Magnet/coil spacing?

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This is a continuation of a previous discussion here: http://fieldlines.com/board/index.php/topic,143423.msg964938.html#msg964938

I would like to know what I could do to improve the efficiency of an axial PMA I am building.
Here are two drawings.
I will include links because they do not meet board guidelines, but scaling them would cause them to probably loose too much detail.

The first:
http://www.comstockcomputing.com/Alt0001.jpg

Is how my current dual rotor axial PMA is made.
3 coils and 8 poles
5.5" rotor OD
4.25" center line for the magnets and coils
0.75" magnets
2.25" coils with a 0.75" center hole

In the top right corner, the dotted circle is an imaginary 4th 2.25" coil.  As you can see it would overlap the two adjacent coils, so if I were to go to 6 coils, I would have to make them smaller or enlarge the rotor.

The second picture:
http://www.comstockcomputing.com/Alt0002.jpg
is what the alternator would look like with six 2.0" coils.  There is a little space between each coil, but not much.

How does the alignment of the coils and magnets look in each of these drawings?
I know the 6 coil arrangement is generally better, but I don't think that it fits the magnets as well.
Does anyone have any suggestions for improvements?

[JeffD]

In 2007 I also built a very small wind turbine, a little bit smaller than yours.  For my first stator build I had also tried 1 coil per phase for a 3 phase star configured alternator.  Performance was very low since I had used 28 awg and phase resistance was high.  Each coil had 225 turns and a resistance of 5.6 ohms. The resistance across two phases was 11.2 ohms.

  I then tried two coils per phase. Each coil was 110 turns of 22 AWG for a coil resistance of 0.65 ohms.  Resistance measured between two phases (4 coils) was 2.60 ohms.

Here is a pic of the 2 coil per phase stator:


The stator is 9/32" thick.  Distance between rotors was 0.5".

My rotors were 4 inches in diameter with 8 Neos measuring 3/4" x 1/4" (actually two 1/8" stacked).

Here is a view of the rear rotor:


The Turbine blade diameter is 20.5", a little smaller than your diameter.
Here is a view of the pieces before assembly on the frame mount:


The biggest improvement was lowering the resistance of the coils by going to a thicker wire.  By using two coils per phase I could use the 22 AWG instead of the 28 AWG to make the coils fit within the area allotted.  I did have to squeeze the circular coils a bit to make them fit but it worked out ok.

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OK, that is very similar to what I am building.
I am using  0.75 x 0.25 N42 magnets.

How much power did you get out of it and at what RPM?

If you have a picture of the stator on top of 1 rotor, I would like to see it.

[jlt]

I think you may have a problem with your blades.also trying to many turns in the coils.try using three separate rectifiers one for. each coil .and then parallel the out puts.also get it up on tall tower, there is very little power at ground level

[Flux]

Use 6 coils for 8 magnets. You are wasting half the available winding space and that means wire that is far too thin.

Flux

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I know my location is FAR from ideal, but for various reasons, I'm stuck with it for now.

I agree that my blades are not very good.

My goal is to get 6 volts, 5~10 watts at 50 RPM.  I may be  a little low on the wattage, but otherwise I'm pretty close.

I tried hooking up a trailer side light to it yesterday when it was putting out about 12 volts and it shined dimly for a moment until it stopped the blades from spinning.

I think that I may have what I want if I go with 6 coils of 450 turns of 26AWG, 2 coils in parallel per phase.  
Anyone have any suggestions as to the coil/magnet spacing shown on my second drawing?
Is it correct that the ideal spacing would have opposing magnetic poles covering opposite legs of 1 coil at the same time?

[jlt]

Take a look at jeffs posts .his amount of turns is very different than yours. a  good 2' blade will turn about 600 rpm at 7mph wind. but there is only about 2 watts available,at that wind speed. Even a3ft needs about 450 to 500 rpm to keep from stalling. 

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If I build it to cut in at 7mph I'll get power for about 2 minutes every other day.
But it will spin 30 to 60 RPM for hours on end.
I know it is never going to make much power.  High wattage isn't what I'm looking for.  I would rather have a usable voltage with 1 or two watts most of the time than a burst of power when a storm blows through and nothing the rest of the time.

Flux, you are of course right, that I need to use 6 coils.

While I'm happy to discuss wire gauge, turns and blade design it does not address my primary question. 

I can't help but think that the magnets I am using should dictate the size of my magnet rotor.  Since they are 0.75" in diameter, do I want a rotor diameter that will cause the magnets to be 0.75" apart?  Then a 2.25" coil with a 0.75" center hole would completely cover 2 magnets.  That is almost what I now, except 6 coils will not fit (see the first link in my original post).

If I go for six 2" coils (same diameter rotor) the coils no longer completely cover 2 magnets (see second link in original post).  I could probably fit six 2.125" coils.  But I can't shake the feeling that there should be a formula for figuring out the appropriate rotor diameter and coil size based on the diameter and number of magnets used.  Is that just wishful thinking?

[willib]

Esc ,
your coils should be 1.5" dia., twice the magnet dia.
and your rotors for a 6 coil alternator should be 0.75 * 5 = 3.75 inches in diameter , tangent to the edge of the magnets at the outer edge. with 8 magnet poles.

[esc]

The coils should be 1.5" in diameter with a 3/4" center hole?
That only leaves 3/8" for wire in each leg.  That isn't very much...

3.75 Diameter = ~ 11.75 circumference that leaves about 1.46" for each magnet and 1.96" for each coil.  Lets say 1.5" and 2" for simplicity.
That is one magnets width between magnets and (with 1.5"  diameter coils) 0.5" of wasted space between the coils.

Somehow, that doesn't seem quite right either.

[Flux]

I didn't see your 6 coil version of the drawing. that looks perfectly sensible. Playing with the alternator probably is not going to make any real difference as long as it has a cut in speed suitable for your area.

I believe your problem is with no wind and a tiny prop. You need a gale to get much out of a 2ft diameter prop, if you are trying it in winds below 10 mph you will need a milliammeter to see any charging current. It will basically just run at cut in speed and slow to that speed until there is enough wind to speed it up.

Assuming you prop could manage a Cp of .3 ( unlikely) then you have .6W available at 5 mph and 4.6 at 10 mph. You need to remove the bearing loss from this and then factor in electrical efficiency. Added to this if it is a bad site the turbulent mess will probably produce no real power, these figures are for a clear area.

So until you get over something like 10 mph you will need some meter that can measure fractions of an amp to see the current into a 12v battery.

Flux

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So the second drawing looks reasonable?   Good.

I can't do much about the available wind, but I can install a larger prop.
There is no reason I couldn't cut out a 3' or 4' diameter "ZubWoofer".  I have 10' of 4" PVC pipe just waiting to be cut into something.
Larger blades would catch more wind...but would they slow it down?  Or are we talking about such low speeds that the larger blades wouldn't really be any slower?


I'm also considering using 0.75 x 0.5 (instead of 0.75 x 0.25) magnets and making my stator 1/2" thick (instead of 1/4").  If I did that with 6 coils I might be able to fit enough 18 AWG wire in to do some good.  But I want to experiment with what I have a little more first.