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 frequ ency drive (VFD) consisting of a pulsewidth modulation (PWM) inverter with insulated gate bipolar transistor (IGBT) power devices, Factors c ontributing to a motor overvoltage transient equal to a theoretical tw ice de bus voltage are first described using existing transmission lin e analysis, A critical cable distance l(c) is defined where this 2-pu overvoltage occurs, However, literature is lacking on how motor voltag e transients >2-pu bus voltage and up to 3-4 pu are generated, This ph enomenon is observed on all PWM inverters with output cable lengths gr eater than l(c) distance, Contributing factors to the >2-pu overvoltag e phenomenon are investigated by exploring the complex interaction bet ween drive modulation techniques, carrier frequency selected, cable na tural frequency of oscillation, cable high-frequency damping losses, a nd, to a lesser extent, inverter output rise time, Theoretical calcula tions of cable frequency and damping are correlated with simulation an d experimental results, Novel modifications to the PWM modulator, as w ell as external hardware apparatus, are proposed solutions to the >2-p u overvoltage problem; both are simulated and experimentally confirmed .