single vs. 3 phase

[Don Randall]

Who is insisting? Not I, and no one else to my knowledge.

You don't need an arbitrary reference point to determine the phase relationship between two equal ac circuits connected in series. You don't even need an oscilloscope. A simple voltmeter will do. As learned in the study of basic ac circuits. If the circuits are series aiding, (120v +120v =240v), they are in phase with each other. If the circuits are series opposing, (120v - 120v = 0v), they are 180? out of phase with each other. A center tapped transformer is electrically equivalent to two identical single output transformers connected as series aiding, (in phase). If you use an oscilloscope to determine phase relationship, you have to connect it correctly to obtain a valid result. Improper connection of test equipment will give invalid results. You have to have opposite polarity between two conductors in order to have a potential difference. This does not make them 180? out of phase with each other. If you understand a sine wave, (graph), then you know that it indicates a changing potential between two conductors, not a changing voltage on one conductor. I believe using a common conductor, neutral, in two circuits causes a lot of the confusion as currents will be out of phase in it. However it ends at the breaker panel. From there you have individual neutral conductors that carry the same current as the so called 'hot' legs. Don

 

[gar]

080425-1320 EST

A conventional transformer fed from a single phase source can only output single phase voltages independent of the number of secondaries and taps. By conventional I am excluding some special circuitry that would modify a transformer, such as capacitors.

Obviously there are small residuals in any real transformer that may provide small phase shifts. For example a resistive 100% load will cause a slight phase shift at the transformer terminals relative to the input phase to the transformer. This results from the transformer leakage inductance.

(edit) A conventional transformer connected from a single phase source can not produce a polyphase output. Obviously this depends upon the definition of polyphase.(end edit)

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[rattus]

You don't need an arbitrary reference point to determine the phase relationship between two equal ac circuits connected in series. You don't even need an oscilloscope. A simple voltmeter will do. As learned in the study of basic ac circuits. If the circuits are series aiding, (120v +120v =240v), they are in phase with each other. If the circuits are series opposing, (120v - 120v = 0v), they are 180? out of phase with each other. A center tapped transformer is electrically equivalent to two identical single output transformers connected as series aiding, (in phase). If you use an oscilloscope to determine phase relationship, you have to connect it correctly to obtain a valid result. Improper connection of test equipment will give invalid results. You have to have opposite polarity between two conductors in order to have a potential difference. This does not make them 180? out of phase with each other. If you understand a sine wave, (graph), then you know that it indicates a changing potential between two conductors, not a changing voltage on one conductor. I believe using a common conductor, neutral, in two circuits causes a lot of the confusion as currents will be out of phase in it. However it ends at the breaker panel. From there you have individual neutral conductors that carry the same current as the so called 'hot' legs. Don

Don, I would argue that any voltage measurement requires one point to be a reference. Your method uses two references, but it is quite common to use a single reference such as a CT. In that case, the line voltages will be nominally equal inverses of each other, and the consensus is that an inversion is equivalent to 180 degrees phase difference.

Simply put, if I choose the CT as a reference, the line voltages will be out of phase by 180 degrees. Clearly, this was the intent of the OP several months back.

PS: Any voltage, AC or DC, can be measured in two ways, and either of the two ways is valid. Just get the polarity and phase angles right.

 

[Rick Christopherson]

and the consensus is that an inversion is equivalent to 180 degrees phase difference.

Don't claim any sort of consensus when there is not a consensus. You have a habit of making it sound like a vast majority of people agree with you, when it is actually the opposite.

 

[Don Randall]

If a person doesn't have basic knowledge of ac circuits such as how the generator produces pulses of voltage that vary as the sine of the angle of rotation of the armature, how sine waves, both in phase and out of phase are added, (algebraically when out of phase, mathematically when in phase), how the graph of a sine wave always portrays the potential difference between two conductors, (both hot relative to each other), etc., then it is difficult to explain a concept with just text. For instance, when you use two phases, in series with each other, of a 3 phase y supply to supply 208 vac, you would know that the two voltages add algebraically to produce a completely new waveform, still a sine wave but only single phase. Easy to illustrate with a drawing, but difficult with text only. Don't they teach this stuff anymore? They drilled it into us in the early 1960's. Don

 

[crossman]

Don: It is fairly easy to post diagrams here on the forum. In fact, there have been quite a few diagrams posted in this very thread. If you have a drawing that can help, I'm sure the forum management would be happy to give an explanation of how to post an image.

