How to avoid stalling

[amiklic1]

I'll list the ones I can remember:

1. Make the air gap bigger- I think it lowers the torque needed to turn the prop, so it would not "brake" the generator as much.
   
2. Put thinner wires from genny to batteries
   
3. Put some resistor in series from the genny to the battery.
   
4. Put smaller prop on the generator.
   
5. Do something with the coils. But I'm still not shure what?
   
   

....

I was thinking ho2w to avoid stalling problem with my 2m diameter, dual disc, 6 coils, 8 mags/rotor generator (12V).

As Hugh wrote, cut in speed would be around 210 rpm, 60 turns of 2mm dia wire, 250 watts at 350 rpm. But it would probably stall before that. I am looking for the ways to avoid that.

[maker of toys]

Um. . . if your problem is stalling, I'd think that you'd want a LARGER prop- more torque at any given RPM; so more torque developed at cut-in, and with the larger power gathering ablility of the bigger disk, better match between alt and prop.

someone PLEASE correct me if i'm wrong-  I'm in the design stages of a mill for a marginal wind area, and I'm planning a big prop on a relatively small alt.

[Waterfront]

Your right, it sounded weird when I read that too... smaller prop will just stall even more badly, you'd need a bigger prop to get more energy, like you said...

amiklic1, If you want to estimate quickly on how much power your prop's gonna have, you might try checking out http://www.alton-moore.net/wind_calculations.html

And then you can compare it to the estimated power input your alternator will require at a given RPM... (Amps into battery X battery voltage + Power lost in lines)

Have fun!

[richhagen]

Larger blades,possibly a lower TSR blade for more torque and lower top end speed tradeoff, larger air gap, more turns of wire per coil (increases the coil resistance), more resistance between the generator and the battery either by smaller wire, or adding resistance in series to give it a little more spring before stall.   That's about all I can think of shorter than less powerfull magnets.  Rich Hagen

You Konw eevn wtih the tpyo (ho2w)I stlil udnerstood waht you were aksing.  I thnik.

[nanotech]

I also seem to remember that one or the other of delta or series enables less stalling.

I could be all just hot air here........

[maker of toys]

seems to me that you could drop a phase (or more, depending on how many you have) and thereby reduce the power output (and therefore power INPUT demand) of the alt. . . the thing will probably vibrate some and the open-circuit phase/s might be a little dangerous to be around. . . . so don't lick the disconnected wire, and don't let it short to anything . . .  <G>

this would probably work better with a machine wired 'star;' my little thought experiment got messy when I tried delta.

 you could add the phase/s back as the wind/prop speed picks up and the prop starts to make more torque.  

it's a thought.  I make no warranty about how well it would work.

<#include std_disclaimer.h>

-Dan

(PS:  I've forgotten, if I ever knew/you ever posted:  how is this troublesome stator of yours wired?)

[DanB]

Here are some of my thoughts about it all.

#  Make the air gap bigger- I think it lowers the torque needed to turn the prop, so it would not "brake" the generator as much.

Yes.  Opening the airgap will do two things.  It will increase the cutin speed, and it will reduce the volts/rpm - since its not changing the resistance it will not decrease the efficiency of the alternator but it will reduce the power you get at any given rpm.  I would open the airgap if the cutin speed is too low.  

# Put thinner wires from genny to batteries

This will not change the cutin speed, and will have a very minor affect on output at low levels, but decrease the efficiency of the system especially at higher output levels.  It will somewhat flatten the power curve so to speak  -but leave things fairly unchanged in low winds/low rpm.  This is what Id do if the machine had the right cutin speed, but started stalling worse and worse as windspeed increased (the alternator has the proper cutin speed, but becomes too powerful for the blades in higher winds).

# Put some resistor in series from the genny to the battery.

Same thing as running thinner wires from teh generator to the batteries really.

# Put smaller prop on the generator.

This would make it stall worse.  Your generator is allready too slow, you would want a larger prop.  The problem is though - if your alternator is sized well for the blades you have, but you wound it such that the cutin speed is too slow, then the resistance of the alternator is such that larger blades would overpower it.  Larger blades would probably startup better/stall less, but then they'd probably overspeed - and overpower the alternator putting your blades, and your alternator at risk.  But sometimes it is a solution.  Alternators Ive made which were too powerful for the blades we either put on slightly bigger blades - or, did a combination of opening the airgap and adding a bit resistance to the line.

