Best coil geometry for round magnets

[DanB]

You maybe taking steps backwards with iron cores, springs in the furling system, fiberglass blades made in china and wiring stators in delta.  But time will tell...  

I may be, according to popular consensus.  But then I've found in the past that popular consensus is not synonymous with wisdom and is most times arrived at thru compromise.  So I throw popular consensus out the window and look at how one of the most successful small wind turbines of all recorded time does it - the Jacobs.  The newest ones, evolved over almost a century from the 31-20 down to the little 23-10, have a delta-wired Winco generator with iron core, springs in the furling system, and fiberglass blades.  If they can do it, so can I.  I'm guessing that they know some things that compromise missed along the way.

Oh - and those Jakes also rotate left and normally furl left (although they can bend to the right too) ---- which is also wrong according to popular consensus.  LOL!
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Chris

It's great to have people like you trying stuff out and reporting to the world Chris, and preventing us getting bored and complacent.  I hope you do not mind when we 'consensus' folk drone on about the best way to do stuff.  My only concern is that homebrew people might get mislead by some of the stuff you are stating such as axial flux alternators never being good for more than 35-40% duty cycle and the weird idea that you seemed to have of parasitic currents in star/wye connected stators.  A good debate helps to bring out and clarify the reasons for our respective views.  Even if it ends up that we have to agree to disagree.

I would never claim that wooden blades are a more suitable material for a manufacturer in the USA or in China compared to wood, but I do suggest that wood is a much more suitable material for homebrew for numerous reasons - it's nicer to work with, it has better fatigue resistance, you don't need to make moulds etc etc...

As for springs, I have seen several people move from using springs to using gravity but never the other direction.  In most cases where a manufacturer uses a spring in a turbine it has been a troublespot - for example the Proven turbines still have problems with the blade springs after nearly twenty years, and the old windseekers, now called Lakotas? suffering spring failures on the tilt-up system.  Anyway it's 'just a materials problem' as the VAWT people would say.  But gravity is free of charge and very reliable.

Star and delta - what can I say?  You can always achieve the same voltage output for the same internal resistance using star rather than delta. (By adjusting the wire size and number of turns to do that.)  In star there is no possibility of parasitic currents arising within the windings (no internal circuits for them).  I and others have measured the mechanical power drain from the parasitic currents in open circuit delta alternators.  We therefore choose to avoid using delta windings.  Your experience has obviously been different.  The world is a strange place.

Iron cores work fine if you don't mind the losses in low winds and you are ready to fabricate them.  I will watch with interest to see how you get on with your iron filings.  Cores can save on magnet material, but magnets have become pretty cheap so I would still recommend to a home-brewer who wants to find the easy road to a machine that works in light winds that he/she uses an axial flux stator without cores.  I used to work with laminated cores in the old AWP turbine that I designed (and there are still ten or more around here working) and it was a breath of fresh air to escape from those cores:  The drag at low speeds, the limited maximum current that prevented effective braking in high winds, the hassle of making them...  Let's say there are pros and cons.

As for which way you furl in relation to the blade rotation - that is a bit of a refinement.  I did it wrong for thirty years without noticing any problem.  If the Jacobs yaws out of the wind slowly (as I suspect) due to excess torque rather than rapidly in response to a gust hitting the blades frontally then it makes no difference, and in any case its best to make sure you have plenty of clearance at the blade tips so it should never be an issue in the first place.

There's more than one way to build a good turbine to be sure, but some of the easier ways are already quite clearly documented and the 'problems' that you find with the 'consensus' ways to do it are not always apparent to me.  Hence my occasional impatience with the solutions.

It's interesting...  yes, I guess I have never seen a new one and I never understood about the torque/furling bit.  They are really neat machines at any rate.

[Flux]

Not sure there is much to add to this. New ideas are always welcome and in many cases new ideas are nothing more than re-invention of old ones.

Much depends on your wind resource and how much you rely on very light winds. There is no doubt that you can do dramatically better in high wind areas if you depart from things designed to work best in winds near cut in.

There seem to be several issues here as I have said before, there can be no doubt that the air gap axial machine designed for cut in at 7mph will be restricted in maximum current capability in high winds because of the inherent lack of cooling. You can devise methods to improve this and similarly it is probably easier to achieve better cooling from radial designs. If you need to make full use of winds above 30 mph and can use these high currents then modifications are likely to be a good idea. Efficiency and current capability are not the same thing and if you use one machine to load from cut in in light wind right up to producing high power in high winds then efficiency has to be lost somewhere. If the efficiency loss has to be in the stator then some better form of cooling is necessary but the inefficiency will still be there. Any attempt to raise this efficiency will stall the blades.

There is probably a strong case for a machine that will survive much higher outputs if it fails to furl, that seems to be the main virtue of motor conversions but no way are they more efficient, they get rid of the internal heat better but you get less effective power into the battery but it is again clouded by the different type of blade matching.

Efficiency is again tied up with duty cycle and you can manage a far higher duty cycle with a high efficiency alternator but the lower your efficiency the better you will need to cool it.

Wind doesn't give 100% duty cycle or anywhere near it so as long as you stay within the limits of the alternator it doesn't have to be rated that way. Commercial large alternators for high speed engine ratings can't be run at the low efficiencies of these wind machines for the duty cycle they use. You can go so far with forced cooling even using hydrogen but in the end the losses have to be kept small. Their type of loading is impossible over a speed range into a voltage clamp.

In the end I suspect it is suitability for the application rather than efficiency that we are really chasing and if you want lots of power from a small alternator then there are indeed many ways worth following.

