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Hysteresis and eddy current losses, reduction

Short answer

Hysteresis loss (P_h ∝ f B_max^1.6) comes from domain reversal; eddy loss (P_e ∝ f^2 B_max^2 t^2) from induced currents. They cause core heating and efficiency drop. Reduce by using thin laminated silicon steel and narrow-hysteresis materials, and lower B_max.

What are hysteresis and eddy current losses? What are their significance in the operation of electric machine? Write down different methods to reduce them.

Hysteresis loss is the power lost as heat in a magnetic core because its domains are repeatedly turned back and forth by an alternating field. Eddy current loss is the I2RI^2R loss caused by circulating currents induced in the core itself by the changing flux.

Expressions

Ph=ηBmax1.6fV W,Pe=keBmax2f2t2V WP_h = \eta B_{max}^{1.6} f V\ \text{W}, \qquad P_e = k_e B_{max}^{2} f^{2} t^{2} V\ \text{W}

where tt is the lamination thickness and VV the core volume.

Origin

  • Hysteresis: energy per cycle equals the area of the BB–HH loop, so loss rises with frequency and flux density.
  • Eddy current: the alternating flux induces emf in the solid core. This drives currents in closed loops inside the iron, which heat it.

Significance in machines

  1. Together they form the core (iron) loss. It is present whenever the core is excited, even at no load, and it is almost constant from no load to full load.
  2. It lowers efficiency and creates heat that must be removed, so it limits the rating.
  3. It fixes the choice of core material, lamination thickness and the flux density used in the design.
  4. In transformers it is the constant loss PiP_i that sets the condition for maximum efficiency (Pi=PcuP_i = P_{cu}).

Methods of reduction

Hysteresis lossEddy current loss
Use materials with a narrow loop (silicon steel, CRGO steel, permalloy)Build the core from thin laminations (0.35 to 0.5 mm)
Add 3 to 4% silicon to the ironInsulate the laminations with varnish or oxide coating
Anneal the steel to remove mechanical stressUse silicon steel, which has higher resistivity
Work at a lower BmaxB_{max}Use powdered iron or ferrite cores at high frequency
Reduce core volumeLaminate along the direction of flux

✓ Answer checked by Claude Opus 5.5 (high effort)

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