Theory and Calculation of Electrical Apparatus (1917) — part 16 of 21

that is, the commutating field is inversely proportional to the square of the speed; for instance, at double synchronism it should be one-quarter as high as at synchronism, etc.

201 . Of the quadrature field, $>', only that part is needed for commutation which enters and leaves the armature at the posi- tion of the brushes; that is, instead of producing a quadrature field, <£', in accordance with equation (20), and distributed around the armature periphery in the same manner as the main field, <$, but in quadrature position thereto, a local commutating field may be used at the brushes, and produced by a commutating- pole or commutating coil, as shown diagrammatically in Fig. 172

Fig. 172. — Commutation with commutating poles.

as Ki and K. The excitation of this commutating coil, K, then would have to be such as to give a magnetic air-gap density <B' relative to that of the main field, <£, by the same equations (19) and (20):

As the alternating flux of a magnetic circuit is proportional to the voltage which it consumes, that is, to the voltage impressed upon the magnetizing coil, and lags nearly 90° behind it, the mag- netic flux of the commutating poles, K ) can be produced by ener- gizing these poles by an e.m.f. e,' which is varied with the speed of the motor, by equation :

where e 0 is its proper value at synchronism.

SINGLE-PHASE COMMUTATOR MOTORS 359

Since (B' lags 90° behind its supply voltage, e, and also lags 90° behind <B, by equation (2), and so behind the supply current and, approximately, the supply e.m.f. of the motor, the voltage, e , required for the excitation of the commutating poles is approxi- mately in phase with the supply voltage of the motor; that is, a part thereof can be used, and is varied with the speed of the motor.

Perfect commutation, however, requires not merely the elimi- nation of the short-circuit current under the brush, but requires a reversal of the load current in the armature coil during its passage under the commutator brush. To reverse the current, an e.m.f. is required proportional but opposite to the current and therefore with the main field; hence, to produce a reversing e.m.f. in the armature coil under the commutator brush a second com- mutating field is required, in phase with the main field and ap- proximately proportional thereto.

The commutating field required by a single-phase commutator motor to give perfect commutation thus consists of a component in quadrature with the main field, or the neutralizing component, which eliminates the short-circuit current under the brush, and a component in phase with the main field, or the reversing com- ponent, which reverses the main current in the armature coil under the brush; and the resultant commutating field thus must lag behind the main field, and so approximately behind the sup- ply voltage, by somewhat less than 90°, and have an intensity varying approximately inversely proportional to the square of the speed of the motor.

Of the different motor types discussed under IV, the series motors, 1 and 2, have no quadrature field, and therefore can be made to commutate satisfactorily only by the use of commutator leads, or by the addition of separate commutating poles. The inverted repulsion motor, 3, has a quadrature field, which de- creases with increase of speed, and therefore gives a better com- mutation than the series motors, though not perfect, as the quad- rature field does not have quite the right intensity.

The repulsion motors, 4 and 5, have a quadrature field, lag- ging nearly 90° behind the main field, and thus give good com- mutation at those speeds at which the quadrature field has the right intensity for commutation. However, in the repulsion motor with secondary excitation, 5, the quadrature field is con- stant and independent of the speed, as constant supply voltage

360

ELECTRICAL APPARATUS

is impressed upon the commutating winding, C, which produces the quadrature field, and in the direct repulsion motor, 4, the quadrature field increases with the speed, as the voltage consumed by the main field F decreases, and that left for the compensating winding, C, thus increases with the speed, while to give proper commutating flux it should decrease with the square of the speed. It thus follows that the commutation of the repulsion motors improves with increase of speed, up to that speed where the quadrature field is just right for commutating field — which is about at synchronism — but above this speed the commutation rapily becomes poorer, due to the quadrature field being far in excess of that required for commutating.

In the series repulsion motors, 6 and 7, a quadrature field also exists, just as in the repulsion motors, but this quadrature field depends upon that part of the total voltage which is impressed upon the commutating winding, C, and thus can be varied by varying the distribution of supply voltage between the two cir- cuits; hence, in this type of motor, the commutating flux can be maintained through all (higher) speeds by impressing the total voltage upon the compensating circuit and short-circuiting the armature circuit for all speeds up to that at which the required commutating flux has decreased to the quadrature flux given by the motor, and from this speed upward only a part of the supply voltage, inversely proportional (approximately) to the square of the speed, is impressed upon the compensating circuit, the rest shifted over to the armature circuit. The difference between 6 and 7 is that in 6 the armature circuit is more inductive, and the quadrature flux therefore lags less behind the main flux than in 7, and by thus using more or less of the field coil in the arma- ture circuit its inductivity can be varied, and therewith the phase displacement of the quadrature flux against the main flux adjusted from nearly 90° lag to considerably less lag, hence not only the proper intensity but also the exact phase of the required commutating flux produced.

As seen herefrom, the difference between the different motor types of IV is essentially found in their different actions regarding commutation.

It follows herefrom that by the selection of the motor-type quadrature fluxes, 4>i, can Be impressed upon the motor, as com- mutating flux, of intensities and phase displacements against the main flux, <h, varying over a considerable range. The main

SINGLE-PHASE COMMUTATOR MOTORS

361

advantage of the series-repulsion motor type is the possibility which this type affords, of securing the proper commutating field at all speeds down to that where the speed is too low to induce sufficient voltage of neutralization at the highest available commutating flux.

VI. Motor Characteristics

202. The single-phase commutator motor of varying speed or series characteristic comprises three circuits, the armature, the compensating winding, and the field winding, which are connected in series with each other, directly or indirectly.

. The impressed e.m.f . or supply voltage of the motor then con- sists of the components:

1. The e.m.f. of rotation, ei, or voltage generated in the arma- ture conductors by their rotation through the magnetic field, <$. This voltage is in phase with the field, <£, and therefore approxi- mately with the current, i , that is, is power e.m.f., and is the voltage which does the useful work of the motor. It is propor- tional to the speed or frequency of rotation, / 0 , to the field strength, 4>, and to the number of effective armature turns, n x .

ei = 2 TfoUx® 10" 8 . (23)

The number of effective armature turns, n h with a distributed winding, is the projection of all the turns on their resultant direc- tion. With a full-pitch winding of n series turns from brush to brush, the effective number of turns thus is :

• I 2

Ui = m [avg cos] 2 = -- m. (24)

T

With a fractional-pitch winding of the pitch of t degrees, the effective number of 'turns is :

n\ = m -- [avg cos] r 2 = ? m sin (25)

7T ~ 2 7T A

2. The e.m.f. of alternation of the field, eo, that is, the voltage generated in the field turns by the alternation of the magnetic flux, <i>, produced by them and thus enclosed by them. This vol- tage is in quadrature with the field flux, 4>, and thus approxi- mately with the current /, is proportional to the frequency of the

362 ELECTRICAL APPARATUS

impressed voltage, /, to the field strength, <£, and to the number of field turns, n 0 .

e 0 — 2j7rfn 0 $ 10“ 8 . (26)

3. The impedance voltage of the motor:

e' = IZ (27)'

and: Z = r -f jx ,

•where r — total effective resistance of field coils, armature with commutator and brushes, and compensating winding, x — total self-inductive reactance, that is, reactance of the leakage flux of armature and compensating winding — or the stray flux passing locally between the armature and the compensating conductors — plus the self-inductive reactance of the field, that is, the reac- tance due to the stray field or flux passing between field coils and armature.

In addition hereto, x comprises the reactance due to the quad- rature magnetic flux of incomplete compensation or overcom- pensation, that is, the voltage generated by the quadrature flux, <£', in the difference between armature and compensating con- ductors, rii — n 2 or n 2 — n-i.

Therefore the total supply voltage, E } of the motor is:

E — Ci -f- 6o T“ d

= 2 7r/ 0 fti<$> 10~ 8 + 2jTfni$ 10~ 8 + (r + jx) I. (28)

Let, then, R = magnetic reluctance of field circuit, thus $ = = the magnetic field flux, when assuming this flux as in

phase with the excitation 7, and denoting:

2wfno 2 10~ 8 R

Xq

(30)

as the effective reactance of field inductance, corresponding to the e.m.f. of alternation:

f

S = ~~ = ratio of speed to frequency, or speed

• as fraction of synchronism,

Ui

c = — - = ratio of effective armature turns to n ° field turns;

SINGLE-PHASE COMMUTATOR MOTORS

363

substituting (30) and-(31) in (28):

E = cSxol + jx 0 I + (r + jx) I

= [(r + cSxo) + j(x + x 0 )] I; (32)

or:

E

' ~ {r -fcSx o) + j (x + x 0 ) and, in absolute values:

y/{r~- 1- cSxo) 2 + (x + xoj 2 ' ^

The power-factor is given by:

a: + Xo ,,, r s

tan 0 = T+Wz (S5)

The useful work of the motor is done by the e.m.f. of rotation :

E± = cSxol,

and, since this e.m.f., E i, is in phase with the current, I, the useful work, or the motor output (inclusive friction, etc.), is:

P = EJ = cSxoi 2

cSx a e 2 , .

_ + (a . 4. Xo y’

and the torque of the motor is :

7, P

D — -g = cx 0 i-

cx 0 e 2

" (r + c&c 0 ) 2 + (x + xo) 2 ' i ;

For instance, let :

e = 200 volts, c = — = 4,

«o

Z = r + jx = 0.02 + 0.06 j, x 0 = 0.08;

10,000

• ~ a/CR- 16 S) F + 49 amp ’.’

cot 6 =

1 + 16 S

32,000 5 ,

^ (1 + 16 > S ) 2 4 - 49 ’

n 32,000

D ~ (l + 16Sj< + i9 5Vn - kW -

364

ELECTRICAL APPARATUS

203. The behavior of the motor at different speeds is best shown by plotting i, p = cos d, P and D as ordinates with the speed, Sj as abscissa*, as shown in Fig. 173.

In railway practice, by a survival of the practice of former times, usually the constants are plotted with the current, I, as abscissae, as shown in Fig. 174, though obviously this arrange- ment does not as well illustrate the behavior of the motor.

Graphically, by starting with the current, 7, as zero axis, 01, the motor diagram is plotted in Fig. 175.

Fig. 173. — Single-phase commutator-motor speed characteristics.

Th e v oltage consumed by the resistance, r, is OE r = ir, in phase with OJ; the voltage consumed fc>y 'the reactance, x, is OE z = ix, and 90° ahead of 01. OE r and OE x combine to the voltage con- sumed by the motor impedance, OE' — iz.

Combining OE' = iz, OEi — and OE Q = e 0 thus gives the terminal voltage, OE — e, of the motor, and the phase angle, EOI = 6.

In this diagram, and in the preceding approximate calculation, the magnetic flux, 4>, has been assumed in phase with the current, I .

In reality, however, the equivalent sine wave of magnetic flux, <£, lags behind the equivalent sine wave of exciting current, J, by the angle of hysteresis lag, and still further by the power

SINGLE-PHASE COMMUTATOR MOTORS

3(15

consumed by eddy currents, and, especially in the commutator motor, by the power consumed in the short-circuit current under the brushes, and the vector, Oh, therefore is behind the current vector, 01, by an angle a, which is small in a motor in which the short-circuit current under the brushes is eliminated and the eddy currents are negligible, but may reach considerable values in the motor of poor commutation.

Fig. 174.-— Single-phase commutator-motor current characteristics.

Assuming then, in Fig. 176, Oh lagging behind 01 by angle a , OE i is in phase with Oh, hence lagging behind 01; that is, the e.in.f. of rotation is not entirely a power e.m.f., but contains a wattless lagging component. The e.m.f. of alternation, OEq, is 90° ahead of Oh, hence less than 90° ahead of 01, and therefore contain^ a power component representing the power consumed by hysteresis, eddy currents, and the short-circuit current under the brushes.

Completing now the diagram, it is seen that the phase angle, 0 , is reduced, that is, the power-factor of the motor increased by

366

ELECTRICAL APPARATUS

the increased loss of power, but is far greater than corresponding thereto. It is the result of the lag of the e.m.f. of rotation, which produces a lagging e.m.f. component partially compensating for the leading e.m.f. consumed by self-inductance, a lag of the e.m.f. being equivalent to a lead of the current.

Fig. 175. — Single-phase commutator-motor vector diagram.

As the result of this feature of a lag of the magnetic flux, by producing a lagging e.m.f. of rotation and thus compensating for the lag of current by self-inductance, single-phase motors having poor commutation usually have better power-factors, and

Fig. 176. — Single-phase commutator-motor diagram with phase displace- ment between flux and current.

improvement m commutation, by eliminating or reducing the short-circuit current under the brush, usually causes a slight de- crease in the power-factor, by bringing the magnetic flux, $, more nearly in phase with the current, I.

204. Inversely, by increasing the lag of the magnetic flux, r $, the phase angle can be decreased and the power-factor improved. Such a shift of the magnetic flux, $, behind the supply current, i, can be produced by dividing the current, i, into components, V

SINGLE-PHASE COMMUTATOR MOTORS 367

and i", and using the lagging component for field excitation. This is done most conveniently by shunting the field by a non- inductive resistance. Let ro be the non-inductive resistance in shunt with the field winding, of reactance, xq + £ 1 , where Xi is

Fig. 177. — Single-phase commutator-motor improvement of power-factor by introduction of lagging e.m.f. of rotation.

that part of the self-inductive reactance, x, due to the field coils. The current, V , in the field is lagging 90° behind the current, i", in a non-inductive resistance, and the two currents have the

ratio v, = — ~ — ; hence, dividing the total current, 01, in this

% Xo t Xi

proportion into the two quadrature components, OP and 07",

Fig. 178. — Single-phase commutator motor. Unity power-factor produced by lagging e.m.f. of rotation.

in Fig. 177, gives the magnetic flux, 0$, in phase with OF, and so lagging behind 01, and then the e.m.f. of rotation is OEi, the e.m.f. of alternation OEo, and combining OE, OEq , and OE f

368

ELECTRICAL APPARATUS

gives the impressed e.m.f., OE, nearer in phase to 01 than with 0$ in phase with 01.

In this manner, if the e.m.fs. of self-inductance are not too large, unity power-factor can be produced, as shown in Fig. 178.

Let 01 = total current, OE ' = impedance voltage of the motor, OE = impressed e.m.f. or supply voltage, and assumed in phase with 01. OE then must be the resultant of OE' and of OE 2 , the voltage of rotation plus that of alternation, and resolv- ing therefore 0E 2 into two components, OE i and OEq, in quadra-

Fig. 179. — Single-phase commutator-motor diagram with secondary excitation.

ture with each other, and proportional respectively to the e.m.f. of rotation and the e.m.f. of alternation, gives the magnetic flux, 0i>, in phase with the e.m.f. of rotation, OE i, and the component of current in the field, OP, and in the non-inductive resistance, OI " , in phase and in quadrature respectively with 0<$>, which combined make up the total current. The projection of the e.m.f. of rotation OE i on 01 then is the power component of the e.m.f., which does the work of the motor, and the quadra- ture projection of, 0E h is the compensating component of the e.m.f. of rotation, which neutralizes the wattless component of the e.m.f. of self-inductance.

Obviously such a compensation involves some loss of power in the non-inductive resistance, r 0 , shunting the field coils, and as the power-factor of the motor usually is sufficiently high, such compensation is rarely needed.

In motors in which some of the circuits are connected inductively in scries with the others the diagram is essentially the same, except

SINGLE-PHASE COMMUTATOR MOTORS

369

that a phase displacement exists between the secondary and the primary current. The secondary current, h, of the transformer lags behind the primary current, Io, slightly less than 180°; that is, considered in opposite direction, the secondary current leads the primary by a small angle, 0 O , and in the motors with secondary excitation the field flux, <£, being in phase with the field current, 1 1 (or lagging by angle a behind it), thus leads the primary current, I Q , by angle 0 O (or angle do — a). As a lag of the mag- netic flux $ increases, and a lead thus decreases the power-factor, motors with secondary field excitation usually have a slightly

Fig. ISO. — Single-phase commutator motor with secondary excitation power-factor improved by shunting field winding with non-inductive circuit.

lower power-factor than motors with primary field excitation, and therefore, where desired, the power-factor may be improved by shunting the field with a non-inductive resistance, r 0 . Thus for instance, if, in Fig. 179, OIo = primary current, Oh = sec- ondary current, OE i, in phase with Oh, is the e.m.f. of rotation, in the case of the secondary field excitation, and OEq , in quadra- ture ahead of Oh, is the e.m.f. of alternation, while 0E- is the impedance voltage, and OE u OE 0 and OE ' combined give the supply voltage, OE, and EOI = 6 the angle of lag.

Shunting the field by a non-inductive resistance, r 0 , and thus resolving the secondary current Oh into the components OI'i in the field and 01" i in the non-inductive resistance, gives the dia- gram Fig. 180, where a = I'\0$ = angle of lag of magnetic field.

370

ELECTRICAL APPARATUS

205. The action of the commutator in an alternating-current motor, in permitting compensation for phase displacement and thus allowing a control of the power-factor, is very interesting and important, and can also be used in other types of machines, as induction motors and alternators, by supplying these machines with a commutator for phase control.

A lag of the current is the same as a lead of the e.m.f., and in- versely a leading current inserted into a circuit has the same ef- fect as a lagging e.m.f. inserted. The commutator, however, produces an e.m.f. in phase with the current. Exciting the field by a lagging current in the field, a lagging e.m.f. of rotation is produced which is equivalent to a leading current. As it is easy to produce a lagging current by self-inductance, the commutator thus affords an easy means of producing the equivalent of a leading current. Therefore, the alternating-current commutator is one of the important methods of compensating for lagging currents. Other methods are the use of electrostatic or electro- lytic condensers and of overexcited synchronous machines.

Based on this principle, a number of designs of induction motors and other apparatus have been developed, using the commutator for neutralizing the lagging ^ magnetizing current and the lag caused by self-inductance, and thereby producing unity power-factor or even leading currents. So far, however, none of them has come into extended use.

This feature, however, explains the very high power-factors feasible in single-phase commutator motors even with consider- able air gaps, far larger than feasible in induction motors.

VII. • Efficiency and Losses

206. The losses in single-phase commutator motors are essen- tially the same as in other types of machines:

(а) Friction losses — air friction or windage, bearing friction and commutator brush friction, and also gear losses or other mechanical transmission losses.

(б) Core losses, as hysteresis and eddy currents. These are of two classes — the alternating core loss, due to the alternation of the magnetic flux in the main field, quadrature field, and arma- ture and the rotating core loss, due to the rotation of the arma- ture; through the magnetic field. The former depends upon the frequency, the latter upon the speed.

(c) Commutation losses, as the power consumed by the short-

SINGLE-PHASE COMMUTATOR MOTORS 371

circuit current under the brush, by arcing and sparking, where such exists.

(d) i 2 r losses in the motor circuits — the field coils, the compen- I

sating winding, the armature and the brush contact resistance. I

(< e ) Load losses, mainly represented by an effective resistance, I

that is, an increase of the total effective resistance of the motor 1

beyond the ohmic resistance. I

Driving the motor by mechanical power and with no voltage I

on the motor gives the friction and the windage losses, exclusive 1

of commutator friction, if the brushes are lifted off the commu- tator, inclusive, if the brushes are on the commutator. Ener- gizing now the field by an alternating current of the rated fre- quency, with the commutator brushes off, adds the core losses to the friction losses; the increase of the driving power then measures the rotating core loss, while a wattmeter in the field exciting circuit measures the alternating core loss.

Thus the alternating core loss is supplied by the impressed electric power, the rotating core loss by the mechanical driving- power.

Putting now the brushes down on the commutator adds the commutation losses.

The ohmic resistance gives the i 2 r losses, and the difference between the ohmic resistance and the effective resistance, calcu- lated from wattmeter readings with alternating current in the motor circuits at rest and with the field unexcited, represents the load losses. I

However, the different losses so derived have to be corrected for their mutual effect. For instance, the commutation losses I

are increased by the current in the armature ; the load losses are 8

less with the field excited than without, etc. ; so that this method 1

of separately determining the losses can give only an estimate of 1

their general magnitude, but the exact determination of the effi- I

ciency is best carried out by measuring electric input and me- 8

chanical output. 1

VIII. Discussion of Motor Types j

207* Varying-speed single-phase commutator motors can be 1

divided into two classes, namely, compensated series motors and I

repulsion motors. In the former, the main supply current is 1

through the armature, while in the latter the armature is closed 1

upon itself as secondary circuit; with the compensating winding 1

372

ELECTRICAL APPARATUS

as primary or supply circuit. As the result hereof the repulsion motors, contain a transformer flux, in quadrature position to the main flux, and lagging behind it, while in the series motors no such lagging quadrature flux exists, but in quadrature position to the main flux, the flux either is zero — complete compensation — or in phase with the main flux — over- or undercompensation.

A. Compensated Series Motors

Series motors give the best power-factors, with the exception of those motors in which by increasing the lag of the field flux a compensation for power-factor is produced, as discussed in V. The commutation of the series motor, however, is equally poor at all speeds, due to the absence of any commutating flux, and with the exception of very small sizes such motors therefore are inoperative without the use of either resistance leads or com- mutating poles. With high-resistance leads, however, fair opera- tion is secured, though obviously not of the same class with that of the direct-current motor; with commutating poles or coils producing a local quadrature flux at the brushes good results have been produced abroad.

Of the two types of compensation, conductive compensation, 1, with the compensating winding connected in series with the armature, and inductive compensation, 2, with the compensated winding short-circuited upon itself, inductive compensation nec- essarily is always complete or practically complete compensa- tion, while with conductive compensation a reversing flux can be produced at the brushes by overcompensation, and the com- mutation thus somewhat improved, especially at speed, at the sacrifice, however, of the power-factor, which is lowered by the increased self-inductance of the compensating winding. On the short-circuit current under the brushes, due to the e.m.f. of alter- nation, such overcompensation obviously has no helpful effect. Inductive compensation has the advantage that the compen- sating winding is not connected with the supply circuit, can be made of very low voltage, or even of individually short-circuited turns, and therefore larger conductors and less insulation used, which results in an economy of space, and therewith an increased output for the same size of motor. Therefore inductive compen- sation is preferable where it can be used. It is not permissible, however, in motors which are required to operate also on direct current, since with direct-current, supply no induction takes place

SINGLE-PHASE COMMUTATOR MOTORS 373

and therefore the compensation fails, and with the high ratio of armature turns to field turns, without compensation, the field distortion is altogether too large to give satisfactory commutation, except in small motors.

The inductively compensated series motor with secondary ex- citation, or inverted repulsion motor, 3, takes an intermediary position between the series motors and the repulsion motors; it is a series motor in so far as the armature is in the main supply circuit, but magnetically it has repulsion-motor characteristics, that is, contains a lagging quadrature flux. As the field exci- tation consumes considerable voltage, when supplied from the compensating winding as secondary circuit, considerable voltage must be generated in this winding, thus giving a corresponding transformer flux. With increasing speed and therewith decreas- ing current, the voltage consumed by the field coils decreases, and therewith the transformer flux which generates this voltage. Therefore, the inverted repulsion motor contains a transformer flux which has approximately the intensity and the phase re- quired for commutation; it lags behind the main flux, but less than 90°, thus contains a component in phase with the main flux, as reversing flux, and decreases with increase of speed. Therefore, the commutation of the inverted repulsion motor is very good, far superior to the ordinary series motor, and it can be operated without resistance leads; it has, however, the serious objection of a poor power-factor, resulting from the lead of the field flux against the armature current, due to the secondary ex- citation, as discussed in V. To make such a motor satisfactory in power-factor requires a non-inductive shunt across the field, and thereby a waste of power. For this reason it has not come into commercial use.

B. Repulsion Motors

208. Repulsion motors are characterized by a lagging quadra- ture flux, which transfers the power from the compensating wind- ing to the armature. At standstill, and at very low speeds, re- pulsion motors and series motors are equally unsatisfactory in commutation; while, however, in the series motors the commu- tation remains bad (except when using commutating devices), in the repulsion motors with increasing speed the commutation rapidly improves, and becomes perfect near synchronism. As the result hereof, under average conditions a much inferior com-

374

ELECTRICAL APPARATUS

mutation can be allowed in repulsion motors at very low speeds than in series motors, since in the former the period of poor commutation lasts only a very short time. While, therefore, series motors can not be satisfactorily operated without resistance leads (or commutating poles), in repulsion motors resistance leads are not necessary and not used, and the excessive current density under the brushes in the moment of starting permitted, as it lasts too short a time to cause damage to the commutator.

As the transformer field of the repulsion motor is approximately constant, while the proper commutating field should decrease with the square of the speed, above synchronism the transformer field is too large for commutation, and at speeds considerably above synchronism — 50 per cent, and more — the repulsion motor becomes inoperative because of excessive sparking. At syn- chronism, the magnetic field of the repulsion motor is a rotating field, like that of the polyphase induction motor.

Where, therefore, speeds far above synchronism are required, the repulsion motor can not be used; but where synchronous speed is not much exceeded the repulsion motor is preferred be- cause of its superior commutation. Thus -when using a commu- tator as auxiliary device for starting single-phase induction motors the repulsion-motor type is used. For high frequencies, as 60 cycles, where peripheral speed forbids synchronism being greatly exceeded, the repulsion motor is the type to be considered.

Repulsion motors also may be built with primary and sec- ondary excitation. The latter usually gives a better commuta- tion, because of the lesser lag of the transformer flux, and there- with a greater in-phase component, that is, greater reversing flux, especially at high speeds. Secondary excitation, however, gives a slightly lower power-factor.

A combination of the repulsion-motor and series-motor types is the series repulsion motor, 6 and 7. In this only a part of the supply voltage is impressed upon the compensating winding and thus transformed to the armature, while the rest of the sup- ply voltage is impressed directly upon the armature, just as in the series motor. As result thereof the transformer flux of the series repulsion motor is less than that of the repulsion motor, in the same proportion in which the voltage impressed upon the compensating winding -is less than the total supply voltage. Such a motor, therefore, reaches equality of the transformer flux with the commutating flux, and gives perfect commutation at a

SINGLE-PHASE COMMUTATOR MOTORS 375

higher speed than the repulsion motor, that is, above synchron- ism. With the total supply voltage impressed upon the compen- sating winding, the transformer flux equals the commutating flux at synchronism. At n times synchronous speed the com- mutating flux should be of what it is at synchronism, and by

impressing of the supply voltage upon the compensating wind- ing, the rest on the armature, the transformer flux is reduced to ~2 of its value, that is, made equal to the required commuta- ting flux at n times synchronism.

In the series repulsion motor, by thus gradually shifting the supply voltage from the compensating winding to the armature and thereby reducing the transformer flux, it can be maintained equal to the required commutating flux at all speeds from syn- chronism upward; that is, the series repulsion motor arrange- ment permits maintaining the perfect commutation, which the repulsion motor has near synchronism, for all higher speeds.

With regard to construction, no essential difference exists be- tween the different motor types, and any of the types can be operated equally well on direct current by connecting all three circuits in series. In general, the motor types having primary and secondary circuits, as the repulsion and the series repulsion motors, give a greater flexibility, as they permit winding the circuits for different voltages, that is, introducing a ratio of trans- formation between primary and secondary circuit. Shifting one motor element from primary to secondary, or inversely, then gives the equivalent of a change of voltage or change of turns, Thus a repulsion motor in which the stator is wound for a higher voltage, that is, with more turns, than the rotor or armature, when connecting all the circuits in series for direct-current opera- tion, gives a direct-current motor having a greater field excita- tion compared with the armature reaction, that is, the stronger field which is desirable for direct-current operating but not per- missible with alternating current.

209. In general, tthe constructve differences between motor types are mainly differences in connection of the three circuits. For instacne, let F = field circuit, A = armature circuit, C = compensating circuit, T = supply transformer, R = resistance used in starting and at very low speeds. Connecting, in.Fig. 181, the armature, A, between field F and compensating winding, C.

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ELECTRICAL APPARATUS

With switch 0 open the starting resistance is in circuit; closing switch 0 short-circuits the starting resistance and gives the run- ning conditions of the motor.

With all the other switches open the motor is a conductively compensated series motor.

T

Fig. 181. — Alternating-current commutator motor arranged to operate either as series or repulsion motor.

Closing 1 gives the inductively compensated series motor. Closing 2 gives the repulsion motor with primary excitation. Closing 3 gives the repulsion motor with secondary excitation. Closing 4 or 5 or 6 or 7 gives the successive speed steps of the series repulsion motor with armature excitation.