If you can do better with a radial or with a delta winding then that is fine as long as it doesn't impact on low wind performance if you must have those few watts in light winds. If a few watts on low wind days are not important then you can drastically improve the existing designs by going for a higher cut in speed. In fact in the end speed is the fundamental issue here, whatever you do the machine becomes bigger heavier and more expensive for a given rating as you drop the input speed. Before the days of neo magnets low speed alternators were massive for a given rating, so much so that gearboxes were common.

As Hugh and Dan have implied the slotted iron core is not the best approach for best low wind results. I have no experience of powdered cores, they made  no inroad in conventional machines but may offer some promise here, the loss will certainly be lower, it remains to see what flux densities they will carry and how much magnet you save over an air gap design and whether the old reactance limiting comes back to haunt you. I suspect it may be a viable idea as long as you avoid teeth. Once you avoid teeth then iron cored machines are very satisfactory with core losses way below the conventional.

I look forward to Chris's experiments, I know it has been tried before but I have seen no really useful figures to base anything on. Radial will be easier to wind effectively and easier to cool. As for the choice of star or delta that is a personal one. For machines operating on mains there is no real difference in performance, motors are available for choices of voltage with star or delta with identical ratings.

There are other issues when feeding a clamped rectifier load and conventional practice may not have been evaluated by many people. It does seem strange that the biggest market for battery charging alternators has virtually standardised on star, I would have thought the car alternator market would have brought to light any real virtue of delta if there was one, but again a car alternator is built for cost size and rating and efficiency is very secondary, a bigger fan is cheap .

There has been a lot of interesting discussion here, no doubt useless to the person who asked the question and as usual there is no real conclusion except that high wind machines may not be the same as low wind ones. I can't contribute any more so it stops here as far as I am concerned.

Flux

[ChrisOlson]

My only concern is that homebrew people might get mislead by some of the stuff you are stating such as axial flux alternators never being good for more than 35-40% duty cycle and the weird idea that you seemed to have of parasitic currents in star/wye connected stators.

Hugh, that is based on looking at the design current and temperature capacity of the winding wire used, and cooling capability, as compared to commercially available units of the same output capacity, but built with much more efficient use of materials.  This is what I mean by popular consensus is often arrived at thru compromise.  You have gone for ease of construction your designs, I prefer to go for a balance of better technology and design for a slight tradeoff in the increased tooling and skills required to build it.
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Chris

[scoraigwind]

My only concern is that homebrew people might get mislead by some of the stuff you are stating such as axial flux alternators never being good for more than 35-40% duty cycle and the weird idea that you seemed to have of parasitic currents in star/wye connected stators.

Hugh, that is based on looking at the design current and temperature capacity of the winding wire used, and cooling capability, as compared to commercially available units of the same output capacity, but built with much more efficient use of materials.  This is what I mean by popular consensus is often arrived at thru compromise.  You have gone for ease of construction your designs, I prefer to go for a balance of better technology and design for a slight tradeoff in the increased tooling and skills required to build it.
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Chris

Which (of my many) current design specifically are you refering to then, and under what exact ambient conditions do you arrive at this duty cycle?

thanks.

[jimovonz]

as compared to commercially available units of the same output capacity

Chris, I am interested in what commercial units you are refering to. It would be handy to have a comparison of commercially available alternators that would be a direct substitute for the dual rotor axial flux alts we make here - especially if they are significantly better. I have not had much luck finding anything that is much better that what I can make - price or performance wise. Sure commercial units invariably manage to use less magnet by substituting more iron but this comes at the expense of performance (iron loss) and complexity. Either way as far as I can tell, any suitably matched alt is either going to have good efficiency at the top end but cut in late or make good power at the bottom end and suffer at the top - due to the miss match in the shape of the power curves.  There was a chap (finsawyer) here on the board some time ago that put some effort into designing an alt that had a power curve that was closer to that of the wind. Perhaps there was potential in his design but unfortunately he seemed to be much more into the theory of it rather than a practical application. As far as I know he never did convince anyone to build one. Surprisingly enough I managed to find a diary entry of his regarding this: http://fieldlines.com/board/index.php/topic,128224.0.html (is it just me or is the search function here not as useful as it could be?) I do think that there is room to further optimise the design of the alts we use - particularly in the area of cost (making the best possible use of your magnet/copper), but I think these will be incremental improvements and not the great jumps you are implying. The long awaited MPPT controller is the one thing I can think of that will make the sort of difference you are looking for.

[dyslexicbloke]

How did this post ever get this long?
Coils over magnets don't make voltage …. Current paths, wires, perpendicular to a moving magnetic field do.
Your end game is to get the maximum flux to intersect with the maximum amount, length, of conductor at the maximum cutting (relative) speed ….. anything is smoke and mirrors.
In resdpect of rotor dimentions ...
magnets too close ....shorted flux.  Magnets too spaced - wasted space .... coil legs, the active two sides, at magnet pitch centers regardless of magnet dimentions.

Fill in the intermidiate space with aditional coil legs ... polyphase ... 3 is well acceptexd for good reason.

Its all about flux dencity interacting with current path dencity.

Not easy but fun ....
I will explain further if anyone wants me to ....

[ChrisOlson]

Which (of my many) current design specifically are you refering to then, and under what exact ambient conditions do you arrive at this duty cycle?

Hugh, please refer to ANSI/NEMA standard MG-1 for motors and generators.  The standard ambient used to rate motors and generators is 40° C (104° F).  I have tested these fiberglass generators against the published standards in MG-1, including temperature rise limitations under full load, etc., and they need some work.  I can build one, as I am sure you can too, that will meet the standard but it ends up being a massive waste of materials due to the cooling limitations of the design.

I don't wish to argue the point - I am confident a better generator can be built.  And I am going to continue experimenting along those lines.
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Chris

[willib]

I and others have measured the mechanical power drain from the parasitic currents in open circuit delta alternators.

