7200 volt to 120/240 volt single phase transformer

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mivey

mivey

Senior Member
Rick,

As part of you helping me clarify my points, could you take a look at this post and see if you can understand what I am trying to say? I'm not asking you to agree with it, I just want to know if you can understand my position.

Any word-smithing advice on how to clarify what I am trying to present in that post would be appreciated. I have tried in many ways to state my position but can't seem to get it across to some. I'm not so concerned that someone wants to argue against my position but what makes it difficult is when they argue against something that is not my position.
 
mivey

mivey

Senior Member
mivey said:
I have a vast number of books and texts in my library dating from the turn of the century to current...
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I guess I have spent too much time in the 20th century. I'm still not used to the fact that "turn of the century" doesn't mean around 1900 anymore.
 
mivey

mivey

Senior Member
jwelectric said:
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.
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Back to the original question. The words "opposing" and "in phase" have to be defined as they can be interpreted different ways.

The forces in the transformer are not opposing each other. The source produces forces in two directions but not at the same time. We can pick either direction to be positive when defining the direction of a voltage rise. That is because there is a voltage rise in one direction that is opposite in direction from the voltage rise that occurs 1/2 cycle later.

If we have any two voltages in the transformer where we take the rise to be in opposite directions, and we do not want the forces to oppose each other, the force produced by the voltage fall of one voltage will push in the same direction as the force produced by the voltage rise of the other (i.e. the rise of one will be in phase with the fall of the other).

Polarity helps us align the forces so they work together. Direction tells us if the force was produced by a voltage rise or a voltage fall.

Here is a summary graphic:

Center-tapDirection.jpg
 
mivey

mivey

Senior Member
jwelectric said:
red inserts are mine and show what you are saying in your last sentende, jusr a different way of seeing the same thing. This is true that both sine waves are twins that look and act alike but in order to see them we have to look at them in two different ways. This in no way means that they are out of phase with each other or at different polarities.
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The meter is showing you exactly what is there. What is important is to not mis-interpret what it is showing. It is indeed showing that Van is 180? out of phase with Vbn because that is the physical reality. Good meters don't lie
 
jwelectric

jwelectric

Senior Member
Location
North Carolina
We did something very simple in the lab. We took and built us a holder for two ?D? cell batteries with a lead coming from each end and one from the center of these batteries.
We installed the batteries in series with the positive of one battery touching the negative of the other. Looking down at the batteries we saw -+,-+ with a lead at each end and one in the middle.
We now connect A to the negative end of the first battery then ground in the middle and B to the positive end of the second battery.

What we saw on the scope was two 1.5 volt batteries that were 180 degrees out of line with each other.
Then we connected the positive of one of the 1.5 cell to the positive of a 6 volt battery and read the voltage of 4.5 from the negative of the 6 volt battery and the negative end of the ?D? cell.
From that point on there was no more discussion about anything being 180 degrees out of phase on the single phase transformer.
 
mivey

mivey

Senior Member
mivey said:
I think what you proved was that you don't know how to read an oscilloscope.
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Here is what I mean.

First you are trying to say that a DC measurement has a phase. DC has positive and negative, that's it.:

ACvsDCScope1.jpg



An AC signal has a positive and a negative as well as phase. You can read several things with your scope (let's assume no time delay between inputs). It can tell you the maximum and minimum you see at the same time reference can be the positive and negative of two in-phase voltages.

What it is NOT telling you is that the maximum and minimum you see at the same time reference is the positive and negative of two voltages with a 180? phase difference. If we were to plot the positive of Van and the negative of Vbn, the peaks would show a maximum at the same time.

Simply put, you need to become more familiar with the operation of your oscilloscope. Here is a graphic with some examples of what you might can see when measuring two AC signals and you get two waves where one is a minimum and the other is a maximum at the same time:

ACvsDCScope2.jpg
 
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