How to select, apply VFDs: Medium-voltage motor-starting application tutorial

Depending on the capacity and impedance of the utility supply transformer, this starting load may result in a significant supply voltage drop, impacting other users on the power system, and potentially triggering the utility to impose financial penalties. Once full 60Hz rated motor speed is achieved, the drive synchronizes the voltage and frequency of its output precisely with the voltage and frequency of the utility supplied line power. Since the voltage and phase have been synchronized, essentially no current flows between the line and the VFD, while both provide current to the motor. Because magnitude and phase are not synchronized and controlled, closing the bypass contactor during an open transition will create a significant mechanical torque transient as the utility supply attempts to pull the motor into sync, as well as corresponding current transients that may result in upstream overcurrent trips or unacceptable supply voltage drop, ce Lucas Paruch is the senior development engineering manager for medium voltage drives at Yaskawa America Inc. Edited by Mark T. Hoske, editor-in-chief, Control Engineering, WTWHMedia, mhoske@wtwhmedia.com.