Questions about Stators

[redfoxx]

Why are the coils all laid out flat horizontally compared to coils in the stator of an alternator which are vertical and stacked on top of each other like bricks? Probably a fairly simple question as I am very new to all this but just can't seem to find the answer. Thanks for any input

[ChrisOlson]

Why are the coils all laid out flat horizontally compared to coils in the stator of an alternator which are vertical and stacked on top of each other like bricks? Probably a fairly simple question as I am very new to all this but just can't seem to find the answer. Thanks for any input

Because the generators most people build for wind turbines are called axial flux, meaning the magnetic lines of flux do their work in an axial fashion from the generator rotor(s). An automotive alternator is radial flux, meaning the lines of flux are in a radial fashion from the rotor.

It should be pointed out that not all axial flux generators use flat coil arrangement. The coils can be overlapped:
http://www.windstuffnow.com/main/dual_rotor_turbine.htm
--
Chris

[tecker]

 I think I know what your talking about  The reason you can't stack up the coils is flux penetration .The coil needs to be the thickness of the magnet or thinner . That way the air gap can setup the impedance with rotor motion as well as warpage in the rotor.

[Ungrounded Lightning Rod]

Why are the coils all laid out flat horizontally compared to coils in the stator of an alternator which are vertical and stacked on top of each other like bricks? Probably a fairly simple question as I am very new to all this but just can't seem to find the answer. Thanks for any input

You'd like to "pave" the section of the stator under the pole path with coil-sides, to get the most out of the magnets.  You could about double the amount of power you generated with a given set of magnets if you filled the center of each existing coil with the sides of two others (for the two other phases of a three-phase setup).  Then you could trade this away down to the original power and coil count by using half as many magnets and the original number of coils now in an overlapping arrangement.  You end up with a smaller alternator and fewer magnets producing the same power.

But if you do it by just stacking flat coils you'd have to move the magnets apart, weakening the field.  Now you'd be "paved" along the magnets' path, but only half the thickness of your stator is wiring and the other half is "empty" - resin or air.  To get the magnetic strength back up to generated the design power you'd have to double the thickness of the magnets - or stack another set of the original thickness magnets on top of the first set.  You're back to having as much magnetic material as before and "wasting" half the space between them is wasted with non-coils, just as you "wasted" the space in the middle of the coils in the one-layer arrangement.  No gain and it's harder to fabricate the double-thick stator.  (The thicker stator also doesn't lose heat as well so you actually lose maximum output.)

Of course you could pave the space between the magnets WITHOUT increasing the spacing - by "bending" the part of each coil that's not between the poles to hop over/under the other coils.  But now your coil isn't flat - it has an arch or s-curve in the inner and outer portions to clear its neighbors.  This makes the coils harder to fabricate.  It also creates a stator where the portion inside and outside the magnet track is thicker than the portion between the poles.  This restricts cooling airflow to the part of the coils between the poles and also reduces cooling of the thicker sections of the stator outside the magnet track.  Fabricating the stator with the recessed track is harder than fabricating a flat-faced stator.  And the "bent" section of the coils increases the length of each turn, raising the resistance a bit.

So we opt for the easy to fabricate design which "wastes" the magnets for half of their travel, and compensate by buying twice as many magnets.  This makes an easy to construct alternator that does the job just fine.

[TomW]

You'd like to "pave" the section of the stator under the pole path with coil-sides, to get the most out of the magnets.  You could about double the amount of power you generated with a given set of magnets if you filled the center of each existing coil with the sides of two others (for the two other phases of a three-phase setup).  Then you could trade this away down to the original power and coil count by using half as many magnets and the original number of coils now in an overlapping arrangement.  You end up with a smaller alternator and fewer magnets producing the same power.

But if you do it by just stacking flat coils you'd have to move the magnets apart, weakening the field.  Now you'd be "paved" along the magnets' path, but only half the thickness of your stator is wiring and the other half is "empty" - resin or air.  To get the magnetic strength back up to generated the design power you'd have to double the thickness of the magnets - or stack another set of the original thickness magnets on top of the first set.  You're back to having as much magnetic material as before and "wasting" half the space between them is wasted with non-coils, just as you "wasted" the space in the middle of the coils in the one-layer arrangement.  No gain and it's harder to fabricate the double-thick stator.  (The thicker stator also doesn't lose heat as well so you actually lose maximum output.)

Of course you could pave the space between the magnets WITHOUT increasing the spacing - by "bending" the part of each coil that's not between the poles to hop over/under the other coils.  But now your coil isn't flat - it has an arch or s-curve in the inner and outer portions to clear its neighbors.  This makes the coils harder to fabricate.  It also creates a stator where the portion inside and outside the magnet track is thicker than the portion between the poles.  This restricts cooling airflow to the part of the coils between the poles and also reduces cooling of the thicker sections of the stator outside the magnet track.  Fabricating the stator with the recessed track is harder than fabricating a flat-faced stator.  And the "bent" section of the coils increases the length of each turn, raising the resistance a bit.

