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 designed since 2000 are variable-speed, primarily for this reason. The need for a variable-speed arrangement in the conversion of a traditional windmill is not clear-cut. One could argue that the sails and wooden and brick components of a traditional windmill are delicate and reducing mechanical stresses is paramount. Alternatively, one might observe that the inertia of the rotating components in a traditional windmill is much greater than a modern turbine of the same diameter, yet the aerodynamic efficiency is considerably lower, and thus the torque fluctuations will be less of a concern.
We favor the more cautious variable-speed arrangement. It has the following additional benefits. • It will allow the maximum speed to be adjusted during commissioning. This may prove valuable since we cannot accurately predict the characteristics of the patent-shutter sails. • It will allow the rotational speed to be controlled to match the wind speed and thus keep the coefficient of performance (Cp) close to its maximum. • It will facilitate starting and stopping of the windmill in a smooth and controlled fashion. The simplest way to vary the speed of an induction generator is by use of a variable-frequency power-electronic converter. Such converters are routinely used in industry and could readily be applied in the conversion of a traditional windmill.
For application with a standard (squirrel-cage) induction generator, two full converters are required back-to-back, which will be expensive compared to the generator alone. Nonetheless, this is our preferred solution. An alternative way to provide variable-speed operation is the doubly-fed induction generator (DFIG), and this is used in a great many modern wind turbines. The size and therefore the cost of the power-electronic converters is greatly reduced, while the cost of the generator itself is only slightly increased. Unfortunately, doubly-fed induction generators are not readily available at sizes appropriate for traditional windmills and therefore cannot be recommended here.
7.3 Gearless generators
Gearboxes in wind turbines are generally larger, heavier and more expensive than their associated generators. Furthermore, they require more maintenance and are more prone to failure. Several modern wind turbine manufacturers, most notably Enercon, eliminate need of the gearbox by using a low-speed generator. These generators are specially made multi-pole synchronous machines and have a very large diameter compared to a high-speed machine of the same power rating.