The Study On Fuzzy Direct Torque Control Schemes For Induction Machine

The conventional Direct Torque Control (DTC) does not require rotating coordinate transformation and any current regulator, it controls torque and flux directly based on their instantaneous errors. In spite of its simplicity, direct torque control is able to produce very fast torque response and is robust with respect to motor parameters changes. However, in the steady-state operation, notable torque, flux and acoustical noise occur. In this paper, aimed at the problem of notable torque ripples in conventional DTC, two improved schemes using newly-emerged strategies are described, and then, a novel scheme that can minimize torque ripples is proposed.Firstly, based on detailed analysis of the mathematical model of induction machine in different coordination, the simulation model of induction machine in two-phase static coordination is set up. It makes a foundation for the research of control schemes.Secondly, principle of conventional DTC is studied deeply. The implement process of conventional DTC scheme is described detailed, and simulation results are obtained. Major factors of notable torque ripples in conventional DTC are analyzed and generalized.Thirdly, two improved schemes of fuzzy direct torque control are discussed and analyzed detailed. In the first improved schemes, a fuzzy logic controller is adopted, instead of hysteresis controllers of torque and flux. It can improve dynamic response speed, but effect is not obvious in decreasing torque ripples. The other scheme of fuzzy direct torque control scheme is based on Discrete Space Vector Modulation (DSVM). Using DSVM, It can generate a number of voltage vectors higher than that used in conventional DTC scheme. And then, a fuzzy logic controller is used to select optimum voltage vector. It can decrease torque ripples increasingly. At last, Simulation result demonstrates effectiveness of two improved schemes.At last, on the basis of discussing and comparing above strategies, a novel fuzzy direct torque control scheme using Space Vector Modulation (SVM) is proposed. Using fuzzy logic controller, anticipated voltage vector can be obtained dynamically in term of torque error, stator flux error and the angle of stator flux. And then, inverter switching can be controlled directly by using SVM. This novel scheme is easily implemented, and the ripples of both torque and flux can be reduced remarkably while maintaining the switching frequency constant.