A yaw drive is the mechanism responsible for rotating the entire upper section of a horizontal-axis wind turbine, known as the nacelle, around the vertical axis of the tower. This movement is necessary because the wind direction is rarely constant, and the turbine’s rotor must be oriented perpendicular to the airflow to operate efficiently. The yaw drive system continuously adjusts the nacelle’s position, ensuring the turbine captures the maximum amount of kinetic energy from the wind.
Purpose of Yaw
The ability to rotate the nacelle is tied to the turbine’s efficiency and structural integrity. When the rotor plane is not perfectly aligned with the wind direction, a condition known as “yaw error” occurs, which reduces the turbine’s power output. Power loss is often modeled by a relationship where the power captured is proportional to the cosine cubed of the yaw angle. For example, a yaw error of 10 degrees can result in a power loss of around 4.5% to 8%.
Yaw movement also manages asymmetrical loads on the rotor blades and tower. A misaligned turbine experiences uneven forces across the swept area, creating a bending moment on the blades and the main shaft. This uneven loading introduces fatigue and excessive stress on mechanical components, potentially leading to premature wear or failure. The yaw drive’s continuous correction prevents these oscillating loads, extending the operational life of the entire wind turbine.
Core Components and Mechanical Function
The physical rotation is accomplished by a mechanical system located between the nacelle and the tower. The entire weight of the nacelle, rotor, and generator is supported by a massive yaw bearing, typically a large-diameter roller or slewing ring. This bearing allows the nacelle to rotate smoothly against the stationary tower flange.
The movement is powered by several electric or hydraulic motors, referred to as yaw drives, mounted on the underside of the nacelle. Each yaw drive incorporates a multi-stage gearbox, often with an input-to-output ratio around 2000:1, necessary to generate the torque required to rotate the nacelle’s mass. This high gear ratio ensures the rotation speed is slow and controlled, minimizing stress on the structure.
The final mechanical step involves a pinion gear attached to the gearbox output, which meshes with a large, stationary ring gear fixed to the top of the tower. When the motor activates, the pinion drives along the ring gear, translating the high torque into a slow rotation of the nacelle. Once alignment is achieved, a set of yaw brakes, often hydraulically or electromagnetically actuated, engage to clamp the nacelle securely in place. These brakes maintain the fixed position against aerodynamic forces until the control system signals another adjustment.
Automated Control and Operation
The intelligence governing the yaw drive’s movement originates from a suite of sensors and the turbine’s main controller. Wind direction is continuously measured by a wind vane and wind speed by an anemometer, typically mounted on the back of the nacelle. The controller receives this real-time data and compares the measured wind direction to the current rotor orientation, calculating the precise “yaw error.”
The turbine controller is programmed with operational parameters that dictate when a correction should be made. To prevent constant, small adjustments that would cause excessive wear, a “dead band” tolerance is employed, often between 3 to 10 degrees of misalignment. If the calculated yaw error exceeds this pre-set tolerance for a specified duration, the controller commands the yaw brakes to disengage and activates the yaw drive motors.
The rotation speed is intentionally low, typically less than one degree per second, to minimize dynamic loads and prevent noise generation. Modern control systems monitor the torque and load on the drives during rotation, often distributing the load uniformly across multiple yaw drives to reduce mechanical stress.
Once the nacelle has been rotated back within the acceptable dead band, the motors stop and the yaw brakes re-engage. The system then returns to a monitoring state, ready for the next shift in wind direction.