Does Southwest Windpower Have the Right Idea?

[SamoaPower]

Earlier, I asked the question about the AIR machine's rather strange alternator configuration and Flux was kind enough to straighten me out. Since then, I've spent a little more time trying to characterize this alternator in more detail.

It appears to be a 12 pole, three-phase radial flux machine with a 36 slot laminated core stator. What first caught my attention was that the coil windings for the three phases were not the same. Unequal wire size and number of turns are used for each phase. The phase configuration seems to be a modified delta where the delta is closed for DC but not for AC. A total of eight rectifiers diodes are used instead of the usual six. My conjecture is that this prevents circulating AC delta currents for this unbalanced configuration.

Flux thinks, and I now concur, that this is their attempt to match the alternator's power response to the wind's V^3 function. Indeed, they advertise that they do this. I wish I had the resources to confirm their degree of success. It would be nice if they were freer with data on performance. I heartedly agree that trying to match system components is the best approach for overall efficiency.

Normally, when I think three-phase, I expect symmetry. Applications needing three-phase AC demand it. However, battery charging is pretty forgiving about charging waveform. The battery doesn't much care if the charge source goes up and down a bit. It really looks at the average current - a giant filter.

The question that immediately comes to mind is why isn't this approach applied to the axial flux machines, so loved on this board? I don't mean the modified delta, but the idea of non-symmetrical phases. I read many posts that bemoan the fact that low cut-in speed and higher wind speed efficiency without stall seem incompatible. Sure it is - if you don't match the alternator to the wind. Has any work been done on this? What's the problem with having coils of different sizes? I would agree that a three-step compensation is pretty coarse but it's a whole lot better than zero.

[oztules]

The question that immediately comes to mind is why isn't this approach applied to the axial flux machines, so loved on this board?......the simple truth may be as awful, that no-one thought about it.

There has been a little discussion about matching the dynamic power source to the "big filter " in the battery box. but mostly involving switching in different configurations, or electronically matching it. The real reason may well have something to do with how well the current imperfect system works.

" I don't mean the modified delta," I think you must mean the modified delta.... why?

Star relies on the tempory and judicious use of the other phase legs to complete its circuit. This would get a bit trickier if we have different length and wire size legs to look through. To digress just a moment, in order to get a split phase motor to start in single phase, we wind a start field. This field is deliberatly wound with thinner wire and different turns. The reason is that the thinner wire and turns presents a different reactance to that of the run winding, and in fact giving a phase change which then gives us effectively a rotating or out of phase rotating field, in what is a single phase environment. The more current, the more the phase differential becomes.

If we translate this to our stator, in star the differing reluctance between the legs of a particular circuit of star (because of the differing turns and thickness of wire), starts going out of phase worse and worse as we draw more current. (If you think jerry wasn't happy with a 1.73 figure, imagine him if we degraded that figure further,)  

So if we change the winding length and thickness,then in star, we will derate the performance  when we want to  be getting more in larger winds.

So that leaves delta and in all probability, jerry delta. Here the circulating currents will be reduced, phase differentials which appear as the current goes up wont matter.

One thing that will have to be reckoned with is the heat sink effect that the iron stator has. The lighter coils we would use for low speed winds will get a bit warmish as the power goes up. As this happens the heavier coil will be cunducting and pushing out the major amps, but the lighter coils will drop their output over their internal resistance until they are held to the larger coils voltage.

So apart from a temperature problem being worth a look, getting people to realise that efficiency is not the be all and end all, (people chasing efficiency will be drawn to star), but that harvesting power at the lower end well, and still being able to take a fair advantage of the less frequent higher winds is more important than looking flash at a particular speed.

Purists will chew their fingernails about the vibration which could arise at the high power end of the scale due to one phase shouldering more of the power than the lesser two, but i assume that this can be held within acceptable limits.

All in all, I see no reason why we should not give it a try.

..........oztules

[henjulfox]

I have less technical knowledge and less experince than most contibutors to the board. However I can and believe do follow this theory.

The first thing that strikes me is that different guage wire would beg for the Jerry rectifier set-up.

