Homebrewed Electricity > Wind

Please discuss stalling and adding resistance to the line.

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Flux:
I was in a bit of a hurry this morning but this is an important issue and it may be worth explaining in more detail . I have covered this before but it may not be easy to find it.

The ideal situation is for the prop to run at ideal ( design ) tsr in all wind speeds. For normal blade profiles this means that the prop speed should track wind speed. There are some profiles where this doesn't seem to apply and they will behave a bit differently but if these are Dan type blades this will be near enough.

A highly efficient alternator will not let this happen, the current will rise very rapidly with speed and it will try to hold the blades at constant speed. If you get the correct tsr at cut in you will go straight into stall.

The best solution is a loading scheme that lets you run at constant tsr and this will be a mppt scheme. The results are the best you can get and you should aim for the highest alternator efficiency to get best results.

Now if you wan t keep clear of electronics and direct connect you have to make compromises. There is very little energy in very low winds, you get most energy in the 10 20 mph band and in very high winds there is so much power in the wind that you can get away with not being very efficient.

Most props have a band over which they work quite well, if we take an example of a prop intended to run at tsr6 then ot will likely work reasonably well over the range of tsr 4 to tsr 8. it may struggle to get much above tsr8 even unloaded so trying to get it to tsr9 may give poor results.

Similarly much below tsr4 it will hit hard stall and the power will drop right off.

We use these points to our best advantage to get the best overall result. if we cut in at ideal tsr we start heading for stall straight away and we run into trouble in the main power capture band.

if we acce0t that there is virtually nothing below 7 mph worth chasing we can set our cut in to this or perhaps 8 mph. The prop will still do well enough if we make cut in at 7mph with tsr 8. We loose a little but it is a little of little anyway but this factor is somewhat site specific, in a very poor wind area you would cut in a bit earlier and in a very good wind area i would raise it to nearer 10mph. I still think most folks go for too low a cut in, it gains you little in light winds and kills performance in better wind where there is far more to gain.

Now having chosen a suitable cut in  we need to do the best we can in the reasonable 10 plus mph band. We can get the prop somewhere on the peak at 10 mph if we chose that cut in right. We have a powerful alternator with good electrical efficiency so that is about as good as we can do.

Now what happens as the wind picks up, we still need the prop speed to rise and our very efficient alternator won't let it so again we hit stall say at perhaps 15mph and the power flattens off and won't go any more.

The ideal solution is to again change cut in speed for this point and a two stage star/ delta or similar machine would change ratio at this point. Our simple scheme is now at a disadvantage because changing the voltage ratio here will mess up the low wind conditions so we can't do anything by altering the gap again. We have got that right for the lower winds.

Now is the time to through away electrical efficiency to make better use of the prop's potential power output. If we had started with an inefficient underpowered alternator we should now not have the prop stalled and it would work much better than our costly efficient alternator and in general this works out to be the best cost effective solution as long a you can furl within the stator heating limitations and that probably means furling not much over 20 mph.

We can now do the series resistor trick with our very efficient alternator and make it behave just like the cheap one, the prop doesn't care where the electrical inefficiency comes from, it just likes being allowed to run at a better speed.

Where we gain now is that we have the same overall heat loss in the system but most of it is in the series resistor rather than the stator. Our better stator can stand more amps for the same temperature rise so we can raise furling point until we reach the new stator limit. This could easily double the power into the battery for the same stator heat and furling speed can be raised, but it still has to furl at a point below the stator heat limit.

Having chosen cut in speed correctly we have the same overall electrical efficiency with a powerful alternator as with the low cost under powered one and the energy capture in the working range is not changed but we extend the working range to higher winds and higher power.

If we don't do the resistance trick the alternator is better not being too powerful or we use much more copper and magnet to build a machine that works well up to about 12 mph then becomes really bad in the higher winds. The only conceivable reason to do this is if you aren't worried about the poor power in high winds and want something that's near bomb proof if you can't understand furling or aren't prepared it get it working properly.

