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What are Losses in Transformer?

by Sonali

The losses in transformer are listed below. Some of them occur due to alternating flux or due to ohmic resistance in the windings.

(1). Core or Iron Loss :

This loss occurs in the core of the transformer. It occurs because of the alternating flux present in the core of the transformer. This includes both hysteresis loss and eddy current loss. Iron or core loss is found from open circuit tests.

Due to exposure to alternating magnetizing force in each cycle and accordingly the hysteresis loop is traced. Then the loss found here is in form of heat given by equation

Hysteresis Loss(Wh) = nB1.6max f V Watt


  • KȠ : It is a proportionality constant.  It depends upon the type of laminations used.
  • f :frequency,
  • Bmax: It is the maximum flux density.

If the core of the transformer is made of silicon steel the iron losses can be reduced. 

Due to flux linkage emf is induced in the coils of the transformer resulting in the flow of current. The amount of current flow depends upon emf induced and also the value of resistance. As the core of the transformer is made up of conducting materials, these circulating currents flow through them. This circulating current is known as Eddy current. 

Hence, the losses which are due to this current are Eddy current losses. The equation of Eddy current loss is 

Eddy current Loss(We) = P B2max f2 t2 Watt


  • Ke – is known as the coefficient of eddy current. This coefficient is dependent on the type of magnetic material.
  • Bm – It is the maximum value of flux density [Wb/m2]
  • T – It represents the thickness of lamination [meters]
  • F – frequency of the magnetic field [Hz]
  • V – It is the volume of magnetic material [m3]

(2). Copper Loss :

This is due to ohmic resistance of the transformer windings. The Cu loss will be in both the windings of the transformer. The equation is 

Total cu loss = I21 R1 + I22 R2

                      = I21 R01 + I22 R02

Hence, the Copper loss Cu loss is directly proportional to I

They are variable losses as they vary according to the load. Then value of cu loss is found from a short-circuit test.

(3) Stray Losses

They occur due to leakage fields. But they are very less compared to the above mentioned losses. Hence, we can neglect them.

(4) Dielectric Losses

These losses occur due to the insulating materials of the transformer. For instance, oil is one of them. When the oil gets deteriorated then the efficiency and working gets affected. 

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