3-Phase Induction Motor starting problems (Locked Motor & abnormal loud sound) 9

[TTojiiLee]

3-Phase Induction Motor starting problems (Locked Motor & abnormal loud sound)

When starting the motor, there is a problem in which a loud noise occurs as soon as power is applied
and the motor rotation is restricted(locked).

The driven machine is a integrally geared centrifugal compressor(3-stage, 2-pinion) and
I guess that the problem is caused by compressor jamming and excessive inrush current at DOL(Direct on line) startup.

If there is no jamming in the compressor(driven machine) or normal conditions, it is possible to start and operate normally without any problems.

When jamming of the compressor (possible seal, thrust collar, impeller, bearing, etc.) occurs,
the inrush current increases more than normal conditions and this problem is occurred a very rare phenomenon.

Can you tell how much the inrush current can cause this problem(Locked Motor & abnormal loud sound) from the attached motor data?

 

[crshears]

Might it be cheaper to just add some sort of quick and dirty turning gear to get the unit spinning slowly before applying starting current? No more dead spots.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[electricpete]

Also I wanted to clarify - was the shaft visually confirmed to be stationary during any of these failed starts? Or is it just a supposition.

I could not see the starting problem situation visually, but based on our engineer said and I checked all compressor logging data for confirming motor locked

Thanks. I'll just mention my thought process in asking the question. IF static friction causes the rotor to completely lock and never move, then it might not be surprising that this kind of event would leave no evidence during inspection of the bearing (energy dissipated is friction torque times relative speed... which is 0 if the rotor doesn't move). On the other hand if motor rolls a little bit but never gets going due to bearing friction, I'd expect you would see a lot of evidence of that event during subsequent bearing inspection.

The coupling is flexible disc coupling.

That makes the scenario of locking due to thrust less likely imo. It doesn't hurt to inspect the diagphrams as edison mentioned. The same logic as discussed above for radial bearings might also apply thrust bearings (it's less credible that thrust bearing could interfere with a startup and not leave evidence during subsequent inspection if the rotor actually moved).

I think any of the above mechanical scenarios is unlikely.

I'll mention and even less likely scenario that was not brought up is rotor / stator slot combinations that can occasionally cause anomalies in motor torque which manifest as locking, cogging or crawling during startup. For the most part these types of problems were solved a long time ago by motor manufacturers. Is this a new motor? Recently redesigned? Is it part of a family of identical motors? (if so how are the others doing)

Trying to think back toward the more likely scenarios and actionable troubleshooting:
[ol 1]
[li]To check condition of the rotor I'd suggest current signature analysis under load. Failing that high-resolution vibration spectrum under load looking for pole pass sideband (whose presence is not particularly conclusive to indicate rotor bar problems but whose absence is more definitive to rule them out imo). At least those are things relatively easy to do on an installed motor. There are other things mentioned that can be done in the shop.[/li]
[li]As far as contactor operating and voltage balance. I'd start with looking at what electrical parameters are available. Do you have indication or trend of voltages (maybe it is low, which decreases torque). All three phase to phase voltages (maybe it is unbalanced, which decreases torque). Also current under load can be an indirect indication of voltage balance but requires some care in interpretation. Do you have digital relays that might have captured any details about the starting transient itself?[/li]
[li]Maybe inspect the contactor.[/li]
[li]If it were our plant we might be inclined to "instrument" the machine so we could catch it in the act the next time this event happens although that's not necessarily easy. You might also gain intelligence during successful starts. In particular does the starting time vary significantly among starts and do those variations correlate with anything else. Also if the contactor is operating irregularly, you might possibly see evidence of that even during a successful start if the current in two phases appears long before the third.[/li]
[/ol]

 

[TTojiiLee]

This abnormal starting problem sometimes occurs unpredictably without mechanical damage.
And after the starting problem occurs, it starts up normally and there is no problem at all with no-load / load operation without mechnical damage (compressor(bearing/gear teeth/pinion thrust collar/shaft/impeller/diffuser/labyrinth seal(just slightly seal touch),etc), flexible disc coupling, and 3-phase squirrel case induction motor(bearing)).
The no-load/load compressor operation logging data at motor rated speed(2pole) was good and , such as pinion vibration, motor winding temp/bearing temp/current.
The presence of abnormalities in the rotor bars mentioned above could not be confirmed.

When this problem occurs, I think it is fortunate that the train, motor/coupling/COMPRESSOR ,did not rotate (or moved very, very slightly), which means that mechanical damage to the compressor parts did not occur.
The motor, coupling and compressor is new with a track record.

I'm checking all possibilities, including the motor factors, compressor factors, electrical factors and combined mechanical and electrical factors. I'll discuss the motor manufacturer next week and I'll continue to inspect and review.

Thank you again.

by TT

 

[waross]

The most probable cause is a faulty contactor that single phases from time to time.

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

 

[zlatkodo]

If there's only one motor involved and if this has been happening from the very beginning since it was installed, it's possible that there's a weak winding design issue, especially on the rotor.
Something might have gone wrong with the custom motor design.

ACW