Failure in 1200 kW 6 kV motor 1

[petronila]

Hello All

I would like to know your inputs about this failure.

In the attachment you will find some pictures.

Thanks and regards

Carlos

 

[Skogsgurra]

No rotor pictures?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[petronila]

No unfortunatelly we only receive the stator, the rotor no.

Regards

Carlos

 

[Skogsgurra]

Looks like something got into the air gap and did a lot of damage. Isn't there even a trace going all around the stator surface?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

What material are the slot wedges made from? Looks like most of them are missing to a greater or lesser degree. Where did they go?


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If we learn from our mistakes I'm getting a great education!

 

[petronila]

Hello Scotty and Gunnar

The stator is earthed, the wedges are magnetical

Regards

Carlos

 

[electricpete]

The observations are:
Observation 1 - Wedges missing. They do not appear to have moved axially out end of slots, since all remaining wedges come to the same axial distance along the end of the stator.
Observation 2 -Evidence of foreign material in airgap damaging both wedges and top of teeth (picture upper right hand corner).
Observation 3 – Some blackening of end turns connection-end upper left hand corner


If we assume items 1 and 2 are related, the obvious ties might be:
Scenario 1: Foreign material caused wedges to come loose.
Scenario 2: Wedges are foreign material which caused the damage.

I have also seen a scenario 3 where rotor contacted stator and abraded the stator tooth tops. This created a lot of localized heating which baked the wedges in that area and made them fall out. But that damage was limited to one area of the stator. And the signs of dramatic overheating don’t seem evident here.


Being far away, any guess is just a guess. My swag would be scenario 2, because:
A – magnetic wedges are notorious for coming loose under vibration, especially if not vpi’d (has this machine been vpi’d)
B – the missing wedges seem very much spread out, might be expected if caused by vibration and poor retention. I would expect if the main problem was foreign material it might be more localized. Granted there is a localized area of higher damage, but …

Followup questions:
Are there any other noteworthy visual observations not described above in observation 1, 2, 3.
Can you describe the machine type? I don't recognize the radial spoke-looking things outside the stator in the upper left hand picture.
What do electrical tests look like on stator (balanced resistances, grounded?).
Was stator core hot-spot test done.
Can you provide closeup picture of damaged area of stator core toothtops.

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(2B)+(2B)' ?

 

[ScottyUK]

There's a fair bit of detail in the top right picture if you zoom in. The slot wedges appear to be GRP, not metallic - I can see the woven pattern of what looks like the glass reinforcing mat showing through the surface where it has been worn down.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Yes, it definitely looks like weave of some kind of glass cloth within those wedges. And the few magnetic wedges I have seen are smooth (like made from powder, similar consistency to ferrite magnets). But I looked in the Gund Electrical Materials catalog and apparently there are at least two types of magnetic wedge material that includes glass cloth (data sheets attached). So I don't think that rules out mag wedges. Maybe Carlos can check with a magnet.

On a different subject, this guy Henk has some interesting discussion of wedge-ology:

http://www.qtime.co.za/Paper_Electric_Motor_Failure_Wedges.htm

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(2B)+(2B)' ?

 

[nawao]

To make an accurate diagnosis is necessary to see the rotor pictures

 

[ScottyUK]

Thanks ePete - hand't seen that slot wedge material before. Interesting.


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If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

Finally got time to read the link too - some good practical insight into wedge design. Some of the photos lower down the page (pic 10 to 13) show similar failure modes to the motor in Carlos' motor shop.


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If we learn from our mistakes I'm getting a great education!

 

[petronila]

Pete: Very good information.

I am with your theory: " Scenario 2: Wedges are foreign material which caused the damage", the wedges are magnetic type and the motor was not VPI. The problem is characteristic of windings with magnetic wedges. Some heavier, dense magnetic wedges can work loose, when the wedges fall out of the slots, with the motor in operation the wedges are contacted and fragmented by the rotor and driven right to the exposed stator coils causing a ground fault.
The pole-pass magnetic pulsation could causes it.

Regards

Carlos