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Multipole radial field machines with permanent magnet excitation can be used as direct coupled generators in wind turbines and can be built to fit within the confined space of a nacelle. High efficiency and power factor are maintained over a wide range of operating power. Ferrite permanent magnets with flux concentrating pole pieces are preferred to rare earth types because of their lower cost. The primary brake for the wind turbine can be electrical since the permanent magnet excitation is available in the absence of an external supply.
Spooner et al. (Mon,) studied this question.