| In the field of AC machines control, Direct Torque Control (DTC) has become a focuswhich has many advantages such as new control method, simple structure, direct controlmeans and fast torque response. The basic idea of DTC is to control the motor torque directlythrough the voltage that acts on the stator flux, so as to control and regulate the rotationalspeed of the motor.This paper described the DTC basic theory of asynchronous motor systematically frommathematical model of asynchronous motor and analyzed the relationship of space voltagevector and stator flux, motor torque. According to different control strategies based on thestator flux, this article intruducs the principle and stucture of the the hexagonal flux trajectoryfirstly. Based on the simulation software Matlab/Simulink, the models of the hexagonalflux trajectory DTC are build. The results are obtained through simulation studies that it haslow flux switching frequency with large torque ripple especially in the low speed range. Inorder to improve the performance of the hexagonal flux DTC at low-speed, a circular fluxstajectory is analyzed and the simulation model of circular flux trajectory control DTC isbuild. It can be seen from the simulation result that the torque ripple is smaller with smothflux path, but the flux switching frequency is much high. Based on the two different controlmethods with their own control performance at low or high speed range, a model of full speedrange is build which uses circular flux control methods at low speed range and switches tohexagonal flux control at high range. The simulation studies show that these two models cancomplete the conversion smothly and reduce the switching frequecy of the flux effectively.In order to solve the problems of variable switching frequency and large flux and torqueripple, an advanced strategy (SVM-DTC) by combining the space vector pulse widthmodulation (SVPWM) and DTC together is researched. The hysteresis controller and voltagevector switching table are respectively replaced by PI controller and SVPWM method. The simulation research is done on the Matlab platform.The results show that the SVM-DTCstrategy can reduce the torque ripple and improve the tanditional DTC performance.An observer of full-order flux based on Layapunov stability theory is designed in orderto indentify the motor speed.The simulation model of speed sensorless DTC is build. It canbe seen from the simulation results that this method can track the speed accurately andcomplete the speed identification. |
