Environmental Engineering Reference
In-Depth Information
1.3.2.2 Induction (asynchronous) generator with gearbox
Induction generators are the most commonly used generators in large wind sys-
tems. They are simple, rugged, and relatively low cost. However they have high
rotational speed compared to DC systems, and therefore necessitate the use of
a gearbox. Small wind turbines rotate at higher speeds than large ones, and it is
therefore easier to use the permanent magnet alternator design approach than on
larger turbines. However it is also true that on small induction generator-based
machines the gear ratio is lower than on large wind turbines, and so this can result
in a somewhat simpler power transmission (fewer reduction stages) than on large
wind turbines.
Small induction generator-based wind turbines operate at close to constant
speed. This means that the wind system only operates at the peak of the C p
l
curve at one wind speed, therefore at other wind speeds the turbine operates at less
than peak effi ciency. Figure 17 illustrates the difference in rotor power between
fi xed and variable speed operation of the same rotor. When the effi ciency of the
inverter is taken into account, it can be expected that variable speed operation can
result in 5
10% more energy capture.
1.3.3 Braking systems
Many small wind turbines have no braking systems at all (except for the rotor
overspeed control discussed above). As mentioned above, permanent magnet gen-
erators are sometimes used as brakes, which can be accomplished by either sim-
ply shorting the windings or connecting the windings through a low electrical
resistance.
It is also possible to use the generator as a brake on an induction machine (e.g.
the AOC 15/50, see Fig. 6). This is sometimes called “electrodynamic braking”.
Figure 17: Fixed vs. variable speed power curves.
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