that is by making the leads between the armature turns and the commutator bars of high resistance or high reactance. Obvi- ously this arrangement can merely somewhat reduce the spark- 1 88 GENERAL LECTURES ing by reducing the current in the short circuited coil, but can not eliminate it ; and it has the disadvantage, that in the moment of starting, if the motor does not start at once, the resistance lead is liable to be burned out by excessive heating ; while when running, each lead is in circuit only a very small part of the time: during the moment when the armature turn to which it connects, is under a commutator brush. As the resistance of the lead must be very much greater than that of the arma- ture coil, and as the space available for it is very much smaller, if remaining in circuit for any length of time, it is destroyed by heat. In direct current motors, commutation may be controlled by an interpole or commutating pole; that is, by producing a magnetic field at the brush, in direction opposite to the field of armature reaction, and by this field inducing in the arma- ture turn during commutation, an e. m. f. of rotation which reverses the current. Such a commutating pole, connected in series into a circuit, would, in the alternating current motor, induce an e. m. f. in the short circuited turn, by its rotation; but this e. m. f . would be in phase with the field of the commu- tating pole, and thus with the current, that is, with the main field of the motor. Therefore it could not neutralize the e. m. f. induced in the short circuited turn by the alternation of the main field through it, since this latter e. m. f. is in quadrature with the main field, and thus with the current; but would simply add itself to it, and so make the sparking worse. A series commutating pole, while effective in a direct current motor, is therefore ineffective in an alternating current motor, due to its wrong phase. To neutralize the e. m. f. induced by the alternation of the main field through the armature turn during commutation, ALTERNATING CURRENT MOTOR 189 by an opposite e. m. f. induced in this turn by its rotation through a quadrature field or commutating field, this field must therefore have the proper phase. The e. m. f. of alternation of the main field through the short circuited turn is proportional to the main field F and frequency N, and is in quadrature with the main field. The e. m. f. induced in the short circuited turn by its rotation (through the commutating field is proportional to the frequency of rotation or speed No, and to the commutating field Fo, and in phase therewith ; to be in opposition and equal to the e. m. f. of alternation, the commutating field must there- fore be in quadrature with the main field, and frequency times main field must equal speed times commutating field. That is : N F = No Fo or in other words, the commutating field must be : or equal to the main field times the ratio of frequency to speed, and in quadrature therewith. Hence, at synchronism: N© = N, the commutating field must be equal to the main field; at half synchronism: No = 2 N, it must be twice ; at double synchronism : No = 2 N, it must be one-half the main field. The problem of controlling the commutation of the alter- nating current motor therefore requires the production of a commutating field of proper strength, in quadrature phase with the main field of the motor, and thus with the current. In a transformer, on non-inductive or nearly non-induc- tive secondary load, the magnetism is approximately in quad- rature behind the primary, and ahead of the secondary current ; transformation between compensating winding and arma- I90 GENERAL LECTURES ture thus offers a means of producing a quadrature field in the alternating current motor for compensation. In the conductively compensated series motor, at perfect compensation, no quadrature field exists; while with over or under compensation, a quadrature field exists, in phase with the current, and therefore not effective as commutating field. In the inductively compensated series motor, the quad- rature field, which transforms current from the armature to the compensating winding, is of negligible intensity, as the compensating winding is short circuited, and thus consumes very little voltage. A quadrature field, however, appears in those motors in which the compensating winding is primary, and the armature secondary, that is in repulsion motors ; since in the armature the induced or transformer e. m. f. is opposed by the e. m, f. of rotation ; so a considerable e. m. f . is induced, and therefore a considerable transformer flux exists. Therefore, when impressing the supply voltage on the compensating winding, and short circuiting the armature upon itself, that is, in the repulsion motor, the voltage is supplied to the armature by transformation from the compensating wind- ing, and the magnetic flux of this transformer is in quadrature with the supply current ; that is, it has the proper phase as com- mutating flux. The repulsion motor thus has in addition to the main field, in phase with the current, a transformer field, in quadrature with the main field in space and in time, and so in the proper direction and phase as commutating field ; thus giving perfect commutation if this transformer field has the intensity required for commutation, as discussed above. As in the repulsion motor, the armature is short circuited upon itself, the voltage supplied to it by transformation from ALTERNATING CURRENT MOTOR 191 the compensating winding equals the voltage consumed in it by the rotation through the main field. The former voltage is proportional to the frequency N and to the transformer field F*, the latter to the speed N© and to the main field F, and it so is : N F^ = No F, that is, the transformer field is : N or equal to the main field times the ratio of speed to frequency. Comparing this value of the transformer field of the repul- sion motor, F*, with the required commutating field Fo, it is seen that at synchronism No = N, F* = F© ; that is, the trans- former field of the repulsion motor has the proper value as commutating field, so that no short circuit current is produced in ithe armature turn under the brush, but the commutation is as good as in a direct current motor with negligible armature reaction. At half synchronism, No = 2 ^' ^^^ transformer field of the repulsion motor : F^ = ^ F, is only one quarter as large as the commutating field required Fo = 2 F, and the short cir- cuit current is reduced by 25% below the value which it has in the series motor ; and the commutation, while it is better, is not yet perfect. At double synchronism : No = 2 N, the transformer field is F* = 2 F, while the commutation field should be: Fo = 2 ^» ^^^ the transformer field thus is four times larger than it should be for commutation ; so that only