Short note: Eddy current loss

Eddy current loss is the power wasted as heat in a magnetic core (or any conducting mass) because of circulating currents induced in it by a changing flux.

Cause: by Faraday's law, an alternating flux induces an emf not only in the windings but also in the iron core. As the core is a conductor, this emf drives small closed currents, called eddy currents, in planes at right angles to the flux. They cause I²R loss.

Expression: for a lamination of thickness t,

Pₑ = kₑ B_max² f² t² V watts

where kₑ depends on the resistivity of the material, B_max is the maximum flux density, f the frequency and V the volume.

Effects

• Heating of the core, which lowers the efficiency.
• Needs extra cooling and reduces the rating.

Methods of reduction

1. Build the core from thin laminated sheets (0.35–0.5 mm).
2. Insulate the laminations from one another with varnish or oxide layers, so that the eddy path is cut into small loops.
3. Use silicon steel, whose higher resistivity limits the currents.
4. Use ferrite or iron-dust cores for high frequency.

Uses: the same effect is useful in induction furnaces, eddy-current brakes and induction heating.