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1 coil, 1 magnet, 1 AC wave
By troy, Section Homebrewed Electricity Posted on Sat Oct 02, 2004 at 01:10:43 AM MST
Understanding the basics, caution long post with pics

When building alternators, it is helpful, and sometimes essential, to understand the theory behind the machine. Failure to understand the theory can lead to wasted time, effort, money and bucketfulls of frustration when the machine doesn't perform up to expectations.
Historically, there has been some confusion among some contributors here as to what exactly happens electrically when a magnet passes a coil, like in the many home built disk brake rotor alternators featured on this web site. I asserted that one magnet passing one coil one time produces a full sine wave. Others suggested that you would not get a full sine wave. In an attempt to clear up some of the confusion, I offer a few fun diagrams and photos. Comments are welcome, as well as constructive criticism if I have gone astray with any of the technical details.

So that more or less covers the theory. Feel free to review any basic physics book to drive that home.
For those that also like some real world evidence in addition to the theoretical conceptual handwaving, I offer the following lab results:
I took a magnet and a coil like so:

I stuck the single magnet to the chuck in my lathe and I taped the coil to a piece of plywood bolted down to the tool holder like so:

I turned the lathe on, hooked up an oscilloscope and got the following results:

As you can plainly see, the oscilloscope has captured and graphically displayed the electrical event associated with the magnet passing the coil one time. Sure enough, a single beautiful full sine wave with both a positive component and a negative component.
This is all very intellectually satisfying, but it is also essential if you want to avoid all sorts of mistakes in the design of stators and rotors and intercoil spacing and leg width and magnet spacing and a boatload of other things.
Respectfully submitted,
troy

1 coil, 1 magnet, 1 AC wave | 16 comments (16 topical)

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#1)
by Norm on Sat Oct 02, 2004 at 06:41:30 AM MST

...and so if you turned the coil on edge you would get a half wave?
( :>) Norm
( :>) Norm

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#2)
by troy on Sat Oct 02, 2004 at 08:25:28 AM MST

Hey Norm, not sure what we would get, but the substance of the original contention was what happens with a regular air core coil in an alternator as usually discussed on this board.

Best regards,

troy

[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#9)
by finnsawyer on Sat Oct 02, 2004 at 11:34:09 AM MST

Most likely you would get a single negative or positive pulse of reduced amplitude. The reason for this is that the field of the magnet drops off quickly with distance and one side of the coil is farther away from the magnet. It will produce a voltage opposite in polarity to that of the close side but much weaker. Should be an easy experiment to do.
GeoM
[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#3)
by wooferhound on Sat Oct 02, 2004 at 08:42:01 AM MST

Can't you trim down the file size on those pictures ?

I'm not sure where your Zero Volt line is on your scope, but
As I understand it . . .

Arranging the magnets North-South-North-South-North-South
is doubling your voltage. The norths passing the coil pull the voltage
to the Plus side of the zero line, then the Souths passing the coil pull
the voltage to the Negative side of the zero line, the combination is
double the AC voltage.

I would be courious to see the scopes waveform using only 2 magnets
and passing the coil as North-South ?

)}=- W o o f -={(

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#5)
by troy on Sat Oct 02, 2004 at 10:11:23 AM MST

The file size for the photos was reduced by 75% so you don't have to scroll to read the text. I also added a warning about the long post and photos for those that have slow connections. Sorry for the inconvenience.

The results of two magnets passing one coil one time would depend on the magnet to magnet spacing. But that was not the bone of contention in the original discussion. There are lots of fun variations you can try with coil leg width, inter coil spacing, coil hole size compared to magnet size, etc etc etc ad infinitum. But I wanted to bring clarity to the most fundamental issue of what happens when one magnet passes one coil one time. If we can't agree on that we're beat.

Good luck and have fun!

troy

[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#4)
by SomeGuy on Sat Oct 02, 2004 at 10:05:31 AM MST

Huzzah!

Thank You Troy for actually measuring something instead of just guessing and building. I have seen a LOT of misinformation posted on this board and I am very happy to see a scope trace! I also salute your choice of fonts, well done.

Some Guy

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#6)
by finnsawyer on Sat Oct 02, 2004 at 10:24:38 AM MST

Troy, I hate to be a spoilsport but that is not a sine wave. A sine wave contains energy at only one frequency. If you were to measure the spectrum of that transient waveform you would find that it contains energy at a range of frequencies. In any case, nice work. I'm glad to see some folks are using o'scopes to good effect. What do you suppose would happen if the magnet was large compared to the coil?

How about changing the sweep speed to show two of the waves?
GeoM

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#7)
by troy on Sat Oct 02, 2004 at 10:53:04 AM MST

Dear Finsawyer,

Technically, you may be correct there. Would you be satisfied if we described the wave form as sinusoidal, since all of the alternators we build fail to produce true faultless definitional sine waves to some degree or another.

But again, the purpose of the post was to refute the supposition that you would only get one hump, no electron flow in the other direction. This clealy has an up (+) and a down (-). In that limited sense, I think it looks pretty sine wavy to me.

Best regards,

troy

[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#8)
by finnsawyer on Sat Oct 02, 2004 at 11:19:16 AM MST

Sinusoidal is probably the best term to use. As you're probably aware, if you move a small coil over the pole of a large magnet you will get a positive and negative pulse separated in time. I thought this would be useful to show in order to demonstrate the importance of getting the proper size relationship between the coil and magnet in an alternator design. The deviation from a pure sine wave in the output can give rise to radio interference. That's probably of not much consequence for the type of projects considered here, but some people might be interested in it. I do like what you've done and it may clarify matters for many.
GeoM
[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#10)
by troy on Sat Oct 02, 2004 at 12:07:06 PM MST

Dear Finsawyer,

Instructive and thoughtful observations are always appreciated whether they "agree" or "disagree". Those two terms are often not helpful anyway, as it's all just a pursuit to more and better knowledge.

best regards

troy

[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#11)
by troy on Sat Oct 02, 2004 at 04:02:37 PM MST

This is a separate experiment that I did some time ago, that answers to some extent what happens when you vary the size of the hole in relationship to the size of the magnet.

In the first test, I compared two coils, one very small, and one medium (A and B respectively). The traces for those two mini coils are shown in the second photo. Coil A produced a big dead spot in the middle of the peak of the sine wave due to the magnet being over both legs at the same time and canceling the voltage. Coil B wasn't so bad, but still a noticable dip.

Now, when I compared coils B and C, (medium and large, respectively) we see more intersting results. Only the biggest coil has a nice full peak because there's very little overlap with the magnet covering both legs at the same time. It's tempting to think that we should make great big holes in our coils.

That's not really true though, because a real coil has many turns, not just two. So while part of two legs may be covered by the magnet, the additional copper you can pack in there still gives you more volts and amps out compared to the great big hole coil. Up to a point anyway. Everything involves some compromise. So I wind coils with holes a little smaller than the magnet and cross my fingers.

Have a lovely day.

troy

[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#12)
by JW on Sun Oct 03, 2004 at 08:02:24 AM MST

Nice write-up Troy,

Honestly, I see nothing wrong with what your saying here. But the phrase "not limited too" comes to mind. I have alot of practical experience with "dc waveforms" and I feel due to the .707-141 conversions I think ac waveforms are sometimes abit more tricky. Perhaps it was un-wise for me you say that you are wrong. Just attempt to envision how my mind interpeates an 'ac' waveform. I believe that the dc waveform is the building block of a true sinewave this includes both positive and negative amplitude.(on the respective positions of the zero-line). Hey, troy Id like to compliment you on your co-gen system that your building in you basement, a very intreging project indeed.

JW

[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#13)
by finnsawyer on Mon Oct 04, 2004 at 10:44:11 AM MST

I suppose we could consider a sine wave as the sum of two dc pulse trains. The problem is to use them in any kind of circuit analysis leads right back to the mathematical properties of sine waves as entities in their own right.
GeoM
[ Parent ]

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#14)
by rpcancun on Wed Mar 22, 2006 at 09:08:28 AM MST

coil hole size and shape i will be using 2x2x1 thick mags so whats the best hole size
for the coils? should the coils be square?, also what about magnet spacing, my cad drawing places 26 mags in a 24" diameter right next to each other, ive seen other rotors where the mags are spaced almost an entire mag space away, in my case it would be about 2 inches, so does the mag need to travel completely over both legs of a coil before the next mag crosses over for it to work properly?
It would appear being so close when mag 1 travels clockwise over leg 1 of coil 1 a voltage is applied when it crosses over leg 2 of coil 1 does it now reverse polarity?
and now at this point the next mag is now over leg 1 of same coil but that mag is reversed polarity,....is there a conflict there?

Thanx

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#15)
by SmoggyTurnip on Wed Mar 22, 2006 at 12:30:33 PM MST

Thanks Troy,
A very nice post, these things cn be confusing.

1 Magnet - 1 coil gives a full 360 degree wave (1 cycle).

but ....

2 magnets and 1 coil also gives only 1 cycle.
When spaced close together as in all of
the alternators shown on this board.

Maybe that's a part of the cause of some confusion.

The sooner you start the longer it takes.

Re: 1 coil, 1 magnet, 1 AC wave (3.00 / 0) (#16)
by DaveK on Sat Mar 17, 2007 at 10:34:03 PM MST

this is a very interesting thread!

What about the idea of using a pair of connected, counter-wound coils, each passing the opposite poles of the same magnet at the same time. Has anyone tried that? Does it double the voltage? My thinking is that it should have approximatley the same effect as two magnet's opposite poles passing the same coil at the same time, as in a dual rotor alternator.

Thoughts?

Dave

1 coil, 1 magnet, 1 AC wave | 16 comments (16 topical)

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