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# Re: [femm] Inductances in electrical transformers

- To: femm@yahoogroups.com
- Subject: Re: [femm] Inductances in electrical transformers
- From: David Meeker <dmeeker@ieee.org>
- Date: Tue, 26 Nov 2002 23:53:11 -0500
- References: <002a01c29401$f71ac4c0$db272dd5@tin.it>
- User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.1) Gecko/20020826

`Fabius wrote:`

> `Hi all.
> I am trying to calculate mutual and leakage inductances in single phase
> trasformer as in attached file.
> I have modeled this transformer with core EI having a cross section of 50x80
> mm, 220 turns at the primary and 117 turns at secondary.
> The core extends 0.08m in the into-the-page direction.
> I also added a small gap between E and I to be sure that there is no
> relevant saturation.
> First i evaluated the self inductance Ls1 = 290.6mH and Ls2 = 82.2 mH using
> A,J integral as explained in tutorial.
> I set primary windings to circuit primary+ and primary- having + or - 10A,
> so the current I1 is 10/220 = 45.45 mA
> Then i turned off primary winding and set secondary to a circiut carring 10
> A.
> Theese results are in according with the classical formulas.`
> `Then i evaluated mutual inductance as described in Femm manual at
> page 34. N2= 117 , I1=10/220 , a2 = 1.139E-3 m², integ A1dV2 = 4.339E-7 but
> the result was M = 77.1 mH
> ( i multiplied the integ x 0.08 , the 3 dimension of core )
> So the cooupling factor k = M / sqr(L1*L2) = 0.498`
> `that is wrong. ( I think k is near 1 )`

>     What is wrong in this model ?
>     Can someone help me ?
>     Best regards
>     Fabio

`You're using the wrong area for a2--this is probably ambiguous the way that I've written it up in the manual. This area is meant to be the cross-section of the coil such that the current density in bulk coil 2 is: J2 = n2*i2/a2. Looking at your geometry, the a2 area should be 9 mm * 61 mm, or 549 mm^2. You've used twice that area, so you calculated exactly half the coupling factor that you should be getting.`
`Dave.`

    --
    David Meeker
    dmeeker@ieee.org
    http://femm.berlios.de/dmeeker

``

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  - Re: [femm] Inductances in electrical transformers
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- References:
  - Inductances in electrical transformers
    - From: "Fabius" <xfabius@email.it>

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