single vs. 3 phase

[weressl]

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.

A three phase circuit have its phases 120 degrees apart from each other. The 120/240V system has its phases 180 degrees apart therefore it IS a 2 phase system. Its primary, however, is a single phase source.:roll:

 

[jim dungar]

2. It is acceptable, even conventional, to use the CT as the reference point in a 120/240V split-phase service. Note that we split a single phase; we do not create a second phase.

I disagree, I do not find it convenient nor is this method conventional in power system analysis university courses. There are many 2-wire 240V systems that do not include a neutral even though the transformer has a center tap.

Just because you can does not mean you should.

The only practical reason I can find for using the CT as a reference is to explain your simultaneous 120V tracings. With this method, almost every other power systems concept needs to be "modified". The most glaring of which is the voltage and current relationship created by a single magnetic field in a transformer core.

 

[rattus]

I disagree, I do not find it convenient nor is this method conventional in power system analysis university courses. There are many 2-wire 240V systems that do not include a neutral even though the transformer has a center tap.

Jim, references have been posted which support this position, and of course one is free to use any method one wishes. Now, one could argue that the CT is a neutral, perhaps grounded, and could be used as a reference.

Obviously, we attended different universities.

The only practical reason I can find for using the CT as a reference is to explain your simultaneous 120V tracings. With this method, almost every other power systems concept needs to be "modified". The most glaring of which is the voltage and current relationship created by a single magnetic field in a transformer core.

I see no connection between the magnetic field in a transformer and the choice of reference. Think black box. We are speaking of voltage, not magnetic flux. The inner workings of the transformer are not relevant.

 

[rattus]

A three phase circuit have its phases 120 degrees apart from each other. The 120/240V system has its phases 180 degrees apart therefore it IS a 2 phase system. Its primary, however, is a single phase source.:roll:

Laszlo,

Technically, you might be correct, but we call it a single-phase system for two reasons:

1. To avoid confusion with the obsolete, 90-degree 2-phase system.

2. The second "phase" is obtained by inversion, therefore it is of no advantage in driving a 2-phase load, and no such loads are available today anyway.

 

[weressl]

Laszlo,

Technically, you might be correct, but we call it a single-phase system for two reasons:

1. To avoid confusion with the obsolete, 90-degree 2-phase system.

2. The second "phase" is obtained by inversion, therefore it is of no advantage in driving a 2-phase load, and no such loads are available today anyway.

Hence the problem is born when people start adopting convenient designations therefore destroying the significance of the meaning of the word or phrase. The problem of course results in confusion and does not aid in understanding the physics behind it.

 

[jim dungar]

Jim, references have been posted which support this position, and of course one is free to use any method one wishes. Now, one could argue that the CT is a neutral, perhaps grounded, and could be used as a reference.

Obviously, we attended different universities.



I see no connection between the magnetic field in a transformer and the choice of reference. Think black box. We are speaking of voltage, not magnetic flux. The inner workings of the transformer are not relevant.

Just because you can think of it as a back box does not mean you should. You have spent a whole lot of time simply defending your mathematics, How about defending your position in the real world. What is the practical reason for your method other than to explain a very specific system's oscilloscope tracings?

You have stated in previous posts that there is importance to the polarity markings of transformer windings, but now you want them to be ignored.

A problem I see with your method of describing a 180? voltage difference, is it is not easily applicable to other power system analyzes as we have seen when describing the voltages and current flow in a single 2-wire circuit. It only works for voltages. You admit that your model is not applicable to describe magnetic flux, but it would be if the voltages were in the same direction

All you have are some mathematical formulas that you can use to prove that 120/240V circuits are tricky (I have lost count of how many times you have said this). Please stop calling me wrong because I disagree with simplistic view that a center tap is the center of power system analyzes.

 

[jim dungar]

1. To avoid confusion with the obsolete, 90-degree 2-phase system.

2-phase systems are part of the cornerstone for brushless servomotors and encoders. Only 2-phase in power distribution systems is obsolete.

 

[rattus]

Hence the problem is born when people start adopting convenient designations therefore destroying the significance of the meaning of the word or phrase. The problem of course results in confusion and does not aid in understanding the physics behind it.

Not sure what that means, but it is a matter of published standards and common usage, e.g., the 3-wire, 120/208 system discussed in this thread is considered to be a single-phase system. Plus, the majority of the Forum members will tell you that the 120/240 split-phase system is single-phase.

And, I can't see that the physics enter into the discussion. We don't really care what the source of the voltage is. We are just looking at the voltages.

