How does a typical residential utility transformer work?

[jim dungar]

I am arguing with Jim because he said that V1n and V2n were in phase when in fact they are inverses of each other.

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.

 

[coulter]

Jim -
Hang in there, you're doing great. I tried and couldn't get the concepts across. You are doing much better.

carl

 

[coulter]

Question to the "singlephase 240V = two 120V vectors 180 out of phase" camp:

The attached sketch shows a standard vector representation of 3 phase 480/277 Y.

Q: What direction does the vector from L1 to L2 point?

Hint: Where this is headed is what does a motor see when it is only connected to the three phases? There is no neutral and the three leads comming at the motor look like 3 phase D to the motor.

Jim - No fair for you to answer.
eric - you neither

carl

whoops - edited to add sketch

 

[rattus]

Question to the "singlephase 240V = two 120V vectors 180 out of phase" camp:

The attached sketch shows a standard vector representation of 3 phase 480/277 Y.

Q: What direction does the vector from L1 to L2 point?

Hint: Where this is headed is what does a motor see when it is only connected to the three phases? There is no neutral and the three leads comming at the motor look like 3 phase D to the motor.

Jim - No fair for you to answer.
eric - you neither

carl

whoops - edited to add sketch

Easy! Phasor V12 points to the left, and phasor V21 points to the right, But the motor does not care.

 

[coulter]

rattus -
Good answer. You got that one right.

carl

 

[LarryFine]

I'm also of the center-tapped-single-secondary-winding-is-a-single-phase camp. If we grounded one end instead of the center tap, what would it be then?

There's no question that a 4-wire Y system is 3-phase; yet if we take a 208v 2-wire circuit from it, there's no question that it's single phase, right?

 

[eric stromberg]

I'm also of the center-tapped-single-secondary-winding-is-a-single-phase camp. If we grounded one end instead of the center tap, what would it be then?

There's no question that a 4-wire Y system is 3-phase; yet if we take a 208v 2-wire circuit from it, there's no question that it's single phase, right?

Yes, it is single phase, but it does have a time displacement from the other phases. Here is a picture:

View attachment 832

Phase A is represents the voltage from A to N. Phase B represents the voltage from B to N. B - A represents the voltage from B to A.

 

[rattus]

I'm also of the center-tapped-single-secondary-winding-is-a-single-phase camp. If we grounded one end instead of the center tap, what would it be then?

There's no question that a 4-wire Y system is 3-phase; yet if we take a 208v 2-wire circuit from it, there's no question that it's single phase, right?

Larry, no one, at least not I, is saying that the 120/240V system is a 2-phase system!

I am saying though that the voltages on L1 and L2 are 180 degrees out of phase. It is implicit that these voltages are measured relative the neutral.

You can however use double subscript notation such as,

Vn2 or Vn1

to specify the "sense" of the AC voltages. Then there is no question about the phase differences.

 

[eric stromberg]

Here is another 'graphic' (i use the word loosely) of a three phase system:

View attachment 833

Hopefully, you can see that a three phase system has an inherent 'rotating' quality to it. Single phase systems do not.

One of the problems with the electrical field (or any field, for that matter) is that inaccurate speech fosters inaccurate thinking which fosters inaccurate understanding by others. It takes far longer to 'unlearn' someone than it takes to teach one. For example, the concept that "current seeks a path to ground." This concept takes over the thought process so much that it is difficult to introduce what is really happening. The incorrect teaching must be unlearned before the mind is open to accept what's really happening.

Yes, one can consider the two single phase legs to be "out of phase" with each other, but why? This is simply inaccurate teaching which, when propagated, confuses people and becomes an impediment to a deeper understanding of what is really happening.

 

[rattus]

Here is another 'graphic' (i use the word loosely) of a three phase system:

View attachment 833

One of the problems with the electrical field is that inaccurate speech fosters inaccurate thinking which fosters inaccurate understanding by others. It takes far longer to 'unlearn' someone than it takes to teach one. For example, the concept that "current seeks a path to ground." This concept takes over the thought process so much that it is difficult to introduce what is really happening. The incorrect teaching must be unlearned before the mind is open to accept the truth.

