Liebert CHALLENGER 3000 - Manual (Page 40)

System Performance with Advanced Microprocessor Controls
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4.2.4 Humidification Operation
System Activation
The humidifier (infrared or steam) is activated when the humidity control calculates a requirement of
100% humidification, and deactivated when the requirement falls below 50%.
4.3 Control Types
4.3.1 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. If propor-
tional control is selected, the gain is factory set and cannot be adjusted by the user. Operator inputs
are the usual setpoint and sensitivity adjustments.
4.3.2 PID Control (Chilled Water or SCR Reheats only)
The PID control combines three individual terms to determine the control output for a given set of
conditions. Note that PID control is used only for temperature. If PID control is selected, humidity
will continue 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.
The derivative (D term) is determined by the rate of change of temperature. This term is expressed in
% cooling (heating) desired for each degree per minute rise (fall) in temperature. It is adjustable from
0% to 100% per degree/min. The purpose of this term is to adjust the control output for quickly chang-
ing temperatures, thus providing an anticipation control.
All three terms are adjusted through the “select control type” menu. If PID control is selected, the
temperature sensitivity setting is not used by the control.
For optimum performance, a PID control must be adjusted or tuned according to the characteristics of
the particular space and load to be controlled. Improper tuning can cause the control to exhibit poor
response and/or hunting. The characteristics of the space and load may change seasonally, so occa-
sional retuning is required for optimum performance.