Two-speed machines start up and run in low winds at a low rotational speed, which makes them aerodynamically efficient and quiet. At wind speeds higher than about 6-7 m/s, the generator windings are reconfigured -- or the output is switched to a larger generator -- and rotational speed increases, usually by 30-50%. Two-speed machines do not incur electrical losses in the AC/DC/AC power conditioning units, which are also used by many variable speed machines.
Meantime, at low wind speeds variable speed machines appear to have the edge over fixed speed, but cannot match the efficiency of the two speed generator. The output per unit swept area from the variable speed Enercon E-66 1.5 MW wind turbine is compared with that of the fixed speed Vestas V42 600 kW machine; the variable speed machine produces 10.5% more energy at a 6 m/s site, down to 5.2% at a 10 m/s site.
The analysis, reported in WindStats, a Windpower Monthly supplement, compares machines of identical power rating -- or rated power per unit area. Such comparisons are tricky, as WindStats points out, because some machines use two versions of their power curve, for example, or their maximum output is greater than their nominal rating. The study also compares direct drive machine outputs with those of "conventional" machines with gear boxes and stall-regulated units. The conclusion: extra energy yields from two speed and variable speed machines is confirmed, but the amount all depends on wind speed.