Research On Direct Torque Control Strategies For Permanent Magnet Synchronous Machines

Taking the project “Research on direct torque control strategies for permanent magnet synchronous machines” as background and based on the study of traditional direct torque control(DTC), this dissertation conducts a depth research about two modified DTC strategies, namely model predictive direct torque control(MPDTC) and duty ratio modulated direct torque control(DDTC), to reduce the torque ripple, stator flux ripple and switching frequency. The main research work are shown as follows.Firstly, the research about MPDTC for permanent magnet synchronous machine(PMSM) is conducted in this paper. The machine prediction model is obtained through discretization of continuous machine equations to predict the stator flux, stator currents and torque. Both single-step MPDTC and multi-step MPDTC are realized and two kinds of multi-step prediction methods, namely multi-step MPDTC with fixed step size and multi-step MPDTC with adjustable step size, are proposed. The multi-step MPDTC with adjustable step size can realize the tradeoff between computation burden and control performance through tunning the step size. The prediction error of the two multi-step prediction methods is studied while the corresponding simplification methods for both single-step prediction and multi-step prediction are presented. Additionally, a control performance evaluation function with adjustable weight coefficients is used to select the optimal voltage vector. Torque ripple, stator flux ripple and switching frequency are choosen as the evaluation criteria, and various control performance can be achieved by tuning the weight coefficients. Experimental results are presented to verify the validity and superiority of the proposed MPDTC.Secondly, the study about DDTC for PMSM is carried out in this paper. The mathematical models of torque and stator flux slopes under different voltage vectors are presented, and the variation of both torque slope and stator flux slope during one control period are analysed. The duty ratio is calculated according to the total error of stator flux and torque through introducing the coefficients of stator flux error and torque error, which can reduce both stator flux ripple and torque ripple. The tradeoff of stator flux ripple reduction and torque ripple reduction can be achived through tunning the weightcoefficients of stator flux error and torque error. Three different kinds of duty ration calculation methods are utilized to achieve various control objectives. And a simple duty ratio determination method based on a PI controller is proposed to reduce the computational complexity and parameter sensitivity. Experimental results are presented to verify the validity and superiority of the proposed MPDTC.Lastly, the comparative evaluation of the proposed MPDTC and DDTC are presented,respectively. The comparison among single-step MPDTC, multi-step MPDTC with fixed step size and multi-step MPDTC with adjustable step size are conducted firstly. The experimental results indicate that MPDTC show better control performance than DTC,while multi-step MPDTCs show better control performance than single-step MPDTC, and the multi-step MPDTC with fixed step size can realize the tradeoff between computation burden and control performance. Then, the comparative analysis among the three different duty ratio determination methods for DDTC are presented. The experimental results show that various control performance are obtained for different duty ratio determination methods, which are all better than that of DTC. At last, the comparison among DTC,MPDTC, DDTC and FOC are carried out through experimental results on the basis of several criteria: steady-state control performance, dynamic process, switching frequency,machine loss, algorithm complexity and machine parameter sensitivity, etc.. Both advantages and disadvantages of each control strategy are fully revealed to provide valuable guidance for practical applications.The experimental setup based on NI 7852 R for a PMSM is built, and the design of both software and hardware are finished. Four control strategies, including DTC, MPDTC,DDTC and FOC, are realized to fully conduct the experimental research.