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Magnetic field/electricity generation


By henjulfox, Section Homebrewed Electricity
Posted on Thu Aug 04, 2005 at 02:46:40 AM MST
Generator basics

Quick easy questions for you folks that know
1) Electricity is generated in a wire when it passes through a magnetic field. In an
axial flux set-up, the electricity is generated when the wire(s) are between 2
magnets, not when they are over magnets. The field is flowing between 2 adjacent
magnets, correct?
2) Larger magnet wire means more amps. The reason we don't use larger gauge wire on
generators is that enough turns for the correct voltage physically wont fit in the
space available, correct?
Thank-you,
-Henry
Magnetic field/electricity generation | 17 comments (17 topical)

Re: Magnetic field/electricity generation (3.00 / 0) (#1)
by benjamindees on Wed Aug 03, 2005 at 09:41:18 PM MST

1) In the dual-rotor axial flux design, the magnetic field "flows" from the face of one magnet, through the stator, to the magnet opposite on the other rotor.  For all intents and purposes, this field is straight.  Also, for most purposes, this is the only field that affects electricity production.  When a wire "cuts" this magnetic field by moving through it in a perpendicular direction, this movement causes electricity to flow through the wire (in a direction determined by the right-hand rule).  As long as the wire is moving in this perpendicular direction, within or through the magnetic field, electricity flows.

I would attempt to confuse you with ASCII art, but I'm sure someone else will have a nice drawing.

2) pass



Re: Magnetic field/electricity generation (3.00 / 0) (#2)
by RP on Wed Aug 03, 2005 at 10:51:03 PM MST

2) That is correct.  More turns means higher voltage but there's only so much space available.

[ Parent ]


Re: Magnetic field/electricity generation (3.00 / 0) (#3)
by tecker on Thu Aug 04, 2005 at 01:13:14 AM MST

  You want to harness the magnetic flow .So you set up a rotation and that rotation
dictates the coil alignment .In this case the coil is postioned  to move the electrons to the suface and chase them out of the leg of the coil around to and  oposeing field .



Re: Magnetic field/electricity generation (3.00 / 0) (#4)
by tecker on Thu Aug 04, 2005 at 02:11:54 AM MST

  Whooo that spelling stinks . My dog is partly to blame he turned the ashes drum over on himself . Let's try this again .

  You want to harness the magnetic flow .So you set up a rotation and that rotation
dictates the coil alignment .In this case the coil is positioned  to move the electrons to the surface and pull them out of the leg of the coil around to the other leg and it's opposing field
 . At that magnetic moment the coil is capable of moving electrons thru your load and
pulling current .  Coils segments need to match magnetic field strength and direction
 as to resistance and mass .

[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#5)
by Ungrounded Lightning Rod on Thu Aug 04, 2005 at 02:21:20 AM MST

Electricity is generated in a wire when it passes through a magnetic field.

Or the field moves through (across) the wire.

In an axial flux set-up, the electricity is generated when the wire(s) are between 2 magnets, not when they are over magnets. The field is flowing between 2 adjacent magnets, correct?

Correct.  The field is between the magnets on opposite rotors (and through the magnets, through the rotors, and back through another pair of magnets and across another gap.)  This field is dragged through one side of the coil (and the return field through the other) as the rotor turns.

Larger magnet wire means more amps. The reason we don't use larger gauge wire on generators is that enough turns for the correct voltage physically wont fit in the space available, correct?

Actually the spacing is adjustable.   But if you open it too much the field gets weaker and you lose more by weakening the magnetic field than you gain by having more turns (more voltage) or thickening the wire (less resistance, more cut-by-moving-field copper producing more current).



Re: Magnetic field/electricity generation (3.00 / 0) (#6)
by Electric Ed on Thu Aug 04, 2005 at 06:11:29 AM MST

These sketches might help illustrate what the other folks have explained.

