Minimization of torque ripple in direct torque control of induction motor at low speed

This paper presents a new technique to minimize the torque ripples inherited in DTC of induction motor. The typical discrete-based DTC imposes delay time which frequently allows the torque to overshoot beyond hysteresis bands. This triggers the selection of reverse voltage vectors which, in turn, cause large torque decrements. The torque ripples become of great significance at low speeds where torque overshoot is most likely to occur due to steep positive torque slope. A multi-level DC link voltage is proposed to vary the DC voltage of Voltage Source Inverter (VSI) according to motor's speed. By varying the DC link voltage, the torque slopes can be controlled and, hence, the torque overshoots can mostly be avoided. Therefore, the torque ripples are significantly minimized. The viability of proposed technique has been validated using MATLAB/Simulink software. Results show the proposed technique yielded over 50% reduction in the RMS torque ripples while maintaining a low switching frequency.