Just to clarify.

[mivey]

I don't like to use "polarity" here because it might be confused with the polarity marks on the transformer which do not change.
...
Simply put, I have used V2n instead of Vn2. These voltages exhibit a 180 degree phase difference, and this is perfectly valid, and it is conventional to use a common reference point where possible.

I thought about looking back to see how polarity had been used in the past but I just don't feel like it. But, in all honesty, it was probably not the best term to use at this point. Granted, I used the term differently than the way it is used when you are referring to transformer polarity. But I think the point that I was trying to make still stands, good or bad terminology. Try to understand what I was saying, not what I said:grin:

 

[mivey]

I thought about looking back to see how polarity had been used in the past but I just don't feel like it. But, in all honesty, it was probably not the best term to use at this point. Granted, I used the term differently than the way it is used when you are referring to transformer polarity. But I think the point that I was trying to make still stands, good or bad terminology. Try to understand what I was saying, not what I said:grin:

For the record, what should I have used "changed the reference direction" or something like that?

 

[rattus]

I thought about looking back to see how polarity had been used in the past but I just don't feel like it. But, in all honesty, it was probably not the best term to use at this point. Granted, I used the term differently than the way it is used when you are referring to transformer polarity. But I think the point that I was trying to make still stands, good or bad terminology. Try to understand what I was saying, not what I said:grin:

"Sense" is used to denote the direction of alternating current. Perhaps we could apply it to voltage as well??

Natheless, I understand your message.

 

[mivey]

quogueelectric,
Could you please clarify what you are saying? If you are weighing in on the discussion, I'm having a little difficulty following where you are coming from. The posts are too short and do not have enough info, at least for me.

You posted this:

I think we should start a mandatory drug testing program.

immediately after Rick posted this:

If Rattus agrees with this summary point...I would be a happy camper! :grin:

Rattus, Do you agree with this?

Are you making some reference to drugs and camping? If so, what is the relevance to this discussion?

Next you posted this:

I am going to play lotto today. The earth is in perfect harmony.

immediately after Rick posted this:

Sorry Mivey, It was a good effort......the minus sign is missing from Rattus' description?

Are you saying this is related to gambling, like someone was taking chances in part of the discussion, or that there is some kind of harmony/dis-harmony or what?

I'm sorry, but I'm just not following you.

 

[jim dungar]

My original post was started because I wanted Rattus to clarify how his choice of voltage directions affected the source currents in a 2-wire resistive circuit. His original position appeared to be that current was not important.

In his example Teaser.doc, Rattus specifically says do not change the polarity dots of the sources. I believe, he as often stated the importance of these dots.

To me as a power engineer the polarity dots of a transformer are very important as they tell me about the relationship of the primary and secondary windings. One important thing they tell me is that as current flows into a primary dot the corresponding secondary current flows out of the secondary dot. Without these dots it would be very difficult to connect transformer secondaries in the proper additive or subtractive voltage connections and we would not know if a CT was looking at a current that was "coming or going".

However, in almost every single circuit analysis Rattus seems to ignore the direction of the current through the transformer as defined by its physical construction (the dots) and instead changes it to match his choice of voltage reference.

I hope that after hundreds of posting over several topics, people recognize the results, both advantages and disadvantages, of choosing any particular reference point (or in my case a method). I don't know of a situation in power systems where making a decision about voltage can be done while ignoring its affect on current.

 

[mivey]

...Rattus specifically says do not change the polarity dots of the sources...

To me as a power engineer the polarity dots of a transformer are very important as they tell me about the relationship of the primary and secondary windings....

However, in almost every single circuit analysis Rattus seems to ignore the direction of the current ...

I don't know of a situation in power systems where making a decision about voltage can be done while ignoring its affect on current.

I think I follow you now. You are saying it is counter-intuitive to have the polarity dots as shown in "teaser", but then pick the intial loop current out of one voltage that is different from the convention you used with the other voltage.

Rattus says the math works. You agree the math works, but it is just counter-intuitive to set it up that way.

