Research On Strategy Of Improved Flux Observer To The Induction Motor Direct Torque Control

The direct torque control(DTC) of induction motors is one of the high performance control strategies, which was proposed after the vector control scheme. It became one of the hot topics in the field of AC speed control because of its novel control method, simple structure and high dynamic performance. The accurate estimation of the stator flux is important to the choice of voltage space vector and the torque estimation. It is the key part for improving the performance of DTC. Using advanced control theory, the stator flux estimation methods of DTC are studied in the thesis.Firstly, the basic working principle of asynchronous motor direct torque control system is studied. The mathematical model of induction motor and inverter has been established, and the regulation of stator flux and electromagnetic torque and how to choose space voltage vector in the direct torque control system are researched.The impact of initial integration and dc errors caused to u-i integrator method are analyzed, and the problem of low-pass filter flux observer method is studied, too. On this basis, a new flux observation model is proposed in this paper which can eliminate the effect of initial value and dc error to stator flux observation, called double low-pass filters(double-LPF).To eliminate the zero drift influence two filters with different cut-off frequency and proportion coefficient are used in the double-LPF flux observer and the output of the filters is subtracted. According to the characteristics of the filters method for compensation of amplitude and phase is proposed, and the influence of the parameters of double-LPF to flux observation is also analyzed.The simulation model of asynchronous motor direct torque control system is established by MATALB/Simulink, the model of low-pass filter flux observer and the double-LPF flux observer are established,too. And the two flux observer methods are applied to the model of asynchronous motor direct torque control system. The two systems running at low speed performance under the condition of the stator current contains the dc error are simulated, and at the circumstance of low speed and low speed with overload of the system based on the double-LPF flux observer method are simulated. The hardware experiment system to direct torque control are established in d SPACE DS1103 platform, and the proposed improved flux observer model has been experimental in the hardware experiment system. The validation and effectiveness of the double-LPF flux observer are verified by simulations and experiments.