Greetings,
I am having issues with a 300HP solid shaft US vertical motor. The motor was sent to our shop for a PM last month. It had been in storage for over a year. Bolted to the shop's isolation pad all velocity amplitudes are <.07in/sec. When coupled to the pump, the levels increase to >.39in/sec, and a periodic, high pitched noise is heard.
The "surge" can be seen with a strobe as speed modulations. The shaft appears to be "cogging" in concert with the noise. The monitor mode shows the amplitude of 1x TS changing, plus variations at close to 2000hz. Alignment has been checked and rechecked. The pump is level. The vibration is not directional around the motor-but the lower end (guide bearing) amplitudes are only .15ips; phase readings are all "normal". Bump test reveals no resonance issues. High Res. shows pole pass sidebands around turning speed. 2X Line frequency was not found (dominate) anywhere.
A visual inspection found brass shavings in the top (single 7226 Thrust) bearing. the motor was sent back to the shop; the bearing was changed, and the motor dismantled and inspected. (found no rubs)The rotor has been growled,tested with the PdMA (RIC)and has been single-phased--all with good results. Thrust is set at .010". The motor was reinstalled with the same results.
Another motor installed on the same pump is running fine. Has anyone come across issues similiar to this? Apologies for the long, rambling post. Data to follow.
Thanks, Jeff

Jeff,

Have you conducted motor current spectrum analysis with pump installed and motor under load? Where did the brass shavings in the top bearing come from? Was it a brass retainer that broke?

Walt

Did it start immediately upon start-up or 2 hrs later w/heat? And get rougher with time? Not in the shop but underload! Loose rotor bar. I had one like this and when mimiced the field condition with heat, it did exactly the same as in the field and was from a rotor bar swinging out causing high 1X --- and, upon power off does the problem drop immediately?

Walt,
No online MCA tests have been performed. The bearing retainer is brass. I did not see any "breaks" in it, just some unexpected wear patterns in the sockets that hold each ball.

Sam,
The vibration/noise begin immediately upon startup. The way it declines at shut down points to an electrical issue.

A little more info to clarify: As mentioned, the base is level now; on initial trip it was noted that the base was off level by .068" per foot. That issue was taken care of, the motor then reinstalled. It was my call that the unit being off level damaged the thrust bearing. Bearing looks good now.
If I have open rotor bar problem, wouldn't I see it reflected in speed? Under load RPM = 1782. Nameplate is 1780. The situation Sam brought up has crossed my mind: perhaps it is not an open bar, just a loose one that does not show itself until there's a load? No major mechanical repairs were performed by the shop; just bearings, cleanup & balance.

Thanks for your responses,
Jeff

This message has been edited. Last edited by: Jeff4,

Jeff,
Vertical motor bearings (top) can be subjected to severe vibration if the coupling distance is not correct. Basically, the motor shaft can be lifted, causing the top bearing to skid. Failures occur quickly. You state that the misalignment was severe. This could have caused the coupling gap to be good on one side and bad on the other (180 degrees).
I have seen large rollers broken in two within a weeks time when the coupling gap was too high (0.090" on a 900 rpm, 2000 hp motor).

Greetings,

I have an update; Sam, you were right on. The pole pass sidebands were caused by a rotor bar issue that only---at first-- made itself known under a load. As mentioned previously, this thing had been growled, single phased, and RIC tested with no faults seen. After it was started under load a few more times, and sent back to the shop, I dismantled it and found obvious rotor bar problems. A coreloss test showed eight open rotor bars. There are four open bars on either side of the two bars that have been hot --see photo(makes sense--they are carrying more current)
The defective bearing was a seperate issue. I feel as though the base being "unlevel" played a part in that.
I have learned to take RIC results with big ol' grain of salt. Still curious as to why the single-phase test showed no current variation.
Sincerely,
Jeff

This message has been edited. Last edited by: Jeff4,

300_hp_Rotor.pdf (195 Kb, 27 downloads) rotor pics

vibration data

300hp_vibedata.pdf (106 Kb, 23 downloads)

That turned into a great case study. Thanks for sharing that info and results.

jank has given us a lot to think about in considering the RIC test in this thread linked here:
http://maintenanceforums.com/eve/forums/a/tpc/f/7161085...831011813#1831011813

In summary, I think he is saying that for cast rotors (like yours), the flux is not high enough to saturate the small "bridge" of iron just above the rotor bars. If you can't saturate that bridge, then the bridge forms a short-circuit for the flux to flow above the bars. Less flux flows below the bars and less current induced in the bars, and broken bars don't show up as well.

I heard an old-timer motor shop guy say that the single-phase test is less sensitive when you use low voltage and more sensitive when you use higher voltage (although you have to watch out against overheating the motor). Maybe the reason is similar... if voltage is too low then the small layer of iron above the bars is not saturated, and the flux bypasses the bars (goes over the top of them).

Do you know what voltage was used for the single phase test?

Thanks EPETE & RON for responding,

Epete; I have talked to the tech that performed the single phase test. He said he put 1200V on it with no deflection on his current! It is a 4160V motor. Thanks for the link.

Ron; The coupling gap was set @ .250" I have seen the problem you described before as well. It doen't take long to damage the thrust bearing when the shaft is being pushed up like that. Thanks again.

Jeff