Furling issues - others here having similar?

[Dan Lenox]

All,

I posted the details about my recent wind 'event' on the otherpower.com discussion board as well as here.

It started up a unique discussion, and recently other wind turbines have experienced the same furling issue that I seem to have.

In relatively constant winds the machine appears to furl just fine and at 28mph is completely furled. However in higher winds 35+mph the output goes up tremendously as if the machine is not furled at all...

I now believe that with relatively constant winds my machine furls fine, it is when tremendous gusts of wind come that it just may not have time to furl and the wind seeking overpowers the furling mechanism.

The discussion is that potentially the offset of the turbine from the yaw bearing may not be enough and probably should be increased slightly.

Wanted people here to know that myself and others are experiencing furling issues and to be on the look out for it yourselves. Any comments?

Dan Lenox

[Steven Fahey]

Hi Dan,

I hope this catches your attention: it's a day late, but hopefully not a dollar short.

I have been crunching numbers on the furling issue. It has always interested me, but I have never seen enough interest from others to get hard numbers. I took some numbers you posted on the "other" forum and put them into a table beside mine.

http://www3.telus.net/faheydumas/Wind_Turbine/Forum/Wind%20Turbine%20Comparison%20Table.html

I still have blanks to fill. Can you help add a bit more data, so that we can at least make estimates. I think I'm getting close to a conclusion.

You didn't know before how much your tail weighs. Maybe just describe the materials and the geomety and we can at least throw a dart at it.

[Dan Lenox]

Steve,

I thought that it was an interesting issue as well, and after being pretty persistant some others have been coming forward and seeing similar issues, helping give my situation some credence. Even some 10'ers are having 'unfurling issues'.

Here is some additional info on my turbine:

The tail was made from 8-1/2' long 1-1/2" pipe,
it also uses 2" x 1/4" x 6' flat steel for reinforcement
the tail vane consists of 3/8" plywood

So I expect total weight of the tail to be on high side

My best guess is that a) I need to replace wood tail vane with light weight FRP and b) increase the offset of the generator at least 1-1/4 to 1-1/2" more.

Interesting to see what your conclusions are.

Just remember that under 'normal/steady' winds I *have* seen it 95% furled in 28mph winds. It is when a 35-40mph gusty wind hits it, that it seems to not furl properly.

Dan

[Sylvain Carrier]

Hi Gents,

Dan,
What exactly is FRP?

Steven,
I will have updated data for your spreadsheet soon. Just to make it as useful as possible. Thanks for all your work, I believe with the accumulated data something will eventually click and we will realize where we went/are going wrong)

Got some good wind yesterday, although gusty i.e. steady 30kmh with gusts to 50kmh I did not see an attempt to furl. So it looks like I might have to do some offset mods thinking 6%. But a stupid question: the turbine only had about 250W of load does that come into play (I know the alternator was not loaded but was just wondering if this affects furling in any way) I'm fairly new at this but did not dare ask in case of getting reamed!!!


sly (same sly has on otherpower)

[Steven Fahey]

Hi Dan, and Sly,

Dan, thanks for adding the info. Earlier today I discovered that you have documented your construction project in great detail on your web pages, and the tail boom info appeared there. At first though, I thought I read 2-1/2" pipe for the tail boom! Going over it again here I see that it's actually 1-1/2" pipe.

Over the weekend I concerned myself mostly with making sensible comparisons and finding good numbers to stick into the table. Today, I've been mulling over what the results mean. I have found two ratios, which I've highlighted in blue, which boil down a lot of the key differences in these windmills.

Reminder: I'm continuously changing this table, and erasing the old copy. Old ones have mistakes in them, so don't bother keeping previous versions saved on your own computer. Today's is dated 17 November, and if we keep discussing it, expect more changes.

Sly, yours is probably under rated, and you probably have room to expect more power. If your power estimate is low because the generator you have mounted cannot deliver more power, then I can say you have a mis-match between the blades and generator. If you are being cautious in your early expectations, and in time improve the output of the generator, either by changing wire connections or removing whatever precautionary measures you currently have in place, then I'd say you have some good times to look forward to.

Dan Lennox, I like the way our turbines compare to each other - in many respects I can multiply the size and power of mine by 4 and there's the specs of yours. Where that comparison breaks down is in the weight of the tail. So that's why it raised a red flag for me. I think the tail has both too much weight and too much inertia, especially considering the turbulence that is getting you into trouble. My numbers back up your assessment that weight must be removed from the tail. One way or the other, aim for a tail with about 3000 inch-pounds of moment.

