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turbines, but two blades and downwind configurations are also quite prominent. One aspect
of small HAWTs that makes them different from larger ones is their frequent reliance on
tail vanes for orienting the rotor into the wind, although downwind turbines offer passive
orienting without any additional devices. A variety of overspeed control methods are found,
including aerodynamic tips, furling of tail vanes, blade pitching, and the use of generator load
control. Direct-drive permanent-magnet generators are most commonly used. A variety of
tower systems continue to be available for these small-scale turbines.
Figures 4-5 and 4-6 show representative small-scale commercial wind turbines.
Figure 4-5. Bergey Excel 0-kW wind turbine utilizes a tail vane for yaw control. ( Cour-
tesy of the National Renewable Energy Laboratory )
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