How does a typical residential utility transformer work?

[LarryFine]

I claim that if I measure V1 and V2 relative to a common reference--say neutral--one voltage is the inverse of the other, that is, 180 degrees out. I think we now agree on that fact.

Absolutely agree.

That is all we are talking about, swapping leads.

Absolutely agree.

I think this is making something fairly simple in our field sound a lot more complex than what it is.

Absolutely agree.


Okay Rattus, here comes a curve-ball:

How would you apply this discussion to the comparison between a corner-grounded Delta and a center-tapped high-leg Delta? Obviously, the only electrical differences would be whether one secondary has a CT, and which point is earthed.

 

[jim dungar]

Each time I swap leads, I see a 180 degree phase shift from my previous reading. That is all we are talking about, swapping leads.

This is why I have called this use of 180? as "fun with numbers". It only exists in 3 wire systems that are center tapped (ie. 2 batteries in series or a 120/240 transfromer connection). As Rattus admits, it only occurs because the "direction" of the measuring leads have been swapped un-noticed by the common electrician.

I contend there is no practical reason to use this term, as it must often be ammended when describing current flow, power, when analysing un-balanced multi-wire circuits, and how single phase relate to poly-phase systems. Also, if we must subtract voltages, why do we then talk about currents "adding in the neutral"?

 

[rattus]

Absolutely agree.


Absolutely agree.


Absolutely agree.


Okay Rattus, here comes a curve-ball:

How would you apply this discussion to the comparison between a corner-grounded Delta and a center-tapped high-leg Delta? Obviously, the only electrical differences would be whether one secondary has a CT, and which point is earthed.

Larry,

I would treat the corner grounded delta just like an ungrounded delta. The only difference is that the B phase is clamped to ground.

Now if I were using a circuit simulator, I would be forced to treat the B phase as common--not neutral.

For the high-leg, I would treat the CT as common for the 120V sources, and treat the other two phases as usual. The circuit diagram would contain 4 sources at various phase angles.

 

[rattus]

Convention:

Convention:

This is why I have called this use of 180? as "fun with numbers". It only exists in 3 wire systems that are center tapped (ie. 2 batteries in series or a 120/240 transfromer connection). As Rattus admits, it only occurs because the "direction" of the measuring leads have been swapped un-noticed by the common electrician.

I contend there is no practical reason to use this term, as it must often be amended when describing current flow, power, when analysing un-balanced multi-wire circuits, and how single phase relate to poly-phase systems. Also, if we must subtract voltages, why do we then talk about currents "adding in the neutral"?

Jim, you are strong on convention, and that is good. However, the convention in writing loop and node equations is to use a common reference where possible, assume the sense of the unknowns, and recognize that some of your assumptions may have been wrong, but that will come out in the wash.

You may solve your problems any way you wish, but there is nothing wrong with assigning a phase shift to L1 or L2. Furthermore, this practice is very descriptive when comparing the two waveforms.

 

[jim dungar]

You may solve your problems any way you wish, but there is nothing wrong with assigning a phase shift to L1 or L2. Furthermore, this practice is very descriptive when comparing the two waveforms.

This practice is limited in its application, it is only very descriptive when comparing L-N waveforms, voltage or current (but not at the same time), in a single phase center tapped 3 wire circuit.

You can not use it when describing single phase loads connected L-N to the high leg in a 240/120 3P 4W system. You cannot use it when analysing a Square D 9070T50D15 transformer that has a 24/120V secondary. I am still trying to visualize using it with a multi-wire circuit consisting of (1) 120V 5A load connected L1-N, (1) 120V 10A load connected L2-N and (1) 240V 5A load connected L1-L2.

 

[johnny watt]

Any two sine wave signals of the same frequency can be measured and described as in phase or else out of phase by a certain angle. I can think of several reasons to use N as the reference point for L1 and L2. Heck, your scope references N by default. I cannot think of a good reason to use another reference point when measuring the power in your home.

Although I follow the inversion vs the time delay I don?t follow which one it has been implied that this is supposed to be. The primary voltage is changed to magnetic and there is a 90 phase shift and then the secondary changes it back to voltage and there is another 90 degrees and so you have a total of 180 degrees. This would be true with many frequencies so it is not a delay it is an inversion.

But were talking about L1 and L2. I don?t follow, how is it, that they are inversions? If you had a transformer that is more saturated in the L2 to N portion, the L1 and L2 waveforms would look different from each other and the L1 to L2 would look like a combination of the two waveforms not half and half.

I don?t understand referring to the sine wave as an entity and that by splitting it in two with a c.t. that you are altering the true sine wave. The sine wave is merely a description of the varying voltage in reference to a given point. It is not a fundamental unit of voltage on a secondary.

