single vs. 3 phase

[mivey]

I think that the electronics gurus would be perfectly happy with the following diagram and the use of the centerpoint as a reference for all voltage measurements. I think the math and current flow and analysis still works.

I suspect that the electronics gurus would call that diagram a two voltage supply not a split voltage supply.

Crossman chose a battery symbol but his point would be the same if he started with a center-tapped transformer and fed into a rectifier and voltage regulators to make a +/- supply.

Since n is in the middle it will be negative to 1 but positive to 2. The terminal "n" is not really neutral or 0, it is simply in the middle.

Not exactly. In the 120/240 single-phase case, it really IS the neutral point for the single phase and can be used as a reference point (but it is NOT the neutral point for the 3-phase delta).

If it were a 2-wire 120 volt circuit, where do you think the neutral point between the grounded conductor and the ungrounded conductor would be? I'll give you a hint, wire two identical heater coils in series and it should be obvious. As another exercise, take this 120 volt circuit and feed a center-tapped transformer (like you might do to feed some electronics circuit board) and find the neutral point. Once you find this neutral point, you can find a way to supply the circuit board with a + and - supply for the components.

This same 120 volt fed circuit can also be fed with a 240 volt center-tapped transformer. This is the same as what we have in the lighting pot of the delta. We do not have access to the real neutral of the 3-phase delta, although we could create one.

By saying that V1n and V2n are 180 out of phase to me you are implying that 1 and 2 both go positive with respect to n. Do a "kick" test with a battery on the winding and you will see that that is not the case.

I think he is saying that one goes positive and one goes negative with respect to n. "kick" tests should show that.

Its only one winding or 2 halves) on one core, only one voltage is induced. We choose to split it for convience

You would get the same result for separate cores as rattus said:

The voltages could come from separate separate transformers, separate generators, or a black box for that matter.

 

[JohnJ0906]

This question is bound to generate a ton of responses. :grin:

You weren't kidding, were you? :wink:

 

[jim dungar]

You would get the same result for separate cores as rattus said:

Every major manufacturer's wiring diagram for a dry type transformer with a 120/240 output has the same wiring connections. One winding is connected between X1 and X2 and the other winding is X3 and X4

If you only want 120/240 3-wire you connect the transformer as L1->X1, N->X2X3, and L2->X4.

One view is that the voltage across coil X1X2 is (or appears to be) 180? different than the voltage across coil X3X4. And that the real world transformer construction is not important.

My personal view is that the connections of the transformer is important and dictates the "direction" of the voltages across the coils forcing X1X2 and X3X4 to be the same.

How is my view incorrect if it can easily explain how the transformer can be reconnected per the manufacturers wiring diagrams to be a single 120V 2-wire output with the terminals L1->X1X3 and L2->X2X4 or as a buck-boost unit?



Oops, I am allowed to express a personal view in this forum?

 

[mivey]

How is my view incorrect if it can easily explain how the transformer can be reconnected per the manufacturers wiring diagrams to be a single 120V 2-wire output with the terminals L1->X1X3 and L2->X2X4 or as a buck-boost unit?

I don't say it is incorrect, only that it is not the only view. Can't you get the same results you just described using two separate transformers with the primaries wired in series?

Oops, I am allowed to express a personal view in this forum?

Just call it a personal TRVTH:grin:

 

[jim dungar]

I don't say it is incorrect, only that it is not the only view. Can't you get the same results you just described using two separate transformers with the primaries wired in series?Just call it a personal TRVTH:grin:

But a 120/240 center tapped transformer is NOT two separate transformers.

And please, point out when I have said mine is the only view. I have stated the problems I see using the 180? difference method.

I have given many reasons why my view is applicable to many different electrical system calculations.What problems are there with using my method except that the oscilloscope tracings referenced to neutral are more difficult to explain?

 

[hardworkingstiff]

What problems are there with using my method except that the oscilloscope tracings referenced to neutral are more difficult to explain?

Isn't that enough?

 

[rattus]

Every major manufacturer's wiring diagram for a dry type transformer with a 120/240 output has the same wiring connections. One winding is connected between X1 and X2 and the other winding is X3 and X4

If you only want 120/240 3-wire you connect the transformer as L1->X1, N->X2X3, and L2->X4.

One view is that the voltage across coil X1X2 is (or appears to be) 180? different than the voltage across coil X3X4. And that the real world transformer construction is not important.

Jim that depends on one's choice of reference. The transformer construction is important, but I see no connection with one's choice of reference.

My personal view is that the connections of the transformer is important and dictates the "direction" of the voltages across the coils forcing X1X2 and X3X4 to be the same.

