Starting Torque

I am using the SWRT furling model as found in the Cert Test folder. I have modified a few dimensions to bring it inline with our turbine and I am running a variable speed analysis using the KP subroutine with the same rotor speed / Torque look up table. The wind input file ramps up from 0 to 18 m/s over 1500 seconds and the initial rotor speed = 0 rpm.

What I am struggling with is the ‘cut-in’ wind speed and torque.

No matter which way I plot the results, the rotor torque = 0 until the generator applies its torque to the rotor shaft even though the rotor is rotating. It has a cut in wind speed of 0 m/s. This is confirmed by the torque vs rotor speed plot which flat lines until 50 rpm, which is the lowest rotor speed on the torque speed look up table in the variable speed subroutine for the generator. The rotor also shoes zero torque when the rotor speed exceeds the maximum speed in the speed / torque look up table.

So my question is…can anyone advise me how to find the torque on the rotor shaft at start up, for example 5 m/s wind speed and 0 rpm rotor speed, even if the electrical generator is not generating power yet? Is there an output variable I have overlooked?

I am outputting ‘HSShftTq, GenTq, RotTorq’ (all of which are perfectly equal throughout the simulation).

Looking forward to any help offered.



Dear Phillip,

The SWRT model has no gearbox (GBRatio = 1), so, I would expect the low-speed and high-speed shaft torques to equal (HSShftTq = RotTorq), but I would expect some difference between these and applied generator torque (“GenTq”). The difference may be small for your case, though, because the generator inertia (GenIner) is small for the SWRT. More information about the drivetrain model in FAST is available in the following forum topic: (in this forum post, RotTorq is identified as LSShftTq; both are equivalent).

I suspect what you want is the applied aerodynamic torque (what the forum post linked above calls “T_Aero”) instead of the torque transmitted through the shaft (“RotTorq”). While applied aerodynamic torque is not currently a standard output of FAST, some guidance on deriving it from the available FAST outputs is given in the following forum topic:

Best regards,

many thanks for your reply. The links you posted were very helpful in understanding how FAST calculated and output the various torque values.

Fortunately I discovered the ‘RotTorq’ output a couple of days after posting this question which is equivalent to ‘LSShftTq’ when at steady state. JrRotAccel must be included if there is transient wind input. In my simulations the wind ramps I use are so slow, for the moment I can neglect the JrRotAccel term.

I ran a simulation with a simple ramp wind input file (4 m/s → 17 m/s → 0 m/s over 1500 seconds) and switched GenDOF to False which appears to have effectively allowed the turbine to run off load. I set ‘RotSpeed’ to zero rpm. I then repeat this simulation but change the RotSpeed from 0 to 1, 5, 10, 25, 50, 75, 100, 200, 300, 350 rpm.

I obtained a family of curves for torque against rotor speed for different wind speeds. =)

I also was able to get a rotor torque vs wind speed plot for 0 rpm from this. My assumption being that if I measure the static torque of the wind turbine rotor shaft in the factory I can then look up this torque value on the zero rotor speed plot and read of the theoretical cut in wind speed.

I look forward to your thoughts on this approach.


Dear Phillip,

I’m not sure I understand what you mean when you say that you would “measure the static torque of the wind turbine rotor shaft in the factory”, but I agree with your other points.

Best regards,

Thanks Jason,
by the static torque, I mean to measure the torque required to start to turn the rotor shaft.

Eg. We put the turbine on the floor with the rotor shaft facing upwards. We then attach a 1 metre long bar to the rotor shaft which has a spring gauge at the end of it. We then, very slowly, apply a force to this bar and read the force off the gauge when the shaft begins to rotate. We do this several times and take an average. This is effectively a measure of the ‘un-sticking’ torque of the rotor shaft which results from generator cogging, seals, bearings etc. This torque is usually slightly larger than the torque required to keep the rotor shaft turning, probably because of the stiction of the seals.

Many thanks again.


Dear Phillip,

Thanks for the clarification. I agree with your approach.

Best regards,