Author Topic: Please discuss stalling and adding resistance to the line.  (Read 38149 times)

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Volvo farmer

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Please discuss stalling and adding resistance to the line.
« on: April 24, 2010, 03:24:02 PM »
I've installed new round-magnet rotors and a "heavy duty" stator on my DanB 10 footer. I had burned out the previous stator in a pretty intense wind storm last fall.  I think this stator is wound with 55 turns of #15. I think my old stator was wound with #14 but I'm not sure how many turns.

So I got to watch this thing in pretty decent ~20mph wind yesterday, and the performance is dismal. This is a 24V machine and I was seeing 6-8 amps sustained with peaks of 10-12 amps. It also seems to fly much slower than my old machine with the 1x2 rectangular magnet rotors and old stator.

So I have read a lot of Flux's ideas on this subject and my understanding of the dilema is this:

I can have a very well behaved machine that operates mostly in stall and doesn't make a lot of power. The alternator is too powerful for the blades and the blades won't turn fast enough to break free of stall. Because of this, I would likely never burn out another stator and furling isn't all that important.

Conversely, I can have a machine that performs well in light winds, that the blades will break out of stall early and operate in lift. This machine needs to furl early because the blades will turn very fast in heavy winds and build lots of heat in the stator, and maybe burn it out.

Now, I think I can improve my dismal performance by adding resistance to the line, but my understanding is that as I move this direction, I also move towards a direction where it is easier to burn out a stator... So it's a balancing act.  However, resistance in  the line is better than resistance in the stator, because some of the heat will go into the line. Please feel free to correct my thinking if I have it wrong.

So... My system is as follows: 24V, danB 10 footer, about 200 ft from the turbine head to the batteries, all #8.  My questions are: What is an easy way to add resistance to the line? What range of resistance should I be experimenting with? How do I know when I have it right?

Last question... What does the air gap have to do with this, and should I play with that? or just add resistance to the line?

Thanks! ;D


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ChrisOlson

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Re: Please discuss stalling and adding resistance to the line.
« Reply #1 on: April 24, 2010, 05:20:38 PM »
Now, I think I can improve my dismal performance by adding resistance to the line, but my understanding is that as I move this direction, I also move towards a direction where it is easier to burn out a stator

I got a 13 footer that (did) the same thing.  I tried adding resistors to the line and that definitely woke it up but I didn't get any more power to the batteries  - all I got was hot resistors.  So I took the resistors out and threw them in the junk drawer.  Then I opened up the air gap until it started to fly.  Now it makes power - into the batteries.  I think I ended up at about .850 air gap and she starts folding up at 1,400 watts and gets fully folded up at 1,600-1,700.

Opening the air gap didn't make one bit of difference at low wind speeds, except it maybe cuts in a 1/2 mph later is all.  With the air gap opened up it spins up quicker and makes about the same power as it did before, running in heavy stall at under 15 mph.  Above 15 mph it's a power pumper now.

It's also a lot noisier than it was before - those blades sing at 20-25 mph wind.  One bad thing about opening up the air gap is that it doesn't shut down very well if I short it.  But it has run in 40 mph winds with the air gap opened up and is well-behaved so I'm too worried about it.
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ChrisOlson

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Re: Please discuss stalling and adding resistance to the line.
« Reply #2 on: April 24, 2010, 05:56:32 PM »
What range of resistance should I be experimenting with? How do I know when I have it right?

Here's what I did:
I took a 2 x 6 about 24" long and drilled three holes in each end of it and put bolts thru the holes.  I stretched common, everyday hardware springs between the bolts, end to end on the board.  I hooked the three legs from the turbine to one end of the board at the tower base junction, and hooked the underground line to the other end with jumper wires.

Then I let it fly.  The resistance end to end on the springs was about 1.7 ohm (the smaller the wire diameter in the springs the more resistance you'll have) and that was too much.  So I moved the jumper wires up the springs about 3/4's of the way and that's where it seemed to fly the best.  Going closer to the end where the drop cord was hooked (less resistance) put it back into stall, going the other way from about 3/4's of the way took power away from the batteries and got the springs really hot.

It's a cheap, quick and simple variable three-phase resistor that you can adjust "on the fly".  If you find the exact place where it seems to work the best, just measure the resistance from the start of the spring to where you hooked on with the jumper wires and that's the resistance you need for a permanent resistor setup.

