Motor Failure 4

[VTer]

All,
Please see attached photo and let me know if you can identify the root cause of the failure.
Motor data is: 10HP, 460V 3-ph RPM 1755 with Class F insulation


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[VTer]

One more picture

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[jraef]

Looks like you had the rotor crash into the stator, then you have winding damage.

I can't think of a way that winding insulation damage could make the rotor crash. So most likely that happened before the winding damage, which is entirely plausible.

But what made the rotor crash? Likely a bad bearing.

So my best guess from the limited info: bearing self destructed and possibly flew to pieces, which caused the rotor to hit the stator, which caused a severe overload and smoked the motor winding insulation. Steps one and two could have transpired in a matter of seconds, but step three would have taken time. Did you not have a properly functioning overload relay?

"Will work for (the memory of) salami"

 

[VTer]

Hi jraef,

The overload relay did operate. It was set to 14.4A with a motor FLA of 12.7A and SF of 1.15.

I believe the marks in the middle on the stator are from the insulation melting through the slots and dripping on the rotor. If actual contact did occur due to bad bearings I would expect to see physical evidence closer to the edges of stator – please correct me if I am wrong. Looking at the overall picture, it seems that the majority of overheating happened on one slot coil that then damaged the surrounding coils. I am just not sure what can cause such overheating in one coil only?




"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[Marke]

Looks like only one coil has failed and this could be due to a shorted turn within that coil. Basically, an insulation breakdown turn to turn within the coil causing an overload in that coil only.

Mark Empson
Advanced Motor Control Ltd

 

[Motorwinder]

It blew to ground in the slot. See this all the time.

 

[electricpete]

Just trying to fill out the details more.
The first picture – we are looking at opposite-connection end of 4-pole motor?
Rotor rotates CCW from this end? (OCE?)
It looks like debris coming from slots at 1:00 and 10:00 position relative to top of picture,... this is same coil?
Blackened area above includes the endwinding for this same damaged coil?
We can sort of form the picture that this one coil carried high current. I’m not particularly familiar with the winding.. I think it’s concentric two layer and the failed coil is bottom layer? (Maybe Ray and Motorwinder and others can talk about the winding some more... I have a hard time understanding what goes where on a concentric double layer winding.)

The scenario that comes to my mind - maybe turn to turn short in the endwinding away from ground so did not trip but drew increased current until slot section heated and caused trip.


Just a thought. At our plant we have ground protection on motors fed from grounded system, so fault in slot doesn’t last long. Of course we have also have ungrounded 480 volt systems so no ground protection on our 480 volt motors.
This motor fed from grounded power system? Did it have ground protection?


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(2B)+(2B)' ?

 

[electricpete]

Should've said: my thoughts are in line with Mark's.

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(2B)+(2B)' ?

 

[powerjunx]

seen this case a lot of time.. looks like worn out bearing. But good point, Mark!



"..the more, the merrier" Genghis Khan

 

[rhatcher]

I think Marke got it right. A single coil shorted and overheated. Also, VTer correct that the marks on the stator are from burned material from the shorted slot being dragged around the stator by the rotor.

 

[VTer]

Thank you all for your replies.

Electricpete, you are correct – it is the same coil. The extent of damage in this particular coil is pretty uniform throughout. The end turns look worse because of overheating in this one particular coil that then damaged surrounding coils.

The motor was part of a cooling unit and it had standard manufacturing protection (fuses). It drove a blower via belt and sheaves. I did notice that the end turns on the most damaged side of the motor contained contaminants from belt dust - not sure why the manufacturer did not choose a totally enclosed motor. Attached is another picture on the end-bell side of the motor – it looks much cleaner than the more damaged drive side.

All evidence points to a turn-to-turn fault. This motor is about 12 years old also, but I would like to try to determine the root cause of the failure. Can bad bearings cause a fault like this? My suspicion is on the combination of age and v-belt dust contamination preventing proper cooling.

Thanks again!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[VTer]

By the way, in the last picture that I attached the bad coils on the back of the motor are also noticeable where leaving and entering their slots.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[electricpete]

Looking for cause of suspect turn to turn short... always a challenge. More data is always better and in the end it is almost always still a best guess.

Electrical measurements may or may not give better picture to double check if rest of winding is intact.

Try to figure out if the failed coil was adjacent to the line terminal. Surges which stress turn insulation are strongest at that locatio/n. Failure in center or neutral of winding is not as likely from electric surge induced turn failure although could be mechanical.

