# Drive train natural frequency

Hi

I’ve been simulating a 2MW wind turbine, with steady wind conditions to allow operation at rated power. The drive train oscillates, which I’d expect it to as I don’t have a cunning drive train damper in my controller yet. It oscillates at 0.93Hz.

I thought I ought to be able to do some hand calculations and get the same result.

My .fsm file tells me that Rotor Inertia (kg-m^2) 42171440.000
My .fst file has
600000000 DTTorSpr - Drivetrain torsional spring (N-m/rad)
97 GBRatio - Gearbox ratio (-)
810 GenIner - Generator inertia about HSS (kg m^2)

I thought that natural frequency = (1/(2*PI))SQRT(DTTorSpr/(Rotor Inertia + GenInerGBRatio^2)), but this comes out at 0.55Hz. Am I daft to expect it to be this simple, or have I got my equation wrong?

Thanks

Alec

Dear Alan,

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

free-free fn (in Hz) = SQRT( DTTorSpr/RotIner + DTTor/( GenInerGBRatio^2 ) )/(2pi),

which is an equation slightly different from yours. Using your numbers, I calculate 1.53 Hz

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

fixed-free fn (in Hz) = SQRT( DTTorSpr/RotIner )/(2*pi).

Using your numbers, I calculate 0.60 Hz.

Neither of these calculations correspond to the 0.93 Hz frequency you are reporting. This could be because the rotor flexibility is impacting the drivetrain natural frequency or because the 0.93-Hz frequency is not the drivetrain mode–it could be e.g. the first blade edgewise mode which you are seeing the impact of in the drivetrain.

I hope that helps.

Best regards,

Thanks John

What is DTTor?

Should that be DTTorSpr?

Alec

Hi Alec,

Sorry for the typo. “Yes,” DTor should be changed to DTorSpr.

Best regards,