For transformers there are a variety of standard shielding cans, which are round and rectangular deep-drawn cans in material according to Mu-metal®. These cans are available in various sizes with or without lids. The thickness range lies between 0.10 to 5.00 mm, as required.
What is Mu-metal®?This is a trade name of an alloy that contains 80 % Nickel, 4.5 % Molybdenum and balance Iron. Other names include Permalloy, Hy Mu80, Magnifer 7904. These alloys all have very high magnetic permeability resulting in the highest possible attenuation ratio.
How does magnetic shield work?There is no known material that can block magnetic fields without itself being attracted to the magnetic force. A magnetic shield acts as a kind of sponge redirecting the magnetic field around the shield instead of passing through the sensitive instrument that is being shielded. To be a good magnetic shielding material it must have a high permeability which means that the magnetic field lines are strongly attracted to the shielding material.
Mu-metal®, Supra 50, and Supra 36 are the most common alloys and are chosen based on the intensity of the magnetic field. If the magnetic field is too high for the material chosen it will saturate and become ineffective. In this case you can use a multi-layer shield with a combination of the above alloys. Alloys should also have a very low remanence to prevent them from becoming permanently magnetised.
What is the difference between RF and Magnetic Shielding?Radio Frequency shielding is required to stop high frequency fields (> 100 kHz) and copper, aluminium, metallised plastics are normally used because they are conductive and have very little permeability. Magnetic shielding is typically found in the 30 – 300 Hz AC range.
What is the difference between DC and AC?DC is direct current that flows in one direction only such as the fields emitted from the Earth or produced by magnets and some motors. AC is alternating current that reverses its direction over a short period and these fields are generated by typical 50-60 Hz electric power equipment. Magnetic shielding is effective for both of these types.
What is the best shape for a shield?
The most efficient shape is spherical but this is very difficult to produce and largely impractical in most shielding applications. The next best is a cylinder with closed ends. These end caps significantly increase the shielding attenuation. This is followed by a box shape but the corners need to have a large bend radius to minimise flux leakage. If possible, do not use a flat sheet .
What is magnetic permeability?It is a materials ability to absorb magnetic flux, a ratio of flux density to field strength. The higher the permeability, the better the magnetic shield attenuation performance.
What is field attenuation?This is also known as the shielding factor (S) and it is a ratio of the magnetic field strength outside of the magnetic shield (Ha) and the resultant field on the inside of the shield, i.e. Ha/Hi (no units) or S=20 x log(Ha/Hi) (Db). There are various formulae based on the permeability of the material, the shape and size of the shield and the material thickness. In most cases these formulae are only approximate and are for DC fields only.
For a closed shielding can:
S = 4/3 X (Mu x d/D)
Mu: the permeability (relative)
d: material thickness
D: Shielding Diameter
For a long hollow cylinder in a magnetic transverse field:
S = Mu x d/D
For a cubic shielding box:
S = 4/5 X (Mu x d/a)
a: box side length.
In the case of multiple layer shields with air gaps provided by insulating spacers, the shielding factors of the individual shields are multiplied together resulting in excellent shielding factors.
For a double layer shield :
S= S1 x (S2 x (2 x change in diameter / diameter) )
Can you use magnetic shielding materials at high vacuum?Mu-metal® is similar to stainless steel so out gassing is minimal.
Do cryogenic temperatures affect the performance of Mu-metal®?Mu-metal® is affected by cryogenic temperatures. The saturation induction remains the same but the permeability decreases. At cryogenic temperatures we need to use a special cryogenic Mu-metal®.
Why is a final heat treatment required for Mu-metal®, Supra 50 and pure iron?After plastic deformation a high temperature heat treatment is required to rearrange the crystal structure as well as allow the grains to grow. Without this final heat treatment the magnetic properties and the shielding attenuation will be much reduced.
Can a shield be re-heat treated?Yes, it can be if it has received knocks or if there are concerns about the shields shielding ability.
Can you weld Mu-metal®?TYes, without a problem but it must be fully heat treated after welding.
Why are both Supra 50 and Mu-metal® used together?Mu-metal® has a very high level of permeability but a relatively low level of saturation, whereas Supra 50 has a lower level of permeability but a level of higher saturation. Supra 50 is used closest to the very strong field to protect the Mu-metal® from saturation.