Infoscience

Thesis

Stabilization of electrodynamic bearings with active magnetic dampers

Electrodynamic Bearings (EDBs) are a kind of passive magnetic bearings that exploits the interaction between the induced eddy currents in a conductor and a magnetic field to provide re-storing forces. They have been regarded as an appealing alternative to Active Magnetic Bearings (AMBs), having the ability to provide positive stiffness passively without introducing negative stiff-ness in any direction. Compared to AMBs, EDBs present advantages such as lower cost and higher reliability due to simplicity of configurations. One of the most interesting features of EDBs is the possibility to obtain stable levitation using standard conductive materials at room temperature, requiring no control systems, power electronics or sensors. Thus EDBs could be suitable solutions for high–speed rotating machinery such as flywheels, small size compressors, centrifuges and vac-cum pumps. Despite these promising characteristics of EDBs, applications are still limited because of instability issues. The main problem is that the effect of the rotating damping force in EDBs causes unstable behavior of the rotor. In existing solutions, stabilization is achieved mainly by introducing non-rotating damping to the rotor with passive ways. Although stable levitation is possible, the effectiveness of the existing methods is still limited. A hybrid solution has been proposed in this thesis, where EDBs are combined with active magnetic dampers (AMDs). Using similar magnetic actuators as those used in classical active magnetic bearings (AMBs), non–rotating damping forces are applied on the rotor supported by EDBs to obtain stable operation. This system is designed to exploit the high reliability of EDBs, overcoming the stability problem by means of controllable AMDs. It results in increased global system reliability. In case of AMBs failure, the EDBs are able to guarantee a stable levitation down to a certain speed considered safe for touch–down. During the operation speed range, the AMDs provide non–rotating damping to stabilize the rotor. This non–rotating damping can be easily tuned during rotor operation phase. At low speeds when the EDB forces are not sufficient to support the rotor, the active magnetic actuators work as AMBs to guarantee stable levitation of the rotor in a wide speed range. Besides, the EDB−AMD configuration also allows characterizeing in dynamic condition, which opens the possibility to establish damping strategy that can in perspective be implemented by totally passive means, such as eddy currents, elastomeric mounts. The combination of EDB and AMD forces are studied both analytically and experimentally. An analytical model of the system, as well as a test rig, has been built. Simulations and experi-mental tests validate the model and characterize the system. The effectiveness of the proposed solution is confirmed. The control strategy of AMDs and stabilizing alternatives of EDBs are dis-cussed consequently. The combination of EDB and AMD can be exploited to investigate easily dif-ferent damping strategies.

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