axial vibrations in generator 2

[rob768]

Hello guys, i posted this thread in the vibration section as well, but was suggested to post it here too.
I am looking for a source of axial vibrations occurring in the bearing support of a diesel driven marine generator. The vibrations occurs at 4x the running speed. We have not identified a mechanical source, but we are suspecting an electrical one, since it occurs at twice line speed. We have encountered this problem in 2 different lay-outs of installations. One is direct driven by a 16 cyl engine (with soft elastic coupling in between), and the other through a PTO of a marine gearbox and by a 9 cylinder engine.
My main question is: can a generator cause axial excitation at 4x operating speed?

 

[edison123]

Is it a 4 pole generator ?

Muthu

http://www.edison.co.in

 

[rob768]

Yes, operating at 1800 rpm (60 Hz)

 

[edison123]

I would suspect uneven air-gap around the periphery / Turn shorts rotor pole coils.

Muthu

http://www.edison.co.in

 

[rob768]

is there an easy method to simply detect if this is the case? The thing is, there are several damages in more or less brand new installations. Can thsi be a manufacturing fault? Sorry for my dumb questions, but i know little on electrickety, so help and reference to perhaps articles is appreciated.

 

[edison123]

If you see a big difference in the vibration levels from unexcited (i.e. no field current) to the excited conditions, then that would confirm the above rotor problems.

Can you post some pictures of the generator rotor and the overall installation here ?

Muthu

http://www.edison.co.in

 

[rob768]

There is a certain dependancy on load.
Sorry but i can't post pictures.
The problem seems to be that the procuced axial vibrations cause resonance of the bearing shields, leading to severe and rapid bearing cage wear, which in turn causes the balls to slip and the bearings to overheat. We were looking for mechanical causes at first, since we were dealing with a installation directly driven by a 16 cylinder engine (which prodcues 4th order vibrations). We now also have installations where this 4th engine order is not present, and mechanical sources for the vibration are not clearly present. Like i said, electricity isn't my field of expertise.
Is the above uneven air gap something that can be checked in the field? I assume this is something like a mis-alignement between center line of the stator and the shaft?

 

[edison123]

I had assumed it was white metal sleeve bearings. The probability of uneven air-gaps in the generator mounted anti-friction bearings is remote. What is the bearing arrangement of the generator ? Roller & Ball or Roller & Roller + Ball ?

Muthu

http://www.edison.co.in

 

[rob768]

Ball bearings on both ends

 

[edison123]

What are the bearing nos. ? What is the generator capacity ?

In a two ball bearing arrangement, the non-locating bearing (usually the ODE) should be free to move axially inside the housing to accommodate thermal growth. The grease retainer plate/cup should not touch this bearing outer race at both sides.

Look at the fig 9 in this SKF page

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_0_79

Check if this arrangement is present.

Muthu

http://www.edison.co.in

 

[rob768]

Don't know the abbr. ODE, but the arrangement is fixed on driven side, and floating on non-driven end. Bearings are type 6232M/c3. Checking of axial play of the bearings is being performed at the moment (or should be anyway)

 

[dcset]

hi, i try to show you my little experience, normally,when you got a problem on short circuits on rotor windings, normally, you got radial vibrations, cause it seems like a unbalance.... for this i think your problem is not windings, i think maybe misalignment...

 

[dpc]

ODE = Opposite Drive End

 

[edison123]

The easiest way to rule out electrical problems (uneven air-gap, eccentricity, misaligned axial magnetic centerer, rotor pole winding shorts etc.) would be to note the vibrations without excitation and with excitation, as I said earlier. Until then, it's all just speculation.

Muthu

http://www.edison.co.in

 

[catserveng]

As noted above, the best first check is to operate the unit at rated speed with no excitation and observe vibration.

Rotor shorts, at least in my experience with the size machines I'm familiar, show up as linear vibration similar to out of balance, and to some degree get worse as load goes up.

Axial vibration problems with gen sets I've usually seen are the result of improper initial setup in alginment and coupling, where the generator magnetic center was not correctly aligned. Or, more likely in your case since you state you have one fixed and one floating bearing, that you're not centered on your crank thrust.

When installing the generator do you measure crankshaft end float before and then after install to assure you aren't pulling or pushing on the crank thrust surface? If the adjustment is off slightly, the axial vibration is usually the result, if it's way off your usually burn the thurst bearing. This is especially the case if the generator coupling is the flex plate style, but I have also seen this in rubber element torsional couplings as well. A properly installed generator should not change the crankshaft end play clearence, would also preform this check both hot and cold to assure thermal growth isn't playing a part.

Hope that helps.

 

[dcset]

I agree with catserveng.... cause my experience shows it...

 

[ScottyUK]

dpc,

Over here we tend to see NDE as an abbreviation for Non-Drive End


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

Scotty,

And I suppose you don't like "counterclockwise" either?

Actually I think NDE is probably more common, even here in the colonies.

 

[rob768]

We have problems in a set running on the PTO output of a gear box. No influence of crankshaft axial vibrations there. Unfortunately, we only have very little information from that set, because at the time axial vibrations weren't suspect. With the set we dit measure extensively, vibratons form the engine in the unloaded conditions decrease too, so difficult to tell what's what.