Wind Turbine Brake Idea

[ChrisOlson]

I came up with an idea that I'm going to try, that should be fairly simple and inexpensive to build.

I'm going to wind a single phase 6 coil stator with 70-80 turns of AWG 12 wire.  I'm going to mount the coils on an aluminum plate so when the coils are energized it creates all sorts of eddy currents in the plate to create more drag and heat.  Then make a brake rotor with six 1/2" thick disc magnets on it.

My thinking is that the cogging effect of a single phase stator, combined with the additional drag created by it, along with the load from the generator, should bring a pretty good sized wind turbine under control in high winds.  The thing that intrigues me with an electric brake is that it can be applied with an rpm sensor (like an automotive rev limiter) to activate a relay that shorts the brake stator.  When the rpm's come back to a safe level the rev limiter can de-energize the relay and turn the brake off.

I have to experiment with this a bit because one thing that could happen is that if you use 3/4" thick coils, for instance, with a .050" air gap the magnets might be close enough to the aluminum plate to create eddy currents and drag even when the brake is off.  After I try it I'll know more about whether or not that happens.

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Chris

[wooferhound]

You wouldn't even need the coils. Just a plate of aluminum is all you would need. But how are you going to get your aluminum plate in and out of the generator without making a huge airgap?

[ChrisOlson]

The turbine I'm going to try this on used to have the generator at the rear.  Here's a photo of what it looked like when I had it that way:

http://picasaweb.google.com/christopher.w.olson/76FtWindTurbine#5430081358178684610

I moved the generator to the front, behind the blade hub.  So I have a stub shaft sticking out the back that used to drive the generator.  I'm going to put the brake on the back since I already have a shaft to drive a rotor, and a flange to mount a stator support.

I experimented with an eddy brake on the lathe.  I made a 10-1/2" aluminum disc and spun it at 600 rpm.  Holding a 2" diameter 1/2" thick neo magnet about 1" from the rotating disc I can start to feel the torque being applied to the magnet.  At 1/16" the torque applied to the magnet becomes quite substantial.

So my original idea is out the window.  I can imagine that if you had several magnets that at even an inch away it would create drag from eddy currents in the plate.

One answer would be to wind a bunch of electromagnets and place them on both sides of the rotating aluminum plate.  Then invent some type of controller where at, say, 650  rpm it would start to apply battery power from your batter bank to the electromagnets to apply the brake.

I'll have to think about this for a bit.  But I like the idea of an eddy brake because there's no brake pads to wear out, it can be applied electrically, the aluminum plate makes an excellent conductor of heat so it won't get too hot, and with the right controller could be made to be completely automatic.

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Chris

[ChrisOlson]

Another update on this:

I tried the same thing except with two 1" x 2" x 1/4" thick bar magnets.  I stuck the magnets together on edge so the north pole of one and the south pole of the other was facing the aluminum disc.  At 600 rpm and holding those magnets at 1/16" inch or so from the disc it becomes quite hard to even hang on to the magnets.  I suspect this is because the magnetic field change creates a stronger eddy current in the disc.

After seeing that my rough guess is that 4 or 6 electromagnets wound with AWG 20 wire, and places on each side of the aluminum brake rotor, would apply a serious enough braking force to bring a runaway turbine back under control.  If you need more braking force, just wind more electromagnets and put them around the disc.

It's a very simple thing to build and there's absolutely no moving parts in it except for the rotating disc.

--

Chris

[ChrisOlson]

Second Update:

I made an electromagnet and tried this.  To make the magnet I welded a 3/4" washer to the head of a 1/2" bolt, welded a 3/4" washer to a 1/2" nut and screwed the two together leaving some threads exposed so the brake coil can be screwed into a brake stator.  I annealed the bolt by heating it red with the torch, then letting it cool slowly.  I then wound it with AWG 18 wire until it was full between the washers.  I didn't count the turns of wire, nor did I pay attention to winding a neat coil.  It took me about 15-20 minutes to make the test brake coil.

This makes a brake coil with a 2" diameter face and it's quite powerful.  It'll pick a 3/4" wrench up off the floor from a distance of 2".  It measures .63 ohm and it draws 24 amps @ 15 volts.  Here's two pictures of the brake coil:

http://www.fieldlines.com/images/scimages/21517/0127102143a.jpg

http://www.fieldlines.com/images/scimages/21517/0127102143b.jpg

I wrapped the coil with duct tape.  But when I build my turbine brake I'm going to put a sleeve around the coil that's slightly larger than the coil and seal it in fiberglass resin.