So we opt for the easy to fabricate design which "wastes" the magnets for half of their travel, and compensate by buying twice as many magnets.  This makes an easy to construct alternator that does the job just fine.

ULR;

Well put,

Maybe we should make this a "coil" sticky for reference.

Thanks.

Tom

[artv]

ungrounded lightingrod you said,..

Of course you could pave the space between the magnets WITHOUT increasing the spacing - by "bending" the part of each coil that's not between the poles to hop over/under the other coils.  But now your coil isn't flat - it has an arch or s-curve in the inner and outer portions to clear its neighbors.  This makes the coils harder to fabricate.  It also creates a stator where the portion inside and outside the magnet track is thicker than the portion between the poles.  This restricts cooling airflow to the part of the coils between the poles and also reduces cooling of the thicker sections of the stator outside the magnet track.  Fabricating the stator with the recessed track is harder than fabricating a flat-faced stator.  And the "bent" section of the coils increases the length of each turn, raising the resistance a bit.............so does this mean the outer and inner parts of the coil don't produce power because their not in the magnet track,just there to complete the flow.......artv

[JeffD]

After building 5 stators the conventional way for axial flux alternators, I built a stator as Ungrounded Lightning Rod described (I think).

Here is the conventional 6 coil star configured stator built back in 2007:


It had a thickness of 9/32" and a resistance of 2.6 ohms between two phases when first build.  22AWG wire was used.  After 8 months of use, I started using a boost converter for the low end (0 to 6 m/s) so removed 30 turns from each coil to bring resistance down to 1.8 and significantly improve top end power production.


The replacement stator built in 2009 is in the attachment. Its 3 phase star configured with each phase having 8 coils (the rotors have 8 magnets) wound using 22 AWg wire. It has a thickness of 4/32" (the magnet track area that is recessed) and a resistance of 0.9 ohms between two phases.  Rotor separation was reduced from 0.5" to 0.275".  It did take a little more time to build than the conventional one but It was the first one and there was the learning curve to deal with.

[artv]

I like the second design better than the first ,but it dosen't look like it was layed out very well ,I'm not trying to be rude just saying that it seems to me that you have to be pretty precise with these things .......how did it preform???.....artv

[willib]

So we opt for the easy to fabricate design which "wastes" the magnets for half of their travel, and compensate by buying twice as many magnets.  This makes an easy to construct alternator that does the job just fine.

ULR , i dont follow what you are saying here ?
Please explain The "buying twice as many magnets", part?

Are you saying to use twice the number of poles?
or are you saying to use a dual rotor?
Or are you saying to double up the thickness of the magnets ?

i understand the "wastes the magnets for half of their travel",part, i just don't believe that this is always the case..
 i have many designs on here which waste very little space .
Because i fill the space with wire.

[scoraigwind]

The second stator photo above does a good job of 'paving the stator with wires', and it keeps the coil turns short to reduce resistance, but the voltage output will not be at all good because the coils hardly enclose any flux so they will not see such a big rate of flux change as coils that are the right shape, so in the end I do not think its such a great idea.

[SparWeb]

Hugh:  ...the coils hardly enclose any flux so ...

- Do you think each coil is really that thin, or do they look overlapped to you?  I'm guessing that there are about 24 coils, in 3 layers.  The top layer has 8, and each of the 8 covers its own 1/8 of the circumference.  Then the next layer behind it is put in, 8 coils, rotated a few degrees relative to the previous layer.

[Flux]

To me it looks as though it is a 24 coil overlapped winding but with the coils having a span greater than the normal full pitch.

These overlapped coil windings work well with thin coils and they suit thin magnets very well.  The coil turns tend to be longer with this method so you struggle to keep the resistance low. In this case matters would be worse by making the coil span wider.

With thick coils the end connections become something of a nightmare. Even with radial construction things start to need formers to form the coils when they are much over 5/16" thick. With axial the space at the inside is very restricted and it may become impossible without very large and resistive end windings.

These overlapped windings were evolved from slotted iron cores where all the flux flips from slot to slot, once you go away from the slotted core they start to become distributed windings and the ideal placement of the conductors ceases to be much advantage.

If you use sector shaped thin magnets then this is probably the best way to go but to make best use of the magnet material you need to work about half way down the demagnetising curve and that means flux density of about 600mT. If you use thick magnets with small air gaps you don't gain that much flux density and it certainly doesn't make up for the great reduction in copper space.

As this has already become quite a distributed winding you don't loose a lot more volts if you wind inside the magnet dimensions, this means that you can use a lot more of the space with single layer coils than seems immediately obvious, so the space wasted is considerably less than it might be. What you save on very short turns and the associated drop in resistance probably outweighs the more theoretical improvement of the overlapped coil.

