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Aug. 27, 1946. G, E, KlNG ' I 2,406,426 CONTROL SYSTEM Filed 001;. 1.7. 1944 bu RU“ 8. » WITNESSES: Z24 8%. ' I ' ' INVENTOR . ’ GeofyeE/(fng M2. BY ATTbRNEY Patented Aug. 27, 1946 2,406,426 UNITED STATES-PATENT OFFICE 2.40am; CONTROL SYSTEM George E. King, Swissvale, Pa., assignor to West inghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania 1 Application October 1'7, 1944, Serial No. 559,067 6 Claims. (Cl. 172.—179) 2 The present invention relates to variable voltage control systems, and more particularly to 7 to provide a variable voltage drive in which ace curate adjustment of the maximum braking cur rents between certain limits is obtained. A further speci?c object of this invention is to a variable voltage drive for operating or auto matically controlling the operation of motors con nected to various types of mechanical loads. In certain of its aspects, this invention is related to a copending application of George E. King and William H. Formhals, Serial No. 559,068, ?led on the same date as this application and entitled Control system (W. E. Case 23,453), to a copend '. provide a variable voltage drive including means for independently adjusting both the accelerat ing and the braking current. Other objects and advantages will become more apparent upon a study of the following disclosure when considered in conjunction with the accom ing application of George E. King, Serial No. panying drawing, in which: The single ?gure thereof illustrates a variable and entitled Control systems (W. E. Case 23,417), voltage drive for a motor embodying the prin and to another copending application of George ciples of this invention. E. King, Serial No. 559,065, also ?led on the same 15 Referring now to the drawing, the variable date as this application and entitled Control sys voltage drive comprises a main motor M which is tems (W. E. Case 23,232). provided with a separately excited ?eld winding This invention provides certain improvements MSF and a regulating ?eld winding MRF. The in variable voltage control systems in which rotat motor separately excited ?eld winding MSF pro ing regulators are employed to regulate certain 20 vides the main excitation for the motor while the electrical quantities of the system, and the inven motor regulating ?eld winding MRF provides the tion as hereinafter described and as illustrated necessary corrective excitation for the motor in the drawing is speci?cally directed to a control under certain operating conditions to maintain for a direct-current motor embodying provisions the motor armature currents within permissible for limiting the motor armature current whether £25 limits. 559,066, ?led on the same date as this application the motor is operating normally as a motor or A main generator G is utilized to energize the motor M. Its armature winding is connected in series with the motor armature winding. The as a generator. main generator G is provided with a separately In the copending applications hereinbefore re 30 excited ?eld winding GSF which provides the ferred to, automatic speed regulation of the motor main excitation for the generator. It also has a is provided in conjunction with current limiting regulating ?eld winding GRF which under cer during regenerative periods when the motor is be ing overhauled by its mechanical load and driven protection. In this application, however, auto matic speed regulation of the motor is not shown in the interest of simplicity. It will be apparent, however, to one skilled in the art that automatic speed regulation in the variable voltage drive hereinafter disclosed may be obtained in a man ner similar to that disclosed in the said copend ing applications. It will also be apparent to one skilled in the art that this invention is not neces sarily limited to motor control, since automatic tain operating conditions of the system provides 35 the corrective excitation for the generator neces sary to maintain the motor armature currents within permissible values. The ?eld system of this generator also includes a commutating ?eld winding GCF and a di?erential ?eld GDF which is utilized near the end of the braking periods when the generator voltage has decreased to a certain minimum value to neutralize the residual voltage of the generator. A control generator C is utilized to provide the regulation of the maximum permissible currents supplied to an electrical load of substantially any corrective currents necessary to maintain the type susceptible of regulation may be had. 45 motor armature currents within permissible A principal object of this invention is to pro values. This control generator is provided with vide a variable voltage drive for a motor in which calibrating ?eld windings, designated CF! and the motor armature currents are automatically CFZ. These windings are differentially arranged, limited within permissible or desired values. that is, they produce