EMF and Coil / Flux orientation question

[CraigM]

I have a question regarding EMF and how it relates to the coil / magnetic flux orientation.

In an axial flux (Hugh P. type) PMA the magnetic flux enters at a right angle to the coil creating EMF. The orientation of magnetic flux to coil is pretty much the same in most motor conversions or in a toroid core machine such as the Proven wind turbines. In all of these instances the magnetic flux is at a right angle to the coil and passes through a portion of the coil.


Motor conversion, example only.

I've seen an axial flux animation on this website that shows the greatest EMF happens when a north and south magnet pole are located on either side of a coil in an axial flux PMA.

In a Fisher Paykel type stator and the small stator shown below, flux enters parallel to the coil and is fed through the steel laminations in the center of the coil. In this instance it appears little if any flux passes anywhere but directly through the center of the coil.



Example Only

I've spent some time on this website http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html that SparWeb pointed me to and I'm unsure what really generates EMF. Is it flux passing through a portion of a coils winding (with the addition of rate of change in polarity) or more so flux passing through the center of the coil? Or is it both as long as there is polarity change and an increasing (speed) rate of change? Example: a magnet placed next to a coil with no change in movement or polarity will not induce EMF.

Just wondering how EMF is generated. Faraday's law seems to suggest it happens in various ways but has one been shown to be better than others?

Thank you,
Craig

[electrondady1]

there could be blood shed on a question like this.
 the way i understand it is
 the lines of flux are sill at right angles to the conductors
but travelling within the iron.
the coil polarity is still reversed
it's like a transformer, the coils are  exposed to only one polarity at a time
there is no left leg right leg
 

[SparWeb]

You get EMF from a change in flux.  Doesn't matter how, and there is no way better than any other.
You have to be really particular with your vocabulary to start, so first, notice the difference between "field" and "flux".
Diagrams usually help.  Try this:
http://www.sparweb.ca/2_Gen_Ax/AXIAL_FLUX_HowItWorks_V4.pdf

The field is there, always, around a permanent magnet, no matter what you do.  The field is represented by the lines drawn in loops from N to S.
Those lines always want to make a complete circuit.  In fact by definition, they are never "broken".
At any given spot, you can measure the intensity of the field with a gaussmeter.  The result is measured in "gauss" or "Teslas" (which is the same, just 10,000x more)

The flux is something else.  Flux is the amount of the field that passes through an area.
The area you use can be almost any size, but it does have to be flat.  Circular, rectangular, triangular, star-shaped, cloud-shaped, it can be any flat shape that is convenient.
You don't need a magnet to have flux through your area, either.  The earth's own magnetic field is passing through it to begin with, so every loop of wire encloses a bit of flux.
When you do hold a magnet up to the area, then you can talk about a sensible amount of flux.  It's the average intensity of the field, pointing through the area, times the area.
Those field lines that don't pass through the area don't count.  If a line goes in and also comes out of the area then it doesn't count either.  This is called "flux leakage".
To be efficient you want as many of those lines from the magnet to go through the area of your coil.  It also doesn't matter if the lines are centered or off to one side, if there are the same number of field lines passing through anyway.  The lines have to go through the area at right angles to be fully counted, too.  Field lines going through the area at a 45 degree angle only create 70.7% of the flux.
The units of flux are in Tesla X square meter (or square inches but then you have to use a conversion).  That's also called a "Weber".

You still don't have an electromotive force, until you change the flux.  The way to change the flux can be ANY way you like.  Change the field, change the area, it doesn't matter.
Move the magnet relative to the area, mov the area relative to the magnet, reverse the magnet, reverse the area, adjust the magnet's field (electromagnet), adjust the size of the area, it all causes a change in flux.

The flux also changes over a certain period of time.  Do it fast, and you get a greater EMF.  So EMF has units of "Tesla X square meter / second".  The combination of those units is also called a "Volt".

Changing the orientation of the magnetic pole through the area of the coil is just one convenient way of rapidly changing the flux.  Causing the change by moving the magnet is electrically the same as causing the change in flux by moving the coil.

[Flux]

I think nearly all the confusion here comes from the days in school physics when you were taught that an emf is induced in a wire moving at right angles to a magnetic field. Yes it's true and things like Flemmings left and right hand rules are useful when you apply them correctly.

In real life you can't have an isolated conductor and if you try to measure the emf between its ends you immediately complete a loop and you are dealing with a turn.

The simplest way is to think of a coil and emf is induced in it when the flux linking it changes.

To understand what flux is linking the coil you need to consider the magnet as part of a magnetic circuit and it is  the different forms of magnetic circuit that lead to the various forms of alternator.

With air cored machines the flux takes the shortest path between the poles of the magnet and that is where you want your coil.

With iron cored machines the reluctance of iron is so much less than air and you can steer the flux to where you want it. In the case of that F & P machine the iron in the cores of those salient poles directs the flux through the coils even though the coil axis is at right angles to an axial flux machine. The motor core is exactly the same but the flux links outwards rather than inwards and it looks rather different.

