Influence of the fan cowl on the acoustic noise radiated from PWM controlled induction machines

The influence of the fan cowl, particularly the material of which it is made, on the acoustic noise emitted from PWM controlled induction machines is investigated. It is shown that there are significant harmonic excitation forces which can cause mechanical resonances of the fan cowl. The mechanical behaviour of both metal and plastic cowls is measured, using impulse force tests and random PWM excitation. The noise spectra and sound pressure level of a 1.5 kW 3-phase induction machine when equipped with a metal and a plastic fan cowl are investigated, and the noise spectra when the machine is supplied from sub-harmonic PWM, space vector PWM and random PWM inverter are compared. It shows that a metal fan cowl can significantly amplify the noise level, whilst the noise level with a plastic fan cowl is effectively the same as that with the cowl removed.