Why is residential wiring known as single phase?
- Thread starter TimWA
- Start date
- Status
- Location
- Connecticut
- Occupation
- Engineer
Yes, both current are entering the node, and none are leaving. What's the issue?
No it shouldn't, and I'm sure you know that.
It isn't MY definition of KCL, it's Kirchoff's (and I haven't messed with it, I've followed it to the letter) - At any node (junction) in an electrical circuit the sum of currentshttp://en.wikipedia.org/wiki/Current_(electricity) flowing into that node is equal to the sum of currents flowing out of that node.
For one node Ia=IL1+IL2 (Ia is flowing into the node and IL1 & IL2 are flowing out.) For the other node Ib+IL2=0 (Ib and IL2 are flowing into the node and no other current is flowing out.)
It don't see that it would be any easier or any more difficult to use Vnb instead of Vbn. The method is the same in each case. By the way, I've shown the math you asked for. Are my Line and Load currents different than what you would expect to see?
- Location
- Connecticut
- Occupation
- Engineer
...
Last edited:
T
T.M.Haja Sahib
Guest
Your reasoning went astray at the very beginning.See below
See 'N' is grounded and its voltage is 0V.So if the voltage vector from V1to N is 120V,the voltage vector from N to to V2 should be -120V,because the the voltage of N is taken as O V which is at higher voltage than -120V.The sum of two current vectors give the voltage at N.Their difference give the voltage at L1-L2.
Last edited:
T
T.M.Haja Sahib
Guest
Do not lose heart.This thread has something even for you to learn........
As for me that it is a single phase system is a given......reinforced from such books as 'American Electrician Hand book which has seen more than 15 editions to date........
kwired
Electron manager
- Location
- NE Nebraska
Wish all you want. To most of us all this discussion is meaningless, and applies to things we never deal with.
A simple answer acceptable to most installers or others that don't need to be too technical to why the typical 120/240 single phase is not called two phase was given somewhere in the first 10 posts (I think) seems like that was years ago.
This is the third or fourth thread that has gone deep into this and no one gives an explanation that just makes everyone else stop and think and then say "you are right". I don't even try to keep up with what has been said or by who I just kind of skim over the replies and find most of them too full of technicalities for the simplicity of the original question. The question is why is it called single phase not what makes it single phase.
T
T.M.Haja Sahib
Guest
It is called so because it can not create a rotating magnetic field by itself.Some phase splitting device such as a capacitor is necessary to create a second phase out of it.......
kwired
Electron manager
- Location
- NE Nebraska
Actually a pretty good answer for those that don't care about being too technical.:thumbsup:
K8MHZ
Senior Member
- Occupation
- Electrician
Excellent explanation!!!
Why does anyone here think the current on N has any importance at all?
The physics of the secondary are:
|240V| potential difference from A to B operating at 60hz
The ONLY relevance of the neutral is for safety and voltage division - NOT phase.
Grounding any circuit establishes the voltage to that ground. It does not establish 0 volts. It doesn't even have to be a local ground. Your volt meter does not measure absolute volts, it only measures potential difference between the leads. The oscilloscope doesn't display absolute volts. It only displays potential difference between the leads. Calling ground, or a grounded conductor "0 volts" is a convention to make the math easier. It's not a fact. It's not a real measurement. It's picked for convenience. It is absolutely correct and acceptable to assign the voltages as:
A = 1000 Vac; N = 1120 Vac; B = 1240 Vac
Now you have NO negative magnitudes.
The physics of the secondary are:
|240V| potential difference from A to B operating at 60hz
The ONLY relevance of the neutral is for safety and voltage division - NOT phase.
Grounding any circuit establishes the voltage to that ground. It does not establish 0 volts. It doesn't even have to be a local ground. Your volt meter does not measure absolute volts, it only measures potential difference between the leads. The oscilloscope doesn't display absolute volts. It only displays potential difference between the leads. Calling ground, or a grounded conductor "0 volts" is a convention to make the math easier. It's not a fact. It's not a real measurement. It's picked for convenience. It is absolutely correct and acceptable to assign the voltages as:
A = 1000 Vac; N = 1120 Vac; B = 1240 Vac
Now you have NO negative magnitudes.
