Why ac current is used in a transformer?

AC current or DC current can be used in transformers. A transformer is made of two coils of wire, the input coil induces a current into the output coil. Transformers change the voltage either up (step up transformer) or down (step down transformer). The amount of change in voltage is dependent solely on the number of windings in both coils in the transformer. DC transformers work exactly the same way AC transformers do.