7200 volt to 120/240 volt single phase transformer

[jwelectric]

A 7200 volt to 120/240 volt single phase transformer has three terminals on the secondary consisting of line 1, neutral and line 2.
Is there two phases of 120 volts that oppose each other at 180 degrees or are the two 120 volt legs in phase with each other and in phase with the total 240 volt secondary?

Please explain your answer.

 

[don_resqcapt19]

That is a single secondary winding with a center tap. You cannot have more than one phase with a single secondary winding. Using the center tap as the point of reference makes it appear that there are two phases 180? out of phase.

 

[jim dungar]

When performing an analysis you do not need to use a single point, however you do need to remain consistent in the application of your reference choice.

My choice, when possible, is to use voltages as defined by the forces (i.e. transformer flux) that create them.
I want to analyze a single winding 3-terminal transformer secondary using the same math, regardless if the 'third terminal' is a neutral point, it is somewhere else on the coil (i.e. a control transformer with both 24V and 120V outputs), or if it is even not used. I cannot see a reason to change, simply because 1-(1<180?)=2, when the result is voltages and currents being out phase with each other when feeding resistive circuits.

Good luck on your discussion.

 

[jwelectric]

Jim I appreciate your answer although it may be over the heads of poor laymen like me that is trying to learn the basic fundamentals of the power coming into their homes. Some of us can barely give change for a dollar let alone doing vector math.
This is why I choose the voltages that I did as they represent the transformer supplying most homes.

The fundamentals of the transformer supplying the premises wiring is the same as the transformer on the inside of the air handler in most homes.

Both are wound around a common core and the voltage of the primary induces the voltage on the secondary be it one or more taps.

There was another thread that raised many questions in a group of us in my area and we now have two opinions as to whether or not the secondary of the transformer supplying our homes has two phases that are 180 degrees out of phase with each other and I am seeking answers to this question.

Does the secondary of a 120/240 volt single phase transformer have two 120 volt phases that are 180 degrees out of phase with each other?

 

[jim dungar]

Does the secondary of a 120/240 volt single phase transformer have two 120 volt phases that are 180 degrees out of phase with each other?

I say according to the laws of physics and the way a single core single winding transformer works - the answer is no.
I have never been able to find the math that shows how a single flux can create opposing voltages in a single conductor wound in a single direction.






For those that want more explanation:
http://www.tabtronics.com/TECHNOLOG...ics/tabid/110/Default.aspx#Flux_coupling_laws
The Universal EMF equation

In the reference section there is a link to http://mysite.du.edu/~jcalvert/tech/transfor.htm, which is less wordy, and shows several diagrams which do have directions associated the secondary voltage(s).

 

[Besoeker]

A 7200 volt to 120/240 volt single phase transformer has three terminals on the secondary consisting of line 1, neutral and line 2.
Is there two phases of 120 volts that oppose each other at 180 degrees or are the two 120 volt legs in phase with each other and in phase with the total 240 volt secondary?

Please explain your answer.

If you measure the voltages as Vline1 - neutral and Vline2 - neutral, then yes they are in anti-phase.
And that, after all, is how most 120-0-120 transformers are used. Two "hots" and a neutral.

 

[Besoeker]

I have never been able to find the math that shows how a single flux can create opposing voltages in a single conductor wound in a single direction.

I don't imagine that anyone with even the most basic knowledge of transformer theory would argue otherwise.
[/QUOTE]

 

[Hv&Lv]

I wonder if this thread will go for 63 pages?

 

[jwelectric]

If you measure the voltages as Vline1 - neutral and Vline2 - neutral, then yes they are in anti-phase.
And that, after all, is how most 120-0-120 transformers are used. Two "hots" and a neutral.

I like using a divided highway to explain what you just said. Using the medium as the neutral point and the traffic as the line voltages, as you have stated from the outside of the lane looking inward to the medium we have traffic that is traveling from left to the right and should we cross over all lanes of traffic to where we are looking back across to the medium the traffic is still going from the left to the right. Nothing has changed.

If we say we are reading our voltages from the neutral point to line 1 and again from the neutral to line 2 then we are standing in the medium looking at one side of traffic and it is moving from right to left and when we turn around we see that that side is also going from right to left and again nothing has changed.

Just because what we see on the screen of a scope looks like it is out of phase does not mean that it truly is out of phase. As I stand in the medium looking at both sides of traffic at the same time it looks like they are coming at me from one side and going away on the other but when I look at them individually it is clear to see that they are going the same way from the right to the left.

