1. When we supply DC to transformers, no magnetic flux will produce and hence no emf induced in secondary on other hand when we supply AC to transformer it produce magnetic flux and induced emf in it by Faraday's law.
2. As we know that DC supply has not frequency and transformers work on constant frequency hence AC is used in transformers.
3. DC supply contains only resistance and transformer contains resistance of low value due to which current is flow through resistance and burn windings of primary but AC contain resistance and reactance.