How does a typical residential utility transformer work?

[cschmid]

As a master student in our field I actually find this thread interesting so feel free to continue as I have learned allot..I will have to brush up on my math ahh what the heck whether this complicates the situation or not it is definitely interesting but my fishing and hunting buddies will not care..

 

[rattus]

As a master student in our field I actually find this thread interesting so feel free to continue as I have learned allot..I will have to brush up on my math ahh what the heck whether this complicates the situation or not it is definitely interesting but my fishing and hunting buddies will not care..

If you have specific questions, please ask. Otherwise, this gas war is about to end.

 

[jim dungar]

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

This is another example of a common transformer that does not use a neutral as the reference point. This is why I don't like using the terms "phase voltage" or 180? with single phase systems.

As I said before, just because you can doesn't mean you should.

 

[rattus]

This is another example of a common transformer that does not use a neutral as the reference point. This is why I don't like using the terms "phase voltage" or 180? with single phase systems.

As I said before, just because you can doesn't mean you should.

Jim, I think we all agree that it is improper to describe L1 and L2 as "phases", but I see no connection between ballasts and reference points. And, the fact remains that it is convention to specify voltages relative to a reference if one is available.

Now explain this to me:

Why is it acceptable to use the neutral as a reference in a 3-ph wye, but not in a CT transformer?

 

[jim dungar]

Why is it acceptable to use the neutral as a reference in a 3-ph wye, but not in a CT transformer?

Could you please show me where I said it was improper to use the neutral as a reference point?

I have said use of the terms of 180? and "out of phase" are not necessary when describing single phase 120/240V systems and often lead to misunderstanding.

 

[rattus]

Depends on your audience:

Depends on your audience:

Could you please show me where I said it was improper to use the neutral as a reference point?

I have said use of the terms of 180? and "out of phase" are not necessary when describing single phase 120/240V systems and often lead to misunderstanding.

On the contrary Jim, I think those terms are necessary for a full description of the system although not necessary for simple problems. I would think a competent engineer would not be confused by such terminology. And, I would teach this in a course on AC Circuits.

Now you seem to agree that V1n and V2n are 180 degrees out of phase irrespective of how you, I, or any else makes their calculations. Am I wrong?

 

[wasasparky]

Now you seem to agree that V1n and V2n are 180 degrees out of phase irrespective of how you, I, or any else makes their calculations. Am I wrong?

I think the hesitancy to flat out admit this (by anyone) is the the fear of the "two-phase" believers chiming in.
All we should care about with 120/240 is magnitude, and that the currents subtract in the neutral...

 

[jim dungar]

On the contrary Jim, I think those terms are necessary for a full description of the system although not necessary for simple problems. I would think a competent engineer would not be confused by such terminology. And, I would teach this in a course on AC Circuits.

Now you seem to agree that V1n and V2n are 180 degrees out of phase irrespective of how you, I, or any else makes their calculations. Am I wrong?

In my very first post I said that V1 and V2 appear to be 180? apart when viewed with the neutral as a reference. However V1-V2 is a single waveform.

Many people other than "competent engineer" discuss AC circuits and in particular 120/240 systems, so why introduce un-necessary confusion.

I have said it many times: Just because you can doesn't mean you should.
Every day we use the unscientific and generic term "water". I have never heard a "competent" chemist complain that we should use the term di-hydrogen oxide (H2O) even though we were all taught the phrase in school.

 

[JohnHurst]

I have never heard a "competent" chemist complain that we should use the term di-hydrogen oxide (H2O) even though we were all taught the phrase in school.

It's dihydrogen-monoxide, thank you very much, and that stuff's dangerous, I tell ya...

 

[rattus]

In my very first post I said that V1 and V2 appear to be 180? apart when viewed with the neutral as a reference. However V1-V2 is a single waveform.

Many people other than "competent engineer" discuss AC circuits and in particular 120/240 systems, so why introduce un-necessary confusion.

Why do you say "appear to be"? They are, and they have to be for the resultant voltage to be 240V. I can't see that V1n - V2n being a single waveform has any bearing on the discussion.

