US2355321A - Motor controller - Google Patents

Description

Aung. 8, 1944. J. M, NEWMAN 2,355,321

MOTOR CONTROLLER VVVVV M 3 M 1N Aug- 8, 1944 QJ. M NEWMAN 2,355,321

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Patented Aug. 8, 1944 UNITED STATES PATENT OFFICE MOTOR CONTROLLER John M. Newman, Wauwatosa, Wis., assignor to Cutler-Hammer, Inc., MilwaukeefWis., a corporation of Delaware Applicationl October 14, 1942, Serial No. 461,961

9 Claims. Ik(31. 20d- 3) stationary die, a traveling carriage having a plier l associated therewith for gripping the stock to be drawn, and a motor driven chain to which the carriage is adapted to be hooked for operation in a direction to draw the stock through the die. It is common practice to employ a compound wound direct current motor for operating the chain and to operate such motor at its base speed or in other words, its full iield speed when the carriage is disconnected from the chain. Thus when the carriage is hooked to the chain for a drawing operation the same is quickly ac-- celerated, and in practice it has been found that with the chain motor operating at base speed the rate of acceleration of the carriage is such that the plier is frequently jerked loose from the stock at the beginning of the draw.

The present invention has among its objects to provide an improved controller for machines of the aforesaid character which overcomes the aforementioned objection.

Another object is to provide a controller for machines of the aforesaid character which provides for idling operation of the chain motor at a speed lower than base speed to thereby provide for cushioning of the shock which occurs at the beginning of a drawing operation.

Another object is to provide a controller of the aforesaid character having improved means for accelerating the chain motor to a predetermined maximum drawing speed upon starting of a drawing operation.

Various other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate an ernbodiment of the invention which will now be described, it being understood that the controller illustrated is adapted for use in connection with machines of various types other than that shown in the drawing, and that the same is susceptible of modification without departing from the spirit and scope of the appended claims.

In the drawings,

Figure 1 is a straight line diagrammatic representation of a controller embodying the invention, and

Fig. 2 is a key illustration of the switches and relays embodied in Fig. 1.

The illustration of the switches and relays in Fig. 2 shows them with their coils and contact members disposed Vin horizontal alignment with their positions in the straight line circuits of Fig. 1 so that thereader may readily determine the position of the coil and contact members of any switch or relay in the straight line circuit. The reference characters `also serve as a guide to the relation between the operating coils of the switches and relays and their associated contacts. For example, switch M is provided with an operating coil MI and contacts M2, M3 and M4.

The switches and relays `are named as follows:

-M-Main switch AS-Armature shunt switch 3A Armature accelerating switches 4A Field accelerating switches lCT 2CT FA FD LR--Load relay VR-Voltage relay UV-Under voltage relay ICR 2CR} Control relays SCR Referring now to Fig. 1, the same illustrates a draw bench of a conventional type including a stationary die block I, a traveling carriage 2 having a plier 3 associated therewith for gripping the stock to be drawn through the die, and a motor driven chain 4 which is adapted to be engaged by a releasable hook 5 associated with carriage 2 to move said carriage away from the die block l for a drawing operation. The draw bench is also provided with a motor drive (not shown) which is adapted upon disconnection of hook 5 from chain 4 to effect return of carriage 2 from the end of its stroke to starting position.

Chain d is driven by a direct current motor i3 having an armature A, a series field winding SF and a shunt eld winding SHF. The supply circuit for said motor is indicated by lines Ll, L2 and main switch M provides for connection of armature A and series field Winding F across said lines in series with a resistance RI. Accelerating switches I A, 2A, 3A and 4A provide for connection of resistancesRZ, R3, R4 and R5, respectively, in parallel with resistance RI and accelerating switch 5A provides for exclusion of all of the resistances Rl to R5, inclusive, from Timing relays the armature circuit of the motor. The armature shunt switch AS provides for connection of a resistance R1 across the terminals of armature A. The shunt field winding SHF is permanently connected across lines LI, L2 in series with a speed regulating rheostat 'I and field accelerating switches IFA and 2FA provide for shunting of resistance sections rl and r2 of said rheostat.

