PMSG for a 20 MW Wind turbine

Dear @Garrett.Barter ,

I am sending this message because I am doing a thesis related to an upscaling from 15 MW to 20 MW. Looking at the 15 MW IEA Wind turbine, the direct-drive generator is PMSG type (Permanent Magnet Synchronous Generator). I am applying a constraint in the 20 MW Wind turbine, which is that the rotor speed has to be the same as the 15 MW rotor speed (I suppose this constraint is reasonable, but I would accept you correct me!).

However, for the 20 MW wind turbine generator design, there are two factors that i am still missing, which are the frequency and pole pairs. In the 15 MW IEA Wind turbine, there are 100 pole pairs as well as 12,6 Hz of frequency. What frequency and pole pairs values would be reasonable to assume for the 20 MW Wind turbine? Thank you very much for your devotion reading this message

Best regards


Hi Alberto,

We wrote a paper that has a number of conceptual generator designs that you may find helpful. We consider these designs higher “quality” than the process we used for the 15-MW reference turbine as they went through more industry scrutiny.


Dear @Garrett.Barter , thank you very much, it was really of help.

Apart from this, i have one concern regarding the tower mass.

In the following figure 8.13 (from Turaj Ashuri PhD thesis), for a 240 meters of rotor diameter, the tower mass is more or less 2750 ton. How can it be possible that there is so much difference between this mass and the tower mass for 15 MW IEA wind turbine(860 tons)?

Additionally, I am running simulations with OpenFAST and i have calculated the following parameters:


Applying the formulas in Turaj Ashuri PhD thesis, the results are as follows:


Why the comparison between the results applying your formula and the results from the simulations differ hugely??


On the tower mass scaling, I found that work in this paper. However, after a bit of searching, I cannot find the tower diameter and the natural frequency assumption (stiff-stiff or soft-stiff) used in the design trends. If you were to use a ~5m diameter tower and/or require a stiff-stiff tower, I can see the mass growing at such an exponential rate. Please let me know if you are more familiar with that prior work and can shed light on these assumptions.

On the parameters that you calculated in OpenFAST and compared to the thesis results, the images of the quantities did not come through, so you’ll have to try again or write in words.

Hi @Garrett.Barter, I have done a bit of searching regarding stiff-stiff/soft-stiff… and i believe the 15 MW IEA is stiff-stiff since the first tower natural frequency is above 3P in the Campbell diagram attached below (Am i right with this assumption?). If there is something you see wrong in the Campbell diagram, I am open for corrections)

Plus, the tower diameter for the 15 MW IEA WT is 10 meters wide. I suppose that this leads to a more exponential rate compared to a 5 meter diameter tower…

Overall, i am familiar with the assumptions you have mentioned, so yeah, you can shed light on those assumptions.

Concerning the parameters calculated in OpenFAST (the maximum values for the 15 MW iea wind turbine, after running simulations from the cut-in to cut-out wind speed):

And these are the parameters obtained in the thesis:

There is a noticeable difference between the parameters using OpenFAST and Turaj Ashuri PhD Thesis.

For the blade flapwise moment and tower fore-aft bending moment, the quantities do not preoccupy me since these are in the order of magnitude. However, regarding blade edgewise moment, tower side to side bending moment and tower side to side displacement, these are completely different.

Was it now better to see the images?

Thank you. Albert

Yes, I can see the images. I do not have enough information on what that analysis looked like to comment. However, I think we’ve established that the work in that thesis used different assumptions and is not a good basis for comparison.

As for the IEA 15-MW tower, it is soft-stiff for the fixed-bottom design and stiff-stiff for the floating design. The lines on your Campbell diagram are a bit hard to see (assuming you generated the plot), but I’m not sure it is fully accurate for the 15-MW monopile. Will have to check on that.

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Hi @Garrett.Barter, thank you very much for the reply.

Hi @Garrett.Barter,

I have done two campbell diagrams , one for the 15 MW tower (without taking the RNA into account) and one more with the 15 MW tower (taking the RNA into account). Below I attach the images for each one:

It turns out that if we take the RNA into account for the tower modal analysis, the natural frequencies corresponding to the 1st side-to-side and 1st fore-aft mode shapes intersect with the 3P straight line. I suppose you said that the tower (for the fixed platform design) is soft-stiff since there is resonance risk between 1P and 3P, right?

Plus, if we did not take the RNA into account, the tower would not be soft-stiff since all the tower natural frequencies do not intersect with both the 1P and 3P straight lines.

Correct me if i am wrong with anything i have said, please

BEst regards. ALberto.

Hi Alberto,

Yes, the full system should be accounted for when doing the modal analysis. The tower was designed with the mass properties of the RNA in mind. You are correct that the first natural frequency of the tower (in the monopile configuration) lies between the 3P range and the 1P range, making in a “soft-stiff” design. Wind turbine controllers account for this with frequency hopping so as not to excite that mode.


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