And BTW - use leather gloves when moving the jumper wires - you'll get a little "tingle" if you happen to touch two legs at once with bare hands if the turbine is running balls out.  Don't ask me how I know but it was hard to crack open a Bud Lite for about two hours after that ordeal without getting suds all over the place.
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« Last Edit: April 24, 2010, 06:38:06 PM by ChrisOlson »

Flux

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Re: Please discuss stalling and adding resistance to the line.
« Reply #3 on: April 25, 2010, 01:39:54 AM »
The first thing is to get the cut in speed right to get the best match in sensible winds. If cut in is too slow you will stall at all speeds.

If you open the air gap too much it will hit the low end performance but the first thing is to open the gap just to the point where your cut in becomes marginal for the lowest winds you get useful power from. It is probably pointless cutting in below 7mph as there is little energy available.

Opening gap doesn't effect electrical efficiency, you are just moving the alternator up on the blade curve where it should be if you got the design right.

Once you have chosen your optimum cut in for your wind area then all that you described applies. If it still stalls then you will get it out by adding resistance. The resistance does affect system efficiency so don't use it as a means of dealing with too low a cut in but once the cut in is right it is your only trick to avoid stall for a simple scheme. It doesn't cause much loss in the 15mph region if you get the cut in right and it will make it far better in the high winds.

Remember that the final temperature is a function of stator current and that is not affected by the added resistance. You will have gained though compared with an under powered alternator where the matching resistance is part of the stator itself and consequently would have been wound with thinner wire.

I would prefer to add the resistance if needed to get the better higher wind performance, once it is out of stall you can prove it is furling and get the furling right and have the best of everything.

As Chris pointed out, if you go up in cut in speed too far for the size of alternator by opening the gap you can no longer guarantee it will remain stalled in the high winds. If it won't brake to a stop in very high winds I suspect it will pull through stall when running in some critical wind speed which you won't have checked the furling at.

Flux

Volvo farmer

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Re: Please discuss stalling and adding resistance to the line.
« Reply #4 on: April 25, 2010, 06:17:50 AM »
Thanks to both of you for the replies. I will attempt to raise the cut in speed by increasing the air gap first.  I will also post back with my results.  Thanks again for the advice!
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Flux

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Re: Please discuss stalling and adding resistance to the line.
« Reply #5 on: April 25, 2010, 09:19:06 AM »
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

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Re: Please discuss stalling and adding resistance to the line.
« Reply #6 on: April 25, 2010, 11:56:30 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.

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.
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Flux

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Re: Please discuss stalling and adding resistance to the line.
« Reply #7 on: April 25, 2010, 03:30:02 PM »
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

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Re: Please discuss stalling and adding resistance to the line.
« Reply #8 on: April 25, 2010, 03:52:51 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.

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.
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SparWeb

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Re: Please discuss stalling and adding resistance to the line.
« Reply #9 on: April 25, 2010, 10:44:21 PM »
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.

No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

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Tinbendr

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Re: Please discuss stalling and adding resistance to the line.
« Reply #10 on: April 26, 2010, 05:42:16 AM »
Flux,
That explanation reminds me of your long and popular explanation a few years ago, in a thread called "Matching the Load".
I found it.

Matching the load

(I had to look at all of Flux's posts and work my way back.  Couldn't find it by searching.)
Nothing is foolproof because fools are so ingenious!

SparWeb

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Re: Please discuss stalling and adding resistance to the line.
« Reply #11 on: April 26, 2010, 08:49:05 PM »
Thanks Tinbender!
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

scoraigwind

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Re: Please discuss stalling and adding resistance to the line.
« Reply #12 on: May 03, 2010, 01:08:17 AM »
Hi everyone, I have not checked in for a while but I see this board is still humming along!

I first encountered this stalling problem back in 2003 shortly after I started to use Neodymium magnets.  Before that time I always tried to design alternators so the machine would cut in at low windspeed and blades would run at their best speed in low winds.  As the wind got stronger, the speed of the alternator would rise pretty fast  allowing the blades to continue running at a nice speed.  Ideally the blade speed should be roughly proportional to windspeed so as to keep the blades at their most productive.  This worked out pretty well using the older ferrite magnets because the alternators were less efficient than they are now.  They needed more speed to produce more volts to overcome the internal resistance of the many turns of thin wire that I had to put into them to cut in at low speeds.  The maximum power was rather lower and the braking torque was pathetic but I have to say that there are quite a few of those older ferrite alternators still working around here and they do the job cheaply and without corrosion headaches.

