single vs. 3 phase

[hardworkingstiff]

Some posters have been able to post nice graphs of sine waves. They all seemed to be referenced to neutral.

Can anyone post a graph of the 3 phases of a 3-phase system without referencing anything to neutral?

 

[hardworkingstiff]

Stiff, it is a matter of definition by the IEEE I believe. See Winnie's post on the matter. No matter, old habits die hard.

What about Mivey's post?

How about IEEE:

single-phase circuit: An alternating-current circuit consisting of two or three intentionally interrelated conductors that enter (or leave) a delimited region at two or three terminals of entry. If the circuit consists of two conductors, it is intended to be so energized that, in the steady state, the voltage between the two terminals of entry is an alternating voltage. If the circuit consists of three conductors, it is intended to be so energized that, in steady state, the alternating voltages between any two terminals of entry have the same period and are in phase or in phase opposition.

See post 36 of this thread.

 

[jim dungar]

Jim it is not my method.

If it is the method you use isn't it yours?

It is simply that the OP used the CT as the reference.

Wrong, there is no mention of a neutral or CT. The OP is

why is 2 phase - 240 volt - called single phase???

And, I say so what? Let's stick to the OP's question and not redesign the entire power grid. In other words, let us stay within the framework established by the OP.

So why did you feel the need to include a neutral?

I don't see any problem with currents either. Where did you get this notion?

From previous discussions.

Jim, you had a lot of help.

All I have done is ask for a practical reason for using your method other than to explain o-scope tracings.

Jim, again let us stick to the framework of the OP's question which does include a CT.

I did. My answer was that there is a single L-L voltage therefore it is single phase (which also works for the IEEE description of 120/208 as single phase).

I didn't think it was necessary, but do you agree with that statement?

When did I say I didn't? In previous discussions I simply wanted you to include the conditions your 180? phase difference rational was based on.

I need to look at the way AC voltages are created, I can not do power system analyzes by thinking of all sources as black boxes. The physics of the secondary of a transformer are defined by the physics of the primary.To ignore the physics is to ignore the real world.



Just because you can doesn't mean you should.

 

[rattus]

If it is the method you use isn't it yours?

Jim, it is a common method, used by anyone who wishes to do so.

Wrong, there is no mention of a neutral or CT.

So why did you feel the need to include a neutral?

The CT/neutral was implied by the OP's wording--from an earlier thread I believe.

From previous discussions.

When did I say I didn't? In previous discussions I simply wanted you to include the conditions your 180? phase difference rationale was based on.

Very basic Jim. L1 and L2 are the opposing ends of a CT (in this case) secondary. Therefore, these two voltages are inverses and exhibit a 180 degree phase difference.

I need to look at the way AC voltages are created, I can not do power system analyses by thinking of all sources as black boxes. The physics of the secondary of a transformer are defined by the physics of the primary.To ignore the physics is to ignore the real world.

That is fine Jim, but we are not doing an analysis. We are merely noting that voltages V1 and V2, as defined relative to the CT, exhibit a 180 degree phase difference. You cannot deny that, can you?

 

[quogueelectric]

Some posters have been able to post nice graphs of sine waves. They all seemed to be referenced to neutral.

Can anyone post a graph of the 3 phases of a 3-phase system without referencing anything to neutral?

I posted a sine wave generator a few posts ago but the most interesting thing is that if you reference the neutral one leg will be -60* to neutral and the other leg would be +60* refrence to neutral. Yet some still insist it is single phase.

 

[rattus]

I posted a sine wave generator a few posts ago but the most interesting thing is that if you reference the neutral one leg will be -60* to neutral and the other leg would be +60* refrence to neutral. Yet some still insist it is single phase.

Quogue, try 120V, 60Hz @ 0 and 120V, 60Hz @ 60. You may be surprised.

 

[quogueelectric]

Quogue, try 120V, 60Hz @ 0 and 120V, 60Hz @ 60. You may be surprised.

Trace the two waves and the resultant wave will be at 60 dead center of the two waves . The resultant wave in the center as reference for the neutral the two windings will have a 60* phase shift in both pos and neg direction with referance to the neutral I think. Cows never give up.

