Page 476 - Wind Energy Handbook

450 COMPONENT DESIGN

The design process outlined above results in an excessive number of lightly-
loaded callipers, because of the limitation on power dissipation per unit area. If the
relative infrequency of emergency braking events allowed this limitation to be
relaxed, then a more economic solution would result.

7.6.5 Two level braking

During normal as opposed to emergency shut-downs, the rotor is decelerated to a
much lower speed by aerodynamic braking before the brake is applied, so the brake
torque required is much reduced. In view of the benefit of reduced loads on the
braking system, and on the gearbox in particular, some manufacturers arrange for a
reduced braking torque for normal shut-downs. This is achieved on the usual
‘spring applied, hydraulically released’ brake callipers by allowing oil to discharge
from the hydraulic cylinder via a pressure relief valve when the brake is applied, so
that the hydraulic pressure drops to a reduced level. After the rotor has come to
rest, the remaining hydraulic pressure can be released, so that the brake torque rises
to the full level.

7.6.6 Low-speed shaft brake design

The procedure for designing a low-speed shaft disc brake is much simpler than that
for the high-speed shaft brake, because the limits on disc-rim speed, pad-rubbing
speed, power dissipation per unit area and temperature rise do not influence the
design, which is solely torque driven. The large braking torque required means that
a brake placed on the low-speed shaft will be much bulkier than one with the same
duty placed on the low-speed shaft. For example the design LSS braking torque of
1800 kNm from the example above would require a 1.8 m diameter disc fitted with
seven callipers.
A study by Corbet, Brown and Jamieson (1993), which investigated a range of
machine diameters, concluded that the brake cost would double or treble if the
brake were placed on the low-speed shaft rather than on the high-speed shaft.
However, when the extra gearbox costs associated with a high-speed brake were
taken into account, the cost advantage of the high-speed shaft brake disappeared.

7.7 Nacelle Bedplate

The functions of the nacelle bedplate are to transfer the rotor loadings to the yaw
bearing and to provide mountings for the gearbox and generator. Normally it is a
separate entity, although in machines with an integrated gearbox, the gearbox
casing and the nacelle bedplate could, in principle, be a single unit. The bedplate
can either be a welded fabrication consisting of longitudinal and transverse beam
members or a casting sculpted to fit the desired load paths more precisely. One
fairly common arrangement is a casting in the form of an inverted frustum which