High frequency switching operation of PWM inverter for direct torque control of induction motor

The paper presents a novel simple method for high frequency switching operation of a PWM inverter for the direct torque control of an induction motor, which makes an acoustically silent drive possible. The conventional method cannot actually raise the switching frequency of the inverter even though hysteresis bandwidths for flux and torque control are reduced because of delay time in the feedback signals. In order to overcome the problem, the proposed method introduces dither signal injection into the conventional system by superposing a high-frequency and small-amplitude triangular, wave on the flux and torque errors respectively. According to simulation and experimental results, the stator flux ripple and torque ripple were reduced to 30 % compared, with those of the conventional operation, and the acoustic noise level was decreased to less than 56 (dBA) over the operating speed range from 100 to 1500 (RPM).