US2014261A - Voltage regulator - Google Patents

Description

T. M. LINVILLE VOLTAGE REGULATOR Filed Feb. 23, 1955 Sept. 10, 1935.

2 SheetsSheet 1 Inventor: Thomas M.Li'nviile his Attovney.

Sept. 10, 1935. T. M. LINVILLE VOLTAGE REGULATOR Filed Feb. 23, 1935 2 Sheets-Sheet 2 Ill"- Inventor:

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Patented Sept. 10, 1935 PATENT OFFICE VOLTAGE REGULATOR Thomas M. Linville. Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 23, 1933, Serial No. 658,011

12 Claims.

My invention relates to voltage regulators, and more particularly to improvements in voltage regulators of the compensating type.

By voltage regulators of the compensating type,

hereafter referred to as compensators. I mean regulators which prevent the occurrence of voltage fluctuations as distinguished from ordinary regulators which cancel voltage fluctuations after they have already occurred.

It is well known that if the load on an ordinary generator is suddenly changed there is produced a transient fluctuation of its terminal voltage. The magnitude of this fluctuation depends upon the magnitude of the load change and it may vary all the way from a fraction of one percent of normal terminal voltage to one hundred percent of normal terminal voltage. The time of recovery of such a voltage fluctuation to ninety-nine percent of normal terminal voltage is typically of the order of magnitude of a few milliseconds and depends upon the constants of the generator circuit in which the transient current change takes place.

The speed of operation of ordinary voltage regulators is for too slow to prevent such voltage fluctuations. Fortunately, however, such fluctuations do not produce any appreciable adverse effects upon the operation of many loads, such as motors, furnaces, or heaters. Nevertheless, there are certain important loads such as radio receiving sets and electric lamps, which are very sensitive to voltage changes and whose operation is adversely afiected by such fluctuations. In the case of ordinary illuminating lamps an annoying flicker is produced by such fluctuations.

An object of my invention is to produce a compensator which substantially completely eliminates voltage fluctuations of the type referred to above.

Another object of my invention is to produce a compensator which eliminates lamp flicker as a result of the sudden application of any reasonable kind of load to a generator which supplies electric lamps.

In analyzing voltage fluctuations of the above described type I have found that they are produced mainly by the inductance voltage drop in the conductors traversed by the transient current resulting from the sudden load change. In accordance with an important feature of my invention, I provide means for substantially exactly compensating for this drop. This means consists essentially of a pair of inductively related windings whose mutual inductance is substantially equal to the inductance of the conductors traversed by the transient current.

My invention should be sharply distinguished from Patent No. 1,313,050 issued August 12, 1919 on an application of John B. Ball and assigned to the assignee of the present application. In that patent the essential principle of operation is the retardation produced by a sell inductance in the generator circuit whereby the rate of change of generator voltage is retarded.

There are several other factors producing components of the resultant voltage fluctuation, such as the resistance voltage drop in all type of generators, certain demagnetizing effects in direct current shunt type generators, or the damping effect produced by the mutual inductance between the field windings of a compound wound direct current generator. In accordance with another feature of my invention I provide means for compensating for the effects of these factors.

Another important feature of my invention has to do with the parallel operation of generators which are provided with compensators. I have found that if a single compensator is provided for a given number of parallel connected generators, a flicker occurs if more or less than said given number of generators are operated in parallel. Similarly, I have found that if each of a plurality of parallel connected generators is provided with an individual compensator that the disconnection of one or more generators without also disconnecting their compensators from the circuit, will cause lamp flicker. Accordingly, I have provided a novel system of parallel operation of generators and compensators. whereby each generator and its compensator are connected and disconnected to the common circuit as a unit.

Thus, a further object of my invention is to provide a new and improved system of parallel operation of generators and. voltage compensators.

My invention will be better understood from the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawings, Fig. 1 is a diagrammatic illustration of an embodiment of my invention as applied to an electric generator; Fig. 2 is a diagrammatic illustration of a modified circuit arrangement to which my invention is applied. Fig. 0 3 illustrates diagrammatically a modification of my invention as applied to a direct current shunt generator; Fig. 4 illustrates another modification of my invention as applied to a direct current compound wound generator, and Fig. 5 illustrates 55 two applications of my invention to generators which are arranged ior parallel operation.

