Reduced torque ripple and constant torque switching frequency strategy for direct torque control of induction machine

The switching strategy of a conventional direct torque control (DTC) scheme which is based on hysteresis comparator results in a variable switching frequency which depends on the speed, flux, stator voltage and the hysteresis band of the comparator. This paper proposes a new switching strategy which compares the torque error from a PI controller with two triangular waveforms (180° phase shift) hence resulting in a constant torque switching frequency. Two triangular waveforms are necessary to ensure four-quadrant operation of the DTC drive. The switching frequency of the DTC drive is determined by the frequency of the triangular waveforms and is almost independent from the speed. Simulation and experimental results are presented to validate this proposed scheme