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Default 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
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Default Brake for small windmill?

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

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.


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, cost matters.


Of course.

All comments and suggestions welcome.


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

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Default Brake for small windmill?

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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Default Brake for small windmill?

Jordan writes:

All comments and suggestions welcome.


http://en.wikipedia.org/wiki/Lenz's_law
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Default Brake for small windmill?

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.


http://www.thebackshed.com/Windmill/Docs/Furling.asp

hth,

Wes


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Default Brake for small windmill?

On Sep 13, 10:21 pm, Jordan wrote:
....
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.

....
Jordan


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

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Default Brake for small windmill?

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?
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Default Brake for small windmill?

Dave Hinz wrote:

alt.energy.homepower might be a really good place to ask this question.

I'll try there, thanks.
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Wes wrote:

http://www.thebackshed.com/Windmill/Docs/Furling.asp

hth,

Wes


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
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Default Brake for small windmill?

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


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Default Brake for small windmill?


"Jordan" wrote in message
u...
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


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.


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Default Brake for small windmill?

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:
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



--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
If you're not the lead dog, the view never changes
Doubt yourself, and the real world will eat you alive
The world doesn't revolve around you, it revolves around me
No skeletons in the closet; just decomposing corpses
--------------------------------------------------------------
Dependence is Vulnerability:
--------------------------------------------------------------
"Open the Pod Bay Doors please, Hal"
"I'm sorry, Dave, I'm afraid I can't do that.."
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Default Brake for small windmill?

Wayne Lundberg wrote:

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.

I think it'd blow the budget.
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Default Brake for small windmill?

JR North wrote:
Viscous fan clutch for cars maybe. Will provide constant drag at low
speeds, and increasing drag at higher rpm. Cheap.

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!
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Default Brake for small windmill?



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!


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Default Brake for small windmill?

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.

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.


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.

Thanks!


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.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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Default Brake for small windmill?

That's a good idea with the fan clutch, I was thinking of making the blades
variable pitch with a fabricated centrifugal controller.


"JR North" wrote in message
.. .
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:
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



--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
If you're not the lead dog, the view never changes
Doubt yourself, and the real world will eat you alive
The world doesn't revolve around you, it revolves around me
No skeletons in the closet; just decomposing corpses
--------------------------------------------------------------
Dependence is Vulnerability:
--------------------------------------------------------------
"Open the Pod Bay Doors please, Hal"
"I'm sorry, Dave, I'm afraid I can't do that.."


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Default Brake for small windmill?

Jordan wrote:

Wes wrote:

http://www.thebackshed.com/Windmill/Docs/Furling.asp

hth,

Wes


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.


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
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Default Brake for small windmill?

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 --



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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
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Jordan wrote:

Wes wrote:

http://www.thebackshed.com/Windmill/Docs/Furling.asp

hth,

Wes


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



Okay, I went over to uncles and here is a picture of his windmill from the
bottom side.

http://wess.freeshell.org/usenet/rec...ll_picture.jpg

He sketched out how it works and I redrew it in cad.

http://wess.freeshell.org/usenet/rec...g/windmill.pdf


HTH,

Wes
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Default Brake for small windmill?

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.


Centrifugal clutch, a la:

http://www.gokartsupply.com/clutches.htm

You put the shaft on the centrifugal part, and tie down the clutch
part. :-)

Good Luck!
Rich

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Wes wrote:

Okay, I went over to uncles and here is a picture of his windmill from the
bottom side.

http://wess.freeshell.org/usenet/rec...ll_picture.jpg

He sketched out how it works and I redrew it in cad.

http://wess.freeshell.org/usenet/rec...g/windmill.pdf


Thanks Wes.
I can't see how we can use it for the vertical shaft rotor though.
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Rich Grise wrote:

Centrifugal clutch, a la:

http://www.gokartsupply.com/clutches.htm


We thought of that, and it might be a solution. Apart of karts, maybe
some from garden tools, chainsaws etc could be source of cheap ones.
For them to work, I think pull-off springs would have to be changed for
very light ones, or maybe no springs at all.


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Joseph Gwinn wrote:

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.


So they're two variations - eddy brake and hysteresis brake?
I think aluminum would be easier to work with, and the speed squared
characteristic of eddy sounds good.
Should the magnets be mounted either side of the disc, with opposite
poles facing each other?
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On Sep 13, 10:33 pm, Dave Hinz wrote:
On Fri, 14 Sep 2007 12:21:17 +1000, Jordan wrote:

....
alt.energy.homepower might be a really good place to ask this question.
Dave Hinz


He did, it isn't, not a single reply there compared to all the ideas
people have posted in RCM.

jw

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In article ,
Jordan wrote:

Joseph Gwinn wrote:

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.