Now it was time to test it on the lathe.  Here's a photo of that ordeal:

http://www.fieldlines.com/images/scimages/21517/0127102135a.jpg

At 600 rpm this one brake coil creates enough drag to deflect the compound tool rest on the lathe with the way I had it oriented.  Holding the brake coil in my hand next to the spinning disc, then energizing the coil, just about pulls it out of my hand.  VERY powerful brake that has no moving parts except for the disc, and nothing that will wear out.

I don't have a good way to measure the torque created by this one coil, but I have no doubt in my mind now that four of these homemade electromagnets, two on each side of the disc attracting each other to get maximum flux thru the disc, will easily bring a runaway 7-8' turbine under control in short order.  My estimate is that the drag that I felt on the electromagnet is about equal to the drag created by a 250 watt stator in a PM generator.

When I build my brake I'm going to use four coils like this wired in series.  The resistance thru the coils should be about 2.5 ohm so the brake coil set will draw around 5 amps @ 12.5 volts.  If I don't get enough braking with the coils in series, I'll rewire them series-parallel so they draw more amps.  AWG 18 is good for about 10-12 amps continuous.

One thing to note about an eddy brake is that it is most effective at high speed and it loses braking power at lower speed.  So the idea is not to stop the turbine with it, but to bring a speeding turbine under control to where the stator can be safely shorted to finish stopping it without burning the stator out.

I've tested this enough now to have no doubts in my mind that it will work.  Eddy brakes have been used in dynamometers, roller coasters and trains for years.  I'm going to put one on a wind turbine.

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Chris

[fluxable]

I am thinking of designing a generator with a separate rotor and stator (I've mentioned this in a separate post). One of the rotor - stator combinations would built heavier, to better handle a fast rotor speed. The other rotor - stator combination would be built to perform at a lower rotational speed. I was also thinking of using the smaller rotor - stator combination as a brake when the rotational speed was high, hopefully extending the usable  power band when the wind is really blowing.

I think this is similar to the approach you are taking Chris, except the second rotor - stator combination would also double up as a low speed generator. If the low speed generator is built with a large diameter rotor, it could potentially put out a higher frequency output at relatively low rotational speeds. And the large diameter rotor would have good leverage for braking when a load is put across the stators. Something I'm thinking of trying on a small scale first.

[TomW]

[wooferhound]

I personally would never push more than 8 amps through 18gauge wire.

[ChrisOlson]

True, but this is an intermittent duty device.  Pushing 24 amps thru the one coil I built last night gets the coil pretty hot after about 30 seconds.  But by that time the brake's work would be done and the turbine back under control.

To follow what's "proper" you shouldn't push more than 8 amps.  But you can push 10-12 amps thru AWG 18 without getting it hot enough to burn the coating off the wire.  For a turbine eddy brake that'll be more than adequate.

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Chris

[ChrisOlson]

I'm not saying that what you're proposing won't work.  Just that the complexity and controls used to operate it become quite complicated.  With wind turbines I've found that the simpler it is, the more durable and reliable it is.

I only got one generator stator and it starts making usable power at 7 mph wind speed and has proven it can put out up to 2 kW at 32 mph wind speed.  I don't need to extend my usable power band because I have one well designed generator.

What I need is a safety device that prevents overspeed and provides control of the turbine if it fails to furl like it should.  I don't trust furling systems because I think there's too many variables involved, and I don't think those variables always fit inside the bounds of the design.  I've experimented with a variable pitch blade control hub and I don't think that's the answer either, for a small turbine.  If you're into big wind turbines where you need to accurately control the speed to make 60 Hz power in varying wind speeds, then variable pitch is the way to go.

I've been kicking this around for a long time.  After burning up one stator trying to control a speeding turbine, then having another one overspeed when all my calculations say it should furl, and static tests using spring scales and torque wrenches to test it indicate that it should - but it doesn't sometimes - I'm going to try a homemade eddy brake.  It's simple, reliable and I can build the whole thing for about 15 bucks.

--

Chris

[DanB]

Good Luck with the project ~ it's not the way I would go but I look forward to hearing more about the project.

There have been a couple postings here lately about machines failing to furl.  Basically my thought is they just were not setup right.  This furling system, done right - is very reliable - the same basic system has been in use with reliable results on both water pumping, and electric machines for over 100 years.  

There are a lot of variables though - they include of course all the angles, offsets and very importantly - how the machine is loaded.

A brake, in my opinion should shut it down in such a way that it stays shut down.  This could be achieved by folding up the tail (that is a common way of doing things) =- I've seen that fail too though.  I've watched a 10kW Bergy track the wind in overspeed with the tail completely folded up.

DaveB's mechanical brake seems to work well and I tend to agree with him, it's certainly not a bad idea especially on larger machines.

My own experience - I've had one 17' machine run away on me with no way of stopping it.  It was neglect on my part, first two leads at the top of the tower shorted out (stopped the machine pretty much)  - then after 6 months of not doing anything about it, they broke off and I had no way of stopping it (short of lowering the tower in high winds.. that was exciting!)

Machines that run in overspeed don't seem to furl well ~ and those same machines tend to have perhaps not quite enough alternator behind the blades (the reason they overspeed) and therefor don't shut down very well.

Most of the machines we've built will stop nicely and stay that way, if we just short 1 phase ~ seemingly in any wind speed.  If we short all three they stop faster.

It's all interesting to me - I've messed with lots of these machines and have had really good results with the furling system and electric braking.  So my thoughts are... go ahead and design any sort of brake you like, it cannot possibly hurt - but try to get it furling right first, furling is critical and should be reliable.

[DanB]

Chris - this is off topic a bit, was just re-reading your post about the machine that failed to furl...

you said you wound the stator w/two strands of #14 wire and 40 turns per coil.  How many coils are in the stator and what magnets did you use?

Am I also correct in understanding that it failed to furl while hooked up to a block heater directly?

[ChrisOlson]

She's wound with 40 turns of AWG 14 two-in-hand, 9 coils, and I'm using the custom wedge magnets running a .005" air gap between the magnets and coils.  My stator is .450" thick.

When I ran the generator on the back I could run a tighter air gap - I used .0025" (that's 25 ten thousandths of an inch) and set the coil to rotor clearance with a piece of paper.  I got a pretty hefty 1-3/8" mainshaft in this turbine, plus using a ball bearings instead of tapered roller, so I'm comfortable with running the clearance at .005" without any rotor/stator contact.  If you use a wimpy mainshaft that can flex then the air gap would have to be widened up.

It failed to furl at 28 mph with it driving a block heater for the load, and that's how it got to 2 kW before I shut it down.  It would've burned the heater out if I would've left it go because the heater element is 24 volt and the turbine was pushing 33.5 volts and 61 amps.  That's also over what the stator should be putting out by a good 500 watts.  If I would've wound the stator with only single wraps, it would've been burned out too.

Shorting all three phases stopped it, but it sure made the generator groan and whine when I threw the shorting switch.

I got that turbine in the shop right now fitting my new brake to it.  Every test I've tried on it using spring scales to pull on the offset point and furl the tail indicate that it should've started furling at around 24-25 mph and be fully furled at 28 mph.  The tail hinge angle, although steep, doesn't appear to be a problem in a static test in the shop.  So I'm going to put the brake on it and fly it again without changing the tail on the chance that it was a condition where the wind coming over the slope of the roof, or some other factor, caused it to not furl.

--

Chris

[ChrisOlson]

Oh, and my stator is .43 ohm per phase in delta and it makes one volt for every 15 rpm (open).  So theoretically it has more capacity than 2 kW.  But I know from experience that running it at 1.5 kW gets the stator pretty darned hot.

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Chris

[Ungrounded Lightning Rod]

Ah - so this is just to brake it down and then you'll short the main genny to keep it stopped.  So you can afford to pull a bunch of current from the battery.  Good idea.

Remember that wire in a coil doesn't lose heat as well (by a LOT) as wire in a straight run.  So you have to derate it a lot compared to its rating as hookup wire when you wind it in a coil.

(I ran into this when I had a travel trailer parked about 20 feet from the outlet - to use my heavy-duty power cord so I used the 50 foot lower-amp cord and didn't overload it for its rating.  But I'd left the extra coiled up in the big Rubbermaid salad crisper I used for storing it.  Luckily I checked it after a couple minutes.  It was WAY hot.  I realized what the problem was and strung the rest out before it melted down and/or caught fire.

