Technical reference

Pitch System

The mechanism that turns each blade about its own axis to set how much power the rotor takes from the wind. It is also the turbine's primary brake: feathering the blades is what stops a machine safely when everything else has failed.

What the pitch system does

Below rated wind the blades sit at a small angle to extract as much energy as possible. Above rated wind the controller turns them progressively into the wind — towards the "feather" position parallel to the flow — so the rotor sheds power and the machine stays within its design loads. The same movement, driven to full feather, is the aerodynamic brake.

This is why the pitch system is safety-critical rather than merely operational. A turbine that cannot pitch cannot protect itself.

Components

• Pitch motor: electric, one per blade on independent-pitch machines. Typical ratings run from 11–15 kW on smaller rotors to 22–25 kW on heavy 3 MW-class blades.
• Pitch gearbox: a high-ratio reduction (commonly in the 1:200 to 1:500 range) between motor and blade bearing, because the blade turns slowly against very large moments.
• Blade bearing: the slewing bearing the blade rotates on, carrying the full root moment.
• Position sensor: an encoder reporting actual blade angle back to the controller.
• Backup supply: battery or capacitor storage able to drive the blades to feather on loss of grid. This is the element that makes the aerodynamic brake dependable.

How the control works

The controller compares the measured output or rotor speed against target, computes an error, and commands a blade angle. The loop is usually PID: the proportional term reacts to the present error, the integral term removes steady offset, the derivative term damps the response. On modern machines each blade can be commanded separately, which lets the controller reduce asymmetric loading as well as regulate power.

The practical consequence is that the controller only knows what the encoder tells it. If calibration drifts, the machine behaves confidently and incorrectly — which is exactly the failure described in Case 07.

Normal operating values

Pitch rate matters as much as angle: typical maximum rates run 3–4°/s on lighter machines and 8–10°/s on larger ones.

Typical faults

• High motor current: the motor working against mechanical resistance — poor lubrication, bearing drag, or gearbox trouble. Worked through in Case 01.
• Pitch asymmetry: one blade at a different angle from the others. Causes rotor imbalance, 1P vibration and lost production. Often a calibration problem rather than a mechanical one.
• Encoder drift: the reported angle and the real angle diverge. The system looks healthy because it believes its own measurement.
• Backup supply degradation: the most dangerous fault on the list, because it is silent until the moment it is needed. This is why backup capacity testing is not optional.
• Blade bearing wear: increasing play or friction at the root; shows up as rising motor current and, eventually, as noise.

Maintenance schedule