Attached are photo's and thermal images of the as-found (pre-burnout) condition of the stator of a 13.2kv 2500hp horizointal sleeve bearing motor.

The repair shop emailed comments to the effect that we have 13 broken or missing magnetic wedges and it looks as if there was a bearing failure at some time that burnt away the wedges.

We have no record of a bearing failure. I didn't realize these are magnetic wedges.

I'm still trying to figure out what to make of this. I guess it does look to me like there has been contact/rub. But I'm pretty sure we've had no bearing failures. I'm thinking it could have been a single momentary contact during a motor start that went unnoticed? Or less likely some foreign material got drug in between the rotor and stator at one point?

Does it look like burning also? Which color represents burning? I guess if there is burning, the cause would be from heating of the shorted laminations during operation since the rub occurred? Does it make any difference if they are magnetic wedges?

Any other thoughts about what it looks like and what could have caused it and any early guesses on the likelihood that the core can be repaired without restack (of course we'll know better after we burn out the core and try some repairs).

77465_as_received.pdf (626 Kb, 76 downloads)

It looks to me like the wedges have failed in the area in the picture and may have raised to contact the rotor. I don't see any evident signs of rubs on the laminations. You should be able to confirm a rub by finding a matching rub on the rotor iron if there were metal to metal contact.
I doubt that there was contact during a start because nothing in the motor should be allowed to flex enough to let that happen.
I don't see anything that really looks like burning to me but the core loss test shows a large area of shorted iron.
I've attached a report I came across about a large motor that experienced magnetic wedge failure. It may shed some light.
Since IR is so good at identifying the shorted area, and there doesn't appear to be any melted iron, My guess would be that the laminations could be cleared by separating each one without restacking. But that is a serious S.W.A.G. and the motor shop will have a better feel what is needed. When I worked in a motor shop it wasn't uncommon to partially unstack a rotor to clear the bad spot or to even replace a section of laminations.

pubsep2005.pdf (387 Kb, 59 downloads) Motor magnetic wedged failure

Wow. Thanks a lot Joe. That is some outstanding information and a lot to think about. I didn't know much about magnetic wedges, but now after reading the article I know a little more. Like how important it is to use a "wet process" so the wedges don't loosen from magnetic forces. I'll have to ask the shop about what kind of process they are planning to use for the rewind.

It's tough to see what's going on in the pictures. The judgement of the shop people is that there has been a rub from rotor to stator, based on the email I got (which said nothing more than what I quoted above... won't be able to get any more info until Monday). I think I can see it in the pictures also but I take their word for it.

One aspect your comments seem right on target: Where the wedges are damaged, they must have swelled up above the core teeth (there is no way that the core teeth were shaved down to the level of the wedges). I didn't think about that before.

IF there was a rub to the stator iron/teeth, THEN the likely scenario is that we had heating from the eddy currents in the smeared/shorted laminations while the motor was running after the rub. The heating may have caused the wedges to swell up and also contact the rotor. This scenario makes sense to me. I'm not sure I can come up with any other scenario which would explain why we have damaged wedges and core hotspots right at the same location.

Something looks weird on this picture. If I follow the line between lamination packs, I can't see the vent ducts. I see some kind of grey stripe between lamination packs... not sure what I'm looking at. I would assume I might be looking at an angle into the vent duct and seeing the side of the teeth... except the same grey color seems to blend into the area where the wedge/coil should interrupt the vent duct. Can anyone explain that?

I thought the vent area looked weird too, but I also thought it may be the camera angle. It looks like they're completely plugged but I thought it might be an optical illusion. Do you have problems with product dust getting into places it doesn't belong? If the ducts are plugged it would definitely worsen any heating issue. You would think the motor shop would have mentioned if the vents were plugged.

quote:

vent ducts

Hello, everyone!
Thanks Joe for very good information about magnetic wedges failure because I didn’t hear about it earlier.I attached some pictures of our exhaust fan motor (6kV, 2MW) after couple days work in dusty surrounding with open covers on the motor for “better” cooling. You can see that abrasive dust was started to damage wages and if it stayed unrevealing the motor would probably broke down. I think that Pet’s motor had maybe similar problem in the past because vent ducts are full with something strange.
Sorry for my bad English!!

Ekshaustor_dust.doc (2,124 Kb, 23 downloads)

I sure hope EPete doesn't have any product dust floating around
Maybe some floor dust

Dave

Thanks for the responses. Interesting case panta. I have never seen wedges wear away at the vent ducts.

Some more information regarding our motor:
1 - I found out from the shop that the core images above were taken after only two minutes on the core tester (and it was being tested at half-current for watts loss test, not full current for hot-spot test). So those hotspots must have been really sizzling during operation.
2 - Research of history on this motor shows that in 1993, we had a lubrication system problem which stopped oil to both bearings. Got bearing temperature alarm and temperatures recorded as high as 300F and smoke reported coming from the bearings. Inspected the bearings at that time and found them damaged. Replaced bearings and dressed up shaft with rotor in place. Never pulled the rotor to check for stator bore or rotor damage at that time. Motor operated ok since then and was only pulled now for a proactive time-based refurbishment/rewind. We also happen to replace rotors proactively on these machines (the reasons for that are a very long story). Removed rotor was sold for scrap.
3 - Winding was stripped, attempts at cracking were unsuccessful at clearing the core hotspots... we're headed for a restack.

I forgot to ask the shop to explain the appearance of the vent ducts to me. I'm sure it is just a matter of a strange angle but I'll try to remember to ask about that.

This message has been edited. Last edited by: electricpete,

Did you end up restacking this stator? I think I would've stacked new laminations instead of restacking the old ones.

We ended up stacking new laminations into the existing frame. Our shop calls that "restacking". I'm not sure if it is correct terminology since as you mention there is also an option to reinsulate and restack with existing laminations.