Generalized harmonic loss curves of AC motors fed from two and three level inverters

Optimized and different sinusoidal PWM methods without and with the addition of different zero-sequence harmonics are generally investigated and compared for both two and three level inverters. The paper examines these PWM techniques from the point of view of the AC motor harmonic losses on the basis of the generalized curves, which are independent of the commutation frequency of the inverter. It is shown that these curves can be given only for the limited region of the fundamental voltage. The accuracy of these curves will be better than 1% if the frequency of the carrier wave is at least forty (without a third harmonic addition) or fifty (with a third harmonic addition) times higher than the fundamental frequency. The comparison of the optimized and the sinusoidal PWM methods show that for the bipolar and dipolar modulations the motor harmonic losses will be 20-30% higher, for the unipolar modulations by several times more than for the optimized method. For unipolar modulation the better results can be obtained by using the three symmetrically shifted carrier waves.