Discussion Forum

Hello,

I have a question about simply modeling permanent magnets; if this has been answered already I will remove this discussion. I walked through the "permanent_magnet" model included in the model library, in which a horseshoe-shaped magnet is modeled, and I noticed that the straight sections of the magnet are made of air, whereas the curved part is made of iron. As usual, the surrounding space is a block of air. The domain condition on the straight parts is Magnetic Flux Conservation using a Magnetization condition of 750[kA/m] in the x-direction.

Now obviously in real life, all parts of the magnet are made of iron (or some other alloy), so I was initially confused as to why the model chooses to make it out of air. I eventually realized that whether it was made of iron or air didn't matter, because the B-field constitutive relation for Magnetization is u0*(H+M), which doesn't depend on the relative permeability of the material used on the domain. Thus, whether air or iron is used, the result is the same. This makes sense to me, mathematically.

However, physically, I am confused. Why couldn't we use a Remanent Flux Density condition on the straight parts instead of a Magnetization condition? The Remanent Flux Density constitutive relation takes into account the permeability of the domain material (in this case, the iron magnet); however, when I tried to use Remanent Flux Density instead of Magnetization by using a B_r value of u0*M and making the straight segment out of iron as in real life, the external H field that results is different than the result obtained using the Magnetization condition and the same numbers. Which one is correct? Am I simply confusing definitions? And what reasons are there for choosing Magnetization versus Remanent Flux Density, ie, when is it good to choose one over the other?


Also, on a side note: when you choose the Magnetization constitutive relation B = u0*(H+M) in Magnetic Flux Conservation, I noticed that the Equation section seems to have a mistake; the second equation listed (Gauss's Law of magnetism) is listed as del*(u0 ur H) = 0 which is the same as what you see for all the different constitutive relations, but since Gauss's Law is del*B = 0, by the Magnetization condition this should instead be del*(u0*(H+M)) = 0. Is this just a formatting mistake, or does the calculation actually use del*(u0 ur H) = 0? Or am I making a mistake?