Theory and Calculation of Electrical Apparatus (1917) — part 12 of 21

148. The usual explanation of the operation of the synchronous machine without field excitation is self-excitation by reactive armature currents. In a synchronous motor a lagging, in a generator a leading armature current magnetizes the field, and in such a case, even without any direct-current field excitation, there is a field excitation and thus a magnetic field flux, produced by the m.m.f. of the reactive component of the armature currents. In the polyphase machine, this is constant in intensity and direc- tion, in the single-phase machine constant in direction, but pul- sating in intensity, and the intensity pulsation can be reduced by a short-circuit winding around the field structure, as more fully discussed under “ Synchronous Machines.”

Thus a machine as shown diagrammatically in Fig. 124, with a polyphase (three-phase) current impressed on the rotating armature, A, and no winding on the field poles, starts, runs up to synchronous and does considerable work as synchronous motor, and under load may even give a fairly good (lagging) power- factor. With a single-phase current impressed upon the arma- ture, A , it does not start, but when brought up to synchronism, continues to run as synchronous motor. Driven by mechanical power, with a leading current load it is a generator.

However, the operation of such machines depends on the existence of a polar field structure, that is a structure having a low reluctance in the direction of the field poles, P — P, and a high reluctance in quadrature position thereto. Or, in other words, the armature reactance with the coil facing the field poles is high, and low in the quadrature position thereto.

In a structure with uniform magnetic reluctance, in which

262

ELECTRICAL APPARATUS

therefore the armature reactance does not -vary with the posi- tion of the armature in the field, as shown in Fig. 125, such self- excitation by reactive armature currents does not occur, and direct-current field excitation is always necessary (except in the so-called “ hysteresis motor ”).

Vectorially this is shown in Figs. 124 and 125 by the relative position of the magnetic flux, <£, the voltage, E } in quadrature to <$>, and the m.m.f. of the current, 7. In Fig. 125, where I and <£ coincide, I and E are in quadrature, that is, the power zero. Due to the polar structure in Fig. 124, I and do not coincide,

Fig. 124. — Diagram of machine with Fig. 125. — Diagram of machine with

polar structure. uniform reluctance.

thus I is not in quadrature to E ) but contains a positive or a negative energy component, making the machine motor or generator.

As the voltage, E, is produced by the current, 7, it is an e.m.f. of self-induction, and self-excitation of the synchronous machine by armature reaction can be explained by the fact that the counter e.m.f. of self-induction is not wattless or in quadrature with the current, but contains an energy component; that is, that the reactance is of the form X = h + jx , where x is the watt- less component of reactance and h the energy component of reactance, and h is positive if the reactance consumes power — in which case the counter e.m.f. of self-induction lags more than 90° behind the current — while h is negative if the reactance produces power — in which case the counter e.m.f. of self-induction lags less than 90° behind the current.

. 149 . A case of this nature occurs in the effect of hysteresis, from a different point of view. In “Theory and Calcuation of Al- ternating Current” it was shown, that magnetic hysteresis distorts the current wave in such a way that the equivalent sine wave,

. REACTION MACHINES

263

that is, the sine wave of equal effective strength and equal power with the distorted wave, is in advance of the wave of magnetism by what is called the angle of hysteretic advance of phase a. Since the e.m.f. generated by the magnetism, or counter e.m.f. of self-induction lags 90° behind the magnetism, it lags 90° + a behind the current; that is, the self-induction in a circuit contain- ing iron is not in quadrature with the current and thereby wattless, but lags more than 90° and thereby consumes power, so that the reactance has to be represented by X = h + jx, where h is what has been called the “effective hysteretic resistance.”

A similar phenomenon takes place in alternators of variable reactance, or, what is the same, variable magnetic reluctance.

Operation of synchronous machines without field excitation is most conveniently treated by resolving the synchronous reactance, x 0 , in its two components, the armature reaction and the true armature reactance, and once more resolving the armature reaction into a magnetizing and a distorting component, and considering only the former, in its effect on the field. The true armature self-inductance then is usually assumed as constant. Or, both armature reactance and self-inductance, are resolved into the two quadrature components, in line and in quadrature with the field poles, as shown in Chapters XXI and XXIV of “Alternating-Current Phenomena,” 5'th edition.

150 * However, while a machine comprising a stationary single- phase “field coil,” A , and a shuttle-shaped rotor, R, shown diagrammatically as bipolar in Fig. 126, might still be interpreted in this matter, a machine as shown diagrammatically in Fig. 127, as four-polar machine, hardly allows this interpretation. In Fig. 127, during each complete revolution of the rotor, R, it four times closes and opens the magnetic circuit of the single- phase alternating coil, A , and twice during the revolution, the magnetism in the rotor, R , reverses.

A machine, in which induction takes place by making and breaking (opening and closing) of the magnetic circuit, or in general, by the periodic variation of the reluctance of the magnetic circuit, is called a reaction machine.

Typical forms of such reaction machines are shown diagram- matically in Figs. 126 and 127. Fig. 126 is a bipolar, Fig. 127 is a four-polar machine. The rotor is shown in the position of closed magnetic circuit, but the position of open magnetic circuit is shown dotted,

264

ELECTRICAL APPARATUS

Instead of cutting out segments of the rotor, in Fig. 126, the same effect can be produced, with a cylindrical rotor, by a short- circuited turn, S, as shown in Fig. 128, This gives a periodic variation of the effective reluctance, from a minimum, shown in Fig. 128, to a maximum in the position shown in dotted lines in Fig. 128.

This latter structure is the so-called “ synchronous-induction motor,” Chapter VIII, which here appears as a special form of the reaction machine.

If a direct current is sent through the winding of the machine,

Fig. 126. — Bipolar reac- Fig. 127. — Four-polar . Fig. 128. — Synchronous- tion machine. reaction machine, induction motor as reac-

tion machine.

Fig. 126 or 127, a pulsating voltage and current is produced in this winding. By having two separate windings, and energizing the one by a direct current, we get a converter, from direct cur- rent in the first, to alternating current in the second winding. The maximum voltage in the second winding can not exceed the voltage, per turn, in the exciting winding, thus is very limited, and so is the current. Higher values are secured by inserting a high inductance in series in the direct-current winding. In this case, a single winding may be used and the alternating-circuit shunted across the machine terminals, inside of the inductance.

151 . Obviously, if the reactance or reluctance is variable, it will perform a complete cycle during the time the armature coil moves from one field pole to the next field pole, that is, during one-half wave of the main current. That is, in other words, the reluctance and reactance vary with twice the frequency of the alternating main current. Such a. case is shown in Figs. 129 and 130. The impressed e.m.f., and thus at negligible resistance, the counter e.m.f., is represented by the sine wave,

REACTION MACHINES

265

E, thus the magnetism produced thereby is a sine wave, <$, 90° ahead of E. The reactance is represented by the sine wave,

Fig. 130. — Wave shape in reaction machine as motor.

varying with the double frequency of E, and shown in Fig. 129 to reach the maximum value during the rise of magnetism, in

206

ELECTRICAL APPARATUS

Fig. 130 during the decrease of magnetism. The current, J, required to produce the magnetism, <&, is found from # and x in combination with the cycle of molecular magnetic friction of the material, and the power, P, is the product, IE. As seen in Fig.

Fig. 131. — Hysteresis loop of reaction machine as generator.

129, the positive part of P is larger than the negative part; that is, the machine produces electrical energy as generator. In Fig. 130 the negative part of P is larger than the positive;

Fig. 132. — Hysteresis loop of reaction machine as motor.

that is, the machine consumes electrical energy and produces mechanical energy as synchronous motor. In Figs. 131 and 132 are given the two hysteretic cycles or looped curves, $, I under the two conditions. They show that, due to the variation of

REACTION MACHINES

267

reactance, x } in the first case, the hysteretic cycle has been over- turned so as to represent, not consumption, but production of electrical energy, while in the second case the hysteretic cycle has been widened, representing not only the electrical energy consumed by molecular magnetic friction, but also the mechanical output.

152. It is evident that the variation of reluctance m ust be symmetrical with regard to the field poles; that is, that the two extreme values of reluctance, maximum and minimum, will take place at the moment when the armature coil stands in front of the field pole, and at. the moment when it stands midway between the field poles.

The effect of this periodic variation of reluctance is a distortion of the wave of e.m.f., or of the wave of current, or of both. Here again, as before, the distorted wave can be replaced by the equivalent sine wave, or sine wave of equal effective intensity and equal power.

The instantaneous value of magnetism produced by the armature current — which magnetism generates in the arma- ture conductor the e.m.f. of self-induction- — is proportional to the instantaneous value of the current divided by the instan- taneous value of the reluctance. Since the extreme values of the reluctance coincide with the symmetrical positions of the armature with regard to the field poles — that is, with zero and maximum value of the generated e.m.f., 1 E 0 , of the machine — it follows that, if the current is in phase or in quadrature with the generated e.m.f., E 0 , the reluctance wave is symmetrical to the current wave, and the wave of magnetism therefore sym- metrical to the current wave also. Hence the equivalent sine wave of magnetism is of equal phase with the current wave; that is, the e.m.f. of self-induction lags 90° behind the current, or is wattless.

Thus at no-phase displacement, and at 90° phase displace- ment, a reaction machine can neither produce electrical power nor mechanical power.

If, however, the current wave differs in phase from the wave of e.m.f. by less than 90°, but more than zero degrees, it is un- symmetrical with regard to the reluctance wave, and the re- luctance will be higher for rising current than for decreasing cur- rent, or it will be higher for decreasing than for rising current, according to the phase relation of current with regard to generated e.m.f., E 0 .

268

ELECTRICAL APPARATUS

In the first case, if the reluctance is higher for rising, lower for decreasing, current, the magnetism, which is proportional to current divided by reluctance, is higher for decreasing than for rising current; that is, its equivalent sine wave lags behind the sine wave of current, and the e.m.f. or self-induction will lag more than 90° behind the current; that is, it will consume electrical power, and thereby deliver mechanical power, and do work as a synchronous motor.

In the second case, if the reluctance is lower for rising, and higher for decreasing, current, the magnetism is higher for rising than for decreasing current, or the equivalent sine wave of magnetism leads the sine wave of the current, and the counter e.m.f. of self-induction lags less than 90° behind the current; that is, yields electric power as generator, and thereby consumes mechanical power.

In the first case the reactance will be represented by X = h + jx, as in the case of hysteresis; while in the second case the reactance will be represented by X = — h + jx.

163. The influence of the periodical variation of reactance will obviously depend upon the nature of the variation, that is, upon the shape of the reactance curve. Since, however, no matter what shape the wave has, it can always be resolved in a series of sine waves of double frequency, and its higher har- monics, in first approximation the assumption can be made that the reactance or the reluctance varies with double freouency of the main current; that is, is represented in the form:

x = a + b cos 2 j8.

Let the inductance be represented by:

L = l + V cos 2 /3,

= Z ( 1 + 7 cos 2 0) ;

where y = amplitude of variation of inductance.

Let :

6 = angle of lag of zero value of current behind maximum value of the inductance, L.

Then, assuming the current as sine wave, or replacing it by the equivalent sine wave of effective intensity, I, current:

i — I a/2 sin (fi — 0).

REACTION MACHINES

289

The magnetism produced by this current is:

Li

$ =

n

where n = number of turns. Hence, substituted:

$ =

= sin (0 — 0) (1 + y cos 2 /?),

n

or, expanded:

n i

cos 0 sin /3 — ( 1 + - J sin 0 cos ft 1

when neglecting the term of triple frequency as wattless.

Thus the e.m.f. generated by this magnetism is:

e = - n it 0 , d$

= ~ 2 * fn dp’

hence, expanded:

e = —2rfll /2 | ^1 — ^ cos 0 cos + ^1 + sin 0 sin 0 and the effective value of e.m.f. :

E =• 2 ir/il - I) 2 cos’ 0 + (l + |) 2 sin 2 0

'V 1

= 2t/ZI A /l -f- — 7 cos 2 0.

Hence, the apparent power, or the volt-amperes :

Q = IE = 2 + ^ - 7 cos 2 0

Jg 2

27r/Z-/l -f-

7 cos 2 0

The instantaneous value of power is:

p = ei

= —AirflP sin (/5 — 0) | (^1 — cos 0 cos /3 +

^1 + ^ sin 0 sin /? ;

270

ELECTRICAL APPARATUS

and, expanded:

V = -2wfir- j (l + sin 2 6 sin 2 0 - (l - ?

sin 2 6 cos 2 0 + sin 2 0 ^cos 2 d — ^ J •

Integrated, the effective value of power is:

P = — 7r/ZJ 2 7 sin 2 0;

hence, negative, that is, the machine consumes electrical, and produces mechanical, power, as synchronous motor, if 6 > 0, that is, with lagging current; positive, that is, the machine pro- duces electrical, and consumes mechanical power, as generator, if 6 > 0, that is, with leading current.

The power-factor is:

V =

Q

1

7 sin 2 6

1 + “7 — 7 cos 2 i 4

hence, a maximum, if: or, expanded :

dp

dd

0 :

cos 2 6 = — and = -

7 2

The power, P, is a maximum at given current, /, if : sin 2 6 = 1 ;

that is :

6 = 45°;

at given e.m.f., E, the pow r er is:

P = -

L

hence, a maximum at:

or, expanded:

E 2 7 sin 2 6

rfl (l +

y cos 2 d)

dP

de

= 0 ;

cos 2 9 =

± 7

REACTION MACHINES

271

154. We have thus, at impressed e.m.f., E, and negligible resistance, if we denote- the mean value of reactance:

Current:

Volt-amperes:

a = 2 7 rfl E

• \f ]

1 4- ^ — 7 cos 2 0

Q =

E 2

V 1

Power:

P =

37 /l 4- — 7 cos 2 6

E 2 y sin 2

2^1 J ~ 4 ” y cos 2 ^

Power-factor:

p = cos (E, I) =

sin 2 (9

a/‘ + ?

2 A /1 d- - 7 cos 2 0

Maximum power at:

cos 2 0 =

Id-

Maximum power-factor at:

2 7

cos 2 6 = and = — 7 L

6 > 0 : synchronous motor, with lagging current,

6 < 0 : generator, with leading current.

As an example is shown in Fig. 133, with angle 6 as abscissae, the values of current, power, and power-factor, for the constants, E — 1L0, x = 3, and y = 0.8.

P

I = -

41

P

V 1.45 - cos 2 6 . -2017 sin 2 d ^

1 .45 — cos 2 s'

cos (E, I) =

0.447 sin 2 6 VL45 - cosl ' S

272 ELECTRICAL APPARATUS

As seen from Fig. 133, the power-factor, p , of such a machine is very low — does not exceed 40 per cent, in this instance.

Very similar to the reaction machine in principle and character of operation are the synchronous induction motor, Chapter IX, and the hysteresis motor, Chapter X, either of which is a gen- erator above synchronism, and at synchronism can be motor as

well as generator, depending on the relative position between stator field and rotor.

155 . The low power-factor and the low weight efficiency bar the reaction machine from extended use for large powers. 8f> also does the severe wave-shape distortion produced by it, and it thus has found a very limited use only in small sizes.

It has, however, the advantage of a high degree of exactness in keeping in step, that is, it does not merely keep in synchronism and drifts more or less over a phase angle with respect to the

REACTION MACHINES

273

impressed voltage, but the relative position of the rotor with regards to the phase of the impressed voltage is more accurately maintained. Where this feature is of importance, as in driving a contact-maker, a phase indicator or a rectifying commutator, the reaction machine has an advantage, especially in a system of fluctuating frequency, and it is used to some extent for such purposes.

This feature of exact step relation is shared also, though to a lesser extent, by the synchronous motor with self-excitation by lagging currents, and ordinarily small synchronous motors, but without field excitation (or with great underexcitation or overexcitation) are often used for the same purpose.

Machines having more or less the characteristics of the reac- tion machine have been used to a considerable extent in the very early days, for generating constant alternating current for series arc lighting by Jablochkoff candles, in the 70’s and early 80’s.

Structurally, the reaction machine is. similar to the inductor machine, but the essential difference is, that the former operates by making and breaking the magnetic circuit, that is, periodically changing the magnetic flux, while the inductor machine operates by commutating the magnetic flux, that is, periodically changing the flux path, but without varying the total value of the magnetic

CHAPTER XVII

INDUCTOR MACHINES

Inductor Alternators, Etc.

156 . Synchronous machines may be built with stationary field and revolving armature, as shown diagrammatically in Fig. 134, or with revolving field and stationary armature, Fig. 135, or with stationary field and stationary armature, but revolving magnetic circuit.

The revolving-armature type was the most frequent in the early days, but has practically gone out of use except for special

Fig. 134. — Revolving armature Fig. 135. — Revolving field al-

alternator ternator.

purposes, and for synchronous commutating machines, as the revolving-armature type of structure is almost exclusively used for commutating machines. The revolving-field type is now almost exclusively used, as the standard construction of alter- nators, synchronous motors, etc. The inductor type had been used to a considerable extent, and had a high reputation in the Stanley alternator. It has practically gone out of use fqr standard frequencies, due to its lower economy in the use of materials, but has remained a very important type of construc- tion, as it is especially adapted for high frequencies and other special conditions, and in this field, its use is rapidly increasing.

A typical inductor alternator is shown in Fig. 136, as eight- polar quarter-phase machine.

07 A

INDUCTOR MACHINES

275

Its armature coils, A, are stationary. One stationary field coil, F, surrounds the magnetic circuit of the machine, which consists of two sections, the stationary external one, B , which contains the armature, A, and a movable one, C, which contains the inductor, N. The inductor contains as many polar projec- tions, N, as there are cycles or pairs of poles. The magnetic flux in the air gap and inductor does not reverse or alternate, as in the revolving-field type of alternator, Fig. 135, but is constant in direction, that is, all the inductor teeth are of the same polarity, but the flux density varies or pulsates, between a maxi- mum, B i, in front of the inductor teeth, and a minimum, though in the same direction, in front of the inductor slots. The magnetic flux, <£, which interlinks with the armature coils, does not alternate between two equal and opposite values, + $o and

Fig. 136 .— Inductor alternator.

-$ 0 , as in Fig. 135, but pulsates between a high value, <f>i, when an inductor tooth stands in front of the armature coil, and a low value in the same direction, $ 2 , when the armature coil faces an inductor slot.

157 . In the inductor alternator, the voltage induction thus is brought about by shifting the magnetic flux produced by a stationary field coil, or by what may be called magneto ' commu- tation , by means of the inductor.

The flux variation, which induces the voltage in the armature turns of the inductor alternator, thus is - $ 2 , while that m the revolving-field or revolving-armature type of alternator is

2 <ho. . . lA , . .

The general formula of voltage induction m an alternator is.

e - /2nrfn$o, G)

276

ELECTRICAL APPARATUS

where :

/ = frequency, in hundreds of cycles, n = number of armature turns in series,

<&o = maximum magnetic flux, alternating through the armature turns, in megalines, e = effective value of induced voltage.

$1 — $2 taking the place of 2 $o, in the inductor alternator, the equation of voltage induction thus is :

e=V2 vfn ' ( 2 )

As seen, <t>i must be more than twice as large as <£ 0 , that is, in an inductor alternator, the maximum* magnetic flux interlinked with the armature coil must be more than twice as large as in the standard type of alternator.

In modern machine design, with the efficient methods of cool- ing now available, economy of materials and usually also effi- ciency make it necessary to run the flux density up to near satura- tion at the narrowest part of the magnetic circuit — which usually is the armature tooth. Thus the flux, <$> 0 , is limited merely by magnetic saturation, and in the inductor alternator, <$>i, would bo limited to nearly the same value as, $o, in the standard machine,

$ i

and ~ — thus would be only about one-half or less of the

permissible value of <f>o. That is, the output of the inductor alternator armature is only about one-half that of the standard alternator armature. This is obvious, as we would double the voltage of the inductor alternator armature, if instead of pulsat- ing between <f>i and <£ 2 or approximately zero, we would alternate between and — <&i.

On the other han , the single field-coil construction gives a material advantage in the material economy of the field, and in machines having very many field poles, that is, high-frequency alternators, the economy in the field construction overbalances the lesser economy in the use of the armature, especially as at high frequencies it is not feasible any more to push the alter- nating flux, $o, up to or near saturation values. Therefore, for high-frequency generators, the inductor alternator becomes the economically superior types, and is preferred, and for ex- tremely high frequencies (20,000 to 100,000 cycles) the inductor alternator becomes the only feasible type, mechanically.

158. In the calculation of the magnetic circuit of the inductor

I

INDUCTOR MACHINES

277

alternator, if <& 0 is the amplitude of flux pulsation through the armature coil, as derived from the required induced voltage by equation (1), let:

p = number of inductor teeth, that is, number of pairs of poles (four in the eight-polar machine, Fig. 136).

pi = magnetic reluctance of air gap in front of the inductor tooth, which should be as low as possible,

p 2 = magnetic reluctance of leakage path through inductor slot into the arma-

• ture coil, which should be as high as possible,

it is:

, . * 1.1 $1 -5- $2 = r - —

; (3)

Pi P2

and as:

cpi — = 2 # 0 ?

