single vs. 3 phase

[mivey]

3-wire 120/208 is 3/4 of a 4-wire 3-phase system not 2/3.

I try to use:
Voltage(s)
Phase = Line-Line voltages not single conductors = P
Wire = number of conductors required for the circuit = W

480V 1P2W
120/240V 1P3W
120/208V 1P3W
240/120V 3P4W
208Y/120V 3P4W

OK. But if I used that type notation, I would have to say that the old "two phase" 1P3W system was actually 3/5 of the old "four phase" 4P5W system. Still looks confusing.

If we take the phase definitions in the same way as used when describing systems, I guess we would have these:
480V 1P2W
120/240V 2P3W
120/208V "2/3"P3W
240/120V 3P4W
208Y/120V 3P4W

The books refer to the old "two-phase" as being one half (2/4) of a 4-phase system and this is why I used the 2/3 in my previous post.

[edit: and the old "two-phase" three wire would be "2/4"P3W]

 

[Rick Christopherson]

Mivey, you?re all over the road here. This has nothing to do with rotating fields or 3-phase. My comment was that Rattus stated that you must consider the two voltages as being 180 degrees out of phase, and that you cannot say that they are inverses.

If:
VAB = 240 sin(ωt)
Then,
VAn = 120 sin(ωt) and,
VnB = 120 sin(ωt)

VnB = -VBn = -120 sin(ωt)

Rattus says that you can?t do this. That you must have:
VnB = VBn @ 180 = 120 sin(ωt+180)

Lead placement is not mentioned at all. We are simply discussing two waveforms, and we don't care how they are measured.

The topic of discussion is about a 120/240 single phase system. When did it suddenly change into some hypothetical system? Only when the original discussion didn't suit your needs?

As I've said, I don't care if you want to call them 180 degrees out of phase. Show me a reference that states that they cannot be inverses. Because you obtained these opposing voltages by reversing the polarity of your scope leads, it is technically more accurate to refer to them as being inverses as opposed to being out of phase. Reversing the polarity of your leads did not result in a time shift from one signal to the other.

 

[mivey]

Mivey, you?re all over the road here.

I promise, it is non-prescription anti-allergy medicine.:smile:

My comment was that Rattus stated that you must consider the two voltages as being 180 degrees out of phase, and that you cannot say that they are inverses....VnB = -VBn = -120 sin(ωt)

Rattus says that you can?t do this. That you must have:

VnB = VBn @ 180 = 120 sin(ωt+180)
...

Then I would have to say he is wrong.

 

[jim dungar]

OK. But if I used that type notation, I would have to say that the old "two phase" 1P3W system was actually 3/5 of the old "four phase" 4P5W system. Still looks confusing.

If we take the phase definitions in the same way as used when describing systems, I guess we would have these:
480V 1P2W
120/240V 2P3W
120/208V "2/3"P3W
240/120V 3P4W
208Y/120V 3P4W

The books refer to the old "two-phase" as being one half (2/4) of a 4-phase system and this is why I used the 2/3 in my previous post.

[edit: and the old "two-phase" three wire would be "2/4"P3W]

This is part of the problem of trying to describe one system as part of a different system.

I have never heard of a 3-wire circuit fed from a 2-phase 4-wire system nor of a 4-phase system. It would be possible to create a 2P 3W system from 4W simply by connecting one conductor from each phase together.

Again it seems we are running into the issue of single Line/hot wires/conductors being called phases and then trying to relate these to the phases caused by "time-differences".

 

[mivey]

Show me a reference that states that they cannot be inverses. Because you obtained these opposing voltages by reversing the polarity of your scope leads, it is technically more accurate to refer to them as being inverses as opposed to being out of phase. Reversing the polarity of your leads did not result in a time shift from one signal to the other.

I don't see that calling them inverses makes any difference. It is the same difference. You did not just "reverse the polarity leads", you have also re-defined the waveform.

You have not redefined the physical points "A" and "B". You have changed the reference frame. When you re-defined the two waveforms, they now have a 180 degree displacment, which is the same as being inverses of each other.

