Under normal operational conditions, yaw movement may occur when the rotor is stopped or when it is turning. When the rotor is stopped, the force applied by the yaw drive needs to overcome only the inertia of the nacelle and the frictional forces at the yaw bearings to cause the nacelle to yaw.
Additional force will be needed for any yaw movement if the rotor is turning. This additional force is known as the gyroscopic force. Depending on the rotational speed of the rotor, and the angular velocity of the yaw movement itself, the magnitude of the gyroscopic force can be extreme. Excessive gyroscopic force may cause failure of the yaw movement system and threaten the safety of the entire wind turbine.
Due to the uneven special mass distribution of the blades as they rotate around the main shaft, the magnitude of this gyroscopic force will vary at a frequency directly proportional to the rotational speed of the rotor.
This variation may cause severe vibration in the nacelle and the tower, which could lead to catastrophic mechanical failure of the wind turbine system
A comprehensive study of the gyroscopic forces at work during yaw movement while the rotor is spinning requires advanced knowledge of solid mechanics, calculus, advanced computer algorithms, and engineering modeling experiments.
However, since rigid bodies can be studied as systems of particles, fundamental physics concepts such as mass particle accelerations, and Newton’s Second Law will help explain the occurrence of the gyroscopic force and the way such force can be minimized during normal wind turbine operations.
For instance, consider a blade that is in the vertical position pointing downward during its rotation around the main shaft. A mass particle at the very end of this blade will not accelerate in the direction parallel to the main shaft, since there will be no change of velocity for that particle in that direction.
Once yaw movement starts, however, acceleration in the direction parallel to the main shaft will occur.
The magnitude of this acceleration is equal to the angular velocity of the yaw movement multiplied by the linear velocity of the particle in the tangential direction as it rotates around the main shaft. According to Newton’s Second Law, this acceleration will cause a force, which is the gyroscopic force.
The total gyroscopic force will be the resultant of all forces acting on all mass particles of the blades at the same instant and at the similar positions. This leads to the following analysis.
First, the gyroscopic force will increase if either the angular velocity of the yaw movement or the linear velocity of the particle increases. Since an increase in angular or linear velocity results in increased acceleration, the gyroscopic force caused by this acceleration also increases. Second, if there is no yaw movement, the angular velocity is zero, and the magnitude of the gyroscopic force is zero because the acceleration is zero. Third, if the rotor is not rotating, the linear velocity is zero, and the magnitude of the gyroscopic force will also be zero, because the acceleration is zero.
We can now see that in order to minimize the gyroscopic force, yaw movement must be very slow when the rotor speed is fast. When the rotor is moving slowly, yaw speed may be higher. In actual practice, yaw speeds are constant, but very slow. This ensures that the gyroscopic force will be at a safe low level at normal rotor speed
Because of the large forces involved, speed controls and brakes for the rotor and yaw system are critical. The inspection and maintenance of these systems are crucial to the safe operation of a wind turbine.
Video : Wind Turbine Yaw Movements and Gyroscopic Forces
Highland Community College as part of WindTechTV.org