As the capacity of refrigeration compressor varies with the system compression ratio so does the power required to drive it. As the suction pressure reduces so does the power required – however this is not in direct proportion (see manufacturers catalogues)

To save costs, manufacturers match motors to suit different applications, say Low, Medium or High Temp /air conditioning ranges.

This means that Low Temp motor /compressor units may not have sufficient power to suit operation when operating with high back pressures during ‘pull down’ or hot gas defrost.

If the overload is of relative short duration the motor would possible be OK. If NOT an automatic ‘crank case regulating valve’ is one solution. (I can remember having to slowly opening the suction vale on large compressors during start-up to prevent the motor overloading and the ‘rattle’ of the compressor sprung heads if opened too quickly so as to allow liquid to came flooding back from flooded type evaporators).

The fitting of a ‘crank case regulating valve’ is only for this purpose and is not intended as a means of capacity control. It should be set to be fully open at or slightly higher than design suction pressure. If selected correctly – with a low pressure drop at normal design conditions it should have little effect on system capacity.

Larger air conditioning water chillers can have the same problems when starting after a prolonged off period when the chilled water temperature can rise to that of the ambient. In these cases the control system modulates the compressor capacity controls
in response to the motor lull load power.

These days electronic expansion valves can be configured to provide the necessary protection.