The re-defining of the "A" & "B" waveforms is the same thing that happens when you change from a "wye" neutral point to a center-tapped neutral point. You do not have a time shift here either, but you still change from a 120 degree displacement to a 180 degree displacement. The new "A" and "B" waveforms have a different reference point.

 

[mivey]

I have never heard of a 3-wire circuit fed from a 2-phase 4-wire system nor of a 4-phase system. It would be possible to create a 2P 3W system from 4W simply by connecting one conductor from each phase together.

I'm not sure about the 2-phase 4-wire system. There has to be a common reference point if you are going to tie two separate 2-wire systems together.

This is before my time but this is my understanding: if you have the old "two-phase" system, you would have 2 coils with a phase displacement of 90 degrees. They can be combined to have a common reference point. This configuration could be "L" shaped with the reference point at the corner of the "L". Without this reference point, you would just have single phase systems as the L-L difference alone would yield a single sinusoidal waveform. With this "L" reference point, two of the wires are at the same point and you have a net 3-wire system.

With two 90-degree displaced coils, you could also have a reference point that was the midpoint of the coils and create a 4-phase system. The 4-phase system was "X" shaped and could have a neutral wire (the 5th wire) coming from the center of the "X". This was the same as joining two 180 degree displaced "L" systems at the corner of the "L".

 

[mivey]

Again it seems we are running into the issue of single Line/hot wires/conductors being called phases and then trying to relate these to the phases caused by "time-differences".

No doubt that this causes much confusion. But you could say that the difference in the phases is not a time difference but a degree difference. If you look at when 3 evenly displaced moving parts of a generator pass one fixed physical point, it is a time difference. If you look at when the 3 different voltages began their existence, it was when the generator first moved and they have the exact same starting point in time, but the points were displaced by 360 degrees.

 

[rattus]

Whoa Nellie!

Whoa Nellie!

What Rattus said is that the statement below is incomplete and misleading:

Originally Posted by Psychojohn
Because the zero crossing point is at the same point in time.
And each leg is in phase with the other, just equal and oposite voltage.

Clearly Psychojohn is referring to V1n and V2n or Van and Vbn in a split phase system as the case may be.

We surely agree that waveforms are said to be "in phase" if and only if:

The waveforms are sinusoidal,
The positive peaks occur at the same instant,
The negative peaks occur at the same instant,
The positive going zero crossings occur at the same instant, and
The negative going zero crossings occur at the same instant.

Waveforms which are inverses of each other do not meet these requirements and are therefore not in phase!

What Rattus never said is that one must describe these voltages in only one way.

 

[Rick Christopherson]

I don't see that calling them inverses makes any difference.

Correct. It is just mathematics, which is why I took issue with Rattus saying it was wrong.

You did not just "reverse the polarity leads", you have also re-defined the waveform.

No, you did not redefine the system. You simply changed how you view the system. Redefining the system would require a physical (electrical) change to the system. This is what I have taken issue with for several weeks. Rattus has been approaching these discussions as though the system has been redefined, and not just viewed from a different perspective. A system definition is an absolute, regardless what point of perspective or reference you take. When you redefine a system to coincide with a particular reference point, then you alter the system for all reference points.

Consider, for example, the Solar System. The system is defined with the planets rotating around the sun. If we set our reference point to be the rotating Earth, then it would appear that the Sun and planets are rotating around the Earth. The movement of the planets can be described from this perspective (and it was). However, if you redefine the system based on this point of observation, then the movement of the planets does not match up with planetary movement as viewed from another point of reference, such as the moon.

That is the difference between changing a reference point, and redefining the system. We can say that the Sun "appears" to be revolving around the Earth from our perspective, but it is incorrect to redefine the movement of the sun such that it physically does revolve around the Earth.

The re-defining of the "A" & "B" waveforms is the same thing that happens when you change from a "wye" neutral point to a center-tapped neutral point. You do not have a time shift here either, but you still change from a 120 degree displacement to a 180 degree displacement. The new "A" and "B" waveforms have a different reference point.

