Research On Electromagnetic Characteristics And Key Technical Problems Of Permanent Magnet Vernier In-wheel Motors

Distributed in-wheel drive is one of the important directions for electric vehicles(EVs)in the future.In-wheel motor is the core power source of EVs,which needs to have high torque capability.Besides,it should also overcome the problems of limited space and harsh heat dissipation.At present,with various types of flux modulation machines are emerging,permanent magnet vernier motors(PMVMs)have attracted much attention in the field of in-wheel direct drive due to their simple mechanical structure and excellent torque performance.If PMVMs are introduced into in-wheel direct drive applications,on the one hand,as one of the flux-modulation machines,permanent manget vernier in-wheel motors(PMVIWMs)need to be investigated in terms of working principle,electromagnetic characteristics,and torque performance;on the other hand,low power factor and severe rotor losses are the key technical problems of PMVIWMs,which will bring great threats and challenges to the safety and reliability of EVs.Hence,the electromagnetic characteristics,torque performance,power factor,and rotor loss issues of PMVIWMs are focused in this article.The main research work includes the following aspects:Firstly,based on classical electromagnetic field theory,the analytical expression of the electromagnetic torque is theoretically derived and summarized by the energy method,and the torque generation conditions of PMVM and PMSM are summarized;according to the distribution of the armature magnetomotive force(MMF)harmonics,the concept of “group” and “family” in armature MMF harmonics is proposed,and the characteristics of armature MMF harmonics belonging to “group” and “family”are summarized;the working principles of PM machines based on “tooth harmonic family” are proposed,and the homology of PMVM and PMSM is clarified,then the concept of generalized PMSM is proposed;the expression of harmonic back-EMF is theoretically deduced,and its distribution under different slot-pole combinations is analyzed,and the selection principle of slot-pole combinations in PMVIWMs with low back-EMF harmonics is proposed.Finally,the theoretical analysis correctness of the back-EMF harmonic characteristics is verified by prototype experiments.Secondly,based on the requirement of high torque capability of PMVIWMs,the analytical expression of the electromagnetic torque of PMVMs is deduced and unified,where the torque amplification factor of PMVMs is proposed,and then the influence of air-gap permeance fundamental component is theoretically anlayzed;the definition of magnetic gear ratio in PMVMs and PMSMs is unified;through the finite element method(FEM),the torque performance is explored when PMSM and PMVM satisfy the dual-magnetic gear ratios,which is further verified by the experiemnts of prototypes;the influence of the number and geometry topology of stator modulated teeth on torque in PMVIWMs is explored;through the Maxwell tension tensor method,the contribution of each air-gap magnetic field harmonic on total torque is explored;the influence of the slot-pole combinations and slot opening width on harmonic torque distribution is then analyzed,respectively.Afterwards,in view of the problem of low power factor of PMVIWMs in the entire operating conditions,a selection guideline of slot-pole combinations of PMVIWMs with high power factor is established.Combined with motor phasor diagrams under typical conditions,the theoretical expressions of power factor in PMVIWMs under typical conditions are deduced,the influence of design parameters and control variables on power factor is analyzed;then,the contribution of each armature reaction reactance component is theoretically calculated,and the root cause of the low power factor in PMVIWMs is revealed;the judgment method of “power factor influencing factor” is proposed.By the calculation of air-gap inductance coefficient,the distribution law of power factor under different slot-pole combinations is proposed.Then,combined wih FEM,the influence of air-gap length and control method on power factor is analyzed;the influence of slot opening width on torque and power factor is studied.Finally,the correctness of theoretical analysis and FEM is verified by the experiments of power factor of the prototypes.Finally,in view of the serious rotor losses in PMVIWMs,the PM loss and core loss models are established,and the influencing factors are analyzed;the contribution of each magnetic field harmonic to PM loss and iron core loss is theoretically analyzed,and the root cause of severe rotor losses is determined;based on the combination of different slot opening widths in split-tooth and open-slot stators and combined with FEM,the influence law of the stator slot opening coefficient on PM loss and torque is analyzed,and the restraints between torque and PM loss in PMVIWMs are revealed;in order to further weaken the rotor losses,an improved scheme of the rotor with flux barriers is proposed,and the influence of flux barrier width on inductance parameters,PM loss,iron core loss,and torque under typical conditions is analyzed,and then the sensitivity of each loss and torque to flux barrier width is calculated,which can prove the feasibility of the proposed rotor with flux barriers.Finally,the correctness of the theoretical analysis and FEM are verified by the experiements of efficiencies and rotor temperature rise.