 

[johnny watt]

When two phase systems are used in servomechanisms are they always 90 degrees out of phase?
Although not as efficient, wouldn?t any angle other then 180 degrees work?
If a servo was being controlled with synthesized voltages could this be done by varying the amplitude or phase angle of the two phases, whichever the designer chooses?

The reason I ask is that I have an old textbook written by Herbert W. Jackson that refers to two phase as being defined by being able to have a rotating magnetic field. And there was also a similar claim in an unmentioned open edit on line encyclopedia. Which kind of implies that two out of three phases could be called two phase.

 

[rattus]

Here we go again:

Here we go again:

Just because you can think of it as a back box does not mean you should. You have spent a whole lot of time simply defending your mathematics, How about defending your position in the real world. What is the practical reason for your method other than to explain a very specific system's oscilloscope tracings?

You have stated in previous posts that there is importance to the polarity markings of transformer windings, but now you want them to be ignored.

Jim,

The practical reason is that you told an OP long ago that the voltages were not out of phase, and I disagreed with you.

Polarity markings are important, but I don't see that they have anything to do with the choice of voltage reference.

A problem I see with your method of describing a 180? voltage difference, is it is not easily applicable to other power system analyzes as we have seen when describing the voltages and current flow in a single 2-wire circuit. It only works for voltages. You admit that your model is not applicable to describe magnetic flux, but it would be if the voltages were in the same direction

No one is dictating that this approach be used by anyone, and the discussion is limited to voltages, phasors if you will. There is no need to consider the flux which induces these voltages.

All you have are some mathematical formulas that you can use to prove that 120/240V circuits are tricky (I have lost count of how many times you have said this). Please stop calling me wrong because I disagree with simplistic view that a center tap is the center of power system analyzes.

Jim, you are putting words in my mouth and greatly complicating this discussion. No one said that the CT is the center of power system analysis.

In a nutshell, I am claiming one point,

If one chooses the CT of a split-phase service as the reference, then the sinusoidal voltages on opposite ends of the secondary exhibit a phase difference of 180 degrees. Whether this is conventional or not; whatever the flux may be, this is true. And, the polarity marks must be right because this is a center tap. No one is even hinting that you should change your way of doing things. I am merely employing a well established principle in circuit analysis, and I am free to do that just as you are free to do things your way.

 

[coulter]

... And, I can't see that the physics enter into the discussion. ...

I see one major difference in the way we look at these systems. To my way of thinking, the physics and the descriptive math models are the entire discussion and the only part that matters.

Jim -
What say you on this issue?

 

[coulter]

... 3. No one cares about this pointless exercise anyway ...

... I see no connection between the magnetic field in a transformer and the choice of reference. Think black box. We are speaking of voltage, not magnetic flux. The inner workings of the transformer are not relevant. ...

My reasoning is the models of should be consistent - not isolated to each system, the inner workings are relavent.

I ask that you please stop calling my views pointless or alluding to my technical stupidity because I disagree with your view.

carl

 

[weressl]

I see one major difference in the way we look at these systems. To my way of thinking, the physics and the descriptive math models are the entire discussion and the only part that matters.

Jim -
What say you on this issue?

It is unlikely that a tradesman(electrician) came up with the phrase "phase", since you needed an scientist to discover it first, then an engineer to make practical use of it and design a system of it and then, only then did you needed and tradesman to install it. It is right to give each its due.

 

[Rick Christopherson]

After several threads, hundreds of posts, a couple of closures, and some excellent help from mivey and winnie, the following TRVTHs have been demonstrated:

Things have only been demonstrated in your shallow little world.

1. Sinusoidal voltages at opposite ends of a center-tapped secondary exhibit a 180-degree phase difference. The fact that this difference is not the result of a time delay is immaterial. Furthermore, it matters not that these voltages are induced in a common winding. If it looks like a phase difference, then it must be a phase difference. What you see is what it is.

You keep stating this as though it was an absolute, and that is what makes it wrong. They only "appear" out of phase because you choose to measure them out of phase, yet you try to state that they are "actually" out of phase.

You can measure them how ever you want, and you can solve your equations with whatever "trickery" you want, but when you state that something is an absolute, then you are forcing your methodologies as though they were absolute too.

That's why this is poor engineering on your part, because without someone else knowing what your viewpoint was to begin with, then they will not achieve the same analytical results.

8. The order of summation of phasors is arbitrary as long as the relative direction of summation is maintained. However, for convenience and clarity, we normally sum them in the order they appear in the diagram.

No, the summation of phasors is always arbitrary and commutative. It is only when you change your rules away from the standards that this is no longer true.

 

[rattus]

My reasoning is the models of should be consistent - not isolated to each system, the inner workings are relavent.

