Патент USA US2408218код для вставки
' Sept 24», W466 G. E. KING 2,42%,217 CONTROL SYSTEM Filed June 27, 1944 INVENTOR . (Geo/ye 5 W099‘. BY C, a~ ATTORNEY Patented Sept. 24, 1946 2,408,217 ‘UNITED STATES‘ PATENT OFFICE George E. King, Swissvale, Pa., assignor to ‘West inghouse ElectrrcyCorporation, ' East‘ Pittsburgh, Pa., in corporation of Pennsylvania 1 Application June 27, 1944, Serial No. 542,402 3Claims. (Cl. 172-479) The present invention relates; to variable volt age control systems and more particularly to-elec tri'c motor control systems for automatically con trolling the operation‘ of motors used to drivema chine-tool elements, paper mills and the: like, ele vators, electrically operated shovels. and so on. ' Thisainvention provides certain improvements in variable voltage control systems inwhich- ro~ toting regulators are employed to regulatea given quantity, and the invention as, hereinafter de scribed and as illustrated in the drawing is- spe ci?cally directed to. the regulation of the speed of rotation of a direct-current motor. However, it will be apparent to‘ one skilled in the art that the invention. is not limited to speed‘ regulation alone since various- modi?cations of the system ‘may be made to control the motor torque or power. It will further be apparent that the in vention is not necessarily limited to motor control since automatic regulation of the voltage current or electrical power supplied to an electrical load of substantially any type susceptible of. regula tion may be had. The improvements provided by the present in- N vention insofar as increased e?iciency and speed of operation are concerned in motor control sys— tems, for example of the type disclosed in ap plicant’s Patent No. 2,205,204, are readily ap parent. In general, such systems comprise a di-. rect~current motor, a main generator connected ’ in series circuit relation with the motor, a. regu lating generator for controlling’ the electrical out put of the main generator, in response to a changing motor characteristic which it is de sired to regulate and an exciter for supplying‘ electrical energy to the electrical system‘, or some portion thereof, depending upon the arrangement 2 the output terminals of t he bridge. The input terminals of the bridge ci rcuit are connected to a suitable source of direct current which may be varied to controlthe excitation of the main generator control ?elds and in view of the series connection of the two regulating generator ?elds with the two main generator control- ?elds the same exciting current fro in the external source ?ows therethrough. The differential magnetoe motive force of the voltage and current energized regulating generator ?elds are opposed to the magnetomotive force resulting from the external excitation of the two regulating generator ?elds in the bridge circuit. Thus when the motor is operating at a proper spe ed as indicated by the excitation of the main generator control ?elds the total ?ux in the regulating generator is zero. The regulating generator armature winding by reason of its connection in the bridge circuit is in such circuit relation with its two ?eld wind ings in‘ the bridge circuit that the generator is substantially self energizing and may therefore have an electrical output independent of the ex ternal excitation of its two windings in the bridge circuit,v or of its voltage and current energized windings, Thus upon a departure in balance of the mentioned differential magnetomotive force against the magnetomotive force of the two regu lating generator bridge circuit windings, an out put of the regulating generator is fed into the bridge circuit in a direction to correct the exist ing unbalance. When a balance in the regulating generator windings is ag ain reached this gen erator maintains: the corre ctive current necessary to keep such balance until a further disturbance occurs. While this practice of connecting ?eld wind of the component parts. ings of the‘ regulating ge nerator in series with The regulating generators used in such sys-. terns are generally provided with a ?eld winding 40 the generator control ?elds provides an accurate measurement of the generator control ?eld cur energized proportionally to the motor current, a rent, it has been found that the system during ?eld winding energized proportionally to the mo tor armature terminal voltage and differentially related to the current energized ?eld, the differ ential magnetomotive force of the voltage and reversing cycles tended slightly vto be sluggish. This sluggishness of op'er atio-n resulted because the currents in the regulating generator ?eld windings could change no faster than those‘ in the generator control ?eld windings, by reason of their series connection. While the generator control ?eld windings are designed to have a fair ly low impedance, the impedance of these wind current energized ?elds is indicative of the coun.