Test and theory of electrodynamic bearings coupled to active magnetic dampers
Electrodynamic bearings exploit repulsive forces due to eddy currents to produce positive stiffness by passive means. Such a feature would make this type of bearing a viable alternative to active and permanent magnet bearings. Although electrodynamic bearings do not violate Earnshaw’s theorem, the open issue remains the stabilization system that is needed to make the rotating body stable, due to the low rotational speeds. Stabilizing solutions proposed in the literature are partially effective and not totally convincing. This limits real industrial applications. The present paper proposes a combination of electrodynamic and active magnetic bearings. At low speed the active part behaves as a conventional active magnetic bearing, while at high speed it provides damping. The readiness of the proposed solution is demonstrated by experimental results obtained using a dedicate test rig.
Active Radial Electromagnetic Damper
A design of a non-contact active radial electromagnetic damper for applications in rotating machinery is presented. The device can effectively damp lateral rotor motions without exerting rotational drag or producing negative stiffnesses. Potential applications include damping and stabilization of rotordynamic modes, stabilization of electrodynamic bearings and limiting unbalance responses. As an example, damper application in a multi-stage down-hole natural gas compressor is discussed.
Ambient-Temperature Passive Magnetic Bearings for Flywheel Energy Storage Systems
Based on prior work at the Lawrence Livermore National Laboratory ambient-temperature passive magnetic bearings are being adapted for use in high-power flywheel energy storage systems developed at the Trinity Flywheel Power company. En route to this goal specialized test stands have been built and computer codes have been written to aid in the development of the component parts of these bearing systems. The Livermore passive magnetic bearing system involves three types of elements, as follows: (1) Axially symmetric levitation elements, energized by permanent magnets., (2) electrodynamic ''stabilizers'' employing axially symmetric arrays of permanent magnet bars (''Halbach arrays'') on the rotating system, interacting with specially wound electrically shorted stator circuits, and, (3) eddy-current-type vibration dampers, employing axially symmetric rotating pole assemblies interacting with stationary metallic discs. The theory of the Livermore passive magnetic bearing concept describes specific quantitative stability criteria. The satisfaction of these criteria will insure that, when rotating above a low critical speed, a bearing system made up of the three elements described above will be dynamically stable. That is, it will not only be stable for small displacements from equilibrium (''Earnshaw-stable''), but will also be stable against whirl-type instabilities of the types that can arise from displacement-dependent dragmore » forces, or from mechanical-hysteritic losses that may occur in the rotor. Our design problem thus becomes one of calculating and/or measuring the relevant stiffnesses and drag coefficients of the various elements and comparing our results with the theory so as to assure that the cited stability criteria are satisfied.« less