# Do something with the coils. But I'm still not shure what?

You can switch to Delta.  I recall talking w/you about this before.  You had thought that by winding a machine for 24 volts that youd pickup power in low winds if you were charging at 12.  This doesn't usually work, because your cutin speed is so slow that the blades can never get up to speed.  If @ 24 volts you were cutting in at reasonable windspeed, like 7 mph or so - then at 12V your 'cutin speed' might want to happen at 3.5 mph - when the power in the wind is so little that it can barely even make your blades turn at all.  In that case - the blades have no chance.  Your alternator, being wound for 24 volts now (lots of turns of thin wire) has resistance way too high for a 12V machine (4 x the resistance if you have twice the windings with wire twice as thick).  In this case - adding resistance to the line would not make sense, and opening the airgap way up might help get things started, but it will be dismally inefficient in higher winds (the blade will run away and youll get very little power - the alternator might overheat).  Your best bet here, with the 2 meter blade is to probably rewind for a proper cutin speed, or - you might try putting what you have into Delta/see how that works.  If I recall, you said you allready had a wide airgap, and you wanted to charge @ 12V and you had a cutin speed around 120 rpm.  Delta would surely improve things, but I think you have way too many windings in there and much too fine of wire.

[dinges]

I think you need to distance yourself from the problem first a bit more;

stalling occurs when the load of the genny is bigger than the prop can deliver (at that windspeed).

So, your options are to increase power available by changing prop parameters (size, nr. of blades, TSR, airfoil,...), or reducing the power the genny can deliver (bigger airgap, resistance in series (increasing internal resistance of stator, thus making a less-than-ideal generator), or simply reducing the load by turning of appliances.

Or you could turn up the windspeed a bit more (now where did they put that thermostat knob :-) )

I think that would be the right way to analyze the problem, before delving into the nitty-gritty individual solutions.

You could even make a fishbone-diagram out of it, if you want. It's often used for summarizing various causes of this failure mode.

Peter,

The Netherlands.

[richhagen]

It would occur to me that since in star you basically are running two coils in series at a time in a sense, it would stall less because the system would see more resistance in the coils.  Not sure, but seems to make sense.  Rich

[MelTx]

  Hello   I built a 12" dia, 2 rotor, three phase.12 on each side, one by two by half neo mags. With 9 coils of # 15 awg 90 turns each...To power the lovely thing a 3 blade prop at 10' dia...Of course it stalled badly...So I cut 1 ft off each blade I thought a smaller prop would turn faster and do the trick [wrong].....At eight foot the blades are still stalling...Up and down the mounting post 16 thousand times and I found an answer...

    I finally opened the gap 3/8 inch on both sides of the 5/8 stator.....

  I now have an 11/8 inch gap between mag faces....But now it starts good, and produces better than before....Just some things I did.

    The one thing that would have been different would be to start with a 12 ft prop.

[dinges]

After re-reading my comment; you can take it to an even higher conceptual level:

stalling occurs when angle of attack (AOA) of the air becomes too big for the profile, causing transition from laminar to turbulent flow. Issues with stability as well (at the 'threshold', laminar flow can occur for a long time, until a small disturbance occurs; conditional stability vs. instability).

The AOA increases because the 'rotational' speed of the prop is too small as compared to the windspeed; the two vectors add up to the relative windspeed (also very known in sailing). When this relative wind reaches the prop at a higher angle of attack, lift decreases while drag increases (you could draw a Cl/Cd-curve, and find out where you are on the curve). As AOA increases, lift increases while drag increases; at a certain AOA, increase of lift becomes minimal, while increase in drag goes faster & faster; soon after, you get transition from laminar to turbulent flow: stall. (or is stall defined as the point where lift decreases at rising AOA?)

How does 'load' of the genny enters into this? It decreases your rotational speed (radial speed) of the prop, causing increase of AOA.

But admittedly, this is taking it perhaps to a too abstract level. On the other hand, if it's understanding of the fundamental problems that you seek, it's the only way.

Hmm, just writing the above message has given me some new insight. Must do a search on the web for some good info regarding props & windturbines... Much more difficult analyzing props than wings though, because of variation in parameters in prop length. Like studying drilling in mech. engineering, I've found it much more difficult than studying the machining process in e.g. turning or milling. And drilling seems such a simple thing to do...

Peter,

The Netherlands.

[dinges]

A typical matching problem...

I was going to state in my previous message that the real solution to stalling would be to match the prop to the genny & load (at a certain windspeed). If your prop stalls, the correct solution would be to change prop parameters.