But, entirely wrong in my opinion.
If you use the centre-tap as the reference point, and that that is eminently logical since it is the common point and usually referred to neutral, you then have two waveforms VLL1-n and VL2-n that are displaced by 180deg. As shown in post #145. A diagram with which TM agreed.
If they are so displaced, you cannot reasonably construe that they in phase.
Further rationale for using the neutral voltage as the reference point....
Take a three phase WYE connected system. Most people would see that as three line to neutral voltages at 120deg intervals. VL1-n is 120deg from VL2-n. It would be perverse to change that into comparing VL1-n to Vn-L2 and then calling it a 60deg interval.
But that's exactly analogous to what the adherents of the one phase model are doing here.
That's why I consider it to be wrong.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Don't you see a conflict in the words I have emphasized?
So explain why one node has current into and out of it, and the other one doesn't. Aren't these nodes A & B simply the opposite ends of the same piece of wire in a center-tapped transfromer?
Yes, the source currents are. My old circuits professor would have had a conniption if I said I had a node where "no current is flowing out" as that is not possible in the real world. He would have pointed out that somewhere I had assumed an incorrect current direction and made me redo the math.
Remove the load #1 so now there is one load a-b. Would you still say that node b still has two currents entering and none leaving? If you do, then wouldn't you also need to say node n has two currents leaving but non entering?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
There is a major difference in the way transformers are physically connected in series versus in wye.
A series connection is actually much closer to a delta connection than it is to a wye. Given (2) transfromers with two terminals X1 and X2 the possible connections are:
For an open delta connection we connect X1 to X2; the effect is one line connected to an X1 terminal, a second line connected to an X2 terminal, and the common point is an X1+X2 terminal.
For an open wye connection we connect all of the X1 terminals together; the effect is one line connected to an X2 terminal, another line to another X2 terminal, and the common point is an X1 terminal.
For a series connection, we connect X1 to X2; the effect is one line connected to an X1 terminal, a second line connected to an X2 terminal, and the common point is an X1+X2 terminal.
Based on this, why shouldn't a single phase series connection be solved the same as that of an open delta?
mivey
Senior Member
LOL. Just stopped by for a visit. What a fun place. Really too busy to get involved but I just can't help myself.
I think I just leave it at that and get back on down the road, except to say:
The differences in preferred definitions, physical interaction and observation methods, and convention preferences all lead to the conflict in terminology.
As for our typical alternating voltages and currents, the original definition of phase is any point on an AC wave. A specific phase of the wave is usually indicated by specifying the electrical degrees between that phase and some reference phase on the wave. That is the only technically correct use of the term "phase".
The other uses of the term "phase" are not technically correct but are common as well as inconsistent. If there is no compelling need to be technically correct, and as long as the user understands what they are trying to say, the technically correct use is not really important, IMHO. In that context, the arguing just becomes minutia, and is really based on what is essentially slang usage the term "phase" anyway.
Why is 120/240 not called two-phase?:
By convention, the term "two-phase" is reserved for the system with 90? displaced waveforms, also known as a "quadrature" system.
The 120/240 volt type systems in the EC world are called single-phase.
The 120/208 volt type systems in the EC world are network systems, but some also call them single-phase.
The naming conventions are what they are, but they are not always consistent between names, nor are they consistent with the physical make-up of the systems they describe. That is just the way it is, and you just have to memorize the names if you want to have a normal conversation with most people in our field.
One could put forth the effort to see why the names are what they are, but it is just an exercise in academia. I have done so but it is just because I like that sort of stuff. The technicalities do not matter to >99.9% of the world and even if you understand it, it will make zero impact on the day-to-day life of an EC.
But to be technically correct:
1) When alternating currents reach their corresponding (but not necessarily equal) zero, max, and intermediate values at the same time, they are said to be "in phase".
2) "Single phase" means that at any point in time, there is only one e.m.f. or current in a circuit, so at any given instant there is only one phase (using the technically correct definition of "phase").
3) A two-wire circuit has only one current and can only have a single-phase e.m.f. impressed on it. It follows that a poly-phase circuit must have more than two wires.
4) FWIW, the 2-phase quadrature system is technically part of a 4-phase system. The 5-wire 2-phase is recognized to more correctly be called a 4-phase system, but naming conventions tend to stick.
