Factors affecting wind turbine cut in speed

[norris]

Hello, could someone explain some of the factors, or the calculation, for determining wind turbine cut in speed? I am talking about small wind turbines using with a single bearing on the rotating axis and a permanent magnet generator. A lot of literature targets a cut in speed of 3 m/s without explaining how to design for this value.

 

[Bystander]

3 m/s

How fast is this in more familiar units (mph, kmph)?