When you position your magnets you want them well out from the center (for good leverage and high relative speed for good induced drag) but not all the way out to the edge (because the eddy current surrounds the pole and you'll cut much of it off if the pole is near the edge).

Think "magnetic disk brake" with the magnet as "caliper and pads".  But the virtual pads are significantly wider than the pole piece.

[Ungrounded Lightning Rod]

Sounds like the offset from the turbine axis to the yaw axis might be too small for the rotor diameter.  (I didn't see it listed - did I miss it?)

[ChrisOlson]

Rotor diameter is 91.5" and the offset is 7.5"

--

Chris

[ChrisOlson]

Yep.  I've found that surface area of the magnet(s) makes a big difference in how effective the brake is.  I thought about winding two big magnets, but that would get quite heavy.  From what I've tested it seems that if the magnet is placed so the outside of it is about an inch from the outside of the disc it provides some really severe braking force.  It'll just about pull the magnet out of my hand on the lathe, and I'm sure it would if it wasn't for the fact that it moves my hand and widens the air gap.  At any rate, I can't hold the magnet there when I energize it.  If I move it out to the edge of the disc it has less power.

I'll bench test the coils before I put it into operation to make sure they won't overheat, or at least get a good idea of how much time they can be energized before they get too hot, and how much cool-down time they need.

Like I said, this is an emergency brake.  I'm not going to put any type of automatic controller on it, at least at first.  I'm going to see how it works.  The idea is to bring a speeding turbine under control without having to use the stator to brake it from an overspeed condition.  If it has the braking power it appears it does, it'll take it less than 2 seconds to stall a runaway 7-8' turbine.  If it does that, it'll be successful and serve its purpose well.

--

Chris

[Flux]

Some interesting points Dan.

I agree that a brake should just stop it. I see little chance of a  brake surviving as a power limiter except the paddle brake of the old Wincharger or power spilling schemes of pitch control. Mechanical brakes will burn out and an eddy current one will get incredibly hot. If the main alternator can't brake the thing to standstill from high speed it probably won't reliably hold it braked in all winds.

My machines all have a means of turning the tail to 90deg as the Bergey does. If the tail is big enough to overcome the seeking force it will stop but a runaway prop does take a lot of torque to pull it round edge on to the wind. It helps to have a long tail boom to get the vane outside the prop swept area to get maximum thrust.

I think you are right in that the most critical factor is the loading, you have come at this from machines that run stalled in high wind and obviously your furling works, others may have longer cables or more resistance in the circuit somewhere and they will run faster. This may change things to the point where the furling doesn't work.

Having come at this from wound field machines I have never gone down this stall operated route and I can say fairly safely that your tails seem far too heavy compared with what I find necessary. Not too easy to get a direct comparison as most of mine don't use the inclined hinge but I have probably the same sort of restoring force.

I have made a couple of smaller ones with the inclined hinge idea and I find that with 15 deg I have to keep my tails very light. I can't imagine 20 deg and a plywood tail working with my type of loading. I suspect that if you add some line resistance to get the speed up you will find that you have to drastically alter the furling.

People can copy a design easily enough but many other things that affect the loading may not be the same. It may also be that the blade profiles that others end up may be significantly different from yours and that again will have a lot of effect on this seeking force.

I agree with you that furling can and does work reliably but I think that some of my faster wound field machines partly furled and partly survived on alternator reactance limiting. I suspect that applies to some commercial machines which I won't name as well.

As long as the alternator doesn't burn out and you can live with the noise most machines will survive but it's a scary experience.

I prefer to have a second method of stopping things if you loose electrical load, I like the tail control but in most climates a very well engineered brake should be fine. In this area it would have to be very well engineered or tested very frequently to stop it rusting up. Possible but I have never done it that way.

Flux

[Shadow]

I'm always happy to see someone working on a  brake system for these type wind turbines!

I know the Dans dont think they are required or necessary but man oh man every second day on this forum is another story of another run away wind turbine.

I agree that alot or most are from furling failure. So that asks the question does the furling designs in the current books available really work? Or  are people deviating that much from the plans that their furling is never going to work?