Not on mine ..
They spin as freely in delta as they do in star. Open circuit.

[scoraigwind]

Which (of my many) current design specifically are you refering to then, and under what exact ambient conditions do you arrive at this duty cycle?

Hugh, please refer to ANSI/NEMA standard MG-1 for motors and generators.  The standard ambient used to rate motors and generators is 40° C (104° F).  I have tested these fiberglass generators against the published standards in MG-1, including temperature rise limitations under full load, etc., and they need some work.  I can build one, as I am sure you can too, that will meet the standard but it ends up being a massive waste of materials due to the cooling limitations of the design.

I don't wish to argue the point - I am confident a better generator can be built.  And I am going to continue experimenting along those lines.
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Chris

My turbines are designed to work in wind, and when the wind blows hard enough to generate power it also blows hard enough to cool the stator so that the rated power output can be obtained as an average.  Surges up to double or treble for brief periods are acceptable.  If you test them in a workshop at 40 degrees ambient then yes they will overheat.  They are not designed for that situation.

I used to make a lot of compromises in my designs when I was forced to use existing machines and modify them.  Now that I can design them myself I don't feel that there are so many compromises.  The hardest thing is that the more efficient you make the alternator, the worse it will match the blades for power and speed, so this puts a limit on the process of making alternators more efficient in high winds unless you can do MPPT like Flux.  So I do compromise over the tip speed ratio matching.  But I do not see this as an alternator design challenge.  It's more of a power conversion challenge.

A lot of hot air gets expended on this board over the issues of stator cooling.  Yes, some of the early designs did have under-sized stators (my 2005 manual had a poorly designed 12 footer) and suffered issues with burn-outs on some sites, and this started a big debate about ways to cool the stator.  But you do not have to build a very big stator to overcome the problem.  I have a lot of worse problems to worry about, so I wonder why this issue of stator cooling rumbles on endlessly like this.  My main focus right now is on preventing magnet corrosion.  If I wanted to squeeze more power out of the alternator in high winds I would not be changing the alternator design, I would be looking at power conversion options to allow the voltage to rise.  This improves the speed matching for the blades at the same time as it reduces the current in the stator, so there is no need to fret about cooling issues down that road either.  But to be honest I am never that excited by high power outputs in strong winds.  My main concern is low wind performance and I reckon the axial flux alternator with no core is likely to be the best option for that.  No compromises whatsoever there.

[ChrisOlson]

But I do not see this as an alternator design challenge.  It's more of a power conversion challenge.

All you have to do is use a delta winding.  The delta winding doesn't build volts as fast as wye for a corresponding increase in rotor rpm.  It keeps the rotor in a more ideal TSR range from low speed to high speed with higher electrical efficiency to-boot.  I have absolutely zero problems with "drag" or cut-in on my delta machines that other people have described, which leads me to believe that if people have had problems with this it is related to poor design and layout of the geometry of the unit.

And cooling IS an issue.  These generators are mounted in a relative "dead" air zone.  If the rotor is doing its job properly it slows the oncoming wind, and further, the generator is mounted inline with the prop hub.  Cooling is not an issue if you use massively big materials in the generator so it doesn't get hot in the first place.  But like I said in another post someplace, that's like using a 10 lb maul to drive a 10 penny nail.  If engineers used pure cross section and mass to solve every stress and cooling problem in design, everything around you would be incredibly expensive, including the alternator in your car.

There's obviously more than one way to skin a cat here.  Like I also said in another post someplace, if there's a mousetrap I always look for ways to make that mousetrap smaller, lighter, faster and more efficient - and still pin that mouse under its bail just as effectively.....and use less cheese to lure him in to-boot.
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Chris

[joestue]

But I do not see this as an alternator design challenge.  It's more of a power conversion challenge.

All you have to do is use a delta winding.  The delta winding doesn't build volts as fast as wye for a corresponding increase in rotor rpm. 

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Chris

I'm liking this new physics.

on the topic of delta waveforms not summing to zero, that's entirely up to the design. you may not notice 5% thd but some of these trapezoidal magnets deliver a nice fat trapezoidal waveform.

[scoraigwind]

But I do not see this as an alternator design challenge.  It's more of a power conversion challenge.

All you have to do is use a delta winding.  The delta winding doesn't build volts as fast as wye for a corresponding increase in rotor rpm. 

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Chris

I'm liking this new physics.

on the topic of delta waveforms not summing to zero, that's entirely up to the design. you may not notice 5% thd but some of these trapezoidal magnets deliver a nice fat trapezoidal waveform.

I don't follow this new physics at all.  But hey what do I know?

As far as I know, the voltage from a delta winding is 1/root(3) times the voltage from a star winding and the resistance is 1/3.  So you get less voltage and you can push more current.  The open circuit voltage is proportional to speed, same as with star and if you use more turns of thinner wire then you can get almost exactly the same performance as a star winding.  The only difference is that there are circuits that can allow parasitic current if the waveform is not a sine wave so it risks some losses there.  No upsides that I can see but the physics is new to me like I said. 

[DanB]

If one must be stuck with either star, or delta, I don't understand a good reason to use delta except for teh fact that in some cases it allows for winding with thinner/easier to handle wire.

I am not sure on this next part - but on very large machines, or very low voltage machines I'm forced to wind with sometimes 3 or more strands of wire in hand.  Multiple strands within the same coil are parallel connections and I do notice parasitic losses (not a big deal - but noticeable drag on the alternator) when I do that.  It could be that for those machines, a delta connection would work as well, or better...  at the very least it would be easier to wind the coils.   I really don't know on this for sure. 

For somebody who is keen to optimize things and make life more complicated, switching from Star to Delta at some point at 'medium' wind speeds may make tons of sense.  So long as it's reasonably inexpensive to do, and reliable.

