Research On Eddy Current Loss And Structural Characteristics Of High Speed Permanent Magnet Motor Rotor

High-speed permanent magnet motors have the advantages of small size and high power density,and have broad application prospects in high-speed grinding machines,air circulation refrigeration systems,and energy storage flywheels.However,its high frequency of magnetic field alternation,rotor eddy current loss is much higher than that of traditional motors,which reduces the energy conversion efficiency of the motor.High rotational speed also brings centrifugal force that permanent magnet materials can hardly withstand,and greatly increases the risk of resonance in the rotor system.Therefore,the study of rotor eddy current loss,mechanical strength and rotor dynamics is of great value for high-speed permanent magnet motors.In this paper,to solve the above problem,the rotor is segmented to reduce the rotor eddy current loss,but the rotor segmentation will have a negative impact on the strength and dynamics of the rotor,so it is necessary to study the changes in the strength and dynamics caused by the axial segmentation of the rotor components.A three-dimensional analytical calculation model of rotor eddy current loss is established for a 150 kW,30,000r/min high-speed permanent magnet motor,and the influencing factors and three-dimensional distribution law of eddy current in the rotor conductor are analyzed.Combined with the electromagnetic field theory,a finite element model is established to analyze the distribution characteristics of eddy currents.In order to reduce the rotor eddy current loss,the sheath and permanent magnet are axially segmented,and the influence law of axial segmentation on rotor eddy current loss is investigated.Finally,according to the characteristics of eddy current loss distribution,the method of unequal segmentation of permanent magnet axially is proposed,which further reduces the rotor eddy current loss with the same number of segments.In order to investigate the influence of rotor axial segmentation on its mechanical strength,based on the theory of thick-walled cylinder in elastic mechanics,an analytical calculation model of rotor strength is established,and the change of permanent magnet stress with radial position is investigated by analytical method,which is verified by three-dimensional finite element method.The sheath and permanent magnet axial segments were taken separately,and the objects of rotor strength calibration after segmentation were determined,as well as the variation law with the number of segments.It was found that the axial segmentation of the sheath significantly increased the maximum local axial stress of the permanent magnet and reduced the rotor strength.Finally,the influence factors of rotor strength,such as sheath thickness,are compared for different number of sheath segments.To address the problem of the influence of the axial segmentation of the sheath and the permanent magnet on the rotor dynamics,the expressions for the critical rotor speed and the amplitude and vibration frequency are derived analytically by combining the rigid rotor dynamics theory.In addition,a finite element model is established to calculate the first three orders of the rotor modal vibration pattern and critical speed,and the variation of critical speed with bearing stiffness.The effect of permanent magnet and sheath segmentation on the critical speed is obtained by taking the axial segmentation of permanent magnet and sheath respectively.Finally,based on the industrial allowable unbalance residuals,the unbalance force is applied to the finite element model to study the variation of amplitude under the segmentation of permanent magnets and sheaths.