opposite ?uxes and are sepa Another and more speci?c object of this inven rately employed to provide the desired excitation tion is to provide a variable voltage drive in for the control generator C during accelerating which accurate adjustment of the maximum ac and braking periods of the motor either in the celerating current between certain limits is ob forward or reverse directions of operation thereof. tained. A regulating ?eld winding for the control gen Yet another speci?c object of this invention is 55 erator designated CRF is excited by the drop 2,406,426 3 across the main generator commutating ?eld GCF. Thus the excitation of the regulating ?eld CRF depends upon the current circulating in the motor generator armature circuit. The control generator C is of the self-energiz ing type and is provided for this purpose with a shunt connected armature current energized ?eld winding C‘SF which is connected in series 4 erator. In its deenergized position circuits are established for energizing the calibrating ?eld winding CF2. In its energized position, the calibrating ?eld winding CFI may be energized. The series coil C5 of this relay is connected in series in the motor generator armature circuit. The shunt coil C6 is connected across a suitable source of potential which may, for example, be an exciter driven at a constant speed along with of the control generator. This resistor is so ad 10 the main generator and the control generator C but which is not shown in the interest of sim justed or selected in its electrical resistance value plicity. The ampere turns of the coil C5 and that the resistance line of this shunt ?eld cir C6 add when the motor is accelerating and sub» cuitpis tangent to the initial straight line por tract during braking periods of the motor when tion of the no~1oad saturation curve of the com trol generator. It then becomes the function of 15 the motor is operating in the forward direction. with a resistor SR across the armature terminals They subtract during acceleration and add dur either of the calibrating ?eld windings depend ing braking when the motor is operated in a ing upon which is energized, operating in con reverse direction. This relay may be adjusted to junction with the regulating ?eld winding to pick up with 10% of full-load current in the select the proper operating point of this gen erator along the tangent curves. With this ar 20 series coil C5 aiding the shunt coil and to drop out with 10% of full-load current in the series rangement and depending upon the net excita coil subtracting from the shunt coil. tion of the control generator, this generator may The VR or voltage relay is used to select the have an electrical output or rather a voltage out generator or motor regulating ?eld and to main put equal to the ordinate of any of the points of tangency of the curve of the resistance line 25 tain full generator voltage when braking due to strengthening of the motor shunt ?eld, the VR and the initial straight line portion of the no relay has its single or shunt coil connected across load saturation curves. The circuit is so ar the generator armature in series with the con ranged, that is, the ?eld windings of the ma tact TRI of the timing relay TR. It is adjusted chine are so arranged when connected, that the ampere turns from the regulating ?eld winding 30 to pick up near maximum generator voltage. It drops out due to the TR relay contact TRI open are always opposed to the ampere turns from the ing and deenergizing its coil. connected calibrating ?eld winding. The TR or timing relay controls the operation Suitable magnetic controllers are provided in of the voltage relay VR. It is an inductive time the system which respond to motor armature currents to connect the control generator in the 35 element relay with an inherent short time ele~ ment when both coils are deenergized. The main system such that the control generator does not or pickup coil C2 is connected in series with the effect the operation of the generator or motor LVB contact and the rheostat interlock RI across until the ampere turns of the regulating ?eld a suitable source of energizing potential. The winding CRF are greater than the ampere turns of the connected calibrating ?eld winding. The 40 rheostat interlock closes when an the generator ?eld portion of the resistor GR has been shorted ampere turns of the calibrating ?eld windings and remains closed during the motor ?eld weak can be preset by proper adjustment of the taps ening portion of the rheostat. This motor ?eld provided on the adjustable calibrating resistors weakening is accomplished over the motor rheo ARI and ARE, respectively, connected in series with the calibrating ?eld windings CFI and 45 stat section MR. The holding coil Cl is con~ nected in series with the motor regulating field CFZ. With this arrangement it is possible to ob so that it will hold the TR relay closed as long tain independent values of