Study On High Efficiency Magnetic Bearing For Flywheel Energy Storage

Magnetic bearing (MB) is a mechatronics system, where rotor suspension is achieved by employing controllable magnetic force. As a high effective energy storage technology, flywheel energy storage system (FESS) has been used in various applications, such as smart grid, renewable energy, aero spacecraft, uninterrupted power supply (UPS) and pulsed power. MB is a key technology of FESS due to its contactless characteristic together with controllable stiffness and damping. The study of MB is meaningful in both theory research and engineer application.Research statutes of the magnetic bearing technology are summarized at home and abroad. Research progress on the working principle of magnetic bearing, electromagnetic field calculation methods and control methods are discussed. Based on the detailed analysis of the FESS rotor supporting requirement, which is that the FESS will accelerate and decelerate frequently with a long working period, thus, the active magnetic bearing is chosen.permanent magnetic (PM) material with high performance is applied in MB to optimize the structure design and minimize the copper loss.the topology, analytical model and control system of radial MB and axial MB are studied in this disseration. The main works of this disseration are as follows:The structure and working principle of PM biased homo-polar radial MB (PMHRMB) is introduced firstly, combined the periodic boundary conditions, the air gap PM magnetic field analytical model is established based on the subdomain method. The electrical excited field is considered as two dimensional field, thus, the analytical model is established with the surface current model. The ending structure of the PMHRMB is simplified as lines with opening boundary conditions. The ending field is divided into pole reign and slot region, meanwhile the axial force calculation model is established with the consideration of slotting effect. The correctness and effective of the established analytical model is validated by the 3D finite element analysis(FEA).The BH curve of the magnetic material used in PMHRMB is tested. Flux path in the PMHMB is assumed, thus, an equivalent lumped magnetic model of the PMHRMB is established considering slot effect, rotor eccentricity and material characteristics. The flux distribution and electromagnetic characteristics is validated by the 3D FEA, furthermore, the two radial freedom of the PMHMB can be treated as decoupled. The maximum electromagnetic force of the PMHRMB is calculated based on the FESS rotor dynamics demand under different rotor space attitude with the consideration of rotor unbalance force and PM machine eccentric force. The ratio of the PM flux and current excited flux is studied for global optimal parameters design. The PMHRB is designed in detail.Based on the comparison of available PM biased axial MB (PMAMB) topologies, a new PMAMB with multiple air gaps is proposed. The working principle of the proposed PMAMB is introduced. The air gap model is established by using the conformal mapping method.based on the Laplace's equation and boundary condition of the leakage flux domain, the expression of the leakage flux is obtained. Based on the analytical model with the aid of the lumped magnetic circuit model, the characteristics of the PMAMB is calculated, the new PMAMB is optimally designed, the length ratio of the inner air gaps is chosen for PM bias force. The PM size and installation position are both optimized. The analytical model and design results are validated by the 3D FEA results.The main current excited flux path is divided into element which contains radial and axial flux separately, thus, the effective AC reluctance can be analytically determined by the element effective reluctance model. The stator mid stator is considered as the secondary winding, meanwhile, the relationship between the various vectors of axial magnetic bearing is established. The dynamic current stiffness is analytical determined and validated by two-dimension (2D) transient finite element analysis on each frequency band. In order to take the eddy current effect into account, the position stiffness of the PMAMB are obtained by the two-dimension (2D) transient finite element analysis with element re-mesh technology. The PM biased force is calculated with AC current exciting or rotor axial movement separately. In order to improve the dynamic ability, the solid stator core is silted in the PMAMB. The dynamic current stiffness of PMAMB is subsequently analytically determined by equivalent process of the effective reluctance model.The output characteristics of the PMHRMB and PMAMB are treated as the liner combination of the current stiffness and position stiffness. The rotor is simplified as the rigid rotor. The PMHRMB and PMAMB model, rotor dynamic model and control system model of FESS are established with the aid of software Matlab/Simulink. The axial starting process is simulated by taking the eddy current effect into account, which indicates that the eddy current will reduce the response speed of the axial magnetic bearing. Meanwhile, rotor radial vibration amplitudes are obtained under different rotating speed, the simulation results indicate the maximum amplitude appears in the resonance frequency point.A 4.8 MJ FESS prototype is manufactured; the FESS prototype contains magnetic bearing, high-speed PM machine, flywheel main body, auxiliary bearings and vacuum chamber shield. The rotor of the FESS is made of high-strength alloy steel together with the flywheel. The displacement and current closed-loop control system of the used MBs are established. The current stiffness and position stiffness of the PMHRMB and PMAMB are measured, which verify the analytical model.The dynamic stiffness of the axial magnetic bearing is tested which verify the effectiveness of the transient finite element analysis.The FESS prototype rotor suspension experiments are carried out under different rotating speed with five freedoms. Experimental results indicate that the rotor vibration frequency component is mainly decided by the fundamental frequency of rotor unbalanced force. Moreover, due to switching device, magnetic Nonlinear saturation factor, there are other orders of rotor vibration harmonic components. The work of this dissertation lays a solid fundament for the related research work in future.