Technical reference
Yaw System
The system that rotates the whole nacelle horizontally to keep the rotor facing the wind. It moves rarely and slowly, which is exactly why its problems tend to go unnoticed for months.
What the yaw system does
A rotor that is not facing the wind loses power roughly with the cosine of the misalignment angle, and it picks up asymmetric loading at the same time. The yaw system exists to keep that error small. It also manages cable twist: the nacelle can only rotate so many turns in one direction before the power cables below must be untwisted.
Components
- Yaw motors: several electric drives working together, typically 3–5 kW each on smaller machines and 10–15 kW on large ones.
- Slewing bearing: a large-diameter ring bearing (roughly 3000–5000 mm) between tower top and nacelle, carrying several hundred tonnes of radial and moment load.
- Ring gear: the toothed ring the drive pinions engage, with a high overall reduction.
- Yaw brakes: hold the nacelle in position between movements so the drive is not fighting the wind continuously.
- Vane sensor: the wind direction sensor on the nacelle roof, typically accurate to a few degrees.
- Lubrication system: often automatic, feeding grease to distributed points around the bearing.
How the control works
The controller does not chase every gust. It averages wind direction over a window, and only commands a movement when the error exceeds a threshold and persists. That deliberate sluggishness is a design feature: a yaw system that reacted quickly would wear itself out and would never settle.
When that balance is upset — by a noisy sensor, a dragging bearing, or a controller gain that is too high — the loop becomes unstable and the nacelle hunts. That combination is the subject of Case 03.
Normal operating values
| Parameter | Typical value | Comment |
|---|---|---|
| Yaw rate | 0.3–0.8 °/s | Deliberately slow |
| Alignment tolerance | ±1° to ±5° | Tighter on newer machines |
| Movements per day | Tens, site dependent | A sharp rise suggests instability |
| Cable twist limit | ±2 to ±3 turns | Triggers an untwist cycle |
| Motor current | Steady, position independent | Variation with position means drag |
Typical faults
- Yaw hunting: the nacelle oscillates instead of settling. Almost always a combination of sensor noise, mechanical drag and controller gain — see Case 03.
- Bearing wear: often starts in one sector rather than around the whole ring, and the usual reason is a blocked lubrication line. Caught without an alarm in Case 06.
- Vane sensor faults: loose mounting, icing or drift. Everything downstream inherits the error.
- Brake problems: worn pads or residual pressure make the drives work against the brake continuously, raising current and heat.
- Ring gear damage: usually a consequence of prolonged bearing trouble rather than a primary fault.
Maintenance schedule
| Task | Interval | Note |
|---|---|---|
| Yaw motor current trend (peaks, not just averages) | Monthly (from SCADA) | Rising peaks are the earliest sign of drag |
| Vane sensor mounting and alignment | 6 months | One minute; poisons everything if loose |
| Slewing bearing lubrication | 12 months | Collect purged grease point by point and inspect for metal |
| Automatic lubrication line verification | 12 months | A running pump is not proof of delivery |
| Ring gear backlash measurement | 12–24 months | Shows whether wear has spread from bearing to gear |
| Yaw brake inspection | 12–24 months | Pad thickness and residual pressure |