Method for controlling the rotational speeds of wind turbines, including determining the rotor rotational speed of a first and a second wind turbine; determining the rotor azimuth angle of the first and the second wind turbine; calculating a speed control value for the first wind turbine, based on the determined rotor azimuth angles and rotor rotational speeds; adjusting the rotor rotational speed of the first wind turbine according to its speed control value, wherein the speed control value is calculated such that the rotor azimuth angle of the first wind turbine is equal to the rotor azimuth angle of the second wind turbine after speed adjustment, and a wind turbine control master unit, a wind turbine control slave unit, a wind turbine control device, a signal light control module, and a wind turbine therefor.