Axial Flux theory questions

[PhillT]

I have been tossing ideas around in my head to try to find a reasonably simple solution to the inherent inefficiencies (discussed here:- http://www.6pie.com/coilsandmagneticflux.php) of the Piggot type Axial Flux alternator.  It seems that the orientation of the coils is the cause of these inefficiencies, and I wonder if re-arranging the orientation might address these problems.
I am completely new to this subject, so don't be too harsh if I have missed or misunderstood some basic principal that makes this un-workable

What I propose is:-
Instead of laying the oval shaped coils flat in the stator disk, what if they were wound as long rectangles,
and laid with one long side against one face of the rotor disk, and the other long side laid flat against the other face of the rotor disk?
The coils would be wound so that their finished width was about the same as the width of the magnets used.

In this design, assuming  magnet rotors with "N" & S" poles opposite one another, with a rotor on each side of the stator,
 one half of a given coil would be passed by a "N" at the same time the other side of that coil would be passed by "S".
This would eliminate entirely the problem of reversing induced currents as the magnetic field approaches, passes over, then exits the two sides of a coil laid flat in the stator.

If this is not clear, what I propose is to basically rotate the coils 90 degrees to the way they are commonly laid now, and squash them into rectangular shapes to keep the stator disk reasonably thin.

Something like this:-
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[Flux]

I am on holiday in a few minutes so this has to be brief.

Your idea is impossible with an air gap alternator but is the basis of a torus machine. it works if you have a laminated iron torus inside the coils to direct the flux in a different way. With no torus you get cancellation on the opposite sides of the coil and net zero out.

I think you have a fundamental misunderstanding of inefficiency, it is nothing to do with the axial design as it stands, it is inherent in any variable speed alternator clamped down to a fixed voltage load.  To maintain high efficiency you must keep the load resistance an order of magnitude higher than the alternator source resistance.

Flux

[PhillT]

With no torus you get cancellation on the opposite sides of the coil and net zero out.

I can see that that would happen if there was only magnets on one side, but with magnets on both sides, i.e. a "N" for one side of a coil, and a "S" for the other side, I cannot see why it will cancel out. I'll have to go and read some more to get a better handle on it.

[tanner0441]

Hi

I don't think you understand the basics. The lines of magnetic flux pass through the windings of the coils this produces alternate half cycles positive/negative 180 degs out of phase, these half cycles go off down your wire, the amplitude is your voltage, these will be at a frequency dependent on the number of times the magnets pass the coils per second.

In what you propose, as the positive half cycle is being produced in one side of your coil the negative half cycle is being produced in the other side of your coil.  At the same time in equal amplitude. The result is the two half cycles cancel each other out. The result no voltage.

This is very very simplified but I hope it helps you. This site is full of explanations of how alternating current and DC generators work, and links to sites and files that will explain what you need to know, search and read.  Flux who replied to you post first would be a good choice of reading.

Brian.

[WindJuggler]

Hi,

Here is another explenation on a magnetic level.

The reason why this won't work without a coil is because the flux will go from 1 side of the coil to the other and not trough the centre of the coil.
If you look at it from the magnet's perspective you will only have straight conductors. If you apply the rule to determine the current in a moving conductor trough a magnetic field you will get a current in one direction. For example the conductor moves right and magnetic field is directed away from you, (north near to you, south at the other side) then the current will move down.

Instead of heaving 1 conductor you propose 2 conductors in nearly the same field. Those 2 conductors will both have a current going down. Meating each other in the lower part of the coil. And making a nice heater.

Maybe a good analogy for flux: Flux is like rain passing trough a bucket without a bottom. Each droplet folows a fluxline and has a direction.

[PhillT]

In what you propose, as the positive half cycle is being produced in one side of your coil the negative half cycle is being produced in the other side of your coil.  At the same time in equal amplitude. The result is the two half cycles cancel each other out. The result no voltage.

That being the case, what would happen if both sides of the coil were exposed to a "S" (or "N") from separate magnets, instead of a "N" & "S"?  Would this not result in a neg. (or pos.) in both halves of the coil, then the following pair of magnets in the revers configuration would produce the other half cycle?

[PhillT]

The reason why this won't work without a coil .........

Thanks for all the replies. I am still trying to get my head around them. I am going to research some more so as to better understand why both sides of my coil, even though exposed to opposite polarities, result in no current produced

[PhillT]

If the magnets in the picture below were flattene out instead of curved, and the coil was moved through the field in a straight line instead of being rotated, why would there not be the same effect as rotating in the curved magnets? The flux lines are basically the same, and the motion of each side of the coil is still at 90 degrees to the flux,....??

