Scroll compressor differential pressure control during compressor startup transitions

1. A method comprising:

determining a cooling value;

comparing the cooling value to an activation point of a lead compressor, wherein the lead compressor is in a tandem set of scroll compressors of a cooling system, and wherein the tandem set of compressors comprises a lag compressor;

activating the lead compressor when the cooling value is greater than the activation point;

activating the lag compressor subsequent to activating the lead compressor;

determining whether a plurality of conditions exist including whether a first condition and a second condition exists, wherein

(i) the first condition is that an alarm associated with the lag compressor is generated, and

(ii) the second condition is that the lead compressor is deactivated; and

deactivating the lag compressor when at least the second condition exists in the cooling system.

2. The method of claim 1 , further comprising turning off the lead compressor when the alarm associated with the lag compressor is generated.

3. The method of claim 1 , further comprising:

while the lag compressor is in an ON state, determining whether a compressor minimum ON time for the lag compressor has expired;

rechecking the plurality of conditions when the compressor minimum ON time has not expired; and

maintaining operation of the lead compressor and the lag compressor when the minimum ON time has expired.

4. The method of claim 3 , further comprising determining a temperature error value based on an ambient temperature and a predetermined set point value,

wherein the cooling value is determined based on the temperature error and a number of activated compressors in the cooling system.

5. The method of claim 3 , further comprising:

starting a delay timer and generating a timer signal when the lead compressor is activated; and

maintaining operation of the lead compressor and the lag compressor in response to the timer signal being greater than a predetermined value.

6. The method of claim 3 , further comprising determining an outdoor ambient temperature,

wherein the lag compressor is activated when the outdoor ambient temperature is less than a predetermined ambient temperature.

7. The method of claim 3 , further comprising determining a suction pressure of at least one of the lead compressor and the lag compressor,

wherein the lag compressor is activated when the suction pressure is less than a predetermined suction pressure.

8. The method of claim 3 , further comprising:

starting a lag timer and generating a timer signal when the lag compressor is activated; and

maintaining operation of the lead compressor and the lag compressor in response to the timer signal being greater than the compressor minimum ON time.

9. The method of claim 3 , wherein:

the lead compressor is a digital variable capacity compressor; and

the lag compressor is a fixed capacity compressor.

10. The method of claim 1 , wherein when the cooling value is less than or equal to a lag compressor activation point and an ambient temperature is less than a predetermined temperature, the lag compressor is activated subsequent to activating the lead compressor.

11. The method of claim 10 , wherein:

the ambient temperature is determined if the cooling value is less than or equal to an activation point of the lag compressor; and

the predetermined temperature is associated with activating the lag compressor.

12. The method of claim 1 , wherein when the cooling value is less than or equal to a lag compressor activation point and a suction pressure of the lag compressor is less than a predetermined pressure, the lag compressor is activated subsequent to activating the lead compressor.

13. The method of claim 12 , wherein the suction pressure is determined if the cooling value is less than or equal to an activation point of the lag compressor.

14. The method of claim 1 , further comprising:

starting a delay timer subsequent to activating the lead compressor;

starting a lag timer subsequent to activating the lag compressor;

maintaining the lag compressor in an activated state while the first condition and the second condition do not exist and the lag timer has not expired; and

determining whether the delay timer has expired if the first condition and the second condition do not exist and the lag timer has expired.

15. The method of claim 1 , wherein both the lead compressor and the lag compressor are activated when cooling for a current operating condition is satisfiable by a cooling capacity of one of the lead compressor and the lag compressor.

16. The method of claim 8 , wherein the lag compressor is loaded when activated.

17. The method of claim 8 , wherein, while maintaining operation of the lead compressor and the lag compressor and prior to the timer signal being greater than the compressor minimum ON time, the lag compressor is loaded.

18. The method of claim 8 , wherein at least one of the lead compressor or the lag compressor is deactivated prior to the timer signal being greater than the compressor minimum ON time.

19. A method comprising:

determining a cooling value;

comparing the cooling value to an activation point of a lead compressor, wherein the lead compressor is in a tandem set of scroll compressors of a cooling system, and wherein the tandem set of compressors comprises a lag compressor;

activating the lead compressor when the cooling value is greater than the activation point;

activating the lag compressor subsequent to activating the lead compressor based on whether a pressure difference across at least one of the lead compressor or the lag compressor is less than a predetermined pressure difference;

determining whether a plurality of conditions exist including

(i) an alarm associated with the lag compressor is generated, and

(ii) the lead compressor is deactivated; and

deactivating the lag compressor when at least one of the plurality of conditions exists in the cooling system.

20. The method of claim 19 , further comprising:

detecting a suction pressure of a fluid received by the at least one of the lead compressor and the lag compressor;

detecting a head pressure of a fluid output from the at least one of the lead compressor and the lag compressor; and

determining the pressure difference across the at least one of the lead compressor and the lag compressor based on the suction pressure and the head pressure.