Take a look back upon the past decade of direct torque control [of induction motors]

More than ten years have passed since the direct torque control strategy of an induction motor was first reported. The strategy has featured direct control of a stator flux vector and an output torque instead of the conventional current control technique which had been employed in the field-oriented control, and has given a systematic solution to improve the operating characteristics of not only the motor but also of the PWM inverter. In this paper, looking back upon the fruitful past decade and looking at the present state of the strategy, several key techniques are reviewed. One of the techniques is improvement of the basic performance which was enabled by means of a double-parallel PWM inverter, and has made ultimately quick torque response possible. Another technique is relevant to robust control, which makes the system insensitive to both stator and rotor resistance. Also, the latest technique, which contributes to enhance system performance further, is also referred to.