If V1n and V2n only appear to be out of phase, then Van, Vbn and Vcn, only appear to be 120 degrees out of phase. Does that mean they are not?

You are not forced to use this convention, and it would be confusing to many people, but that is not the question. The only question is,

Are V1n and V2n really, really, really, out of phase, or do they just look that way? The thought that the latter is the case confuses me!

Come on Jim, just come out and say it along with several others who have posted on the matter. After all, we are just after the TRVTH.

 

[winnie]

IMHO the 'appears to be' is an important part of this discussion.

To a large extent, we as a group hold to the idea that a phase shift means a displacement in time. Very clearly, a single phase center tapped coil has absolutely no displacement in time.

As I have previously argued that for a _pure_ sinusoid, the 'appears to be' is in fact reality. A single sine wave inverted _is_ exactly the same as a single sine wave time shifted by half a cycle. Because an inverted sine wave cannot be distinguished from a 180 degreed delayed sine wave, they are the same.

In particular, if you have a true polyphase system, you can use the inverted versions of each of your phases to provide additional phase angles in your system. Most common three phase motors use _six_ phases of stator current flow; the supplied phases and their inverse, which is simply the supply phases wired in the reverse direction. As physically embodied in most all motors, the inverse of a phase can be used in exactly the same way as a 180 degree delayed phase.

However this is still an 'appears to be', and that appearance falls apart as soon as you introduce an even harmonic.

-Jon

 

[rattus]

Almost missed this one:

Almost missed this one:

I did not say that V1n and V2n were not different.

I object to the general practice of talking about 180? phase differences on single phase systems. I am fighting for consistency in terminology. In three phase systems it is not standard practice to refer to L-N voltages as PHASE voltages, therefore I do not believe it should be done with single phase systems.

Jim, I missed this one in the smoke and dust. Isn't "phase voltage" defined as the voltage across the secondary? In that case, the L-N voltages in a wye are indeed phase voltages. Each "phase", in effect, is generated by a separate alternator and delivered through a separate transformer. Inverting a single phase does NOT, as we all know, create another phase.

I have tried to make this point several times, but it is ignored:

Since 3-ph wye voltages are specified relative to the neutral, why is it inconsistent to specify L1 and L2 the same way? Now we know you don't like it, but that is not a valid reason.

 

[rattus]

Now Winnie:

Now Winnie:

IMHO the 'appears to be' is an important part of this discussion.

To a large extent, we as a group hold to the idea that a phase shift means a displacement in time. Very clearly, a single phase center tapped coil has absolutely no displacement in time.

As I have previously argued that for a _pure_ sinusoid, the 'appears to be' is in fact reality. A single sine wave inverted _is_ exactly the same as a single sine wave time shifted by half a cycle. Because an inverted sine wave cannot be distinguished from a 180 degreed delayed sine wave, they are the same.

In particular, if you have a true polyphase system, you can use the inverted versions of each of your phases to provide additional phase angles in your system. Most common three phase motors use _six_ phases of stator current flow; the supplied phases and their inverse, which is simply the supply phases wired in the reverse direction. As physically embodied in most all motors, the inverse of a phase can be used in exactly the same way as a 180 degree delayed phase.

However this is still an 'appears to be', and that appearance falls apart as soon as you introduce an even harmonic.

-Jon

This whole discussion is based on ideal sinusoids--no harmonics please.

 

[winnie]

This whole discussion is based on ideal sinusoids--no harmonics please.

I differ. The discussion is about a physical system: the secondary of a transformer. Ideal sinusoids are a very good approximation of what is going through that physical system.

-Jon

 

[rattus]

No Winnie:

No Winnie:

I differ. The discussion is about a physical system: the secondary of a transformer. Ideal sinusoids are a very good approximation of what is going through that physical system.

-Jon

Not so Winnie: Steady state analyses using phasors are based on single frequency sinusoids. You cannot speak of the phase shift in a square wave because each harmonic is at a different frequency. Therefore the same millisecond of delay produces twice the angular phase shift in the second harmonic as it does in the fundamental. Reactances assume a single frequency, and we must assume that in our calculations. There is distortion, however small, in all electric and electronic circuits. We just have to accept it and compensate for it as is neccessary.