I have a feeling that Jim is going to have something to say about this statement.

 

[Rick Christopherson]

We surely agree that waveforms are said to be "in phase" if:

The waveforms are sinusoidal,
The positive peaks occur at the same instant,
The negative peaks occur at the same instant,
The positive going zero crossings occur at the same instant, and
The negative going zero crossings occur at the same instant.

Waveforms which are inverses of each other do not meet these requirements and are therefore not in phase!

What Rattus never said is that one must describe these voltages in only one way.

The mistake you are making here is that you are using a definition of when two signals are "in phase" to extend to the statement that they are "not in phase".

That's like saying that two objects have exactly the same weight if they both have the same density and volume. This is true. However, you are extending this to mean that two objects cannot have the same weight if one is smaller, but more dense. You are using a definition of "inclusion" into a group, as though it also applied to "exclusion" from the group. A pound of feathers does not have the same volume and density as a pound of lead, yet they both still have the same weight.

 

[mivey]

I have a feeling that Jim is going to have something to say about this statement.

I don't think Jim will disagree that with one reference, you call points "A" and "B" 120 degrees out of phase and in another definition, you can call them in phase, and in another definition, you can call them 180 degrees out of phase. In all 3 cases, we are still talking about the same 2 physical points that have been doing the same thing all along.

 

[mivey]

A pound of feathers does not have the same volume and density as a pound of lead, yet they both still have the same weight.

What? I'm saying a pound of feathers weighs a pound whether I measure it on a balance beam or a spring scale.

 

[mivey]

No, you did not redefine the system...Consider, for example, the Solar System. The system is defined with the planets rotating around the sun...

But I did redefine the waveform. That is by definition. The way I describe a point does not have to be an absolute.

Vab is Vab in either system. That does not change. The voltage at A and B can be defined differently because I need a reference point to define them. I can have Vab, Van, Van', etc. These definitions will give me different waveforms.

Point A is still point A but I want you to give me an absolute definition of the voltage at point without a reference point. Give me the absolute sinusoidal function that describes the voltage at point A without a reference. If you give me an absolute formula, I contend that it will have an absolute reference. It sounds like you are arguing that there has to be an absolute reference.

[edit: I guess I should clarify that Vab magnitude is the same in either system and the vector direction will be the same as long as the zero direction is the same. What changes is the waveform I use to show Va and Vb. In one case, I use Van where n is the delta neutral point, and in the other case I use Van' where n' is the center tap point. I could also graph Vnn' which would tie the two descriptions of Va and Vb together. We would have to decide which one of n vs n' was the sun and which was the earth, or who was rotating around who]

 

[rattus]

Make no mistake about it:

Make no mistake about it:

The mistake you are making here is that you are using a definition of when two signals are "in phase" to extend to the statement that they are "not in phase".

There is no mistake. The corollary of the statement is obviously true. To wit:

If any one of the conditions is not true, then the waveforms cannot be in phase.

In other words, to be in phase, two sinusoids must be identical with the possible exception of amplitude.

 

[Rick Christopherson]

We surely agree that waveforms are said to be "in phase" if and only if:

The waveforms are sinusoidal,
The positive peaks occur at the same instant,
The negative peaks occur at the same instant,
The positive going zero crossings occur at the same instant, and
The negative going zero crossings occur at the same instant.

There is no mistake. The corollary of the statement is obviously true. To wit:

If any one of the conditions is not true, then the waveforms cannot be in phase.

In other words, to be in phase, two sinusoids must be identical with the possible exception of amplitude.

Really?!? So this is the official definition of signals that are in-phase?

Hmmm. So according to your unilaterally imposed definition of being in-phase, the following two wave forms are not in phase?

V1(t) = 120 sin(ωt)
and
V2(t) = 10 + 120 sin(ωt)

The DC bias on the second waveform causes it to have zero crossings which do not directly line up with the first wave form, so clearly, by your definition, these two waveforms cannot be in-phase. I think there might be quite a few Electronic Engineers who would find this revelation of yours to be rather unsettling.

