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Re: [femm] Motion induced eddy currents

C F Cheuk wrote:

Hi David,

Can you tell me where the following link has been moved to?

<http://groups.yahoo.com/group/femm/message/1260>



All of the messages are on the new site, but unfortunately, the message numbering from the Yahoo list wasn't preserved. Generally, it is possible to use the search on the main list page at http://femm.foster-miller.net/mailman/listinfo/femm to find old messages by their content. I think that the particular one you are after is at:
http://femm.foster-miller.net/pipermail/femm/2002-May/001186.html

I am trying to use your FEMM to design a linear motion damper using eddy
current induced on a shaft oscillating in a sinusoidal manner. The amplitude
of the oscillation is 1.7mm. Have you done similar damper before? Any
guidance that you can give me will be most appreciated.

C F Cheuk


I've used FEMM before as part of the solution to this sort of problem. The way you'd do it sort of depends on the design of the damper. In one past project, I'd specifically looked at damping the lateral vibrations of a rotating shaft via an eddy current damper. The damping was due to the motion of a circular damper plate in a magnetic field produced by an array of ring magnets. I used FEMM to compute the field linking the plate at the nominal position. In that particular sort of device, the magnetic field of the induced currents can often be ignored and a thin plate can be assumed, leading to a 1D differential equation, driven by the flux linking the damper plate as computed by FEMM, that is solved for the current distribution in the damper plate. The damping predicted by this method was in pretty good agreement with the observed behavior when the device was actually built.

A good paper about analyzing this sort of damper is:
Kligerman, Y., Grushkevich, A., and Darlow, M. S., 1998, "Analytical and Experimental Evaluation of Instability in Rotordynamic System with Electromagnetic Eddy-Current Dampers", ASME, J. of Vibration and Acoustics 120(1), 272-278.

Other sorts of dampers would use other approaches, e.g. if the damper had wound coil(s), if motion is axial instead of lateral, if there are moving magnets instead of moving plates, depending on the frequency range of interest, and so on.

Dave.