# Single Phase or Polyphase?

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#### Volta

##### Senior Member
So if windings AB and CD are connected in series, or are the center-tapped secondary of a single winding primary transformer, and one could call them 'two-phase', it is because:
1. AB and AD both exist? (Two diferent voltages.)
2. And not because AB and CD exist? (Two duplicated voltages.)

In the preceeding post, does #3 allow a duplicated phase angle with a different voltage?

I guess my only difficulty with this remains terminology. How do you verbally distinguish between a center-tapped winding as 'two-phase' and a traditional 90 degree angle 'two-phase' system?

#### jim dungar

##### Moderator
Staff member
Even if you had not removed one of the loads, why would you think both halves of the winding would have the same current?
The number of phase does not depend on the current magnitude.

Are you saying the X23-X1 current would be the same as the X23-X4 current?
X23-x1 is not the same as X1-X23, you have moved the reference point. as I have said all along you 2-phase depends on using the center tap as a reference. If any other point is used it is not 2-phase.

They are not the same. My source may be a three-phase transformer bank. I may use it to supply single-phase to a load. That does not mean I no longer have a 3-phase source.
Here you go again, moving the discussion away from the single primary winding 240/480V transformer. Seeing how all interconnected POCOs in the US generate 3-phase, it appears the only true single-phase sources would be stand alone generators that do not have neutral connections.

The number of phases that can be taken out is dependent on whether or not the center-tap neutral is used
This is the entire issue, I do not agree that the number of phases should depend on if a neutral point is used.

Do you think the currents in a split winding would be any different than the currents in two separate windings using separate but synchronized fields?
With a center tapped single winding, the primary current creates the magnetic field, that creates the direction of the secondary current. Choosing to analyze the portions as X23-X1 and X23-X4 instead of X1-X23 and X23-X4 is a mathematical gimmick it does not change how the current is created.

#### gar

##### Senior Member
100314-2010 EST

A single audio oscillator is fairly clearly a single phase source when it has only two output terminals. We will make these the secondary of a transformer.

Next I add a second similar oscillator with the unique characteristic that it is precisely synchronized to the first oscillator. This second oscillator has an additional knob that allows adjustment of its phase relative to the first from slightly less than 0 to slightly more than 2*Pi . I describe this as a two phase system, even at 0, Pi, and 2*Pi phase angle, and even if the voltages are not the same amplitude.

I have two different sources that are precisely related to each other.

.

#### mivey

##### Senior Member
The number of phase does not depend on the current magnitude.
:-? But they are indeed two different currents.
X23-x1 is not the same as X1-X23, you have moved the reference point. as I have said all along you 2-phase depends on using the center tap as a reference. If any other point is used it is not 2-phase.
I think I have made it clear that to get two-phase from a center-tap transformer you have to use the neutral as a reference.
Here you go again, moving the discussion away from the single primary winding 240/480V transformer. Seeing how all interconnected POCOs in the US generate 3-phase, it appears the only true single-phase sources would be stand alone generators that do not have neutral connections.
By George, I think he's got it. That would be an example of a source that can supply only single-phase. That does not mean a higher-order source could not also supply single-phase.
This is the entire issue, I do not agree that the number of phases should depend on if a neutral point is used.
That is one point we disagree on. Your own labeling methodology would show that your premise does not hold water for a 5-wire four-phase system.

With a center tapped single winding, the primary current creates the magnetic field, that creates the direction of the secondary current. Choosing to analyze the portions as X23-X1 and X23-X4 instead of X1-X23 and X23-X4 is a mathematical gimmick it does not change how the current is created.
I can take a DC battery and produce 3-phase current. I will have gone from a steady, unidirectional current to a system of 3-phase alternating currents. That is no mathematical gimmick.

I can take two-phase current on the primary side and produce 3-phase current on the secondary side. That is no mathematical gimmick.

The primary is not the system being evaluated. The secondary is the system being evaluated. How I define that secondary system determines what I have.

#### mivey

##### Senior Member
100314-2010 EST

A single audio oscillator is fairly clearly a single phase source when it has only two output terminals. We will make these the secondary of a transformer.

Next I add a second similar oscillator with the unique characteristic that it is precisely synchronized to the first oscillator. This second oscillator has an additional knob that allows adjustment of its phase relative to the first from slightly less than 0 to slightly more than 2*Pi . I describe this as a two phase system, even at 0, Pi, and 2*Pi phase angle, and even if the voltages are not the same amplitude.

