極間通風回転子を用いた大容量風力発電向け永久磁石発電機の検討;極間通風回転子を用いた大容量風力発電向け永久磁石発電機の検討;Permanent Magnet Generator with Ventilation Flues between Poles in the Clover Leaf Structure Rotor for a Large Wind Turbine Generator System