Center-tap Transformer Voltages

[ronaldrc]

No, the scope can't trace backward.

No it can't but you know exactly what I am talking about positive becomes minus and minus
becomes positive. And the scope traces downward instead of upward but left to right.


And all through this site you see post refering to line 1 as phase one and line two as phase two
when taliking about single phase. And if the two secondary windings are a 180 degrees offset
that is the same thing as being two phases.

And if the two winding are hooked in series and out of phase 180 degrees there would be no
voltage between line #1 and Line #2.

 

[Hv&Lv]

No it can't but you know exactly what I am talking about positive becomes minus and minus
becomes positive. And the scope traces downward instead of upward but left to right.


And all through this site you see post refering to line 1 as phase one and line two as phase two
when taliking about single phase. And if the two secondary windings are a 180 degrees offset
that is the same thing as being two phases.

And if the two winding are hooked in series and out of phase 180 degrees there would be no
voltage between line #1 and Line #2.

Isn't a difference in phases a difference in time also?

 

[Besoeker]

No, the scope can't trace backward. But if you use a dual trace scope, clip both grounds to the neutral, clip probe A to one line, clip probe B to the other, and sync on probe A, you will see two sinusoids separated by 180 degrees.

I agree. It will look like this:

No ifs, no buts, that's what you get.
I don't understand what's difficult to comprehend about that.
You probably feel the same way.

 

[rattus]

I agree. It will look like this:

No ifs, no buts, that's what you get.
I don't understand what's difficult to comprehend about that.
You probably feel the same way.

ronald, et al, please note that the DIFFERENCE between the two traces is 340V peak to peak.


340Vpp/sqrt(2) = 240Vrms

 

[rattus]

And all through this site you see post refering to line 1 as phase one and line two as phase two
when taliking about single phase. And if the two secondary windings are a 180 degrees offset
that is the same thing as being two phases.

Not quite ronald. A true two phase system provides two phases separated by 90 degrees. It would be more proper to refer to the two legs of the single phase system.

 

[ronaldrc]

Rattus and Besoeker I agree from the neutral it would look just like that.

Hu&Lv and that what another phase means in the electrical trade
another 60 cycle sine offset in time.

Glad were all on the same page anyway

 

[gar]

111119-2121 EDT

ronaldrc:

Consider two phases of a three phase wye system originating from the same alternator. The two sine waves of these two phases are not a result of a time shift, but of a phase shift created by the electro-mechanical construction of the alternator. To relate the two phases relative to each other we need to describe their relative voltages in some fashion. One way is to display them on a dual beam or two channel oscilloscope. An instant of time is one point on the horizontal axis (that is when a linear sweep voltage vs time is applied to the horizontal axis). What you see at one one instant of time is two different signals with one having a phase displacement relative to the other but not a time displacement. Another way to see a phase relationship on an oscilloscope is to apply one sine wave to the x axis, and the other to the y axis. The result is a Lissajous figure from which you can derive the phase difference and frequency ratio for harmonically related sine waves.

http://en.wikipedia.org/wiki/Lissajous_curve
http://mathworld.wolfram.com/LissajousCurve.html
http://www.2dcurves.com/higher/higherli.html

For a time shifted waveform see my photo P1 at
http://www.beta-a2.com/cat-5e_photo.html
This shows a step rise input waveform to a 150 ft length CAT-5 cable, and the delayed signal, 200 nS, at the far end of the cable. Since the far end is an open circuit energy is reflected back on the cable and reaches the input at 400 nS and the phasing is such that the reflected energy adds voltage to the original input, and you see an additional rise in the input signal level. Had the far end been shorted, then at the input end the reflected energy would produce a negative change in voltage.

.

 

[jumper]

No, the scope can't trace backward. But if you use a dual trace scope, clip both grounds to the neutral, clip probe A to one line, clip probe B to the other, and sync on probe A, you will see two sinusoids separated by 180 degrees.

Yep.

CAUTION: Never connect a probe ground clip to a hot line!

Want to know what happens if you do? See I know this fella......:angel:

I agree. It will look like this:

No ifs, no buts, that's what you get.
I don't understand what's difficult to comprehend about that.
You probably feel the same way.

