Unusual stats for a transformer?

[rhamblin]

We have a small transformer, it has Magnetek Jefferson Electric on the label. Unfortunately the rest of the label has disappeared. I'm thinking it's a 15 KW size transformer, single phase 480V-120/240V. One of the mechanics asked me to check it out, because it was hot. I told him, transformers normally run in the 120°F range and up. Later when I had time, I checked the transformer with a clamp meter. I found the Primary side of both phases to be drawing 1.86 amps. But the secondary lines were drawing 0.06 amps, keeping in mind that my clamp meter reads 0.06 amps when its not wrapped around any wires. I'm quite confident the small panel that is being fed with the secondary line has no load on it. So my question is "why was the transformer so hot, with no load?" "Why was it drawing 1.86 amps @ 480V, when there was no load?" The temperature of the transformer was quite warm (could hold hand there, but was uncomfortable). Seems like with no load it shouldn't have been anywhere near that hot. I understand that wires heat up when we pass current through them, and I've heard its not good to have transformers running with little or no load. Any thoughts?

 

[GoldDigger]

The only drawback to running transformers unloaded is the constant energy loss. It is less than the measured VA since the power factor is low. The energy loss corresponds to the heat you feel coming from the transformer, and results from both magnetic and resistance losses.

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[Ingenieur]

1.86 x 480 ~ 900 va
900/15000 ~ 6%
but as was mentioned pf is low so <1/2 may be real power 300-400 W or 2-3%
that will make some heat

 

[Sahib]

I've heard its not good to have transformers running with little or no load. Any thoughts?

Switching off the transformer for long period is no good either because moisture will condense on winding and there will be risk of failure on re-energisation.

 

[electrofelon]

We have a small transformer, it has Magnetek Jefferson Electric on the label. Unfortunately the rest of the label has disappeared. I'm thinking it's a 15 KW size transformer, single phase 480V-120/240V. One of the mechanics asked me to check it out, because it was hot. I told him, transformers normally run in the 120°F range and up. Later when I had time, I checked the transformer with a clamp meter. I found the Primary side of both phases to be drawing 1.86 amps. But the secondary lines were drawing 0.06 amps, keeping in mind that my clamp meter reads 0.06 amps when its not wrapped around any wires. I'm quite confident the small panel that is being fed with the secondary line has no load on it. So my question is "why was the transformer so hot, with no load?" "Why was it drawing 1.86 amps @ 480V, when there was no load?" The temperature of the transformer was quite warm (could hold hand there, but was uncomfortable). Seems like with no load it shouldn't have been anywhere near that hot. I understand that wires heat up when we pass current through them, and I've heard its not good to have transformers running with little or no load. Any thoughts?

any idea how old it is? Very generally, the older it is the lossier it will be. A modern pad mount will lose about .5% unloaded. modern dry types seem to be about twice that. I once measured a 1950's 10 KVA dry unit and it was losing 3% - that is quite warm to the touch.

Switching off the transformer for long period is no good either because moisture will condense on winding and there will be risk of failure on re-energisation.

Failure how? moisture can certainly lower the resistance of the windings temporarily (and probably not significantly in modern units), but I have never heard of it damaging the windings either directly or by causing severer current draw upon re-engergization.

 

[Sahib]

electrofelon:I based my last post on NFPA 70B sec 10.3.7.4.

 

[electrofelon]

electrofelon:I based my last post on NFPA 70B sec 10.3.7.4.

fair enough, but here is what it actually says (bolding mine):

10.3.7.4 Best service life results if the windings are maintained above the
ambient temperature level. For that reason, transformers operating in high humidity should
be kept energized, if feasible. If a transformer is to be deenergized long enough for it to cool,
special drying procedures might be necessary before the transformer is reenergized. Refer to
the manufacturers' recommendations for the drying procedures to be followed.