Application problem of PWM AC drives due to long cable length and high dv/dt

Abstract

Advances in power electronics technology have improved the performance and output waveforms of PWM voltage source inverters. Switching frequencies of 2 to 15 kHz with 0.1 mus rise times are common with the current IGBT technology while allowing for power levels over 200 kW. While the high switching speeds and zero switching loss schemes drastically improve the performance of the PWM inverters, the high rate of voltage rise (dv/dt) of 0 to 600 V in less than 0.1 mus has adverse effects on the motor insulation and bearings and deteriorates the waveform quality. Long cables contribute to a damped high frequency ringing due to the distributed nature of the cable leakage inductance and coupling capacitance (L-C) at the motor terminals resulting in over voltages which further stress the motor insulation. Voltage reflection is a function of inverter output pulse rise time and the length of the motor cables, which behave as a transmission line for the inverter output pulses.