Research On Direct Torque Control Of Asynchronous Traction Motor And Its Control Strategies In Low Speed Range

The development history of the electric drive system for locomotives is presented, the tendency of which is discussed through analyzing the requirement of railway traction to the drive system. The direct torque control of two-level voltage source inverter-fed asynchronous traction motor is an important branch, and because of the problems of big toque and flux ripple related to the low speed range, the control strategies in low speed rang is a hot topic now. So the control strategies of asynchronous traction motor in low speed range are of special consideration in this thesis.According to the methodology, which runs between system and parts time by time, various control methods of asynchronous motor are discussed. The flux ripple, the current ripple and the torque ripple of the traditional direct torque control (DTC), indirect torque control based on three phases asynchronous sinusoidal pulse width modulation (ITC) and the direct torque control based on space vector modulation (SVM-DTC) are studied by using of Matlab/Simulink, the advantages and disadvantages of the methods are pointed out. Based on the simulation results and the analysis of the cause of torque ripple a new direct torque control method based on flux hysteresis comparator and space vector modulation is presented which comprises the advantages of the traditional DTC, which has minimum torque ripple, and the SVM-DTC, which has constant switching frequency. The new method has the capacity to control the frequency used to control the stator flux by using of the flux hysteresis comparator and can operate at quasi-constant frequency by using of space vector modulation. The request of the traction applications to limits the torque ripple con be matched by set a proper flux hysteresis limits. The cardinal principle of the new method is presented and Matlab simulations have made to verify the performance of which. Finally, a summary of the achievements of the thesis and problems needed to study are presented.