the Otherpower.com Discussion Board || hole size in air core coils, o'scope reveals all...

Front Page - [Homebrewed Electricity-- (wind) (solar) (hydro) (steam) (controls) (storage) (mechanical)] - Classifieds - Site News
Everything - Newbies - [Remote Living-- (housing) (heat) (light) (water)] - Rants & Opinion - Diaries - Our Products

hole size in air core coils, o'scope reveals all...
By troy, Section Homebrewed Electricity Posted on Thu May 22nd, 2003 at 03:00:15 PM MST
I was always curious how small I could make the hole in the coil. Now I know, sort of...

Howdy out there in electron land,
The rule of thumb that I've seen a bunch of times on the board is that the hole in the coil should be about the same size as the magnet. Being the curious type, I wanted to see what happened to the waveform as I changed coil core size in relationship to the magnet size.
To generate the wave forms, I made three test coils. The hole in test coil A was half the width of the magnet. The hole for coil B was about the same size and test coil C had a hole 25% bigger than the magnet. The vertical legs of the coil (the effective part) all had three turns of 18 AWG magnet wire for all three coils.
Since I have only a two channel o'scope, I first compared coil A to coil B, with the following results:

Then I compared coil B and coil C with the following results:

I have superimposed the bottom of the two waveforms for easy comparison. In the A vs B pic, the very short AC wave with the pronounced dip in the middle is the output from coil A, the little skinny coil. It's obvious that you really start losing efficiency when the magnet can cover both legs of the coil at the same time, causing a voltage cancellation.
In the B vs C wave form comparison, C definitely outperformed B, with virtually no dip as the magnet passes over the center of the coil. This demonstrates that the flux doesn't just shoot straight out of the magnet, but rather, spreads out somewhat, so we still get a little voltage cancellation in coil B as the FLUX of the magnet can cover both legs at the same time, briefly.
Like everything though, it's a compromise. If you make really big holes in your coils, you'll have really great "efficiency" and pretty wave forms, but less total output than if you pack more copper in with a smaller hole size. However, this is definitely a case of diminishing returns. If you fill the air core hole up with wire completely, the very middle part of that will generate basically no useful electricity and just add to total resistance, reducing your top end generating capacity.
I intentionally used very few turns in the coils, to eliminate the averaging of the wave form from a full coil. A full solid coil with no central gap might still show a decent wave form as the outer layers are compensating for the cancellation of the very center.
So yeah, I'm back to making the holes in the coils just a little smaller than the magnet.
Voltage and amperage measurements with a Fluke DMM produced a similar outcome, with the small "A" coil producing about half of the watts of the the "B" coil, despite having the same amount of copper in the effective part of the legs.
Best Regards,
troy

hole size in air core coils, o'scope reveals all... | 18 comments (18 topical, 0 editorial)

oops, forgot... (none / 0) (#1)
by troy on Thu May 22nd, 2003 at 03:08:02 PM MST
(User Info)

Oh yeah, this was a dual rotor setup, with 12 pairs of the pictured neo magnet on 11" rotors. 294 rpm driven by a lathe checked with a tach. Obviously, no laminations, though I would expect similar results.

troy

Very informative (none / 0) (#2)
by Chuck on Thu May 22nd, 2003 at 03:54:46 PM MST
(User Info) http://www.greeleynet.com/~cmorrison

Troy,
This is a very well thought out and executed experiment. It should answer a lot of questions. Dans, shouldn't this have a dedicated page in the experiments section somewhere ?
chuck

Great work troy! (none / 0) (#5)
by windstuffnow (elenz(at)windstuffnow(dot)com) on Thu May 22nd, 2003 at 06:57:47 PM MST
(User Info) http://www.windstuffnow.com/main

Quite interesting, its always nice to be able to compare different outcomes of a unique test. What was your magnet spacing? ( basically the gap between them). I tend to run them close or even touching at times but don't tend to use as many turns of wire as everyone else does...
Anyway, Keep up the great work... all will learn from this.

Have Fun!
Ed

Have Fun! Windstuff Ed
[ Parent ]

mag spacing (none / 0) (#6)
by troy on Thu May 22nd, 2003 at 07:27:43 PM MST
(User Info)

Hey thanks Ed,

The neos were spaced pretty far apart. I could fit 24+ around the outer edge of each rotor, and have only 12 in there now. It's a money thing, hmmm, 24 big neos on each rotor at so much per magnet, and pretty soon we're talkin' some real money.

I think I will end up with a compromise of 14 or 16 neos per rotor.

Best Regards,

troy

[ Parent ]

Magnet spacing (none / 0) (#9)
by Electric Ed on Fri May 23rd, 2003 at 07:09:16 AM MST
(User Info) http://www.electric-ed.com

Great work, Troy. This is the kind of work that really advances this science.

