Acoustic noise radiated from direct torque controlled induction motor drives

Acoustic noise emissions from a direct torque controlled (DTC) induction motor drive are investigated. Due to the hysteresis control of the flux and torque, DTC results in a variable switching frequency and dispersed voltage and current harmonic spectra. It is shown that both the bandwidths of the flux and torque hysteresis controllers and the equivalent modulation indices can have a significant influence on the switching frequency and the spread spectrum of the harmonic content. The acoustic noise is compared with that which results with space vector PWM (SVPWM) and random PWM (RPWM), and is shown that, in contrast to SVPWM, which results in cluster harmonic spectra and tonal acoustic noise, DTC and RPWM controlled drives have similar spread spectrum features and atonal noise emissions which are generally regarded as less irritating. However, the wideband of their harmonic spectra is more likely to induce mechanical resonances, and thus may sometimes result in higher noise emissions. The influence of the load and the flux level on the emitted noise is investigated, and it is shown that the noise level increases with the load, but there is an optimal flux level for minimal noise.