Phase A and Phase B in single phase wiring diagrams

[George Stolz]

If you used A and Neutral for a 120-volt circuit, would you say they were 180 out?

No, I think it would be more correct to say that for a line-to-neutral load, then the neutral is the 0V "x" axis of the graph.

If you use A & B for a 240-volt circuit, I don't think it is correct to say they are 180 out. (They appear 180 out when referenced to the mid-point, aka neutral).

But 180? could be verified a different way: At any given instant, the "A" phase will be exactly opposite "B". For example, when "A" is on the upswing of the cycle (say at 45V), then "B" will be on the downswing of it's cycle, at -45V. Without a neutral reference, then it would look like one wave (not seperate A & B waves) that measure 240V at their peak.

But then, does a change to the reference change the nature of the waves? Does standing on phase A change the nature of the pair's interaction? No, it just changes the perspective. If you're standing on phase "A", and watching phase "B" from your vantage point, without any other references your mind would perceive that you're standing still, and "B" is moving 240V away from you.

Whether both phases are oscillating between 120V+ and 120V- at opposite times, or one phase is oscillating between 240V+ and 240V- depends entirely on your frame of reference.

If you'll excuse me, I'm going to go make a tin foil hat now. :lol:

I have to disagree with you. The phasing perspective is all about reference to ground (neutral).

I agree, I mis-spoke. I'm going to mull this over some more.

 

[hardworkingstiff]

I think most people look at a single phase service as two separate sources of power, A and B. This is actually not accurate. It is one source only, with a center tap that is grounded so you look to A and look to B from the center point and they appear to be 180 degrees out of phase from the reference point.

I try to think of the sine waves not as being two different waves 180 out, but as one source expanding and contracting then expanding and contracting in the opposite direction to complete a cycle. When the reference point (center tap, aka neutral) is thrown into the picture, it then appears to be two sine waves.

This may not make sense, but thinking of the graph expanding and contracting as one source is what helped me understand why single phase is exactly that, single phase.

 

[jwelectric]

The wonderful thing about a simulator is you can do things that can?t be done in the field.
Here is a picture of a center tap transformer showing the opposing sine waves. Notice that Channel ?A? if connected to the center tap with the ground of the scope. Channel ?B? is also using the center tap ground connection.

Now look at the scope where Channel ?A? is connected to one end of the secondary and the Ground from the scope is connected to the other end. Here we have only one sine wave across the phase.

In the first picture we are reading both halves of the single sine wave at the same time which gives the affect of two opposing sine waves. There is NOT two opposing sine waves when the center tap is being used we are only reading both sides of the SINGLE sine wave at the same time.

 

[johnny watt]

Re: Phase A and Phase B in single phase wiring diagrams

I think most people look at a single phase service as two separate sources of power, A and B. This is actually not accurate.

I disagree, a center tap transformer acts like two voltage sources. We can apply loads to just one leg and the current flows in just one half of the transformer essentially independent of the other half

Another way of looking at it is to draw a Thevenin equivalent circuit of a split phase service feeding a home with a few loads in it. You cannot do that with just one voltage source. You must use two and this image is a good attempt:

But there are a few things wrong with it.
First, I believe phasors are supposed to use the peak voltage, which means instead of 120V it should say 169.7V but 120V is a familiar number so no sense in changing it.
Second, the diagram ignores that we call earth zero volts. If we assume earth is zero volts then the voltage on leg 2 has to be changed to ?120V in order to keep the +- signs the same as they are on the rest of the diagram.
Now in phasor notation negatives are to be avoided and so the ?120V @ 0 degrees should be changed to the equivalent +120V @180 degrees.

So there you have it: 120V @ 0 degrees and 120V @ 180 degrees
But if you want to disregard ground is zero volts then go ahead and call them ?in phase?

The main point is that in circuit theory you have to use 2 voltage sources to describe the service.
In my mind, the way we hook up loads in our home is the deciding factor in what the reference point should be when comparing the phase relationship of those two voltages.

