The Simple free-body diagram of the drivetrain equation

Wind & Water Controls
1

Dear Dr. Jonkman,

I realized a difference between the equation (7-5) of the motion of the drivetrain in your 5-MW offshore wind turbine document (nrel.gov/docs/fy09osti/38060.pdf) and other books and journal papers.
The difference is that other books eliminated “Ngear” that is multiplied by generator torque in your document.
Your equation:

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The other equation I saw is as follows:
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Why are they different in only “Ngear”?

Thanks,
Best Regards,
Sina

2

Dear Sina,

In Eq. (7-5), the generator torque is expressed relative to the high-speed shaft side of the drivetrain. In the other equation, the generator torque is expressed relative to the low-speed shaft. (The low-speed shaft torque is larger than the high-speed shaft torque by a factor of the gearbox ratio.)

Best regards,