Technology evolves: although today's turbines are much larger and more powerful than traditional windmills, the principles on which they operate still revolve around many of the same technique; quite literally when it comes to the yaw system.
The cap (top of the mill) of traditional windmills could turn in relation to its body and attics (the superstructure). This turning of the cap was called yawing', and in the old mills this system was known as the yaw mechanism. There used to be two types of yaw system: sliding and rolling.
The sliding technique involved the mill's cap sliding over metal-clad wooden blocks. This block yaw mechanism is fairly vulnerable to wear and the drag from friction was also considerable.
Drawing of a block yaw mechanism.
The second system involved the use of rollers, which were made of wood or metal according to the design. This 'English yaw mechanism' resulted in less friction than the sliding technique, as well as being easier to maintain.
Drawing of an English yaw mechanism.
Photograph of an English yaw mechanism.
Then – infinite rotation
The cap had no fixed connection to the mill's attic and body other than a rotating shaft (king spindle) running through it. The cap could therefore theoretically keep rotating indefinitely without any parts becoming entangled.
Now – limited rotation…
The nacelle (upper part) of a modern wind turbine can also turn in relation to the turbine tower. This action is still referred to as yawing and, depending on the make of wind turbine, takes places by means of gearboxes or oil-assisted hydraulics. The two parts slide one on top of the other on the yaw plate, which may be clad in friction-reducing plastic and is lubricated with grease or oil.
…with a position sensor
The nacelle can make three rotations without any difficulty, but the cables running through the turbine may well pose a problem: they run the risk of being damaged by overtwisting. A position sensor monitors the position of the nacelle in relation to the zero point – the point at which the nacelle has made zero rotations around its shaft – keeping track of the actual position. Ideally, in conditions with little wind and at low production levels, the computer steers the nacelle so that it yaws back to the zero point. From this pause position, the wind turbine can continue to rotate.
Photograph of a yaw block.