Single Phase Theroy

[jim dungar]

Yes, the choice of reference only makes a difference in the phase angle of the measured voltage.

Absolutely, the only thing that changes is the appearance on the scope.

 

[rattus]

A little thought experiment:

A little thought experiment:

Consider an open wye where the voltages Van and Vban are provided by identical generators driven from a common shaft and let,

Van = 120V @ 0
Vbn = 120V @ -120

Clearly Van and Vbn are out of phase.

Now let the coupling between generators slip to the point that

Vbn = 120V @ -180

Can anyone argue that Van and Vbn are suddenly in phase??

Does anyone think that this is a 2-phase system?

 

[K8MHZ]

Consider an open wye where the voltages Van and Vban are provided by identical generators driven from a common shaft and let,

Van = 120V @ 0
Vbn = 120V @ -120

Clearly Van and Vbn are out of phase.

Now let the coupling between generators slip to the point that

Vbn = 120V @ -180

Can anyone argue that Van and Vbn are suddenly in phase??

Does anyone think that this is a 2-phase system?

"In a 2-phase system where no single conductor is out of phase with the other, all of the conductors shall be in phase with each of the other conductors of the system."

 

[LarryFine]

I'm not picking on you Larry, I'm just making the point that calling that 120/208 "single-phase" service single-phase is....... well....... just not quite right. I've argued in the past on this forum (and a lot of the posters have said I'm wrong) that this is not a single-phase service.

Really, it is a single-phase service tapped from a 3-phase source. Each connection, whether L-N or L-L, has a single waveform.

But, there definitely is a phase angle between the lines when the neutral is considered, which is why 120 + 120 ≠ 240v.

 

[LarryFine]

I would too! But we can still swap leads and see a phase shift, but not another phase.

I would call it a polarity shift, which resembles a 180 degree shift, but isn't one, because a phase shift requires a timing change.

180 degrees is the only phase shift that resembles a swap in polarity. Nobody would call any other phase angle a polarity swap.

 

[hardworkingstiff]

Really, it is a single-phase service tapped from a 3-phase source.

What are your definitions of single-phase and 3-phase?

 

[LarryFine]

Let us connect batteries in such that we have a total of 240Vdc. If we add a center tap and call this "N" and call the positive and negative terminals L1 and L2, we would have,

V1n = +120Vdc
V2n = - 120Vdc

Just like many electronic circuits where,

Vcc = +5V
Vgg = 0V
Vee = -5V

Agreed, but we call the two lines from the bi-polar DC supply a polarity difference, because we use chassis as the reference. The 240vdc or the 10vdc from those supplies is the same whether we use (or even have) the Vgg to reference or not.

Of course, this is a semantics discussion. I'm just saying that it's improper to say that a CT'ed AC supply has two phases just because it happens to be AC. It has two lines of opposing polarity relative to the neutral, just like a DC supply.

 

[LarryFine]

Can anyone argue that Van and Vbn are suddenly in phase??

I say they are.

Does anyone think that this is a 2-phase system?

I say it is not.

 

[LarryFine]

What are your definitions of single-phase and 3-phase?

In what context?

 

[rattus]

I would call it a polarity shift, which resembles a 180 degree shift, but isn't one, because a phase shift requires a timing change.

180 degrees is the only phase shift that resembles a swap in polarity. Nobody would call any other phase angle a polarity swap.

Me neither! Never heard of a polarity swap! Besides, the argument that an inverted sinusoid merely resembles a 180 degree shift, difference, separation, or whatever is illogical. Bet you can't support it with a solid reference!

However you look at it, the phasors for V1n and V2n must indicate a 180 degree phase difference.

 

[rattus]

Of course, this is a semantics discussion. I'm just saying that it's improper to say that a CT'ed AC supply has two phases just because it happens to be AC. It has two lines of opposing polarity relative to the neutral, just like a DC supply.

Larry, for the 470th time, I am NOT saying there are two phases! I am only saying it is quite proper, even conventional, to express V1n and V2n as phasors with a 180 degree phase difference. There is no rule that one must express V1 and V2 in such a way that they are in phase.

 

[hardworkingstiff]

What are your definitions of single-phase and 3-phase?

In what context?

Basically, all context. I'm just trying to understand what is the determining factor for you to call a power supply 3-phase or single-phase. I'm not trying to trick you (I'm in no way smart enough to do that), I'm just trying to understand since you told me I'm wrong to consider 120/208 (2 ungrounded and 1 grounded) something other than single-phase.

 

[rattus]

I say they are.
/QUOTE]

Larry, how can they be in phase when the peaks do not coincide?? To paraphrase K. Y. Tang, "Alternating Current Circuits", two waves are in phase if both waves reach their maximum positive values at the same instant. Clearly, this is NOT the case!

If V1n and V2n were in phase we could safely short them together because there would be zero potential between them. Not even an old hand like Larry would do this on purpose.

Now V1n and Vn2 are in phase, but the discussion centers on V2n!

 

[LarryFine]

Bet you can't support it with a solid reference!

Nope. I'm just expressing my take on it.

However you look at it, the phasors for V1n and V2n must indicate a 180 degree phase difference.

Okay, if you're talking about drawings, I can't argue with you.

Larry, for the 470th time, I am NOT saying there are two phases! I am only saying it is quite proper, even conventional, to express V1n and V2n as phasors with a 180 degree phase difference.

The problem my logic has with that is you're saying that a single phase has two phases. I really do grasp what you're saying, of course.

There is no rule that one must express V1 and V2 in such a way that they are in phase.

I dig, but there's no real rule that we must express them in any one way, as far as I know. I understand why we use the grounded conductor, but it's a choice.

