Define 'stalling'

[bruce]

    I thought stalling was an issue at low speeds, like when the wind generator starts to charge but is turning too slow to have enough momentum, the blades stall out and the unit has a hard time charging. Now reading this on otherpower, where they offer a 24v heavy duty stator which is wound different, they state  

     "This stator has much lower resistance than our other 24 Volt stator. It must be used with the larger disc magnets. With that setup it will overpower your blades and they may stall in higher winds, depending on your battery voltage and the losses in the line."

     Unless there is a typo there, they speak of stalling in high winds. Can someone clarify what 'stalling' really means?

[electronbaby]

"stalling" means that a blade set cannot reach its optimum operating RPM. It is caused by the increase of kinetic energy blowing into the blades, and the blades failing to "speed up" to follow the incoming power increase. This usually happens in two ways...

The first is when the shape of the airfoil actually is altered. This is cause by ice buildup on the blade and changes its aerodynamic properties. It will no longer track the proper power curve based on your intended airfoil design. It may not even work at all. From the blades point of view, this will make the alternator seem "stiffer" and there no longer is a match between the alternator and the blade set, and performance suffers.

The second way is that the alternator is built too large for any particular blade set. This will have the effect of there not being enough mechanical force (speed/RPM) on the input of the prop to carry the alternator into its proper operating window. The alternator will "bog down" the prop and prevent the blades from reaching their proper operating RPM once a load is applied.

If you expect any performance, you must match the alternator curve somewhat to the input curve (prop curve). You will never get it exact, or perfect. Matter of fact, this in reality, is VERY hard to do unless you are familiar with the principals involved. If you can get your alternator cut in voltage to happen at the bottom of the proper prop RPM window, then you should be headed in the right direction.

The next thing to think about is getting it to furl properly shortly thereafter.

[Airstream]

Somehow I think there is another question not being asked - like which of these stators is wasting potential power; answer is neither as they are designed for different wind class sites or near/far transmission line requirements to the power house.

From the same page(s) with the 'stalling' info there is a graph ...

In the stator descriptions they call for:

 2-inch diameter 1/2-inch thick magnets with volume of 1.571 cu in for the HD Stator;

 2 x 1 x 1/2 block magnets w/ volume of 1 cu in for the Standard stator.

So, given an identical drop-line, rectifier and battery the 10-foot blade simply finds its swept area & airfoil output peak at a lower RPM as the HD Stator is spec'd for larger magnets & coil winding gauges (lower internal resistance)

Look on the graph for a particular wattage; say 500 watts - the HD stator achieves that at 180~ rpm while the standard stator will match it at 240~ rpm.

(I believe) a turbine blade set in full stall throws greater oblique force back onto the furl pivot so it will allow it to point out of the airstream earlier..

[Flux]

Stall is when the angle of attack becomes too great for the type of aerofoil and in most cases occurs with an angle of attack of much over 12 deg.

At start up the wind is nearly perpendicular to the blade, the angle of attack is ridiculously high and it is hard stalled. As the blades speed up the apparent wind rotates as a vector to the real wind and comes into a reasonable angle of attack to the aerofoil. If you have things right you will be up to this speed before cut in and you will not be remotely stalled at cut in. As the emf exceeds the battery volts the alternator load comes on very rapidly and the prop will slow down relative to its intended tsr so although you start with tsr well above design at cut in you will reach the design figure rapidly as the wind picks up so if you cut in at 7mph then you will likely be at design tsr at 10 mph and you will be running near peak efficiency.

With direct battery loading things go very wrong in higher winds as the alternator is trying to maintain constant speed with its terminal voltage clamped at battery voltage and the prop will stall at quite modest wind speeds. If the alternator is less efficient then the prop will hold its speed up better than if you have a very efficient alternator so in this case the machine with small magnets will develop more power from the prop in higher winds, The electrical output will be higher for the same conditions even with lower efficiency as the gain from the prop is very considerable.

If you have to deal with a long line and high resistance cables then you will not get a high overall electrical efficiency so the larger alternator will then run as the smaller one does with a short cable run. If you can't avoid a long cable run then you can get better results with the bigger magnets and more efficient alternator.

For a short cable run then compared side by side the big magnet alternator will perform considerably worse and the maximum power out in reasonably high winds below furling will be well down. The issue can easily be solved by adding the equivalent of the extra cable resistance of a long line in as a series resistance. The performance of the big magnet machine will then be similar to the small magnet one ( and it cost a lot more)

Now the important bit, in very high wind areas the small magnet machine with thinner wire will be seriously limited by stator heating so you must make every effort to get it furling at a safe power output ( below 700W continuous). The big magnet machine will produce the same electrical loss overall but only part will be in the stator, the part in the resistance of the line will not heat the stator so you have much more chance to keep it furling safely in high winds and it will be much more durable. That is why it is a better choice for high wind areas.

