Reduction of acoustic noise emission in AC-machines by intelligent distributed random modulation

AC-machines driven by PWM-VSI emit acoustical noise. The dominating frequencies in the acoustical noise emission spectra are placed around the switching frequency of the inverter and multiples of this. To reduce the emitted noise a quasi random modulation strategy is proposed. Quasi random modulation means that the switching frequency is varied within a prespecified frequency band. The modulation gives the emitted noise a more `acceptable´ sound for the human ear due to the distinct tones being less significant. The modulation strategy is improved in order to distribute the output voltage spectra more uniformly. Measurements show an almost uniform distribution of the voltage spectra when a special weight function of the distribution of the random modulation frequency is used. Further improvements can be done by considering the mechanical resonance frequencies of the AC-machine. Tests on a pump system shows that omitting the switching frequencies around estimated mechanical resonance frequencies of the AC-machine and load reduces the emitted acoustical noise significantly. It is concluded that an almost silent AC-machine is obtained even at low switching frequencies