Theory and Calculation of Electrical Apparatus (1917) — part 20 of 21

251. The unipolar machine is not free of armature reaction, as often believed. The current in all the armature conductors (Fig. 224) flows in the same direction, and thereby produces a circular magnetization in the magnetic return circuit, S, shown by the arrow in Fig. 224. While the armature conductor mag- netically represents one turn only, in the large machines it repre- sents many thousand ampere-turns. As an instance, assume a peripheral speed of a steam-turbine-driven unipolar machine, of 12,000 ft. per minute, at 1800 revolutions per minute. This gives an armature circumference of 80 in. At in. thickness of the conductor, and 2500 amp. per square inch, this gives 100,000 ampere- turns m.m.f. of armature reaction, which probably is sufficient to magnetic- ally saturate the iron in the pole faces, in the direction of the arrow in Fig. 224.

At the greatly lowered permeability at saturation, with constant field excita- tion the voltage of the machine greatly drops, or, to maintain constant voltage, p IG> 22 6. Multi-con-

a considerable increase of field excita- ductor unipolar machine ,* -i it* * i t with compensating pole

tion under load is required. Large face winding, cross-section.

unipolar machines thus are liable to

give poor voltage regulation and to require high compounding.

To overcome the circular armature reaction, a counter m.m.f. may be arranged in the pole faces, by returning the current of each collector ring li, 2i, 3i, 4 h of Fig. 225, to the collector rings on the other end of the machine, 2, 3, 4 in Fig. 225, not through an external circuit, but through conductors imbedded in the pole face, as shown in Fig. 226 as 1', 2', 3', 4'.

The most serious problem of the unipolar machine, however, is that of the high-speed collector rings, and this has not yet been solved. Collecting very large currents by numerous collector

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ELECTRICAL APPARATUS

rings at speeds of 10,000 to 15,000 ft. per minute, leads to high losses and correspondingly low machine efficiency, high tempera- ture rise, and rapid wear of the brushes and collector rings, and this has probably been the main cause of abandoning the develop- ment of the unipolar machine for steam-turbine drive.

A contributing cause was that, when the unipolar steam-tur- bine generator was being developed, the days of the huge direct- current generator were over, and its place had been taken by turbo-alternator and converter, and the unipolar machine offered

no advantage in reliability, or efficiency, but the disadvantage of lesser flexibility, as it requires a greater concentration of direct- current generation in one place, than usually needed.

252. The unipolar machine may be used as motor as well as generator, and has found some application as motor meter. The general principle of a unipolar meter may be illustrated by Kg. 227.

The meter shaft, A, with counter, F, is pivoted at P, and carries the brake disk and conductor, a copper or aluminum disk, I), be- tween the two poles, N and S, of a circular magnet. The shaft, A , dips into a mercury cup, C, which is insulated and contains the one terminal, while the other terminal goes to a circular mercury trough, G. An iron pin, B, projects from the disk, D, into this mercury trough and completes the circuit.

CHAPTER XXIII

REVIEW

& 53 . In reviewing the numerous types of apparatus, methods of construction and of operation, discussed in the preceding, an alphabetical list of them is given in the following, comprising name, .definition, principal characteristics, advantages and dis- advantages, and the paragraph in which they are discussed.

Alexanderson High-frequency Inductor Alternator. — 159. Comprises an inductor disk of very many teeth, revolving at very high speed between two radial armatures. Used for producing very high frequencies, from 20,000 to 200,000 cycles per second.

Amortisseur. — Squirrel-cage winding in the pole faces of the synchronous machine, proposed by Leblanc to oppose the hunt- ing tendency, and extensively used.

Amplifier. — 161. An apparatus to intensify telephone and radio telephone currents. High-frequency inductor alternator excited by the telephone current, usually by armature reaction through capacity. The generated current is then rectified, be- fore transmission in long-distance telephony, after transmission in radio telephony.