Reierring now to Fig. 1, there is shown therein an electric generator I, which may be a direct or alternating current generator oi any type, to which are connected a variable power load, such as an electric motor 2, and a relatively steady load which is sensitive to voltage fluctuations, such as a lamp bank I. Motor 2 is adapted to be connected and disconnected irom generator I by means oi any suitable switch or circuit controller I, so that by opening or closing this switch the motor load may he suddenly removed or applied to generator I. Connected in series relation with loads 2 and I are a pair oi inductively related windings I and I, respectively. windings I and I are preierably mounted on a core oi magnetic material, such as an'iron core I. Also, in order to minimise the likelihood oi magnetic saturation oi core I, it is preierable to provide this core with an air-gap, which is shown at I.

Windings I and I are so related that their mutual inductance is substantially equal to the inductance oi generator I. The inductance oi generator I may be determined by calculation or by empirical methods which are well known to those skilled in the art, and the turns oi windings I and I are then so proportioned that their product is proportional to the inductance oi generator I. The windings I and 8 are also so arranged on core I that when a change in current occurs in one oi the windings it produces a flux change in such a direction in core I as to induce a voltage in the other winding which is opposite in direction to the normal voltage drop in that winding. With such an arrangement oi the windings, any change in current required by either oi the two loads will cause a voltage to be induced in the circuit of the other load which is substantially equal and opposite to the inductance voltage drop in generator I. As a practical matter, changes in load current required by lamp bank I, such as are occasioned by the switching on or oi! oi one or more oi the individual lamps, are so small as to produce no appreciable inductance voltage drop in generator I, and consequently no appreciable voltage fluctuation is produced. However, motor I will usually be a relatively large current consuming device and the switching on or oi! oi this motor will produce relatively heavy transient currents through generator I which will produce substantial voltage fluctuations. These fluctuations ordinarily produce an objectionable flicker oi the electric lamps. These fluctuations are largely composed oi the inductance voltage drop in the generator and consequently by the use oi the particular arrangement oi windings I and I described above, the efl'ect oi the inductance voltage drop In generator I may be substantially compensated for with the result that voltage fluctuations produced by the intermittent operation of motor I are greatly minimized and consequently the flicker oi lamp 3 is greatly reduced.

Winding I will oi course have some seli-inductance. but this self-inductance plays no part in my invention and theoretically it is preferable to have this sell-inductance as low as possible. This is because the lower the seli-inductance oi winding I is, the sooner the voltage applied to motor 2 will recover to substantially its normal value after the motor is suddenly connected to the generator. Thereiore, the lower the seliinductance oi winding 5, the shorter the duration oi the voltage fluctuations which are applied to motor 2 when it is connected to the generator.

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The seli-inductance oi winding I may conveniently be made very low by providing this winding with a minimum number oi turns. In such a case the turns oi winding I are increased so as to bring the product oi the turns oi the two wind- 5 ings up to a value which is proportional to the seli-inductance oi generator I.

From the standpoint oi efllciency, however, it may sometimes be desirable to design the compensator in such a way that the amount oi copper used in the two windings is proportional to the average load currents traversed by these windings. It may therefore happen that ii eiilciency is a criterion for the design oi the compensator, winding I will have more than a pracl5 tical minimum oi seli-inductance, An increase in the seli-inductance oi winding I does have the advantage oi reducing the severity oi the voltage fluctuations occurring in the circuit oi motor I when it is switched on or oil, but at the same time the duration oi these fluctuations is increased. It should be emphasized, however, that the seliinductance of winding 5 plays no part in the regulating or compensating action oi my invention.

The operation oi Fig. l-has been largely described above. It may be helpiul, however, to review briefly its basic principle oi operation. The mutual inductance oi coils I and 6 may be deflned as the voltage induced in coil I divided by the rate oi change of current in coil I. Ii, thereiore, the value 0! this mutual inductance is made equal to the value oi the seli-inductance oi generator I, any change oi current in generator I will produce a voltage boost in winding 6 which is substantially exactly equal and opposite to the inductance voltage drop in generator I, and consequently the voltage fluctuation-oi generator I caused by its inductance will be substantially exactly compensated for.

