Is it Single or Two Phase?

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rattus said:
Matt, you have pulled ONE phase from the transformer. You no longer have a three-phase system. Va, Vb, and Vc are separated by 120 degrees, but since you only have Va for example, you can no longer reference Vb and Vc.

V1 and V2 add to 240V because the phase angle is 180 degrees. If the angle were 120 degrees, they would add to 208V.

If you look at L1 and L2 relative to the neutral, you will see a separation of 8.33 milliseconds--half a cycle, 180 degrees.
Click to expand...


First, how are you getting V1 and V2 from just Va? I am thinking delta here, so there needs to be Va-Vb, or such.

Second,

rattus said:
V1 and V2 add to 240V because the phase angle is 180 degrees. If the angle were 120 degrees, they would add to 208V.
Click to expand...

I think you are incorrect here. For the delta, a 240 V voltage drop is across the whole winding (I agree that this is from the number of turns, and is basic, and that if you measure the center of any of those windings you get 120 V).

The reference angle is still 120 (between Va-Vc) for me because it still relates to generation. The 120 deg phase angle is still supported for the delta case, take a look at my drawing...

http://mpross.public.iastate.edu/delta wye.bmp

I think there is still confusion on just what exactly is a "phase angle".

-Matt
 
180 degrees?

180 degrees?

kingpb said:
I have watched the debate go back and forth regarding the voltage angle between lines L1 and L2 on a 120/240V system. (I have purposely not mentioned phases, damn, I just did!)

Anyway, has it FINALLY been accepted that they are 180 deg apart? "If so, let's put that to bed and just stick with the phase issue ( A-rrr, did it again!) Phase, phase, phase, when will it ever end?
Click to expand...

Take a look at my drawing.

-Matt
 
jwelectric said:
What is the difference between a "phase" and the sine wave I see on a scope?
Are they the same thing?
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Hey Mike,

Depends on what you are thinking about. What is the scope measuring and at what points are the measurement taken?

What are you thinking of when you say "phase"? That term has a lot of varying meanings in this thread.
 
Matt,

Your drawings are of voltage systems derived from three phase sources, and therefore are not applicable to this topic. The topic is single phase or two phase.

I believe everyone should be in agreement that on a three phase system the line-to-neutral voltages are in fact 120 deg apart, when measured from the same reference point, with respect to each other. This also applies to the relationships between the line-to-line voltages.

Agreed everyone?
 
huh?

huh?

I think understanding the three phase relationship has everything to do with single phase, and this discussion!

Also,

kingpb said:
I believe everyone should be in agreement that on a three phase system the line-to-neutral voltages are in fact 120 deg apart, when measured from the same reference point, with respect to each other. This also applies to the relationships between the line-to-line voltages.
Click to expand...

I can see that you are one of the confused ones about the phase relationship. "line to neutral" being 120 deg apart? It has nothing to do with the neutral!

Single phase is derived from three phase, are you forgetting this?

-Matt
 
Matt,

It is apparent that you are trying to understand something, that is good. However, in order to do so, you have to be willing to accept some things that are possibly outside your current capabilities. By your last post it is obvious that you do not completely understand line versus phase voltages, in relation to wye and delta systems.

As far as being confused, since you do not know of where my education comes from, or my line of work, it would better for you to keep your statements of whom is confused, to yourself.

Believe me, there is no confusion in my understanding of power, or power related calculations.
 
lets see what others have to say :)

with respect to line and phase voltages, I thought the same thing about you.

Let me restate... "I think that you are one of the confused". There, now it is my opinion :)

Sorry for any offenses :)

-Matt
 
kingpb said:
I believe everyone should be in agreement that on a three phase system the line-to-neutral voltages are in fact 120 deg apart, when measured from the same reference point, with respect to each other. This also applies to the relationships between the line-to-line voltages.
Click to expand...
Matt,

Remember, the phasor diagram representing the voltages on A, B & C has a center reference point.

The lower half of your diagram ( http://mpross.public.iastate.edu/delta%20wye.bmp ), while suggesting a phasor diagram, is really tied to the transformer secondary physical configuration.

The reference at the center of the voltage phasor diagram rests in the "middle" of the triangle in the lower half of your diagram, not in the middle of the line between V1 and V2.
 
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Who is confused?

Who is confused?

Matt,

Be assured that you are confused. Your phase diagrams do not have arrows, therefore, they are incorrect. Your lines with arrowheads indicate currents, not voltages. You apparently have added two phase voltages of a 240V delta to obtain the negative of the third phase. This does not apply.

You need to realize that once you pull a 240V service from a delta secondary the 3-phase angles are meaningless. We can arbitrarily assign any phase angle to L1-N, then the phase angle of L2-N is 180 degrees out.

