Shaping the SPL spectra of the acoustic noise emitted by inverter-fed induction motors

Acoustic noise emissions from PWM-VSI driven induction motors is a serious environmental problem. This paper reports on a novel approach to alleviating this problem by influencing the spectra of the sound pressure level (SPL) of the emitted noise. The main objective of the method is a reduction of the effect of mechanical resonances on the resultant SPL spectra. In the first step, the resonances are being identified by using a random pulse width modulation (RPWM) with a uniform probability density function (PDF) of the carrier frequency or, alternatively, by using the sigma-delta modulation (SDM); in the latter case, the current spectrum is inherently flat. Then, the target PDF is being constructed so as to account for the machine properties (notably by avoiding mechanical resonances). The initial results obtained using this approach are promising, particularly in terms of lower acoustic power concentrated in the vicinity of resonance frequencies (as compared to the case of uniform PDF). The resultant noise profile is less annoying to the human ear. Moreover, the total SPL can also be reduced.