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# Electromagnetic shielding

electromagnetic shielding FEA finite element analysis, field attenuation by magnetic shielding

Uniform external magnetic field is produced by the electric magnet. The shield with the measuring coil inside is placed between its poles. EMF in the coil is measured: in case of DC current in the coil - by ballistic galvanometer (in the moment of switching on), in case of AC current - by use of voltmeter.

Geometry
The shield consists of two halves. The gap could be enlarged up to 2 mm by sheets of non-magnetic materials.

Given
Electrical conductivity of copper 57 MS/m.
Electrical conductivity of steel 10 MS/m.
Relative magnetic permeability of steel μ=1000.
External magnetic field flux density B = 0.139 T (peak value for AC magnetic problem).

Find the level of magnetic field reduction inside the shield. Shields made of steel and copper of the same geometry are analyzed.

Solution
Due to symmetry of the model the simulation domain is requced to a quater of a full model.
External magnetic field is specified by the boundary condition A=0.0695 Wb at the top boundary.

Results
Current density in cylindrical and spherical shielding:

Shielding coefficient - relation of magnetic flux densities outside and inside the shield.

• Time-harmonic magnetic field, f=50 Hz
Shield type Flux density inside, mT Problem file
Steel cylinder 0.013 lab4cFe.pbm
Steel sphere 0.082 lab4Fe.pbm
Steel sphere with gap 36 lab4Fe+.pbm
Steel cylinder with gap 40 lab4c_Fe+.pbm
Copper cylinder 69 lab4cCu.pbm
Copper cylinder with gap 71 lab4cCu+.pbm
Copper sphere 98 lab4Cu.pbm
Copper sphere with gap 100 lab4Cu+.pbm
• DC magnetic field
Shield type Flux density inside, mT Problem file
Steel sphere 1.5 lab4_f.pbm
Steel cylinder 2.1 lab4c_f.pbm
Steel sphere with gap 39 lab4_f+.pbm
Steel cylinder with gap 45 lab4c_f+.pbm

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