What you think?

 

[jim dungar]

I don't imagine that anyone with even the most basic knowledge of transformer theory would argue otherwise.

Are you saying the voltages in the two halves of a single winding are really in phase?
Even though we can view/use them based on what we choose as a common point.

 

[ceb58]

jwelectric

Jim I appreciate your answer although it may be over the heads of poor laymen like me that is trying to learn the basic fundamentals of the power coming into their homes. Some of us can barely give change for a dollar let alone doing vector math.
​

I pay the electric bill on line,dont have to make change:angel:

 

[Besoeker]

Are you saying the voltages in the two halves of a single winding are really in phase?
Even though we can view/use them based on what we choose as a common point.

If you take end to end on the primary and end to end on the secondary, then yes, if you measure with respects to the starts and finishes, the dots, you will get two in-phase voltages.
If you measure the two hots with respect to the common centre point, usually taken as the neutral and commonly grounded in your part of the world, you will get voltages mutually displaced by 180deg.

 

[kwired]

I wonder if this thread will go for 63 pages?

Consider it an extension of the recent one that did. What was it, just about 700 posts? I read about the first 200, gave up after that.

 

[mivey]

A 7200 volt to 120/240 volt single phase transformer has three terminals on the secondary consisting of line 1, neutral and line 2.
Is there two phases of 120 volts that oppose each other at 180 degrees or are the two 120 volt legs in phase with each other and in phase with the total 240 volt secondary?

Please explain your answer.

Before we get to that, may I say it would be better use the term voltage instead of phase. Some people just don't understand and that confuses the issue with the debate over system naming conventions. A whole other topic.

That is a single secondary winding with a center tap. You cannot have more than one phase with a single secondary winding. Using the center tap as the point of reference makes it appear that there are two phases 180? out of phase.

Call it phase if you want but there is no doubt that there are two voltages with a 180? phase displacement.

My choice, when possible, is to use voltages as defined by the forces (i.e. transformer flux) that create them.

As we all must do. The problem is that the flux does not define the direction we must use when taking the voltages from the source. No way, no how.

...when the result is voltages and currents being out phase with each other when feeding resistive circuits.

The source voltage is not out of phase with its current. I don't know why you keep thinking that. It just ain't so.
I believe the reason you think that is because you think the voltage direction is somehow picked for you by some other means than your own choice.

Does the secondary of a 120/240 volt single phase transformer have two 120 volt phases that are 180 degrees out of phase with each other?

Vbn is indeed 180? out of phase with Van. If you let anyone convince you otherwise, all I can say is I feel sorry that you can't see the truth of the matter and I really do wish I could help you. I do have one other approach I can try so bear with me.

I say according to the laws of physics and the way a single core single winding transformer works - the answer is no.

I say the laws of physics do not tell you that one direction is for the "real" voltages and the other is for the "math tricks". The laws do not say what you are implying that they say.

I have never been able to find the math that shows how a single flux can create opposing voltages in a single conductor wound in a single direction.

Who said they did? The opposing voltage comes 180? later. It has to do with how the voltages are taken. All I can figure is that we are just having a major mis-communication because I know we both know the theory and laws backwards and forwards.

Are you saying the voltages in the two halves of a single winding are really in phase?

They are in phase if we are taking the voltages in the same direction.

 

[jim dungar]

They are in phase if we are taking the voltages in the same direction.

The OP was about what really exists. That should be a yes or no answer based on the physics of the source, a single core single winding transformer. How we view/treat/manipulate them as a 'black box source' is an entirely different discussion.

ANSI has standards are a method to define voltage direction based on the construction of the transformer. The method they provide is Positive/<0?/upward or Negative/<180?/downward, but not both ways at once.

 

[mivey]

Mike,

In the other thread you said

From line 1 to line 2 of a single phase transformer there is a potential and only one sine wave. Why would a sine wave need to oppose itself to have potential? Think of a two cell flashlight that has the batteries reversing at the rate of 60 times a second. Now we have a two cell AC flashlight. Should we reverse one of the batteries it wouldn?t matter how fast we reversed them no current would flow. The same is true with the AC transformer.

If you understand that the polarity is reversing, then perhaps we can use the battery analysis but with batteries that are reversing polarity every 1/120 of a second. Granted, the voltage won't be sinusoidal because the magnitudes are steady, but maybe it will be easier for you to understand what is being said.

