Single Phase Theroy

[LarryFine]

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

You sure it wasn't two-phase/5-wire?

 

[LarryFine]

A typical arrangement is the "hexaphase"
Meaning six phases.

That's what it's called, and that's what it means, but it's really three center-tapped single-phase windings.

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?

Same discussion, different numbers. I still call it 3-phase, just like the primary.

 

[rattus]

You sure it wasn't two-phase/5-wire?

Exactly, but this author called it 4-phase. Goes to show you can't believe everything you read.

 

[LarryFine]

Exactly, but this author called it 4-phase. Goes to show you can't believe everything you read.

Even here.

 

[hardworkingstiff]

Help the world

Help the world

Some of you smarter guys may want to help the world with better information by getting wikipedia straightened out. From what I read here, their definition of single-phase would not included 120/208 (2 ungrounded and 1 grounded conductors) as a single-phase service.

In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison.

http://en.wikipedia.org/wiki/Single-phase

A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field.....

I believe we once had a discussion about this and if I remember right, I believe someone showed that with some creative connections, you could run a 3-phase motor from 2-hots and a neutral of a wye system (also being touted as a single-phase service by a lot of posters here).

I'd also like to point out that you cannot take a single-phase service and using transformers create a 3-phase service. PoCo does exactly that whenever they supply a 120/240V Delta 3-phase service.

 

[rattus]

Even here.

You can believe half of it--the right half that is; ignore the wrong half.

 

[rattus]

Some of you smarter guys may want to help the world with better information by getting wikipedia straightened out. From what I read here, their definition of single-phase would not included 120/208 (2 ungrounded and 1 grounded conductors) as a single-phase service.



http://en.wikipedia.org/wiki/Single-phase

I believe we once had a discussion about this and if I remember right, I believe someone showed that with some creative connections, you could run a 3-phase motor from 2-hots and a neutral of a wye system (also being touted as a single-phase service by a lot of posters here).

I'd also like to point out that you cannot take a single-phase service and using transformers create a 3-phase service. PoCo does exactly that whenever they supply a 120/240V Delta 3-phase service.

First request is impossible!

Second request is doable: The clean way is to isolate one phase with a transformer, and then connect to the other in an open delta. Or, you might be able to connect the motor leads in such a way to do the same thing.

Third item: You have it bassackwards. The POCO takes one phase from one transformer in a 4-wire delta.

 

[hardworkingstiff]

PoCo does exactly that whenever they supply a 120/240V Delta 3-phase service.

Third item: You have it bassackwards. The POCO takes one phase from one transformer in a 4-wire delta.

That was very poor wording by me. What I was trying to say is that PoCo (often) takes 2 legs and the neutral of a wye distribution to create a delta secondary to provide 3-phase service to a building. If 3-phase cannot be created using transformers by a single-phase distribution, and 2-phases and a neutral from a wye distribution can create a 3-phase distribution by using transformers, then a 120/208 "single-phase" service really is poor wording (and arguable wrong terminology).

It's like someone calling the dispenser you use to fill your gas tank a pump when the actual pump is in the underground storage tank. We all know what you are saying, but what you are saying is just not quite right.

 

[LarryFine]

It's like someone calling the dispenser you use to fill your gas tank a pump when the actual pump is in the underground storage tank.

If I'm not mistaken, the pump is in the dispenser, and the gas is brought up by vacuum through a dip tube.

 

[hardworkingstiff]

If I'm not mistaken, the pump is in the dispenser, and the gas is brought up by vacuum through a dip tube.

It's a better than 98% chance you are getting fuel from a dispenser. Almost always at modern retail gas stations, the units used for gas are dispensers. The pump is in the tank and the fuel pushed to the dispensing unit.

 

[Besoeker]

Not really. I would argue that the extra "phases" derived through inversions do not count.

If the connection was the more common arrangement of the windings at their ends you would have just three voltages at 120deg intervals.
Hexaphase has six at 60deg intervals.
So, it is different in that respect.

Here's another scenario.
Variable speed drives produce harmonics. Those supplied from three phase are six-pulse and the predominant harmonics are 5th, 7th, 11th 13th etc. (6n+/-1). For larger drives it is not uncommon to use a 12-pulse system. This is usually derived from a transformer with a Ddyn0 or similar arrangement - delta primary and two secondary windings, one star (wye) the other delta.
Three phase primary but how many phases on the secondary?

 

[ggunn]

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!

It all depends on your point of reference, doesn't it? For example, take a two wire, single phase, 120VAC supply. Connect two (high value) resistors in series from hot to neutral. Connect the ground terminal of your meter to the connection point between the resistors and read the voltages on hot and neutral. You'll see 60VAC on each, 180 degrees out of phase, right? The service hasn't changed, only your reference point has.

 

[mivey]

As I'm laid off I had nothing better to do this afternoon so I think I can settle this once and for all.

Good luck. :grin:

Being that there in only one expanding and collapsing electric field from the primary there is no way physically this could cause a lead or lag of a "phase."

The point that there is only one flux in the single-phase transformer was not questioned. The point was that the secondary voltages are exactly the same as the voltages you can get by joining two phase-opposed voltages.

