single vs. 3 phase

[rattus]

Not really:

Not really:

This sounds like you agree that the voltages in the two halves of a center tapped transformer coil must be in phase with each other.

Jim, can not must.

It depends on how you look at it, that is, your choice of reference. And, I am unaware of any rule that says you must do it one way or another. In a word though, it is quite proper and logical to use the CT as a reference, then the two voltages must be out of phase since they are inverses.

 

[rattus]

I don't think so:

I don't think so:

They are not in phase they are 180* out of phase to each other.

One half is a lefthanded turn winding and the other is a righthanded turn winding when you look at from the tap point and you have to since that is the common reference point for the voltage measurements, it is the same potential for both circuits. If you would have two windings one top on the other going in the same direction, then you would have no voltage difference between the coil ends on the same extremes and if you connect one end and ground it, you would have 115V to each opposite end and NO voltage difference between the two open ends instead of 230V in the case of a single winding, center tap.

Laszlo, not sure what you are saying, but I think it is wrong. Give us a diagram please.

 

[jim dungar]

They are not in phase they are 180* out of phase to each other.

One half is a lefthanded turn winding and the other is a righthanded turn winding when you look at from the tap point and you have to since that is the common reference point for the voltage measurements, it is the same potential for both circuits. If you would have two windings one top on the other going in the same direction, then you would have no voltage difference betwen the coil ends on the same extreemes and if you connect one end and ground it, you would have 115V to each opposite end and NO voltage difference between the two open ends insted of 230V in the case of a single winding, center tap.

Lazlo,

Choosing an arbitrary point for reference cannot change the physics of the transformer. A center tapped winding is a single coil how can you have different voltages in a single piece of wire cut by a single flux? The voltages across two individual windings on a common core must also be in phase if the windings are intended to be wired in parallel as are most 2 winding transformers. Two completely separate transformers can be wired any way you want.

And if you have not read my previous discussions on this topic, I do understand that the math can be worked using the neutral point as your reference, I just see little reason to so. My preference is to define my voltage directions based on the direction of the emf created by the transformer.

 

[weressl]

Lazlo,

Choosing an arbitrary point for reference cannot change the physics of the transformer. A center tapped winding is a single coil how can you have different voltages in a single piece of wire cut by a single flux? The voltages across two individual windings on a common core must also be in phase if the windings are intended to be wired in parallel as are most 2 winding transformers. Two completely separate transformers can be wired any way you want.

And if you have not read my previous discussions on this topic, I do understand that the math can be worked using the neutral point as your reference, I just see little reason to so. My preference is to define my voltage directions based on the direction of the emf created by the transformer.

If the windings on the same core are wound in the SAME direction. The centertap makes the windings go to the OPPOSITE direction, that is why they are out of phase 180*. If you are to CUT the same winding at the point of the tap AND connect the left sides of each section together, then you would have 2 single phase circuits IN phase with each other and no voltage difference between the two open ends.

 

[winnie]

Laszlo,

The particular point that you are discussing with Jim is one of the core misunderstandings of this entire thread. Jim is _not_ arguing with the physics that you are describing; he is arguing with the most sensible presentation of that physics. I think that you are going to be forced to agree to disagree.

What you are calling the direction of the windings depends upon the selection of the reference point. Jim has argued strongly that there is no good reason to select the center tap as the reference point, and that the reference point should be selected to match the transformer coil polarity. For a single phase coil, this puts the reference point at one of the end terminals, and the winding direction (as you have described it) remains constant throughout.

However he has also agreed that the math is consistent and correct if you select the center tap as the reference point.

I belive that we have all agreed on the basic physics: call the three terminals of the single phase coil A, N, B, with N being the center tap. When A is positive relative to N, then N will be positive with respect to B, and that when Van crosses zero, Vbn will also cross zero. Clearly Van is exactly in phase with Vnb.

Much of the rest of the debate is about personal preference, about why it might make sense to select the center tap rather than an end terminal as the reference.

As stated previously, I prefer to use the center tap as the reference, and use Van and Vbn, which are 180 degrees out of phase. I do not consider this a 2 phase system, any more than I consider a common motor winding a 6 phase winding.

-Jon

 

[charlie b]

I am afraid of two things about this thread. One is that I am afraid to try to read it. :roll: The other is that I am afraid that if left unchecked, it might exceed in length the famous record setter, the ?Big Oops Thread,? of which I was the number two contributor. That one was closed out at 781 posts, so there is still some margin remaining.

