15' mill question

[cdog]

Hello all, It has been a while since I have asked a stupid question so I need to get it out of my system!

My 10`mill has been working great, and I think I have the itch to build a bigger one(you buggers!!).

On the otherpower wind section the 14 and 15`mills use large wedge magnets that are very close together so it seems.

I have a bunch of 14ga wire left over from my 10`build and was wondering if there could be a suitable combination using the 14ga wire and a bunch of the 2x1x.5 inch magnets(they seem to be the most magnet for the money is why I am asking this).

The mill would need to be wound for 24 volt.

Assuming that a 15 footer would give double the power could I simply make everything double... as in 24 mags per rotor and 18 coils....

I am sure this could possibly have drawbacks but I am looking fer the bang for my buck, thanks as always,

Cdog.

[Flux]

I think your #14 wire is going to be a limitation.

If you can make sure it furls early then it will be ok.

You can manage with less magnets than the 24, things don't go op linearly. 20 magnets per disc should be perfectly ok but I would be looking at something like 2 in hand #13 wire. The #14 should be ok and make a nice machine with good results in lower winds but I wouldn't want to let it do much over the 1kW in high wind ( depends on how far you want to push your luck).

If you can get it to furl reliably at this power level then go for it.

With 16" discs you would probably need about 35 turns per coil depending on magnet grade. I doubt that the 2 in hand #14 will fill the winding space so you may end up with space between the coils.

You could probably manage something with coils with more turns, in parallel rather than series connection and this may give you more from the #14 wire but it is messy and I wouldn't consider it for 24v. To avoid circulating currents you would need a rectifier for each parallel circuit and that would mean rectifiers up the tower or lots of leads coming down.

Flux

[oztules]

If you have plenty of #14 wire, and are not committed on magnets yet, I would feel inclined to use round 2" x 1/2" magnets.

When the Dans switched from 2x1 to the rounds on the 10', I recall they almost halved the resistance.

In this case, the extra magnet could either allow less turns of 2 in hand and be happy, or maybe even allow you to use the extra space and get 3 in hand at less turns.

I think this may be a better compromise.

Using the rounds on a 14" disk,  (12 mags each), my stator seems to be more efficient than the Dans at 12"

If I look at the advertisment on Otherpower for the h/duty 48v stator we find that with the same no of the same size magnets, Danb's advert on their heavy duty stator goes like this:

"This stator is wound with #15 gage wire. It has 9 coils and each coil has 105 turns in it. " (mine has 100turns #13)

"After you rectify the output to direct current, this stator should produce 48 Volts at 140 rpm. The blades should be 10 foot in diameter." (Mine cuts in around the 120-125rpm and uses 13 footers)

"In our tests we believe this stator will be about 50% efficient at 1000 Watts. Sustained output above about 1000 Watts may overheat this stator so you should design your machine to furl before that point."... (mine barely feels warm to the touch after 5mins above 1000 and below 1500watts... 5 mins is a long time for sustained power in lightish winds)) My star resistance is .8ohms, I'm guessing theirs is about 1.2 ish

Could this be a better use of current resources?... unless he already has the magnets. I would guess that with the larger disk, his winding space would then allow him better scope to get resistance down better (comparatively .. mine is 48v)

I have placed this after Flux, in the hope he won't let me lead you astray if I'm on the wrong track with this.

..........oztules

[oztules]

Darn, I forgot, ... and still use 16" disk

.......oztules

[Flux]

I don't know how the price compares between the common rectangular magnets and the round ones for the same result.

keeping the same number of magnets, then the round ones produce a more robust machine. I am not in a position to know how the same result would compare in magnet price. Here the only sensible priced magnet is the metric 46 x 30 x 12mm and that seems to me to be possibly better than the 2 x 1 x 1/2" equivalent but again grade can alter the result. In reality I suspect they are quite similar. I am convinced that here the cost of round magnets for the same total flux would be way up.

It's possible to achieve exactly the same results by using more of the rectangular ones than round ones. It certainly isn't fair to compare rectangular against round for a given number of magnets.

Much depends on how cautions you want to be, typically running at 50% efficiency at full load seems to be accepted, it is way hotter than anything I have ever tried so I am not in a position to judge. You are relying on the mean power being well below peak and that is normally the case for all wind set ups. This is where the limitation comes down to furling and how well you can control the thing in a gale.

In terms of energy capture the very low resistance winding will be no better and probably it will be worse unless line resistance is added to drop the overall efficiency back to the 50% figure as the prop will stall badly.

The main virtue of the very low resistance winding is that you can keep the internal losses low and reduce the heat, you decide if you want to push the top limit or have a greater safety margin.

I also suspect that resistance is not the whole story, the heat you can dissipate is a function of surface area. If you got the same resistance from small coils of thinner wire as from large coils of thicker wire, I can't help think that the large coils will have a larger cooling surface and will cope better.

I am not sure that you can really put a limit on power out for a given wire size. It is typical to run wire at current densities of 3000A square inch for common electrical machines. Things like welders rated for short duty cycle may reach 5000A sq inch. These wind alternators are pushed way beyond this so it becomes a compromise between maximum output life span and how well you control the duty cycle of the full load current. Coil size, type of potting and the effects of wind cooling are all variables that interact and I am afraid much of this is not based on anything that has been carefully studied.

What works for years in one wind region may fry in the first storm on a very windy site.

Flux

[cdog]

Great replies guys, thanks a bunch.

It seems as though the 14ga wire may not be the best way to go, so in your opinions what would be the ideal 24 volt set-up?

I would like to have some safety margin and have a 60 amp morningstar tristar handle the dump load with no worries.

The whole point of the idea is to capture more juice in light winds, I really have no need for a steady 1500 watts.

Thanks again,

Cdog.

[Flux]

If you are after more energy capture in light winds then the #14 wire should be ok especially as you have it.

The only factor is that you should be able to control the thing in high winds and keep to a safe top limit. It is also reassuring to have a winding that will hold the machine stopped in any wind ( even if it will not stop it from full output)

All the machines I have ever built tend to drop power in high winds as they go hard into furling so they are safer in a very high wind than at the furling point. I get the impression from lots of comments here that others don't find this happening. It may be something to do with this idea of running fairly well stalled. In moderate winds these machines work in stall control mode and I suspect that many are fooling themselves into believing that they are furling when in reality they aren't. I have never run anything in a mode where it is stall dominated in high winds so I am not in a good position to comment, but To be safe I think you need to consider reducing the tail pivot inclination to something less than the 20deg if you want to use big sheets of plywood for tail fins.

The only real advantage of using a heavier winding in the alternator is that it will survive the 1500W and more in high winds far better and for longer. This is fine if you need that high wind power, otherwise it is largely a substitute for accurate control of furling.

There is always the chicken and egg situation. A powerful alternator will take bigger blades than you first intended in lower winds so there is the temptation to do this and capture more low wind energy. If you don't watch it you end up back in the same situation where it is fine if you can tame the high wind output.

It is less cost effective to use a monster alternator for a given size of blade, the energy capture is not going to be increased as blade size dictates that. All you do is gain ability to push the high wind power up with a margin of safety.

You know your wind conditions and from your original machine you should know how well you can control it within it's intended limits, this should give a good idea of what will happen if you scale it up.

Flux