With the above described arrangement, the compensating action is entirely independent oi the constants of the load on the power circuit. This means that load 2 may be largely inductive or largely resistive, or have equal eiiective values of both inductance and resistance without in any way changing the compensating action oi my arrangement.

The modiflcation of my invention illustrated in Fig. 2 represents an application thereoi which is more likely to be met with in actual practice than is the arrangement shown in. Fig. 1. This flgure diiiers from Fig. 1 in that both the power load 2 and the lighting load I are connected to generator I through common circuit conductors 9. Such a circuit arrangement is likely to be met with in practice because both the power load and the lighting load are likely to be relatively remotely situated with respect to the generator, whereas they are themselves relatively closely situated. With such an arrangement, ii the circuit conductors 9 are oi appreciable length they, themselves, will have an appreciable seliinductance, which is illustrated diagrammaticab 1y by coil La, so that transient currents required by the operation of power load 2 will result in t5 inductance voltage drops in the conductors I which themselves will be an appreciable iactor in voltage fluctuations suflicient to produce flicker of the lamps 3. Consequently, windings I and I are so arranged that their mutual inductance is substantially equal to the inductance oi generator I plus the inductance of conductors I. On the drawings this relation is indicated by the equation M=L+ a With such an arrangement, the inductance drop of both the generator and the conductors 9 is compensated so that the effect of these drops does not aid to produce flicker of lamps 3.

In Fig. 3 is illustrated an additional modification of my invention as applied to a direct current shunt type generator i0. Generator I is provided with a shunt field winding Ii and the compensator of Fig. 3 differs from the compensators of the preceding figures in that additional means are provided for inserting a compensating voltage or current in the circuit of shunt field winding II. This additional means comprises inductively related windings l2 and I3 connected respectively in the load circuit of generator l0 and in series with the shunt field winding ll. These windings are preferably mounted on a core [4 provided with air-gaps l5 and I6. In addition, core I of the compensator for compensating for the inductance drop in the generator, is shown as provided with an additional air-gap 8'.

As has been previously stated, the inductance voltage drop is merely one of a number of factors going to make up the resultant voltage fluctuation when the current or load on the generator changes suddenly. Other factors are the resistance voltage drop in the generator and its leads or connections. Still another factor is certain demagnetizing effects produced by the current change and these effects are substantially pro portional to current. By properly adjusting the mutual inductance between windings l2 and I3 the effects of these various factors may be substantially eliminated so that the result is that lamps 3 will show no appreciable flicker when load 2 is suddenly connected or disconnected from the generator. The mutual inductance between windings i2 and I3 is arranged so that just enough current is induced in winding I3, and consequently in field winding H, by a change in main load current flowing through winding l2 to build the flux of the generator up during the change in generator current so that there will be produced just enough change in generated voltage to cancel the effect of the factors enumerated above.

In the operation of Fig. 3, when the power load on generator l0 changes windings 5 and 6 act to compensate for the fluctuation produced by the inductance drop in the generator Ill and windings l2 and I3 act to compensate for the resistance drop and the effect of demagnetization, both of which tend to produce voltage fluctuation.

It should be understood that the portion of the total voltage fluctuation which is compensated for by windings l2 and I3 is produced by factors which cause a characteristic droop in generated voltage of shunt generators as the load increases. Consequently, the compensating means including windings l2 and i3 only acts to hold the voltage constant while the current of the generator changes. However, some of the compensating current induced in the circuit of field winding i I by the windings l2 and i3 dies out so slowly that, although there is a final change in voltage on the lamps 3 due to the characteristic droop in the voltage of shunt generators as the load increases, the transition from the initial to the final voltage is so slow as normally not to produce flicker, However, by the use of series winding on the generator so as to produce a compound machine, this drooping voltage characteristic may be changed to a substantially flat voltage characteristic, in which case there will be substantially no voltage change whatever. Such an arrangement is illustrated in Fig. 4.

In addition to providing generator H) with a series field winding I'I, Fig. 4 difiers from Fig.