The point (I think) "our side" has been making is that the use of the neutral itself as the focus (right word?) of the vectors is arbitrarily chosen, in the grand scheme of things.

I say they are.

Larry, how can they be in phase when the peaks do not coincide?? To paraphrase K. Y. Tang, "Alternating Current Circuits", two waves are in phase if both waves reach their maximum positive values at the same instant. Clearly, this is NOT the case!

If he's the authority, I can not argue. I though simultaneous, opposing (polarity ()) peaks could occur in a single-phase supply, too.

Hey, wait! Now, I'm arguing your point. :roll:

If V1n and V2n were in phase we could safely short them together because there would be zero potential between them. Not even an old hand like Larry would do this on purpose.

Now V1n and Vn2 are in phase, but the discussion centers on V2n!

Well, if you get to choose whether you're using VN2 or V2N, then of course, you get to choose whether they're in phase or not.

I'm always right when I get to answer my own questions. Don't believe me? Just ask me.

 

[rattus]

Finis I hope:

Finis I hope:

Larry: Nope. I'm just expressing my take on it.

Rattus: That is like the tail wagging the dog, and that dog won't hunt!

Larry: Okay, if you're talking about drawings, I can't argue with you.

Rattus: The drawings represent complex numbers--phasors that is, and you can't argue with the numbers.

Larry: The problem my logic has with that is you're saying that a single phase has two phases. I really do grasp what you're saying, of course.

Rattus: Never, ever said that! Said the opposite many times!

Larry: I dig, but there's no real rule that we must express them in any one way, as far as I know. I understand why we use the grounded conductor, but it's a choice.

Rattus: No hard and fast rule, but for me and most engineers I think, it is conventional.

Larry: The point (I think) "our side" has been making is that the use of the neutral itself as the focus (right word?) of the vectors is arbitrarily chosen, in the grand scheme of things.

Rattus: "Reference" is the right word, but you were claiming V1n and V2n were in phase.

Larry: If he's the authority, I can not argue. I though simultaneous, opposing (polarity ()) peaks could occur in a single-phase supply, too.

Rattus: He is one of many authorities.

Well, if you get to choose whether you're using VN2 or V2N, then of course, you get to choose whether they're in phase or not.

Rattus: That is the crux of the matter.

 

[winnie]

I know that I am going to need a mortuary licence to keep beating this horse, but the word 'phase' is being used in different ways here.

A single phase service has _two_ phase angles available. The fact that there are two phase angles available does not change that it is a single phase service. The fact that there are two phase angles available does not make it a two phase service.

'Phase' as it applies to service has a meaning that pretty much describes how such a service will be used. You still have a single phase service even if the two supply legs are derived from a wye source, with a 120 degree phase angle difference.

'Phase' as it applies to a sinusoidal waveform has a meaning that simply tells the time displacement of that waveform relative to a reference. Under this meaning, an inversion _is_ a 180 degree phase angle difference. A waveform and its inverse are 180 degrees out of phase. When you look at a waveform and its inverse, you have two phase angles. When you look at Van and Vbn of a _single_ phase service you have _two_ phase angles.

The fact that you have _two_ phase angles when you use the word 'phase' in the phase angle context does not change the fact that you have a _single_ phase service when you use the word 'phase' in the service context.

-Jon

 

[rattus]

I know that I am going to need a mortuary licence to keep beating this horse, but the word 'phase' is being used in different ways here.

A single phase service has _two_ phase angles available. The fact that there are two phase angles available does not change that it is a single phase service. The fact that there are two phase angles available does not make it a two phase service.

'Phase' as it applies to service has a meaning that pretty much describes how such a service will be used. You still have a single phase service even if the two supply legs are derived from a wye source, with a 120 degree phase angle difference.

'Phase' as it applies to a sinusoidal waveform has a meaning that simply tells the time displacement of that waveform relative to a reference. Under this meaning, an inversion _is_ a 180 degree phase angle difference. A waveform and its inverse are 180 degrees out of phase. When you look at a waveform and its inverse, you have two phase angles. When you look at Van and Vbn of a _single_ phase service you have _two_ phase angles.

The fact that you have _two_ phase angles when you use the word 'phase' in the phase angle context does not change the fact that you have a _single_ phase service when you use the word 'phase' in the service context.

-Jon

Thank you Winnie. I think this horse is really, really, really dead now!

 

[Besoeker]

Are any of you familiar with low voltage high current rectifiers?
A typical arrangement is the "hexaphase"
Meaning six phases.
Typically you have a step down transformer with a three-phase primary. The secondary winding arrangement is star (WYE) but with the windings connected at a centre point rather than the ends. The result looks like six spokes of a wheel. If you look from that centre point to the end of each winding you get six waveforms displaced at 60 degree intervals.
Isn't that six phase?

 

[jumper]

According to IEEE it is.......... 120/208Y is a modification of a three phase, 4 wire, system, where two legs are used to create a single-phase, 3-wire, open wye application.

Works for me. All the rest of this stuff is fascinating, but this all I really gotta know. :grin:

 

[rattus]

Hey, a new dead horse!

Hey, a new dead horse!

Are any of you familiar with low voltage high current rectifiers?
A typical arrangement is the "hexaphase"
Meaning six phases.
Typically you have a step down transformer with a three-phase primary. The secondary winding arrangement is star (WYE) but with the windings connected at a centre point rather than the ends. The result looks like six spokes of a wheel. If you look from that centre point to the end of each winding you get six waveforms displaced at 60 degree intervals.
Isn't that six phase?

Not really. I would argue that the extra "phases" derived through inversions do not count. Suppose you build a full rectifier by connecting diodes to L1 and L2 in your residential service--still single phase.

But I have seen a "4-phase" service made this way described in a textbook.