Losses in transformers:

Losses in a transformer are power loss taking place in transformer. An ideal transformer is loss free. But in practice transformer there are following losses taking place.

(I) Core Losses Or Iron Losses

Eddy current loss and hysteresis loss depend upon the magnetic properties of the material used for the construction of core. Hence these losses are also known as core losses or iron losses.

Eddy current loss in transformer: In transformer, AC current is supplied to the primary winding which sets up alternating magnetizing flux. When this flux links with secondary winding, it produces induced emf in it. But some part of this flux also gets linked with other conducting parts like steel core or iron body or the transformer, which will result in induced emf in those parts, causing small circulating current in them. This current is called as eddy current. Due to these eddy currents, some energy will be dissipated in the form of heat.

(II)Copper Loss In Transformer

Copper loss is due to ohmic resistance of the transformer windings.  Copper loss for the primary winding is $I_1^2R_1$ and for secondary winding is $I_2^2R_2$. Where, $I_1 $and $ I_2 $ are current in primary and secondary winding respectively, R1 and R2 are the resistances of primary and secondary winding respectively. It is clear that Cu loss is proportional to square of the current, and current depends on the load. Hence copper loss in transformer varies with the load.

Why is transformer rated in kVA or VA and not in W or kW :

1. The VA or kV rating of the transformer simply indicates how much maximum apparent power or total power a transformer can supply.
2. It includes the active power (true power) and the reactive power (useless power).
3. Output of the total supplied by the transformer (VA or kVA), a part or full will be useful, depending on the load. For a purely resistive load, all the power will be reactive.
4. At the time of manufacturing of a transformer , the nature of load is not known. Hence the maximum total(apparent) power supplying capacity is given as the rating.

Hence the transformer is rated in VA or kVA and not in W or kW.