You need to listen to your elders, those with BS, MS, and PE after their names and with decades of experience. Until you do, you will flounder in a sea of confusion.

One of my old profs told me recently that AC Circuits is not taught in the engine schools anymore. Pity!
 
I am comfortable with the concept that 120/240V 3-wire circuits can be thought of as (2) single phase 120V 2-wire circuits with a common connection. However if they are said to be 180 degrees apart how can they be added to any value other than 0? (1cos(0) + 1cos(180) = 0).

Much of this discussion seems to have been caused by the incorrect/inappropiate use of the term "phase". Try restating your arguements by substituting "Line to Line" instead of Phase, if you can do this then you have created a possible mis-understanding. For example: "A 3-phase system has 3 line to line voltages" is less confusing than "A 3-phase system has 3 phase voltages".

Most of this mis-use is because of the 3-phase terminology for delta to wye conversions that incorporate short hand references like VP = VL *1.73. This formula is wrong because it identifies a voltage by using a single reference instead of a more accurate two reference version like VL-L =VL-N *1.73 or even VP-P =VL-N *1.73.
 
Jim,

In answer to your first question: We SUBTRACT L2-N from L1-N, not add!

For example, 120 @ 0 -120 @ 180 = 120 @ 0 +120 @ 0 = 240 @ 0.
 
1 at 0 degrees = -1 at 180 degrees. (It's just a math thing)

Three phase is three generated "outputs"/phases, connected in delta or wye.

Single phase is one generated "ouput"/phase. (Center tapping or not)


Envision a vector pointed up at 240.
All center tapping does, is break it into two 120 vectors, pointing up end to end.

Now we can take that bottom one and make it point down, AND note it is 180 degrees out of phase with the other.

That does not change it's physical nature, just a double-negative convention.
 
Jim,

I agree with your assessment of mis-use. I think that for so long, they have been inappropriately applied, that the line has become blurred.

I was able to find in the 13th Edition of Electrical Engineers Handbook, where they specifically made a point of distinguishing between phases and line voltages. This provided the best explanation of how the different systems are known, i.e. 1ph, 2ph, and 3ph, and clarified the argument for me that the 120/240V system should be considered a single phase system. It's kind of disturbing to learn through this discussion how many text books are using it out of context.
 
Sorry Matt, but in your lower diagram, one of the arrows should be reversed, so they're both either clockwise or counter-clockwise.

That would show the proper relationship of current in the shared neutral. You have it as additive, when it should be subtractive.
 
rattus said:
Jim,

In answer to your first question: We SUBTRACT L2-N from L1-N, not add!

For example, 120 @ 0 -120 @ 180 = 120 @ 0 +120 @ 0 = 240 @ 0.
Click to expand...

If there two 120V transformer windings connected in series (winding X1-X2 and X3-X4) the only reason you would say that the voltages are subtracted from each other is to justify your 180 degree logic. Because when the discussion is 3-phase wye circuits the common statement becomes "add the voltages vectorially". How about some consistency?
 
While all the math is fascinating to watch, it doesn't answer the original question. As I look out my window I see a single POCO primary feeding the pot that ultimately feeds my house, 120/240v service. How many phases can there possibly be on one line at 60hz?
 
LarryFine said:
Just to point it out, notice that your own attachment shows that, even with four "phases" (hot wires) shown, the drawing refers to it as two phase power. The point is that adding a center tap doesn't create an additional "phase". ;)
Click to expand...
Actually, mid-tapping both phase windings and connecting them together "derived" four additional phases (not counting the original two/four) though they were to my knowledge never used (I think four was more than enough). These created the voltages denoted as 155V in the diagram, and they are at [generator] armature angles 45?, 135?, 225?, and 315?.

View attachment 56
 
RHaggie said:
One phase; two legs in your example.
Click to expand...

Seems like the answer to the OP if I'm not mistaken. The excursions into other phase derivations look like smoke screens to me. I don't see where the additional pahses have any relevance. Residential services are normally single phase in rural areas how is the derevation that different in a more urban area?
 
confused

confused

first,
i have updated my drawing, as i never intended it to be a phasor diagram. i meant for it to be a depiction of secondary windings, and instantaneous voltage values.

so i plan to talk with rattus about this issue personally :) as i will admit that i am confused, and having some issues with this topic.

last, and in this case most important. i am sincere in saying that i have the highest level of respect for my seniors. i think i lose track of who is on this forum, and that is not a good thing. so for now, i am bowing out of this topic and will listen to my seniors.

i hope that i did not offend anyone, especially kingpb :). i really do want to learn!

-mpross
 
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