The wind turbine yaw mechanism is used to turn the wind turbine rotor against the wind.
The wind turbine is said to have a yaw error, if the rotor is not perpendicular to the wind. A yaw error implies that a lower share of the energy in the wind will be running through the rotor area, the share will drop to the cosine of the yaw error.
If this were the only thing that happened, then yaw control would be an excellent way of controlling the power input to the wind turbine rotor. That part of the rotor which is closest to the source direction of the wind, however, will be subject to a larger force, bending torque, than the rest of the rotor. On the one hand, this means that the rotor will have a tendency to yaw against the wind automatically, regardless of whether it is an upwind or a downwind turbine. On the other hand, it means that the blades will be bending back and forth in a flapwise direction for each turn of the rotor. Wind turbines which are running with a yaw error are therefore subject to larger fatigue loads than wind turbines which are yawed in a perpendicular direction against the wind.
Almost all horizontal axis wind turbines use forced yawing, i.e. they use a mechanism which uses electric motors and gearboxes to keep the turbine yawed against the wind. The image shows the yaw mechanism of a typical 750 kW machine seen from below, looking into the nacelle. It can be seen the yaw bearing around the outer edge and the wheels from the yaw motors and the yaw brakes inside. Almost all manufacturers of upwind machines prefer to brake the yaw mechanism whenever it is unused. The yaw mechanism is activated by the electronic controller which several times per second checks the position of the wind vane on the turbine, whenever the turbine is running.
Cable twist counter
Cables carry the current from the wind turbine generator down through the tower. The cables, however, will become more and more twisted if the turbine by accident keeps yawing in the same direction for a long time. The wind turbine is therefore equipped with a cable twist counter which tells the controller that it is time to untwist the cables.
Therefore, occasionally a wind turbine may be seen which looks like it has gone berserk, yawing continuously in one direction for five revolutions. Like other safety equipment in the turbine there is redundancy in the system. In this case the turbine is also equipped with a pull switch which is activated if the cables become too twisted.