High vibrations on Alternator NDE

Hello Team,

Once again I am coming here with similar kind of machine and vibration problem.

EMD engine with an Alternator (with single bearing at NDE)

RPM : 900 RPM

Alternator: 600V, 2525KVA, 1400kw, 60 HZ

  • Engine vibrations are within range (around 8-9 mm/s)
  • Alternator NDE is vibrating with High vertical (24 mm/s , purely 1X)
  • & Axial (16 mm/s with 1X = 8.6, max peak 12 mm/s @7182 cpm and minor 2X,3X, 4X) vibrations. 

(due connectivity issue at offshore I counldn't upload the spectrum here. Will upload soon once I reach onshore.)

  • vibration velocity (mm/s, RMS) with phase:

                                    V                                  H                                     A

Alt DE                   5.4@ 170                      5.7@ 86                         16@ 8

Alt NDE                24@ 15                        4.1@ 140                       16@190

 

I have also taken phase readings at NDE axial plane. (attached.)

I am still fighting with an issue.

  • Done with laser-alignment with all readings ≤ 0.04 mm (within tolerance)
  • foundation base frame checked & found ok.
  • Started with balacing trial and will come with conclusion by tomorrow. 

 

Please suggest guys. 

 

 

 

 

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Original Post

As this is your second similar machine with similar symptoms I think it could be some sort of structural characteristics of those bearing supports. I would say the NDE bearing attachment lacks of stiffness. It could lead to resonance. Every small eccentricity or other issue could result in higher vibration levels than expected. 

Thanks for reply becar.

I gone through previous shop-balancing reports of machine.

Alternator's were balanced by 2-plane method at 350RPM, whereas we are operating machine at 900 RPM. 

I also observed today that vibrations were normal at ideal speed up to 450 RPm and then drastically increased  as we increase in speed to a rated 900 RPM. 

once an alternator reaches to 900 RPM, the vibrations were fluctuating in cycle of like 9.5 ,11 , 13.4 , 15 , 17, 19.2 , 21  within a period of 8 to 9 sec but the phase readings which I got remains constant. 

Does it because of dynamic unbalance? 

(1x is dominant at vertical as well as axial )

From the information given so far I am quite sure the issue is some crutial part resonating at the rotation frequency. Most likely the holding cover/cap. It could be some sort of original construction issue. Will wait for other experts to say their opinion. 

 

Alternator's were balanced by 2-plane method at 350RPM, whereas we are operating machine at 900 RPM. 

I also observed today that vibrations were normal at ideal speed up to 450 RPm and then drastically increased  as we increase in speed to a rated 900 RPM. 

once an alternator reaches to 900 RPM, the vibrations were fluctuating in cycle of like 9.5 ,11 , 13.4 , 15 , 17, 19.2 , 21  within a period of 8 to 9 sec but the phase readings which I got remains constant. 

 

Steady phase with changing amplitude,  i do not think that it is dynamic unbalance but i  agree with Becar that it could be due to resonance issue.

Besides, could you take a look with load. Is it changing with?

I was tried 1-plane balancing here.

as we don't have direct access to weld balance weight at alternator NDE rotor (because it comes under vicinity of stator and welding nearby stator was not allowed by customer) so, 2-plane balancing is not possible to carry out on field site. 

With 1-plane balancing trial I experinced, there was reduction in vibration amplitude from 20 mm/s to 12 mm/ s but still its fluctuating  in cycle. (12, 13,14 , 16,17)

When we synchronized an alternator with load the vibrations increases again i.e again to 21 -22 mm/sec. 

The 4 axial values are in-phase. The 2 horizontal values and 2 vertical values  are in-phase with each other after correcting for the sensor orientation (180-degrees). I still believe you need to conduct tests for natural frequency near 1xSS. These are the 4 typical tests:

Variable speed with engine overspeed, as possible

Operating Deflection Shape (ODS) with engine at normal speed

Impact test during operation

Impact test during shutdown

Walt

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