High radial vibrations in uncoupled vertical motor

[edison123]

Hi

We are having severe radial vibrations in a 3 MW, 375 RPM, vertical cage induction motor. The motor has a thrust cum guide bearings on the top and a guide bearing at the bottom. All the bearings are white metal and have adjustable pads.

The motor was test run in its bed but uncoupled to the pump.

I have attached the spectrum for 90 deg radial (mentioned here as MNDE-H and MNDE-V) vibrations for the 1st and 2nd trial runs.

Would appreciate your inputs on what causes such high radial vibrations.

edit: I see that I can only upload only one file per post. So I will upload all the four files separately below.

edit 2: A major point which I should have said in the very beginning, sorry. The vibrations were not due to electrical reasons. We monitored the vibrations at both DE and NDE simultaneously and when the motor power was disconnected, the vibrations did not drop immediately. They slowly reduced during the coast down.

Muthu
www.edison.co.in

 

[edison123]

1st trial - MNDE - Horizontal

Muthu
www.edison.co.in

 

[edison123]

1st trial - MNDE - Vertical

Muthu
www.edison.co.in

 

[edison123]

2nd trial - MNDE - Horizontal

Muthu
www.edison.co.in

 

[edison123]

2nd trial - MNDE - Vertical

Muthu
www.edison.co.in

 

[Tmoose]

What was different between the "trial" runs ?

I'm having difficulty reading what frequency of the vibration(s) of interest is.
Other than it is not 375 rpm / 6.25 Hz.

 

[electricpete]

Maybe it is harmnonics of 1x (6.25) with highest peak at 2x (12.5hz)? Needs a closer look at the frequencies to confirm the pattern. Preliminary swag would be looseness.

Can you tell us more about the bearing configurations?
On top, rolling or sliding beairngs?
If rolling, what type? As you know, some types of rolling element thrust bearings misbehave when they don't have enough downthrust.

Problems with attachment of machine to the test bed maybe (looseness)?

=====================================
(2B)+(2B)' ?

 

[edison123]

Thanks Tmoose and epete. Sorry for the delayed reply since I was away.

Yes, Tmoose, the signatures, as sent to me, were not very clear. Nothing was done between run 1 and run 2 and yet we had different signatures. So upon my recommendation, the thrust collar was changed yesterday and the whole problem went away. Since today morning, the motor is running on load with vibrations less than 1 mm/sec and 25 microns.

epete, as I mentioned, they are white metal thrust and guide bearings.

Muthu

http://www.edison.co.in

 

[ronfrend]

Just to add a little input here, I had a similar problem way back in the 80's in Oman on a 3MW shipping pump motor after overhaul. In this case the highest vibration was axial and the cause was that the thrust pads had been put back on the wrong sides. In other words the motor was now running out of magnetic centre. Took a lot of figuring out but the upshot was that we swapped the thrust pads over and the problem went away.

Ron Frend

http://www.predicon.net