Motor Failure Cause

[nightfox1925]

If I have a motor failure and the breaker was tripped and one pole contact of the 3pole contactor is welded, what would be the possible cause? ground fault? single phasing?

If a motor trips during running and at normal current, I am under the impression that it is caused by excessive heating due to mechanical failures like bearing temperature...any other possible causes?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[dpc]

The breaker tripped or the overload relay tripped?

I'd suggest a basic megger test on the motor windings as a first step.

It could be a lot of things. The welded contact could indicate that it tried to open under fault conditions, or maybe it welded closed the last time it was started.

 

[LionelHutz]

The motor likely had a phase to ground failure on the same phase as the shorted contactor pole. The fault current likely welded the contactor pole.

The motor overload relay should not trip if running with normal current. So, something like an interlock or a winding temperature relay had to cause the trip.

 

[jraef]

Assuming 3 phase because you asked about single phasing, but define "breaker". MCCB? Magnetic only, or Thermal-magnetic, or Electronic? Or one-piece Motor Starter Protector with built-in adjustable OL settings? Or was there a separate OL relay and did it trip/not trip?

Generally though, LionelHutz would be right. Single phasing or a phase-to-phase fault would likely show 2 poles damaged. Contactor chattering can sometimes cause contacts to weld, but if only one pole was welded as a first point of failure, that would not usually cause the breaker to trip because there would not be a complete circuit path for anything to cause the trip.

 

[ELEP]

If motor failure means ground or phase fault. Simultaneous fault and single phasing is also possible. Analyze DFR data.

If during normal running condition single phasing occures motor will damage as one of pole is stuck.

 

[nightfox1925]

Thanks for the valuable insights gentlemen. I will look deeper into the issue at hand here with all the guidance I got from all of you.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[electricpete]

If a motor trips during running and at normal current, I am under the impression that it is caused by excessive heating due to mechanical failures like bearing temperature...any other possible causes?

Sure: moisture intrusion (did the failure occur shortly after startup, during a storm, failure point near air inlet), vibration, poor end-turn blocking allowed excessive movement eventually leading to turn-to-turn short (is the evidence of end-turn movement), inadequate coil spacing on end-turns of a high-voltage motor 6kv and above (is the white powder at various places around the endwindings), voltage surges (was the failure point at a line-end coil), manufacturing defect (do you know which coil is the throw coil... often extra stresses during manufacture on that one).

You might make a more educated guess with careful testing and inspection of the failed motor along with gather details such as whether motor had just been started, any available trends of winding temperature, vibration trends etc. Cut the coils apart at the location of the failure and other similar locations to check for voids in insulation etc.

And there really is a lot more you can look at and analyse. And even gathering all that data, the best you can do is usually a guess.

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[cranky108]

I see some discussion of a possible phase to ground fault, but it is also possible to have a turn to turn fault where the motor would draw excessive current on one phase.
One case I've seen was tripped on a 87 relay.

 

[jraef]

I see some discussion of a possible phase to ground fault, but it is also possible to have a turn to turn fault where the motor would draw excessive current on one phase.

Yes, but it wasn't one phase, it was one pole of the contactor that welded. Usually in that scenario you would have damage on 2 poles of the contactor.

 

[cranky108]

Are you by chance confusing phase-to-phase with turn-to-turn?
A turn-to-turn fault can involve only one phase.

 

[jraef]

Ooops, read something into that which wasn't there...

 

[cranky108]

Actually, I don't think the 87 should have opperated, maybe it was a saturated CT that made the 87 opperate.

 

[LionelHutz]

Well, I somewhat agree with Jraef there. You really can't get a turn to turn fault that causes high current on one phase without also seeing higher currents on the other phases. And, a fault that causes higher current but not fault level currents is not as likely to cause welded contactor tips.

 

[powersoff]

I recently had experience with 1 pole of a Nema style Motor Starter melting. The problem in this case was the wire terminating at the coil was loose,causing the starter to chatter.