Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 220

outer end of each blade, normally oriented parallel to the blade's path. During an emer-
gency they operate centrifugally to deploy perpendicular to the blade's path, slowing the
rotor by drag. They are simple and effective, and have saved many small fixed-pitch rotors
from destruction. However, tip brakes have two significant drawbacks. They are not aero-
dynamically part of the blade, and when not deployed they add significant drag, lessening
the efficiency of the rotor. When deployed, they do not reduce the blade's lift, which is the
principal force causing the overspeed. Buckets are semicircular plates which, like tip brakes,
are centrifugally deployed to create drag and slow the rotor.
Pitchable tips , Figure 4-(b), differ from tip brakes in that they are an integral part of
the blade airfoil. When deployed, they rotate from within the plane of the blade to an out-of-
plane position. This not only creates drag near the tip but also takes about 0 percent of each
blade out of operation. Since the outer third of the blade creates most of the lift, the deployed
tips dramatically reduce rotor torque. The Nordic Windpower N000 (Fig. 4-2) and the
Aerostar 6 Meter wind turbine (Fig. 4-6) are examples of commercial turbines today that use
stall regulation with pitchable tips along with two-bladed and teetered hubs.
Drive Train, Nacelle, and Yaw Drive
Figure 4-2 illustrates the common mechanical components within the drive trains of
wind turbines. These are a turbine shaft (often called the low-speed shaft), a gearbox (speed
increaser), a brake (located on either side of the gearbox), a generator shaft (often called the
high-speed shaft ), and a yaw drive.
Figure 4-2. Representative components in the drive trains of medium- and large-scale
wind turbines. The drive train shown is that of the Vestas V39/500 wind turbine. ( Courtesy
of the Vestas Wind Systems A/S )
Turbine Shaft and Gearbox
The gearbox increases the turbine shaft rotational speed to the speed required for the
generator to produce AC power at frequencies of 50 or 60 Hz. The vast majority of all com-
mercial wind turbines employ two- or three-stage gearboxes using planetary gears in combi-
nation with parallel, helical, or spur gears. Some variations are evolving to handle the large