The voltages at the secondary are the same as you will get from connecting phase-aligned voltages head-to-tail or by connecting phase-opposed voltages tail-to-tail. We have adopted the convention of calling it single-phase but must not ignore the fact that the voltages are the same as you would get from a two-phase 180-degree displacement.

It seems some want to argue that any hot wire constitutes a phase.

No. A phase needs two points (i.e. a voltage).

It seems others what to philosophical define what a phase is. That is useless.

It must be defined if you want to use the classic definition of a poly-phase system because there are so many uses of the word "phase"

The question really is do you define phase order by the primary or the secondary? For all practical purposed and using standard names for systems it seems clearly that defining them by the primary is correct.

And will result in the wrong name for cases where you make phase conversions.

And using the proper names; most anyone in the field should know what you are talking about.

I agree with sticking with the conventional names. I'm just proposing that the user understand why the system was named and how it relates to the other possible names.

 

[mivey]

Question for Sparkys that insist 120/240 is not single phase:

If the nameplate requires 120/240 1PH 60Hz and you have it in your brain that 120/240 has two phases, what kind of phase converter are you going to use?

Hmmm???

Did anyone say it was not single-phase? I said the voltages can represent both.

 

[mivey]

For me I think it just makes sense to call things by their common names and all this 'stuff' about it being single phase or 2 phase amounts to people just trying to prove they have the biggest brains.:roll:

:roll: It is about trying to help people understand the common link between poly-phase systems and what appears to be a discontinuity in system definitions. It is because we have an overlap of systems and we have chosen one name above the other.

However, it is incorrect to imply that this is a 2-phase system because the extra "phase" is created by a simple inversion--not a second generator.

But it is incorrect to say that these voltages are not the same as we would get from using two time-displaced generators.

If you took two individual, but synchronized 120v 1ph sources, and connected them "in phase" in such a way that they'd form a 120/240v source, like we get from two 120v windings in series, I'd still call that single phase.

And that is where we differ. At what point between 90 degrees and 180 degrees does the transformation happen that makes two phases suddenly become one and only one phase?

How about by counting the number of L-L voltages and not the number of L-N ones. After the voltages have been combined the result is a single waveform.

Because that method has flaws. There are 4 L-L voltages in the old 5-wire 2-phase.

In the 120/208 service case, the loads are single-phase but the source is called a network. We also call a two-legged wye with a neutral 2-phase in the utility industry.

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

That would go back to your definition of a phase. In the classic sense, the two voltages represent two phases. With the series reference frame, they are two individual phases that remain separate in two individual system classifications. In the neutral reference frame, they are two individual phases grouped into one system classification.

 

[mivey]

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.

What is your definition of phase? Why do you think these waveforms are functionally different in steady-state than the ones you described in #62?

 

[mivey]

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.

I'm saying the two voltages from the single-phase will be exactly the same as we can get from 180-degree two-phase. They can represent either, but we call them single-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.

Either reference is a choice. I say both are valid.

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.

No more arbitrary than the other valid choice. In fact, I would say you will find the neutral used as the reference more often.

 

[mivey]

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

Very nicely put.

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?

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.

Not really. I would argue that the extra "phases" derived through inversions do not count.

And I would disagree with you, as would many text books. Some would call the old 5-wire 2-phase a 4-phase system, some would not. I would call it capable of representing both.

Exactly, but this author called it 4-phase. Goes to show you can't believe everything you read.

I think it depends on the context. IEEE calls it 4-phase.

That was very poor wording by me. What I was trying to say is that PoCo (often) takes 2 legs and the neutral of a wye distribution

We call this 2-phase in the utility industry (or "V"-phase).

then a 120/208 "single-phase" service really is poor wording (and arguable wrong terminology).

The utility industry calls it a network service. Probably to distinguish it from the historic 2-phase.

Hexaphase has six at 60deg intervals.
So, it is different in that respect.

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.

It all depends on your point of reference, doesn't it? For example, take a two wire, single phase, 120VAC supply. Connect two (high value) resistors in series from hot to neutral. Connect the ground terminal of your meter to the connection point between the resistors and read the voltages on hot and neutral. You'll see 60VAC on each, 180 degrees out of phase, right? The service hasn't changed, only your reference point has.

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

The set of voltages and what systems they can represent can also be a consideration. When someone starts discussing the voltages and how the angles relate to a poly-phase system of voltages, we must look beyond the service label to understand the link between the two.

 

[LarryFine]

If you took two individual, but synchronized 120v 1ph sources, and connected them "in phase" in such a way that they'd form a 120/240v source, like we get from two 120v windings in series, I'd still call that single phase.

And that is where we differ. At what point between 90 degrees and 180 degrees does the transformation happen that makes two phases suddenly become one and only one phase?

Because the resemble, as in they measure like, and can be used like they are a single phase. Just as the single, center-tapped secondary resembles two distinct sources that are connected as, measure as, and can be used as a single, center-tapped source.

When two sources, connected end-to-end, suddenly 'snap' into being 180 degrees apart, and are now additive, they have indeed combined to become a single-phase source, as far as the end-to-end voltage is concerned, as well as each from the neutral reference.

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.

 

[LarryFine]

I'm saying the two voltages from the single-phase will be exactly the same as we can get from 180-degree two-phase.

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

They can represent either, but we call them single-phase.

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.

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.