Page 206 - Fluid Power Engineering

178 Chapter Nine

Rack and pinion in which rack is circular and meshes with

pinions of the two or three blades. The rack is driven by a
motor through a worm gear
Individual motor to control each blade. This is the most com-

mon method of controlling pitch.

The pitch mechanism is at the front of the rotor hub, as shown in
schematics Figs. 9-3 and 9-6. A toothed gear is at the interface of the
rotor hub and the root of blade. An external motor operates the pitch
with a gear (see Fig. 9-3) or a toothed belt (see Fig. 9-6). The external
motors and any other part of the pitch mechanism are attached to the
hub and rotate with it.

Nacelle
The primary components of nacelle are the main shaft, bearings, gear-
box, generator, brake, nacelle frame, hydraulic systems for brakes and
lubrication, and cooling systems.

Gearbox
Gearbox is a standard component in nondirect drive turbines. The
purpose is to increase the rotational speed. There are different types
of gears in a gearbox: Planetary gears and spur/helical gears.
As an example, consider a turbine with speed of rotor hub = 24
revolutions per minute (rpm) and speed of a generator = 1,800 rpm.
This speed conversion will require a gearbox of 1:75 ratio, which can
be accomplished in a three-stage gearbox. Stage one and two are plan-
etary gears in series, each with ratio of 1:5. This yields speed ratio of
1:25. An additional gear stage consisting of spur gear of ratio 1:3 will
further increase the speed ratio to 1:75. Planetary gears are compact,
typically about one-third the size of spur/helical gears. Two disadvan-
tages of planetary gears are: (i) Overheating due to reduced surface
area to release heat generated because of friction, and (ii) higher noise
because the teeth are not beveled.
A schematic of gearbox is in Fig. 9-8. The life of a wind turbine
gearbox is about 10 years and unexpected failure can lead to signifi-
cant downtime. Therefore, gearbox condition monitoring systems are
recommended. A comprehensive condition monitoring system will
measure vibration, temperature, properties of lubricating oil, wear
particles in oil, and others in order to provide early warning.

Yaw Drive
The yaw mechanism allows the turbine to face the wind, that is, align
the plane of rotation to be perpendicular to the direction of wind.