I agree with the comparison to batteries but I disagree with the characterization of the batteries. If you have a device that requires +12V and -12V you could not power it with a single 24V battery. Two 12V batteries are a complete different animal.

The original question: ?How does a typical residential utility transformer work?.
Circuit analysis for a house cannot be done without using two phasors. You could do it without using N as the reference but that doesn?t sit well in my mind. I want to have a common reference if I can and then I can refer to voltages by a single point. Imagine making L1 your reference and then referring to the voltage on N as a single point and it would be 120V. Not very conventional. Maybe I?m off base here but on paper any point can be the reference. But is that reality? A mole of copper can only have a finite number of electrons forced from it before it can?t become anymore positive. The electrons are all gone. Chemistry teaches us of the stable octet. In spite of the ph of the soil and such things, my intuition says that the ground/earth on average is neutral by this I mean on average an atom or molecule from the earth would be no more likely to attract or repel an electron.

So why not make the ground the reference when part of the voltage source is attached to the ground? I can?t think of a good reason not to. What?s more, I can?t think of a good reason to use any other point as the reference.

 

[rattus]

You asked for it!

You asked for it!

This practice is limited in its application, it is only very descriptive when comparing L-N waveforms, voltage or current (but not at the same time), in a single phase center tapped 3 wire circuit.

You can not use it when describing single phase loads connected L-N to the high leg in a 240/120 3P 4W system. You cannot use it when analysing a Square D 9070T50D15 transformer that has a 24/120V secondary. I am still trying to visualize using it with a multi-wire circuit consisting of (1) 120V 5A load connected L1-N, (1) 120V 10A load connected L2-N and (1) 240V 5A load connected L1-L2.

Jim, this is the way it is done:

Refer to the attached diagram please:

I1 = (120V @ 0)/24 = 5A @ 0
I2 = (120V @ 180)/12 = 10A @ 180
I12 = (120V @ 0 - 120V @ 180)/48 = (240V @ 0)/5 = 5A @ 0
In = I1 + I2 = 5A @ 0 + 10A @ 180 = 5A @ 180

Source currents are found in the same way,

Iv1 = 10A @ 0
Iv2 = 15A @ 180

This is the general approach to writing loop and node equations and is not limited as you say. However, it is not necessarily the easiest solution especially for a simple problem such as this.

 

[jim dungar]

So why not make the ground the reference when part of the voltage source is attached to the ground? I can?t think of a good reason not to. What?s more, I can?t think of a good reason to use any other point as the reference.

When looking at the voltage waveform of a 240V 2-wire motor, do you use two sets of probes connected L-N and combine the results or do you use a single set of probes connected L-L?

In the majority of residential loads connected to a multi-wire circuit the currents are in phase with the voltages. If you use the neutral as your reference point and you say that V1 and V2 are 180? apart then the currents, I1 and I2 must also be 180? apart. If the line currents flow in different directions, how can a 240V 2-wire load be justified?

For some reason people have no problem understanding batteries connected in series so why do we need to something different for 120/240 3-wire circuits. This is why I do not agree using with the term 180?, just because it can be done doesn't mean it should be.

 

[jim dungar]

However, it is not necessarily the easiest solution especially for a simple problem such as this.

Rattus,
I never said it could not be done. I have said that saying the voltages are out of phase makes describing the currents more difficult.

 

[coulter]

...In the majority of residential loads connected to a multi-wire circuit the currents are in phase with the voltages. If you use the neutral as your reference point and you say that V1 and V2 are 180? apart then the currents, I1 and I2 must also be 180? apart. If the line currents flow in different directions, how can a 240V 2-wire load be justified? .....

...I never said it could not be done. I have said that saying the voltages are out of phase makes describing the currents more difficult. ...

Jim - That is excellent.

carl

 

[rattus]

The Big Picture:

The Big Picture:

Rattus,
I never said it could not be done. I have said that saying the voltages are out of phase makes describing the currents more difficult.

Jim, you told the OP that V1 and V2 were in phase, but to say that you have to assume a reference, and the logical choice is the neutral, and in that case, V1 and V2 are out of phase.

For the MWBC I would agree. However, for more complex problems one cannot eyeball the schematic and determine the sense of the unknowns. The convention is to define the sources relative to the neutral if possible and assume the sense of the currents as leaving the "hot" end of the source. This convention is followed in the 3-ph wye system where the phase voltages are defined relative to the neutral. So, if we are to be consistent, why not use the same convention for the two legs of a single phase service?

I would argue, even insist, that the proper way to express the voltages on L1 and L2 is to use the neutral as a reference. That is to say, the voltages on nodes L1 and L2 are V1n and V2n, and they are out of phase as the OP said.