Whatever you do with a simple direct connected mill you have got to accept that you loose out badly in high wind power if you want to keep some decent low end results. This can normally be justified by assuming that in periods of high wind your batteries are full and dumping. If you have means of using the power in those very windy days then i really can't see why even slightly more complicated schemes have to be ruled out at all cost to keep the KISS principle.

For those completely afraid of any electronics then it is virtually hard luck but for the slightly more adventurous there is considerable gain from such things as star delta or the much easier star/jerry or whatever they choose to call it now.

Even that is a poor compromise to a boost converter for the low winds and a machine wound to suit the high wind end but I concede that this is possibly not suitable without some electronic experience.

I hope this goes some way to explaining the best way to get a good compromise from a simple direct connected machine, I am sure half of these machines are performing very badly and are stall regulated, the day the big wind comes they pull out of stall and the furling doesn't work ( never did but you thought it did) and there is a fry up.  The very big and seriously over powered alternator will stay stalled and survive bit the one that is over sized for normal winds but not a monster is the one that is going to fry.

Flux

ChrisOlson:

--- Quote from: Flux on April 25, 2010, 11:19:06 AM ---if we acce0t that there is virtually nothing below 7 mph worth chasing we can set our cut in to this or perhaps 8 mph. The prop will still do well enough if we make cut in at 7mph with tsr 8. We loose a little but it is a little of little anyway but this factor is somewhat site specific, in a very poor wind area you would cut in a bit earlier and in a very good wind area i would raise it to nearer 10mph. I still think most folks go for too low a cut in, it gains you little in light winds and kills performance in better wind where there is far more to gain.

--- End quote ---

I'd like to point out another phenomenon I've discovered with axial generators.  First you have to understand that the power they put out is dependent on how many rpm/open AC volt they make, the difference between that and loaded volts, how many rpm it's turning, and how much resistance it has to push against.  An overview of this can be found on Ed Lenz's website here:
http://www.windstuffnow.com/main/generator.htm

Basically, if the generator is too powerful for the blades, the blades will increase rpm (and generator output) to where the generator simply overwhelms the blades, they go into stall and that's all you get - this is what's happening to Volvo Farmer.  It's not a "soft stall" like you should get just before furling - it goes into what I call "hard stall" because the blades simply don't make enough power to turn that oversized generator any faster.

I used to build my generators with a very tight air gap as this makes the rpm/volt stay closer to the same at lower rpm as it is at higher rpm.  I'd wind them with fewer turns of wire and they were screamers but I kept burning them up because they never put the blades into stall in high winds.  So then I decided that putting real amounts of wire in a turbine generator is the way to go - multiple strands of the biggest wire that will fit, and wind them with a couple extra turns so the volts/rpm is lower and puts those blades into stall in high winds.

Well, the last couple I built, I overwound them pretty badly and ended up with the same problem Volvo Farmer has.  They'd start trickle charging the batteries at 6.5 mph wind speed but they'd peak at 15 mph and that's all they'd put out because the blades were already stalled by then.  That's when I started playing with air gap.  Thru some bench testing I found that running a wider air gap than "normal" changes the rpm/volt you get at different speeds.  With my old tight air gap generators I'd get the same rpm/volt at 150 as I got at 450 rpm.  But with an overly wide air gap the overwound generator will make less rpm/volt at 150 than it does at 450 - it becomes increasing more powerful as the rpm's pick up.  Instead of a linear power curve like most generators have, it has more of a parabolic power curve.

This actually works great.  It lets the blades spin up and start putting out as much power as the tight air gap generator did at 10 mph, puts out dramatically more power at high rpm's/wind speeds than the tight air gap generator, and puts the blades into "soft stall" right at furling speed.  As an example of what sort of air gap I'm talking about, I lowered the tower for my 13 foot this morning to turn the furling down on it a bit, and while I had it down I measured the air gap so I could write it down to remember it for later - .933" with 1/2" thick mags.

