User manual

Operation

VSD Setpoint (VSD Fan Speed Setting)

If the VSD Fan Speed Control (Service Menu, Setpoints) is set for Manual, the VSD Fan Speed

Setpoint (Service Menu, Setpoints) may be set for the desired speed of the variable speed motor.

Depending on the product control design, there may be an internal minimum speed, as defined by

that specific product operation, while the customer input may be set for 0-100%:

• Fan speed may be set locally at the unit using the Liebert iCOM display.

• Fan speed may be set remotely via a BMS signal (sent via Modbus using an optional Liebert

IntelliSlot 485 card), which then transmits to the unit local control.

3.1.2 General Compressor Requirements

Low-Pressure Time Delay

When the compressor starts, the low-pressure input is ignored for a selected period of time based on

the setting of the Low Pressure Alarm Delay (Service Menu, Options Setup). This time is usually set

to 3 minutes on air-cooled units, and to 0 or 1 minute on water cooled units. When this time is

expired, a second timer starts to operate if the low-pressure input is active. This second timer is active

during normal compressor operation to avoid compressor trips due to bubbles in the refrigerant or

other influences creating short trips of the low-pressure switch. The low-pressure device input is

ignored if the compressor is not operating. Exception: Pump Down (see Pump Down).

Pump Down

Pump Down is applicable to compressorized systems if equipped for pump-down operation, which is

set at the factory. This operation prevents the compressor oil from being diluted with liquid

refrigerant to ensure that the compressor is properly lubricated for the next startup.

The Pump Down operation operates in the following manner:

Whenever the control determines that no more cooling is required and a compressor needs to be shut

off, the liquid line solenoid valve (LLSV) is closed (de-energized). The compressor will continue to

operate until the low suction pressure device (LPS or LPT) opens, which shuts off the compressor. If

the LP device does not open within a specified time, the LLSV is turned On, then back Off (the

assumption is that the LLSV is stuck). If, after three times, the LP device does not open, the

compressor and LLSV are locked off and an alarm “Pump Down not completed” will appear.

There is a re-pump down if the LP device opens again after the compressor has been already

stopped—a maximum of six re-pump-down cycles per hour are allowed. At the seventh request of re-

pump down the alarm “Comp 1 Pumpdown Fail” or “Comp 2 Pumpdown Fail” will appear and the

compressor will be locked out.

Pump down is always performed loaded (for compressors with unloaders: unloaders off, digital scroll:

control solenoid valve disabled).

For digital scroll only: when pump down has finished successfully (LP device opened), pump down will

be continued for another half-second with the control solenoid valve energized.

NOTE

Low-pressure condition could be read through contacts or through pressure transducers with

threshold setting.