Doubly fed electric machine

A doubly fed electric machines, doubly fed induction generator (DFIG), or slip-ring generator is an electric motor or electric generator where both the field magnet windings and armature windings are separately connected to equipment outside the machine. By feeding adjustable-frequency AC power to the field windings, the magnetic field can be made to rotate, allowing variation in motor or generator speed.

Source: Wikipedia — Doubly fed electric machine (CC BY-SA 4.0)