5.2 Basic Concept and Principles of DTC

5.2.1 Basic Concept

The DTC principle was introduced in the late 1980s [11–13]. In contrast to vector control, which became accepted by drive manufacturers after 20 years of extensive research, DTC needed only just over a decade to really take off. A direct torque controlled induction motor drive has been manufactured commercially by ABB since the mid-1990s [5]. In the direct torque controller developed by ABB, the optimum inverter switching pattern is determined in every sampling period (25 µs). The core of the control system in DTC is the sub-system containing torque and flux hysteresis controllers and optimal inverter switching logic. An accurate machine model is also important, since estimation of the stator flux and motor torque is based on the machine model and the measurement of the machine input stator voltages and currents. The measurement of actual speed is not required [5]. A machine model in stationary reference frame is used to develop DTC theory. Detailed discussion regarding mathematical modeling of induction machines is presented in Chapter 6.

In the stationary reference frame, the stator flux linkage is the integral of the stator emf. If the stator voltage drop on stator resistance can be neglected, then the stator flux is the integral of the applied voltage. Hence, in a short period of time the increments of stator flux are proportional to the applied voltage. It therefore follows that the inverter output voltage space vector ...

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