Applications of induction motor drive based on DTC in railway traction

In this paper, schemes of direct torque control (DTC) for railway traction over the whole speed range are proposed. In order to observe the phase angle and amplitude of stator flux accurately, a flux observer based on a U-N model is presented. In the low-speed range, the locus of stator flux is controlled to be a circle to reduce torque ripples, and the schemes of start-up and the selection method of voltage vectors are proposed in this paper. In medium-speed and high-speed range, the locus of stator flux is controlled to be hexagonal to reduce switching frequency. In order to obtain good steady state and dynamic performance, a DTC scheme under square wave conditions in the high-speed range is proposed. All of the methods and schemes are implemented on a 500 kW converter and a 190 kW induction motor, and are proved to be effective by experiment results.