Variable speed compressor control with sound-controlled defrost mode

1. A drive controller for a motor of a compressor, the drive controller comprising:

a drive circuit that applies voltages to windings of the motor;

a speed control module that controls the drive circuit to rotate the motor at a requested speed;

a speed determination module that generates the requested speed; and

a defrost module that enables a defrost mode in response to a defrost command received from a system controller, wherein:

in response to the defrost mode being enabled, the speed determination module ramps the requested speed down from a speed demand to a defrost speed,

while the defrost mode is enabled, the drive circuit continues to rotate the motor and the system controller actuates a refrigerant reversing valve to a first position,

in response to the requested speed reaching the defrost speed, the speed determination module maintains the requested speed at the defrost speed until conclusion of the defrost mode, and

in response to conclusion of the defrost mode, the system controller actuates the refrigerant reversing valve to a second position and the speed determination module ramps the requested speed up to the speed demand.

2. The drive controller of claim 1 wherein the defrost speed is set according to a programmed value that reduces audible noise generated by the compressor.

3. The drive controller of claim 2 wherein the programmed value corresponds to a speed at which the audible noise generated by the compressor during defrost mode is at a minimum.

4. The drive controller of claim 2 wherein the defrost speed is set to the greater of (i) the programmed value and (ii) a minimum operating speed of the compressor.

5. The drive controller of claim 4 wherein:

the minimum operating speed of the compressor is 1500 rpm,

the speed determination module maintains the requested speed at the defrost speed for 60 seconds,

the defrost module causes the speed determination module to ramp the requested speed down from the speed demand to the defrost speed at a ramp rate of 500 rpm per second, and

the defrost module causes the speed determination module to ramp the requested speed up to the speed demand at a ramp rate of 500 rpm per second.

6. The drive controller of claim 2 further comprising:

an envelope determination module that stores an operational envelope for the compressor, wherein the operational envelope specifies a range of acceptable speeds for varying operating conditions of the compressor,

wherein the envelope determination module determines a minimum speed corresponding to present operating conditions of the compressor.

7. The drive controller of claim 6 wherein the operating conditions include (i) a refrigerant pressure at a condenser in fluid communication with the compressor and (ii) a refrigerant pressure at an evaporator in fluid communication with the compressor.

8. A method of operating a drive controller for a motor of a compressor, the method comprising:

applying voltages to windings of the motor;

controlling the voltages to rotate the motor at a requested speed;

selectively enabling a defrost mode in response to a defrost command;

in response to the defrost mode being enabled, ramping the requested speed down from a speed demand to a defrost speed;

while the defrost mode is enabled, actuating a refrigerant reversing valve to a first position and the motor of the compressor continues rotating;

in response to the requested speed reaching the defrost speed, maintaining the requested speed at the defrost speed until conclusion of the defrost mode; and

in response to conclusion of the defrost mode, actuating the refrigerant reversing valve to a second position and ramping the requested speed up to the speed demand.

9. The method of claim 8 further comprising setting the defrost speed according to a programmed value that reduces audible noise generated by the compressor.

10. The method of claim 9 wherein the programmed value corresponds to a speed at which the audible noise generated by the compressor during the defrost mode is at a minimum.

11. The method of claim 9 wherein the setting the defrost speed includes setting the defrost speed to the greater of (i) the programmed value and (ii) a minimum operating speed of the compressor.

12. The method of claim 11 wherein:

the minimum operating speed of the compressor is 1500 rpm,

the requested speed is maintained at the defrost speed for 60 seconds,

the ramping the requested speed down from the speed demand to the defrost speed is performed at a ramp rate of 500 rpm per second, and

the ramping the requested speed up to the speed demand is performed at a ramp rate of 500 rpm per second.

13. The method of claim 9 further comprising:

storing an operational envelope for the compressor, wherein the operational envelope specifies a range of acceptable speeds for varying operating conditions of the compressor; and

determining a minimum speed from the stored operational envelope according to present operating conditions of the compressor.

14. The method of claim 13 wherein the operating conditions include (i) a refrigerant pressure at a condenser in fluid communication with the compressor and (ii) a refrigerant pressure at an evaporator in fluid communication with the compressor.