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: Resistant moment of the rotor and of the electric generator (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,