Main switch I and certain other of the aforementioned switches are controlled by the under voltage relay UV which in turn is controlled by a snap switch marked Safe On, a normally open push button switch marked Start and a normally closed push button switch marked Stop The armature shunt switch AS is controlled by the voltage relay VR and armature accelerating switches 2A to 5A and field accelerating switches IFA and 2FA are controlled by the timing relays ICT, 2CT, FA and FD, control relays ICR, 2CR and SCR and by the load relay LR. The controller also includes a normally open push button marked Fastf a normally closed push button marked Slow and two limit switches LSI and LS2 which are associated with the draw bench carriage 2.

The aforementioned switches and relays with the exception of voltage relay VR and load relay LR are operated by current from lines L3 and L4 which are energized from lines LI, L2 through the medium of knife switches KSI and KS2. The voltage relay VR is provided with an operating winding VRI which is permanently connected across the terminals of armature A. Load relay LR is provided with a series operating winding LR! and a shunt operating winding LRZ. series operating winding LRI is connected in the armature circuit of the motor and the current in the shunt operating winding LRZ is controlled by two adjustable rheostats S and Ill and a plurality of resistances r3, r4 and T5. The adjusting arm of rheostat 9 is mechanically connected to the adjusting arm of speed regulating rheostat 1 and winding LR2 is connected between the adjusting arms of rheostats 9 and I0. One terminal of rheostat 9 is connected to line L3 and the opposite terminal thereof is connected to line L4 through the medium of resistance r3. One terminal of rheostat I is connected to line L3 through the medium of resistance r4 and the other terminal thereof is connected to line L4 through the medium of resistance 1'5. Winding LR2, rheostats 9 and I0 and resistances r3, r4 and f5 form a Wheatstone bridge circuit by means of which winding LR2 is controlled to assist the series winding LRI of the load relay upon setting of speed regulating rheostat I in low speed positions and to oppose said series winding upon setting of said speed regulating rheostat in high speed positions.

Each of the relays FA, FD, ICT and 2CT has a condenser C and a resistance r6 connected in series across the terminals of its associated operating winding. Each of said condensers is charged upon establishment of the energizing circuit for its associated relay and each acts to delay dropping out of its associated relay upon interruption of the energizing circuit therefor.

The function and operation of the aforedescribed controller and the circuit connections thereof will now be more fully described.

Upon closure of knife switches KSI and KS2 supply lines L3 and L4 are energized and field timing relay FA, armature timing relays ICT and 2CT and control relay 3CR all respond, the operating windings thereof being energized from The ` contacts 5A5.

said supply lines by the following circuits. Operating winding FAI of timing relay FA is energized by a circuit extending through control relay contacts ICR4 and accelerating switch Operating winding ICTI of timing relay ICT is energized by a circuit extending through accelerating switch contacts IA4. Operating winding 2CTI of timing relay 2CT is energized by a circuit extending through accelerating switch ccntacts 3A3. Operating winding 3CRI of control relay 3CR is energized by a circuit extending through control relay contacts ICRB and timing relay contacts FD4.

Assuming now that motor 6 is to be started, hook 5 of the draw bench carriage 2 is disconnected from chain 4 and the safe on switch is moved into closed position. The start push button is then momentarily depressed and upon closure thereof field accelerating switches IFA and 2FA and under voltage relay UV respond, the operating windings thereof being energized from lines L3, L4 by the following circuits. Operating winding IFAI of field accelerating switch IFA is energized by a circuit extending through field timing relay contacts FA3 and control relay contacts ICR2 and 2CR2, Operating winding 2FA! of field accelerating switch 2FA is energized by a circuit extending through contacts FAZ and FDZ of timing relays FA and FD. Operating winding UVI of under voltage relay UV is energized by a circuit extending through contacts ICT2 and 2CT2 of timing relays ICT and 2CT. Upon release of the start push button the aforedescribed energizing circuits for eld switches IFA and 2FA and under voltage relay UV are maintained through the medium of the under voltage relay contacts UVZ. Upon response of the under voltage relay UV switch M responds, the same being energized by a circuit extending from line L3 through the safe on switch, through contacts UV3 of said under voltage relay, through accelerating switch contacts 2A3, 4A3 and 5A3 and through the operating winding MI of said main switch to line L4.

