What would happen if a DC voltage is applied to a transformer coil?

Transformer is a device used to step up or step down AC voltages.

Transformer windings have a fixed resistance which is very low and their reactance depends on the frequency of the applied voltage.

The frequency of DC power is zero and the flux generated by it is always constant.

When a DC voltage is applied to the winding,  due to low resistance, the winding acts as a short circuit across the  terminals of the DC source.

DC source drives a large current through the winding, overheating and eventually damaging it.

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Never connect DC supply to a transformer. But during testing, current controlled source can be connected.