Author Topic: windmill braking system  (Read 10579 times)

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hallmgr

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windmill braking system
« on: August 04, 2005, 02:27:13 PM »
I have a homebuilt 1kw windgenerator and I am looking for some kind of braking system.  The generator has a furling system which turns it out of the wind when it is very strong, but I would like to be able to actually stop the thing from turning in  an emergancy.

Any ideas would be much appreciated.


Hallmgr.

« Last Edit: August 04, 2005, 02:27:13 PM by (unknown) »

kitno455

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Re: windmill braking system
« Reply #1 on: August 04, 2005, 08:54:46 AM »
short the coils briefly. the circulating current will heat the stator quite a bit, but it will act as an electro magnetic brake. after that, i would suggest a friction brake or sliding pin to hold it still. dont leave the coils shorted for too long and expect the stator to last :)


allan

« Last Edit: August 04, 2005, 08:54:46 AM by kitno455 »

stm

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Re: windmill braking system
« Reply #2 on: August 04, 2005, 09:22:06 AM »
You could use a hydralic disc brake from a motor cycle - if they can bring a 300 km/t motor cycle to a compleate stop, then a wind generator should be an easy task.....


A disc brake would also spinn frealy without limiting the rotation of the blades under normal/unbreaked conditions.


/Steffen

« Last Edit: August 04, 2005, 09:22:06 AM by stm »

Flux

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Re: windmill braking system
« Reply #3 on: August 04, 2005, 09:53:35 AM »
It all depends on your alternator whether you can stop it in a high wind or not, you need a fair bit of magnet material to be able to stop it in a high wind. If it has a slotted iron core it may reactance limit and never stop.


This is no use if you have a cable or alternator fault.


I prefer to include a mechanism in the furling system that pulls the tail at right angles to the normal direction. It is unlikely that you will be able to adapt your present furling system easily. This adds quite a bit of complication but it is not so bad if you do it from the start.


I am sure you can devise a brake but I have always felt that it would be an awkward and troublesome idea in the long run.


Flux

« Last Edit: August 04, 2005, 09:53:35 AM by Flux »

Ungrounded Lightning Rod

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Re: windmill braking system
« Reply #4 on: August 04, 2005, 11:55:13 AM »
after that, i would suggest a friction brake or sliding pin to hold it still. dont leave the coils shorted for too long and expect the stator to last :)


Why not?


With the rotor slowed to almost nothing you have very little power collected from the wind to be dissipated in the rotor in keeping it slowed - especially as compared to the power dissipated during normal operation (let alone the pulse when you brake it).  The blades stall and the force drops quite low due to the loss of lift.  Your current is what fights the force, so with low force you have low current and 'way lower current SQUARED resistive heating.  Further, with the mill's tail held in furling position there's also little power available even if it were spun up.


Of course there's no reason, once the blades are turning very slowly if at all, not to tie a cord to one of them and tie it down - preferably to something that yaws with the mill, unless you also tie the mill against yaw.


So I'd leave the short there.  If nothing else, it provides you with safety.  If you manage to knock your other tiedowns loose while up the tower with the mill it will keep it from spinning up to more than a gentle motion.


Just be sure that your short is on the genny side of the diodes, so you don't pop 'em when initially braking the mill.


Note that this argument about stalling and low current applies only to lift-based mills - essentially all HAWTs and the darreius-style VAWTs.  A drag turbine like a Savonius will give you more force (and thus more current) once it's reached crawl-speed equilibrium with the electric braking.  It will keep heating your genny, even more than under normal operation.  Thus you want to use electric braking only for slowing it and provide some alternaitve braking (or parking braking if it's small enough that you can slow it the rest of the way by hand) once you've gotten it down to a sane speed.

« Last Edit: August 04, 2005, 11:55:13 AM by Ungrounded Lightning Rod »

Jon Miller

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Re: windmill braking system
« Reply #5 on: August 04, 2005, 12:04:50 PM »
How about throwing a rope over the back and pulling it totaly out of the wind then once its slowed down throw a rope into and around one blade and tie it of then once your finished doing what ever your doing slide the ropes of???
« Last Edit: August 04, 2005, 12:04:50 PM by Jon Miller »


cr8zy1van

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Re: windmill braking system
« Reply #6 on: August 04, 2005, 12:15:44 PM »
I plan on using a downhill bike brake... They are hydraulic and can stop a rhino, (and since I have them laying around from my other hobby)
« Last Edit: August 04, 2005, 12:15:44 PM by cr8zy1van »

old55olds

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Re: windmill braking system
« Reply #7 on: August 04, 2005, 12:53:22 PM »
I have been there and done that. IF the shorted stator leads stop your machine quickly you are ok. In my case it was wired so that it would short the stator leads when the batteries were full. While at work a good wind came up. Batteries full. shorted leads to stop stator. That all worked fine. Except stator did not stop. FRIED  windings. Rewound it. Just a freak of nature. Math says this cannot happen. 1 Year later it did it again. Now I just let it freewheel if the wind gets too strong.  It furls. max prop speed gets to 350 RPM. Makes a lot of noise. But it's better than rewinding the stator again.  BTW Make sure that the prop can take it.