[WindJuggler]

In the picture you gave us the coil will turn around its own axle. The result is that both sides of the coil move in a different direction thus the induced voltages are summed. Move both of the legs of the coil in the same direction, the induced voltages are subtracted.

Maybe look up right hand and left hand rules. They are used to explain current, forces, velocity and magnetic flux

[ghurd]

That being the case, what would happen if both sides of the coil were exposed to a "S" (or "N") from separate magnets, instead of a "N" & "S"?  Would this not result in a neg. (or pos.) in both halves of the coil, then the following pair of magnets in the revers configuration would produce the other half cycle?

That would result in a horribly inefficient torus machine.
Efficiency will increase with an iron core, which would be a typical torus machine.

In DanB's new post, check out the photo and text about the Proven, about half way down, here,
http://fieldlines.com/board/index.php/topic,143480.0.html

G-

[PhillT]

In the picture you gave us the coil will turn around its own axle. The result is that both sides of the coil move in a different direction thus the induced voltages are summed. Move both of the legs of the coil in the same direction, the induced voltages are subtracted.

Maybe look up right hand and left hand rules. They are used to explain current, forces, velocity and magnetic flux

My crude drawing has been mis-interpreted.  The coil is moulded into the Stator disk. (stator = stationary) The magnets either side are on rotor disks (rotor = rotating)

The picture of the basic generator in the later posting is, if the magnets are flattened, exactly what I was trying to depict. If you imagine the armature is fixed in that position, and the coloured magnets are one pair of several pairs mounted on rotating disks each side of the armature coil, then as the pairs of magnets pass the coil (armature) there will be induced voltage. If there are many of these coils moulded into a stator, would we not have an axial flux alternator?

[ghurd]

It would be a torus...

[tanner0441]

Hi again

I for one would not want to sit for hours passing the correct length of wire through the middle of a laminated ring of soft iron.

A test coil if it doesn't work on a conventional turbine can just be discarded and binned. Up to a hundred plus turn through a toroidal former would mean the same number being unwound.

Add to the fact there is tons of information on successes and failures all well documented to help avoid the pitfalls with conventional stators. 

Brian.

[wpowokal]

G if you now explain the effect that the laminations have as opposed to an open wound coil I suspect he will understand.

allan

[PhillT]

Thanks Guys.  Ghurd, that is exactly it, but I din't think it would need a core.

Brian, I can assure you that I will not be going that route. It is a pity it will not work without the core, though the reason why is beyond my limited understanding.

WindJuggler, following your advice re the rules, it appears that the direction of the movement of flux, in relation to the coil, has a bearing on the polarity of the induced current, which I did not know then.
Just as impractical, but for the sake of it, what would happen if it was built without the core, BUT, had contra-rotating rotors?

[PhillT]

In DanB's new post, check out the photo and text about the Proven, about half way down, here,
http://fieldlines.com/board/index.php/topic,143480.0.html
G-

That is definitely what I was thinking of, but the core was not. Awesome looking machine.

[ghurd]

G if you now explain the effect that the laminations have as opposed to an open wound coil I suspect he will understand.

allan

The laminations complete the flux path.
Flux goes out all over the place unless there is a path of less resistance, which the laminations provide.

People tend to think the lines of flux travel directly to the next magnet, as if all the flux travels left and right in an orderly fashion.  They do not.  Flux lines are going every which way.
They must travel from their N to S in the shortest path with less traffic (like people driving to work?).  That is why magnets on a standard axial flux PMA are mounted on iron.  Just because the coils are on the other side of the magnet does not mean the other side of the magnet is not important.

A torus, without an iron core, has flux from each magnet going up, down, left and right.   A large portion of the flux is not acting on the coils.
With an iron core, the path of least resistance is through the core, forcing the flux through the coils.

There was a recent post about a single disk not containing the flux very well.  It would hold up a screwdriver at the OD.
When the second disk was installed, it would hardly hold up a paper clip.  IIRC.
It is the same principle.  Without the second disk the flux went in every direction, much like a torus without a core.  Except a coreless torus would be worse.
(edit- the post about 1 disk leakage vs both disks, by Eric esc
http://fieldlines.com/board/index.php/topic,143394.msg964680.html#msg964680 )

A coreless torus would work, but it would not work very well.

Can get a voltage measurement by waving a magnet near a spool of wire.
That does not make it a good PMA.
G-

[PhillT]

Thanks for the comprehensive explanation Ghurd. It begins to make sense at last.

