DTC-based strategies for induction motor drives

Direct torque control (DTC) is an emerging technique for controlling the PWM inverter-fed induction motor (IM) drives. It allows the precise and quick control of the IM flux and torque without calling for complex control algorithms. In principle, moreover, DTC requires only the knowledge of the stator resistance. The tutorial starts by reviewing the basic operation of an IM and of a PWM inverter using the space vector theory. The field-oriented (FO) control is also reviewed. Then the concept of DTC is illustrated and three DTC-based strategies (i.e. switching table (ST), direct self control (DSC), space vector modulation (SVM)) are described. The ST strategy is dealt with in depth, illustrating by means of experimental data the flux and torque responses generated with different choices of the switching table and the influence of the amplitude of the hysteresis bands of the flux and torque controllers on the drive performance. Furthermore, an improved stator flux estimator is presented which allows the system under to be kept under control even at standstill. Finally the sensitivity of the control system to parameter variations is presented