The present invention relates to a system for controlling rotational speed on a rotating process machine, which for example is a turbine or a propeller, where the rotating process machine is connected to at least one motor and arranged to rotate with a given rotational speed given by the motor, the motor is connected to a control system and the rotational speed of the motor is arranged to be controlled by a control system. The rotating process machine comprises a load control arranged to be adaptable, the control system is connected to the rotating process machine. With the invention, one achieves soft transitions between several selectable rotational speeds and lower energy consumption by step by step adjusting the size and the rotational speed of the motor to varying energy requirement.