# Thread: May I ask a question about the single vs two phase stuff

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## May I ask a question about the single vs two phase stuff

The following may be at the heart of what he was going on about. Note that the author admits it can just as easily be depicted as two sources 180 degrees apart. After reading the article I am finding it hard to call him dead wrong, but I would have before.

I suppose it is because I learned far more about transformers building tube audio amps. There it is quite common to use a center tapped transformer to either.....output two identical signals, 180 degrees apart......or to sum to signals 180 degrees apart into one "phase".

Gar, than you for helping me see the part about a pulsing field vs a rotating one. Probably was something I knew in the past but had forgotten.

Anyhow, curious how some of you see that article in terms of the statement that the two 120 volt sources must be in phase.

2. Careful - this topic is probably more likely to end up closed than a topic with "gun" in the title.

3. Originally Posted by electrofelon
Careful - this topic is probably more likely to end up closed than a topic with "gun" in the title.

4. To me 120/240 is single phase because it can not start a motor unless said motor has something to create phase displacement such as a cap or pole shading. 3 phase and 2 phase (90*) are rightfully called that because they can start a motor by themselves.

Just my 2 cents.

5. Originally Posted by mbrooke
To me 120/240 is single phase because it can not start a motor unless said motor has something to create phase displacement such as a cap or pole shading. 3 phase and 2 phase (90*) are rightfully called that because they can start a motor by themselves.

Just my 2 cents.
You have two phases 180° apart. Or in anti-phase. That such a supply can't start a motor is a non sequitur.
Just my two pence.

6. Originally Posted by Besoeker
You have two phases 180° apart. Or in anti-phase. That such a supply can't start a motor is a non sequitur.
Just my two pence.

Technically be definition, yes, one could call 180* apart 2 phase as it meets the literal definition. However from a practical point it gives no rotation. Keep in mind the reason why 2 phase 90* was invented by Tesla: to start and run induction motors. As latter 3 phase power. Split phase power originally came from DC and latter AC with the intent to supply more power using less wire and less voltage drop.

7. I was going to reply in recently closed thread to post linking to dictionary definition of phase but thread was closed about the time I was going to reply The problem is that dictionary definition of "phase" is too generic. And even in the electrical trades it is sometimes too specific, so you need to know the context in which it is being used. It is not a one term fits all situations kind of thing.

From a portion of that dictionary link:

— in phase
: in a synchronized or correlated manner
— out of phase
: in an unsynchronized manner : not in correlation
For normal three phase supply with no abnormalities going on:

The voltages of each leg are "in phase" from the perspective of where we expect them to be at any moment in time.

They are "out of phase" from one another from their relationship to a particular reference point at any given moment in time.

Bottom line is "phase" is somewhat meaningless on it's own, one needs to know what it is in relation to before it has any meaning.

Voltage wave on each side of a center tapped source winding is "in phase" with other side with respect to voltage above or below zero crossing point, but is always opposite polarity on each side, exception is they both always cross the zero point at same time. They are "out of phase" with respect to which side of zero they are on - except at that instant when they both cross zero.

There is no "phase" relationship that creates a rotating field for an ac induction type motor without additional methods being used to create such rotating field on such "split phase" source.

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An important reason not to call split-phase 'two-phase' is that two-phase actually means something else.

Otherwise, mathematically sin(n)=-sin(n+180), and thus it's a bit pointless to argue over which is a 'better' mathematical representation of what you read with your meter. Just specify which probe on your scope is being used as reference.

From a conceptual point of view, if the two sources are actually derived from the same transformer primary, it makes more sense to think of it as 'single phase with a center tap'. If they are something else (say two inverters that can use software to have various relationships) then perhaps speaking about mutliple phases is more justified. Ultimately, however, one's obligation is only to employ words to accurately represent what is being done physically, which can be done multiple ways as long as one defines terms clearly. Trying to settle what it 'is' is relatively pointless.

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The morning coffee hasn't kicked in yet, so I'll take a stab at this, too .

This song is called "Alice's Restaurant", and it's about Alice, and the restaurant, but Alice's Restaurant is not the name of the restaurant. That's just the name of the song, and that's why I called the song "Alice's Restaurant"
We have a system called "3-phase" because it has 3 separate wires/leads, they have a definite and understood relationship (any two leads are 120 electrical degrees apart*), and as a bonus, they can't be created without at least separately excited two coils.

*when referenced a third lead on connected to another point (electrically neutral point or the third lead).

We also have a system called "2-phase", it could be wired with 3 wires but usually has 4 or 5. They also have the definite/understood relationship (any two leads are 90 degrees*) and also can't be generated without at least separately excited two coils.

Then we have a system called "single phase".... it comes from a single coil with as many taps as you want, but no matter how many taps you have, the phase relationship from one end of the coil to any other terminal, well, it doesn't matter because there isn't a third reference point. Adding a third reference point, like a center tap, makes it look like there are now distinct "phases" but they still came off the same coil that excited by another single coil.

IMNSHO, call it "single phase", call it "split-phase", but do not call it "two phase" because that's completely different system. Whether that makes sense doesn't really matter .

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But from a pratical point....I could have two motors, both called single phase, feed from a 3 phase 480/277 supply. Both would have run caps, but one would get 277 and one would get 480. Seems the one that is 480 is a true two phase motor (albeit 120 degrees apart) and therefore should be able to run without the cap.....but it doesn't seem to.

I feel I'm failing to understand something here.

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