 

[rattus]

Trace the two waves and the resultant wave will be at 60 dead center of the two waves . The resultant wave in the center as reference for the neutral the two windings will have a 60* phase shift in both pos and neg direction with referance to the neutral I think. Cows never give up.

Point is that the sum of these two waves is 208V which is the DIFFERENCE in 120V @ -120 and 120V @ 0. We merely shifted the phase of the first by 180 degrees and added the two waves.

 

[quogueelectric]

Point is that the sum of these two waves is 208V which is the DIFFERENCE in 120V @ -120 and 120V @ 0. We merely shifted the phase of the first by 180 degrees and added the two waves.

Not convinced. I took calc 2 in 77 so i am a little rusty but it will come back to me when prodded. The resultant single wave of the additive a/b is at 60* so if you reference the neutral at this time reference it is pretty safe and a COMMON point of the additive waves A/B

 

[hardworkingstiff]

Jim Dungar

Jim Dungar

....... (which also works for the IEEE description of 120/208 as single phase).

Jim,

Can you post that IEEE description/definition of single-phase?

In other words, is this a definition of single-phase supply or of using 2 of 3 phases to feed a single-phase service? (I believe there is a difference).

Thanks,

 

[rattus]

Works for me:

Works for me:

Not convinced. I took calc 2 in 77 so i am a little rusty but it will come back to me when prodded. The resultant single wave of the additive a/b is at 60* so if you reference the neutral at this time reference it is pretty safe and a COMMON point of the additive waves A/B

Works for me. What values did you put in?

If you put in equal amplitudes, same frequency, 120 degrees apart, you will get the third phase as in a delta.

 

[rattus]

Some posters have been able to post nice graphs of sine waves. They all seemed to be referenced to neutral.

Can anyone post a graph of the 3 phases of a 3-phase system without referencing anything to neutral?

Stiff, think of the zero value of these waves as their baseline which is not necessarily a neutral. I would expect though that the waveforms in a floating delta would tend to "float" around ground potential.

 

[jim dungar]

The CT/neutral was implied by the OP's wording--from an earlier thread I believe.

Can you answer the exact OP question without adding a CT?

Very basic Jim. L1 and L2 are the opposing ends of a CT (in this case) secondary. Therefore, these two voltages are inverses and exhibit a 180 degree phase difference.

So are you saying that I am wrong because I can't agree to use what you consider a basic concept? And again you have simplified your explanation. L1n and L2n are opposites, L1n and Ln2 are not.

That is fine Jim, but we are not doing an analysis.

Are you agreeing that your method has a limited use? And since when is solving for voltages around a circuit not an analysis?

We are merely noting that voltages V1 and V2, as defined relative to the CT, exhibit a 180 degree phase difference. You cannot deny that, can you?

I don't' believe that I have said it is not possible to represent them this way. My point has been there is no practical reason to do so.

Using the CT as your reference is a choice not a requirement, but it seems that you bring it into every discussion about single phase. Instead of always justifying that the math can be done, why not present a reason why it should be done. I will start, the use of L1n and Ln2 allows: the circuit to represent the real world way most transformers are built: allows the (2) source AC circuit to be solved simliar to that of (2) batteries in a DC circuit; and the current in a 2-wire circuit leaves and enters the "expected" terminals of the (2) voltage sources.

 

[Local26 Sean]

My two cents:

The word "phase" itself is used as a reference to electrical characteristics over TIME. Hence the terms "in phase" and "out of phase", which both refer to a comparison of two like proporties that are not equal over time.

ie: The voltage in "A" phase is rising, at the same time "B" phases voltage is falling. The purpose of refering to the different legs of an electrical system as phases is so that we can compare thier differences at any given point in time.

in a 120/240 volt system, the voltage across the coil is equal everywhere at all times. True, the center tap to a 0V (earth) potential gives the appearance of two seperate voltages, but the reality is that we are using half of one single source. No phase shift means single phase.

