single vs. 3 phase

[quogueelectric]

PETA won't allow it.

I will sign a release.

 

[jim dungar]

If he is calling them opposites, then the point would be midway between the two outputs. The two outputs can't be the same unless they are at the same node.For a "T-T", I would think they would be the same. For the Scott, I'm not sure how that applies.

When making a statement like "they are opposite" the conditions used to establish that relationship need to be stated.

 

[jim dungar]

Jim, I am perhaps pickier than thou, and my book describes a "T-connection (Scott connection)" My book is older than you as well.

A "Scott" connected transformer is a T connection. Saying Scott-T is simply being redundant.

For more information:
http://www.mikeholt.com/newsletters.php?action=reply&inResponseTo=258&letterID=53

... some of that power was converted to two-phase using "Scott" type transformers

 

[rattus]

Uncle!

Uncle!

A "Scott" connected transformer is a T connection. Saying Scott-T is simply being redundant.

For more information:
http://www.mikeholt.com/newsletters.php?action=reply&inResponseTo=258&letterID=53

... some of that power was converted to two-phase using "Scott" type transformers

I concede. Jim is pickier than I.

 

[rattus]

1000 Words:

1000 Words:

Lest we become more confused than usual, the attachment provides a diagram of the Scott connection. Polarity dots are mine.

And, in so many words, the author states that V20 and V30 are out of phase by 180 degrees.

 

[mivey]

I will sign a release.

With what? A cow pen? :smile:

 

[quogueelectric]

With what? A cow pen? :smile:

I will make an X with my hoof. :wink:

 

[peter52]

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.

This is an excelent answer, and you can proof it with math.

 

[rattus]

OK then:

OK then:

This is an excelent answer, and you can proof it with math.

I can't see that, so let's see the proof. Maybe you can add something new.

 

[Rick Christopherson]

I can't see that, so let's see the proof.

Geeze you are obstinate!

You so desperately want to state that the two voltages are out of phase to suit your previous arguments that you close off all other views. You are doing this whole "absolutes" thing again. It is fine if you choose to call them out of phase, but it is not fine that you imply that they cannot be in-phase, but inverses.

The world does not revolve around "Rattus' viewpoints". There are other viewpoints, and they are equally correct!

You never proved that the two voltages were out of phase, as you seem to think you did. All that you did was demonstrate that they could be mathematically out of phase. The distinction is important, because if you could prove that they were out of phase, then you could also prove that they were not inverses of each other. For a proof to be true, you can't have both, and you cannot disprove the latter, and therefore, you cannot prove the former.

 

[rattus]

Where is the proof?

Where is the proof?

I don't see any proof, therefore I will prove my own point with a paraphrased reference:

'Two sinusoidal waves, of the same frequency, are said to be in phase if their positive peaks occur at the same instant.'

[Tang, K. Y., "Alternating Current Circuits," Intl. Textbook Co., 1960]

The positive peaks of a wave and its inverse do not occur at the same instant, therefore, they cannot be in phase. They are 180 degrees out of phase.

 

[winnie]

Arrrrrrrrgggggggggghhhhhhhh.

This exact issue has been repeated several times, by several different people. We are dealing with multiple meanings of the word 'phase'.

A waveform and its inverse cannot produce a rotating magnetic field without additional phase shifting components. The zero crossings coincide. A standard center-tapped 'single phase' service will produce a sine voltage and its inverse. By this meaning, a waveform and its inverse provides only a single phase.

At the same time, a separate meaning of 'phase' is to describe the time displacement between two periodic waveforms. At 60Hz, a phase difference of 180 degrees means 8.33 milliseconds. A 60Hz sine function delayed by 8.33 milliseconds is equivalent and indistinguishable from a 60Hz sine function inverted. By this meaning it is entirely reasonable to say that the two legs of a single phase center tapped service are 180 degrees apart.

_One_ phase, described _correctly_ with two different phase angles. There are clearly two different phase angles because if there were only one phase angle than the waveform would coincide at _all_ points, not just the zero crossings.

-Jon

 

[Rick Christopherson]

The positive peaks of a wave and its inverse do not occur at the same instant, therefore, they cannot be in phase. They are 180 degrees out of phase.

They do occur at the same time, Einstein! You reversed the polarity of your measuring probe, hence, you created the inverse.

I am not saying they can't be considered 180 degrees out of phase mathematically, but for the same reason, you can't say that they are not inverses either. You assume that the entire world has to view everything exactly the same as you do, and heaven forbid if they don't, because then they are wrong.

 

[Buck Parrish]

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.

I was going to say that, You nailed it

 

[mivey]

...A waveform and its inverse cannot produce a rotating magnetic field without additional phase shifting components. The zero crossings coincide. A standard center-tapped 'single phase' service will produce a sine voltage and its inverse. By this meaning, a waveform and its inverse provides only a single phase...There are clearly two different phase angles because if there were only one phase angle than the waveform would coincide at _all_ points, not just the zero crossings.

Winnie is correct. I think one of the problems is that some fail to recognize that a waveform is defined by its reference points. For example, Rick may not be saying exactly that, but it could be read to sound that way:

...You reversed the polarity of your measuring probe, hence, you created the inverse.

I am not saying they can't be considered 180 degrees out of phase mathematically,...