[ChrisOlson]

If one must be stuck with either star, or delta, I don't understand a good reason to use delta except for teh fact that in some cases it allows for winding with thinner/easier to handle wire.

I am not sure on this next part - but on very large machines, or very low voltage machines I'm forced to wind with sometimes 3 or more strands of wire in hand.

What I have found is that the power requirement (input power) curve on delta wound generators is not as steep for the same increase in rpm compared to a similar unit wound wye.  I don't know why this is because when you look at watts output they are very close to the same with the delta having a slight advantage.  But on my 10 foot machine that I recently put a new delta generator in, it cuts in at the same rpm as the old wye one did but at 20 mph it's running at 7 TSR instead of 6 - and it puts more power down the pole than it did with the wye generator.  Plus I used one size smaller wire in it for the delta unit - AWG 16 instead of AWG 15.

On my 13 footer I used a different approach.  I wound that one for a wye/delta switch so it has too few turns to cut in at a decent speed in delta but too many turns for the right cut-in in wye.  So it starts putting power down the pole at 5-5.5 mph and by the time the wind picks up to ~12 or so it has to be switched to delta.  When it switches to delta it's pretty much like pressing the nitrous button in your drag car.  Because it's actually under-wound for delta it spins at 8.5 TSR until it gets in the 20-22 mph wind speed range where it comes down to around 7.5.  But it's still running too fast.

As far as I'm concerned the jury is still out on this deal - I like the way I did the 10 because it's hassle free - it just runs in delta and it performs very well from cut-in to full throttle.  The other one is a bit of a hassle because it needs an automatic way to switch it back and forth.
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Chris

[Flux]

I will just reply to Dan's point.

There is an issue with slotelss machines with thick section wire, this was known in the era when they were tried commercially and for large machine it was necessary to build the copper sections from transposed wire strands and they were pressed into the shape of rectangular bars. When slotted cores were adopted the flux snapped from tooth to tooth and the bars were linked quickly, there was no longer a normal limit on copper section. Only in multi-megawatt alternators was it found necessary to revert back to transposed copper bars.

We are only playing with very tiny machines but even so we easily reach the point where thick copper wires suffer eddy loss, fortunately it is very close to the point where the wire becomes very difficult to wind anyway for such small coils.

Winding two strands in hand usually causes no trouble at all but with a lot of strands in hand the flux linkage per strand is sufficiently different for circulating currents to take place. Ideally you need transposed conductors or litz wire but in small machines it tends to waste a lot of space.

Certainly at 12v there are issues building star machines if you use all coils in series, it can still be done but you need to consider things that don't normally matter. Delta lets you use a bit thinner wire and may make life a bit easier. I have tended to divide the star winding into parallel sections with only 3 coils per section ( one coil in series) and parallel the sections with separate rectifiers.

I actually believe that this is a case where IRP ( Jerry) makes much more sense. Whatever you do you have lots of leads to deal with and lots of parallel rectifiers.

Flux

[joestue]

But I do not see this as an alternator design challenge.  It's more of a power conversion challenge.

All you have to do is use a delta winding.  The delta winding doesn't build volts as fast as wye for a corresponding increase in rotor rpm. 

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Chris

I'm liking this new physics.

on the topic of delta waveforms not summing to zero, that's entirely up to the design. you may not notice 5% thd but some of these trapezoidal magnets deliver a nice fat trapezoidal waveform.

I don't follow this new physics at all.  But hey what do I know?

As far as I know, the voltage from a delta winding is 1/root(3) times the voltage from a star winding and the resistance is 1/3.  So you get less voltage and you can push more current.  The open circuit voltage is proportional to speed, same as with star and if you use more turns of thinner wire then you can get almost exactly the same performance as a star winding.  The only difference is that there are circuits that can allow parasitic current if the waveform is not a sine wave so it risks some losses there.  No upsides that I can see but the physics is new to me like I said. 

the voltage from a delta winding is 1 and current 1.73; relative to a star winding at 1.73 and current is 1. its exactly the same thing.
if you prefer to use delta so you have more turns of thinner wire it makes no difference to me.

What I have found is that the power requirement (input power) curve on delta wound generators is not as steep for the same increase in rpm compared to a similar unit wound wye.  I don't know why this is because when you look at watts output they are very close to the same with the delta having a slight advantage.  But on my 10 foot machine that I recently put a new delta generator in, it cuts in at the same rpm as the old wye one did but at 20 mph it's running at 7 TSR instead of 6 - and it puts more power down the pole than it did with the wye generator.  Plus I used one size smaller wire in it for the delta unit - AWG 16 instead of AWG 15.

cuts in at the same rpm, but less copper = more resistance, which would explain the tsr 7 at 20 mph.
your blades generate more power at tsr 7 than they do at 6, which more than compensates for the higher resistance.

[scoraigwind]

For a delta winding to have the same cut-in speed as a wye or star winding, it needs to have 173% of the number of turns/coil.  So I would have expected the wire size to go at least 2 places smaller - from 15 to 17 rather than to 16.  Ratio of 15 to 16 is only 126% (in terms of wire cross sectional area) so actually the resistance of the (fat) delta winding is lower than the wye/star in this case, if the star used 15 and the delta used 16 with 73% more turns.  Chris does not tell us the number of turns though, and I wonder if it is 173% of the number used in the star phases?  If it is less then this would explain the higher speed.  But the cut-in would not be the same, in which case the whole comparison is dubious.

[joestue]

heh
had that backwards.

doesn't explain the higher tsr however.

[ChrisOlson]

For a delta winding to have the same cut-in speed as a wye or star winding, it needs to have 173% of the number of turns/coil.

