HT MOTOR HAVING HUMMING NOISE 3

[loadangle]

we have BOILER FEED PUMP MOTOR OF 300kw HT(3.3kV)directly connected to VCB.Recently after overhauling we are encountering an abnormal humming noise from the motor.Temp of DE & NDE side bearing is very normal as both side bearing has been changed & sleeving done at both side shied bearing housing.No trace of neither abnormal temp rise nor vibration.Checked both side bearings on loaded condition with bearing analyzer & found no issue with it.The noise decreases noticeably under decoupled condition.So,the only problem is to find the origin of the humming noise.
please feel free to ask for more information.

 

[krisys]

Core lamination stack getting loose may be one cause which I can imagine. Higher than the rated voltage could be another reason.

 

[melspuds]

If motor was rewound, many things could make the humming sound. The shape of a coil, coil being loose, missing wedges or wrong type of material(magnetic or non magnetic). Also loose brazed winding connections. First check to see amperage is the same on all three phases.
Good luck,
Dave

 

[loadangle]

dear no rewinding work done in that motor only varnishing done.

 

[VTer]

Not sure exactly what you mean by the noise decreases when decoupled? If you have roller bearings, they sometimes tend to make strange noise on larger units when the motors is unloaded but that should go away under load. Not sure if that is your problem but I would also try to use a good quality meter and analyze all the electrical data and look for abnormalities. Good luck and let us know what you find.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[melspuds]

If there was not rewound, only a varnishing and a bearings replaced. The only things I can think of are.
1) With that size of motor, the rotor could have damaged the laminations of either rotor, stator, or both by removing and reinstalling the rotor.
2) Damaging the rotor laminations when removing the old bearing or by pressing on the new bearings.
3) The bearings are not the correct bearings, for the needed end-thrust of the driven end-thrust.
4) The bearings were not pressed on correctly, to match the driven end-thrust.
5) Is the coupling on the motor shaft set at right spot to match driven end-thrust.
6) Is the coupling flexible or rigid type? Make sure shafts are alight correctly.
7) First check to see if the voltages and amperage are equal on all three phases.
Good luck,
Dave

 

[krisys]

I feel melspuds has summarized almost all the possibilities. Thanks melspuds.

Just to add to the above, if the motor is medium voltage, check for any circulating current through the bearing. Normally to avoid the shaft circulating current, in the medium voltage motors, one side bearing will be insulated. This is more critical, if the motor is supplied through Variable Speed Drive (VSD or VFD).

 

[loadangle]

The possible causes mentioned above are been taken into account while checking.WE have checked the bearing with BEARING analyzer which I have mentioned earlier & all the values ware satisfactory.There is no temp. rise on DE or NDE side of the motor.So we can rule out the possibility of wrong or damaged bearing or any kind of friction on both bearing sides.The core lamination damage thing can be a cause or rotor unbalance maybe is a maybe the criminal,I am not sure because we cannot check for these on running condition.The Motor is directly connected to 3.3kV vcb without any VVFD.so is there any chance for circulating current on bearing & how will I measure that on running condition?Meanwhile the according to the bearing expert its the sound of bearing CAGE.he advised to run the motor for a week & then thake all the bearing values again. The roller bearing & ball bearing on DE side has brass cage & the ball bearing on NDE side has steel cage,so
can it cause the noise,as there is little weight difference between steel & brass cage?please let me know any of your valuable ideas.

 

[loadangle]

at last we had a shut down & the motor was overhauled.While taking measurements of both end shield covers we found offset of 0.10mm in front shield & 0.15mm offset at rear end shield covers.Due to this ovality of both end shield the rotor was not in magnetic center,thus causing humming noise.Corrections made & dynamic balancing done of rotor where mass got adjusted.trial taken & guess what?? no humming noise. hope you all are doing well.thanks for your valuable suggestions. & shaft voltage is under 120mv which is very much under the level.

 

[itsmoked]

Thanks for the update!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[krisys]

Well done!!!