Open loop control of hysteresis band amplitudes in direct torque control of induction machines

In direct torque control (DTC) of induction machines there are flux and torque ripples, since none of the selected inverter´s vectors are able to generate the stator voltage required to produce the desired changes in the electromagnetic torque and stator flux linkage in most of the switching instances. Also, when direct torque control is implemented in a fully digital form these ripples will increase due to some characteristics of discrete signal processing. In this paper, the influence of the amplitudes of flux and torque hysteresis bands and the sampling time of the control program on the torque and flux ripples are investigated. New formulas are then derived for hysteresis band amplitudes to reduce flux and torque ripples and prevent the inverter switching frequency exceeding a desired limit. Extensive simulation results confirm the superiority of the DTC under the proposed method over the conventional DTC