Abstract
A novel random pulse width modulator with variable switching frequency but constant sampling frequency is proposed for application in voltage source inverters for adjustable-speed AC drives. The decoupling of these two frequencies allows the optimal sampling ratio for closed-loop control purposes to be maintained, while the randomly varying switching ratio results in the transfer of harmonic power of the inverter output voltage into the continuous power density. This mitigates the undesirable acoustic, vibration, and EMI phenomena associated with fixed-frequency switching. A novel, so-called harmonic spread factor is introduced for quality evaluation of voltage spectra of inverters with random PWM. Practical realization of the modulator using the TMS320F240 DSP controller is described, and the results of simulations and experiments are presented.
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