I do have a suspicion, however, (a blind suspicion I should admit, given that I have not studied the discussion). It is that the essence of this thread can be characterized by the following ?argument?:
? Person One says, ?I tell you that the sky is blue.?
? Person Two replies, ?No, you are wrong. The grass is green.?

Am I close? :smile:

 

[weressl]

Laszlo,

The particular point that you are discussing with Jim is one of the core misunderstandings of this entire thread. Jim is _not_ arguing with the physics that you are describing; he is arguing with the most sensible presentation of that physics. I think that you are going to be forced to agree to disagree.

What you are calling the direction of the windings depends upon the selection of the reference point. Jim has argued strongly that there is no good reason to select the center tap as the reference point, and that the reference point should be selected to match the transformer coil polarity. For a single phase coil, this puts the reference point at one of the end terminals, and the winding direction (as you have described it) remains constant throughout.

However he has also agreed that the math is consistent and correct if you select the center tap as the reference point.

I belive that we have all agreed on the basic physics: call the three terminals of the single phase coil A, N, B, with N being the center tap. When A is positive relative to N, then N will be positive with respect to B, and that when Van crosses zero, Vbn will also cross zero. Clearly Van is exactly in phase with Vnb.

Much of the rest of the debate is about personal preference, about why it might make sense to select the center tap rather than an end terminal as the reference.

As stated previously, I prefer to use the center tap as the reference, and use Van and Vbn, which are 180 degrees out of phase. I do not consider this a 2 phase system, any more than I consider a common motor winding a 6 phase winding.

-Jon

There is absolutely good reason to take the ceter tap as the reference point.

Actually there are two reasons.

1./ That is the common point between the two part of the system,
2./ That is the grounded point of the circuit, it is grounded so it becomes a COMMON REFERENCE point.

 

[rattus]

Well, yes.

Well, yes.

If the windings on the same core are wound in the SAME direction. The centertap makes the windings go to the OPPOSITE direction, that is why they are out of phase 180*. If you are to CUT the same winding at the point of the tap AND connect the left sides of each section together, then you would have 2 single phase circuits IN phase with each other and no voltage difference between the two open ends.

Sure Laszlo, but the big yow yow seems to be about the choice of reference which is purely arbitrary. Some, including me, prefer the CT/N/G. One reason I is that all source currents can be defined as in phase more or less with their source voltages.

 

[mivey]

I am afraid of two things about this thread. One is that I am afraid to try to read it. :roll: The other is that I am afraid that if left unchecked, it might exceed in length the famous record setter, the “Big Oops Thread,” of which I was the number two contributor. That one was closed out at 781 posts, so there is still some margin remaining.

I do have a suspicion, however, (a blind suspicion I should admit, given that I have not studied the discussion). It is that the essence of this thread can be characterized by the following “argument”:
? Person One says, “I tell you that the sky is blue.”
? Person Two replies, “No, you are wrong. The grass is green.”

Am I close? :smile:

While the basic argument is probably a simple preference, there has been a lot of good discussion on theory, standards, etc that has been posted as to why each one has their preference. I have enjoyed the technical refresher and have learned some things as well. I think we have also found that some of our "facts", when polished up, were actually gold-plated opinions. These things might be hard for an individual to realize without some discourse.

[edit: besides, we are a long way from iwire's prediction of 2,000 responses - see post #2]

 

[JohnJ0906]

I do have a suspicion, however, (a blind suspicion I should admit, given that I have not studied the discussion). It is that the essence of this thread can be characterized by the following ?argument?:
? Person One says, ?I tell you that the sky is blue.?
? Person Two replies, ?No, you are wrong. The grass is green.?

Am I close? :smile:

Who knows? I haven't understood anything since page 1....

 

[mivey]

180 degree 2-phase impossible?

180 degree 2-phase impossible?

Rattus, given the 360/n rule and taking the case where n=2, it would appear you would disagree with this description and would say that a 2-phase system cannot exist.

This 2-phase notion was the OP's original question. It would appear that we call it single-phase by convention, but how would the center-tap reference that we call single-phase be different than a "real" 2-phase system?

 

[rattus]

Rattus, given the 360/n rule and taking the case where n=2, it would appear you would disagree with this description and would say that a 2-phase system cannot exist.

Maybe that is why the real 2-phase system has all but disappeared? I have seen a 4-phase system described in which 2 of the phases were inverses. Doesn't count though.

This 2-phase notion was the OP's original question. It would appear that we call it single-phase by convention, but how would the center-tap reference that we call single-phase be different than a "real" 2-phase system?

It would differ by 90 degrees.

 

[cadpoint]

why is 2 phase - 240 volt - called single phase???

