Hermetic motor cooling and control

1. A compressor assembly comprising: a compressor housing; a compressor positioned within the housing; a compressor inlet, to input refrigerant gas to the compressor; a motor, the motor comprising a motor housing, a stator positioned within the motor housing, the stator having windings and cooling channels, a rotor positioned within the stator, the rotor having a shaft having a first end connected to an impeller the compressor and a second end, a rotor gap located between the stator and the rotor, wherein when electrical current is applied to the stator windings, the rotor rotates, causing the compressor to operate; electromagnetic bearings positioned at both ends of the rotor to support the rotor when current is applied to the stator windings; the motor further including a temperature monitoring device positioned within the stator; a stator inlet and a stator outlet, the stator inlet receiving liquid refrigerant for cooling the stator, a rotor inlet, a rotor liquid drain and a rotor gas vent, the rotor inlet receiving refrigerant from the stator outlet and sending it through the rotor gap; a flow control device positioned within the stator to control the flow of refrigerant in response to the temperature of the stator and the rotor as provided by the temperature monitoring device; and wherein the stator outlet provides refrigerant to the rotor inlet via a flow passage formed entirely within the motor housing.

2. The compressor assembly of claim 1 wherein the compressor is a centrifugal compressor having an impeller connected to the rotor shaft and positioned within the compressor housing.

3. The compressor assembly of claim 1 wherein the temperature monitoring device is a thermistor.

4. The compressor assembly of claim 1 wherein the liquid refrigerant to cool the motor is provided from a subcooler.

5. The compressor assembly of claim 1 further including a controller to regulate the flow of liquid refrigerant through the flow control device in response to the temperature of the stator as provided by the temperature monitoring device positioned within the stator.

6. The compressor assembly of claim 5 further including a rotor temperature monitoring device positioned within the rotor to provide the rotor temperature to the controller to regulate the flow of liquid refrigerant through the flow control device in response to the temperature of the stator and the rotor.

7. The compressor assembly of claim 6 wherein the rotor temperature monitoring device is positioned adjacent a rotor gap.

8. The compressor assembly of claim 6 wherein the rotor temperature monitoring device is a thermistor.

9. The compressor assembly of claim 1 wherein the flow control device is an expansion valve.

10. A method for cooling a compressor motor in a refrigeration system comprising the steps of:

providing a compressor motor having a stator and a rotor hermetically sealed within a motor housing, a rotor gap located between the stator and the rotor, the rotor having a shaft, and the compressor motor in fluid communication with a source of high pressure subcooled, refrigerant liquid;

providing a flow of high pressure refrigerant liquid to the stator;

monitoring the temperature of the stator;

circulating the refrigerant liquid through the stator to remove heat from the stator to maintain the temperature of the stator within a predetermined range;

lowering the pressure of the high pressure refrigerant exiting the stator from a stator outlet prior to providing the refrigerant to an inlet of the rotor;

providing the refrigerant from the stator outlet to the rotor inlet after lowering the pressure of the refrigerant;

circulating the refrigerant through the rotor inlet and through the rotor gap to remove heat from the rotor;

adjusting a flow of refrigerant to the stator to maintain the temperature of the stator within the predetermined range;

returning liquid refrigerant exiting the stator and not entering the rotor to an evaporator;

returning refrigerant exiting the rotor to the evaporator; and

wherein the stator outlet provides refrigerant to the rotor inlet via a flow passage formed entirely within the motor housing.

11. The method of claim 10 further including the steps of

monitoring the temperature of the rotor; and

circulating the refrigerant through the rotor to remove heat from the rotor to maintain the temperature of the rotor within a predetermined range.

12. The method of claim 10 wherein the step of circulating the refrigerant through the stator to maintain the temperature of the stator within the predetermined range includes maintaining the temperature of the stator between a preselected maximum temperature, the preselected maximum being a temperature below a safety device activation temperature, and a preselected minimum temperature, the preselected minimum temperature being a temperature at which the motor housing will not be below a dew point of air surrounding the motor.

13. The method of claim 12 further including the steps of

providing a controller;

providing a flow control device in communication with the controller;

providing a stator temperature monitoring device in communication with the controller;

providing a rotor temperature monitoring device in communication with the controller; and

wherein the step of adjusting the flow of refrigerant to the stator further includes providing a signal from the stator temperature monitoring device to the controller indicative of the stator temperature, providing a signal from the rotor temperature monitoring device to the controller indicative of the rotor temperature, the controller determining whether the stator temperature is within a predetermined temperature range an whether the rotor temperature is within a predetermined temperature range, the controller sending a signal to the flow control device to adjust the flow of refrigerant when the stator temperature or the rotor temperature is not within the predetermined temperature range.

14. The method of claim 10 further including the step of providing a subcooler in fluid communication with a condenser and the stator, the subcooler providing high pressure subcooled refrigerant liquid to the stator.