Interaction of drive modulation and cable parameters on AC motor transients

This paper investigates overvoltage transients on ac induction motors when connected through a cable of arbitrary length to a variable frequency drive (VFD) consisting of a pulsewidth modulation (PWM) inverter with insulated gate bipolar transistor (IGBT) power devices. Factors contributing to a motor overvoltage transient equal to a theoretical twice dc bus voltage are first described using existing transmission line analysis. A critical cable distance [l.sub.c] is defined where this 2-pu overvoltage occurs. However, literature is lacking on how motor voltage transients >2-pu bus voltage and up to 3-4 pu are generated. This phenomenon is observed on all PWM inverters with output cable lengths greater than [l.sub.c] distance. Contributing factors to the >2-pu overvoltage phenomenon are investigated by exploring the complex interaction between drive modulation techniques, carrier frequency selected, cable natural frequency of oscillation, cable high-frequency damping losses, and, to a lesser extent, inverter output rise time. Theoretical calculations of cable frequency and damping are correlated with simulation and experimental results. Novel modifications to the PWM modulator, as well as external hardware apparatus, are proposed solutions to the >2-pu overvoltage problem; both are simulated and experimentally confirmed. Index Terms - Cable impedance, pulse elimination technique (PET), transient overvoltages, voltage reflection.