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

This is definitely of interest. Alum would work? What about mild steel?
Need to keep cost down. It'll only be up and running a few weeks.
Thanks!

Aluminum will work only as an eddy brake, and the drag will vary as the
square of speed (rpm).

A sheet of hard steel will in addition act as a hysteresis brake,
increasing the drag at lower speeds. Mild steel is too soft
magnetically to get much hysteresis effect, but a flat sheet of hardened
spring steel ought to work. It doesn't need to be pretty.

One thing to worry about is corrosion if the disk will be exposed to the
elements.

Joe Gwinn