Arc Machines. — 138. Constant-current generators, usually direct-current, with rectifying commutators. The last and most extensively used arc machines were :

Brush Arc Machine. — 141-144. A quarter-phase constant- current alternator with rectifying commutators.

Thomson-Houston Arc Machine. — 141-144. A three-phase Y-connected constant-current alternator with rectifying commu- tator.

The development of alternating-current series arc lighting by constant-current transformers greatly reduced the importance of the arc machine, and when in the magnetite lamp arc lighting returned to direct current, the development of the mercury-arc rectifier superseded the arc machine.

Asynchronous Motor. — Name used for all those types of alternating-current (single-phase or polyphase) motors or motor couples, which approach a definite synchronous speed at no-load, and slip below this speed with increasing load.

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ELECTRICAL APPARATUS

Brush Arc Machine. — (See “Arc Machines.”)

Compound Alternator. — 138. Alternator with rectifying com- mutator, connected in series to the armature, either con- ductively, or inductively through transformer, and exciting a series field winding by the rectified current. The limitation of the power, which can be rectified, and the need of readjusting the brushes with a change of the inductivity of the load, has made such compounding unsuitable for the modern high-power alterna- tors.

Condenser Motor. — 77. Single-phase induction motor with condenser in tertiary circuit on stator, for producing starting torque and high power-factor. The space angle between pri- mary and tertiary stator circuit usually is 45° to 60°, and often a three-phase motor is used, with single-phase supply on one phase, and condenser on a second phase. With the small amount of capacity, sufficient for power-factor compensation, usually the starting torque is small, unless a starting resistance is used, but the torque efficiency is high.

Concatenation. — III, 28. Chain connection, tandem connec- tion, cascade connection. Is the connection of the secondary of an induction machine with a second machine. The second machine may be:

1. An Induction Machine . — The couple then is asynchronous. Hereto belong:

The induction frequency converter or general alternating-current transformer , XII, 103. It transforms between alternating-cur- rent systems of different frequency, and has over the induction- motor generator set the advantage of higher efficiency and lesser capacity, but the disadvantage of not being standard.

The concatenated couple of induction motor s, 9, 28, 111. It permits multispeed operation. It has the disadvantage against the multispeed motor, that two motors are required; but where two or more motors are used, as in induction-motor railroading, it has the advantage of greater simplicity.

The internally concatenated motor ( Hunt motor), 36. It is more efficient than the concatenated couple or the multispeed motor, but limited in design to certain speeds and speed ratios.

2. A Synchronous Machine . — The couple then is synchronous. Hereto belong:

The synchronous frequency converter, XII, 103. It has a defi- nite frequency ratio, while that of the induction frequency con-

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461

verter slightly changes with the load, by the slip of the induction machine.

Induction Motor with Low-frequency Synchronous Exciter. — 47. The synchronous exciter in this case is of small capacity, and gives speed control and power-factor compensation.

InductionGenerator with Low-frequency Exciter . — 110, 121. Syn- chronous induction generator. Stanley induction generator . In this case, the low-frequency exciter may be a synchronous or a commutating machine or any other source of low frequency. The phase rotation of the exciter may be in the reverse direc- tion of the main machine, or in the same direction. In the first case, the couple may be considered as a frequency converter driven backward at many times synchronous speed, the exciter is motor, and the generated frequency less than the speed. In the case of the same phase rotation of exciter and main machine, the generated frequency is higher than the speed, and the exciter also is generator. This synchronous induction generator has peculiar regulation characteristics, as the armature reaction of non-inductive load is absent.

3. A Synchronous Commutating Machine. — 112. The couple is synchronous, and called motor converter. It has the advantage of lower frequency commutation, and permits phase control by the internal reactance of the induction machine. It has higher efficiency and smaller size than a motor-generator set, but is larger and less efficient than the synchronous converter, and therefore has not been able to compete with the latter.