So they're two variations - eddy brake and hysteresis brake?
I think aluminum would be easier to work with, and the speed squared
characteristic of eddy sounds good.
Should the magnets be mounted either side of the disc, with opposite
poles facing each other?


That would work. You will need some iron too, to make a complete
magnetic circuit.

One variation is an aluminum cylinder with the magnetic field being
radial.

I would look into the patent literature for ideas.

Joe Gwinn
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Jordan wrote:

Thanks Wes.
I can't see how we can use it for the vertical shaft rotor though.



Maybe I'm misunderstanding things. Is mount the windmill vertical or are
the blades oriented like a helocopter?

Wes
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On Sun, 16 Sep 2007 14:01:18 -0400, Wes wrote:

Jordan wrote:

Thanks Wes.
I can't see how we can use it for the vertical shaft rotor though.



Maybe I'm misunderstanding things. Is mount the windmill vertical or are
the blades oriented like a helocopter?

Wes

The SHAFT is vertical, so like a helicopter, but with REAL FUNNY
blades. Look up "Savonius"

--
Posted via a free Usenet account from http://www.teranews.com



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Joseph Gwinn wrote:
In article ,
Jordan wrote:


Joseph Gwinn wrote:

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.


So they're two variations - eddy brake and hysteresis brake?
I think aluminum would be easier to work with, and the speed squared
characteristic of eddy sounds good.
Should the magnets be mounted either side of the disc, with opposite
poles facing each other?



That would work. You will need some iron too, to make a complete
magnetic circuit.

One variation is an aluminum cylinder with the magnetic field being
radial.

I would look into the patent literature for ideas.

Joe Gwinn





Easy demo. Drop a magnet through a chunk of Al. pipe. Slow-mo drop.

Slide same magnet down an inclined sheet of Al. Slow-mo slide.

Pretty sure this works with about any metal, but having the magnets
sticking to the disk is problematic.

Faster it goes, the bigger the eddy currents formed and resisting
movement.

Think simple.

Simple disk. Simple magnet holders under the disk, out of sight.

My two bits worth, anyways.

If complicated works, a bit, anyway, I would look at gearing up the
output shaft with a couple sets of bicycle gearing/chains, and having
the brake at the end of the train of gearing to do its thing with a
geared advantage. Salvaged bike parts are cheap to aquire.

Or a generator with a variable load, rpm dependant.

Or running the LEDS planted up the blades.

Or...:-)

Just ideas.

Cheers
Trevor Jones

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Simple disk. Simple magnet holders under the disk, out of sight.

My two bits worth, anyways.

If complicated works, a bit, anyway, I would look at gearing up the
output shaft with a couple sets of bicycle gearing/chains, and having
the brake at the end of the train of gearing to do its thing with a
geared advantage. Salvaged bike parts are cheap to aquire.

Or a generator with a variable load, rpm dependant.

Or running the LEDS planted up the blades.

Or...:-)

Just ideas.

Cheers
Trevor Jones


Thanks, good ideas.

What do you think about aluminum-rimmed bicycle wheels? Big diameter
should be good?

For maximum effect, does the magnetic pathway need to be "through" the disc?
That is, with a horseshoe magnet over the edge, or 2 linked magnets?

A single magnet would be simpler. In that case, would both poles need to
be near the disc? Or just one, the other end remote from the disc?

Sorry to pester, but I haven't been able to find details by web-searching.
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Default Brake for small windmill?

clare at snyder.on.ca wrote:

The SHAFT is vertical, so like a helicopter, but with REAL FUNNY
blades. Look up "Savonius"


Oh, a S-rotor. Darn, somewhere I have a book on those but I don't think
I've seen it in 25 years.


Wes
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Default Brake for small windmill?

According to Jordan :
Simple disk. Simple magnet holders under the disk, out of sight.


[ ... ]

Thanks, good ideas.

What do you think about aluminum-rimmed bicycle wheels? Big diameter
should be good?


Yes -- but the spokes and the nuts for adjusting them may cause
a cogging effect -- unless both parts are a non-magnetic stainless
steel.

For maximum effect, does the magnetic pathway need to be "through" the disc?
That is, with a horseshoe magnet over the edge, or 2 linked magnets?


That depends on the speed. Above a certain speed, the field
would not penetrate very far in thick aluminum.

A single magnet would be simpler. In that case, would both poles need to
be near the disc? Or just one, the other end remote from the disc?


