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 Fia. 134. — Revolving armature alternator Fig. 135.— Revolving field al 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 for 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. 274 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\, in front of the inductor teeth, and a minimum, Btl 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, + *0 and Fio. 136. — Inductor alternator. — *», as in Fig. 135, but pulsates between a high value, *i, when an inductor tooth stands in front of the armature coil, and a low value in the same direction, *,, when the armature coil faces an inductor slot. 167. fn 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 #i — *», while that in the revolving-field or revolving-armature type of alternator is 2 *„. The general formula of voltage induction in an alternator is: (1) ! - y/2 «/«*„, 27G ELECTRICAL APPARATUS where : / = frequency, in hundreds of cycles, n = number of armature turns in series, *0 = maximum magnetic flux, alternating through the armature turns, in megalines, e = effective value of induced voltage. *i — *s taking the place of 2 *0, in the inductor alternator, the equation of voltage induction thus is: W2rt«< (2) As seen, *, must be more than twice as large as *o, 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 (he 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, *o, is limited merely by magnetic saturation, and in the inductor alternator, $,, would be limited to nearly the same value as, 4>0, in the standard machine, *i — *i and — i, — thus would be only about one-half or less of the permissible value of *0- 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 4>, and *2 or approximately zero, we would alternate between *i and — *i. On the other hand, 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 higli frequencies it is not feasible any more to push the alter- nating flux, $0, 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, 168. In the calculation of the magnetic circuit of the inductor INDUCTOR MACHINES , 277 alternator, if 3>o 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, P2 = magnetic reluctance of leakage path through inductor slot into the arma- ture coil, which should be as high as possible, (3) (4) it is: and as: $! -f- 2 = — -f- Pi P2 $1 — $2 = 2 $0, it follows: < 4*1 — £t 4?o y P2 — Pi * — 9 * Pl 4^2 — 4 ™0 ) P2 — Pi (5) and the total flux through the magnetic circuit, C, and out from all the p inductor teeth and slots thus is : $ = P ($i + $2) P2 + Pl = 2 p$o = 2 p$o P2 — Pl 1 + -^-}- (6) P2 — Pl 1 In the corresponding standard alternator, with 2 p poles, the total flux entering the armature is : 2 po and if pi is the reluctance of the air gap between field pole and armature face, p2 the leakage reluctance between the field poles, the ratio of the leakage flux between the field poles, $', to the armature flux, $0, is: 0 - *' = A + } ; (7) Pl P2 hence: $' = $„ -> (8) Pa 278 ELECTRICAL APPARATUS and the flux in the field pole, thus, is _2pA *„ + 2 *' = .(i + ! hence the total magnetic flux of the machine, of 2 p pole* * = 2p«„(l -J-2")- 2pL As in 16), pi is small compared with p,, — in (6) differs 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 Vu;. |:>7. Siimli'v iti'iiirdir iiltt'Tiinlrtr. alternator, and thereby ia 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 unsyrametrieal cycle, between two values of the same direction, Bx and B%, and the loss therefore is materially greater than it would be with a symmetrical cycle of the same amplitude. It is given by: /B, -Ba1'6 ' = *°( 2 ) n-»P +eB"]. 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 of the machine may be no larger, or may even be smaller than that of the standard type, in spite of the higher hysteresis coefficient, 170. 169. The inductor-machine type, Fig. 136, must have an £—21 \f\j\j\/\r\/\j\r ^ :f-A J fttfMtai«4**Aft« ! I >U Fig. 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 10 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 two 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. 2SU ELECTRICAL APPARATUS A mollification 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, /, 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, S. The armature winding is a single-turn wave winding threaded through the armature faces, as shown in section .S' ami 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, 7, built to stand the enormous centrifugal stresses nf the great peripheral speed. We must realize that even with an armature pitch of less than l fa 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 m long distance radio communication, 20,000 to 30,000 cycles, such' ma- chines have been built for large powers. 160. Fig. 139 shows the Eieke- 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 ductor alternator. the field coil, as seen in Fig. 139, sur- rounds the inductor, /, 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. In the types, Figs. 136 and 137, magnetic flux passes, and the field exciting coil magnetizes INDUCTOR MACHINES 281 longitudinally to the shaft, arui thus magnetic stray flux tends ) pass along the shaft, closing through bearings and supports, and causing heating of bearings. Therefore, in the types 136 ,nd 137, magnetic barrier coils have been used where needed, that is, coils concentric to the shaft, that ia, parallel to the field coil, and outside of the inductor, that is, between inductor and bearings, energized in opposite direction lo 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- ti of it, adopted as converter, is used to a considerable extent: he inductor, /, is supplied with a bipolar winding connected to a ■ommutator, and the machine therefore is a bipolar commutating machine in addition to a high-frequency inductor alternator (I6-polar in Fig. 130). 