Equivalent 3-Level PWM: An Improved Technique to Reduce Torque Ripple in DI-WRIM Analyzed as DI-OWIM

In the supersynchronous mode of double-inverter-fed wound rotor induction machine (DI-WRIM) drive, torque ripple is dependent on the sum of the quadrature (q) axis stator and rotor flux ripples (termed as overall flux ripple). Therefore, in the existing method, the torque ripple is reduced by merely changing the slope of one of the flux ripples such that their sum is reduced. However, the torque ripple can be further reduced, as the overall flux ripple in DI-WRIM resembles the q-axis stator flux ripple of a double-inverter-fed open-end winding induction machine which is generally analyzed as a three-level neutral-point-clamped inverter fed squirrel cage induction machine (SQIM) (3L-SQIM) drive. Thus, in this article, the stator and rotor voltage source inverters are switched together and the resemblance is explored to develop stator-rotor pulsewidth modulation (PWM) combinations, such that the overall flux ripple is equivalent to the q-axis flux ripple of 3L-SQIM. The effect of phase shift between the stator and rotor voltage vectors on torque ripple is analyzed and a hybrid PWM combination is also proposed to minimize the same. The reduction in torque ripple, with the proposed combinations, is validated in terms of the estimated instantaneous and RMS torque ripples. The simulation and experimental results further validate this reduction.