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Losses in magnetic materials, reduction

Short answer

Alternating flux causes hysteresis loss (P_h ∝ B_max^1.6 f V) reduced by silicon steel with narrow loop, and eddy current loss (P_e ∝ B_max² f² t² V) reduced by laminating core and using high-resistivity material.

Explain the losses which occur when magnetic materials are subjected to an alternating flux. How can we reduce these losses practically?

Two losses occur in a magnetic core carrying alternating flux: hysteresis loss and eddy current loss. Together they are called the core (iron) loss.

Hysteresis loss

  • Each cycle of flux reversal turns the magnetic domains of the core against internal friction. The energy used is lost as heat.
  • It equals the area of the B–H loop multiplied by the volume and the frequency.
  • Pₕ = ηₕ B_max^1.6 f V (Steinmetz's law), in watts.

Eddy current loss

  • The alternating flux induces emf in the core itself, which drives circulating currents. These produce I²R loss.
  • Pₑ = kₑ B_max² f² t² V, in watts, where t is the lamination thickness.
  • Since the loss varies as f² and t², thin laminations are very effective.

Both depend on B_max and frequency, so they are present in the core of every transformer, motor and generator, and they do not change much with load.

Practical reduction

LossMethod of reducing
HysteresisUse silicon steel (3–4 % Si) or other soft magnetic alloys with a narrow B–H loop
HysteresisAnneal and use cold-rolled grain-oriented (CRGO) steel
HysteresisDo not operate at too high a B_max
Eddy currentBuild the core from thin laminations (0.35–0.5 mm)
Eddy currentInsulate each lamination with varnish or oxide coating
Eddy currentUse material of high resistivity (silicon raises it)
Eddy currentUse ferrite or powdered-iron cores at high frequency

✓ Answer checked by Claude Opus 5.5 (high effort)

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