Hi guys,

I am facing a serious motor problem & would like your opinion. It's a 2000 kW, 6.6 kV, 2-pole, brand new motor with plain bearings. The problem is high vibration, primarily at 3X. A series of 1X harmonics can be seen. The vibration gets amplified in coupled condition, but the harmonic pattern remains the same even in uncoupled condition. Motor Circuit Analysis did not reveal any fault.

A coast-down check indicated the problem is mechanical. Phase measurements indicate a rocking motion at both bearings. A bump test revealed a natural frequency at 150 Hz, which would coincide with the 3X component.

We stripped the motor, checked run-out on shaft & core, re-burnished the shaft seat, checked stator & rotor core lengths, checked rotor balance, replaced the bearings after blue matching with the housing & physically inspected the motor. Some rubbing was observed in the seals, these too were replaced. The rotor core is definitely tight on the shaft. Fans are shrunk fit & seem to be fine.

After completion of this activity, a trial run was conducted. Vibration has now increased, pattern remains as earlier (1X harmonics with 3X dominant)! Also, the axial float has drastically increased. We are planning to put the motor on a vertical borer to check stator core concentricity (with respect to spigots), confirm that stator & rotor positions are aligned and also inspect the bearings again.

I'm going nuts on this one. Any ideas or suggestions would be greatly appreciated.

Thanks,

Aditya

It sounds like you are covering all the angles.

It sounds like you likely have some problem in addition to your resonance (maybe looseness somewhere?) but since that other problem seems elusive it would be interesting to try to reduce the resonance and see if that also somehow reduces the other harmonics. Maybe try temporary bracing or changing the mounting configuration.

Maybe prox probe orbit analysis would help to narrow the problem?

How high was the 3x? The 1x?

What did you mean the axial float has increased dramatically? Are you talking about the endplay? (that shouldn't change). Or are you saying that during uncoupled run you are seeing more and more axial motion?

IT SEEMS MOST OF THE CHECKS DONE AND REMAINING ARE GOING ON.

ONE THING, I COULD NOT SEE IS "SOFT FOOT" CHECK FOR THE MOTOR. IN CASE OF SOFT FOOT 3X WILL BE PRESENT AND GETS AMPLIFIED BECAUSE OF NATURAL FREQUENCY RESONANCE.

ALSO ENSURE ALIGNMENT OF BOTH BEARINGS WITH RESPECT TO EACH OTHER. IF BOTH BEARINGS ARE MISALIGNED ROCKING MOTION IS EXPECTED.

Hi guys,

I took data today with the heat exchanger removed, then again with the bearings loosened & finally with the fan removed. No significant change.

We managed to check the relative stator-rotor position, this is fine. During running, the axial movement of the rotor is high.

If soft foot or misaligned bearings were to be present, shouldn't a twice line frequency peak show up? Its not there. What is your experience?

Regards,

Aditya

Since this is a new motor, I'm curious as to why you didn't have your vendor make it right. That aside, it's obvious that since the readings look similar, but the amplitude increased after you took the motor apart and did your checks, then one of the components you effected must be causing the initial problem. It is possible to have misaligned bearings without misaligning the rotor/stator enough to cause 2X line freq. You said that some rubbing was observed in the seals. You may have a seating problem with your motor endbells(brackets).

Hi Don,

I am sitting with the vendor, trying to sort it out. Had a breakthrough! The stator bore was checked, it was found to be out of centre. Wonder how that happened. Hopefully, this should sort things out.

Regards,

Aditya

From the charts you would think this would give twice line frequency vibration. Possibly 1x vibration of mechanical origina might get reinforced. Hard to imagine series on 1x harmonics. Do you have any explanation in mind? The only thing I can think of is that the machine didn't read those charts ;-)

It will be interesting to hear your results when the stator bore is corrected.

This message has been edited. Last edited by: electricpete,

Sometimes it makes you feel like the weatherman, all this science and look what happens.

I think 3xRPM on motor could be due to:
> Bearing misalignment, if it was occured...seals could be as the first victim.
> If it was coincide with natural frequency, vibration amplification will appear..it make problem more seriously.
Softfoot isn't as predominanly (due to Soffoot will make 2xLF dominant)

Regards,

Rahardi

This is getting out of hand!

We got the stator bore & frame properly aligned. Out of curiousity, we also put the rotor on the balancing machine. Vibrations were high & we had to put on 100 gms. to balance it. So why did this not give any 1X component?

Assembled the motor & tested, vibration has gone up & the spectrum looks the same! Now, NDE axial vibration is higher than DE (3X domiant with all 1X harmonics present).

