# Drivetrain 2.2Hz frequency and natural frequency 0.625Hz def

UPDATED: the formulas were wrong.

Hi to everyone again,

I have a really short question that is killing my collegues and me during the last days. When calculating the natural frecuency of the equivalent Drivetrain of the 5MW NREL Baseline Wind Turbine Onshore configuration:

Equiv_Iner = GenIner*(97^2)RotorIner/(GenIner(97^2)+RotorIner)

Undamped Freq = sqrt(DTTorSpr/Equiv_Iner)1/(2Pi) = 2.23 Hz

DTDampingRatio = 0.66

DampedDTfrequency = Undamped Freq * (1-sqrt(DTDampingRatio^2)) = 1.68 Hz

Why none of these is 0.625 HZ, the same as in the 5MW definition? We think that is because a changing on the coordinate system? If I’ve made some mistakes please correct them. Thanks!

Dear Oriol,

The equation for the natural frequency of the torsional mode of the drivetrain in a free-free condition (generator free to rotate) with a rigid rotor is:

free-free fn (in Hz) = SQRT( DTTorSpr/RotIner + DTTorSpr/( GenInerGBRatio^2 ) )/(2pi),
which is an equation slightly different from yours. I’m also not sure where you drivetrain damping ratio is coming from.

If the generator DOF is disabled (so that the generator is parked or spins at a constant speed) than the drivetrain essentially behaves in a fixed-free condition, with the natural frequency:

fixed-free fn (in Hz) = SQRT( DTTorSpr/RotIner )/(2*pi).
For the NREL 5-MW turbine, the free-free drivetrain-torsion mode is around 1.7 Hz and the fixed-free mode is around 0.6 Hz.

When the rotor is made flexible, the blade flexibility will impact these equations/frequencies.

I hope that helps.

Best regards,

Dear Jason,

thanks for your answer, but sadly I’m not able to obtain a 1.7 Hz frequency for the drivetrain mode free-free. I’m doing the calculations myself, not simulating. I leave here my excel data so you could look it up. I hope is self explaining.

Structdynamics_drivetrain.xlsx (13.9 KB)

Few numbers:

Jrotor = 38.832E6 kg·m2