-' ter emf of the motor and hence indicates the speed ofv the motor, and two ?eld windings each connected in series with. a control ?eld winding for the main‘ generator. The series connected 50 in'gs is, none the less,‘ considerably higher than regulating generator and main generator ?eld that ‘required for the regu lating generator ?elds. windings are usually arranged in opposite legs Thus,v even- though the ?eld windings of‘ the regu of a conventional bridge circuit having resistors Ilating generator ' are :of a character that permits in the remaining two opposite legs and the regu-: a faster'change of current than those of the. main lating generator armature is connected across generator,‘ the series conn ection of the windings 2,408,217 of these two machines prevented such a condition. It is highly desirable that machine tools have a high production capacity. On such machines, for example as metal planers, as the machine is being operated with fairly short strokes, the re versing cycle of the motor may comprise a sub stantial portion of the time required for a com plete operating cycle. For this reason, it is neces» sary that the motor be decelerated, stopped and 10 controller which provides certain desirable auto matic control features and which comprises the cut and return directional contactors C and R, a control relay CR, an automatic relay AR, a break relay BR, a voltage relay VR, 3. ?eld weakening relay FR. and a ?eld weakening contactor FW; and (6) The limit switches which comprise the cut and return limit switches CLS and RLS and the cut and return slowdown limit switches CSD and accelerated in a reverse direction in as little time as the electrical and mechanical characteristics ' of the equipment will permit. This has been accomplished in part, in the past, by designing faster control ?elds for the main generator and also by controlling the resistance of the discharge circuits for these generator ?elds so that the flux RSD. ' The motor and the main generator in Fig. 1 are connected with their armatures in series circuit decay in the generator ?elds may be as rapid as can be attained without producing current peaks on deceleration near or above the maximum limits that the motor energized by the generator can commutate. With the present scheme of control, a forcing action of the generator control ?elds is obtained by removing the regulating generator ?elds from their series circuit relationship with the generator relationship. As will be seen more conveniently from Fig. 2, the ?eld windings GFI and GF2 of the main generator are connected in opposite legs of a conventional Wheatstone bridge circuit, hav ing for its other two opposite legs, the bridge balancing resistors RESI and RESZ. The input terminals of this bridge circuit are identi?ed as 3 and 4 and the output terminals of the bridge are identi?ed :as I and 2. The armature winding of the regulator generator and the winding RSF in series therewith, are connected in series with the resistor elements RESE' and RESI l which are of predetermined ?xed value and the resistor RESB which is varied by means of the contacts control ?elds and placing a single ?eld elsewhere ARI and ARZ across the galvanometer terminals in the system that it may yet be energized by i and 2 of the bridge circuit. The control ?eld currents proportional, if not equal, to those ?ow winding RCF is connected in series with the sen ing through the generator shunt ?eld windings. sitivity adjusting resistor RES9 across the con With such an arrangement, the current ?ow 30 ductors supplying the bridge circuit. The dif through the newly located ?eld of the regulating ferential ?eld winding RDF is connected with its generator may change as rapidly as the imped series sensitivity adjusting resistor RESB across ance of the ?eld will permit and produce correc the motor armature as will be seen from Fig. 1 tive currents far in advance of those which were and the separately excited ?eld winding RF is attainable with the system of control previously connected in shunt relationship with the main described. A principal object of this invention is to in crease the speed of regulation of a generator em generator series ?eld winding GSF and, hence, is excited in proportion to the current ?owing therethrough. ployed in a variable voltage control system. The regulating generator RG is preferably Another object of this invention is to provide a 40 provided with operating characteristics which, variable voltage control system in which the rate while unfavorable vfrom an operating point of of change of currents in the main generator sup view insofar as a machine such as the main plying the system is increased. generator G is concerned, are most favorable Other objects and advantages will become ap for the regulating generator. Normally stable parent upon a study of the following disclosure operation of a direct-current generator is ob when considered in conjunction with the accom tained only if the slope of the resistance line panying drawing in which: of the ?eld circuit is less than that of a line Figure 1 is a schematic diagram of a complete tangent to the initial substantially straight line system of control for a reversible motor operat portion of the no-load saturation curve of the ing a planer platen or other tool actuating ma 50 machine. If the resistance is less, the generator chine embodying the novel features of this inven can have an open circuit voltage which is de termined by the intersection of the resistance tion; Fig. 2 is an elemental diagram of a portion of line with the saturation curve. If the resistance the system of Fig. 1 and illustrates the novel fea 55 is higher, and, consequently, the slope of the re tures of this invention. sistance line higher than the initial straight line Referring now to Fig. 1 of the drawing, the portion of the saturation curve, the generator speci?c system illustrated therein is for control voltage cannot build up. If the slope of the ling a direct-current motor of the type used, for resistance line just equals the slope of the in example, in operating a planer platten. This sys