US Patent 7642684 Nested variable field dynamoelectric machine

A dynamoelectric machine comprises a first rotor shaft, a second rotor shaft and a central element. The first rotor shaft rotates about a central axis of the machine and has a first magnetic drive element disposed about an outer circumference of the first rotor shaft. The second rotor shaft rotates about the first rotor shaft and has a second magnetic drive element disposed about an inner circumference of the second rotor shaft. The central element is disposed between the first rotor shaft and the second rotor shaft and is configurable to remain stationary while the first rotor shaft and the second rotor shaft rotate about the central axis. The central element also includes a third magnetic drive element for interacting with the first magnetic drive element, and a fourth magnetic drive element for interacting with the second magnetic drive element.