the Otherpower.com Discussion Board || Air gap vs test coil voltage

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

Air gap vs test coil voltage
By commanda, Section Wind Posted on Sat May 10th, 2008 at 04:31:56 AM MST
A question of extrapolation, varying the air gap

So I wound a test coil for my baby PMA;
http://www.fieldlines.com/story/2008/4/30/3552/30035
250 turns of 0.6mm wire. Turned out a little thicker than I wanted, but was easy enough to open the gap for testing purposes.
Now, my question is this. Can I extrapolate a new figure for volts per 100 turns per 100 rpm from the gap used for the test coil, as against the final, tighter, air gap?
Amanda

Air gap vs test coil voltage | 13 comments (13 topical, 0 editorial)

Re: Air gap vs test coil voltage (3.00 / 0) (#1)
by Flux on Sat May 10th, 2008 at 01:19:57 AM MST
(User Info)

Yes, you can extrapolate it over a small range.

With no gap the flux density is Br for the magnet ( 1.3T ish). It then falls with the ratio of gap length to magnet length. With gap equal to total magnet length it should be 600mT but will be less due to leakage etc.

If your final gap is smaller than total magnet length you can roughly do it by drawing a line from 1.2T with no gap to .6T at magnet length and assume that you work up and down this line. Just ratio the test gap to required gap.

Flux

Re: Air gap vs test coil voltage (3.00 / 0) (#2)
by Flux on Sat May 10th, 2008 at 01:25:01 AM MST
(User Info)

That bit in brackets should have been (1.2T ish) but the actual value depends on magnet grade. It's the slope of the line from Br to half Br at magnet length that really matters so if you used Br of 1.3T then at magnet length it would be 650mT.

Flux

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#4)
by vawtman (vawtman(at)charter(dot)net) on Sat May 10th, 2008 at 10:30:57 AM MST
(User Info)

Flux, Did you mean to say depth instead of length?

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#5)
by commanda (alwynne at unwired dot com dot au) on Sat May 10th, 2008 at 03:04:08 PM MST
(User Info)

Perhaps I can answer this;

Distance through the magnet from the North face to the South face.

You can, of course, get magnets which are magnetised lengthwise, with the poles at the end. Useless for an axial flux pma. But it does show that the term "length", which could be taken as a physical dimension, could be confusing. Used in context of the discussion, it refers to the length of the internal magnetic path.

Amanda

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#6)
by vawtman (vawtman(at)charter(dot)net) on Sat May 10th, 2008 at 03:28:11 PM MST
(User Info)

Thanks Amanda and have fun with your neat little project.

I was thinking in terms of say a 1x2 magnet with 2 being the length.

I'm o.k now i think :)

Mark

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#7)
by martin1 on Sat May 10th, 2008 at 03:34:43 PM MST
(User Info)

In a dual rotor would the "magnet length" be the thickness of two magnets?

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#8)
by commanda (alwynne at unwired dot com dot au) on Sat May 10th, 2008 at 03:49:41 PM MST
(User Info)

Yes. Realised I should have made that clear after I hit the post button.

Amanda

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#3)
by commanda (alwynne at unwired dot com dot au) on Sat May 10th, 2008 at 01:59:35 AM MST
(User Info)

Thanks Flux.

With a total magnet length of 20mm, changing the gap from 17mm down to 14mm should give approx 13% more volts.

Amanda

Re: Air gap vs test coil voltage (3.00 / 0) (#9)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Mon May 12th, 2008 at 01:04:55 PM MST
(User Info)

I suppose this is a bit late, but I thought I could contribute just one picture:
I've done this test before:

My best guess was an inverse-square law. Maybe an inverse cube...

When the magnets get really close, there's no further increase in flux, hence I believe the curve flattens out close to the y-axis.

Steven Fahey

Re: Air gap vs test coil voltage (3.00 / 0) (#10)
by ghurd on Mon May 12th, 2008 at 02:51:43 PM MST
(User Info)

How thick / long were the magnets?
Measured from one disk to the other disk.

I am guessing they were 0.5" or so.
G-

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#11)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Mon May 12th, 2008 at 04:58:01 PM MST
(User Info)

Yes, didn't give much info, did I?

This was from the axial I built a year ago, RIP. It had 1/4" thick magnets, each 1" diameter. The two rotors had 16 poles, and the stator was 3/8" thick. I only tested down to 1/2" air gap, where things could start scraping together. I was doing the test to correlate what I could see in FEMM.

If you want to know more, I can dig up details about the coil, RPM, etc., or you can find it in my diaries and postings from about 2006.

Steven Fahey
[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#12)
by Flux on Tue May 13th, 2008 at 01:19:41 AM MST
(User Info)

I think that if you neglect leakage and consider it over large gaps it is inverse square as the thing is based on the ratio of gap to magnet length ( magnet length constant)

Beyond gaps greater than total magnet length you will likely see increased leakage and with close magnet spacing you may see leakage effects with gaps at magnet length.

The linear approximation I gave is not really applicable with large changes in gap but works well enough if you don't stray too far from the magnets best operating point.

Flux

[ Parent ]

Re: Air gap vs test coil voltage (3.00 / 0) (#13)
by bj (jackbp996@msn.com) on Fri May 16th, 2008 at 10:41:35 AM MST
(User Info)

Amanda--have lots of info on small P.M.A's, but mostly based on
H.D GM. conversions. Any info that is helpful is yours. Good Luck.
bj
jackbp996@msn.com
thanks all bj

Air gap vs test coil voltage | 13 comments (13 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

��172 Scoop users have viewed this posting.

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!