Design and Characterization of a high-speed Active Magnetic Bearing System

Abstract

Active magnetic bearings use controlled electromagnetic forces to support rotor shafts without any physical contact. Compared to conventional bearings, active magnetic bearings offer system advantages in high-speed applications. The contact-free operation eliminates wear, the need for lubrication and minimizes friction. Therefore, these bearings have proven successful in commercial applications such as turbo compressors, flywheel energy storage systems, and turbomolecular pumps. However, due to their inherent instability, active magnetic bearings must be integrated into a control loop for stabilization, increasing the complexity and size of these systems. Additionally, operating high-speed machines can be challenging and requires the implementation of advanced control algorithms. This thesis focuses on designing and experimentally validating a test stand of an active magnetic bearing system to provide a platform for future investigations on rotor dynamic effects and advanced control algorithms. In particular, the emphasis is also on a test stand that is as modular as possible in order to achieve flexibility for different research objectives. The design, layout, and alignment of the individual components will take mechanical and electromagnetic aspects into account. The challenge is to not only achieve flawless functionality but also to ensure a system behavior as linear as possible. This enables the use of simple, linear control algorithms to stabilize the rotor shaft. Accompanying validation ensures that the magnetic bearing meets the desired requirements and corresponds to the mathematical model. In particular, attention is paid to the force-current and force-displacement characteristics of the active magnetic bearing. Additionally, a mathematical model of the rotor is created, and model-based position control is implemented. Finally, measurements are taken with the levitating rotor shaft. Notably, measuring the output sensitivity shows that robust position control of an exemplary rotor shaft is possible with simple, linear control structures.