I had 45 turns in the star wired gen, the delta unit has 70 turns.  Same magnets, same air gap.  The cut-in speed is identical and the number of turns was based on a running a test coil in the air gap.  It is not a 1.73:1 ratio on the number of turns required in this gen, even though that's theory.  And the first test coil I ran in it had 77 turns and got too much voltage.

There was a basic change in the coil shape, however, and I'm told by you guys that doesn't make any difference.  The way I had it before was very similar to DanB's layouts with bar magnets.  The way I have it now is what I call an "Ed Lenz" design that makes better use of space and fills voids with copper, but using the same bar magnets.  It is obviously more efficiently using the available flux and not wasting magnet area.
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Chris

[joestue]

I had 45 turns in the star wired gen, the delta unit has 70 turns.  Same magnets, same air gap.  The cut-in speed is identical and the number of turns was based on a running a test coil in the air gap.  It is not a 1.73:1 ratio on the number of turns required in this gen, even though that's theory.  And the first test coil I ran in it had 77 turns and got too much voltage.

There was a basic change in the coil shape, however, and I'm told by you guys that doesn't make any difference.  The way I had it before was very similar to DanB's layouts with bar magnets.  The way I have it now is what I call an "Ed Lenz" design that makes better use of space and fills voids with copper, but using the same bar magnets.  It is obviously more efficiently using the available flux and not wasting magnet area.
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Chris

You are comparing two machines that have had 3 variables changed. There is no way i'm going to accept that one variable is better than the other given the other two have changed. it would be trivial to run said tests with a three phase wye-delta isolation transformer feeding the rectifier. switch the coils and flip the transformer around and see what difference there is/isn't.

[ChrisOlson]

You are comparing two machines that have had 3 variables changed. There is no way i'm going to accept that one variable is better than the other given the other two have changed. it would be trivial to run said tests with a three phase wye-delta isolation transformer feeding the rectifier. switch the coils and flip the transformer around and see what difference there is/isn't.

With AWG 15 the internal generator resistance on this unit was .62 ohm in star with 50.6% power efficiency.  With AWG 16 the internal resistance came in at .34 ohm with 56.5% power efficiency in delta.  Hugh says I should've used AWG 17 instead (but AWG 16 fit fine in the space I had).  But........calculating the use of AWG 17, and knowing how much wire I used in it, with the same turns the internal resistance would come in at .43 ohm and I have no way to know what the power efficiency would be because I've never built that and tested it.  But even with AWG 17 the internal resistance of the generator winding is less in delta than it is in star with AWG 15.  So I suspect it's going to be more efficient.

I've seen it claimed here that if you make the generator too efficient it will stall the rotor.  This is pure horse-hockey.  The blades are going to make the same shaft power at a given wind speed and TSR no matter what.  The more efficient generator, properly matched, will convert more of that shaft power to electricity and it won't stall it.

And that's the whole point here - I got a better match with delta thru the prop's total speed range than I did with star.  You combine that with the efficiency advantage of the delta winding and you get more power down the pole.

A lot of people don't like delta generators because of the dreaded "circulating currents" and there must be a phobia with putting more turns of smaller wire in the stator for some folks.  But I'm just not seeing justification for this fear of using a delta winding.  My machines perform MUCH better since I've gone to delta generators than they did in star.  And granted, I made some changes to the coil geometry when I built them.  But in this thread I've learned that you can use virtually any coil shape you want, overlap legs, don't have to pay attention to not having two poles over one leg at the same time - and none of it makes any difference anyway.  But unfortunately, I'm not buying it.  It is simply NOT what I have found in building and testing these things.

Maybe with star-wound generators you can get away with anything and it works.  I suspect DanB and Hugh have done WAY more testing along those lines than I have.  But my interest in all this was getting a delta winding to work because of the inherent efficiency advantages.  And after fiddling with it, trying a bunch of different things, and getting input from people who have made it work, I've come to the conclusion that you can't wrap wire any shape you want and pass big discs over it and expect a delta winding to work right.
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Chris

[DanB]

Oh dear...
Hi again Chris 

I've seen it claimed here that if you make the generator too efficient it will stall the rotor.  This is pure horse-hockey.  The blades are going to make the same shaft power at a given wind speed and TSR no matter what.  The more efficient generator, properly matched, will convert more of that shaft power to electricity and it won't stall it.

Properly matched is the key here.  Before I did post the power curves (they must be several pages back by now) of a couple alternators, showing one that is 'too powerful' for 10' blades.  I could've perhaps put it even more obnoxiously as 'too efficient'.  In other words, the cutin speed is just about dead right, but the power curve climbs so steeply that even a 'perfect' set of 10' blades will not be able  to put enough power into the shaft to satisfy the power that the alternator will put out at the rpm those blades woudl have to be running to maintain a reasonable TSR.  So - that particular alternator can only work well if we introduce inefficiency by adding resistance down stream... or perhaps some sort of clever MPPT control.

We could consider the extreme case.  We used to actually own a small company that built superconducting materials.  I once built the same alternator and wound the coils just the same (it was wired in star) with the same number of turns per coil and everything... but instead of copper wire I wound the coils with superconducting wire and cooled the stator with liquid nitrogen.
(all those tubes were a real pain and the cost of the liquid nitrogen started to add up after a few days so I took this machine down) The cutin speed was just right (about 140 rpm) for the 10' blades, but the damned thing took an 'infinite amount of additional torque'  (infinite) to get above 140 rpm.  So the wind would come up...  and the blades would spin right up to to 140 rpm but then after that they just could not go any faster at all, no matter how hard the wind blew, they would stay at exactly 140 rpm.
That alternator was *way too efficient* for 10' blades - even worse than 'alternator 2' in my earlier posting with the graph.