A picture is worth a thousand words:smile:

Edit: Welcome to the forum!

 

[gar]

080426-0620 EST

A fundamental characteristic of sine waves of the same frequency is that mathematically for steady state conditions the instantaneous sum of any number and any phase angles is a sine wave of the same frequency with a resultant amplitude that is a function of the individual amplitudes and phase angles, and the resultant phase angle is also a function of the individual amplitudes and phase angles.

The easy way to solve for the resultant is by means of vector diagrams.

There are cases where the resultant amplitude is zero. I will still call this a sine wave because it originated from sine waves.

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[rattus]

No matter:

No matter:

Don't claim any sort of consensus when there is not a consensus. You have a habit of making it sound like a vast majority of people agree with you, when it is actually the opposite.

It really doesn't matter, because it is an obvious fact which is easily proven by a simple trig identity.

 

[mivey]

I stopped following this thread hours ago, but something keeps gnawing at me, so I must ask......

So why is it that these long complicated threads always seem to be interrupted by rick claiming he is representing the majority of the electrical engineering world, and his good ol' boys mission to find rattus in error? Is it that rick and his boys have some sort of great insight into the engineering world that the rest of us do not posses? No, of course not.

It is because rick and his boys take the stance that their way is the only way, and all other applications are therefore wrong.

The majority of rick's posts would never even exist if the good ol' boy's club were not trying to trap rattus in an error.

:grin: (this is tongue-in-cheek pick at rick in response to #516 and #544, and I have no ill will towards rick, as I think he can be a generous and helpful fellow, but he just can't seem to get over this rattus thing.)

 

[Don Randall]

L1 and L2 are never out of phase with each other in a standard center tapped, single phase, transformer.. It's not possible. Just an illusion You get by incorrectly using the neutral as a reference point for your observations. For instance,You could stand beside a one way road and observe that the cars are coming towards you when you look to the left and going away from you when you look to the right. Then you may conclude that, from your reference, the cars are going in two directions, towards you and away from you. An observer with a distant perspective would know that they are all going in one direction. You can choose the illusion, I prefer the reality. Don

 

[crossman]

Wait a second! If I am standing by a one way street, and I look left, and cars are approaching, and I look right, and cars are going away, then there is nothing wrong with my perception, in fact, it is MY reality! To say it is wrong is, well, wrong.

If someone else is looking from far away, they see a different perspective. Not necessarily the "correct" one. Just another perspective.

I can give you an inertial reference frame that would have the cars going BACKWARDS!

 

[winnie]

L1 and L2 are never out of phase with each other in a standard center tapped, single phase, transformer.. It's not possible. Just an illusion You get by incorrectly using the neutral as a reference point for your observations.

You vastly limit your ability to understand the world if you claim that a particular reference point is _incorrect_.

This exact point has been touched on already, for aircraft flying over the North Pole. It is entirely correct to say that the aircraft is flying north, crosses the North Pole, and is then flying south. It would only be an incorrect application of this reference point if you were to then conclude that the aircraft experienced a sudden acceleration upon crossing the north pole.

The center tap of the transformer is one of several reasonable and correct reference points. From this reference point there is an apparent phase angle difference of 180 degrees. As long as the rest of your mathematics correctly accounts for the selection of reference point, then this is acceptable. It would only be an incorrect application of this reference point if, because of the apparent 180 degree phase difference, one were to conclude that there was a time difference in the outputs or the ability to produce a rotating field.

You might consider this a poor choice of reference point that confuses some aspect of the system (Jim has given good reasons, and the airplane flying over the North Pole is an example of confusion caused by a poor reference point), or you might consider it a good choice of reference point (I have my reasons), but it is not 'incorrect'.

