A variable electromotive-force generator (VEG), which is a modified generator with an adjustable overlap between the rotor and the stator, is proposed to improve the efficiency and/or expand the operational range of a conventional generator, with particular applications to wind turbines, hybrid vehicles, and so on. A mathematical model of the VEG is developed, and a novel prototype is designed and fabricated. The performance of the VEG with the active control system, which adjusts the overlap ratio based on the desired output power at different input speeds, is theoretically and experimentally studied. The results show that reducing the overlap between the rotor and the stator of the generator at low speeds results in a reduced torque loss of the generator and an increased rotational speed of the generator rotor.

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