Relationship between flux and coils to generate power

[whitehawk]

I am quite green to wind power generation and alternator design, but I have a question about the relationship between the magnets and the coils. Do the coils get excited when the magnet passes over the parallel legs of the coil? About a minute ago a realization occurred, when I noticed the words "air core" when browsing posts to search for this answer. So does the coil get excited when the magnet is in the very center (or core) of the coil? When people talk of iron cores, does that mean they just put iron into the hollow coils of the standard air cores like the 10 footer's design?

I appreciate any help and apologize if this question has already come up. I am designing my first alternator right now and need to clarify some of this info.  I may deviate from the "3 coils-to-4 magnets" formula and need to make sure the magnets will line up with the correct spot on the coils.

[SparWeb]

Hi

welcome aboard!

This might help:

http://www.sparweb.ca/2_Gen_Ax/AXIAL_FLUX_HowItWorks_V1a.pdf

Read it through and maybe some of your questions will be answered.

The term "core" would be equivalant to the "stator", in the context you're reading about.  Motors usually use an "iron core".  The axial-flux alternator, using rare-earth permanent magnets, does away with all the iron, but there's still all that copper in between the faces of the magnet rotors.  The alternator turns smoothly when the magnets aren't pulling toward iron stator poles.

If you don't use the 3:4 ratio, you won't get a 3-phase alternator.  Fine, but do so because you know you need something else; such as 1-phase, 5-phase, 7-phase, etc.  There are reasons to go with a 3-phase alternator, when you're a newbie, and I covered a bit of that in my write-up, too.

Good luck!

[whitehawk]

With the ASSUMPTION when the magnet is in the middle of the coil winding, this will be where the voltage peaks for that specific phase.  With that, taking a 3 coil with 4 magnet 3-phase design I have drawn in a CAD program, this gives me the following order of pole changes when I rotate the magnets about the stator coils:

1. Phase A (N)
   
2. Phase B (S)
   
3. Phase C (N)
   
4. Phase A (S)
   
5. Phase B (N)
   
6. Phase C (S)
   
   

*Repeats

So my next ASSUMPTION is that any combination of magnets, within a 3 phase design, that produces that same order of pole changes will work.  I can't see why a 3 coil 16 magnet 3-phase wouldn't work because it gives the same exact pole changes.

Thank you for that PDF link, I will read it again, but it is still not clear to me.

[Tritium]

Here is a link to a virtual model,

http://www.scoraigwind.com/CABLE/stator.html

Thurmond

[whitehawk]

Ok, here is representation of the "10 footer" alternator. The blue phase has peaked since both magnets are above each leg of the coil. So each leg of the coil of wire has to be charged with opposite polls.

[wooferhound]

This may help . . .

http://www.windstuffnow.com/main/3_phase_basics.htm

[southpaw]

Hi Whitehawk

Your assumption that a 3 coil 16 magnet setup will work is correct but the maximum power output will be about the same as a 3 coil 4 magnet setup. The coils will only handle a certain amount of current before overheating.  The only advantage to spending 4 times more on magnets is that you will only have to turn your rotors at about 1/4 the speed to get the same output.

Also if you look at the scoraigwind model link you will see the voltage peaks, positive and negative occur when the magnet is centered over the positive and negative legs of the coil. The 3:4 ratio is the most efficient for $ spent on wire and magnets and for space limitations, but theres more than one way to skin a cat.  

[whitehawk]

Southpaw, this speed advantage is what I am pursuing because of the region I live in may not produce high average wind speeds.  I would also like to design some active pitch blades for low RPM high torque and speed control.  At this point I am definitely experimenting and learning the fundamentals, as well as being grateful that many innovative people have already done a ton of experimenting and documented the work (ehem, otherpower.com) to help me hit the ground running.

I appreciate all the feedback from everybody on this thread!

[SparWeb]

quote: ...the ASSUMPTION when the magnet is in the middle of the coil winding, this will be where the voltage peaks...

Which isn't correct.

It's a bit surprising, but the peak voltage actually occurs when the magnetic field is most rapidly changing, and that occurs when the coil is centered over the space between two opposite-pole magnets.

Another point - it's not all about the voltage.  You produce power by producing current.  Power=voltsXamps.  If you use a battery with relatively constant voltage, eg 12v, then the only way to increase power output is to produce more and more amps.

[whitehawk]

Ok, I broke out the physics book!!!  The literature keeps referencing "the magnetic field through the loop", not above the legs. The current is produced by the delta change in the amount of flux lines through the loop versus the delta change in time (so the faster you go increases the potential emf).  The book did provide a picture showing max/min emf voltage when the flux lines are removed from inside the coil, so that previous animation in this thread is correct (when the PM flux is removed from the center of the coil, this produces the emf peak).

If you have a single straight wire with current flowing through it, it will produce a magnetic field axially around the wire...so if you make a thumbs up with your RIGHT hand and fingers curled in, the thumb points in the direction of the current flow in the wire (typically from + to -) and the direction of the flux lines will be in the direction of your curled fingers.

So coiling many loops of wire together enhances and concentrates the magnetic flux lines towards the center of the loop arrangement (thats why we make coils).  In our alternators, there is a sort of back emf that cause the system to "load down" under heavy current draws.  This is because the coils act as magnets themselves when there is increasing current flow, that effect resists the permanent magnets on the rotor, therefore, requiring more external power (wind) to make more current.

Another cool thing are AC/AC transformers, its just input wires wrapped around an iron core (laminated of course) and the output wires wrapped around the same iron core, BUT, the output wires will either have more or less loops.  More loops on the output side steps UP the voltage, less loops steps DOWN the voltage. The alternating current creates an ever changing magnetic field in the input loops(greatly enhanced by the laminated core), this changing magnetic field induces the current in the secondary output loops.

I'm still studying...

[TomW]

Quote:

the thumb points in the direction of the current flow in the wire (typically from + to -)

I think there is some confusion here. It may depend on your background / age but current is commonly acknowledged to flow from - to + [Electron Theory]. Some older theories stated current flows from plus to minus [Hole Theory]. Functionally the same, but current flow from negative to positive seems to be the "current" way to describe current flow.

Just tossing that on the pile this stuff is confusing enough that it is important to keep it accurate. Be careful what you cherry pick from old text books. I saw one once that listed pi as "3". The flux induced voltage in the loop is crude and not really applicable in the systems we see. Generally I think the loop bit is more to make the circuit so current flows. Otherwise nothing happens if there is not a return path for the electrons. With the same flux polarity on both legs you get nothing in a single loop as it cancels out.

I could be wrong.

Tom

[mdntdncr]

okay . . . this does intrigue me.  I understand that using 16 magnets will increase

the power, but which part of it is it likely to increase . .the voltage or current?

I would have assumed the voltage, but since you're mentioning the overheating,

I thought I'd ask for clarification.

[mdntdncr]

So  . . . to clarify . . .

flux lines going through the coil will have more impact than flux lines

going _across the coil, right?  This would be why dual rotors with steel

backplates would produce more power, yes?