The second is that the axial machines seem to operate near their limits regarding heat. I've seen posts metioning warm, hot, and cooked coils. I don't think I've seen a post, however, regarding heat being a problem with motor conversions. I know the motor I'm working on has the wire wrapped around and through some heavy steel and is enclosed in a nice large aluminum case with fins. Perhaps a re-wound motor conversion would be the best use of this idea.

-Henry

[DanB]

My guess is there's not a lot to be gained with this approach and the drawbacks are obvious (more heat in one or two phases at high output and vibration - and..  what I beleive is a less efficient 'delta' sort of arrangement where we rectify each phase seperately).

I did mess with this a few years ago with a two phase machine where I was trying to stack coils between one another...  it worked but had lots of other problems.

I think it's handy for SW windpower - here are the benifits for them:

it allows them to get a low cutin speed and a fairly impressive 'rated output' (in a high 30mph wind) from a tiny machine with a tiny/cheap alternator with blades that must run at very high speeds (TSR greater than 9) or they wont work at all.  In their case I think it's to save cost and to make the machine more marketable (bragging about 400 watts in high winds).

For bigger slower machines, our blades can be reasonably happy running at a TSR anywhere between 5 and 8.  Our alternators are much heavier and - if the TSR slips a bit (slows down as windspeed increases) then the alternator can do a reasonably good job of being fairly efficient over the range of windspeeds - and we can keep our star connection and avoid vibration.

I don't believe the Air 403 is doing that good a job of matching the cube of the windspeed.  They're basicly just taking a tiny alternator and winding one phase so that it can cutin in lower winds - it is costing them efficiency in higher winds though.  If we did this - in order to gain in higher winds, we'd need to build the alternator even bigger than we do now and the stator would still be getting hotter in high winds unless we shut off the 'low wind' windings.  

I think if we're really worried about maximizing output in high winds then a star delta switch makes more sense - although my opinion is that we can build a very simple thing without any switching at all - and shoot to work well in low/average winds.  Then in high winds we have plenty anyhow.  If not - it's an easy matter to build a touch larger.

[Flux]

An interesting question and one that some of us have given some thought to.

It is largely a question of how you compromise. The Air unit does undoubtedly produce a high output for its size in high winds and may be a good choice for use at sea in many areas.

It achieves this at a serious cost to its low wind performance( it doesn't have any)

The unbalanced winding compromise is one of several that makes it useless in low winds.

At start up it is effectively single phase with all the problems associated with it, in higher winds it behaves a lot more as 3 phase but still has potential problems with loss in the low speed coil. It relies on good cooling and reactance limiting to survive.

We could do similar things with axial machines but for most land based sites it is desirable to have the best low wind performance and to compromise this to any great extent is not in the best interest for maximum overall power production.

Star/ delta is a much better compromise if you want to go that way and if you can use the high power on the occasions of high winds could be well worth the trouble.

for normal battery charging with reasonable sized machines the batteries are often dumping in high winds and it may not be worth the complication.

For small machines the benefit is greater and makes more sense. It is a nice idea for manufacturers to be able to quote output figures way above their competitors and the purchaser is not likely to realise that a machine of half the rated power may well produce more energy and do it more consistently.

There is indeed a place for a simple scheme that tracks the cube law adequately especially if it can do so with high efficiency in low winds but this unbalanced winding method may not be the best starting point.

Flux

[Jerry]

oztules.

"Star relies on the tempory and judicous use of other phase legs to complete its circut."

So if one of the star phase legs is producing 10 volts it is in sires with "complete the circut" with another phas leg that is producing what voltage? Less ofcource but the 2 are never 10 volts at the same time. Even though they will produce 10 volts within a short time of each other. But never at the same time.

I'm thinking while one phase leg is at 10 volts these other phase leg it "complets the circut" will be 5.8 volts.

This is as per the answers offered to my post yesterday (thanks Zubbly) So tell me how again its better to get 15.8 volts from coils that have a potential of 10 volts.

better then wire two twenty volt coils in perelell that still equal 20 volts in perelell.

I know its been discused to death here. So no responce will help me here till after the small alt test.

