Research On Key Technologies Of Multi-layer Flux-barrier Less-rare-earth Permanent-magnet Synchronous Machine For Electric Vehicles

Electric vehicles(EVs),which own the merits of energy conservation and environment protection,are getting more and more attention as the energy shortage and environment pollution become more and more serious.The rare-earth permanent-magnet synchronous machines are widely used in EVs as the drive machine for their advantages of high efficiency and high power density.However,the less-rare-earth permanent-magnet synchronous machine has been widely investigated recently for the violent price fluctuations of the rare-earth permanent-magnet(PM).The multi-layer flux-barrier less-rare-earth permanent-magnet synchronous machine(MLFB-LRE-PMSM),which can take full advantage of the reluctance torque and reduces the usage of rare-earth PM material,is investigated in this paper.Meanwhile,as the traditional permanent-magnet synchronous machine,the MLFB-LRE-PMSM also has the advantages of high efficiency and high power density.Hence the MLFB-LRE-PMSM has a promising application prospect in the field of EVs.This paper focuses on the key problems of the MLFB-LRE-PMSM and the major work can be summarized as follows:Firstly,the accurate magnetic circuit model of the MLFB-LRE-PMSM is investigated.The accurate no-load and demagnetization magnetic circuit models of the MLFB-LRE-PMSM are established.The expressions of the no-load flux and the PM flux density with the demagnetization current applied are derived.The results calculated by the magnetic circuit model and the finite-element analysis(FEA)are analyzed and compared,which verifies the accuracy of the magnetic circuit model.The effects of the key structural parameters including the PM size,the PM position and the bridge width on the no-load flux and the PM flux density with the demagnetization current applied are investigated,which provide the reference for the structural parameter selections of the MLFB-LRE-PMSM.Secondly,the torque characteristics and the optimization design of the MLFB-LRE-PMSM are investigated.The reluctance torque and PM torque of the MLFB-LRE-PMSM are obtained by the frozen permeability method.The torque characteristics versus the current angle and the torque harmonics under different winding currents are analyzed,and the influences of the structural parameters on the electromagnetic torque and torque ripple are studied.The influence of the PM position on the torque contribution and the influence of the PM size on the proportion of reluctance torque and PM torque are investigated.The optimized design scheme of the MLFB-LRE-PMSM that reduces the PM usage is obtained.The key performances of the initial design and the optimized design are compared and analyzed.Thirdly,the mechanical strength of the MLFB-LRE-PMSM rotor is investigated.The rotor stress analysis model of the MLFB-LRE-PMSM is established,and the effects of the centrifugal force and electromagnetic force on the rotor stress under different working conditions are analyzed.The influences of the fillet radius and the bridge width on the rotor stress are investigated.The design method of the rotor structure that owns the high mechanical strength is explored.The matching laws of the bridge and the central rib versus the safety factor are investigated.The effects of different bridge and central rib schemes on the electromagnetic torque are analyzed.Afterwards,the thermal characteristics of the MLFB-LRE-PMSM are investigated.The thermal field model of the MLFB-LRE-PMSM is established and the thermal parameters are calculated and analyzed.The temperature distribution law of the MLFB-LRE-PMSM under different working conditions at natural cooling condition is investigated.The influences of the water temperature and speed on the cooling effectiveness are analyzed,and the cooling scheme that is suitable for the MLFB-LRE-PMSM is chosen.The effects of the bridge safety factor and the central rib safety factor on the losses and temperature rise under different working conditions are investigated.Finally,the prototype of the MLFB-LRE-PMSM is manufactured and the test bench is established.The no-load performances,torque characteristics,efficiency map,mechanical strength and temperature rise of the prototype are tested.The experimental results verify the correctness of the previous theoretical analysis and provide the experimental basis for the engineering application of the MLFB-LRE-PMSM.