(4)

it follows:

$1 = 2 $0 — ^ >

P 2 — Pi

(5)

$2 = 2 $ 0 — — — ;

P2 — Pi J

.

and the total flux through the magnetic circuit, C, and out from all the p inductor teeth and slots thus is :

4? == p (#i ^ 2 )

= 2 p<K

P 2 — Pi

= 2 p<f>o 1 1 H — ]■ • (6)

l p2 — Pi J

In the corresponding standard alternator, with 2 p poles, the total flux entering the armature is :

2 p$o

and if p x is the reluctance of the air gap between field pole and armature face, p 2 the leakage reluctance between the field poles, the ratio of the leakage flux between the field poles, <£', to the armature flux, $ 0 > is:

4>o 4 - (7)

Pi P2

hence:

¥ = 3>o-> (8)

P2

I

278

ELECTRICAL AFP A It A THE

and the flux in the field pole, thus, is :

f ho -f* 2 ( I >/ = c f) |l -f P ) )

hence the total magnetic flux of the machine, of 2 p poles :

<3? = 2p<3? o ^1 + ■ ' (9)

2 pi

As in (6), pi is small compared with p 2 , in (6) differs

P2 — pi

little from — 1 in (9). That is:

P 2

As regards to the total magnetic flux required for the induc- tion of the same voltage in the same armature, no material difference exists between the inductor machine and the standard machine; but in the armature teeth the inductor machine requires more than twice the maximum magnetic flux of the standard

alternator, and thereby is at a disadvantage where the limit of magnetic density in the armature is set only by magnetic saturation.

As regards to the hysteresis loss in the armature of the in- ductor alternator, the magnetic cycle is an unsymmetrical cycle, between two values of the same direction, B i and B 2) and the loss therefore is materially greater than it would be with a symmetrical cycle of the same amplitude. It is given by:

where:

770 = ij [1 + P B v ].

INDUCTOR MACHINES 279

Regarding hereto see “Theory and Calculation of Electric Circuits, under “Magnetic Constants.”

However, as by the saturation limit, the amplitude of the magnetic pulsation in the inductor machine may have to be kept very much lower than in the standard type, the core loss ot the machine may be no larger, or may even be smaller than that of the standard type, in spite of the higher hysteresis coefficient., rj 0 .

159. The inductor-machine type. Fig. 136, must have an

Fig.

I A

POOOOOBOOBOOOOt —.

NfNnu\rjArsj\r

lOOOOOOOOOOOOOO/

i __=h A

138. — Alexanderson high frequency inductor alternator.

auxiliary air gap in the magnetic circuit, separating the revolving from the stationary part, as shown at S.

It, therefore, is preferable to double the structure, Fig. 136, by using two armatures and inductors, with the field coil between them, as shown in Fig. 137. This type of alternator has been extensively built, as the Stanley alternator, mainly for 60 cycles, and has been a very good and successful machine, but has been superseded by the revolving-field type, due to the smaller size and cost of the latter.

Fig. 137 shows the magnetic return circuit, B , between the tw r o armatures, A, and the two inductors N and S as constructed of a number of large wrought-iron bolts, while Fig. 136 shows the return as a solid cast shell.

280

ELECTRICAL APPARATUS

A modification of this type of inductor machine is the Alex- anderson inductor alternator, shown in Fig. 138, which is being built for frequencies up to 200,000 cycles per second and over, for use in wireless telegraphy and telephony.

The inductor disc, I, contains many hundred inductor teeth, and revolves at many thousands of revolutions between the two armatures, A , as shown in the enlarged section, S . It is surrounded by the field coil, F, and outside thereof the magnetic return, B . The armature winding is a single-turn wave winding threaded through the armature faces, as shown in section S and face view, Q. It is obvious that in the armature special iron of extreme thinness of lamination has to be used, and the rotat- ing inductor, I, built to stand the enormous centrifugal stresses of the great peripheral speed. We must realize that even with

an armature pitch of less than in. per pole, we get at 100,000 cycles per second peripheral speeds approaching bullet velocities, over 1000 miles per hour. For the lower frequencies of long distance radio communication, 20,000 to 30,000 cycles, such ma- chines have been built for large powers.