Just kidding... I never built such a machine really, but the theory there is right...  that is the extreme case of an alternator that is too efficient.  For a gas powered generator you really can't be too efficient because you're only looking at 1 speed to run at - and if it's got an electromagnetic field, short of field losses - you can have efficiency quite high because you can play with the flux density.  But on a PMA that's directly clamped to a certain batttery voltage, there *has* to be a certain amount of inefficiency in the alternator or else you will stall the blades and it really just has to be about dead right.  You know all this stuff but again, you posted the quote above and I think it's a bit wrong.   We rely on a certain amount of resistance in the system (either in the stator, or the line) to match the engine (the blades) to the battery.

[bob g]

i have been following this topic with some interest, and now have a question

(or two or three)

lets take for example an ideal alternator, 100% efficient (i know one does not exist but bear with me)

this alternator will consume 1000watts of mechanical  power to produce 1000 watts of electrical power?

if another alternator is only 50% efficient, then for the same 1000watt mechanical input power it will produce
500watts electrical and 500 watts of heat?  (neglecting windage, etc)

so how does a lower efficient alternator take less power from the rotor and allow it to spin up faster and not stall
as quickly?

in example #1 the 100% efficient alternator it produces 1000 watts for the 1000 watts of input

in example #2 the 50% alternator still produces 1000 watts (500 electrical + 500 heat) for the same 1000 watts of input

so both should require the same input power to produce the same output power, its just that example #1 produces more electrical
power and no heat, and example #2 produces half the electrical power with the other half going to heat.

so i don't see how decreasing efficiency via increased resistance which makes more heat relates to there being a reduced need for power
from the rotor blades allowing them to run up faster?

there has to be something else in play here, and i am missing it?

bob g

[ChrisOlson]

But on a PMA that's directly clamped to a certain batttery voltage, there *has* to be a certain amount of inefficiency in the alternator or else you will stall the blades and it really just has to be about dead right.

Right - and that's my point too - I got it dead on with my 10 footer and I am extremely pleased with that unit.  I went overboard with the 13 and I got a problem on my hands with that one - it runs way too fast.  The third one I built - the one with the flippy tail on it and the rear mounted gen - was a revert to what I used in the 10 and that one works really good too.  But I'm still flying it with 9.4 foot WindMax blades.  I got my new 13 foot blades the other day but haven't put them on yet.  I'm hoping to get that done this weekend, close the air gap down and see how close of a match I got to a 13 with that one.

But regardless, I'm not seeing the "bogging down" with a delta generator vs what they do with a star.  For some reason I have had to use slightly less turns when I run a test coil for a delta build, as compared to what it takes in star x 1.73.  Maybe you can explain to me what's going on here - would it have to do with the fact that the copper density is higher or something?  Wider coil legs?  Whatever it is, it takes less than the 1.73x turns to get the right cut-in.

I'm just wishing right now that I would've done my 13 different - I still haven't heard from that guy that has that star/delta controller that I talked to you about.  That thing might fix the problem I got with it, and it might just prove to be more problems again.  But I'm about ready to rip that generator out and do another one that's more like the 10 footer because I'm sick of it.  I got it running in delta today and it only puts out power about half the time because the wind isn't strong enough to keep it wound up.  If I throw it to star it slows way down and just does a trickle charge when it could be putting out 20 amps in the stronger gusts.  That's an example of having about the poorest match you could possibly have - but it was an experiment, hoping I could come up with an innovative way to do an automatic star/delta switch  
--
Chris

[Flux]

This all comes back to matching the prop.

At any spot the best result will be at the peak of the prop tsr curve and with the highest possible efficiency, there is no issue here, if you make your 100% efficient alternator and load it at this point then that is the nearest to betz than you can get.

This is where things have gone off the rails from the start. This ideal situation only works at one point unless you have a variable voltage generator to let the prop track the peak of the curve at all wind speeds.

You can pick any point on the power curve and match it correctly and you will get the best possible result with the highest alternator efficiency.

What you can't do is run a variable voltage alternator into a clamped rectifier and keep the prop on the peak of its curve over any significant range of wind speed.

If you choose the best match at low wind near cut in speed you will seriously compromise the high wind end, if the alternator is efficient then the prop will stall and you loose from input power. If you make the alternator inefficient you can keep the peak prop power but mainly make heat. There is a compromise and you have to base this compromise on the wind speed that you value most.

Almost invariably people go for too low a cut in, the normal fix is to increase air gap, this increases cut in speed and gets the prop away from stall, a small departure from the ideal cut in in low winds will make only a small difference in the low wind but the improvement in higher wind is very considerable.

If you go far enough to get the best high wind results the prop can't go fast enough in low winds. Once you have optimised the cut in speed ( first logical step) then if you are still stalling in high wind then you will do better to drop the electrical efficiency and more than make up for it in prop output.

If you must have the best of both worlds then you meed to change the voltage of the alternator with wind speed.

I am not re entering this debate about star and delta and coil shape simply because the things are not scientifically compared, there are far too many variables and assumptions are being made about changes that are influenced by factors not considered. I can't deny that at any ideal load point the highest electrical efficiency will give best output but that will not be over a range of speeds.

Wind measurements and power curve plotting is a nightmare and fraught with troubles, it would take months with very special data monitoring to prove or disprove these points. Bench efficiency or load tests will clear upthe electrical side of the debate, that's easy, the wind bit is little more than inspired guesswork.

Flux

[ChrisOlson]

If you must have the best of both worlds then you meed to change the voltage of the alternator with wind speed.

That's what I (am) trying to do with my 13 footer and it ends up being a nightmare too.  I'm going to tally the Doc Wattson readings after a month of operation using manual star/delta switching to change the voltage at a certain wind speed.  But my gut guess at this point is that it's going to make less total kWh than if I had left it alone.  Without an automatic way to do it I'm seeing situations like today where I can't baby sit the thing and it ends up spinning in a 10 mph breeze making no power, but puts out good power when the wind picks up.  If I switch it the other way (to star) it puts out power all the time but goes into stall when the wind picks up and I lose total kWh output that it could be making in the higher wind.

