Acoustic noise radiated from direct torque controlled induction motor drives

Acoustic noise emissions from a direct torque controlled (DTC) induction motor drive arc 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 aiid torque hystcresis controllcrs and the cquivalent modulation indices can have a significant influence on the switching frequency and tlic spread spectrum of thc harmonic content. The acoustic noise is comparccl 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 spcctra and tonal acoustic noise, DTC and RPWM coiitrollcd drives have similar spread spectrum features and atonal noise emissions which are generally rcgarded as lcss irritating. However, the widcband of their harmonic spectra is more likely to induce mechanical resonances, and thus may sometimes result in higher noise emissions. The inlluence of the load and the flux lcvel on the einittcd noisc is investigated, and it is shown that the noise level increases with the load, but there is an optimal flux level for minimal noise.