I have two different sources that are precisely related to each other.

.
You definitely have two single-phase systems. The question becomes what happens when you combine these two. What type loads can they serve when used together?

Do you have identical systems that are connected in parallel and can serve bigger single-phase loads?

Do you have identical systems that are connected in series that can serve higher-voltage single-phase loads?

Or do you have non-identical systems connected in series that can serve two-phase loads?

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
How do you verbally distinguish between a center-tapped winding as 'two-phase' and a traditional 90 degree angle 'two-phase' system?
By not calling the former two-phase, because it ain't.

#### mivey

##### Senior Member
So if windings AB and CD are connected in series, or are the center-tapped secondary of a single winding primary transformer, and one could call them 'two-phase', it is because:
1. AB and AD both exist? (Two diferent voltages.)
2. And not because AB and CD exist? (Two duplicated voltages.)
It is not just because you have two voltages, it is because you have two equal magnitude voltages that have a different phase angle when measured relative to the reference point.

It is because you have the option of using BC as the reference. Then you can get voltages AB and DC. The voltages will be equal in magnitude have a 180 degree phase difference.

AB would be considered in a lower-voltage phase count and AD would be considered in a higher-voltage phase count.
In the preceeding post, does #3 allow a duplicated phase angle with a different voltage?
Per #1, a different voltage would not be included in the tally for this system. It would be considered in a tally for a system of the same voltage
I guess my only difficulty with this remains terminology. How do you verbally distinguish between a center-tapped winding as 'two-phase' and a traditional 90 degree angle 'two-phase' system?
If you are being technical, the center-tapped would be called a two-phase system with 180 degree displacement. The traditional would be called two-phase with 90-degree displacement.

Labels that could be used:
90 degree displacement: historic two-phase
180 degree displacement: split-phase
120 degree displacement: network

#### mivey

##### Senior Member
By not calling the former two-phase, because it ain't.
It sure ain't how we make use of it anyway. :grin:

#### gar

##### Senior Member
100314-2153 EST

mivey:

I can do anything I want with those two phases. A Lissajous of a straight line at many different angles, ellipses, and circles. With appropriated phase angles drive two phase motors. Create diffraction patterns in the audio range or in the visible spectrum. The two phases do not even need an electrical connection.

Also I can have phase relationships with signals having a harmonic relationship and very interesting Lissajous figures.

.

#### mivey

##### Senior Member
Certainly more diverse loads than most people here will see. Most here will only see typical single and three phase loads.

#### jim dungar

##### Moderator
Staff member
I think I have made it clear that to get two-phase from a center-tap transformer you have to use the neutral as a reference.
Thank you for finally clearly stating that the number of phases is dependent on the presence or absence of a neutral.

Your own labeling methodology would show that your premise does not hold water for a 5-wire four-phase system.
Remember I call this a 2-phase 5-wire system.

What problem? I have never said to treat a grounded conductor differently.

The primary is not the system being evaluated. The secondary is the system being evaluated. How I define that secondary system determines what I have.
By treating the primary and secondary of transformers differently, I believe, your system is simply a mathematical means to explain why there is an appearance of 2-phases when a center tapped point is used as a reference. After all, you have stated that the 'second phase' disappears if the neutral is not used.

#### gar

##### Senior Member
100315-1044 EST

Four wires from two separate secondaries constitute two phases. These two secondaries might be on the same leg of the same core and be in phase or 180 deg out of phase. If the correct two wires are connected together and the voltages of each secondary are the same, then these two secondaries become a center tapped secondary.

The two secondaries could be on separate transformers and if the frequencies are identical, then any arbitrary phase could exist between the two secondaries.

It is entirely possible to measure a phase relationship between the secondaries whether a conductive path exists between them or not.

.

#### winnie

##### Senior Member
IMHO much of this argument stems from the varied uses of the term 'phase'.

Phase can be used to describe the time displacement between two waveforms of the same frequency. With this usage, the units of measurement are angular units with a basis of a complete cycle. Under this usage of the term phase, a waveform and its inversion are 180 degrees out of phase.

Phase can be used to describe the particular portion of a waveform being considered.

Phase can be used to describe the number of _independent_ voltage sources of a system.

