Vertical 700HP,3.3KV rotor problem?

[suntop]

We have experienced a noise problem with the motor that drives a vertical turbine can pump.After disassembling the motor it appeared that the rotor was touching/scratching the stator.Some opinions say that its because of the improper sized thrust bearing,but no clear cause is finalized.
My worry is from a similar breakdown of other identical motors that operates the same water facility.
Please advise what shall I do?

Thank you

 

[smms]

Ask the motor manufacturer for the max. thrust which is allowed for these motors. You will get an answer. Compare it with the real thrusts.

 

[GusD]

Hello Suntop

You don't seem to be sure if rotor did in fact contacted stator or not.The only way rotor can contact stator would be due to bearing assembly problems "thrust"or a long rotor that was subject to excessive "Bowing" caused from
umbalanced ot overhung conditions.These conditions are not as rare as one would think.Upthrust on vertival pumps may very well cause rotor to stator contact.
I think it is important to ascertain if in fact rotor made contact or is there another cause for the apparent rub.If rotors come in contact with stators, the friction heat developed is rapid and destruction of rotor/stator a real possibility.Bearing housing assembly and inner/outer bearing caps should be inspected for proper installation .

GusD

 

[RichJ]

Another way the rotor could contact the stator, assuming this is an induction motor, is for the rotor bars to move within the rotor. Large motors, especially verticals, are designed for a fixed number of starts -- generally 10,000. If these motors cycle frequently you can get up to that number quickly. When you exceed the number of starts the most probable failure point is the rotor bar attachment to the end rings.

 

[suntop]

To GusD
Thanks much for the response
Please note that yes I am sure that the rotor was rubbing inot the stator,this was very clear after the disassebly of the motor in the workshop.Can you please advise from your experience that the thrust bearing are appropriate or not.The motor specs. are700HP, 3 phase, 1480 RPM, 50 Hz, Frame 5810PH, 3300Volt, 107A, Type HV4, Encl DP, Design Code G, 40C amb, SF 1.15, Insulation Class F, Upper Bearing 7228-BCB, Lower Bearing 6222-J, Oil capacity; upper bearing 24 quarts, lower bearing 3 quarts, ID B10 97067932-001R-1 M?RR 175% EHT, Over Temperature Protection 2; 94.1 NEMA Nom Eff., High thrust, Special Features; 80C at 1.0 SF by RES
Thanks again for your effort.

 

[suntop]

Hellow smms
Thanks much for your idea,and yes the manufacturer confirmed that its o.k .But the problem is it might be as you mentioned incomatability betwwen whats there and what was supposed to be there,the manuf.will not accept any manuf. defect claim.Please advise how we can make sure of the compatability?
Thank you.

 

[suntop]

Hello RichJ
Thank you for your comment.Its diffecult to know how many starts did this motor experienced.This motor has been in operation since June 2000.Is there any preventive maintenace recommendation after a certain hours of operation especially for the mechanical parts(replacements,lubrication,other routine tests and inspection).
Thanks again

 

[rotman]

Hello Suntop.
What was the bearings condition stated after disassembling the motor (ie. if any traces of bearing failure was noticed)?.
If bearings themselves are OK, you should check possible axial clearance (play) of the rotor in relation to stator. In some operating states of the pumps, axial forces acting upwards occur; they can even be of higher value than rotor weight, causing 'loosening' of thrust bearing (single angular contact ball bearing - which require certain axial load for proper operation).
If traces of rubbing rotor vs stator are in upper part of the core, it may confirm such process.

 

[GusD]

Hello suntop.

Thanks for your comments.I can only rely on our history in dealing with similar motor bearing arrangements.
It appears you have an Angular Contact ball bearing at the top (thrust bearing).I would expect that a 700 hp motor would probably have 2 bearings on a B/B arrangement.I guess the motor manufcturer didn't see the need for it.Your lower bearing is a deep groove ball and takes most of the radial load on the drive end as well as some axial thrust loading capability.If the cause of rubbing was from improper seatting of the top bearing,I would think that your rotor should have contacted the stator at the upper region of the stator core.The lower bearing should
be stable in its housing.The movement on the lower bearing would be limited to its end float (thermal growth).
If the area of contact is not on the upper area of the stator I would have to look at other reason for the rubbing.
The following example may be a long shot, but a few years ago we had 6 -800 HP vertical motors from "unknown" manufacturer.We experienced 2 motor failures in what initially appeared to be a bearing problem causing the rotor to contact stator.We did not find a bearing problem,what we did find was rotor laminations that in certain area of the rotor core were developing excessive thermal growth causing it to rub stator.The 2 motors that experienced similar failure had Serial#s that indicated they came out of the factory at the same time.
Suntop,this is not the most common failure,however;if you don't find a problem with Top bearing assembly,or its housing,you might have to look for other cause for your rubbing.

GusD

 

[jbartos]

Suggestion: If the manufacturer does not accept any defect claim, all what is left is go back to the drawing board. Ask the manufacturer for the motor parameters including the bearing thrust. Then, model motor-load, and test/measure the field condition including the thrust. Contact the bearing manufacturer for the bearing solution.

 

[electricpete]

Good comments.

It makes sense as Gus said that the axial location of the rub should give you some clues. Near the top indicates problem with the thrust bearing. Near center might indicate the bow he mentioned or perhaps pullover from magnetic force if airgap was not proper.

Were the rub marks 360 degrees around stator? If not it might just be contacting during starting. Was it 360 degrees around rotor? If not might suggest a bow.

If the angle thrust bearing should lose downward thrust for a period of time then it also loses its ability to provide axial positioning. Does the pump have the capability to generate momentary upthrust?