Motor control circuit

• US 5,350,992 A
• Filed: 09/17/1991
• Issued: 09/27/1994
• Est. Priority Date: 09/17/1991
• Status: Expired due to Fees

• Current Assignee: Micro-Trak Systems, Inc.
• Original Assignee: Micro-Trak Systems, Inc.
• Inventors: Colter, Jeffrey B.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): Cabeca, John W.
• Application Number:
  US07/761,328
• Time in Patent Office: 1,106 Days
• Field of Search: 363/96, 318/805, 318/806, 318/807, 318/798, 318/810, 318/802, 318/803, 318/808
• US Class Current: 318/807
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: H02P 1/44 by phase-splitting with a capacitor

Abstract

Power from a DC power source, such as a 12 volt RV or boat battery, is boosted and converted to provide an AC signal suitable for operating an induction motor such as a refrigerator compressor motor. The converter provides voltage pulses having a substantially constant width. The converted voltage signal frequency is initiated at a low frequency and ramps up to 60 Hz. A start winding control circuit is provided for activating the start winding for a limited initial period. A thermostat for controlling the activation of the compressor motor is connected in series with the motor load such that shutting off the compressor disconnects the motor from the circuit. Control circuitry monitors under voltage, over voltage, overcurrent, AC switching, and adequate delay before restart and controls the circuitry accordingly.

Claims

1. A circuit for providing AC power to an induction motor from a DC source, comprising:

pulse width control means for generating a constant pulse width control signal;

voltage control means for generating a constant peak voltage control signal;

frequency control means for generating a variable frequency control signal;

output means, responsive to the constant pulse width control signal, the constant peak voltage control signal and the frequency control signal, for providing an AC power signal having a substantially constant peak voltage, positive and negative pulses having substantially constant pulse widths, and a varying frequency comprised of a first, lower frequency, a second linearly increasing frequency and a third, higher frequency to the induction motor such that the AC power signal applied to the induction motor has a constant volts per hertz ratio.

2. A circuit in accordance with claim 1 wherein the induction motor is a single-phase motor.

3. A circuit for controlling the power provided to an induction load by a converter comprising:

(a) ramping voltage generating circuit means for generating a voltage output signal which provides a first voltage signal for a first selected time period, a linear ramping voltage signal which changes from a first voltage level to a second voltage level at a substantially constant rate over a second selected time period, and the second voltage signal;

(b) voltage-controlled oscillating circuit means connected to and substantially controlled by the ramping voltage generating circuit means for generating a pulse signal having a first, lower frequency substantially corresponding to the first voltage signal generated by the ramping voltage generating circuit means for a time corresponding to the first selected period of time, a linearly increasing frequency which increases from the first, lower frequency to a second higher frequency at a rate substantially proportionate to the rate at which the ramping voltage generating circuit means voltage output signal changes from the first voltage signal to the second voltage signal, and a second, higher frequency substantially corresponding to the second, voltage signal generated by the ramping voltage generating circuit means; and

(c) constant-pulse-width circuit means connected to the voltage-controlled oscillating circuit means output for generating a control signal to a circuit providing AC power to the induction load such that the waveform provided across the induction load by the circuit providing the AC power has a substantially constant pulse-width, a frequency substantially proportionate to the frequency of the signal generated by the voltage-controlled oscillating circuit means and a substantially constant peak voltage.

4. A circuit in accordance with claim 3 wherein the first voltage level generated by the ramping voltage generating circuit means is at a lower level than the third voltage level generated by the ramping voltage generating circuit means.

5. A circuit in accordance with claim 3 further comprising a delay means connected to the input of the voltage-controlled oscillating circuit means for delaying an increase in the frequency of the pulse signal generated by the voltage-controlled oscillating circuit means for a predetermined period of time.

6. A circuit in accordance with claim 3 further comprising start winding activation circuit means receiving the AC power controlled by the constant-pulse-width circuit means for activating a start winding for an induction motor for a substantially constant, predetermined period.

7. A circuit in accordance with claim 3 wherein the induction motor is a single-phase motor.

8. A circuit in accordance with claim 3 wherein the induction motor is a three-phase motor.

9. A circuit for powering a single phase induction motor with a DC source comprising:

(a) an AC converter circuit means for receiving DC power and periodically providing AC power across an induction motor load, said AC converter circuit means including means for switching the connection of the DC power across the induction load to provide a first positive voltage across the induction load, a second substantially null voltage across the induction load, a third negative voltage across the induction load, and a fourth substantially null voltage across both ends of the inductive load, wherein the first positive voltage provided across the induction load and third negative voltage provided across the induction load by the AC converter circuit means are provided for substantially constant time period, and wherein frequency with which the first positive voltage, second substantially null voltage, third negative voltage and fourth null voltage are provided across the induction load varies; and

(b) AC converter circuit control means for providing a periodic control signal to said AC converter circuit means for switching said AC converter circuit means to alternatively provide the first positive voltage, first null voltage, the first negative voltage, and second null voltage across said induction load.

10. A circuit in accordance with claim 9 wherein said AC converter circuit control means provides a control signal to said AC converter circuit means such that one of said null voltage signals is provided by connecting a conductive path to both ends of the induction motor load from one end of the DC power source, and the other of said null voltage signals is provided by providing a conductive path to both ends of the induction load to the other end of the DC power source.

11. A circuit in accordance with claim 9 wherein the AC converter circuit means comprises a first transistor connected between a first end of the DC power source and a first end of the induction motor load, a second transistor connected between the first end of the DC power source and a second end of the induction motor load, a third transistor connected between a second end of the DC power source and the first end of the induction motor load, and a fourth transistor connected between the second end of the DC power source and the second end of the induction motor load.

