Research On Control Strategy Of Torque Ripple Suppression For PMSM

The tendency of modern AC servo system can be summarized as high efficiency,high performance and low cost.As the important part of servo system,permanent magnet synchronous motor(PMSM),widely used in industrial production,features simple structure,high efficiency,reliable operation,wide speed range,good dynamic and static characteristics.However,the existence of torque ripple limits the application of PMSM in high precision operating condition,at the same time,torque ripple can lead to the fluctuation of speed.Thus,it is of significance to research suppression strategies on torque ripple,and then modify the smoothness of torque.In this thesis,from the perspective of control strategy,the electromagnetic torque ripple caused by the ripple torque and the cogging torque are suppressed to reduce the fluctuation of the electromagnetic torque.First of all,the fundamental mathematical model and harmonic mathematical model of PMSM are deduced in this thesis,moreover,the relationship between electromagnetic torque and the harmonics is analyzed.The SVPWM principle and its realization method are introduced subsequently.Secondly,the current of PMSM contains a large number of harmonics caused by the non-linear characteristics of the inverter,resulting in the distortion of the current.On the condition that PMSM is operating in a certain frequency range,the current distortion will lead to the electromagnetic torque pulsation and ripple torque,which would effect the stable operation of the system.Through analyzing the current harmonics caused by the dead time in the inverter,the current prediction method and the current vector method are proposed to judge the current polarity,and the time pulse compensation method is used to compensate the error voltage caused by the dead time,thereby suppressing the effect of dead time on current harmonics.The simulation model of the PMSM control system,based on time pulse compensation method,is built.In the case of the same speed,the comparisons of dead-zone compensation on phase current,speed and electromagnetic torque are demonstrated.The conclusion that the dead zone compensation algorithm can reduce the harmonic components of current and weaken the electromagnetic torque ripple is obtained.Thirdly,considering that the cogging torque is the inherent characteristic of PMSM,theelectromagnetic torque fluctuations also exists when PMSM is operating at low speed.In this thesis,the cogging torque of PMSM is analyzed by Ansoft,and the periodic fluctuation principle of the cogging torque is obtained,besides,the harmonic components of the cogging torque are calculated by Fourier decomposition,and the relationship between the cogging torque and current is obtained by derivation and calculation of the formula.Based on the construction of the nonlinear model of PMSM and the completion of vector control simulation system,the cogging torque suppression module is introduced to suppress the torque ripple caused by the cogging torque.The injection of quadrature axis harmonic current leads to the additional electromagnetic torque generated by current is equal to the amplitude of the cogging torque and the phase is opposite meanwhile,thus to achieve the purpose of compensation and reduce the influence of cogging torque on the electromagnetic torque ripple.Finally,on the basis of establishing the simulation model,the dead-zone compensation algorithm and injection quadrature axis harmonic current method are introduced in the model.The simulation results show that the proposed method can not only suppress the current harmonics caused by the dead time,but also inhibit the impact of the cogging torque,which further weakens the electromagnetic torque ripple.