If you use the overlapped coil arrangement the classical gap = half magnet width works well for radials, the nearest thing for axials is wedge ( sector ) shaped magnets with gap about half the mean magnet width.

If you go to the single layer method then you can push the spaces up a lot to advantage and I don't think sector magnets are best any more.

In the specific example that Jeff shows he said he reduced the turns of the original single layer coils, he doesn't say if he increased the wire size to keep the coils touching, I suspect not and doing so would almost certainly have removed any advantage that the 24 coil version may have showed.

If you have slots then the overlapped coil method is easy to wind but once you remove the iron core you have to make some sort of former to hold the coils in place while you form them if you aim for any reasonable thickness. Up to about 1/4" thick you can just place the coils on top of each other and squash the centre part down to coil thickness.

There is no perfect method, a lot depends on your skill and personal preference and what may be optimum for one method will not be so for the other. As long as you don't do anything too silly I really don't think there is much point in chasing the perfect method.

In the end if you want more out you put more magnet and copper in and if you don't go silly the end result will be much the same for any given quantity of magnet and copper.

In the end it will be the alternator that matches the blades best that gives best output and it may not necessarily be the most efficient electrically.

I really don't see how we got here from a question concerning the difference between radial and axial construction but that is another matter.

Flux

[JeffD]

Hmm... I think the photo of the overlapping coil stator might be misleading people in how the coils are formed.  Attached is a picture of the stator but with colouring of the coils (using Gimp) so its easier to see phases and individual coils that belong to each phase.  The red and pink coils belong to the first phase and are in series with each other.  The pink coils are wound opposite in direction to the the red coils.  The blue coils make up the second phase and the green coils make up the third phase.  The number of coils in a phase equals the number of magnet poles which was 8.

I'm doing up a diary entry right now to show photos on how the stator was constructed.  Will post back with a link when the diary entry is posted.

[Flux]

Thanks, that shows it much better. It is fact full pitched and not over pitched as as I thought. That is the conventional way to do a 3 phase winding with overlapped coils. There is another method where you follow a 2 layer lap winding with two coils sides on top of each other but twice width, one coil side is on top and the other on the bottom.To do it with no core slots is a real challenge but again it can be done and all coils are identical.

With quite thin coils you can do as Jeff has done and get away with identical coils and squeeze the ends in somewhere but if you try this with thick coils you will need a mixture of flat coils and bent ended ones for this layout.

Flux

[JeffD]

Here is the link to the construction photos in a diary entry: http://fieldlines.com/board/index.php/topic,143444.0.html

[Ungrounded Lightning Rod]

ungrounded lightingrod you said,..

(Quote deleted)

so does this mean the outer and inner parts of the coil don't produce power because their not in the magnet track,just there to complete the flow.......artv

Yep.

The the parts of the wires that the lines of force "cut" are where the voltage is generated.  The rest of the wire in the coils is just to hook these active sections together.  They're a necessary nuisance - like the wires from the generator to the rectifier and from the rectifier to the battery.

[Ungrounded Lightning Rod]

So we opt for the easy to fabricate design which "wastes" the magnets for half of their travel, and compensate by buying twice as many magnets.  This makes an easy to construct alternator that does the job just fine.

ULR , i dont follow what you are saying here ?
Please explain The "buying twice as many magnets", part?

Are you saying to use twice the number of poles?
or are you saying to use a dual rotor?
Or are you saying to double up the thickness of the magnets ?

I'm saying that the typical non-overlapping coil version, if spacings are optimized, "paves" half the path of the magnets.  So it uses twice as many magnet poles (and thus twice as many magnets) as an overlapping-coil version with the same number of coils of the same number of turns of the same size wire, arranged to fully pave the path to the same thickness.

Separately, I talked about  just stacking up the coils so they overlapped, but not "bending" the coil sides to keep the sections between the poles the same thickness and full of copper.  That would also reduce the number of poles.  But it would thicken the stator and any given magnetic path through the stator would have half its length through a coil and half its length through a gap above or below it.  You'd have to move the rotors apart to receive this thicker stator, doubling the gap, which would weaken the field.  To get the field back to full strength you'd have to strengthen the magnets - by doubling their thickness.  And you're back to a doubling of the amount of magnetic material.

So your choices are essentially:

1) Overlapped coils, sides bent:  Magnetic path is completely full of copper.  N magnets.
2) Overlapped coils, sides not bent:  Magnetic path is half full of copper with gaps "above" or "below" each coil-side (assuming the axis is oriented up/down).  Stator is twice as thick as the individual coils.  N poles, but double- thick magnets to get the field strength up in the larger gap.
3) Non-overlapped coils:  Magnetic path is half-full of copper, with the other half being the empty centers of the coils.  2N magnets of regular strength.