Upon response of the field accelerating switches IFA and 2FA the shunt field winding SHF of the motor is connected directly across lines LI, L2 and upon response of main switch M, armature A and series field winding F are connected across lines LI, L2 in series with resistance RI. Motor 6 is thus started and at a predetermined low counter E. M. F. of the motor operating winding VRI effects response of the voltage relay VR. Voltage relay VR in responding establishes an energizing circuit for the armature shunt switch AS extending from line L3 through the safe on switch, through under voltage relay contacts UV3, through contacts 5A4 and IA3 of accelerating switches 5A and IA, through the Voltage relay contacts VR2, and through the operating winding ASI of said accelerating switch to line L4. Upon response of armature shunt switch AS the contacts AS2 thereof connect resistance rl across the terminals of armature A.

Upon starting of the motor, winding LRI is energized to effect response of the load relay LR. With carriage 2 of the draw bench in starting position limit switch LS2 is closed, and upon response of the armature shunt switch AS control relay ICR is energized by a circuit extending from line L3 through the limit switch LS2,.through the armature shunt switch contacts AS4, through field accelerating switch contacts 2FA3, through the slow push button and through the operating Winding ICRI of said control relay to line L4. Upon response of control relay ICR a maintaining circuit for its operating winding ICRI is established through the medium of its contacts ICR'I, the load relay contacts LRB and the slow push button. Also control relay ICR in responding interrupts the aforedescribed energizing circuit for control relay 3CRI by opening of contacts ICRS.

The resistances RI and R'I are designed to provide for operation of motor 6 at a relatively low speed, as for example 50% of full field or base speed when the motor is merely idling to operate chain 4. These resistances are also designed to provide a steep torque characteristic so that even a relatively light load on the motor will cause an appreciable drop in the speed thereof.

Assuming now that hook 5 of carriage 3 is dropped into engagement with chain 4 to start a drawing operation. Carriage 3 is almost instantly jerked to a speed corresponding to the speed of the chain and when the stock is drawn into the die motor E is subjected to an increased load which reduces the speed thereof. If the load imn posed on the motor at the start of the draw is extremely light the motor does not slow down appreciably, but for heavier loads there is a very considerable drop in the motor speed. As will now be set forth, the limit switch LSI operates upon extremely light load conditions at the start of a draw to initiate acceleration of the motor and under heavier load conditions at the start of a draw voltage relay VR operates to initiate acceleration of the motor.

Voltage relay VR is calibrated to drop out upon a relatively small reduction in the speed of the motor at the start of a draw and upon dropping out of said relay the contacts VRZ thereof interrupt the aforedescribed energizing circuit for the armature shunt switch AS. Upon deenergization of the armature shunt switch AS the contacts AS2 thereof open to exclude resistance R1 from the motor circuit and the contacts AS3 thereof close to establish an energizing circuit for the armature accelerating switch IA extending from line L3, through control relay contacts ICR3, through main switch contacts M4, through shunt switch contacts ASS and through the operating winding I AI of said accelerating switch to line L4. v

If the load imposed on motor 6 at the start of a draw is extremely light, voltage relay VR remains in its attracted position and when the draw bench carriage 2 has moved a short distance away from die block I limit switch LSI closes to establish an energizing circuit for the armature accelerating switch IA extending from, line L3, through control relay contacts IC3, through main switch contacts M4, through the limit switch LSI and through the operating winding IAI of said accelerating switch to line L4. Upon response of accelerating switch IA the contacts IA3 thereof open to interrupt the aforedescribed energizing circuit for armature shunt switch AS.

Thus under either extremely light or heavy load conditions at the start of a draw resistance R1 is excluded from the motor circuit and resistance R2 is connected in shunt with resistance RI by accelerating switch contacts IA2 to effect acceleration of the motor. As will now be set forth, upon response of accelerating switch IA armature accelerating switches 2A, 3A, 4A and 5A close and field accelerating switches IFA and 2FA open in the order named to accelerate the motor and bring the same up to a selected maximum. drawing speed determined by the setting of rheostat 1. Upon response of accelerating switch IA `the contacts IA4 thereof open to interrupt the aforedescribed energizing circuit for the operating winding ICTI of timing relay ICT and said relay then drops out after a given period of delay caused by the discharge of its associated condenser C. Upon dropping out of relay ICT contacts ICTS close prior to contacts ICT4 and said contacts in closing establish energizing circuits for the operating windings 2AI and 3AI of accelerating switches 2A and 3A through the medium of control relay contacts ICR3 and main switch contacts M4. Accelerating switch contacts 2A2 and 3A3 are thus closed sequentially to connect resistances R3 and R4 in parallel with resistances RI and R2.

