According to Jordan :
I've volunteered to help an artist, who wants to place a number of small
vertical rotor windmills in a seaside park, on top of picnic sheds. (He
has permission!)

The rotors will be about 6 ft diameter, 3 ft high, and free-spinning -
won't drive anything.
The challenge is how to control the things so they don't spin too fast,
but still be able to start in a light breeze.

Of course, cost matters.

Ideas so far:
1. Centrifugal-operated "air brake".
2. Permanent-magnet motor, with speed-sensing switch to short out for
back-emf braking.
3. Sails attached with Velcro, to rip out in big wind.

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.

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