3 in that all of the compensator windings are r, mounted on core 1 thereby eliminating the use of core M. For convenience of designation, the windings 5, l3 and 6 cf the compensator will hereafter be referred to as the power winding, the field winding, and the lighting winding, re- 10 spectively, although it should be understood that my invention is not limited to use in connection with lighting circuits, and that it may equally well be applied to any circuits or loads which are sensitive to voltage fluctuations. The grouping of all of the compensator windings on a single core will, of course, also suggest that this may equally well be done for Fig. 3, in which case the winding I! could be omitted and the mutual inductance between winding l3 and winding 5 be made equal to the mutual inductance which existed between winding I2 and winding iii.

In a compound generator, the series field winding produces the change in excitation with changes in current which is necessary to give substantially constant terminal voltage of the generator with varying loads upon it. Therefore, in a compound wound generator the magne'tcmotivc force necessary to produce compensation for the resistance drop and. the demagnetizing effects which are substantially proportional to current is already present and consequently the series winding produces the same equivalent magnetomotive force as the winding l3 in Fig. 3. Therefore, the only purpose of winding l3 in Fig. 4 is to compensate for the damping eifect produced by the unavoidable mutual inductance between the shunt and series field windings of the generator. This mutual inductance is always present because the windings 40 are on the same magnetic circuit. In order to compensate for this damping of the field windings and permit series winding I'I to produce flux changes unhampered by damping by the main shunt field winding H, the mutual inductance between the power winding 5 and the field winding I3 is made substantially equal to the mutual inductance between the field windings H and I1.

When this is done the fiux through the magnetic circuit of the generator can change very rapidly in proportion to the armature current because the flux through the coil I3 is changing in the opposite direction in similar proportion, thereby keeping the net change in flux linkage in the shunt field circuit zero.

As a practical matter, the field coil l3 should have somewhat greater mutual inductance with the power coil so as to induce some current in the circuit of shunt field winding ll so as to produce an effect assisting the series field winding. This is needed to overcome the damping of iron paths and certain saturation effects which latter require that the flux be not quite proportional to armature current. However, this change in the mutual inductance between wind- 5 ings 5 and I3 is small so that, broadly speaking, the mutual inductance between windings 5 and I3 is substantially equal to the mutual inductance between windings II and i1. With such an arrangement, substantially complete compensation is secured and the sudden application or removal of load of any type and of any reasonable value will produce no voltage fluctuation on the lighting circuit or flicker of the lamps 3.

The description of my invention has thus far been conflned to its application to single generators. However, another important feature of my invention has to do with its application to parallel operated generators. Due to the fact that the mutual inductance between the power and lighting windings of a compensator bears a dcflnite relation to the inductance of the generator associated with the compensator, it is impossible to use the same compensator for two parallel connected generators. Thus, if another similar generator is connected in parallel with the generator having a compensator, the inductanoe of the generator circuit including the parallel generators will be one half the inductance of one generator and consequently the voltage of the lighting circuit will be over-compensated and flicker will occur. Similarly, if a compensator is designed so as to have the mutual inductance between its power and lighting winding equal to the inductance of a plurality of parallel connected generators, then the disconnection of one of said generators or the addition of another generator will disturb the relation between the generator inductances and the mutual inductance of the compensator windings, and consequently voltage fluctuations will occur with their attendant flicker. I have therefore found that it is necessary to provide each generator with an individual compensator, for only in this manner may the number of parallel connected generators be changed. However, I have discovered that, if a plurality of generators are connected in parallel to power and lighting loads through individual compensators for compensating for the fluctuation produced in each generator, flicker will still occur if one or more generators is disconnected without also breaking the circuit connections of the power and lighting coils of its associated compensator with the power and lighting circuits. This is because the power and lighting windings of the compensator of the disconnected generator serve to conduct current from the lighting circuit or bus to the power circuit or bus, so that when a load change occurs on the power bus the compensator or compensators for the connected machines tend to maintain voltage on the lighting bus, while the voltage on the power bus varies. The result is that power flows through the windings of the compensator of the disconnected generator from one circuit to the other, depending upon which is at the higher voltage, with the resuit that voltage fluctuations on the lighting bus are produced and therefore flicker is produced.