So far, I've tried a shaft speed governor to switch it and that was too problematic with too many moving parts, and a switch on the tail but that didn't work very good because as soon as it switches from star to delta it unloads the prop, the tail swings back and it switches back to star.  Hugh suggested putting a pilot coil in the stator, which is a good idea but I didn't wind it with one.  This fellow had an electronic controller that I'd like to get my hands on to try:
http://www.dsgnspec.com/StarDelta.html

I've emailed him and he never answers back.  So I'm back to the good old toggle switch that has to be flipped manually.
--
Chris

[kurt]

that guy used to post to the old old otherpower.com message board all the info on that switch is on that board that sadly the owners of this site took down the archive of that board a couple years ago do to security reasons the format the the archive was in was a large security risk for the rest of the server it was on. i doubt any of the images/schematics etc. would still be there anyway as they were hosted off site and it was up to each individual user to find hosting for his photos on that board.

edit: i doubt if he ever open sourced the programing for the chip so even if the old old board archives were available i doubt it would do you any good.

but these days a picaxe or such chip should be able to do the same thing and i hear they are pretty easy to program never messed with them myself.

[ChrisOlson]

but these days a picaxe or such chip should be able to do the same thing and i hear they are pretty easy to program never messed with them myself.

Sadly, when it comes to programming any chips, I'm totally lost.  In fact, I couldn't build the circuit board either if it can't be built with a chunk of steel and the welder.
--
Chris

[scoraigwind]

You are comparing two machines that have had 3 variables changed. There is no way i'm going to accept that one variable is better than the other given the other two have changed. it would be trivial to run said tests with a three phase wye-delta isolation transformer feeding the rectifier. switch the coils and flip the transformer around and see what difference there is/isn't.

With AWG 15 the internal generator resistance on this unit was .62 ohm in star with 50.6% power efficiency.  With AWG 16 the internal resistance came in at .34 ohm with 56.5% power efficiency in delta.  Hugh says I should've used AWG 17 instead (but AWG 16 fit fine in the space I had).  But........calculating the use of AWG 17, and knowing how much wire I used in it, with the same turns the internal resistance would come in at .43 ohm and I have no way to know what the power efficiency would be because I've never built that and tested it.  But even with AWG 17 the internal resistance of the generator winding is less in delta than it is in star with AWG 15.  So I suspect it's going to be more efficient.

I've seen it claimed here that if you make the generator too efficient it will stall the rotor.  This is pure horse-hockey.  The blades are going to make the same shaft power at a given wind speed and TSR no matter what.  The more efficient generator, properly matched, will convert more of that shaft power to electricity and it won't stall it.

And that's the whole point here - I got a better match with delta thru the prop's total speed range than I did with star.  You combine that with the efficiency advantage of the delta winding and you get more power down the pole.

A lot of people don't like delta generators because of the dreaded "circulating currents" and there must be a phobia with putting more turns of smaller wire in the stator for some folks.  But I'm just not seeing justification for this fear of using a delta winding.  My machines perform MUCH better since I've gone to delta generators than they did in star.  And granted, I made some changes to the coil geometry when I built them.  But in this thread I've learned that you can use virtually any coil shape you want, overlap legs, don't have to pay attention to not having two poles over one leg at the same time - and none of it makes any difference anyway.  But unfortunately, I'm not buying it.  It is simply NOT what I have found in building and testing these things.

Maybe with star-wound generators you can get away with anything and it works.  I suspect DanB and Hugh have done WAY more testing along those lines than I have.  But my interest in all this was getting a delta winding to work because of the inherent efficiency advantages.  And after fiddling with it, trying a bunch of different things, and getting input from people who have made it work, I've come to the conclusion that you can't wrap wire any shape you want and pass big discs over it and expect a delta winding to work right.
--
Chris

I get a sense of going around in circles here so I am pretty much ready to give up.  I am not sure if anybody is learning anything now.  Chris's tests could be very revealing if he changed one thing at a time but as it is I can make no sense of them.  It's not true to say that coil shape does not matter - it does affect the output voltage - but there are numerous tradeoffs between the possible choices of good shapes (and there are numerous bad shapes too).    Some shapes will give slightly more voltage and some will have slightly lower resistance, for the same number of turns.  Often what you gain on the roundabouts you lose on the swings.  Waveform can also be a critical issue if you choose to connect in delta.

I honestly do not believe there is a radical improvement in efficiency to be gained by fiddling around with coil shape and certainly none to be got by going from star to delta (with the same cut-in speed).  If I am wrong on this then I'd like an explanation of the 'new physics' otherwise I am none the wiser to be honest.  I do know that it's very easy to improve the efficiency quite radically by just making the whole thing a bit bigger so if I need a more efficient alternator that's what I am inclined to do.  No big mystery there.

I am depressed that Chris still says that rotor stalling is 'horse hockey' because it highlights the lack of communication, and that's why I am ready to give up trying to explain this nuance.  Flux and DanB have both been stressing this aspect from the beginning and I agree with them.   I will give it one more shot.

If you have a perfect generator with no resistance at all then it will run at a fixed speed regardless of power output.  However the blades need to run at a range of speeds.  As the wind gets stronger the blades have to run faster or they will stall.  If you have a super-efficient alternator that is still running at cut in speed when the wind is up to 30 mph then the blades will be stalling badly.  If that's too hard to understand then I really don't know what to say.  Calling it horse hockey will not make it go away - it's one of the most basic lessons of wind turbine design.