I'm starting to wonder if the current furling design works for 10 foot turbines but  not reliable for larger sizes.

I've preached this before, shorting out the stator may work for a brake IF... and thats a big IF  the stator is still working and intact. I lost one turbine from the tail flying around too far and striking the blades, and a second one from the stator overheating..while furled ,until a coil dropped out and then its a runaway. Those blades reached supersonic speeds I'm sure . I found one 300 feet away

So its like having a car with brakes that only work when the engine is running!

I say keep working on a reliable brake system for these! Everyone! Before someone gets hurt or theres a lawsuit or just to save your blades if the stator burns out.

[Dave B]

I agree Shadow. A copied version of say the 10' machine in the book and to the letter is not the end of it in regards to if it will perform exactly as previous machines have.

 Variables such as site location and most important .. what will the alternator see as the load ? These variables can make the difference between a happy predictable machine that makes other people wonder what's the problem and a burn out and self destruction posting.

 We hear very little about the specifics of what people are actually connecting the alternator to and it is a HUGE factor in the total performance of the machine and most critically how the furling performs.

 Unless ALL the variables are copied exactly as written by a published design then we will continue to see results posted good bad or otherwise that will reflect the outcome from any change in the complete system.

 These machines are electro-mechanical and you can destroy the machine just as easily with an electrical change as well as a mechanical one. I think all the talk about changing the furling angles, weight, bent fins, off-set etc. etc. might be more a result of what we can "see". If we see the machine over speed and we observe the furling didn't move enough we then might assume the furling is at fault.

  It's tough to "see" that the electrical loading may be the greatest contributing factor to over speed. I am very certain that we all do not have the same loading. New batteries, old batteries, under rated wire, long runs, short runs, too much battery, too little battery, dump load maybe, maybe not, grid tied, maybe not etc.

  When we are talking about very small voltage levels possibly making a very large difference in the loading such as with battery charging then these are not minor things to be over looked when asking ourselves why would or could this happen ?

 I know it's usually much easier to visualize and change the mechanics and sometimes this can work but lately I think we have to ask why so much attention to the furling and some changes to it that aren't helping much. I think it could be more of the other half of the equation but the not so apparent, the electrical load.

  Dave B.

[ChrisOlson]

It's past midnight here, but I got my eddy brake built tonight and installed on the turbine.  The brake works, just spinning the turbine by hand on my test stand in the shop.  It works better at slow rpm than I thought it would but it's not as strong as the stator being shorted.  If I short two phases in the stator I can barely turn it at maybe 20 rpm.  With the eddy brake on I can spin it at maybe 60 rpm by hand and it drags pretty good.

I wired the electromagnets series/parallel and they draw 10.6 amps when hooked to a battery.  I left them hooked up for about 3 minutes and they got pretty hot - hot enough that it's uncomfortable to lay your hand on one.

I maybe should have wound the electromagnets with smaller wire - more turns, less amps, less heat, and probably a stronger magnetic field.  AWG 18 was the smallest wire I had in the shop.

I'll know more after I get the turbine flying again, which is not going to happen for a couple of days, but I think it's going to work.  I'm going to bed and get some sleep.

--

Chris

[bj]

   Very interesting idea Chris, Thanks.

[TomW]

Dave B;

Well said! I tend to agree that, possibly just changing one variable such as how it feeds to the batteries [cable size,  run distance, etc] could skew things enough to not work the same as the one you otherwise directly copied. And once you start tweaking the mechanical configuration, all bets are off!

I also doubt any 2 of a dozen home brewed blade sets are exact duplicates which adds more "windage" to the matching.

Thanks for the input.

Tom

[ChrisOlson]

Dan wrote:

There are a lot of variables though - they include of course all the angles, offsets and very importantly - how the machine is loaded.

I tried something else this morning, when I usually have my best ideas.

The tail hinge angle in relation to the yaw shaft is not relevant.  The tail hinge angle in relation to the tail boom angle is.  I left my tail hinge at 35° but whacked a slice out of the tail boom with the chop saw and bent it so that instead of the tail boom running at a 90° angle to the yaw shaft, it now runs at 75° with an upward incline to the tail boom from the hinge to the fin.  I would suspect this makes the effective hinge angle 20°.  So I welded the slice shut and checked it.

Pulling on the end of the tail with my spring scale it reduced the amount of pull force required to furl it by about half.

