Research On Rotor Eddy-Current Loss And Rotor Stress Of High-speed Permanent Magnet Brushless DC Motors

High-speed permanent magnet(PM)brushless direct-current(BLDC)motors are increasingly used in many applications due to their high efficiency and high power density.However,the temperature rise caused by the rotor eddy-current loss constrains the performances of the high-speed PM motors.Excessive temperature rise can even result in irreversible demagnetization in the PMs.In addition,the elastic moduli of the alloy materials equipped in the rotor also decrease as the temperature rise increases.The reduction of the moduli could deteriorate the mechanical performance of the rotor operating at high speed.On the other hand,the PM has a small tensile strength,and it is difficult to withstand the stress due to the high rotational speed and the high temperature rise of the rotor,thus the safe operation of the motor is affected.In order to effectively reduce the rotor eddy-current loss of high-speed PM BLDC motors,this thesis focuses on two methods for reducing the loss,including split-phase winding and copper shielding.An analytical model for the rotor harmonic eddy-current loss is proposed.The mechanism of drastically reducing the harmonic eddy-current losses of PM BLDC motors by using split-phase winding is revealed.The influence of copper shield on rotor eddy-current loss and rotor stress of high-speed PM BLDC motors is studied.The influence of the non-uniform thermal expansion of the magnet materials on the rotor stress of high-speed PM BLDC motors is investigated.An analytical model for the rotor harmonic eddy-current loss of high-speed surface-mounted PM motors in consideration of eddy current reaction fields,harmonic currents,stator slotting and different rotation states of the rotor is proposed,which can be utilized to reliably calculate and evaluate the rotor harmonic eddy-current loss.The accuracy and effectiveness of this analytical model are validated by using FEM(finite element method).Based on the theory of heat transfer and electromagnetics,a method for experimental measurement of rotor harmonic eddy-current loss is proposed.The analytical model is validated by the experiment.Based on the study of the distribution law of the MMFs(magnetomotive forces)of split-phase winding,associated with the characteristics of harmonic currents of PM BLDC motors,the mechanism of drastically reducing the harmonic eddy-current losses of PM BLDC motors by using split-phase winding is revealed and validated by FEM and experimental measurement.The results calculated by the analytical method indicate that the rotor harmonic eddy-current losses generated by low-order harmonic currents,which inherently exist in PM BLDC motors,are eliminated by the split-phase winding.Thus the rotor harmonic eddy-current loss is significantly reduced.The problems of high temperature rise and poor heat dissipation in the rotor of high-speed PM motors are solved.The use of split-phase windings in PM BLDC motors is essential to broaden their applications in the field of high-speed drive.The influence of copper shield on eddy-current loss and rotor stress of high-speed PM BLDC motors is investigated.Firstly,the effect of copper shields with different thicknesses on reducing rotor eddy-current loss is verified,and then the effect of the split-phase winding and the copper shield on reducing rotor harmonic eddy-current loss is compared.The stress calculation procedure for the rotor with multi-layer rings including copper shield is established.A method for calculating the presetting interference fittings at the inner and the outer interfaces of the copper shield is proposed.On this basis,the influences of copper shield thickness on the maximum stress of each part of the rotor are investigated.The influence of the non-uniform thermal expansion of the magnet materials on the rotor stress of high-speed PM BLDC motors is studied.For the rotor with radial magnetization,the analytical model of rotor stress under the condition of tangential-radial non-uniform thermal expansion of the magnet materials is established and validated by FEM.For the parallel magnetized rotor,the fluctuation law of the rotor stress distribution is revealed,and the influence of copper shield on rotor stress under the condition of non-uniform thermal expansion of magnet materials is further investigated.The effectiveness of stress analysis is verified by the experiment of high-speed operation of the rotor with copper shield.