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In an actual transformer the iron loss remains practically constant from no-load to full-load because
1. value of transformation ratio remains constant
2. permeability of transformer core remains constant
3. core flux remains practically constant
4. primary voltage remains constant

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Correct Answer - Option 3 : core flux remains practically constant


Eddy current losses:

  • When an alternating magnetic field is applied to a magnetic material, an emf is induced in the material itself according to Faraday’s law of Electromagnetic induction.
  • Since the magnetic material is a conducting material, these EMFs circulates current within the body of the material. These circulating currents are called Eddy currents. They are produced when the conductor experiences a changing magnetic field.
  • The process of lamination involves dividing the core into thin layers held together by insulating materials.
  • Due to lamination effective cross-section area of each layer reduces and hence the effective resistance increases.
  • As effective resistance increases, the eddy current losses will get decrease.

Mathematically, the eddy current loss is given by:

Eddy current loss in the transformer is given by:

Pe = KBm2. t2. f2. V Watts


K - coefficient of eddy current. Its value depends upon the nature of magnetic material

Bm - Maximum value of flux density in Wb/m2

t - Thickness of lamination in meters

f - Frequency of reversal of the magnetic field in Hz

V - Volume of magnetic material in m3

From the above formula, we conclude that the Eddy current loss is proportional to the square of the frequency.

Hysteresis losses: 

These are due to the reversal of magnetization in the transformer core whenever it is subjected to the alternating nature of magnetizing force.

\({W_h} = \eta B_{max}^xfv\)

\({B_{max}} \propto \frac{V}{f}\)


x is the Steinmetz constant

Bm = maximum flux density

f = frequency of magnetization or supply frequency

v = volume of the core

At a constant V/f ratio, hysteresis losses are directly proportional to the frequency.

Wh  f

Total iron losses Wi = W + We

At constant V/f ratio, Wi = Af + Bf2


The flux in the core is almost constant by the principle of the transformer.

The core flux in the transformer depends mainly on supply voltage and frequency.

Therefore, the iron losses in the transformer are almost constant by maintaining the V/f ratio as constant.

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