Page 201 - Fluid Power Engineering

W i nd T urbine Generator (WTG) Components 173

Bending moment in the plane of rotation along the tangential direc-

tion, also called the flapwise bending moment, M t . This bending
moment arises because of drag forces that are the highest at
the tip of the blade and lowest at the hub. The frequency of the
moment depends on the frequency of wind speed. Measured
at the root of the blade, this oscillates with much smaller am-
plitude and stays positive. This moment is dependent on the
aerodynamic load and, therefore, it varies with the variation
in wind speed.
Twistingmomentalongthelongitudinalaxisofblade.Thistwisting

moment is due to the lift force. The frequency of the moment
depends on the frequency of wind speed. Measured at the
root of the blade, this is a smaller component of moment.

No mention of loads is complete without the mention of fatigue
loads. Wind turbines are subject to extreme levels of cyclic loads. For
instance, the root of the blade is subject to load that cycle every ro-
tation. At 30 rpm and 20-year life of turbine, the number of cycles is
9
∗
∗
∗
30 60 8760 20 = 0.3 × 10 cycles.
Rotor Hub
The next major component of the rotor is the hub. Blades are radially
bolted to the hub. On the axial end, the rotor hub is connected to the
drive train, which may be one or more of the following: Main shaft,
gearbox, and generator. The hub is made of high-quality cast iron. It
transfers load from the blades to the nacelle frame and to the drive
train. The manner of transferring loads from the hub to rest of the
components in the nacelle depends on the turbine configuration—
direct drive or with gearbox.

Alternative Configurations of Turbines
In the beginning of this section, two configurations are discussed: With
and without gearbox. At the end of the section, a two-blade turbine
with a rotor lifting and lowering mechanism is described.
A typical turbine with gearbox has a fixed-speed generator that
produces power at grid frequency. Although newer turbines allow
some variation in speed of generator, nevertheless the rotor hub rpm
is stepped up from about 20 to 30 rpm to generator speed, which is
50 to 75 times. Generator speed is typically 1,000 rpm or more.
Figure 9-3 is a schematic of a traditional turbine with gearbox.
Label1intheschematicillustratestwotypesofpitchdrives:Motorand
hydraulic. Rotor hub is labeled 2, which is connected to the main shaft
(labeled 3). Main shaft is a forged component made from hardened