Research On Flux-modulation Linear Permanent Magnet Machine Topology With High Thrust Density

Due to the advantages of high positioning accuracy,good dynamic response and simple transmission structure,linear machines(LMs)are widely used in equipment manufacturing,rail transit,national defense and military fields.The linear machine is directly connected with the load in the vast majority of application occasions,so it is necessary to limit the volume of the electric motor to avoid the whole movement unit is too large,and to reduce the weight of the actuator as far as possible in the design to ensure the optimal dynamic response when the thrust is fixed.Consequently,how to obtain a sharp rise in LM's thrust density becomes one of the most main research topics.Flux-modulation linear permanent magnet machine(FMLPMM),which operates in a "magnetic-gear-like" working principle,has the potential to greatly increase the thrust density,and little difference from conventional LMs in structure,thus are of good research significance and engineering application value.Since the FMLPMM has yet been in existence for long and the related research has just begun,the research of this kind of machine is not so comprehensive as that of conventional LMs,such as basic theory,new topology,design methodology and control strategy,and so on.To be specific,the intrinsic electromagnetic properties of FMLPMM still needs to be solved,and the performance potential of high thrust density,low thrust ripple remains to be further exploited.Aiming at further improving the thrust density of FMLPMM,this article has proposed the topological research and development methodology from the aspect of increasing effective magnetic load via magnetic field enhancement,multiple working magnetic field and so on.Accordingly,a series of novel FMLPMM with high thrust density has been proposed.Besides,the electromagnetic properties of FMLPMM has also been studied in-depth through the theoretical derivation,finite element analysis(FEA)and prototype test.The obtained research achievements are shown as bellow:1.The existing research work of FMLPMM is sorted out and summarized.According to the principle of flux modulation,the existing FMLPMM is classified into two categories:modulation unit static type and excitation unit static type.The development and function of related researches are briefly analyzed.By comparing the performance of the existing machine topologies,the characteristics of two types of FMLPMM are summarized.With the help of the classical flux modulation principle,the electromagnetic characteristics of FMLPMM are analyzed and derived by using the magnetomotive-force(MMF)-permeability model,including the expressions of permanent magnet magnetomotive force,air gap flux density,no-load backEMF and electromagnetic thrust,which improves the theoretical framework for the analysis and design of electromagnetic characteristics of this kind of machine.2.Based on the comparison of the topological performance of multiple permanent magnet LMs and the analysis of the characteristics of FMLPMM such as inductance,power factor,end effect and cogging effect,the performance improvement strategy is proposed,which is mainly based on increasing thrust density,improving power factor and reducing the back-EMF asymmetry as a complement.Combined with the practical engineering application requirements,three research objectives are proposed,namely,to maximize the thrust density,to increase the thrust density while reducing the amount of permanent magnet,and to increase the thrust density without increasing the amount of permanent magnet.In view of the above research objectives,three research directions of excitation enhancement,compound modulation and harmonic utilization are proposed respectively.3.With the aim of maximizing the thrust density and from the perspective of excitation enhancement,a method of promoting the thrust density is proposed by using the permanent magnet array which can enhance the fundamental amplitude of MMF and weaken the high order harmonic.Based on this method,two new topologies,the three-modular Halbach-PM-type linear vernier permanent magnet machine(LVPMM)and the bilateral Spoke-PM-type LVPMM,are designed,which increase the thrust density by 30%.In addition,the optimization method of three-modular winding transposition and magnetic circuit series is introduced,which can effectively reduce the thrust ripple caused by the asymmetry of back-EMF,improve the power factor,which makes the related topology have better comprehensive performance.Aiming at the three-modular Halbach LVPMM,a 2000 N prototype was manufactured and tested.The results show that the proposed performance improvement method is effective and some performance indexes of the proposed topologies have reached the international advanced level.4.In order to reduce the usage of permanent magnets and increase the thrust density,a dual modulation method based on consequent-pole secondary structure is proposed from the perspective of compound flux-modulation,and the working principle of the method is deduced analytically.On this basis,two new topologies of modular dual-modulation LVPMM and improved flux switching type dual-modulation LVPMM are proposed.Thanks to the dualmodulation method,both machines can increase thrust density by more than 10% while reducing permanent magnet usage by half.In addition,a 200 N modular dual-modulation LVPMM is designed and manufactured,and the feasibility of the dual-modulation method is verified by experimental tests.5.In order to improve the thrust density without increasing the amount of permanent magnet,a thrust enhancing method of introducing multi-MMF working harmonics is proposed from the perspective of harmonic utilization.The related working principle is expounded through analytical derivation,and the performance improvement effect of this method for machines with different pole pair/slot combination is analyzed by FEA.Based on this,a kind of bilateral modular multi-MMF harmonic linear flux reverse permanent magnet machine is proposed.Under the premise of not increasing the usage of permanent magnet and electromagnetic load,the machine is able to improve up to 15?25% thrust density only relying on the utilization of harmonics,which is of great application significance.