DYNAMIC BRAKING OF INDUCTION MOTORS

• US 3,581,168 A
• Filed: 03/10/1969
• Issued: 05/25/1971
• Est. Priority Date: 03/19/1968
• Status: Expired due to Term

• Current Assignee: Donald Kirkby
• Original Assignee: Donald Kirkby
• Inventors: Kirkby, Donald
• Application Number:
  US04/805,409
• Time in Patent Office: 806 Days
• US Class Current: 318/759
• CPC Class Codes: H02P 3/18 for stopping or slowing an ac motor

Abstract

The disclosure of this specification relates to a system for braking an induction motor comprising normally nonconducting switch means operable, when conducting, to connect capacitance means to the primary winding of the motor to effect braking of the motor and means responsive to the interruption of the supply to the motor to render the normally nonconducting switch means conducting for a period of time sufficient to bring the motor to rest.

Claims

1. A system for braking an induction motor comprising normally nonconducting switch means operable, when conducting, to connect capacitance means to the primary winding of the motor to effect braking of the motor and means responsive both to the interruption or failure of the supply to the motor, when running, and to the currents induced in the primary winding by continued inertial rotation of the motor to render the normally nonconducting switch means conducting for a period of time sufficient to bring the motor to rest.

2. A system according to claim 1, in which the supply to the motor is a polyphase AC supply, the motor has a number of primary windings corresponding to the number of phases of the supply and the capacitance means includes a separate capacitor for connection across each primary phase winding of the motor.

3. A system according to claim 2 in which the means responsive to interruption of the supply comprises circuit means including a first electromagnetic relay coil energizable from said supply upon operation of a start switch to render nonconducting a first normally conducting switch and to render conducting first normally nonconducting switch means connecting the primary winding to the AC supply and second normally nonconducting switch means rendered responsive to said induced currents by the conducting condition of said first normally conducting switch to assume the conducting condition and connect said capacitors across the primary phase windings.

4. A system according to claim 3 in which the second normally nonconducting switch means comprises a second electromagnetic relay coil connected in series with said first normally conducting switch across a unidirectional current supply derived from said induced currents and a first normally open contactor having a number of poles corresponding to the number of phases of the supply.

5. A system according to claim 4 in which the unidirectional current supply is derived from a rectifier bridge connected across a primary phase winding of the motor.

6. A system according to claim 5 in which a smoothing capacitor is connected across the rectifier bridge.

7. A system according to claim 4, including a third electromagnetic relay coil energized by said unidirectional current supply a predetermined interval of time after deenergization of said first relay coil to connect additional capacitance means across the primary phase windings of the motor.

8. A system according to claim 7 including a second normally open contactor having a number of poles corresponding to the number of phases of the supply and responsive to operation of said third electromagnetic relay to connect said additional capacitance means across the primary windings of the motor.

9. A system according to claim 4 including means responsive to deenergization of the first relay coil and energization of the second relay coil to inject DC current into the primary phase windings a predetermined time interval after the energization of the second relay coil.

10. A system according to claim 3 in which the second normally nonconducting switch means comprises a full-wave rectifier bridge circuit operable by the conducting condition of said first normally conducting switch to connect said capacitance means to the primary phase windings.

11. A system according to claim 10 in which the supply is a three-phase AC supply and the full-wave rectifier bridge circuit comprises two oppositely connected half-wave three phase bridge rectifier circuits having their neutral points connected by a thyristor, the base and firing electrode of the thyristor being connected by said first normally conducTing switch.

12. A system according to claim 3 in which the first normally nonconducting switch means is a further normally open contactor having a number of poles corresponding in number to the number of phases of the supply and operable upon energization of the first relay coil to connect the primary phase windings of the motor to the AC supply.

13. A system according to claim 3 in which the first normally nonconducting switch means comprises a separate thyristor connecting each phase of the supply to a corresponding primary phase winding of the motor and a separate normally nonconducting switch associated with each thyristor and connected between the base and firing electrodes thereof and arranged to assume the conducting condition upon energization of the first relay.

