A transformer cannot work on d.c. supply. If a rated d.c. voltage is applied across the primary, a flux of constant magnitude will be set up in the core. Due to constant flux, there will not be any self induced emf in the primary winding to oppose the applied voltage. According to the Faraday’s law of electromagnetic induction, whenever a conductor cuts by the varying magnetic flux an emf is induced in the conductor. But when transformer is connected with dc supply, a flux of constant magnitude is set up. As the resistance of the primary winding is very low, the primary current will be very high as given by the relation;
This current is much more than the rated full load current of primary winding. Thus it will produce large heat in core (due to copper losses) and burns the insulation of the primary winding, this results the transformer will be damaged. That is why, transformer is not operated with dc supply.