Control strategies of direct torque control of induction motor applied to traction

A control strategy of a direct torque control (DTC) system for an induction motor fed with a GTO voltage source inverter applied to railway traction is introduced. Because the switching frequency cannot be high in the condition of high power GTO inverter applications, different control methods are adopted for different speed ranges: (1) when the speed is higher than the rated speed, a weakened flux DTC is used; (2) when it is lower than 30% of the rated speed, a kind of indirect torque control is employed to reduce the ripple of electromagnetic torque; and (3) if the speed is from 30% to 100% rated speed, the ordinary DTC with hexagon stator flux trace is applied to reduce the switching frequency. Both simulations and experimental results are presented in this paper.