Influencing the speed of rotation
A system for adjusting the angle of the sails had not been invented when traditional windmills were designed, so it was not possible to change the speed of rotation by adjusting the angle of vanes. The rotation speed of traditional windmills was altered by simply tightening more or less canvas on the four sails of the mill. The wind-catching surface could thus be influenced by means of this 'swifting' method. At a later stage, the sails were fitted with a self-swifting system whereby vanes adjusted the angle to the wind automatically.
Cap less directly into the wind
Another option to handle the peaks of fierce gusts of wind was to position the entire windmill cap so that it faced less directly into the wind (i.e. backing from the wind). A third way of stopping the sails rotating too fast was to brake the main shaft of the mill mechanically and then roll up the sailcloth; this also reduced the wind-catching surface.
From streamlining and efficiency…
Improvements, including the Van Bussel system, were introduced in the 1960s, which optimised the streamlining of the sails. This could increase efficiency by as much as 10% for more effective milling, pumping or sawing, among other things.
One disadvantage and increased safety risk of this modification was that the mill was more prone to running too fast and getting out of control. In response to this issue, Van Bussel developed integrated vanes in the wings that would open if the mill speeded up too much.
Wing extremity with closed and open brake vane
…to intelligent measuring and control systems
Today's wind turbines have changed completely both inside and out. For example, they feature control systems that can measure the wind speed, register and adjust the angle of the blade, and check rotation values, as well as monitoring output power. If the anemometer on the turbine registers too high a value, the blades are automatically tilted to a less efficient angle by hydraulic rams or electric motors, so if a storm develops with wind speeds in excess of 9 BFT, the controls of the wind turbine will engage and the blades will be tilted to an angle of 90 degrees (vane position).
Two additional safety checks
The monitoring and control systems described above may not operate satisfactorily in extreme conditions. Modern turbines feature two systems that can operate in such an eventuality. These focus on the following aspects.
- The rotation speeds of the different shafts. If a rotation speed exceeds the threshold value, the system will turn the blades to a less efficient angle (vane position). The mechanical brake will also engage to stop the wind turbine.
- The power generated in relation to wind and rotation speeds.
If any parameter is not in keeping with the correct ratios, the system will turn the blades to a fully ineffective angle (vane position).
The engagement of either of these systems may imply that a breakdown has occurred, in which case the wind turbine must be repaired.
Innovation in a nutshell
Wind power has brought society many good things. It is good to know that we can operate the wind turbines of our own time in a safe and responsible manner based on a combination of historical knowledge, state-of-the-art technology and new insights.
With due thanks to Emiel Kuster, miller of the Walmolen in Doetinchem.