As for the use of a single-phase transformer, loads are almost always single-phase in nature. That is to say, most loads are two-wire in nature (or groups of two-wire type loads). Nevertheless, it is technically correct to say that there are two lower-voltage sources and one higher voltage source. It is also technically correct to say that the two lower-voltages can be taken in phase or with 180? phase displacements. However, the circuits that make use of the 180? displacement are in the over-whelming minority.
One can easily see that the two-wire transformer is, by default, feeding a single-phase load. That is, the current leaving one terminal is exactly the same as the current entering the other terminal and means there is only one phase in the circuit at a given point in time. Also, the current through the winding is the same all the way through. In practice, this is the only way the currents in the winding halves will be the same because as soon as you use another wire from the transformer, the load through the halves can be different. Even with perfectly matched loads, there will be a slight difference in the phase due to imperfections, although that difference is negligible in most cases.
With a three-wire transformer, you can obviously have two different currents and two different values for the currents in the winding halves. However, what you usually have is two different single-phase circuits that are in phase.
But there are some circuits that make use of the 180? displacement in the available output voltages of the three-wire transformer, and the two currents are part of the same circuit and are displaced by 180?. This is technically one of the cases where the transformer voltages are used not as separate single-phase sources but as a two-phase source with a 180? displacement between voltages.
In all reality, the voltage outputs can be used in-phase or with 180? displacements but we still call it single-phase by convention.
An easy way to picture the fact that the 0? system and the 180? system map to the same physical space has been illustrated many times:
Consider two 120? displaced voltages or currents. Allow the phase displacements to shift to 120.001?. Continue this progression until you reach 179.999?. There is nothing physically magical about the final 0.001? shift to 180?. The only thing that really changes is what we call it based on how the angle changes.
The physical reality is that both voltage sets exist and both can be used. One way or the other is not the only "correct" way because both are valid. This fact has been illustrated with practicel real-world applications many times over during these discussions.
For the minutia of minutia, I will simply refer to my previous postings on this topic as I don't feel like going through it again.
PS: The battery analogy is a poor one because DC does not have cycles that will yield a phase difference. Also covered many times in prior posts.
Truer words were never spoken
I think that is one of the biggest issues. Too many "standards" in use.
:thumbsup: A very susinct way to illustrate the problem with the naming conventions we use.
Person 1: "2+2 is 4."
Person 2: "No, no, NO; 2-(-2) is 4."
Person 3: Well yeah - you both have that part of the math right; but it doesn't have anything to do with what a phase is.
Nope. That is just another reason in the pile. Other contributors:tradition, the fact that almost every load served is a single-phase load, the transformer primary is single-phase, etc.
I think I just leave it at that and get back on down the road, except to say:
The differences in preferred definitions, physical interaction and observation methods, and convention preferences all lead to the conflict in terminology.
As for our typical alternating voltages and currents, the original definition of phase is any point on an AC wave. A specific phase of the wave is usually indicated by specifying the electrical degrees between that phase and some reference phase on the wave. That is the only technically correct use of the term "phase".
The other uses of the term "phase" are not technically correct but are common as well as inconsistent. If there is no compelling need to be technically correct, and as long as the user understands what they are trying to say, the technically correct use is not really important, IMHO. In that context, the arguing just becomes minutia, and is really based on what is essentially slang usage the term "phase" anyway.
Why is 120/240 not called two-phase?:
By convention, the term "two-phase" is reserved for the system with 90? displaced waveforms, also known as a "quadrature" system.
The 120/240 volt type systems in the EC world are called single-phase.
The 120/208 volt type systems in the EC world are network systems, but some also call them single-phase.
The naming conventions are what they are, but they are not always consistent between names, nor are they consistent with the physical make-up of the systems they describe. That is just the way it is, and you just have to memorize the names if you want to have a normal conversation with most people in our field.
One could put forth the effort to see why the names are what they are, but it is just an exercise in academia. I have done so but it is just because I like that sort of stuff. The technicalities do not matter to >99.9% of the world and even if you understand it, it will make zero impact on the day-to-day life of an EC.
