Variable frequency transformer (VFT) is a controllable bidirectional transmission device that can transfer power between asynchronous networks. The construction of VFT is similar to conventional asynchronous machines, where the two separate electrical networks are connected to the stator and rotor respectively. Electrical power is exchanged between the two networks by magnetic coupling through the air gap of the VFT. This paper describes the basic concept of the VFT and presents an overview of the mechanical and electromagnetic design of a unit having four poles and rated at 1.00 MW, 17 kV/17 kV, 60 Hz.