Predicting Thermal Effects in Magnetic Bearings via Multiphysics Models - Theory and Experiment

Climate goals are catalyzing the need for more efficient and sustainable turbomachines. Magnetic bearings use magnetic forces to provide a contactless solution to support rotors. Such a force can be produced by permanent magnets (passive magnetic bearings – PMB) or by electromagnets controlled by a feedback system (active magnetic bearings – AMB). Since AMBs do not need lubricants, greases (including the additional machinery necessary), and with reduced friction between the moving parts, the maintenance in AMB systems is considerably less frequent, making these bearings a candidate for working in harsh environments, including at high temperatures. In this context, this work investigates a multiphysics methodology for predicting the thermal effects on the dynamics of rotors levitated by magnetic bearings. Multiphysics models are derived, coupling the rotodynamics, the electromagnetism, the control, and the heat transfer domains, leading to a set of nonlinear differential equations that is simultaneously solved using numerical methods. The thermal effects predicted by the multiphysics models are validated using one dedicated test rig mimicking a flywheel consisting of one vertical rotor suspended radially by AMBs and axially by one PMB. The increase in temperature occurs due to the vacuum environment, and its influence on the system dynamics can be predicted by accounting for the temperature increase’s influence on i) PMBs demagnetization, ii) AMBs coil resistance, iii) expansion of components, and iv) drift of eddy current sensors.