High-performance off-line pulse width modulation without quarter wave symmetry for voltage-source inverter

Half-wave symmetry (HWS) and quarter-wave symmetry (QWS) are traditionally associated with high waveform quality of the inverter output voltage, particularly when the ratio of switching frequency to the modulating frequency (i.e. pulse number, P) is quite low. This paper investigates and establishes the possibility of high-quality pulse-width modulated waveforms with HWS, but without QWS, for P = 7. The performance of proposed PWM without QWS is evaluated and compared with that of optimal PWM with QWS. The total harmonic distortion (THD) of line current and the weighted total harmonic distortion (WTHD) of line-line voltage, pertaining to the two PWM methods, are compared through simulation and experimental studies on a 3kW induction motor drive. The theoretical and experimental results establish that the proposed PWM is superior to the optimal PWM with QWS at any modulation index (M) below 0.6 and also in the range of M between 0.88 and 0.94, where M = 1.0 corresponds to six-step operation.