Author Topic: Balancing Wind Turbine Blades  (Read 8329 times)

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BrianEllul

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Balancing Wind Turbine Blades
« on: November 07, 2007, 07:53:18 AM »
Hi


Just bought a set of 3 blades --> 60" 403 Wind Turbine Generator Blades Rotor airx AIR-X. I measured the weight of the blades on my digitial kitchen scale and 2 blades weigh 322 grams while the third one is reading 320grams. Is this acceptable or should I add some weight to it? I was thinking of dropping some paint with hardner next to the hub to add 2 grams!


I also noticed that the blades are not very smooth! Shall I sand them down? Will this improve on noise?


Regards

Brian

« Last Edit: November 07, 2007, 07:53:18 AM by (unknown) »

hiker

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Re: Balancing Wind Turbine Blades
« Reply #1 on: November 07, 2007, 02:49:44 AM »
awhile back their was a post on how to make noisy blades quiet..

the remady was quite simple--just glue on a piece of trimmer wire to the back side of the airfoil..it didnt hinder the blades performance in anyway..but killed the noise..did a google search but did not find the post.maybe another member out their can??..............................
« Last Edit: November 07, 2007, 02:49:44 AM by hiker »
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electronbaby

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Re: Balancing Wind Turbine Blades
« Reply #2 on: November 07, 2007, 08:43:33 AM »
« Last Edit: November 07, 2007, 08:43:33 AM by electronbaby »
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etownlax

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Re: Balancing Wind Turbine Blades
« Reply #3 on: November 07, 2007, 11:33:22 AM »
I'm not sure if this is true but... I thought of this and it makes sense.


Total weight of a blade isn't the critical part. Its where the most mass is on that blade. So 2 blades can weigh the same but the tip be heaver on one than the other(then the root is lighter than the other). This would make it possible for 2 blades to weigh the same but yet be different. What I think should be done rather is to weigh using 2 scales, 1 on each end. This would eliminate that and you can tell the relitive weight of each end.


Also.. just putting them on the hub and then balancing them will help you even them out. Adding weight or taking it away weight.


2 grams isn't much. I doubt it will make a big differenc. But then again I could be wrong.


-Randy

« Last Edit: November 07, 2007, 11:33:22 AM by etownlax »

waynep712

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Re: Balancing Wind Turbine Blades
« Reply #4 on: November 07, 2007, 12:37:52 PM »
look up how to balance  engine connecting rods... balance shops use a single scale with a swinging pivot for one end and a swinging pivot for the other... this devices goes through the holes in each end of the rods so it comes to rest at the tops of the bored holes..  each end is measured and marked..  once the set is weighed the heaviest is trimmed to match the lightest... there is more to it on an engine but that is not needed here...


the idea is each blade needs to be measured at the exact same point...and the blade as level as possable..


hope this helps....

« Last Edit: November 07, 2007, 12:37:52 PM by waynep712 »

dinges

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Re: Balancing Wind Turbine Blades
« Reply #5 on: November 07, 2007, 08:54:25 PM »
What's the accuracy of your weighing scales ? It should say that somewhere on it, or in the manual. And, has it been calibrated recently ? If not, I'd take any result with a grain of salt.


The simple fact that it has a digital readout doesn't necessarily mean it is accurate.


0.6% (2 gram on 322 gram) accuracy I would consider to be extremely good for a cheapo weighing scale (but, not sure how accurate yours is).


Could test yourself for repeat accuracy: weigh the same blade 10 or more times, write the results down and average. Do the same for the other 2 blades.


You could make all blades the same weight. But after they're installed on the hub, you'd likely have to rebalance again if your mounting is less-than-perfect.


Personally, I'd consider 0.6% difference in weight to be a very good result and would simply mount them, then balance the entire hub assembly.


I'd wait with any sanding of the blades till you have installed them and found out if (and how much) noise they make. You can always sand a little off later. It would also offer the benefit of having a reference to compare your actions to, to determine how much sanding/smoothing the blades helped in reducing noise. That is, of course, assuming it's easy to lower the genny, work on it a little and raise it again.


Peter.