With regard to the common axial flux alt., I've seen some that use large disk magnets, and others using bar magnets.
One article I read expressed concern that the magnet induces a current as it approaches a coil, then as it passes over the centre of the coil and begins to approach the other half of the coil it induces an opposite current, not to mention the period spent around the middle section where it does not induce much at all.

Would the bar magnets (suitably spaced) do a better job than disk magnets because they would spend less time in the central area of the coil, inducing both halves of the coil (resulting in nil).
They would be more likely to pass over one coil and their influence on that half would be approaching zero, before approaching the second half, etc. etc.?

[artv]

as mag enters coil produces current flow in say clockwise direction , hits hole stops, hits other leg reverses....but as mag leaves first leg,  collapsing current flow sends current in other direction,thereby adding to current flow, or working agianst original ??.............sorry for question in a question ,...........been looking but can't seem to find the answer..artv

[wpowokal]

Artv, another good question about a complex subject, with regard to the normal duel rotors as discussed on this site the "collapsing" current does not have an effect on the current in the other leg IMHO.

Why is this so, well you may ask, we need to speak of voltages ( for it is a voltage that is created when there is a current flow) and for what you ask it is called a back EMF (opposing voltage), when a current flows there is a emf built up around that conductor who's voltage lagges (in this case) the voltage that created it, when the conductor is suspended in air as in the duel rotor there is little effect because air does not hold a magnetic flux.

To give an example where this is significant, take the example of a transformer wound on a laminated core, there is a far greater EMF built up within that core (lamination's, as they are able to be magnetized as opposed to air which is not) than in an open coil, your question is more appropriate to windings on a laminated core such as within a small AC gen set or large commercial unit as  in a power station or indeed a "conversion" who's windings are on lamination.

allan

[wpowokal]

Phill the magnet induces a voltage in the coil which will cause a current to flow if there is a complete circuit. It is more about the amount of magnetic flux than the actual magnet shape, as the coil is wound to accommodate this shape. The length of wire in the coil is an important consideration because this introduces resistance to a greater or lesser degree, the thickness of the magnet contributes to allowable optimal air gap.

There are many ways to skin a cat, personally I prefer to just shoot them but that aside I suggest you draw on a piece of graph paper different coil/ magnet shapes and calculate the wire length/coil taking into account magnet area. There is no single simple answer which is basically why it is usually suggested that first time builders stick to a proven plan.

The magnet needs to be in a neutral position (the middle of the coil) at zero crossing point(when the voltage is neither positive or negative) if it is over both legs it will cancel the induced voltage.

allan

[PhillT]

The magnet needs to be in a neutral position (the middle of the coil) at zero crossing point(when the voltage is neither positive or negative) if it is over both legs it will cancel the induced voltage.
allan

Thanks Allan. This is exactly my point.  Take a look at the magnets used on the rotor here: http://www.otherpower.com/listeraxialflux.shtml
They are 2" in diameter, and therefore would not ever be on a "neutral" position between the legs of a coil. I am wondering why he used such wide magnets, considering that the same guy used bar magnets here: http://otherpower.com/steamengine.shtml
These are only 1.5" wide, and would most likely be in a near neutral position.
Having said that, the round magnet unit seems to perform quite well, and is basically what I want to build, once I decide the best configuration and components to use.

I would also ask if you know why he used 4 strands of 15g wire, instead of 1 strand of thicker wire?

Cheers.

[artv]

allan............I really don't understand how these things work ,..but why build a system thats going to create cancilation.....dose'nt make sense to me ......build it so every strand of copper produces power............rectifie'diode ,every individuale strand..........this can't be impossible...............Jerry did it with each phase ........I say do it with every strand ......anybody ever tried this...............artv.......diode ,cap in line

[wpowokal]

Phill round magnets work just fine, ordinarily the hole in the middle of the round coil is the same size as the magnet so there is a 'neutral' position. Why were they used, I suspect it's what they had, the builders are the board hosts.

Two or more strands are commonly used as it is easier to work than a single thick strand and it makes a neater coil with less air gaps formed by cross layers, if you have the time you could use a single wire just fine but it's hard work by hand to get each layer side by side and neat.

