Root cause for motor failure
Root cause for motor failure
(OP)
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
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





RE: Root cause for motor failure
RE: Root cause for motor failure
There's the culprit......
RE: Root cause for motor failure
1. Is this being run from an inverter?
2. Have you disassembled and inspected the bearings yet?
The rotor crash may be a secondary effect of bearing failure. EDM damage to motor bearings is a well known phenomenon in motors run from VFDs, but even if not, there are other causes of bearing EDM damage that can be considered. It can be mitigated, but the proper steps must be taken, which means identifying the problem correctly.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Root cause for motor failure
This results in bearing housing wear,
Eventually the rotor catches the stator.
RE: Root cause for motor failure
To validate whether it is enough to cause contact, have you done measurement of the as-found bearing housing bore? How loose is it?
How many motors are we talking about... one motor or several motors with ideantical symptoms?
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(2B)+(2B)' ?
RE: Root cause for motor failure
Only in one case we have seen EDM damage in bearing. Please see attached image. The image is from SKF catalog, mentions current passage when the bearing is not running. A similar observation is seen in Drive end inner ring, outer ring and balls of ball bearing.
How will the current pass when the bearing is not running and later cause the rotor to touch stator.
Thanks,
RE: Root cause for motor failure
But to address this new question, EDM damage on a stationary bearing, as shown in that photo, is typically the result of something like welding taking place on a machine with inadequate grounding, or the grounding has too much resistance, so the current travels through the bearing to ground.
EDM damage in a ROTATING motor bearing is called "fluting" and usually looks like this:
Obviously once your bearing races look like this, it becomes much more easy for the rotor to rub on the stator.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Root cause for motor failure
Thanks for that post Jeff, often times bearings are only replaced and not taken apart and examined for this type of failure. Nice heads up!
Chuck
RE: Root cause for motor failure
It is usually a welding-like current that causes a damage like that.
But it is not very often caused by welding as such. There is a presentation that can be found if you google "ge bearing currents" and in that presentation frame 16 shows an example of such a failure and the failure mechanism behind such a damage is shown in frame 9.
An insulated coupling is one way to prevent such damages. Two insulated bearings is another way to avoid it. Yes, you need two. If you insulate one bearing, the current goes through the uninsulated one.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Root cause for motor failure
Found your link, it is very informative. http://www.gke.org/presentationer/files/GE%20Beari...
I don't understand your two bearing theory though, wouldn't the second non-insulated bearing self destruct essentially presenting another problem?
Thanks, Chuck
RE: Root cause for motor failure
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Root cause for motor failure
http://www.timken.co.uk/en-us/Knowledge/ForMainten...
RE: Root cause for motor failure
jraef Question:
Is this being run from a VFD or not? I do not know. The connection is from a control unit behind generator. We do not have access and information to this.
EDM failure mode is seen in one motor. The EDM damage is predominantly seen in DE bearing. Non drive end bearings have EDM damage but to less extent.
My question is when the rotor touches the stator windings does the current flow from the stator windings through rotor shaft to ball bearings?
RE: Root cause for motor failure
The stator iron is solidly connected to the motor frame. And the end-bells/bearing housings are also solidly connected to the motor frame. So, there is not much that can happen electrically when rotor contacts stator iron. I would answer you question with a NO, it cannot.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Root cause for motor failure
1 - Poor initial centering.
2 - Wear at housing ID.
3 - Wear internal to the bearing.
4 – Rotor bending (result of unbalanced magnetic pull or operating near flexible rotor critical). I tend to discount this cause for your case because the rub was near the end of the machine rather than the center.
5 – Small airgap by design can make machine more susceptible to small variations above.
It doesn’t seem we have full info available. You’ve mentioned “motors’ many times. It’s not clear how many motors have rubbed and which symptoms are seen on those that rub.
A methodical approach would be to take a motor that rubbed and do some checks:
1 – Before even disassembling the motor, push pull on the rotor shaft extention and measure shaft movement radially with dial indicator. This is just extra info
2 - Remove rotor from stator but don’t remove bearings from rotor.
3 - Measure bearing internal clearance in radial direction. i.e. with bearings still mounted on rotor, push/pull on the bearing and measure movement in both directions with dial indicator (rotor solidly mounted on test fixture to prevent movement).
4 – Measure bearing OD. *
5 – Measure housing ID. *
* to be thorough, stand procedure would be to explore variability with 6 measurements: 3 angular locations (60 degrees apart) and 2 axial locations (1/4 distance in from each end).
6 – Measure rotor ID in vicninity of the rub. 3 locations 90 degrees apart. Measure at other locations for comparison.
7 – measure stator bore in the vicinity fof the rub. Measure at other locations for comparison.
8 – Attempt to determine deviation from concentricity of housing bore and stator bore. This is tough for a number of reasons and certainly complicated by rough geometry of stator bore. I’d talk to my machine shop about what they can do. Some options:
8A – with drive end endbell mouted to stator: Put on mill capable of rotating both. Center the fixture using one surface and indicate off the other surface.
with drive end endbell mouted to stator: with drive end endbell mouted to stator:
8B – with drive end endbell mouted to stator: use 3Dcoordinate measuring device.
8C – if cannot do either of above, might measure stator and endbell separately, comparing each one to the rabbet fit of endbell to stator.
With above info, you should have a good basis for making a determination.
If the cause does not jump out at you, you can take similar measurements at the ODE for comparison to help highlight measurements that are out of whack on the DE.
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(2B)+(2B)' ?
RE: Root cause for motor failure
Assuming your motor is already disassembled, I’d start with:
housing ID checks compared to bearing od to see how much clearance is there.
Compare to airgap measurements based on od of rotor and id of stator in area of rub.
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(2B)+(2B)' ?
RE: Root cause for motor failure
9 – check for clearance at rabbet fit between stator and endbell: Specifically, with endbell inserted onto stator, but not bolted tight (bolt loosely if needed), attempt to push endbell radially and check for movement with dial indicator.
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(2B)+(2B)' ?
RE: Root cause for motor failure
My reference was to your comment in the post just above mine, third paragraph
" An insulated coupling is one way to prevent such damages. Two insulated bearings is another way to avoid it. Yes, you need two. If you insulate one bearing, the current goes through the uninsulated one."
Are you suggesting this from field experience and found this technique to bleed off current without damage to either bearing?
By the way, could using the wrong grease contribute to the problem the OP has?
Thank's Chuck
RE: Root cause for motor failure
No. This is not about bleeding off any current. It is about stopping a welding current or, more likely, a ault current caused by an unwanted potential difference between the PE system and the bulk of a plant. Picture 9 in the presentation shows such a case.
There is a fault current flowing from failure to the earthing bar. From the earthing bar, most of the fault current flows to next ground point (or to main ground) while a lesser part (arrow width is proportional to current) flows to the motor's frame and then via the DE bearing to the grounded machine.
There are two ways to avoid damage in such a situation: The first is to use an insulated coupling and the second method is to use two insulated bearings. One insulated bearing will not work because the current then will flow through the non-insulated bearing.
Remember that the presentation shows all four fundamental failure mechanisms. The one in the example is the so-called "Frame Voltage" mechanism. It has nothing to do with inductive or capacitive coupling.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Root cause for motor failure
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.
RE: Root cause for motor failure
I also forgot to suggest checking rotor TIR.
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(2B)+(2B)' ?
RE: Root cause for motor failure
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?
RE: Root cause for motor failure
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)
RE: Root cause for motor failure
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)' ?