14. A system according to claim 3 including short circuit means responsive to deenergization of the first relay coil and energization of the second relay coil to apply an effective short circuit across the primary phase windings of the motor a selected interval of time after the energization of the second relay coil.

15. A system according to claim 14 in which the supply is a three-phase AC supply and the short circuit means comprises two oppositely connected half-wave three-phase bridge rectifier circuits having their common star points connected by a thyristor the firing and base electrodes of which are connected by a normally closed switch controlled by an additional electromagnetic relay coil which is energized upon energization of the first relay coil to open the normally closed switch and which is deenergized a selected interval of time after the first relay coil is deenergized to release the normally closed switch to the closed condition.

16. A system according to claim 14 in which the short circuit means comprises an additional normally open contactor having a number of poles corresponding to the number of phases of the supply and an additional electromagnetic relay coil energizable a selected interval of time after the energization of the second relay coil to close the contactor and short circuit the primary phase windings.

17. A system according to claim 1 in which the supply is a polyphase supply, the motor has a number of primary windings corresponding in number to the number of phases of the supply, first normally nonconducting switch means connects the phases of the supply to the primary phase windings, first control means is provided to respond to actuation of a start switch to render the first switch means conducting and to respond to actuation of a stop switch to render the first switch means nonconducting, second normally nonconducting switch means connects the capacitance means to the primary phase windings of the motor, second control means is provided to respond to actuation of the stop switch to render the second switch means conducting for a limited period of time under the control of said induced currents, third normally nonconducting switch means is connected across the primary phase windings, when in the conduction condition, and third control means is provided to respond to operation of the second control means and render the third switch means conducting for a selected period of time commencing after the commencement of, but before termination of said limited period of time.

18. A system according to claim 17 in which the first switch means is a normally open contactor having a number of poles corresponding to the number of phases of the supply and the first control means includes a first electromagnetic relay coil electrically connected in series with the normally open start switch and the normally closed stop switch across the supply for energization by the closure of the start switch to close the contactor.

19. A system according to claim 17 in which the first switch means comprises a separate thyristor connecting each phase of the supply to a primary phase winding and a separate normally open switch connecting the base and firing electrodes of each thyristor and the first control means includes a first electromagnetic relay coil energizable by closure of the normally open start switch to close the switches connecting the base and firing electrodes of the thyristors.

20. A system according to claim 17 in which the second switch means comprises a normally open contactor having a number of poles corresponding in number to the number of phases of the supply and the second control means comprises a second electromagnetic relay coil energized upon operation of the stop switch to close the contactor.

21. A system according to claim 17 in which AC supply is three-phase and the second switch means comprises two oppositely connected half-wave three-phase bridge rectifiers having their common star points connected by a thyristor the base and firing electrodes of which are connected by a normally conducting switch and the second control means comprises a second electromagnetic relay coil operable to maintain the normally conducting switch in the conducting condition while the first switch means is conducting.

22. A system according to claim 20 in which the second relay coil is energized at least in part by electric currents induced in the primary phase windings by continued rotation of the motor after operation of the stop switch

23. A system according to claim 17 in which the third switch means comprises a third normally open contactor having a number of poles corresponding to the number of phases of the supply and the third control means comprises a third electromagnetic relay coil energizable after the response of the second control means to close the contactor.

24. A system according to claim 17 in which the supply is a three-phase AC supply and the third switch means is provided by two oppositely connected three-phase half-wave rectifier bridge circuits having their common star points connected by a thyristor having its base and firing electrodes connected by a normally conducting switch and the third control means comprises a third electromagnetic relay coil energizable by the first control means to maintain the normally conducting switch in the nonconducting condition while the primary windings are connected to the supply and for a delay interval after the primary windings are disconnected from the supply.