HV Motor damage - Root cause analysis 2

[jmmeunier]

Hi,

We discovered damages on a HV Motor having the following parameters:
Rated voltage: 6kV
Power: 5500 kW
Freq: 50 Hz
RPM: 1490
Operation: water pump (power station)
Airgap: 3 mm

Pictures from the stator, the rotor and the bearings are presented in attachement.

- This motor was rewound, the bearings were machined and the rotor pressure plates were replaced.
- The damages were discovered in 2011 during a minor overhaul, after 1000 hours of operation.
- Nothing was noticed during operation.
- The vibration analyses did not indicate any eccentricity, broken rotor bars or abnormal vibration.
- No problem with the temperature of the oil was registered during operation.
- The Ring test indicates hot spots on the damaged zone of the stator core.
- no element is missing inside the machine.
- the melted metal on the rotor is made up of same composition as the metal of the stator and the rotor core.

The measurement results (motor is not permantly monitered) do not indicate any problem during operation, but it is clear that the rotor has rubbed the stator.

Do you have an idea about the possible root cause?
- eccentricity not always present?
- bad lubrification of bearings?
- ...


Thank you for your help.

 

[electricpete]

I think krsh suggested a pretty good scenario. Let’s say the initial cause was excess clearance in the non-coulping-end bearing. Then the rotor operates with more motion on the non-couplng end and rubs the stator toward that end. The non-coupling-end bearing degrades and puts debris into the oil That debris goes into the oil system where it is pumped into the coupling end bearing which gives the streaks that are seen on the coupling-end bearing. (most of the coupling-end bearing damage looks like streaks from debris... there is only one small place in a corner of the coupling end bearing top which looks like a babbit smear).

It is a pretty good scenario. One question that would remain is why it didn’t show on vibration. (That is a question for any of the scenarios).


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(2B)+(2B)' ?

 

[jmmeunier]

Electricpete,

Q1: see attachement
Q2: only a derbis
Q3: yes it is (mistake from my part the stratches are only present on the bottom part of the bearing)
Q4: yes

Indeed a problem with oil could damage the bearing in such a way.

 

[jmmeunier]

Also, the accelerometers are not permanently installed, but are used for routine measurements. The model is A0760GP from CSI.
Regards,

 

[jmmeunier]

ElectricPete,

I agree with the faxct that the scenario of krsh is good.
You also suggest that the initial cause could be excess clearance in the non-coulping-end bearing.
However the clearances of both bearings which were measured in 2009 by the repair shop were within the tolerances (10% of the airgap). Moreover the backs of both bearings are not damaged. In this case, where could the excess clearance in the non-coulping-end bearing come from?

Do you exclude an unbalanced magnetic pull (UMp) as a possible cause? The pressure fingers of the rotor were replaced. Do you think that a (local) bad restack of the rotor laminates have could reduce the airgap?
Again the vibrations provoked by the UMP should have been measured. Could UMP be higher a the start of the motor?

Thank you

 

[electricpete]

You also suggest that the initial cause could be excess clearance in the non-coulping-end bearing. However the clearances of both bearings which were measured in 2009 by the repair shop were within the tolerances (10% of the airgap). Moreover the backs of both bearings are not damaged. In this case, where could the excess clearance in the non-coulping-end bearing come from?

One thing to mention, 10% of the airgap is not really a specification that should be used for bearing clearance. EASA has some limits... as a ballpark, the diametrical clearance in mils should be around the shaft diameter in inches plus two. For example 6” shaft should have around 8 mils diametrical clearance (plus or minus a mil or two).

Also, you mentioned back of bearings... clearance can be measured between bearing and housing at the clamping surface using plastigage. Usually no more than 1 or 2 mils allowed here and prefer tigher.

How can it be relevant... obvioiusly in all but extreme cases these clearances are far less than the air gap dimension. But sleeve bearing clearance has a profound effect on the bearing stiffness which affects rotordynamics. Ordinarily I would expect those effects to show up in steady state vibration, but you never know...

Do you exclude an unbalanced magnetic pull (UMp) as a possible cause?

Nope. Most likely it played some role. UMP gets stronger as the rotor gets further off-center, so it can easily combine with other problems to push you over the edge. Some motors are more susceptible: single circuit wye (vs multiple parallel paths), small airgap (so small movement constitutes larger fraction of the airgap), rotor that acts flexible at operating speed or some speed encountered during startup/shutdown.

The pressure fingers of the rotor were replaced. Do you think that a (local) bad restack of the rotor laminates have could reduce the airgap?

I guess they could move off-center, and also could change stiffness of rotor which again feeds into the rotordynamic picture.

Could UMP be higher a the start of the motor?

** Most definitely. ** The side pull is much higher when starting than running. Most rubs occur during starting.

If you have more detailed information about your rotor, like a drawing or perhaps view from the end with rough description of dimensions, I can do some quick checks that may (or may not) shed some light on whether the rotor can act flexibly at your running speed.


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(2B)+(2B)' ?

 

[jmmeunier]

Hi,
Herewith attached is the description of the scenario which, I suppose, occured on this motor. This based on the consulted measurements reports and on the visual inspection (see pictures on the former post).

Regards,