I am having trouble making sense of some simulation results with FAST:
I am using the Simulink interface to do some very simple simulations of a wind turbine. All control inputs are set to fixed values, i.e., generator torque, power, pitch angles and yaw command. The wind input file is a simple hub-height wind with a constant value. After some time, the wind signal jumps to a higher value. Thus, since all control inputs are kept constant, the wind turbine accelerates to a higher rotational speed. So far, everything works out fine.
However, I have a question regarding the simulation results. I would expect the rotor torque (RotTorq or LSShftMxa) to jump to a higher value the instant the wind signal suddenly increases, since aerodynamic torque is suddenly increasing. In my simulation, with my turbine model, this is not the case. The increase in rotor torque is only miniscule. If this is the case, what causes the wind turbine to accelerate as it does?
I checked the scenario using the NREL 5MW reference turbine model. There, the increase in rotor torque is more apparent. I think there is one important difference between the two turbines: The NREL reference turbine has a gearbox with i=97, whereas the model I am working on has no gearbox, i.e., i=1. Could this be an issue?
I am using FAST 8.10, but I also checked with FAST 7, there is no change.
Am I looking at the wrong output signal? Is there another way to obtain the aerodynamic torque on the rotor?