At wind speeds below rated, the goal is to capture as much energy as possible by
maximising the Power Coefficient. To do this, the tip speed ratio must be kept constant, at the Optimal value. This is only possible if the speed of the rotor can change as wind speed changes. To explain how this works, we need to show that when the power coefficient is
maximised, the ratio of generator torque to the generator speed squared has a
certain constant value. This constant value is called the Optimal Mode Gain. Equation (1) is our original mechanical power formula. Substituting the new
expressions for power and wind speed into equation 1, we have a new expression for
rotor torque in terms of rotor speed and tip speed ratio (equation 2)
Equation (1) is our original mechanical power formula. Substituting the new
expressions for power and wind speed into equation 1, we have a new expression
for rotor torque Constant rotor speed is achieved by pitching the blades progressively towards
feather as the wind speed rises. Pitching the blades ensures that despite the increasing wind speed, the net aerodynamic torque (about the rotor rotational axis) is held at a constant
value equal to the torque demand. With torque balanced, rotor speed – and therefore power output – both remain
constant. The usual way to pitch the blades is to use motors called pitch actuators, These pitch actuators turn the blades by powering a pinion which rotates the blade
root pitch bearing.