Phase can be used to describe the number of ungrounded terminals in a system.

By some of these usages, it is clear that multiple taps on a single transformer core can be described as being 'different phases'.

With that said, IMHO there is a key feature that distinguished 'polyphase' systems from 'single phase' systems of any number of terminals:

When you have a polyphase system, the time displacement of the various available waveforms is such that, with a suitably wound stator, or with suitable transformers, and with no time delay elements, you can develop a rotating magnetic field in a motor.

It doesn't matter if your single phase transformer has 2 terminals, 3 terminals with a center tap, or 100 terminals with a tap changer; if you want to run a motor with this transformer it will have to be a single phase motor, and will require some sort of phase displacement element such as a capacitor in order to develop a rotating field.

I am happy to say that legs A and B of a center tapped transformer are 180 degrees out of phase; but as far as rotating fields are concerned, any n*180 degree phase displacement is the equivalent of a 0 degree phase displacement.

When you look at a stator winding for developing a rotating field, you will also see that any supply legs that are 180 degrees out of phase are essentially redundant. The stator of a 5 wire system (neutral and 4 'phases' 90 degrees apart) is exactly the same as a stator of a 3 wire 2 phase system (neutral and 2 'phases' 90 degrees apart). As far as the development of a rotating field is concerned, they are both _2_ phase systems.

-Jon

#### mivey

##### Senior Member
Thank you for finally clearly stating that the number of phases is dependent on the presence or absence of a neutral.
I'm glad you finally recognized it. I was wondering how many times I was going to have to say it. To be clear, it is not just the presence or absence but how we use it.
Remember I call this a 2-phase 5-wire system.
Then you are in conflict with your "count the number of L-L voltages" rule because there are also four L-L voltages.
What problem? I have never said to treat a grounded conductor differently.
Based on your posts, you most certainly have. Here is a recent occurrence:
...Neither a 120/240V nor a 120/208V circuit meets the above definition of two-phase, as there is only a single L-L current, therefore the L-N currents are not distinct from each other.
I also know that you have said in the past that the 3-wire wye is not two-phase. Be kind and just admit it so I don't have to look it up. When you say the 3-wire wye is not 2-phase, you are treating the grounded conductor differently.
By treating the primary and secondary of transformers differently, I believe, your system is simply a mathematical means to explain why there is an appearance of 2-phases when a center tapped point is used as a reference. After all, you have stated that the 'second phase' disappears if the neutral is not used.
The phase does not "disappear" if the neutral is not the reference, it just becomes redundant in the reference frame of the system we have defined.

Why is your arbitrary choosing of X1 as a reference point any less arbitrary than choosing X23?

I just remembered something else you did not address:
And all this time, there is a single current flowing through a single transformer winding.
Why do you think the same current would flow in both sides of the transformer?

A two-phase load would be a perfect example of where the currents are not the same. That would make the neutral conductor a grounded conductor, not just a connection to a neutral point. By your own rules, it would then be included when counting.

So we at least these un-revoled issues:
1) The issue with four L-L voltages in the 5-wire 90 degree system.
2) Ignoring the grounded conductor in the 3-wire wye.
3) Arbitrary selection of an X1 reference.
4) Claiming a single L-L current through a coil with a neutral

#### mivey

##### Senior Member
By some of these usages, it is clear that multiple taps on a single transformer core can be described as being 'different phases'.

With that said, IMHO there is a key feature that distinguished 'polyphase' systems from 'single phase' systems of any number of terminals:

When you have a polyphase system, the time displacement of the various available waveforms is such that, with a suitably wound stator, or with suitable transformers, and with no time delay elements, you can develop a rotating magnetic field in a motor.

It doesn't matter if your single phase transformer has 2 terminals, 3 terminals with a center tap, or 100 terminals with a tap changer; if you want to run a motor with this transformer it will have to be a single phase motor, and will require some sort of phase displacement element such as a capacitor in order to develop a rotating field.

I am happy to say that legs A and B of a center tapped transformer are 180 degrees out of phase; but as far as rotating fields are concerned, any n*180 degree phase displacement is the equivalent of a 0 degree phase displacement.

When you look at a stator winding for developing a rotating field, you will also see that any supply legs that are 180 degrees out of phase are essentially redundant. The stator of a 5 wire system (neutral and 4 'phases' 90 degrees apart) is exactly the same as a stator of a 3 wire 2 phase system (neutral and 2 'phases' 90 degrees apart). As far as the development of a rotating field is concerned, they are both _2_ phase systems.

