Investigation of Critical Speed Increase by Five Degrees of Freedom Magnetic Levitation System with Combined Radial-Axial Magnetic Bearing

Magnetic bearings (MBs) can support a rotating shaft without any contacts using electromagnetic force. A typical five-degree-of-freedom (5-DOF) magnetic levitation system uses two radial MBs and one axial MB. However, the size of this system may become large due to the combination of three MBs, posing the issue of reduced critical speed with increased shaft length. Therefore, combined radial-axial MBs (CRAMBs) have been proposed to address these challenges. In this study, reluctance-type CRAMBs capable of providing support in three-degree-of-freedom with a high suspension force density are proposed. As the CRAMBs do not use permanent magnets, they are easy to assemble and can operate in cryogenic and high-temperature environments. This paper presents the structure designed to increase the opposing surface area where the magnetic attractive force acts, to achieve a high suspension force density. The suspension force and bearing stiffness of the proposed structure are analyzed using a three-dimensional finite element method (3D-FEM). Further, a 5-DOF magnetic levitation system is constructed using the proposed CRAMB and radial MB. The proposed system is then compared to two types of conventional 5-DOF magnetic levitation systems (Separated Type A and B) based on axis vibration analysis using 3D-FEM. Critical speed maps for the first, second, and third (bending first) modes of each system, along with their corresponding deformations, are displayed. The analysis shows that reducing shaft length in the proposed system leads to an increase in the critical speed of the first bending mode. In addition, loss characteristics when the rotor is rotated are evaluated.