8.4 Compressor Control
The compressor will run only when the circuit is in a run or pumpdown state. This means the compressor should not be
running any time the circuit is off or during preopening the EXV.
Cycle Timers
A minimum time between starts of the compressor and a minimum time between shutdown and start of the compressor
will be enforced. The time values are set by global circuit set points.
These cycle timers are enforced even through cycling of power to the chiller.
These timers may be cleared via a setting on the controller.
Compressor Run Timer
When a compressor starts, a timer will start and run as long as the compressor runs. This timer is used in the alarm log.
Compressor Capacity Control
After starting, the compressor will be unloaded to the minimum physical capacity and no attempt to increase compressor
capacity is made until the differential between evaporator pressure and oil pressure meets a minimum value.
After the minimum differential pressure is met, compressor capacity is controlled to 25%.
Compressor capacity will always be limited to a minimum of 25% while it is running, except for the time after compressor
start when the differential pressure is being built and except when changes to capacity are performed as needed to meet
unit capacity requirements (see unit capacity control section).
Capacity will not be increased above 25% until discharge superheat has been at least 12°C for a time of at least 30
seconds.
Manual capacity control
The capacity of the compressor can be controlled manually. Manual capacity control is enabled via a set point with choices
of auto or manual. Another set point allows setting the compressor capacity from 25% to 100%.
The compressor capacity is controlled to the manual capacity set point. Changes will be made at a rate equal to the
maximum rate that allows stable circuit operation.
Capacity control reverts back to automatic control if either:
• the circuit shuts down for any reason
• capacity control has been set to manual for four hours
Slide Control Solenoids (Asymmetric compressors)
This section applies to the following compressor models (asymmetric):