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Building A New Stator
By egreen, Section Wind Posted on Mon Nov 02, 2009 at 12:35:31 PM MST
Not what I expected...

Current turbine stats: 5.5ft 3 wood blade HAWT, dual 8" rotors, 16 neo magnets (2" x 0.5" x 0.5") per rotor, 12 coils (3 phase, 4 coils per phase, #16 wire, 72 turns/coil).
My current stator is a little bit larger than 0.75" thick because of the 72 turns of #16 wire per coil and the small 8" rotors. Each coil produces about 1.6vac (stator is 12vdc). Because of the small diameter of the rotors and the thickness of the wire I made the coil centers so that they were slightly smaller than the dimensions of the face of the magnets.
I came across a spool of #22 magnet wire so I began testing some coils for a new thinner stator. Because of the existing coils, I figured that I'd need at least 72 turns to get the 1.6vac out of the coil. However because the wire is much thinner I was able to make the center of the coil quite a bit larger than the size of the magnets and I was also able to close the air gap from 1" to just about 0.5625". Turns out that with the new coil dimensions and the smaller gap the 72 turns of #22 produced close to 2.9vac. What is going on here? At 2.9vac I could go 24vdc instead of 12vdc (2.9 x 4 x 1.73 x 1.4) - 1.5 = 26.6vdc. With another #22 coil having the same dimensions and at 45 turns instead of 72 turns I get the same 1.6vac that I got from the #16 coil with 72 turns and a smaller center. Is the size of the center of the coil making the difference? The smaller gap should only result in more current not more voltage, correct?
The RPM that I ran the tests were always between 220rpm and 230rpm. I have a digital tachometer so I know that speeds are consistent between coil tests.
How much current can I safely put through the #22 wire if I pot the stator using poly resin and talc? I put 6amps of current through the coil and it became VERY hot after about 1 minute. Almost too hot to pick up. I'm just not sure how hot is too hot for a coil cast in resin.

Building A New Stator | 6 comments (6 topical)

Re: Building A New Stator (3.00 / 0) (#1)
by Flux on Mon Nov 02, 2009 at 01:06:40 PM MST

Halving your air gap has virtually doubled the flux in the gap. The snag is that your thin wire has a much lower current handling capacity and will have far more resistance.

You gain from a lower cut in but your overall power out will be way lower.

You have probably not gained from the increase of hole size, the gain has been form flux increase.

Not sure what your original problem is, I certainly don't see why you are doing this unless you want to change to 24v. The current rating is going to be way down on the #16 wire at 12v.

Without knowing how it performs now I can't help much, but just getting more volts is not going to solve anything except possibly for a slightly better performance at cut in.

You really don't have a lot of magnet to handle a 5ft 6" prop so choose a winding that cuts in about 300 rpm for a fairly normal tsr prop.

If your tests are at 230 rpm then your cut in is a bit low unless it is a slow prop but you are not far from the mark.

If you just want to change to 12v then just double the turns with #19 wire or go for a bit thinner stator with a smaller air gap ( 1/2" stator perhaps) with less than double the turns of #19.

Using #22 is going to get you up to a 48v stator for the same sort of power out.

Flux

Re: Building A New Stator (3.00 / 0) (#3)
by Ungrounded Lightning Rod on Mon Nov 02, 2009 at 03:16:53 PM MST

Agree that almost all of the improvement is from the increased flux.

However, the turns in the center on the old coil resulted in some cancellation - so they approximately might as well not have been there. Comparing the turn count of that coil to the count on the new one where all the turns are outside the magnet hole is apples-oranges. Compare the turns on the new one to the fraction of the turns outside the hole in the original coil. (Remember that the field spreads a bit, too, so the "hole" should actually be slightly broader than the pole.)

Finally, narrowing the coil sides steepens the waveform, increasing the fraction of it where the voltage is max (at the cost of narrowing it so the area remains the same). This will show up as a slightly higher voltage on some meters. (The actual peak may be slightly higher, too, if the curve of the field strength over the poles is humped rather than having a flat plateau as wide as the side of the coil.)

[ Parent ]

Re: Building A New Stator (3.00 / 0) (#4)
by egreen on Tue Nov 10, 2009 at 09:06:24 PM MST

OMG, I can post a reply again!

There actually isn't a problem with the current stator. I just wanted to play around with some other stator configurations to see what kind of improvements I may be able to make. I like to tinker :) I just figured that since I've never seen more than ~200w from this turbine for very short periods of time in moderate winds that perhaps #16 wire is overkill, which would mean that the current gap that I have is unnecessarily large and wasting much of the flux that my small magnets are producing.

I'm not in a high wind area (averages only about 10 to 12 mph) so 230rpm has worked pretty well for me. I could try a bit higher rpm I guess. The blades have a tsr of 6. I'm not trying to lower the cut-in at all.

I'd change to 24v except then I'm not sure what I'd do with my 12v solar panels :)

I didn't realize that by halving the gap that the voltage would increase. I assumed that the voltage was just a function of the number of turns. I thought that the gap affects the amount of current. If I'm effectively cutting the gap in half should the current be twice as much?

If I try 2 in hand #22 (#19) with 40 turns (12v) then I can make the gap almost half the size it was before and get twice the current (if I can get twice the current from halving the gap) than I'm getting now with the #16 wire. So I guess the question is, will the increased resistance and heat of the #19 cancel out the gains of the increased voltage and current due to the smaller gap?

[ Parent ]

Re: Building A New Stator (3.00 / 0) (#2)
by DanB on Mon Nov 02, 2009 at 01:27:43 PM MST

I expect the #16 wire should handle 15 amps once cast in the stator. I wouldn't expect #22 AWG wire to stand up to much more than 3 (maybe 4).

Re: Building A New Stator (3.00 / 0) (#5)
by SnickersFS on Sun Nov 15, 2009 at 09:43:45 PM MST

@egreen

What is the area of your coil in the original machine?

Based on what you have written, I assume the area to be 1.875" squared.
Convert to meters squared, .0012.

72 turns * B * .0012 / .3 seconds = 1.6v
Solve for B @ Webmath

B=5.5T

72*5.5T*.0012/.3=1.6v

Only change your coil. Currently it is 15 turns deep and 5 wide.
Instead, make the coil 5 turns deep and 15 wide.
Will change the size of the rotor though.
This will give you an area of 7 square inches or .0045 sq meters.

72*5.5T*.0045/.3 = 6.0vac

That will make it a 48v wind generator.

Food for thought,

SnickersFS

Re: Building A New Stator (3.00 / 0) (#6)
by egreen on Thu Nov 19, 2009 at 12:44:22 PM MST

I finished the new stator using the #22 wire. I wound the coils with 2 in hand which effectively results in #19 wire. I used 40 turns per coil to maintain a similar cut-in as the previous stator (~230 RPM). After casting the coils, the new stator is less than 1/2 inch thick. Now that the gap between the rotors is about 9/16 inch the current output seems to rise faster than it did with the gap at 1 inch with the previous 12V stator (~0.75 inch thick w/ #16 wire @ 72 turns per coil). With the new stator mounted I ran the alternator using a hand drill for a few minutes with the current around 6 amps and didn't feel much heating from the stator casting. I did notice that when I short out the alternator phases that the new stator has MUCH better braking. I'm going to put the turbine back up this weekend with the new stator and see how it performs with the wind instead of my drill. Hopefully the new stator will be more efficient (if I can keep it from buring up).

Building A New Stator | 6 comments (6 topical)

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