?A phase? as a noun, has a stricter definition in the electricity industry then do ?in phase and ?out of phase?

Mike, I don?t understand the sign wave demonstration. No one would argue that those waveforms are incorrect. But there are no sin waves per se, there are only voltages and currents. There is certainly a voltage between each leg and neutral and those voltages certainly vary in a sinusoidal waveform. Those voltages are relevant because they are used both individually and combined to power loads. If you take sin wt and divide it in two you get ? sin wt. You still have a sin wave not half a sin wave.

 

[hardworkingstiff]

Re: Phase A and Phase B in single phase wiring diagrams

I disagree, a center tap transformer acts like two voltage sources.

Acts, that is the operative word. It acts like two sources, but actually is not. I agree that it acts like and looks like two sources, but actually is one source that you are allowed to use in different portions.

I think this is degrading to semantics, so I will not post on this thread again (but reserve the right to join in a new thread, lol).

 

[jwelectric]

Johnny

Please explain what you mean by, ?the diagram ignores that we call earth zero volts. If we assume earth is zero volts?

Earth plays no role with the voltage.

Notice the secondary winding has a center tap. The current is returning from the outside leggs to this center tap.

I have a computer that is supplied from the receptacle with 120 volts. What is the power source for this computer?

Once I open the computer up I will find several different values and types of voltages on the inside. Does this change the supply voltage?

A house is supplied by a center tap 240 volt single phase system. After I enter the house I start breaking this voltage down. Some Countries do not supply the center tap and the end user will provide the equipment with a means to achive their desired voltage.

In your example digram you show the center tap as carring both positive and negevitive current at the same time. We know that this can not happen as one will cancel out the other. What is being carried through the center tap is the difference or unblanced portion of the total current.
The voltage is different because we have two differences of potential over this center tapped winding.

If I have a flashlight that uses two ?D? cell batteries I will have a three volt flashlight. If I want to connect a motion dector to this light and the dector required one and a half volts I could achive this by hitting the middle of these two batteries and using only one battery. Whould this change my power source for the flashlight?

 

[johnny watt]

Acts, that is the operative word.

I agree, ?if it quacks like a duck? proves nothing.
I hate to belabor you with semantics, but since you exclude split phase from the realm of two sources of power, how would YOU define two sources of power? IMHO Thevenin is the deciding factor.

Please explain what you mean by, ?the diagram ignores that we call earth zero volts. If we assume earth is zero volts? Earth plays no role with the voltage.

I copied the diagram from an earlier post (and I don?t approve of the way the currents are depicted either).

Reversing the scope leads allows you to see the waveforms in phase with each other as the author intended, but the + and ? leads on the scope ?mean something?. If a voltage is indicated on the negative lead of the scope then the voltage must be named negative. Voltages on the positive lead must be named positive.

In this case the voltage indicated by the scope is either saying there is +120V on the neutral or else the L2 is negative 120V. Defining neutral as zero volts makes the choice for us. And as you know a negative phasor is the same as a positive phasor at 180 degrees.

Mike, I take all your points (to a point) but in the case of split phase, isn?t it grounded out at the pole before it even gets to the user. As I said, IMHO Thevenin is the deciding factor.

 

[jwelectric]

Johnny

Using your theory explain the next four pictures to me.

This first picture is a Delta three phase generator @ 480 volts. It is connected to a single resistive load of 1000 ohms.
An oscilloscope is connected to both ends of the resistor. The scope is displaying two sine waves that are 120 degrees apart. The circuit is not grounded.



In the next picture I grounded the circuit but did not change the leads of the scope. Now the scope is displaying one sine wave. Why?




In a series circuit the resistance will add up to a total, so, this one 1000 ohm resistor would be the same as two 500 ohm resistors. This circuit is ungrounded so it displays the same sine waves as the first picture.




