axial vibrations in generator
axial vibrations in generator
(OP)
Hi all,
is it possible that electrical problems in a generator cause high levels of axial vibrations at a frequency of 2x line speed? Any reference to articles or books would be highly appreciated. I am examining a case in which high axial vibratios seem to cause the roller bearing cages to wera out rapidly, introducing failure of the sets without prior warning (other than smoke coming from the seized bearing). (sets are rated 2300 kW, running at 1800 rpm)
is it possible that electrical problems in a generator cause high levels of axial vibrations at a frequency of 2x line speed? Any reference to articles or books would be highly appreciated. I am examining a case in which high axial vibratios seem to cause the roller bearing cages to wera out rapidly, introducing failure of the sets without prior warning (other than smoke coming from the seized bearing). (sets are rated 2300 kW, running at 1800 rpm)





RE: axial vibrations in generator
2x line frequency is the dominant frequency from magnetostriction force in electric motors, generators and transformers. It is typically low in the axial direction, so I would suspect a resonant structure that causes amplification. It could be the generator end bell/plate, bearing pedestal, or foundation depending upon the machine design. A natural frequency test could confirm this. Stray current passing through bearings in another cause of rapid bearing failure from electrical discharge machining (EDM). EDM typically shows in the higher frequency range and is not associated with 2xLF.
Walt
RE: axial vibrations in generator
RE: axial vibrations in generator
I am examining a case in which high axial vibratios seem to cause the roller bearing cages to wear out rapidly
a) Is the axial vibration present at FSNL? If it is, then electrical issues might not be the cause
b) How is the axial position of the gemerator maintained? Most designs of roller bearings cannot accomodate anything other than light axial loads. Have you eliminated the possibility that high static axial loading (e.g thermal gorwth or alignment) is overloading the bearing and causing failure?
RE: axial vibrations in generator
RE: axial vibrations in generator
RE: axial vibrations in generator
RE: axial vibrations in generator
RE: axial vibrations in generator
You didn't indicate that an engine was driving the generator in your original post. Apparently the vibration is minimal at full speed no load, so the engine can be ruled out as the source. I said previously that 120 Hz is an an excitation force generated within the generator. You have confirmed this by saying vibration is load dependent. That is exciting the resonant structure. Detune the resonant structure by changing mass or stiffness or adding a dynmaic mass absorber. I have worked on several generator s with this type of vibration problem.
Walt
RE: axial vibrations in generator
To Strong: Vibration isn't absent at low loads, just less. Detuning has been tried by the end user, as he was very pressed for time and wanted to use the sets. Stiffening the bearings shields gave some improvement.
It can still be engine related, but the probablility with two different types and speeds of engines in use is much much lower.
A solution may not be the most difficult part, if only i know what the source is. I am mechanical, and no little about electrickety. Hence my post..
And yes, we have considered tuned mass dampings, but I'd rather solve it completely.
RE: axial vibrations in generator
RE: axial vibrations in generator
You mentioned:
"we now have a similar damage in a pto generator"
Does this mean you are using a universal joint connection between the engine and the generator? If so, what is the angle of universal joint. The steaper the angle of a universal joint the larger the axial thrust loading.
Just a thought,
Steve
Stephen Seymour, PE
Seymour Engineering & Consulting Group
www.seymourecg.com
RE: axial vibrations in generator
Stephen Seymour, PE
Seymour Engineering & Consulting Group
www.seymourecg.com
RE: axial vibrations in generator
Gonna take BobM's advice and will cross-post this in an electrical forum.
RE: axial vibrations in generator
Johnny Pellin
RE: axial vibrations in generator
When generators are mounted on sleeve bearings that can be seen and measured. We then take this in consideration when ajusting-aligning the couplings. It acts just as a thermal growth would do.
RE: axial vibrations in generator
RE: axial vibrations in generator
2FL indicates a variable air gap. it can come from a soft foot condition, but it can also come from a stator which is not concentric with the bearing journals or bearings in the end cover of the housing. That condition originates at the manufacture of the generator. The air gap can be measured physically, but depending on how it's built, it can be a hard thing to do.
You say that you have a measurable axial movement at start-up. This force is constantly applied during operation and it refers to the relation mag-center to mech-center. It means that the 2FL signature is always there and since you have amplitudes that are strongest in the axial direction, that's where it shows the most.
Fix that and it will go away. It is a magnetic force it comes from the function of the generator.
There could be a way to check this. On the reverse thinking of an electric motor, where you simply cut the power and witness the disapearance of the phenomenon. You could cut the demand and try to see if it goes away with real time analysis from your instrument.
If the problem is electric, it will go away instantly and you will be able to see it. You must program a sample that takes no time to renew itself. One that takes only a second to sample, but continuously. I think it would be the primary cause to fix.
You may still need to fix the resonance problem, but at least it would be less excited in my opinion.
Let me know what you think.
Yvan
RE: axial vibrations in generator
RE: axial vibrations in generator
Did this gen set just have plain roller bearings?
And there were no thrust bearings?
I don't understand.
RE: axial vibrations in generator
Vibration levels on the shields are all lowered to within specs (some weren't, which was one the suspect causes, but not mine) and the customer and the manufacturer figured that was enough. I hope they are right..
A generator does not need an thrust bearing, as axial forces are low.
RE: axial vibrations in generator
If not how is it connected?
RE: axial vibrations in generator