ASK THE EXPERTS: Wind Generator Governing

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I used centrifugal feathering with my wind generator. It starts at 30°, then goes to 0° at high speeds. However, I’ve seen the feathering used in some big wind systems go the other way—starting at 7° and stalling at 30°. Could you tell me which system would be most convenient with a three-blade, 7-meter-diameter wind turbine?

Erik Barney • Misiones, Argentina

Changing the angle of wind turbine blades, usually called “pitching,” is a good strategy for protecting the turbine from high winds that could otherwise overload and damage the machine. The wind’s angle of attack on the blades determines both lift and drag forces, so pitching can be used to optimize a turbine’s performance—or to spoil it when needed.

Pitching a blade changes its angle relative to the plane of rotation. This blade angle is not the same as angle of attack. Changing the blade angle has the opposite effect on its angle of attack to the relative wind. The angle of the relative wind itself depends on the blade’s speed and on the air velocity reaching it, which will already be slowed and diverted by the lift and drag forces.

For maximum working efficiency, start by optimizing the blade’s geometry. (Optimum angle depends on the “tip-speed ratio;” how to find it would require a much longer explanation.) In high winds, you can pitch the blades to “spoil” this optimum setting.

Most designers choose to govern by changing the pitch to a smaller angle. This increases the angle of attack, and brings on stall. This is the simplest strategy because stall creates drag, slowing the blades’ motion and further increasing the angle of attack. Once stall is started, the wind angle increases, pushing the machine deeper into stall. While a small pitching movement is sufficient to do this, making the mechanics easier, the downside is that the thrust loads remain quite high in stall and the blades and tower are not completely protected from the violence of a storm.

Pitching the blades the opposite way—by increasing blade angle—is known as feathering. This reduces the angle of attack, which slows the turbine by reducing the lift forces that drive it. In this case, the range of angular motion needs to be much larger, because the wind angle tends to increase as the blades move into feather. This approach is the better one for extreme winds (provided the blades can pitch far enough) as the loads on the turbine are reduced. But it is harder to engineer a larger angle of movement and its control, so it is more common to see blades that are designed to pitch into stall.

Blade pitching is a great way to control wind turbines, provided the mechanism itself is correctly designed and robust. The pitch mechanism can be a weak point in the turbine’s design, leading to poor power output or maintenance issues as the moving parts (springs, bearings, etc.) wear out and fail. Careful design and experimentation can find the right balance, and lead to long-term performance.

Hugh Piggott • Scoraig Wind-Electric

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