400 HP Amps fluctuating +/- 200 amps/sec 1

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
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
[ul]
[li]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).

[/li]

[li]It might be interesting to consider whether your winding failures could be tied to a torsional scenario....[/li]

[li]And it might also be interesting to consider whether a posulated torsional resonance could be affected (tuned) by stator support stiffness.[/li]
[li]It leads us back to the "what changed" question. Could this stator support characteristics be part of the what changed?[/li]
[/ul]

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(2B)+(2B)' ?

This is a difference motor than the one updated on 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 -

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

Muthu