Damaged Dry-Type Transformer Lug

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hbendillo

Senior Member
Location
South carolina
Please see the attached photograph. I was called out by one of my clients to trouble shoot a problem they were having with some pump motors. Brief summary of installation:

Transformer in the picture is being use to step up voltage from 208 to 480 volts (Three-Phase) to provide a feeder to a set of well and irrigation pumps on a large residential property. The transformer rating is a 240-600 volt, three-phase with 7 taps on the 600 volt side to vary the output from 540 to 630 amps. The transformer was fed with a 208 volt circuit and the taps changed on the high side to produce the 480 volt output. Measurements confirmed the output to be roughly 485 volts.

The length of the feeder is about 800 to 100 feet. At the pump site another transformer, 480-240 volt dry-type , is used to step voltage down to feed two 240 volt, three-phase pumps. The owner was having trouble with the starter for the pumps kicking out and/or cycling until at some point one of the existing starters feeding a 30 HP pump failed completely and had to be replaced. The Owner and his Contractor thought is was a voltage problem because of the transformer configuration.

So we began our investigation by removing the cover of the transformer in question so we could look at the connections and measure voltages and found the damage to the connection shown. Apparently at some point the wiring connection to the lug in the picture worked loose through heating and expansion and began arcing. As you can see by the picture part of the lug on the transformer is completely burned off and the wire and lug connected to it barely making contact. It also shows a lot of damage to a portion of feeder the wire. So apparently this is their problem. The arcing connection playing havoc with the voltage supplying the pumps, possibly intermittent single phasing.

The owner wants to look into repairing the connection but I am wondering, what other damage may have been done to the transformer as a result of this condition besides the visible physical damage. How can you repair something like this? The entire transformer lug needs to be replaced but that won't be easy because wires from the transformer connected to the lug are welded to it. We may be able to figure something out but my instinct is to replace the transformer. Are there any other issues I need to consider besides the damaged lug?
 

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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I would not expect any additional damage to the transformer, but I don't think there is a way to repair that.

You might be able to make a temporary repair by bolting the lugs to the bottom of the transformer connection pad. Not sure if there is enough flat contact area to make the work, and you would have to use the mounting bolt to both support the connection pad and make the connection.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
The damage probably only extends as far as the
burnt insulation. I can't tell for sure but could a
crimp connection with heat shrink on the output
wire, and a multi-tap lug be sufficient (as don has
suggested above)? I'm sorry if this is wrong because
my screen resolution isn't that good. But try to get back
to the part of the wire with good insulation and splice
from there. If the transformer was sized properly, this
should be localized heat damage (leaving the main coils
of the transformer ok, at least until a replacement or
rebuild can be done with a knowledgable rewind shop.)
Where I work we have this quite often, but the majority
of our transformers are slightly undersized from
installation. The only ones fully rated (110-125%) are
at our generation sites and are delivered by railroad.
So we really dread changing those out.

In the future, let them know to keep the lugs tight
or use the bellvue washers to accomidate the thermal
expansion.

If they can afford it, it would be wise to replace if possible,
but the temporary solution could last years. That may give
enough time to fund the replacement and if there is a competent
winding shop, they could have this one as a spare.

Spare parts are always good if there is somewhere to
store them.

Whichever way this goes, hopefully it will work out for
you and your client.

Jimmy
 

fmtjfw

Senior Member
Thank you both for your help. I think I will advise him on a temporary connection and recommend that the transformer be replaced.

Why do you think the transformer is damaged? Is the insulation on the wires on the xfmr side of the connection damaged? If so Megger the xfmr. If not, why worry?

It just looks like a poorly torqued connection that overheated.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
It just looks like a poorly torqued connection that overheated.
I think the over sized holes, as compared to the bolt size, in the wire connector and maybe on the transformer connection pad played a big part in this problem.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Looks like wrong fasteners for the purpose.
It looks to me like you can make a repair.
I'd bolt a new bar to a good portion of the old where the flat conductors are soldered. Bolt on new proper lugs.
I don't know how that affects its listing but is should work.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I wonder if that is true? The damaged pad looks like it left the factory with its corner cut off.
There is no melted aluminum 'slag' in the bottom of the transformer.
Jim,
After a closer look, I think you are correct. That, along with the over sized hole in the wire connector would not have left much contact area for the connection.

Maybe he could clean the tab up, drill and new hole and install a new wire connector
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Why not test it before replacing? TTR, DAR, PI, IR, ....
I agree. I'd test it (or really, HAVE it tested by a NETA shop) and if the transformer is OK, solder on a new tang for a little more meat and use it again. Most likely a good NETA shop could do that too, for far far less than the cost of a new transformer.

But if the insulation is damaged, it might not be worth saving. So you have to start there.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
Safety / Liability

Safety / Liability

If after the repairs (temp. to semi-permenant) are complete,
the suggestions to have it tested at an International Electrical
Testing Assoc. (NETA) recognized shop would add a great deal
of credibility to the repairs ( if they are chosen to be made )
I apologize for not mentioning this above, but if this transformer
could cause harm to staff/guests at the facility it will show that
due diligence has been fulfilled. This transformer may be a good
candidate for repair and testing as stated by those above.
It really can be impressive what can be salvaged under the
proper knowledge and skill of a good rebuild and testing shop.
When specialized motors or transformers come back from our
rebuilders, the records kept on them show no significant
difference in longevity in its "second" or "third" life than when
it was initially installed.
Depending on the rebuild vs. replacement cost, which of course
varies according to type/size/configuration unless there is a melt-
down of the core or windings it is fairly impressive what can be
saved.
I have to admit that my responsibility is to keep our equipment
(some of which is 90+ years old) functioning and dependable.
So that is my disclaimer for suggesting repair/rebuild rather
than replacement. Much (not all) but most of the older equipment
in our system had been well maintained and the new replacements
are ( in my personal opinion ) maybe between fifty to seventy
percent the quality of original equip.

And thanks to those above that suggested testing ( a fairly
disgraceful oversight on my part because that is a major
part of what I do, mental overload at the moment I guess )

Jimmy
 

hbendillo

Senior Member
Location
South carolina
Jim,
After a closer look, I think you are correct. That, along with the over sized hole in the wire connector would not have left much contact area for the connection.

Maybe he could clean the tab up, drill and new hole and install a new wire connector

This is what the Contractor ended up doing. The pump is running now and they are going to periodically check the transformer. I also asked them to examine and tighten all the other lug connections in the transformer.
 

hbendillo

Senior Member
Location
South carolina
Why do you think the transformer is damaged? Is the insulation on the wires on the xfmr side of the connection damaged? If so Megger the xfmr. If not, why worry?

It just looks like a poorly torqued connection that overheated.

I worry because I worry. Didn't think it was likely the transformer was otherwise damaged. Just throwing it out there.
 
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