Motor voltage spectrum and torque pulsations of optimized three-level inverters

The harmonic loss minimized PWM strategy for three-level inverters is investigated from the point of view of the motor voltage spectrum and torque pulsations. It is shown that for the optimized PWM, the motor voltage spectrum is “cleaner” (the low order components are absent) and there are smaller torque pulsations and smaller speed fluctuations in comparison to the case of the dipolar without or with third harmonic injection (low speed region) or the unipolar (for high speed region) modulations. For the relatively low value of the number of the commutations the low order torque components can be virtually eliminated. But for the rated speed and field weakening area, the low order voltage (and consequently torque) components can not be eliminated if the fundamental voltage is higher than 91% of its highest possible value