Braking force measuring method

A method of measuring a braking force of a brake device in a wind turbine, the wind turbine including:

• a ring gear fixed to one of a tower of the wind turbine or a nacelle of the wind turbine;
• a yaw drive device driving a pinion that meshes with the ring gear to turn the wind turbine in a yaw direction, the yaw drive device being fixed to the other of the tower or the nacelle;
• and the brake device pressing a friction body against the ring gear to keep the nacelle from turning in the yaw direction, the method comprising:
• a brake step of braking the ring gear by the brake device to keep the nacelle from turning in the yaw direction;
• a measurement step of measuring a load on a rotating shaft of the pinion that mesh with the braked ring gear;
• a determination step of determining a maximum value of the load during a time period that begins when input torque is applied to the pinion from the yaw drive device or an external power device with the nacelle held still, and ends when the friction body begins to slip and the nacelle that has been held still begins to turn in the yaw direction;
• and
• a conversion step of converting the determined load to the braking force.