Environmental Engineering Reference
In-Depth Information
The prototype had a compact design with the rotor installed
directly on the output shaft of a gearbox with reinforced bearings.
The service brake was an electromagnetic brake mounted on the
rear side of the generator. As a novelty for small turbines the turbine
has electric yawing, controlled by a wind vane. In Karl Erik's opinion
yaw rotors, which at the time were the norm, made a wind turbine
look visually disfigured. In the end Karl Erik made the yaw bearing
for the yaw system himself. It was a clear proof of his mechanical
competencies.
The prototype wind turbine was erected in early summer of
1978, and it soon began to run. In general, it was surprisingly easy,
considering how innovative the product was.
However, a sharp break in our delight occurred in early autumn
of 1978. While the wind turbine was operating during a gale there
was a short grid failure, the turbine ran away and threw of the
blades. Afterwards we were analysing what happened. We found
out that the violent acceleration of the rotor in the short interval
between the disconnection of the generator by power failures and
activation of the brake, was sufficient to increase the rotor torque
to a level that could not be handled by the mechanical brake. In
a few seconds the rotor reached such a speed that its output was
multiplied, and the brake became red-hot without being able to stop
the rotor. And when the brake had been burned of the rotor really
began to run fast and threw of the blades. Two of them hit various
objects near the wind turbine whereas the third followed a ballistic
curve and hit the ground 475 metres from the wind turbine.
This event made us realise that we needed to design blades
with built-in air brakes. For this we liaised with Erik Grove-Nielsen,
and he quickly came up with a solution with turning blade tips.
Under normal operation the blade tip was kept in place by a spring.
However, if the rotor speed exceeded a certain limit the centrifugal
force on the tip would cause it to compress the spring and move
axially outwards, and with a guide pin system the tip would turn
to act as a brake. The first blades with air brakes were completed
late in 1978. The set for the prototype had outside springs, because
one could not get a standard spring that would work. All production
wind turbines had special-design inside springs, but the funda-
mental principles were unchanged. Nowadays such blades are
made with tips that are kept in place with hydraulic cylinders; this
allows the air brakes to be used also during normal shutdown.
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