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

The authors introduce a new method for estimating the mechanical resonances from acoustic noise measurements. By random modulation of the output voltage of the inverter, the voltage spectrum can be spread over a wide frequency. By FFT-analyzing the measured acoustic noise, the resonances can easily be estimated as the dominant frequencies of the acoustic noise spectrum. The method is 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 are calculated by the Holzer method. The calculations show some agreement with the sound measurements. The influence of the fan cowl is investigated and it is concluded that this can be an important factor in the acoustic noise emission. Finally, it is observed that new mechanical resonances appear at load operation when the induction motor is mounted on a pump system and they are estimated by the measurement technique