12. A circuit in accordance with claim 11 wherein the converter circuit control means comprises a dual D flip-flop controlled by first and second clock signals, wherein the first clock signal is inverted to produce the second clock signal.

13. A control circuit for controlling the activation of an induction motor comprising:

(a) a thermostat, connected to monitor an external temperature, and connected in series between a first end of the induction motor and a power source for providing a closed circuit connection between the induction motor and the power source when the temperature detected by the thermostat is within a preselected range and an open circuit between the induction motor and the power source when the temperature detected by the thermostat is within a preselected range and an open circuit between the induction motor and the power source when the temperature detected by the thermostat is within a second range;

(b) first voltage generating circuit means connected to the thermostat and induction motor for generating a first voltage signal when the thermostat provides a closed circuit connection;

(c) a second voltage generating circuit means connected to the thermostat for providing a second voltage signal when the thermostat provides an open circuit connection; and

(d) voltage comparator means, connected to receive the first and second voltage signals, for generating a first control signal when the first voltage signal is received and a second control signal when the second voltage is received;

(e) control means, connected to said voltage comparator means, for activating the induction motor power source upon receipt of the first voltage signal.

14. A control circuit in accordance with claim 13 wherein said first and second voltage generating circuit means comprise current sources.

15. A circuit for controlling an induction load powered by a DC source comprising:

(a) a converter circuit means for receiving DC power and providing AC power to the induction load, said converter circuit means including means for switching the connection of the DC power across the induction load to provide a first positive voltage signal across the induction load, second substantially null voltage across the induction load a third negative voltage across the induction load, and a fourth substantially null voltage across both ends of the induction load, said second and fourth substantially null voltages provided by providing a conductive path between one of the two wires providing the DC power and both ends of the induction load;

(b) converter circuit control means for providing a periodic control signal to said converter circuit means for switching said converter circuit means to alternately provide a positive voltage, first null voltage, negative voltage, and second null voltage across said induction load; and

(c) start winding activation circuit means for the single phase induction load comprising;

(i) ramp voltage generating circuit means for generating a voltage output signal which provides a first voltage signal for a first selected time period, a ramping voltage signal which increases at a constant rate from the first, voltage signal to a second, higher voltage signal over a second selected time period;

(ii) voltage-controlled oscillating circuit means connected to and substantially controlled by the ramp generating circuit means for generating a pulse signal at a first, lower frequency substantially corresponding to the first, lower voltage signal generated by the ramp voltage generating circuit means, an increasing frequency which increases from the first, lower frequency to a second, higher frequency at a rate substantially proportionate to the rate at which the ramp voltage generating circuit means output voltage changes from the first voltage signal to the second voltage signal, and a second, higher frequency substantially corresponding to the second, higher voltage signal generated by the ramp voltage generating circuit means; and

(iii) constant-pulse-width circuit means connected to the voltage-controlled oscillating circuit means output for generating a control signal to the converter circuit means such that the signal provided across the induction load by the converter circuit means has a substantially constant pulse-width, a frequency substantially proportionate to the frequency of the signal generated by the voltage controlled oscillating circuit means and a substantially constant peak voltage.

16. A circuit in accordance with claim 15 wherein the induction motor is a single-phase motor.

17. A circuit in accordance with claim 15 wherein the induction motor is a three-phase motor.

18. A control circuit in accordance with claim 15 further comprising a circuit for activating the start winding of the single phase induction motor comprising rectified voltage generating means for generating a rectified voltage signal from an AC or inverted DC power source, constant voltage generating means connected in parallel across said rectified voltage generating means for converting the rectified voltage signal providing a substantially constant voltage signal, and activating circuit means interconnected with the constant voltage generating means for providing a starter winding circuit activation signal for a selected period of time substantially independent of the level of the substantially constant voltage signal generated by constant voltage generating means.

19. A temperature control circuit for an induction motor load power circuit comprising:

(a) a thermostat means, connected to monitor an external temperature, and connected in series with the induction motor load and a power supply and having first, open and second, closed, states corresponding to off and on settings;

(b) a first and second current source means electrically interconnected with said thermostat means such that the current source means are connected in parallel when said thermostat means in one of the first and second states;

(c) a comparator means having an input interconnected to said first and second current source means, such that a first voltage is detected by the comparator means when said thermostat means is in one of the first and second states and a second voltage is detected by the comparator when said thermostat means is in the other of the first and second states;

(d) control means interconnected to the output of said comparator means for activating the induction motor load power circuit when the first voltage is detected by the comparator; and

(e) control means for sensing the rising DC bus voltage which is indicative of loss of induction motor load on DC converter due to the opening of the thermostat.

20. A method for starting an induction motor having a start winding, comprising the steps of:

(a) controlling the pulse-width of an AC signal provided to the induction motor to maintain a substantially constant pulse-width;

(b) controlling the amplitude of the AC signal provided to the induction motor to maintain a substantially constant peak amplitude;

(c) providing the controlled AC signal to the induction motor at a first, lower frequency for a predetermined period of time;

(d) activating the start winding of the induction motor during the predetermined period of time, wherein activating step (d) occurs substantially simultaneously with providing step (c);

(e) linearly ramping the frequency of the controlled AC signal such that the frequency of the controlled AC signal being applied to the induction motor increases linearly from the first, lower frequency to a second, higher frequency, wherein linearly ramping step (e) occurs after the predetermined period of time of steps (c) and (d); and

(f) providing the controlled AC signal to the induction motor at the second, higher frequency.

21. The method according to claim 20 wherein activating step (d) further includes the steps of:

(d1) generating a rectified voltage signal from an AC source;

(d2) converting the rectified voltage signal into a substantially constant voltage signal;

(d3) applying a start winding activation signal to the start winding for the predetermined period of time, wherein the predetermined period of time is sufficient to start the induction motor but not substantially longer.