Of course, but that is a given and that fact is reflected by the position of the polarity dots. Again it does not affect one's choice of reference. That is a matter of personal preference. Bet none of you knew that reference rhymes with preference.

How is my view incorrect if it can easily explain how the transformer can be reconnected per the manufacturers wiring diagrams to be a single 120V 2-wire output with the terminals L1->X1X3 and L2->X2X4 or as a buck-boost unit?

Never, ever, and never would say it is incorrect, but I will say that L1 corresponds to terminal X1, and L2 corresponds to the terminal X4, not to the entire coil.

Now since Jim won't say yay or nay, I will echo Engy's statement that correct results are obtained in either case. That alone should make it permissible.

 

[mivey]

And please, point out when I have said mine is the only view.

Maybe I misunderstood you, but I thought that is what the following quote implies:

Just because you can doesn't mean you should.

 

[coulter]

....What problems are there with using my method except that the oscilloscope tracings referenced to neutral are more difficult to explain?

Isn't that enough?

Single phase analysis is near trivial. All of us can handle either model.

However, there are other problems associated with "the arrows point away from each other" model.

For the 120/240 model, some of us like the current to go out the top and in the bottom. Unbalanced current is handled by:
1. that which goes out the top and does not go in the bottom, goes in the neutral.
2. That which goes in the bottom and does not come out the top, comes out of the neutral.
The arrows point away model doesn't do this well

But , the most challenging I see is the reversal of current through the point marked "N". That is non-intuitive to any methods of circuit analysis. Carl likes the current in a loop to not change direction. Mivey is good enough at the math to keep the signs straight.

This model also does not extend to fit with transformer connections, or magnetic fields. Rattus says this (the physics) really doesn't matter and we really aren't doing an analysis. Jim wants the model to be consistent with the rest of the world. Carl thinks the physics and the analysis is the only point - what else matters.

Single phase 208 open wye is different from 120/240. The model is different. 208 open wye rotates, 120/240 does not - which does not matter to some, does to others. The "others" are seeking consistency

So, no, using a particular model just because it explains an o-scope tracing caused by ignoring the polarity of the input connections - is not enough of a reason.

carl

 

[jim dungar]

Maybe I misunderstood you, but I thought that is what the following quote implies:

Just because you can (this means it is possible to do) doesn't mean you should (there are issues to be dealt with if you do it).

In my opinion these issues include:

1) If you make a blanket statement that L1 and L2 are 180? apart you are misrepresenting fact. You are forgetting to include the important clarifier that this occurs only when referenced to a neutral position.

2) If you say the voltages are out of phase, how do you explain that they combine to be a larger number, but that out of phase resistive currents combine to be a smaller number?

3) It becomes difficult to explain how connecting the exact same transformer coils into 2-wire and auto transformer circuits is possible if 2 of the voltages are out of phase with each other and therefore the primary.

4) Focusing Sly on the neutral as your reference point totally ignores the ANSI standard for transformer coil connections. If you use the transformer connections as your reference the voltage and current concepts remain the same regardless of the transformer configuration.

5) Ignoring the transformer construction ignores the "laws of physics" that allow the voltage sources to be created in the first place.

6) If you accept that these voltages are out of phase then you must also accept that 120/208 is also single phase, after all the math proof is the same)

The only issue with saying they are in phase and in series, that I can see, is explaining the oscilloscope tracings when referencing the neutral.

These issues have been the basis of my discussions not the exact math:
Value1 - (-Value1) = Value1 + Value1 = 2*Value1 regardless if done using trigonometric, algebraic, vectorial or any other mathematical process.

I have listed 5 issues against and 1 issue for saying that 120/240 voltages are 180? out of phase, which is why I think this method should not be used. I have asked for additional reasons why my method (two 120V sources in series) should not be used but people would rather complain about my personal views instead.

Please note: I have been saying "should not be" instead of "can not be".

 

[jim dungar]

Never, ever, and never would say it is incorrect...

Rattus I have previously supplied your quotes where you specifically called me incorrect.

... but I will say that L1 corresponds to terminal X1, and L2 corresponds to the terminal X4, not to the entire coil.

I am sorry if real life transformer connections are confusing. A 2-wire transformer is created by joining the coil terminals X1 and X3 and then connecting them to L1, likewise the other ends of the coils X2 and X4 are joined along with L2. Where did I say an "L" corresponded to an entire coil?

Now since Jim won't say yay or nay, I will echo Engy's statement that correct results are obtained in either case. That alone should make it permissible.

I don't remember saying it is not permissible, I have been warning against doing it.

