Cycling Vibration on 1800 RPM Vertical Motor 1

[RM12]

I am running a vertical motor 4160 V and 1800 RPM unloaded. I checked the mechanical integrity of all parts, bearings housings, stator and rotor. When I measure the vibrations, I get cycling vibration at 2 x the rotation between 0.07 to 0.22 inc/s. Can anyone help understand why this maybe?

 

[BrianE22]

By "cycling" do you mean it seems to come and go at a certain frequency? If so then I'd guess you have a beat frequency.

 

[RM12]

By Cycling I mean that the values increase to 0.22 gradually, 0.07,0.075,0.089,0.01.....0.22 and then the reverse happens. It isn't a beat frequency with the way it eventually increases or decreases.
Also, these values are being measured radially.

 

[GregLocock]

so let's see a graph of vibration vs time

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Tmoose]

Note the time or start a stop watch when the vibration is .22 ips. Note the time the vibration next reaches .22 ips again.
Does it repeat ?

 

[Strong]

I would look for a structural natural frequency close to 60-Hz (2xSS) by conducting an impact-resonase test at various suspect locations. Is motor being operated in a shop or at the plant with pump and support pedestal?

Walt

 

[electricpete]

As Greg said, more data would help. trend over time. Spectra, TWF, photo, all of that can help in possibly unexpected ways.

I have seen Tmoose's idea used productively when evaluating beating on an unloaded motor. Typically the beat frequency is so low (due to low slip) that you'll never find it with high res spectrum, you just need to time it. BUT typically the beat is at 4x running speed for a 4-pole or 2x for a 2-pole. You wouldn't get anything electrical at 60hz close to your your 4x unless there were electronics in the power supply (and even then there would have to be additional phase asymmetries).

If no electronics, my best guess: what is the thrust bearing configuration in this motor? (part number if possible). I have seen unloaded (lower than expected downthrust) vertical motors do a lot of strange things including oscillating vibration levels and oddball frequencies. Have seen that with spherical roller thrust bearings and with angle contact bearings (these bearings depend on downthrust to develop their radial stiffness). I'm led to believe it is more likely when the thrust bearing is far underloaded. In one of our vertical motors, it is necessary to reduce endplay in order to near zero before running the motor uncoupled. Typically the abnormal vibration goes away when you add downthrust by coupling a pump or whatever to them.

A little further out into the wild guess territory, assuming you're running this motor uncoupled, is there a coupling hub mounted? If so is it interference fit, or slip fit with split rings. The slip fit coupling might possibly wander although I'd expect 1x more than 2x.

 

[GregLocock]

Measuring beating (heterodyning) is a bunch of laughs. Typically I ended up using envelope analysis of the time signal. It's a very good idea to synthesise two sine waves in excel or octave or whatever and study how the envelope and waveform changes as you mess around with the frequency and amplitude of the second signal.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[RM12]

Sorry for not responding earlier to the thread.
This thread turned out more informative than I expected.

Coming to the conclusion to the original question (OP), I had to do a bump test and low-behold the natural frequency was close to the running speed between 1659 - 1750 and it created a varying vibration frequency. I left it on a rubber base and ran like a Cadillac.

We then arranged the pump head associated with that motor and had the same results as having it on a rubber.

Had I read @Strong 's post earlier could have solved it sooner, but I was hit with the same idea right after.

 

[electricpete]

Yes, it is a standard NEMA/EASA requirement for shop testing that the mounted natural frequency should be checked to ensure sufficient separation from running speed enough to meet the requirements of either a resilient mount or a rigid mount.

But I have to admit I've never heard of resonance causing oscillation of the magnitude of the vibration (although I can see a theoretical mechanism if machine speed varies over time or stiffness varies over time)

And I don't understand what you're suggesting

the natural frequency was close to the running speed between 1659 - 1750

1800 RPM unloaded. I checked the mechanical integrity of all parts, bearings housings, stator and rotor. When I measure the vibrations, I get cycling vibration at 2 x the rotation between 0.07 to 0.22 inc/s

It doesn't make sense to me that bump test natural frequency near running speed would explain cycling vibration at twice running speed. I suspect there must be more to the story somewhere.

 

[RM12]

I have to admit to it also that I am yet to understand completely why it would make the vibration vary between a range.

For now, my theory is that the motor since it's not a synchronous motor, the motor rpm oscillates as well which oscillates the vibration with its natural frequency.
It's a farfetched idea because the interval between varying vibration was quite high. For averaging of 5 seconds, the values seemed to have changed after at least 30 seconds plus the varying of motor speed unloaded is said in other words having no slip or 1799-1800 in this case.

Also, although I placed the motor on the rubber base or the pump head, I still noticed some cycling (0.015-0.03) but not as bad without them. Based on this it would shatter the premise of natural frequency affecting the cycling or oscillating vibration.

I will try to collect the existing data, waveforms etc. for us to visualize and analyze here.

 

[electricpete]

since it's not a synchronous motor, the motor rpm oscillates as well
A large induction motor at no load is effectively at sync speed. In the US grid, that would mean a rock stable speed (there may be notable grid frequency variations in other places).

Also, although I placed the motor on the rubber base or the pump head, I still noticed some cycling (0.015-0.03) but not as bad without them.

That raises a question for me whether initial test (where you saw cycling from 0.07 to 0.22ips) was done in a shop or in the field.
[ul]
[li]In the field, for vertical motor mounted on top of pump I would oridinarily expect resonance to manifest itself with high directionality (big difference in resonant vibration magnitudes in the two radial directions) due to the asymmetries introduced by the cutout within the motor stool in vicinity of the packing/coupling, and by the attached piping.[/li]
[li]In the field, I wouldn't be as surprised to see a (postulated) varying stiffness over time as I would in the shop (there may be theoretical mechanisms for a pump casing to be affected by changes to the system and the attached piping, in contrast to a shop mounting configuration)[/li]
[/ul]

Based on this it would shatter the premise of natural frequency affecting the cycling or oscillating vibration.

I wouldn't pay as much attention to cycling at lower magnitudes like 0.015 ips to 0.03 ips. Those low magnitude oscillations are not as notable / significant imo, at least without a lot closer look. (btw do you know if that was still predominantly 2x.. I'm guessing it wasn't).

And sometimes things end up happening we just can't fully explain. It's a learning opportunity for everyone.

 

[RM12]

This time it wasn't cycling 2 times when on rubber or pump head.
It was 1 time and 2 times cycling without the pump head and it did not matter where I measured it radially around the motor.

The initial and final test were both done at the shop.
Initially we had the motor run in the shop without the pump head and directly mounted on its flange. This is where we had the varying vibration consistent and repetitive.

After the bump test was conducted, we decided to place it on rubber, and it worked. Finally, we asked the pump head to be sent to us and it worked just the same as on the rubber with low vibrations.
I do understand the structure of the pump head stool can make it highly directional, and we tested the natural frequency to find different frequencies in the direction where there is no cutout.

I am waiting until the motor is installed back in the field and loaded to see difference in the vibration. The bearing configuration is two thrusts down and the endplay set at 1 thousandth.