So back in 2003 I designed my 8 foot turbine to cut in at about 150 rpm and it worked beautifully in light winds.  But with the new strong magnets and low resistance windings, the speed did not change much at all as the wind got stronger and so the blades stalled.  Stalling is when the blade speed is too low compared to the wind speed and so the angle that the wind hits the blade is too large and it fails to get a good grip of the wind.  Blades are nice and quiet but output is pathetic.  (Higher battery voltage helps to counteract this by allowing the blades to run faster.)

Anyway at that time I posted some suggestions for fixing the stalling issue here http://www.scoraigwind.com/axialplans/update.htm

There are basically 4 options:
"1. One is to increase the alternator rpm. This can be done by increasing the space between magnet rotors so that there is a larger gap each side of the stator. Or wind the stator with fewer turns in each coil. This prevents stall but it will also mean a higher cut in rpm, resulting in some slight loss of performance in low winds (around 3 m/s or 7 mph). These are both good, simple solutions.

2. You can also increase the range of speed of the alternator, by increase the circuit resistance. Using a longer or thinner cable is one way to do this. This allows you to put the machine further away without excessive cost. Choose cables sizes (on page 40) for 30% loss at 500 watts. Or use a resistor (12V=0.15ohms, 24 volts=0.6 ohms, 48 volts = 2.4 ohms). These values are only suggestions. The resistor needs to be able to handle 500 watts when the machine output is 700 watts. Maybe you could use it to heat water? It may seems crazy to burn off power, but it improves the blade efficiency over the whole range, has very little impact on efficiency in low winds and is a relatively simple solution too.

3. Or you can put a larger diameter set of blades on the machine say 2.7 metres/9 feet. Scale up all the dimensions (length, width, drop and thickness). Or use the dimensions lower down this page.   I only recommend going up to 9 feet if you seriously want a larger wind turbine. I have tried it and it seems to work well. But there may be extra loads on everything due to the larger rotor.

4. Finally you can use some sort of electronic voltage converter between the input from the windmill and the battery. This is only for electronic wizards. this promises to be the only 'real' solution but it is still under development and I worry about the consequences for cost and reliability."
 
I basically adopted the first option and designed the machine to cut in at 200 rpm and have not really had a problem with stalling since then.  However it can happen if the battery voltage is rather low and somebody makes a turbine with a very small air gap, and uses short thick transmission wiring, and is not aware of the potential problem.

The issue that you have with burning out the stator, Volvofarmer is really not an essential consequence of getting good power output.  The furling tail can and should work well to protect the turbine when it is running at a good speed for power production.  It is true that a machine that stalls will protect itself better from overload, but that doesn't mean that it has to stall in order to furl properly.  You can make adjustments to the tail hinge angle and the tail weight and get a good furling behaviour.  If it's not working out then increase the offset of the alternator on the frame and reduce the tail hinge angle and make sure that the tail is large, and both light and strong.

I hope this helps.

Hugh


Hugh Piggott scoraigwind.co.uk

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Re: Please discuss stalling and adding resistance to the line.
« Reply #13 on: May 03, 2010, 02:54:01 AM »
Has anyone tried using diodes in series to raise the apparent voltage the mill sees or at least parallel them with the resistor to limit the maximum voltage drop?   The approximate .7V forward drop of each diode would seem to give an easy way to fine tune the cut in speed.

Flux

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Re: Please discuss stalling and adding resistance to the line.
« Reply #14 on: May 03, 2010, 03:08:18 AM »
To get the necessary rise in speed you need something approaching half battery volts across the line resistance.

With a 12v mill you may be able to clamp the maximum resistor drop with diodes but it becomes a bit impractical with higher voltages. 12v machines in general are less inclined to stall with the already existing diode drop and the high currents in the connecting leads making the resistive loss quite high so it may be worth trying a few clamp diodes across any added resistor.

Flux

Volvo farmer

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Re: Please discuss stalling and adding resistance to the line.
« Reply #15 on: May 03, 2010, 06:48:14 AM »
Thanks for the clarification Hugh.  I believe I'm actually starting to understand the way these machines work. :)

In my particular case, I wish I had saved some money  as I ran my whole 200 ft run in #6 copper. I opened up my air gap and my machine runs better now.  I am still not real happy about how it performs at lower battery voltage in moderate winds.  at 25 volts, the machine struggles to push over 10 amps, even in 15-20 MPH winds.  Once the batteries get up to 28 volts though, I am seeing good power in the same winds, 15-20 amps with peaks to 30-40.