120/208 or 277/480 have true seperate phases, each being 120 electrical degrees apart. The angular difference results in the apparent loss of 32V when using two ungrounded conductors as a feed as opposed to one ungrounded conductor and a grounded wire. The loss is actually the result of the vectoral addition of 2 values not rising and falling in sync.

ie; "A" phase is peaking at 120V RMS while "B" Phase is at a lower value. therefore the numbers cannot add directly. Pythagoreans theorum needs to be used.

that my take on the issue. I'm sure there are other opinions.

 

[crossman]

I think that the electronics gurus would be perfectly happy with the following diagram and the use of the centerpoint as a reference for all voltage measurements. I think the math and current flow and analysis still works.

 

[winnie]

I can't answer for Rattus, but my _preference_ is to use the center tap reference in the following cases:

1) when describing how MWBCs work. This permits very similar descriptions to apply to both single phase and three phase MWBCs.

2) when describing rotary converters that develop three phases from single phase input.

-Jon

 

[wasasparky]

Freeze the 120/240V system in time.
Put the black lead on the neutral, red on L1.
Lets say the reading is +120V
Move the red lead to L2.
What is the reading? ... -120V
Advance the waveform 180 degrees.
Now what is the reading? +120V
Why is Rattus wrong?


The confusion starts when people start calling L1 and L2 "phases"...they are not. We are arguing semantics not physics...I guess that what makes it so fun!

 

[johnny watt]

I suspect that the electronics gurus would call that diagram a two voltage supply not a split voltage supply. I think that the center tap is a preferred reference because it is grounded. Doesn?t the scope reference ground by default? I think that although you can choose any point as reference for circuit analysis purposes and the math will work the ground is a preference. From a chemical point of view is there not a limit to how much charge can be removed from a mole of copper? Isn?t charge and voltage closely related?

How can you get 2 of 3 phases? Vab and Vbc, wait that's all three isn't it. I know, do like Rattus does Van and Vbn, but wait these are really nothing more than Vab which is 1 phase. I don't care how you do it, you can have two of three L-L phase conductors but you can not have two of three L-L phases.

Yes of course I would be talking Van and Vbn. I understand you cannot have two of three L-L phases what I was asking or stating is that you can derive two voltages that might fit the definition of two phase using the neutral. These voltages can and are utilized to power loads in the real world.

 

[rattus]

Jim, you have complicated the OP's question beyond recognition. Said question is reproduced below:

Does anyone know of a thorough, reliable article that explains this? I understand how transformers work in general, but I don't quite get how a single high-voltage wire goes in, and two low-voltage wires, 180 degrees out of phase with a neutral center tap, come out (on top of that, I'm sure there are plenty of different configurations out there - the one I describe is how I believe it works in Hawaii). Thanks very much for any help.

It is not a matter of what you or I or anything else would do or should do. The OP correctly identified V1n and V2n as being out of phase by 180 degrees, but you claimed that was not so.

I don't see how you or anyone else can deny that V1n and V2n are not out of phase. We are not speaking of V1n and Vn2 which are in phase. It is that simple, so please, let's keep it simple.

 

[jim dungar]

Jim, you have complicated the OP's question beyond recognition. Said question is reproduced below:

It is not a matter of what you or I or anything else would do or should do. The OP correctly identified V1n and V2n as being out of phase by 180 degrees, but you claimed that was not so.

I don't see how you or anyone else can deny that V1n and V2n are not out of phase. We are not speaking of V1n and Vn2 which are in phase. It is that simple, so please, let's keep it simple.

What original question are you quoting?

Post #1 says

why is 2 phase - 240 volt - called single phase???

I am not saying that V1n and V2n are not opposite. I am saying who cares. The only practical reason I can see for describing a 180? difference is to validate an oscilloscope voltage tracing.

And why is that I am complicating things by not agreeing with using your method, yet you are not complicating things by not agreeing with me that the method you use has a limited purpose?

1) when describing how MWBCs work. This permits very similar descriptions to apply to both single phase and three phase MWBCs.

2) when describing rotary converters that develop three phases from single phase input.

So you find it easy to say that the voltages in a single phase MWBC are out of phase and therefore combine to equal a higher voltage, and the currents are out of phase but they combine to equal zero? Or have I misrepresented your description?

I cannot figure how you use this to describe phase conversion from single to three phase