Torque ripple minimization for induction motor drives with direct torque control

Direct torque control (DTC) schemes can produce quick and robust responses, but they usually behave like hysteresis controllers. Consequently, there are similar problems like non-constant switching frequencies and high torque ripple, especially when operation conditions result in low switching frequencies. In this article, the basic concept of DTC will be improved by modifying the conventional method. Problems that arise when using standard DTC methods are explained and improvements are derived to overcome these problems. Two theoretical approaches are verified and compared with standard DTC by measurements and experiments. The proposed methods provide a constant switching frequency and solve the starting problem without dither signal. Further results are a reduction of torque pulsations, as well as a new method for stator flux estimation.