In article ,
Jordan wrote:

An aluminum disc on the back end of the shaft running between a
pair of high strength permanent magnets recovered from dead hard disk
drives (thus cheap). The faster it goes, the more drag from the
magnetic field generating eddy currents in the disc.

It may take some playing to determine what radius and how many
magnets will be needed to get sufficient speed control.

Good Luck,
DoN.


Thanks Don
I don't know much eddy currents. Does this tend to allow the rotor to
spin easily at first?

Yes. To flatten it out, use a disk of a magnetically hard metal, such
as hardened steel. This is called a hysteresis brake.

Joe Gwinn