The rotor and the generator get the attention, but two much smaller systems decide whether a turbine survives its design life. Pitch rotates each blade about its own axis. Yaw rotates the whole nacelle so the rotor faces the wind. Neither produces any electricity. Both are load bearing in the sense that matters, because they are how the machine protects itself.
Pitch, the primary control and the primary brake
Each blade sits on a large bearing at the hub and can be rotated through a wide angle. Changing that angle changes the aerodynamic angle of attack, which changes how much energy the blade extracts from the wind and how much force it transmits into the structure.
Below rated wind speed, pitch stays near the angle that captures the most energy, and power is regulated by varying generator torque instead. Above rated wind speed, pitch does the regulating, rotating the blades to spill the surplus so that output stays flat while the wind keeps rising. At the cut out wind speed the blades go to the feathered position, nearly edge on to the wind, which is also the position they take for any emergency stop.
That last function is why pitch is a safety system and not merely a control system. A large turbine has no mechanical brake capable of stopping the rotor against full aerodynamic torque. Feathering the blades is the brake. The mechanical disc brake on the high speed shaft is for holding the rotor once it has slowed and for parking, not for arresting it from operating speed.
Hydraulic and electric pitch
Two implementations are in common use. Hydraulic pitch uses cylinders fed from an accumulator, which is compact, delivers high force and stores the energy to feather the blades even with the supply lost. Its drawbacks are oil, seals and the leaks that accompany both.
Electric pitch uses a geared motor on each blade, typically with a battery or capacitor backup to guarantee feathering during a grid loss. It is clean, individually controllable per blade and diagnosable in detail. Its weak point is the backup energy store, which ages and must be tested, and which is the component most likely to be quietly degraded when it is finally needed.
Whichever is fitted, the independence of the three blade systems is a design principle. Feathering any one blade substantially reduces rotor torque, so a single failure should not leave the machine unable to slow itself.
Yaw, slower and more consequential than it looks
The nacelle rests on a large bearing at the tower top and is rotated by several geared yaw drives engaging a ring gear. A wind vane and anemometer on the nacelle roof tell the controller where the wind is coming from, and the controller corrects the heading when misalignment persists beyond a threshold.
The correction is deliberately sluggish. Yawing a nacelle creates gyroscopic loads on the rotor and drivetrain, so fast correction is itself damaging. The controller therefore ignores brief shifts and acts on sustained ones, accepting some misalignment as the lesser cost.
That accepted misalignment has a price. Power capture falls with the cosine of the yaw error cubed in the simplest treatment, so a persistent error of a few degrees costs a small but continuous percentage of output across every hour of operation. Because the error is invisible from outside and the loss is gradual, miscalibrated wind vanes are among the most common and most overlooked sources of underperformance in operating wind farms.
Cable twist
Power and signal cables run from the nacelle down through the tower, and every yaw rotation twists them. Turbines count their accumulated rotation and perform an untwist manoeuvre when it reaches a limit, unwinding the loop. A fault in that counting, or a cable guide that has worn, produces a failure that is expensive and entirely preventable.
Why these systems dominate fault statistics
Field reliability studies repeatedly find pitch and yaw systems among the most frequent sources of turbine faults, though usually with shorter downtime per event than a gearbox or generator failure. The pattern makes sense. These are actuation systems with bearings, seals, gears, motors, slip rings, position sensors and backup energy stores, operating continuously in a vibrating, temperature cycling environment at the top of a tower.
They are also where condition monitoring pays most readily, because a pitch system that has begun to respond slowly, or a blade whose commanded and actual angle no longer agree, is visible in data long before it causes a stoppage. The broader maintenance picture is covered in our note on operations and maintenance of a wind farm, and the drivetrain these systems protect is described in our piece on how a wind turbine drivetrain works. General background is summarised in the overview of wind turbine design.
Frequently asked questions
How does a large turbine stop?
By feathering the blades, which removes aerodynamic torque. The mechanical brake holds the rotor once slowed and parks it, but it is not sized to arrest the rotor from operating speed.
Is hydraulic or electric pitch better?
Both are in volume production. Hydraulic systems are compact and store feathering energy inherently in an accumulator. Electric systems are cleaner and more diagnosable but depend on a backup energy store that must be maintained and tested.
Why does the nacelle correct its heading so slowly?
Because yawing creates gyroscopic loads on the rotor and drivetrain. Rapid correction would cause more damage than the misalignment it removes.
How much output does yaw misalignment cost?
A persistent error of a few degrees costs a small percentage of production continuously. Because it is invisible and gradual, wind vane calibration is a frequently missed source of lost energy.
What happens to the cables when the nacelle keeps turning?
They twist. The controller tracks accumulated rotation and performs an untwist manoeuvre at a set limit. Failures in that tracking cause expensive and avoidable cable damage.