Research On Internal And External Magnetic Field Of Permanent Magnet Synchronous Motor Based On Subdomain Model

Permanent magnet motor(PMSM)has the advantages of high efficiency,high power density,flexible shape and structure design,and has been more and more used in industrial,commercial and military fields.The air gap magnetic field analysis of permanent magnet motor is the basis and key of all kinds of motor performance calculation,and it is an important step in the process of motor design.At present,the calculation methods of air gap magnetic field mainly include analytical method and finite element method,and the sub domain model method with high calculation accuracy is a typical representative of analytical method.The magnetic field distribution of permanent magnet motor is complex,in addition to the internal main flux and leakage flux,there is also external leakage magnetic field“ The "leakage" magnetic field has no effect on the operation performance of the motor,but it will have an adverse effect on the safety and concealment of the special ship driven by the motor,so the calculation of the external magnetic field is also necessary in some special fields.In this paper,the internal and external magnetic fields of the marine propulsion multi-phase permanent magnet direct drive motor are studied.The analytical model of the internal and external magnetic fields of the motor is established by the subdomain method.The accuracy of the analytical method is verified by two-dimensional finite element simulation and some experiments.Due to the unequal thickness of the poles,the traditional subdomain model can not be used to solve the surface mounted eccentric pole cutting permanent magnet motor.In this paper,the surface current is introduced to equivalent the permanent magnet,and the magnetic field produced by the current carrying conductor is used to replace the magnetic field produced by the permanent magnet.Based on the subdomain model of a single current carrying conductor,the magnetic field generated by all current carrying conductors is synthesized by the superposition theorem,so as to obtain the magnetic field generated by the whole permanent magnet.Based on the subdomain model,the air gap magnetic fields under no-load state and only considering armature reaction are calculated respectively.The load magnetic field can be obtained by superposition of no-load magnetic field and armature reaction magnetic field,and the induced electromotive force,cogging torque and electromagnetic torque are solved analytically.Compared with the results of finite element method,the subdomain model based on equivalent surface current method has high accuracy.In order to solve the external magnetic field of the motor,based on the equivalence of the internal magnetic field,the non slot sub domain model including the external region of the motor is established.The analytical formula of the external magnetic field of the non slot model motor is obtained by derivation,and the effect of the stator slot is considered by introducing the magnetic field correction coefficient.The effectiveness of the subdomain model considering the external magnetic field is verified by comparing the external magnetic field obtained by the subdomain model including external magnetic field with the finite element model including external magnetic field.In order to find out the factors that affect the external magnetic field of the permanent magnet motor and the magnitude of the effect,this paper analyzes and calculates the external magnetic field of the motor by changing the number of poles,the thickness of the iron core,the thickness of the shell and the relative permeability of the iron core.The analysis results show that the magnetic density of the external magnetic field decreases exponentially with the distance between the location and the axis of the motor The thickness of the shell and the thickness of the core are inversely proportional,and the influence on the external magnetic field decreases with the increase of the number of poles.The research on the external magnetic field of permanent magnet motor in this paper can provide ideas for the analysis of the external magnetic field of other types of motor,geomagnetic field compensation,motor fault diagnosis and so on.