Transformer Question

[Southern Charm]

Hello, called out to a building to look at a transformer that was "very hot" . Transformer spec: 208 primary 120/240 secondary. 7.5kva. Went into panel fed by transformer. Load on first phase 1amp. Load on second phase 7amps. This panel feeds some IT equipment about 35ft away in another room. Literally cannot keep hand on transformer for more than a second or so, its pretty hot. Was told unit is 5 years old. Maker on name plate was Jefferson. Is it possible this thing is so hot because its way underloaded??? Any thoughts?

 

[augie47]

Happen to have any other nameplate information ? Some of them run VERY hot to the touch by design

 

[LarryFine]

Seems like the transformer was not really needed. But, no, running unloaded won't make it run hot.

 

[gar]

190124-2025 EST

Southern Charm:

You need to look at primary voltage, primary current, and is the transformer excessively noisy? An IR temperature measurement would be useful.

You are so lightly loaded that load current is not a problem. And as stated some transformers run quite hot to touch even with no load. They will run even hotter under load. Somewhere in the 110 to 120 deg F range gets quite hot to touch.

.

 

[milldrone]

Hello, called out to a building to look at a transformer that was "very hot" . Transformer spec: 208 primary 120/240 secondary. 7.5kva. Went into panel fed by transformer. Load on first phase 1amp. Load on second phase 7amps. Any thoughts?

Is the neutral on the 208 primary connected?

 

[LarryFine]

Is the neutral on the 208 primary connected?

Shouldn't be. Good catch!

 

[augie47]

I thought 7.5 kva were only readily available as single phase units so I discounted the "neutral" connection occasionally found on 3 phase transformers.

 

[jim dungar]

Literally cannot keep hand on transformer for more than a second or so, its pretty hot.

Hands are not a good temperature measuring device. IMO, if you can touch it at all it is probably not running too hot.

A 7.5 kVA transformer like this might be a sealed unit (so heat transfer is only through the case) with a 115C temperature rise (at full load) above ambient. Granted the 115C will be deep inside the windings of the transformer, but a case temp of 60C above ambient might not be unreasonable.

 

[gar]

190124-2228 EST

If the specified secondary is 120/240 why would any of you expect this to be a three phase transformer?

.

 

[texie]

190124-2228 EST

If the specified secondary is 120/240 why would any of you expect this to be a three phase transformer?

.

That plus the fact that it is 7.5 KVA which is not a standard size in 3 phase.

 

[ptonsparky]

190124-2228 EST

If the specified secondary is 120/240 why would any of you expect this to be a three phase transformer?

.

That plus the fact that it is 7.5 KVA which is not a standard size in 3 phase.

The first phase and second phase references in the OP, but you are correct.

Danged Assumptions.

 

[GoldDigger]

The very first thing I would do is measure the primary current. It should be low (the vector sum of the reflected load current and the magnetizing current.) If the primary current is higher than the expected magnetzing current even with the secondary unloaded or lightly loaded, I would suspect something like a shorted turn, causing extra current and local heating.

 

[Southern Charm]

Happen to have any other nameplate information ? Some of them run VERY hot to the touch by design

Jefferson Electric catalog # 411-0131-208
Also found out that 6 other units like this are in the facility, all feeding the same sort of IT gear. And they are all hot. The facility manager didnt realize that fact. Im thinking you guys nailed it with they typically run hot theory.

 

[Southern Charm]

190124-2025 EST

Southern Charm:

You need to look at primary voltage, primary current, and is the transformer excessively noisy? An IR temperature measurement would be useful.

You are so lightly loaded that load current is not a problem. And as stated some transformers run quite hot to touch even with no load. They will run even hotter under load. Somewhere in the 110 to 120 deg F range gets quite hot to touch.

.

Didn't hear any noise from the unit. Will have to go back for IR measurement

 

[texie]

Seems like the transformer was not really needed. But, no, running unloaded won't make it run hot.

I wonder why as well. Maybe this is supplying 240 volt equipment? Or they had no neutral available? If it were me I'd be analyzing why it is there.

 

[augie47]

I was unable to find that catalog number on Jefferson's website but the Cu Alliance states:
Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C.
Depending on the transformer, Jefferson shows similar numbers.
With an ambient of 80°F if you add a rise of 115°C (239°F) you get a temperature of around 300°F.. not going to layn hands on that long,.

 

[retirede]

I was unable to find that catalog number on Jefferson's website but the Cu Alliance states:
Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C.
Depending on the transformer, Jefferson shows similar numbers.
With an ambient of 80°F if you add a rise of 115°C (239°F) you get a temperature of around 300°F.. not going to layn hands on that long,.

That would be the max internal insulation temperature. No way the case will be that hot.

But I agree, too hot to touch is not abnormal.

 

[Andy Delle]

Well you said it was feeding an IT load. Remember that IT gear, especially older stuff, can have a high harmonic current. Depending on the load current, that could overheat the transformer.

 

[winnie]

Transformer spec: 208 primary 120/240 secondary. 7.5kva. Went into panel fed by transformer. Load on first phase 1amp. Load on second phase 7amps.

This is a single phase transformer. If you are measuring 7A on one side of a single phase load, and 1A on the other side of the same load, then you have 6A going somewhere it doesn't belong.

The question about 'neutral' is pertinent because it is the sort of mis-connection that you might be dealing with...if the transformer had a primary side neutral (very unlikely). Perhaps the primary and secondary were swapped. Perhaps the primary is connected to the secondary.

Or perhaps the load changed between measurements, and there is no current going the wrong way.

-Jon

 

[Jraef]

Class H insulation, then they go on to say that design is rated for 275 deg F temperature RISE over ambient, so assuming 30C ambient rating, that's 361 degrees F. You can leave your hand on something that is 140 F for about 5 seconds before receiving a 2nd degree burn, that is not even 1/2 of what would be problematic for this transformer...

And I agree, it's likely because the load is all non-linear, lots of odd order and triplen harmonics coming from all the power supplies in the IT equipment that, in a single phase transformer, are not cancelling each other out.