The Research Of Direct Torque Control Method

The motor is the main device which converts electric energy into mechanical energy.Speed governing performance of the motor affects product quality, production efficiency andutilization rate of electric energy. It is necessary to control the running speed of motor. Directtorque control has the advantage of simple structure, involved less motor parameters, andbetter dynamic, static performance. But at present the direct torque control theory is not yetmature, there are still some problems remain to be solved, such as large torque ripple, variableswitching frequency and large following torque and speed errors at low speed. It has practicalsignificance for further research on direct torque control.The space vector analysis method is used in direct torque control technique. The torqueand flux linkage of AC motor is calculated and controlled in two phase stator coordinate.According to stator field orientation, torque and stator flux can be traced directly. Based onthe two-point regulator, voltage vector is selected to control switch state of the inverter, andobtain the torque dynamic performance is obtained. However, hysteresis control results inlarge torque and flux ripples. At the same time, direct torque control depends on a linearmodel of the motor, whose speed controller uses PI controller with fixed value to outputtorque. Therefore，the tracking result of torque and speed are undesirable when the load orspeed changes suddenly.Because of the large torque and flux linkage ripple of the traditional direct torque control,the space vector modulation technology is introduced. By using PI controller to replace thehysteresis controller and replacing the switching table by SVM module, the reference voltagevector is obtained in every sampling cycle. The reference voltage vector is synthesized fromtwo adjacent basic working voltage vector and zero voltage vector which can exactlycompensate current flux error and torque error.Therefore, this methods can achieve thepurpose of reducing flux and torque ripples.By using the fuzzy PI control, the problem of the poor tracking performance of speed and torque is resolved in the sudden load or speed. The PI control of the traditional speedregulator control is improved into the fuzzy PI control, which reasonable choice of differentPI parameters in the full range to adjust performance of control system.The simulation model of the torque, speed and flux closed-loop control system isestablished, and the performance of the closed-loop control including DTC, SVM,FUZZY-SVM is compared. The experimental result shows that the torque, flux and currentripples of traditional DTC system are reduced by SVM-DTC, and the performance of thetraditional DTC system is improved. Compared with the SVM control, the speed response ofthe fuzzy SVM control is faster in full-speed range with lower overshoot. It can be indicatedthat the design is reasonable.