Optimized gearbox design - Wind Power Generation and Wind Turbine Design - page 524

Reaction
M/C1

Output gears

Generator

Epicyclic
gears

Reaction
M/C2

Figure 12 : Variable ratio gear arrangement.

found that at this size, it has worked quite successfully without heat dissipation
problems with a limited slip of up to 5%.
For larger powers, to provide better control and a bigger speed range with greater
energy capture without excessive losses, it is necessary to control the reaction with
a closed loop bypass branch comprising either a hydraulic pump and motor or an
electrical equivalent to recover the power which would otherwise be lost. With
such a system, torque may be monitored to enable transient referred inertia effects
to be eliminated. Variable ratio gears using this principle have been successfully
developed for powers up to 3.6 MW with synchronous generators driven by tur-
bines with speeds ranging from 60 to 100%. In effect, such gears allow turbine
speeds to increase when subject to a transient torque increase so that the excess
torque is absorbed by the increased kinetic energy in the rotor while the excess
speed is absorbed by the reaction member. Conversely, when the turbine torque
has a transient decrease its speed can be reduced by a ratio change to recover the
kinetic energy. For more sustained changes the gear ratio is changed accordingly
(see Fig. 12 ).

7 C onclusions

The purpose of this chapter is to show how the transient torque/speed charac-
teristics of a wind turbine affects the volume/weight of the drive train and the
benefi ts that accrue due to the use of epicyclic gears not only for reducing weight
and increasing compliance but also for their differential torque limiting properties.