3 Phase Motor premature bearing failure 5

[elfman]

We have a 480VAC 3 phase, 1/2 HP motor that has failed in a very short period of time. The bearing are making a lot of noise. The site is telling us that the motor current is within the normal range. We don't think it is gear drive to which it is connected. My question. I have been putting phase monitors in a lot of our controls as required by spec. I understand the concept of the 3 power phases potentially getting out of phase, but what does that do to the motor? What would the symptoms be?

Thanks in advance!

"To be or not to be, that is the question" - William Shakespeare via Hamlet

 

[SceneryDriver]

elfman,
Is this motor on a VFD by chance? If so, you may have a bearing current issue that needs addressing through the use of an insulated bearing and/or a shaft grounding brush arrangement.

I have read mention of bearing currents being a problem even with DOL started motors in facilities with lots of non-linear loads (there was another thread here recently that addressed this, but I can't find it right now).

To answer your question regarding phase monitors, they will tel if you loose a leg to the motor; an already rotating motor can still continue to turn with a dropped phase, albeit with lower torque and much more generated heat. I don't believe a dropped leg would damage bearings.

Have you taken the bearing apart? Does it appear to have "fluting?" Have a look at the attached white paper from Baldor regarding bearing currents and the issues that arise.


-SceneryDriver

 

[electricpete]

I understand the concept of the 3 power phases potentially getting out of phase, but what does that do to the motor?

Never heard of it. Can you explain what you are referring to.

There are many possible causes of bearing failure. Best approach is look at the bearing for starters.

=====================================
(2B)+(2B)' ?

 

[elfman]

I guess my understanding is flawed then. I thought the ??% that is set on the phase monitor was to detect when the incoming phases are no longer 120° apart. Does is simply monitor if you loose a phase and thus alarm? I do agree we need to have a full bearing analysis done. We currently do not have access to the motor, but when we gain that access, I am sure a bearing analysis will be performed. Thanks.

"To be or not to be, that is the question" - William Shakespeare via Hamlet

 

[sibeen]

elfman, I suspect you've got it in the back of your mind, or heard somewhere that an unbalanced voltage causes problems with motors. The major problem here is with heating effects. When a three-phase induction motor is supplied by an unbalanced system the resulting line currents show a degree of unbalance that is several times the voltage unbalance. The motor acts as an induction generator and tries to balance the voltages. Hence the high currents when motor voltages are unbalanced. Motors will also draw excess current trying to correct any phase shifts in the supplied voltage.

I've pulled the below figures from some notes I had lying around.

% Voltage Imbalance ------- % Temperature Rise
2% ------------------------------ 8% hotter
3% ------------------------------- 18% hotter
4% ------------------------------- 32% hotter
5% ------------------------------- 50% hotter

 

[jraef]

You still have not answered SceneryDriver's question pertaining to VFD use. There are well known and documented issues with regards to bearing damage from VFD use, but there are also several ways to mitigate it.

If there is no VFD in use, then the second most common cause of bearing failure in an otherwise well maintained motor is misalignment. The longer the motor runs, the more critical the alignment becomes and for motors that must run continuously for long periods, the best practice is to get a laser alignment now.

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[elfman]

Sorry. No VFD - disconnect, contactor, overload.

"To be or not to be, that is the question" - William Shakespeare via Hamlet

 

[mikekilroy]

no vfd, no bearing leakage current issue. besides, I never heard of bearing damage on a motor less than 50 or 100hp with bearing currents - you gotta have a BIG motor with lots of current to get bearing damage; I think if you current damage on a 1/2hp motor you would have the worlds first.

you said you don't think the gearbox attached is the issue.... as Jraef said, you most likely have a shaft alignment issue.

if it were phase, voltage, or any other electrical thing, the motor would get hot and damage would likely be to windings, not a mechanical noise issue. mech noise = bearings.

only other likely reason is you have one of those $ 20.00 Cheap motors with cheapo bearings that cannot survive more than a few hundred hours.

You have an alignment issue.