Scroll compressor differential pressure control during compressor shutdown transitions

1. A method comprising:

while a lag compressor is in an ON state, determining whether an OFF criterion is satisfied to transition the lag compressor of a tandem set of scroll compressors from the ON state to an OFF state, wherein the tandem set of scroll compressors comprises a lead compressor;

initiating at least one process in response to the OFF criterion being satisfied; and

maintaining the lag compressor in the ON state for a predetermined period subsequent to the OFF criterion being satisfied,

wherein the at least one process comprises at least one of

overriding a called for opening percentage of the lead compressor including operating the lead compressor at a maximum level,

overriding a motor overload protection method, wherein the motor overload protection method protects motors of the lead compressor and the lag compressor, and

overriding a proportional integral derivative (PID) method to reduce a speed of a condenser fan, wherein the PID method controls the speed of the condenser fan.

2. The method of claim 1 , wherein the maximum level is equal to 100%.

3. The method of claim 1 , wherein:

the at least one process comprises a scroll method; and

the scroll method comprises

determining a loading duration of the lead compressor,

operating the lead compressor at the maximum level,

starting a load duration timer and generating a timer signal, and

deactivating the scroll method and ramping the lead compressor to a predetermined value when the timer signal is greater than the loading duration.

4. The method of claim 3 , further comprising:

determining a cooling value; and

determining the predetermined value based on the cooling value, wherein the predetermined value is a pulse width modulation percentage for the lead compressor.

5. The method of claim 4 , further comprising:

determining a temperature error value; and

determining a number of the compressors in the tandem set that are activated,

wherein the cooling value is determined based on the temperature error value and the number of the compressors that are activated.

6. The method of claim 1 , wherein the at least one process comprises:

copying an overload temperature threshold used by the motor overload protection method from a first memory location to a second memory location;

overwriting the overload temperature threshold stored in the first memory location with a predetermined value;

controlling an opening percentage of an electronic expansion valve based on the predetermined value;

starting an overwrite timer and generating a timer signal;

comparing the timer signal to an expansion valve duration; and

changing the predetermined value at the first memory location back to the overload temperature threshold when the timer signal is greater than the expansion valve duration.

7. The method of claim 6 , wherein the changing of the predetermined value at the first memory location back to the overload temperature threshold comprises ramping the predetermined value to the overload temperature threshold at a predetermined rate.

8. The method of claim 6 , further comprising:

maintaining the opening percentage of the electronic expansion valve at a predetermined percentage based on a proportional integral derivative (PID) method prior to initiating the at least one process;

decreasing the opening percentage of the electronic expansion valve subsequent to initiating the at least one process; and

increasing the opening percentage of the electronic expansion valve to the predetermined percentage when the timer signal is greater than the expansion valve duration.

9. The method of claim 1 , further comprising:

controlling a speed of the condenser fan based on a first maximum speed prior to turning OFF the lag compressor; and

controlling the speed of the condenser fan based on a second maximum speed after turning OFF the lag compressor,

wherein the at least one process comprises determining the second maximum speed based on the first maximum speed, a first predetermined value, a second predetermined value, and a condenser fan reduction exponent.

10. The method of claim Si the at least one process further comprises ramping up the speed of the condenser fan (i) subsequent to controlling the speed of the condenser fan based on the second maximum speed, and (ii) based on the first maximum speed.

11. The method of claim 9 , further comprising:

determining a cooling value;

determining the first predetermined value based on the cooling value, wherein the first predetermined value is a first pulse width modulation percentage for the lead compressor before the lag compressor is turned OFF; and

determining the second predetermined value based on the cooling value, wherein the second predetermined value is a second pulse width modulation percentage for the lead compressor after the lag compressor is turned OFF.

12. The method of claim 11 , further comprising:

determining a temperature error value; and

determining a number of the compressors in the tandem set that are activated,

wherein the cooling value is determined based on the temperature error value and the number of the compressors that are activated.

13. The method of claim 1 , further comprising:

controlling a speed of the condenser fan based on (i) the PID method, and (ii) a first maximum speed prior to turning OFF the lag compressor, wherein the speed of the condenser fan is at an original speed prior to the lag compressor being turned OFF;

reducing the first maximum speed to a second maximum speed after turning OFF the lag compressor;

adjusting the speed of the condenser fan to the second maximum speed; and

ramping up the speed of the condenser fan from the second maximum speed to the original speed.

14. The method of claim 1 , wherein the at least one process comprises:

operating the lead compressor at the maximum level;

overriding the motor overload protection method; and

overriding the PID method.

15. A system comprising:

a lag compressor module configured to, (i) while a lag compressor is in an ON state, determine whether an OFF criterion is satisfied to transition the lag compressor of a tandem set of scroll compressors from the ON state to an OFF state, and (ii) maintain the lag compressor in the ON state for a predetermined period subsequent to the OFF criterion being satisfied, and wherein the tandem set of scroll compressors comprises a lead compressor; and

a lead compressor module configured to, in response to the OFF criterion being satisfied, at least one of,

override a called for opening percentage of the lead compressor including operating the lead compressor at a maximum level,

override a motor overload protection method, wherein the motor overload protection method protects motors of the lead compressor and the lag compressor, and

override a proportional integral derivative (PID) method to reduce a speed of a condenser fan, wherein the PID method controls the speed of the condenser fan.

16. The system of claim 15 , further comprising:

a load duration timer configured to determine a loading duration of the lead compressor; and

an overwrite timer configured to generate a timing signal,

wherein the lead compressor module is configured to, if the OFF criterion is satisfied, perform a scroll method including

determining a loading duration of the lead compressor via the load duration timer,

operating the lead compressor at the maximum level, and

deactivating the scroll method and ramping the lead compressor to a predetermined value when the timing signal is greater than the loading duration.

17. The system of claim 15 , further comprising an expansion valve module configured to, if the OFF criterion is satisfied:

copy an overload temperature threshold used by the motor overload protection method from a first memory location to a second memory location;

overwrite the overload temperature threshold stored in the first memory location with a predetermined value;

control an opening percentage of an electronic expansion valve based on the predetermined value;

start an overwrite timer and generating a timer signal;

compare the timer signal to an expansion valve duration; and

change the predetermined value at the first memory location back to the overload temperature threshold when the timer signal is greater than the expansion valve duration.

18. The system of claim 15 , further comprising:

a condenser fan module configured to control (i) a speed of the condenser fan based on a first maximum speed prior to turning OFF the lag compressor, and (ii) the speed of the condenser fan based on a second maximum speed after turning OFF the lag compressor; and

a control module configure to, if the OFF criterion is satisfied, determine the second maximum speed based on the first maximum speed, a first predetermined value, a second predetermined value, and a condenser fan reduction exponent.

19. The system of claim 15 , further comprising a condenser fan module configured to:

control a speed of the condenser fan based on (i) the PID method, and (ii) a first maximum speed prior to turning OFF the lag compressor, wherein the speed of the condenser fan is at an original speed prior to the lag compressor being turned OFF;

reduce the first maximum speed to a second maximum speed after turning OFF the lag compressor;

adjust the speed of the condenser fan to the second maximum speed; and

ramping up the speed of the condenser fan from the second maximum speed to the original speed.

20. The system of claim 15 , wherein the lead compressor module is configured to:

operate the lead compressor at the maximum level;

override the motor overload protection method; and

override the PID method.