As per "people chasing efficiency are drawn to star". I'm not in that group. I am chasing efficiency but by the simple math 2 times 10 should equal 20 not 17.3.

                   JK TAS Jerry

[Nando]

The CUBE Law

So easy to follow and so difficult to implement.

Difficult, because the builders "DO not want" to follow what nature says to do to be able to follow the curve.

Difficult, because the "builder" says I refuse to do it that way because it is too expensive and I make do " this way".

EASY if the proper steps are followed.

With the present technology one can produce or manufacture a GOOD and LONG Lasting Wind mill, if the proper steps are followed.

Electromechanical design MARRIED to Electronic design.

Pitch Control is more expensive YES -- but long lasting and greater energy harvester.

Wind Mill with Pitch control and high voltage generation and a good Charge controller with MPPT = = High generator efficiencies, good low cut in, excellent higher FLAT LEVEL power generation when high wind regimes are available.

South West solution trying to "circumvent" the proper steps and the poor decision of not adding a simple circuit to attain a much better performance system.

Heck I would love to re-design the AIR with the proper "follow the nature path requirements"

Nando

[SamoaPower]

Many thanks to all that have contributed so far.

The purpose of the post was not necessarily to produce a critique of The AIR machine so much as it was to address the issue of use of unbalanced phases in an axial flux machine to enhance it's match to the wind. As yet, no one has come forward who has actually done it for three phases.

Concerns raised seem to revolve around high speed efficiency and possible vibration issues. These would seem logical yet not borne out by measurement. I calculate that the power available from the wind for a 46 inch diameter air rotor (1.1 m2 swept area) at the Betz limit would be 770 watts at a wind speed of 28 mph (12.5 m/s). If we believe the claimed output for the AIR of 400 watts at that speed, it means an overall efficiency of 52%, which isn't bad at all. For an air rotor of 42% efficiency, 545 watts would be available from the air rotor and again, a 400 watt output would represent 73% alternator efficiency, again not bad. Do the axial flux machines do as well at the high end? Could they do better?

For those interested, the DC phase resistances for the 12 volt AIR Industrial, measured by the known resistor/current method, are 1.30, 0.283 and 0.173 ohms.

I certainly agree with Flux that the low-end performance is more significant. For my low wind area of 10.3 mph mean wind speed, the AIR is little more than a toy. That small 46 inch rotor only produces about 25 watts at 10 mph and 102 watts at 16 mph. Of course, actual output would be even less. Since I have two new AIR Industrials, I intend to modify one of them for better low-end performance and set them both up on a test range for comparative measurement. The modifications I'm considering are:

1.     Increasing the air rotor swept area to1.82 m2 (60 inch diameter) with simple blade extensions.
   
2.     Changing the phase configuration to star (still unbalanced).
   
3.     Replacing the eight-diode rectifier with six Schottky diodes for less loss.
   
   

Some will ask what about the high end? Frankly, I'm not much concerned about winds over 24 mph - they seldom happen here. I'll probably just shut it down above that speed.

My main interest is still the evaluation of the unbalanced stator scheme as it might apply to my contemplated large (16 ft) axial flux machine.

[oztules]

this is paul gipe's figure work on the air 403. http://www.wind-works.org/articles/air403pc.html

perhaps you may be able to find similar machines on this site who's power curve may be graphed and the slopes of the curves compared. I say slopes,because this may give you a better indication of how the gennie is tracking the winds dynamics. eg, if the slope compares favourably with what you want, then perhaps change the turns ratio to reflect earlier cut in.

"I certainly agree with Flux that the low-end performance is more significant. For my low wind area of 10.3 mph mean wind speed, the AIR is little more than a toy. That small 46 inch rotor only produces about 25 watts at 10 mph and 102 watts at 16 mph."

Looking at the figures for a prop this size, i,m not sure that flux is completly fair in saying there's no performance down low with respect to the alternator topology. There's barely any power in the wind to convert to electricity. At 10mph, it still may be getting as much as 40-50% return on the wind power available to the system. This isnt  disgraceful by any means. It says more about the swept area at this speed than the alt I would contend. At least with that system, a few more turns on the low and middle coils, may help push that part of the curve. At least it does present options without unbalancing the other higher speed collecting capability.

