2-phase power

[johnny watt]

Okay, I have a problem that I need everyone to help rectify
I have 2-phase power in my house. Not only that, but about every house I go into has 2-phase power.

The problem is that no one in the industry has ever bothered to take note of this and therefore it isn’t even recognized much less commonly thought of or named in that way.

It seems we skip from single phase and go directly to 3 phase when it comes to nomenclature.

Let me assure you if you take an oscilloscope and measure the two 120volt legs you can see that they are 180 degrees out of phase and that they are two distinct phases.

Or if you don’t have a scope you could find two wires that both measure 120volts to ground and touch them together and see what happens. If it were single phase then nothing would happen. But in most houses you have a 50/50 chance of making some sparks.

So please everyone, from now on refer to 120volt 200-amp single phase as 2-phase.
Thank You

[GregFretwell]

I hope you are burning the red mast head light as vessels dragging fishing gear are required ;-)

[jwelectric]

Johnny Watt

Your said

Let me assure you if you take an oscilloscope and measure the two 120volt legs you can see that they are 180 degrees out of phase and that they are two distinct phases.

For a residential panel this is very true. There is a reason that it is true. There is two different ways to achieve this. One is to center tap one phase. This will give you two sine waves that are 180 degrees out of sink. The other is to use two transformers.

Think about three people standing side by side. The one on the left is person (A) the one in the middle is person (N) and the one on the right is person (B).
Now person (A) walks 120 steps to the left and person (B) walks 120 steps to the right. Both (A) and (B) are 120 steps from person (N) but (A) is 240 steps from (B).

Now let’s look at a delta transformer. It is drawn as a triangle with three corners that can be labeled (A), (B) and (C) from (A) to (B) is one phase of the three phases of the delta transformer, thus when it is taped in the center it is 120/240 volt single phase. Only one phase is being used.

When we refer to two phase electrical we are referring to two phases that are 90 degrees out of phase to each other. These will read the same to ground but this will not be doubled when reading phase to phase as in the residential panel.

Let’s say that the polyphase circuit (two phase) is 120 volts to ground for each phase then the voltage between the two phases would be about 170 volts.

Ceck out this link
http://en.wikipedia.org/wiki/Two_phase

[johnny watt]

Mike,
Thanks for taking the time to entertain my thoughts on this.

I just don’t get why they adopted a convention that says that 2 phase has to be 90 degrees out of phase with each other in order to be called 2 phase. Must have been the same people that told Tesla that an ac motor was impossible to invent. As I said, and you know, if you look at them with a scope (120v/240v) the two legs are not in phase with each other. So they must be out of phase with each other. Seems like you would have to have 2 phases to have one be out of phase with the other. And if you have two phases, it seems like you should call it 2 phase power.

Makes sense to me that a better naming convention would be:
2 phase is….2 legs 180 degrees apart
3 phase is….3 legs 120 degrees apart
4 phase is…4 legs 90 degrees apart

If you give the voltages such as 120v/240v then the phase relationship has been defined
and therefore no phase angle is necessary to describe the power.
And so you would have:
120v/240v 2 phase…is 180 degrees
120v/208v 2 phase…is 120 degrees
120v/268v 2 phase…is 90 degrees

Back to reality, are you saying the naming convention in use today is referencing the phases at the Generation plant?

[GregFretwell]

Your house is single phase, center tapped. There is only one winding of one transformer doing the work. Don't let the location of the grounded conductor confuse you.

[jwelectric]

posted by Johnny
Back to reality, are you saying the naming convention in use today is referencing the phases at the Generation plant?

This is correct

posted by Greg
Your house is single phase, center tapped. There is only one winding of one transformer doing the work. Don't let the location of the grounded conductor confuse you.

This is correct


Remember this; in order to get 120/240 a delta transformer is used. When a wye transformer is used we end up with 120/208
The 120 is derived from reading from one point on the transformer to ground. What is ground?

Article 100 NEC
Ground. A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some conducting body that serves in place of the earth.

When we take a voltage reading from line (A) to line (B) in the 120/240 volt house panel we are reading the voltage of one phase not two.

When dealing with a triangle we can take away one side of the angle and leave the rest behind, but when we take two sides of the triangle away we have taken away the whole triangle or all three points.

[johnny watt]

Okay I’m starting to understand. If you would just bear with me a little longer.

First some points:
Why do we name the phases based on what happens at the generator plant? We don’t call it 13.2kv simply because that was the output at the generator. We rename it based on a description of what is delivered to the panel. If there was an inverter that ran off one phase but delivered 3 120V legs 120 degrees apart would that be considered single phase?

Center tapping a transformer is essentially two transformers. If there were no center tap, ground was never referenced to the secondary and no current were ever present on the neutral then I would quickly agree with the terminology that it is single phase.

Maybe what confuses me is what I always inferred the meaning of phase to be. The word phase has to do with the timing of a cycle. In a residential panel you have 2 legs that are basic building blocks. Since the two legs are not in phase it makes since to me to call them phase 1 and 2. But since that isn’t the practice, how do we describe the two legs in the trade to make sure that everyone knows that leg one is not the same as leg two?

