ASK THE EXPERTS: Wind Concepts

Angle of Attack and Apparent Wind
Angle of Attack and Apparent Wind
Angle of Attack and Apparent Wind

I enjoyed the article on wind turbines (“Wind-Electric Systems Simplified” in  HP148). When it comes to governing, the concepts of “furling” and “pitch control” make sense. But I don’t understand “stall,” and what it has to do with making sure the turbine does not overspeed.

The article also indicates that for induction motor turbines, rotational speeds are constant, and thus stall is inherent. So in effect, do the windings act as electric braking? Does extra torque try to break away from the fixed speed, increasing voltage and generation, which also increases the electrical braking?

Don Turner • via email

You pretty much got it right: As the wind grows stronger, the machine will produce higher current and hence higher torque, but the rpm do not increase enough to maintain a usable angle of attack, so “stall-regulated” turbines limit their output and protect themselves against strong winds without any fancy mechanisms.

Wind turbine blades get a shove from the wind, and slow the wind down. The lift force on the blade, which drives the shaft to rotate, produces this mutual interaction. The airflow over a wind turbine is made up of two parts—the actual wind and the rather larger headwind caused by the blades’ own motion, which add together to produce a diagonal “apparent wind” that strikes the blade and creates both lift and drag. Lift is at right angles to this diagonal wind direction, and drag is aligned with the apparent wind.

Only a small part of the lift force acts in the direction of the blade’s movement, so it is very important that this small driving force is not countered by too much drag force. Efficient blade design therefore requires a high lift-to-drag ratio.

Lift and drag both depend on the “angle of attack” of the apparent wind on the blade. This is the angle between the apparent wind and the chord line (which is the longest line in the blade section, from leading to trailing edge). A wind turbine should be designed to load the blades carefully over a range of wind speeds so that the power consumed at each wind speed will let them run close to the ideal rpm, and hence the ideal angle of attack.

As far as the windings acting as electric braking, the turbine’s working rpm depends on the balance between what the wind can do and what power is required of it. If the shaft spins without resistance, the blades will overspeed. But if there is a high torque on the shaft, the blades will slow. This reduces the “headwind,” raising the angle of attack. If the angle of attack increases much beyond the optimum, the blades will stall and the lift–drag ratio will drop drastically. 

By careful choice of gearbox ratio and blade pitch angle, it is possible to use the stall point to prevent overload, but stall regulation like this is only effective up to a point, after which a good brake is needed to stop the turbine in stronger winds. Passive stall is still an attractive option for some turbines when designers want to use the electrical load rather than mechanical means to protect the machine from overload. Often, in utility-scale turbines a second generator winding was used to allow generation at a lower fixed rpm in low winds.

Hugh Piggott • Scoraig, Scotland

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