Series AC Inverters User's Manual

Section 8: PI Controller8.1 IntroductionE-trAC X2C inverters have a built-in PI (Proportional-Integral)Controller that makes it possible to control a process by adjusting motorspeed using a reference input and a feedback input. When PI control isenabled, several new parameters (as well as new data codes for someexisting parameters) become available to support PI control.This section Þrst provides an overview of how PI control operates.Following this discussion, the new parameters and data codes aredescribed.8.2 Overview of PI ControlFigure 17 on the next page provides a ßowchart of PI control. Thecharacteristics of PI control are set with the 81-PRGNO parameter (seepage 81). The data codes for parameter 81-PRGNO allow you to select:¥ Direct- or reverse-acting loopIn a direct-acting loop, a positive error will cause an increase inoutput frequency. Conversely, in a reverse-acting loop, a positiveerror will cause a decrease in output frequency.¥ Slow or fast rate of integrationA slow rate of integration is usually selected for processes withlong time constraints (for example, thermal and ßuid levelcontrols). On the other hand, a fast integration rate is utilized forprocesses with short time constraints (such as mechanicalsystems and pressure loops).¥ Whether feed-forward is activeFeed-forward is useful in situations where the reference valuehas a direct relation to the feedback signal, such as controllingmotor speed in a closed loop. Note that feed-forward should beenabled when attempting to close a speed loop.¥ Whether the PI Controller is turned on and off via the PS3 inputSeparate parameters are also available for adjusting proportional gain(parameter B3-KP), integral gain (parameter B4-KI), and scaling for thefeedback terminal VIN2 (parameter B5-KIN). These parameters may beadjusted while the inverter is operating.© 1998 TB Wood’s All Rights Reserved 73