How and why do they change the pitch of the blades on large wind generators?

Answer 1

Although there is generally some kind of *steady* wind at the location of wind generators, even a steady wind changes speed constantly . The wind generators must be kept at a constant speed, however, to operate on a 60 Hertz power grid. By changing the pitch of the blades, the generator can be made to operate at a steady speed when the wind speed changes. When the wind speed slows down the pitch is raised to keep the windmill going at the same speed, with a corresponding decrease in torque. When the wind speeds up the pitch is lowered so, the windmill speed remains the same and the torque goes up.

Note: that's not all there is to it -- keeping the generator running at 60 Hz -- but that's most of how its done.

As for how it is done. Rotary actuators located at the hub change the pitch of the blades automatically (under computer control). This system is similar to what helicopters and propeller planes use.

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Answer 2

to keep the output frequency as close to 60 Hertz as possible.

typically the pitch is controlled by a small microprocessor.

Answer 3

To maximize the the RPMs in order to produce the most electricity.