Ed





Re: Magnetic field/electricity generation (3.00 / 0) (#14)
by kevindtimm on Fri Aug 05, 2005 at 08:13:59 AM MST

Wow, this stuff is such a great reminder of what I knew 30 years ago :(

Anyway, what's the equation for determining voltage produced?  I know it's a function of turns/spacing/field strength and rpm, but how do I get there from here?

TIA

[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#7)
by henjulfox on Thu Aug 04, 2005 at 06:55:11 AM MST

Excellent answers all. Technical enough to be correct but simple enough for me to
understand. If I have it straight:

Dual rotor - juice flows when a coil leg is directly over a magnet, making it between 2 magnets on opposing rotors. The field flows in a straight line between
the 2 rotors.

Single rotor - the oposite. Electricity flows when the coil leg is centered between
magnets. The field is an arc between 2 adjacent magnets.

From there, Electricity flows through the wire as it passes through a field in the
direction dictated by the right hand rule (been a while since physics 101). If a
section of straight wire passed through a N-S field while a different section passed
through a S-N field the 2 would cancel out. The coil works because the wire turns
180 degrees before encountering the other, opposite field. From the wire's point of
view, both legs of the coil are passing through an identical field. As it moves to
the next set of magnets the 2 fields are again the same, but opposite, thus creating
the AC.

Thanks again,
-Henry




Re: Magnetic field/electricity generation (3.00 / 0) (#8)
by kitno455 on Thu Aug 04, 2005 at 07:15:15 AM MST

henry, the electricity flows in the wire whenever the flux around the wire is changing. not just when the wire is a one physical location in relation to the magnet. we are making AC here, so electricy is pretty much always flowing, just at differing strengths and directions at all times.

there are multiple ways to change the flux, one is to move the magnet in relation to the coils (permanent magnet axial flux mill for example), another is to vary the strength of an electromagnet (regular AC transformer), another is to do both (regulated alternator or generator)

so your single/dual rotor statement might be better stated as the point of maximum voltage is at the times you mention, cause the wire still has a current in it at other times too, just a smaller one. though i personally am not sure about the max point on a single rotor...

allan

[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#9)
by Electric Ed on Thu Aug 04, 2005 at 08:01:55 AM MST

[quote][Single rotor - the oposite. Electricity flows when the coil leg is centered between magnets]

Not really. There is normally a "dead" zone between two adjacent magnets.

In the sketch below, which was originally drawn to show magnets that are too close together, see the note referring to voltage cancellatation.

Ed



[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#10)
by henjulfox on Thu Aug 04, 2005 at 08:29:24 AM MST

Got it! Thus proving that a picture is worth a thousand words. Smack me upside the
head with a 2 by 4 long enough and I'll start to pay attention. Thank-you!
-Henry

[ Parent ]


Re: Magnetic field/electricity generation (3.00 / 0) (#11)
by MountainMan on Thu Aug 04, 2005 at 09:12:28 AM MST

Great diagram Ed,
Would there be any significant advantage to placing an anit-magnetic material (like maybe Bismuth) in between adjacent magnets to "force" the leakage flux to go up and around and thus through the coil?

thanks,
jp
MountainMan, Julian California
http://www.real-home-based-business-opportunity.com
My ultimate hobby project is to build a ten foot tall robot.
[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#12)
by henjulfox on Thu Aug 04, 2005 at 06:49:49 PM MST

Great idea! I'm setting up a testing setup to experiment with some different
configurations. I have a 12" steel disk mounted to a low speed motor (240 volt motor
running on 24 volt), some 1" square neos and lots of magnet wire. Previous question
was to get some parameters on what might be interesting to try. I know I'm re-
inventing the wheel but somehow I feel beter about the wheel knowing I have re-
invented it.
I happen to have some bismuth laying around. I'll try sliding some between magnets and see what happens. Give me a week or two to post results.
-Henry

[ Parent ]


Re: Magnetic field/electricity generation (3.00 / 0) (#15)
by Victor on Fri Aug 05, 2005 at 05:43:11 PM MST

Hi JP,

 "Would there be any significant advantage to placing an anit-magnetic material (like maybe Bismuth) in between adjacent magnets to "force" the leakage flux to go up and around and thus through the coil?"