I was going to say too that 'worsening' your alternator (getting further from the ideal situation of lowest internal resistance in stator) by switching from delta to star (for higher internal resistance; but at the same time it reduces the cut-in speed of the genny!), or adding long power wires from genny to battery, or even ballast resistance in series, is a 'fix' that works (i.e., you get more power with it than without it). However, it's not the solution to the underlying problem (prop & genny not properly matched at your intended windspeed).

However, if it works...

But then, imagine what extra power you would get with your 'ideal' genny (low internal R, small airgap) with a properly matched prop. Yes, this means more extra work. But it's something I would do myself, after having invested lots of time, money and effort in building a genny, tower, electronics, battery, etc. Your situation may differ.

The pity about windgennies: EVERYTHING is a (design) compromise. However, it's hard to build a genny that will not generate at least a bit of electrical energy. Reaching the optimum situation however... 'Good enough', as opposed to 'the best', I think.

Peter,

The Netherlands.

[dinges]

Think of it this way: there's a certain amount of power present at every 1m^2 of prop surface (at a certain windspeed). If the genny load is bigger than the power present in the air, the thing stalls. Now if you start clipping its wings, reducing surface area of the prop, even less power of the wind is available to the genny; it will stall sooner... You need MORE surface area of the prop, to extract more power from the wind; but not TOO much, or the prop won't see the load from the genny and overspeed.

I'm guessing that if you left the prop in its original (10ft) state, and increased airgap just a little (less than the present situation), you would have gotten more energy out of your genny. Lessons learned for the next genny.

Peter,

The Netherlands.

[maker of toys]

just my humble opinion, and I know that this stalling question doesn't come up much, but DanB's nice, concise explaination of how the various electrical parameters affect cut-in and output curves might be a candidate for the FAQ.

-Dan

[SparWeb]

Dinges,

You seem to see clearly the relationship between turbine blades and propellers such as those on aircraft.  Since the latter have been studied intensely to improve aircraft performance, there is much to read on props from sources such as the old NACA reports, which is as relevant and valuable today as it was 70 years ago.  If you (or any other members here) are looking for recommendations on the topic, I could dig up quite a few.

I've seen some pretty crude wings on the site - and when the purpose it just to get some electricity to flow there's no big deal.  I'm not too picky about this stuff, yet, myself.  But when folks are looking for every last milliAmp, then there's one often overlooked place to look, and it's not the actual diameter of the blades (though that's important).  The profile, taper and twist can boost performance, either by higher RPM in a given wind speed, or more torque holding the same speed.  There are some fine examples of craftsmanship visible on this site that will only stall out after delivering high amounts of power, unhindered by surface flaws or incorrect spanwise proportions.

[maker of toys]

the place I've been digging lately in in the Human Powered Aircraft/ Human Powered Hovercraft arenas. . .  there they are trying to couple 250 watts (max) into the air as efficiently as possible.  (they're also trying to save weight, which confuses things. . . )

but it seems a good match:  low rpm, big swept areas, low airspeeds, low (relatively) power and high efficiency.  

unfortunately, most HPA teams seem to be a little reticient about thier prop parameters.  I'll post if I find anything worth while. . .

[wdyasq]

Take a look at the DAE series of airfoils.

http://www.ae.uiuc.edu/m-selig/ads/coord_database.html#D

They would be difficult to carve by hand for a lot of folks.  Seli and company from University of Illinios group has doen a lot of windturbine work and publishes a lot.

Ron

[wooferhound]

I was thinking that this whole thread should be in the FAQs

[elvin1949]

I agree

later

elvin

[elvin1949]

Thank's Ron

 I needed that [had it and lost it]

later

elvin

[dinges]

Hmm, looks a bit like the 'superlaminar' wingsections that are nowadays used on gliders; they've got their thickest point at about 45-50% of chord, instead of the usual 30-35%.

Carving isn't as easy as the usual profiles; the hollow section makes it more difficult/more work. But can be done, if you've got the patience. I wonder if it's worth the extra effort though; how much extra power would you get in low winds?

Now, if anyone had a mold and started churning them out...

Peter,

The Netherlands.