Money issues aside, the wider spacing will actually give better results than would more magnets with closer spacing, due to leakage flux and voltage cancellation.

See my sketch at[http://www.fieldlines.com/story/2003/5/8/113141/6796]

Ed

[ Parent ]

I second that (none / 0) (#3)
by Geek on Thu May 22nd, 2003 at 05:05:29 PM MST
(User Info)

I think this would be a nice addition to the exp. page. Good Work! Geek
Alway have a dream. Those with out dreams are just waiting to die.

many thanks! (none / 0) (#4)
by troy on Thu May 22nd, 2003 at 05:16:41 PM MST
(User Info)

Praise from my peers on this board is the creme de la creme.

Good luck and keep having fun!

troy

YES! That's more like it! (none / 0) (#7)
by iFred (ifred2006@yahoo.com) on Thu May 22nd, 2003 at 08:43:29 PM MST
(User Info)

Good work Troy! I see you finnaly got a scope, nice yes.. now you can see what is really going on rather then just a volt meter. The experiment shows everything very nice.
>> all energy used to produce this comment or post came from solar and wind energy! It works!

A man, a dream and a scope... (none / 0) (#8)
by xeroid (centurion27@lycos.com) on Fri May 23rd, 2003 at 06:48:05 AM MST
(User Info)

Hi Troy.

Excellent work! I was sure that the most efficient design was to have the hole in the coil to be the same size as the magnet, but your experiment clearly shows that this was a mistaken assumption.

In the continuing quest for more efficient alternators, this is very useful information. Thank you!

Hey Dans, I wanna put my vote in to have this one on the experiments page too!

Regards,

Xeroid.

Cool! (none / 0) (#10)
by DanB (danb@*no spam*otherpower.com) on Fri May 23rd, 2003 at 08:08:04 AM MST
(User Info) http://www.otherpower.com/

I tried a similar experiment - but I was getting near perfect sine waves every time. In my case the magnets were touching...

also though - in my case I tested the "whole" coil...never thought to simplify it like you did.

My guess, from your experiments - is ideally (with space between the magnets) you'd want the hole the same size, or slightly larger than the magnet. But - since coils normally have many windings, I don't think this is so much determining the size of the hole in the center of the coil, but rather the "average" diameter. If you use that as your "average" diameter, then the hole should obviously be slightly smaller than the magnet and the outer dia of the coil slightly larger.

My newest alternator (this weeks project) has holes somewhat smaller than the magnets diameter, and the OD of the coils is larger than the magnets - so that one coil occupies the space of 1 and 1/3 magnets (so 3 coils fit over 4 magnets). I've tested this with a couple rotors, one where the magnets (1" dia) were touching, and then one where the magnets (1.8" diameter) have about 1/2" between them. In both cases it yeilded the most power to actually run coils larger than the magnets and have fewer coils than magnets.

I think it should be pointed out that ... the rules change with different types of rotor/stator setups. With slotted laminates and very thin airgaps things would probably change a bit over using flat laminates and a fairly thick airgap. Dual rotors might change things some too.

life is complex (none / 0) (#11)
by troy on Fri May 23rd, 2003 at 10:32:27 AM MST
(User Info)

Yes, it gets complex and compromises have to be made. Here's another way to look at these results: We are seeing what contribution each layer of a full coil actually makes. I think the final wave form from a full coil is the result of the average diameter of the coil versus the magnet to magnet spacing, just as you describe. But the real take home message is that you do want some hole in the coil, bigger hole being somewhat more efficient (in volts per inch of wire so to speak) but smaller hole giving higher total output up to a point.

I'm with you, I want the coils bigger than the magnets, so some space between magnets, but not 50% space.

And just to clarify, you're still doing single phase with the same number of magnets and coils, or is this three phase with less magnets than coils or something else entirely? I eagerly await output numbers on your latest project.

Best Regards,

troy

Assumtions can be fatal (none / 0) (#12)
by Dave B on Fri May 23rd, 2003 at 12:23:27 PM MST
(User Info) http://www.madbbs.com/users/bruggelog

Great work from all you guys out there and with the scope comes a sense of certainty but .... It is very time consuming for sure to build a complete scale "unit" for testing but watch what happens. I think Dan is seeing it and Troy besides. Things don't always add up or react the way it is assumed they will when testing a single coil or 2 and then just filling in the blanks on paper. Add up the coils yes but I think what we are all hitting on is the "time" or frequency factor which relates directly to the spacing / magnet / coil size relationaship. Again it's all what you need for your application. If a clean wave at a certain voltage / speed is important then shoot for it with the "ideal" spacing, size etc. (I'll call it Ed's clean machine) Some of us less fortunate who have been working with meters can see (measure) a higher output at the same given speed by packing in the coils and magnets, not as clean or ideal electrically but if it fits your own application it maybe a more efficient "bang for your buck"(thanks Dan) especially at a lower windspeed. What a great collection of knowledge here and I am having fun which is most important to me. Thanks guys, Dave B.

