回复: [femm] Re: Calculation of self and mutual inductances

["si hang" <stsc_sh@xxxxxxxx>]

Before you caculate inductance or mutul inductance, you have to formulate
your problem in such a way that all the currents (space, surface or linearly
distributed) but one are set to zero. There must be no permanent magnets in
your model. In that case equation above becomes xtremely simple and you can
get inductance value as :
E
L = 2 * -----
I^2
where E is stored magnetic energy and I is the only current(amp-turns).
This question is used for selfinductance caculation. It's more exact than
flux linkage caculation.

Also, you can get mutul inductance use flux linkage method by the below
equation:

φ
L = ----
I
where φ is the flux linkage with the coil excited by current I(amp-turns).

for plannar problem:

∫A ·dS ∫A ·dS
φ = RightSide(--------- ) - LeftSide(---------)
∫dS ∫dS

where A is the vector magnetic potential; RightSide and LeftSide denote the
right and the left side of the coil accordingly.

The difficult to use this equation is that we can't get dS from femm. But if
we choose a uniform mesh density in coil cross section and the mesh is dense
enough,
we can assume that each dS is a constant in the cross section. So the
equation become easy to caculate by femm:

φ = RightSide( ∫A ) - LeftSide( ∫A )

Hope this can help you.

si hang
-----Original Message-----
·¢¼þÈË: Rob <rtickle@xxxxxxxx>
ÊÕ¼þÈË: femm@xxxxxxxxxxx <femm@xxxxxxxxxxx>
ÈÕÆÚ: 2000Äê12ÔÂ29ÈÕ 16:58
Ö÷Ìâ: [femm] Re: Calculation of self and mutual inductances


--- In femm@xxxxxxxxxxx, P.Neumaier@xxxx wrote:
> Hello,
>
> can anybody help me? When I calculate the self inductance of e.g. a
> coil encircling a copper rod using the "A.J" integral I get the
> correct unit "Henry * A?quot;. But this integral yields incorrect
> results, which can be prooved when defining a problem, which can
also
> be solved analytically. However, using the integral "A" for the
> mutual inductance, the value is correct, but the unit is given
> as "A*m?quot; which is obviously not the unit for an inductance.
>
> Thanks for any answer in advance.
>
> Peter

Are you dividing the result FEMM gives you by the current in the coil
squared? If so are you using the correct value for the current? If
you have multiple turns the current scales with N... I did some
calculations like this when I started using FEMM and got some
incorrect answers due to faulty current values. Be sure to add up all
cross sectional areas of conductor multiplied by their respective
current densities. This solved the problems I was having...