A limited-pool multi-stage random PWM method optimized for adjustable speed drives

This paper proposes a new technique, the so-called limited-pool multi-stage random PWM (RPWM) technique, to optimize the selection of the random switching frequencies throughout the speed range typical of adjustable speed drives (ASD). To this end, the speed range is divided into four different stages based on the changes of the voltage spectra as function of the inverter fundamental frequency. For each stage, a novel equation selects the optimal limited pool of switching frequencies that should be used randomly within the stage. The feasibility of the proposed method is illustrated by analyzing experimental voltage spectra for the fixed-switching-frequency PWM, single-pool RPWM and proposed limited-pool multistage RPWM cases