# Deriving Rotor's RPM from Aero. Torque

Hello all,

Newbie question over here. We’re a bunch of amatures trying to figure out a project. Any help/guidence is -highly- appericate.

We are trying to derive the Rotor’s RPM using a dataset of wind speed and aerodynamic torque.

How is it possible to derive the rotor’s rpm from the windspeed as shown in the graphs.

Kind regards,
Emir

Dear Emir,

You can’t derive the rotor speed from just wind speed and aerodynamic torque (there is no unique solution). Do you have other information, e.g., the pitch angle or torque coefficient surface of the rotor as a function of tip-speed ratio and pitch?

Best regards,

Hello Jason,

Thank you for taking your time to answer our question. We thought the same thing, however, we lacking some of the parameters.

We are working with this wind turbine, NORWIN 47-ASR-750kW Datasheet

In the technical datasheet, NORWIN specificed the thrust cofficient (Ct) as a function of wind speed. Please see the data sheet.

Unfortunately we do not have the torque coefficient surface of the rotor as a function of tip-speed ratio and pitch. We have the Cp vs Lambda graph, however, we are unsure on how to transform the windspeed and lambda to get the RPM yet.

An insight on how to derive the rotor’s speed with the information we have will be highly appericated!

Kind regards,
Emir

Dear Emir,

The tip-speed ratio (TSR, Lambda) = (rotor speed in rad/s)*(rotor radius in)/(wind speed in m/s). Your data sheet shows the TSR as a function of mean wind speed, so, if you know the wind speed, you can calculate the mean rotor speed without knowledge of the aerodynamic torque.

Your original question assumes that you know the aerodynamic torque, but how do you know this? Aerodynamic torque is not typically measured.

Best regards,

Dear Jason,

Thank you for your time!

You mention this;

By this, you mean calculating the TSR using the thrust cofficient to find the rotor speed? I am a bit lost on how to go from mean wind speed to TSR. I know TSR = Tip speed/wind speed, but our given tip speed of 63m/s is at full load, how can we derive the tip speed at not full load?

We utilized this formula, since Ct was given by NORWIN. Do you think this approach is invalid?

Is it possible for you to give me the meaning/use of this parameter? (7.5deg/s).

Kind regards,
Emir

Dear Emir,

Oops, I misread your data sheet during my quick skim. If thought I saw that TSR was listed in the table as a function of wind speed, along with power and Ct, but now I see that is not the case. I don’t see that you have enough information to calculate the TSR or rotor speed.

I’m not sure I understand your response about aerodynamic torque, which is different than the thrust force.

The nominal pitch speed is the rate at which the pitch angle is controlled by the pitch controller.

regards,

Dear Jason,

Thank you for your time and effort!

We came to a conclusion that there must be some kind filter involved. The filter is either filtering the windspeed or the rotor’s speed to get the two graphs of RPMs as mentioned above.

I assume this omega R was an output from FAST? The researchers specifced that they used FAST.

Do you have any valuble input on what could it be?

Kind regards,
Emir

Dear Emir,

Wind turbine controllers typically have filters; e.g., the baseline controller for the NREL 5-MW wind turbine calculates generator torque and power from the low-pass filtered generator speed.

Best regards,

Dear Jason,

Thank you for your time!
Do you mind sharing the Simulink architecture of how the Fast model is built up? We are struggling to define the right parameters.

Off topic question, why are the generator power and torque are used as an input to many FAST modules instead of the power and torque generated by the wind via the rotor blades? We are trying to investigate torsional vibrations in the drivetrain of a windturbine. Do you have any experience in modelling such problems by any chance?

Kind regards
Emir

Dear Emir,

I’m not sure I understand your question regarding the Simulink architecture. Are you referring to the Simulink interface with FAST v8 or OpenFAST? This is documented in the FAST v8 ReadMe file (which is nearly identical in OpenFAST): drive.google.com/file/d/17LDwMn … sp=sharing.

Regarding the generator torque and power, as similar question was discussed recently in the following forum topic: http://forums.nrel.gov/t/why-is-generator-power-an-input-into-the-sfunc-mex/2474/1. In FAST / OpenFAST, the aerodynamic torque and power are computed within the aerodynamics module, AeroDyn.

FAST / OpenFAST are tools aimed at wind turbine loads analysis, including torsional vibrations of the drivetrain. So, yes, we’ve looked into such matters before.

Best regards,