In Fig. 5 of the drawings, I have illustrated a flicker compensated system which includes two parallel connected generators, and which is so arranged that either generator may be connected or disconnected from the circuit without producing fluctuations and flicker on the lighting circuit. In this flgure there are shown a power bus I! to which is connected a variable power load 2 and a lighting bus I! to which is connected a voltage sensitive lamp load 3. Connected in parallel to these two busses are two direct current compound-wound generators 20 and 2|. As shown. generators 20 and 2| are of the well known three-wire type having a neutral or midvoltage tap brought out from the armature which is adapted to be connected by means of conductors 22 and 23, respectively, to the neutral of the lighting bus i9. The purpose of this arrangement is to permit the operation of power loads at substantially double the lamp voltage. In this manner, medium size motors for power purposes can be operated at their usual rated voltage of approximately 220 to 230 volts, while the lamps can be operated at their rated voltage of 110 to 115 volts. The armatures of generators 20 and 2| are connected respectively substantially directly in parallel with each other by means of the usual equalizer bus 24. However, in circuit with each of the armatures are commutating windings 25 and 26, respectively, for generators 2B and 2|. For controlling the connections of generators 20 and 2| to the various buses there are provided any suitable controlling means, such as circuit breakers 21 and 28, respectively. Circuit breaker 21 has contacts 29 and 30 which control the connections of the armature of generator 20 to the equalizer bus 24 and a set of contacts ll for controlling the connection of the neutral of the generator 20 to the neutral of the lighting bus is. Similarly, circuit breaker 28 has contacts 42 "and ll which control the connections oi the armature of generator 2| to the equalizer bus 24 and contacts 34 for controlling the connection of the neutral of generator 2| to the neutral of lighting bus l9.

Associated with generator 20 is a flicker compensator 35 which is adapted to be remotely situated with respect to the generator, such, for example, as at the back of the switchboard on which is mounted the circuit breaker 21 which controls the connections of generator 20. In this manner, compensator 35 may be made to compensate for the voltage drop in the electrical connections of generator 20 to the various buses. compensator 35 is essentially similar to the compensator of Fig. 4, in that it is provided with a power winding 36, a fleld winding 31, and a lighting winding. However, the lighting winding is split into two halves, l8 and 38, which are connected respectively to the positive and negative sides of the lighting bus. This is necessary because the lighting circuit is a three-wire circuit, so that in order always to maintain equality between the voltage to neutral of each of the outside conductors of the lighting bus, it is necessary that the compensating voltage of the lighting coils be divided in half and applied equally to the outside conductors of the lighting bus.

The connections between the generator 20, the compensator 25 and the various buses is as iollows: From the right hand side of the armature of generator 20 through commutating winding 25, a series field winding 40, conductor 4|, contacts 42 on circuit breaker 21, contacts 43 of a suitable relay or circuit controller, power coil 26, to the positive side of a power compensator bus 44 and then through a conductor 45 to the positive side of the power bus ii. The return circuit is from the negative side of the power bus I! through a conductor 48, contacts 41, 0! circuit breaker 21, through another series field winding 48 which is a duplicate of series field winding 40 and back to the negative side of generator 2|! through commutating winding 25.

The lighting coil 29, which is adapted to be connected to the positive conductor of the lighting bus I9, is connected thereto as follows: from the terminal of the power winding 36, which is connected to the positive side of the generator 20 through a circuit already traced, through the lighting winding 39, a conductor 49 to the positive side of a lighting compensator bus 50. From the positive side of this bus through a conductor II to the positive side of the lighting bus. The lighting winding 38 of the compensator 25, which is adapted to be connected to the negative side of the lighting bus, is connected thereto as follows:

has already bcentraced, through aconductor 52 to the'negative sideof the bus 44,then through a conductor 53' to, contacts 54= on a circuit cont roller of, any suitable type and then through the winding '38 to the negative side of the lighting compensator bus 50. From there the connection is through a conductor 55 to the negative side of the lighting bus. The purpose of compensator power bus 44 and compensator lighting bus 50 is to facilitate the paralleling of compensators of other generators which might be connected to the power and lighting buses, as are generator 20 and compensator 35.