With direct battery connection there is no way you can avoid some potential power loss in higher winds if you choose a very low cut in speed and for the same cut in speed the big magnet machine will some how have to have its high wind rotational speed increased to keep the prop from stalling or the performance will be way down on the small magnet version. If you have a long cable run then it will sort out but if you don't have a long cable run then you need to do something to raise the speed.

If you aren't worried about performance then you can of course run the big magnet machine stalled and it will then become fairly self protecting even if the furling is miles out and with plenty of wind in a high wind area this could be the safest option but you pay a heavy penalty in terms of total energy capture.

I don't agree that the stalled prop will try to furl earlier but this is a very complex area indeed and probably beyond simple analysis. In simple terms the thrust depends on the power extracted so the thing furls easiest at optimum tsr and will take more wind to develop that thrust if stalled. This is very much complicated by the seeking force, which is less when stalled and higher at best tsr and may be higher still during run away so what finally happens is in the lap of the gods.

You finally have to decide whether to use the cheap small magnet version, match the performance of the big magnet version with line resistance or have a near bomb proof but poor performing machine for high wind areas. Whatever you do stall in the higher wind region will limit the performance to a fraction of a properly matched scheme.

Not sure this will help Bruce as it may be a bit too detailed but this stall issue is complicated from the aerodynamic point of view and far worse when you include the alternator effect.

Flux

[bruce]

     Thanks for the info, I'm trying to digest all the information. The reason I was looking at the heavy duty stator is that I smoked my 3rd stator in 4 yrs. Wind storms have torn them up. I do have the larger magnets in my rotors. My last windings were 64 turns of #13 wire, I believe that suggestion came from the Dans. This last one I'm building is off the website, 70 turns of #14, I'm pouring the resin tonight. I have the batteries at the bottom of the turbine, large 24v forklift batteries. Was considering winding this last one the way the heavy duty stator was wound. Honestly, still not sure if it would be an advantage or not.

     I believe the blades are 11' or so, I will trim those down to 10'. I think the offset is alittle off, possibly more than I thought. Will check all those out before hoisting back up.

          Thanks again, Bruce

[luv2weld]

Bruce,

Wait!! STOP!! ,

Please tell us that you did NOT pour the stator already.

According to the website and the book, the coils for the Heavy Duty stator

are 55 turns of 15 gauge wire (for 24 volt).

You did say you have the larger magnets. The 2 inch round, right???

The standard coils for the 1 x 2 inch rectangle magnets are either 66 turns

of 14 gauge or 70 turns of 14 gauge depending on where you get the info (for

24 volt).

Ralph

[bruce]

     No I did not pour the mold yet, and your right, I did not notice in the directions that they were using different magnets, my mistake. I built this several yrs ago when the design was calling for the larger round magnets. That sucks, I've got 9 coils wound, wired and ready to go. Good call on your part, Ralph.

     OK, here's a thought. What if I open my air gap further, to offset the mismatch between the stronger magnets and smaller wire in the coils? Not saying thats the ideal way to go, but I've basically built this stator just short of pouring resin, I may as well finish it. I may as well do what I can to make it work this time, then in the meantime I can build a spare with the right coils in them. When/if this one smokes, I can replace it with the proper one.

              Thoughts?

[Flux]

There may be more than one issue here but I think the problem in the end comes down to furling.

My first thought is that if you cooked a winding with #13 wire you are going to be worse off trying something with #14.

The only thing that adds confusion is that the #13 winding would have had a higher cut in speed and would most likely have run more away from stall. With more turns in the #14 winding you will more likely stall and with the batteries close and little line resistance you may run most of the time stall limited.If that is the case you may stand a better chance of surviving with ineffective furling.

With an oversized prop and possibly a smaller offset I suspect you have never furled at all and with a winding that was less inclined to stall then you probably well exceeded any sensible output.

In many ways I would agree with using less turns of the thicker wire ( #13). It would loose you a little bit at cut in but it would give significantly more energy capture on a decent wind site.  Whatever winding you use you will fry it unless you can hold it in hard stall up to the highest winds before it furls ( if it ever does). Even with the #13 wire you will not sustain output over 800W for any length of time and unless you can make it furl to keep below this level you will eventually fry it although it may stand up for a year or two ( they run burnt out until you get a shorted turn or it fails mechanically and rubs the magnets).

I seriously believe the furling is questionable anyway and a lot relies on stall as a control. If your prop is bigger or the offset is smaller then I think you will be relying entirely on stall to limit the thing and it may do so up to a certain wind speed. Beyond a critical point it may pull through stall and will go well beyond a safe output in a high wind.

I would imagine that the coils you have wound will be perfectly ok if you can control the output and messing with a few turns here and there and one wire size will not save you if you can't get it to furl.

Flux