Optimum PWM Waveforms of an Inverter for Decreasing Acoustic Noise of an Induction Motor

Pulsewidth modulation (PWM) waveforms for decreasing the acoustic noise of an induction motor are presented and relate to earlier work concerning the loss of minimum PWM waveforms. The proposed control scheme combines merits of PWM control with those of pulse amplitude modulation (PAM) control to achieve silent operation of the induction motor. Calculation of the optimum PWM waveforms is complicated because the function representing the PWM wave contains many variables to converge and many modes in initial values. For example, when the pulse number of the PWM wave is 225, the number of variables is 112 and the initial values would be about 4 x 1023. Therefore, to get the optimum PWM wave, the convergence calculation of 112 variables using steepest-hill-gradient procedure must be tried with every mode of the waveforms. It is shown how to solve such difficult problems, and examples are presented of solved PWM waveforms. Through use of experimental means, acoustic noise is measured and better noise reduction of 10-15 dB can be achieved, compared with the noise of PAM operation.