Rotor eddy current loss in high speed permanent magnet motors for flywheel energy storage system

In order to reduce the rotor eddy current loss,which is mainly caused by temporal harmonics in the excitation current,spatial harmonics in the pulsating stator magnetic motive force,and the airgap permeance variation due to slot-openings,and improve the efficiency of high speed permanent magnet motor,the spatial harmonics of the airgap flux density and the temporal harmonics of the excitation current were first analyzed by ANSOFT software,The influences of slot opening width and airgap length on the rotor eddy current loss were studied considering the skin effect.The mechanism and the effectiveness of a thin unmagnetized metal shielding ring around the magnets,which uses the shielding effect of the eddy current magnetic field,to reduce the eddy current loss in the rotor core,the magnets,and the retaining sleeve were analyzed.The effects of the electrical conductivity and thickness of the shielding ring on the rotor eddy current loss were analyzed.The results show that a thin unmagnetized metal shielding ring around the magnets only with a properly chosen slot opening width,airgap length,electrical conductivity and thickness of the shielding ring can significantly reduce the total eddy current loss of the rotor.