Design of an Active Magnetic Bearing for the Identification of Static and Dynamic Performances of a self-Aligning Hydrodynamic Journal Bearing

Large Self-Aligning Hydrodynamic Journal Bearings (SAHJB) are used in many heavy-duty rotating machines such as steam turbines. In an EPR2 nuclear power plant, their diameter can reach up to 800mm. To test a representative small scale of such bearings (100mm diameter), a test rig is being designed where the SAHJB is mounted in the middle of a shaft supported by two active magnetic bearings (AMB) at its ends and operated up to 5000 rpm. The role of the AMBs is to apply enough static load to the SAHJB to obtain a Sommerfeld number on the tested SAHJB equivalent to the one used on the real turbine. The AMBs will also be used to impart small perturbations to the rotor movement in order to identify the stiffness and damping coefficient of the SAHJB for different bearing-rotor misalignment. To evaluate the forces in the magnetic bearing, a finite element FE approach wasimplemented using the inhouse software code_Carmel. Both a simplified structure of an electromagnet (a U shape core attracting and an I shape core) and a realistic magnetic bearing (8 magnetic poles) have been modelled. On the simplified structure, results of FE calculations with linear hypothesis on magnetic materials (7000 times vacuum permeability) are in very good agreement with the results of an equivalent analytical model. These theoretical results are also in good agreement with experimental data. The same FE model was utilized to evaluate the effect of the non-linearity of the magnetic core. When the saturation of the magnetic core occurs, the forces available within the airgap between the fix part and the mobile part of the electromagnet, decrease. For low saturation of the magnetic core, the obtained forces are in good agreement with the analytic model results. Considering the actual magnetic bearing, the same type of calculations was carried out, allowing evaluations of the effect of magnetic non-linearity and geometrical dimensions on bearing performances. These elements are used to evaluate both the actual forces available within the bearing and the optimal domain of use of the AMB in a pre-design stage.