Design and development of megawatt wind turbines - Wind Power Generation and Wind Turbine Design - page 227

Figure 25: LSS rated torque for the industry study set.

transient operational considerations, max design torque of 2.5 3.0 times the

steady-state level is provisioned, so that the shaft and reaction confi guration would
actually be designed to accommodate 30,000 kNm or about 48.6 tank-meters.
Future large turbine main shaft and bearing technology will likely need to be
more integrated and consider cast or alternative approaches to today's typical main
shaft forging. Taken together with the requirements for larger and heavier rotor
hubs, the fi xed axle arrangement should also be evaluated.

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4.4.5 Mainframe or bedplate
The rotor and drivetrain support structure is a direct compliment and enabler to the
main shaft and bearings. The design goal is to achieve as light a structure as pos-
sible, with allowable defl ections within the overall materials and system specifi ca-
tion, yet strong and capable of resisting fatigue damage with a maintenance free
life. The amount of rotor overhang relative to the tower top centreline (see OH 1
and OH 2 in Fig. 10) directly affects the size and weight of the mainframe required
for a given turbine design.
Figure 26 shows (1) today's industry study set and (2) moving from today's
fabricated or cast bedplate towards a canard or spaceframe structure may save
signifi cant mass [28]. Further system benefi ts should be possible for integrating
the drivetrain within these types of spaceframe structures. The heavy trend line
shown in Fig. 26 represents the conventional fabricated or cast bedplate from the
industry study set. This represents today's technology, and is more than 20%
lighter than the double T frame used in the earliest MW WT designs. Even so,
the bedframe alone would be more than 140 tonnes at the 10-MW size if today's
technology were to remain unchanged.
Moving towards spaceframe type designs may reduce bedplate mass on the
order of half, but this will also depend on the amount of stiffness reduction that can