So you cannot make a very efficient wind turbine just by making very efficient blades and a very efficient alternator.  You will either have to compromise on efficiency somewhere or use some kind of voltage conversion device.   For battery charging you are pretty much stuck at one voltage but the priority is to get good efficiency in low winds - in high winds you tend to be dumping anyway.  If you are heating or grid connected then you will do well to use a variable voltage load so as to optimise both the blades and the alternator.  This matching process tends to be more important than the efficiency of the individual parts of the machine.  I have often seen an improvement in output by deliberately reducing the efficiency of an alternator that cuts in at too low speed.  It's not whizzkid engineering but it brings me in the amphours and so I am content.

[Flux]

Star/delta switching does in fact give worthwhile results but it will need to be switched automatically and even then you will spend a lot of time in the wrong mode. In the ideal world we would have high wind days and low wind days and a manual switch would be fine.

There are a few low wind days when it would be best in star and a few high wind ones that were best in delta but on most days the wind goes through cyclic variations and the thing will need to change often to be much use. This is why I gave up on the method and went to progressive methods that change volts smoothly with speed.

Switching from speed works fine, my first attempt was with a centrifugal switch and it worked fine but it was a high speed device on a geared alternator, low speed ones will be messy. The switch needs to snap from one mode to the other or it will chatter and burn.

The 2955 tacho chip gives an easy way to make a speed sensitive switch sensing the alternator frequency and I used that in later versions. I would imagine you could persuade a board member with electronic skills to build one, it's really very simple. If I was in your part of the world I would build you one but it makes sense to get one locally.

Once you get a decent automatic switch working you will probably find you need to settle on the best high wind cut in in delta for best compromise. If you have good results in delta without the switch I suspect your cut in in star is too high for best low wind over a reasonable range. Similarly reducing turns will make the delta performance even better in high winds. I tended to find the best switch point was in the 12/14 mph region and you get a very good high end match with dramatically better high wind output.

I tried sensing from voltage and current and it can be done but speed is the most satisfactory as it is inherently stable.

Flux

[joestue]

here's some speculation.

odd harmonics suppress output voltage on wye, and on delta cause circulating current.

but we're not letting the terminal voltage get high enough to cause current to flow through the other coils.
so it's quite possible that the delta connection, or god forbid rectify each separately results in higher efficiency than a wye connection.

[Flux]

Certainly getting into the unknown here.

If the phase voltage is a perfect sine wave then into a resistive load star and delta will be identical and there will be no circulation in delta.

If the phase volts have harmonics then the line voltage will be a sine in star so if you check the relative cut in for star 7 delta ( allowing for the root 3 factor)
then the dc volts may be suppressed in relation to the delta case. If you close the delta with a non sine phase voltage then harmonic current will circulate on no load.

When you change to a clamped rectifier load the waveform is distorted beyond recognition, in delta harmonics will circulate even if the open circuit volts was a sine. It is virtually certain that the star case will also suffer losses from a change in form factor due to the chopped load.

I have always found the efficiency lower into a rectifier and battery than into a pure resistance load, I suspect this is inevitable. I failed to see any significant improvement with low distortion rectifier circuits but there were other factors at play during these tests so that is not conclusive.

Even so on all bench tests I have done there is not a great deal of difference between star and delta on load, if anything delta is lower efficiency but may give a few more watts out. If you are not worried about losses at cut in use either but in low winds you will do better with star.

IRP removes the circulating harmonics at cut in but they come back when the rectifier conducts. In that respect for start up it is better than delta.

On load I found it to be mid way between star and delta for normal loads, being a bit more efficient than delta. Some discussion with Jerry in the past hinted that when you push the machine down on its knees with very low efficiency the IRP scheme does have benefits but I can't consider running at such low efficiencies. Certainly the rectifier conduction mode will be different from star or delta.

Anyway from my tests these effects are small and only imply that delta is not wise in low winds unless you take precautions to have a near perfect sine wave below cut in.

In other respects there is something to be said for 6 phase star, the rectifier conduction changes from the single phase case at cut in to near 3 phase case at full load. The volts at cut in are 2 x phase volts but revert to root 3 in higher winds.

The overall efficiency is a bit lower than 3 phase star but the curved power curve could easily more than compensate for it. You can change the characteristics from parallel star to 6 phase simply by linking the neutral points. I haven't done much with this because it happened at the time when I changed to some forms of mppt but at best I suspect 6 phase is a better bet for direct connection and I doubt that it would prove worth removing the link to get back to parallel star but you certainly have this option. I have no need for delta, any virtues it seem to posses areas a result of not a true comparison with star and it comes from blade matching gains.  If you wind a star machine and it has a higher resistance than the equivalent delta then there is something wrong with it and direct comparison is not valid.

For a given winding the resistance will be 1/3 of the star value in delta but id you wind to allow the voltage ratio it will have the same resistance.

Flux
 

[ChrisOlson]

I am depressed that Chris still says that rotor stalling is 'horse hockey'

Well, I have to apologize Hugh, because many times I don't explain myself well.  I certainly don't want you to be depressed   

I guess my point was that matching the load to available shaft power is very important to optimize output of your turbine.  Using the star/delta switch is one way to do that.  I really like the sound of this setup flux has on his turbine but I would not have the slightest clue on how to go about building such a thing.  If someone here with the appropriate skills could build one of these things flux has and sell them to other members it would probably be one of the greatest advancements in enhancing output of fixed pitch turbines yet.

So, due to my poor skills at communicating what I mean, it didn't come out right.  I was just saying that increasing the efficiency of your generator is not a bad thing - and if you match that increased efficiency with the available power you will get more power down the pole.  There is no doubt you have to wind and build for peak efficiency at the average wind speed on your site and accept the compromise on either side of that.  But that's why this thing flux has is so interesting to me.
--
Chris