Moreover, is it your assertion that non-sinusoidal waveforms can never be in-phase with each other under any condition?

Either you pulled this definition of being in-phase out of thin air, or you choose to rely on very unreliable sources. Having a strong understanding of mathematics is a key qualification in the engineering discipline.

More aptly put from a mathematical standpoint:

V1(t) = 120 sin(ωt) and V2(t) = 120 sin(ωt+180) are 180 degrees out of phase

V1(t) = 120 sin(ωt) and V2(t) = -120 sin(ωt) are in-phase with each other, but inverses.

 

[rattus]

Hmmm. So according to your unilaterally imposed definition of being in-phase, the following two wave forms are not in phase?

V1(t) = 120 sin(ωt)
and
V2(t) = 10 + 120 sin(ωt)

The first requirement is that the waveforms be sinusoidal, therefore the requirements apply only to the sinusoidal portion of the waveform.

Moreover, is it your assertion that non-sinusoidal waveforms can never be in-phase with each other under any condition?

It is improper to assign phase angles to non-sinusoidal waveforms. You may do so with the individual harmonics, but not the waveform as a whole.

More aptly put from a mathematical standpoint:

V1(t) = 120 sin(ωt) and V2(t) = 120 sin(ωt+180) are 180 degrees out of phase

V1(t) = 120 sin(ωt) and V2(t) = -120 sin(ωt) are in-phase with each other, but inverses.

These two expressions describe the same waveform pairs, and indeed they are out of phase just like the waveforms seen on L1 and L2 in a split phase service. Note that we are talking about waveforms. It matters not how we describe them mathematically.

 

[rattus]

Redundancy:

Redundancy:

I might add that three of the last four requirements are redundant. If any one of these requirements is satisfied, the other three are satisfied as well.

 

[Rick Christopherson]

The first requirement is that the waveforms be sinusoidal, therefore the requirements apply only to the sinusoidal portion of the waveform.

By whose requirement? You are inventing these rules as you go, so by whose requirement is it that non-sinusoidal waveforms cannot be in-phase?

Furthermore, the V2(t) waveform is still sinusoidal by mathematical definition with a simple offset. The only reason you are contesting it is because it cannot fit your made-up definition. It isn't the waveform that doesn't fit the principle, it is your invented definition that fails.

There is a good reason why your definition of "in-phase" is not a published definition outside of your own mind, and that is because a high school math student could knock holes in it. As I said, you either invented it yourself, or you rely on unreliable sources.

It is improper to assign phase angles to non-sinusoidal waveforms.

Improper? Show me the reference. You need to broaden your horizons a little before you tell other people absolutes. The engineering world does not revolve around your limited knowledge base.

These two expressions describe the same waveform pairs, and indeed they are out of phase just like the waveforms seen on L1 and L2 in a split phase service. Note that we are talking about waveforms. It matters not how we describe them mathematically.

No kidding they are describing the same waveforms--d'ya think that was accidental?

You want to use mathematics to claim they are out of phase, but now you want to say that we can't use mathematics to state they are inverses? What's good for the goose is good for the gander. Pick your approach and stick to it, but don't waffle back and forth when someone uses that same approach against you.

If you want to play the math versus electrical card, then you prove that the two signals are electrically out of phase. For this to be true, then there needs to be a time-shift, which there is not. You can show that they are mathematically out of phase, but that is not the same as being electrically out of phase. So I challenge you--prove this electrically.

Mathematically, sin(x+180) = -sin(x). Electrically, sin(x+180) ≠ -sin(x)

(just in case the symbol does not come through, it is the not-equal sign)

 

[rattus]

No more:

No more:

Rick,

I am not going to waste any more time arguing these fundamental points with you.

 

[Rick Christopherson]

Rick,

I am not going to waste any more time arguing these fundamental points with you.

ROFLMAO!!! :grin: :grin: :grin: And there you have it folks. The standard Rattus suckout when he gets backed into a corner that he can't dig his way out of. You can get by with your voodoo engineering on some of the people here, but you know it will not fly past me.:smile: