This section discusses various generator arrangements and their suitability for use in the conversion of a traditional windmill. The generator arrangement may include a speed-increasing gearbox or belt-drive and/or a power electronic frequency converter. See also the Appendix: Background information on induction generators. 7.1 Fixed-speed induction generator
Many modern wind turbines (especially those manufactured during the 1990s) use a four-pole induction generator, which is directly connected to the grid and therefore runs at an almost constant speed: a little over 1500 rpm. A speed-increasing gearbox is used to couple the aerodynamic rotor to the generator. Thus, the aerodynamic rotor also runs at an almost constant speed, which leads to the term: fixed-speed wind turbine. A very similar arrangement could well be used in the conversion of a traditional windmill. A belt drive could be considered in place of the gearbox, and the required speed ratio could be reduced by use of a six or eight-pole generator, which would run at a little over 1000 rpm or 750 rpm respectively. Lower-speed generators are proportionally larger, heavier and more expensive, but this may be acceptable, if, for example, it enabled use of a single-stage belt-drive. A major consideration with a fixed-speed arrangement is that the drive train has to withstand large and rapid torque fluctuations caused by the turbulence of the wind.
7.2 Variable-speed induction generator
The induction generator described above can be adapted to operate at variable speed by the application of power-electronic frequency converters. Similar converters are widely used in general industry to provide variable speed drives (usually motors). In wind turbines, variable-speed operation has the big advantage that the inertia of the rotating parts serves to smooth out the torque fluctuations and significantly reduces stresses on the mechanics. The majority of large wind turbines...