Research On Direct Torque Control System Of Asynchronous Motor Based On Sliding Mode Variable Structure

Direct torque control (DTC) is a prospective AC variable speed control scheme for its novel control thought, simple system structure, fast dynamic response, small motor parameter influence and strong robustness. However, the disadvantages of large torque ripple and unfixed switching frequency are still unsolved. Consequently, it is of a great significance to do more researches on DTC system.The sliding mode variable structure (SMVS) theory, which is the representative of the nonlinear robustness control theory, has received much attention in recent years and it is also applied increasing in industry. Sliding mode variable structure control is essentially a special kind of nonlinear control, When the system state in the sliding mode surface, the system are highly robust to internal parameters change and external disturbances.This thesis introduces the basic principles and the composition of DTC system for AC asynchronous motor. The disadvantages of conventional DTC system mentioned above are analyzed and reasons are elaborated in detail. Afterwards, this thesis combines the sliding mode variable structure with DTC system, and uses the space vector pulse width modulation (SVPWM) technology, establishes a new control scheme which is the direct torque of asynchronous motor based on sliding mode variable structure (SMVS-DTC).The new control scheme adopts the speed controller based on SMVS instead of PI speed controller, uses the flux and torque controller based on SMVS to replace the two hysteresis controllers of traditional DTC system, and uses SVPWM instead of voltage vector switching table to control inverter. Theoretical analysis shows that, SMVS-DTC system can maintain a constant switching frequency and control the continuous smooth on the flux and torque, can effectively reduce the torque ripple.Simulation models of the conventional DTC system and the SVM-DTC system are established separately based on MATLAB/Simulink platform. Simulation results prove that the SMVS-DTC system can remarkably overcome the large torque ripple and current distortion while maintaining the switching frequency constant.