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Turbines start producing power at a windspeed called the cut-in windspeed, typically about 4 metres/sec. As windspeed increases, power output increases until the maximum power output is reached, typically at a windspeed of about 12-13 metres/sec. The turbine design or its control system ensures that if the windspeed increases above this level, power output will not increase. This is to protect turbine components from forces larger than those for which they are designed - the additional energy available in higher windspeeds over a year would not justify the additional costs of using a larger gearbox, generator, bearings etc.

At a higher windspeed, sometimes called the cut-out or furling windspeed and typically about 25 metres/sec., the control system stops the turbine. The maximum windspeed the turbine is designed to stand is called the survival windspeed. The levels of these windspeeds vary with turbine design and the sites for which the turbine is designed.

Turbines may be stopped on an instruction from the operator, when the windspeed exceeds the cut-out windspeed, or if the control system detects a fault. Most turbines have a mechanical brake mounted on one of the shafts in the nacelle [the generator housing]. In addition, many turbines have aerodynamic brakes in which the blade tips, or the entire blades, twist to a position perpendicular to the plane or rotation, so slowing the turbines down.

Turbines may be upwind or downwind of the tower. Advantages of downwind design are that the blades can cone or flex in the wind without any danger of hitting the tower. This reduces the stresses in the blades and allows cheaper, lighter blades and a lighter structure to be used. Disadvantages are that a sudden drop in windspeed each time a blade passes behind the tower may cause vibration and fatigue of the blade and structure, and may also cause noise.

The capacity factor of a wind turbine is the ratio of average power output to rated power output for a given period. Turbines on windier sites tend to give higher capacity factors. Typical capacity factors are in the region of 20-30%.

from an article by Ciaran King in In the Wind, the newsletter of the Irish Wind Energy Association.

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