Analytical Calculation Of Rotor Eddy Current Losses In High Speed Permanent Magnet Machines

High speed permanent magnet motor has become one of the research hotspots in the field of electrical machines because of its advantages of high power density,high efficiency and direct connection with high speed load.However,high speed permanent magnet motor rotor is small in size.Therefore,the high loss density and poor cooling conditions easily cause excessive temperature rise of the rotor and affect the permanent magnet performance.This paper mainly researches on rotor eddy current losses on high speed permanent magnet motor.The work is as follows:Firstly,according to the theory of time-varying electromagnetic field,a two-dimensional electromagnetic field model of high speed permanent magnet motor based on magnetic vector potential is established.The motor is divided into five sub-domains: stator slot,slot-opening,air gap,retaining sleeve and permanent magnet.Poisson equation is established in the stator slot to consider the actual stator current waveforms and the actual distribution of the winding.Laplace equations are established in slot-opening and air gap.Diffusion equations are established in the retaining sleeve and permanent magnet to consider the rotor eddy current reaction.With the help of the boundary conditions between subdomains,the equations are solved.The relationship between rotor eddy current losses and magnetic vector potential is obtained by Poynting's theorem.Secondly,according to different magnetic field sources,the open-circuit model,the stator magnetic motive force model and the load model is respectively established,and the equations satisfied by magnetic vector potential in different sub-domains under different models are deduced and calculated in detail.The magnetic field distribution and rotor eddy current losses are obtained.The calculating results are verified by finite element method.Finally,based on the analytical model established,the factors affecting the rotor eddy current losses are analyzed from the aspects of stator and rotor.In the aspect of stator,the slot-opening width,stator slot number and short-distance distribution is considered.On the rotor side,the rotor speed,the materials of the retaining sleeve,the thickness of the retaining sleeve and the materials of permanent magnet are considered.The analysis results are basically consistent with those obtained by finite element method.In the aspect of experiment,the open-circuit experiment on a 2-pole 6-slot permanent magnet motor was done.The rotor eddy current losses were separated from the losses of the motor by means of loss separation.The experimental results and the analytical calculation results are contrasted to prove that the analytical model is right.