Research On Direct Torque Control System Of PMSM Based On Improved Field Weakening Method

Direct torque control(DTC)method of permanent magnet synchronous motor(PMSM)is one of the most advanced control technology,which has the features of strong robustness,fast dynamic response and simple structure.By weakening the magnetic field of the motor,flux weakening(FW)method can provide a wide speed range,which has great significance to improve the performance of PMSM.While,the load angle will increase gradually and exceed the linear range of principle of DTC during FW operation,which degrades the performance of motor and even results in the instability of the control system.The FW scheme for DTC system is mainly discussed and improved in this paper.In consideration of the instability of DTC system in deep FW region,a novel FW algorithm based on safety margin is proposed to make sure the load angle and torque work in a linear region.In addition,a new torque limitation method is presented to confirm the stability of PMSMFirst,the common control strategies,controller algorithms and flux weakening techniques are introduced briefly.Next analyses the rotor structure and establishes the mathematical model of PMSM in the rotating reference frame under the principle of coordinate transformation.The principle of DTC is further studied,where the selection of space voltage vector and estimation method of stator flux and electromagnetic torque are illustrated in details.Normal stator current trajectory control strategies are analyzed systematically,the optimal FW control is gotten next.The instability of system is pointed out after analyzing the output characteristics and load angle of PMSM under different FW level.Therefore,in order to improve the stability of the motor,a FW method based on the limitation of load angel and a new torque limitation method are proposed in this paper.In consideration of the strong coupling and non-linear characteristics of the PMSM drive system,slide mode control theory is adopted to design the controller.Exponentially reaching law is introduced to restrain chatter effectively.The validity and feasibility of proposed method is verified by simulations at last.It is proved that the improved FW method has a better stability than the traditional method.A semi-physical simulation platform is further constructed and the validity of improved method is also verified by experimental result.