stall

[Jason Wilkinson]

Could someone explain "stall" to me ? from reading the board it appears stall means different things to different people, to me it is when the turbine stops turning  due to a load, ie  like  a car going up a very steep hill

    jason

Moved to Newbies because it is likely a common question for new folks. TW

[CmeBREW]

Good question. I recall that was confusing to me a couple years ago. People seem to vary on this because "stall" can come from so many different sources.  I will tell you the way I view it--others may describe it differently and better.

To me, normally, a "Stall" is not a "stop". It IS possible though that the blades are so badly "stalled" by the stator coils , that the blades are actually "stopped" from turning in the lower winds. But this is more rare I think.

But to me, a "Stall" is when the blades ARE still turning, but they are being "HELD BACK" from picking up enough momenum to reach an appropriate or OPTIMAL speed (rpm) to generate good power.

I view it like driving a 'stick shift' car down the freeway. If you Had to pick ONLY ONE gear to use, which gear would it be? It would probably be 3rd gear , since you will still have barely enough torque to get going (to avoid an actual stall 'n stop), and yet still get some decent speed (MPH // compare to WATThours) when you get going.

The Stator coils in an alternator is  like the "transmission". You are chosing the ONE "gear" that it will be 'stuck in' by how you make your coils (what GAuge and # of turns) and the configuration of the wiring of the coils.

DanB, calls it "picking a happy medium".

So alot depends on WHAT WIND SPEED (MPH) you wish to make your generator reach it's OPTIMAL generating range.(and peak)    -Anyway, thats my way of looking at it at this time.

[harrie]

Yes I agree with that, I perfer to keep my turbines in stall, to prevent overspeeding and damage. I use very heavy blades to act as kind of a flywheel that puts alot of torque on the input shaft and will put out a reasonable amount of power in low winds and at very low RPMS. I can also get away without a furling mechinism because they will not break stall.( At least they havent yet) My 18 foot is in stall at 45 RPM and the 22 foot is at 60 RPM.

[SparWeb]

Great analogy, Brew, I never thought of explaining it that way.

[Flux]

Yes the analogy with the car stuck in the wrong gear gives you the general idea. Unlike the car engine , it will not stop, it will just run below an effective speed.

The true analogy is with stall on an aircraft wing, it is the same effect. If the angle of attack of the wind to the blade is increased, in the early stages you see an increase in lift, but beyond some critical angle ( 12 to 14 deg or more) the air separates from the blade surface and becomes turbulent and the lift drops dramatically.

With the aircraft the situation is disastrous,but with the wind generator the blades just loose power, you come off the peak of the power curve and there is a severe loss of power. The result is that the blades slow down to the point where the limited power they produce is enough to supply the load.

You end up running very near to cut in, where the alternator power is small. If it is badly stalled you will not break out of this region as the alternator load increases rapidly with speed and you just drive the blades harder into stall as the wind speed increases and the blade speed doesn't.

When running it will never stop as the alternator load vanishes below cut in with a battery charging load. With other loads then it might stop.

Start up is always in stall and any slight drag will delay start up until the torque is great enough to overcome the drag. Once started and out of stall the slight drag has little effect.

Flux

[CmeBREW]

The best way to learn about all the different causes of "Stall" is by researching here on "Fieldlines" everything that "Flux" has commented about it over the years. Go to the Google search on the front of this website and choose "Fieldlines.com" and type in "Stall" and "Flux" ,and you will have ALOT of good info to study. He has given some incredible answers on this very subject countless times. Someone should write a book from what all he says, I think.

He just highlighted yet another aspect of "stall"--which can also be caused by the blades. He has covered all the causes MANY times.

Of course, I was highlighting yet another main possible cause of "stall", which is from the stator and the coils we choose.

My illustration of the car transmission may have been somewhat misunderstood. I didn't quite mean that an alternator can get "stuck in the wrong gear". Rather, I was trying to say that ALL alternators HAVE TO BE "stuck" within the EFFICIENCY RANGE AND LIMITATIONS of only ONE "gear". (by how we choose to make the stator)

Alternators do not have the luxury of 'changing gears' like a car does in order to maintain a very good efficieny in all conditions. The air-core alternators we make here are LIMITED to being in only ONE "Efficiency range" (or, "Gear")  in all the different wind conditions.

