guitarboyled

Back to basics,

For standard residential service I understand that both lines are 180 degrees out of phase and this is why we can get a difference in potential of 240v when both lines are used.

120v + (-120v) = 240v.

But what device creates the out of phase and where is it located?

InPhase277

In this case, the 180 degree shift is nothing magical. It is simply polarity. It means that the ends of the coil are equal and opposite at any instant of time. In this exact same sense, the poles of a battery are "180 degrees" opposite. North and south are "180 degrees" opposite. In other words, somebody used the term 180, and it happens to have the same phrasing as the 120 degree shift used in 3-phase terminology, except it really doesn't mean a phase difference any more than north or south are out of phase.

The device that creates it is the transformer. At any instant of time, current is flowing in one direction, and therefore the magnetic lines of flux have a specific polarity, driving a current in the secondary a certain direction, creating a positive potential on one side and negative on the other. In the next part of the cycle, the polarity changes, so negative becomes positive and positive becomes negative.

In other words, at any instant, one wire is positive and one wire is negative, thus the idea that they are "180 degrees out of phase".

Loose Neutral

I believe the source of the phase difference comes from the generator producing the electricity. I don't remember the exact terms but taking a stab. I think the 2 magnetic poles on the generator are mounted 90 degrees apart. If it were three phase they would be mounted 120 degrees apart.

steelersman

I don't think so.......

MDShunk

There isn't really any such thing as 180 degrees out of phase. It's just the difference from one hot wire to another. That's just a sine wave. The 120 degrees is when you're talking about the whole consist of 3-phase generation. In the single phase, you don't really have any reference point like you do with the whole 3-phase supply.

The 120/240 in a house is more like the output of a potentiometer. We can only measure from one end to the midpoint (120V), or one end to the other end (240V) but if the transformer midpoint were shiftable like a pot, we could make that measurement from one endpoint to the middle anything from 1V to 240V.

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Peter D

There have been very long debates about "phase" as it relates to 120/240 derived from a single phase of a wye or delta distribution system. As for my feeble mind, I can't understand how you can have a phase relationship with something derived from only a single phase, but surely better minds will prevail in this discussion.

InPhase277

I don't think there is a single phase distribution system. All single phase is derived from one leg and a neutral, or two legs, of a three phase network. Even if the three phase sine waves are 120 degrees apart, the current in just two wires can only flow in one direction at a time. Which means the magnetic poles of any coil attached to those two wires have to be "180 degrees" opposite of each other.

MDShunk

No!! Not 180 degrees out of phase for the same reason that 120/240 is not "2 phase electric".

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Loose Neutral

What do you think.......

Loose Neutral

Of course there is a phase difference, put it on an o scope and you can see that. If not your 2 wire circuit would turn in to a phase to phase fault. With them being 180 out we are able to have 240 volt circuits.

InPhase277

That's why I put it in quotes. It is 180 degrees out in the geometric sense, in the same sense that the poles of a battery or magnet are 180 degrees opposite. That is, they are equal and opposite at any instant in time. It is unfortunate terminology that comes from 3-phase systems, and it keeps popping up.

I'm trying to clarify the difference between the electrical degrees that apply to 3-phase circuits, and actual geometric degrees that mean "equal and opposite"

MDShunk

Got it. I guess we're saying the same thing in different terms.

If the supply was actually 180 degrees out of phase, the voltage would be ZERO!

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MDShunk

Sure, go ahead a post those pictures... all you'll find is a change in amplitude.

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InPhase277

But it's not a phase difference. It is a single phase, so the ends of the coil are "in phase", but opposite. Like I said in the other posts, you are mixing terminologies that have phrases that are "homophonic". In other words, saying that the wires in a 3-phase system are 120 electrical degrees "out of phase" has a particular meaning. But saying the wires of a single phase system are "180 degrees out of phase" is meant in a completely different context, even if the terminology is similar.

In the latter usage, your left and right hands are "180 degrees out of phase". Day and night are "180 degrees out of phase". Blue and yellow are "180 degrees out of phase". Do you see what I'm saying here? I know it is hard to grasp, due to the similarity of the wording. These are geometric degrees, not electrical degrees.

Loose Neutral

Man that was a fast response. Don't have an o scope handy that was a long time ago in school. But just to clarify your saying that there is not an 180 degree difference between Leg A and Leg B on a single phase service.

MDShunk

Absolutely not. They are "in phase". (∠ 0 degrees), otherwise they wouldn't add to get 240. Electronics guys might call this "split phase". Google that, if you care to. The secondary of the 120/240 transformer is just like a potentiometer with a fixed wiper... fixed in the middle.

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steelersman

you were saying that the 2 poles on a generator were mounted 90 degrees apart. I'm saying I don't think so. I'd say if their mounting has anything to do with it I'd say that they are mounted 180 degrees apart since 120/240 is 180 apart from each other. That's all.

Loose Neutral

Where's the electrical engineer when you need one. I'm simply saying i think the phase difference is derived from the generator and leg A's sine wave goes up and leg B's goes down. But I could be wrong it has been a while since school.

InPhase277

I don't possess the finesse with words that will make it clear, but let's see...

Single phase is just that: "a single phase". The ends of the coil are opposite in polarity at any instant, so in a geometric sense, they are "180 degrees" opposite. However, because it is a single sine wave, it makes no sense to speak of its opposite peaks as being "out of phase". How can something be out of phase with itself? A single sine wave is, in fact, perfectly "in phase", because any point you pick will have an equal and opposite value.

In 3-phase, you have three separate sine waves, so it is perfectly legitimate to speak of them as being "out of phase", because you are comparing them to each other.

heel600

I thought it was called single phase because 120/240 comes from just one phase, and it's exactly 180 because each phase is wound in opposite directions