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If the primary of a transformer is connected to the DC supply, the flux produced in the primary will be of constant magnitude because of DC current. So no EMF will be induced in the primary, because emf is induced only when if either the flux is changing or the conductor is moving i.e. the emf will be induced in the winding only when if there is any relative motion between flux and conductor.
Hence no emf will be induced either in the primary or in secondary.
Secondly, in case of AC supply, the current in the primary is given by -
I1 = (V1 - E1) / Z1 , where,
V1 = supply voltage
E1 = emf induced in the primary and
Z1 = impedance of primary winding
But, when primary is fed with DC supply, then as stated above, no emf will be induced in primary. So, E1 = 0; and Z1 = R1, where, R1 is resistance of primary winding ( reactance of the winding is zero in case of DC). So in this case the value of primary current is -
I1 = V1 / R1
The magnitude of this current is very high as E1 is zero and R1 is small.
So the transformer can be damaged due to this high current, if it is connected to DC supply.