In a 120v/208v 3 phase system is 120v to neutral considered a phase? Or does it have to be 208v to be considered a phase. Are devices that run on 2 legs considered to be running on 2 out of 3 phases and therefore 2 phase devices?

I guess I could clear everything up with the answer to this: what is the definition of “a phase”?

[GregFretwell]

I see your point of confusion. Electricians have 2 definitions of "phase".
Properly they define phase as referring to the angular relationship of transformer windings but confusingly they also call each ungrounded conductor a phase.

[johnny watt]

Originally Posted by GregFretwell

I see your point of confusion. Electricians have 2 definitions of "phase".
Properly they define phase as referring to the angular relationship of transformer windings but confusingly they also call each ungrounded conductor a phase.

Yupper!

But the root of the problem is even more deeply ingrained in my brain, from years ago in high school math and science classes.

In these classes, when studying sinusoidal waves they would use PHASOR DIAGRAMS, (not to be confused with Captain Kirks weapon schematic) and VECTORS.

More often than not (but not always) the waves would be referring to voltages or currents.

The waves would be described with a magnitude and a phase angle (usually in radians).

The “phase angle” was also referred to as just plain “phase”

And so “phase” came to be known as the angle of a wave as referenced to t = 0

But the reason for my rant on this subject is “how do we describe the two legs in the trade”. We have two 120 volt legs and they have two different phase angles with respect to neutral but we can not call them phase 1&2 or phase A&B or two phase.

On another thread “12/3 w/ ground used for 2 curcuits” it is answered, “it's fine as long as the breakers are on separate busses.”

Is that the answer, are we relegated to calling two out of phase voltages “separate busses”?

[littlebluetruck]

Kiind of like the pedals on a bicycle I suppose...

[jwelectric]

Johnny
You answer you own question with this statement;

“how do we describe the two legs in the trade”.

We describe them as two legs of a single phase.

[fmclaren]

There is a common error committed when placing an oscilloscope on a residential 120 / 240 Volt system. People usually connect one lead of the scope to Line 1 and the other to Neutral. This shows a sinusoidal wave with an RMS value of 120V.
When looking at the Line 2 to Neutral voltage they remove the Line 1 scope lead and connect it to Line 2 while leaving the neutral connected scope lead on the neutral. The scope now shows a sinusoidal wave that appears to be 180 degrees out of phase with the first wave.
The L1 to N wave and the L2 to N wave are actually in phase with each other. If they were 180 degrees out of phase the L1 to L2 voltage would be 0 Volts - but, it is not, it is 240 V. (The vector sum of 2 - 120V in phase voltages is 240V. The vector sum of 2 - 120 voltages, out of phase by 180 degrees, is 0 Volts.)
The problem is the scope connection. To see the true relationship of these waves you must reconnect BOTH leads of the scope and not just ONE.
L1 scope lead should go to N, and the N scope lead should go to L2. If this is done then both waves will appear to be in phase and a true representation of the wave relationship will be obtained.
Residential services are single phase 240V services that have been split by a center tapped Neutral conductor. They should be called split-phase services.
Two-phase power is traditionally 2 voltage vectors 90 degrees apart.

Be careful when changing the scope connection. Plug the scope into an isolation transformer to avoid connecting a grounded scope lead to a hot conductor.

I hope this helps

[johnny watt]

Fmclaren,
Thanks for sharing your thoughts on this. Correct me if I’m wrong but I believe you are mistaken with the scope measurements you are describing.

The best way to view it would be to use a duel channel scope. If you put CH1 on one leg and CH2 on the other simultaneously, you would see two sinusoidal waves 180 degrees out of phase. Neutral is the nominal point of reverence. If neutral/ground were never established, or even if neutral simply never had current on it, then it would be fair not to use it as the reference. From the loads point of view neutral is the reference. Even many 220V devices still have some current on the neutral.

You are correct that the vector sum of two 120V voltages, 180 degrees out of phase is zero. However, unless you connect both legs together with a dead short you are not “summing them”, you are just measuring them. If you did connect both legs together, you would indeed see zero volts. (Provided you are not consumed in a cloud of plasma)

To name the service “split phase” implies that a phase has been split. And since we know that it is split in the center, we could say that it was split in two…I think you know where I’m going with this but I’ll stop…

The traditional semantics used in the industry are now clear to me. Personally, I think they are intellectually dishonest because they appropriated the word “phase” to have a particular meaning that is much more restrictive then when the word is used in the sciences. Indeed, if a circuit analysis of a house were studied in a science class, the supply would probably be labeled Vsub1 and Vsub2 and the voltages would be described as “out of phase with each other”.


I guess the biggest problem is that science and electricity work closely with each other. If they were not associated so closely I would not have been confused in the first place. For instance, I was never confused from the first day that I heard the term “homerun”.

[littlebluetruck]

Thanks Johnny... I'm with you but I wasn't sure how to articulate it. There is indeed 240 v between phase 1 and phase 2, but the phases have got to be 180* apart or else they wouldn't cancel. That is what makes possible the 12/3 homerun which we've been talking about in the other threads.

edit - inaccurate language

[fmclaren]

Appreciate your responses Johnny & lbt. Before I get back to you can you tell me if there is any way to post graphics along with text on this site? Sometimes a picture paints a thousand words.
thanks