Dear Forum members,

Attached is the FFT profiles of Alternator end and engine end profiles Horizontal, vertical and Axial profiles.

Q1. on engine side at 0.41x /300rpm 50.51mm/s RMS is noted.

Q2. The overall vibration also showing high on vertical on engine end and marine classification societies recommended maximum satisfactory value 18 mm/s RMS.

Appreciate your comments and countermeasures suggestions.


Chary Tatta


Original Post

Dear Team members,


I am sending the clarifications for my above query as following-

1. This is 5 Unit HIMSEN- Ssangyong Korean make, 900KW, 720 rpm diesel generator.

2. Attached the FFT profiles in pdf format as the word document may not be opening for some members.

3. Please read Point 1 is Vertical, point 2 is Horizontal, Point 3 is Axial and point 4 is Foundation pad(resilient type) vertical direction.

4. In engine side, non driving end, Vertical direction reading, at 0.41X non synchronous, 50.57mm/s RMS amplitude is dominant and question is, indicating as Oil whirl issue or any other disturbing force. At 720 rpm can it generate oil whirl in the shaft and bearing system. All bearings are Journal/ Half shell type.

5. Further any clarifications required feel free to put in this forum and glad to give the required details.

kind regards




Usually orbits and SACL plots are very useful to diagnose oil whirl/whip issues. But maybe in your case these data cannot be generated, can they?


Is the amplitude at 300 cpm stable?

Can you take high resolution data on the low frequency side to confirm the exact frequency?


Usually whirl is seen at higher speeds than yours (720 rpm). So is whip and possibly at even much  higher speeds. Are there any other components in the machine that could generate this frequency? Any rolling element bearings for instance? Do you know the critical speeds for the machine?

Sorry for asking so many questions.

Regards-Ali M. Al-Shurafa

Thanks Mr.Shurafa

Pleased to note your remarks.

1. Yes there should not be so high sub synchronous components other than journal bearings at sub harmonic and at low rpm 720 oil whirl/ oil whip not anticipated.

2. The other 2 generators not shown any such sub synchronous frequency amplitude.

3. I used single channel accelerometer so orbits and SACL plots not possible and at later stage next inspection I will take the plots and verify.

4. There are no roller bearings in the generator driving gears all are bush and journal bearings.

5. As Generator normally runs at 720 rpm normally and no critical speeds identified.

Thanks for all your questions I have put this case in forum as other generators not shown any such disorders.

If you wish I will post the time wave forms in velocity and acceleration modes also any other information you need are welcome.

kind regards

Chary Tatta

If the frequency of the system is too high, the speed of the generator will increase, which will increase the centrifugal force of the generator rotor. When it is serious, it can cause damage. There are few cases where the system frequency is too high during normal operation. It is easy to run when the frequency of the system is reduced and will be maintained for a period of time. If the frequency is too low, the generator's output will be limited. When the rotating speed of the generator is reduced, the ventilation at the end of the generator will be reduced and the cooling conditions will become bad, which will increase the temperature of the winding and iron cores and reduce the generating force of the genset.

Low Frequency of Diesel Generator Set

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