4. A direct-current commutating machine , as exciter, 41. This converts the induction machine into a synchronous machine (Danielson motor). A good induction motor gives a poor syn- chronous motor, but a bad induction motor, of very low power- factor, gives a good synchronous motor, of good power-factor, etc.

5. An alternating-current commutating machine , as low-fre- quency exciter, 52. The couple then is asynchronous. This permits a wide range of power-factor and speed control as motor. As generator it is one form of the Stanley induction generator discussed under (2) .

6. A Condenser .— This permits power-factor compensation, 55, and speed control, 11. The power-factor compensation gives good values with very bad induction motors, of low power- factor, but is uneconomical with good motors, Speed control

462

ELECTRICAL APPARATUS

usually requires excessive amounts of capacity, and gives rather poor constants. The machine is asynchronous.

Danielson Motor. — 41. An induction motor converted to a synchronous motor by direct-current excitation. (See “ Con- catenation (4).”)

Deep-bar Induction Motor. — 7. Induction motor with deep and narrow rotor bars. At the low frequency near synchronism, the secondary current traverses the entire rotor conductor, and the secondary resistance thus is low. At high slips, as in start- ing, unequal current distribution in the rotor bars concentrates the current in the top of the bars, thus gives a greatly increased effective resistance, and thereby higher torque. However, the high reactance of the deep bar somewhat impairs the power- factor. The effect is very closely the same as in the double squirrel cage. (See “Double Squirrel-cage Induction Motor. ”)

Double Squirrel-cage Induction Motor. — II, 18. Induction motor having a high-resistance low- reactance squirrel cage, close to the rotor surface, and a low-resistance high-reactance squirrel cage, embedded in the core. The latter gives torque at good speed regulation near synchronism, but carries little current at lower speeds, due to its high reactance. The surface squirrel cage gives high torque and good torque efficiency at low speeds and standstill, due to its high resistance, but little torque near synchronism. The combination thus gives a uniformly high torque over a wide speed range, but at some sacrifice of power- factor, due to the high reactance of the lower squirrel cage. To get close speed regulation near synchronism, together with high torque over a very wide speed range, for instance, down to full speed in reverse direction (motor brake), a triple squirrel cage may be used, one high resistance low reactance, one medium resistance and reactance, and one very low resistance and high reactance (24).

Double Synchronous Machine. — 110, 119. An induction ma- chine, in which the rotor, running at double synchronism, is connected with the stator, either in series or in parallel, but with reverse phase rotation of the rotor, so that the two rotating fields coincide and drop into step at double synchronism. The machine requires a supply of lagging current for excitation, just like any induction machine. It may be used as synchronous induction generator, or as synchronous motor. As generator, the armature reaction neutralizes at non-inductive, but not at inductive load,

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463

and thus gives peculiar regulation characteristics, similar as the Stanley induction generator. It has been proposed for steam- turbine alternators, as it would permit higher turbine speed (3000 revolutions at 25 cycles) but has not yet been used. As motor it has the disadvantage that it is not self-starting.

Eickemeyer Inductively Compensated Single-phase Series Motor. — 193. Single-phase commutating machine with series field and inductive compensating winding.

Eickemeyer Inductor Alternator. — 160. Inductor alternator with field coils parallel to shaft, so that the magnetic flux disposi- tion is that of a bipolar or multipolar machine, in which the multitooth inductor takes the place of the armature of the stand- ard machine. Voltage induction then takes place in armature coils in the pole faces, and the magnetic flux in the inductor re- verses, with a frequency much lower than that of the induced voltage. This type of inductor machine is specially adopted for moderately high frequencies, 300 to 2000 cycles, and used in in- ductor alternators and inductor converters. In the latter, the in- ductor carries a low-frequency closed circuit armature winding connected to a commutator to receive direct current as motor.

Eickemeyer Rotary Terminal Induction Motor. — XI, 101. Single-phase induction motor with closed circuit primary winding connected to commutator. The brushes leading the supply cur- rent into the commutator stand still at full speed, but revolve at lower speeds and in starting. This machine can give full maxi- mum torque at any speed down to standstill, depending on the speed of the brushes, but its disadvantage is sparking at the com- mutator, which requires special consideration.

