US1546900A - Control system for electric motors - Google Patents

Description

July 21, 1925.

B. W. JONES CONTROL SYSTEM FOR ELECTRIC MOTORS Inventor- Beqjarnin W. Jones, y fi wnhf His Attorney.

' Patented July .21, 1925.

UNITED STATES 1,546,900 PATENT OFFICE.

BENJAMIN W. JONES, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC- TRIO COMPANY, A CORPORATION OF NEW YORK.

. CONTROL SYSTEM FOR ELECTRIC MOTORS.

Application filed November 23, 1923. Serial No. 676,660.

To all whom it may concern.

Be it known that I, BENJAMIN W. JoNns,

a citizen of the United States, residing at Schenectady, county of Schenectady, State a of New York, have invented certain new and useful Improvements in Control Systems for Electric Motors, 'of which the following is a specification.

My invention relates to improvements in control systems for electric motors and has for its principal object the provision of means for insuring the starting of the motor with a predetermined initial torque. My invention is of particular utility when embodied in a control system for an adjustable speed motor such as is commonly used in clevator and hoisting service, although my invention also is applicable to the control system of any motor driven apparatus where a smooth and easy starting and control of the apparatus is desirable. In passenger elevator service, a. smooth and easy starting of the elevator car in either direction of travel is essential to avoid serious discomfort to the occupants as well as severe strains and shocks to the elevator mechanism. In order that the driving motor may effect such a smooth and easy starting of the elevator ear, the system of control should limit the initial starting torque exerted by the motor within a very narrow range of the torque required merely to balance the carand subsequently increase the to bring the motor up to full running speed. I Furthermore, such a motor control system is readily adapted for dynamically braking the elevator car in stopping by providing means for varying the resistance in shunt with the motor armature, which then serves as a dynamic braking circuit for the motor, .and applying full field to the motor to increase the dynamic braking efi'ect.

motor torqueand speed to accelerate the elder full field, and finally weakening the field;

However, in the operation of an elevator provided with such a motor control system it frequently occurs that the. car either overruns or under-runs the selected floor while stopping under the full field dynamic braking action of the motor. To bring the car to the selected floor the operator may then hastily apply power tothe motor with the field at full value and the armature shunt resistance at an improper value for smooth starting. This results in the motor exerting a starting torque much greater than desired and the passengers and equipment consequentlyexperience asevere jar or jolt.

My invention contemplates eliminating the possibility of produclng such a jolt or ar by providing a control system in which the motor starting switch cannot be operated to connect the 'motor to the power source until after the field is weakened and the armature shunt resistance is set at the pro er value, thus at all times insuring the starting of the motor with weak field and an initial startlng torque of a predetermined value.

For a proper understanding of my invention reference is had to the accompanying drawing in which the single figure diagrammatically shows the invention embodied in 'a master controller type of motor control system for the purpose of explaining the principles thereof. The novel features which I believe to be characteristic of my invention are pointed out with particularity in the appendedvclaims.

Referring'to the drawing, the motor 10 having an armature 11 provided with a suitable armature winding and a shunt field winding 12 maybe connected by mechanism, not shown, to both drive and dynamically brake an elevator or other apparatus. Motor 10 may be provided with a suitable commutating field 13 if desired, although such a field winding is not essential in carrying outmy invention. As indicated in the drawing the shunt field winding 12 is connected in series circuit with a field regulating resistor 14 and the operating winding of a relay 15 across the current suppl lines 16 and .17. Relay 15 is biased to the c osed position shown and is desi ned to respond to a field current of afpre eterinined value to open the relay contacts, and the resistor 14 is proportioned to weaken the field current below this predetermined value while it remains in the field circuit.

The resistors 18, '19, and 20 are connected through the contacts of electromagnetic switch 21 in a shunt circuit across the motor armature 11 and the commutating field winding 13, to supply a variable resistance load during the dynamic braking operation of the motor and also when properly controlled furnish an armature by-pass res1stance of a predetermined value in order to tion by a suitable operating winding as will more fully appear in connection with the operation of, my invention. The resistors 19 and 20 are controlled respectively by the electromagnetic switches 22, and 23,'each of which is biased to the closed position to establish a shunt circuit around the resistor as shown, and is provided with a suitable operating winding for actuating the respective switchcontacts to the open positions. Resistors 24, 25, and 26 comprise the usual motor starting series resistance and are controlled respectively by the electromagnetic switches 27 28, and 29, which are biased to the open position, as shown, and are operated to the closed position to short circuit the resistors by suitable operating windings. The several resistors connected in series and shunt with the motor armature are so proportioned that when the armature winding is connected to the supply lines 16 and 17 with the corresponding electromagnetic controlling switches in their respective unenergized positions shown, the armature current is limited to a predetermined Value. This value of armature current is such that with the motor field current at the weak field value, the initial starting torque exerted by the motor is just sufficient to move the elevator car slowly and smoothly from a position of rest.

