The transformer winding is equivalent to a large inductor, Inductors have a property that prevents the current from changing (that is, when the current changes, the inductor will generate an induced electromotive force to prevent the current from changing). When the current is small, the inductor will generate an electromotive force to make the current not so fast, which is to release the energy stored inside the inductor. When the current becomes large, the inductor prevents the current from becoming larger Both ends of the inductor create an induced electromotive force that prevents the current from increasing and that's what you call charging.