MAGNETIC FIELD TRAPPING

[Ethnan]

I have about 60A of current flowing in a 300 turn coil. I need the magnetic flux from this coil totally trapped in a soft iron or Mumetal or Metglas or any other soft magnetic material that is affordable.
So I would wrap the coil toroidally around the soft magnetic material. Now I designed with metglas of 35 x 35mm ring thickness using FEMM and got the flux density in the metglas to be more than 400 Tesla! What is the practical implication of this design? (Magnetic Saturation of Metglas occurs at about 2Tesla or less.)

 

[MacGyverS2000]

Totally trapped? No chance.

What problem are you trying to solve?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

Totally trapped in a practical sense, i.e., >90%, is certainly doable, but so what? You can have fairly high densities with just a core, which is why practical solenoids have such things.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[MagMike]

Ethnan, do you believe the 400 Tesla value is realistic?

 

[argotier]

It looks like the material characteristics of the FEMM model is off. Do you have the proper B/H or relative permeability curves?

400 T could be the value for a non-saturated magnetic material, but it will get saturated at that NI (300 turns x 60 A).

Once saturated, the induction in the material will fall (to less than 0.1 T, doing some quick calculation "by hand").

 

[Ethnan]

Many thanks guys. I appreciate your valuable time.

I need to shield a nearby electrical system and I have control ONLY IN TRAPPING THE MAGNETIC FIELD FROM THE 60A AC coil.

Maybe I should look at the BH curve again of metglas. I will appreciate any data that can help on this.

Just thinking, the permeability of metglas should be more than 400times that of pure iron. Shouldn't it make sense for its flux density in this design, for same cross-sectional area as iron, have about 400times flux density value that of pure iron?

 

[IRstuff]

Then you shield it with an appropriate material and thickness and be done. You don't need perfect shielding.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Gymnast]

You can not get a magnetic field anything near 400 T. As you write. Metglass saturates at about 2 T so you cannot get above that.

In air you can get higher magnetic field strength. Posibly quite high with pulse currents, but I doubt 400 T in normal applications. Super conducting magnets used in MR scanners produce a field strength of about 3 T used for full body scanning and about 20 T for small samples. The energy stored in such fields are substantial.

For some physical experiments, metglass ring cores can be used as high power fast switches, because they saturate fast. They are able to switch faster and higher power than spark gabs.

 

[MagMike]

Ethnan: From the FAQ in the FEMM wiki ( Link ):

FEMM returned unexpectedly high flux densities for my problem. What's going on?
FEMM will only give reasonable results if you are interpolating between defined points on the B-H curve for the material. Depending on your particular problem, you may need to add points to your B-H curves so that you are always interpolating rather than extrapolating.

You pushed beyond the defined BH curve in the material file.

 

[waross]

A sheet of aluminum between the magnetic field source and the affected objects is effective.
This method has been used to reduce the magnetic field surrounding reactors to avoid inducing circulating currents and damaging heat in nearby reinforced concrete walls.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ethnan]

Dear All,

I am sorry I have been offline for quite a while due to family concerns. But I do appreciate all your responses and time you spent on my question. I will return to my work soon and update you all on what I was able to do. Thanks once again

 

[Ethnan]

Yes, I am back to my project.! I loaded the correct BH data for metglass and other materials I have been trying. Indeed that was the problem. I got a flux density of about 1.2T for the metglas. I have used iron of thickness of about 20mm and got very good shielding.

Thanks everyone. I really appreciate.

 

[Comcokid]

Torodial shapes do a good job of containing the magnetic field even if the core saturates. It goes along with the good self-shielding characteristics of toroids. When you do fully saturate, additional dI/dt will be controlled only by the resistance of the wire. Metglas has a pretty sharp BH loop, so the transition into saturation will be abrupt.

Some of the newer nanocrystalline cores have a higher flux density - up to around 2 Telsa, if flux density is what you are after. VACUUMSCHMELZE GmbH, or VAC for short, is one of several companies making cores with this material.

 

[Ethnan]

@ Comcokid. I would love to check out this material for my other projects. Thanks

 

[Comcokid]

Metglas and Nanocrystalline are similar technologies. A metal melt quenched at 1,000,000 C/sec into a thin ribbon. Both are metallic glass (no large scale crystalline structure). Metglas was developed in the 1970's. Nanocrystalline developed in late 90's with additional annealing steps to create uniform grain size of about 10um.

I find the nanocrystalline used for some common mode noise chokes are pure magic. Just this month I was called in to another engineers project with a emission issue at 500 MHz. Two turns on the proper ferrite for that frequency didn't do the job. Two turns on a similar size VAC core was 30db better! Now, this issue was 100x the frequency range of nanocrystalline, but the fact that it corrected the issue so totally showed that there was current imbalance at a very low frequency that was saturating the ferrite making it not work at the high frequency. The design will ship with only one nanocrystalline for the choke. Go figure!