In order to control the field regulating resistance 14, a relay 30 is provided with an operating winding 31, one terminal of which is connected to the resistor 18 and the other terminal is connected to the resistor 26 as.

indicated. Relay 30 is biased to the open positlon, as shown, and the operating windmg 31 is designed to actuate the relay 30 to close a shunt circuit around resistance 14 when energized by an appreciable voltage from either the current supply lines 16 and 17 during motoring or from the motor during dynamic braking operation.

The electromagneticallyactuating reversing switches 32 and 33 are arranged to con nect the motor armature winding 11 to the.

current supply lines 16 and 17 for operation in either direction and are provided with operating windings 34, and 35 respectively. These switches are biased to the open position, as shown, and'are equipped with the customary mechanical interlock 36 for preventing interference in the control of the motor. It will be observed that one termi nal of each of the operating windings 34 and 35 is connected through a common resistance 37 to the current supply line 17. Theresistor 37 limits the current supplied to either of the operating windings 34 and 35 to a value which is insufiicient to operate the respective swiches 32 and 33 from the open position but which is suflicient to maintain either switch in the closed position. A shunt circuit of negligible resistance around the resistor 37 is arranged to be controlled jointly by the contacts of relay 15 and an auxiliary contact mechanism 38 carried by electromagnetic switch 22 in order to permit operation of the reversing switches 32 and 33 from the open positions.

For controlling the energizing circuits of the reversing switch operating windings 34 and 35 as well as the respective operating windings of the electromagnetic switches 21, 22, 23, 27, 28, and 29, a manually operated master controller 39 is provided with the cooperating contacts and segments 40, 41, 42, 43, 44, 45, 46, 47, and 48. These cooperating contacts and segments are arranged so as to engage in a predetermined sequence to energize from the supply lines 16 and 17 first, either the operating winding 35 of reversing switch 33 or the operating winding 34 of reversing switch 32 and then successively energize the operating windings of electromagnetic switches 22, 27 23, 28, 21, and 29, when the controller 39 is operated from the off position in which it is shown through either the control positions 1 to 7 -or 1 to 7 respectively.

Operation.

With the parts as thus constructed and arranged and in their respective positions shown in the drawing the operation of my improved motor control system is as follows:

Assume that the supply lines 16 and 17 are connected to a suitable source of current and the motor 10 is at rest. The motor shunt field in energized through a circuit extending from supply line 16 through the field winding 12, resistance 14, the operating winding of relay 15 to the other supply line 17. With the relay 30 in the open position, resistance 14 reduces the current through the shunt field winding 12 below the predetermined value previously mentioned and the motor field strength will be at the corresponding weak field value. The relay 15, being operatively; energized only bya field current below the predetermined value, will remain in the closed position shown. In order to operate the motor 10 in one direction, assume itto be the up direction of the elevator car for the purpose of clarity, the master controller 39 is. first operated to control position 1. In this position an energizing circuit for the operating windin 35 of reversing switch 33 is completed roin supply line 16 through contact and segment '40, contact and segment 41, operating winding 35, auxiliary contact 38 car. As the motor is thus started, the operating winding 31 of relay 30 is operatively energized by the voltage of the supply lines 16 and 17 and effects the operation of the Closure relay contacts to closed position. of relay 30 establishes a shunt circuit about the field re ulating resistor 14 and the field current is t ereby gradually increased from the weak field value to the -full. field value to correspondingly increase the motor torque and accelerate the car. It should be noted that the gradual increase in the motor fieldstrength begins immediately after the motor is started by closure of the reversing switch 33 and continues during an appreciable time interval due to the inductive action of the shunt field winding. The-increased value of field current energizes the operating wind-j ing of relay 15 and causes the circuit controlling contacts of the relay to move to open position.

