The braking system in the Dan's book

[Ronnn]

Or does it? I'm assuming [ dangerous ] that's what happens. Or does the short just keep the stator from melting while the prop has a wind wet dream? I have a retired electronic engineer friend who was unable to explain this to me. I thought there should be some change in the quality of the fields [ like a ton of resistance to rotation ] to make the prop stop turning if that's what actually happens.

Ron

[ghurd]

He will understand this...

Open voltage is dependent on RPM.

Isc = Vopen/R

Watts = I^2 x R

R is very small.

Airplanes can not take off straight up.

Shorting the PMA causes a huge drag on the blades.

The blades slow down.  They stall, about the same as a 747 trying to fly or take off with a lawn mower engine powering the forward momentum.

The blades slow down enough they can hardly make any power.

It will not stop turning.  It will still turn very slow.

"does the short just keep the stator from melting while the prop has a wind wet dream?"

Not sure what you mean.

'while the prop has a wind wet dream' to me would imply a run away prop, resulting in something bad.

A run away prop, while shorted, results in a melted stator.

G-

[Flux]

The short must bring the blades hard into stall. If it does so the blade input power is very small indeed and the power you still produce is dissipated as heat in the stator but it is only a few watts.

If for some reason the short doesn't manage to stall the blades then it certainly won't stop the stator from melting, in fact it is a sure method of inducing a burn out.

Unless the alternator is powerful enough to stall the blades under any wind condition you had better not try braking it in high winds. In the marginal case it may hold stalled if you can get the blades in stall during a lull and it may stay stopped but unless you have proven that it will not break away again in a storm it would be safer to let it running.

When shorted all the blade power will heat the stator windings so you are critically relying on the blades producing very little power.

If you build an Otherpower machine it will be capable of being stopped in high winds but not all designs will survive this treatment.

Flux

[Dave B]

  "If you build an Otherpower machine it will be capable of being stopped in high winds but not all designs will survive this treatment."

  Very true, changing the blade profile, alternator design, loads and or furling weights and angles can make it very "unsafe" to assume the shorting switch will safely brake and hold a machine from over speeding.

  When you are convinced it will hold and you watch it start fully shorted, pass through stall and start flying you have 2 options (if no mechanical options).

  Neither are very relaxing, leave the switch shorted and hope the winds and furling allow for the braking to have some effect without burning up the stator and leading to self destruction or turn off the shorting switch and let it run normally hoping that the furling doesn't allow it to overspeed to the point of self destruction.

 I will never put up a machine without a mechanical assisted backup to the shorting switch. An open in the stator or line any where before the switch cries run away. Check the Dan's files, seems I remember them losing a phase or two with some high speed excitement a while back.

  Dave B.

[DanB]

We had an interesting failure last week Dave.  All the machines I've built for a long time would stop, in any wind (and stay that way) even if 1 phase (two leads) are shorted.

On big machines, there is surely merit to your feelings about a mechanical backup in  my opinion - perhaps something I should start doing.  I wouldn't likely bother with it on smaller ones however.

But - last week, the 10' machine we put up a couple months ago with your blades on it burned up while it was shorted out.  One of the lugs that attached the line to the alternator snapped off (they were not high quality lugs and I had a bad feeling about it when we put them on there).  I think your blades like to run a bit more slowly than ours, with a lot more torque.  Tom (the owner of the system) observed that the machine was slow to stop when shorted (the blades are also quite heavy compared to what Im used to) and at one point (after the lug broke) he could not stop it in high winds.  He did finally get it shut down in lower winds and thought everything was OK...  the wind came back up and the stator burned out on its own while it was shorted at the base of the tower!

A good example of blades that can overpower the stator when it's shorted out...

I'll try to post about this in more detail soon...

[dbcollen]

I got a call from a customer the other day in a storm, thier new whisper 200 was furling, and they were scared of the sound it was making. So I drove over and listened to it and explained it was normal, I walked up to the tower, leaning into the wind, it must have been 40+ with gusts over 50, the turbine was going in and out of furl, mostly furled. One of the upper guy wires was a little looser than I liked, and it was of course the up wind guy that needed tightening. I installed a shorting switch at the bottom of the tower, like I do on all the windmills I put up. I waited for a lull in the wind, threw the switch, counted to 5 and opened the switch back up, it didn't even slow the blades down. I tightened the downwind guy even though it made the tower a little off plumb, I would rather have tight guys and a slightly bent tower, than loose guys in high winds.

Dustin

[Ronnn]

My engineer friend will you're right. I don't even know what some of the symbols mean... ISC?

^ ? What I was asking is why they go into stall, why mechanically? what changes to make the blades go slower? I know there is math underlying this phenomenon. It underlies everything.

The only thing we have here is magnets and coils interacting.... fields interacting right? I'm trying to apply limited common sense here and may be totally wrong.  But it seems to me, something about the fields changes to create the resistance to stall the blades. I hated algebra, my teacher in the 7th grade had green deposits around his teeth and his breath would tarnish metal. I'm looking to understand the physical mechanics of what your formula probably explains very elegantly.

