# Discussion Forum

## Simulation of the magnetic fied

 Topics: magnet, 4.2a, AC/DC Module, Physics, Geometry
RSS feed   |   Turn on email notifications   |   3 Replies   Last post: December 7, 2012 6:24am UTC

Petr Koutný

December 3, 2012 8:32pm UTC

Simulation of the magnetic fied

Hi,
I have one physical NdFeB magnet and I need to simulate this magnet in Comsol 4.2a, but I dont know exactly how to do it.

I know these parameters of the magnet:

Remanence Br = 1380 mT
Coercivity Hcb = 876 kA/m
Intrinic coercivity Hcj = 876 kA/m
Maximum Energy Product BHmax = 334 kJ/m3

Dimensions of the magnet (cylindric magnet): height = 5 mm; weight = 8,5 mm
The magnet is magnetized axially.

So I used 2D axisymmetric geometry for this shape of the magnet, entered dimensions and now... I tried used mfnc physics, but I dont know exactly, which of these physical parameters (Br, Hcb ...) should be used for modelling magnetic field around this magnet.

I wanted to use this formula for magnetization

M = (1/u0)*B-H

but I dont know, if the B and H in this formula is the same as Br and Hcb or Hcj of my magnet, or if I can simulate magnetic field without this parameter.

Please can anybody help me? Thank you very much.

Peter.

Ivar Kjelberg

December 4, 2012 7:44am UTC in response to Petr Koutný

Re: Simulation of the magnetic fied

Hi

that is a good question,
I have a way, but I have never really managed to validate it through detailed measurements, from the few measurements I have I trust it to 10%.
So my way is to define a specific "Magnetic Flux Conservation 2" for the magnet and set the magnetisation M to the Hc value of the magnet. Assuming that it is a hard magnet with a constant mu_r close to 1. This is not fully your case as Br/Hc = 1.25 quite high from my experience but OK.

You have hereby a model, where the magnet is in free air, and is shortened with a perfect iron mu_r=4000 and an air gap you can adjust. If you set it to 0.5 or 0.1 mm you will see that the Hz value in the magnet and the airgao has a difference getting close to Hc, and the Bz is getting close to the Br, which for me is showing that the model is quite OK

--
Good luck
Ivar

Petr Koutný

December 6, 2012 9:24pm UTC in response to Ivar Kjelberg

Re: Simulation of the magnetic fied

Hi Ivar,

thanks for you answer. I tried open your model, but I cant. Comsol said that the model was created in latest version. I have version 4.2a. Second problem, I dont understand the part, where you speak about shortened with perfect iron. I have only the NdFeB magnet and no iron with ... or if it is for the model, that I cant open... Please for clearing.

Thank you
Petr

Ivar Kjelberg

December 7, 2012 6:24am UTC in response to Petr Koutný

Re: Simulation of the magnetic fied

Hi

sorry and I do not have the older versions up running

The story of shortening with iron is just my way of validating the model, not any need for your final model. But when you define a magnet the Br corresponds to a magnet in free air/vacuum, the Hc to a magnet "short circuited" by a fictitious INF mu_r material. So one I have defined my magnet in 2D axi in my case I check that the filed just above the magnet reproduces the Br and Hc values.

the magnetisation is a material property, but the true field around a magnet depends also on the magnet shape, you see that in the field concentration on all corners in ACDC (and elsewhere) if the absolute values you calculate by FEM on sharp corners might be wrong, the field concentration effect is a true physical effects (lightening goes or arrives on sharp edges etc) therefore I always round off all my sharp edges (whenever possible in ACDC

--
Good luck
Ivar

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