Single Phase Theroy

[LarryFine]

Unfortunately, there are some here that would say that you can't have two phases with a 180 degree separation and you would be forced to have only 3 phases. BTW, I'm not one of those.

I'm not, either. I'm one who says you can't have either two or three phases with a 180-degree separation, only one.

But they are forced to call it 3-phase to make their arguments work in the 120/240 or the 5-wire 90-degree cases.

Forced? No, I call it that willingly.

A voltage is defined by its reference point and there is no universally "correct" reference point.

Agreed, but we call our 1ph supply 240v center-tapped, not +/- 120v.

 

[mivey]

Let's be fair, now. If "your side" can claim (with the agreed mutual understanding that we're really talking semantics here) that two phases exist from one when using the neutral reference, "our side" can certainly claim the opposite can also be true.

In all fairness, I claim both the single-phase case and the two-phase case can be true. In fact, they are both true and happen to map to the same voltages in this case.

 

[mivey]

I guess I'm saying that there is no such thing as 180 degrees 2 phase.

I think that is where we won't be able to come together, not that we have to.

If I'm not mistaken, we all agree that two individual 1ph transformers (with a single source), a single unit with two individual secondaries, and a unit with one center-tapped secondary (when the first two are connected additively), are electrically identical.

Yes.
How are you on two individual 1ph transformers with separate sources that are displaced in time by 1/2 cycle or does your first answer say this can't exist?

To me, regardless of what you do with the secondary conductors, all of them can only be single phase setups. You cannot create poly-phase or multi-anything with any of them. Only the first (the two separate units) are capable of being used in any other way.

Do you now say that there can exist two independant primary sources that can supply 180 degree 2-phase?

 

[mivey]

Forced? No, I call it that willingly.

But that would not sound near as good in the context of my post. :grin:

 

[LarryFine]

How are you on two individual 1ph transformers with separate sources that are displaced in time by 1/2 cycle or does your first answer say this can't exist?

That's the same thing as flipping the polarity of one, which means the secondaries are no longer additive, nor directly comparable to a single, center-tapped secondary. It can't exist in my comparison.

Do you now say that there can exist two independant primary sources that can supply 180 degree 2-phase?

No, I was merely pointing out that only two independent transformers could be use to supply other than single-phase, such as 120/208v, or two-phase.

 

[LarryFine]

But that would not sound near as good in the context of my post. :grin:

True.

 

[goldberg]

same idea with a 3 phase transformer. the old single phase pots had the secondary windings in series and tapped a neutral from the center. with alternating current, as line1 is "comming" line 2 is "going". single phase.

 

[mivey]

as line1 is "comming" line 2 is "going".

Sez who? It is a matter of reference preference said the poet know-it.

 

[mivey]

same idea with a 3 phase transformer. the old single phase pots had the secondary windings in series and tapped a neutral from the center. with alternating current, as line1 is "comming" line 2 is "going". single phase.

Sez who? It is a matter of reference preference said the poet know-it.

The physics of the voltages present and the names we use are not always consistent. We have reserved the name "two-phase" to mean the 90? quarter-phase system. We call the 180? system single-phase.

The names we choose do not change the physical nature of the voltages. You can't always perfectly match the naming conventions we use to the physical system, so ultimately you have to say it is called what it is because we choose to call it that.

I contend that we should stick with the conventional names we are all used to or we will create confusion. But, for those that desire a more complete understanding, we should also recognize the physical nature of the systems involved. Considering most of what we deal with in the EC world, most people could care less about the physics of the system.

From a broader scope of things, if we ignore the physics as the ultimate reference and let the names be the ultimate reference, we will be trying to constrain the physical system to the description given by the naming conventions. The naming conventions we use are not consistent across different physical systems and therefore can't be the ultimate reference for the nature of the physical systems.

In other words, by convention we call the 180? system single-phase. And this name describes almost everything we will encounter. However, it is incorrect to say that there are not also two different phase voltages present. It just so happens that in this particular case, the single-phase configuration and the two-phase configuration have exactly the same EMFs.

Handbook for Electrical Engineers-1917
Strictly, the so-called single-phase system is a star-connected two-phase system, since the currents from the two terminals are in opposite directions at any instant, the current leaving by one and entering by the other. However, in practice the name two-phase system is used for a system supplied from a generator or other source of e.m.f. having two windings in which are developed two e.m.f.'s differing in phase by 90?; i.e., a two-phase system is in reality two distinct single-phase systems each with two terminals.

note: the star connection referenced is a common-point connection like a wye as opposed to the mesh connection or end-to-end connection like in a delta

 

[LarryFine]

note: the star connection referenced is a common-point connection like a wye as opposed to the mesh connection or end-to-end connection like in a delta

My opinion differs from that Handbook reference where it says "... the currents from the two terminals are in opposite directions at any given instant ..."