In this last picture I have grounded the circuit but I grounded it between the two resistors and now the scope is displaying two sine waves that are 180 degrees out of phase with each other.
What has changed?
Have any phases been added or taken away?
Why do I get three different displays from the same connection to the circuit?
From beginning to end in all four pictures channel ?A? and ?B? leads have not been moved.




Notice that I grounded one side of this transformer. Only one sine wave is displayed.




This next picture has the transformer center tapped and grounded. What we have is one half of the coil being read at one time. Notice how much smaller the sine waves are.
Again there were no phases neither added nor taken away.




All that has changed in any of the examples above is the point of reference on the circuit.

 

[johnny watt]

Mike, I have no disagreement with your waveforms. Although I must admit, in your first example, I was surprised to see such clear waveforms, in reference to ground, from an ungrounded source.
I agree, it?s all about the reference point.
But why is one reference point more relevant than another?
I have given several reasons why the neutral is a relevant reference point.

My chief criticism was with the diagram that places the positive lead of the scope on the zero potential point.

If you had to hook the secondary of two transformers together in parallel you could either hook them up in phase or out of phase. The center tap is essentially what you would have if you started to hook them up out of phase. If you then hooked the L2 and L1 together, that would complete hooking them up out of phase.

As far as how this should be taught, I think that it should be stressed to a student that in the electric power industry, as a noun, ?phase? has a very specific meaning, as do 2-phase and 3-phase.

In addition it should be taught that L1 and L2 ARE out of phase by 180, but only with respect to neutral. And that they are called legs not phases (even if phases might sound like an intuitively good name for them.)

 

[jwelectric]

Johnny
I want to teach this correctly so I am asking you to help me to explain this to my students.

What I have here is a 120 volt circuit that consists of two 1k ohm resistors.

I have attached my scope to the circuit with the ground from the scope connected in the middle of the resistors and the two channel leads connect on the power source ends.

Now do I tell the class that I have a 120 volt power source that is supplying a voltage divider and the voltage divider is then giving us two 59.95 volt phases that are 180 degrees out of phase with each other?

Where do I tell them the ?zero potential? is?

 

[iwire]

I will add fuel to your fire. :lol:

If I recall correctly Charlie B has stated that the two legs of single phase are indeed 180 degrees out of phase with each other.

I think it might depend on your use of the word phase.

The moon has 'phases' but that has nothing to do with single or 3 phase.

Look up phase http://www.m-w.com/dictionary/phase.

It does not have a specific meaning that only applies to '3 phase'

 

[winnie]

I'd like to suggest that some of you'all are arguing that the answer is six, while others of you are arguing that the answer is a half dozen.

This is a discussion about terminology and sign convention, not about the actual physics. As long as _you_ (any individual working with these numbers) are internally self-consistent with your usage, such that you get correct answers when you do the math, then any of these 'representations of reality' is fine.

Sometimes it makes sense to use one representation or another. It is entirely reasonable to use different representations to solve different calculations, as long as you remain consistent throughout a given calculation, or carefully include additional calculation terms to include your 'change of reference'.

Voltage is always measured between two points. If the voltage measured from A to B is 10V, then the voltage measured from B to A will be -10V. You could also simply say that the voltage is 10V with B positive. Voltage versus time for AC is similarly measured between two points, and you must specify which point you are starting from. While the RMS voltage from A to B is equal to the RMS voltage from B to A, the _instantaneous_ voltages will be of equal and opposite polarity, and the voltage versus time graphs will be inverted relative to each other.

Mike, in your simulation, set up a single phase secondary and _two_ scopes. Connect the first scope ground to X1, and its terminal A to X2. Connect the second scope ground to X2 and its terminal A to X1. The two scopes should display sine waves with the same frequency but opposite polarity.

In a single phase center tapped system, the voltage as measured from X0 to X2 is 180 degrees out of phase from the voltage as measured from X0 to X1. The voltage as measured from X2 to X0 is at 0 degrees relative to the voltage as measured from X0 to X1.