« Last Edit: November 07, 2007, 08:54:25 PM by dinges »
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picmacmillan

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Re: Balancing Wind Turbine Blades
« Reply #6 on: November 08, 2007, 04:02:12 AM »
In the past i had an opportunity to watch folks fly those remote control planes..it occured to me that they do almost exactly what we do, just in a smaller scale..now to the point on blade balancing...they have a small unit with a pin on it to check each blade to make them identical..they also have leading edge tape and a variety of ways to make the blades balanced and true...if you get the chance, go see someone who is into this hobby of flying and check out their balancing equipment and how they do it...i found it very educational and useful to what we do here and our blades...pickster

« Last Edit: November 08, 2007, 04:02:12 AM by picmacmillan »

TomW

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Re: Balancing Wind Turbine Blades
« Reply #7 on: November 08, 2007, 05:17:51 AM »


Uh, I think what Peter is trying to say in a nutshell is:


Do not confuse resolution with accuracy. Just because it has X places right of the decimal point does not mean it can accurately distinguish between .001 and .002 units.


My advice from limited experience is gross blade weight has very little to do with balance. Where the weight is impacts what it "looks" like to the hub more than gross weight.


Cheers.


TomW

« Last Edit: November 08, 2007, 05:17:51 AM by TomW »

seanchan00

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Re: Balancing Wind Turbine Blades
« Reply #8 on: November 08, 2007, 03:50:24 PM »
I find proper balancing the blades very critical to its performance espcially in light winds. Actual weight of the blades even if all identical will still have to be tested when mounted on the generator. Once installed I first check static balance by spinning the blades repeatedly and see if it favours any side by stopping with that blade down gravity. If it does I add counter weights on the opposing side. If all seems balanced you still have the more labourious task of checking dynamic balance, ie check if all three blades are in the same plane.This is adjusted by tweaking the nuts of the allthreads holding the blades. This inbalance causes the whole tower to vibrate and rob you of valuable wind power esp in low wind areas. This second task always takes 2 to 3 times the time I spent in balancing. perfect balance will get it going even in seemingly no wind conditions.


Hope that helps.


SeanChan

« Last Edit: November 08, 2007, 03:50:24 PM by seanchan00 »

neilho

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Re: Balancing Wind Turbine Blades
« Reply #9 on: November 09, 2007, 04:35:15 AM »
Quote: "If all seems balanced you still have the more labourious task of checking dynamic balance, ie check if all three blades are in the same plane."


What you're referring to is tracking. Dynamic balancing involves spinning the rotor at operating speeds and looking at it with strobes, then doing some really complicated (well, for me anyway) math. It's unnecessary. Get the static balance and tracking right and you'll be fine.


Here's an easy way to statically balance blades:


Weigh each blade. Then balance each blade on a knife edge, or near knife edge that extends perpendicularly across the blade. Measure the distance of that knife edge from the root end of the blade and add the distance from the root end to the center of rotation of the hub. For each blade, multiply the resulting distance by the weight of the blade to get the moment of each blade. Moments of each blade should be, conservatively, within 2% of each other, though a 5% spread can work.


This method does ignore hub imbalance, though a carefully built hub/magnet rotor is unlikely to have significant imbalances. Even if it does, imbalances close to the center of rotation have much less effect than differences in weight at the blade tip.


Neil

« Last Edit: November 09, 2007, 04:35:15 AM by neilho »

Usman

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Re: Balancing Wind Turbine Blades
« Reply #10 on: November 11, 2007, 03:38:41 PM »


I always considered balancing involves; making sure that all the blades are equal in weight and testing involves making sure that one side is not favoured than the other two (or one incase of two bladed machine).


But reading this post, I have to rethink.


I suppose that 0-0 balancing is more crutial to smaller sized rotors - that spin in excess of 500 rpm?


I am invovled in building a 5KW machine with 6.7m rotor (20 footer) that would spin at 200 rpm max. The way I accomplished balancing:


A-Using a wire tied to the center of the hub with all the three blades installed, and then adding weight to the blade or blade(s) until the rotor is 100% horizontal and levelled (tested by using a spirit level).


B-By spinning the rotor, to ensure that no side is favoured when all the three fiberglass blades are on that hub.