--

Chris

[DanB]

Hi Shadow ~ good discussion here for sure.

'I'm always happy to see someone working on a  brake system for these type wind turbines!'

Me too, there can surely be no harm in it and I tend to agree that for larger machines especially, it's probably wise.

'I know the Dans dont think they are required or necessary but man oh man every second day on this forum is another story of another run away wind turbine.'

It does seem a common topic these days.  Although in most cases it seems to me that folks are deviating quite a bit from past designs.  Which-  is good and only leads towards progress in my mind, but I should hate for folks to get the idea that the furling system is unreliable.  In my mind, it is one of the most reliable governors for a wind turbine ever.  And deviations include those in the alternator, those in the basic frame/geometry of the wind tubine itself - and loads.  Using a single resistor as a load with no regulator is pretty risky.  Having a very long/resistive line may also be risky... to a point.  Sometimes a very long/thin line can save things I suppose.  One 'odd' variation that I often think of is perhaps the first dual rotor axial flux machine we ever built.  It's on a ridge top (pretty good site) about 400' from the battery (12 Volt) and the line is thin (8 or 9 gauge wire).  His runs in overspeed a lot and doesn't furl as early as it should but the line itself has so much loss in high winds that it seems to protect the stator and the machine has been reliable with no real problems now for 7 years or so!  In average / low winds, his line loss is not so bad and overall I don't think the long/thin line costs him too much.  

'I agree that alot or most are from furling failure. So that asks the question does the furling designs in the current books available really work?'

I tend to think it works quite well.  The only failures I've had direct experience with involve machines I've built that simply dont furl early enough and burn out in high sustained winds - or - machines that are not loaded correctly (one case involved somebody hooking it directly to a single resistor, another involved a rectifier burning out).

' Or  are people deviating that much from the plans that their furling is never going to work?'

I think about any deviation will change things enough so that it'll likely take a bit of time/tuning to get things right.

'I'm starting to wonder if the current furling design works for 10 foot turbines but  not reliable for larger sizes.'

It seems quite reliable on all of the larger machines I've built so far.  The first run of my 20' turbine was 8 months straight and I never shut it down once no matter what the wind speed.  It did finally fail in very high/gusty winds (measured windspeed nearby was over 100mph) when the blades struck the tower.  

'I've preached this before, shorting out the stator may work for a brake IF... and thats a big IF  the stator is still working and intact.'

Agreed.  And I know there are exceptions, but usually when a stator overheats it warps enough/comes apart enough to jam up the machine anyhow.  I know that's not always the case though.

' I lost one turbine from the tail flying around too far and striking the blades, and a second one from the stator overheating..while furled ,until a coil dropped out and then its a runaway. Those blades reached supersonic speeds I'm sure . I found one 300 feet away'

Yikes...

Well I'm surely not arguing against having a brake.  I think it's quite a good idea done right.  I just want to stress my opinion, that setup right... furling should / can and is well proven to be a very reliable governor.  Furthermore, any 'new' wind turbine design is likely going to take a lot of time to get well sorted out.  To really know if it's problematic or not - it likely takes a lot of machines in different environments and a lot of time and even then there are bound to be failures.  We've been building about the same 10' machine for a long time now and I still make little changes all the time.  I try not to change too many things at once though.  

'So its like having a car with brakes that only work when the engine is running!'

yes...  in this case the engine is a pretty simple beast but agreed, if it becomes disconnected or burns out then some other sort of brake (other than a shot gun) would sure be nice ~ especially on the larger machines.

[DanB]

Hi Chris - sorry to bring this off topic, now we're discussing furling again and it's supposed to be about your brake.  That said...

I maybe misunderstanding you but if not:

'The tail hinge angle in relation to the yaw shaft is not relevant. '

by this, do you mean the angle between the tail pivot, and the yaw bearing?  I would say it's very relevant.  

There are two angles that are important there, one is the angle between the yaw bearing (and the tower) and the pivot on which the tail can swing.  That angle is critical - the steeper it is the later it will furl and 35 deg seems very extreme to me.  The most I ever do is 20 deg, usually we're somewhere between 15 and 20. The less this angle is, the heavier the tail can be...  if it's too small then furling gets picky if the tower is not perfectly plumb, which - in high winds it may not be.

