Setting Up Pi Control; Compressor Control; Figure 22 Digital Scroll Capacity Modulation, 10-100% Variable; Table 6 Pi Control Troubleshooting - Emerson Liebert iCOM Manual

3.2.1

Setting Up PI Control

Three to 5 minutes integration time is usually good for most applications. The proportional band may
depend on the amount of heat load and other factors. Use Table 6 as a guide for making the PI
control settings.
Table 6 PI control troubleshooting
Problem
Unit is too slow to activate
cooling.
Compressor short cycle alarm.
Excessive valve oscillation or
hunting.
Temperature Deadband
The deadband range is used to widen the setpoint and prevents small temperature changes from
activating compressors and valves. When the air temperature falls within the deadband, the control
operates the same as if the temperature equaled the setpoint exactly. This setting helps maximize
component life by preventing excessive component cycling. Figure 21 illustrates how the
temperature proportional band and deadband is evenly divided on either side of the temperature
setpoint.
The AutoSet Enable parameter, found in Service Menu, Setpoints, automatically sets the
proportional bands for temperature and humidity, and both the integration time factors according to
the type of unit (chilled water, single or dual compressor).
NOTE
Before the proportional or integral setpoints can be changed, the Auto Set Enable must be
changed to NO.
3.2.2

Compressor Control

Depending on its type, a Liebert cooling unit may have a different quantity of compressors with or
without unloaders or variable capacity.
Digital Scroll Compressors
A digital scroll compressor can modulate its capacity anywhere between 10-100%. This variable
capacity modulation allows cooling units to control an environment more precisely.
Digital scroll capacity modulation is achieved by energizing and de-energizing a solenoid valve on the
compressor. When the solenoid valve is de-energized, the compressor capacity is 100%. When the
solenoid valve is energized, the compressor capacity is zero. Therefore, the capacity of the compressor
depends on how long the solenoid is de-energized for. If the solenoid is de-energized for 10-seconds,
then energized for 5 seconds during a 15-second cycle, the resulting capacity will be 66% as shown in
Figure 22.

Figure 22 Digital scroll capacity modulation, 10-100% variable

Call for Cooling: 66%
100% Loaded
0% Loaded
® ®
Liebert iCOM for Liebert DSE
Solution
Decrease the proportional band slightly and monitor operation.
Increase the proportional band slightly and adjust the integration time to 3-5 min.
Monitor compressor run time. Run must be higher than 3 minutes to avoid short
cycle from occurring.
Widen the temperature proportional band and/or consider increasing the
integration time.
Solenoid Solenoid
De-energized Energized
0 5 10
15-Second Capacity
Modulation Cycle
™
Solenoid
De-energized
15 20
15-Second Capacity
Modulation Cycle
28
Operation
Solenoid
Energized
25 30