[edit: I agree that it is counter-intuitive]

 

[rattus]

My original post was started because I wanted Rattus to clarify how his choice of voltage directions affected the source currents in a 2-wire resistive circuit. His original position appeared to be that current was not important.

In his example Teaser.doc, Rattus specifically says do not change the polarity dots of the sources. I believe, he as often stated the importance of these dots.

To me as a power engineer the polarity dots of a transformer are very important as they tell me about the relationship of the primary and secondary windings. One important thing they tell me is that as current flows into a primary dot the corresponding secondary current flows out of the secondary dot. Without these dots it would be very difficult to connect transformer secondaries in the proper additive or subtractive voltage connections and we would not know if a CT was looking at a current that was "coming or going".

However, in almost every single circuit analysis Rattus seems to ignore the direction of the current through the transformer as defined by its physical construction (the dots) and instead changes it to match his choice of voltage reference.

I hope that after hundreds of posting over several topics, people recognize the results, both advantages and disadvantages, of choosing any particular reference point (or in my case a method). I don't know of a situation in power systems where making a decision about voltage can be done while ignoring its affect on current.

Jim, Certainly one must consider the senses of the various currents to be in accord with the phases of the sources, but in more complex situations, we may choose the direction incorrectly. Then a negative sign appears in the solution.

Now consider the simple circuit in teaser.doc. I2 can be defined as CW or CCW.

If the current arrow is drawn CCW, then

I2 = 120/R2 @ 180 (in phase with the source)

If the current arrow is drawn CW, then

I2 = -120/R2 @ 180 (out of phase with the source)

Now, you may prefer one choice over the other, but both choices are valid.

The same argument applies to I3 and I4.

 

[mivey]

found it

found it

...Then a negative sign appears in the solution...

There you go Rick. We found it!:grin:

 

[mivey]

...but in more complex situations, we may choose the direction incorrectly...

rattus, Jim's frame of reference appears to be the power engineering/protective relaying arena. We can draw any number of complex circuits, but can you provide a real world example of a complex situation like you were referring to? It might help establish a different frame of reference.

 

[rattus]

rattus, Jim's frame of reference appears to be the power engineering/protective relaying arena. We can draw any number of complex circuits, but can you provide a real world example of a complex situation like you were referring to? It might help establish a different frame of reference.

Maybe I could dig one out of my ancient texts, but I fear that would just set us off on another tangent.

I hope though that I have made the point that the choice of current direction is arbitrary.

 

[mivey]

...the choice of current direction is arbitrary.

...when drawing the loop currents used to determine the actual circuit currents.

At one time, I thought you were talking about the actual circuit currents and I was trying to come up with a concept to get on board with that idea without violating mother nature.

 

[Rick Christopherson]

...when drawing the loop currents used to determine the actual circuit currents.

At one time, I thought you were talking about the actual circuit currents and I was trying to come up with a concept to get on board with that idea without violating mother nature.

I believe your perception is correct. When this discussion started, I believe he was referring to the actual voltage/current with an incorrect sense, which is why I engaged in this discussion to begin with. I wouldn't have cared what direction someone wanted to put on a label.

By the way, that's where the missing minus sign comes from that I have kept referring to, and that is why Rattus was so reluctant to acknowledge it.

 

[coulter]

Quote:
Originally Posted by mivey
rattus, Jim's frame of reference appears to be the power engineering/protective relaying arena. We can draw any number of complex circuits, but can you provide a real world example of a complex situation like you were referring to? It might help establish a different frame of reference.

Maybe I could dig one out of my ancient texts, but I fear that would just set us off on another tangent.

I hope though that I have made the point that the choice of current direction is arbitrary.

rattus -
I'm okay with the math being able to be worked out regardless of the chosen direction for a KCL loop current. I'm still not clear on why one would want to pick out a current direct different than that matching with the voltage sources. I'm also not clear on this phase shift at the currents pass through the N point. I Perhaps it would be more accurate to say I don't understand why anyone would ever want set up for a solution to do it

I'm with mivey, I think a real world problem illustrating why one would pick this orientation will help. But don't use a text problem. Pick something you have worked on in the last year or two (or ten) - or even a current problem not yet solved.