The other angle that's involved is that between the spindle (and the direction from which the wind is coming) and the tail bracket.  I usually position the tail bracket (the tail bracket is the piece where we cut the other angle... about 20 deg or in your case 35 I think) at 135 deg, or... 45 deg from the plane in which the blades rotate if you look down at it.  This angle affects the power curve of the machine after it starts to furl.  If its dead inline with the wind (parallel to the spindle) it will furl and once it does the power will likely drop off quickly.  If its the other extreme (90 deg to the spindle) it will likely furl but the power curve will probably continue to climb after it starts furling and I doubt it'll protect itself.  At 135 deg as we do it, it seems the power curve falls off some after it starts furling but the machines continue to produce reasonably well in high winds.

' The tail hinge angle in relation to the tail boom angle is.  I left my tail hinge at 35° but whacked a slice out of the tail boom with the chop saw and bent it so that instead of the tail boom running at a 90° angle to the yaw shaft, it now runs at 75° with an upward incline to the tail boom from the hinge to the fin. '

Again I may misunderstand you - but I would say the angle of the tail boom in relation to the yaw bearing is insignificant - I tip them  upwards for cosmetic reasons only.  I guess tipping it up some would have a similar affect to lightening the tail slightly, which would cause it to furl a bit earlier.

 'I would suspect this makes the effective hinge angle 20°.  So I welded the slice shut and checked it.'

again, unless I misunderstand..  that doesn't make sense to me.

[ChrisOlson]

Hi Dan,

Firstly, I think furling is part of this discussion and is not necessarily off-topic because if my furling would've worked I probably wouldn't be working on an eddy brake.  The two sort of go together - overspeed control of a wind turbine.

There are two angles that are important there, one is the angle between the yaw bearing (and the tower) and the pivot on which the tail can swing.

You may have misunderstood what I did, so I'll give an extreme example;  furling is based purely on gravity keeping the tail from folding up, and the wind pushing on the fin to counteract gravity's force.  So, for argument's sake, let's say you put the tail hinge on at 45° in relation to gravity's force (from the true vertical) and it always stays there no matter what.  If you weld the tail boom to the hinge tube so the boom is perpendicular to gravity's force it will take a lot of wind to get this design to furl.  On the other hand, if you weld the boom to the hinge tube so the boom is at a 45° angle to gravity's force, or in other words the boom swings in the same plane as the tail hinge, it will flop over on its side to the furled position with zero wind.  You didn't change the tail hinge angle, the plumb of the tower, the inclination of the yaw shaft from vertical - the only thing you changed was how the boom was welded on to the hinge tube and it radically changes the furling result.

I'm a mechanical engineer by trade, so I like to use force vectors to examine what happens in a situation like this.  And it's apparent to me that there's more to furling than just the hinge angle and the weight on the tail.  For instance, you can set up a furling system to furl at exactly 24 mph with the hinge angle set just so, the length and tip weight of the tail set just so.  In the real world, one day you get a strong wind.  That wind deflects the tower 2° which is not even perceptible to the naked eye.  The yaw shaft flexes another degree.  But what did this do to your carefully prepared furling setup?  It increased the tail hinge angle by 3° in relation to gravity's force, and it decreased the boom angle in relation to gravity's force by the same 3°, for a total change of 6° at the tip of the tail in relation to gravity.  Suddenly your carefully prepared design that was supposed to furl at 24 mph, and has in the past, doesn't furl in a 30 mph wind.

That's why I'm working on a brake system.

--

Chris

[ChrisOlson]

My wife and I had to go to Minnesota today so I didn't back to working on my brake until late this afternoon.  I'm meeting with some disappointing results.

I put the blades on and spun the turbine up in my shop on the test stand with my fans.  This gets it up to about 170 rpm unloaded.  I apply the eddy brake and it does slow the turbine down to about 60 rpm and then it hangs right in there.  And this is just with two high-speed fans blowing on the turbine that simulate, maybe at the most, a 8 mph wind.  After three minutes the brake coils get hot - really hot.

If the brake is really working like it should, it has to convert mechanical energy to heat in the aluminum brake rotor.  After the maximum 3 minute application time of the brake, I stop the turbine and use my infrared pyrometer that I use to check exhaust port temperature on diesel engines to check the temperature of the disc, and it's only 2° C warmer than rest of the steel in the turbine structure.  And this is a very accurate pryo, so that means the brake is creating some drag - enough to slow a turbine that's running in its most inefficient Cp band, but not enough to create the heat that should be created if it's really working.