 It's likely both cheaper and more effective to use blocking poles. Blocking poles, in this case, can be implimented by simply making the magnets wider even to the point of touching. This would result in less output per unit of magnet material but more output per turn and per unit of volume.

 I know everyone (well maybe not everyone) is going to jump on me about the flux leakage where the two magnets touch, but the leaking flux, is flux that is supplied by the extra magnet material and the flux density in the area of the original magnet will be higher.

Victor

[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#13)
by benjamindees on Thu Aug 04, 2005 at 11:00:19 PM MST

Ah, that's the picture I was looking for.  This shows nicely that, in a single-rotor design, the magnetic flux lines are not straight, but curved.  This causes inefficiencies.

By the right-hand rule, only the component of the magnetic field that (in the diagram) is straight up and down contributes to electricity production.  This is at a maximum near the middle of each magnet.

To demonstrate the right-hand rule, point at the picture above with your (right!) hand.  Your index finger represents the direction of electricity flow, and corresponds with the wires in the diagram.  Now, stick your thumb straight up (making a 'gun').  This represents the direction of the component of the magnetic field that influences the flow of electricity.  You can see this is at a maximum near the middle of the magnets, where the magnetic 'field' is closest to being straight up and down.  Last, but certainly not least, sticking your middle finger out to the left, perpendicular to the other two fingers, represents the direction of motion.  When a coil leg is moving past a magnet in this direction, electricity will flow in the direction of your index finger.  If you turn your hand around to point at yourself, you can see that reversing the direction of motion will also reverse the direction of electricity flow.


[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#16)
by hobot on Fri Aug 05, 2005 at 11:11:00 PM MST

Could yo'all continue in this productive vien
with the dual rotor design
BUT sustituting an iron disc instead of a 2nd mag rotor,
WITH abutting magnets,
WITH Mu metal flux conductor or enough ferric behind mags
on rotor to contain flux so undetectable at rear surface.

WHAT would be difference of coil winding output if INSTEAD
of the pancake type where ~half the wire mass is useless
non flux cutting resistence, rather wound so
one leg of coil goes up in front of magnet and its
downleg goes down close to the iron backup disc?
Picture stator as flat washer with wire wrap around as
in high school 6V electro pencil magnets but in narrow
sections of course to match magnet pole widths.

Don't know if to picture this as both a N/S thru same
coil conflicting its current flow OR
as one polarity field entering a leg with RH rule upward
then exiting other leg of coil with RH rule flipped also
for the push pull one direction current desired/

hobot

[ Parent ]



Re: Magnetic field/electricity generation (3.00 / 0) (#17)
by benjamindees on Mon Aug 08, 2005 at 01:24:55 AM MST

BUT sustituting an iron disc

This thread goes through pretty much all of the reasons not to do this.  It seems to be better just to leave it out entirely.  Basically, my understanding is that, while a second rotor with magnets acts to draw flux in the correct direction, a ferrous rotor without magnets would not really alter the flux path, but would induce losses.  A better idea (and what is commonly done in commercial motors) is to use laminated sandwiches between the coils instead of air/epoxy.  That design isn't normally used because it causes cogging.

rather wound so one leg of coil goes up in front of magnet and its
downleg goes down close to the iron backup disc?
Picture stator as flat washer with wire wrap around as
in high school 6V electro pencil magnets but in narrow
sections of course to match magnet pole widths.

I'm not sure what you're saying, something like the torogen perhaps?

If I'm understanding correctly, this sounds like it would increase the gap.  Which is an awful, awful idea.  Magnetic field strength decreases cubically with distance.  Reducing the magnetic gap is the best way to make more efficient use of your magnets.

Also, if both legs of a coil are in the same field at the same time, the current will cancel out (unless, as in the torogen, the magnet is surrounded by the coil).  Use the right-hand rule above or search the board for the experiment that shows this on a scope.

[ Parent ]



Magnetic field/electricity generation | 17 comments (17 topical)
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