Here is our two-battery stack oscillating back and forth in polarity:

Now this is what Jim & Rick and some others would have you believe:

Please note that there is nothing wrong with taking the voltages to be positive in one direction. The problem comes in when you say it is the only right way and taking them a different way does not give you real voltages but just "math tricks". This is what they are calling math tricks:

They try to validate their point by claiming that doing things different from what they say is breaking laws and physics. This is what they claim:

The truth is that the laws do not dictate in what direction we must take the voltages. In fact, the truth is:

These are the facts and we know them to be the facts because the real working circuits work in agreement with these facts. Here is an example:

I hope this clarifies what Besoeker, David, Laszlo, and I (I apologize if I forgot someone) have been saying. Whether or not you agree is up to you, but at least maybe you can stop mis-stating what we are saying.

 

[mivey]

The OP was about what really exists. That should be a yes or no answer based on the physics of the source, a single core single winding transformer. How we view/treat/manipulate them as a 'black box source' is an entirely different discussion.

A simple thought experiment would show that what you think is not true.

I have already shown how two 180? displaced voltages can combine and feed a circuit the same way as the center-tap source. In fact, we could join the two separate cores (not perfect as if they were manufactured together of course) and wind up with the same flux and voltage that we have with the single core. The two 180? displaced sources would combine their forces. The difference is what direction we started with. Even though we would have the same two phase-displaced sources working together on the primary, all the laws of flux, physics, etc. would work just fine.

That is because no laws are being broken. The simple fact is that our choice of reference is arbitrary. As long as we let our voltage forces work in unison, all will be well. Polarity tells us what to look for when we are comparing two different parts of the circuit. It is what keeps us from trying to join two different potentials at the same point. But polarity does not tell us that one direction is right and the other is wrong.

If we look at the world of sources as a whole, the reality is that they were all started at different points of a "universal" phase reference. If we were to truly resolve all of the primary sources into voltages that began during the positive vs negative half of a "universal" phase reference, statistics and natural distribution would tell you that 50% of the signals started in the positive half and 50% started in the negative half. So if you want to trace everything back to the source, you will find that 50% started in one direction and 50% started in the other direction.

ANSI has standards are a method to define voltage direction based on the construction of the transformer. The method they provide is Positive/<0?/upward or Negative/<180?/downward, but not both ways at once.

That is the difference in polarity vs direction. The simple fact remains that Vbn and Van are real voltages and have a 180? difference.

 

[mivey]

FYI,

I just noticed that I copied and pasted the wrong blocks in CAD. I should have printed in and proof-read it first. Figures. I will fix the drawings.

 

[mivey]

FYI,

I just noticed that I copied and pasted the wrong blocks in CAD. I should have printed in and proof-read it first. Figures. I will fix the drawings.

Here are the corrected drawings:

 

[ronaldrc]

Hello thought I would put my two cents in.

Every since I have been in the Electrical trade myself and other Electricians knew for a fact with
no doubt that in the Single Phase Electrical system feeding a house was just that single phase.

There is nothing out of Phase about it. It is only a polarity difference.Nothing is 180 degrees out.
Line one is line No. one and line two is line two and the center tap is a neutral between those two lines.

This misconception only started after the forums on the internet became popular.


It has always been my opinion that all this stuff about referring too Line no. 1 as phase one and line no. two
phase 2 was brought about by the people posting here with a strong Electronic field. They are and have
always been taught when they reverse the polarity with a so called phase inverter they are shifting the
phase instead of the polarity. I don't think there is a thing wrong with calling this electronic stage a phase
inverter. But it just reverses the polarity also.

Please in the Electrical trade lets not confuse Single Phase with 2 phase because of this belief.

Electricians do not look at there voltages with oscilloscopes Electronic techs do.
Yes if you measure from the neutral two line 1 and neutral to line 2 it will appear as if you have two
sine waves 180 degrees out of phase with each other. That is because you are using the neutral as
the common. If this where an Electronic circuit they would probably be using the Line one as the
common. It would really all depend on the application.

But if you use line one as the common when using a oscilloscope instead of the neutral you will
get two 120 volt sine waves in phase with each other and the polarities will also be the same.

Using the neutral as common only makes it appear as you have two 180 degrees out of phase. The only
difference is the polarity they both rise and fall at the same time at the same rate.

The reason the Electrical field uses the neutral on a Single Phase system as the common instead of line
1 or line 2 is for safety this gives you only a 120 volts to line 1 or line 2 if you come in contact with them.

Thanks:Ronald