Determination of mechanical resonances in induction motors by random modulation and acoustic measurement

Acoustic noise emission from PWM-VSI inverter driven induction motors is a well-known problem. The generated noise depends mainly on two factors: the harmonic content of the supply voltage waveform; and the mechanical resonance frequencies of the induction motor. It is necessary to know the mechanical resonances to optimize the switching scheme of the inverter to reduce the noise emission. Measuring the resonance frequencies is normally a laborious process. This paper introduces a new method for estimating the mechanical resonances from acoustic noise measurements. Employing random modulation of the output voltage of the inverter, the voltage spectrum can be spread over a wide frequency range. Using the spectrum analysis of the measured acoustic noise, the resonances can easily be estimated as the dominant frequencies of the acoustic noise spectrum. The method was tested on a 2.2 kW induction motor at no load and full load. Some of the mechanical resonances of the motor at no load were calculated by the Holzer method. The calculations showed good agreement with the sound measurements. The influence of the fan cowl was investigated and it is concluded that this is an important factor in the acoustic noise emission. Finally, it was observed that new mechanical resonances appeared when the induction motor was loaded by a pump drive system, and they were estimated by the measurement technique