accelerating and as it it is energized by the control generator dur braking current. ing the braking cycle. The CR or current relay is used to connect the The FW or motor separately excited ?eld weak control generator armature to the generator and 60 ening relay is used to insert the motor rheostat motor regulating ?elds at the proper time. If resistor MR and its conducting segment MRI in it is desired to limit the accelerating current, for series with the motor shunt ?eld. Its shunt coil example, to 150% of full-load current, then the Cl is connected across the generator armature CR relay should pick up near this value. The CR relay has a series coil C4 connected in series 5.5 in series with the LV‘! contact. The other is connected across the positive and negative ter relation with the motor and generator armatures minals indicated which again may be eXciter and a shunt coil C3 excited according to the elec buses, in series with either the F! and con trical output of the control generator. The tacts or the RI and R2 contacts depending upon ampere turns of these coils are in opposition when the control generator voltage output is due to the 60 which of the F and R relays are energized and the generator rheostat GR or the contacts VRI ampere turns of either of the calibrating ?eld if the VR relay is energized. This relay FW is windings CF! or CR2 being greater than those adjusted to pick up at a different generator volt of the regulating ?eld winding and the ampere age which may, for example, be somewhat less turns of the two coils of the CR relay are cumu lative when the voltage output of the control 65 than that required to pick up the VR relay. Energization of both coils is required for pickup. generator C is due to the ampere turns of the The FW relay drops out when the coil C3 thereof regulating ?eld Winding being greater than those is deenergized or the generator voltage decreases of either of the calibrating ?eld windings. Un su?lciently. der this last mentioned condition, the CR relay The LV or low voltage relay is used to energize picks up and establishes certain circuits for en 70 the generator shunt ?eld GSF in conjunction ergizing the generator regulating ?eld or the with either of the forward or reversing contactors motor regulating ?eld or under certain operating F and R depending upon whether the forward or conditions both of these ?elds. ' reversing push button Fwd or Rec is depressed. The RC or reverse current relay is used to select , This relay in a sense e?ects the operation of the the correct calibrating ?eld ‘of the control gen 2,406,426 5 timing relay TR and the ?eld weakening relay FW. Energization of this relay is initiated by depressing either the forward or reverse push 6 potential adjacent the contacts RI through the contact members Fl, the field winding GSF, the contact members F2 through the generator rheo button. stat GR to a point of negative potential. Thus The motor and generator ?eld rheostats, desig Or a voltage is generated by the generator, and this nated respectively, MR and GR, together with the voltage is applied across the motor armature rheostat interlock RI are controlled from a single terminals. Since the motor ?eld winding MSF handle or handwheel H. The arrangement of is connected from its point of positive potential the motor and generator rheostats is such that through the back contacts FWZ of the ?eld weak it is ?rst necessary to strengthen the generator 10 ening relay to a point of negative potential, this separately excited ?eld GSF before weakening ?eld winding is excited at its maximum voltage. the motor separately excited ?eld and vice versa. The motor thus accelerates under the in?uence When the generator rheostat GR, is completely of a fully excited ?eld winding. The contact shorted, the contact arm CA for the rheostat members Fl and F2 also complete an energizing interlock engages the arcuate conducting segment 15 circuit for the coil C8 of the ?eld weakening and thus completes an energizing circuit for the relay .FW. As shown, this ?eld is connected in coil C2 of the timing relay TR. parallel with the generator ?eld GSF. Hence, The forward and reversing contactors respec~ its energization will be understood without fur tively designated F and R are required to reverse ther tracing of the circuit through the Fl and the main generator separately excited ?eld GSF 20 F2 contacts. The contact members F3 open while and the motor regulating ?eld MRF. It is neces the contact members F4 close. Closure of the sary to reverse the generator ?eld GSF in order contact members F4 inserts a large portion of to reverse the direction of rotation of the motor the adjustable calibrating rheostat ARI in series armature. The current through the motor ar with the control generator calibrating ?eld CFl. mature reverses and consequently is in the re 25 This ?eld winding is arranged to produce a mag verse direction through the control generator netic ?eld