Research On Direct Torque Control System For Six-Phase Induction Motor

Compared with three-phase speed control system, multi-phase speed control system has many advantages like higher power, lower torque ripple, better steady and dynamic performance. With the characteristic of high reliability, the motor can still be running with one or more open-circuited phases by adjusting control strategy. Thus, in the applications with high-reliability requirement like ship propulsion, multi-phase system is becoming more widespread. Direct torque control and vector control are the most commonly used strategies in the modern high-performance AC speed control field. Compared with vector control, direct torque control avoids rotating aixs conversion and the torque response is more rapid.Research content in this thesis is direct torque control of six-phase induction motor under the normal condition and the fault condition. First of all, mathematical models of six-phase induction motor under the normal condition and one phase open-circuited condition are built, by using the space vector decoupling transformation theory. Method to control the motor under one phase open-circuited condition is found by analyzing mathematical model, in which a set of three-phase stator windings needs to be removed. And mathematical model of six-phase induction motor is built, with a set of three-phase stator windings removed.Moreover, for the stator harmonic current is large in the six-phase induction motor, space vector pulse width modulation(SVPWM) algorithm based on the theory of space vector decoupling transformation is utilized to synthesize virtual voltage vector. And this thesis expands application of the fundmental voltage liner-output over-modulation algorithm, from three-phase motor speed control system to six-phase motor speed control system. Validity of the algorithm is verified by simulation.Once more, the traditional direct torque control theory is researched and a novel direct torque control algorithm is proposed in the thesis. The realization process of traditional direct torque control is given, and in order to improve the performance of system, this thesis proposes a novel direct torque control algorithm based on fuzzy control theory and changing duty cycle technology, the validity of this algorithm is also verified by simulation.At last, hardware circuit of direct torque control system is given and experiments have been carried out to test performance of algorithms. Experiments include traditional direct torque control algorithm and the novel direct torque control algorithm, with the motor operating in normal condition and a set of three-phase stator windings removed condition, respectively. Experimental results verified the validness of algorithms in this thesis.