If voltage relay VR has dropped out the same recloses at some point during acceleration of the motor and upon reclosure thereof establishment of the aforedescribed energizing circuitfor lthe operating winding ASI of armature shunt switch AS is prevented by opening of contacts IA3 of accelerating switch IA. Upon response of accelerating switch 2A the aforedescribed energizing circuit for the operating winding MI of main switch M is interrupted by opening of accelerating switch contacts 2A3 and said main switch operating winding is then maintained energized through the medium of main switch contacts M3.

Upon response of accelerating switch 3A the contacts 3A3 thereof interrupt the aforedescribed energizing circuit for timing relay 2CT and said relay then drops out after a given period of delay due to the discharge of its associated condenser C. Upon dropping out of relay 20T contacts 2CT4 close prior to closure of contacts 2CT3. Closure of contacts 2CT4 establishes an energizing circuit for the operating winding 4AI of accelerating switch 4A through the medium of control relay contacts ICR3 and main switch contacts M4. Closure of timing relay contacts 2CT3 establishes an energizing circuit for the operating winding 5AI of accelerating switch 5A through the medium of the safe on switch, under voltage relay contacts UV3, control relay contacts 3CR2 and eld accelerating switch contacts (FAS. Accelerating switch contacts 4A2 and 5A2 are thus closed in the order named to first connect resistance R5 in parallel with resistances RI, R2, R3 and R4 and to thereafter exclude all of said resistances from the armature circuit of the motor.

Upon starting of the drawing operation limit switch LSZ is opened by the carriage 2 and upon opening of said limit switch the operating winding ICRI of relay CR is maintained energized through the medium of load relay contacts LR3, control relay contacts ICRI and the slow push button. With relay ICR energized the aforedescribed energizing circuit for timing relay FA is controlled by accelerating switch contacts 5A5. Thus upon response of accelerating switch 5A timing relay FA drops out after a given period of delay due to the discharge of its associated condenser C. Upon dropping out of timing relay FA the contacts FAZ and FA3 thereof open in the order named to interrupt the aforedescribed ener-.- gizing circuits for the operating windings ZFAI and IFAI of iield accelerating switches 2FA and IFA. Contacts 2FA2 and IFA2 of the eld accelerating switches are thus opened to include speed regulating rheostat I in the shunt eld circuit of the motor.

From the foregoing it is apparent that upon starting of a drawing operationr resistances R2,

R3, R4 and R5 are successively connected in parallel with resistance RI and thereafter all of said resistances are excluded from the armature circuit for acceleration of the motor. Also upon exclusion of all of the armature resistances field regulating rheostat 'I is included in the shunt field circuit of the motor to bring the motor up to a selected maximum drawing speed determined by the setting of said rheostat.

Upon dropping out 0f timing relays ICT and 2CT during acceleration of the motor the contacts ICTZ and 2CT2 are opened and the operating winding UVI is then maintained energized through the medium of a resistance r1 to hold under voltage relay UV in closed position. Upon dropping out of timing relay FA, during acceleration of the motor, contacts IFA3 are opened and the operating winding 5AI is then maintained energized through the medium of resistance f8 to prevent dropping out of accelerating switch 5A. Upon dropping out of field accelerating switch IFA the contacts IFA4 and control relay contacts ICRE establish an energizing circuit for the operating winding FDI of timing relay FD. Timing relay FD in responding closes its contacts FD3 to establish an energizing circuit for the operating winding ZCRI of control relay 2CR and upon response of said control relay the operating winding FAI is energized through the medium of contacts 2CR3 to effect reclosure of timing relay FA. Upon reclosure of timing relay FA, control relay contacts ICR2 and 2CR2 are in open position to prevent energization of the operating4 winding IFAI of accelerating switch IFA and the timing relay contacts FD2 are in open position to prevent energization of the operating winding 2FAI of accelerating switch ZFA.

Assuming now that the drawing operation is completed or that the stock breaks or slips out of engagement with pliers 3 during the drawing operation, the motor becomes unloaded and load relay LR drops out to interrupt the aforedescribed energizing circuit for control rela-y ICR by opening of its associated contacts LR3. The current in the shunt operating winding LR2 of load relay LR is controlled as hereinbefore set forth and said winding acts in conjunction with the series winding LRI to insure dropping out of said relay at various running speeds immediately upon unloading of the drawing mechanism and u regardless of the fact that there may be only a slight reduction in load when the stock breaks or leaves the die. Depression of the slow push button also interrupts the aforedescribed energizing circuit for the operating winding ICRI of control relay ICR. As will now be set forth, upon dropping out of relay ICR resistance RI is included in the armature circuit of the motor, speed regulating rheostat l is excluded from the shunt field circuit of the motor and resistance R1 is connected across the motor armature to provide for operation of the motor at its slow idling speed.

