Wow, this is fantastic everyone. Thanks!
I knew I came to bthe right place! Rob On Fri, Sep 9, 2011 at 12:41 PM, John Doty <[1][email protected]> wrote: 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. [2]http://www.noqsi.com/ [3][email protected] _______________________________________________ geda-user mailing list [4][email protected] [5]http://www.seul.org/cgi-bin/mailman/listinfo/geda-user References 1. mailto:[email protected] 2. http://www.noqsi.com/ 3. mailto:[email protected] 4. mailto:[email protected] 5. http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
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