Control Types; Proportional Control; Pi Control - Emerson Liebert Challenger 3000 Operation & Maintenance Manual

Evaluate the start operation of the unit (with pump down).
1. Close LLSV
2. Wait for LPS to make or ignore for WSK time
If LPS is made before WSK has expired, allow the compressor to continue running, if not then
stop compressor and alarm low pressure alarm.
3. Once LPS makes start compressor.
4. Verify the alarm phrase for the WSK and the low pressure switch.
5.2

Control Types

5.2.1

Proportional Control

This is a standard control method that maintains the room at a temperature proportional to the load.
The temperature maintained increases as the room load increases. At full load the room would be con-
trolled at a temperature equal to the temperature setpoint plus the temperature sensitivity.
5.2.2

PI Control

The PI control combines two individual terms to determine the control output for a given set of condi-
tions. Note that PI control is used only for temperature. If PI control is selected, humidity will con-
tinue to use proportional control.
The proportional (P term) is determined by the difference between the current temperature and the
control setpoint. This term is expressed in % cooling (heating) desired for each degree above (below)
the setpoint. It is adjustable from 0% to 100% per degree. The purpose of this term is to adjust the
control output for any deviation between the current temperature and the control setpoint.
The integral (I term) is determined by two things: the difference between the temperature and control
setpoint and the amount of time this difference has existed. This term is expressed in % cooling (heat-
ing) desired for each minute and degree above (below) the setpoint. It is adjustable from 0% to 100%
per degree-minute. The purpose of this term is to force the control to maintain the temperature
around the setpoint by slowly but continuously adding (subtracting) a small amount of cooling (heat-
ing) to the total control output until the temperature is at the setpoint.
A suggested tuning procedure is as follows:
1. Initially adjust the integral setting to 0%.
2. Starting with 20%, adjust the proportional setting in small increments (10% steps) until the
control sustains a constant hunting action (the temperature swings are approximately the same
amplitude from one peak to the next).
3. Note the time in minutes between peaks of adjacent temperature swings and the amplitude of the
temperature swing (degrees above the setpoint).
4. Adjust the proportional control setting to about 1/2 the value obtained in Step 2.
5. Adjust the integral setting to a value calculated by the following equation:
NOTE
If this calculation results in a value of less than 1%, then set the integral to 1%.
The tuning procedure above is only an approximation for an initial set of adjustments and are based
on the "average" room characteristics. Your particular settings may need to be further adjusted for
optimum PI control performance. Some suggestions for additional tuning are as follows:
• If cooling output overshoot is occurring on load changes, decrease the proportional setting.
• If system hunting occurs with constant room load, decrease the integral setting.
• If the control responds too slowly, resulting in large temperature excursions on a load change,
increase the proportional setting.
• If a constant temperature deviation exists between the temperature and setpoint, increase the
integral setting.
Approximate room load (in % full load)
Time between peaks x peak amplitude x 4
30
Operation