Upon dropping out of control relay ICR the contacts ICR2 thereof close to re-establish the aforedescribed energizing circuit for the operating winding IFAI of field accelerating switch IFA and said switch in responding shunts resistance section rl of speed regulating rheostat 1. Also upon dropping out of contro-l relay ICR the contacts ICR3 thereof interrupt the aforedescribed energizing circuits for accelerating switches IA, 2A, 3A and 4A to provide for dropping out of said switches. Upon dropping out of accelerating switch IA contacts IA4 reestablish the energizing circuit for the operating winding ICTI of control relay ICT and upon dropping out of accelerating switch 3A contacts 3A3 re-establish the energizing circuit for the operating winding 2CTI of control relay 2CT. Upon response of field accelerating switch IFA the contacts IFA4 thereof open to interrupt the energizing circuit for operating winding FDI of timing relay FD and said timing relay then drops out subject to a given period of delay due to the discharge of its associated condenser C. Upon dropping out of timing relay FD contacts FD2 close prior to opening of the contacts FD3 thereof. Closure of contacts PD2 re-establishes the aforedescribed energizing circuit for the operating winding ZFAI of field accelerating switch 2FA and said switch in responding closes its contacts 2FA2 to shunt resistance section r2 of speed regulating rheostat 1. Opening of timing relay contacts FD3 interrupts the energizing circuit of winding 2CRI of control relay 2CR and closure of the timing relay contacts FD re-establishes the energizing circuit of operating winding 3CRI of control relay 3CR. Control relay SCR in responding opens its contacts 3CR2 to interrupt the aforedescribed energizing circuit for the operating winding SAI of accelerating switch 5A and upon dropping of said accelerating switch resistance RI is re-included in the armature circuit of the motor. Upon dropping out of accelerating switch 5A the contacts 5A4 thereof re-establish the aforedescribed energizing circm't for the operating winding ASI of armature shunt switch AS and said armature shunt switch in responding re-connects resistance RI across the terminals of armature A. The motor is thus decelerated and brought to its low idling speed under no load conditions.

The controller also operates to effect automatic acceleration of the motor from its slow idling speed to its selected maximum drawing speed upon depression of the fast" push button. Closure of the fast push button establishes an energizing circuit for the operating winding ICRI of control relay ICR through the medium of the slow push button. With the motor operating at slow idling speed load relay LR is in closed position and upon release of the fast push button the operating winding I CRI of control relay ICR is maintained energized through the medium of load relay contacts LR3 and control relay contacts ICRI. Upon response of control relay ICR contacts ICR3 close to establish the aforedescribed energizing circuit for the operating winding IAI of accelerating switch IA. Upon response of accelerating switch IA the armature shunt switch AS is opened and the timing relays ICT, 2CT and FA then operate as hereinbefore set forth to effect sequential response of armature accelerating switches 2A to 5A and field accelerating switches IFA and ZFA for acceleration of the motor to the speed determined by the setting of rheostat 1.

Upon depression of the stop push button the aforedescribed maintaining circuit for the operating winding UVI of the under voltage relay is interrupted and upon dropping out of said relay the contacts UVE and UV3 thereof interrupt the aforedescribed energizing circuits for main switch M, accelerating switch 5A, armature shunt switch AS and field accelerating switches IFA and 2FA. Upon dropping out of main switch M the contacts M2 thereof open to effect stopping of the motor and the contacts M4 thereof open to insure deenergization of accelerating switches IA, 2A, 3A and 4A.

What I claim as new and desire to secure by Letters Patent is:

l. The combination with a direct current motor and a machine driven thereby, of means for starting said motor and for causing the same to normally operate at a predetermined slow speed below its base or full field speed with said machine in an unloaded condition, and means associated with said former means for accelerating said motor to a selected maximum speed when the load on said machine exceeds a predetermined value.

