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Failure in 1200 kW 6 kV motor

Failure in 1200 kW 6 kV motor

Failure in 1200 kW 6 kV motor

(OP)
Hello All

I would like to know your inputs about this failure.

In the attachment you will find some pictures.

Thanks and regards

Carlos  

RE: Failure in 1200 kW 6 kV motor

No rotor pictures?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Failure in 1200 kW 6 kV motor

(OP)
No unfortunatelly we only receive the stator, the rotor no.
 
Regards

Carlos

RE: Failure in 1200 kW 6 kV motor

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

RE: Failure in 1200 kW 6 kV motor

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?
  

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

RE: Failure in 1200 kW 6 kV motor

(OP)
Hello Scotty and Gunnar

The stator is earthed, the wedges are magnetical

Regards

Carlos

RE: Failure in 1200 kW 6 kV motor

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.

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

RE: Failure in 1200 kW 6 kV motor

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.
  

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

RE: Failure in 1200 kW 6 kV motor

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

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

RE: Failure in 1200 kW 6 kV motor

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

RE: Failure in 1200 kW 6 kV motor

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

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

RE: Failure in 1200 kW 6 kV motor

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.
  

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

RE: Failure in 1200 kW 6 kV motor

(OP)
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

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