Encyclopedia Magnetica™

Skin depth vs. frequency for some materials. The equation used for skin depth calculation was:

where: δ - skin depth, ρ - resistivity of the material, ω - angular frequency = 2·π·f, μ - absolute magnetic permeability (μ_r·μ₀).

The materials are as follows:

• Mn-Zn - soft ferrite 3C90 from Ferroxcube, initial permeability = 2300 (initial, 25°C, 10 kHz) and resistivity = 5 Ω·m based on catalogue data (3C90), such ferrite is not a good conductor so the curve is shown here as dashed, just for demonstration how much better such materials are at higher frequencies
• Al - aluminium, relative permeability = 1, resistivity = 2.7e-8 Ω·m based on data from MatWeb (Al)
• Cu - copper, relative permeability = 1, resistivity = 1.7e-8 Ω·m based on data from MatWeb (Cu)
• steel 410 - magnetic stainless steel 410, permeability = 1000 and resistivity = 5.7e-7 Ω·m based on data from MatWeb (steel 410), the highest value of quoted permeability (1000) is used for “worst case” scenario
• Fe-Si - grain oriented electrical steel (around 97% Fe and 3% Si) from AK Steel manufacturer, permeability = 29,000 and resistivity = 5e-7 Ω·m based on manufacturer bulletin (Fe-Si), maximum value of permeability is read out from the radial plot and it is used to demonstrate the “worst case” scenario
• Fe-Ni - permalloy (trade name of VACUUMSCHMELZE GmbH & Co. KG is Mumetall), alloy mainly of Fe (around 20%) and Ni (around 80%), values of permeability = 250,000 (Fe-Ni permeability) and resistivity = 5.5e-7 Ωm (Fe-Ni resistivity) are based on the manufacturer data.

Red vertical line indicates 50 Hz frequency.

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