linking the rotor of the control gen regulating ?eld C-RF. The polarity of the con erator which is in opposition to that produced trol generator armature is, therefore, reversed, by the regulating ?eld CRF for forward opera and it is necessary to reverse the motor regulat tion of the motor. Since the excitation of the ing ?eld to keep its polarity correct with respect 30 calibrating ?eld CF! upon closure of the contact to the motor shunt ?eld. Analogous considera members F4 is lower than that obtained when tions apply to the coil 08 of the FW relay. the contact members F3 are closed by reason of Starting of the system for operation of the the setting of the rheostat taps, the currents in motor in the forward direction is accomplished the motor armature circuit are limited at a lower by pressing the forward push button Fwd. This 35 value during acceleration of the motor. Such immediately closes the back contacts of the for a feature, for example, may be desirable if the ward push button which in conjunction with the motor is driving a high inertia mechanical load stop push button now closed completes an ener through a suitable friction drive. Under such a gizing circuit for the coil of the low-voltage re condition, it is desirable to accelerate this high lay LV. This relays picks up opening its contact 40 inertia mechanical load without causing slipping members LVI and disconnecting the generator of the friction drive. Hence, it is desirable to differential ?eld from its position across the gen limit the motor armature currents more as a erator armature terminals. The contact mem means to prevent excessive accelerating torque bers LVZ close and complete an energizing cir of the motor than to limit the motor armature cuit for the coil C1 of the ?eld weakening relay 45 currents to prevent damage to the electrical cir FW across the main generator armature. The cuits. Both the accelerating and braking re contact members LV3 close and in conjunction sistors, of course, may be adjusted to permit high with the stop push bottom complete a holding armature currents during initial periods of ac circuit for the coil of the low-voltage relay LV. celeration and braking if desired. The contact When the contact members LV4 close, an ener members F5 and F6 when closed function to par gizing circuit for the coil of the forward con tially complete energizing circuits for the motor tactor F is completed. This circuit includes the regulating ?eld winding MRF across the control reversing push button designated Rev, the con generator. Th'ese contact members determine tact members LV4, the coil of the forward con the polarity of the motor regulating ?eld for tactor F and the back contact members R8 of forward operation of the motor to provide the the reversing contactor. The contact members cumulative relationship of the motor ?elds. The LV5 also close. These contact members normal contact members F1 in conjunction with the ly complete an energizing circuit for the coil of contact members VRll when closed, establish a the reversing contactor. This circuit, however, holding circuit for the coil of the F contactor at this stage of operation is not completed, since 60 which is independent of the reversing push but~ the forward push button is depressed and its ton. This eliminates any possibility of reversing back contact members which are essential to this the motor at high generator voltages. It is first energizing circuit are open. Closure of the con necessary to depress the stop button and deen tact members LV6 completes a partial energizing ergize the LV relay thus decelerating the motor circuit for the coil C2 of the TR relay. This cir 65 under the influence of its ?eld windings. This cuit, however, ‘is open at the rheostat interlock provides a decelerating cycle substantially the RI, since the conducting segment and its con~ reverse of the accelerating cycle. This circuit, tact arm CA are not in mechanical engagement. however, is not established until the generator Energization of the LV relay as previously de voltage has approached its maximum values since scribed energizes the coil of the F relay for the the VR relay does not pick up until such a volt assumed forward operating condition of the mo age obtains. Opening of the contact members tor. This relay picks up and closes its contact F8 prevents the coil of the reversing contactor ‘ members Fl and F2. This establishes an ener R from being energized when the forward push gizing circuit for the generator ?eld winding GSF button is released and its back contact members which may be traced from the point of positive closed. 