Eddy brakes are proven devices.  But for this project it's whether or not one can be built economically and simply enough to be effective.  It's looking like I'm not getting strong enough flux and eddy currents in the disc for the brake to have the effectiveness I want.  I might have to do some research on how to create stronger eddy currents in a non-ferromagnetic metal.  But if it involves stronger, bigger magnets the weight and complexity starts to outweigh the simplicity of a mechanical disc brake.

We were at Northfield, MN today and looked a couple Jacobs 20 kW turbines (sold by Wind Turbine Industries Corp in Prior Lake, MN) and those use a centrifugal variable pitch governor for speed control, and a simple mechanical disc brake on the gearbox to stop the turbine in the event of control system failure.  The Jacobs doesn't have a service brake on the yaw drive as it doesn't use one, but most of the utility-scale turbines on the planet have service and emergency brakes in them made by Stromag in France.

One thing I've found over the years is that if you over-engineer something it can be pretty exotic and cool like a BMW.  But just like that BMW, it can be a nightmare to keep it running too

--

Chris

[Dave B]

Chris,

  Good luck with the brake, I built mine before to use as a parking brake after watching my machine start up and fly completely shorted. There is no better feeling than to have complete control from the ground over limiting the rotation of the machine.

 Everyone's system is different and those who accept that tweak their system to work and life goes on. I agree with Dan B 100% that the tilt tail furling is a great system and is primary to over speed protection.

 Those of us who are confident of the furling system and have it working properly do not see the brake as an over speed protection device. To us it is a governor or parking brake when we want to manually limit the stresses on the machine and or tower in extreme conditions. This or just knowing we can do this if we are going to be gone for a time and don't want to worry or when lowering the machine for maintenance.

 Just like the fix for stator burnout is not higher temp materials, cooling fins, exotic design etc. neither is a brake the fix for over speed.

 All the components are there for a safe design but we must accept the fact that for what ever reason we might have to work at it to get it right.

  Dave B.    

[fluxable]

With all this discussion about maintaining control of a turbine in windy conditions, it sounds to me like it may be a good idea to go with a heavier generator. That's the approach I plan on when building my turbine.

Definitely a great idea to have some kind of emergency braking system, and would even be better if it could be applied automatically, in the event the turbine began to over speed. Sounds like you're onto a good thing Chris, just needs to be built much larger in order to be effective IMO.

[Flux]

Yes, much as I expected. You will need much better electromagnets with a far better flux path, even so you will find it turns out heavier and more expensive than the equivalent neo magnet field which you can't use in this case.

An effective brake will be significantly bigger than the main alternator and cost rather more. In the end you will get a better brake with a wound stator and external resistors than you will get from an aluminium disc so you effectively build a separate wound field alternator bigger than the pmg just to act as a brake.

Done properly it will certainly work but no better than a more powerful main alternator with sufficient capacity to brake the thing under any wind condition.

Yes indeed you can devise some very clever things and many will work well most of the time but with wind power it must be capable of sitting there for months without being used and be 100% reliable on the day it is needed.

Flux

[ChrisOlson]

Flux wrote:

Done properly it will certainly work but no better than a more powerful main alternator with sufficient capacity to brake the thing under any wind condition.

My basic instincts tell me that the generator can't be relied on to stop it in an emergency.  It's like a fighter pilot flying without wearing a parachute.  That fighter aircraft is a high-performance, state-of-art machine that should never fail.  But in the real world a bird can turn that $12 million machine into a smoldering pile of scrap in less than 15 seconds.  And many pilot's lives have been saved because they wear that 'chute.

It's just my opinion, but I think that what's good for the big boys (utility scale turbines) should be good for the small ones.  It'd be suicidal to fly a big turbine without a brake on it.  They don't rely on their generators or furling systems in an emergency - every single one is fitted with emergency brakes.  I think a little 8' overspeeding turbine is just as dangerous as a big one - somebody could get seriously hurt or killed by a failed blade even on a little turbine.  So when your furling system fails, and many have, and you short the generator but the wind just won't give up and the generator drops a coil - where's your parachute?

--

Chris