Root cause for motor failure 2

[paramathma]

Hello experts,

We are receiving failed electric motors from field. The motor is connected to a gear pump.
Winding open is observed after checking the connectivity.

Important observations after disassembly of motor
Contact marks on drive end side of rotor and stator see attached figure
Outer ring of Drive end bearing rotating in the housing
Dark color on rotor and stator only on the drive end

With the above information, please help me in identifying the root cause

 

[theode]

I notice from you first motor pic,That the motor itself is a cheap "off the shelf" Job.
They are made in Italy and different companies attach their own nameplates to them.
In effect,a throw away motor.
The casing and endsheilds are made from aluminium (brit spelling) which is quite soft and expands easily when warm.
Its often cheaper to replace them than repair.
I wouldn't recommend for heavy duty work or constant running.
Cast iron is more robust but it depends on your environment.
Just a thought.

 

[electricpete]

Good points. Movement due to distortion of a flimsy frame under external load (example: belt drive) would not show up during the checks I suggested.

I also forgot to suggest checking rotor TIR.


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(2B)+(2B)' ?

 

[paramathma]

Thank you all for the information.
In this particular case along with rotor touching the stator, one wire in stator is open. Please see attached picture. Is it possible for the current to flow through this open wire to bearings via rotor shaft?

 

[theode]

The open wire as you call it,is a broken circuit in one of the motor coils.
Depending on how the coils are wound gives a different result.
If single wire wound,it will show a shorted coil or an open phase.
If open phased there will be difficulty in starting and rotation.
The two other phases will take excess current and will not last.
If parallel wound coil,part of the coil will be open causing imbalance in that phase.
Either way ,it's not healthy.
It wouldn't cause your bearing failure.
Why is the wire broken? Caught when removing rotor? Blown?(look for burn marks)

 

[electricpete]

You’re identifying one lead open on the endturns. Current shouldn’t flow to ground unless it were in contact with the rotor. It doesn’t look like rotor would contact this location. But if it did, then I’d think much more likely that the loss of geometry control caused rotor to contact winding and cause the electrical fault, rather than electrical fault cause bearing degradation and loss of geometry control.

In theory, an open circuit causes an unbalance in flux among three phases which (if continue to operate) can drive circulating currents through the bearing. The potential for bearing current due to flux unbalance is one of the reasons that some large non-vfd motors have one bearing insulated. But it is a long-term mechanism, typically evident in vibration and noise. Unless there is something to steer in that direction, it seems like a longshot to me given that you have already seen plausible explanation (loose bearing in housing, as yet not quantified). It’s not clear to me whether you actually saw the fluting during inspection of bearings of this particular motor or some other motor.

Just my reaction fwiw and I may be missing something. I do appreciate the thought process trying to connect together the two different symptoms (rotor/stator rub and open circuit) and it’s a good question to ask whether/how these two symptoms are related to each other.


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(2B)+(2B)' ?