2,406,426 7 8 Since the motor is now operating at its full field speed, its speed of rotation is fairly low. If it is desired to increase the speed of the motor, contact members VR3 ClOSe and shunt the con tact members FWI which originally connected the motor regulating ?eld across the control genera tor. Thus irrespective of the position of the con tact members FWI the motor regulating ?eld is connected across the control generator. Under the present operating conditions, the motor regu lating ?eld now functions by itself to limit the motor armature currents. The contact members VH4, in conjunction with the contact members Fl, form a holding circuit for the coil of the F contactor which is independent of the position of either of the forward or reversing push but tons. As the motor approaches the preestab lished speed the accelerating armature current decreases and the current relay CR drops out due to the fact that the excitation of the control generator is now predominantly due to the cali brating ?eld CFI. This disconnects the control generator from the motor regulating ?eld. The motor armature current decreases to the value required to drive the load at the selected speed. Should the operator desire to increase the speed of the motor and thus turn the rheostat further in the counterclockwise direction to insert more of the motor rheostat in series connection with the motor shunt ?eld winding, the excitation of the motor separately excited ?eld MSF will be weakened thereby causing the motor to acceler ate. If under this accelerating condition the mo tor armature current increases su?iciently, the CB relay will again close and the control gener~ ator will act on the motor regulating ?eld MRF the handle or handwheel H may be rotated in a counterclockwise direction. This will shunt in creasingly larger portions of the generator rheo stat GR to increase the excitation of the gener ator ?eld GSF and hence increase the electrical output of the main generator. Thus, the motor will tend to increase its speed. Eventually when 10 all of the generator ?eld rheostat is shunted, the contact arm CA closes the rheostat interlock and completes an energizing circuit for the pick-up coil C2 of the timing relay This timing relay is thus energized and closes its contact 15 members TRI, thus connecting the coil of the voltage relay VR across the generator armature circuit. The voltage relay, however, may not at this time pickup, since the generator voltage is more than likely not up to or closely approach ing its maximum value. 20 As the generator volt age builds up, it causes increasingly larger cur rents to ?ow through the motor armature, the control generator regulating ?eld CRF and series coils C5 and Ct. respectively, of the RC and CR 25 relays. As soon as the current builds up, for ex ample, to 10% of the motor full-load amperes, the reverse current relay RC picks up and ener gizes the calibrating ?eld CFI of the control gen orator. If the adjustable calibrating rheostat A35 has been set to limit the accelerating cur rent to say 150% of full-load current, then the current relay CR will close when the current approaches 153% of the full-load current since then the coil C3 then aids the coil C5. The relay CR upon closing connects the generator regulat ing ?eld GRF to the control generator armature through the back contact VRZ and the front con tact CR1, respectively, of the VR. and CR relays, so that the ampere turns of the regulating ?eld GRF are in opposition to the ampere turns of the shunt ?eld GSF. This action of the control gen erator on the main generator regulating ?eld prevents the generator voltage from changing too rapidly and so limits the accelerating current to 1.50% of the motor full-load current. As the motor speed approaches the full ?eld speed, the l'ilW relay picks up and closes its contact members FW I. This, through the medium of the contact members F5 and F6, connects the motor regulat ing ?eld MRF across the control generator to be energized in the proper direction. When the back contact members FWZ open, these contact members which’ normally shunt the motor rheo to limit the current as hereinbefore described. Assume the operator desires to decrease the motor speed and turns the rheostat to a low-speed setting at reduced generator voltage. The motor ?eld is strengthened causing the motor to regen erate through the‘ generator. The motor gener ator armature current reverses and the ampere turns of the coil C5 of the reverse current relay are now in opposition to those of the shunt coil C6. This causes the RC relay to drop out. The control generator calibrating ?eld CFI is thus ' deenergized at the contacts ROI and the cali stat from the circuit now insert the motor rheo stat in series with the motor shunt ?eld. The motor regulating ?eld by reason of the contact members F5 and F6 is now connected to brating ?eld CFZ is energized at the contacts RC2 functioning in conjunction with the closed back contacts R3. Most of the adjustable calibrat ing resistor ARZ is shunted by the contacts R3 to thus provide, for example, a higher permissible decelerating current than the previous accelerat ing current. When the braking current ap proaches the limit established by the setting of the adjustable calibrating rheostat