[finnsawyer]

Hmm, it makes sense that the flow would tend to be laminar as long as the profile causes the air to concentrate and speed up.  That is, to the thickest part of the wing.  The thing is that windmill blades are much narrower than airplane wings and the flow characteristics may be quite different.  While wind tunnel tests may shed light on the differences, one is still left with the question of whether the fact that the air flow is actually in control and moves the blade affects the pattern.  That is, the induced motion of the blade through the air may cause the air over the back (convex side) of the blade to lie down and hence keep the flow laminar over the entire width of the blade.  If that were the case, many of the issues with wing design would not apply to blade design.  The question is, how could one verify this experimentally?

[msloat]

Would vortex generators help prevent stalling or at least delay it? or are they a waste of time on the airfoils we are using?

[dinges]

There are similarities and differences between props/turbines and wings. Wings are usually much easier to analyze, because there's a few variables less (e.g. varying rotational speed vs. radius).

No need to re-invent the wheel. Plenty of research should be available on a/c propellors and ship props. I haven't yet looked into this though.

And even props and turbines differ, as someone else stated; didn't know about the differing efficiencies (Betz vs. 75% of props). But even on windmills lots of research have been done. It's just a matter of finding it, studying it and understanding it. Oh yeah, and applying it :-)

This is left as an exercise for the reader. Should be easy enough.... ;-)

Peter,

The Netherlands.

[maker of toys]

while vortex gens would keep the flow attached a little longer, a mill that runs close enough to stall to benefit from them isn't working very well in the first place.  Better to find the root cause (here, too much gen for the blades) and fix that than to clutter up the airfoil.

BTW: for you non-pilots in the home audience, from a pilot's standpoint 'Stall' is when the flow 'detaches' from the curved upper surface of the airfoil;  the airfoil then transitions suddenly and completely to a high-drag, low lift apparatus.  this event announces itself with a slight 'bump' that's perceptable and diagnostic if you're tuned-in to your aircraft.  The aircraft will slow and develop a higher sink rate, but is still under control; normal flight (with attached flow) can be re-established by simply reducing the angle at which the wing meets the air (usually by pointing the nose lower).  The goal of a pilot trying to make a smooth landing is to stall the wing just as the wheels touch the ground, then to keep the wing stalled as long as possible to slow the aircraft and prevent 'bouncing' down the runway.  

flow over the wing becomes turbulent considerably before the stall  (indeed, in some cases, it is always turbulent: ie a 'dirty' profile, with, say, raised rivets or other imperfections), but the flow is still more-or-less conforming to the surface of the wing.

watching an airfoil stall in a smoke tunnel was one of the best things to come out of the practical side of my fluids class. . . a stalled airfoil has much the same flow pattern as a parachute; you can see the streamlines off the tail of the airfoil trying to wrap around the sharp edge and get into the low pressure region.  Fascinating.

[dinges]

Good explanation. Only have seen pictures of windtunnel texts on stalling, but still very educative.

In a/c (at least the gliders I've flown) you could feel the entire a/c starting to shake when stalling, because the turbulence hit the stabilo and elevator.

Generally, the goal is to keep airflow laminar for as long as possible. E.g. by means of a good profile, smooth & clean surface, correct airspeed, etc. etc. At some technical universities in NL (Delft) & Germany they are also doing research on 'boundary layer suction (?)', whereby they suck air (by means of a small pump) through tiny holes in the wing, at the point where the airflow becomes turbulent. This causes the airflow to stay 'stuck' to the airfoil for longer.

With these kinds of (experimental) designs, they've managed to go from a glide ratio of say 60 to almost 100... (the ones I flew never were better than 28 :-( )

In a/c this is difficult enough, but how you'd do it in a turbine...?

Think I'll stick to the KISS principle for now. Fun reading about these innovations though.

BTW, from all the info on this board, it should be very easy for me to cut a prop; however, until recently (a few weeks ago), I would have probably mounted it reverse, i.e. like an aircraft prop. Funny; all the detailed stuff is explained & understood, yet such a major thing I had almost missed...

Peter,

The Netherlands.

