Core Loss Calculation Method And Thermal Field Simulation Of Radial Active Magnetic Bearing

Active magnetic bearing is a new type of multiply-advantages electromechanical integrated bearing that uses a controllable non-contact electromagnetic force to suspend the rotor in space stably to achieve no mechanical contact between stator and rotor.it has been widely used in industrial fields for its superior performance such as no contact,no lubrication,high speed,low power consumption,simple maintenance,etc.Although the power loss(namely copper loss and core loss)of active magnetic bearings is much lower than the friction loss of the mechanical bearing,when working under special occasions such as ultra-high speed and vacuum,the copper loss and core loss may cause the stator and rotor to thermally expand,and affect its positioning accuracy and working performance.In the extreme case,it may even cause the iron core to deform,reduce and damage the insulation performance of the coil,and shorten its life.Therefore,in order to ensure the performance and service life of active magnetic bearings,it is necessary to study the calculation method of the loss of active magnetic bearings in order to explore the method and way of reducing the loss.This paper focuses on the electromagnetic field modeling method and loss calculation method for eight-pole radial active magnetic bearings.On the basis of the basic reluctance calculation method,the calculation method of stator and rotor reluctance is given.The interpolation method is used to fit the BH curve of the ferromagnetic material to consider the nonlinearity of the magnetic permeability of the core.The magnetic field segmentation method is used to calculate the edge effects of the magnetic poles,the total reluctance of the air gap is obtained through its series-parallel relationship,thereby establishing a nonlinear equivalent magnetic circuit model of the radial active magnetic bearings.Comparing the calculation results of the nonlinear equivalent magnetic circuit with the results of the finite element,the error is about 3%directly opposite the magnetic extreme surface,which verifies the accuracy of the equivalent magnetic circuit model.Based on the calculation of the equivalent magnetic circuit model and the improved loss model,the loss of active magnetic bearings is calculated.Based on the classic three-term constant coefficient loss model,the hysteresis loss modifying factor and the eddy current loss modifying factor are introduced to establish an improved loss model,and the coefficient of the modifying factor is obtained by fitting method.The calculation results show that,compared with the classic three-term constant coefficient loss model,the improved loss model greatly reduces the error in the calculated speed range,and after the speed exceeds 5000rpm,the relative error is basically reduced by more than 10%,and the accuracy is improved.When active magnetic bearing is running under ideal conditions,the center of the stator and the center of the rotor coincide.However,in actual engineering,due to the imbalance of the rotor mass and the external excitation force,the rotor will vibrate,resulting in uneven distribution of the air gap of active magnetic bearings and affecting the magnetic field distribution.It is necessary to model the magnetic field under the rotor vibration of active magnetic bearings.The calculation formula of the eddy current of the lamination core of the bearing is derived according to the electromagnetic field theory,and the equivalent magnetic circuit model is introduced to analyze the influence of the eddy current of the core caused by the rotor vibration on the magnetic field and electromagnetic force of active magnetic bearings.Compared with the model without considering the eddy current,after considering the influence of the eddy current,the magnetic flux and electromagnetic force of active magnetic bearings are attenuated,and the higher the rotor vibration frequency,the greater the attenuation.In the example of this paper,the rotor vibration When the frequency reaches 2000Hz,the electromagnetic force attenuation is about 5.95%.Based on the loss calculation,the electromagnetic field-temperature coupling analysis of active magnetic bearings was carried out.When the rotation speed is low,the heat source of active magnetic bearings is mainly the copper loss of the stator coil,the rotor core loss is relatively small,and the circumferential temperature of the rotor is not much different.The highest temperature point in the air gap is concentrated at the position directly opposite the magnetic extreme surface,The lowest point of the core temperature is at the stator yoke;when the speed is higher,the iron loss ratio increases,and the temperature difference of the rotor in the circumferential direction increases with the speed,and due to the existence of the eddy current skin effect,with the speed rises,the temperature distribution on the rotor will also vary greatly at different locations.This paper lays the foundation for the magnetic circuit modeling,loss calculation,eddy current analysis of active magnetic bearings,and the subsequent exploration of loss calculation method in rotor vibration and loss reduction method.