itial portion of the saturation curve, that is, is 60 tem comprises generally: (1) A direct-current tangent to this portion of the saturation curve, motor M having a separately excited ?eld wind the generator can theoretically have an open ing MF; (2) A variable voltage direct-current circuit voltage’ equal to any of the points of generator G having a series connected ?eld Wind tangency. It is the latter of the three mentioned ing GSF, two separately excited control ?eld conditions for which the regulating generator is windings GFI and GFZ, and a shunt connected 65 preferably adjusted. The action of the ?eld differential or suicide field winding GDF; (3) A windings RDF and RF together with that of regulating generator RG provided with a series the control ?eld winding RCF select the proper connected ?eld winding RSF, a separately excited control ?eld winding RCF, a separately excited, operating point of this generator along the tangent curve and maintain this operating point differentially connected ?eld winding RDF and a 70 constant for any ‘setting of the generator rheo separately excited ?eld winding RF; (4) An ex citer E which supplies the direct-current potential As previously noted, the ‘armature winding of of a constant value to the system and which has the regulating generator, the series ?eld winding a ?eld winding EF connected in shunt relation 75 RSF and a group of resistors RES5, RESB, and stat GR. ship with .the armature thereof ; (5)‘ A magnetic ' ’ ' r > ' 5 72,408,217 6 RES“ are connected in series across the gal ation of the cut and return limit switches CLS and RLS. The control relay CR, during periods when the vanometer terminals I and 2 of the Wheatstone bridge circuit. Normally, the resistor RESB is substantially entirely shunted from the series circuit by the contact AR! of the automatic relay AR. The value of the resistance in this series motor generator system is being started and dur-. ing periods when the planer platen is desired to be stopped with the generator system still oper ating, provides circuits to control the break relay circuit is preferably such, that when adjusted to obtain the desired tangent relationship of the BR to connect the differential or suicide ?eld resistance line of the regulating generator ?eld circuit with the no-load saturation curve, that 10 GDF of‘the main generator across the armature thereof to prevent creeping of the motor, and the total voltage across the galvanometer circuit provides a circuit for the coil of the ?eld weaken ing contact FW' across the positive and negative conductors 3+ and B- to close the contacts FWI of this contactor and short the motor rheostat MR from the circuit of the motor ?eld winding is zero for a predetermined operating condition. Since in the present system of control it is desired to regulate the speed of the motor M, an indication of the counter voltage of the motor is desired. rFhis is obtained by connecting the MF, thus applying full ?eld excitation to this differential ?eld winding RDF across the motor winding. armature terminals and by connecting the ?eld ?eld winding GSF. In this manner, the differ the ?eld circuit of the motor and ‘when open, removes the shunt circuit and thus connects this ential ?eld Winding is excited by a voltage pro portional to the motor armature terminal volt~ age, and the ?eld winding RF has a voltage ap rheostat in the circuit. — eration beyond predetermined speeds of the motor position, the differential magnetomotive force produced by these ?elds is an indication of the speed of the motor. The control ?eld winding RCF, by reason of its connection across the input 30 terminals to the bridge circuit, is energized by a- voltage proportional to the voltage applied to the ?eld windings GFl and GF2 of the main generator and, hence, produces a magnetomotive force which is an indication of the selected speed 35 of operation of the motor. The magnetomotive forces produced by the control ?eld winding RCF and the electrically opposed ?eld windings RDF and RF are in opposition and the differential of these magnetomotive forces is an indication of the departure of the motor speed from the selected speed of operation, as determined by the setting of either of the out or return leads GCL or GRL of the generator rheostat GR. The differential magnetomotive force of the three regulator generator ?eld windings causes a cor rective current, produced ‘by the regulator gen erator, to ?ow in the Wheatstone bridge circuit in such a direction as to produce the necessary change in excitation in the main generator ?eld ' windings GF! and GFZ, that the motor speed may be brought to its selected value. Before proceeding with a discussion of the operation of the system as shown in Fig. l, a description of the various contactors and relays comprising the magnetic controller will prob ably be desirable. The cut and return directional relays C and R are provided primarily to establish the proper polarity of the Wheatstone bridge circuit and, hence, the main generator by controlling the di » The ?eld weakening relay FR is provided with the contacts FR! which during periods of accel plied thereacross proportional to the load current of the series motor generator circuit. Since the ?eld windings RDF' and RF are in electrical op - The ?eld weakening contactor FW when closed, as just explained, shunts the motor rheostat from winding" RF across the main generator series are open, thereby permitting the ?eld weakening contactor to drop out and weaken the motor ?eld