I ask that you please stop calling my views pointless or alluding to my technical stupidity because I disagree with your view.

carl

Carl,

It is quite common to model electrical sources and loads with no thought about the way the sources are generated. It is also quite common, no recommended, to keep the problem as simple as possible.

And, the derivation of the trig identities was pointless because,

1. The trig identities are well established as you well know. It would be a vastly complicated world if we could not use well established formulas without proving them.

2. The derivation thereof was in no way necessary to answer the question at hand. You demanded it more or less, and I refused to do it.

I don't recall calling you or anyone else "stupid".

This is simply a matter of choosing a voltage reference. There is no need to model the entire system all the way from the coal mine to someone's hair dryer. All this talk of magnetic flux, polarity dots, etc., does not prevent one from choosing the CT as a reference point.

You are making this simple issue overly complicated with these comments.

 

[jim dungar]

When two phase systems are used in servomechanisms are they always 90 degrees out of phase?
Although not as efficient, wouldn?t any angle other then 180 degrees work?
If a servo was being controlled with synthesized voltages could this be done by varying the amplitude or phase angle of the two phases, whichever the designer chooses?

The reason I ask is that I have an old textbook written by Herbert W. Jackson that refers to two phase as being defined by being able to have a rotating magnetic field. And there was also a similar claim in an unmentioned open edit on line encyclopedia. Which kind of implies that two out of three phases could be called two phase.

Two phase does have a rotating magnetic field.

How can you get 2 of 3 phases? Vab and Vbc, wait that's all three isn't it. I know, do like Rattus does Van and Vbn, but wait these are really nothing more than Vab which is 1 phase. I don't care how you do it, you can have two of three L-L phase conductors but you can not have two of three L-L phases.

 

[jim dungar]

The practical reason is that you told an OP long ago that the voltages were not out of phase, and I disagreed with you.

So the only practical reason for your method is because you disagree with me.

Polarity markings are important, but I don't see that they have anything to do with the choice of voltage reference.

I keep saying your math only works with voltages, your concept cannot be directly applied to any other power system analysis method which makes it impractical for most applications.

No one is dictating that this approach be used by anyone, and the discussion is limited to voltages, phasors if you will. There is no need to consider the flux which induces these voltages.

You have consistently said that your method of phasor analysis is the only correct way to analyze voltages. You dismiss anyone who tries to use your phasor analysis for currents. I want to define my voltages based on the flux that creates them but you have just said that there is no need to.

Jim, you are putting words in my mouth and greatly complicating this discussion.

So hundreds of posts are because I am complicating this discussion. I have been asking for a practical reason for performing math that does not need to be done.

No one said that the CT is the center of power system analysis.

This must have been a poor assumption on my part, based on your reluctance to ever discuss circuits that do not include a CT or neutral conductor.

In a nutshell, I am claiming one point,

If one chooses the CT of a split-phase service as the reference, then the sinusoidal voltages on opposite ends of the secondary exhibit a phase difference of 180 degrees. Whether this is conventional or not; whatever the flux may be, this is true. And, the polarity marks must be right because this is a center tap. No one is even hinting that you should change your way of doing things. I am merely employing a well established principle in circuit analysis, and I am free to do that just as you are free to do things your way.

I can not remember you ever so clearly stating your position.

 

[rattus]

So the only practical reason for your method is because you disagree with me.

Jim it is not my method. It is simply that the OP used the CT as the reference.

I keep saying your math only works with voltages, your concept cannot be directly applied to any other power system analysis method which makes it impractical for most applications.

And, I say so what? Let's stick to the OP's question and not redesign the entire power grid. In other words, let us stay within the framework established by the OP.

You have consistently said that your method of phasor analysis is the only correct way to analyze voltages. You dismiss anyone who tries to use your phasor analysis for currents. I want to define my voltages based on the flux that creates them but you have just said that there is no need to.

Never claimed to employ the only correct method. I don't see any problem with currents either. Where did you get this notion?

So hundreds of posts are because I am complicating this discussion. I have been asking for a practical reason for performing math that does not need to be done.

Jim, you had a lot of help.

This must have been a poor assumption on my part, based on your reluctance to ever discuss circuits that do not include a CT or neutral conductor.

Jim, again let us stick to the framework of the OP's question which does include a CT.

I can not remember you ever so clearly stating your position.

I didn't think it was necessary, but do you agree with that statement?

 

[hardworkingstiff]

the 3-wire, 120/208 system discussed in this thread is considered to be a single-phase system.

Not by all. It does serve a single-phase service, but that does not make it a single-phase system.

 

[rattus]

Not by all. It does serve a single-phase service, but that does not make it a single-phase system.

Stiff, it is a matter of definition by the IEEE I believe. See Winnie's post on the matter. No matter, old habits die hard.