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tvrgeek 4th January 2012 10:42 PM

Shielding metal
 
I was looking to add some shielding around a power transformer. So a bit of research and sure enough, what I want is a can made of mu-metal. It is about 5000 times as effective as plain steel. Well, you can't just get a sheet and form a can as it has to be annealed after forming in a pure hydrogen environment. So, I guess that reduces me to electrical steel, also known as the laminating steel used for transformer cores as it is a good 400 times better than plain steel.

Has anyone had any success in adding shielding around transformers? Anyone know where to get electrical steel sheets or boxes made of it?

gmphadte 5th January 2012 03:19 AM

Copper may be easier to find and is used as a belly band around a transformer.

Gajanan Phadte

jcx 5th January 2012 03:26 AM

low carbon steel is so cheap, available that you just use it, may take more, a couple of air -spaced layers but its not worth getting special material unless you know what you're doing, have insane space, weight constraints - and it usually means knocking down the level with cheap steel, air space 1st to avoid saturating the mu metal anyway

Conrad Hoffman 5th January 2012 04:26 AM

My experience is that thin mu-metal made into a box around a transformer is not terribly effective and not as good as heavier plain steel. Data sheets can be misleading. You'll get more field reduction for your money by reorienting the transformer; don't feel it has to be mounted parallel to the chassis walls. Distance is also a better friend than fancy metals. Depending on your construction, and this may be hard to believe, a heavy aluminum plate, about 5/8" minimum, will kill fields very effectively by dissipating them as heat from eddy currents. Strange but true.

tvrgeek 5th January 2012 07:00 PM

Yes, eddy currents cause an opposing field provided there is a place for the currents to flow, i.e shorted winding from copper around the windings. Aluminum is considered in the literature as useless as a magnetic shield for LF as it has the same permeability as air. I do have some half inch plate so I may play a little. For power transformer shielding, we are talking about less than 1Khz.

I guess mild steel it is. Most of my issues are to retrofit existing problems where I have very little leeway on position, or it is impracticable to remote locate the transformer.

Turns out, mu-metal is not suitable for a DIY. Although it has a permeability of 50,000 vs 100, if you bend it at all, it is then just as poor as soft steel. It has to be annealed as it's magic is dependent on the grain structure. I was hoping to find a source of electric laminating steel in less than hundred ton lots.

Conrad Hoffman 5th January 2012 09:13 PM

I've mentioned the aluminum trick before and had it dismissed, but don't be too quick. Long ago I was working with amplifiers for atomic force microscopes where the slightest hum will destroy the image and feedback stability. It took reorienting the power transformers at an angle, putting the low level circuitry in the lowest field direction, and then putting in the 5/8" aluminum plate between the transformers and amplfier circuitry. No box, just a flat plate. The effect requires very thick plate; I seem to remember that 1/2" isn't effective. Some alloys are better than others, but I think plain old 6061 is fine. The plate brought the residual 60 Hz field down to a negligible level. All this assumes you've already done everything else correctly regarding star ground and such.

tvrgeek 5th January 2012 09:41 PM

As I have no 3/4, I will try two halves. Books are one thing, direct experience another.

Yea, lots of other tricks. Ground lift, Adding a belly band, hexfred bridge and total tri-star wireing, better placement of bypass caps, lower esr caps etc.

jcx 5th January 2012 10:51 PM

you can search "skin depth", a calculator:

Ness Engineering Tech Data - Skin Depth and Skin Effect

gives 1/2" for 6061 @60 Hz - and one skin depth is only ~40% attenuation

Conrad Hoffman 6th January 2012 12:19 AM

That explains why 1/2" isn't enough. I hang around machine shops, so heavy plate isn't uncommon, and given the crazy stuff audio people do, putting a plate in the box isn't that radical. OTOH, if shopping at the local home store, steel will be more available. :)

jackinnj 6th January 2012 12:23 AM

I thought with mu-Shield you can bend the material, but you can't kink it. Supposedly, you can also drill mu-Shield with little ill effect.

Will try the "Aluminum trick" -- where had you posted it previously?

One of the Linear Tech application notes had a nice graphic of the emf profiles of different transformer configurations, but I can't find where I put the dang'd thing.


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