It should be noted that if, atthis point in the acceleration of the elevator car, the controller 39 should be returned to the ofi position, the operating winding 35 of reversing switch 33 is deenergized and 'disconnects the motor from the supply lines 16 and 17. The motor 10 then operates as a dynamic brake to retard the movement of the elevator car. During braking the operating winding 31 of relay 30 is energizedby the voltage generated by the motor-and maintains the shunt circuit around the field regulating resistor14 closed to efiect full field dynamic braking action of the motor.'.

It will be evident that before either of the reversing switches 32 and 33 can again connect the motor armature 11 to the supply contacts and again establish the shunt circuit around the resistor 37. When the motor 10 has brought the elevator car almostto a standstill under full field-dynamic braking, the voltage generated by the motor then becomes insufficient to maintain the relay 30 in the closed position. Relay 30 there must be.

upon' opens the shunt circuit around the field regulatin resistor 14 and reduces the field current from the full field value to the weak field value, thus permitting the relay to close according to its bias. Closure of relay 15 again establishes the shunt .circuit around the resistor 37 and ermits either of the reversing switch windings 34 and 35 to be operatively energized when the master controller 39 is operated from the ofi position.' 2

However, if after operation of the master controller 39 to the position 1 as previously described, the controller 39'is then successively moved through theintermediate control positions 2, 3, 4, 5, and 6 to the running position 7; the electromagnetic switches 22, 27, 23, 28, 21, and 29 are successively energized to alternately insert resistance in shunt with the. motor armature 11 andremove the resistance in series with the armature, thus gradually accelerating the motor. It will be observed that upon the energization of the operating winding of electromagnetic switch 21 the shunt armature resistance circut is opened, thus in effect, inserting infin to resistance in shunt with the motor armature. Although upon the opening of switch 21 the operating winding 31 of relay is no longer energized by the full .voltage ofthe supply lines 16 and 17, the voltage drop across the remaining starting resistor field circuit and the motor 10 is accelerated to full running speed under weak field.

To stop the elevator car by dynamlc;

braking at. any selected floor, the master erating winding of switch 21, this switch closesaccording to its bias and again estab lishes the resistance circuit in shunt with the armature; thus causing the motor to operate as a dynamic brake. Closure of switch 21 also impresses the voltage across the motor armature upon operating winding 31 0f .relay .30. This relay'then closes the shhnt circuit around the field regulating resistor 14. Since the electromagnetic switch 22 .was-the first to be energized, it will be released last to short. circuit the resistor 19 v .as well as close the auxiliary. contact 38.

The motorv 10 then operates as a dynamic brake under full field and with a maximum resistance load to bring the elevator loo car almost to a standstill. When the voltclosing position, the latter opens accordingto its bias and once more inserts the resistance 14 in the shunt field circuit to reduce the field strength to weak field value. This permits the relay 15 to close according to its bias and again cooperate with the auxiliary contact 38 on switch 22 to establish the shunt circuit around the resistor 37.

The starting of the motor to operate the elevator car in the down direction is accomplished in substantially the same manner as that just described, with the single exception that in control positions 1' to 7 the cooperating contact and segments 42 are brought into engagement to complete an en ergizing circuit for operating winding 34 of reversing switch 32 extending from supply line 16 through con-tact and segment 10, contact and segment 42, operating winding 34, contact 38, relay 15 to the other supplyline 17. Upon response of reversing switch 1 32 the motor armature, with the resistor 18 connected in shunt circuit therewith, is connected in series circuit with the series starting resistors 24, 25, and 26 across the supply lines 16 and 17 for starting with weak field and the same predetermined initial starting torque in the opposite direction. The deceleration of the motor under dynamic braking to stop the elevator car at any selected floor is likewise similar to that previously described.

