Research On Axial-magnetic-fieldmodulated Brushless Double-rotor Machine For Hybrid Electric Vehicles

The series-parallel hybrid electric vehicle(HEV)attracts extensive attention with the advantage of high fuel economy and remarkable driving performance.So far,the most successful series-parallel HEV,i.e.Prius,employs a planetary gear,a generator and a motor as electronic continuously variable transmission(ECVT).It enables the engine operating at high-efficiency area independent of road conditions,resulting in the fuel economy improved and emission reduced.As a promising direction of the HEV field,ECVT based on the pure machine scheme can realize the function with the advantage of compact structure and flexible control.However,the early brush machine scheme requires brushes and slip rings for the rotating windings,reducing the reliability of the system.Therefore,an axial-magnetic-field-modulated brushless double-rotor machine(AMFM-BDRM)is proposed in this dissertation.It can be connected with a conventional permanent-magnet synchronous machine to form the magnetic-field-modulated brushless compound-structure machine,which can solve the problems of low reliability,dynamic balance and overheating of inner-rotor windings of the early brush machine.Moreover,as the flat structure can increase the utilization of the axial space,the AMFM-BDRM has the potential advantage of torque density.The research work is mainly summarized as follows:First of all,the operating principle of the AMFM-BDRM and the basic electromagnetic design method is investigated.The influence of the modulating effect on the air-gap flux distributions is analyzed with the analytical method.The mathematic model of speed decoupling and torque transmission is established.The cogging torque is mathematically deduced and the effect of the interaction of the modulated PM and stator armature harmonics to the contribution of the electromagnetic torque is discussed.Based on the Maxwell stress tensor method,combined with three-dimensional(3D)finite element method(FEM),the effect of the combination of the pole-pair number of the stator ps,the PM rotor pp and the number of ferromagnetic segments pm on the unbalanced axial magnetic force is investigated.After that,the basic electromagnetic design method of this kind of machine is given.Secondly,to solve the problem of the complex 3D FEM analytical model and increase the flexibility of the design and optimization process,two 2D analytical models,i.e.simplified 2D analytical model based on equivalent current method and precise 2D analytical model in consideration of stator slot-opening effect,are established.Considering the similar regularity of the flux density distribution along the radial direction,simplified 2D analytical model based on equivalent current method of the armature winding.No-load back electromotive force(EMF),torque and axial force is mathematically formulated.The formulation is modified based on precise 2D analytical model in consideration of stator slot-opening effect.In order to evaluate the feasibility and scope of application,the analytical methods are compared with 3D FEM simulations.Thirdly,The special electromagnetic problems,i.e.power factor,one-side axial magnetic force the dissymmetry problem of the machine and eddy loss,is investigated.To solve the problems of low power factor,the influence of turn number of the winding,diameter ratio and inner power factor angle on power factor is discussed.The effective approaches to higher power factor is proposed.Therefore,the influence of key geometrical parameters on axial magnetic force is studied based on Maxwell stress tensor method.Three kinds of dissymmetry of the machine are discussed,i.e.offset,tilt and position deviation of PM rotor.The impact of three dissymmetry situation on air-gap flux distribution,axial magnetic force,no-load back EMF and electromagnetic torque is evaluated.The loss composition and corresponding proportion are investigated.The effect of the speed of two rotors and PM segmentation on PM eddy loss is analyzed.To solve the problems of low power factor and high PM eddy loss for the existing scheme,two novel MFM-BDRM with improved structure are proposed,i.e.improved PM-rotor scheme and improved double-rotor scheme The validity of two improved schemes is evaluated.Lastly,a prototype of the AMFM-BDRM is manufactured.The test bench based on the HEV background is built up to carry out the experiment.The accuracy and feasibility of the theoretical analysis are verified by the tested results of no-load back EMF,no-load loss,cogging torque,output torque,power factor and efficiency.The operating mode of the AMFM-BDRM in the hybrid system is conducted,validating the power-split characteristics of the machine.The experimental results establish the theoretical and technical foundation of the AMFM-BDRM for application in HEVs.