Direct torque control of induction machine with variable amplitude control of flux and torque hysteresis bands

In this paper, effects of the hysteresis bands on the direct torque control (DTC) of an induction machine are investigated, and a method to control the switching frequency of inverter by a variable amplitude of the hysteresis band is proposed for DTC of an induction machine. A major drawback of the conventional DTC is an unpredictable inverter switching frequency which varies due to operating speed, load condition and parameters of the induction machine. Therefore the amplitude of the hysteresis band should be large enough to avoid excessive inverter switching at any operating region, which inevitably causes relatively large torque ripple especially in the low speed region. This paper proposes an effective switching frequency regulation method which consists of hysteresis band controller and switching frequency command generator. The effectiveness of the proposed strategy is analyzed and compared with the conventional method.