But to be technically correct:
1) When alternating currents reach their corresponding (but not necessarily equal) zero, max, and intermediate values at the same time, they are said to be "in phase".
2) "Single phase" means that at any point in time, there is only one e.m.f. or current in a circuit, so at any given instant there is only one phase (using the technically correct definition of "phase").
3) A two-wire circuit has only one current and can only have a single-phase e.m.f. impressed on it. It follows that a poly-phase circuit must have more than two wires.
4) FWIW, the 2-phase quadrature system is technically part of a 4-phase system. The 5-wire 2-phase is recognized to more correctly be called a 4-phase system, but naming conventions tend to stick.
As for the use of a single-phase transformer, loads are almost always single-phase in nature. That is to say, most loads are two-wire in nature (or groups of two-wire type loads). Nevertheless, it is technically correct to say that there are two lower-voltage sources and one higher voltage source. It is also technically correct to say that the two lower-voltages can be taken in phase or with 180? phase displacements. However, the circuits that make use of the 180? displacement are in the over-whelming minority.
One can easily see that the two-wire transformer is, by default, feeding a single-phase load. That is, the current leaving one terminal is exactly the same as the current entering the other terminal and means there is only one phase in the circuit at a given point in time. Also, the current through the winding is the same all the way through. In practice, this is the only way the currents in the winding halves will be the same because as soon as you use another wire from the transformer, the load through the halves can be different. Even with perfectly matched loads, there will be a slight difference in the phase due to imperfections, although that difference is negligible in most cases.
With a three-wire transformer, you can obviously have two different currents and two different values for the currents in the winding halves. However, what you usually have is two different single-phase circuits that are in phase.
But there are some circuits that make use of the 180? displacement in the available output voltages of the three-wire transformer, and the two currents are part of the same circuit and are displaced by 180?. This is technically one of the cases where the transformer voltages are used not as separate single-phase sources but as a two-phase source with a 180? displacement between voltages.
In all reality, the voltage outputs can be used in-phase or with 180? displacements but we still call it single-phase by convention.
An easy way to picture the fact that the 0? system and the 180? system map to the same physical space has been illustrated many times:
Consider two 120? displaced voltages or currents. Allow the phase displacements to shift to 120.001?. Continue this progression until you reach 179.999?. There is nothing physically magical about the final 0.001? shift to 180?. The only thing that really changes is what we call it based on how the angle changes.
The physical reality is that both voltage sets exist and both can be used. One way or the other is not the only "correct" way because both are valid. This fact has been illustrated with practicel real-world applications many times over during these discussions.
For the minutia of minutia, I will simply refer to my previous postings on this topic as I don't feel like going through it again.
PS: The battery analogy is a poor one because DC does not have cycles that will yield a phase difference. Also covered many times in prior posts.
Last edited:
I think that misses my point. Neutral is routinely use the common reference. the 120-120 system has the merit that currents cancel in the neutral. As they do in a three phase WYE.
Van and Vbn on a centre-tapped transformer have neutral as the common point. It is logical to reference the voltages to that point. In which case they are in anti-phase.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I am missing something, doesn't the use of Van and Vnb, also use the neutral as a common point?
Are you saying my method is illogical? Or incorrect? Or unusable? Or totally without merit?
I have already stated the use of only the word 'phase' is technically undefined as regards this discussion, so there is no way I am going to deal with 'anti-phase'.
my emphasis
:thumbsup:
For some of us, it's important to be technically correct most all the time. The rest treat the subject differently
Yes it is, but it doesn't change the physical reality of a circuit. It just changes how it looks on a meter or oscilloscope. A point of view not a change in nature.
The circuit really doesn't care. It's gonna cancel at the neutral no matter what value you assign. Cause the power is traveling between A & B not AN or BN. That's just a bus stop on the hill.
Any point on the secondary coil can be considered common. Move the neutral somewhere else on the coil. You'll get two new magnitudes but it'll still be common.
TECHNICALLY the are "In-phase" and "Opposite in polarity" but as Mivey said above: If there's no need to be technically correct ...
Two hots. One neutral.
If they are going it opposite directions at every point in time, how can you reasonably construe that as being in phase?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Because, as I have said several times (most recently in Post 179), the phase of a function has nothing to do with polarity.
- Status