While the above described operation is the usual and normal manner in which the motor control system is intended to operate, it should be understood that my improved control system insures at all times that the motor is started with weak field and an initial starting torque of a predetermined amount even under unusual and abnormal operating conditions. For instance, suppose that while the motor is running at full speed in the down direction the operator should instantly shift the master controller 39 from the control position 7 to the control position 1 to quickly effect the reversal of the motor.' In this event the operating winding 34. of reversing switch 32 as well as the operating winding of the shunt and series armature resistance controlling switches would be deenergized and the operating winding 35 of reversing switch 33 would be energized. However, it will be seen that the resistance 37 prevents sufiicient current from passing through operating'winding 35 to ef-' fect the closure of reversing switch 33 at this time. Thus, the motor operates under 'full field as previously described to dynamically brake the elevator car until the speedhas been reduced to a value at which the voltage generated by the motor is insuflicient to sustain relay 30 in the closed position. T' hen relay 30 opens as before and inserts the resistance 1 1 in the shunt field circuit to reduce the field strength to the weak field value. As soon as the field current has decreased to the predetermined value at which the relay 15 is no longer operatively energized, this relay will close according to its bias. After the relay 15 has completed the shunt circuit around resistor 37, winding 35 is energized by the full voltage across the supply lines 16 and 17 and operates switch 33 to start the motor with weak field and the predetermined resistance in shunt to the armature. Thus it will be seen that a control system embodying my invention always insures that the motor is connected to the source of current with weak field and limited starting torque.

As has been previously pointed out, when the master controller 39 is operated from either direction to the off position, the electromagnetic switch 22 is the last of the armature resistor controlling switches to be deenergized to permit operation according to its bias. Due to this fact, I have found in practice that the single auxiliary contact 38 on the electromagnetic switch 22 is suflicient under all ordinary conditions to insure that the proper armature shunt and series resistors are in circuit for starting the motor with a predetermined armature current. But it will be obvious to those skilled in the art that additional auxiliary contacts may be provided for any selected ones of the other electromagnetic switches and included in the shunt circuit around the resistor 37 should it be desired to positively prevent operation of the reversing switches 32 and 33 from the open position until after the particular switch or switches selected have been deenergized and operated according to their bias. Also it will be understood that after the regulating resistor 14 is inserted in the shunt field circuit upon completion of the dynamic braking operation of the motor, an appreciable time may be required for the current in the shunt field circuit to decrease to the weak field value due to the inductive action of the shunt field windings. But until the motor field current is below the predetermined value, the relay 15 will be maintained in the open position and prevent completion of the shunt circuit around resistance 37 and consequently the operative energization of winding 35 of reversing switch 33 even thou h the contact 38 on switch 22 is in close position. This feature is of great practical importance since it insures that the motor is always started with weak field.

In accordance with the provision of the patent statutes I have described the principles of my invention together with the apparatus which I now consider to represent to have it understood that the apparatus shown is only illustrative and that the in-- vention may be carried out by other means.

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

1. In a control system, the combination with an electric motor, of current controllin'g means operable to weaken the motor field and to limit the motor starting current under weak field to a predetermined value, switch mechanism for connecting the motor to a source of current, -and means for preventing operation of said switch mechanism until after operation of said current controlling means to insure starting of the motor with weak field and limited starting current of a predetermined value.

2. In a control system, the combination with an electric motor having armature and field windings and regulating resistors in circuit therewith, of switchmechanism for connecting the motor to a source of current and controlling the said resistors to vary the motor field strength and starting torque,

and means cooperating with the said switch mechanism for insuring the starting of the motor with minimum field strength and starting torque. v

3.- A control system for an electric motor having armature and field windings and regulating resistors in circuit therewith, comprising a starting switch mechanism for connecting the motor to a source of current, electromagnetic means for controlling the said starting switch mechanism and the said armature and field resistors, and means cooperating with the said electromagnetic means for insuringthe starting of the motor 'with weak field and starting torque of a predetermined value. p

4. A control system for an electric motor having armature and field windings and regulating resistors in circuit therewith, c'omprising switchingmeans for controlling the saidresistor's to limit the motor starting torque under weak field to a predetermined value, electrically actuated switch mechanism for connecting the motor to a source of ourrenty'and means for operatively energizing the said electrically actuated switch mechanism controlled by the said switching means to insure the starting of the motor with weak field and starting torque of a predetermined value.

5. A control system for a reversible. electric motor having armature and shunt field windings and iegulating resistors in circuit therewith, comprising electromagnetically actuated switch mechanism for connectmg the motorto a source of current, a plurality of mechanically independent switches for;

controllin the said resistors to vary the motor fie'd strength and starting torque between minimum and maximum values, and

means dependent upon the operation of said independent switches to the minimum field and starting torque positions for operatively energizing the said electroma'gnetically actuated switch mechanism.

