Improving pulse width modulation in order to get a silent AC motor

When using sinusoidal PWM for controlling inverters feeding induction motors, the inverter operates with fixed switching frequency. If this frequency is under 20 kHz, the motor emits noise which is annoying for human beings. One solution is to use a triangular waveform with a frequency higher than 20 kHz, but this will result in increased losses in the electronic devices and the motor. Another solution is to spread out the harmonic spectrum. This paper describes two strategies for decreasing the acoustic noise of an induction motor by reducing the concentration of harmonic energy in distinct tones. The strategies implemented are generation of a triangular waveform with randomized frequency for each fixed subcycle and for each random subcycle of the output frequency. Simulation and experimental results show the absence of acoustic noise concentrated at specific tones. The motor emits a whistling sound which is not annoying to human beings.