An AC transformer having a cylindrical core configurable for single or polyphase power input and/or output transformer applications. The transformer core structure is capable of being configured to provide for single or polyphase inputs or outputs by varying the transformer primary and secondary winding configurations. A polyphase input configuration can be utilized in polyphase output transformers, such as for variable frequency drive (VFD) applications. Additional methods for winding transformer cores minimize the quantity of core winding wire necessary for transformer manufacture.