Research On Improved DTC Strategy For Permanent Magnet Synchronous Motor In Electric Vehicles

With the gradual improvement of key technology of electric vehicles,electric vehicles will become a green transportation of zero pollution.And as one of the four key technologies of electric vehicles,motor drive system is also widely concerned.Currently vector control technology and direct torque control technology have been mainly adopted in the motor speed control system for electric vehicles.But DTC technology is simple in structure and less dependent on the motor parameters,which has obvious advantages over vector control.Furthermore this method has been widely concerned and deeply studied in recent years,it is completely suitable for the operational requirements of electric vehicles.Therefore,DTC strategy of PMSM used in electric vehicles is studied in this paper.Firstly,the structure and characteristics of PMSM are analyzed,simultaneously the coordinate systems commonly used in motor analysis and mutual transformation formulas between distinct coordinate systems are given in this paper.The mathematical model of PMSM under distinct coordinate systems is established,the principle of traditional DTC technology is expounded.In addition,a control scheme of complete system and simulation results are given,which lays a theoretical foundation for the subsequent research.Then,aiming at the problems of traditional DTC algorithm,such as large flux linkage and torque ripples,poor low-speed performance.The PMSM SVM-DTC strategy based on torque angle for electric vehicles is proposed,the motor torque angle control is mainly used as a starting point in this algorithm.The amplitude of the required voltage vector is directly obtained by closed-loop regulation of electromagnetic torque.According to the closed-loop regulation of stator flux linkage and the torque angle feedforward,the phase angle of the desired voltage vector is calculated.And the required drive signal is obtained by means of space vector modulation technology.On the basis of analyzing the regulation principles of electromagnetic torque and stator flux linkage,the block diagram of system and the principle of each part are given in this paper.And the mutual coupling effect between control variables is studied.The simulation results show that the steady-state and dynamic characteristics of the system are markedly ameliorated than the traditional DTC algorithm.Meanwhile,the flux linkage and torque pulsations are effectively reduced and the speed dynamic and static performance of system is improved in this strategy.Finally,in order to further improve the above problems of the traditional DTC algorithm.A DTC strategy of PMSM in the MT coordinate system used in electric vehicles is proposed,the algorithm is implemented in the MT coordinate system that is synchronized with the stator flux linkage.The improved MTPA algorithm is used to calculate the reference stator flux linkage.In order to meet the limit requirements of voltage and current,a limiting measure is taken on the torque and flux linkage to achieve high-speed and stable operations.The flux estimation is based on the integral of flux errors generated by the stator flux controller.The flux estimation errors generated by the voltage saturation of the inverter are compensated with variable voltage vectors in the proposed system.At the same time,the voltage vector calculation module combined with SVPWM technology is used to replace the hysteresis controller and switch table,and then the target voltage vector is synthesized.The simulation results show that the steady-state and dynamic characteristics of the system are remarkably upgraded than the above two methods,the flux linkage and torque pulsations are significantly reduced and this algorithm has a good speed dynamic and static performance.