Direct torque control of induction motor for low speed drives considering discrete effects of control and dead-time of inverter

The direct torque control of an induction motor is an outstanding method based on instantaneous space vector theory. However the low-speed performance of this method is unsatisfactory. Some papers have been published to improve the performance of DTC, but they are often too complicated or not applicable to very low-speed areas. This paper investigates the torque characteristics of an induction motor in the low-speed range in a fully digital control system. The discrete effects of the control have been analysed in the paper. One simple scheme based on predictive control of the torque is proposed. Another problem is the dead-time effect in PWM inverters which influences the low-speed performance. A method of compensation by adding a dead-time vector to the flux calculation is realised. Simulation results show that, with the proposed schemes, very good dynamic response and steady state precision can be obtained and this is confirmed by experiments with a digital control structure using a TMS320C25 digital signal processor.