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 *t :o. 140. — Voltage wiive of inductor niter nth -jitiB.il- [ill.- , an alternating current, passed through the field coil, F, of a requency low compared with that generated by the machine as inductor alternator, then the high-frequency current generated .• 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, ,nd a voltage and current wave about as shown in Fig. 140. The power required for excitation obviously is small compared the power which the machine can generate. Suppose, icrefore, that the high-frequency voltage of Fig. 140 were ftified. 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. 13(i. 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 niany 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 antennae, rectified, gives wireless telephonic communications. As seen, the power, which hereby is sent out from the wireless antenna?, 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 II -iimlc- 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 anil 3, and 2 ami 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 Fio. 142.— Induction type of higb -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. • HICH WrfumJ 1 Z17" ffmlms\ 1S0' Fig. 143. — Diagram of connection of induction type of inductor alternator. 163. A type of inductor machine, very extensively used in smalt 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, J, 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 INDUCTOR MACHINES 285 small machines the field excitation is not by direct current, but by permanent magnets. This principle of magnetic commutation, that is, of reversing 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 Fig. 145.— Magneto com mutation voltage regulator. regulators, the so-called " magneto regulator*)." It is illustrated in Fig. 145. P is the primary coil (shunt coil connected across the alternating supply circuit), 8 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- 280 ELECTRICAL APPARATUS ratio flux produced by the primary coil, through the secondaf] coil, in the direction opposite to the direction, in which it would send the magnetic flux through the secondary coil when in the position /', shown in dotted lines. In vertical position, the inductor, /, 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, /, from position I over the vertical position to the position /', the voltage induced in the secondary cod. S, is varied from maximum boosting over to zero to maximum lowering. 164. lrig. 146 shows a type of machine, which has been EHri still is used to some extent, for alternators as well as for direct- i fiTiT]* current commutating machines, and which may be called an inductor machine, or at least has considerable similarity with flu- inductor type. It is shown in Pig. 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 coil,/1, concentric to the shaft. The poles overhang the field coils, and all poles of one polarity, N, come from the mic side, all poles of the other polarity from the other side of the field coll. The magnetic structure thus consists of two parts which interlock axially, as seen in Fig. 146. The disadvantage of this type of field construction is the high flux leakage between the field poles, which tends to impair the regulation in alternators, and makes commutation more difficult for direct-current machines. It offers, however, the advantage INDUCTOR MACHINES 287 of simplicity and material economy in machines of small and moderate size, of many poles, as for instance in small very low- speed synchronous motors, etc. 166. In its structural appearance, inductor machines often have a considerable similarity with reaction machines. The characteristic difference between the two types, however, is, that in the reaction machine voltage is induced by the pulsation of the magnetic flux by pulsating reluctance of the magnetic circuit of the machine. The magnetic pulsation in the reaction machine thus extends throughout the entire magnetic circuit of the machine, and if direct-current excitation were used, the voltage would be induced in the exciting circuit also. In the inductor machine, however, the total magnetic flux does not pulsate, but is constant, and no voltage is induced in the direct- current exciting circuit. Induction is produced in the armature by shifting the — constant — magnetic flux locally from armature coil to armature coil. The important problem of inductor alternator design — and in general of the design of magneto com- iriutation apparatus — is to have the shifting of the magnetic flux from path to path so that the total reluctance and thus the total magnetic flux does not vary, otherwise excessive eddy- current losses would result in the magnetic structure. It is interesting to note, that the number of inductor teeth is one-half the number of poles. An inductor with p projections thus gives twice as many cycles per revolution, thus as syn- chronous motor would run at half the speed of a standard syn- chronous machine of p poles. As the result hereof, in starting polyphase synchronous machines by impressing polyphase voltage on the armature and using the hysteresis and the induced currents in the field poles, for producing the torque of starting and acceleration, there frequently appears at half synchronism a tendency to drop into step with the field structure as inductor. This results in an increased torque when approaching, and a reduced torque when passing beyond half synchronism, thus produces a drop in the torque curve and is liable to produce difficulty in passing beyond half speed in starting. In extreme cases, it may result even in a negative torque when passing half synchronism, and make the machine non-self-starting, or at least require a considerable increase of voltage to get beyond half synchronism, over that required to start from rest.