I find that if I am solving a series circuit (using the Kirchoff voltage law to figure out the voltage applied to a load) that it is simpler to order my voltage measurements going in series. If I were looking at a residential 240V circuit, then I would calculate X2 to X0 and X0 to X1 in series.

I find that if I am solving a parallel circuit (say using the Kirchoff current law to figure out the current flowing out of a node) then I find it simpler to arrange everything so that all of my sources are defined as going 'in' to that node. If I were trying to figure out the current flowing through a neutral conductor, then I would use X0 to X1 for one leg, and X0 to X2 for the other leg.

-Jon

 

[jwelectric]

Jon

I have had a similar discussion in another forum about a 240 volt single phase service.
What I am trying to get across is why it is called a single phase panel.

In the last illustration that I posted I posted a voltage divider not a phase divider although if I connect a multi channel scope to the circuit and use the mid point of the two resistors as the ground point for the scope I will display two sine waves that are 180 degrees out of phase.

The one thing that is consistent, all across this great nation most residential services are called 240 volt single phase and they are not called 120 volt two phase.
Another thing that is consistent is the last illustration that I posted would be called a 120 volt circuit not a 60 volt two phase circuit.

I have stood in front of classrooms for over five years now and one of the biggest problems I run into especially with students that transferred from electronic technology courses is explaining the difference between 240 volt single phase and 120 volt two phase.

When displaying the sine waves on a scope the size (value of the voltage or current) will be proportionate to the points of reference. When using a multi channel scope the sine waves displayed on screen will also be proportionate to the points of reference.

The one thing that cannot happen is for the applied current to oppose itself anywhere in the resistive circuit that I posted although I can see two sine waves that are 180 degrees out of phase with each other. The sine waves that are displayed on the scope are 60 volts but the applied voltage is 120 volts. The current across the circuit will always travel in the same direction and can never reverse itself half way through the circuit.

 

[charlie b]

If the voltage measured from A to B is 10V, then the voltage measured from B to A will be -10V.

Quite true. I would have put it this way: Hang a picture on the center of a wall, and then walk to the left side of the wall. I ask you where the picture is, and you point to the right. Then you walk to the right side of the wall. Now I ask again where the picture is, and you point to the left. I asked the same question, and you gave me two different answers. So, which of your two answers was the right answer?

 

[jwelectric]

If the voltage measured from A to B is 10V, then the voltage measured from B to A will be -10V.

Quite true. I would have put it this way: Hang a picture on the center of a wall, and then walk to the left side of the wall. I ask you where the picture is, and you point to the right. Then you walk to the right side of the wall. Now I ask again where the picture is, and you point to the left. I asked the same question, and you gave me two different answers. So, which of your two answers was the right answer?

I shall use this in the class room, thanks Charlie

 

[charlie b]

You're more than welcome, Mike.

 

[winnie]

Mike,

There is some terminology used in the "high phase order motor" field which may be of use to you.

If you look at an ordinary three phase motor, there are _six_ distinct 'phase bands' which are used. In order these are A, C', B, A', C, B'. Of course, this is simply the three supply phases, connected in either the 'forward' or 'reverse' direction (forward and reverse being arbitrary). A and A' are related as being 180 degrees out of phase, and quite frequently the A phase band and the A' phase band are simply opposite haves of the same coil.

This terminology becomes important when you need to distinguish a three phase motor with six phase bands from a true six phase motor. I bet that it is also used in ordinary three phase machines; I simply learned it when I was looking at machines with higher phase count.

With this in mind, in a three phase panel, you would have phases A, B, and C. In a _two_ phase panel (used historically for driving motors), you would have phases A and B. In a single phase center tapped panel you have phases A and A'.

With phases A and B (or more) you can continuously deliver power to a load, and you can generate a rotating magnetic field without phase shifting. With phases A and A' the available power is continuously varying, falling to zero twice per cycle, and you cannot generate a rotating magnetic field without phase shift hardware.

