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# Re: [FEMM] Solid block vs laminations - does not compare

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- Subject: Re: [FEMM] Solid block vs laminations - does not compare
- From: David Meeker <dmeeker@ieee.org>
- Date: Sat, 29 Nov 2003 23:26:23 -0500
- In-reply-to: <web-59265788@california.com>
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`The bulk lamination model only applies when the lamination is thin enough so that all of the eddy currents are flowing in the plain of the lamination. The eddy current induced in the "usual" direction (i.e. into and out of the screen) can be neglected if the laminations are thin. In the lamination model, the flux tends to get confined to a thin layer adjacent to the insulation between laminations, and flux in any skin region near the interface of the laminated region with air is neglected. Conversely, a solid block model assumes that all of the current is flowing in the into and out of the page direction, neglecting the impedance of currents that must travel in the plane of the simulation to enforce current constraints. In the solid block case, all of the flux flows in a thin skin regions near the surface of the material (i.e. the mode which is neglected in the bulk lamination model). If you use the solid block model to model sections that are physically short in the into-the-page direction, or if you use the bulk lamination model to model sections that are thick relative to the other dimensions of the problem, you'll get nonsense. Solid and laminated results don't converge in the "nonsense" region, because they are making different assumptions as to where the eddy currents and flux are flowing.`
`robert Macy wrote:`

>     I really need help on this problem.  Trying to understand
>     how femm characterizes eddy current losses in an inductor.

>     The results of the the following analyses suggest that it's
>     better to use a solid block than large laminations.
>     Somehow, that does not seem intuitively correct.

>     First, use a huge slab of magnetic material
>      femm predicts fairly good inductance with fairly high
>      losses from eddy currents.

> `BUT! the block is not an infinite length so set the core's current to a circuit value of zero.`
> `femm then predicts better inductance and lower eddy current losses.`
> `Second, use laminations to simulate limited block length
> Make the core's current go zero due to laminations, but
> make the laminations as thick as the block, or half
> the block.`
> `BUT! the effective permeability has gone down to 11 !!`
> `femm predicts a 50 times drop in inductance!`

>     For laminations femm applies Stoll's equation (as shown in
>     the femm manual) to predict a "new" effective permeability
>     versus frequency versus lamination thickness.

>     Problem is that the difference between analyses using
>     laminations and using block material are *WILDLY*
>     different!

>     As femm uses Stoll's equation, once the laminations are
>     more than 3 skin depths thick; the effective permeability
>     becomes simply divided by the ratio of lamination thickness
>     to skin depth at an angle of 45 degrees.  It is possible to
>     drive the effective permeability arbitrarily to zilch.  It
>     seems it should have converged to solid block permeability.

> ``
> `Question: How do you correlate femm's solutions using
> Stoll's equation for effective permeability to solutions
> one gets using giant blocks?`

``

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  - [FEMM] Solid block vs laminations - does not compare
    - From: "robert Macy" <macy@california.com>

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