In a 1400 KW, 6.6 KV,1500 rpm motor where both bearins are sleeve bearings, an axial movement of approx 2mm in motor shaft was observed but the motor was in coupled condition. There are 4 motors in a row for the same operation and normally three remains in running while one in standby mode. The phenomena took place on changing over the standby motor to running motor. According to motor manufacturere's data sheet, it should be coupled by a rigid coupling with .05 to .1 mm tollerance. In this case also the motor was coupled with a similar coupling having "SIM" in between. The axial play which was observed in the motor shaft was not observed in the pump shaft. Later when the motor was run without coupling for inspection, it was observed that the motor runs exactly at its magnetic neutral centre as marked by the manufacturer. There was no damage or abnormality observed in coupling. The motor is coupled again and now in standby mode.
I would like to know the cause of such behaviour. If any more data is needed for the analysis, I can provide to my best.

Sounds as though coupled it is not on magnetic center. Run uncoupled and scribe shaft and a convenient increment such as 1". Also with the pump running scribe its hydraulic center as well. Now with both coupled move the motor so centerlines are set to where they operate.

However, you said a rigid coupling - are you sure? A rigid coupling in running state will require 0.002" maximum tolerance radially and only 20 seconds angularity. And as far as axial spacing goes, you will want to be within +/- 0.015" from exact zero from the exact settings of the motor and pump set at their centers.

If you have a disc (Thomas) coupling make sure it is the right model for the application. Certainly a rigid coupling seems incorrect without knowing more.

Just a thaught, check motor shaft or motor base where you can get the best reading for proper level( with precision level or machinist's level)Rotor tends to roll back and forth to compensate for the grade. If your pump shaft has antifriction bearings ,the pump shaft will not show any visible axial movement. Only motor rotor is free floating.
Rai

Thanks Mr. Rai for your reply. But the motor is properly leveled which was checked once again. Had there been a level problem, then I think the motor would not run at its magnetic centre on decoupled run. In decoupled condition the motor runs at its absolute magnetic centre as marked by the manufacturer.

Thanks Mr. Sams also. There may be a deviations in coupling. It was observed that, when motor came to stand still after switching it off , its pointer for magnetic centre was showing a shift of the rotor axially by about 2 mm. But then, why should it oscillate to and fro even if it is not coupled in its magnetic centre. Primarily, in such cases, the abnormality should have been reflected in the current drawn by the motor, but in this case current was fully balanced.
As for your doubt on the coupling type, it is “Metastream TSKS type coupling having Flexible diaphragm”
Just a thought is that, due to a variation in the process parameters, there was some impact load on the pump impeller. As the pump is having antifriction bearings, so its axial movement can not be more then 0.3 mm. But the impact load can get transmitted through the coupling to the motor. The coupling is having “ Flexible Diaphram” in between. When the impact load force is transmitted through the coupling, due to the flexible diaphragm, the motor shaft moves backward. But the magnetic pull of the winding pulls the rotor to its centre and in doing so the rotor gets an small forward overshoot. This causes the oscillation. Can it be correct to think in this way??

"According to motor manufacturere's data sheet, it should be coupled by a rigid coupling with .05 to .1 mm tollerance. In this case also the motor was coupled with a similar coupling having "SIM" in between."

What is SIM? I don't know kind of coupling you have but if motor shaft is moving relative to pump shaft it must bel flexible.

Here is a case from our plant that sounds identical.

Horizontal 1200rpm 2500hp sleeve bearing motors coupled to pump through Thomas Shim Pack coupling.

When operating at lower flow per pump (more pumps with same total flow), we see axial movement of the motor shaft 0.125" and no axial movement of the pump shaft.

I am pretty sure that the vibration originates in the pump (since it is related to flow). The pump thrust bearing has tight axial clearance maybe 10 mils. But the pump moves within those limits (at low flow only) and provides exciting displacement for a mass spring system: the spring is that Thomas Shim pack coupling and the mass is the whole motor rotor mass. The excitation I believe is broadband and the resulting frequency of movement is the natural frequency of that mass spring system. With flexible coupling (low k) and large motor mass you can imagine the natural frequency is a few hertz which is in the neighborhood of how fast we see the motor shaft moving. When we shift pumps to a condition of higher flow per pump it goes away.

I am very confident the source of the vibration originates in the pump and not magnetic centering force. Like you we have checked motor magnetic centering sat.

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

Thanks Electricpete for your reply. Your explanation sound very convincing.

Ujjwal

I have never seen a rigid coupling in that applicaton that was the correct application or can't recall one anyway. The pump in all probability woul have ~40 mils axial float but the mtr/pmp coupler should have a floaat limiter which would only allow for 40 mils max. An eight of an inch may be acceptable but I would go to the drawing board to confirm and not rely on OEM.

Electricpete,
Your assumption could be wrong. Higher flow would require higher HP what leads to denser magnetic flux and higher axial stabilizing forces on the motor rotor. Also higher transmitted HP would change coupling axial siffness.

[QUOTE]Originally posted by Sam Pickens:
Sounds as though coupled it is not on magnetic center. Run uncoupled and scribe shaft and a convenient increment such as 1". Also with the pump running scribe its hydraulic center as well. Now with both coupled move the motor so centerlines are set to where they operate.

QUOT/
I think Sam is correct. While coupling, the motor rotor was not at the pointer position marked by OEM. Normally there is a pointer near the DE bearing pointing to a mark on the rotor which should be matching with the pointer at the time of coupling. Otherwise, the motor rotor will keep floating to and fro. I also think , it is a flexible coupling with a pack of shims (commonly known as metaflex or discoflex manufactured by John Crane and others) which provides required axial float and can take considerable misalignment. Since, the motor is already on standby, re couple it properly . I have myself faced such problem in the past with 1600 MWe motor driving a BFP. the problem was of magnetic centre.
Regrads.
Irshad Akhtar

quote:

Originally posted by electricpete:
I am very confident the source of the vibration originates in the pump and not magnetic centering force. Like you we have checked motor magnetic centering sat.

Pete, slightly off the track. Will you pl. explain how we can determine magnetic center of the motor or generator in installed position in field?
Regards
Irshad Akhtar

Short answer: For a sleeve bearing motor (*), the magnetic center can be checked by running the motor uncoupled and noting the axial shaft position while running uncoupled.

Long answer: For marking purposes, we spray a blue developer on the shaft while uncoupled with the shaft pulled all the way inboard before the uncoupled run. Then while running we carefully place a metal ruler flat on the housing and lower it to just make contact with the shaft and scrape away the blue developer. (keep personnel safety in mind). Then we secure and tagout the pump and during the coupling process we make adjustments so that when coupled to the driven equipment the motor shaft runs at the same position (usually the position of the hub on the motor shaft adjusted to put the motor in desired axial position, and then position of hub on pump shaft adjusted to maintain "coupling gap").

In addition to magnetic center, it is more important to make sure the motor is not coupled up so far away from mechanical center that it is near either limit of travel.

* There is no way to check the endplay if the motor has rolling bearings that don't allow axial movement.