As an aside, on two phase motors. I belive that two phase motors were used historically simply because they are the _simplest_ polyphase motor, and thus the first invented. It was quickly learned that three phase machines make more sense; better winding distribution, and better power transmission, and so things were very quickly changed to three phase use. It takes a minimum of three wires to transmit two phase power, so two phase power could be considered very poorly balanced three phase power

-Jon

 

[Sparky Joe]

With AC current there is no +/- meaning your neutral is + 50% of the time. 120/240 3 wire is only dealing with one coil(1 phase) of a 3 phase delta system(3 coils). It doesn't matter what order your wires are connected on your breaker until you get into all 3 phases.

The kicker though, where my first sentence doesn't hold true is in a wye system because you still have single phase while your using 2 coils even though the 2 coils are 120 degrees 'out of phase'.

With your 120/240 they are only 180 degrees out of phase when refferrencing from the neutral, but from A to B the are 100% in phase, thus single phase.

Maybe someone can explain your single phase 208, or single phase 480?

 

[johnny watt]

The one thing that is consistent, all across this great nation most residential services are called 240 volt single phase and they are not called 120 volt two phase.

Agreed!

Another constant is that all across this great nation even professionals in the industry confuse the two.
You might say because they are lacking understanding or that they are reading the voltages from the wrong reference when judging them to be out of phase. But I disagree.

I think that Winnie touches on the genesis of the confusion. It seems our electrical forefathers developed poly phase primarily for driving motors. When creating three phases they simply divided the circle into 3 parts to get 120 degrees.

However, for two phases, if you divide the circle into two parts, you get 180 degrees, which offers no benefit for driving a motor. This is because, generally, you can simply reverse the wires on any voltage source to obtain voltage 180 degrees out of phase with the original voltage. And so two phase skips ahead to 90 degrees.

In practice, when two stators generate voltages that are 180 degrees out of phase, they are simply combined together to create one voltage. And when a motor stator requires a voltage that is 180 degrees from another stator they simply use the same voltage and reverse the wires.

The end result is that 180 degree voltages have never been counted when numbering phases.

I must concede that although it is not conventional to describe neutral as 120V with respect to L2 it is technically correct. I?m not sure if I would say that it is a description of the voltage on L2 though, but rather, a description of the voltage on neutral. But I?m sure someone would assert that it describes both points. At any rate, the original contention that L2 and L1 are out of phase implies a common reference. And the original diagram that I was finding fault with does not use a common reference.

Mike, I?m not sure it?s fair to compare a voltage divider with two secondary coils because loading the voltage divider would affect both voltages. However if high resistance loads were used then I could go along with calling it a power supply.

I do find your scope emulator a little troubling. In the last waveform the scope ground is shorting out the one resistor and the waveform should be a flat line but it is not. In an earlier waveform the same thing happened and the flat line was invisible.

I think the students should be taught to never hook the ground to anything but a ground.
Do not defeat the scope grounds, as it is hazardous
and to use two probes in the differential mode, when trying to take measurements such as the ones you are depicting.

 

[jwelectric]

Mike, I?m not sure it?s fair to compare a voltage divider with two secondary coils because loading the voltage divider would affect both voltages. However if high resistance loads were used then I could go along with calling it a power supply.

At least we are making headway.
First there is NOT two secondary coils in a center tap 240/120 transformer there is only one secondanary coil that is divided in half.
The 240/120 center tap transformer is nothing more than a voltage divider just as the two resistors.

The two resistors in a series circuit will have one total resistive load on the circuit. When the center point of the two resistors is used for a reference point (the ground on the scope) then we will see two different smaller voltages. When referencing the outside points of these resistors with a two channel scope using the center as the common point we will also see two different sine waves that are 180 degrees apart.

The confusion is coming not from the applied voltage but from the point of reference on the circuit with the scope.