As you imply, just about any calculation we make is an approximation, but usually it is a rather good one. The argument that the sinusoid is not perfect can be carried all the way back to the generator if you so wish. The transformer draws a non-linear magnetizing current but it reproduces the voltage waveform rather well.

Even with harmonics, the inversion produces a 180 degree phase shift in the fundamental, and that is the sticking point with some.

 

[jim dungar]

It's dihydrogen-monoxide, thank you very much, and that stuff's dangerous, I tell ya...

Not according to my chemist son. The Hydrogen Oxygen co-valent bond does not require the use of the prefix "mono".

Edit: and by the way H2O can also be written as H-O-H which is how is often drawn.

 

[ronaldrc]

Just for the record my ballast staement was in fun.

But I knew it was just a matter of time before someone would bring
harmonics into play on this subject.

Whats that got to do with the price of eggs in China?

We will have harmonics on any ac circuit.

 

[jim dungar]

Rattus,

Go back and read my post #10. You are conducting an argument that I am not participating in. My point is to not do something just because you can.

It is a matter of where you place your test leads. That is,

V1n and V2n are out of phase.
V1n and Vn2 are in phase.

This is one of those facts that are often mis-used causing easily accepted myths. There are very few reasons to use this fact except to simply the reason that the two 120V L-N currents add to zero, but this then creates the problem of explaining how the two 120V L-N voltages add to 240V.

Not every voltage measurement uses a neutral as a reference point.
If you want to look at a voltage waveform to a single L-L 2-wire load, do you look at V1n and V2n and combine them, or do you look simply at V12? Does it make a difference if the supply is 120/240 1PH 3W, 240/120 3PH 4W, or 208Y/120 3PH 4W?

In reducing your analysis in post #109 to a single 2-wire circuit you said:
I12 = (120V @ 0 - 120V @ 180)/48 = (240V @ 0)/5 = 5A @ 0

This means that the current in IV1 is in phase with the current IV2 . But because you insist on always using the neutral as your reference, the effect is you are saying the current in a purely resistive circuit is out of phase with its voltage. While this is true in this circuit, it is not standard terminology.

 

[rattus]

Rattus,

Go back and read my post #10. You are conducting an argument that I am not participating in. My point is to not do something just because you can.



Not every voltage measurement uses a neutral as a reference point.
If you want to look at a voltage waveform to a single L-L 2-wire load, do you look at V1n and V2n and combine them, or do you look simply at V12? Does it make a difference if the supply is 120/240 1PH 3W, 240/120 3PH 4W, or 208Y/120 3PH 4W?

In reducing your analysis in post #109 to a single 2-wire circuit you said:
I12 = (120V @ 0 - 120V @ 180)/48 = (240V @ 0)/5 = 5A @ 0

This means that the current in IV1 is in phase with the current IV2 . But because you insist on always using the neutral as your reference, the effect is you are saying the current in a purely resistive circuit is out of phase with its voltage. While this is true in this circuit, it is not standard terminology.

No Jim, I do not insist, but the OP did use the neutral as a reference. Now you well know that to fully describe a sinusoidal voltage, you must specify the magnitude and phase angle, and to do this you must specify a reference point. This point need not be the neutral. Likewise, to fully describe a sinusoidal current, we must specify the magnitude and phase angle, and to do this, we must specify the direction or "sense" of the current.

That being said, the phase angle of a sinusoid can be shifted 180 degrees by changing the reference of the voltage measurement or sense of the current measurement.

But back to the basic question:

Leave the CT floating. Now are V1ct and V2ct, as they are defined by the double subscripts (which is convention), in phase or out of phase?

Answer this yes or no, unless you are shooting for 200 posts.

BTW, I have several who agree with me on this question already.

 

[jim dungar]

Rattus,

Do you agree that using your circuit analysis for current I12 in post #109, a 2-Wire only load would have the source current in coil 2 (your IV2) with a different sense or direction than your reference voltage V2n?

And to remind you again, I am not debating that V2n = -Vn2. I am saying that it is not common practice to say that in a resistive circuit the voltage and current are "opposite" each other.