The principal components of a typical modern wind energy convertor (WEC) are: turbine rotor, gearbox, generator, tower. WECs with turbine rotors up to 80m diameter have been built but those in series production are mostly in the 25 - 45m range. Three-bladed designs are the most common, but two-bladed designs are also in use and even single-bladed machines have been built. The rotational speed is fixed, usually in the range 30 - 50rev/min, and the rated power is typically 200 to 600kW. The gearbox increases the speed to just over 1500rev/min (or 1800rev/min in areas with 60Hz elec- trical supply) and drives an induction generator which feeds power into the grid. Recent years have seen a gradual movement to higher power ratings with larger diameter, lower speed turbines. As a consequence, two developments have become desirable and have been introduced in some commercial models