If you took the tail boom off the windmill, hung the vane from the ceiling of the garage, and rested the hinge on a table with the boom level, a spring scale will tell you the exact weight of the tail. Then you can go about figuring out how much you have to lighten it.

?
|
O spring scale
|
|
x
-------______________________________________________O hinge
vane ______________________________________________ ___________
__________________________________________________ _ | table

I haven't drawn ASCII art in a long time. It shows doesn't it!?

For the sake of interest, not because Dan Bartmann volunteered the information for this, I included the Otherpower's 10 foot windmill. I discovered that it probably furls later and in higher winds than they think it does. By under-promising on the performance, their customers get a windmill that exceeds "expectations". The blades and alternator seem hardy enough to handle higher power output, despite the occasional failure over the years.

Janne's windmill is there, too. It was Janne and Dan Lennox both reporting problems that provoked me to start this project.

If there's anything else I can add that could help, just ask. After considering it more tomorrow, perhaps I'll have some new opinions to share, too.
Steven

[Rob Beckers]

Dan, a few questions in an attempt to wrap my head around this: When you say that for high wind gusts it doesn't furl properly, does that mean it never goes from running to furled position, or that it furls and comes back out of furl? The process of furling; is this a 'smooth' process in terms of the forces required, or does it have a 'hump' in that you need to apply more force to get furling started, and forces then drop off when moving further into furl?

-RoB-

[Sylvain Carrier]

Steven,

Mine is like the otherpower's 10' with slight variations. Yes I am intentionaly holding it in stall via 38v battery bank wanted to make sure furling worked prior to releasing it into 48v bank (Nicads). But that was my original question could this be affecting at what point or how it furls?

Dan,
Your plywood vane weight seems high. I seem to recall regular plywood at 3lb per sqft based on 1" thickness. 1/2" would be 1.5lbs etc..

Tks
Sly

[Steven Fahey]

Sly,

I suspected you were doing something to de-rate it, but I expected it would be resistors or something. How does using a lower battery bank voltage affect furling? Well we have to go through several steps of logic and math to get there.
Lower voltage means lower cut-in, right off the bat. Cut-in for 36v will be 75% of cut-in for the 48V bank. From there, increasing wind speed will turn the generator faster and produce current. This output power is collected in a speed range lower than intended in the design of the generator. Losses due to resistance add up the way they would otherwise, but you're getting less power for your trouble. Overall, the windmill is loaded almost the same, but is less efficient. It is also forced to run slower (a little bit slower), so in that way it is protected from overspeeding by using lower system voltage.
Since the windmill is loaded almost the same as it was, you will not see much change in thrust on the prop. Assuming the thrust doesn't change, it will furl at the same wind speed. If it really is turning slower, then there will be less gyroscopic inertia and perhaps even less of the wind-seeking moment that would otherwise turn it to the wind.

Now that I've crawled that far out one branch of the logic tree, I would say that furling wind speed is slightly reduced, if it is at all, by lower system voltage. Can you hear any cracking yet?

[Dan Lenox]

Hi Gents,

Dan,
What exactly is FRP?



Sylvain,

Fiberglass Reinforced Plastic (FRP) or some derivative

Dan

[Dan Lenox]

Rob,

The furling process is *anything* but smooth! It's a a bit of a herky-jerky motion, but have nothing else to compare it too... Seems to furl reliabily with constant speed air, but have not been outside to see it with the 35+mph gusty winds - typically it has been dark. Your description seems pretty close.

Sylvain,
I did not state the weight, only thickness of plywood and square area. I don't know exactly what the weight is.

My main reason for starting this thread is that on 'other' boards a number of people seem to have similar events. I wanted to see if others here have seen this as well, if I did not put the question out there I don't think that I/others would be aware of it.

I suspect others also have either an offset problem (from yaw bearing to center of turbine) or the angle of the 'wedge' that supports the tail can easily be too large in angular degrees, not to mention tail weight. Three things to easily gotten wrong during construction.

Dan

[Dan Lenox]

Sly,

I also have a stall issue and occasionally use a .6ohm 6kw power resistor inline to help with mid-performance.

Perfectly honest I like the stall, even if I get less output from the turbine! I would much rather have a 17' that typically hovers around 125rpm than 250rpm anyday!!! I think that keeping down the rpm's will help with longevity and maintenance.