winding. This relay is provided with two coils FRCI and FRC2. The coil FRCZ, as will be seen more clearly from Fig. 2, is energized by the ex citer voltage and the coil FRCi, as will be seen in l, is energized by a voltage proportional to the voltage applied across the motor armature. Both coils of this relay must be energized before this relay picks up. The coil FRCZ, immediately upon operation of either of the contactors C and R, has the exciter voltage applied thereacross. The coil F‘RCI during periods of acceleration is energized by the increasing voltage across the motor generator series circuit. Thus, a slight time delay is obtained during acceleration, as well as deceleration, before this relay picks up or drops out. The voltage relay VR together with the CR contactor controls the energization of the coil of the break relay BR through its contacts VRl. This relay, in a manner similar to the ?eld weak ening relay m, is provided with two coils VRCI and VRCZ, the coil VRCS being responsive to the voltage across the motor generator series circuit and the coil VRC2. responding to the exciter volt— age. The coil VRCZ of this relay when energized causes the relay to pick up. Coil VRC! alone will hold the relay up. The automatic relay AR varies the resistance of the discharge circuit for the generator ?elds GFl and GFZ and for the regulating generator ?eld RSF. As previously explained, the purpose of changing this resistance in the ?eld discharge circuit is to obtain a more rapid rate of decay of the generator ?eld ?ux. This automatic relay rection of flow through the ?eld windings GF! has two coils. The main coil ARC?! is connected and GF2 thereof, and to establish portions of across the exciter and responds to eXcit'er voltage. the control circuits for the ?eld weakening con The‘ other coil or holding coil ARC! responds to tactor W, the ?eld weakening relay FR and the the voltage across the motor generator series cir voltage relay VR. These contactors also control cuit.‘v This relay picks up immediately upon the the degree of excitation of the generator and energization of the coil ARC?! when either of the motor ?eld winding through the medium of the contact-ore C or R operate, and it is held in by generator ?eld winding rheostat GR and the motor ?eld winding rheostat MR. Each of these 70 the holding coil ARC! during reverse cycles until the generator voltage has declined to a suf?cient contactors is provided with a single coil which value to permit the insertion of more resistance is interlocked with the other contactor and which in the ?eld discharge circuits, thus tending to are each selectively connected across the eX force the deceleration and acceleration at the citer busses B+ and B— through operation of and the beginning of each direction of move either the cut and return push buttons or oper 75 end ment of the planer platen. ' _ 2,408,217 7 It will be understood that the generator system of Fig. l is to be driven by a constant speed prime mover connected to the common shaft indicated by the dotted line intersecting the axes of the three generators. Such prime mover together with its control circuits have not been shown for the purpose of simplifying the illustration of the invention. Similarly, it will be understood that inching operation of the planer platen may be obtained by installing suitable inching push but tons in the control system in the manner shown, for example, in the applicant's Patent No. 2,205,204. These have also been omitted from the system in an effort to simplify the illustra tion of the invention. To operate the system the prime mover (not accelerated under the influence of a high torque. The voltage relay VR picks up immediately when the main generator shunt ?elds are energized, thus opening the contacts VRI and open circuit ing the coil of the break relay BR‘. Thus the differential or suicide ?eld is instantly removed from the generator circuit. When the main gen erator voltage builds up to a predetermined value, the ?eld weakening relay picks up opening‘its 10 contacts FRI thereby opening the circuit to the ?eld weakening contactor FW, This contactor immediately drops out, opening its contacts FWI and inserting the motor rheostat in the circuit of the motor ?eld MF. The resistance now in the 15 motor ?eld circuit is determined by the setting of the motor rheostat cut lead MCL. The motor thus accelerates to the speed determined by, the rheostat setting. Immediately upon closing of the contacts C4 of the contactor C, the coil ARCZ shown) driving the system of generators is brought up to speed. A voltage is thus applied by the exciter across the conductors B+ and B—~. of the automatic relay is energized and this relay The BR relay thus picks up, since the coil thereof 20 picks up, closing its contacts ARI and decreasing is connected across the busses 13+ and 13 the resistance across the galvanometer terminals through the now closed contacts VRI and CR1 of the bridge circuit to the predetermined value and closes its contact BRI to connect the differ for normal operation. ential ?eld winding GDF of the main generator Near the end of the cut stroke, the cut slow 25 across this generator. Thus, any voltage which down limit switch CSD is operated, This opens tends to build up in the generator due to residual its contacts CSDZ and closes its contacts CSDI. magnetism. is opposed by the action of the differ Closing of the contacts CSDI establishes a circuit ential ?eld winding and hence, no voltage or at through the