AC 1250kW 6000V 3 phases, Vibration 29 mm/s during the starting time (1-2 sec.) at full load 4

[Creator_87]

Hello everyone.

I have a AC motor 1250kW 6000V 3 phases. It has a high horizontal vibration of 21-29 mm/s on a non-drive end bearing during the starting time (1-2 sec) at full load.
Then, the vibration is ok , about 1.9 mm/sec.

According to an operation manual the motor can withstand up to 18mm/sec (1-2 sec) during the starting time at full load.

I don't know what it might be.

Could anyboby tell what reasons might be? Or sources (books, web) where similar broblem was described.

Thank you in advance.

 

[electricpete]

My thoughts from most likely to least likely
[ol 1]
[li]Most likely it's a resonance.[/li]
[li]Less likely could be a rub (the electromagnetic forces pulling the rotor toward the stator can be higher during start). Disassembled inspection would confirm / disprove whether there is evidence of rotor/stator rub.[/li]
[li]Very remote could be related to lubrication conditions which might take awhile to stabilize after start (sleeve bearing with oil ring lubrication). This would probably also leave evidence that could be seen during disassembled inspection[/li]
[/ol]

You can try to get more detailed vibration measurements during start to try to get more clues. For example try to get a waterfall plot of spectra vs time and polar plot of magnitude/phase vs time. These will clearly show whether resonance exists. If rub exists there might be some clue about that although it's not as straightforward.

Similar measurement during coastdown may also provide a clue (especially for resonance) because the speed change is typically a lot slower during coastdown than DOL start (particularly slow during uncoupled coastdown). Or if you have VFD they may be able to slow the ramp rate to assist your troubleshooting (with suitable precautions of monitoring/communication to make sure you don't dwell in a resonance too long).


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(2B)+(2B)' ?

 

[FacEngrPE]

If the vibration occurs strongly on acceleration, and not at all once power is removed, you might have rotor bar problems. A FFT amplitude vs frequency plot can make a diagnosis of this and many other causes of excessive vibration.
AC Induction Motor Problems: Broken / Cracked Rotor Bars

Here are other plots of possible diagnosis

http://www.vibrationschool.com/mans/SpecInter/Charts.htm

Understanding The Basic Theory Behind Vibration Analysis

Source = Vibration School

 

[Mr_Curious]

Vibration vs time at full load. There are rolling bearings

 

[Mr_Curious]

Thank you for advices!

I will measure the vibration during uncoupled coastdown.

 

[Mr_Curious]

FacEngrPE Hello.

As I know 1-2 sec is not enough time to perform FFT analysis, isn't it?

 

[edison123]

A loosely fitted sensor that vibrates on its own during starting in-rush current forces on the motor frame? I doubt it is rotor bars problems since the vibration lasts 1-2 seconds at start up.

Muthu

http://www.edison.co.in

 

[FacEngrPE]

If you have a Rotor bar problem, it will show up in the steady state FFT, Spin up vs spin down is only to tell if power vs no power is different. I would look at the data coupled first. The trace above seems to indicate Start and stop are different, so a complete FFT may be in order.

I was not thinking that an FFT for a one second period would be usable, but an expanded trace with a large number of samples during spinup and spindown could reveal something about resonance.

The reason for suggesting rotor bar problems is that at locked rotor the circulating currents in the rotor bars are highest. A rotor having a cracked end weld will carry less current, which would upset the magnetic field somewhat. This will be most noticeable under starting conditions.

Does your data from some time ago look the same? or has the vibration on start increased over time?

Also please observe the maximum starts limit in your motor data sheet. Starting too often will overheat the rotor and is one of the causes of rotor bar trouble.

 

[dArsonval]

What is mounted to the output shaft of the motor?

John

 

[Mr_Curious]

dArsonval
A pump.

 

[Mr_Curious]

to FacEngrPE

It is the new motor. So the data from some time ago doesnt exist.

 

[electricpete]

I can rule out 2 of the 3 that I mentioned before
[ul]
[li]Lack of peak during coastdown seems to rule out resonance.[/li]
[li]Anti-friction bearings rules out the type of lubrication problems I had in mind.[/li]
[/ul]

That leaves only rotor / stator rub from the three I started with.

Since you don't have any prior history, I wonder if it could be a normal result of the starting transient:
1 - motor starting torque and transient electromagnetic torque pulsations
2 - change in fluid system conditions (change in fluid momentum and pressures, check valve goes open etc).

I have seen some of our large vertical motors visibly rock during start. We don't have continuous vibration monitors so I don't know the vibration but it must've been large if I could see it. Is this motor vertical? Do you have any sister pumps in similar service with vibration monitoring during start?


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(2B)+(2B)' ?

 

[Creator_87]

to electricpete
It is the horizontal motor.
I reckon, the reason why in the end it doesn't have peak because it slowed down by the pump quickly.
I will measure vibration without load.