Frequency Converter or General Alternating-current Trans- former. — XII, 103. Transforms a polyphase system into another polyphase system of different frequency and where desired of differ ent voltage and different number of phases. Consists of an induc- tion machine concatenated to a second machine, which may be an induction machine or a synchronous machine, thus giving the induction frequency converter and the synchronous frequency con- verter. (See “Concatenation.”) In the synchronous frequency converter the frequency ratio is rigidly constant, in the induction frequency converter it varies slightly with the load, by the slip of the induction machine. When increasing the frequency, the second machine is motor, when decreasing the frequency, it is generator. Above synchronism, both machines are generators

464

ELECTRICAL APPARATUS

and the machine thus a synchronous induction generator. In concatenation, the first machine always acts as frequency con- verter. The frequency converter has the advantage of lesser machine capacity than the motor generator, but the disadvantage of not being standard yet.

Heyland Motor. — 59, 210. Squirrel-cage induction motor with commutator for power-factor compensation.

Hunt Motor. — 36. Internally concatenated induction motor. (See u Concatenation (1).”)

Hysteresis Motor. — X, 98. Motor with polyphase stator and laminated rotor of uniform reluctance in all directions, without winding. Gives constant torque at all speeds, by the hysteresis of the rotor, as motor below and as generator above synchronism, while at synchronism it may be either. ,Poor power-factor and small output make it feasible only in very small sizes, such as motor meters.

Inductor Machines. — XVII, 156. Synchronous machine, gen- erator or motor, in which field and armature coils stand still and the magnetic field flux is constant, and the voltage is induced by changing the- flux path, that is, admitting and withdrawing the flux from the armature coils by means of a revolving inductor. The inducing flux in the armature coils thus does not alternate, but pulsates without reversal. For standard frequencies the inductor machine is less economical arid little used, but it offers great constructive advantages at high frequencies and is the only feasible type at extremely high frequencies. Excited by alter- nating currents, the inductor machine may be used as amplifier (see “Amplifier”) ; excited by polyphase currents, it is an induc- tion inductor frequency converter , 162; with a direct-current wind- ing on the inductor, it is a direct-current high-frequency converter . (See “Eickemeyer Inductor Alternator.”)

Leading current, power-factor compensation and phase control can be produced by:

Condenser.

Polarization cell.

Overexcited synchronous motor or synchronous converter.

Induction machine concatenated to condenser, to synchronous motor or to low-frequency commutating machine.

Alternating-current commutating machine with lagging field excitation.

Leblanc’s Panchahuteur, — 145. Synchronous rectifier of many

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465

phases, fed by polyphase transformer increasing the number of phases, and driven by a synchronous motor having as many cir- cuits as the rectifier has phases, each synchronous motor circuit being connected in shunt to the corresponding rectifier phase to byepass the differential current and thereby reduce inductive sparking. Can rectify materially more power than the standard rectifier, but is inferior to the converter.

Magneto Commutation.— 163. Apparatus in which the induc- tion' is varied, with stationary inducing (exciting) and induced coils, by shifting or reversing the magnetic flux path by means of a movable part of the magnetic circuit,, the inductor . Applied to stationary induction apparatus, as voltage regulators, and to synchronous machines, as inductor alternator.

Monocyclic. — 127. A system of polyphase voltages with essen- tially single-phase flow of power. A system of polyphase vol- tages, in which one phase regulates for constant voltage, that is, a voltage which does not materially drop within the range of power considered, while the voltage in quadrature phase thereto is of limited power, that is, rapidly drops with increase of load. Monocyclic systems, as the square or the triangle, are derived from single-phase supply by limited energy storage in inductance or capacity, and used in those cases, as single-phase induction motor starting, where the use of a phase converter would be uneconomical.