C-Lastly, measuring & equaling the displacement between the tips of:



  1. -the first blade and the second,
  2. -then second and the third,
  3. -and then third and the first one again.


So, all the three readings are 99% similar.


In my understanding, these steps would ensure that the three blades are in accurate position/solidity to each other, equal distribution of weight.


Is there still something else needed to be mesured and corrected? or is it that the 200 rpm speed is going to forgive any remaining flaws?


I didn't quite understand the term "all the blades are in the same plane".


Furthermore, there is a remaining question that comes accross my mind every now and then, looking at the turbine/ rotor from one side (at 90 degrees) the tips or even the whole blades may not seem staright, i.e. one looks leaning a bit forward and the others backward, and the tips may pass at different points i.e. one tip might cross at a point and the other may not.

This can occur, I suppose and I haven't tested it, even if all the three steps A,B & C mentioned above are followed? Would this also needed to be corrected or wouldn't make any difference? Or is it just that I am over-thinking it?


Thanks.


 

« Last Edit: November 11, 2007, 03:38:41 PM by Usman »

neilho

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Re: Balancing Wind Turbine Blades
« Reply #11 on: November 12, 2007, 08:10:14 AM »
Quote:


"I didn't quite understand the term "all the blades are in the same plane"."


Yes, it's usefull and intuitively understood by some, but isn't really accurate. Blades have thickness, so can't all be in the same plane. The term means that the planes defined by points at similar locations on the blades are perpendicular to the axis of rotation.


I'm not sure what you mean by 0-0 balancing.


Quote:


 "Furthermore, there is a remaining question that comes accross my mind every now and then, looking at the turbine/ rotor from one side (at 90 degrees) the tips or even the whole blades may not seem staright, i.e. one looks leaning a bit forward and the others backward, and the tips may pass at different points i.e. one tip might cross at a point and the other may not."


You've got it! That's tracking, and it IS important. Tracking errors will also make the tail "wag", usually at moderate rotor speeds. On turbines of 3 meters diameter or so, the tolerance between blades at the tip is about 1/8", if I recall correctly. Closer is better, but unless it's done on a stationary fixture with a fixed reference point to measure from it's difficult to be more accurate than that. The tracking can be changed by shimming or removing material at the hub, or sometimes by variable tightening of the blade mounting bolts.


And while you're checking tracking, take a look at the tips to make sure that they're pitched to spec, and that they're pitched the same.


Your balancing method (A) can work well, or not. It's similar to balancing the assembled rotor on a point, ala balancing a lawnmower blade. The sensitivity of both methods depends on where the wire is attached in relation to the "vertical" Center of Gravity of the rotor, or if using the point, where the point contacts the rotor in relation to the vertical CG. The higher the contact or attachment point in relation to the CG, the less sensitive the method. So it's hard to say if you've got a balanced rotor or not from doing just A. B verifies A if the rotor was repeatedly allowed to come to a stop in varying orientations, didn't display any "heavy spots" and there was very little bearing drag. With what you've done already and making sure the tracking and the pitch are right, your rotor should be good to go.


If I've made you nervous about your balancing, it's easy enough to check your balancing by the moment method. A scale accurate to 1% of blade weight, a blunt knife edge, tape measure and a calculator are all that's required. The information gained is usefull, too. If, for instance, blade replacement is necessary, a replacement blade with the same moment (+ or - 2%) can be substituted without taking the entire rotor down for rebalancing. All because the results of the moment method are quantifiable. No other static balancing method is.


Neil

« Last Edit: November 12, 2007, 08:10:14 AM by neilho »

Usman

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Re: Balancing Wind Turbine Blades
« Reply #12 on: November 13, 2007, 08:29:42 AM »
Neil's explanation is very handy and he seems to be well familiar with various balancing methods. Just if he could further illustrate the following points:


*"I'm not sure what you mean by 0-0 balancing."

I was referring to a term used in tire balancing machines, which confirms the item is perfectly balanced.


*" On turbines of 3 meters diameter or so, the tolerance between blades at the tip is about 1/8", if I recall correctly."

But how do we measure this? (Perhaps in a turnery method by placing a pin attached to stand?)