Here is what DanB has said about "Efficiency":

"Inefficiency--Every generator has a certain speed at which it runs most efficiently. But since the wind is not constant, we must try to design to a HAPPY MEDIUM. As the wind speed rises, the raw power coming into the generator from the wind becomes more than the generator can effectively use, and it gets more and more inefficient. This power is wasted as heat in the stator coils. Alternators with wound fields can adjust the magnetic flux inside to run most efficiently, but PM alternators cannot. An alternator that uses many windings of thin wire will have better low-speed performance than one that uses fewer windings of thicker wire, but higher internal resistance. This means it will become inefficient more quickly when producing higher amperage as wind speeds and power output rise."

So One of the main factors in getting to this "happy medium" (or,3rd gear in my illustration) is determined by the coils in the stator and the literal, MECHANICAL "resistance" they (coils) will have in OPPOSITION to the wind blowing on the blades when current begins to flow thru the coils.

If this "opposition" is TOO GREAT, then you will get this "STALL" condition in the lower winds. (the blades will only turn so fast and get "stuck" ,and not spin any faster)

I think this also causes confusion with beginners. (It did me)

This "Literal, MECHANICAL resistance" I just mentioned is exactly the OPPOSITE to what is commonly referred here to "resistance OF the coils".(Or; resistance to CURRENT trying to flow in the coils)

So RAISING the (current)resistance of the coils --actually LOWERS the literal, MECHANICAL resistance of the coils in relation to the blades turning by the winds.

And, LOWERING the resistance of the coils --actually RAISES the literal, "MECHANICAL resistance" of the coils (MORE current trying to flow) in relation to the blades trying to turn by the winds.

So one of the causes of a "STALL" condition can be caused by lowing the resistance of the coils alittle TOO MUCH, and the blades cannot reach a torque momenum to over-come it in the lower winds.

--I hope I confused everybody!

Quess I should have went with the "10-speed bicycle" illustration!!      

[Jason Wilkinson]

Thanks to all who responded,i'm still not out of the woods,but a little light is shining. Now another query. I had to take down the TB (the nuts that hold the blades had slackened off causing the holes to be enlarged ,blade wobble/noisy allthread bent) DanB built 10 footer  Repairs completed, raised, spinning well until the wind dropped later in the day. The wind speed picked up next day much higher than usual (much higher than when the TB was raised) but it was bearly turning. It laboured for 2 days in that condition,on the 3rd day i was about to take it down when late in the afternoon it started to spin rappidly with little wind. Though i'm glad it's working again i'm at a loss to what happened

    jason

[Flux]

It rather looks as though you had a case of stall, but I am not sure what caused it.

It may have been a mechanical problem, when you tightened the blades you may have made something tight on the alternator and it had enough drag to hold the blades stalled until the wind was strong enough to get it away. When running at full speed it may have settled what was causing the trouble.

The other possibility is that you had a short somewhere and that has cleared itself. If this was the case then I can't help feeling that it will return at some point, but while it is gone you will probably not find where it was.

Just keep an eye on things for a while. If you hear any rubbing or scraping noises you had better investigate in case the magnets are rubbing the stator. If it is a short and it comes back it will either go into stall and crawl round or it will burn something.

Flux

[finnsawyer]

The only way that I can see to keep the blades in stall would be to use an alternator with such a low resistance that the curve of power to the battery stays above the wind power curve out to some (hopefully) unattainable wind speed.  Can you elaborate on what you have done?  Alternator resistance, cut-in voltage, angle of attack, design TSR, etc.

[Jason Wilkinson]

Thanks for your swift response Flux,  i do hear a slight scraping noise when it is turning slow (like one scrape/rev but it goes away as the blade speeds up )i'm not all together satisfied with it's performance, from reading the board i think i'm losing flux due to the mag.disc being too thin. It should've been 1/4" as called for but it's only 3/16 and the most i've seen is 4amps going into the batteries

[seanchan00]

Hi Harrie,

Your 18 foot stalls at 45 rpm and 22 foot at 60 rpm. At these rpms how much amps are you charging at what voltage ? At what rpms do yours cut in for each blade ? I am also trying to harvest the max from low wind conditions and I have also made a 12 foot blade to take over from the recommended 9 foot. I am also trying to cut in and start charging at just over 60 rpm say at 70 or 75 rpm max. My 12 footer has a twist, tip at half the drop compared to the root and tapers in thickness from the root to the tip but no taper in width so it is the same width at root and tip. Really catches the slightest wind and start rotating. It is 8 inches wide and 3 inches or 7.5cm thick made of laminated pine strips.

