Research On The Direct Torque Control Strategy Of High Power Asynchronous Motor

Currently,there are two kinds of high-efficiency control scheme of an induction motor: field oriented control(FOC)and direct torque control(DTC).Compared with field oriented control,direct torque control strategy has many advantages,such as simple structure,quick torque response and the robustness of the motor parameters.In high power traction,switching frequency of the inverter should be controlled at a low level on account of the economic cost of the system.The thesis focuses on the direct torque control strategy in high power applications at low switching frequency.Firstly,the basic theory of the conventional direct torque control system,the mathematical model of two-level voltage inverter and asynchronous traction motor is discussed.Next,the basic structure of direct torque control system are thoroughly analyzed,focused on the basic control algorithm of circular,eighteen-corner,and hexagonal flux,and the switch of strategies different flux trajectory strategies is presented.Then,in order to limit the switching frequency in high power traction,a simple algorithm is presented.The control scheme is based on the dynamic adjustment of torque hysteresis width by the PI control of the error between the given switching frequency and the calculated switching frequency.For the high peak current on start-up and failure of field excitation during the restart-up of the motor,several start-up scheme under zero-speed condition are comparatively investigated with its MATLAB simulation,a new start-up scheme,based on hybrid start-up method,which can effectively limit the peak current on start-up,even when the speed of the drive is not zero,is presented in this thesis.The thesis also analyzed the reason why there is always some difference,which varies according to the speed of the motor,between mean torque and given torque in traditional direct torque control,especially in high power traction.A new scheme with torque compensation,based on feedforward method is proposed.Experiments are conducted on a 1200 kW traction asynchronous motor in this thesis.The control system design is completed based on Digital Signal Process(DSP)and Field-Programmable Gate Array(FPGA),and direct torque control algorithm is mainly implemented by FPGA.Simulation and experimental results demonstrate that phase peak current on start-up is well restricted,restart-up of an already rotating drive is also included;and there is no current shock on the switch of different flux trajectory strategies.Torque offset in low switching frequency is eliminated effectively with the proposed compensate scheme.