ARZ, the CR relay is su?iciently energized to close and con» nects the control generator to the motor regu lating ?eld. Since the voltage of the control gen the control generator so that its ampere turns are cumulative with the ampere turns of the motor shunt ?eld MSF. The action of the con trol generator on the motor regulating ?eld now erator is now reversed due to the reversal of motor armature current, current is circulated through the motor regulating ?eld so that its ampere turns are in opposition to the ampere turns of the motor separately excited ?eld, there~ prevents the motor ?eld flux from changing too by preventing the motor ?eld ?ux from building rapidly and so limits the accelerating current to the prcestablished value. It should be noted that now the control generator is connected to both up too rapidly and so limits the braking current. Since the rheostat was turned to a speed set This closes the con ting at reduced generator voltage, the rheostat interlock RI opened and deenergized the main or pickup coil on the relay TR. The short in herent time element of the TR relay prevents it tact members VRI, thereby shunting the genera from opening immediately, thus holding it closed tor’ rheostat from the circuit and exciting the generator separately excited ?eld GSF at its maximum voltage. The contact members VRZ open and disconnect the generator regulating ?eld from the control generator at the same time the 75 until the control generator has sufficient time to the motor and generator regulating ?eld. As the generator voltage approaches its maximum value, the voltage relay VR closes. generate a current in the reverse direction through the motor regulator ?eld and the TR holding coil C l. The TR, relay is now held closed by its holding coil. As the motor approaches its ‘2,406,426 full field’ speed, the braking current decreases until the relay CR opens and deenergizes the mo tor regulating ?eld and the TR relay holding coil. The TR relay drops out deenergizing the voltage relay. The voltage relay upon opening .10 tations are to be determined from the scope of the appended claims. ‘I claim as my invention: 1. In a variable voltage drive, the combina tion of, a motor, a main generator for supply ing electrical current to the motor, a regulating ?eld winding for the motor, a regulating ?eld winding for the generator, an auxiliary genera tor, ?rst electrically operated means responsive to the current of said motor and to the electrical output of said auxiliary generator, second elec— lating ?eld. The motor regulating ?eld, how trically operated means responsive to the elec~ ever, yet remains connected through the contacts trical‘output of said main generator, third elec FWI if the relay FW is yet su?‘iciently energized trically operated means responsive to the electri by the main generator voltage. The braking cur rent again increases due to the ?eld flux of the 15 cal output of said main generator, said ?rst means when electrically operated cooperating with said generator decreasing. The current relay CR. thus second means when not electrically operated to again picks up connecting the control generator electrically connect the regulating ?eld winding to the generator regulating ?eld. Since the con of the main generator to the auxiliary generator, trol generator voltage is still reversed, current is circulated through the generator regulating ?eld 20 said third means when electrically operated co of its back contact members VRI inserts a portion of the rheostat GR in series with the generator shunt ?eld and at its contact members VRZ and VR3 transfers the control generator circuit from the motor regulating ?eld to the generating regu operating with said ?rst means when electrically operated to connect the regulating ?eld winding direction as those of the separately excited ?eld for the motor to said auxiliary generator, and before it was deenergized. The action of the con said second means when electrically operated dis trol generator is, therefore, to slow down the decay of ?ux in the generator and so limit the 25 connecting said main generator regulating ?eld winding from said auxiliary generator and elec braking current. As the motor approaches the trically connecting the motor regulating ?eld lower set speed, the braking current decreases. winding to the auxiliary generator independently The CR relay drops out deenergizing the gener of said third means. ator regulating ?eld. The motor armature cur‘ 2. In a variable voltage drive, the combination rent reverses itself and settles down to the load 30 of, a motor, a main generator for supplying elec current in a direction to operate the motor in the trical current to the motor, an auxiliary gen forward direction'. erator, and magnetic controller means respon~ Now assume the motor is running at some weak sive to the electrical quantities of said motor and ?eld speed and the stop push button is pressed. so that its ampere turns