CONTROL STRATEGIES FOR DIRECT TORQUE CONTROL USING DISCRETE PULSE-MODULATION

This paper presents a scheme for direct torque control of ac machines using discrete pulse-modulated inverters such as the resonant dc link inverter. A unique feature of the control method presented is that only one current sensor in the dc link is required. The scheme is of particular importance for the realization of traction drives rated in the 10-100 kW power range for electric cars as well as a potential replacement for conventional open-loop constant V/f drives. Excellent control of torque, stator flux, and motor current waveshapes is seen to be obtained without affecting the high dynamic performance associated with direct self control (DSC) of machines. By defining a control that does not use the zero voltage vector, an observer is purposed for estimation of the motor current using only one current sensor in the dc link. A direct bang-bang (sliding mode) torque controller is then derived on the basis of this current observer. Experimental verification of simulation results are included on a resonant dc link inverter drive with a switching frequency of 27 kHz. The inverter switching scheme is implemented using a real-time Motorola DSP56000 digital signal processor-based controller.