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1. Originally Posted by Sahib
So the only 240/120V with or without neutral is incapable of producing a rotating magnetic field by itself and so must be a single phase system.
That's always been my take.

Two lines from a wye supply with the neutral can supply two transformers and create an open delta, but two lines alone are definitely a single-phase source.

2. Originally Posted by LarryFine
That's always been my take.

Two lines from a wye supply with the neutral can supply two transformers and create an open delta, but two lines alone are definitely a single-phase source.
The 120-0-120 arrangement is two lines phase displaced by 180° plus a neutral.
I think we can all agree on that.

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Originally Posted by Besoeker
The 120-0-120 arrangement is two lines phase displaced by 180° plus a neutral.
I think we can all agree on that.
Actually, I am afraid we can't.
Some say that 120-0-120 has two line to neutral voltages, of which one is the negative of the other. Others say that it has two line to neutral voltages offset in phase by 180 degrees. The two are equivalent for pure sine waves and ones with symmetry about the 90 degree time point. But if the basic waveform is sufficiently far from ideal (a sawtooth wave is a good example to look at) the two descriptions (negative versus 180 degree phase shift) are not identical.

[/B] equivalent.

4. Originally Posted by Besoeker
The 120-0-120 arrangement is two lines phase displaced by 180° plus a neutral.
I think we can all agree on that.
I agree that we can't. The only application of 180 degrees is to refer to instantaneous polarity, and that's only because of the choice to use the center tap as a stationary reference point. There is no displacement.

Added: If viewed on a dual-channel o-scope, the two waves might appear displaced in time, but the fact is they rise and fall in sync in opposite directions from zero simultaneously; thus, it's a matter of polarity.

You really need to find and read that incredibly long thread we did.
Last edited by LarryFine; 01-11-19 at 06:53 AM.

5. Originally Posted by GoldDigger
Actually, I am afraid we can't.
Some say that 120-0-120 has two line to neutral voltages, of which one is the negative of the other. Others say that it has two line to neutral voltages offset in phase by 180 degrees. The two are equivalent for pure sine waves and ones with symmetry about the 90 degree time point. But if the basic waveform is sufficiently far from ideal (a sawtooth wave is a good example to look at) the two descriptions (negative versus 180 degree phase shift) are not identical.
[/B] equivalent.
Yes. Two. Displaced in phase.
i.e. not the same phase.

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Originally Posted by Besoeker
The 120-0-120 arrangement is two lines phase displaced by 180° plus a neutral.
I think we can all agree on that.
I wonder if you will get anyone to agree with that.

The two hot legs brought by themselves without the neutral have a voltage which isn't displaced from anything.

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Originally Posted by Besoeker
The 120-0-120 arrangement is two lines phase displaced by 180° plus a neutral.
I think we can all agree on that.
You're new around here, aren't ya'?

(I know you aren't)

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Originally Posted by Sahib
So the only 240/120V with or without neutral is incapable of producing a rotating magnetic field by itself and so must be a single phase system.
Originally Posted by mivey
...

Think about it: The 3-wire quadrature two-phase we get from a transformer bank is just the 120/208 system shifted from 120d to 90d. We could also call 120/208 two-phase except that the name has already been taken by the quadrature two-phase system.
Here we are (finally?) getting back to what the OP was actually asking about. How can 120/240 be considered more than one phase if it is incapable of producing a rotational field? If that's what single phase means (no rotational field) then 120/240 is single phase. But I doubt that we can get Boeseker to agree that that's the meaning.

To that point, Mivey, you're exactly correct that the three wire quadrature two-phase is qualitatively similar to the 120/208 3-wire system. But I don't agree that the latter is not called 'two-phase' because the name is already taken. After, all, a lot of utilities call it 'single phase' and that name was already taken, too. Which just proves that people have been using disparate conventions for too long, and this will never get resolved. At least not in a free society. I concede the point on that.

9. Originally Posted by jaggedben
I wonder if you will get anyone to agree with that.

The two hot legs brought by themselves without the neutral have a voltage which isn't displaced from anything.
But that wouldn't be 120-0-120, would it..................

10. Originally Posted by jaggedben
Here we are (finally?) getting back to what the OP was actually asking about. How can 120/240 be considered more than one phase if it is incapable of producing a rotational field? If that's what single phase means (no rotational field) then 120/240 is single phase. But I doubt that we can get Boeseker to agree that that's the meaning.
With respect, the capability of producing a rotational field is not of relevance. I designed multi-phase high current rectifiers. No rotation involved.

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