It’s Because of two main reasons behind that transformer can’t be used with DC source or DC ( Direct current ) supply. So we discussed the question of Why can’t transformers use DC?
First point is transformer works on principle of electromagnetic Induction you know very well . It According to this principle a change in current (keep in mind Ac current) of primary side of transformer make changing a magnetic flux or field in the transformer core and this induces varying or changing magnetic field in the secondary side of transformer (KVA)
it will produce EMF (full form is Electromotive force ) on the secondary side of transformer.It is working principle of transformer.
You can see here give AC sinusoidal waveform .it’s changing every and each cycle .But second condition if we use DC supply that time transformer,
You can see in the above given DC source of waveform .it has constant with time not any changing ,its cycle at the time magnitude characteristics.
Therefore, there will not be changed in current and also it will not be change in flux so there induced EMF will be zero in secondary side of transformer.
So finely according faraday’s law rate of change of flux linkages is equal to the induced emf,
E= N dΦ/dt
If will not be change in flux in transformer