Balancing large 8600hp electric motor with fluid film bearings

[Sparky4598]

Hello all, looking for some advice, tips, and suggestions here. We have two large 8600 horsepower motors where I work and we are having some vibration problems from them. Have been for a long time, but nobody has ever bothered to try to fix it because it worked good enough and ran without tripping vibration limits. The motors have oil journal bearings.

The motors have vibration sensors that read in velocity and we have a very good data acquisition system they are connected to. Everything is ISO calibrated as we are a testing facility. The mentality has been well it's good enough to run what we've been doing so we're not going to bother with it. But now we have a test coming up that is going to require higher rotational speeds and we tripped due to high vibrations when trying to reach those speeds. Forgot to mention the motors are variable speed, VFD controlled.

I have a bit of experience balancing rotors mounted with ball or roller bearings, but not any experience with fluid film bearings. We tried balancing the motor using a trial weight and the 4-run method. We only have a speed encoder on the motors, not a position or index encoder so we can't use the 2-run method. The results were lower vibrations, but not nearly as good as expected.

My suspicion with my limited knowledge and experience of journal bearings is the oil in the journal bearings is absorbing a considerable amount of the vibration. I am suspecting the vibration readings we are getting are significantly lower than they would be for the exact same equipment with roller bearings. Like I guess I'm saying I'm thinking the vibration with fluid film bearings doesn't necessarily correspond to the imbalance linearly as it would in a roller bearing.

So I guess my question is what would be the best method to balance the rotor without removing it from the motor housing?

I have thought about using lasers and monitoring shaft displacement, but I have read that displacement vibration monitoring is only good up to about 10Hz(600rpm) and these motors typically run at 20Hz but the test we have coming up will run at 30Hz(1800rpm). They are rated for up to 2000rpm.

Thanks for your time!

 

[GregLocock]

Frankly I'd be calling in a large motor balancing guy.

Did you 2 plane balance? Since automotive crankshafts are oil bearings I don't remember any problems balancing them.

Depending on the natural frequencies of the rotor you may be operating above the whirl speed. Certainly possible, definitely exciting.

Watch out for a couple of sub critical rpms, at half the critical rpm, due to oil film whirl and gravity 2/rev excitation. So mount an accelerometer to each bearing housing (if you can get to it) or on the frame in line with each bearing, all pointing the same way. Then for each plane add an imbalance at each bearing in separate runs.

This gives you the response at each plane to an oob at each plane. With just 2 planes it is easy to do by hand, more than that you probably want to build a matrix.

I prefer the 3 trial run method rather than just 1 trial run, for robustness, but my runs are cheap.

B&K do a nice series of free books, the blue vibration one covers balancing in some detail.

 

[Sparky4598]

Frankly I'd be calling in a large motor balancing guy.

Did you 2 plane balance? Since automotive crankshafts are oil bearings I don't remember any problems balancing them.

Depending on the natural frequencies of the rotor you may be operating above the whirl speed. Certainly possible, definitely exciting.

Watch out for a couple of sub critical rpms, at half the critical rpm, due to oil film whirl and gravity 2/rev excitation. So mount an accelerometer to each bearing housing (if you can get to it) or on the frame in line with each bearing, all pointing the same way. Then for each plane add an imbalance at each bearing in separate runs.

This gives you the response at each plane to an oob at each plane. With just 2 planes it is easy to do by hand, more than that you probably want to build a matrix.

I prefer the 3 trial run method rather than just 1 trial run, for robustness, but my runs are cheap.

B&K do a nice series of free books, the blue vibration one covers balancing in some detail.

Supposedly they called in a motor repair and maintenance company and they attempted to balance it after they did other maintenance like replacing the bearing journals and some tuning on the drives, but did not do it properly because the vibration were not any better. Or so I have been told, that was before my time here.

Normally I haven't had much trouble balancing things before so I figured we should have no problem doing it since we already have all the sensors, but I've never done oil bearings before.

We have only done the balancing of 1 plane so far. The other plane is going to require a lot more effort to install the trial and balance weights so we did the easy one first. When we did not get the improvement we expected, I started looking into why and if something may be different about journal bearings that I am not familiar with. Maybe we do just need to go ahead and do the 2nd plane and see how it is after that. Just didn't want to spend a lot of effort doing so if it needs to be done differently than I have done in the past.

I do believe the motor is running above the critical speed though, had not considered those effects yet. Good point.

 

[mfgenggear]

Supposedly they called in a motor repair and maintenance company and they attempted to balance it after they did other maintenance like replacing the bearing journals and some tuning on the drives, but did not do it properly because the vibration were not any better. Or so I have been told, that was before my time here.

Normally I haven't had much trouble balancing things before so I figured we should have no problem doing it since we already have all the sensors, but I've never done oil bearings before.

We have only done the balancing of 1 plane so far. The other plane is going to require a lot more effort to install the trial and balance weights so we did the easy one first. When we did not get the improvement we expected, I started looking into why and if something may be different about journal bearings that I am not familiar with. Maybe we do just need to go ahead and do the 2nd plane and see how it is after that. Just didn't want to spend a lot of effort doing so if it needs to be done differently than I have done in the past.

I do believe the motor is running above the critical speed though, had not considered those effects yet. Good point.

I never have been involved with balancing of electric motor.but did with aerospace gear shafts. Dynamic 2 plane balancing.
The machines used not that complex.
But it was permissible remove material with a hand grinder.

I had a 383 stroker Chevy engine. The crank shaft was from ac400 Cid Chevy engine. The crank have had to be dynamic balanced with heavy metal installed on the flywheel.
350 Chevy engine crankshaft was they called internal balancing. The crank shaft it self was added or stock remove. Hot rod only not sure about production. All bearings usage
It should make a difference.
But the work I was involved with was very stringent.
So based on that, the balance has to be with rotor removed. As said to verify contact the manufacture.

 

[GregLocock]

2 plane balancing is more complex than 2 1 plane balances, as you need to consider the influence coefficients from plane1 to plane 2 and vice versa.