Research On Torque Ripple Suppression For Interior Permanent Magnet Synchronous Machines

As the global automobile industry is gradually transforming to intelligence,the development of automatic parking,unmanned driving and other technologies have put forward higher requirements for electric power steering(EPS)motor.Permanent magnet synchronous machines(PMSM)have gradually replaced brushless DC motors with their advantages such as no commutation torque ripple and higher positioning accuracy,and have become an important development trend of the EPS motor.Compared with the surface-mounted permanent magnet machines,the interior permanent magnet machines have higher power density,reliability and lower manufacturing cost.However,their application in the EPS system is limited by the large torque ripple.Therefore,this paper focuses on the low torque ripple problem of the interior PMSM used as the EPS motor.Firstly,based on the energy method(virtual displacement method),the theoretical expression of the cogging torque is derived.The corresponding relationship between the harmonic components of the cogging torque,the harmonic components of the air-gap magnetomotive force and the air-gap permeance is determined,and the theoretical explanation for suppressing cogging torque by the offset elimination method is given.Three kinds of offset elimination methods are compared and analyzed,and their strengths and weaknesses are pointed out respectively.Then,an improved stepped slot-opening shift method is proposed,and the stator structure parameters are optimized by using the Taguchi method.Compared with the initial model,the cogging torque and torque ripple of the optimal scheme decrease by 87.87% and 67.79%,respectively.Although the average torque is slightly reduced,the overall torque characteristics meet the design requirements of the EPS motor.In order to further reduce the cogging torque,a combined method of stator tooth chamfering and stepped slot-opening shift is proposed.The theoretical expressions of the harmonic components of the cogging torque,the harmonic components of the air-gap permeance and the air-gap flux density are derived in detail.The effects of the stator tooth chamfering on the cogging torque are identified from the aspects of qualitative analysis and quantitative calculation.Compared with the initial model,the cogging torque of the optimal scheme decrease by 97.10% and 96.71% respectively,and the torque ripple decreases by 86.91%and 75.95% respectively which not only meets the design requirements of the EPS motor but also has a large design margin.Whereafter,the influences of four manufacturing errors on the torque ripple are analyzed.In order to reduce the influence of manufacturing error,Taguchi robust design and multiobjective robust design based on radial basis function model and non-dominated sorting genetic algorithm(NSGA-Ⅱ)are studied.The signal-to-noise ratio of the cogging torque and the torque ripple of the robust design scheme increase by 6.15% and 7.59% respectively,and the robustness of the prototype with low torque ripple is significantly improved.Finally,two prototypes are designed and manufactured,and the cogging torque is measured by using the electronic scale method.In view of the deviation between the measured values and the theoretical values of the prototype,a detailed analysis and demonstration process is given.The measured values of the prototype verify the effectiveness of the proposed design method in suppressing the cogging torque of the interior PMSM.