Research On Electromagnetic Characteristics And Topologies Of Linear PM Vernier Machines

As an electromagnetic device for the conversion between the linear motion mechanical energy and the electric energy without any other intermediate transmission,the linear machine is widely used in aerospace,CNC machine tool,laser processing,semiconductor processing,rail transit and other advanced manufacturing industries because of its advantages of high precision,high acceleration,high efficiency and simplicity.It is one of the core components in many advanced manufacturing industries.However,at present,the development of the linear machine industry in China is still in the initial stage,and there is a wide gap with foreign products,which holds a large section of market share in our country.Therefore,it is very important to study the high-performance linear machine,especially the linear permanent magnet synchronous machine(LPMSM),which is the first choice in the linear servo system and has a large market share around the world.From the point of view of machine design,the performance of the linear machine is mainly measured by two indexes: thrust density and thrust ripple.In recent years,in order to obtain high thrust density and low thrust ripple,a novel kind of linear machine named as linear permanent magnet vernier machine(LPMVM)has been proposed and attracted a lot of attentions.The LPMVM comes from the rotary vernier permanent magnet machine(VPMM),which is verified to has both high torque density and low torque ripple at the same time.However,there are many differences between the linear and rotary vernier machine,such as the special end effect and the general large air gap length of the linear machine.As a result,the current LPMVM still suffers from large thrust ripple and its thrust density is supposed to be further improved.At the same time,the research of the LPMVM is still in the stage of finite element(FE)simulation and verification,lacking of comprehensive analysis of its internal electromagnetic mechanism and thrust ripple characteristics.Therefore,by the analytical formula derivation,FE simulation and prototype experiment,this paper mainly focus on the analysis of the basic electromagnetic characteristics and the study of new topologies of the LPMVM to give a clearer explanation of the internal mechanism of the LPMVM.A series of results have been obtained in the paper.The content of this paper is mainly consisted of three parts.Firstly,existing literatures about the LPMVM are reviewed in the foregoing part of the paper.With the help of the classical magnetic field modulation theory,the concept of LPMVM is clarified in this part,and it is also explained that the existing secondary permanent magnet(PM)structure is the LPMVM corresponding to the standard VPMM in the rotary machine.Secondly,analytical equations of the LPMVM's electromagnetic characteristics are derived in the paper,and thus the framework for the analysis and design of the LPMVM is improved.Based on the mathematical model construction and analytical formula derivation of the classical surface permanent magnet LPMVM(SPM-LPMVM),analytical expressions of the air gap flux density,no-load back electric motive force(EMF),average thrust,inductance and power factor of the machine,are obtained in the paper.Those analytical expressions are also verified by their relative FE simulations.On this basis,the characteristics of high thrust density and low power factor of the LPMVM are explained,and the size equation and general design process of the LPMVM are also obtained.Thirdly,analytical equations of the detent force,thrust ripple and vertical force ripple of the LPMVM are derived,and a series of methods on thrust ripple reduction are proposed in the paper.Although the end shape optimization and modular design are commonly used in the LPMVM to reduce its thrust ripple,but the internal mechanisms haven't been clearly explained by existing literatures.Therefore,based on analytical equations,the theoretical analysis of the thrust ripple reduction in the LPMVM is improved,and an important conclusion that the end force cannot be completely eliminated by the end shape optimization is given in the paper.Furthermore,through the analysis of slot and pole combination of LPMVM,the novel concept of the fractional pole-pair LPMVM(FP-LPMVM)is proposed and analyzed in the paper.It is proposed to analyze the asymmetry of the three-phase back EMF,which is caused by end effect,by the symmetry component method.Based on the analysis,it is proved that arranging the two end slots to two phases is better than one phase for the thrust ripple reduction.Finally,based on the analytical research results described above,in order to further improve the thrust density and reduce the thrust ripple of the LPMVM,four new LPMVM topologies,which are the interior PM LPMVM(IPM-LPMVM),double side PM consequentpole LPMVM,FP-LPMVM and double side PM modular LPMVM(DPMM-LPMVM),are proposed and comparatively studied in the last part of the paper.Two prototypes,i.e.the 12-slots/20-poles LPMVM and the 18-slots/29-pole FP-LPMVM,have been designed and manufactured to be tested in the paper.The experimental results match well with the previous design results,and thus the effectiveness of the analysis in the paper is validated.