Dan

[Steven Fahey]

Dan, I'm not so sure about the stall you referred to in your last posting, there.
I worked out your mill's TSR on my table, and came up with 7.6 before furling.
Unless your blades are cut for TSR>10 and have virtually no twist, I wouldn't expect that to be your problem. Feel free to check my math.

I revised the table the other day, too, so if you look at it now, I've filled in more blanks and tidied it up, too.

[Dan Lenox]

Steven,

the stall is because my generator overpowers the blades, probably need more air gap between the rotors.

Dan

[Dan Lenox]

Steven,

I also went back over my logs, and found that I weighed the steel for the tail (sans wood vane) was 35lbs. So if the wooden vane was ~8lbs the total weight of the tail is ~43lbs, not 27 total as you have in your spreadsheet

Dan

[Steven Fahey]

Hi again Dan,
I still need to check my math then. Is this a case of garbage in-garbage out? I started with 250 RPM, 20 mph, and 8.5' radius on your blades, and that's how I got a TSR of 7.6. Maybe I should not assume that in 20 mph wind, your mill turns at 250 RPM.

I updated the tail data in the table for you. Now the furling moments balance at 2:1, like mine do.

Stepping back from it, I think I should have made the ratios of aerodynamic moment and force moments to divide the same way. One of the fractions should be turned upside-down so that they both express "Prop-vs-Tail" or "Tail-vs-Prop" in the same order.

Nitpicking.

Back to the problem at hand. Your tail is lighter than I thought it was. I'm still wondering if inertia plays a role, but only because you are getting gusts, not because I think the tail is too light or heavy or anything else.

Could the gusts be changes not just in wind speed, but in direction, too? Assuming the windmill furls by turning the prop to the right, a sudden change from 20mph NW to 40mph NE, and suddenly the mill is face-on? It accelerates the RPM, but gyroscopic inertia prevents the prop from turning around right away...

If that was the case, more surface area on the tail would help, but I won't jump to that conclusion if you don't have observations to back up the hypothesis.

[Dan Lenox]

Steven,

No my turbine does not turn 250rpm in 20mph winds, it *rarely* turns that fast. It rarely gets above 200rpm

Below is somewhat close output at various wind speeds:

MPH RPM Watts
6 105 250
10 120 400
14 145 900
18 165 1400
22 200 2200
24 220 3000
-- -- --
34 240 4400
38 255 6400

The wind measurement were from the top of the tower, there is an area between 24 and 34mph that is a bit elusive. The tower wind speed is typically about 2X that of the ground wind speed.

I am sure you will run through the numbers, but essentially I have a pretty high stall condition, and I have no problem with it, I like the low rpm's as it should result in less maintenance...

Dan

[Mel Tyree]

Hi Dan,

I have been looking more carefully at your thread. Please take a look at my thread about energy loss. There are two solutions I see.
(1) Improvement to the inverter so it can put a cap on power production and deal with the over-voltage that results.
(2) Make furling of the Bergey Excel turbine more sensitive to wind gusts.

Does anyone know how adjustable the furling mechanism is in a Bergey? How is it done?

Based you your experience, Dan, is this a reasonable adjustment to attempt?

Sorry, my posted reports a so long-winded so take your time to reply!
I won't try anything new for a few months.

--Mel

[Rob Beckers]

Dan, in high or gusty winds does the tail maybe oscillate in and out of furl? I can see that the combination of weight/gravity and wind forms a second-order system that has the potential to set up an oscillation. If that is the case then maybe a bit of dampening could help. There are small dampers (for example, motorbikes use them on their front end to prevent oscillation in their steering system), bolting one on might smooth thing out a bit.

Re-reading your messages you seem to say that the turbine speeds up while already furled, when the winds pick up. It would seem that furling doesn't take the turbine far enough 'out' of the wind (still too much surface area presented in the direction of the wind). If that is the case, and if I understand the furling mechanism on your genny correctly, too small a tail surface could cause that.

-RoB-

[Dan Lenox]

Rob,

When the turbine has gone into overdrive mode it typically is dark out and can't visually observe the event.

I believe that reducing weight at the tail vane will help with this issue, furling appears to be sporatic and at best is certainly not a smooth motion as it jerks itself into furling position. I certainly believe that it may not furl at all with gusty winds as there is a lot of mass to this beast.

Since this is my first wind turbine was not sure what others see as far as furling goes.

Have been looking into damping mechanisms for the tail motion, just want to be sure that at any point it will not impeed furling.

For now I am happy in degraded output by keeping the rpm's low, and turning off in high winds.

Dan