Multi-stage compressor with variable speed drive and method of use

1. A control system for a multi-stage compressor, said control system comprising:

an alternating current (AC) line voltage source configured to operate the multi-stage compressor;

a variable-voltage variable-frequency drive coupled to said AC line voltage source and configured to operate the multi-stage compressor at a variable speed; and

a processor coupled to the multi-stage compressor and said variable-voltage variable-frequency drive, said processor configured to:

selectively couple each of said AC line voltage source and said variable-voltage variable-frequency drive to the multi-stage compressor to power the multi-stage compressor; and

transmit a capacity control signal to the multi-stage compressor to operate the multi-stage compressor in one of a high-capacity setting to provide a high cooling output, and a low-capacity setting to provide a low cooling output, responsive to the capacity control signal,

wherein a combination of the selective coupling and transmitting the capacity control signal enables said processor to operate the multi-stage compressor: (a) under the high-capacity setting and powered by said AC line voltage source that bypasses said variable-voltage variable-frequency drive, (b) under the high-capacity setting and powered by said variable-voltage variable-frequency drive, (c) under the low-capacity setting and powered by said AC line voltage source that bypasses said variable-voltage variable-frequency drive, and (d) under the low-capacity setting and powered by said variable-voltage variable-frequency drive.

2. The control system of claim 1 , wherein said processor is further configured to transmit a speed control signal to said variable-voltage variable-frequency drive to set the variable speed at which the multi-stage compressor operates.

3. The control system of claim 2 , wherein said processor is further configured to select the variable speed at which the multi-stage compressor operates according to a measured cooling load that is based on a function of a temperature set point for an interior space and an ambient temperature of an exterior space measured by a temperature sensor.

4. The control system of claim 1 , wherein said processor is further configured to selectively couple said AC line voltage source and said variable-voltage variable-frequency drive to the multi-stage compressor based on an estimated cooling load.

5. The control system of claim 1 , wherein said AC line voltage source comprises a switch network through which an AC line voltage is switched, the AC line voltage comprising a 60 Hertz 240 volt signal.

6. The control system of claim 1 further comprising a bypass circuit coupled to said variable-voltage variable-frequency drive, said bypass circuit configured to supply power to the multi-stage compressor using said AC line voltage source coupled in series with a capacitor, and bypass said variable-voltage variable-frequency drive.

7. A heating ventilation and air conditioning (HVAC) system, comprising:

a multi-stage compressor configured to operate selectively at a high-capacity setting and a low-capacity setting to provide a high and a low cooling output, respectively;

a variable-voltage variable-frequency drive coupled to said multi-stage compressor and configured to operate said multi-stage compressor at a variable speed; and

a processor coupled to said multi-stage compressor and said variable-voltage variable-frequency drive, said processor configured to:

select one of the high-capacity setting and the low capacity setting at which said multi-stage compressor should operate based on a cooling load determined for said multi-stage compressor, the cooling load based on a function of a temperature set point for an interior space and an ambient temperature of an exterior space measured by a temperature sensor;

transmit a capacity control signal to the said multi-stage compressor to operate said multi-stage compressor in the selected one of the high-capacity setting and the low-capacity setting, responsive to the capacity control signal;

selectively couple said multi-stage compressor to said variable-voltage variable-frequency drive to power said multi-stage compressor at the variable speed to match the cooling output to the cooling load; and

selectively couple said multi-stage compressor to said AC line voltage to bypass said variable-voltage variable-frequency drive when operating said multi-stage compressor for a maximum cooling output;

wherein a combination of the selecting one of the high- and low-capacity settings, transmitting the capacity control signal, and selectively coupling said multi-stage compressor to said variable-voltage variable-frequency drive and said AC line voltage enables said processor to operate said multi-stage compressor: (a) under the high-capacity setting and powered by said AC line voltage source that bypasses said variable-voltage variable-frequency drive, (b) under the high-capacity setting and powered by said variable-voltage variable-frequency drive, (c) under the low-capacity setting and powered by said AC line voltage source that bypasses said variable-voltage variable-frequency drive, and (d) under the low-capacity setting and powered by said variable-voltage variable-frequency drive.

8. The HVAC system of claim 7 , wherein said processor is further configured to employ said variable-voltage variable-frequency drive when the low-capacity setting is selected.

9. The HVAC system of claim 7 , wherein said processor is further configured to:

determine the variable speed based on the cooling load; and

transmit a speed control signal to said variable-voltage variable-frequency drive to operate said multi-stage compressor at the variable speed.

10. The HVAC system of claim 7 further comprising:

a heat exchanger coupled to said multi-stage compressor configured to transfer energy between a compressed refrigerant from said multi-stage compressor and an airflow; and

a fan configured turn at a fan speed to draw the airflow through said heat exchanger, the airflow having a volume defined as a function of the fan speed, wherein the volume is sufficient to transfer the energy.

11. The HVAC system of claim 10 further comprising a variable speed drive coupled to said fan, said variable speed drive configured to operate the fan at a variable speed to draw the airflow.