Single Phase or Not.

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donnie

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Kentucky
Am I correct by saying that even though a panel that has 2 hots and 1 neutral for the feed, that this is considered a single phase instead of a 2 phase, I know 3 ph has 3 hots but I was thinking I read somewhere that 2 hots is still considered single ph because of the windings in the transformer. And If so this would bring another question to mind for Art 210-8 (b).
 
Re: Single Phase or Not.

Try not to lok at the individual conductors as phases. A better term would be poles. In a single phase system, you will have two poles 180 degress out of phase with each other producing currents exactly 180 degrees out of phase. This is your "A" leg and "B" leg of a typical 3-wire system. 3-phase differs in that the poles are 120 degrees out of phase with each other. This is your three legs.

2-phase is a completely different animal that is practically obsolete. This system consists of four poles 90 degrees out of phase. :)

[ May 12, 2004, 05:49 PM: Message edited by: bphgravity ]
 
Re: Single Phase or Not.

Many times I have noticed, in discussions involving 120/240 volt three-wire circuits, statements that say, in effect, that the currents in the two hot (ungrounded) conductors are opposite in direction because they are "180 degrees out of phase with each other."

It is accurate to say that a current can be "out of phase" with one or more other currents, a voltage can be out of phase with one or more other voltages, or a current can be out of phase with a voltage.

But, the current flowing in the two hot wires of a 3-wire circuit is actually the same current. (If they are not equal, the difference flows on the neutral wire.)
At any instant the current is flowing towards the load in one of the wires, while flowing back to the source in the other wire. During the next cycle the current flow is reversed.

This is why a three-wire service or feeder supplied from a three phase distribution system is called single phase.

Ed
 
Re: Single Phase or Not.

In single phase systems the current/voltage in the two hot conductors are not out of phase with each other. The two hot conductors are out of phase only when referenced to a neutral conductor.

Single phase and three phase describe the relationship between hot conductors without any reference to a neutral. Three phase means three hot combinations (L1-L2, L2-L3, and L3-L1), single phase means one hot combination (L1-L2).

In a two wire 240V circuit there is only one current and therefore only one phase. The addition of a center tap does not change the relationship of the hot conductors.
A two wire 480V circuit also has only one current and therefore is still referred to as single phase even though even though the system is three phase.
 
Re: Single Phase or Not.

I do not agree that the phrase ?180 degrees out of phase? does not apply to a single phase 240 volt circuit. It may just be a matter of ?mathematical semantics,? but I submit that the phrase does apply.

Let?s look at if from the perspective of a person monitoring each breaker in a panel. But let us first define current to be ?positive? when it leaves the panel, and define current to be ?negative? when it enters the panel. Also, let us define voltage at the wire?s connection to the breaker to be ?positive? during that half-cycle when the wire has a higher voltage than the neutral point, and let us define voltage at this same point to be ?negative? during that half-cycle when the wire has a lower voltage than the neutral point.

Now have the ?breaker monitor person? take a look the wires leaving positions 1 and 3. Never mind whether the ?breaker monitor person? is aware that a single breaker takes up both positions, but have him only look at the wires. You and I, looking at the whole circuit, will notice that a stream of electrons leaving position 1 will go through a 240 volt load and return to the panel via position 3. But the ?breaker monitor person? does not need to know that.

We ask the ?breaker monitor person? to measure all voltages and currents related to positions 1 and 3 at one specific point in time. Ready? Go! He takes his measurements, and informs us that the current at position 1 is positive, the voltage at position 1 is positive, the current at position 3 is negative, and the voltage at position 3 is negative. Now we all know that this is one circuit, one load. But the ?breaker monitor person? is telling us that he sees the two wires showing opposite characteristics.

It is in this sense that we can legitimately say that positions 1 and 3 are opposite in phase, or that they are ?180 degrees out of phase with each other.? It is our attempt to create a mathematical representation that causes this to happen. We had to chose a convention for positive and for negative. Having done that, we are forced to recognize that a single circuit has current leaving one position and entering another position, all during the same half-cycle.
 
Re: Single Phase or Not.

You can safely call a 240/120V voltage a "single phase" voltage. I don't think I have ever heard anyone use the term dual-phase or two-phase, and it definately is not three phase.

210.8(B) specifically lists 125 volts, so 240V outlets do not need GFCI protection (it doesn't matter if you call them 1 phase or 2 phase).

Steve
 
Re: Single Phase or Not.

Charlie B,

Using your example then the positive pole of a DC system is 180 degrees out of phase with the negative pole.

Before you can define something as positive or negative you need a reference point. Once you chose the point (in your case position 1) then you can monitor the other point (in your case position 3). Now with only two points, it is not possible to say one point is out of phase with the other point.
 
