Transformers

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Jubert Lowe

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I am installing temporary power on a site I have a 277/480v 3-phase step down transformer that I would like to use, the problem that I am having is I have 480v single phase from the utility company, so when I hook up to H-1 & H-2 from the utility company and my secondary X-1 & X-2 between X-1 and X-0 it read 120v but between X-2 and X-0 it read 78v between both it reads 198v, what could I do to get 120v on each I don't want to purchase a single phase transformer.

Thanks
Jubert
 
I have a hunch, one that needs careful analysis and confirmation:

Presuming that you have a three phase delta 480 to wye 208/120 transformer, you could do the following:

Connect incoming L1 to both H1 and H3.
Connect incoming L2 to H2
_disconnect and insulate_ the jumper to the primary coil connected between H1 and H3.
Get the desired 120/240 single phase using X1, X0, and X2.

Please note: I am a motor theory guy, not an electrician. Do not use this suggested connection without confirmation from a suitably experienced and educated professional in the field.

-Jon
 
Yup.

I've been thinking about it some more, and would now call my hunch a sound theory.

But we all know the difference between theory and practice. (In theory, there is no difference.)

-Jon
 
winnie said:
I have a hunch, one that needs careful analysis and confirmation:

Presuming that you have a three phase delta 480 to wye 208/120 transformer, you could do the following:

Connect incoming L1 to both H1 and H3.
Connect incoming L2 to H2
_disconnect and insulate_ the jumper to the primary coil connected between H1 and H3.
Get the desired 120/240 single phase using X1, X0, and X2.
...
-Jon
Click to expand...
Jon,
Do you really have to disconnect the primary coil connection? You would be applying the same voltage to both ends of that coil, so it would not be doing anything. You would have 480 volts on two of the coils and 0 volts on the third.
 
480 V Transformer

480 V Transformer

Hi Jon that was what I was thinking myself, but I put it out there to see if some else has the same Idea.
 
don_resqcapt19 said:
You would have 480 volts on two of the coils and 0 volts on the third.
Click to expand...

That was my first thought, but on a common 3 phase transformer, all three core legs act as return flux paths for each other. In this modified configuration, the two energized cores should balance out, but I figure that there is a chance of some residual alternating flux on the supposedly 'unenergized' core.

Alternating flux through the 'unenergized' core would induce voltage in the shorted coil; probably not that much, but possibly enough to be a problem. I figured that it would be a safe bet to make the extra effort to not short this coil.

-Jon
 
Jubert Lowe said:
I am installing temporary power on a site I have a 277/480v 3-phase step down transformer that I would like to use, the problem that I am having is I have 480v single phase from the utility company, so when I hook up to H-1 & H-2 from the utility company and my secondary X-1 & X-2 between X-1 and X-0 it read 120v but between X-2 and X-0 it read 78v between both it reads 198v, what could I do to get 120v on each I don't want to purchase a single phase transformer.

Thanks
Jubert
Click to expand...

I would expect you to get 120V between X-1 and X-0 and 60V between X-2 and X-0. I would then expect to see 180V between X-1 and X-2. The actual values that you recorded seem close. I'm not sure if phase angles play into this at all?
 
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