Dear GukSun,

FAST does have a gearbox friction model available when input parameter GBoxEff is set less than 100%. However, in the NREL 5-MW model, GBoxEff is set equal to 100% for simplicity and the friction loss is lumped in with the electrical power loss.

Your equations look OK, but in FAST the aerodynamic torque is rotor-speed-dependent, meaning that what you call “external_rotor_damping” is included in “aerodynamic_torque” (which is calculated by AeroDyn in FAST). Likewise, the generator torque in FAST is generator-speed-dependent, meaning that what you call “external_generator_damping” is included in “generator_torque” (which equals GenTq in FAST). In your equation, it looks like you’ve linearized these torques into a constant + linear variation with speed; in general, they can vary nonlinearly in FAST/AeroDyn.

“external_generator_damping” can be found by the partial derivative of GenTq with respect to GenSpeed. You maybe be able to identify “external_generator_damping” from the FAST input file depending on what generator torque model is chosen. For example, if the simple-induction-generator (SIG) model is selected (VSContrl = 0, GenModel = 1), then

GenTq = SIG_RtTq*( GenSpeed – SIG_SySp )/( 0.01*SIG_SlPc )

for torques less than SIG_RtTq*SIG_PO. In this case:

external_generator_damping = SIG_RtTq/( 0.01*SIG_SlPc ).

For more advanced torque models, this partial derivative may be harder to calculate, but the process is similar.

A similar partial derivative is needed to compute “external_rotor_damping” from “aerodynamic_torque” with respect to RotSpeed. In this case, the result may be a bit harder to get from FAST because the aerodynamic torque is not directly output from FAST. That is, while the aerodynamic torque is certainly computed within FAST, the value itself is not output directly. Instead, FAST outputs the rotor torque (RotTorq. what you call “restoring_torque”), which is the torque transmitted through the low-speed shaft. This torque equals the aerodynamic torque only when the rotor is not accelerating or decelerating. Thus, the best way of obtaining “external_rotor_damping” would be to output the rotor torque from a few steady-state simulations at different rotor speeds and compute the derivative numerically.

I hope that helps.

Best regards,