I am in, all these other arguments are confusing.

 

[Besoeker]

[/B]

Want to know what happens if you do? See I know this fella......:angel:

On where you can and can't connect ground clips....
I was testing this last week:

It's a high current low voltage hexaphase controlled rectifier. (SCRs) It needs six equally displaced firing pulses per cycle. And they need to go to the correct SCR, of course.
Unfortunately, they were not. There are a lot of permutations possible..........
It takes a bit of logical thinking and no interruptions to sort out the correct one. However it's on the low voltage side and not and not connected to ground at any point so connecting the ground clip of the scope isn't a problem. Except if you connect two of them to two different points. Which is what I did. Nothing spectacular happened. I had one lead looking across a SCR and the other on the DC voltmeter. Luckily, the voltmeter was fused on both sides at 2A. One of them blew, that's all.

On the subject of centre-tapped windings, the hexaphase, hex from six, is just a three-phase version of a centre-tapped arrangement. It's often used on LV rectifiers where the voltage drop across the SCR is significant proportion of the output voltage.

Each limb looks like this:

 

[pfalcon]

pfalcon,

The equations presented simply make the point that a phase shift of 180 is seen between the ends of a secondary which point has been argued for and against ad nauseum. The equations are universally true. The actual design of the transformer is of no concern as is the application.

(emphasis added). Which was EXACTLY my point. It only matters when the actual design of the transformer IS of concern.

The equations you're using are ONLY true when the PHYSICAL transformer is neglected in favor of an EQUIVALENT CIRCUIT. The 240/120 transformer holds both secondaries in phase and in polarity through magnetic coupling. This can't be repealed any more than gravity.

Objects don't fall upwards just because you're standing on your head. They just look that way. The bottom leg doesn't actually shift just because you decided to measure from the middle. It just looks that way. And as long as you correct your signs, then for all practical purposes you can do the math any way you want. Except on an engineering exam.

Science and Engineering have a term for what you're seeing. It's called apparent phase shift. And the only people that really care about the difference between real and apparent phase shift are people that design transformers and model magnetic field induction. Which would not be you.

So feel free to do your math on the secondary side any way you want. It won't matter as long as you're consistent to your arbitrary reference point.

... Divorce your thoughts from the specifics of a transformer. ...

Yes, because without the specifics of a transformer, the equations promoted by rattus et al are absolutely correct.

 

[rattus]

pfalcon,

How would you define the voltages on L1 and L2 in a split-phase transformer?

 

[ronaldrc]

(emphasis added).
Objects don't fall upwards just because you're standing on your head. They just look that way. The bottom leg doesn't actually shift just because you decided to measure from the middle. It just looks that way. And as long as you correct your signs, then for all practical purposes you can do the math any way you want. Except on an engineering exam.

Pfalco that quote was great

My girl I think is the smartest person I have ever known. She really has a way
with words. She can explain any thing to anybody and make them understand it.

You come in second.

Here is my illustration of what we are talking about it is
a pictorial. I have changed it.

Click here for illustration

 

[gar]

111121-1019 EST

ronaldrc:

Your illustration needs to define how the scopes horizontal sweeps are synchronized. I know what the assumption is, but it needs to be defined.

Why is the word "apparent" used to described the phase shift when it actually shows a phase shift.

Consider two voltages, both defined with reference to the same x=0 time base.

v₁ = A sin (x + 0)
and
v₂ = A sin (x + B)

Is there no phase shift when B = N * 180, where N is an integer, but any other values of B are considered to be a phase shift? No good logical reason except for maybe N = 0. I am actually willing to call a phase difference angle of 0 a phase shift of 0 when it serves a useful purpose.

There seems to be an attitude against thinking in consistent generalized terms. Why create unnecessary discontinuities in definitions?

.

 

[pfalcon]

pfalcon,

How would you define the voltages on L1 and L2 in a split-phase transformer?

Split-phase transformers are what we've been discussing. RonaldRC shows a great picture after the click (and not just because of the compliment )
X1-X4 is in-polarity and in-phase. That's the real part of a transformer.

Below in the drawing is an example use of oscilloscopes. They appear out-of-phase only cause you're looking at it funny.

Mathematically there are all sorts of EQUIVALENT circuits. Typical three-phase doesn't have to be described as <0, <120, <240 on an equivalent circuit. We COULD go with <0, <60, <120 or <180, <240, <120. We don't do that cause that creates all sorts of double negatives in the math. Why make life harder on yourself?

You keep wanting to take X3-X4 and call it <180. Fine. Just don't put it on the drawing with the transformer cause then the polarity matters. When connected to a primary coil the entire secondary MUST BE in-phase and in-polarity. Feel free to leave the primary coil off then you can keep resolving your vectors like (120+j0) - (-120+j0) and deal with all the double negatives in the math you like.

Pfalco that quote was great

My girl I think is the smartest person I have ever known. She really has a way
with words. She can explain any thing to anybody and make them understand it.

You come in second.

Here is my illustration of what we are talking about it is
a pictorial. I have changed it.

Click here for illustration

 

[pfalcon]

... There seems to be an attitude against thinking in consistent generalized terms. Why create unnecessary discontinuities in definitions?

This entire thread grew because of a lack of those definitions.

Problem 1) For all practical purposes you may model the 240/120 split-phase transformer in numerous ways. You can even set up scopes to demonstrate your models. All of these models are called equivalent circuits. All the measurements for phase shift you get are called apparent phase shifts. As long as you pay attention to your vectors, your math will work. Such as Vab<0 = Van<0 - Vbn<180.

Problem 2) Some of us engineering types prefer to use equivalent circuits that most closely conform to the actual circuit. In an actual transformer the magnetic coupling guarantees that both legs of the secondary are in-phase and in-polarity. By convention we assign <0 to the primary phase to keep the math simple. Therefore the correct equation for engineering purposes is Vab<0 = Van<0 + Vnb<0.

Either set of math works.

Contention 1) Some of the earlier issues being argued were without question because the engineering folks didn't recognize that the non-engineering folks were only discussing Problem 1 and didn't care about Problem 2.
Contention 2) Some of the later issues have been that the non-engineering folks insist that the presumptions under Problem 1 apply to Problem 2.

 

[Besoeker]

Here is my illustration of what we are talking about it is
a pictorial. I have changed it.

Click here for illustration

The words at the start are interesting:

"How a balanced neutral was created by simply merging two 120 volt circuits of opposite polarity"
In an AC circuit what would you interpret opposite polarity to mean other than anti-phase?
i.e displaced by 180deg.

V_an reaches its maximum positive value at the same point that V_bn reaches its maximum negative value.
In other words, opposite polarity. As indeed they are at all points in time.

 

[ronaldrc]

The words at the start are interesting:

In an AC circuit what would you interpret opposite polarity to mean other than anti-phase?
i.e displaced by 180deg.

Just that opposite polarity , I said from day one a few yrs. back that I don't think
polarity and Phase are the same thing.

Ronald

 

[Besoeker]

Just that opposite polarity , I said from day one a few yrs. back that I don't think
polarity and Phase are the same thing.

Ronald

If the two voltages, V_an and V_bn are, at all times, of equal magnitude and and opposite polarity, how could that be without them being in anti-phase?
i.e mutually displaced by 180deg?

 

[rattus]

Split-phase transformers are what we've been discussing. RonaldRC shows a great picture after the click (and not just because of the compliment )
X1-X4 is in-polarity and in-phase. That's the real part of a transformer.

That is not the answer. I want to know the magnitude and phase of Vx1x2 and Vx4x3 in a split phase system.

 

[ronaldrc]

If the two voltages, V_an and V_bn are, at all times, of equal magnitude and and opposite polarity, how could that be without them being in anti-phase?
i.e mutually displaced by 180deg?

Darn were beating each other to a pulp over this I call it opposite polarity and you can call it
anti-phase. It don't mean a hill of beans to me. I just hate to confuse everybody else.

I like to discuss this but I need a break.
see you guys later: Ronald :bye: