Influence of rotor structure on rotor eddy-current loss in high-speed permanent magnet brushless DC motors

Rotor eddy-current loss appears significant in high-speed permanent magnet brushless DC(PM BLDC) motors.The effect of segmenting magnets on rotor loss was analyzed with finite element analysis.Only when the circumferential width of the magnet segment is smaller than the skin depth,the eddy-current loss in the magnets can be reduced by segmenting,otherwise the eddy-current loss will increase.By using the shielding effect of the eddy-current magnetic field,a copper shield with high conductivity was used between the retaining sleeve and magnets.Although eddy-current loss is induced in the copper shield,this eddy-current magnetic field reduces the harmonic field in the air-gap,so that the loss in the magnets,rotor yoke and retaining sleeve are cut down dramatically,resulting in a reduction of the overall rotor eddy-current loss.