Research And Analysis Of Direct Torque Control System Of Ac Induction Motor

During the recent 20 years, Direct Torque Control (DTC) for induction motor has been developed greatly because of its simple system scheme and excellent dynamic and static performances. In this scheme, the torque of motor is calculated and controlled in the stator reference frame. By using stator field orientation, the optimum switching mode is achieved to control the inverter.The digital DTC system is implemented easily using TMS320F240, which is a fixed-point DSP and developed specially for motor&motion control by Texas Instruments Inc. The DTC scheme is studied in the paper, and the software of control system with speed sensor is complied in DSP assembly language. This software has realized functions of the sampling of stator phase current, the calculating of the phase voltage, the observing of stator flux and the output of switching signal etc. In this paper, performances have been analyzed when the parameter of torque hysteretic comparator or the hysteretic band of stator flux is changed. The system of circle flux control is completed and the feasibility of the scheme is proved. In addition, the traditional PI regulator and the observer of stator flux are improved in this paper.According to the analysis of the theory of DTC scheme and the design of the control software, some primary experiments of the close-loop circle flux control system have been done using the DSP test-bed for experimentation and development of motor control. Experiments of motor with variable load or at a variable given speed are completed in this paper. These results are compared and analyzed. The feasibility of the scheme is showed through these results.Fluctuations of torque and stator flux are the main factors, which affect the performance of the traditional DTC system. So reasons of fluctuations of torque and stator flux are analyzed in this paper and some suggestions are raised.