What I meant about the wet dream of the blades was that the energy created by the spinning might be somehow negated as a result of the short somehow shunting the current elsewhere. symbols

 Thanks for taking the time G-. I know you have more on your plate than answering emails. I should have gotten that if the prop spins too fast bad things are going to happen regardless. I hate not knowing stuff.

Ron

[Ronnn]

I'm glad you said that about building the Dans' design, but it was also stated that variance in the blade parameters can override in some cases, the shorting brake. Now I'm worried again. I'm planning to build my own blades. I hope to make a set of smaller blades first. [ like for a seven footer ] just to get an idea of what's involved. I have moderate wood working and wood carving experience. it was also my plan to go with laminated blades. So I'm wondering from reading Dan's post in this thread, how critical is the pitch and how it varies along the length of the blade? Also, will using western red cedar laminates change the weight enough to possibly cause trouble. Shouldn't they have about the same mass if I follow the dimentions close enough?

I'm very grateful for the education I'm getting here. Thank you all for helping.

Ron

[ghurd]

Isc is 'short circuit amps'.

Watts = I^2 x R is 'watts equals amps squared times ohms'.

"What I was asking is why they go into stall, why mechanically?"

If the blades try to go faster, and faster enough to make the alternator try to make more power than there is available in the wind,

then it is something like trying to start a Chevette moving in 5th gear, while pulling a 5 ton trailer.

G-

[Dave B]

Hi Dan,

  Sorry to hear of the burnout but thank you for posting this in support of what might happen with changing any number of the variables in the design including blades of different specifications. Keep us posted as you get this back up flying again, maybe already the way you guys go at it.

  Again, it's all a matter of knowing what it is you want the machine to do and matching it to that application. Easier said than done when you deviate from the norm as Dave and I both have some wall art to prove it.

  It's easy to under estimate the power in the wind and Mother Nature can humble us in a hurry. The numbers don't lie, 10'- 20' we are dealing with 1 1/2 - 10 HP of power in decent winds.

  We know this but I think it's good information for those new to this to consider, respect the power and be safe. Thanks again Dan for the interesting information.

  Dave B.    

[ghurd]

And something here may help.

http://www.fieldlines.com/story/2009/2/10/205613/991

G-

[Ronnn]

Hey Dave can you point me to some posts about your, or others, back up braking system. I'm assuming this would be something of a more mechanical nature. Can this type of brake be adapted to the Dans' turbine?

Ron

[Dave B]

Ron,

  I had a mechanical scrub brake on my last revision of my 16' that I was heating water with. This was cable activated from the base of the tower up through the yaw pipe sleeved inside a length of brake line adjacent to the power cable. Where the cable exited it was connected to a lever arm which pushed a large hard rubber pad (a piece of tractor mud flap) against the back rotor. This worked very well but it was quite a piece of engineering with a couple hinge pins and change of direction of force etc.

  There was a large mechanical advantage to give considerable force against the rotor. I used large extension springs at the base of the tower to keep the pull on the cable constant. Yawing full turns would wrap the brake cable around the power cable which really was not too much of a problem as I would be watching to unwind the power cable at times anyway and with 85' and an open lattice tower this was easy to monitor.

  My new set up relies on a cable activated manual furling tail, basically you can pull the tail furled from the tower base and doing this along with a shorting switch is a great thing.

  I have learned that it is well worth the effort to figure something else out along with the shorting switch to shut the machine down. It is a very helpless and frightening thing to watch your machine run away. With the dual rotor the back rotor lends itself well to a scrub brake, many commercial units use the manual furling tail.

  Dave B.

[Ronnn]

Thank you Dave, that scrub brake does sound pretty complex. Seems the manual furl seems like it would be less complicated. Did you post any pictures of either of those? I'd love to take a look. Your explanation was very clear but...well you know.....

Ron

[Dave B]

Ron,

 I had posted photos a long time ago of the scrub brake, they are gone from the board as some where in the mix of all the changes here either I accidently deleted them from my uploads or they just vanished.

 Basically a lever arm pulling up on another lever that pushed the rubber pad onto the back surface of the rear rotor. The first lever was long with the cable connected to it right at the top of the yaw tube. This gave the pull a large mechanical advantage applying a lot of pressure to the pad.

 Every system can be a bit different but that rear rotor is exposed very nice for this type of brake. The mechanical furling is nothing more than connecting a cable again down through the yaw tube to the base of the tower. You need to isolate the cable from your power cord at the top and then connect it to the tail boom a little ways out to get enough pull to furl it. Use some pulleys to get the pull right. I use a hand boat winch at the base to pull it in, this works great but it is a trick to get everything in the right place.

 It is well worth the effort to figure something out, there is no more helpless of a feeling than when the big winds come and you have no control over what happens, it is a scarey thing.  Dave B.