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1. Senior Member
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Originally Posted by Carultch
It is called single phase, because it is most commonly produced from a single phase of a three phase system. It is center-tapped on the transformer secondary to produce equal and opposite line voltages, relative to neutral.

Assuming ideal waveforms, it happens to be mathematically equivalent to a hypothetical system made from two separate phases, that would be 180 degrees out of phase. But this doesn't represent how it is supplied in practice, so that is why it isn't called "two phase".
Plausible. Though I think that it may have more to do with how the service is used than how it is derived. Consider that the following are all called 'single phase':

1) two hot legs 180 degrees out of phase supplied by a single core transformer with a center tap
2) two hot legs 120 degrees out of phase 'derived' from a three phase wye service in multi-occupancy residential situations
3) two hot legs 180 degrees out of phase supplied by a generator connected 'double delta' to make best use of all of the generator windings when supplying single phase loads.

-Jon

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Originally Posted by winnie
Plausible. Though I think that it may have more to do with how the service is used than how it is derived. Consider that the following are all called 'single phase':

1) two hot legs 180 degrees out of phase supplied by a single core transformer with a center tap
2) two hot legs 120 degrees out of phase 'derived' from a three phase wye service in multi-occupancy residential situations
3) two hot legs 180 degrees out of phase supplied by a generator connected 'double delta' to make best use of all of the generator windings when supplying single phase loads.

-Jon
That 'out of phase' phrase is also just colloquial though. A lot of us think it suggests that the sum of the voltages would be zero and not 240. That's the reason for thousands of posts, perhaps even more that the number of phases. On a high leg delta, are the 120V waveforms 'out of phase'? That seems wrong to us intuitively when considering it as a three phase supply. And it's not because we don't know the math.

3. Originally Posted by jaggedben
A lot of us think it suggests that the sum of the voltages would be zero and not 240
So with that logic you would think that the two 120 volt phases on a 120/208 system yield 120 volts instead of 208 volts.

4. Originally Posted by winnie
I know my thinking on this subject has changed over the course of the thread(s).

I started in the camp of 'it is a single phase' and that is why we call it 'single phase'.

I've shifted to 'there are clearly two discernible phases present', but it is still a 'single phase' supply because that is the naming convention.

-Jon
Was it the 2 diode full wave bridge? Seems like I remember a comment to that effect but I'm not positive if that was you way back then.

5. Originally Posted by mivey
So with that logic you would think that the two 120 volt phases on a 120/208 system yield 120 volts instead of 208 volts.
No, because there genuinely IS a phase shift between the two 120v waves, which are derived from more than a single-phase primary.

6. Originally Posted by ggunn
Also, there is a single phase across the terminals of the center tapped transformer; it only looks like two phases because a reference point is chosen between the terminals. I think I'm the first person to say that in this thread... NOT!
There is a single phase with any two terminals, by definition. The two end terminals of a 120/208 only have one 208 volt signal.

To get a 3rd phase from an open wye-wye bank you have to recognize the other half of the winding is 180 degrees out of phase. If you do not recognize that difference, you will think you have 0d, 120d, and 60d voltages instead of what you really get which are 0d, 120d, and 240d voltages.

7. Originally Posted by LarryFine
No, because there genuinely IS a phase shift between the two 120v waves, which are derived from more than a single-phase primary.
If you use the same erroneous logic proposed you will get the same erroneous conclusion I stated. It's a math error that I was pointing out, nothing to do with the rest of the debate.

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Originally Posted by mivey
So with that logic you would think that the two 120 volt phases on a 120/208 system yield 120 volts instead of 208 volts.
No, you're mischaracterizing. Like I said, it's not because we don't understand the math.

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The 'logic' applies only to a 180deg phase shift that becomes a 0deg phase shift when references are reversed (or vice versa!). Since the math doesn't produce that result with any other phase shift, it doesn't apply to any other phase shift value.

Also, it wasn't a claim about logic. It was a claim about semantics.

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