Still worth exploring further.

.......... oztules

[Flux]

I think the only way you will get any low wind output from the Air is to increase blade size, iron loss probably takes its output at 8 mph. With larger blades it will need an even higher tsr and that may again defeat you.

It was designed as a cheap machine to extract good power in high winds from a modified car alternator. It relies on very high speed and as such is unsuited to producing power in low winds.

I thought you were considering the concept in general rather than improving the Air in low wind.

For low wind results on a tiny machine you need a lower tsr, low cogging and more important low iron loss ( better none really). With a small iron core saturated with large neos and having to turn at high speed to achieve cut in you have no chance. If you can break it away from cog it will not pull those blades out of stall and supply its iron loss below 10 mph.

Yes I have tried unbalanced 3 phase many years ago in a different form and the conclusion I come to is that at starting and low wind it is single phase. I have never had any luck with single phase even in the case I mentioned, it did alter the slope of the power curve and match the prop better, but the overall output was lower despite the better match.

In a small machine vibration is not a significant issue and if it was the only limitation I am sure we could live with single phase for bigger ones.

Digressing for a while, it is possible to remove most of the vibration from single phase by adding an inductor on the dc side of the rectifier so that conduction is continuous rather than when peak ac exceeds battery volts. I had hoped that this would improve single phase efficiency but the gain is not worth the losses in the choke, so single phase is dead as far as I am concerned.

As Dan said, it is not that obvious that the Air thing does make much of an effort to track the cube law in terms of power out, it might be close on input power and most of the gain may come from holding the speed up on those fast blades.

For a tiny machine the way Nando hinted at will give you a far better result and it can be done with an air gap alternator with no iron loss. If you want to play with frantic high speed and noisy blades the air gap machine gives you an advantage that you will have no trouble with start up, but I have never found that tsr 9 or above will give good results in low wind.

Schottkys on an air may degrade its low wind performance by holding it harder into stall, but will help at the top end.

I have never found progressive paralleling of windings to be worthwhile, it is better to dump the low speed windings in high wind, they contribute mostly loss.

Flux

[SamoaPower]

Flux

I was afraid this misunderstanding might happen.

"I thought you were considering the concept in general rather than improving the Air in low wind."

I guess you must have missed the first and last paragraphs of my last comment. I am indeed primarily interested in the general concept. The AIR machines I have only provide a convenient exploratory test bed that already have an unbalanced stator. I don't have an axial flux machine at the moment to work with but magnets are on the way. Any improvements I may make to the AIR's are secondary to the major thrust.

My interest in the "low-end" means wind centered around 16 mph since that is the energy peak for my area. Start-up is an issue but not as significant as the 12 - 20 mph region.

You may be interested in a test I ran this afternoon. I changed the phase configuration of one of the AIR's to star and changed out the rectifiers to Schottky. Running at 500 RPM (measured), the stock machine provided 15 watts into a 2 ohm load. The modified machine did 38.3 watts under the same conditions. Tomorrow, I'll run the same test into the battery. It's the middle of the night now in the South Pacific and I don't want to wake the whole family with the racket.

Your interest and comments are appreciated.

 

[Flux]

Yes the results will be interesting, my guess at present is that it will stall.

I am not sure what you have in mind for your 16 mph wind, that seems very high for an average, I assume it is the peak point of your wind distribution.

If you can afford to set the cut in as high as 10 mph you will get a very good match and may gain more energy than choosing cut in for a lower speed. If you want to get the low end as well I would suggest that you use a dc converter over at least part of the speed range, you seem to understand these things and should be able to build something.

Can't remember what voltage you are operating at, but if it is 24v then I would set cut in for 10 mph or a bit more and use a boost converter to capture the lower winds.

For 12v it will be a bit lossy but with schottky diodes it will still be better than any unbalanced winding.

If you are up to it, design the alternator for cut in at 7 mph and use a buck converter to let the voltage rise and keep efficiency up in the higher winds.

