On Sep 9, 2011, at 5:29 AM, Peter Clifton wrote: > If you're after significant resistance, I would go for a copper disk, > about 5mm or thicker, with strong magnets - either an electromagnet on > an iron core - placed quite close (within a few millimetres) of the > spinning disk, OR - some neodymium hard-disk magnets (for example). > > You could use an aluminim disk (much cheaper, and easier to obtain I'd > imagine) - but I would up the thickness.
The torque will fall off at low speed, when the magnetic diffusion depth exceeds the thickness of the disk. The diffusion depth is given by: sqrt(dm*t) where dm is the magnetic diffusion coefficient, about 130 cm^2/s for Cu, 230 cm^2/s for Al. The time parameter, t, is essentially the time a point on the disk remains in the vicinity of the magnet. This is, of course, just dimensional analysis: a detailed model of the field configuration is needed if you need to be more quantitative. The same physics leads to the concept of "skin depth". John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ j...@noqsi.com _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
