using a flux coil for in field motor diagnostics

[micjk]

Can anyone lead me to some informational links involving flux coil analysis of motors. Is this a practical approach to motor analysis or should we stay with current and vibration analysis. What do we look for. New to this type of analysis.

 

[electricpete]

http://www.compsys.com/drknow/aplpa...4232a7bed9852569d0007a0bbb/$FILE/fluxtech.pdf

by monitoring axial flux leakage patterns, we may detect:
broken rotor bars
unbalanced voltage
turn-to-turn shorts
other assymetries

There may be more info in some f CSI's other papers at:

http://www.compsys.com/drknow/aplpapr.nsf!OpenDatabase&Start=1&Count=50&Expand=8

 

[vanstoja]

To Electricpete,

In your second reference he lists internal misalignment as the #1 source of 2E but says all the cases involve babbitted oil-lubricated sleeve bearings. Plane sleeve fluid film bearings are designed with the intention of running misaligned , ie, eccentrically. The optimum eccentricity has been stated as 0.7 in several books or papers. Is plain sleeve bearing eccentricity what is being called "internal misalignment"? If so, it's total nonsense to be calling a bearing design characteristic a deficiency or malfunction. Pivoted-pad fluid-film radial bearings, prized for their stability characteristics, can have operating eccentricities approaching 2 times the "nominal" clearance when they translate "between the pads" depending on the number of pads employed.

 

[electricpete]

Greetings, John (vanstoja) !

I believe that for horizontal motors the shaft will be off-center low within the bearing, but this is compensated by setting the bearing off-center high within the end-bell, such that the rotor is roughly centered within the stator.

You are right that centering of the rotor shouldn't change by itself, only if not set up properly from the factory or during subsequent dissasembly/reassembly of the motor, so malfunction is probably not a good term. Normal bearing wear might allow the rotor to drop down no more than a few mils.

My impression is that foot-related conditions are more common source of 2*Electrical frequency for small horizontal motors perhaps 100hp and below. For larger horizontal motors with stiffer frames and perhaps more care in installation, foot-related distortion does not seem to be a big problem and the less-common air gap problems from other causes would be more likely the cause of 2E.

I have not encounted many electrical-related vibration problems in vertical machines using the pivoted-pad radial bearings. The closest I have seen is one example of large vertical motor with pivoted pad bearings where I had a very strange pattern of 2*LF with running speed sidebands

http://www.reliability-magazine.com/ubb2000/ubb/Forum2/HTML/000605.html

I was aware that the general reason for using these tilted-pad radial bearings was to avoid oil-whirl type problems in vertical machines where there is no radial load to enhance stability. On our large vertical machines there is typically somewhere around 5 mils clearance at each pad (machines have air gap on the order of 100 mils). Can you explain what you mean in your last sentence? Are you saying that we expect around 10 mils of radius of shaft movement during normal operation?

 

[electricpete]

The machines I described above have 6 pads.

 

[electricpete]

regarding my statement that "bearing is off-center high within the end-bell".... I guess I'm not 100% sure if that is a universal practice. Does anyone else have any comments?

I see in EASA Principles of Large Motors 2000 page 15-33 it says "At least one manufacturer... deliberately bored babbit bearings off-center (the bore was not concentric to the OD), calling them high-lift bearings". This appears to be another approach to compensate and keep the rotor in the center of the stator.

For lower-speed motor it may be acceptable to make no correction for shaft riding low within the bearing but I think for 2-pole horizontal sleeve bearing motor it is required.

 

[jbartos]

Suggestion: Bearings aspects are also dealt with under Mechanical Engineering.

 

[jbartos]

Small encore: Visit

http://www.cedrat.com/software/flux/flux.htm

http://www.cedrat.com/software/software.htm

http://www.ansoft.com/support/workshops/emworkshop/Trends_in_finite.pdf

etc. for flux coil analyses.

 

[vanstoja]

To Electricpete,
Hadn't seen your responses to my remark before. Sorry about that. The eccentricity limits of pivoted pad radial bearings depend on the number of pads employed. For 3-pad bearings it is nearly 200% of nominal clearance. For 6 pads the maximum would be something like 120%. We calculated this eccentricity for various pad numbers (it's a simple geometry problem) but I don't have the results at hand. Conceivably, our calculated eccentricities could be even higher with very heavy bearing loading if the pad holder stresses are high enough to elastically or plastically deform the pads so as to increase the arc at the pad edges.
Regarding 1X sidebands around 2LF sidebands to Rotor Slot Passing (RSP) peaks, we see them frequently in our tests of vertical rotors with 3-pad PPRBs. We attribute this to the rotor orbiting about the eccentric centerline of the loaded rotor. Occasionally, there are more than one set of +/- 1R sidebands. We attribute multiple 1R sidebands around +/- 2LF sidebands to distortion of the orbit by interference with the pads during "deep" between pad loading. We have some limited orbital data from an instrumented machine that shows triangular and nearly square rotor orbits at very high eccentricities indicative of either rotor to pad contact or "close approach" with squeeze-film separation.

 

[electricpete]

Thanks John. So what you're saying is that even though the tilted pad bearings are stable (resistance to fluid instabilities), they still allow a lot movement during normal operation. That makes sense.

The particular case that I linked to was not 1X (running speed) sidebands around 2LF sidebands of RBPF. It was 1X sidebands around 2LF. It makes some sense that either of these might be caused by rotor orbiting within the bearing clearances. And the number of running speed sidebands was very large.

This particular motor had a typical setup of tilted-pad radial bearings on top and ball-bearing guide bearings on the bottom. The guide bearing was loose within it's housing. The symptom did not reappear after we did maintenance on the motor, which included restoring lower bearing housing clearances to within tolerance as well as many other adjustments (and a complete rewind).

 

[vanstoja]

Tilting pad radial bearings (TPRB) only stabilize bearing half frequency whirl. They may not stabilize subsynchronous whirl from other instabilities like plain annular seal whirl, shrink fit induced whirl, "packing rub" (contact) whirl,etc. We've even seen one rare example of water-gap-induced rotor whirl in a water-cooled induction motor operating with TPRB. Another motion possible with TPRB is vertical rotor "wander" from one between-pad position to another at very low frequencies like 1.4 Hz. Considering that, among "stabilizing" radial bearing designs, TPRB have the worst damping properties making them vulnerable to dynamic loading, there is good reason to consider other stable bearing designs like two and three lobe designs or herringbone groove bearings.

 

[jbartos]

Suggestion: Visit

http://cisd.yonsei.ac.kr/research/researches/overview/AH_Team/detail.htm

etc. for Development of Herringbone groove spindle bearing system leads.