Synchronised PWM strategies based on space vector approach. II. Performance assessment and application to V/f drives

The harmonic distortion due to the four space-vector-based synchronised PWM strategies discussed in Part I of this paper is studied in the whole linear modulation range. Different strategies give the best performance for different ranges of fundamental voltage V₁. A hybrid PWM using the different strategies at appropriate voltages is proposed. Theoretical and experimental results show that this hybrid PWM results in a significant reduction in harmonic currents in V/f drives with a base frequency of 50 Hz and different maximum switching frequencies f_sw(max) of the inverter. Experimental results show that the current transients during change of pulse number are also reduced. Compared to the other real-time, open-loop, synchronised PWM techniques with a uniform sampling rate, the proposed hybrid PWM shows superior performance in the medium-voltage range. In the high-voltage range, its performance is inferior only to that of the more complicated high-voltage suboptimal (HVSO) strategy. With f_sw(max)=450 Hz, the harmonic distortion in the whole speed range of the drive is brought down to less than that of a six-step operation, which is regarded as an acceptable level for such low switching frequencies. This has not been possible with other comparable strategies