The one and only disadvantage I've found to doing this is that the generator is so "loose" at lower rpm that the shorting switch is virtually useless.  It's so "loose" that with the generator shorted it comes out of stall in 25-30 mph wind and with an ammeter on the lines it's putting out 18-20 amps shorted.  So unless you have a Mountain Generator with big wire and lots of turns-in-hand, running a really wide air gap like that will lead to a burn-up if you try to shut it down at speed.  I've never seen it put out more than 20 amps shorted if it was shorted before the wind started blowing, but shorting it when it's running balls out will turn even my big generator into a glowing, smoking mass in short order.
--
Chris

Flux:
It sounds as though you have increased leakage reactance to the point where it dominates the characteristic in the same way as it does in iron cored machines.

Your air gap is not that big for those magnets but if you have a thin stator there could be a fair bit of leakage flux. Also winding turns into the area that is not normally wound may cause more armature reaction effect.

Certainly if you can get the speed up without adding extra resistance you will keep a better efficiency.

The braking effect of normal air gap axials is something that comes about mainly as a result of the resistance dominated characteristic, iron cored machines in general don't respond well to brake switches so that is to be expected.

Some will find this a problem but I use other methods of stopping the things.If it does reactance limit within the winding ratings it won't burn out even if it doesn't furl properly but you may have a lot of noise to contend with.

I have not found this phenomena with any air gap that I have tried so I can only guess that it is something to do with the extra wire you have got into the space where the hole in a conventional coil is.

It would be interesting to see the results of a bench test into a battery and into a short circuit. The same effect could be obtained with added line reactances on a conventional machine but I always assumed it was something to avoid as it seems to put iron cored machines at a disadvantage. Maybe there is a critical point where it helps and too much gives the run away characteristic of most iron cored machines.

Flux

ChrisOlson:

--- Quote from: Flux on April 25, 2010, 05:30:02 PM ---Your air gap is not that big for those magnets but if you have a thin stator there could be a fair bit of leakage flux. Also winding turns into the area that is not normally wound may cause more armature reaction effect.

--- End quote ---

I ended up with a fairly thick stator in that machine because I stuck one turn of AWG15 and three turns of AWG18 into it.  I did that because I was out of the right size of wire, and it seems to work fine.  In order to get it to fit I used a single 1/4" bolt in the bottom of the coil winder - so the hole in the coil is only 1/4" across at the bottom.  I don't remember the exact dimension at the top of the coils, but they're also pretty narrow - something like 5/8".  And then when I put the coils in the mold they were kind of a tight fit on the inside and I used the hydraulic press and two 1/2" thick steel plates to sort of smash everything into place and compress the coils.

So the stator is pretty much solid copper with 12 narrow slots (the coil holes) in it.  It ended up way more powerful than I imagined it would.  It's also very smooth running with only a humming noise in the tower structure at high output.  But even so, with the air gap at almost an inch, it doesn't stator brake all that well.  I'm going to fit a fabricator-style brake (see the pics of his 17 footer) to it when time permits so I can shut it down and keep it parked if I want.  But at this point it's not a pressing need.  I've left the thing running in pretty high winds (gusts to 60) with no problems, so far.  But I know that if I did need to shut it down and keep it shut down, it would be a problem without a mechanical brake.
--
Chris

SparWeb:
Flux,
That explanation reminds me of your long and popular explanation a few years ago, in a thread called "Matching the Load".  I can't seem to find it on the board any more (after 1/2 hour of trying out the new search tool).  Fortunately I did download a copy and filling in all the diagrams so that I would never lose it.

Volvo Farmer,
If you want I can e-mail you the text and diagrams from his thread because it was probably the best explanation of the steps to take to judge how to match blades and alternators when they don't seem to be working at the same speed.

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