Question

.
Large transformers, when used for some time, become very hot and are cooled by circulating oil. The heating of the transformer is due to
[A]. the heating effect of current alone
[B]. hysteresis loss alone
[C].
both the heating effect of current and hysteresis loss @
[D]. intense sunlight at noon

Answer 1

its 'c' .
both the heating effect of current and hysteresis loss..

Answer 2

c

Heating effect of current due to resistance in the transformer coil. 
Also due to the hysteresis losses in the ferromagnetic material used inside the transformer.  Due to the reversal of the current and directions of magnetic fields, there is energy lost in magnetizing, demagnetizing and magnetizing again...