Variable Switching Frequency PWM Technique for Induction Motor Drive to Spread Acoustic Noise Spectrum with Reduced Current Ripple

Acoustic noise is one of the undesirable consequences of the harmonic currents fed into an induction motor by a voltage source inverter. If the inverter is modulated at a fixed switching frequency, the resulting current harmonic components are located in narrow bands around integral multiples of the switching frequency. This causes discrete tonal noise which is unpleasant to human ear. This paper experimentally evaluates a variable switching frequency PWM method on an 8-hp motor drive both from acoustic noise and total harmonic distortion (THD) perspectives. Experimental results show that the acoustic noise is spread uniformly over a range of frequencies without introducing low-frequency current harmonics. The variable frequency PWM is further shown to reduce the line current THD at high speeds of the drive, compared to fixed frequency PWM.