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

Electric motors operated on fixed period PWM waveforms emit objectionable, predominantly tonal, acoustic noise in addition to the more usual motor noises. Spread spectrum switching is presented 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 preferable, 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