Speed of a wind turbine

If a wind turbine is spinning too slowly, most of the wind just passes through the blades and therefore wasting the energy that it could be producing. So the minimum speed for a wind turbine to work most efficiently is about 5mph (8 km/h).
On the other hand, if a wind turbine is spinning too fast it could one cause mechanical damage; and two create a "wall" against the wind to safely spin the turbine to create electricity. Most wind turbines have a maximum speed that they can work at properly. They usually shut down if wind speeds get higher than about 55mph (88.5 km/h). The reason they have an automatic shut off is to prevent any mechanical damage.
The optimal speed of a wind turbine where it works the best is between 30mph and 50mph.
A wind turbine can also be calculated using rotations. The blades of the turbine are connected to the rotor that tuns at about 30-60 rotations per minute. Then that rotor is connected to the whole gear box that raises the rotation speed to about 1000-1800 rotations per minute. This is the average speed that is required to produce electricity in rotations per minute.