Ideally -- both poles near the disc -- which would make those
recovered from dead hard disk drives very good choices, since that is
how each side is arranged in my experience.

Sorry to pester, but I haven't been able to find details by web-searching.


You *won't* find information to the level you want. This is the
kind of thing which you answer by experimenting, especially since the
magnets from the disk drives are somewhat of an unknown quantity. But
they typically are *very* strong -- so beware getting pinched between
them while separating them. They are typically bonded to mild steel
backing plates, and you might as well leave them on that for ease of
mounting.

Go to a place which repairs computers and pick up one or two
*dead* disk drives. (A couple of bucks each at worst, or free if you hit
the right place.) Or -- if you are willing to pay more, you can buy the
magnets already extracted from someone who does this as a business,
probably selling on eBay.

Go to a store which has specialty screwdriver bits (a good
electronics place is a good choice) so you have the necessary medium and
small Torx drivers to take apart the disk drives. Extract the magnets
and play with them to answer your own questions.

And note the brand and model of the disk drive which provides
the easiest to use set of magnets so you know what to look for when
collecting those for the final workpieces.

The greater the radius the greater the effect, of course, so you
may wind up spending more to have discs of aluminum cut to the size
which you need.

Good Luck,
DoN.
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Default Brake for small windmill?

IIRC, Torx from T6 to T10 should do you for everything.

Go to a store which has specialty screwdriver bits (a good
electronics place is a good choice) so you have the necessary medium and
small Torx drivers to take apart the disk drives. Extract the magnets
and play with them to answer your own questions.



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Default Brake for small windmill?

Test report:

I got a 9 inch diameter thin sheet of Al (ex- pie dish), spun it with a
hand drill. I've got this seriously powerful magnet from an early
voice-coil HDD (much bigger than typical now), that I held up to one
side of the disc.
The magnet slows the drill somewhat - I can hear the change in pitch.
The magnet tends to want to "push off" the disc.
As proof of principle it's encouraging. It'd need a much larger
diameter, especially if I can only get smaller magnets, but multiples
would help of course (I think?).
Also I'm guessing the thickness of the sheets would need to be at least
1/8 inch, as the push-off pressure would likely warp a thinner disc.

But hey - all that aluminum is threatening the purse limit again!
I wonder - would it work if a number of pieces were attached to a sheet
of ply? Then we could make a number of "rings" from less material.
I've gone cool on the bicycle rim idea, thinking there's not enough
brake area.

Jordan
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Default Brake for small windmill?

According to Jordan :
Test report:

I got a 9 inch diameter thin sheet of Al (ex- pie dish), spun it with a
hand drill. I've got this seriously powerful magnet from an early
voice-coil HDD (much bigger than typical now), that I held up to one
side of the disc.


Good -- experimentation is the way to go.

The magnet slows the drill somewhat - I can hear the change in pitch.
The magnet tends to want to "push off" the disc.
As proof of principle it's encouraging. It'd need a much larger
diameter, especially if I can only get smaller magnets, but multiples
would help of course (I think?).
Also I'm guessing the thickness of the sheets would need to be at least
1/8 inch, as the push-off pressure would likely warp a thinner disc.


Also -- I would expect you to get greater braking from thicker
aluminum -- up to a certain limit at least. Yes -- 1/8" sounds
reasonable.

But hey - all that aluminum is threatening the purse limit again!
I wonder - would it work if a number of pieces were attached to a sheet
of ply? Then we could make a number of "rings" from less material.
I've gone cool on the bicycle rim idea, thinking there's not enough
brake area.


You would lose braking effect as you approached the seams,
because it depends on the ability to conduct electricity in a zone
around the magnet's field. If you have someone who can do a MIG or TIG
job of welding them together, the effect should be better.

But your plywood would be the limiting factor on lifetime. How
long do you need this to work before it is taken down? If you're
talking over a year, I would strongly suggest that you avoid the
plywood, especially since with a tight budget, the odds of someone doing
regular maintenance are pretty poor.

See whether you can find a local shop which needs to make discs
of aluminum from sheet. You may be able to get them to cut from the
center of the sheets and sell you the "drops" -- squares with round
holes.

Enjoy,
DoN.

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Default Brake for small windmill?

DoN. Nichols wrote:

But your plywood would be the limiting factor on lifetime. How
long do you need this to work before it is taken down?


Only 2 months life needed.


See whether you can find a local shop which needs to make discs
of aluminum from sheet. You may be able to get them to cut from the
center of the sheets and sell you the "drops" -- squares with round
holes.

Great idea.
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