 

[mivey]

...You are forgetting to include the important clarifier that this occurs only when referenced to a neutral position

I don't recall saying that we should ignore the reference point. Any method has to have a reference point. I think your method uses L1 as the reference point. I am no more ignoring the L1 reference than I am ignoring the N reference.

I have asked for additional reasons why my method (two 120V sources in series) should not be used but people would rather complain about my personal views instead.

I have not said your method should not be used. It actually makes more sense in some cases. Also, I am not meaning to complain about your personal views as I highly respect your opinion and appreciate your input. I think your expertise brings value to the table outside of just facts alone.

Please note: I have been saying "should not be" instead of "can not be".

Duly noted.

Jim, if you don't mind, could you read the following five pages (I promise it only takes a few minutes). I think it pretty much describes how I see the situation. I think it shows the circuits both ways. I welcome your feedback.

http://www.allaboutcircuits.com/vol_2/chpt_2/3.html
http://www.allaboutcircuits.com/vol_2/chpt_2/7.html
http://www.allaboutcircuits.com/vol_2/chpt_2/8.html
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

 

[rattus]

Shall we try for a new record?

Shall we try for a new record?

Just because you can (this means it is possible to do) doesn't mean you should (there are issues to be dealt with if you do it).

In my opinion these issues include:

1) If you make a blanket statement that L1 and L2 are 180? apart you are misrepresenting fact. You are forgetting to include the important clarifier that this occurs only when referenced to a neutral position.

Jim, in a split-phase, 3-wire service, we are describing the AC voltages on L1 and L2, that is two wires; there is only one wire left, the neutral. Plus, we have emphasized many times that we are using the CT/neutral/ground as the reference.

2) If you say the voltages are out of phase, how do you explain that they combine to be a larger number, but that out of phase resistive currents combine to be a smaller number?

Jim, it is in the math. We are describing a potential difference, therefore we take the difference between V1 and V2 which results in the larger result.

For the neutral current, the load currents combine algebraically which results in a smaller, sometimes zero, current

3) It becomes difficult to explain how connecting the exact same transformer coils into 2-wire and auto transformer circuits is possible if 2 of the voltages are out of phase with each other and therefore the primary.

We are not discussing those circuits plus there is no neutral.

4) Focusing only on the neutral as your reference point totally ignores the ANSI standard for transformer coil connections. If you use the transformer connections as your reference the voltage and current concepts remain the same regardless of the transformer configuration.

I cannot see that ANSI dictates my choice of reference.

5) Ignoring the transformer construction ignores the "laws of physics" that allow the voltage sources to be created in the first place.

I see no connection here. If you do the math, the results are the same.

6) If you accept that these voltages are out of phase then you must also accept that 120/208 is also single phase, after all the math proof is the same)

I don't see that argument at all. No one is claiming two-phase.

The only issue with saying they are in phase and in series, that I can see, is explaining the oscilloscope tracings when referencing the neutral.

These issues have been the basis of my discussions not the exact math:
Value1 - (-Value1) = Value1 + Value1 = 2*Value1 regardless if done using trigonometric, algebraic, vectorial or any other mathematical process.

I have listed 5 issues against and 1 issue for saying that 120/240 voltages are 180? out of phase, which is why I think this method should not be used. I have asked for additional reasons why my method (two 120V sources in series) should not be used but people would rather complain about my personal views instead.

Please note: I have been saying "should not be" instead of "can not be".

[/quote]
I will list one reason that this method is general and is therefore permissible:

We use the symbols, V1 and V2, to denote the voltages on L1 and L2. We are NOT denoting the voltages on X2 or X3. The only option is to measure V1 and V2 relative to the third wire which is the CT/neutral/ground/X2X3.

That being the case, V1n and V2n are indeed 180 degrees out of phase. It is not a matter of appearing to be out of phase; they ARE out of phase. No tricks, no magic, they are out of phase. If we observe these voltages as they are described by their subscripts, they appear out of phase--and they are, really. I mean REALLY.

Personal preferences are not the issue here. People will continue to do things their way, and that is just fine.

 

[rattus]

Rotation?

Rotation?

Single phase 208 open wye is different from 120/240. The model is different. 208 open wye rotates, 120/240 does not - which does not matter to some, does to others. The "others" are seeking consistency

carl

Carl, explain to me what rotates in an open wye. I can't see this at all.

 

[mivey]

Sez who?

Sez who?

...208 open wye rotates, 120/240 does not...

Sez who? I contend that I can define a reference point for the 240 that will show that V1 and V2 rotate. I can also define a reference point that makes the V1 and V2 of the 208 appear to not rotate.

