Actuators Propel Wind Turbines

Wind turbines have two axes of control — yaw and pitch — plus braking systems. Yaw control keeps the turbine pointed into the wind, while pitch control continuously adjusts the angles of the blades to ensure as high and constant power output as possible, while protecting the equipment from being damaged by overspeed. Some turbines manage to get by with fixed-pitch blades, using clever design to ensure that if wind speed gets too high the blades will go into aerodynamic stall and lose lift, but most of the larger units have adjustable pitch.

Depending on a number of design factors, pitch control can be done electrically or hydraulically; in electric control a motor with an attached brake is coupled to the pitch-control mechanism through a gearbox. A set of backup batteries ensures that the blades can be moved to a safe position in case of power failure. Hydraulic control uses push-pull cylinders that extend or retract to control blade angle.

These cylinders tend to be large, says Choudhary. “You’re looking at up to a meter and 1.5 meters stroke and you’re looking at bore sizes of anywhere north of 10 cm,” he says “and that’s for each blade.” Backup power is provided by a hydraulic accumulator. The accumulator also helps to reduce the duty cycle of the hydraulic power unit, he adds. An electric system, he explains, must use electric power every time it adjusts blade pitch, whereas “in a hydraulic system you charge up the accumulators and then you can keep using those accumulators until they discharge down to almost 50, 40%, and then you just start your hydraulic power unit, charge it back up to 100% and go dead.”

There are two sets of brakes on a wind turbine, one for yaw and one to stop the rotor from turning during an emergency or to keep it stationary during servicing. These brakes are generally spring-applied and either electrically or hydraulically released, both because it’s better not to use power while the turbine has no output, and because a spring-applied brake is inherently failsafe.

Both electric and hydraulic systems have advantages and disadvantages for wind turbines. Hydraulic systems have to include rotary unions, and it’s difficult to prevent these from leaking. Electric systems don’t leak, which means not only a cleaner installation but ensures that nothing slippery drips onto the brake pads or causes an environmental problem.

On the other hand, says Choudhary, hydraulic systems don’t use gears, while electric ones do. “The turbines are not static and the wind is not static,” he points out, “so as these blades are vibrating, they are causing fretting issues. The bigger, heavier the blade, the bigger the fretting issues.” And in areas where low temperatures or salt air are a problem, they’re more subject to corrosion and poor battery performance. Hydraulics also have greater power density than electric, he continues; for this reason he feels that the newer generations of ever-larger turbines will increasingly use hydraulics.