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McQuay OM 751
Description of Operation
PI Control Parameters
Associated with each PI loop is a set of two adjustable parameters: Proportional Band and
Integral Time. When the unit ventilator is properly sized for the space, the factory settings for
these parameters provides the best and most robust control action (see Figure 20).
If field problems arise, first ensure these parameters are set back to the factory default settings.
If adjustment is required, only make small adjustments to one parameter at a time. After each
adjustment, allow enough time for the system to stabilize before making further adjustments.
If you do not have the means to graph the space performance, record the actual measured
value and set point for several minutes and then plot the results using a spreadsheet to
determine the correct action to change the PI parameter.
Figure 20: Optimized PI loop control
Proportional Band
The proportional band, or proportional action, causes the controlled output to changes in
proportion to the magnitude of the difference between the sensor value and set point.
A proportional band setting that is too small (see Figure 21) causes control oscillations that go
fully above and below the set point.
Figure 21: Proportional bands
A proportional band setting that is too large (see Figure 21) causes an offset between the actual
measured oscillation center and the set point. A small offset is not necessarily a problem since
most systems have a small “natural” offset and the integral function automatically works to
eliminate or reduce this effect.
CAUTION
Adjusting PI parameters can cause erratic unit operation,
and potentially damage the equipment.
PI control parameters should only be adjusted by trained
personnel having a complete understanding of how these
parameters affect system operation. Generally these parameters
do not need to be adjusted from the factory default settings.