Either way if you do it well you can hold 70% efficiency over your wind speed range up to 24 mph. If you drastically over build the alternator you may get 80% but the cost is never worth it, far cheaper to increase prop size a few inches and gain over the whole speed range.

Star delta may be an option if you have high wind days and low wind days. Here we don't and even on a high wind day the thing is constantly changing from star to delta and is rarely doing the best possible.

Flux

[Ungrounded Lightning Rod]

Are the phases really unbalanced?  Or does each phase have two winding, one with more turns of finer wire, one with fewer turns of heavier?  Are they rectified separately - two bridge rectifiers?

In the latter case, and if they're rectified separately, you'd have an alternator with a nonlinear load curve.   The high-turns, thin-wire windings would give you some power at cutin, at the cost of extra heating at higher speeds.  The heavier windings would cut in at a higher speed, providing a heavier load.  While the thinner windings would be producing a disproportionate amount of the heating, the heavy windings would be supplying most of the load so it wouldn't be all that bad.

Somewhat like the deat-wye switch hack, but less effiient and without needing active circutiry to flip relays.

And of course you wouldn't get the high circulating currents and vibration you'd get with the individual PHASES wound in distinct ways.  This is just multiple windings for EACH phase.

[Ungrounded Lightning Rod]

You probaby wouldn't want to try this with a radial flux no-core design, since the concentrated heat from the fine-wire windings at high speeds couldn't be distributed easily.  (you'd want to cut them out at high speeds, and that gets you back to active circuitry, like the star-delta switch)  But it should work just fine on a radial flux machine with a metal core and good heat dissipation - like a motor rewind.

It might be a bit cheezy.  On the other hand, it might be a cute hack.  Or both at the same time.  B-)  I'd need more info about the details of the wind to attempt making a call.

[SamoaPower]

"Yes the results will be interesting, my guess at present is that it will stall."

I agree. That's why I propose to increase the swept area by 70%.

"I am not sure what you have in mind for your 16 mph wind, that seems very high for an average, I assume it is the peak point of your wind distribution."

No, 16 mph is not the average. Here, this explains it better.

The mean annual is 10.3 mph. The yellow curve is the Rayleigh TIME distribution in units of hours/year. The light blue is the ENERGY distribution in kWh/year. It's my belief that a system should be optimized for the peak of the energy curve (green line). The other curves are for my 16 ft air rotor.

"For 12v it will be a bit lossy but with schottky diodes it will still be better than any unbalanced winding."

THIS is what I want to confirm. I haven't yet seen enough convincing evidence to believe one way or the other. Converters are okay but you have to give up 10-15% in loss and give up KISS. If an unbalanced stator can get within 10% of a converter rig, OVER MY SPEED RANGE OF INTEREST (10-24mph), I'd go with KISS and as a bonus, it wouldn't cost a penny more compared to an uncompensated system.

[Flux]

Yes that is what I suspected you meant by the wind speed. Not a bad wind area, average just over 8 mph here.

If you want to experiment with unbalanced windings then start with 6 phase, it has a wider speed range than star, almost as good an efficiency and will possibly tolerate a difference in the diametric arms. you may be able to tolerate one phase larger than the rest to lower cut in to gain that bit below 10 mph. The trick will be to make the odd phase quite weak as you will need a low slope in low winds and also that will help that phase from hogging too much load in the higher winds.

Even without any trickery you should manage to track from 10 to 24 mph with quite a good efficiency, the trick is not to aim for silly low speed cut in. If you cut in at 10 you will do quite well and if you can trick the winding to come in at 7 without messing up things at 16 mph you will be on a winner.

Flux

[oztules]

a possible workaround may be to put a small resistance (few coils of lighter guage wire) in the output line of the lighter phase. At low currents this would be fairly invisable, at higher loads as the lighter coil/s goes significantly over voltage, this may help distribute the heat away from the stator into this external loop.

Yes it brings with it losses, but they will occur when it does not become as important. as we may be wishing to shed some power at these higher levels. By judiciuos tuning, i think a fair compromise may be attained between temp, contribution to help negate blade stall and low speed output.

This may go some way to tuning up the low end whilst still keeping a lid on core heat.

........oztules