The connection of the field coil 31 of compensator 35 to generator 20 is as follows: From the positive side of the generator through windings 25, and a main shunt field winding 56, the winding 31, a conductor 51, a field rheostat 58 and back to the negative side of the generator through conductor 46 and series field winding 48 and commutating winding 25.

The pu pose of contacts 43 and 54 is to control the connections of the compensator 35 to the power and lighting buses HI and I9. As shown, these contacts are controlled by electromagnetic contactors or relays, but any other equivalent circuit controllers might be employed. As illustrative of one way of controlling these contactors, they are shown connected in series across generator 20 and this series circuit is controlled by contacts 59 on circuit breaker 21.

From the above description, it will be seen that when generator 20 is in operation and circuit breaker 21 is closed, all of the various circuits heretofore traced will be completed, the circuits through contacts 43 and 54 being closed by reason of the fact that the voltage of generator 20 energizes the electromagnetic relays and closes the contacts 43 and 54, this energizing circuit being completed through contacts 59 of circuit breaker 21. However, when it is desired to take generator 20 out of service, the opening of circuit breaker 21 will break the energizing circuit for the relays having contacts 43 and 54 consequently the circuit through the compensator 35 will be broken.

Generator 2| has associated therewith a compensator 63. This compensator has a power winding 6|, a field winding 62 and lighting windings 63 and 64 which correspond respectively with the similar windings of compensator 35. Similarly, generator 2| has series field windings 65 and 66 and shunt winding 61, which correspond respectively with the similar windings of generator 20.

The connection of the power winding 6| between generator 2| and power bus I8 is as follows: From the positive side of generator 2| through commutating winding 26, series field winding 66, power winding 6|, a conductor 68, contacts 69 on circuit breaker 28 to the positive side of power bus I8. The return circuit is from the negative side of power bus |B, contacts 10 of circuit breaker 28, series field winding 65 and commutating winding 25 of generator 2| to the negative side of this generator. The connection of lighting coil 64, which is adapted to be connected to the positive side of the lighting bus I9 is as follows: The positive side of the generator through commutating winding 26, series field winding 66. winding 64, contacts 1| of circuit breaker 28 to the positive side of the lighting bus IS, The connections of the lighting winding from the'negative side ofthe-power bus, the connection of which to thenega'tive sideof generator.

63 to the negative side of the lighting bus is as follows: From the negative side of the generator through commutating winding 26, series field winging 65, winding 83, and contacts 12 of circuit breaker 28 to the negative side of the lighting bus IS. The field winding 62 of the compensator is connected from the positive side of the generator 2| through commutating winding 26, series winding 66, shunt winding 01, the field winding 52, a field rheostat 13 and back to the negative side of the generator through the series field winding 65 to the commutating winding 26.

From the above description of the circuit, it will be seen that by opening and closing circuit breaker 28 both the connections of the generator 2| to the lighting and power buses, and also ,the connections of the compensator 6|] to the lighting and power buses may be opened or closed simultaneously at will. Consequently, when the circuit breaker 28 is closed and generator 2| is in parallel with generator 29 compensator 60 will also be in parallel with compensator 35. Also, when generator 2| is shut down, as by the opening of circuit breaker 28, not only will generator 2| be disconnected from the power and lighting buses, but also compensator 60 will be disconnected from these buses.

It will be obvious to those skilled in the art that if desired, compensator 66 might be connected between generator 2| and the power and lighting buses in the same manner as compensator 35 is connected, and also that if desired, compensator 35 might be connected between the generator 20 and the power and lighting buses in the same manner in which compensator 60 is connected between these elements. In the case of compensator 35, the compensator is adapted to be an element of the switchboard and to be relatively remotely situated from the generator 20, and is adapted to be a separate unit from generator 20, whereas in the case of compensator 63, this compensator is adapted to be mounted closely adjacent to generator 2| and to operate more or less as a unit with the generator 2|.

