Torque ripple minimization for induction motor drives with direct torque control (DTC)

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 paper, the basic concept of DTC will be improved by modifying the classical method. Problems which arising 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.