Crankcase heater systems and methods for variable speed compressors

1. A system comprising:

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

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;

wherein the control module receives the compressor temperature signal, determines a desired compressor 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.

2. 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 and determines the desired compressor temperature based on the ambient temperature.

3. The system of claim 2 , wherein the control module determines the desired compressor temperature as a sum of the ambient temperature and a predetermined temperature threshold.

4. The system of claim 3 , wherein the predetermined temperature threshold is between ten and twenty degrees Fahrenheit.

5. The system of claim 1 , further comprising:

a variable frequency drive temperature sensor that generates a variable frequency drive temperature signal corresponding to a variable frequency drive temperature;

wherein the control module receives the variable frequency drive temperature signal and determines the desired compressor temperature based on the variable frequency drive temperature.

6. The system of claim 5 , wherein the control module determines the desired compressor temperature as a sum of the variable frequency drive 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 , wherein the compressor temperature sensor measures at least one of a temperature of a lubricant in a lubricant sump of the compressor, a temperature of the compression mechanism, and a temperature of a stator of the electric motor.

9. The system of claim 1 , wherein the variable frequency drive varies the voltage delivered to the stator to achieve the determined amount of electric current supplied to the stator in the off state.

10. The system of claim 1 , wherein the control module determines a temperature difference between the compressor temperature and the desired compressor temperature and determines the amount of electric current to supply to the stator in the off state based on the temperature difference.

11. A method comprising:

controlling, with a control module connected to a variable frequency drive, a speed of an electric motor that drives a compression mechanism of a compressor by varying, with the variable frequency drive, a frequency of a voltage delivered to the electric motor in an on state;

controlling, with the control module, electric current supplied to a stator of the electric motor by the variable frequency drive to heat the stator of the electric motor in an off state, the compression mechanism not being driven by the electric motor in the off state;

receiving, with the control module, a compressor temperature signal generated by a compressor temperature sensor and corresponding to a compressor temperature;

determining, with the control module, a desired compressor temperature;

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

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

12. The method of claim 11 , further comprising:

receiving, with the control module, an ambient temperature signal generated by an ambient temperature sensor and corresponding to an ambient temperature;

wherein the control module determines the desired compressor temperature based on the ambient temperature.

13. The method of claim 12 , wherein the control module determines the desired compressor temperature as a sum of the ambient temperature and a predetermined temperature threshold.

14. The method of claim 13 , wherein the predetermined temperature threshold is between ten and twenty degrees Fahrenheit.

15. The method of claim 11 , further comprising:

receiving, with the control module, a variable frequency drive temperature signal generated by a variable frequency drive temperature sensor and corresponding to a variable frequency drive temperature;

wherein the control module determines the desired compressor temperature based on the variable frequency drive temperature.

16. The method of claim 15 , wherein the control module determines the desired compressor temperature as a sum of the variable frequency drive temperature and a predetermined temperature threshold.

17. The method of claim 16 , wherein the predetermined temperature threshold is between ten and twenty degrees Fahrenheit.

18. The method of claim 11 , wherein the compressor temperature sensor measures at least one of a temperature of a lubricant in a lubricant sump of the compressor, a temperature of the compression mechanism, and a temperature of a stator of the electric motor.

19. The method of claim 11 , further comprising varying the voltage delivered to the stator, with the variable frequency drive, to achieve the determined amount of electric current supplied to the stator in the off state.

20. The method of claim 11 , wherein the comparing the compressor temperature with the desired compressor temperature includes determining a temperature difference between the compressor temperature and the desired compressor temperature and wherein the determining the amount of electric current to supply to the stator in the off state includes determining the amount of electric current to supply to the state based on the temperature difference.