I liken the center-tapped source to two batteries in series. Even from the "center tap," nobody could claim that the currents flow in opposing directions.

There has to be a polarity difference to induce current, but the currents must be going "in the same direction" for the voltages to be additive.

The system works, so I agree with you, the discussion is one of semantics. "A rose by any other name ..."

 

[rattus]

My opinion differs from that Handbook reference where it says "... the currents from the two terminals are in opposite directions at any given instant ..."

I liken the center-tapped source to two batteries in series. Even from the "center tap," nobody could claim that the currents flow in opposing directions.

The system works, so I agree with you, the discussion is one of semantics. "A rose by any other name ..."

Larry, the key word is "from". If we define the load currents to be coming from or coming out of the terminals, then the statement is correct. Then if one current is positive, the other is negative. This is an acceptable practice in circuit analysis.

For example, you would define the phase currents of s wye as coming out (from) the free ends of the secondaries. Why not single phase CT? You can do this with batteries too, then the value of one of the currents will be negative. It's just algebra.

 

[LarryFine]

That's why I called it semantics. We're talking about the difference between adding a negative and subtracting a positive.

The only real difference to me is that I prefer to call it a polarity difference, not a 180-degree phase shift.

 

[rattus]

That's why I called it semantics. We're talking about the difference between adding a negative and subtracting a positive.

The only real difference to me is that I prefer to call it a polarity difference, not a 180-degree phase shift.

Then you admit that V1 and V2 relative to the CT exhibit a 180 degree phase shift even if you call it something else?

 

[mivey]

I liken the center-tapped source to two batteries in series. Even from the "center tap," nobody could claim that the currents flow in opposing directions.

The problem with the battery reference is that it kind of works but has some built-in references that makes it not work completely. A positive battery would have a tip on the positive side and a negative battery would have a tip on the negative side. Proper orientation would produce the same system output, regardless if the batteries were positive or negative batteries.

To make the analogy to AC, you would have to start flipping these batteries back and forth in the holder at regular intervals to get cycles like we have with AC.

You would also have to have smooth ends on the batteries because we have no tip with AC unless we go back to the moment of creation of the original generation-supplied wave (and time-shifting is not the phase-shift we are talking about).

Now take our two batteries (four possibilities: could be 2 positive batteries, 2 negative batteries, 1 positive battery + 1 negative battery, 1 negative battery + 1 positive battery) that are flipping back and forth at regular intervals. We were not there at the moment of creation so we just look at the voltage output waveform and pick a start point for our measurement.

The voltages that we get from our system are exactly the same voltages we would get with any of the four possible battery configurations. It would be silly to say that only one of the four source configuration possibilities is represented by the voltage output.

 

[LarryFine]

Then you admit that V1 and V2 relative to the CT exhibit a 180 degree phase shift even if you call it something else?

No, only that the opposing polarity resembles a 180-degree shift.

 

[LarryFine]

To make the analogy to AC, you would have to start flipping these batteries back and forth in the holder at regular intervals to get cycles like we have with AC.

I agree with that.

We were not there at the moment of creation so we just look at the voltage output waveform and pick a start point for our measurement.

We basically do that with AC.

It would be silly to say that only one of the four source configuration possibilities is represented by the voltage output.

For any one given moment, it's accurate.

 

[rattus]

No, only that the opposing polarity resembles a 180-degree shift.

You mean like a CZ and a diamond. Even the jewelers can't tell one from the other!

 

[rattus]

No, only that the opposing polarity resembles a 180-degree shift.

Translation: - 120V @ 0 resembles 120V @ 180

Now I must admit that one could generate a sinusoidal voltage if one had an infinite number of batteries and a rotary switch with an infinite number of positions, but wait, we could cut the number of batteries in half by realizing that half the values are duplicates, and we could halve that again with a single polarity reversing switch. Then the number of batteries required is only one fourth of infinity--approximately that is because the positive and negative peaks occur only once per cycle.

 

[Mayimbe]

Translation: - 120V @ 0 resembles 120V @ 180

Now I must admit that one could generate a sinusoidal voltage if one had an infinite number of batteries and a rotary switch with an infinite number of positions, but wait, we could cut the number of batteries in half by realizing that half the values are duplicates, and we could halve that again with a single polarity reversing switch. Then the number of batteries required is only one fourth of infinity--approximately that is because the positive and negative peaks occur only once per cycle.

1/4*infinity=small infinity :grin:

No offense intended.

 

[mivey]

For any one given moment, it's accurate.

Which is accurate? My statement that an exclusive identification is silly, or the idea that an exclusive representation is made by the voltage waveform?

If you are thinking the latter: Given a waveform that is already cycling steadily and that the batteries could be any of the four combinations mentioned before, please identify any moment along that waveform that would allow you to identify which of the four configurations is exclusively represented. Hint: it would be impossible.