A.C. MOTOR SPEED CONTROL SYSTEM FOR TWO INDIVIDUAL LIMITING CIRCUITS THEREIN

• US 3,748,556 A
• Filed: 11/10/1971
• Issued: 07/24/1973
• Est. Priority Date: 11/10/1971
• Status: Expired due to Term

• Current Assignee: GEC Automation Projects Incorporated
• Original Assignee: Jimmie Douglas Gillett
• Inventors: Gillett, Jimmie Douglas
• Primary Examiner(s): Rubinson, Gene Z.
• Application Number:
  US05/197,403
• Time in Patent Office: 622 Days
• Field of Search: 318/227,230,231
• US Class Current: 363/34
• CPC Class Codes: H02P 27/047 V/F converter, wherein the voltage is controlled proportionally with the frequency
• CPC Class Codes: H02P 27/06 using dc to ac converters or inverters H02P27/05 takes precedence

Abstract

An A.C. motor speed control circuit is disclosed in which a three-phase A.C. input signal is converted to a D.C. link voltage by a controlled rectifier bridge and filter. The D.C. link voltage is converted back to a three-phase output voltage, of controllable frequency, by a three-phase switching bridge inverter. The amplitude of the link voltage provided by the controlled rectifier bridge and filter is controlled by a control signal which also sets the frequency of the three-phase output signal provided by the switching bridge inverter. This insures that the amplitude versus frequency characteristic of the threephase A.C. output is relatively linear. A slow acting current limiting circuit monitors the output current of the switching bridge inverter. A fast acting current limiting circuit monitors the integral of the D.C. link voltage over preselected time intervals to adjust the frequency of operation of the switching bridge if the integral exceeds a threshold value.

Claims

1. A system for providing a drive signal to an AC induction motor in which the amplitude and frequency of said drive signal are controlled by a speed control signal;

said system including;

a circuit responsive to said speed control signal for providing a DC voltage having an amplitude dependent upon said speed control signal;

an inverter bridge responsive to said DC voltage and a frequency signal for providing said drive signal; and

means responsive to said control signal for providing said frequency signal;

said system being characterized by;

means responsive to the integral of said amplitude of said drive signal exceeding a predetermined value over an interval determined by said frequency of said drive signal for increasing said frequency of said drive signal.

2. The system as defined in claim 1 in which said interval has a duration which is less than or equal to one half cycle of said frequency of said drive signal.

3. The system as defined in claim 1, in which said frequency signal is normally provided by a first resettable relaxation oscillator responsive to said control signal for providing a first pulse train having a first repetition rate related thereto.

4. A system for providing a drive signal to an AC induction motor in which the amplitude and frequency of said drive signal are controlled by a speed control signal;

said system responding to charges in said speed control signal at a first rate to change said amplitude of said drive signal and at a second rate to change said frequency of said drive signal, said second rate being faster than said first rate;

said system being characterized by;

means responsive to the integral of said amplitude of said drive signal exceeding a predetermined value over an inTerval determined by said frequency of said drive signal for increasing said frequency of said drive signal;

said drive signal is generated from a fixed frequency power source and includes;

a conrolled rectifier circuit responsive to said speed control signal for providing a DC voltage having an amplitude dependent upon said speed control signal;

a filter circuit responsive to said DC voltage for providing a DC link voltage;

an inverter bridge responsive to said DC link voltage and a frequency signal for providing said drive signal;

means responsive to said control signal for providing said frequency signal;

said frequency signal is normally provided by a first resettable relaxation oscillator responsive to said control signal for providing a first pulse train having a first repetition rate related thereto;

said frequency increasing means includes a second resettable relaxation oscillator reset by said first pulse train, responsive to said DC link voltage for providing a pulse time period after being reset if not reset again by said first pulse train; and

said time period being determined by said DC link voltage.

5. The system as defined in claim 4 in which said resettable relaxation oscillator resets said first resettable relaxation oscillator.

6. The system as defined in claim 5 in which said pulse from said second resettable relaxation oscillator is combined with said first pulse train to provide said frequency signal.

7. The system as defined in claim 6 also including:

means responsive to the current output of said system exceeding a threshold valve for limiting said current output.

8. The system as defined in claim 7 in which said current output is limited by decreasing said frequency signal.

9. The system as defined in claim 8 in which said frequency signal is decreased by bleeding current from said first resettable relaxation oscillator.

10. The system as defined in claim 3 in which said frequency increasing means includes a second resettable relaxation oscillator reset by said first pulse train, responsive to said DC link voltage for providing a pulse time period after being reset if not reset again by said first pulse train;

said time period being determined by said DC link voltage.

11. The system as defined in claim 10 in which said second resettable relaxation oscillator resets said first resettable relaxation oscillator.

12. The system as defined in claim 11 in which said pulse from said second resettable relaxation oscillator is combined with said first pulse train to provide said frequency signal.

13. The system as defined in claim 12 also including:

means responsive to the current output of said system exceeding a threshold value for limiting said current output.

14. The system as defined in claim 13 in which said current output is limited by decreasing said frequency signal.

15. The system as defined in claim 14 in which said frequency signal is decreased by bleeding current from said first resettable relaxation oscillator.