Asynchronous Motor Direct Torque Control System For Starting And Low Speed Performance Improvements Of The Method,

At first, the direct torque control (DTC) theory is analyzed in this paper. Then a scheme of grading start-up control with current hysteresis band is proposed on the base of analysis of three different start-up methods including of serial start-up, parallel start-up, hybrid start-up in DTC of Asynchronous Motor. The problems of large ripples of torque and distortion of stator flux owing to using zero voltage vector when stator current is large in start-up process are solved by grading start-up control with current hysteresis band method. So the amplitude limitation of flux and torque are respectively grading controlled that torque and flux increase slowly in start-up process. The instability of start-up process, as much as the failure of start-up are avoided in the DTC Asynchronous Motor. In a word, the start-up performance of IM DTC system is greatly improved through the scheme of grading control with current hysteresis band in start-up.Secondly, an improved integration algorithm with amplitude compensate is proposed to solve the problems of large ripples of torque and stator flux depression at low speed using the single integration algorithm in the UI model. The modified strategy takes place of the single integration to overcome the problem of the integrator drift because of direct current offset in the UI model. The twelve-section control method of voltage select table is presented to deal with the defects of traditional six-section control of voltage select table. The defects are depression of stator flux and distortion of stator current when stator flux is near the section line. The improved integration algorithm method and the twelve-section voltage vector select table method comprise an improved method which can improve the performance of the system markedly.At last, simulation models of the conventional DTC, and the improved DTC are separately established based on the MATLAB/ Simulink platform. Simulation results can prove that these improved methods can decrease ripples of torque and improve the performance of start-up, stator flux and whole system in DTC system effectively.