transformer is based on the principal of electromagnetic
induction. 
when flux related to a coil changes with time it produces
voltage across da coil ..same funda used here ...when AC
current flows through da transformer coil it produces
varying magnetic field tat magnetic field interact with da
secondary coil of transformer and produces voltage
...whereas in case of DC there is no magnetic field produces
in coil ....

That is true there is no Back EMF also. while if we inject
DC in a transformer. and if doing so the transformer will
act like a short circuit to DC. so that the coil will be burnt.

In a transformer, when ac current flows then due to its 
frequency an emf is induced in it due to electromagnetic 
induction.
But when a DC current flows through the transformer, then 
no emf is induced in it because there is no frequency in dc.
Thus dc current is not used in transformer.

dc current has no frequency

transformer works on two principals 
1 elctric current produces magnetic field
2 varying magnetic field induces voltage across the 
terminals of wire..
so if there will be no change in current as in DC
no magnetic field will be produced and neither the current 
will be induced in the seconday...transformer action will 
not take place..

1) transformer works on the principle of Electromagnetic
Induction when Ac supply is fed to Xmer due to Alternating
nature it Induces EMF in the Secondary winding.
2) When Dc supply is fed to primary as frequency is 0 in DC
supply No Emf s induced in Secondary & total input power
loss takes place in Core only Core burnt out Finally.

because the ac supply has a frequency but dc supply has 
there is no frequency.