Motor Converter. — 112. An induction machine concatenated with a synchronous commutating machine. (See ‘ ‘ Concatenation (3).”) The latter thus receives part of the power mechanically, part electrically, at lower frequency, and thereby offers the ad- vantages incident to a lower frequency in a commutating machine. It permits phase control by the internal reactance of the induc- tion machine. Smaller than a motor-generator set, but larger than a synchronous converter, and the latter therefore preferable where it can be used.

Multiple Squirrel-cage Induction Motor. — (See “Double Squirrel-cage Induction Motor. 77 )

Multispeed Induction Motor. — 14. Polyphase Induction Motor with the primary windings arranged so that by the opera- tion of a switch, the number of poles of the motor, and thereby its speed can be changed. It is the most convenient method of producing several economical speeds in an induction motor, and therefore is extensively used. At the lower speed, the power- factor necessarily is lower.

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ELECTRICAL APPARATUS

Permutator. — 146. Machine to convert polyphase alternating to direct current, consisting of a stationary polyphase trans- former with many secondary phases connected to a stationary commutator, with a set of revolving brushes driven by a syn- chronous motor. Thus essentially a synchronous converter with stationary armature and revolving field, but with two armature windings, primary and secondary. The foremost objection is the use of revolving brushes, which do not permit individual observation and adjustment during operation, and thus are liable to sparking.

Phase Balancer. — 134. An apparatus producing a polyphase system of opposite phase rotation for insertion in series to a polyphase system, to restore the voltage balance disturbed by a single-phase load. It may be :

A stationary induction-phase balancer , consisting of an induc- tion regulator with reversed phase rotation of the series winding.

A synchronous-phase balancer , consisting of a synchronous machine of reversed phase rotation, having two sets of field wind- ings in quadrature. By varying, or reversing the excitation of the latter, any phase relation of the balancer voltage with those of the main polyphase system can be produced. The synchronous phase balancer is mainly used, connected into the neutral of a synchronous phase converter, to control the latter so as to make the latter balance the load and voltage of a polyphase system with considerable single-phase load, such as that of a single- phase railway system.

Polyphase Commutator Motor. — Such motors may be shunt, 181, or series type, 187, for multispeed, adjustable-speed and varying-speed service. In commutation, they tend to be inferior to single-phase commutator motors, as their rotating field does not leave any neutral direction, in which a commutating field could be produced, such as is used in single-phase commutator motors. Therefore, polyphase commutator motors have been built with separate phases and neutral spaces between the phases, for commutating fields: Scherbins motor. '

Reaction Machines. — XVI, 147. Synchronous machine, motor or generator, in which the voltage is induced by pulsation of the magnetic reluctance, that is, by make and break of the magnetic circuit. It thus differs from the inductor machine, in that in the latter the total field flux is constant, but is shifted with re- gards to the armature coils, while in the reaction machine the

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467

total field flux pulsates. The reaction machine has low output and low power-factor, but the type is useful in small synchronous motors, due to the simplicity resulting from the absence of direct- current field excitation.

Rectifiers. — XV, 138. Apparatus to convert alternating into direct current by synchronously changing connections. Rec- tification may occur either by synchronously reversing connec- tions between alternating-current and direct-current circuit: reversing rectifier , or by alternately making contact between the direct-current circuit and the alternating-current circuit, when the latter is of the right direction, and opening contact, when of the reverse direction: contact-making rectifier . Mechanical rectifiers may be of either type. Arc rectifiers, such as the mer- cury-arc rectifier, which use the unidirectional conduction of the arc, necessarily are contact-making rectifiers.

, Full-wave rectifiers are those in which- the direct-current cir- cuit receives both half waves of alternating current; half-wave rectifiers those in which only alternate half waves are rectified, the intermediate or reverse half waves suppressed. The latter type is permissible only in small sizes, as the interrupted pul- sating current traverses both circuits, and produces in the alter- nating-current circuit a unidirectional magnetization, which may give excessive losses and heating in induction apparatus. The foremost objection to the mechanical rectifier is, that the power which can be rectified without injurious inductive spark- ing, is limited, especially in single-phase rectifiers, but for small amounts of power, as for battery charging and constant-current arc lighting they are useful. However, even there the arc recti- fier is usually preferable. The brush arc machine and the Thomson Houston arc machine were polyphase alternators with rectifying commutators.

