Research On Direct Torque Control Technique For Asynchronous Induction Motor

Direct torque control technique(DTC) debuted in the AC drive field in 1980's. It is a new efficient AC drive control technique, which followed field-oriented control technique and intended to exceed field-oriented control technique. After analysing and comparing with advantages and drawbacks of classical DTC by studying mathematic models of induction motor, a new control strategy-SVMDTC combining SVM and variable-structure control was presented to ensure high-performance operation in steady state and under transient conditions. For the purpose of quick and accurate control of torque and flux, the new scheme apply sliding control algorithm to DTC system and drive stator flux trajectory as an approximate circle. In order to validate the feasibility of the new DTC control system, simulating experiment model was established by using MATLAB/SIMULINK, and also searched the characteristics and problem of DTC.The accuracy of stator flux observer is critical to steady and transient behavior of the whole DTC system. The paper absorbed advantage of i-n flux observer: accurate flux observation in full range of speed and introduced variable-structure control idea to enhance the robustness in face of parameter uncertainties and other disturbance. Thus adequate balance between accuracy and robustness was achieved by the method. DTC feature low switching frequency and larger torque ripple, SVM is employed to reduce the torque, current and flux ripple and to ensure constant switching frequency where SVM used existing space vector voltage to synthesize new space vector voltage of any amplitude and direction.The speed estimation without speed sensor is another popular trend of AC drive. Through weighing advantages and disadvantages of many kinds of speed estimation, the paper adopts a model reference adaptive system based on popov hyperstability theory to identify rotor speed. The simulating results proved the feasibility of this method.