Re: Single Phase or Not.

Originally posted by steve66:
I don't think I have ever heard anyone use the term dual-phase or two-phase, and it definately is not three phase....

...(it doesn't matter if you call them 1 phase or 2 phase).
Two-phase is a real and separate system from single phase and three phase. :)
 
Re: Single Phase or Not.

Originally posted by jim dungar: Using your example then the positive pole of a DC system is 180 degrees out of phase with the negative pole.
The term ?phase? cannot be applied to a DC system. Neither, then, can the phrase ?out of phase.?
Before you can define something as positive or negative you need a reference point.
The point of reference that I selected is the enclosure. Let?s call it a train station. Trains leaving the station, along any track, constitute ?positive train flow.? Trains entering the station, from any location, constitute ?negative train flow.?
Now with only two points, it is not possible to say one point is out of phase with the other point.
I?m not. What I am saying is that the current ?leaving? (i.e., defined as positive?) one point (i.e., breaker position #1) is 180 degrees out of phase with the current that is ?leaving? the other position (i.e., breaker position #3). That is because the one is positive (by definition) at the same time that the other is negative (by definition).
 
Re: Single Phase or Not.

Phase, means time domain. A point of a period when something occurs. A customary minute or second when an event happens.

An activity that happens at the same time is one phase. Any activity that happens in two time domains is two phase. Any activity that happens in three time domains is three phase.

Domain, means; A set of elements to which a mathematical or logical variable is limited. Specifically , the set on which a function is defined.

To put it plain(I hope) If it starts and stops at the same time, it is single phase.

Only in lektricty can one from three equal one.
 
Re: Single Phase or Not.

First of all let me give you the types of services we had in our country(asia) for clarification purposes only:

1.Single Phase: 2 wire service drop (both hot or one hot and one neutral) depending on local electricity supplier.
: 3 wire single phase (2 hot and one neutral

2. for 2 phase we never use this as it is already obselete although in some industrial plant they used it.

3. 3 phase 3 wire ( all 3 conductro are hot)
4. 3 phase 4 wire (3 hot and one neutral)

Note: we used the term live rather that hot.although neutral is consider live as well but instead of saying live we used the term neutral or ground.

Single phase transformer:2 wire or 3 wire curent in 2 hot is not out of phase.why? when you look a sinle phase winding it has only one iron core(meaning the E core as commonly used it is placed in the center)even the neutral willnot be out of phase because to get neutral the usual connection is just like the center tap.

Three phase: the constructions is that 3 winding in 3 different iron core( as for example the E core each of the leg of that E core has winding and have six lead out for connection fo wye or delta) here the current is 120 degree out of phase in each other.If you add the neutral the neutral current will be 90degrees out of phase with each phase current.

In the above the 3 wire sigle phase service can not said 3 phase service drop just because it has 3 wire.3 wire single phase service cannot also to be said 2 phase just because someone will said transformer construction was 2 windings(meaning phase a and B)connected with center tap as neutral(if this is the reasoning, let see the voltage output)

Lets used nominal voltage 230VAC:
1 phase 2 wire(both hot) Voltage= 230V
1 phase 3 wire (2 hot, one neutral=connected as center tap) voltage both hot=230V with respect to neutral it is 115V.

3 phase 3 wire (all 3 hot, voltage=230V with each phase)
3 phase 4 wire (same as above except with respect to neutral it is 115V phase to neutral. NOTE: only if wye(star) connected for delta its another story and voltage reading different.

If transformer construction is just like mention above 3 wire two winding trnasformer where in there is center tap voltage reading will be 460 for two hot and 230V for neutral to hot.

For us who has engineering background the above is cheap but for those electrician who don't have enough theories it will be easier to understand what i have writen( i do hope i did it easier).
 
Re: Single Phase or Not.

Wyedelta: With due respect to your engineering background, your explaination and description is based on fundamental technology, that is included in apprentice programs.

Separation of time, or simply time sharing, is the background of two and three phase power.

Three people using an outhouse at the same time is one phase, three people using the outhouse at three different times is a three phase outhouse.
 
Re: Single Phase or Not.

As Bennie said, "If it starts and stops at the same time, it is single phase."

I agree. "Out of phase" is the vernacular for the engineering term "phase displacement" meaning "displaced in time", or occuring at different times.

Think of a single phase circuit as a continuous train track, with a train that fills the complete track. In other words, the engine is up behind the caboose. :p
(The train cars are analogous to the electrons in an electrical circuit.)

If the engine starts moving at a specific time, all of the cars begin moving at that same time (assume that there is no lash in the couplings).

