| After direct torque control (DTC) technology was invented, it has been given wide concern and has got rapid development because of its original control notion, concise system structure and excellent capability on activity and immobility. In brief, using an analytical method on space vector, DTC technology directly calculates and controls the torque of asynchronous motor under the stator coordinate. On the other hand, in order to get best capability on activity, adopting the stator-magnetic-field-oriented way, DTC technology directly applies optimum control on the switch conditions of inverter and generates PWM signal taking advantage of discrete Band-Band adjustment.DTC has many merits, but as a new theory and technology, it still has some disadvantages. Some of them become the main obstacles during the development of DTC. Because of this reason, this paper discusses the two main research domains, which include improvement on low speed performance and speed sensorless estimation in DTC system.In this paper, a study of asynchronous motor model is made; according to the faults of traditional direct torque control and the requirement of an asynchronous motor's control strategy, the author changes the method of regulating stator flux to make its track approximate to a circle as possible as could. To get quickly torque control, a three-value regulator is adopted. To demonstrate the validity of the asynchronous motor's direct torque control system, simulation system of DTC is designed through the simulation software MATLAB/SIMULINK. The result of the test is analyzed. At the other hand, after changing the parameters in direct connection with flux-regulation and torque-regulation, this paper gives the effect of changing parameters to motor ability.The speed sensorless estimation is an important trend of modern AC drive. This paper has discussed several primary algorithm of speedestimation and emphasized on the algorithm of speed estimate based on model reference adaptive system ( MRAS ).
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