Research On Key Technical Problems Of Axial Flux Permanent Magnet Synchronous In-wheel Motors

At present,the distributed electric drive system has become one of the key development directions of electric vehicles due to its unique advantages.As the main power component of the system,the in-wheel motor should face the problems of limited space and harsh heat dissipation environment.The axial flux motor has a flat structure and is easy to match the inner space of the rim,which have a potential application in the wheel hub system.However,the 3D finite element calculation speed of the axial flux motor is quite slow.The motor is easily affected by machining and assembly errors,resulting in mechanical asymmetry Therefore,this article focuses on the key technical issues about the application of axial flux permanent magnet synchronous motors in wheel hub systems.The main research work includes the following aspects:Firstly,the key geometrical parameters including motor split ratio and motor aspect ratio are defined based on the space limitation and the heat dissipation constraints in the wheel hub system.The expressions of torque and torque volume density ratio of axial flux motors are derived.The influence of key geometrical parameters and the plate number on the torque volume density ratio are analyzed.The critical aspect ratio is proposed to distinguish the dominant regions of axial flux and radial flux motors.The influence of the outer diameter of the motor,the air-gap length,and the pole-slot combination on the critical aspect ratio is studied,followed by the pole structures on electromagnetic parameters,constant power characteristic and torque performance of the axial flux motor.The motor with stepped poles is proposed,while the influence of the stepped pole on motor performance optimization is studied with constant restrictions on the amount of permanent magnet and the axial length of the rotor.Secondly,the complex structure and a large number of meshes will reduce the calculation efficiency of the 3D finite element analysis of axial flux motors.The calculation accuracy of 2D equivalent models is compared and analyzed when the axial flux motor has different magnetic pole structures.A 3D equivalent model is proposed which is fit for the in-wheel motor which has high electromagnetic load and high saturation operating conditions.Based on the principle of equivalent equations and boundary conditions,the variation rules of the material property of the air-gap region are derived.The influence of equivalent air-gap length on the calculation accuracy of motor performance is investigated.The calculation time of2 D equivalent models,the 3D equivalent model,and the traditional 3D finite element model are compared.The prototype is manufactured and tested.The feasibility of the3 D equivalent model in axial flux motor design and optimization is proved.Afterward,according to the material properties and heat source distribution,the internal area of the double-stator-single-rotor axial flux motor is divided.Based on the T-type equivalent heat circuit model,the accurate 3D thermal network model of the double-stator-single-rotor axial flux motor is established.The internal heat transfer path of the motor is analyzed.A mixed cooling structure combining epoxy and metal with high thermal conductivity is proposed.According to the properties and positions of different materials at the end thermal conduction region,the equivalent thermal conductivity of the end thermal conduction region is deduced.The influence of the thickness and thermal conductivity of the filling material on the cooling effect is further analyzed.The temperature distribution of the mixed cooling structure,the cooling structure with and without epoxy potting are compared by computational fluid dynamics simulation.The cooling effect of the mixed cooling structure is also verified by experiments.Lastly,the characteristics of common mechanical asymmetry problems of double-stator single-rotor axial flux motor are summarized.The expression of the back-EMF in a tooth coil of the axial flux motor with stepped poles is derived.To further establish the correspondence between each mechanical asymmetry problem and the waveform information in amplitude and phase of the back-EMF in a tooth coil,the influence of mechanical asymmetry on the back-EMF is analyzed.The detection method of the mechanical asymmetry problem based on the back-EMFs in search coils is proposed.Through the detection of back-EMF waveforms and the usage of Fourier analysis and Clark transform,the information of each mechanical asymmetry can be acquired.For both single and mixed mechanical asymmetry problems,the precision of the detection method is verified by the 3D finite element analysis and the experiments on the prototype.