Similarly, when a switch in a single phase circuit is closed, the electrons start moving at the same instant in all parts of the circuit. There is no way that the current in one part of the circuit can be out of phase with the current in another part of the circuit.

It is quite common for the current to be out of phase with the voltage in a circuit that contains inductive or capacitive reactance.

The term used to describe the fact that one point in a circuit can have a negative charge while another point in the same circuit has a positive charge, is polarity, not phase.

Ed
 
Re: Single Phase or Not.

Charlie B,

You can not use the enclosure as your point of reference. If the enclosure is not bonded then the reference point is invalid, and if it is bonded to one of the hot conductors you only have two conductors, and finally if the enclosure is bonded at a point between the conductors you need to look at the circuit in it's entirety.

If you use Thevenin's Theorem for analyzing your three wire circuit you will find there is a single current direction in the winding of the source (either a transformer or a generator) of your L1 and L2 conductors and two currents (flowing in opposite directions) in your "neutral". This single current direction gives us a single time domain for the AC, and therefore it is a single phase source.

The fundamentals of analysis of a circuit do not depend on whether it is AC or DC. The source must always be examined.

So, it is okay to say a single phase system has two conductors 180 degrees out of phase with the neutral. But it is not proper to say they are out of phase with each other.
 
Re: Single Phase or Not.

What I am saying is that, viewed in isolation, they can be described as being two circuits that are out of phase with each other. By that I mean if you connect a voltage probe between line 1 and neutral, and a second voltage probe between line 2 and neutral, keeping the convention that ?positive? means the line is higher than the neutral, and show them on the same oscilloscope, you would see two different traces, 180 degrees out of phase with each other. You could do the same with two amp probes, and get the same picture.

The only reason that I want to be able to describe line 1 and line 2 as being 180 degrees out of phase is that I want to use the same description for two separate circuits, separate loads, say on positions 1 and 24 on the same 120/240V panel. In this case, there is no sharing of current between the two loads. But those two currents are 180 degrees out of phase with each other, in the same sense that I have described. That is the reason that I made the statement, in my first post, about the ?breaker monitor person? not having to know whether the two wires he was evaluating were connected to a 2-pole breaker or to two single-pole breakers. It is, as I have said, a matter of mathematical semantics. I don?t think you and I have a fundamental disagreement.

That being said, I do believe that I can use the enclosure as a ?node,? or point of reference. I can draw a circle around any portion of a circuit, however complex, and look at the currents leaving that circle and the currents entering it.
 
Re: Single Phase or Not.

bennie:
with all due respect as well, i agree with you that it was base on fundamental technoly and that is why i made it easier for electrician to understand it.If we all talking here engineering theories we don't need to anology for we know already the theory but there are some who don't that is they asked.

Secondly I agree that it is time.In my example the single phase was in house thetwo two hot L1 L2 if you like,while the three phase was in three seperate house although I use the E core.

In single phase L1 and L2 cannot be out of phase although L1 might be posetive in half cycle and L2 is negative in half cycle because both L1 and L2 had started at the point let say 0 degree and will leave at 180 degrees.While in three phase L1 enter(begin) in 0 degree and L2 in 120 degree while L3240 degrees tey are out of phase as its time start not all the same time but with variation.(Im sure we all know the theories behind this and need not to explain.

there are somebody here who are not theoritically incline therefore we must show in the easiest way they can understand because if we talk the way like in school they will be in lost they can't cope up with our discusion.

thanks for the comment anyway.
 
Re: Single Phase or Not.

Charlie B

I am not convinced that we are not in disagreement.

You are picking and chosing terms that you want to use. It is the source of the voltage and current that determines the phases not the circuit wiring.

In the electrical industry the terms single phase and three phase refer to the oscillograph plots (time domain differences) of the "hot" conductors in relation to each other. The presence or abscence of a neutral does not and can not influence the number of phases.

If you have a two wire circuit (either L-L or L-N), the current waveform has nothing to do with any other circuit, and there can not be any meaningful comparision. If you have a multi-wire circuit (L-N-L) you have a center tapped source, not two seperate circuits.

As I said in my previous post, I think it is acceptable to talk to lay-people about the single phase lines being out of phase with the neutral. But you must emphasis the word "neutral" otherwise how do you talk about the current in a 240V two wire water heater circuit?
 
Re: Single Phase or Not.

why can't we all just get along ?....

buy an ocilloscope with 3 or more channels
set the time base and bandwidth accordingly
calibrate to "0"
measure all 3 phases at the same time


now tell me what you see if you try this with 3 wire dual-phase
or 3 wire single phase
or dual dc
or chicken soup......

now, do you see the point ?....

exactly !!!!....in the end , it's chicken soup that is plotting our demise !!!

stop the madness !!!

Mike
 
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