In reference to a wind energy conversion device's blades, the tip speed ratio is the ratio between the rotational speed of the tip of the blade and the actual velocity of the wind. High efficiency 3-blade-turbines have tip speed ratios of 6–7. On the whole, a high tip speed ratio is better, but not to the point where the machine becomes noisy and highly stressed. The tip speed ratio determines how fast the wind turbine will want to turn and so has implications for the alternator that can be used.

 

Modern wind turbines are designed to spin at varying speeds. Use of aluminum and composites in their blades has contributed to low rotational inertia, which means that newer wind turbines can accelerate quickly if the winds pick up, keeping the tip speed ratio more nearly constant. Operating closer to their optimal tip speed ratio during energetic gusts of wind allows wind turbines to improve energy capture from sudden gusts that are typical in urban settings.

 

In contrast, older style wind turbines were designed with heavier steel blades, which have higher inertia, and rotated at speeds governed by the AC frequency of the power lines. The high inertia buffered the changes in rotation speed and thus made power output more stable.

 

The speed and torque at which a wind turbine rotates must be controlled for several reasons:

 

To optimize the aerodynamic efficiency of the rotor in light winds.

To keep the generator within its speed and torque limits.

To keep the rotor and hub within their centripetal force limits. The centripetal force from the spinning rotors increases as the square of the rotation speed, which makes this structure sensitive to overspeed.

To keep the rotor and tower within their strength limits. Because the power of the wind increases as the cube of the wind speed, turbines have to be built to survive much higher wind loads (such as gusts of wind) than those from which they can practically generate power. Since the blades generate more downwind force (and thus put far greater stress on the tower) when they are producing torque, most wind turbines have ways of reducing torque in high winds.

To enable maintenance; because it is dangerous to have people working on a wind turbine while it is active, it is sometimes necessary to bring a turbine to a full stop.

To reduce noise; As a rule of thumb, the noise from a wind turbine increases with the fifth power of the relative wind speed (as seen from the moving tip of the blades). In noise-sensitive environments, the tip speed can be limited to approximately 60 m/s.

 

Overspeed control is exerted in two main ways: aerodynamic stalling or furling, and mechanical braking. Furling is the preferred method of slowing wind turbines.