Research On Direct Torque Control Strategy Of High-speed Square-wave Permanent Magnet Synchronous Motor

Compared with the constant-speed motor,the high-speed square-wave permanent magnet synchronous motor（PMSM）saves the speed change device and improves the operation efficiency and reliability of the motor.Compared with the sine wave PMSM,it has the advantages of high torque density,simple control system and low cost.High-speed square-wave PMSM has broad application prospects in important fields such as CNC machine tools,defense equipment,and automobile manufacturing.Therefore,it is of great significance to construct a high-speed square-wave PMSM control system with high precision and strong stability.The high-speed square-wave PMSM control system studied in this paper adopts the direct torque control（DTC）strategy.Compared with others control strategies,the DTC strategy has the advantages of fast response speed and simple control structure.The excessive torque ripple of the traditional high-speed square-wave PMSM DTC system and the inherent drawbacks of the PI control have a great impact on the performance of the system.In view of the above problems,the main research contents of this paper on the DTC system of high-speed squarewave PMSM are as follows:Firstly,the structure and working principle of the high-speed square-wave PMSM are expounded,and its mathematical models in the three-phase static coordinate system and the two-phase static coordinate system are established.The calculation of stator flux linkage,torque calculation and space voltage vector selection of high-speed square-wave PMSM DTC system are deeply studied,and its feasibility is verified by simulation.Secondly,in view of the problem of large torque ripple in the traditional DTC of highspeed square-wave PMSM,the torque ripple is deeply studied according to the operating characteristics of the system,and the reasons for the existence of torque ripple during commutation are analyzed.A space voltage vector selection table using "three-two conduction" during commutation is proposed.The problem of torque ripple in non-commutation period is studied,and a non-commutation torque ripple suppression scheme with optimal space voltage zero vector is proposed,which can reduce non-commutation torque ripple while ensuring low commutation torque ripple.Simulation models using different space voltage vector selection tables were built on the Matlab/Simulink platform,and the simulation results were compared and analyzed.Finally,aiming at the problems of large overshoot and poor robustness of PI controller in the speed loop of the control system,an improved variable structure sliding mode speed controller is proposed.Through the combination of fuzzy control,Luenberger load torque observer and variable structure sliding mode control,the sudden change of load torque is compensated in time.At the same time,the gain coefficient of variable structure sliding mode control is adjusted through fuzzy rules,so as to reduce the chattering of sliding mode control while ensuring no overshoot and strong robustness of motor control system.The simulation results of MATLAB/ Simulink show that the proposed sliding mode controller is superior to the traditional sliding mode controller.