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# Re: [femm] Permanent magnets

- To: femm@yahoogroups.com
- Subject: Re: [femm] Permanent magnets
- From: David Meeker <dcm3c@earthlink.net>
- Date: Wed, 23 Jan 2002 00:41:53 -0500
- References: <20020122115742.8841.qmail@zet.net>

steudner@surfeu.de wrote:

> `Dear all,`

> `I want to solve the problem with a metal disk rotating`
> `in the air gap of a magnet. In a first step I want to`
> `show the flux-density in the neighbourhood of a permanent`
> `magnet. I added to the properties of a block the properties`
> `of a material from the library connecting SmCo. The results`
> `for a 25x13x3 permanent magnetized in the direction of 90`
> `degrees to the 25x13 plane show in the 25x3 cross-section`
> `high density only near the 3 mm sides. I think that this`
> `is not real and connected with the model of permanent`
> `magnets used in the program and showed in figure 31 of the`
> `users manual. Is that right? What can I do, to obtain better`
> `results for permanent magnets?`

> `Regards`
> `Reinhold Steudner`

If I understand what's going on, you have a magnet that has a big pole area but is very short. In this case, the flux should be much higher near the edges of the magnet. The lengths of the paths along which flux travel are much shorter out near the edges, increasing the flux density.

Anyhow, think about it in terms of Amperes's loop law: The integral of H around a flux path equals the amp-turns enclosed by the path. Since all the flux paths, either near the edges of the magnet or near the center of the magnet, enclose more or less the same number of amp-turns, the H must be much higher on the shorter flux paths near the edges of the magnet to satisfy the loop law.

Dave.
--
David Meeker
http://femm.berlios.de/dmeeker

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