[richhagen]

I'll have to order some of Prof. Selig's books if I want to have a chance of understanding the theory behind this.  I have an 18" blade carving as I type this.  It is not the more advanced airfoil, but a plain NACA 4415.  The script I put together allows me to change airfoils provided I have the profile data, and generate the G-codes for it and the parameters I entered.  I still have to toy with the output a little, but I can have a new length, TSR, and airfoil generated in about an hour or so and ready to carve.  I saved some of those airfoils when you pointed me to them before.  I haven't carved any of them yet though.  The blades carve really slow though, as the machine runs from point to point around the profile, and comes to a stop at each point for both the roughing and finishing.  I have the maximum acceleration and decceleration turned down as the mill in use has steppers and it is prone to get out of sink if the torque required is greater than the motor can provide, or the momentum of the motor causes it to spin past its stopping point.  Rich Hagen

[seanchan00]

Hi Guys,

This topic is interesting and I can understand most of the points. Will someone please tell me how to diagnose if the problem of not getting enough power from my genny is due to stall or just too marginal wind site. I am desperately trying to get enough power (50 to 60 AH a day is all I ask for) from my Wind Gen 8.6 feet prop over dual rotor 24 mags(1x2x0.5") on 12 inch steel disc. I am really in a marginal wind area and although I see the prop turning easily I have never seen it really speed up till the blades blur.

SeanChan from Malaysia.

[rotornuts]

lets not forget the fundemental difference here. A prop is a driver and a turbine is being driven. Very different I think when it comes to predicting flow aft of the transition  point and also aft of the trailing edge. The "work" is being done at higher AoA's as well.

Examining props and wing sections etc. is very very usefull but there is certainly a departure that must be remembered.

A turbine blade in my mind should be able to function over a range of angles that a prop would never be asked to work in. I don't think it would be offensive ask a turbine blade to function well at the brink of stall so I don't think it would be wise to dismiss flow augmentation.

Mike

[Flux]

Unless you have some reasonable measurement of the wind speed this can be difficult but here are a few ideas.

If you are generating power in light winds and there is an obvious increase in wind speed with little increase in power you may be stalled. The power normally rises rapidly as the wind picks up.

You don't give a lot of information such as cut in speed voltage etc so I have to generalise.

If you have the rectifier at ground level you can wait for a reasonable wind and remove one ac connection from the rectifier to single phase the alternator, this will cause the prop to speed up and if you get as good or better results then you can be sure you are stalled.

If you can't do this you can try running it temporarily into a higher voltage battery, such as 24v if you have 12 now or 36v if it is 24v now. If the POWER increases then you have stall ( the current may actually not increase, it's power you want)

You can also get some idea from the noise of the prop. If you are stalled the noise will be low and will not increase significantly with wind speed. If it is noisy and the noise increases a lot with wind speed they you will not be stalled.

You could also add some resistance to the line to speed the prop up and see if things improve. The resistance needed will depend on the voltage but a rough guide is to add enough resistance to get about double the battery volts on the alternator side of the resistor.

If you have the option to connect in delta that would also prove things.

Even in a poor wind area I would expect that sometimes you would have peaks to over 500W from an 8ft 6" prop, if you never get near this I would strongly suspect stall.

Some of these ideas should be possible for you, if it is stall you may gain a lot.

[DanB]

There are compromises to be made with blade profiles I think.  

A big one is how hard is it to carve.

I also expect that the 'most efficient' airfoil may not actually startup very well.  When stopped - we still need it to act like a 'drag' machine for a bit- lots of these 'fat' airfoil sections wouldn't startup well I don't think.

[IntegEner]

"I have listened respectfully to all the experts but I am going to go ahead and say it anyway" and "the problems that were created can't be solved by the level of thinking that created them". I think the "stalling" here is a general term meaning "a wind turbine not performing well". All this aviator talk is fine and dandy for aviation, otherwise known as the "miracle of flight", but the answers to these questions are at least partly found from the sailors and their ability to trim their sails. Has anything at all resulted from this discussion about wing stalling that can be applied to making this wind turbine blade produce more energy? It all seems more force of habit in always getting into the same discussion no matter the subject. I appreciate the time and attention and the giving of time here but I have to at least add the point that a "Newtonian Mechanics" is available for providing answers as well.

Knucks the "Knuckleheaded Know-It-All"

IntegEner-W

www.integener.com

[seanchan00]

Thanks Flux,

I have bad stalling. In moderate winds I see the blade speed increase and the amperage start to rise and the moment it reach 2 to 3 amps it immediately falls to less than half an amp. I removed the three cables to the rectifiers and then saw the blades really fly fast. When it has reached speed enough to be heard I reconnected and immediately the blades slowed down. I definitely have to open up the air gap between the rotors. Right now each rotor is just about 1/8 inch from the coil panel. Do I double up or just adjust by trial and error empirically till it no longer stalls? Going to be very tedious lowering and raising the tower repeatedly. Any suggestions?

Thanks for your input and Happy New Year.

SeanChan.