By means of the circuit arrangement disclosed in Fig. 5, I have produced an electrical distribution system comprising a plurality of parallel connected generators which supply both power and lighting loads and this system is so arranged that regardless of how many generators are connected or disconnected from the various loads, it is impossible to produce flicker of lamps connected to the l ghting bus when loads are suddenly applied or removed from the power bus l8.

While I have shown and described various embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made therein, and consequently I aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, an electric generator, a pair of loads connected to said generator, and means comprising two inductively related windings connected respectively in series with said loads for substantially exactly compensating the voltage of one load for the inductance voltage drop only in said generator produced by a relatively sudden change in value of said other load.

2. In combination, a direct current generator, a pair of loads connected to said generator, a pair of inductively related windings connected respectively in series with said loads, said windings having a mutual inductance substantially equal to the inductance of said generator.

3. In combination, a direct current generator, a variable load power circuit connected to said generator, a lighting circuit connected to said generator, two inductively related windings connected respectively in said power circuit and in said lighting circuit, the mutual inductance between said windings being substantially the same as the inductance oi said generator.

4. In combina'tion, a direct current generator, a variable load power circuit connected to said generator, a lighting circuit connected to said generator through a common portion of said power circuit, a pair of inductively related windings connected respectively in said circuits, said windings having a mutual inductance which is substantially equal to the inductance of said generator plus the inductance of said common portion said circuits.

5. In combination, a direct current generator having a shunt i'ield winding, 9. pair of load circuits connected thereto, inductively related wind ings connected respectively in said circuits, said windings having a mutual inductance which is substantially equal to the inductance of said generator, and another pair oi inductively related windings connected respectively in one of said circuits and in series with said shunt field winding.

6. In combination, a direct current generator having shunt and series field windings, a pair 0! load circuits connected to said generator, a pair oi inductively related windings connected respectively in said load circuits, the mutual inductance 01' said windings being substantially equal to the inductance of said generator, another pair of inductively related windings connected respectively in one of said circuits and in circuit with said shunt winding, the mutual inductance between said last mentioned pair of inductively related windings being substantially equal to the mutual inductance between said field windings.

7. In combination, a three-wire generator having main terminals and a mid-voltage tap, a twowire power load connected to the main terminals 01' said generator, a three-wire lighting circuit having its outer conductors connected to the main terminals of said generator and its neutral conductor connected to said mid-voltage tap, a voltage compensator for said generator, said compensator having two substantially identical windings connected respectively in series with the outer conductors of said lighting circuit.

8. In combination, a plurality of parallel load circuits, a plurality of generators arranged to be connected in parallel to said circuits, individual voltage compensators connected to each generator, said compensators normally interconnecting said circuits, and means for insuring that a generator cannot be connected to or disconnected from the remainder of said generators without also connecting or disconnecting its associated compensator from the remainder of said generators and compensators.

9. In combination, a lighting circuit, a power circuit, a plurality of generators connected in parallel to said circuits, individual flicker compensators for each generator, said compensators normally interconnecting said circuits, and circuit controlling means for selectively disconnecting any generator together with its flicker compensator from said circuits.

10. In combination, a pair of load circuits, a plurality oi generators, individual circuit controllers associated with each generator for connecting their respective generators to said circuits whereby said generators may be connected in parallel to said circuits, an individual voltage compensator associated with each generator, said compensators being connected to interconnect said circuits, and individual circuit controllers for making and breaking the interconnections between said circuits produced by said compensators.

11. In combination, a pair oi! load circuits, a plurality of generators, individual circuit controllers associated with each generator for conmeeting their respective generators to said circuits whereby said generators may be connected in parallel to said circuits, individual voltage compensators associated with each generator, said compensators being connected to interconnect said circuits, and means controlled by each of said circuit controllers for breaking the interconnection between said circuits produced by the compensator associated with the generator which in turn is associated with any particular circuit controller when said circuit controller disconnects said generator from said circuits.

12. In combination, an electric generator, a pair of loads connected to said generator, a pair 01' inductively related windings connected respectively in series with said loads, said windings having a mutual inductance substantially equal to the self inductance of the conductors including said generator which are traversed by the current of both of said loads.

THOMAS M. LINVILLE.

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