Influence Of Various Rotor Structures On Rotor Eddy Current Loss In High-speed PM Brushless Motor

A rotor retaining sleeve is usually essential in high-speed permanent magnet(PM)brushless DC motors to firmly retain the magnets and ensure the rotor integrity.The structure and material of retaining sleeve have a significant effect on rotor losses,especially on the rotor eddy current loss.On the other hand,heat dissipation from the rotor is difficult.Therefore,any excessive rotor loss may lead to over temperature and cause irreversible demagnetization of the magnets.In this thesis,a new type of laminated sleeve is first proposed,which aims to block the path of eddy current and reduce the eddy current loss in the sleeve.Then the influence of the retaining sleeve with different structures and materials on the rotor eddy current loss in high-speed PM brushless DC motors is comprehensively investigated,with both 3-D finite element method(FEM)simulations and experiments.Particularly,the proposed experiments are carried out with a stationary rotor,by applying equivalent current harmonics into the stator windings to ensure that the dominant stator harmonic fields observed from the rotor are the same as those of practical operation.The study reveals that the equivalent conductivity of the retaining sleeve has a significant influence on the rotor eddy current loss,and countermeasures can be made accordingly so as to reduce the rotor eddy current loss.Also,the influence of PM resistivity on the total rotor eddy current loss in the high-speed PM brushless motors is investigated in detail.Finite element analysis shows that,for a given prototype,a lower PM resistivity is beneficial to reduce the total rotor eddy current loss,and vice versa.This result can have reference value when studying the PM materials for the high-speed PM brushless motors.