The crux of the matter is the reference point. With the 240 delta, we have picked a reference point that is the midpoint of a line drawn between V1 and V2. With the 208 wye, I could also draw a line between V1 and V2 and use the midpoint as a reference and claim they do not rotate.

 

[jim dungar]

Jim, if you don't mind, could you read the following five pages (I promise it only takes a few minutes). I think it pretty much describes how I see the situation. I think it shows the circuits both ways. I welcome your feedback.

http://www.allaboutcircuits.com/vol_2/chpt_2/3.html
http://www.allaboutcircuits.com/vol_2/chpt_2/7.html
http://www.allaboutcircuits.com/vol_2/chpt_2/8.html
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

Excellent references, but this just is a personal opinion.

The author constantly points out why polarity dots are important. in Chapter 2/10.1

Here is where the (+) and (-) polarity markings really become important. This notation is often used to reference the phasings of multiple AC voltage sources, so it is clear whether they are aiding (“boosting”) each other or opposing (“bucking”) each other. If not for these polarity markings, phase relations between multiple AC sources might be very confusing.

But remember rattus continually says the real world connections are not important. And Rattus changes his reference point but not his polarity dots.

I believe that rattus prefers to use an unconventional method as described in Chapter 2/7.

 

[jim dungar]

We are describing a potential difference, therefore we take the difference between V1 and V2 which results in the larger result.

For the neutral current, the load currents combine algebraically which results in a smaller, sometimes zero, current

So, are you using different math for currents than you do for voltages?

I cannot see that ANSI dictates my choice of reference.

It doesn't. It dictates how our industry uses polarity dots to indicates the "direction" of voltages across transformer windings. You have repeatedly stated that the dots are important but the direction you chose is not. A center-tapped transformer winding forces the dots to be in a series and additive, however (2) individual transformers may be connected with different polarities.

I have stated that I see 5 negative issues with your method, in particular the ignoring real world connections. And I have stated that your math proves that 120/208 is also single phase.

I have asked for anyone to present negative issues of my method (other than the oscilloscope tracings).

 

[rattus]

So, are you using different math for currents than you do for voltages?

No Jim, same math, different laws. In the wye, the convention is to reference the phase voltages to neutral. Then to compute L-L voltage we sum around a 3-voltage loop. One of those phasors points against the direction of summation, therefore we subtract. You know that.

Then to compute the neutral current, we sum the phase currents algebraically. You know that too.

It doesn't. It dictates how our industry uses polarity dots to indicates the "direction" of voltages across transformer windings. You have repeatedly stated that the dots are important but the direction you chose is not. A center-tapped transformer winding forces the dots to be in a series and additive, however (2) individual transformers may be connected with different polarities.

Jim, we must assume that the transformer is wired correctly. Then we no longer need to consider the dots. If I choose the CT as a reference and I assign a phase angle of zero to V1n, I know the phase angle of V2n is 180. That is true however you draw your sources. You know that.

I have stated that I see 5 negative issues with your method, in particular the ignoring real world connections. And I have stated that your math proves that 120/208 is also single phase.

Jim, please expand on that last statement.

I have asked for anyone to present negative issues of my method (other than the oscilloscope tracings).

Jim, I for one do not care how you do your work. If it works for you it is fine with me--and I assume everyone else as well.

 

[jim dungar]

No Jim, same math, different laws. In the wye, the convention is to reference the phase voltages to neutral.

When did this discussion change to 3-phase?

Are you saying that voltage and current "laws" are different between single phase and three phase?

Which physical or mathematical laws changed? I thought vectors only cared about magnitude and angular displacement.

I set up my source directions based on physical connections (the dots).
In a single phase circuit I connect my vectors head to tail.
In a three phase wye I connect my vectors tail to tail.
In a three phase delta I connect my vectors head to tail.
I use the same vector formulas whether I am dealing with currents or voltages.

 

[rattus]

When did this discussion change to 3-phase?

In the last hour or so.

Are you saying that voltage and current "laws" are different between single phase and three phase?

Not at all.

Which physical or mathematical laws changed? I thought vectors only cared about magnitude and angular displacement.

We change from KVL to KCL.

I set up my source directions based on physical connections (the dots).
In a single phase circuit I connect my vectors head to tail.
In a three phase wye I connect my vectors tail to tail.
In a three phase delta I connect my vectors head to tail.
I use the same vector formulas whether I am dealing with currents or voltages.

Yes, but to compute L-L voltages in a wye, you must subract one of your tail to tail phasors. That fact is no doubt built into your formulas. You violate convention if you flip one of the voltage phasors. If you added them the magnitude and phase angle would be off. This is in response to your earlier question.