I'm drifting the topic a bit here, but having watched my machine a little, I think I might need to work on furling. I have watched the ammeter go to 40-50 amps at 29 volts and have yet to see it fully folded up. I wonder if the paint I put on the tail made it too heavy. 

Since changing tail offsets and angles appear to be modifications that need cutting and welding,  I think my first course of action is to build a lighter tail. After that, I think I'll open up the air gap a little more and see what happens.  I'm not too concerned about losing 50 watts in 7 mph winds and manually adding resistance seems to be a bit of a pain.




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ghurd

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Re: Please discuss stalling and adding resistance to the line.
« Reply #16 on: May 03, 2010, 07:03:37 AM »
at 25 volts, the machine struggles to push over 10 amps, even in 15-20 MPH winds.  Once the batteries get up to 28 volts though, I am seeing good power in the same winds, 15-20 amps with peaks to 30-40.

manually adding resistance seems to be a bit of a pain.

That may be a candidate for adding a diode drop?
Take the 25V up to 25.7V.

Let me know if you want to try that.  I think I have some serious industrial  "diode packages" around here somewhere.
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Re: Please discuss stalling and adding resistance to the line.
« Reply #17 on: May 03, 2010, 10:40:08 AM »


That may be a candidate for adding a diode drop?
Take the 25V up to 25.7V.

Let me know if you want to try that.  I think I have some serious industrial  "diode packages" around here somewhere.
G-

VF;

Yeah, I got some nice Siemens "semipacpack" diode packs from Dean "Drives" and I would certainly send you one no problem. Number SKKD 46/16. I think I have a couple of these but they need a heatsink.

I think that means  46 amp 1600 volt and it has 2 diodes in it.

Let me know and send your address in a private message and I will send it this week. I will not likely need them and would rather they get used than tossed when they clean my shop after the inevitable...

Tom
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Volvo farmer

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Re: Please discuss stalling and adding resistance to the line.
« Reply #18 on: May 03, 2010, 10:50:22 AM »
Thanks for the offers. Let me cogitate on it a while.

It really takes about 27V before this thing "wakes up" I just don't know how helpful .7V would be

Also, at 46A, I'd be a little worried about letting the magic smoke out of those diodes in a big gust.

Less bark, more wag.

scoraigwind

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Re: Please discuss stalling and adding resistance to the line.
« Reply #19 on: May 19, 2010, 12:30:22 AM »
I don't see the advantage of using diodes compared to using a simple resistance.  In both cases you are simply losing power.  But with a resistance, you lose less power in low winds when the turbine is not stalling and you do not need a voltage drop  (and you really want it to be efficient).  As the wind increases you might need a voltage increase to raise the speed and avoid stall. The resistance gives you that increased voltage.  The resistance gives you variable voltage which is the ideal thing to match blade speed to wind speed.  The only downside is that more and more of the power goes into heating instead of into the battery.  With a grid-tie inverter you can adjust the DC voltage and use that power.  With a battery charger & resistance you simply see more heat in the series load.  But battery charging systems often dump power into heat in stronger winds so that's not such a big problem and maybe the heat is doing something useful.

Having said all of this I rarely resort to anything like this.  I prefer to set the gap so the speed is high enough to prevent stall, and there is no problem after that.
Hugh Piggott scoraigwind.co.uk

dyslexicbloke

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Possible alternator solution.
« Reply #20 on: July 15, 2010, 08:28:50 AM »
I am about to build a new alternator, single halbatch rotor with 60 poles on a 470mm dia marine ply disk.
This not a high power machine, only about 1.6m swept area but the theory would scale.

The idea is to use many small magnets with relatively few turns over each pole and the stator divided into 4 sets of 15 poles.
The rational is to vary the config of the 4 stator sections depending on the wind available.

Light wind - all sections in series - 60 pole alt - maximum available voltage.
Medium wind - parallel series pares of 30 poles - half the voltage, half the resistance.
Strong wind - all 4 in parallel - minimum voltage / maximum current.

I hope to be able to extract a small amount of power from light wind whilst still making good use of strong wind because I will effectively be changing the characteristics of the alternator to match the conditions.

