Torque Pulsations in Induction Motors with Inverter Drives

Torque pulsations in induction motors would have an average value of zero and therefore are generally neglected by inverter induction machine system designs. A simple equivalent circuit method is presented for estimating the magnitude of torque fluctuations under steady-state operating conditions based on single-phase equivalent circuits. The method indicates that torque fluctuations are due mainly to the interaction of fundamental flux in the air gap at harmonic rotor currents. Inverters producing a 6-stepped voltage waveform, the predominant pulsating torque is at the sixth harmonic and the magnitude of the fluctuation is independent of operating frequency, provided that constant volts per Hz is maintained. The method is extendable to PWM inverter-machine systems with similar conclusions with respect to variation of torque over the input frequency range. However, for the PWM inverter with a fixed number of pulses per cycle, the torque pulsations can approach or exceed full-load average torque. The method outlined is simple and amenable to hand calculations.