are now in the same The LV relay is deenergized and drops out deen 35 said main generator for electrically connecting said auxiliary generator during operation of said ergizing the main coil of the TR relay as well as motor from zero speed to running speed, to ef the coil of the F contactor. As the generator fect a control of ?rst, said main generator, sec voltage declines the W relay drops out shorting ond to e?ect a control of both said main genera out the portion of the rheostat in series with the motor ?eld MSF at its back contacts FW 2. The 40 tor and said motor and third, to effect a control only of said motor. action is the same as though the rheostat were 3. In a variable voltage drive, the combination turned back as previously described. However, of, a motor, a main generator for supplying elec when the VR relay opens it now deenergizes'the trical energy to the motor, means for controlling generator ?eld GSF completely, since, substan tially at this time the forward contactor F drops 45 the speed of said motor, an auxiliary generator, out opening its contact members FI and F2 re quired to complete the energizing circuit for the ?rst means responsive to the motor current and the electrical output of the auxiliary generator, second means responsive to the main generator voltage, third means responsive to the main gen braking current and the generator voltage also 50 erator voltage, said ?rst means and said second means cooperating under certain operating con decrease. The current relay CR drops out de ditions of said motor to connect said auxiliary energizing the generator regulating ?eld and then generator to said main generator, said ?rst, sec~ the braking relay BR drops out connecting the 0nd and third means cooperating under different generator differential ?eld across the generator armature, since the contacts LVI are now closed, 55 operating conditions of said motor to connect said auxiliary generator to both said generator and thus bringing the motor to rest and preventing it said motor and said ?rst and second means co from creeping. operating under other operating conditions of The reversing cycle of the system is substan said motor to connect said auxiliary generator tially the same as that described in conjunction with forward operation, the reversing contactor 60 only to said motor. 4. In a variable voltage drive, the combination R as previously mentioned reversing the connec of, a motor, a main generator for supplying elec tions of the main generator separately excited trical energy to the motor, an auxiliary generator ?eld GSF and the motor regulating ?eld. The for effecting a control of the motor current, ?rst coil C1 of the ?eld weakening relay FW is also 65 means for selectively exciting the auxiliary gen reversed in polarity ‘during reversing operation of erator with a predetermined electrical quantity for the motor, since the motor armature voltage is accelerating and braking of said motor, second reversed. Thus to make the action of the two means for exciting the auxiliary generator ac coils cumulative, it is necessary to reverse the coil cording to the motor current in opposition to C8 to excite it in the same direction as the re 70 either of the ?rst mentioned selectively obtained versely excited coil C1. excitations, and means operable when the ex The foregoing disclosure and the showings citation resulting from said second means pre dominates that of said ?rst means for connect made in the drawing are merely illustrative of the ing the auxiliary generator to said main gen principles of this invention and are not to be interpreted in a limiting sense. The only limi 76 erator. generator ?eld GSF. The motor now continues to brake down to near zero speed. Meanwhile the 11 2,406,426 5. In a variable voltage drive, the combination of, a motor, a main generator for supplying elec trieal energy to the motor, an auxiliary generator for effecting a control of the motor current, means for exciting said auxiliary generator in accord 12 of, a motor, a main generator for supplying elec trical energy to the motor, an auxiliary genera tor for effecting a control of the motor current, means for exciting said auxiliary generator in ac cordance with the motor current, means for pro ducing a standard excitation in opposition to the excitation according to the motor current, and means operable when the excitation according to the motor current is the predominating excitation ance with the motor current, means for produc ing a standard excitation in opposition to the excitation according to the motor current, and means operable when the excitation according to the motor current is the predominating exci 10 for selectively connecting the auxiliary generator tation for connecting the auxiliary generator to to the main generator, to both the main genera the main generator. tor and the motor and only to said motor. 6. In a variable voltage drive, the combination GEORGE E. KING.