We have a Synchronous,1750Hp, 327 RPM motor, with sleeve bearings, driving six Nash vacuum pumps that I was told sounded a little different from normal. The only thing that I see in my data are multiples of 2x line freq with some turning speed sidebands. From what I have read in my books and from earlier post, this is pointing at loose stator coils.
If this is the case(my first experience of this type)are these amplitudes high enough to cause concern?
Thanks for any input,
Scott

Vacuum_Pump_mtr.pps (44 Kb, 59 downloads)

It's my understanding that loose stator coils in a synchronous motor would create energy at a peak equal to the # of stator coils with 1X sidebands. In this case you have a peak at 22X with 1X sidebands. Since this is a 22 pole motor I would suspect a loose coil or pole on the rotor. (if I'm thinking right there should be 22 pole pieces/coils on the rotor but there would be many more stator coils, like 144 or more on a motor this size). Like I said, this is the way I understand it but I could be wrong.Has the 1X vibration gone up? That could indicate some unbalance from a loose piece on the rotor.

Warning - rambling post ahead. I don't have much of a clue on this one but I'll throw in my two cents.

Looking at the time waveform, I'd say it doesn't look like impacting/looseness because it is so regular and symmetrical with no evidence of any ringdown. But even if it were impacting/looseness, I would say more likely it was looseness of the core than looseness of slots within the core. The reason is the peak of the 7200 harmonics peaking around 850 hz seems to suggest that if it were impacting it's impacting something with 850hz resonance... and I don't think coil impacting could possibly excite as high as 850 hz since the coils are not metal.... they can be very hard but not metal. So the bottom line, it doesn't look like looseness, but if was looseness I would say not looseness of the coils but remotely possible looseness of the core.

I'd say there is a 90%+ probability it is something else altogether, but it beats me what. We don't have any sync motors or motors that drive 6 compressors (if I understood right this single motor drives 6 compressors?). That low speed must be a salient pole rotor (vs smooth pole).

Some wild ideas that come to mind for no particular reason:
1 - As mentioned it's a 22-pole machine. If one pole is different (for example shorted turns), that difference shows up every time that rotor pole lines up with a stator pole which is 22 times per revolution 7200cpm. Why that higher frequency stuff would also show up (7*2*lf), I don't know. Easiest pole check I know is offline ac pole voltage drop test.
2 - Sort of smells electrical like 2*LF. On faster speed machines and smaller airgap machines, 2*LF would make us think of airgap problems or foot problems. Those don't seem as likely here with slow machine and big airgap, but you never know.
3 - Stator current unbalance for whatever reason (supply problem or machine assymetry) can upset the symmetry of the fields and cause 2*LF radial vibration and I think maybe also 2*LF torque oscillation. Stator current checks are usually pretty easy, so that's the first thing I'd check.

Thanks Joe and Pete, This motor is already scheduled to be inspected in three weeks during our annual outage.
I will post the findings after the outage.
Thanks again for your input.

Greetings,

We had high G's (@3) at high CPM (150K) in acceleration. Which was equeal to .16 IPS at the same range in velocity. Found a book that said .1 IPS in a motor at bar/stator freq's is bad news. 1X Displacement before/after was < .5 mil. Not sure how it applys to your request for info. But I had a neat picture I wanted to show you.

3600 rpms motor W/40 bars was inspected and repaired.

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