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Anyone with insight on blocking a magnetic field?

  1. Feb 13, 2008 #1
    I know RF signals can be blocked using a copper screen, for instance lining the walls of a room in order to block out cell phone signals. What I'd like to find out is if it's possible to similarly block a magnetic field, something generated by an electric motor for example. Any feedback would be appreciated.
     
  2. jcsd
  3. Feb 13, 2008 #2

    NoTime

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    You could try plain sheet steel.

    They do make specialized material for magnetic shielding.
    MuMetal is one, but this tends to be $$$.
     
  4. Feb 13, 2008 #3
    Yea, mumetals are best magnetic shields around due to their unusually high magnetic permeability.
     
  5. Feb 14, 2008 #4

    f95toli

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    Most types of steel are not very good for shielding (the "impurieties" in the iron that makes it into a steel also lowers the permeability), a sheet of iron is usually a much better choice. Just make sure you find a piece which is not ferromagnetic.
    Mumetal, permalloy and other specialised materials are much more efficient than iron but unfortunately also quite expensive; although if it is a demanding application and you have the money I would say mumetal is probably worth it.
    Note that you can't just buy sheets of mumetal and bend/weld it into shape; bending it (or even knocking it) can significantly lower the permeability (or even make it ferromagnetic) so the shield must be heat-treated (controlled heating to about 1100 degrees C) once it has the right shape.
     
  6. Feb 14, 2008 #5

    NoTime

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    You are going to be looking for a very long time.
    I might suggest that you look up the definition of "ferromagnetic".
     
  7. Feb 14, 2008 #6
    A Faraday cage won't block out a magenetic field right?

    Is there a cheap way to measure a magnetic field...other than just seeing if one exists by messing around with a compass?
     
  8. Feb 14, 2008 #7
    Yes. Hall effect sensors are used to the measure magnetic field strength accurately. They are cheap, and easy to implement.
     
  9. Feb 14, 2008 #8

    OmCheeto

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    Made out of the right material, I don't see why not. Magnetic shields work not by blocking the magnetic fields, but by redirecting the lines of flux. Kind of like quantum kung fu. The effect is basically the same though.
    DC MAGNETIC FIELD VIEWER CARD (Cat. #A1120)
    $2.75
    http://www.lessemf.com/dcgauss.html

    Though I don't know why you just don't use a compass if you already have one.
     
  10. Feb 14, 2008 #9

    NoTime

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    That card thing looks interesting. Never saw one before.
     
  11. Feb 14, 2008 #10
    #1 is all about frequency of the field. A common rule of thumb is to use a Faraday cage above 100 kHz, high mu shielding below 100 kHz. Since you're talking about a motor here, most of the noise should be below 10 kHz - not much of a challenge in theory. In practice, the trick is to really, really watch gaps in the high mu metal.

    #2 If you're not too much about accuracy, you can buy a decent magnetometer gauge for less than $40. Lots of machine shops have them - in fact I'm pretty sure I've got one down in my shop.
     
  12. Feb 15, 2008 #11

    f95toli

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    Sorry, what I meant to say was that one should be carefull not to use a piece that has been magnetized :redface:
    Iron is of course alway ferromagnetic, regardless if the domain are lined up or not.
     
    Last edited: Feb 15, 2008
  13. Feb 15, 2008 #12

    f95toli

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    Not quite true, the mechanisms are quite different. Materials with high permeability are good for shieldning mainly because of their domain structure which "adapts" to resist being lined up when it is exposed to an external field, i.e. it is not a question about inducing currents as in the case of a Faraday cage. The reason why these materials do not work at high frequencies (above a 1 MHz or so) is simply that the domains are too "slow" and can't keep up.
    In demanding applications it is therefore neccesary to combine many types of shields that work well in different frequency ranges, one should preferably also use several layers of mu-metal separated by spacers. I often use two-layer mu-metal shields as the first room-temperature layer (followed by other shield, including two layers of superconductors) and as far as I remember the shielding factor for two 0.5 mm layers of "standard" mu-metal is something like 10 000 at low frequencies (up to 100 kHz or so).

    Also, even if there are opening is the shield it is still possible to get a reasonably good magnetic shielding factor. The "trick" is simply to keep the openings as far away as possible from whatever you are trying to measure. Magnetic shields are often made in the form of cylinders open at one end only, and as long as the distance between opening and whatever you are trying to measure is a more than 2-3 times the diameter of the cylinder it will still shield quite well at low frequencies (note that this does NOT work at high frequencies, then you DO have to close all openings).



    (despite my rather embarrasing misstake above, I do have quite a lot of practical experience with magnetic shielding since I spent several years working with dc-SQUIDs).
     
  14. Feb 15, 2008 #13

    OmCheeto

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    Well..... perhaps when I said "using the right materials" I meant a solid superconductor sphere. Given the OP's nick, I didn't think building a Meissner effect device should be the first choice to shield his electric motor.

    Or was it my quantum kung fu analogy that you were referring to when you said "not quite true"?
    Joule'ian Jujitsu perhaps?

    On a more serious note, the saturation of the shielding material now brings up a paradox, in my feable mind anyways, that a spherical object is supposed to completely shield either internal or external electric and/or magnetic fields.

    Or did something I thought I understood in physics class about 30 years ago completely steer me wrong?
     
  15. Feb 16, 2008 #14
    I wasn't very clear when I mentioned gaps. I didn't mean openings - I've used open ends as well - but gaps similar to what you would get with a few spot welds along the roll seam rather than a continuous weld. Sorry about the confusion.

    I am interested by your sucess in going to 1 MHz with a shield; if ever again I need to do high frequency shielding, I'll look at how that works. Thanks.
     
  16. Feb 16, 2008 #15
    Thank you all very much for your feedback. Since I posted my question I've also continued researching for an answer on my own and have learned that my citing a motor's magnetic field as an example in my question, was completely in error. What I'm actually needing to find out instead, is wether or not there is a way to shield against the magnetic field of a permanent magnet, which I've come to understand is something of a different animal. I'm working with magnets that have field strengths in the 2T - 5T (2000G - 5000G) range. Can anyone elaborate?
     
    Last edited: Feb 16, 2008
  17. Feb 19, 2008 #16
    That thing brings back memories of a magic thingy that 3M used to make for use with 2 inch professional video tape...

    You placed it on the surface of the tape and it showed the tracks recorded by the video head... I wish I'd snaffled a couple when I had the chance...

    It was used in the dim & distant past when tape editing was done with a razor blade...
     
  18. Feb 19, 2008 #17
    The control system of my rectiformer started behaving strangely when subject to stronerg magnetic field. Relays and solenoids in a 50 gauss DC field. How can I shield effectively to get down to 5-10 gauss? Racks with components ( 50 cm x 50 cm) in cubicle (1 x 2 m). Mu metal sheet? Steel plates? Any experience on this?
     
  19. Feb 20, 2008 #18
    OmCheeto,

    Thanks for the link. Though I have a gaussmeter, that viewer card looks handy.
     
  20. Feb 21, 2008 #19
    2" video tape! OMG! Another OF. Remember when the 40 pound Ampex units were called "portable"?
     
  21. Feb 22, 2008 #20
    This card you can use for free. Just display it on a CRT monitor and watch the change of colour as you put the magnet nearby. :wink: :biggrin:
     
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