228 Actual signals and parameters
Typically two different PID controller sets are used when the load of the motor
changes considerably from one situation to another.
You can use External PID (PID2), defined in Group 42: Ext / Trim PID, in 2 different
ways:
• Instead of using additional PID controller hardware, you can set outputs of the
ACS320 to control a field instrument like a damper or a valve. In this case, set
Parameter 4230 to value 0. (0 is the default value.)
• You can use External PID (PID2) as an additional PID controller to Process PID
(PID1) to trim or fine-tune the speed of the ACS320.
An example of the trimming is a return fan that follows the speed of the supply fan. As
the return fan needs to run faster or slower then the supply fan in order to create
under- or overpressure, correction factors to the supply fan speed are needed. Use
External PID (PID2) in the return fan drive to provide these corrections.
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