Yes, one can consider the two single phase legs to be "out of phase" with each other, but why? This is simply inaccurate teaching which, when propagated, confuses people and becomes an impediment to a deeper understanding of what is really happening. If, however, a person is comfortable with a particular level of understanding, there is nothing further that can be done.

Eric,

How can you call this inaccurate teaching when it is absolutely true. To fully understand phasors, one must understand them upside down, right side up, forward, and reverse. That is what this is all about.

Why? You ask. Because the OP posed his question this way and this is perfectly valid. In fact it is a fundamental concept in electrical engineering. Furhermore, 3-ph wye voltages are defined this way, so why not L1 and L2 in a single phase system? So what is the beef?

As far as understanding is concerned, I would expect students to understand the significance of double subscripts and phase reverals even if they don't understand the math.

I cannot understand the reluctance of engineers to accept the concept that phase reversals can occur in single phase systems. There are numerous ways to obtain phase reversals--transformers, phase splitters, push-pull amplifiers, op-amps. Just because we say there is a phase reversal does not imply we have a two phase system.

In another thread, someone said, "it may look like a phase reversal, but it is not" But I say, "If it looks like a duck,...."

 

[coulter]

...Furhermore, 3-ph wye voltages are defined this way, so why not L1 and L2 in a single phase system? ...

I have difficulty associating the concept of rotation with single phase. Part of the reasoning is the difficulty I'm having is with associating the symmetrical components model with single phase. I'm not saying one can't, I just can't make the model fit. The easiest way to show it is with physical devices:

1. Single phase motors don't have a rotating field. They don't know which way to turn. (Chapman, Electric Machinery Fundamentals, 10.2)

2. A 12 lead 120V 3ph gen connected for single phase is connected as shown in Sketch 1. It is certainly not connected as shown in sketch 2

...I cannot understand the reluctance of engineers to accept the concept that phase reversals can occur in single phase systems. There are numerous ways to obtain phase reversals--transformers, phase splitters, push-pull amplifiers, op-amps. ...

The concepts you are using generally refer to small signal models. I've not seen this to be common practice power applications. But that's okay, I'll reach back to 1972 and dig out my understanding of small signal models. I don't know about other engineers, but from my point of view, the secondary of a single phase house transformer is one coil, one vector, L1 to L2. Yes. I used the word "vector" not "phasor". It isn't rotating. (see Chapman again)
If you like small signal models and dot convention, then I would contend the xfm is connected as shown in Sketch 3. As I understand your argument, you would agree with sketch 3, even though you show the two phasors (your word) pointing in opposite directions. (Alert - engineer has stalled brain process:-? )
Hold on - I'll connect this all up in a minute

no one, at least not I, is saying that the 120/240V system is a 2-phase system! ...

I would gently suggest you may well be inferring this is a two phase system - even if you are not saying it.

1. You are comparing single phase to 3phase - which has rotation
2. Your calling the single phase vectors "phasors" - which means rotation.
3. 1 and 2 implies the symmetrical components model fits.
4. Since there are two fault modes, positive sequence and zero sequence that implies two phase.

My reasoning:
1. there is one secondary coil, one vector (yes, it is center tapped)
2. This vector doesn't rotate, it pulses (see Chapman again)
3. power system conventional notation puts the two vectors making up the L1 - L2 vector nose to tail. (see 12 lead gen connection)

What you are advocating certainly works as long as one does not go any further than connecting up the O-scope as you suggest. I can?t keep your model from falling apart right after that.

...As far as understanding is concerned, I would expect students to understand the significance of double subscripts and phase reverals even if they don't understand the math....