If I were to do it again I would use a brake shoe like thay did on the old fairbanks morse windmill.

Ken
« Last Edit: August 04, 2005, 12:53:22 PM by old55olds »

hallmgr

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Re: windmill braking system
« Reply #8 on: August 04, 2005, 03:41:27 PM »
Thankyou for your suggestions.

At the moment I have a length of rope tied to the windmill tail which allows me to manually turn it out of the wind.  When the rotor stops I climb up a ladder and tie the rope around it so it stays still.

I plan to mount the generator on a higher pole so I will not be able to climb up to the top easily to stop it turning.

The rotor bearing is simply a car rear wheel hub bearing, so I could possibly mount a hub with the brakes attached.  however this would increase the weight of the generator and be very complicated.

Stopping the windmill by shorting the phases would work unless there is a problem with the cable or the alternator.

I have tried simply throwing a length of rope at the rotor when it is spinning to stop it but this does damage to the blades.

I would like to be able to stop the generator in emergencies and also if I need to do maintenance.


Hallmgr.

« Last Edit: August 04, 2005, 03:41:27 PM by hallmgr »

MelTx

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Re: windmill braking system
« Reply #9 on: August 04, 2005, 04:29:58 PM »
  BRAKE? PatPend... Would something like this help? MelTx

« Last Edit: August 04, 2005, 04:29:58 PM by MelTx »

georgeodjungle

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Re: windmill braking system
« Reply #10 on: August 04, 2005, 10:19:36 PM »
put the tail on a actuator.

or a pin and hinge system like the old timey wind mills do.
« Last Edit: August 04, 2005, 10:19:36 PM by georgeodjungle »

DanB

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Re: windmill braking system
« Reply #11 on: August 05, 2005, 08:15:03 AM »
"dont leave the coils shorted for too long and expect the stator to last :)"


I'd have to disagree there.. once its stopped then having the coils shorted should pose no risk unless the alternator is very resistive, in which case the machine probably needs some re-thinking anyhow.


Any time I want my machine shut down, I keep it shorted and there is no risk to the alternator.  The biggest risk in using the alternator as a brake in this way is the moment at which you short it... if its running fast and producing significant current, but even then - I've killed mine (Shorted it) at full output with no problems (the tower usually makes a clanging sort of sound.  Once it comes to a stop - or near stop, the blade cannot produce nearly enough power to damage anything.


In cases where folks short their machines and burn up their stators I would expect the resistance in the stator is too high, perhaps their blades are too large for their alternators - or... perhaps they have steel laminates.  In machines with steel laminates I dont believe you can always count on shorting the alternator to serve as a good 'brake' to stop it while it's running, but in most cases - if its allready stopped, I believe shorting the alternator should keep it from starting.

« Last Edit: August 05, 2005, 08:15:03 AM by DanB »
If I ever figure out what's in the box then maybe I can think outside of it.

DanG

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Re: windmill braking system
« Reply #12 on: August 05, 2005, 09:00:06 AM »
The physics behind a disc brake count the disc's surface as near infinite in length - if the wind continues to blow it wont be near infinite, it WILL be infinite.


Everyone seems to overlook vehicle brakes are not designed to overcome anything but inertia of the vehicles weight & then have a certain rest period before they are reapplyed - I've seen a dump trucks rear tires ignite from an unnoticed stuck brake pad, complete w/ firey explosion when the hulls failed w/ the loaded truck laying over on its side, crushing a guardrail, and tumbling down a 35-yard embankment. Quite a sight.


In severe weather all bets are off - 80mph gusts will have their way with your mill & tower for sure, I'm sure I'd not want to be anywhere near the mill trying to stop it while a squall came through. Old55olds told us alot, it's the fair weather w/ a strong wind that will be 99.5% likely to cause problems. Furling and having hardware built to take sustained furling is best insurance.


Severe heating will change every tolerance causing stretched or snapped bolts, charring of wooden components, over temperature of the bearings / bearing lube. If you are planning  to use a friction brake have it isolated by an extended shaft and use heat-proof hardware AND have a mechanical lock pin so the shorted stator / friction brake is brief duration. A brake disc will be glowing red hot within minutes or seconds of use IF the wind energy is still present!

« Last Edit: August 05, 2005, 09:00:06 AM by DanG »

hallmgr

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Re: windmill braking system
« Reply #13 on: August 12, 2005, 03:08:13 AM »
For now I think I will stick to turning the generator out of the wind using a piece of rope tied onto the tail, I can then tie the end to my parked quad bike which should be able to hold it for as long as needed.


Hallmgr.

« Last Edit: August 12, 2005, 03:08:13 AM by hallmgr »