Regulating Pole Converter. — Variable-ratio converter.. Split- pole converter, XXI, 230. A synchronous converter, in which the ratio between direct-current voltage and alternating-current voltage can be varied at will, over a considerable range, by shift- ing the direction of the resultant magnetic field flux so that the voltage between the commutator brushes is less than maximum alternating-current voltage, and by changing, at constant im- pressed effective alternating voltage, the maximum alternating- current voltage and with it the direct-current voltage, by the superposition of a third harmonic produced in the converter in

468

ELECTRICAL APPARATUS

such a manner, that this harmonic exists only in the local con- verter circuit. This is done by separating the field pole into, two parts, a larger main pole, which has constant excitation, and a smaller regulating pole, in which the excitation is varied and reversed. A resultant armature reaction exists in the regulating pole converter, proportional to the deviation of the voltage ratio from standard, and requires the use of a series field. Regulating pole converters are extensively used for adjustable voltage service, as direct-current distribution, storage-battery charging, etc., due to their simplicity and wide voltage range at practically unity power-factor, while for automatic voltage control under fluctuating load, as railway service, phase control of the standard converter is usually preferred.

Repulsion. Generator. — 217. Repulsion motor operated as generator.

Repulsion Motor. — 194, 208, 214. Single-phase commutator motor in which the armature is short-circuited and energized by induction from a stationary conpensating winding as primary. Usually of varying speed or series characteristic. Gives better commutation than the series motor at moderate speeds.

Rotary Terminal Single-phase Induction Motor. — XI, 101. (See “Eickemeyer Rotary Terminal Induction Motor.”)

Shading Coil. — 73. A short-circuited turn surrounding a part of the pole face of a single-phase induction motor with definite poles, for the purpose of giving a phase displacement of the flux, and thereby a starting torque. It is the simplest and cheap- est single-phase motor-starting device, but gives only low start- ing torque and low torque efficiency, thus is not well suited for larger motors. It thus is very extensively used in small motors, almost exclusively in alternating-current fan motors.

Single-phase Commutator Motor. — XX, 189. Commutator motor with alternating-current field excitation, and such modi- fications of design, as result therefrom. That is, lamination of the magnetic structure, high ratio of armature reaction to field excitation, and compensation for armature reaction and self- induction, etc. Such motor thus comprises three circuits: the armature circuit, the field circuit, and the compensating circuit in quadrature, on the stator, to the field circuit; These cir- cuits may be energized by conduction, from the main current, or by induction, as secondaries with the main current as pri- mary. If the armature receives the main current, the motor is

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469

a series or shunt motor; if it is closed upon itself, directly or through another circuit, the motor is called a repulsion motor . A combination of both gives the series repulsion motor .

Single-phase commutator motors of series characteristic are used for alternating-current railroading, of shunt characteristic as stationary motors, as for instance the induction repulsion motor, either as constant-speed high-starting-torque motors, or as adjustable-speed motors.

Lagging the field magnetism, as by shunted resistance, pro- duces a lead of the armature current. This can be used for power-factor compensation, and single-phase commutator motors thereby built with very high power-factors. Or the machine, with lagging -quadrature field excitation, can be used as effective capacity. The single-phase commutator motor is the only type which, with series field excitation, gives a varying-speed motor of series-motor characteristics, and with shunt excitation or its equivalent, give speed variation and adjustment like that of the direct-current motor with field control, and is therefore exten- sively used. Its disadvantage, however, is the difficulty and limitation in design, resulting from the e.m.f. induced in the short- circuited coils under the brush, by the alternation of the main field, which tends toward sparking at the commutator.