one-quarter of the transformer field is used to neutralize the e. m. f. of alter- nation in the short circuited turn; the other three-quarters induces an e. m. f., thus causing a short circuit current three times as large as it would be in a series motor. That is, the short circuit current under the brush, and thus the sparking, in 19* GENERAL LECTURES the repulsion motor at double synchronism is very much worse than in the series motor, and the repulsion motor at these high speeds is practically inoperative. Hence, as regards commutation, a repulsion motor is equal to the series motor at standstill where no compensation of the short circuit current is possible — ^but becomes better with increasing speed : as good as a good direct current series motor at synchronism; and then again becomes worse by over com- pensation, until at some speed, at 40% above synchronism, it again becomes as poor as the alternating current series motor ; above this speed, it becomes rapidly inferior to the series motor. To produce right intensity of the transformer field, to act as commutating field, it is therefore necessary above syn- chronism to reduce the transformer field below the value which it would have when transforming the total supply voltage from compensating winding to armature. This means, that above synchronism, only a part of the supply voltage must be trans- formed from' compensating winding to armature, the rest directly impressed upon the armature. Thus at double syn- chronism, where the transformer field of the repulsion motor is four times as strong as is required for commutation, to re- duce it to one-quarter, only one-quarter of the supply voltage must be impressed upon the compensating winding, three- quarters directly on the armature. To get zero short circuit current in the armature turn under the brush, below synchronism more than the full supply voltage would have to be impressed upon the compensating winding, which usually cannot conveniently be done. At syn- chronism the full supply voltage is impressed upon the com- pensating winding, while the armature is short circuited as repulsion motor ; and with increasing speed above synchronism, ALTERNATING CURRENT MOTOR 193 more and more of the supply voltage is shifted over from com- pensating winding to armature; that is, the voltage impressed upon the compensating winding is reduced, from full voltage at synchronism, while the voltage impressed upon the armature is increased, from zero at synchronism, to about three-quarters of the supply voltage at double synchronism. Such a motor, in which (the transformer field is varied in accordance with the requirement of commutation, is called a "series repulsion motor," The arrangement described here eliminates the short cir- cuit current induced in the commutated armature turn by the alternation of the main field, and that completely above syn- chronism, so that during commutation, no current is induced in the armature turn. This, however, is not sufficient for per- fect commutation: during the passage of the armature turn under the brush, the current in the turn should reverse; so that in the moment in which the turn leaves the brush, the current has already reversed. For sparkless commutation, it therefore is necessary, in addition to the neutralizing e. m. f . of the trans- former field, to induce an e. m. f. which reverses the current. This e. m. f., and thus the magnetic flux which induces it by the rotation, must be in phase with the current. That is, in addi- tion to the "neutralizing" component of the commutating field (which is in quadrature with the current), to reverse the cur- rent, a second component of the commutating field must exist, in phase with the current; this component so may be called the "reversing field". The total commutating field required to eliminate the short circuit current due to the alternating main field by the "neutralizing" flux, and to reverse the arma- ture current by the "reversing flux", must therefore be some- what less than 90° lagging behind the main field and thus the main current. 194 GENERAL LECTURES While in a transformer with non-inductive load on the secondary, the magnetic flux lags nearly 90° behind the primary current, in a transformer with inductive load on the secondary, the magnetic flux lags less than 90° behind the primary current ; and the more so the higher the inductivity of the secondary load. Therefore, by putting a reactance into the armature circuit of the motor, and so making the armature circuit inductive, the transformer flux is made to lag less than 90° behind the current, and act not only as neutralizing but also as reversing flux ; and so, if it be of proper intensity, it gives perfect ccmmu- tation. An additional reactance would in general be objectional, in lowering the power factor of the motor. The motor, how- ever, contains a reactance: its field circuit, which has to be excited, can be used as reactance for the armature circuit. That is, by connecting the field coils into the armature cir- cuit, or in other words, using secondary excitation, the trans- former flux of the motor is given the lead ahead of quad- rature position with the main field, which is required to act as reversing field. In this manner, it is possible in the alternating current commutator motor, to get at all speeds from synchronism upwards, the same perfect commutation as in a direct current motor with commutating poles, by varying the distribution of supply voltage between compensating winding and armature, and exciting the field in series with the armature circuit ; that is, in the series repulsion motor B of the preceding table. Obviously, this distribution of voltage would for all practical purposes be carried out sufficiently by using a number of steps, as shown diagrammatically by the arrangement in Fig. 46 : Pig. 46. 196 GENERAL LECTURES T is the supply circuit, F the field winding, A the arma- ture, and C the compensating winding. Closing switch i, and leaving all others open, the motor is a repulsion motor. Closing switch 2, and leaving i and all others open, the motor is a repulsion motor with secondary excitation. Closing switch 3, or 4, 5 . . . and leaving all others open, the motor is a series repulsion motor B, with gradually increasing armature voltage and decreasing voltage on the compensating winding. By winding the armature for half the voltage and twice the current of the compensating winding, when changing from position I, the field in the compensating circuit, to the next position, with the field in the armature circuit, the field current and the field strength becomes double the value it had in start- ing, where no compensation exists, and which it would have to maintain in a series motor; and thus a correspondingly greater motor output is secured, than would be possible in a motor in which the commutation is not controlled.