When you do transient simulations of an induction motor driving a load, there can be a torsional natural frequency which somewhat resembles a mechanical torsional resonance, except the motor field also plays a role in the resonance. If the torque variation frequency of the load happens to land close to that natural frequency, then you can get large angle swings of the rotor and accompanying wild swings in power between the motor and the power system.

Here is one case on eng-tips where I believe that's what happened.
thread237-249262: Low power factor, recip pump
It took awhile to get to the conclusion in that thread, but in the end a transient model for torsional natural frequency agreed almost perfectly with the waveforms that were posted by that thread's op.

I saw another example of a motor / belt driven low-speed air compressor presented at an EPRI LEMUG conference which seemed to follow the same pattern (the presenter didn't view it exactly that way... his main point was that the current oscillation they were seeing changed the way the overload relay responded, but all the data that he provided lined up looking like a torsional natural frequency to me...including angle between current and voltage swinging enough to indicate reversal in power).

I'm not particularly familiar with the torque-variation characeristics of ball mills, but it appears they can excite torsional resonances.

So to me your (op's) symptom of wildly oscillating current matches the torsional natural frequency scenario. But it doesn't explain why it changed when you swapped back in the original motor. Perhaps it was coincidence that something else changed either the voltage (which affects the natural frequency) or more likely what was going through the mill may have changed either it's apparent inertia or it's tendency to generate torque variations.

If you can catch it in the act doing this again, I would use a strobe to check for angular oscillation of the shaft (which if present would tend to confirm the theory/). High-resolution vibration might show sidebands at the same frequency as current oscillation. Also try using a different current meter, perhaps analogue.

Edit - I presume this is the same motor that you just updated us on: thread237-482813: 400 HP Failure Yearly

• It appears you had seen repetitive winding failures related to apparent overload (based on inspection) when no overload was known to exist, and apparently the failures were fixed by addressing a vibration problem due to machine support and something to do with rotor (I don't follow what was going on with the rotor).
  
  
• It might be interesting to consider whether your winding failures could be tied to a torsional scenario....


....certainly torsional resonance can cause apparent overload of the machine. Torsional resonant oscillation may also come and go (depending on what's in the mill) and escape your detection if the machine doesn't trip (although I would expect it to trip if it were thermally challenging the windings unless your overload settings are too high). Also torsional resonant oscillation may create unique vibratory stresses on the coils, while I'd think rigid body stator vibration of the stator frame doesn't stress the coils very much, even at 1.0 ips.


• And it might also be interesting to consider whether a posulated torsional resonance could be affected (tuned) by stator support stiffness.


....It might be a longshot, but it doesn't seem implausible that problems with base grouting could impact the rigidness of the stator support in resisting stator horizontal rocking mode and could tune a torsional resonance. Or I may be waaaay off.


• It leads us back to the "what changed" question. Could this stator support characteristics be part of the what changed?


... Maybe over time the grout put in during 2012 degraded in 2021 bringing back the torsional resonance. And perhaps during process of replacing the motor, that same stator support stiffness was changed in some way related to holddown bolt tightness of soft foot or other unknown factors. Or considering you had changes in behavior at two different points in time (one when the motor started oscillating and another when the reinstalled motor was working fine), then it could be that support stiffness explained one of them and something else affecting torsional resonance (mill contents, voltage) changed another time. My gut says mill contents change is most likely but with the information available to us here at this point in time there's no reason to associate mill contents change with these particular points in time... while on the other hand it's plausible that stator support stiffness may have changed at these particular times at least when new motor was installed. I apologize, that's a lot of rambling. And it's also a lot of speculation on my part.

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(2B)+(2B)' ?

This is a difference motor than the one updated on https://www.eng-tips.com/viewthread.cfm?qid=482813. Completely different Feed Mill this happened at.

Thanks, so not the same exact motor position. But the similar application and similar motor? (As I understand it both 400hp motors driving ball mill).

So some of the connections that I drew are not correct (in particular the hypothesis of grout fix loosening up).

But I'd still say torsional resonance could play a role in the recent current oscillation that you saw. And when you put the original motor back in, it may have gone away due to either coincidental ball mill contents change or coincidental voltage change or change in stator support conditions (shimming, bolt tightness) which was directly related to the swap.

And for the other thread with repetitive winding failures apparently solved by some mechanical changes (rotor swap and stator grouting), I'd still say torsional oscillation could play a role. But if I have any more comments about that I'll put them in the other thread.

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(2B)+(2B)' ?

Nicely summed up Pete.

All VFDs have forbidden frequencies you can program in to avoid any and all kinds of damaging resonance speeds. Often you can program about 10 of them in. I suspect one would've prevented the motor failure described by the OP.

Keith Cress
kcress - http://www.flaminsystems.com

Loose cable connection at the motor pecker head the first two times and got fixed the 3rd time around in a happy coincidence?

Muthu
www.edison.co.in