-Jon

 

[weressl]

Don't claim any sort of consensus when there is not a consensus. You have a habit of making it sound like a vast majority of people agree with you, when it is actually the opposite.

He can claim conscensus in the sense that he stated is a fact that every person who talks the same language of physics and the underlying mathematics will agree. Albeit is not a dogma itself, but pretty darn close to it.

 

[rattus]

Semantics?

Semantics?

Just what do we mean when we speak of the voltages on L1 and L2 of a split-phase service? A summary of my interpretation follows:

V1 and V2 are the voltages on nodes L1 and L2 and are nominally 120V. Note that L1 and L2 are nodes; they cannot be part of the transformer which is a circuit element.

A reference is required to properly describe V1 and V2, and the CT/N/G is the only node left. Then we have V1n and V2n where even my harshest critics admit are out of phase by 180 degrees.

Now no one is expected to change their methods. I am just trying to clarify this rather basic point.

 

[rattus]

In other words:

In other words:

He can claim consensus in the sense what he stated is a fact that every person who talks the same language of physics and the underlying mathematics will agree. Albeit is not a dogma itself, but pretty darn close to it.

Let me paraphrase Laszlo's statement, "If one knows the math, one will understand and be part of the consensus".

I would add, "If one don't know the math, that doesn't make one a bad person."

 

[weressl]

Let me paraphrase Laszlo's statement, "If one knows the math, one will understand and be part of the consensus".

I would add, "If one don't know the math, that doesn't make one a bad person."

However if one does not know math and argues about it, it makes one a fool....... still not neccessarily a bad person though:grin:

 

[Don Randall]

I will agree with Crossman and Jon about perspective and just stick to the electrical side of things. (If I don't forget) Don

 

[Don Randall]

You vastly limit your ability to understand the world if you claim that a particular reference point is _incorrect_.

This exact point has been touched on already, for aircraft flying over the North Pole. It is entirely correct to say that the aircraft is flying north, crosses the North Pole, and is then flying south. It would only be an incorrect application of this reference point if you were to then conclude that the aircraft experienced a sudden acceleration upon crossing the north pole.

The center tap of the transformer is one of several reasonable and correct reference points. From this reference point there is an apparent phase angle difference of 180 degrees. As long as the rest of your mathematics correctly accounts for the selection of reference point, then this is acceptable. It would only be an incorrect application of this reference point if, because of the apparent 180 degree phase difference, one were to conclude that there was a time difference in the outputs or the ability to produce a rotating field.

You might consider this a poor choice of reference point that confuses some aspect of the system (Jim has given good reasons, and the airplane flying over the North Pole is an example of confusion caused by a poor reference point), or you might consider it a good choice of reference point (I have my reasons), but it is not 'incorrect'.

-Jon

Jon--Can you give me a little more information about the apparent phase difference of 180?? Is this a phase difference between the two halves of the coil, or between two conductors of the coil? I notice you use the term apparent, does that mean that you don't consider it is an actual phase difference? Is it possible to have a phase difference between two conductors? If so, then wouldn't that apply to all 2 conductor ac circuits, that is one conductor is always of opposite polarity of the other? How do you double check your conclusion of a 180? phase difference? How about the series aiding, in phase, and series opposing, 180? out of phase, rules. How do they fit in with your conclusion? Don

 

[winnie]

I use the term 'apparent' simply to recognize the fact that this phase shift will appear simply with a change of point of view (or swapping of meter leads). It is produced by a simple inversion.

I consider this a _real_ phase angle difference, in the sense that I can _use_ these two different phase angles in a system. For example, in an electric motor, consider the timing of the current flow in the various motor phase belts. Phase belt A is maximally positive, then phase belt C' hits its positive peak 60 degrees later, then phase belt B at 120 degrees, then belt A' at 180 degrees... In other words, this 'simple' inversion is producing a real usable different phase angle.

However this phase angle difference doesn't meet several criteria for defining a separate 'phase'. Essentially, each 'phase' (transformer core and associated coils) has _two_ phase angles associated with it.

-Jon

 

[mpross]

Wow

Wow

it is going to take a long time to read this whole string