Variable speed wind turbine

A variable speed wind turbine is one which is specifically designed to operate over a wide range of rotor speeds. It is in direct contrast to fixed speed wind turbine where the rotor speed is approximately constant. The reason to vary the rotor speed is to capture the maximum aerodynamic power in the wind, as the wind speed varies. The aerodynamic efficiency, or coefficient of power, ${\displaystyle C_{p}}$ for a fixed blade pitch angle is obtained by operating the wind turbine at the optimal tip-speed ratio as shown in the following graph.

Tip-speed ratio is given by the following expression,

${\displaystyle \lambda ={\frac {\omega R}{v}}}$

where ${\displaystyle \omega }$ is the rotor speed (in radians per second), ${\displaystyle R}$ is the radius of the rotor, and ${\displaystyle v}$ is the wind speed. As the wind speed varies, the rotor speed must be varied to maintain peak efficiency.