If you find a diagram of a wound field alternator it will have the salient poles of the F & P to provide the flux and a stator like the induction motor you show

The only flux that generates emf is that which links your coil and changes. The emf depends on the number of turns, the flux and the rate of change.

There are complications in other than the simple cases as not all the same flux links every turn so the total emf is not always directly related to the number of turns but not much need to worry about those things until you grasp the basic context of changing flux linking a coil.

With an iron cored coil you can more or less assume that the flux that matters is that passing through the iron core, without iron cores things are not so simple if the coil has multiple turns as they can't all be in the same place.

Flux

[definitionofis]

I did ten or more videos on my youtube channel about magnetic field geometry without iron cores.

I smiled when I read Flux say, in his last paragraph, that it gets more complicated without iron cores, because I see it the opposite way, as., "it gets more complicated when you add iron cores".   

I just add that the right hand rule still applies and flux cutting wire at right angles still applies with iron cores, except the iron core completely changes the geometry so in effect the entire circle of wire is cut at the same right angle simultaneously. That might be tricky to visualize since those are neat and tightly wound around a core. You have to see it as right angles radiating in all directions from the center, although the rotor motion is across the coil. 

You do not see neat close near circles in an air core generator and you don't see them in a TV yoke either.  Here is my TV yoke video: 

there could be blood shed on a question like this.

Bwahahaha 

[CraigM]

Thank you everyone for the comments.

I don't have much more to add to this thread except to say the physics of electricity and magnetism is a very new and fascinating subject for me. I'm peeling the onion so to speak, one layer at a time and realize I've only just scratched the surface.

Definitionofis; I'll be watching more of the videos on your youtube channel. Great stuff, and thank you for presenting this information.

I'm in information gathering mode right now with a long term goal to help myself understand every aspect of sustainable living while still embracing modern technology and not reverting back to the dark ages.

Now where did that onion go? I've got some peeling to do.

Thanks again,
Craig

[artv]

Hi Craig,excellent thread.
Definitonofis a few questions if I may, In your video with the pennies around the wire coming out of the page towards me , you say that one side is north orientated and the other south, correct?
 This depends on the direction of flow in the conductor, in a coil ,the direction of flow is all the same in the leg of the coil that is being subjected to a magnetic polarity ,regardless of pole polarity,polarity just determines direction of flow ,corrrect?
So if a coil had say ten turns ,would  each individual wire of the coil be repelling, since all the ten wires have their own pennies and the flow is the same, all the norths pointing in ,all the souths point out, but in reality is'nt the magnetic field around the pennies spherical like a ball as opposed to flat like a pancake, and in the ball shaped pattern where do you draw the line between north and south?
Thanks for your time....artv

[definitionofis]

Hi Craig,excellent thread.
Definitonofis a few questions if I may, In your video with the pennies around the wire coming out of the page towards me , you say that one side is north orientated and the other south, correct?

Yes, one flat side is N and the other S.

This depends on the direction of flow in the conductor, in a coil ,the direction of flow is all the same in the leg of the coil that is being subjected to a magnetic polarity ,regardless of pole polarity,polarity just determines direction of flow ,corrrect?

Yes, you are talking about a generator coil subjected (that's your word) to magnetic field now, I think, and magnetic polarity means N or S. So I answer yes.

So if a coil had say ten turns ,would  each individual wire of the coil be repelling, since all the ten wires have their own pennies and the flow is the same, all the norths pointing in ,all the souths point out, but in reality is'nt the magnetic field around the pennies spherical like a ball as opposed to flat like a pancake, and in the ball shaped pattern where do you draw the line between north and south?
Thanks for your time....artv

You should watch my series of videos. I answer exactly those two questions.
This one is fun, Two Coil Forces: http://www.youtube.com/watch?v=Nb6NOGixm7Y (start at 4:06 to see the short demo about a loose coil)
It demonstrates that the ten turns, in your example, attract each other, not repel. You have the right general idea, though.

The second question is in this video: http://www.youtube.com/watch?v=yAFmB6frePc
It shows how a circle of magnetic field contour lines look toroid shaped in a coil. It could be a flattish pancake toroid or rounder and bigger in a long coil, but not exactly a sphere.

[tecker]

 Electrons don't Flow in a coil of copper if they did you would have an ionic transformation and the copper would breakdown . The spin is polarized and a B field is started that has magnetic properties that are the same as those of material that can be magnetized .  With out  getting down  to the bond length and quantum view of copper a true picture of what is going on is not completely clear but we take advantage of the rotation of the B field with resistance and impedance etc .
 In a flat coil it's easy to start a Ac wave of that B field with magnets raking the electron spin and rotating in a like like direction through out a closed loop . The magnets work best when they rake the coil at the same magnetic moment IE the geometry of the Axial magnet rotor or a conversion for that matter .
It's said that the B field moves along outside the wire and this can be seen best in a high voltage circuit when the insulation properties of air break gown and a plasma arc happens . You can see that B field Change to E field at that moment where an E field develops around the arc .
Also when you modulate a coil fast enough you can see the repulsion between turns of the coil and a scalar field happens which transmits in all directions
To complete  an understanding of the whole magnetic phenomenon  make a simple magnetizer that blasts a High magnetic field through some Neo material that has been heat demagnetized . When the neo material is demagnetized its crystals are in natural order when you hit it with the field the bonds deform and a static scaler transmission starts  as the electron spin is also set with this deformation  and that scalar transmission id what magnetism is .

