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g5tran
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Join Date: Aug 2012
Posts: 3

Delta - Wye Transformer Question on the 480/277v Setup as Opposed to 120/208v

08/09/2012 2:43 PM

Hi all,

I'm having trouble understanding the differences in setup between the Delta/Wye used for a 120/208v arrangement compared to the 277/480v arrangement. In particular, I'm looking at it with 2400v being applied to the primary windings.

If 2400v is applied to each coil on the Delta side of the transformer, the line to line voltage is clearly 2400v on the primary side since the coil is connected between phases. Because the secondary coils on the Wye side are each connected from the phase to neutral(lets assume grounded neutral), the secondary voltage, 120v for example, should appear across the coil from phase to neutral, making each phase voltage 120v and the line to line voltage 208v. This much I understand.

The 277/480v on a Delta-Wye transformer has a 277v phase voltage and a 480v line to line voltage. Let's still say we have 2400v line to line on the Delta primary side like before. I'm having trouble comprehending how the voltage across the primary winding(2400v) transforms into 277v across the secondary winding(phase to neutral), considering 277v is not a scalar from 2400v. 480v is a scalar of 2400v, but that voltage is the line to line voltage in a 277/480v system.

In the first scenario, 120v, the scalar multiple of 2400v appears as the phase voltage. In the second scenario, 480v, the scalar multiple of 2400v appears as the line to line voltage. As far as I know, the Delta-Wye transformers are used in the same way in each case.

What is the difference in implementing 120/208v compared to 277/480v to obtain the different voltage ratios? Or is there a mistake I'm making in my reasoning?

Thanks in advance, and I appreciate any insight anyone can offer!
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bigg
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#1

Re: Delta - Wye Transformer question on the 480/277v setup as opposed to 120/208v

08/09/2012 3:17 PM

From a theoretical standpoint, the only difference in 2400 to 120/208 and 2400 to 277/480 is the ratio of the transformer windings. You seem to be hung up on the winding ratios as if there is only one loop on the secondary winding, so that the primary winding would be x loops, giving the voltage ratio an integral multiple. But what if the secondary had more loops, say 277, and the primary had 2400 loops? Then, the voltage ratio would be 2400:277, giving 277 phase to neutral in a wye secondary configuration. If the secondary had 120 loops and the primary had 2400, you'd have 120 phase to neutral (in a wye secondary). In the real world, you have voltage losses everywhere, and the theoretical voltages are not what you get anyway. That's why real transformers often have extra taps.
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g5tran
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#2

Re: Delta - Wye Transformer Question on the 480/277v Setup as Opposed to 120/208v

08/09/2012 3:34 PM

Thanks for the input! That makes sense, I always thought typical transformers were implemented in a simple ratio, such as 4:1.

Is that typically how transformer windings would be used in industry? With some winding ratio that would obtain a voltage close to 277v across the secondary winding?
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bigg
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#5
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Re: Delta - Wye Transformer Question on the 480/277v Setup as Opposed to 120/208v

08/09/2012 5:04 PM

Shucks, it was nothing glad I could shine a little light.

Yeah, that's generally how transformers are used.

I meant to say in my first reply, I don't have a clue how many actual loops (turns) are in real power transformers. If I were to actually try to build a transformer, I'm sure there would be a lot of smoke involved. I've taken apart a lot of stuff in my day, but never had a chance at a big transformer. I tell people if I had kept my childhood toys instead of taking them apart, I could be a wealthy man.
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SolarEagle
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#3

Re: Delta - Wye Transformer Question on the 480/277v Setup as Opposed to 120/208v

08/09/2012 3:59 PM

This sort of explains it, the seemingly odd voltage relationships of 277/480, and 120/208, result from the timing of the individual output waveforms ....

http://www.nfphampden.com/xftheory.pdf

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Codemaster
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#6
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Re: Delta - Wye Transformer Question on the 480/277v Setup as Opposed to 120/208v

08/10/2012 6:11 AM

There's nothing odd about those ratios, they're all √3 = 1.732, from basic 3-phase theory.

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g5tran
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#4

Re: Delta - Wye Transformer Question on the 480/277v Setup as Opposed to 120/208v

08/09/2012 4:21 PM

Thanks Solar, I do understand the inner workings of a transformer somewhat. I was more confused about how the 277 phase voltage was achieved but Bigg's answer makes perfect sense.

Thanks again!
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