evaluate core test thermography results 2

[electricpete]

We have a core loss test that failed the 10C / 18F degrees rise requirement. I’m interested in understanding the significance (beyond just degraded lamination insulation… where is it degraded and how much risk would it pose to rewind without core replace/restack). I occasionally review core loss tests with the help of our shops, but I’m not an expert in figuring out what is causing the patterns and what is the significance and my usual go-to guys for this type of question are unavailable. So that’s why I’m asking opinions here.

Details
[ul]
[li]50 hp 2 pole motor. Manufactured in the 1970’s by Siemens Allis[/li]
[li]Test was conducted with an excitation equivalent of 105% rated backiron flux density calculated as per IEEE Std. 432.[/li]
[li]As-found pre-burnout test (not included) hottest spot after 30 minutes was 24F rise.[/li]
[li]Burnout was conducted at/below 650F.[/li]
[li]Post burnout thermography test (attached):[/li]
[li]...Slides 1- 4 10 minute rises 13/14/14/31 (see 1 below)[/li]
[li]...Slides 5-8 20 min rises 19 / 26 / 18 / 18[/li]
[li]...Slides 9-13 30 minute rises 24 / 30 / 29 / 26[/li]
[li]The post burnout core loss is 3.38 W/Lb.[/li]
[/ul]

My initial thoughts:
[ul]
[li]1. I’m inclined to discount the 31 on page 4. I don’t see similar number on page 8. I’m not sure what feature on page 4 it represents. Is it the white spot around 3… maybe that’s a reflection of something?[/li]
[li]2. Looking at the bore (tooth tips), it looks cooler on one end and hotter on the other end with a distinct stripe. However when they swap viewing ends it always look like you’re looking from the hotter end. Example compare slides 5 and 7… opposite ends based on positioning of the large and small cables, but they both look like the cooler part of the bore is on the far end from the viewer. Is this a viewing angle problem or is the imager averaging the top of the teeth with the empty slots which always looks cooler on the far end?[/li]
[li]3. The hottest spots are show in the slots where some degree of cavity effect is expected. I don’t really see any pattern there, just uniformly hot in the slots is all I can discern.[/li]
[li]There is no obvious surface distress of the bore (teeth ends) or the slots[/li]
[li]The temperature continued to rise between 20 minutes and 30 minutes=[/li]
[li]Looks like just uniform degradation of the bulk lamination insulation as far as I can tell. But doesn't look particularly severe to me. [/li]
[/ul]

The motor I’d say is on the lower/middle end of criticality (not high).

What do you see in the images and what does it tell you about the core condition and risk of rewinding without replacing/restacking the core.

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(2B)+(2B)' ?

 

[Gr8blu]

electricpete:
1) If all I saw was slide 4, I would be looking at the piece to see if there was some broken/cracked/missing lamination material around the wedge groove at that 0300 (relative) position. However, having reviewed all the other slides, I'm inclined to agree with you - it's an aberration, in that there might have been something somewhat reflective at that particular point when the image was taken.
2) The angle of incidence and the distance between the camera and the surface observed makes a HUGE difference in the ability to reliably detect temperatures. In this case, you certainly have an oblique angle when looking at the opposite end, as well as increased distance. With that in mind, the opposite end is ALWAYS going to read cooler than something up close and/or oriented more perpendicularly to the lens.
3) The higher temp in the slot portion is not necessarily going to be a result of deformation of the lamination in the area - at least in terms of new deformation. It is often a result of the burr between laminations at the bottom/sides of the slot (created by the lamination manufacturing process). For punched laminations, a physical burr is left which is often not sanded away and can result in higher surface temperatures where the burr is located. For laser-cut laminations, the high-energy cut often changes the magnetic properties of the material - and incidentally changing the thermal properties as well.
4) Unless the process driven by this machine is critical (which is a bit hard to believe, given the power rating), I'd agree with the basic premise of middling deterioration, uniformly distributed throughout the core area. Re-use the core for a rewind, rather than restack ... UNLESS there is obvious physical evidence of lamination damage in the slot area that could result in a premature failure of the strand insulation (something like a sharp edge or burr that might scrape the enamel off the strand during coil insertion, or might tear the liner and reduce the insulation to ground).