160 . Fig. 139 shows the Eicke- meyer type of inductor alternator. In this, the field coil F is not con- centric to the shaft, and the inductor teeth not all of the same polarity, but the field coil, as seen in Fig. 139, sur- rounds the inductor, J, longitudinally, and with the magnetic return B thus gives a bipolar magnetic field. Half the inductor teeth, the one side of the inductor, thus are of the . one, the other half of the other polarity, and the armature coils, A , are located in the (laminated) pole faces of the bipolar magnetic structure. Obviously, in larger machines, a multipolar structure could be used instead of the bipolar of Fig. 139. This type has the advantage of a simpler magnetic struc- ture, and the further advantage, that all the magnetic flux passes at right angles to the shaft, just as in the revolving field or revolving armature alternator. Ip the types, Figs. 136 and 137, magnetic flux passes, and the field exciting coil magnetizes

I

INDUCTOR MACHINES 281

longitudinally to the shaft, and thus magnetic stray flux tends to pass along the shaft, closing through bearings and supports, and causing heating of bearings. Therefore, in the types 136 and 137, magnetic barrier coils have been used where needed, that is, coils concentric to the shaft, that is, parallel to the field coil, and outside of the inductor, that is, between inductor and bearings, energized in opposite direction to the field coils. These coils then act as counter-magnetizing coils in keeping magnetic flux out of the machine bearings.

The- type, Fig. 139, is especially adapted for moderate fre- quencies, a few hundreds to thousands of cycles. A modifica- tion of it, adopted as converter, is used to a considerable extent: the inductor, I, is supplied with a bipolar winding connected to a commutator, and the machine therefore is a bipolar commutating machine in addition to a high-frequency inductor alternator (16-polar in Fig. 139). It thus may be operated as converter, receiving power by direct-current supply, as direct-current motor, and producing high-frequency alternating power in the inductor pole-face winding.

161 . If the inductor alternator, Fig. 139, instead of with direct current, is excited with low-frequency alternating current, that

Fig. 140. — Voltage wave of inductor alternator with single-phase excitation.

is, an alternating current passed through the field coil, F, of a frequency low compared with that generated by the machine as inductor alternator, then the high-frequency current generated by the machine as inductor alternator is not of constant ampli- tude, but of a periodically varying amplitude, as shown in Fig. 140. For instance, with 60-cycle excitation, a 64-polar in- ductor (that is, inductor with 32 teeth), and 'a speed of, 1800 revolutions, we get a frequency of approximately 1000 cycles, and a voltage and current wave about as shown in Fig. 140.

The power required for excitation obviously is small compared with the power which the machine can generate. Suppose, therefore, that the * high-frequency voltage of Fig. 140 were rectified. It would then give a voltage and current, pulsating

282

ELECTRICAL APPARATUS

with the frequency of the exciting current, but of a power, as many times greater, as the machine output is greater than the exciting power.

.Thus such an inductor alternator with alternating-current excitation can be used as amplifier. This obviously applies equally much to the other types, as shown in Figs. 136, 137 and 138. ?

Suppose now the exciting current is a telephone or micro- phone current, the rectified generated current then pulsates with the frequencies of the telephone current, and the machine is a telephonic amplifier.

Thus, by exciting the high-frequency alternator in Fig. 138, by a telephone current, we get a high-frequency current, of an amplitude, pulsating with the telephone current, but of many times greater power than the original telephone current. This high-frequency current, being of the frequency suitable for radio communication, now is sent into the wireless sending antennae, and the current received from the wireless receiving antennse, rectified, gives wireless telephonic communications As seen, the power, which hereby is sent out from the wireless antennse, is not the insignificant power of the telephone current, but is the high-frequency power generated by the alternator with telephonic excitation, and may be many kilowatts, thus permitting long- distance radio telephony.

It is obvious, that the high inductance of the field coil, F , of the machine, Fig. 138, would make it impossible to force a tele- phone current through it, but the telephonic exciting current would be sent through the armature winding, which is of very low inductance, and by the use of the capacity the armature made self-exciting by leading current.

Instead of sending the high-frequency machine current, which pulsates in amplitude with telephonic frequency, through radio transmission and rectifying the receiving current, we can rectify directly the generated machine current and so get a current pulsating with the telephonic frequency, that is, get a greatly amplified telephone current, and send this into telephone circuits for long-distance telephony.

