Research On A Novel DTC Of Asynchronous Motor Based On SVM

Direct Torque Control (DTC) is another kind of promising AC speed regulationscheme, after Vector control, which attracts wide attention because its simple structure,ideal dynamic performance and less motor parameters involved. But the application ofclassical DTC was confined due to its large torque ripple and variable switching frequency.The main intention of this thesis is to alleviate the torque ripple of classical DTC and tomake the switching frequency constant.In this thesis, the basic principle and existing problems of classical DTC aredescribed at first. The mathematical model of Three-phrase Asynchronous Motor isdiscussed and the relation among space vector, stator flux and torque is analyzed in detail.A DTC theory and structure based on SVPWM is investigated on the basis of the sectionabove. The basic strategy of SVM-DTC is to calculate the reference space vector which isused for SVPWM control according to torque error and flux error. More specifically,through a series of calculations on the torque error, the increment of torque angle can beobtained, added which the phase-angle of the reference flux can be known, and then bycomparing the reference flux and its feedback, flux increment can be reached. Finally thereference space vector can be calculated according to the relationship between stator fluxincrement and space vector.In addition, the speed pulse processing of low resolution speed sensor is discussedand the model of SVM-DTC system, on which to base the design of the torque controllerand speed controller, is built. Moreover, a variable proportional gain PI controller oftorque is introduced.Basing on the analyses above, simulation under MATLAB environment is conductedand the result indicates that SVM-DTC could alleviate the torque ripple and reduce currentharmonic effectively. At last, an experiment involving flux and torque is conducted withDSP-based multi-CPU device to compare the performance of SVM-DTC with classicalDTC’s. The correctness of the strategy presented in this thesis is verified by theconsistency of experimental and simulation results.