Power captured from the wind is P = ½·ρ·A·v³·Cp(λ,β), where ρ is air density, A the rotor-swept area, v the wind speed, and Cp the fraction of that kinetic-energy flux the blades actually convert — never more than the Betz limit of 59.3%. Cp itself depends on the tip-speed ratio λ = ωR/v (how fast the blade tips move relative to the wind) and the blade pitch angle β (how much the blades are twisted out of the wind).
Cp(λ,β) = c1(c2/λi − c3β − c4)e^(−c5/λi) + c6λ
1/λi = 1/(λ+0.08β) − 0.035/(β³+1)
- Below rated wind speed — the generator's torque control lets the rotor speed track the optimal λ that maximizes Cp, squeezing maximum power out of light-to-moderate wind (maximum power-point tracking).
- At rated wind speed — the rotor reaches its rated rotational speed and the generator reaches rated power; this is the "knee" of the power curve.
- Above rated wind speed — rotor speed is held constant and blades pitch progressively toward feather (larger β), deliberately spilling excess aerodynamic energy so power stays flat instead of overloading the drivetrain.
- Above cut-out (~25 m/s) — blades fully feather and the rotor is braked to protect the structure from extreme loads.
Switch off auto-pitch to fix β yourself: at low wind a shallow pitch keeps λ near optimum, but the same fixed pitch at high wind lets power run past the rated line — exactly the loading auto-regulation exists to prevent.