If you have round magnets or desire to use then the do so it will work if you follow a design, coil shapes are made to accommodate the magnet shape.

allan

[wpowokal]

Artv I don't understand what you are getting at, cancellation is not designed into any of these alternators, there may be small amount caused by slight layout imperfections but nothing to write home about.

allan

[oztules]

Code: [Select]

Thanks Allan. This is exactly my point.  Take a look at the magnets used on the rotor here: http://www.otherpower.com/listeraxialflux.shtml
They are 2" in diameter, and therefore would not ever be on a "neutral" position between the legs of a coil. I am wondering why he used such wide magnets, considering that the same guy used bar magnets here: http://otherpower.com/steamengine.shtml
These are only 1.5" wide, and would most likely be in a near neutral position.
Having said that, the round magnet unit seems to perform quite well, and is basically what I want to build, once I decide the best configuration and components to use.

I would also ask if you know why he used 4 strands of 15g wire, instead of 1 strand of thicker wire?
Phill, the world is not a perfect place. Danb has introduced some cancellation by way of design sizing for the following reason.

RESISTANCE!...... and layout constraints...

(A):
Resistance is the killer for an axial flux.  Dan has deliberately gone inside the magnet size with his coil because not all the turns are linked  by the flux at the same time in this flat coil design. (with iron core slots, we can link them all at once as the field flips from tooth to tooth)  Because of this, we can use quite a bit of the hole for winding a few more turns... I have used up to 15% of the hole in this manner. You don't get quite the volts/turn you get with coils outside the magnet diameter, but you do have a lot less resistance introduced, as the turns are made up of shorter length turns.

So what he lost due to slight cancellation compared to the same turns outside the magnet diameter, he makes up for with less resistance per turn, and wack in a few extra to make up for the less than perfect volts per turn by being inside the magnet diameter...... in the real world he wins.... (it won't help with the waveform however.... delta could become problematic)

This is a game of diminishing returns, and filling the hole is only good for so much. At some point, the cancellation effects outweigh the shorter wire resistance gains. It is a balancing thing.

He wins two ways. He will probably have more turns, but less resistance for the same/better emf at X rpm, and the coil may be made to fit in a less than desirable space.....

(B):
Wire in hand. ......1 lump of wire that will replace 4x#15 will be a difficult thing to wind,... or even bend,  and secondly, the thicker wire will likely get into difficulty with eddy currents..... so easier to wind, and avoids eddy currents.

An interesting game I played was to use 500grams of #13 wire, and wound it in all kinds of hole size etc, and tested each one for EMF@ Xrpm. The differences are published on this site somewhere, but I would no longer be able to find them here now.... but the gains are worthwhile using the hole a bit. (from memory 42mm for a 50mm magnet was starting to be the useful limit.... down hill from there. I call it volts/gram)



.............oztules

[PhillT]

Thanks for the great explanation. It all makes sense now I think

One last (I hope) question. Danb is getting about 2,000 watts from this machine, which is about what I am looking for, however I don't know at what voltage that is.
How would I go about working out the wire size & number of windings to achieve 2,000+ watts at about 30 volts (for charging 24 volt battery, after rectifier losses etc.), or am I going to have to settle for a far higher voltage and use a voltage regulator to get my 28-30 volts?

 

[Flux]

I only skimmed through the article but they were testing it with 4 car batteries so it would be 48v.

If you want to use it at 24v then you need to wind with half the number of turns of twice cross sectional area wire. This may prove challenging and you are effectively looking at using twice as many strands in hand and winding with half the number of turns.

Adapting these larger machines for low voltage is always a challenge and it may be better in the end to depart from the standard winding. If you used the same winding and brought all 6 leads out instead of making the star point internal you could rectify each phase individually with its own bridge. This means bringing 6 leads down to the rectifier at the base of the tower or mounting rectifiers up the tower.

An alternative method for a 12 coil stator is again to keep the same winding but split it into two separate star connected windings with half the number of coils. You can keep them in phase as parallel 3 phase or reverse the starts and finishes of one section and make it 6 phase and link the star points. Either way you need two 3 phase rectifiers paralleled on the dc side. You still need 6 leads down or rectifier at the top.

The other thing to watch with 24v is that you will need quite heavy connecting cable to the battery and this may be an issue if it is a long run.

Flux

[PhillT]

Flux,
Yes, I suppose for testing purposes, there would be no reason to use four configured as 24 or 12 volts. (unless for loading purposes??)

Your suggestion of bringing the 6 wires out will work OK for me because I am doing exactly what he did, i.e. A/F alt. on
a 3/1 Lister for charging only. (My lister will run veg oil) Batteries will be next to it.

Are there any advantages to your alternative,..."An alternative method for a 12 coil stator is again to keep the same winding but split it into two separate star connected windings with half the number of coils. You can keep them in phase as parallel 3 phase or reverse the starts and finishes of one section and make it 6 phase and link the star points."

Phill