Put a scope on it and then get back to us...

 

[jim dungar]

I do have a suspicion, however, (a blind suspicion I should admit, given that I have not studied the discussion). It is that the essence of this thread can be characterized by the following ?argument?:
? Person One says, ?I tell you that the sky is blue.?
? Person Two replies, ?No, you are wrong. The grass is green.?

Am I close? :smile:

Charlie,

Jon has the stated the argument pretty close with:

What you are calling the direction of the windings depends upon the selection of the reference point. Jim has argued strongly that there is no good reason to select the center tap as the reference point, and that the reference point should be selected to match the transformer coil polarity.

But my point has been I can only think of two reasons (oscilloscope tracings and manufacturing convenience)to chose the neutral as the basis of voltage direction. But I have listed more than 2 reasons why I think it shouldn't.

 

[mivey]

Maybe that is why the real 2-phase system has all but disappeared? I have seen a 4-phase system described in which 2 of the phases were inverses. Doesn't count though.

It would differ by 90 degrees.

If talking about 2 phase systems, how do you get 90 degrees with the 360/n rule?

Are you saying to have an n-phase system, the phases cannot be derived from a lower order system because you must have n independent sources?

 

[Rick Christopherson]

I stopped following this thread several weeks ago, but something keeps gnawing at me, so I must ask......

So why is it that these long complicated threads always seem to boil down to Rattus (and his troupe) versus the rest of the electrical engineering world? Is it that Rattus and his rodents have some sort of great insight into the engineering world that the rest of us do not posses? No, of course not.

It is because Rattus and his rodents take the stance that their way is the only way, and all other applications are therefore wrong.

The majority of these discussions would never even exist if the rat-trap was not trying to state that one method is the only method.

 

[rattus]

If talking about 2 phase systems, how do you get 90 degrees with the 360/n rule?

I don't think the old timers knew about the 360/n rule, so they came up with the 90 degree system which was better suited for driving motors..

Are you saying to have an n-phase system, the phases cannot be derived from a lower order system because you must have n independent sources?

Not exactly independent because they must be linked in such a way that separation is maintained, and they cannot be inverses.

 

[mivey]

...and they cannot be inverses.

What is the difference? If you can use a transformer to manipulate the sources into deriving a higher order of phases (say 3 to 6), why can't you do the same for 1 to 2 or is it just a coincidence that they are inverses?

Don't get all crazy with the "you can't make 3-phase with 1-phase using a transformer" explanation, as I know you have to have a non-linear reference to get 3+ (non-linear) phases.

I just wonder why it is valid to use transformer manipulations to derive higher-order phase systems when you have 3-phases, but it becomes invalid when you have only 1-phase. It would seem to me that in both cases you are shifting reference points around to create something different.

 

[mivey]

I stopped following this thread several weeks ago, but something keeps gnawing at me, so I must ask......

So why is it that these long complicated threads always seem to boil down to Rattus (and his troupe) versus the rest of the electrical engineering world? Is it that Rattus and his rodents have some sort of great insight into the engineering world that the rest of us do not posses? No, of course not.

It is because Rattus and his rodents take the stance that their way is the only way, and all other applications are therefore wrong.

The majority of these discussions would never even exist if the rat-trap was not trying to state that one method is the only method.

Do you know any other songs?

 

[winnie]

From my point of view, the 360/n rule doesn't work all that well.

In an electric motor, what matters is the direction of current flow in the slots. I can always get a 180 degree phase difference by simply running the wire in the opposite direction; if you look at an ordinary 3 phase, 2 pole motor, you will find _6_ phase bands; A, C', B, A', C, B'. The difference between 'primed' and the 'not primed' phase bands is the direction of the wire in that part of the winding. So with a conventional _three_ phase supply, I can define _6_ different stator phase angles.

If you were to use the 360/n rule for 6 phases, then you would have 6 phases spaced 60 degrees apart. Call these phases A, B, C, D, E, F. If you wound such a '6 phase' motor, then you would find that the phase D belt was redundant with the A' belt. In other words, a 360/n 6 phase motor would be no different than a normally wired 3 phase motor.

IMHO 'legs' 180 degrees apart should be considered the same phase.

In the 18 phase motors that we've built, we used the phase angles 0,10,40,50,80,90,120,130,160,170,200,210,240,250,280,290,320,330.

On the other hand, I recall that EPRI did some research on high phase order power transmission; and they did use 360/n phase displacement for n=12 and n=18. This had something to do with pushing more power through the same right of way without raising voltage or current.

-Jon