REDUCED COMPRESSOR CAPACITY CONTROLS

Abstract

A method of controlling the frequency of a compressor in a chiller system is disclosed. The method includes determining a nominal minimum frequency of a compressor, and providing a control algorithm for controlling the period of time and the deviation of the actual operating frequency below the nominal minimal frequency that the compressor may operate without shutting down the chiller system. The time period of operation below nominal minimum frequency may be predetermined as a fixed time period of operation below nominal minimum frequency, or as a variable time period based on the amount by which the frequency deviates below nominal minimum frequency. Also, an absolute minimum operating frequency is provided that results in the control algorithm shutting down the chiller system and the compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a general system configuration of the present invention.

FIG. 2 illustrates schematically one embodiment of a variable speed drive used in the present invention.

FIG. 3 illustrates schematically a refrigeration system that can be used with the present invention.

FIG. 4 illustrates a flow chart of one embodiment of the present invention.

FIG. 5 illustrates a partial flow chart of a preferred embodiment of the present invention.

Claims

1. A method of controlling the frequency of a compressor in a chiller system comprising the steps of: monitoring an operating frequency of a compressor;comparing the operating frequency to a nominal minimum frequency for the compressor;comparing the operating frequency to a predetermined absolute minimum frequency for the compressor in response to the operating frequency being less than the nominal minimum frequency, the predetermined absolute minimum frequency being less than the nominal minimum frequency;operating the compressor for a predetermined time period in response to the operating frequency being less than the nominal minimum frequency and the operating frequency being greater than or equal to the predetermined absolute minimum frequency, andshutting down the compressor in response to an expiration of the predetermined time period of the operating frequency being less than the nominal minimum frequency.

2. The method as set forth in claim 1, also comprising the step of: preventing unloading of the compressor below the predetermined absolute minimum frequency.

3. The method as set forth in claim 1, wherein the predetermined time period is a fixed time period of operation below the nominal minimum frequency.

4. The method as set forth in claim 3, wherein the fixed time period is in the range of five to ten minutes.

5. The method as set forth in claim 1, wherein the predetermined time period is a variable time period based on an amount the operating frequency is below the nominal minimum frequency.

6. The method as set forth in claim 1, wherein the predetermined absolute minimum frequency is a predetermined amount below the nominal minimum frequency.

7. The method as set forth in claim 6, wherein the predetermined amount below the nominal minimum frequency is about 10 Hz.

8. The method as set forth in claim 1, wherein the absolute minimum frequency is a predetermined percentage of nominal minimum frequency.

9. The method as set forth in claim 8, wherein the predetermined percentage of nominal minimum frequency is about 80%.

10. The method as set forth in claim 1, comprising the additional steps of: monitoring the time period during which the actual operating frequency is less than the nominal minimum frequency, in response to the actual operating frequency dropping below the nominal minimum frequency; andcontrollably shutting down the chiller system in response to the monitored time period exceeding a predetermined interval.

11. The method as set forth in claim 5, comprising the additional steps of: controlling the predetermined time period inversely with a magnitude of a deviation of the operating frequency below the nominal minimum frequency, wherein for a small amount of deviation, the predetermined time period is greater than the predetermined time period for a large amount of deviation.

12. The method as set forth in claim 1, comprising the additional steps of: aggregating fluctuations in operating frequency for an intermediate time period between zero and a maximum time period, wherein the intermediate time period being computed based on a fundamental maximum parameter.

13. The method as set forth in claim 12, wherein the maximum parameter is a function of Hz-seconds.

14. The method of claim 1, wherein the chiller system includes at least one additional compressor; and the method also includes the steps of controlling a capacity of the chiller system by selectively operating the at least one additional compressors until the compressor is the last operating compressor and overriding a shutdown control of a capacity control program, in response to the capacity control program having an operating frequency below the minimum frequency corresponding to a decreased capacity demand.

15. A chiller system comprising: a refrigerant circuit comprising a compressor, a condenser arrangement and an evaporator arrangement connected in a closed loop;a drive arrangement connected to the compressor to power the compressor, the drive arrangement comprising a motor and a variable speed drive;a controller for controlling the frequency of the compressor, the controller configured to: monitor an operating frequency of the compressor;compare the operating frequency to a nominal minimum frequency for the compressor;compare the operating frequency to a predetermined absolute minimum frequency for the compressor in response to the operating frequency being less than the nominal minimum frequency, the predetermined absolute minimum frequency being less than the nominal minimum frequency;operate the compressor for a predetermined time period in response to the operating frequency being less than the nominal minimum frequency and the operating frequency being greater than or equal to the predetermined absolute minimum frequency; andshut down the compressor in response to an expiration of the predetermined time period of the operating frequency being less than the nominal minimum frequency.

16. The chiller system of claim 15, wherein the predetermined time period is a fixed time period of operation below nominal minimum frequency.

17. The chiller system of claim 15, wherein the predetermined time period is a variable time period based on an amount the operating frequency deviates from the nominal minimum frequency.

18. The chiller system of claim 15, wherein the predetermined absolute minimum frequency is about 10 Hz below the nominal minimum frequency.

19. The chiller system of claim 15, wherein the absolute minimum frequency is about 80% of nominal minimum frequency.

20. The chiller system of claim 15, wherein the controller is further configured to: monitor the time period during which the actual operating frequency falls below nominal minimum frequency, in response to the actual operating frequency dropping below the nominal minimum frequency; andcontrollably shut down the chiller system in response to the monitored time period exceeding a predetermined interval.

21. A computer program product embodied on a computer readable medium and executable by a microprocessor for controlling the frequency of a compressor in a chiller system, the computer program product comprising computer instructions for executing the steps of: monitoring an operating frequency of a compressor;comparing the operating frequency to a nominal minimum frequency for the compressor;comparing the operating frequency to a predetermined absolute minimum frequency for the compressor, the predetermined absolute minimum frequency being less than the nominal minimum frequency;operating the compressor for a predetermined time period in respect to the operating frequency being less than the nominal minimum frequency and the operating frequency being greater than the predetermined absolute minimum frequency.; andshutting down the compressor in response to an expiration of the predetermined time period wherein the operating frequency is less than the predetermined absolute minimum frequency.

22. The computer program product of claim 21, wherein the computer program product further includes computer instructions for executing the step of: preventing unloading of the compressor below the predetermined absolute minimum frequency.

23. The computer program product of claim 22, wherein the step of preventing unloading is executed by setting the operating frequency of the compressor equal to the absolute minimum frequency.

24. The computer program product of claim 21, wherein the predetermined time period is a variable time period based on an amount the operating frequency deviates from the nominal minimum frequency.

25. The computer program product of claim 24, wherein the computer program product further includes computer instructions for executing the step of: controlling the predetermined time period inversely with a magnitude of a deviation of the operating frequency below the nominal minimum frequency, wherein for a small amount of deviation, the predetermined time period is greater than the predetermined time period for a large amount of deviation.

26. The computer program product of claim 21, wherein the computer program product further includes computer instructions for executing the steps of: aggregating fluctuations in operating frequency for an intermediate time period between zero and a maximum time period, wherein the intermediate time period being computed based on a fundamental maximum parameter.

27. The computer program product of claim 26, wherein the maximum parameter is a function of Hz-seconds.