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Brake for small windmill?

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Posted by Jordan on September 14, 2007, 2:49 am

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.


Posted by bealiba on September 17, 2007, 1:10 pm

Okay. Take a motorcycle drum brake. Now normally the drum would spin
and the shoes on the backing plate would be stationary.

Proposal; Spin the backing plate with the shoes and have the drum
fixed. You will need to replace the shoe springs with lighter springs.

The faster the backing plate spins the more the braking effect. May
require weight on shoes to increase effect and twin leading shoe brake
would be better.


A chain type governer.


As seems the case you are talking savonius, they are generally self
regulating and by tuning the angle of the blades can be speed limited.

Posted by Jordan on September 18, 2007, 12:58 am
 bealiba@gmail.com wrote:

Yes, we considered a centrifugal clutch (eg chainsaw), which is similar
to your suggestion. Motorcycle parts are too bulky, alas.

Not sure what that is, but we're trying to avoid chains, belts, etc if

Sounds like serious engineering - could blow the budget.

I'm investigating an "eddy current brake", which sounds simple to
organise - aluminum disc and magnets only.

Thank you very much for the suggestions.


Posted by Neon John on September 18, 2007, 2:32 am
 If you don't want to use any gearing then your choices are very limited.  Most
braking devices including simple eddy current brakes require more speed than your
vertical windmill will generate.

Given your constraints, I'd probably go with surplus permanent magnet motors
operating as generators and equipped with 'crowbar' circuit.  No speed sensing

A crowbar circuit can be as simple as an SCR and something to provide the gate
trigger - a couple of resistors and maybe a zener diode.  Google for "crowbar
circuits".  The circuit would work thus:

The windmill is essentially free wheeling normally.  The PM generator produces a
voltage directly and linearly proportional to the speed.  The crowbar circuit is
connected across the generator's terminals.  When the speed and therefore voltage
rises to the trip point, the SCR fires, shorting the generator and braking it to
immediate almost-halt.  It will still rotate at a low speed proportional to the
leakage flux in the generator and the voltage drops across the various
Once fired, the SCR continues to conduct until the voltage is removed - until the
windmill stops.

By bringing the windmill almost to a halt, the mill stops harvesting wind energy
therefore you don't have much energy to dissipate.  Most everything else you're
considering will require you to dissipate considerable heat.

If you don't want to nearly stop the windmill then you can include a series power
resistor in the circuit.  Depending on the size of the windmill, this may involve
dissipating a LOT of power.

An ordinary induction motor functions nicely as an eddy current brake by simply
applying DC current to the input.  If a source of DC power is available then that
might be an option.  The energy is dissipated in the rotor though, so cooling
be a consideration.  If you brake the windmill almost to a stop, again, the
involved won't be great.

If these mills are to be very large then I'd probably include a fail-safe purely
mechanical backup.  Say, a centrifugally operated mechanical pawl that extends to
catch a stationary boss.  Perhaps backed by a coil spring so the stop won't be so


John De Armond
See my website for my current email address
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
Serenity: That feeling of knowing that your secretary will never tell either of
your wives.

Posted by Jordan on September 18, 2007, 3:36 am
 Neon John wrote:

I've seen how effective shorting a PM generator can be at braking a
friend's AIR400 wind generator. That would be a great result, with cost
again the only constraint. We're talking about seven of these, no time
to go scrounging for individual surplus units, so I've got my fingers
crossed for an "off the shelf" solution. Many thanks!


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