MV Motor Runs Uncoupled, Locked Rotor Fault When Coupled 3

[Plum_Point1]

We have a Siemens 1750 HP 3600 RMP 4000V motor that is used on an air compressor. We recently had the windings replaced. All motor shop test results looked fine. We got the motor back, done an uncoupled run to identify magnetic center, aligned and coupled the motor. We get a "locked rotor fault" when attempting a coupled run. We uncoupled the motor and it will find magnetic center and run fine, no issues. It is only on the coupled run that we have an issue. We have realigned and coupled multiple times with oversight from our local air compressor rep even and are still dealing with the issue. We have done extensive motor testing and have found nothing wrong. Any ideas on what we could be dealing with here?

 

[BrianE22]

Just curious, what type of coupling has problems with too much grease?

 

[TugboatEng]

Splined couplings such as used in Cardan shafts can behave as a hydraulic cylinder when pumped with grease.

 

[electricpete]

We were able to successfully start the air compressor motor after removing a plug in the coupling to relieve grease and allow for axial movement. We also moved the shaft out to magnetic center prior to the start attempt. It seems the culprit was us being hydraulically locked from the grease in the coupling. Thanks for all the input everyone.

First, kudo's to Bill who said "Does the coupling allow for expansion?"

But the scenario isn't clear to me. I'd like to circle back and figure out how exactly does axially locked coupling cause a trip during start?

[ol 1]
[li]it puts so much load on the thrust bearing that increased torque is required to drive it?[/li]
[li]change in motor rotor axial positioning changes the motor magnetic behavior based on axial misalignment of rotor and stator iron? (could create decreased torque due to decreased effective length of overlap and increased starting current due to decreased leakage reactance)[/li]
[/ol]


For scenario 1, it would have to result in significant thrust bearing damage in order to result in a a non-negligible increase in torque loading. I'd think that would leave some evidence.

For scenario 2 it is also a small effect for a normal sleeve motor with 1/4" or at most 1/2" endplay. The fractional changes in lengths are pretty small unless there's something tricky going on with vent alignment (But I think most 2-pole rotors don't even have vents). So I think any torque effects associated with decrease in effective length are neglibible. But leakage reactance effects where end of stator core extends past rotor core or vice versa might be a little bigger.

Either scenario seems somewhat surprising / unlikely to me, although personally I'd lean towards 2.

 

[waross]

Will you consider a guess, Pete?
The coupling is holding the rotor off of magnetic center, resulting in an extended or truncated flux in the air gap.
The ratio of core permeability to air gap permeability is so great, that for many calculations and almost all estimates, the effect of the core may be ignored.
So, a 10% stretching or clipping of the air gap may lead to a 10% decrease in inductive reaction, a corresponding increase in current and reduced torque.
Comments?
Discussion?



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Don't get it. Can you post some coupling photos here showing the problem area?

Muthu

http://www.edison.co.in

 

[electricpete]

Bill - I think that is similar to my 2nd scenario. We can just lump it into a bucket of "magnetic effects" which could in theory include decreased locked rotor torque and increased locked rotor current although I'm not sure by how much.

I agree more info from op would be helpful. Some things I'd be interested to know:
[ol 1]
[li]What type of radial and thrust bearings on the compressor? How does oil get to these bearings (oil ring, oil pump, other)[/li]
[li]Was the compressor manually rotated prior to start?[/li]
[li]... if not, how long had the compressor sat since last rotation?[/li]
[li]Was any attempt at manual rotation performed after the trip but before uncoupling?[/li]
[li]What is your confidence level that the machine actually rotated, and have you reconciled that with the relay?[/li]
[li]Have you or do you plan to do inspections of compressor thrust bearings[/li]
[li]Have you or do you plan to do inspections of the motor for unusual rub at the bearing thrust shoulders and possible rotor rubbed on a stationary piece due to abnormal axial location. [/li]
[li]EDIT - I would be interested to know more about your motor protection relays, both the one that did trip and the ones that didn't trip. [/li]
[/ol]


Part of the motivation for some of my questions is one thought that it may have been heavy loading of a compressor thrust bearing that never "broke loose". This scenario agrees with the relay label "locked rotor" but requires us to assume eyewitness reports of rotation were wrong. I do know that indeed sliding bearings (radial or thrust) can exhibit some very strange behavior in the static condition before they begin to move. As we all know they have boundary lubrication during start and they don't develop a hydrodynamic film until they start rotating. But it goes way beyond that, because the means for delivering oil to the bearing may be dependent on the rotation itself (like an oil ring that drags oil from below the bearing to above). If the machine has not been rotated in awhile, then it may be trying to start dry. That creates very high static friction, and the torque necessary to break it loose depends on product of static friction and loading (which might be high due to this unusual coupling condition). This type of scenario may not leave a lot of visible evidence because friction heating is product of friction torque times speed of relative motion... if no relative motion than no heat. No doubt some readers may be skeptical of me for even proposing this. I do have some experiences / analysis to somewhat support this type of scenario as plausible but I don't think its worth spending much more words on unless anyone is really interested (which i kind of doubt). No doubt I am way into the realm of speculation but just trying to describe one possible scenario (of many).

I would suggest depending on your reliability posture, the last two inspections of compressor, motor (6,7) might be worth more than just figuring out the cause, they may be worthwhile to see if there is latent damage from the event that might possibly come back to haunt you later. Available vibration and temperature indications might be worth reviewing, although in theory a damaged thrust face might not show if during normal operation it acts as the passive/unloaded side of a double acting thrust bearing.