6. A control system for an electric motor having armature and field windings and resistors incircuit therewith, comprising, an electromagnetically actuated switch mechanism for connecting the motor to a source of current, a switch for controlling the armature resistor to vary the starting current through the said armature winding, a second switch for controlling the field resistor to vary the current through the said field winding, a relay connected in circuit with the said field winding and operatively energized responsively to a predetermined current, and electrical connections for energizing the said electromagnetically actuated switch mechanism jointly controlled by the said relay and the said armature resistor controlling switch to insure the starting of the motor with weak field and limited starting torque of a predetermined value.

7. A control system for an electric motor having armature and'shunt field windings starting switch mechanism for connecting the motor to a source ofsupply, a resistor in circuit with the operating winding of said mechanism for preventing operation thereof, switching means for controlling the said armature and field resistors to weaken the motor field and limit the motor starting torque under weak field to a predeterminedvalue, and switch mechanism and electrical connections controlled by the said switching means for establishing a shunt circuit around the said operating winding resistor to permit starting the motor with weak field and limited starting torque of a predetermined value.

8. A control system for an electric motor having armature and shunt'field windings comprising a resistor in shunt circuit with the said armature winding, a reslstor n series circuit with said field winding and a source of current, electromagnetically operated starting switch mechanism for con necting the motor to the said source of current, a resistor in series circuit with the operating winding of said mechanism for preventing operation thereof, switching means for opening a shunt circuit around the said field resistor and closing a shunt circuit around the said armature resistor to weaken the motor field and limit the starting torque of themotor under weak field to a predetermined value, and switch mechanism .and electrical connections controlled by said switching means for establishing a shunt circuit around the said operating winding resistor to permit starting the motor with weak field and starting torque of a predetermined value.

9. A control system for an electric motor having armature and shunt field windings, comprising, a resistor in shunt circuit with the said armature winding, a resistor in series circuit with said field winding and a source of current, electromagnetically actuated switch mechanism for connecting the motor armature winding to the said source of current, a switch for closing a shunt circuit around the said armature resistor, a second switch for opening a shunt circuit around the said field resistor to reduce the motor field current below a predetermined value, a relay connected in the said field circuit and operatively energized responsive ly to current above the said predetermined value, and electrical connections for energizing the said electromagnetically actuated switch mechanism jointly controlled by said armature resistor controlling switch and said relay for preventing actuation of said switch mechanism until after both the closure of said shunt circuit around the said armature resistor and the reduction of the motor field current below the said predetermined value.

10. A control system for an electric motor having armature and shunt field windings, comprising, a resistor in shunt circuit with the said armature windin a resistor in series circuit with said field winding and a source of current, electromagnetically operated switch mechanism for connecting the motor armature winding to the said source of current, a resistor in circuit with the operating winding of said mechanism for preventing operation thereof, a switch for opening a shunt circuit around the said field resistor to reduce the motor field current,a relay connected in the said field circuit and operatively energized responsively to current above a predetermined value, a second switch for closing a shunt circuit around the said armature resistor to limitthe motor starting torque under reduced field to a predetermined value, and switch mechanism and electrical connections jointly controlled by said relay and said armature resistor controlling switch for establishing a'shunt circuit around the said operating winding resistor after both the closure of said shunt circuit around the said armature resistor and the reduction of the motor field current below the said predetermined value to permit starting the motor with predetermined field and starting torque.

11. A control system for a reversible electric motor having armature and shunt field windings and resistors in circuit therewith, comprising electromagnetically actuated reversing switches for connecting the motor to a source of current for operation in either direction, an electromagnetically actuated switch for closing a shunt circuit around the said armature resistor, a second electromagnetically actuated switch for opening a shunt circuit around the said field resistors to reduce the motor field current, a master controller for controlling the actuation of the said electromagnetically actuated switches, and means, including a relay responsive to a predetermined current through the shunt field winding and auxiliary switch mechanism operated-by said ,armature resistor controlling switch, for preventing actuation of said reversing switch mechanisms until after the closure of the said shunt circuit around the said armature resistor and the reduction of the shunt field current to the said predetermined value.

12. A control system for a reversible electric motor having armature and field windings adapted for dynamic braking, comprising, a resistor in shunt circuit with the said armature winding, a field regulating resistor, switching means for controlling the said resistors to stop the motor under full field dynamic braking action and to limit the motor starting torque under weak field to a predetermined Value, reversing switches for connecting the motor armature winding to a source of current for operation in either direction, and means cooperating with the said switching means and the said reversing switches to insure the starting of the motor with weak field and a limited torque of a predetermined value.

