PERFORMANCE EVALUATION OF A DIRECT TORQUE CONTROLLED DRIVE IN THE CONTINUOUS PWM-SQUARE WAVE TRANSITION REGION

This paper investigates the operation of a direct torque controlled dr ive when operating under transient conditions and when operating in ov ermodulation conditions or in the ''transition region'' to six-step op eration, The direct torque control is a dead-beat control of the torqu e and flux magnitude, in the steady-state, the stator voltage vector w hich drives the torque and flux to the reference value is calculated d uring each fixed switching period. Under transient or overmodulation c onditions, an alternative switching algorithm must be used since dead- beat control is no longer possible. Two alternatives are presented for operation in overmodulation. The first involves a determination of th e switching state a priori, and calculating the duty cycle for each ph ase based on the torque and flux error. A much simpler scheme is prese nted which utilizes the voltage reference vector from the direct torqu e control algorithm. This scheme, although not resulting in dead-beat control, is shown to provide very satisfactory performance in overmodu lation. The direct torque control method shows great promise for light traction applications where a large quasi-constant power region is re quired. The scheme operates very satisfactorily in overmodulation, com pared with existing current regulated PWM-based schemes, due to the fa ct that the voltage space vectors are directly controlled. A complete experimental evaluation of the proposed scheme operating in the transi tion region is also given.