162 . Suppose, now, in the inductor alternator, Fig. 139, .with low-frequency alternating-current excitation, giving a voltage wave shown in Fig. 140, we use several alternators excited by low-frequency currents of different phases, or instead of a single-

INDUCTOR MACHINES ■

283

phase field, as in Fig. 139, we use a polyphase exciting field. This is shown, with three exciting coils or poles energized by three- phase currents, in Fig. 141. The high-frequency voltages of pulsating amplitude, induced by the three phases, then super- pose a high-frequency wave of constant amplitude, and we get, in Fig. 141, a high-frequency alternator with polyphase field excitation.

Instead of using definite polar projection for the three-phase bipolar exciting winding, as shown in Fig. 141, we could use a distributed winding, like that in an induction motor, placed in the same slots as the inductor-alternator armature winding. By

Fig. 141. — Inductor alternator with three-phase excitation.

placing a bipolar short-circuited winding on the inductor, the three-phase .exciting winding of the high-frequency (24-polar) inductor alternator also becomes a bipolar induction-motor primary winding, supplying the power driving the machine. That is, the machine, is a combination of a bipolar induction motor and a 24-polar inductor alternator, or a frequency converter.

Instead of having a separate high-frequency inductor-alter- nator armature winding, and low-frequency induction motor winding, we can use the same winding for both purposes, as shown diagrammatically in Figs. 142 and 143. The stator winding, Fig. 142, bipolar, or four-polar 60-cycle, is a low- frequency winding, for instance, has one slot per inductor pole, that is, twice as many slots as the inductor has teeth. Successive turns then differ from each other by 180° in phase, for the high- frequency inductor voltage. Thus grouping the winding in

284

ELECTRICAL APPARATUS

two sections, 1 and 3, and 2 and 4, the high-frequency voltages in the two sections are opposite in phase from each other. Con- necting, then, as shown in Fig. 143, 1 and 2 in series, and 4 and 3 in series into the two phases of the quarter-phase supply cir- cuit, no high-frequency induction exists in either phase, but the high-frequency voltage is generated between the middle points

Fig. 142. — Induction type of high-frequency inductor alternator.

of the two phases, as shown in Fig. 143, and we thus get another form of a frequency converter, changing from low-frequency polyphase to high-frequency single-phase.

FREQUENCY

(420)

ram!/™

u> .u I

LOW

FREQUENCY

(60)

Fig. 143. — Diagram of connection of induction type of inductor alternator.

163 . A type of inductor machine, very extensively used in small machines — as ignition dynamos for gasoline engines — is shown in Fig. 144. The field, F, and the shuttle-shaped armature, A , are stationary, and an inductor, I , revolves between field and armature, and so alternately sends the magnetic field flux through the armature, first in one, then in the opposite direction. As seen, in this type, the magnetic flux in the armature reverses, by what may be called magnetic commutation , Usually in these

Fig. 144. — Magneto inductor machine.

the magnetic flux produced by a stationary coil, in another stationary coil by means of a moving “magneto commutator” or inductor, has been extensively used in single-phase feeder

regulators, the so-called “magneto regulators.” It is illustrated in Fig. 145. P is the primary coil (shunt coil connected across the alternating supply circuit), S the secondary coil (connected in series into the circuit which is to be regulated) the magnetic inductor, I, in the position shown in drawn lines sends the mag-

286

ELECTRICAL APPARATUS

netic flux produced by the primary coil, through the secondary coil, in the direction opposite to the direction, in which it would send the magnetic flax through the secondary coil when in the position I', shown in dotted lines. In vertical position, the inductor, I, would pass the magnetic flux through the primary coil, without passing it through the secondary coil, that is, with- out inducing voltage in the secondary. Thus by moving the shuttle or inductor, I, from position I over the vertical position to the position I', the voltage induced in the secondary coil, S : is varied from maximum boosting over to zero to maximum lowering.

164. Fig. 146 shows a type of machine, which has been and still is used to some extent, for alternators as well as for dircct-

Fig. 146. — Semi-inductor type of machine.

current commutating machines, and which may be called an inductor machine, or at least has considerable similarity with the inductor type. It is shown in Fig. 146 as six-polar machine, with internal field and external armature, but can easily be built with internal armature and external field. The field contains one field coiljF, concentric to the shaft. The poles overhang the field coils, and all poles of one polarity, JV, come from the one side, ail poles of the other polarity from the other side of the field coil. The magnetic structure thus consists of two parts which interlock axially, as seen in Fig. 146.