High Generator bearing temperature with increase in terminal voltage

I would check the shaft ground, and also the shaft for eccentricity...and check bearing for vibration levels....

https://www.powermag.com/controlling-shaft-voltages/

https://www.stiweb.com/kb_results.asp?ID=140#:~:text=Eccentricity%20is%20the%20measurement%20of,Fixed%20mechanical%20bow

http://www.sumatron.com/dl/Utilization%20of%20Repetitive%20Surge%20Oscillograph%20(RSO).pdf

How do shorted-turns make themselves noticeable?

 Mostly by increased vibrations

 Vibrations sensitive to field current (VAR) changes

 Sometimes by reduced excitation capability

 Often they DON’T cause any obvious performance changes and are only found by monitoring (e.g.: flux probe and shaft voltages) and/or by testing (RSO, Open Circuit voltage, impedance, poledrop, etceteras)

The Sumatron article is using RSO to test a rotor, they have exchanged the rotor so probably not the best diagnostic for this situation, certainly enlightening though.

Dear seniors

Thank you for support

I ld like to highlight that rated machine rpm is 1800 rom and we are doing open ckt test after winding repair...

Now when voltage is 10kv then nde bearing temp remains 60 degree c while at voltage 11kv the temp rise upto 425 degree c and even a rtd on bearing is burnt out

Wat relates bearing high temp with increasing terminal voltage

It sounds like your rotor has too many shorted turns...

We installed new spare rotor

Rso is also ok so inter turn are not there

I work in instrumentation and the highest voltage I've ever handled is 440V so anything I say here might not be correct.

I suspect that at high voltages, you are getting insulation breakdown and current is flowing through the bearings. You might be getting sparks between the balls and the races, which can cause the high temperatures.

If that is the case, I would say, your bearings are already damaged and you need to replace them.

regards,

Vulcan

Maybe check that your excitation current is not returning through the shaft, rather than the sliprings. A ground fault could find a nice easy return path through the huge shaft & bearings rather than the brush rigging. You should be able to verify this with a clamp on dc ammeter, like a Fluke 469.

Sir we have brush less excitor...the generator nde bearing is insulated....

The shaft connected to coupling generator is inly ground for complete train

Then the only other explanation I can think of is that the repairs changed the location of the magnetic center of the rotor, so you need to move your thrust bearing. That is the only way I can think that rated air gap flux could cause a shift in rotor position.

It seems like at this point the force on the thrust bearing is increasing suddenly...do you have any way to measure this thrust force?

I m thinking from another view

Due to winding repair we bypassed coio from all 3 phases and it has caused asymmetry in magnetic field distribution that cause spike in voltage and ultimately breaking down nde bearing insulation

I think highly unlikely, especially since only reduced excitation has been applied, but easy enough to check.

You can roll the generator set at low speed allowing the shaft to float on the bearing film, and monitor the shaft axial position with a displacement laser as you apply excitation. Since it is a brushless exciter, you may have trouble getting enough field current at low speed, but it might be enough.

We have started machine by degausing turbine side shaft and replacing pmg

Now no bearing temp abnormal observed

But since to handle emergency we bypassed stator coils so we ve to increase excitation to reached rated terminal voltage...now machine tripped on stator neutral over voltage as excitation voltage increased beyond 20 percent

Please confirm if the hot NDE bearing is a thrust bearing? If not, then axial shaft position should not cause heat, I jumped to conclusion. This is the bearing on the exciter end of the generator shaft, also insulated?

A ground fault somewhere within 20% of the bottom of the stator windings could easily give you that neutral voltage. Machines that small I usually see medium resistance grounded, but you have high resistance or reactance grounded? You definitely have a ground fault, since the machine is not even connected to anything, I assume?

Before, your excitation was over 20% before observing high bearing temperature, so not sure how you think degaussing the shaft fixed anything, plus changing exciter should not have created a ground fault.

It is also difficult to believe that the pilot exciter magnetic center would develop enough force to shift the main generator shaft enough to be noticeable.

You may be forced to disable the neutral voltage relay to allow further measurements as your excitation is increased. My question is why didn’t it trip before? It makes no sense.

So your theory about high neutral voltage (because of your high reactance grounding) breaking down the bearing insulation seems like the root cause, good work!

Not enough voltage behind the ground fault to cause a problem until excitation was built up, and perhaps the voltage relay wouldn’t actuate because you in essence were solidly grounded through the bearing insulation?

Please confirm the original equipment manufacturer is involved with, at least, advising on this repair.

Then there will be a commercial warranty as part of that process. If it isn't right, get the OEM back in to sort it out.