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.
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.
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.
30 | McQuay OM 751 |