Abstract:
Nowadays, the most popular control method to effectively control both the speed and the torque of an induction motor is the vector control. However, the inverter which supplies the power to the motor can produce only voltages with limited patterns and amplitudes, and this limitation of the inverter's capability has been neglected in the vector control system. This leads to the development of the direct torque control (DTC) method which integrates the inverter limitation into the torque and flux control of the motor. Since the switching timing of the inverter in the DTC is determined by the hysteresis comparators, the resultant switching pattern is irregular or unpredictable, and the implementation needs a very high speed hardware comparable to that of the analog circuit. To overcome these problems, a novel DTC method is proposed in this thesis. The new DTC generates a regular switching pattern with a predetermined sequence similar to that of the conventional PWM, and owing to the predictive-algorithm-based DTC scheme, it is suitable for digital implementation. By calculating the appropriate time intervals of the active vectors and zero vectors generated by the PWM inverter, both the stator flux and the torque can be regulated within their specified limits. Simulation and experimental results confirm the feasibility of the proposed DTC method