Research On The Direct Torque Control For Asynchronous Motor Based On The Sliding Mode Variable Structure

In AC drive fields, direct torque control (DTC) is a new control strategy, which followed the vector control technique. The traditional direct torque control has control of simple structure, rapid response and strong robustness to the system internal parameter perturbation and external disturbance, but there are also the inaccurate observation of stator flux and larger torque ripple.Sliding mode variable structure control with fast response, has strong robustness to the change of system parameters and outside interference, so the advantages of sliding mode variable structure control strategy is employed to asynchronous motor direct torque control, not only can improve the system robustness and rapid response, at the same time also can reduce the torque ripple and keep switching frequency constant. In view of the above, this paper made the research work:(1) The accuracy of stator flux observation performance has a direct influence on the whole direct torque control system, in order to make the performance of the stator flux accurate observation, the widespread used model of the flux observation is improved, make the stator flux can accurate observation at full speed range;(2) This paper puts forward a control scheme which combined sliding mode variable structure theory with Space Vector PWM technology (SVPWM) to reduce torque ripple and keep switch frequency constant, The scheme has designed sliding mode variable structure controller based on flux error and torque error, to make the torque and flux can fast and accurate adjustment, controller outputs get inverter switch pulse signal by implementing SVPWM modulation, realize the constant frequency inverter switch;(3) In order to reduce the steady-state error of stator speed, this paper using sliding model variable structure control, compared to the traditional PI control, the system response speed faster than before, and steady-state error has obvious decrease. Finally, This paper has built the DTC for induction motor based on sliding mode variable structure system simulation model by using Matlab/Simulink. The simulation results show that the system control stator flux movement with circle trajectory, reducing torque ripple and keep inverter switch frequency constant at the same time, the system represent strong robustness to parameter perturbation and external disturbance.