PS I am aware that real world problems are not always determinant - so that's okay if your example isn't.

carl

carl

 

[rattus]

Quote:
Originally Posted by mivey
rattus, Jim's frame of reference appears to be the power engineering/protective relaying arena. We can draw any number of complex circuits, but can you provide a real world example of a complex situation like you were referring to? It might help establish a different frame of reference.




rattus -
I'm okay with the math being able to be worked out regardless of the chosen direction for a KCL loop current. I'm still not clear on why one would want to pick out a current direct different than that matching with the voltage sources. I'm also not clear on this phase shift at the currents pass through the N point. I Perhaps it would be more accurate to say I don't understand why anyone would ever want set up for a solution to do it

I'm with mivey, I think a real world problem illustrating why one would pick this orientation will help. But don't use a text problem. Pick something you have worked on in the last year or two (or ten) - or even a current problem not yet solved.

PS I am aware that real world problems are not always determinant - so that's okay if your example isn't.


carl

Carl, look at I1 and I2 in the teaser circuit. They are both in phase with their sources--I1 @ 0, I2 @ 180. If we reversed the I2 arrow, I2 would be @ 0.

On the line to line loads, the currents are leaving one source and returning to another. One would expect a phase difference between the return current and the return source.

I never said the current changes phase as it passes N. The phase of each loop current cannot change.

One would set up a problem to yield the easiest solution. We have been going through this yow-yow because I insisted that V1n and V2n are really 180 degrees out of phase; they don't just appear to be.

I have found a diagram, but it is late, and old men need their sleep. Manana!

I have found a diagram which might answer

 

[mivey]

...I'm also not clear on this phase shift at the currents pass through the N point....

This was my doing. Picking the loop current in the "wrong" direction is one thing. The solution will just show the "wrong" current to have a sign opposite to the "correct" currents.

If you say the ACTUAL circuit currents are in the "wrong" direction (which is what I thought rattus was trying to say at one time) you must have some way that the currents can have a sign change, the same way the voltages had a sign change because the voltage and currents must stay in phase for a resistive load.

The voltage was easy, when you cross the reference point (the ground lead on the volt meter), the sign changes. With the current, you have to play a different game because you don't have a real ground lead to stick in a "reference point". The only way I could picture it was a "forced" shift as modeled by orienting the clamp-on meter in one direction above the neutral point, and reversing the clamp meter when below the neutral point. Reversing the clamp meter direction is the same as adding a negative sign. Now the current can have a "reference point" like the voltage, and all is well.

[edit: use neutral "point" instead of neutral]

 

[winnie]

When solving these problems, I tend to stick with the _same_ set of sense designations rather than switching for different problems.

I stick with the source neutral as the voltage reference, and use 'current flowing _out_ of each source terminal' as my current sense.

This view is more complex then necessary when solving for a single resistive load connected to a single phase center tapped transformer. But I find that it makes life easier when solving for (say) multiple loads connected to a wye source. And in my research (the stuff I get paid for) I work with 'high phase order' inverters and motors where the source phase angle is not 120 degrees and changes under computer control; sticking with a single generalizable approach works much better for me than adjusting for each problem.

Carl: In the single phase center tapped system, the same current is flowing through the entire system. There is _no_ change in the current flow as it passes through the neutral point. The only thing that changes is the _description_ of the current flow.

You want to describe your voltages and your currents using the same 'sense', so that when you multiply current times voltage you get a positive number for components that supply power, and a negative number for components that consume power. (Or the reverse; the selection of positive for supply is arbitrary, but you want to pick one or the other convention and then stick with it.)

If you elect to describe all of your voltages in a loop (clockwise or counterclockwise, take your pick), then the sense that you use to describe current around the loop remains constant.

If you elect to describe all of your voltages relative to the neutral point, then the sense that you use to describe current must also be oriented with the neutral point. If (as I do) you always describe current in terms of the flow _out_ from the neutral along the various branches of the circuit, then the sign or phase of the _description_ of the current flow will flip as you go through neutral. This change in description of the current matches the description of the voltage, and maintains the convention that (voltage * current) is power supplied by that component (transformer coil, inverter half bridge, battery, etc).

