Evaluation and minimization of low-order harmonic torque in low-switching-frequency inverter fed induction motor drives

Low-order harmonic pulsating torque is a major concern in high-power drives, high-speed drives and motor drives operating in over-modulation region. This paper considers the specific case of eliminating or minimizing the sixth-harmonic torque (τ₆) with a PWM waveform having only two switching angles per quarter. A method for evaluation of harmonic torques in the synchronously revolving dq reference frame is presented. This expression is used to propose an optimal PWM which minimizes τ₆. This paper also evaluates the frequency-domain based approximate solutions for elimination of τ₆ (both theoretically and experimentally), and brings out the divergence in performances of the different solutions. A condition to identify the best among these approximate solutions is presented. The best solution obtained through the frequency-domain approach is found to be close to the above optimal PWM.