A Reduced Torque Ripple Controller for Direct Torque Control of Induction Machines

Direct Torque Control (DTC) of induction machines (IM), based on hysteresis comparators and selection table, suffers from variable switching frequency and large torque ripple, especially at low speed. This is because the rate of change of torque and flux and hence the time taken for the torque and flux to reach their upper and lower bands, varies with operating conditions. In this paper a new torque controller with fixed switching frequency is proposed. The structure of the new controller is very simple, with its output similar to that of the conventional three-level hysteresis comparator. The approximate frequency domain analysis of the proposed controller is presented, which served as a guide for the controller's parameters selection. The simulation of the DTC of IM with the proposed torque controller is presented, which is verified by experimental results. The results indicate that the proposed controller is capable of maintaining the torque switching frequency constant and reducing the torque ripple, particularly at low speed.