Research Of Direct Torque Control For Nine-phase Induction Motor

Compared with the conventional three-phase variable-speed drive system, multiphase system with phase numbern≥5 has many advantages, such as the capability of using the existing low voltage power devices to realize high power drives, rich control sources and multi-control freedoms, better fault tolerance so higher reliability, etc. Due to these advantages and the increasing demands on high performance and high power variable-speed drives, the research on multiphase system has experienced a significant growth. Multiphase system has many similarities with three-phase system in principle, but also many new features. So there are still some problems in theory and practices which are worth research and need to be improved. This paper has researched deeply on the direct torque control strategy of nine-phase induction motor on the basis of a lot of reading.First of all, the MMF of multiphase motor is briefly analyzed using the theory of winding functions, and the advantages of multiphase motors like multi-control freedom, rich control sources and higher reliability are revealed. A multiphase motor with phase numbern≥5can be decoupled to multiple orthogonal planes and each orthogonal plane can be controlled individually. The mathematical model of a multiphase induction motor is established, laying the mathematic and theoretical foundations for later research.The principles and structure of direct torque control strategy are elaborated. The voltage space vectors of nine-phase machines are described according to the topology of the multi-phase voltage source inverter. And the role of the voltage vectors in the changes of torque and stator flux is analyzed. The u-n flux observer model used in this paper is introduced. On this basis, the method of direct torque control and the principle of selecting vectors are presented, and a detailed simulation of a DTC controlled nine-phase system is carried out using the Matlab7.1 software.The conventional direct torque control strategy is a hysteresis control method based on switching table, can only control the direction of the changes of torque and flux (increase or decrease), but not the accurate variation, leading to torque ripple. Besides, there is no voltage modulation in the structure of the direct torque control system. And the control of torque and stator flux is carried out in the stator coordinate system. When used in multiphase system, only the basic plane, namely theα1-β1 plane can be controlled. When a needed voltage vector is selected in the basic plane, vectors with non-zero amplitudes are generated in other orthogonal planes, leading to harmonic currents, resulting in the phase current distortion and electrical noise. This is the inherent defects of the conventional DTC. A torque predictive control method based on SVPWM is presented in this paper to solve these problems. In this method, the required voltage vector is derived from torque difference and flux difference according to the mathematical model of the multiphase motor. And multiphase SVPWM is used to synthesis the required voltage vector. The accurate and rapid compensation of torque difference and flux difference can be realized, and stator harmonic currents can be eliminated in this method. It can also be easily extended to non-sinusoidal power supply technology.Finally, the theory is verified on a multiphase variable-speed drive system.