On Fri, 14 Sep 2007 12:21:17 +1000, Jordan
wrote:

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.

All comments and suggestions welcome.

The old style water pump windmills had a wind-speed trigger gidge
rigged up - if a hurricane came up the tail had an over-center spring
and folded over, and the tail vane directed the fan wheel aim
90-degrees to the oncoming wind. That tended to stop the wheel.

After the storm you climbed up and reset it by hand IIRC.

The aluminum disc magnetic hysteresis brake (mentioned elsewhere)
would work to keep them from spinning to destruction speed in normal
stiff breezes, but even that has it's limits.

You do NOT want the unloaded windmill fan wheels going fast enough
to throw off a fan blade, car radiator fan blades coming loose at high
RPM's have been known to punch nice crescent shaped holes through
steel car hoods. If you are in the way they can maim or kill.

You can put a disc brake pad on the side of the tail boom, so when
the tail triggers over the pad contacts the hysteresis flywheel.

Myself, I'd get real wind generator windmills made (with the old
style multi-blade wheels instead of a three-blade propeller) and rig
them up with an inverter to feed into the power grid, then you
decorate them up to look like old-style farm windmills. The credit on
the power bills can easily pay the water bill and the park maintenance
costs...

-- Bruce --