Motor won't balance

[sii]

We have a 460 VAC, 1800 rpm, 75 hp Lincoln motor that drives a vertical mill in our plant. While PMing the machine last Friday I thought the ODE bearing sounded tough while running. I asked one of my other guys to take a look at it (without telling him what I thought was wrong), he came to the same conclusion I did. BTW this motor has sealed bearings.

We pulled the motor Saturday morning and took it to the motor shop. Disassembly showed wear on the DE bearing location so it was sent to the machine shop. I got a call today saying they had reassembled it but that they were having trouble balancing it. I went out to the shop tonight and listened to it run. It doesn't sound bad (but not great either) at speed but as it decelerates, it sounds absolutely terrible and vibrates quite badly around 1400 rpm and below about 500. The balance report shows less than .25 mm/sec.

I am stumped as are they. They are the best motor shop around and I think if anyone could figure this out, they could. Any insights?

 

[GoldDigger]

duplicate....

 

[GoldDigger]

We have a 460 VAC, 1800 rpm, 75 hp Lincoln motor that drives a vertical mill in our plant. While PMing the machine last Friday I thought the ODE bearing sounded tough while running. I asked one of my other guys to take a look at it (without telling him what I thought was wrong), he came to the same conclusion I did. BTW this motor has sealed bearings.

We pulled the motor Saturday morning and took it to the motor shop. Disassembly showed wear on the DE bearing location so it was sent to the machine shop. I got a call today saying they had reassembled it but that they were having trouble balancing it. I went out to the shop tonight and listened to it run. It doesn't sound bad (but not great either) at speed but as it decelerates, it sounds absolutely terrible and vibrates quite badly around 1400 rpm and below about 500. The balance report shows less than .25 mm/sec.

I am stumped as are they. They are the best motor shop around and I think if anyone could figure this out, they could. Any insights?

Well, taking a cue from the famous Steinmetz anecdote ("Drawing X on casing $1, knowing where to draw it $999.") I would suggest this:


If the static balance of the motor is OK (and we can pretty much count on that) and the dynamic balance is OK (almost certainly tested by spinning the motor up with external power, yes?)
Then either there is something which is changing position relative to the rest of the rotor when it is decelerating or else you are seeing an electromagnetic effect.

What sort of electromagnetic effect would be small when the motor is not at operating speed and also when accelerating (the last is a guess, but since you did not mention it, I will venture that it is true) then it is something which involves an interaction among the three phase windings only when they are not connected to an external power source which holds the voltages constant and balanced.
This could be something a simple as a single shorted turn in one winding or infinitely more complex.

If you have a way to break the connection between the windings at the points of the delta (it is delta, yes?) then you can test to see whether that removes the vibration. Or else you could connect it up to a VFD which would keep control of the winding voltages during deceleration.)
Or have your motor shop do an AC impedance test of all of the windings if they have not done that already.

Pure SWAG, but has some logic behind it.

 

[Jraef]

...
What sort of electromagnetic effect would be small when the motor is not at operating speed and also when accelerating (the last is a guess, but since you did not mention it, I will venture that it is true) then it is something which involves an interaction among the three phase windings only when they are not connected to an external power source which holds the voltages constant and balanced.
This could be something a simple as a single shorted turn in one winding or infinitely more complex. ...

Pure SWAG, but has some logic behind it.

Added SWAG to some already good SWAG, another possibility is a broken rotor bar or end ring connection. Experienced motor shops often use vibration analysis to sniff that out, it has some distinct effects that can be seen by a trained eye. Often times this can then be pinpointed by doing whats called a "growler" test. A growler is a magnetic device in which the rotor is suspended that just induces current flow, and thus magnetic fields, into the windings of DC motors or cages in AC. Its called a growler because if there is a problem in an armature winding in a DC motor, it makes a nasty growling sound. So growlers are often thought of by some motor people as a test only for DC motors therefore they don't do them for AC. But I attended a presentation a while ago showing how a growler can be used to find rotor bar problems in AC squirrel cage motors using iron filings in a plastic bag being ignored when there is a broken connection. A lot of motor shop people are unaware of this technique, or they don't think to use it. they rely solely on visual inspection, which can miss early failures.

There are several web articles on using what is called "side band" vibration analysis to discover rotor bar issues, then even a few that go into using current waveform analysis as well if you have a good scope. Here is an article on the growler technique discussion I attended at an EASA convention when it was here in my home town.
http://www.pumpingmachinery.com/pump_magazine/pump_articles/article_29/rotortesting.pdf