This is where the 'crossing the North Pole' analogy comes into play. Imagine that you are flying from Anchorage in Alaska to Kiev in the Ukraine, on a direct flight going right over the North Pole, as you cross the pole, your path does not change; the plane does not suddenly flip around. But the description of your direction of travel changes; one moment you were headed North, and the next instant you are headed South. If you describe the path of travel in terms of 'Anchorage to Kiev', then nothing changes; if you describe your path of travel in terms of North or South, there is a flip when you pass the reference point.

-Jon

 

[jim dungar]

I stick with the source neutral as the voltage reference, and use 'current flowing _out_ of each source terminal' as my current sense.

Jon,

What is your normal method for systems that do not have a neutral like an ungrounded delta?

In reality, you have chosen the source polarity dot as your reference. Standard wye connected transformers have their polarity dots away from the neutral point. This results in the current having a sense of flowing away from the neutral when in fact it is simply flowing out of the dot. This is part of my problem with Rattus's descriptions and diagrams even though he correctly draws a center tapped transformer.

 

[rattus]

Jon,

What is your normal method for systems that do not have a neutral like an ungrounded delta?

In reality, you have chosen the source polarity dot as your reference. Standard wye connected transformers have their polarity dots away from the neutral point. This results in the current having a sense of flowing away from the neutral when in fact it is simply flowing out of the dot. This is part of my problem with Rattus's descriptions and diagrams even though he correctly draws a center tapped transformer.

Jim, I too would draw the polarity markings away from the neutral in a wye. If you place the dots at the neutral, the phase of each secondary voltage would be reversed, relative to the primary, and the sequence would be reversed as well.

If you are ready to talk 3-phase, let's start a new thread.

 

[winnie]

Jon,
What is your normal method for systems that do not have a neutral like an ungrounded delta?

I don't really have a normal method for that situation, and would hope to come up with something reasonable.

In reality, you have chosen the source polarity dot as your reference. Standard wye connected transformers have their polarity dots away from the neutral point. This results in the current having a sense of flowing away from the neutral when in fact it is simply flowing out of the dot. This is part of my problem with Rattus's descriptions and diagrams even though he correctly draws a center tapped transformer.

I guess that I'd just prefer to think of a center tapped secondary in a fashion more similar to a wye secondary, even though the polarity dots will be arrayed differently in the two.

-Jon

 

[coulter]

Carl, look at I1 and I2 in the teaser circuit. They are both in phase with their sources--I1 @ 0, I2 @ 180. If we reversed the I2 arrow, I2 would be @ 0....

rattus -
This is not new information. Already been stated. I understood what you said the first time. As I already said one can make the math work.

..On the line to line loads, the currents are leaving one source and returning to another. One would expect a phase difference between the return current and the return source. ...

Still no news. As I have agreed, with your solution setup, yes this happens. As I said, with my solution setup, it does not.

...I never said the current changes phase as it passes N. The phase of each loop current cannot change. ...

Yes, mivey already explained that it was he that had alluded to that.

...One would set up a problem to yield the easiest solution. ...

I agree with this.

... We have been going through this yow-yow because I insisted that V1n and V2n are really 180 degrees out of phase; they don't just appear to be. ...

I agree that, as you have chosen to set up your solution, and as you chose to measure V1n and V2n, you see V1n and V2n as 180 out of phase.

We have beat the teaser way past death. I don't see that we have any more to get from it - unless you have an insight that has not already been stated.

... I have found a diagram which might answer

A textbook diagram may well be enlightening, I certainly would encourage you to post it.

However, I agree with mivey, that a real world scenerio illustrating why one would choose the setup you propose, would be far more enlightening. As I suggested, pick one that you have worked or are working. Again it need not have a determinate solution. Most of the stuff I work does not. The setup and methodology is the important part - I think this is what leads to understanding the installation.

carl