[ Parent ]

total out put (none / 0) (#13)
by Johnny Cool Pants on Fri May 23rd, 2003 at 10:43:20 PM MST
(User Info)

Quote:
"Like everything though, it's a compromise. If you make really big holes in your coils, you'll have really great "efficiency" and pretty wave forms, but less total output than if you pack more copper in with a smaller hole size."

Troy what if everything was bigger? I mean even the armature too? What if everything was scaled up to accomidate the new hole size, so the diamitor of of the armature was bigger too, would the output increase without losing the efficiency?

I don't know, asking.
Damn it Spoc, I'm a motivator, not a DJ.
Nice job by the way.

DTT Johnny Cool Pants

Wow, an olive branch. (none / 0) (#14)
by troy on Sat May 24th, 2003 at 11:16:08 AM MST
(User Info)

I agree that larger rotors and stators offer some advantages like less crowding of the coils and magnets, which may provide better efficiency compared to cramming the same wire and magnets into a smaller alternator. The downside is bigger/heavier/more expensive construction. I'm still pondering the implications of the data, as far as the ideal mix of coil size vs magnet spacing. Will keep you posted on the outcome..

Best Regards,
troy

[ Parent ]

Martini Olive Stick (none / 0) (#18)
by Johnny Cool Pants on Sun May 25th, 2003 at 09:25:17 PM MST
(User Info)

Credit where credits due, even for you. Hey here's one that may or may not already exist, shape magnets to be airodynamic, have them on the outer most tip of the windmill blades. Not too heavy but flying past a coil at high speed, 'while' it turns the hub genny too.

Just an idea I want to try but too many irons in the fire. "emagination is more important as knowledge" said the man in the funny hair.

I think some of the minds on this board should just form a mini "corporation" (with out the yuck usually associated with that word) and just pump out patents. Working alone can have benifits but a team often produces more. You could be the "The Copper Head Gang" Copperhead Inc.

Give me the word, I'll chair it, tee hee.
DTT
Johnny Cool Pants

[ Parent ]

how many phases are you wiring? (none / 0) (#15)
by scoraigwind (magnet@scoraigwind.co.uk) on Sat May 24th, 2003 at 02:55:30 PM MST
(User Info) http://www.scoraigwind.co.uk

I have checked out the comments on this thread and I think you need to look more closely at the magnet pitch. That is the spacing between magnet centres. The magnet pitch is the most important variable here. When you make optimally shaped coils and pace them side by side in the stator you don't get single phase output. And that is a good thing because you get better power factor output to the battery and less vibration. So I wonder how many phases you guys are using in the circuit?
Hugh Piggott http://www.scoraigwind.co.uk

three phase (none / 0) (#16)
by troy on Sat May 24th, 2003 at 03:28:21 PM MST
(User Info)

If this is Mr. Piggot, then I'm honored that you stopped by to join in the discussion. In any case, we're happy to have you.

Personnally, I hope to make a three phase mill next year, and am fiddling with a single phase right now just to play with.

No question, three phase is more efficient, and gives you the star/delta option to better match the alternator to the avg. wind speed.

Best Regards,

troy

more about phases (none / 0) (#17)
by scoraigwind (magnet@scoraigwind.co.uk) on Sun May 25th, 2003 at 09:45:22 AM MST
(User Info) http://www.scoraigwind.co.uk

Thanks for the welcome Troy. There's a lot of interesting stuff on this board. To be honest I have done very little work with single phase, and I found it vibrated quite badly. The other thing that bothers me about single phase is that it should only have wires in certain positions, spaced at the same pitch as the magnets. Wires in other places produce very little power in that particular circuit (phase) but could be useful if connected to a different phase. So it seems like a waste to only be using the flux of the passsing magnets at certain times and at certain positions.
Hugh Piggott http://www.scoraigwind.co.uk
[ Parent ]

hole size in air core coils, o'scope reveals all... | 18 comments (18 topical, 0 editorial)

Menu

� create account
� How to use the board
� FAQs
� search the board
� Google search the board
� Old Otherpower Board

Login

Make a new account

Username:

Password:

Total Views

��134 Scoop users have viewed this posting.

Related Links

� magnet
� Also by troy

You must be a registered user to post here. It's easy and free, and the link is on the upper right side of your page.
All trademarks and copyrights on this page are owned by their respective companies. Postings are owned by the poster, but may be deleted or moved at the ADMIN's sole discretion. The Rest � 2003 Forcefield.
You can Email the board ADMIN here. PLEASE include the username you signed up with!