Trouble modeling magnets

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JP

JP

June 18, 2012 8:25pm UTC

Trouble modeling magnets

Hello, I am modeling a tool that is two opposing magnets (i.e., north faces north or south faces south), which have a solenoid (copper wire wrapped around a soft iron core) between them.

I have attached my model. (I surrounded my device by rectangles/cylinders of air so I can implement a moving mesh.)

My magnets correspond with Ampere's laws 2 and 3 under the Magnetic Field physics, and their magnetic fields are described by the constitutive relation of remanent flux density. My questions are:

1. How can I model them so the like poles face on another? I've been doing it by making one B_r positive and the other negative.

2. My material is anisotropic, but for mur, when I select "User defined" and then "Anisotropic" and then click on any other node in the Model Builder window, the selection changes back to "Isotropic". Why can't I model them as being anisotropic magnets? Do I have to change that matrix? Sorry, I don't understand what that is.

Thanks! Any input would be much appreciated.

Attachments:   comsolmod.mph  

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Ivar Kjelberg

Ivar Kjelberg

June 19, 2012 4:59am UTC in response to JP

Re: Trouble modeling magnets

Hi

I mostly use the magnetisation as material properties for permanent magnets, you need to define one node per magnet orientation, except if there are easy ways to define the relative orientation i.e. with a sign(y) operator to inverse the one one ht positive Y side w.r.t the negative Y side. With anisotropy you mostly should work with one node per orientation.

As I have not yet received the 4.3 CD (it's in the post somewhere) I cannot open your model

--
Good luck
Ivar

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JP

JP

June 19, 2012 1:10pm UTC in response to Ivar Kjelberg

Re: Trouble modeling magnets

Thanks for your input, Ivar! One question though:

If i'm just given the properties:
- Max Energy Product (BH)max
- Residual Induction Br
- Coercive Force Hc
- Intrinsic Coercive Force

along with some physical properties, do you know how I can use these to realize the magnetization in Comsol?

Thanks!

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Ivar Kjelberg

Ivar Kjelberg

June 19, 2012 2:03pm UTC in response to JP

Re: Trouble modeling magnets

Hi

as first guess I would try to use the Coercive Force Hc in [A/m] as the magnetisation value, provided you have a high quality Nd or SmCo type magnet with a straight Hc-Br line

Then check the Br you get when you put the magnet almost in short circuit via an unsaturated iron in a small 2D model, you might want to consider to use a mur of Br/Hc too

By the way, somebody else has a better approach ?


--
Good luck
Ivar

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JP

JP

June 19, 2012 2:37pm UTC in response to Ivar Kjelberg

Re: Trouble modeling magnets

Thank you for the quick reply!

Since I have two Alnico magnets, which don't have a similarly straight demagnetization curves as Nd or SmCo magnets, it would be less accurate to use Hc as the magnetization right? Could you please explain the process of making a 2D model to check the Br? I don't really understand the set-up or the motivation.

Also, do you know why I might be getting errors when I change my material type of the magnets (in the Ampere's Law node) from the default "non-solid" to "solid"? The error is saying that it failed to evaluate variables mod1.Aphiz, mod1.mf.BR , and mod1.mf.HR.

Thanks!

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Ivar Kjelberg

Ivar Kjelberg

June 19, 2012 8:24pm UTC in response to JP

Re: Trouble modeling magnets

Hi

indeed I suspect you need then to define your magnetisation with a lookup table, which means also making your model highly non linear.

When I have a doubt I often make a simple model, i.e. the one of a Hc to Br curve measurement device, by looping my magnet poles around with a perfect iron core + a small air gap that I then increases just to verify that I get a flux response with reasonable values w.r.t a ideal magnet Hc to Br load curve

Your error seems strange, it could come from some update errors internally to COMSOL (when you change the physics), sometimes its worth to reset the moel save it and reload it (useful in 4.2a at least, could still be valis in v4.3 ;)

--
Good luck
Ivar

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Ehsan Asadi

Ehsan Asadi

April 21, 2013 7:07pm UTC in response to Ivar Kjelberg

Re: Trouble modeling magnets

Hi,

I have 2 questions:


1-What is the difference between solid and non solid ?
Does non solid means that it can not get deformed?

2- For the domain which we define the coils(using multi-turn coil domain), should we also define cooper and assign it as the material for that domain? This is what have done in the above example, but I highly doubt it!

Thanks,
Ehsan

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Ivar Kjelberg

Ivar Kjelberg

April 22, 2013 7:17am UTC in response to Ehsan Asadi

Re: Trouble modeling magnets

Hi

the "material type", from what I understand is particularly useful in "Solid" and FSI, normally you can leave it to its default with little risk, and in ACDC alone, I would say no risk ;)

I agree it could be better explained in the doc. Probably the best place to read is a search for " "material type" feature "
all three words with inner """

It defines on which of the material or spatial frames the material is locked to. These two frames dissociate in Solid when non-linear geometry is selected (by default or by the user) and for FSI or when mixed with CFD modules

For the multi turn domain it's easier to give it a material definition, by default you will see that epsilon_r and mu_r are read from the material node, but NOT the resistivity or conductivity sigma, that is defined in the multi-turn BC node, you can also set the other material properties to user defined whence no need for materials for that domain, or you can leave "air" also for the domain, the multi-turn BC node will pick the desired values for the ACDC current calculation part

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Good luck
Ivar

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