Yes I have another pupms nearby with same motors these motors have starting vibration lower than 18 mm/s.
Also, I will try to play with motor - base frame bolts may be an uneven tightening skewed the body.

 

[FacEngrPE]

I am with electropete on most of his answers.
Is the "SAME" motor really identical or is it the same rating / frame, but possibly internally different? Different manufacturer perhaps? This could create a different pulsation condition at runup.

Is the motor still under warranty? Call customer support and ask them about vibration during runup. The limit during runup might be higher than during normal operation, and just not stated in the manual.

Make sure you do not have a soft foot condition (this would show in a FFT, and can be easily detected with shaft alignment tools). If you loosen a base frame bolt (another way to detect soft foot), set up a dial indicator between the base and the foot first, it will detect if tightening/loosening the bolt allows the motor to move with respect to the base.

Rotor dynamics is a subject that can be just about any depth you care to take it to, I did not think to mention this before, but after considering the soft foot idea, I found this discussion, which talks about possible dynamic interactions between the base and the rotating element Reliability.com "Vibration Analysis Motor Testing A Better Understanding of Rotor Dynamics and Support Stiffness" This can be explored with an FFT analysis, and an expert.

 

[edison123]

Like rotor bar problems, soft foot issues would not go away after the initials kicks. You are starting a big machine DOL and the inrush currents (6 to 8 times) will rock the motor. I test open shaft LV motors DOL and have seen them jump at start if they are not bolted down. The OEM specs say the motor will have such high vibrations during the start. imo, it's not a big deal.


FacEngrPE

Thanks for the link.

Yup, we have had a few such vibration issues on site after perfectly good runs in our shop. In all cases, it turned out to be site issues (poor beds, poor installations, poor alignment etc). And a few of the customers are not beyond lying about how the motor was perfectly running before servicing.

Muthu

http://www.edison.co.in

 

[electricpete]

I reckon, the reason why in the end it doesn't have peak [after stopping] because it slowed down by the pump quickly.

But it starts at least as quickly as it stops, doesn't it? If it was a resonance, I don't see why it would peak during start but not during stop.

You are starting a big machine DOL and the inrush currents (6 to 8 times) will rock the motor. I test open shaft LV motors DOL and have seen them jump at start if they are not bolted down. The OEM specs say the motor will have such high vibrations during the start. imo, it's not a big deal.

In support of that, I'm remembering that the late great skogsgurra posted a slo-mo video of a horizontal motor during DOL start and then during reversing: thread237-246142

I think the video is no longer accessible but I remember the motor dancing around, and you can tell from the comments there was visible movement both in the start and the reversing. His motor was not bolted down. Most people don't start a motor DOL without bolting it down but this was a tiny motor.

Visible with naked eye is a pretty darned high vibration. So even if other configurations (bolted down) don't result in visible movement, it's still not surprising the vibration can get very high. The motor starting torque reaction torque will be applied to the stator frame (regardless of whether the rotor torque is going towards accelerating rotating inertia or delivering torque to a load).

I can't explain the difference in behavior from your other motors. But maybe there is some unknown factors associated with support stiffness, bolting, motor internals as FacEngrPE alluded. In which case maybe inspection or calibrated-hammer bump test would reveal such differences. Also there might be a difference in signal processing among the instrument channels (over how long a period do they average the vibration). Gathering more detailed transient vibration measurements (time waveform, spectral waterfall, polar plot, even time waveform) could shed light in a number of ways.

The only competing theory in my mind (something other than a response to starting torque) is rotor/stator rub and that is pretty rare especially on rolling bearing motors. As mentioned disassembled inspection would show evidence if that happened but of course that's a lot of work. If you're lucky (it's not very common) your motor might have some airgap checking ports built in which would make it easy to check airgap / centering without disassembly (of course requires motor tagged out for safety). A satisfactory airgap check (ruling out starting rub due to off-center rotor) combined with no vibration during coastdown (ruling out a flexible rotor resonance below running speed) would move the rotor/stator rub scenario even closer to implausible imo.

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(2B)+(2B)' ?

 

[electricpete]

I guess the one thing that makes the torque reaction explanation a little strange is the fact that you're only reporting elevated vibration on the ODE.

Out of curiosity,
how high does the vibration get on the DE of this motor during start
how high does the vibration get on similar motors during start
does the motor support look stiffer at the DE than the NDE?

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(2B)+(2B)' ?

 

[waross]

Is the pump impeller supported on both sides or is it cantilevered?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BrianE22]

Do you know what the frequency is? A hammer tap with an accelerometer and scope would probably do it for you. I'm curious if it is related to the large drive current during startup.

 

[Creator_87]

Hello everyone.
Customer support is comming.
I will check all links and advices left above. Thank you.

DE side pic below. Startup max vibration is 2.2 mm/sec

Another motors show different startup vibration but lower than 18 mm/sec.
Pump impeller is supported on both sides. Picture below.