SeanChan.

[harrie]

Finsawer and Seanchan. Yes, the 18 foot Cut in is about 30 RPM at 12 volt, and beleive me it rarely stops turning. It is on the edge of the Missisippi River, so it does get good wind. Again, I think the reason it works so well is the heavy blades. It useally puts out from 3 to 7 amps in low winds and high winds. I mainly use this one to power a boat house on the river, plus keep my trolling motor batterys charged . as I said, it stalls at 45 RPM and I love it It has not been shut down since Sept of 2006, but only adds up to around 2000 AMP hours a month.

The 22 foot is in 48 volt, has 18 coils 85 turns of #15 wire and rotors 24 2'diaX1/2 ' N38 mags.IT cuts in at 48 volt, and charges at 58.4 volts 60/RPM at 5 to 10 amps. When the bat. are full, it will divert at as much as 30 amps. Again, I feel The reason it works so well is the weight of the blades which are 40 lbs each.

I would like a little more charging amps, and have already opened MY air gap to 1X1/8" I think my next plan, is run another three wires down the tower so I can switch to Delta when the wind is right. As it is, it gives me around 200 KWH a month for use in the house. I am very happy with its performance.

However, I live in a area that is class 2 for wind, so Im not saying this would work so good in areas that have 60 70 mile winds, at least without a furling tail. Im not really worried about it braking stall, because I have disconected the load in about a 30 mile wind, let it spin up to about 150 RPM, and reconected the load which makes the amp gage jump up to almost 80 Amps, but very rapidly goes back to stall RPM. almost like shorting it out.

You can see the projects in my files under authors.

[harrie]

Had to claifiy that coils were 85 wraps of 2 in hand #15. sorry

[ZooT]

So, if the blades never make it out of stall would that mean that they never achieve lift?

And if so, then is it a lift machine or a drag machine?

[finnsawyer]

It's still a lift machine.  The drag always acts to stop the blades.  Stall actually occurs around the attack angle of maximum lift (12 to 14 degrees).  As the blade goes into stall, the lift drops and the drag increases until a balance is reached.  Oh, yeah, and you get some power out as part of that balance.

[CmeBREW]

Jason,,

     Do you mean 40 amps or 4 amps?  I believe A 10 foot diameter Hawt would get closer to 40 amps for a 12volt system. (or 20 amps for a 24volt system)

The thinkness of the thinner steel plates would not cause THAT big of losses. If you only get 4 amps something gotta be wrong.  

[CmeBREW]

I have a little better understanding of the term 'Stall' now as used for wind turbines.  It is quite confusing and not so simple to understand and especially to explain. My previous comment(s) were somewhat confusing I believe.  

As Flux said below it is an Aeronautical term refering to the 'Angle of attack' on the blades changing in certain ways which affects the lift and/or drag on the blades.

Here is a decent link to a pdf file that discusses "Stall" very well I think. I am still learning though, since it is quite deep to grasp. I wish I had known this earlier.

http://www.windmission.dk/workshop/BonusTurbine.pdf

Chapter 4 discusses lift, stall, changing 'angle of attack' on the blades, etc.

I learned there are TWO different winds to consider with wind turbine blades ,not just ONE obvious wind direction like with an aeroplane flying.

Evidently then, with  wind(mill) turbine blades, the two winds to be aware of are the 'normal' (or, obvious) wind direction AND the second is the 'Head wind' direction that the blades actually make in rotation.

You sorta mix the angles of both of these 'winds' together and you will get a varriing 'angle of attack' on the blades that Varies (in degrees) and is determined by the 'normal wind' speeds (mph) AND the speed (RPM) the blades are traveling (moving) at.

Evidently then, 'Stall' can be at BOTH sides of the power curve. You can have unwanted Stall at too low of rpm, AND you can get WANTED 'Stall' at higher rpm (maximum load) to keep the mill controlled at a certain point.

OR, with a big mill, you can Stall at some 'medium point' to keep things safe as Harrie did above.