Transformer temperature rating (high secondary output)

J

journeyman0217

Senior Member
Location
Tennessee
Occupation
Electrician
I recently came across a transformer that has a high secondary output voltage. I believe the xfmr is failing. It's a 480hv - 120/208lv. Input voltage is 483. Output is 234. It does not have multiple taps and is wired correctly. As part of my troubleshooting , I used a infrared camera on it and had temperature readings of 213F on the coils and the same on the conductors coming off of the xfmr taps. I'm not quite sure what a normal operating temperature would be and I know there are multiple variables that can change the temp. The xfmr is 220C insulation and 150C rise. If the insulation is rated at 220C (428 F) then 213F on the coil doesn't seem bad. Does anyone know if the insulation rating is 220C what the max operating temp would be? 150C is 302F so the rise alone would say that 213F on the coil isn't alarming?? Has anyone taken temp readings on a normal operating xfmr? Trying to figure out what a normal temp reading is and why my secondary is so high.
 
213F (100°C) doesn't sound too hot for the transformer coils themselves. But it is probably not good for the conductors.
What is the loading% of the transformer. What is the ambient temperature.



Transformers run hot. I know of one situation where the painters put cardboard tents over the transformer while they were working. Luckily someone notice the cardboard smoking before it burst into flames.
 
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jim dungar said:
213F (100°C) doesn't sound too hot for the transformer coils themselves. But it is probably not good for the conductors.
What is the loading% of the transformer. What is the ambient temperature.



I know of one situation where the painters but cardboard tents overthe transformer while they were working. Luckily someone notice the cardboard smoking before it burst into flames.
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Yeah, agreed temp is not good for conductors being rated at 90C but that was measured directly off the tap and seemed to dissipate quickly. When you say loading %, is that a rating on the xfmr or are you asking what the actual amp reading was?
 
journeyman0217 said:
When you say loading %, is that a rating on the xfmr or are you asking what the actual amp reading was?
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The % loading of the transformer full rating.
A loading impacts voltages regulation and heat generation.

It is not uncommon to see above normal voltages on a lightly loaded transformer.
Fully loaded transformers produce so much heat they should be considered during the HVAC design.
 
jim dungar said:
The % loading of the transformer full rating.
A loading impacts voltages regulation and heat generation.

It is not uncommon to see above normal voltages on a lightly loaded transformer.
Fully loaded transformers produce so much heat they should be considered during the HVAC design.
Click to expand...
I see. I did read that about light loads can affect output voltage but 234 output seems too high.
 
About 135V to neutral or 191 peak to peak.
I have read online most power supplies and stuff for the 120V market can survive 200V peak to peak or about 141VAC RMS.
I have not tested it at work LOL.
 
Can we get all the phase to phase & phase to ground measurements? And possibly some type of loading situation? Like are the legs balanced or is one heavily loaded on line to neutral loads while the others are not.
 
pipe_bender said:
Thats odd what are your readings L-L and L-N on all phases?
is it powering any 120V loads?
Most I see are around 218/125.
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L-L 234 ish volts L-N 135 volts. The reason we started to look into this transformer is because we had a 120volt timer blow 2 times in a short period of time. We took voltage reading at timer and had 135 volts L-N. Traced the volatge back to the source which was the transformer. Coming right off the transformer is 135 L-N 230 some volts L-L. That's why I chalked it up to being a bad transformer. For reference the input voltage was around 483 volts.
 
I seem to remember years ago an old electrician telling me about a similar situation - hot transformer with elevated secondary voltage. They dissected the transformer and found an insulation failure that effectively reduced the number of turns in the primary.
 
retirede said:
I seem to remember years ago an old electrician telling me about a similar situation - hot transformer with elevated secondary voltage. They dissected the transformer and found an insulation failure that effectively reduced the number of turns in the primary.
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interesting. This sounds like a possibility. I was thinking about meggering the xfmr...I wonder if the test would reveal that situation
 
retirede said:
I seem to remember years ago an old electrician telling me about a similar situation - hot transformer with elevated secondary voltage. They dissected the transformer and found an insulation failure that effectively reduced the number of turns in the primary.
Click to expand...

I think this is it.
 
journeyman0217 said:
interesting. This sounds like a possibility. I was thinking about meggering the xfmr...I wonder if the test would reveal that situation
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A normal megger will not find turn to turn shorts.

What size is this transformer? I don't think I have ever seen a 30kVA and larger without some type of taps.
 
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