Most devices such as wind turbines or electric fans have three leaves, why?

In short, it is - single blade type: under the same size, in order to obtain the same amount of energy, the single blade type requires a higher speed. At the same time, this means more noise and visual pollution. , greater ecological impact; the rotation is unbalanced, requiring counterweight, and the benefits of high speed - lower ratio gearbox and lighter mass are weakened; efficiency is low. Therefore, there is no cost advantage. Double-blade type: The pros and cons are similar to the single-blade type; Three-blade type: It can achieve its own dynamic balance, has lower speed, low noise, and is not easily hit by birds. More blades: low speed, requiring a high ratio gearbox, heavy, causing trouble in manufacturing, installation, operation, and maintenance; easy to hit birds, oh, no, it’s bird hits. (The blades are relatively large in size, with a diameter of up to tens of meters.) In summary, three-leaf blades are the most suitable.