Converting energy to motion for more than half a century

 

[waross]

A couple of points that I found confusing, Pete.
Pre burn-out. Information given as temperature rise.
burnout temp less than 650 degrees, How valid is this information?
Post burnout: Temp appears to be absolute, not rise. hard to compare.
loss as Watts per unit mass. No information to compare to pre burnout.

But my next question may render previous questions moot.
How much does the core weigh?
At a core loss is 3.38 W/Lb., what will the total core loss be and is this acceptable. (Consider the motor duty and normal loading. Many motors are loaded to less than 100% and there is some headroom to accept additional losses safely.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

> Pre burn-out. Information given as temperature rise.
> Post burnout: Temp appears to be absolute, not rise. hard to compare.

All temperatures in my post were "rises"

> burnout temp less than 650 degrees, How valid is this information?

Motor shop keeps a chart record of temperature vs time during the burnout. Not foolproof but good enough for me.

> How much does the core weigh?

Your guess is as good as mine for a 50hp 2-pole motor. Watts per pound is generally the quantity of most interest in judging core condition. The measured as-left watts/pound is well within normal repair spec limit of 4 watts per pound, which typically we interpret as having no significant overall thermal impact. We do have some extra thermal margin because the insulation class used for rewind is higher than original. In general when judging the thermography deviation from spec (>10C rise), I'm not as concerned about what the overall motor post-rewind thermal snapshot would be as much as I'm concerned about what the test tells us about the "health" of the core... after rewind during service do we expect localized hotspots or core lam insulation degradation at an abnormal rate. Gr8blu's comments tended to confirm my thoughts that there is not much reason to be concerned about this core

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(2B)+(2B)' ?

 

[waross]

Thanks for the clarifications Pete.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[macmckim]

Pete;

A 1970 vintage Siemens Allis (Allis Chalmers) motor laminations, may not be a C5 core plate material, I am thinking it may have C3 core plate.
Going from memory, C3 lamination material require a lower burn out temperature (around 500F), otherwise damage may occur to the organic insulation on the lamination.
In the past 20 years, burnout temp have been standardized at 650F.

https://www.eng-tips.com/viewthread.cfm?qid=174387

regards
Mac

 

[dArsonval]

There is no, and there never has been, a "standardized" burnout temperature at 650 degrees F.

It does not exist.

Burnout temperatures are tweaked all the time depending on what's being burned out.

Pete, your thermography imagines look pretty reasonable.
Its as you wrote in your original post...

"Looks like just uniform degradation of the bulk lamination insulation as far as I can tell. But doesn't look particularly severe to me."

I agree.

John

 

[electricpete]

Thanks Mac and John

> There is no, and there never has been, a "standardized" burnout temperature at 650 degrees F.

I think what Mac was referring to is a customer spec for maximum burnout temperature. The EPRI rewind specs (which we base our specs off of) uses 650F as a maximum burnout temperature. We have in the past occasionally been asked by the shop to go above that temperature (spec deviation) if they are not getting results at that temperature. If they don't go up to 650, that's not something I've heard about (they wouldn't have to ask permission for that).

Btw it looks like an older version of the EPRI rewind spec for medium voltage motors is available for free. The rewind spec for low voltage motors has the same max burnout oven temperature but that's $15,000 (for non members)

Attached is a bit more info on this particular motor. There was some spreading of the end lamination teeth during the winding removal process even though the winding shop said they were careful. I asked if was bent back before the thermography, they said no. I would have rather they bent it back before thermography (so any resulting damage would show... you'd never see it with lams spread because that prevents any circulating currents). But at this point in time that's a moot point, there is a rush on this motor, rewind has already begun.

The reason the shop sent me the photos was not to show those bent lamination teeth. It was to show a change in lamination appearance around the middle of the core (and a small associated change in temperature). I have no idea about that, must have come that way from the oem (the motor was installed new in our plant in the late 1980’s and never removed until now.


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(2B)+(2B)' ?