Single-phase Generation. — 135.

Speed Control of Polyphase Induction Motor. —

By resistance in the secondary , 8. Gives a speed varying with the load.

By pyro-electric resistance in the secondary, 10. Gives good speed regulation at any speed, but such pyro-electric conductors tend toward instability.

By condenser in the secondary, 11. Gives good speed regula- tion, but rather poor power-factor, and usually requires an un- economically large amount of capacity.

By commutator, 58. Gives good speed regulation and per- mits power-factor control, but has the disadvantage and com- plication of an alternating-current commutator.

By concatenation with a low-frequency commutating machine as exciter, 52. Has the disadvantage of complication.

Stanley Induction Generator. — 117. Induction machine with low-frequency exciter. (See “Concatenation (2).”)

Stanley Inductor Alternator— 159. Inductor machine with two armatures and inductors, and a concentric field coil between the same. (See “Inductor Machine.”)

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ELECTRICAL APPARATUS

Starting Devices. — Polyphase induction motor:

Resistance of high temperature coefficient , 2. Gives good torque curve at low speed and good regulation at speed, but requires high temperature in the resistance.

Hysteresis device , 4. Gives good speed regulation and good torque at low speed and in starting, but somewhat impairs the . power-factor.

Eddy-current device , 5; double and triple squirrel-cage, 18, 20, 24; and deep-bar rotor, 7. Give good speed regulation combined with good torque at low speed and in starting, but somewhat impairs the power-factor. (See “ Double Squirrel-cage Induction Motor” and “ Deep-bar Induction Motor.”)

Single-phase induction motor:

Phase-splitting devices , 67. Resistance in one phase, 68. In- ductive devices, 72. Shading coil, 73. (See “Shading Coil.”) Monocyclic devices, 76. Resistance-reactance device or mono- cyclic triangle. Condenser motor, 77. (See “ Condenser Motor.”)

Repulsion-motor starting .

Series-motor starting.

Synchronous-induction Generator. — XIII, 113. Induction machine, in which the secondary is connected so as to fix a definite speed. This may be done:

1. By connecting the secondary, in reverse phase rotation, in shunt or in series to the primary : double synchronous generator. (See “Double Synchronous Machine.”)

2. By connecting the secondary in shunt to the primary through a commutator. In this case, the resultant frequency is fixed by speed and ratio of primary to secondary turns.

3. By connecting the secondary to a source of constant low frequency: Stanley induction generator. In this case, the low- frequency phase rotation impressed upon the secondary may be in the same or in opposite direction to the speed. (See “Con- catenation (2).”)

Syuchronous-induction Motor. — IX, 97. An induction motor with single-phase secondary. Tends to drop into step as syn- chronous motor, and then becomes generator when driven by power. Its low power-factor makes it unsuitable except for small sizes, where the simplicity due to the absence of direct- current excitation may make it convenient as self-starting' syn- chronous motor. As reaction machine, 150.

Thomson-Houston Arc Machine. — 141-144. Three-phase Y -

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471

connected constant-current alternator with rectifying commu- tator.

Thomson Repulsion Motor. — 193. Single-phase compensated commutating machine with armature energized by secondary current, and field coil and compensating coil combined in one coil.

Unipolar Machines. — Unipolar or acyclic machine, XXII, 247. Machine in which a continuous voltage is induced by the rotation of a conductor through a constant and uniform magnetic field. Such machines must have as many pairs of collector rings as there are conductors, and the main magnetic flux of the machine must pass through the collector rings, hence current collection occurs from high-speed collector rings. Coil windings are impossible in unipolar machines. Such machines either are of low voltage, or of large size and high speed, thus had no application before the development of the high-speed steam turbine, and now three- phase generation with conversion by synchronous converter has eliminated the demand for very large direct-current generating units. The foremost disadvantage is the high-speed current collection, which is still unsolved, and the liability to excessive losses by eddy currents due to any asymmetry of the magnetic field.