Where does one look next? Tomorrow's the deadline, we're welding some stiffeners on the end-shields & also changing the bearing housing. Let's see what happens.

Any chance the rotor could be loose on the shaft? This would give 1X multiples (3X being amplified by resonance). However, it should generally not be possible to balance such a rotor. Also, wouldn't phase readings be erratic?

Regards,

Aditya
Regards

Saw another strange thing. The Bode Plot of the 3X harmonic shows that it passes through a no. of resonances (180 degree phase shifts) during coastdown. The significant one is one that happens when the motor speed is down to 2800 RPM. Does anyone know the interpretation of the 3X Bode plot (or is it the same as a 1X Bode)?

Regards,

Aditya

Interesting headache case.
Super synchronous instability problem...
There are several thins to study :
-Relation between :
-Whirl speed
-Shaft speed
-Threshold speed
-Now you found phase instabiliti in run down at 3x frquency.
-You have checked bearing aligment
- Let me think but reply us the following :
-Shaft diameter?
-Aproximate mass of the rotor?
-Distance from bearing to bearing?
-Lenght of the bearing?
-In the rudown did you observe some critical speed less than 3x2800 RPM?
-Have you some little peak near 1/2x?

I will think probably solution and please send us answer to last questions

Regards

This message has been edited. Last edited by: Miguel Kovac,

I am trying to understand the magnitude of 100g. Was it significantly worse than the previous balance? If so it seems you are right it would be strong proof of a problem with the rotor. Another possibility for rotor problem would be bars loose in the slots.

If it were a rotor unbalance or looseness problem creating 1x and harmonics with 3x amplified by resonance, one would expect that 3x would not remain a dominant peak during coastdown. Did 3x remain one of the biggest peaks during coastdown or did 1x dominate?

This message has been edited. Last edited by: electricpete,

It is worth checking the 'roundness' of the journal areas to see if the manufacturing/final grind has introduced 3 high points (or lobes) on the circumference - you can do this with a dial gauge (down to 0.0001 inches) or, if your vibration is being measured from prox probes, looking at the runout content just before the machine comes to rest - does the 3X content remain right down to 100rpm during a shutdown?

If available, what do your orbits and timebase waveforms look like at both 3000rpm and again at 100 rpm during shutdown?

I haven gotten answer to my questions.
Harmonics of running frequency are probably originated by the nonlinear response.
Please check the following,
-Loose part to a dynamic input from the rotor, fix firmly the motor
-Shaft scratch near from a non contact displacement probe
-Shaft bow wich can produce magnetic forces at 3X between rotor and stator
-You mentioned rubs in the bearing cap, check it again
-Deficient lubricant. Try to increase oil viscisity. It can produce non linear response
-Worse case , shaft crackdue to increase stress introduced by poor machining techniques or microdiscontinue quality at a percent depth to diameter ratio. Change of 1X amplitude and phase maybe early warning of a shaft crack. Ive observed it in several little centrifugal pumps shaft.

Another question ,
Does the wave form predominant is 1X with little discontinues of 3 times or 2 times?

Found something else!

The shopfloor guys had said that the fans couldn't be loose as they were welded. We've now forced them to take the fans off. These should have an interference fit of 90 microns. Well, one had an interference of 20 microns while the other had a clearance of 40 microns. We should get the new fans tomorrow & will check again.

We've also welded some stiffeners on the end-shields to move the natural frequency away from 3X.

Lets see what happens tomorrow. Will keep everyone posted. I still wonder though, why does putting the coupling on (which is nicely balanced) increase the vibration? Any ideas?

Regards,

Aditya

P. S. Miguel, I'll get those values tomorrow & let you know.

Putting the coupling hub on can change the resonant speeds and possibly push one slightly closer to operating speed.

This motor is getting weirder still, if possible. Vibration is still high.

We pulled the rotor core, it was nice & tight on the shaft. However, the rotor bars were loose. They are normally hammered into the slots; in this case we could slide them in & out.

We're getting new bars, but why should they have caused 3X vibration? The amplitudes at rotor bar pass frequency & its harmonics are very low.

A rotor problem in a motor often will not show up at bar pass frequency. Sometimes it will have multiples of running speed with pole pass sidebands on most of the multiples of running speed. The 3 times being dominate is most likely related to the natural frequency problem.

If the rotor problem is severe it will have a hard time reaching nameplate speed, even when lightly loaded.

What does the vibration spectrum look like during coasting down?

A dominant 3x component usually indicates wear/clearance problems that exaggerate minor unbalance and/or misalignment problems.

Good hunting!

CZ

This message has been edited. Last edited by: Christos,