Permanent Magnet Biased Three Degrees Of Freedom Magnetic Bearing:Structure,Modeling,and Analysis

Magnetic bearings levitate the rotor by electromagnetic force without mechanical contact.Compared with the mechanical bearings,Magnetic bearings have many advantages such as no mechanical wear,small loss,no lubrication,long-life and adjustable dynamic characteristics.Hence,they have been widely concerned and applied in the industrial fields of flywheel energy storage,turbomachinery,aeronautics and astronautics,medical equipment and so on.Under this background,this dissertation studies the topology,electromagnetic model,design method,electromagnetic performance and cross-coupling effect of the permanent magnet(PM)biasd three degrees of freedom(3-DOF)magnetic bearing.The main resach contents of this dissertation are as follows:(1)An improved magnetic circuit model of the PM biased 3-DOF magnetic bearing is proposed.The PM bias field and radial and axial control fields are analyzed together.Hence,the cross coupling between the axial and radial MB parts is included;all the magnetic flux leakage paths are taken into consideration;and the nonlinear magnetic permeability of the iron material is taken into account.By comparing the results predicted by the traditional magnetic circuit model,the improved magnetic circuit model and the three-dimensional finite element(FE)software,the accuracy of the improved magnetic circuit model is verified.It provides a basis for further structural design and coupling analysis.(2)A new PM biased 3-DOF magnetic bearing with toroidal radial control coil is proposed.By changing the radial control coil from wound arould the tooth to arould the yoke,the copper loss can be reduced.Taking the new bearing as an example,an improved design method combining with the improved magnetic circuit model for magnetic bearing is proposed.It can take into account the influence of magnetic flux leakage and iron pearmeance and save the required time.The design method is used to design the specific structural parameters,and the electromagnetic performance is calculated and analyzed.The results are verified by the three-dimensional FE method.(3)The cross-coupling effect of the PM biased 3DOF magnetic bearing is analyzed.The results show that the axial control current has a significant influence on the radial force and stiffnesses,while other force characteristics and stiffnesses do not show the cross-coupling effect.On this basis,three restraining methods are proposed:adding compensation coil,increasing the axial length of magnetic ring and increasing the axial length of the PM.The principles of these methods are explained in detail,and their performance are analyzed and compared.The results are verified by three-dimensional FE method.(4)Based on the analysis of the cross-coupling effect,a new PM biased 3-DOF magnetic bearing with symmetrical structure is proposed.The toroidal coil is applied to reduce the copper loss.The radial stator is located in the center position of the axial stator,therefore the overall structure is symmetrical about the radial stator in the axial direction and the cross-coupling effect is eliminated fundamentally.The specific structural parameters are designed,and the electromagnetic performance is analyzed by magnetic circuit model and FE method.A prototype is designed and tested to verify the accuracy of the magnetic circuit model and FE model.