Surge test non-failed portion of the motor. If phase comparison is not possible, use a waveform comparison while increasing test voltage. If surge test of non-damaged portion fails at low voltage, that would tend to feed a view that you have weak turn insulation. In contrast if it passes through to very high voltages, seems to suggest you have strong turn insulation. Actually endwinding contamination is a subset of poor/strong turn insulation. It is certainly possible that conductive endwinding contamination contributes to turn fault by bridging between expected defects of the magnet wire insulation (NEMA allows so many defects per 100 feet). Ordinarily I'd think belt dust would not be a problem unless perhaps it combines with moisture (and even then I'm not sure if it would be conductive).

As a general principle (at least for large form wound motors... not sure about random)- vibration is somewhat of an enemy of turn insulation along with looseness of the coils and blocking. There is no obvious visual evidence of endwinding movement.... certainly no broken ties other than the fault location. Also tightness in slot tends to support endwindings against movement. Working with large form wound motors, it's a little surprising to me that I can see a big gaping hole between the coil and the slot liner in your motor. But I think the nature of the winding is that some slots house two coils and some slots have only one (correct?). In that case it is unavoidable to have big gaping hole in the one-coil slot.


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[petronila]

Hello Vter

Looks like Inverter duty motor, so this kind of windings are manufactured with special inverter resistant spikes wire enamelled with special coating .

I.M.O the failure is not turn-turn short circuit, the insulation is melted and looks like single phase operation, the pattern failure is not the habitual for single phase operation

and the winding don ´t burns off totally due to the overcurrent Protection trip, the marks on the stator belongs to the contact of the exploded wires with the rotor in movement.



Regards


Carlos

 

[electricpete]

I didn't think about single phasing. But wouldn't that result in more coils overheated than just one?

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[rhatcher]

I would definitely rule out single phasing. If this was the case you would see four coil groups, single phase for a delta connected winding, or eight coils groups, single phase for a wye connected winding, burned out.

In my opinion Pete has nailed it. I agree with his recommendations. A surge test of the other phases and a determination of whether this coil was a lead coil could give an indication of whether this was a general insulation failure due to age or whether this was a surge related failure. Also, a 'surge related' failure on a lead coil could also include VFD related failure as well as being caused by lightning or utility switching/reclosing on a motor that operates "across the line".

It is interesting to note the following:
- the failure appears to be on the end turns of the winding that is shown in the first set of pictures. It is not apparent whether this is the drive end or the opposite drive end.
- the second photo (10 Jun 13 9:44) does not appear to show any end turn damage and also shows lead connections. I have occasionally seen this before with two circuit motors (2-wye or 2-delta) where one circuit is connected on the drive end and the other is connected on the opposite drive end.
- the conclusion is that a surge related failure (vfd or utility surge) would affect the end turns of two coils, being the lead coils of the two circuits (2-wye or 2-delta). If only one coil was affected then that implies that the cause may be something else. A likely cause that would affect only one end of an open motor would be abrasion caused by intrusion of foreign material into the winding. The end that is affected could (would) be influenced by the proximity of the driven equipment/load or by the direction of the flow of the internal cooling fan.

 

[rhatcher]

BTW Pete, LPS for you.

 

[VTer]

I agree with Pete as well on the single phasing. Gentlemen, only one coil is damaged and the damage seems pretty uniform throughout the coil.

Rhatcher, majority of the damage that occurred is visible on the drive end of the motor end turn, but I think that is only because that coil is more visible. The end bell winding end-turns appear much cleaner but the bad coil is buried below other coils and I could not get a clear view of it to take the photo. Also the back view is also much cleaner in terms of belt dust contamination on the windings. However in the last photo it can be seen that the conductors exiting the slots on that side are pretty bad as well. This motor is not driven by a VFD – but surge related failure is definitely a possibility in my opinion. I also agree that the drive side looks much worse than the back side of the motor but I will try to get to the end-turn of the bad coil on the back side and take some more photos for you.

Also, if it was a turn-to-turn fault I do not think that GFP, even if we had it, would have picked it up until it escalated into a full ground-fault. Not sure if the overloadrelay or GFP would be more sensative in this case? We have multiple units with this setup and I would like to see if we can add some other form of protection to help prevent this type or extent of failure.

Thank you all for your input!



"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[VTer]

As promised, attached is a picture of the back side of the motor of the same coil– the damage in this end turn is almost identical to the end-turn on the drive side of the motor.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[electricpete]

Thanks Ray, good comments from you as well.

One other thought comes to mind, if you have turn to turn short, there is surely somewhere in the coil where melted copper will be obvious. It doesn't seem obvious from recently-posted closeups of connection end. It was harder to see what was going on in the pics from earlier-posted photos of the opposite connection end. If you haven't found that location of melted copper yet looking at the endwindings, look for it when you pull the windings out of the slot for rewind. And look around that area of melted copper for any obvious clues (chances are you won't find any, but worth looking).

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