13. A control system .fora reversible electric motor having armature and field windings adapted for dynamic braking, comprising a resistor in shunt circuit with the said armature winding, a field regulating resistor, switching means for controlling the said resistors to stop the motor under full field dynamic braking action and to limit the motor starting torque under weak field to a predetermined value, electrically actuated reversing switches for connecting the motor armature winding to a source of current for operation in eitherdirection, and means for operatively energizing the said electrically actuated reversing switches controlled by said switching means to insure the starting of the motor, with weak field and limited torque of a predetermined value.

14:. A control system for a reversible electric motor having armature and field windings adapted for dynamic braking, comprising, a resistor in shunt circuit with the said armaturewinding, a field regulating resistor, switching means for controlling the said resistors to stop the motor under full field dynamic braking action and to limitthe motor starting torque under weak field to a predetermined value, electromagnetically operated reversing switch mechanism for connecting the motor armature winding to a source ofcurrent for operation in either direction, a resistor in circuit with the operatin winding of said mechanism for preventing operation thereof, and switch mechanism and electrical connections controlled by the said switching means for establishing a shunt circuit around the said operating winding resistor to permit the starting of the motor with weak field and limited torqueof a predetermined value.

15. A control system for a reversible electric motor having armature and shunt field windings adapted for dynamic braking, comprising, a resistor in shunt circuit with the said armature winding, a resistor in series circuit with said field winding and a source of current, electromagnetically actuated reversing switch mechanism for connectin the motor to the-said source of current or operation in either direction, a switch for closing a shunt circuit'around the said armature resistor to regulate the dynamic braking eflect of said motor and to'limit the motor starting torque, a second,

switch foropening a shunt circuit around the said field resistor to reduce the motor field current below a predetermined value, a .relay connected in the said field circuit and operatively energized responsively to current above the said predetermined value,

and electrical connections for operatively energizing the said electromagnetically actuated reversing switch mechanism jointly controlled by said first switch and said relay for preventing actuation'of said reversing switch mechanism until after both the closure. of said shunt circuit around the said armature resistor and the reduction of the motor field current below the said predetermined value.

16. A control system for a reversible electric motor having armature and shunt field windings and adapted for dynamic braking,

comprising, a resistor in shunt circuit with the said armature winding,a resistor in series circuit with said field winding and a source of current, electromagnetically operated switch mechanism for connecting the motor armature to the said source of current for .and operatively energized responsively to current above a weak field value, a second 7 switch; for closing a shunt circuit around the said armature resistor to regulate the dynamic braking effect of said motor and to limit the motor starting torque under weak field to a predetermined value, and switch mechanism and electrical connections jointly controlled by the said relay and the said armature resistor controlling switch for establishing a shunt circuit around the said resistor to permit the starting of the motor with weak field and limited torque of a predetermined value.

17. In a control system, the combination with an electric motor having a shunt field winding and-a resistor in circuit therewith, of switching means for controlling the said resistor to vary the field current of said motor, 'an electrically actuated switch for connecting the motor armature to a source of supply, a relay for controlling the energizing circuit of said switch, and electrical connections whereby the said relay is operatively energized responsively to the motor field current for preventing actuation of the said starting switch.

'18. In a control system, the combination with an electric motor having a shunt field winding and a field resistor in circuit therewith, of switching means for controlling the said resistor to vary the field current of said motor, an electromagnetically operated starting switch for connecting the motor armature to a source of supply, a resistance in circuit with the operating winding of said switch for preventing operation thereof, and a relay connected in'the said field circuit and responsive to current below a predetermined value for closing a shunt circuit around the saidresistance to permit operation of thesaid starting switch.

19. In a control system, the combination with an electric motor having a shuntfield windin and a dynamic braking circuit, of a switch mechanism for connecting the motor to a source of supply for operation in either direction and for disconnecting the motor therefrom for dynamic braking operation, of a resistor in circuit with the said field winding, an electromagnetic switch biased to open position and operable to closed position for controlling the said resistor to vary the field current of said motor and having an operating winding energized responsively to the voltage of the source during 'moto ring operation and to the voltage generated by the motor during dynamic braking 0 ration, and a relay connected in the said eld circuit and operatively energized responsively to field current above a predetermined value for preventing operation Ill of the said switch mechanism to connect the motor to the source.

In witness whereof, I have hereunto set my hand this 22nd day of November 1923.

BENJAMIN w. JoN s.

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