Incompatibility of Power Factor Correction Capacitors with AC PWM Drives

Usage of power factor (P. F.) correction capacitors is commonplace on many induction motors used on constant speed (across-theline) applications. A fixed KVAR (Kilovolt-Amp-Reactance) value of capacitors is sized to counteract the motor's inductive reactance, thereby raising the overall power factor as seen by the user's line. A bank of these oil-filled or dry type AC capacitors usually reside in a conspicuously placed metal enclosure nearby the motor. Beware of retrofitting any AC controllers (including VVI and CSI types) to existing induction motors! If the installation is not thoroughly examined and found to be free of P. F. capacitors between the motor and controller, unsatisfactory performance may result. As viewed by a controller working into a capacitor bank, the capacitors appear as a "short circuit" every time a switching transition occurs in the inverter section. This results in excessive peak currents, which will either cause electronic trips in the controller or stress the power semiconductors, leading to subsequent drive failure.