13.1 Background
Almost all wind turbines in Denmark are producing electricity.
Electricity produced by an asynchronous generator is sent to
the public grid at 50 Hz with the related speed of rotation. The
windmill is, thereby, locked by the electricity grid. With constant
rotation speed, the blades will stall with increasing wind speed,
on wind speed, the blade tip speed will remain constant. Average
tip speed of older windmills is about 40 m/s and with increasing
size of wind turbines the tip speed increases to about 70 ...