Any comments welcome
Al

TomW

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Re: Possible alternator solution.
« Reply #21 on: July 15, 2010, 08:43:01 AM »
I am about to build a new alternator, single halbatch rotor with 60 poles on a 470mm dia marine ply disk.
This not a high power machine, only about 1.6m swept area but the theory would scale.

The idea is to use many small magnets with relatively few turns over each pole and the stator divided into 4 sets of 15 poles.
The rational is to vary the config of the 4 stator sections depending on the wind available.

Light wind - all sections in series - 60 pole alt - maximum available voltage.
Medium wind - parallel series pares of 30 poles - half the voltage, half the resistance.
Strong wind - all 4 in parallel - minimum voltage / maximum current.

I hope to be able to extract a small amount of power from light wind whilst still making good use of strong wind because I will effectively be changing the characteristics of the alternator to match the conditions.

Any comments welcome
Al


My comment is:

This is a classic example of a thread hijacking. It has nothing to do with the original post.

Please refrain from doing this.

Thanks.

Tom
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dyslexicbloke

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I am sorry ....
« Reply #22 on: July 15, 2010, 08:50:01 AM »
I thought the discussion was about adding impedance to perform alternator matching ....
Granted the impedance change is within the alternator if you were to follow my thought process but I did not intend to hijack anything ...
Please remove my post and I will start a new thread to discuss this topic.

Sorry
Al

rogeriko

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Re: Please discuss stalling and adding resistance to the line.
« Reply #23 on: August 10, 2010, 04:45:08 PM »
I have added 2 1 ohm resistors to a 48 volt turbine and have seen quite a remarkable increase in power. I was thinking this just increases the voltage at the turbine so why not just add another 2 volt cell instead. 48v SMA inverters run on any battery voltage up to 60 volts , selectable on startup. Now where can i find one more cell???  Maybe everyone should have 26v systems to get these turbines spinning instead of 24v most inverters work fine up to 32volts no problem just have to limit battery voltage 2v higher than normal.
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scoraigwind

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Re: I am sorry ....
« Reply #24 on: August 20, 2010, 03:24:01 AM »
I thought the discussion was about adding impedance to perform alternator matching ....
Granted the impedance change is within the alternator if you were to follow my thought process but I did not intend to hijack anything ...
Please remove my post and I will start a new thread to discuss this topic.

Sorry
Al

I agree that the thread is about matching, and I did find the post to be on topic myself.  Certainly did not deserve to be shouted down like that.

I don't think its a great idea because the speed changes involved are too drastic though.  You are looking for changes in speed/impedance of around 20-50% rather than doubling.  Even the star/delta 73% change is a bit radical.  You need smaller steps.
Hugh Piggott scoraigwind.co.uk

sbotsford

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Re: Please discuss stalling and adding resistance to the line.
« Reply #25 on: January 07, 2011, 01:17:01 PM »
I'm a novice at this.

In an ideal world as the wind speed rose, you would continue to produce the same voltage, but the available current would rise.

In practice you get a rising voltage, and the resulting current also rises.  Power is disapated in the stator by I^2R and you end up with molten copper and that lovely eau de burnt plastic.

A car alternator uses a powered field.  In older cars the voltage regulator works by sensing the output voltage of the alternator, and opening the contacts when the voltage is too high, closes them when it's too low.  The impedance of the coils on the rotor was such that it was opening and closing a few times per second only.

By adjusting the field current, you adjusted the output voltage.  I'm sure with modern electronics there is a better way to do this.

Measuring the turbine speed isn't sufficient.  For a given wind it could be in stall, or flying.

So your field controller has to have two inputs:  The current wind speed, and the current turbine speed.

To get current wind speed, you need an anemometer.  This just a tiny free wheeling turbine that swings a small magnet by a small coil, producing a pulse every rotation.  Since it's small, you can make it withstand whatever wind your tower stands.

Now your field controller compares the speed of the anemometer to the speed of the turbine.  That in turn is used to adjust the alternator field.

This means you're making an alternator instead of a generator -- TWO sets of coils to wind.  And there is waste power for the field coils.  However you could make a bastard compromise with magnets and coils.  The coils are used to reduce the field when it gets going too fast. 

Am I making any sense?

ghurd

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Re: Please discuss stalling and adding resistance to the line.
« Reply #26 on: January 07, 2011, 01:26:40 PM »
Reducing the field when it gets going too fast will result in more speed.
G-
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