Enlarging switching frequency in direct torque-controlled inverter by means of dithering

This paper presents a novel, simple method to enlarge switching frequency in the direct torque-controlled inverter for an induction motor drive, which makes an acoustically silent motor operation possible. It is actually impossible to raise the inverter switching frequency in the conventional system, although the hysteresis bandwidths for the stator flux and torque control are sufficiently diminished, because of the delay in estimating the flux and the torque. In order to overcome the problem, the proposed method introduces a dithering technique into the conventional system by superposing triangular waves with high frequency and minute amplitude on the flux error and the torque error, respectively. According to the simulation and experimental results, the stator flux ripple and the torque ripple were suppressed to approximately 30% compared with those of the conventional system. Also, the acoustic noise level of the motor was reduced to less than 56 dB over the whole speed range from 100 to 1500 r/min.