We chose 3) as the easy one to build.

i understand the "wastes the magnets for half of their travel",part, i just don't believe that this is always the case..
 i have many designs on here which waste very little space .
Because i fill the space with wire.

As you can see, 1) has no "magnet waste".  It's just a bitch to fabricate and has cooling problems because the coils are not simple shapes and the stator may have a recessed track.  It's easier to spend a few bucks more for extra magnets and use 3) than to fabricate some version of 1).

Now if you want to spend some extra days making goofy coil winding and casting forms, just to avoid buying a few extra magnets, that's fine.  But I prefer the other tradeoff.

[willib]

I'm saying that the typical non-overlapping coil version, if spacings are optimized, "paves" half the path of the magnets.

But while one phase is producing no power  the other two are gaining and loosing power respectively.
And if one was to make the holes smaller than the magnets the downtime becomes even less.

So it uses twice as many magnet poles (and thus twice as many magnets) as an overlapping-coil version with the same number of coils of the same number of turns of the same size wire, arranged to fully pave the path to the same thickness.

you just can't generalize like that.
using the simplest arrangement as an example , a three coil 4 pole machine , you are saying to use 8 poles 
I don't see how that is possible.

[Flux]

I think it is less confusing to look at this from a different angle.

Take an 8 pole machine then the obvious single phase winding will have 8 coils, one under each pole and obviously each adjacent coil will have to be flipped or reverse connected for the voltages under opposite poles to add.

Assemble 3 of these windings at 120 electrical degrees and you have the most obvious 3 phase winding with 24 coils.

Now go back to the single phase case and obviously you can leave out alternate coils ( say those under the S poles). you now have 4 coils in series all producing the same polarity and no coils need reversing. This is a common method used in iron cored machines and is the coincident pole scheme where the poles of the stator that are not wound are formed in a return path from those poles that are present. having halved the number of coils you can now wind them with twice as many turns in the extra space and get exactly the same effect.

Put three of these at 120 elect degrees and you have an 12 coil winding. Both this and the 24 turn version have to be wound with overlapped coils.

Now go back to the single phase case again, take the second case with 4 coils and it is fairly obvious that you can leave out 2 of these and still connect them in series and it will still work. You will have to double the number of turns to get the same voltage and it seems a little unreasonable to expect that you can wind these bigger to get back to the original state but you can go part way as long as you don't go too far under the wrong poles.

The three phase version has 6 coils and these will fit on a single layer without overlap ( seen this idea before?, no normal sane engineer brought up on conventional machines will have but it has become common practice for wind powered alternators).

It is not a case of changing the number of poles but more one of leaving out coils. The fundamental thing is that you must stick to the basic concept of the original single phase coil for the full winding, it must more or less sit with one leg under a N pole and one leg under a S pole. You can move a little as this basic arrangement only really applies to a single turn, with multiple turns you can't get all the turns in the perfect place at once unless you use a slotted iron core to flip the flux from slot to slot.

You can of course go the final stage and leave out all but one of the original single phase coils and end up with the 3 coil single phase version but this will have to have lots of unwound space to keep to the coils not coming under opposite magnet criteria and you can't hope to get much power out of a winding that is mostly missing. it is very wasteful of magnet but if you do it correctly it still works.

With a radial machine with overlapped coils you can wind the whole of the surface but with axials, if your coils touch on the inside you will have unused space at the outer part unless you can find tapered wire and wind it with that. Add the complexity of overlapping thick coils on the limited inner circumference and you soon find that you are forced to use thin coils. If you want to use thick coils then you have enormous trouble with the overlaps and you loose out over the simple single layer 4 magnet / 3 coil simplification.

If you stick to the original single phase concept and a suitable coil then you can adapt it to any multiphase winding in almost any way you choose but you can't dream up coil dimensions at random or you stand a good chance of producing a resistor with no generated emf and a load of expensive copper scrap.

I hope this helps explain how these various windings evolved.

Flux

[Ungrounded Lightning Rod]

I'm saying that the typical non-overlapping coil version, if spacings are optimized, "paves" half the path of the magnets.

But while one phase is producing no power  the other two are gaining and loosing power respectively.

Yep.  They are the other two phases of a three-phase machine.

And if one was to make the holes smaller than the magnets the downtime becomes even less.

Not so.   If the hole is smaller than the magnet pole, the pole will spend part of its time overlapping both sides of the coil.  The voltage induced in one side is in the opposite direction to that of the voltage produced in the other, so they cancel.  The windings within the footprint of the pole add no power (on the average).  But they CONSUME power due to resistive losses.  So they're a net loss and a waste of copper.

(I think Flux has answered the other question.  But I only have time to skim it right now.)