B and H in a solenoid with and without iron core 1

[PaulaSilva72]

I’m modeling a permanent magnet machine using a commercial FEA software. I had found some unexpected values for the magnetic field H related with magnetic flux density B, so I tried to track how the software deals with it using simple examples.

When I calculate the magnetic field H and the magnetic flux density B due to a straight wire carrying a current, the results are coherent with theory, given the same H independent of the medium where the wire is located and different values for B, such that B=absolute permeability*H.

In the case of a solenoid, I found results that I do not understand: consider a long solenoid, with a small cross section area, such that in the center of it we may consider H=NI/h (h is the height of the solenoid, N the number of turns, I current); solving FEA when the solenoid has no magnetic material in the interior (case1), results for B and H are coherent with the expected ones, but with a linearized soft ferromagnetic material inside the solenoid (case2), I found that the results for H are no longer equal to NI/h. H should be the same independent of the solenoid core material.
I believe I’m failing somewhere, but I cannot figure out what.
Thanking in advance any consideration about the results in attach.

 

[electricpete]

If you change the permeability of the flux path non-uniformly (i.e. increase the permeability inside the core but leave it low outside the core), then surely you will change the H everywhere.

Remember the magnetic circuit is like an electric circuit. H plays the role of voltage and flux plays the role of current. You have a certain fixed amount of H to go around the whole flux loop (ok it's a simplification to consider only one flux loop, but it tells the story). If mu is uniform, then H is uniform along the path. If mu changes, than you tend to get higher H in the areas of low permeability and lower H in the areas of high permeability.

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(2B)+(2B)' ?

 

[2dye4]

I don't think H should be the same. The inclusion of the core material shifts H outside of the core.

I think of the magnetic circuit analogy with H being volts and B being A of current.

If you lower the resistance in one part of the circuit with all else being equal the V drop across this resistance decreases.

It is still true that the line integral of H around a closed contour is linear with the current it is just that the H in the core is lower and the H in the air return path is higher.

 

[PaulaSilva72]

First of all, thank you both for your replies. I am new in this forum and I’m grateful for your help.
I believe I have already reached a comprehensive understand of the problem; you are right about the analogy with electrical circuits, voltage drop - magnetic field (I was confusing with the magnetomotive force). Thanks again!