2. The combination with a direct current motor and a machine driven thereby, of means for starting said motor and for causing the same tc normally operate at a predetermined slow speed below its base or full eld speed with said machine in an unloaded condition, and means associated with said former means and responsive upon slowdown of said motor below said normal due to loading of said machine for eiiecting acceleration of said motor to a selected maximum speed.

3. The combination with a direct c rrent motor and a machine driven thereby, of means for start-l ing' said motor and for causing the same to normally operate at a predetermined slow speed below its base or full eld speed with said machineI in an unloaded condition, and means associated with said former means for automatically accelerating said motor to a selected maximum speed upon loading of said machine and for automatically decelerating said motor to said predetermined slow speed upon unloading of said machine.

4. The combination with a direct current driving motor and a machine driven thereby, of a resistance to be connected in series in the armature circuit of said motor, a second resistance to be connected across the armature of said motor, means for establishing power connections for said motor and for including said resistances in the armature circuit thereof to provide for normal operation of said motor at a relatively slow speed below its base or full eld speed with said machine in an unloaded condition, and accelerating means for automatically excluding said resistances from the armature circuit of said motor when the load on said machine exceeds a predetermined Value.

5. The combination with a direct current motor and a machine driven thereby, of a resistance to be connected in series in the armature circuit of said motor, a second resistance to be connected across the armature of said motor, means for establishing power connections for said motor and for including said resistances in the armature circuit oi said motor to provide for normal operation of said motor at a speed below its base or full eld speed with said machine in an unloaded condition, accelerating means for automatically excluding said resistances from the armature circuit of said motor when the load on said machine exceeds a predetermined value and decelerating means for re-including said resistanccs in the armature circuit of said motor when the load on said machine drops below said predetermined value.

6. The combination with a direct current motor and a machine driven thereby, of a series resistance to be connected in the armature circuit of said motor, a shunt resistance to be connected across the armature of said motor. means for establishing power connections for said motor including said series resistance, means for including said shunt resistance in the motor circuit upon starting of said motor to provide for normal operation of said motor at a predetermined slow speed below its base or full eld speed with said machine in an unloaded condition, and for excluding said shunt resistance from the motor cir.Y cuit upon slowdown of said motor due to a loaded condition of said machine, and accelerating means responsive upon exclusion o1 said shunt resistance from the motor circuit for automatically excluding said series resistance from the motor circuit.

7. The combination with a direct current motor and a machine driven thereby, of a series resistance to be connected in the armature circuit of said motor, a shunt resistance to be `connected across the armature of said motor, means for establishing power connections for said motor cluding said series resistance, control means for said shunt resistance including a relay responsive to counter E. M. F. conditions in the motor circuit, said relay acting upon starting oi said motor to include said shunt resistance in the motor circuit for normal operation of said motor at a predetermined slow speed below its base or full eld speed with said machine in an unloaded condition, said relai7 also providing for automatic exclusion of said shunt resistance from the motor circuit upon slowdown of said motor below said normal speed due to loading of said machine, and means acting upon exclusion of said shunt resistance from the motor circuit for automatically excluding said series resistance for acceleration of said motor.

8. The combination with a direct current motor and a machine driven thereby, of a series resistance to be connected in the armature circuit oi said motor, a shunt resistance to be connected across the armature of said motor, means for establishing power connections for said motor including said series resistance, means acting upon starting of said motor for including said shunt resistance in the motor circuit to provide for normal operation of said motor at a predetermined slow speed below its base or full iield speed with said machine in an unloaded condition and for excluding said shunt resistance from the motor circuit upon slowdown of said motor below said predetermined speed due to loading of said machine, means acting automatically upon exclusion of said shunt resistance from the motor circuit to exclude said series resistance from the motor circuit for acceleration 0f the motor and decelerating means responsive to load conditions in the motor circuit for including said series and shunt resistances in the motor circuit upon unloading of said machine.

9. The combination with a metal drawing machine and a driving motor therefor, of means for starting said motor and for causing the same to normally operate at a predetermined slow speed below its base or full field speed when said drawing mechanism is unloaded and in an initial position for starting of the drawing operation, means associated with said former means for accelerating said motor to a selected maximum speed when said drawing mechanism is started out of initial position and subjected to a load exceeding a predetermined value, and means acting under load conditions below said predetermined value for accelerating said motor to said selected maximum speed upon a given movement of said drawing mechanism out of initial position.

JOHN M. NEWMAN.

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