Motor drive control using pulse-width modulation pulse skipping

1. A control circuit for a motor of a compressor, the control circuit comprising:

an inverter control module configured to control power switching devices of an inverter to generate output voltages from a DC power supply, wherein the output voltages are applied to windings of the motor;

a current control module configured to generate voltage signals based on a torque demand, wherein the inverter control module controls the power switching devices according to the voltage signals;

a selector configured to output one of an open loop torque value and a closed loop torque value as the torque demand; and

an open loop torque module configured to generate the open loop torque value, wherein the open loop torque module is configured to apply an upper limit to the open loop torque value, and wherein the upper limit is based on a voltage of the DC power supply.

2. The control circuit of claim 1 , wherein the open loop torque module is configured to determine the upper limit based on the voltage of the DC power supply and a measured temperature of the power switching devices.

3. The control circuit of claim 2 , wherein the open loop torque module is configured to look up the upper limit from a table indexed by the voltage and the measured temperature.

4. The control circuit of claim 1 wherein the open loop torque module is configured to output a lesser one of a predetermined startup torque value and the upper limit as the open loop torque value.

5. The control circuit of claim 1 , further comprising a power factor correction converter configured to produce the DC power supply from AC input power.

6. The control circuit of claim 1 , further comprising:

a transition module configured to generate a transition signal instructing a transition from closed loop mode to open loop mode,

wherein the selector is configured to output the open loop torque value as the torque demand prior to generation of the transition signal, and

wherein the selector is configured to output the closed loop torque value as the torque demand subsequent to generation of the transition signal.

7. The control circuit of claim 6 further comprising:

an estimator configured to calculate an estimated speed of the motor based on measured current from at least one of the windings of the motor,

wherein the transition module is configured to generate the transition signal in response to convergence of the estimator.

8. The control circuit of claim 1 further comprising:

a pulse-width modulation module configured to generate duty cycle values based on the voltage signals, wherein the inverter control module is configured to control the power switching devices with pulse-width modulation according to the duty cycle values;

a pulse skip determination module configured to selectively generate a pulse skip signal; and

a duty cycle adjustment module configured to selectively prevent the power switching devices from switching during intervals specified by the pulse skip signal.

9. The control circuit of claim 8 further comprising:

a mode determination module configured to set a mode signal to (i) a first state in response to a speed of the motor being less than a predetermined threshold and (ii) a second state otherwise,

wherein the pulse-width modulation module is configured to prevent the power switching devices from switching during intervals specified by the pulse skip signal only while the mode signal is in the first state.

10. The control circuit of claim 8 wherein:

the pulse skip determination module is configured to generate the pulse skip signal based on a series of integer values such that the pulse skip signal has a first state in response to a present one of the series of integer values being nonzero;

the series of integer values is a random sequence; and

the duty cycle adjustment module is configured to selectively prevent the power switching devices from switching in response to the pulse skip signal being in the first state.

11. A drive circuit for a motor of a compressor, the drive circuit comprising:

a power factor correction converter configured to produce a DC power supply from AC input power;

an inverter including power switching devices controlled according to voltage signals, wherein the power switching devices pulse-width modulate the DC power supply to generate a plurality of output voltages that are applied to a plurality of windings of the motor, respectively;

a current control module configured to generate the voltage signals based on a current demand;

a torque mapping module configured to generate the current demand based on a torque demand;

an estimator configured to calculate an estimated speed of the motor based on measured current from at least one of the plurality of windings of the motor;

a speed loop control module configured to calculate a closed loop torque value based on a difference between the estimated speed of the motor and a commanded speed of the motor;

a transition module configured to generate a transition signal instructing a transition from closed loop mode to open loop mode in response to convergence of the estimator;

a multiplexer configured to:

from initial startup of the motor until the transition signal is generated, output an open loop torque value as the torque demand; and

subsequent to the transition signal being generated, output the closed loop torque value as the torque demand; and

an open loop torque module configured to:

determine a torque limit by indexing a lookup table with both (i) a voltage of the DC power supply and (ii) a temperature of the power switching devices;

in response to a predetermined open loop torque value being less than the torque limit, output the predetermined open loop torque value as the open loop torque value; and

in response to the predetermined open loop torque value being greater than the torque limit, output the torque limit as the open loop torque value.

12. A method of controlling a motor of a compressor, the method comprising:

determining an upper limit based on a voltage of a DC power supply;

generating an open loop torque value, including applying the upper limit to the open loop torque value;

selecting one of the open loop torque value and a closed loop torque value as a torque demand;

generating voltage signals based on the torque demand; and

controlling power switching devices of an inverter according to the voltage signals to generate output voltages from the DC power supply, wherein the output voltages are applied to windings of the motor.

13. The method of claim 12 , further comprising determining the upper limit based on the voltage of the DC power supply and a measured temperature of the power switching devices.

14. The method of claim 13 , further comprising looking up the upper limit from a table indexed by the voltage and the measured temperature.

15. The method of claim 12 further comprising setting a lesser one of a predetermined startup torque value and the upper limit as the open loop torque value.

16. The method of claim 12 , further comprising:

generating a transition signal instructing a transition from closed loop mode to open loop mode,

wherein the selecting includes selecting the open loop torque value as the torque demand prior to generation of the transition signal, and

wherein the selecting includes selecting the closed loop torque value as the torque demand subsequent to generation of the transition signal.

17. The method of claim 16 further comprising:

using an estimator, calculating an estimated speed of the motor based on measured current from at least one of the windings of the motor; and

generating the transition signal in response to convergence of the estimator.

18. The method of claim 12 further comprising:

generating duty cycle values based on the voltage signals;

controlling the power switching devices with pulse-width modulation according to the duty cycle values;

selectively generating a pulse skip signal; and

selectively preventing the power switching devices from switching during intervals specified by the pulse skip signal.

19. The method of claim 18 further comprising:

setting a mode signal to (i) a first state in response to a speed of the motor being less than a predetermined threshold and (ii) a second state otherwise; and

preventing the power switching devices from switching during intervals specified by the pulse skip signal only while the mode signal is in the first state.

20. The method of claim 18 further comprising:

generating the pulse skip signal based on a series of integer values such that the pulse skip signal has a first state in response to a present one of the series of integer values being nonzero, wherein the series of integer values is a random sequence; and

selectively preventing the power switching devices from switching in response to the pulse skip signal being in the first state.