Crankcase heater systems and methods for variable speed compressors

1. A system comprising:

a compressor including a shell housing a compression mechanism driven by an electric motor in an on state and not driven by the electric motor in an off state;

a variable frequency drive that drives the electric motor in the on state by varying a frequency of a voltage delivered to the electric motor and that supplies electric current to a stator of the electric motor in the off state to heat the compressor;

a control module connected to the variable frequency drive that controls a speed of the electric motor in the on state and that controls the electric current supplied to the stator of the electric motor in the off state;

a compressor temperature sensor that generates a compressor temperature signal corresponding to a compressor temperature;

wherein the stator heats the compressor for a first time period and the control module receives the compressor temperature signal, determines a rate of change of the compressor temperature over a second time period, after the first time period, and calculates an amount of current to supply to the stator based on the rate of change.

2. The system of claim 1

wherein the control module controls the electric current supplied to the stator of the electric motor in the off state to maintain the temperature of the compressor above a predetermined temperature threshold.

3. The system of claim 2 , wherein the temperature sensor measures a temperature of a lubricant in a lubricant sump of the compressor.

4. The system of claim 2 , wherein the temperature sensor measures a temperature of the compression mechanism.

5. The system of claim 1 , further comprising:

an ambient temperature sensor that generates an ambient temperature signal corresponding to an ambient temperature;

wherein the control module receives the ambient temperature signal, determines a desired compressor temperature based on the ambient temperature, compares the compressor temperature with the desired compressor temperature, and determines an amount of electric current to supply to the stator in the off state based on the comparison.

6. The system of claim 5 , wherein the control module determines the desired compressor temperature based on a sum of the ambient temperature and a predetermined temperature threshold.

7. The system of claim 6 , wherein the predetermined temperature threshold is between ten and twenty degrees Fahrenheit.

8. The system of claim 1 , further comprising:

a second temperature sensor that generates a second temperature signal corresponding to at least one of a temperature of an inverter board of the variable frequency drive, a temperature of a power factor correction module of the variable frequency drive, and a suction tube temperature;

wherein the control module receives the second temperature signal, determines a desired compressor temperature based on the second temperature, compares the compressor temperature with the desired compressor temperature, and determines an amount of electric current to supply to the stator in the off state based on the comparison.

9. A method comprising:

driving a compression mechanism of a compressor with an electric motor by driving the electric motor with a variable frequency drive that varies a frequency of a voltage delivered to the electric motor in an on state, and not driving the compression mechanism with the electric motor in an off state;

heating the compressor by supplying electric current to a stator of the electric motor with the variable frequency drive to heat the stator of the electric motor in the off state;

controlling a speed of the electric motor in the on state with a control module connected to the variable frequency drive;

controlling, with the control module, the electric current supplied to the stator of the electric motor in the off state;

generating a compressor temperature signal corresponding to a compressor temperature with a compressor temperature sensor;

heating the compressor with the stator for a first time period;

receiving the compressor temperature signal with the control module;

determining, with the control module, a rate of change of the compressor temperature over a second time period, after the first time period;

calculating, with the control module, an amount of current to supply to the stator of the electric motor based on the rate of change.

10. The method of claim 9 , further comprising:

controlling, with the control module, the electric current supplied to the stator of the electric motor in the off state to maintain the temperature of the compressor above a predetermined temperature threshold.

11. The method of claim 10 , wherein the predetermined temperature threshold is zero degrees Fahrenheit.

12. The method of claim 11 , wherein generating the temperature signal includes measuring a temperature of a lubricant in a lubricant sump of the compressor.

13. The method of claim 10 , wherein generating the temperature signal includes measuring a temperature of the compression mechanism.

14. The method of claim 9 , further comprising:

generating an ambient temperature signal corresponding to an ambient temperature with an ambient temperature sensor;

receiving, with the control module, the ambient temperature signal;

determining, with the control module, a desired compressor temperature based on the ambient temperature;

comparing, with the control module, the compressor temperature with the desired compressor temperature;

determining, with the control module, an amount of electric current to supply to the stator of the electric motor in the off state based on the comparison.

15. The method of claim 14 , wherein the determining the desired compressor temperature is based on a sum of the ambient temperature and a predetermined temperature threshold.

16. The method of claim 9 , further comprising:

generating a second temperature signal corresponding to at least one of a temperature of an inverter board of the variable frequency drive, a temperature of a power factor correction module of the variable frequency drive, and a suction tube temperature, with a second temperature sensor;

receiving the second temperature signal with the control module;

determining, with the control module, a desired compressor temperature based on the second temperature;

comparing, with the control module, the compressor temperature with the desired compressor temperature;

determining an amount of electric current to supply to the stator of the electric motor in the off state based on the comparison.