To reduce the cost of energy for extreme-scale wind turbines,
especially those off-shore ones, a modification of the power generation
is proposed as shown in Fig. 1. In the conventional concept
(Fig. 1a), the rotor shaft is converted to electrical power using an
electrical generator. A gearbox is generally used to convert the lowspeed
rotor rpm to the higher rpm required for the electric
generator to operate most efficiently. For extreme-scale systems,
the rotor rpm is reduced even further because of increased blade
radius combined with a limit on blade tip speed (ca. 80e100 m/s).
The increased gear ratio may require multiple gearbox stages that
increase complexity and probability of failure [1,2]. As such newer
turbines generally eliminate the gearbox by replacing them with
direct-drive Permanent Magnet Generator (PMG), which are more
expensive and heavier