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My picture below shows a coil carrying AC current, generating an alternating magnetic field. On top of the coil I've drawn a disk of high-permeability metal. Such as carbon steel, or mu-metal.

  • If the disk is made of carbon steel, the oscillating magnetic field induces eddy currents which, thanks to the metal's high permeability, are "concentrated" in a thin layer close to the bottom, which is perfect for heat generation; it's an induction cooker.
  • But if the disk is made of mu-metal, the field high permeability of the material provides a "tighter" path for the magnetic field, and decreases the strength of the magnetic field beyond it, but without inducing currents, eddy currents, or an opposing magnetic field; it's a magnetic shield.

Mu-metal and carbon steel have many different material properties. Please can someone help me understand what the relevant differences are, and exactly how this results in such different behaviour? Thanks!

My drawing showing the two situations above.

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  • \$\begingroup\$ Why do you think there aren't eddy currents in the mu metal? \$\endgroup\$ Jun 17, 2021 at 12:17
  • \$\begingroup\$ A higher permeability material will still produce eddy currents providing it has conductive properties. However a ferrite shield won't produce any significant level of eddy currents so, maybe the picture source is giving poor information? \$\endgroup\$
    – Andy aka
    Jun 17, 2021 at 12:18
  • \$\begingroup\$ Perhaps this mu-metal shield is made of enameled rolled strip, to prevent eddy currents? The ferrite itself is made of bonded dust, that's why it has low eddy currents. \$\endgroup\$ Jun 17, 2021 at 12:54
  • \$\begingroup\$ @MarkoBuršič Being made of bonded ferrite dust doesn't matter for eddy currents--it's simply that ferrite as a material has very low electrical conductivity. If you were to do the same thing with iron, you'd still see eddy currents. (metal powder cores combine the powder with an insulating epoxy that means relatively few particles are in contact with each other, and also provides a distributed air gap for better energy storage) \$\endgroup\$
    – Hearth
    Jun 17, 2021 at 14:40

1 Answer 1

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Mu Metal has a very high permeability, meaning that it saturates very quickly in the presence of a strong magnetic field, and also exhibits low hysteresis loss. For cooking, a lossy magnetic material is desired, because the losses from eddy currents and hysteresis are converted to heat. In your induction cooker "high permeability" is relative; cast iron has a high permeability compared to non-ferritic alloys, but would not be considered a high permeability alloy by magnetics designers. The very features that make it a poor magnetic material in our world are just what is needed for induction cooking. Remember that designers of magnetic alloys are trying their very best to supply products that do not get hot when subjected to alternating current.

The extremely high permeability makes Mu Metal ideal for a shield for weak, slow moving or even static magnetic fields. Once saturated, Mu Metal is no longer as effective as a shield, and so "weak" is the operative word. But it is a very effective shield when used properly. We use concentric tubes of Mu Metal, which effectively eliminate the earth's field in the center, to provide a zero point for fluxgate magnetometers.

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