Parasitic Effects of PWM-VSI Control Leading to Torque Harmonics in AC Drives

Abstract

Precise torque control without pulsations is one of the major quality issues in pulse-widthmodulated voltage-source inverter (PWM-VSI) drives. Theoretically, it could be postulated that atfrequencies of some kHz, the machine’s inertia absorbs switching frequency torque harmonics, andthe resulting torque becomes smooth; though, in reality, parasitic effects in voltage source invertersmay cause additional torque harmonics of low order. In particular, first, second and sixth torqueharmonics are observed. Such torque harmonics are especially dangerous for normal drive operation,since they may be amplified by drive train resonances at corresponding rotational velocities. Newparasitic effects in PWM-VSI control, leading to torque harmonic of low order, are described in thepaper, and recommendations for their compensation are given.