It is not just a mathematical manipulation, it is that way by definition. Take the corners of a 3-wire delta...please (ba-dump-bump) . I believe most would be happy to say (using a rough example) that the electron sitting on corner "A" is 120 degrees out of phase with the electron on point "B" (please read Winnie's note on the use of the word phase). Now center tap a coil between "A" and "B" and they want to say these must be in phase.

What has made the difference? The answer is that the reference point changed. The reference point is included in the definition of the waveform. The same definition that allows you to say the points "A" and "B" changed from being 120 degrees out of phase to being in phase is the same one that allows you to use the center tap and say "A" and "B" are 180 degrees out of phase.

So, "in-phase" or "out-of-phase" all depends on your reference point (again, see Winnie's note on the use of the word "phase"). Winnie stated that you had to use a phase shifting component to create three phase out of single phase. This is true. When we only have access to the three points provided by the center-tap, we have lost our original reference to the third leg of the delta (or the original neutral point). These original reference points gave us the phase shift we needed to create rotation with the same "A" and "B" legs that do not produce rotation as a single phase pair.

Without a phase shifting component, our new reference has to be one of the legs, or the center tap. If you choose one of the legs, "A" and "B" are in phase. If you choose the center tap, "A" and "B" are out of phase by 180 degrees.

Now, as Winnie has also stated, the 3-wire 120/208 does not act the same because it can create rotation because it did not lose its original reference which gave the phase shift needed for rotation. It is called a single-phase service by some because only single phase loads are served.

Lately, I think I am liking the term "3-wire network service" because it makes a distinction between the 3-wire 120/240 (two line conductors of a delta, center-tapped, and rotation not readily available), and the 3-wire 120/208 (two line conductors of a wye, with the original neutral point, with rotation readily available).

[edit: Perhaps we can say that a service fed by conductors with linear voltage relationships is called a single phase service. We might could also say that a service that is intended to serve only single-phase loads, and is fed by conductors with non-linear voltage relationships, may also be referred to as a single-phase service.]
[edit: changed "legs" to "line conductors"]

 

[rattus]

No lead swapping here:

No lead swapping here:

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.

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

The simple fact is that the preceding quote is incomplete and misleading. "Legs" do not carry a phase angle, voltages and currents do, but the implication is that the two line voltages are in phase even if they are inverses of each other. The statement contradicts itself.

 

[mivey]

refresher

refresher

It has been a while, and lest we forget the original question:

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

In terms of polyphase systems, an "n" phase system will have "n" voltages of essentially equal magnitude displaced by 360/"n" degrees.

Most everyone is familiar with the 360/3 or 3-phase system with its 120 degree displacement between voltages of equal magnitude.

2-wire 240, while fed from two original "A" and "B" voltages, now only has either the Vab voltage or the Vba voltage, depending on which conductor you use as a reference. This makes it a single phase system.

What used to be called "two-phase" was actually 1/2 of a "four phase" system with a 90 degree displacement between the voltages.

If you center tap a 240 volt coil and use the tap as a reference, you really have a real 2-phase system because the voltages are displaced by 360/2 or 180 degrees. But, by convention, we normally do not refer to this as 2-phase, although in terms of how poly-phase systems are defined, it really is.

If you will read Jim Dungar's responses, you will see he promotes looking at the L-L voltages to tie to today's terminology. This lines up nicely with the present conventions and gets away from the problems created when we consider the neutral. It is a nice solution on one hand but, the problem is, if we are not just talking about convention but want to talk about poly-phase systems, we must include all conductors.

When you have 3-wire 120/208, you really have 2/3 of a 3-phase system. With 3-wire 120/240, you really have a 2-phase system. This does not line up with current convention, and as Jim has essentially stated, we have made a mess of things over the years in the way we have defined things plus have multiple uses for the same word. I guess we get away with it because we say we are talking about a "service" definition and not a "system" definition.

[edit:typo]

 

[weressl]

Geeze you are obstinate!

You so desperately want to state that the two voltages are out of phase to suit your previous arguments that you close off all other views. You are doing this whole "absolutes" thing again. It is fine if you choose to call them out of phase, but it is not fine that you imply that they cannot be in-phase, but inverses.

The world does not revolve around "Rattus' viewpoints". There are other viewpoints, and they are equally correct!

You never proved that the two voltages were out of phase, as you seem to think you did. All that you did was demonstrate that they could be mathematically out of phase. The distinction is important, because if you could prove that they were out of phase, then you could also prove that they were not inverses of each other. For a proof to be true, you can't have both, and you cannot disprove the latter, and therefore, you cannot prove the former.

1* - out of phase
27* - out of phase
134* - out of phase
180* - out of phase AND in mathematical terms can be expressed as inverse.
181* - out of phase

 

[mivey]

1* - out of phase
27* - out of phase
134* - out of phase
180* - out of phase AND in mathematical terms can be expressed as inverse.
181* - out of phase

So simple, yet so hard for some to grasp.

 

[jim dungar]

When you have 3-wire 120/208, you really have 2/3 of a 3-phase system. With 3-wire 120/240, you really have a 2-phase system. This does not line up with current convention, and as Jim has essentially stated, we have made a mess of things over the years in the way we have defined things plus have multiple uses for the same word. I guess we get away with it because we say we are talking about a "service" definition and not a "system" definition.

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