Research On Speed Sensorless Direct Torque Control Of Asynchronous Motor

Direct Torque Control (DTC) is a high-performance AC speed control technology which was brought in the middle of 1980s, with its advantages, such as simple control, well dynamic performance, robust. But the problem of large flux and torque ripple in the DTC have limited its development. The speed sensorless technique is a very hotspot of current research works. It can simply the system structure and reduce the system cost. In this thesis, the improvement of low speed performance, the design of speed-sensorless schemes were studied detailedly.Firtsly, based on detai1ed analysis of the mathematical mode of asynchronous motor, equivalent circuit of asynchronous motor, mathematical model of converter, space voltage vector, the principle of the conventional DTC was studided deeply and the facts that affect the performance of conventional DTC scheme were analyzed. After deduced the model of asynchronous motorгtype equivalent circuit in detail, the motor voltage equation was established which provided the basis for the establishment of the observer.Secondly, to solve the problem of large flux and torque ripple in the conventional DTC, a new control strategy to reduce the pulse was adopted. With full order flux observation and the increment of stator flux calculation, the stator reference voltage vector was calculated. According to the current stator reference voltage vector which was modulated by Space Vector Modulation (SVM), optimum combination of the fundamental voltage space vector was obtained in order to accurately compensate the error of stator flux and torque.Thirdly, based on model reference adaptive control system theory, a new speed-adaptive flux observer was designed. The observation of stator flux and speed were applied in the direct torque control system. The basic idea of the scheme were: according to the model of asynchronous motorгtype equivalent circuit, the motor state space equation was built, which used stator flux and rotor flux as state variable, asynchronous motor itself as reference model, the designed observer as modulation model, speed observer as modulation parameter; obtained the gain matrix G by using the pole-placement technique, according Lyapunov's stabi1ity theory the speed self-adaptive scheme was confined, while the stator flux observed involved in system control.Finally, synthesize the proposed improvement strategies in the dissertation,a high performance speed sensorless asynchronous motor direct torque control scheme was proposed.The sensorless asynchronous motor DTC simulation model was built which introduced the SVM technique to replace the traditional swithed scheme of DTC. Simulation results show that the control strategy adopted in this paper had good performance.