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

Pulse-width modulation (PWM) of a motor drive at a fixed switching frequency leads to harmonic currents in narrow bands around integral multiples of the switching frequency resulting in tonal frequency acoustic noise components, which are irritating to the human ear. Random variation of carrier frequency to spread the spectrum poses challenges in device loss calculation, thermal design, and closed-loop controller design. Random pulse positioning is not quite effective at high speeds of the drive, where the problem of acoustic noise is most pronounced. This paper proposes a simple variable-frequency PWM and evaluates the performance of the same experimentally on a 6-kW induction motor drive. The experimental results show that the acoustic noise is spread uniformly over a range of frequencies without introducing low-frequency current harmonics. Compared with fixed-frequency PWM, the dominant acoustic noise is reduced significantly over a wide range of speeds. The variable-frequency PWM is further shown to reduce line current total harmonic distortion at high speeds of the drive. The computational effort required by the variable-frequency PWM is only marginally higher than that required by fixed-frequency PWM.