contacts R5 and the contacts CSDI least insufficient voltage to cause operation of for the coil of the ?eldvweakening relay. This the motor is applied across the motor armature 30 relay immediately picks up and shunts the motor terminals. At the same time the ?eld weakening rheostat MR from the circuit and thus near the contactor FW is connected across the positive end of the cut stroke causes the planer motor and negative busses through a circuit which in separately excited ?eld MF to build up to full cludes the now closed contact CR2. This closes strength. The motor now slows down to full ?eld motor rheostat ‘ the contacts FW! and shunts the speed. At the same time, the coil FRCZ of the MR thereby applying that portion of the exciter ?eld weakening relay FR is deenergized and this voltage to the generator ?eld winding MF as relay drops out closing its contacts FRI, As the determined by the value of the permanent resistor planer platen reaches the extreme limit of its RP2, The initial direction of the planer platen out stroke, the cut limit switch is operated. This 40 can be selected by the cut and return push but deenergizes the coil of the cut relay C which drops tons, provided the planer table or platen is be out and closes its contacts C1 causing the return tween the limits of its travel. ‘ contactor R to pick up. This closes the contacts Pressing the out push button closes the back RI and R3 and reverses the polarity of the bridge contacts thereof and connects the coil of the con circuit. At the same time, the excitation of the trol relay CR through the stop push button across " bridge circuit is increased since the current sup the exciter busses 13+ and B—. This relay im plied to the bridge circuit is now controlled by mediately picks up, closing its contacts CR3, the setting of the return lead GRL of the gen CR5, CR6 and CR1, and opening its contacts erator rheostat and less resistance is in series CRI, CR2 and CR4. An instant thereafter. while with the bridge circuit. Also a circuit is estab the cut push button is yet depressed and the cir lished to prevent the ?eld weakening contactor cuit through the contacts CR1, now closed to the FW from dropping out. This circuit includes the coil of the return contactor R, open, the cut con contacts R3, generator return lead GRL, a por tactor C picks up since its coil is energized tion of the generator rheostat GR, the generator through a circuit across the positive and nega cut lead GCL, contact members R4, contact tive busses which includes the return push but members RSD2 of the return slowdown limit ton, the now closed contact CR6. back contact switch, contact members FRI and a coil of the R1 and the return limit switch RLS, This opens ?eld weakening contact FW. The motor is thus the circuit for the coil of the return contactor R accelerated in the reverse direction at full ?eld at contacts C1 thus preventing this contactor speed and when the generator voltage builds up from picking up and at the same time establishes sufficiently, the contact members FRI open and a second circuit for the coil of the ?eld weaken the planer platen is ‘driven in the return direc ing contact FW to hold this contactor in. which tion at a speed determined by the setting of the includes the contacts C3, C4. contacts CSD2 for rheostat leads GRL and GCL. the cut slowdown limit switch CSD and the con With the system just described, when the tacts FRI of the ?eld weakening relay FR which 5 speed of the planer motor is correct for the set are yet closed. Since the contacts CI and C3 of ting of the rheostat, the three sets of regulator the contactor C are now closed and the contactors generator ?elds balance each other and no volt CR3 of the control relay are closed, a voltage is age is generated in the generator by the action applied across the Wheatstone bridge circuit and of these ?eld windings. The voltage across the this voltage is determined by the setting of the output terminals of the bridge circuit is there lead GCL for the generator rheostat. The ?eld fore zero. Should the motor speed attempt to windings of the main generator are thus ener change, the regulator generator ?elds become un gized and current begins to flow through the balanced and the regulator generator generates a motor armature. The speed of the motor thus voltage causing current to ?ow through the builds up to its full ?eld speed and the motor is 75 2,408,217 Wheatstone bridge circuit in the direction re quired to hold the planer motor speed constant and in accordance with the setting of the rheo stats. Because the main generator is separately ex cited, variable voltage is easily obtained by means of a rheostat such as GR in series with the main 10 in an amount proportional to the motor armature terminal voltage, and the third separately excited ?eld Winding being connected in a circuit dis posed across the input terminals of said bridge circuit. 