*"The sensitivity of both methods depends on where the wire is attached in relation to the "vertical" Center of Gravity of the rotor, or if using the point, where the point contacts the rotor in relation to the vertical CG."

Do you mean that how far above is the wire attached to the wire or the length of the wire? I fabricated a steel disc that has a small hole in the center to accommodate a 150cm x 6mm steel rope and lifted the assembled rotor using a fork-lift.

*"If I've made you nervous about your balancing, it's easy enough to check your balancing by the moment method."

This seems interesting but I am not familiar with that! Knife edge? Can you please supply a link to this method? Does this method cover all the tracking and other steps mentioned here or is it just a way to equalize blade-weight.


Thanks.

« Last Edit: November 13, 2007, 08:29:42 AM by Usman »

zeusmorg

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Re: Balancing Wind Turbine Blades
« Reply #13 on: November 14, 2007, 04:51:26 PM »
  Here's my cheap method of balancing, First acquire a length of allthread i use 4'.

Then hang it from a wire strong enough to support two of your blades in the center, the balance of the allthread can then be adjusted by screwing towards the lighter end until it's perfectly level. then hang two same length wires from both ends,make sure they are

at the same distance(count a few turns in from the ends). then hang two blades form hooks on the wires. you're balanced when level. to find difference of weight add weight to the lighter blade.

 To determine if your blades all have the same center of gravity use a wedge and balance each blade on them (tedious).measure from one end to the wedge and if they're all the same, you're good to go.

 The next step is radial alignment of your blades. First mount them on your rotor then

support solidly your windmill where the tips just clear the floor. slowly turn them and use a solid movable block to find where one blade will contact it at the tip one one side. Rotate until the next is near the block. difference from the block is your runout. Also you want tip to tip distance equal on all blades.

 From this point you can get fancy with a strobe, static or dynamic balancing, but i think you'll find this gets you well within a good balanced system.

« Last Edit: November 14, 2007, 04:51:26 PM by zeusmorg »

neilho

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Re: Balancing Wind Turbine Blades
« Reply #14 on: November 15, 2007, 10:21:50 AM »
Usman,


Quote:

*"I'm not sure what you mean by 0-0 balancing."

I was referring to a term used in tire balancing machines, which confirms the item is perfectly balance.


Got it. I just hadn't heard that before.


Quote:

*" On turbines of 3 meters diameter or so, the tolerance between blades at the tip is about 1/8", if I recall correctly."

But how do we measure this? (Perhaps in a turnery method by placing a pin attached to stand?)


The most dependable way I know to measure tracking is to mount the rotor shaft (or substitute) vertically on a bench, attach the rotor and blades, measure the distance from a convenient reference point on the floor (or the bench if it's long enough) to the tip of the blade closest to the floor. Turn the rotor to measure each blade from the same reference point. The difference between those distances should fall within the tracking tolerance. If not, carving and/or shimming the blades at the blade mounting surfaces can adjust the tracking.


A setup like this can also be used to compare blade pitch. A piece of wood or cardboard or perhaps a carpenter's bevel gauge between the blade and the bench or wood or cardboard held against the end of the blade can identify the pitch of the blade. Turn the rotor so the next blade ends up over the same point the first measurement was made from, etc, etc.


Lots of people measure tracking on the tower. Seems convenient, but machine yaw, wind force making the blades move in relation to the tower, the rotor turning out of position, not holding the measuring device in the same place and way every time makes it a not very repeatable method, in my opinion. Also, IMHO, tracking should be verified well before the blades are painted or oiled- saves time having to repaint or oil after tracking changes are made.


Quote:

Do you mean that how far above is the wire attached to the wire or the length of the wire?


I'm referring to the point at which your wire is attached to your fabricated plate.


Quote:

This seems interesting but I am not familiar with that! Knife edge? Can you please supply a link to this method? Does this method cover all the tracking and other steps mentioned here or is it just a way to equalize blade-weight.


See my reply (in this thread) to Seanchan's post. The moment method is just a simple and accurate way to balance a rotor. It doesn't do anything for tracking.


Neil

« Last Edit: November 15, 2007, 10:21:50 AM by neilho »