T

Fig. 182. — Alternating-current commutator motor arranged to operate either as series or repulsion motor.

Connecting, in Fig. 182, the field, F , between armature, A , and compensating winding, C , the resistance, R } is again controlled by switch 0.

All other switches open gives the conductively compensated series motor.

SINGLE-PHASE COMMUTATOR MOTORS 377

Switch 1 closed gives the inductively compensated series motor.

Switch 2 closed gives the inductively compensated series motor with secondary excitation, or inverted repulsion motor.

Switch 3 closed gives the repulsion motor with primary excitation.

Switches 4 to 7 give the different speed steps of the series re- pulsion motor with primary excitation.

Opening the connection at x and closing at y (as shown in dotted line), the steps 3 to 7 give respectively the repulsion motor with secondary excitation and the successive steps of the series repulsion motor with armature excitation.

Still further combinations can be. produced in this manner, as for instance, in Fig. 181, by closing 2 and 4, but leaving 0 open, the field, F } is connected across a constant-potential supply, in series with resistance, R , while the armature also receives con- stant voltage, and the motor then approaches a finite speed, that is, has shunt motor characteristic, and in starting, the main field, F , and the quadrature field, AC, are displaced in phase, so give a rotating or polyphase field (unsymmetrical) .

To discuss all these motor types with their in some instances very interesting characteristics obviously is not feasible. In general, they can all be classified under series motor, repulsion motor, shunt motor, and polyphase induction motor, and com- binations thereof.

IX. Other Commutator Motor

210 . Single-phase commutator motors have been developed as varying-speed motors for railway service. In other directions commutators have been applied to alternating-current motors and such motors developed :

(a) For limited speed, or of the shunt-motor type, that is, motors of similar characteristic as the single-phase railway motor, except that the speed does not indefinitely increase with decreasing load but approaches a finite no-load value. Several types of such motors have been developed, as stationary motors for elevators, variable-speed machinery, etc., usually of the single-phase type.

By impressing constant voltage upon the field the magnetic field flux is constant, and the speed thus reaches a finite limiting value at which the e.m.f. of rotation of the armature through

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ELECTRICAL APPARATUS

the constant field flux consumes the impressed voltage of the armature. By changing the voltage supply to the field different speeds can be produced, that is, an adjustable-speed motor. The main problem in the design of such motors is to get the field excitation in phase with the armature current and thus pro- duce a good power-factor.

(i b ) Adjustable-speed polyphase induction motors. In the secondary of the polyphase induction motor an e.m.f. is gener- ated which, at constant impressed e.m.f. and therefore approxi- mately constant flux, is proportional to the slip from synchron- ism. With short-circuited secondary the motor closely ap- proaches synchronism. Inserting resistance into the secondary reduces the speed by the voltage consumed in the secondary. As this is proportional to the current and thus to the load, the speed control of the polyphase induction motor by resistance in the secondary gives a speed which varies with the load, just as the speed control of a direct-current motor by resistance in the armature circuit ; hence, the speed is not constant, and the opera- tion at lower speeds inefficient. Inserting, however, a constant voltage into the secondary of the induction motor the speed is decreased if this voltage is in opposition, and is increased if this voltage is in the same direction as the secondary generated e.m.f., and in this manner a speed control can be produced. If c — voltage inserted into the secondary, as fraction of the voltage which would be induced in it at full frequency by the rotating field, then the polyphase induction motor approaches at no-load and runs at load near to the speed (1 — c) or (1 + c) times syn- chronism, depending upon the direction of the inserted voltage.

Such a voltage inserted into the induction-motor secondary must, however, have the frequency of the motor secondary cur- rents, that is, of slip, and therefore can be derived from the full- frequency supply circuit only by a commutator revolving with the secondary. If cf is the frequency of slip, then (1 — c)/ is the frequency of rotation, and thus the frequency of commuta- tion, and at frequency, /, impressed upon the commutator the effective frequency of the commutated current is / — (1 — c)/ = cf } or the frequency of slip, as required.

Thus the commutator affords a means of inserting voltage into the secondary of induction motors and thus varying its speed.