# Rotor inertia

Hello everybody ,

i face a problem when trying to modelize the WindPac 1.5MW by a two mass model :

• the rotor inertia calculated in the summary file , includes hub and blade inertia or not ?

• I want to know a simple methode for extraction of external Rotor and Generator external Damping

Thanks ,

Sincerely

Dear Younes,

The following forum topic is likely of interest to you: http://forums.nrel.gov/t/state-of-linearized-model-rotors-speed/811/1.

Yes, the rotor inertia written to the FAST summary (*fsm) file includes contributions from the hub and blades.

The rotor (aerodynamic) damping can be derived through a FAST linearization analysis. The generator damping is discussed in the forum topic linked above.

Best regards,

After i made a manual computation , i guess that rotor inertia includes also generator inertia about LSS (Jg*Ng²) . Is this true ? And if it was , so the above two mass model is not suitable :

Jr dωr = Ta − Tls −Krωr ,
Jg dωg = Ths −Kgωg −Tem,
Tls = Kls(ωr −ωls)+Dls(θr − θls)

• If you can advise me how to extract Rotor aerodynamic damping from linearization file because it was not evident for me .

Best Regards ,

Sincerely

Dear Younes,

No, the rotor inertia written to the FAST summary file does not include the generator inertia.

Have you performed a FAST linearization analysis before? I suggest you read the “Linearization” chapter of the FAST User’s Guide for more information. Of course, you’ll want to linearize a FAST model with the generator azimuth and drivetrain torsion degrees-of-freedom (DOFs) enabled.

Best regards,

Thank you Jason ,

Yes , i’ve performed 4 linearizations and so i get 3 rotor dampings and i’ve done the mean , but results was not coherent because ive found 2 Dr near of each others and the third was very far from them .

After reading the topic you advise me and other topics , i’ve understood that : GenTq and RotTorq provided from FAST are equal to :

GenTq = Thss - Dg*wg

RotTorq = Tlss - Dr*wg

with Thss and Tlss are respectively : High Speed shaft torque and Low Speed Shaft .

is is true ?

Sincerely .

Dear Younes,

I’m not sure I understand all of the terms in your equations, but:

*GenTq in FAST is the torque applied to the high-speed shaft from the generator
*RotTorq in FAST is the torque transferred through the low-speed shaft between the rotor and gearbox. This should equal Tls from you post above, dated Mar 04, 2014.

More information about the drivetrain model in FAST is available in the following forum topic: http://forums.nrel.gov/t/resistant-moment-of-the-rotor-and-of-the-electric-generator/408/1 (in this forum post, RotTorq is identified as LSShftTq; both are equivalent).

I’m not sure how your linearizations differ to comment on whether the results make sense.

Best regards,