Then again, it could be that I am just dumber than the average student..:smile: I thinking that has to be true - I'm still in this conversation

carl

PS to whizzes in symmetrical components: Symmetrical components is not my strong suit. Please feel free to jump in and correct my thinking

 

[wasasparky]

Lets consider just a single 120V ?phase? with terminals labeled ?a? and ?b?

It?s perfectly acceptable to say Vba is 180 degrees out of phase with Vab.
( -120 at angle 180 degrees = 120 at angle 0 degrees )

Lets not forget that Vba = -Vab. The above is a double-negative which actually proves the fact that we are talking about, dare I say, a single phase system?

The confusion begins when one only looks at half of the double-negative (usually the 0 & 180 degree part)

 

[rattus]

Wow!

Wow!

Carl, you are making this thing 10 times as complicated as it should be.

First, any sinusoidal voltage or current can be described by a fixed phasor or a rotating phasor, single phase or polyphase. e.g.,

v1(t) = 170V x exp(-jwt) -- rotating

V1 = 120V @ 0 -- fixed

This discussion has nothing to do with rotating magnetic fields or phase sequencing in 3-ph systems. The comparison between single and 3-phase merely points out that the voltages are measured relative to the neutral in both cases.

If you wish to measure it differently, that is your choice. But to look at L1 and L2 simultaneously on a scope, the neutral is the obvious choice for the ground clip especially so since the ground clip is usually connected to the EGC!

It boils down to this:

According to convention,

The voltages (relative to the center tap) at the outer ends of a center tapped secondary are inverses of each other. If these voltages are sinusoids, that is tantamount to 180 degrees phase shift. It is that simple!

And for the umpteenth time, this does not imply a 2-phase system.

Jim and Eric have grudgingly admitted as much, so why are you holding out?

 

[rattus]

Thanks:

Thanks:

Lets consider just a single 120V ?phase? with terminals labeled ?a? and ?b?

It?s perfectly acceptable to say Vba is 180 degrees out of phase with Vab.
( -120 at angle 180 degrees = 120 at angle 0 degrees )

Lets not forget that Vba = -Vab. The above is a double-negative which actually proves the fact that we are talking about, dare I say, a single phase system?

The confusion begins when one only looks at half of the double-negative (usually the 0 & 180 degree part)

Thanks for pointing out the obvious.

 

[coulter]

...Jim and Eric have grudgingly admitted as much, so why are you holding out?

I don't know what jim and eric grudgingly admitted, but I am pretty sure they are smarter than I am - as is evidenced by me still being in the conversation.

You are right, I am making this way too complicated.

carl

 

[coulter]

rattus -
Just curious, what's a "fixed phasor"? That would represent a completely unknown to me vector opeeration.

carl

 

[rattus]

rattus -
Just curious, what's a "fixed phasor"? That would represent a completely unknown to me vector opeeration.

carl

Va = 120 @ 0
Vb = 120 @ -120
Vc = 120 @ -240

are fixed phasors in polar form. They may be easily converty to complex form.

 

[eric stromberg]

Va = 120 @ 0
Vb = 120 @ -120
Vc = 120 @ -240

are fixed phasors in polar form. They may be easily converty to complex form.

Rattus,
Correct. There are, in fact, phasors in three phase systems. A "Phasor" can be thought of as a "Vector rotating with a constant frequency." There are no "Phasors" in single phase systems, there are only "Vectors."

And, by the way, i haven't admitted any kind of defeat. :smile: :smile:
Keep it coming, we'll get through it. I've often heard Mike Holt say "I've never learned anything from someone who agrees with me all the time."

 

[don_resqcapt19]

rattus,

I am saying though that the voltages on L1 and L2 are 180 degrees out of phase.

How can conductors from the same transformer coil be out of phase with each other?

 

[rattus]

rattus,

How can conductors from the same transformer coil be out of phase with each other?

Don, it depends on your choice of reference. For example,

If V1n = 120 @ 0, then

Vn2 = 120 @ 0 as well, but

V2n = 120 @ 180

All we are doing is swapping leads in our minds. Either way gets the right answer with phasor calculations.