Variable speed wind turbine

Original models of wind turbines were fixed speed turbines; that is, the rotor speed was a constant for all wind speeds. The tip-speed ratio for a wind turbine is given by the following formula:

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

where ${\displaystyle \omega }$ is the rotor speed (in radians per second), ${\displaystyle R}$ is the length of a blade, and ${\displaystyle v}$ is the wind speed. That is to say, for a fixed-speed wind turbine, the value of the tip-speed ratio is only changed by wind speed variations. In reference to a ${\displaystyle C_{p}}$-${\displaystyle \lambda }$ graph, which illustrates the relationship between Tip-speed ratio and efficiency, it is evident that only one value of ${\displaystyle \lambda }$ yields the highest efficiency. That is, the fixed speed wind turbine is not operating at peak efficiency across a range of wind speeds. This was a motivator for the development of variable speed wind turbines.

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Wikipedia