Direct torque control of induction motor drives

Direct torque control (DTC) is an emerging technique for controlling 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, it 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 of an IM drive is also reviewed. Then the concept of DTC is illustrated and three DTC-based strategies [i.e. switching table (ST), direct self control (DCS), space vector modulation (SVIM)] are described. The ST strategy is dealt with in detail, discussing the results which can be obtained with different choices of the switching table. Problems associated with the selection of the amplitude of the hysteresis bands of the flux and torque controllers are illustrated by means of experimental data. Merits and limits of DTC are reported and a comparison with FO control is given