# Drive Train DOF for Direct Drive WT's

This may be a silly question but I’m an electrical engineer, so you’ll have to forgive my ignorance of most things mechanical. Would it be valid to model drive train torsion in a direct-drive wind turbine? In the FAST users manual the equation given (Tres = DTTorSpr•( RotorPos – GboxPos ) + DTTorDmp•( RotorSpeed – GboxSpeed )) is in terms of two gearbox quantities. In a direct-drive turbine would RotorPos=GboxPos and RotorSpeed=GboxSpeed or is that only valid if the shaft is stiff? The reason I ask is that I’m seeing some fairly high-frequency (>1kHz), low magnitude shaft torque oscillations in my FAST simulations for the SWRT, which doesn’t have data for drivetrain damping or spring constant. I’m trying to decide whether those are realistic or if they would normally just be damped away by the shaft (or something else). Thanks very much.

David Ochs

Dear David,

It is valid to model shaft torsion in a direct-drive wind turbine. In the equation you refer to, the “Rotor” position and speed refer to the hub-side of the shaft and the “GBox” position and speed refer to the generator-side of the shaft for a direct-drive wind turbine. If the shaft were infinitely stiff (as is the case when the drivetrain DOF is disabled; DrTrDOF = False), then both positions/speeds are equal.

Hoewever, I’m not sure what is causing the >1KHz oscillation in your shaft torque and whether this would normally be damped by drivetrain torsoin.

Best regards,