2. In a system of control for a motor, the com bination of, a generator having a pair of ?eld generator control ?eld circuits. Part of the speed windings, a motor directly connected to be ener range is obtained by varying the main generator gized :by the generator, an electrical bridge cir voltage and maintaining full ?eld on the planer cuit including said generator ?eld windings in motor, and the rest of the speed range is obtained opposite legs thereof and having electrical bridge by weakening the planer motor ?eld and main balancing elements in the remaining two opposite taining full voltage on the main generator. legs, a source of electrical energy for energizing Tests made with a system according to this invention indicated that measurable increases in 15 the bridge circuit, a regulating generator includ ing a series connected ?eld winding and three the speed of reversing the motor could be ob separately excited ?eld windings, said regulating tained. With a system connected according to generator including the series ?eld winding being that mentioned in the preceding pages, in which connected across the output terminals of said the control ?eld windings of the regulating gen bridge circuit, two of said separately excited ?eld erator were connected in series with the control windings being differentially connected to the ?eld windings of the main generator, it was found motor-generator circuit one winding being con that the motor required 1.76 seconds to accelerate nected to be energized in an amount proportional to ‘765 R. P. M. or 90 per cent of the full testing to the current in the motor-generator circuit and speed of 850 R. P. M. With the system provided the other winding being connected to be ener by this invention, the: motor accelerated to 765 gized by a voltage proportional to the motor ar R. P. M. in 1.25 seconds or 71 per cent of the time mature terminal voltage, and the third separately required with the standard connections. With excited ?eld winding being connected to be ener the connections according to this invention, it was found that the regulating generator was ac gized in an amount proportional to the Voltage ~ applied across said bridge circuit by said source of electrical energy. connections of the previously described prior art 30 3. In a system of control for a direct current system, the regulating generator current was neg motor, the combination of, a direct current motor ative for a large portion of the reverse cycle thus having an armature winding and a ?eld winding, tending to retard if not actually retarding the a main generator having an armature winding acceleration. and a pair of ?eld windings, circuit means elec Further comparative tests made with the for trically connecting the armature winding of said mer system and the system according to this generator and the armature winding of said motor invention, while reversing the motor from 1,200 in series circuit relationship, an electrical bridge R. P. M. in one direction to 1,080 R. P. M. in the circuit, said pair of generator ?eld windings being opposite direction, indicated that with the system connected, in opposite legs of the electrical bridge according to this invention, the motor accelerates circuit, a pair of electrical bridge balancing ele from rest to 1,080 R. P. M. in 77 per cent of the ments connected in the remaining opposite legs time required with the former system and re of the bridge circuit, a regulating generator hav verses within the speed limits mentioned in 81 ing an armature winding, a ?eld Winding in series per cent of the time required with the former with the regulating generator armature winding system. and three separately excited ?eld windings; cir The foregoing disclosure and the showings made cuit means connecting the armature winding and in the drawing are merely illustrative of the prin series ?eld winding of the regulating generator ciples of this invention and are not to be inter preted in a limiting sense. The only limitations 50 across the output terminals of said electrical bridge circuit, means for adjusting the resistance are to be determined from the scope of the ap of the series ?eld circuit of the regulating gen pended claims. erator such that the resistance line thereof is I claim as my invention: tangent to the initial straight line portion of the 1. In a system of control for a motor, the com no-load saturation curve of the regulating gen bination of, a generator having a pair of ?eld erator, circuit means connecting one of said sepa windings, a motor directly connected to be ener rately excited ?eld windings of the regulating gized by the generator, an electrical bridge cir generator to the motor-generator armature cir cuit including said generator ?eld windings in cuit to be energized in an amount proportional to opposite legs thereof and having electrical bridge the current ?owing in said circuit, circuit means balancing elements in the remaining two opposite differentially connecting another separately ex legs, a source of electrical energy for energizing -60 cited ?eld winding of the regulating generator the bridge circuit, a regulating generator includ with respect to the current energized ?eld wind ing a series connected ?eld winding and three ing, to the motor-generator armature circuit to be separately excited ?eld windings, said regulating energized in an amount proportional to the motor generator including the series ?eld winding being armature terminal voltage, circuit means conconnected across the output terminals of said necting the third separately excited ?eld winding bridge circuit, two of said separately excited ?eld of the regulating generator across the input ter windings being differentially connected to the minals of the electrical bridge circuit, and means motor-generator circuit, one winding being con for supplying direct current to said bridge circuit nected to be energized in an amount proportional and said motor ?eld winding. 70 to the current in the motor-generator circuit and tually forcing the acceleration, whereas with the the other winding being connected to be energized GEORGE E. KING.