480v Feeding a Pri/277v Sec/24v transformer?

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NicC

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My Mind Can't stop thinking about this transformer. I cannot find an answer.

If anyone could explain this situation would be great!! I will be able to sleep better lol.

I Ran into this on a site.
A Single Phase 277v Fan Box. A 277vac Primary and 24vac secondary 45va transformer installed for the control circuit.

Verified Secondary 24vac power as Control boards where acting in a way as if there was a Volt Amp issue.

On secondary I had 20vac. removed loads from the transformer and still 20vac. Across And to ground.

Verified Primary Voltage with transformer energized. L1-N 277v ...L1-Ground 277v
N-Ground 110v .

Now when I shut the disconnect off isolating any loads. I verified Line Voltage into the Disconect. I got L1-N 480v !!...L1-Ground 277v ...N-Ground 277.
L1-N 480v tells me there are two 277v lines a phase out from each other comming in. Very possible Some one missed wired this Nuetral to the Unit.

this scenario acts like the transformer is created its own Nuetral like some 2 secondary Wye transformers I have read about. But I don't understand how.
I would assume 480v feeding a 277v transformer would burn it up.

Now my Questions are.
Why Does this 480v supplied to a 277v transformer not burn the windings up? Thinking somthing to do with Alternating current . helps me sleep.

And why is the Secondary 24vac output less then 24vac.? would assume it would be more?? More Volts into transformer more volts out right? Thinking this also has to do with Alternating current. and Transformer really still only getting 277v applied? and this lack of a true Nuetral causing less Volts Out?

Last question since where here.
Why on single phase 277v systems do I see the 110vac on the Nuetral side of things? does This all have to do with How Wye transformers work?

Some how this is all related. Just can't put it all together. typing this Post I feel like I'm Close.
But I may be completely going the wrong way on it.

If Anyone could explain this would be Amazing!
 
... when I shut the disconnect off isolating any loads I verified Line Voltage into the Disconect. I got L1-N 480v !!...L1-Ground 277v ...N-Ground 277.
L1-N 480v tells me there are two 277v lines a phase out from each other coming in. Very possible Some one missed wired this Neutral to the Unit.

Perhaps there is a load from L2 (or L3) to N upstream of the disconnect. And then upstream of that load the neutral conductor has a break in it. Then when you open the disconnect, that L2-N load pulls the N feeding the disconnect very close to the L2 voltage. That way when you measure the L1-N voltage going into the disconnect you get 480V.

Verified Primary Voltage with transformer energized. L1-N 277v ...L1-Ground 277v
N-Ground 110v .

With the disconnect closed, the transformer primary will present a load across the L1 and N terminals going into the disconnect. Now the transformer load and the L2-N load mentioned above will share the 480V that's across L1 and L2, with the N at the disconnect somewhere in between the L1 and L2 voltages. It's possible you could have close to 277V on L1-N and 110V N-Ground depending on the impedances of the loads.

No matter what, when you have the disconnect open or closed your N-Ground voltage is quite large, and this is consistent with having an open neutral upstream.

Perhaps the 20 VAC you are measuring after the transformer is due to voltage harmonics on the open neutral that feeds it? Or maybe more likely, the transformer has been damaged from an overvoltage due to the open neutral and changing L-N loads.
 
Perhaps there is a load from L2 (or L3) to N upstream of the disconnect. And then upstream of that load the neutral conductor has a break in it. Then when you open the disconnect, that L2-N load pulls the N feeding the disconnect very close to the L2 voltage. That way when you measure the L1-N voltage going into the disconnect you get 480V.



With the disconnect closed, the transformer primary will present a load across the L1 and N terminals going into the disconnect. Now the transformer load and the L2-N load mentioned above will share the 480V that's across L1 and L2, with the N at the disconnect somewhere in between the L1 and L2 voltages. It's possible you could have close to 277V on L1-N and 110V N-Ground depending on the impedances of the loads.

No matter what, when you have the disconnect open or closed your N-Ground voltage is quite large, and this is consistent with having an open neutral upstream.

Perhaps the 20 VAC you are measuring after the transformer is due to voltage harmonics on the open neutral that feeds it? Or maybe more likely, the transformer has been damaged from an overvoltage due to the open neutral and changing L-N loads.
Thank you for the reply!
This makes alot of since.
Very possible just a bad Connection in a Junction then sounds like.
 
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