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Originally Posted by robeward
Yes most commonly used for step down. Can you explain how you came up with 138v please?
The trick is actually easier than it should be. You represent each voltage and phase by a vector that has length equal to the voltage and a direction equal to the phase angle.

To add two voltages (of arbitrary phase) together, you simply add the vectors (connect them head to tail).

It is rendered very easy since your transformer diagram already puts the coils in the appropriate directions.

Draw your symmetric 'wye' and call the length of each leg 277 'units'. The angle of each leg of the wye is the phase angle for that phase. The length between any two 'tips' of the wye will be 480 units, and its angle will be the phase angle of the 480V connection. Draw this 480V line, and find its midpoint. Measure the distance from the neutral of the wye to the midpoint of the 480V leg.

-Jon

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raleigh
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seem to be a lot of discussion over the designation. couple things. you can't just rely on how many HV bushings you see for determining voltage. if you go back and look of the picture of the xfmr there are actually two HV bushings but they are internally tied. we use one for source and usually install an elbow MOV on the other.

i have an attachment provided by cooper that explains the designation. essentially the designation determines how the xfmr is built. either one coil/mid tap or two coils connected. internally grounded or not. some can be used in wye configurations and some can't. it references an ANSI standard that governs this but doesn't actually give the name. the file is too big. once i figure out a way to make it available i will post it.

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Originally Posted by winnie
The trick is actually easier than it should be. You represent each voltage and phase by a vector that has length equal to the voltage and a direction equal to the phase angle.

To add two voltages (of arbitrary phase) together, you simply add the vectors (connect them head to tail).

It is rendered very easy since your transformer diagram already puts the coils in the appropriate directions.

Draw your symmetric 'wye' and call the length of each leg 277 'units'. The angle of each leg of the wye is the phase angle for that phase. The length between any two 'tips' of the wye will be 480 units, and its angle will be the phase angle of the 480V connection. Draw this 480V line, and find its midpoint. Measure the distance from the neutral of the wye to the midpoint of the 480V leg.

-Jon
Dag. I was stuck in single phase mode. Forgot to take into account angles from the source. let me ponder on this.

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document compressed and split. hope it helps explain things.

5. Originally Posted by robeward
document compressed and split. hope it helps explain things.
Good article, thanks. Similar to the one Smart\$ posted but this one is more specific for single phase and gives a little more detail with examples.

Yes the loop feed bushing can throw you off, it did the same to me. I have a step up step down setup serving my house, and I was also planning on using the loop feed bushing for an arrestor elbow.

Out of curiosity, did they supply a different transformer for the step up an down or are they both the same?

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Aug 2014
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raleigh
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Originally Posted by electrofelon
Good article, thanks. Similar to the one Smart\$ posted but this one is more specific for single phase and gives a little more detail with examples.

Yes the loop feed bushing can throw you off, it did the same to me. I have a step up step down setup serving my house, and I was also planning on using the loop feed bushing for an arrestor elbow.

Out of curiosity, did they supply a different transformer for the step up an down or are they both the same?
Yes technically there is a difference but without the order form showing which one is which, I don't think you could tell the difference just by looking at them. Physically there is not difference and I honestly don't know what would make them different. The impedance numbers are different but I am not sure it is enough to say they build them in a different way. Maybe someone up here could shed some light.

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