Brake for small windmill?

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.
Jordan
Reply to
Jordan
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What RPM is "too fast"? What wind speed is needed to reach this given the design (Savonious, right?), and, how often is it exceeded locally?
Of course.
Depends on which type of rotor you're building. A drag-driven one (Savonious) will have a very different answer from an, sorry, can't remember real name, google "eggbeater wind turbine" design.
alt.energy.homepower might be a really good place to ask this question.
Hope this helps, Dave Hinz
Reply to
Dave Hinz
According to Jordan :
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.
Reply to
DoN. Nichols
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's_law
Reply to
Richard J Kinch
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hth,
Wes
Reply to
Wes
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I built several rock-tumbler windmills when I was a kid. They were set up out back behind the garden where the noise bothered only the birds. The blades were very thin sheetmetal attached at the leading edge and they bent back as the wind increased. Typical life was about 3 months before the "bearings" seized.
If this is a Savonius type, maybe you could shock-cord the sail at the inner edge so a strong wind opens it up to bleed air and lose efficiency.
jw
Reply to
Jim Wilkins
Thanks Don I don't know much eddy currents. Does this tend to allow the rotor to spin easily at first?
Reply to
Jordan
I'll try there, thanks.
Reply to
Jordan
Thanks Wes, that's a great description of controlling horizontal shaft 'mills. The current design is vertical shaft though, so alas the method can't be used.
Jordan
Reply to
Jordan
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
Reply to
Joseph Gwinn
Take a look at the centrifugal speed controllers on the old steam engines. The perfect solution in that they are simple, dependable, cost effective, and will not hinder low breeze operations.... Two balls rotating and the faster they go the tighter the pull on the brake lining. You can see the same effect on small motor drives like in a scooter. Rev up the motor and the clutch engages to drive the pup.
Reply to
Wayne Lundberg
Viscous fan clutch for cars maybe. Will provide constant drag at low speeds, and increasing drag at higher rpm. Cheap. JR Dweller in the cellar
Jordan wrote:
Reply to
JR North
I think it'd blow the budget.
Reply to
Jordan
Are these heat-sensitive? I'm not sure if viscous fan is the same as the type with bi-metallic spring. Interesting though, and I think I have a couple around here I can try, thanks!
Reply to
Jordan
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!
Reply to
Jordan
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Reply to
RLM
According to Jordan :
Both will work, though I think that Aluminum would give more braking difference between slow and high speeds.
Get a disk of each of the same size, and a pair of magnets out of a dead disk drive (be *very* careful not to get pinched when separating the magnets -- those are killers. :-) Once you have both disks, try each to see what behaves more like what you want. At a dead stop, the aluminum will produce zero drag. The faster it turns, the more drag. I've not tried it with steel, but I would expect more braking at slow speeds, and less at high speeds.
You're welcome. I hope that it works out for you.
Note -- the more wind power going into the prop, the hotter the disc will get, so weld or secure with screws or rivets, not glue, or it might let loose with a stiff breeze -- just the time you don't want it to do so.
If you want to get really tricky -- add a centrifugal weight assembly to change the pitch of the props, so you get less drive at higher speeds.
Good Luck, DoN.
Reply to
DoN. Nichols
That's a good idea with the fan clutch, I was thinking of making the blades variable pitch with a fabricated centrifugal controller.
Reply to
Tony
Both should produce 0 drag at 0 speed. The lower the resistiviity the higher the drag when moving, all other things being equal. jk
Reply to
jk
IIRC, the tail on a vertical shaft windmill is set off axis to blade spindle to turn the disk out of the wind. I meant to take a look at my uncles windmill today but I totally forgot it. I'll try to remember to bring a camera with me to take a picture of his mill tomorrow.
Wes
Reply to
Wes

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