[Ungrounded Lightning Rod]

Electrons don't Flow in a coil of copper if they did you would have an ionic transformation and the copper would breakdown .

SURE they do flow.  But when some of them move off others move in.  You maintain the sea of electrons in the conduction band, binding the nuclei together by electrostatic attraction, and the metal remains a metal rather than breaking into ions.

If you could somehow haul off the bulk of the conduction band electrons the metal would come apart into ions.  But hauling off electrons without replacing them leaves the charge of the nuclei uncanceled, raising it to a high positive voltage.  The voltage where it would start shedding ions is so hysterically high that you'd pull electrons from miles away and discharge it with a lightning bolt long before you reached it.  (And once the incoming electrons impact it, shedding ions would be the least of your worries.)

The spin is polarized and a B field is started that has magnetic properties that are the same as those of material that can be magnetized .  With out  getting down  to the bond length and quantum view of copper a true picture of what is going on is not completely clear but we take advantage of the rotation of the B field with resistance and impedance etc .

But the pauli exclusion principle keeps half the electrons in the conduction band, along with half the bound electrons in lower orbitals, pointing the opposite way from their brethren.  So it all cancels out in a metallic conductor.  If you want to see any practical effect from electron magnetization you need unpaired electrons - which means a semiconductor or a magnetic material.

[Ungrounded Lightning Rod]

I have a question regarding EMF and how it relates to the coil / magnetic flux orientation.

In an axial flux (Hugh P. type) PMA the magnetic flux enters at a right angle to the coil creating EMF. ... In a Fisher Paykel type stator and the small stator shown below, flux enters parallel to the coil and is fed through the steel laminations in the center of the coil. In this instance it appears little if any flux passes anywhere but directly through the center of the coil.

The math is easiest when the field is moving at right angles to the conductor, so that's what you see in textbook explanations of the Faraday effect.  But all that matters in a practical machine is how much flux transitions from one side of the conductor to the other (passing through it in the process), not what angle the flux is at with respect to the conductor as it passes through.  While the force on the electrons from the field line's passage may not be aligned exactly with the wire, the electrons are constrained so they (on the average) only move along the wire.  The induced EMF (think "voltage") is from one side of the penetrating field line to the other.  So it adds up the same along the wire whether the field line is at right angles or some slant.

The permeability of an iron core is so much higher than the permeability of air that virtually all of the flux through a coil in a cored design is in the core.  But as the field through the coil increases it isn't additional flux lines coming into existence in the core - it's additional flux lines being dragged through the coil to settle in the core.  Similarly, when the field is reduced it happens by dragging the flux back out of the core, again passing through the coil in the process.

Field lines are always closed loops.  When they ARE created or destroyed, it happens by infinitesimal loops being created and expanded, or loops shrinking to nothing and disappearing.  In our machinery this only happens inside a conductor when the current through it is increasing or decreasing, and the passage of the magnetic field through the conductor (and the other turns beside it) as the loops grow or shrink induces a voltage along the wire, resisting change in current flow.  This is the mechanism creating the electrical phenomenon of inductance.

In cored designs like the F&P you're dealing with hairpins of flux lines sinking into the crack in the core between the coils (or being pulled back out), with the bulk of the "legs" in the outer part of the cores while the "loop" is bridging them somewhere down the slot and passing through the coils as it moves in or out, or being dragged from one pole piece to another, passing through the coils in the process.  Ditto with the transition of the flux in a wound rotor or stator of something like a motor conversion.  Again what matters is not the angle of the flux with respect to the wire as it passes, but which side of each turn it's on.  The flux that hairpins part way down and never makes it to the bottom before you move on to the next half-cycle and drag it back out has only "cut" some of the turns and thus only contributed partially to the generated voltage.

[gotwind2]

"With air cored machines the flux takes the shortest path between the poles of the magnet and that is where you want your coil."

Wise words as ever from Sir Flux.

[tecker]

The fact electrons move from bond to bond is an old theory . If you pass current through a coil the similarities to a magnet are almost identical  . In electroplating whole atoms move . Electrostatic fields , E field in general,  batteries etc  remain static for long periods of time .When those bonds break down with excess charge part of the material goes with it and sound light and plasma happen . All these happen in or around light speed . The color doesn't chance in copper unless there is oxidation .Coils made over hundreds of years work with no appreciable change . When a stream of electrons is ejected from a source there's also radiation . I Know it's off track but the only model that works in all cases is changing the spin , alignment and position in the bonds with out movement .