Research On The Direct Torque Control Strategy At Low Switching Frequency

With the development of power electronics technology, people’s expectation of theperformance of AC speed regulation system becomes higher. Vector control and directtorque control are the two kinds of mainstream control method. Vector control has widespeed regulation range and good dynamic performance, but it is influenced deeply by thevariation of motor parameters. Direct torque control can compensate for this shortcomingof vector control, moreover, it has simple control structure and faster torque response.These characteristics make it more suitable for traction power applications. Meanwhile,low switching frequency should be guaranteed in high power applications for safety,promoting the research on direct torque control strategy at low switching frequency.The research is mainly about direct torque control strategy of induction motor at lowswitching frequency. By Analyzing the equivalent circuit of induction motor, the basicprinciple and control structure of direct torque control are studied. Effect of voltage spacevector on stator flux and torque are researched by analytical method. Meanwhile, animproved stator flux model is designed for the accuracy of the stator flux detection. Animproved hysteresis controller maintains the switching frequency at a low level bychanging the hysteresis bandwidth. A synthetical direct torque control strategy, which isdesigned to reduce the switching frequency further, uses circle flux control strategy at lowspeed and hexagon flux control strategy at high speed.On the basis of theory analysis and the design of control strategy, simulation researchon the control system is developed based on MATLAB/SIMULINK, the result shows thevalidity of the control strategy. This direct torque control strategy is realized on a digitalcontrol system with DSP as the core. The experimental results validate the feasibility ofthe strategy and system.