Re: [femm] Inductances in electrical transformers

[David Meeker <dmeeker@xxxxxxxx>]

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@xxxxxxxx
http://femm.berlios.de/dmeeker