 

[rattus]

In the majority of residential loads connected to a multi-wire circuit the currents are in phase with the voltages. If you use the neutral as your reference point and you say that V1 and V2 are 180? apart then the currents, I1 and I2 must also be 180? apart. If the line currents flow in different directions, how can a 240V 2-wire load be justified?

QUOTE]

Jim, If you follow the conventions and rules, everything comes out in the wash.

 

[jim dungar]

Jim, you told the OP that V1 and V2 were in phase, but to say that you have to assume a reference, and the logical choice is the neutral, and in that case, V1 and V2 are out of phase.

What I said was

The legs are not actually 180 degrees apart. They appear to be 180 degrees apart only when measured using the neutral as a reference point.

If you look at the diagram, posted by stickboy, you will see a single winding on the primary which creates a single magnetic flux which in turn creates a single output of 240V when measured between the two outside legs of the secondary. Adding the center tapped 120V neutral point does not magically create a different "phase".

I talk about a 120/240 supply as (2) 120V supplies in phase and in series.
I have never had to use vectors/phasors to explain different combinations of 120V 2-wire, 240V 2-wire, and 120/240 3-wire circuits to a non-technical person. I feel that using the term 180? unnecessarilly complicates a discussion and often leads to future mis-understandings.

 

[rattus]

What I said was

I talk about a 120/240 supply as (2) 120V supplies in phase and in series.
I have never had to use vectors/phasors to explain different combinations of 120V 2-wire, 240V 2-wire, and 120/240 3-wire circuits to a non-technical person. I feel that using the term 180? unnecessarilly complicates a discussion and often leads to future mis-understandings.

Jim, of course you wouldn't talk phasors to an amateur. However, I would think that an engineer in the power field would understand such terms with no problems.

The crux of the matter is you like to use Vn2 which is fine, but any engineer understands that V2n not only appears to be 180 degrees out from Vn2, but it is.

With all due respect to the guys who do the hard and dirty work, they don't worry about it because they don't need to.

 

[jim dungar]

Jim, of course you wouldn't talk phasors to an amateur. However, I would think that an engineer in the power field would understand such terms with no problems.

The crux of the matter is you like to use Vn2 which is fine, but any engineer understands that V2n not only appears to be 180 degrees out from Vn2, but it is.

With all due respect to the guys who do the hard and dirty work, they don't worry about it because they don't need to.

Do you believe I am debating that V2n does not equal -Vn2?

The use of 180? has resulted in over 100 posts of defence, clarification, and explanations. My point is that this is unnecessary complexity when a 3-wire circuit can be explained simply and accurately as (2) sources connected in series.

 

[rattus]

Sure:

Sure:

Do you believe I am debating that V2n does not equal -Vn2?

The use of 180? has resulted in over 100 posts of defence, clarification, and explanations. My point is that this is unnecessary complexity when a 3-wire circuit can be explained simply and accurately as (2) sources connected in series.

Of course Jim. I have admitted as such, but there are still those who balk at the idea of a phase shift in a single phase system. I am trying to convince them that the voltages at the ends of a CT secondary, relative to the neutral, are 180 degrees out of phase, and indeed, there is no second phase voltage, only a phase shift.

I am also trying to convince them that the fact that these voltages derive from a single winding does not preclude a phase shift.

Along the way, I have had to dispel the notion that single phase voltages cannot be described with phasors because the magnetic field does not rotate in a single phase motor--nonsense! I have also tried to get across the fact that the basic definition of a phasor describes a fixed complex number. These are the phasors we use in steady state analyses. Rotating phasors are similar but they are variable functions of time.

Yes, this thread is about 10 times too long, and I hope you see why.

 

[ronaldrc]

Just got home

Worked all day on a Ford Dealerships Lot lights changed
26 1000 watt metal halide bulbs, and changed out 4
of them there 5 phase ballast. These ballast had 5 taps
120,208,240,277 and 480 that would make them 5 phase wouldn't
it ?
Ain't really sure anymore

 

[rattus]

Just got home

Worked all day on a Ford Dealerships Lot lights changed
26 1000 watt metal halide bulbs, and changed out 4
of them there 5 phase ballast. These ballast had 5 taps
120,208,240,277 and 480 that would make them 5 phase wouldn't
it ?
Ain't really sure anymore

Ronald, you wouldn't tease an old man would you?

 

[winnie]

These ballast had 5 taps
120,208,240,277 and 480 that would make them 5 phase wouldn't
it ?
Ain't really sure anymore

Depending upon which tap you pick as your reference, I get a total of 15 different magnitudes and two phase angles *grin*

-Jon

 

[ronaldrc]

No Rattus

I've just about got this figured out, but its going to take me a while.
Lets see now 240/2 = 180 degrees my ballasts are 480 so my 120 tap
will be 90 degrees, 240 tap would be 180 degree, its goin to take me a while to figure that 208 and 277 but I will get back to you.