Winter-Eichbery-Latour Motor. — 194. Single-phase compen- sated series-type motor with armature excitation, that is, the exciting current, instead of through the field, passes through the armature by a set of auxiliary brushes in quadrature with the main brushes. Its advantage is the higher power-factor, due to the elimination of the field inductance, but its disadvantage the complication of an additional set of alternating-current commu- tator brushes.

CHAPTER XXIV

CONCLUSION

254. Numerous apparatus, structural features and principles have been invented and more or less developed, but have found a limited industrial application only, or are not used at all, be- cause there is no industrial demand for them. Nevertheless a knowledge of these apparatus is of great importance to the elec- trical engineer. They may be considered as filling the storehouse of electrical engineering, waiting until they are needed. Very often, in the development of the industry, a demand arises for certain types of apparatus, which havQ been known for many years, but not used, because they offered no material advan- tage, until with the change of the industrial conditions their use became very advantageous and this led to their extensive application.

Thus for instance the commutating pole (“interpole”) in direct-current machines has been known since very many years, has been discussed and recommended, but used very little, in short was of practically no industrial importance, while now practically all larger direct-current machines and synchronous converters use commutating poles. . For many years, with the types of direct-current machines in use, the advantage of the commutating pole did not appear sufficient to compensate for the disadvantage of the complication and resultant increase of size and cost. But when with the general introduction of the steam-turbine high-speed machinery became popular, and higher- speed designs were introduced in direct-current machinery also, with correspondingly higher armature reaction and greater need of commutation control, the use of the commutating pole became of material advantage in reducing size and cost of apparatus, and its general introduction followed.

Similarly we have seen the three-phase transformer find gen- eral introduction, after it had been unused for many years; so also the alternating-current commutator motor, etc.

Thus for a progressive engineer, it is dangerous not to be famil- iar with the characteristics .and possibilities of the known but

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CONCLUSION

473

unused types of apparatus, since at any time circumstances may arise which lead to their extensive introduction.

255. With many of these known but unused or little used ap- paratus, we can see and anticipate the industrial condition which will make their use economical or even necessary, and so lead to their general introduction.

Thus, for instance, the induction generator is hardly used at all today. However, we are only in the beginning of the water- power development, and thus far have considered only the largest and most concentrated powers, and for these, as best adapted, has been developed a certain type of generating station, compris- ing synchronous generators, with direct-current exciting circuits, switches, circuit-breakers, transformers and protective devices, etc., and requiring continuous attendance of expert operating engineers. This type of generating station is feasible only with large water powers. As soon, however, as the large water powers will be developed, the industry will be forced to proceed to the development of the numerous scattered small powers. That is, the problem will be, to collect from a large number of small water powers the power into one large electric system, similar as now we distribute the power of one large system into numer- ous small consumption places.

The new condition, of collecting numerous small powers — from a few kilowatts to a few hundred kilowatts — into one sys- tem, will require the development of an entirely different type of generating station: induction generators driven by small and cheap waterwheels, at low voltage, and permanently connected through step-up transformers to a collecting line, which is con- trolled from some central synchronous station. A cheap hy- draulic development, no regulation of waterwheel speed or gen- erator voltage, no attendance in the station beyond an occasional inspection, in short an automatically operating induction gen- erator station controlled from the central receiving station.

In many cases, we can not anticipate what application an unused type of apparatus may find, and when its use may be economically demanded, or we can only in general realize, that with the increasing use of electric power, and with the intro- duction of electricity as the general energy supply of modern civilization, the operating requirements will become more diver- sified, and where today one single type of machine suffices — as the squirrel-cage induction motor — various modifications thereof

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will become necessary, to suit the conditions of service, such as the double squirrel-cage induction motor in ship propulsion and similar uses, the various types of concatenation of induction machines with synchronous and commutating machines, etc.

256. In general, a new design or new type of machine or apparatus has economically no right of existence, if it is only just as good as the existing one.