Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 309

Stator retaining ring

Rotor brake

Electric switchboard
and control system

Stator lamination
Pol pieces
Rotor retaining ring
Disk brake

Rotor blade
adjusting
device

Rotor hube with
blade adjusting
device

Ge a r bo x

Anemometer

Anemometer

Electric switchboard
and control system

Ge ne r a t or

Rotor bearing

Nacelle bearing

Nacelle adjusting
device

Nacelle bearing

Rotor blade Rotor blade

Tow e r

To w e r

Grid connection Grid connection

Foundation Foundation

Fig. 7.11 Schematic representation of commercially available horizontal axis converters
equipped with gearbox (right) and gearless (left) (see /7-1/, /7-3/)

To prevent exaggerated gear transmission, the number and shape of rotor
blades are designed for a relatively high numbers of revolutions to suit the com-
mon high-speed generators. Their revolutions generally vary between 1,000 and
1,500 r/min. Alternatively, also low-speed generators directly driven by the rotor
(i.e. gearless converters), especially designed for wind energy converters may be
applied.
There is an optimum number of revolutions (i.e. turbine design point) for every
wind speed at which maximum rotor power is achieved; wind power decreases at
lower and higher numbers of revolutions (Fig. 7.12 and 7.16). To maximise useful
power and thus the extraction of wind power at a high number of revolutions (ra-
tio of rotor blade tip speed to incident wind speed) (see Fig. 7.16) rotors equipped
with a few and narrow-shaped blades are required. They also help keep torsion
and friction losses to a minimum. This is why modern wind energy converters are
equipped with one to three rotor blades; whereas rotors of more than three blades
are virtually not applied for grid-connected power generation. Besides a high
number of revolutions (Fig. 7.12) converters provided with only a few rotor
blades also save material.
− Three is the lowest number of rotor blades that are still dynamically controlla-
ble. Thanks to favourable mass distribution less vibration-dynamic problems
are encountered with three-blade rotors than with two or one-blade rotors. Due
to tip speed ratios between 6 and 10, blade tip speeds are not particularly high
and exaggerated noise is prevented. The smooth running properties also help