Spread spectrum switching: low noise modulation technique for PWM inverter drives

Abstract

Electric motors operated on fixed period PWM waveforms emit objectionable, predominantly tonal, acoustic noise in addition to the more usual motor noises. These additional components essentially reflect the harmonic structure of the applied PWM waveform and occur as a series of tonal ‘combs’ centred on multiples of the PWM switching frequency. The tonal nature of the noise significantly increases its perceived annoyance. Spread spectrum switching is presented here as a technique which effectively ‘breaks up’ the characteristically tonal noise of a PWM drive, replacing it with a wideband atonal noise. This is achieved by pseudorandomly varying the instantaneous PWM switching frequency from one cycle to the next, resulting in the generation of broadband (‘whitish’) voltage, and consequently acoustic, spectra. Compared to a fixed period modulator, the emission of a broadband atonal noise from the driven motor is definitely prefeable, from a listener's perspective, with additional advantages arising from the minimal excitation of natural resonances in the drive system due to the broadband nature of the harmonic energy.