An input shaft rotates at variable rotation rates and is driven by a variable speed source of energy or power such as by the propeller of a wind-driven electrical generator. The input shaft is connected to a first rotor which electromagnetically interacts with a second rotor to create a torque to cause the second rotor to rotate. The interaction of the first rotor and the second rotor may be varied electrically to vary the torque and in turn the rotation rate of the second rotor. The second rotor interacts with a stator to produce AC electrical power. The rotation rate of the second rotor is controlled so that the generator supplies AC power at an essentially constant frequency.