Short note: Eddy current loss

Eddy current loss is the power wasted as heat in a magnetic core when the alternating flux induces emf in the iron itself and this emf circulates currents inside the core.

Cause: by Faraday's law, a changing flux induces an emf in any conducting path linked with it. A solid iron core has low resistance, so closed current loops (eddy currents) flow in planes at right angles to the flux, and they cause $I^2R$ heating.

  Solid core          Laminated core
  +--------+          +--+--+--+--+
  | (  )   |          |  |  |  |  |
  | (  )   |  flux    |  |  |  |  |
  +--------+          +--+--+--+--+
  large loops         thin insulated layers,
                      small loops

Expression:

$$P_e = k_e B_{max}^{2} f^{2} t^{2} V\ \text{W}$$

where $t$ is the thickness of a lamination, $f$ the frequency, $B_{max}$ the peak flux density and $V$ the volume. The loss is proportional to $t^2$ and $f^2$.

Effects: lower efficiency, heating of the core, and a demagnetising effect of the eddy currents on the main flux.

Reduction:

• Build the core from thin laminations (0.35 to 0.5 mm) insulated from each other by varnish.
• Add silicon to the steel to raise its resistivity.
• Use ferrite or powdered iron cores at high frequency.

Uses: eddy currents are useful in induction furnaces, eddy current brakes and energy meters.