-Jon
Thank you Jon. That is a great example of how one addresses a variance from the general methodology. You clearly laid out a common-sense and logical explanation. You do not ignore the physics of the situation but show why you would not consider the other phases unique for a motor load.

It is not that they do not exist, they are just not unique enough in that situation for them to be put in separate buckets.

It is a labeling method that has been modified from the general case with a reasonable explanation as to why you would do so. It seems to be a good fit for motor loads.

#### mivey

##### Senior Member
...Why do you think the same current would flow in both sides of the transformer?...
That should have been: "both sides of the winding"

#### mivey

##### Senior Member
That should have been: "both sides of the winding"
In other words:
With a neutral, the current on one side of the coil can be out of phase with the current on the other side of the coil.

So by the current criteria I think you specified: two currents out of phase with each other = two-phase.

#### jim dungar

##### Moderator
Staff member
Why is your arbitrary choosing of X1 as a reference point any less arbitrary than choosing X23?
It isn't, but if any reference other than neutral is chosen, you have said, the 2-phase no longer exists. So why must the neutral point be the reference in order to determine the number of phases?

I just remembered something else you did not address:Why do you think the same current would flow in both sides of the transformer?
I was discussing the single current on the primary side of the transformer creating a corresponding current 'direction' in a single center tapped secondary winding.

A two-phase load would be a perfect example of where the currents are not the same. That would make the neutral conductor a grounded conductor, not just a connection to a neutral point. By your own rules, it would then be included when counting.
I did not say that a grounded conductor must be counted, only that it may be. It is you who is putting requirements on which conductors must be used. I am looking for a definition that does not change based on which reference is used.

So we at least these un-revoled issues:
1) The issue with four L-L voltages in the 5-wire 90 degree system.
There are two pairs of two L-L voltages. L1-L2 and L1'-L2'. L1-L1' and L2-L2' were never considered as valid connections (similar to the high leg in a 240/120 connection). This is why I refer to this as 2-phase 5-wire.

2) Ignoring the grounded conductor in the 3-wire wye.
With your logic, the presence or absence of a center tapped neutral changes the number of phases, but for some reason it doesn't affect a wye connection. With my logic it makes no difference.
3) Arbitrary selection of an X1 reference.
Not all grounded connections use a neutral, not all 'non-end point' taps are neutrals. For example, there is a standard control power connection of 24/120V (x1-X23= 24V, and X1-X4=120V).
4) Claiming a single L-L current through a coil with a neutral
I believe I have said there is a single current direction, created by a single magnetic field direction, such as X1->X23->X4. But through the magic of math, X1->X23 and X23->X1 can be interchanged, by following proper 'signing' rules, giving the appearance of two currents.

Do you say a high-leg 4-wire delta (one winding is center tapped) has these 3-phases: 2@180?, and 1@90? while ignoring the 3@120?? If you mention them all is this a 6-phase transformer?

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#### mivey

##### Senior Member
It isn't, but if any reference other than neutral is chosen, you have said, the 2-phase no longer exists. So why must the neutral point be the reference in order to determine the number of phases?

I was discussing the single current on the primary side of the transformer creating a corresponding current 'direction' in a single center tapped secondary winding.

I did not say that a grounded conductor must be counted, only that it may be. It is you who is putting requirements on which conductors must be used. I am looking for a definition that does not change based on which reference is used.

There are two pairs of two L-L voltages. L1-L2 and L1'-L2'. L1-L1' and L2-L2' were never considered as valid connections (similar to the high leg in a 240/120 connection). This is why I refer to this as 2-phase 5-wire.

With your logic, the presence or absence of a center tapped neutral changes the number of phases, but for some reason it doesn't affect a wye connection. With my logic it makes no difference.
Not all grounded connections use a neutral, not all 'non-end point' taps are neutrals. For example, there is a standard control power connection of 24/120V (x1-X23= 24V, and X1-X4=120V).
I believe I have said there is a single current direction, created by a single magnetic field direction, such as X1->X23->X4. But through the magic of math, X1->X23 and X23->X1 can be interchanged, by following proper 'signing' rules, giving the appearance of two currents.

Do you say a high-leg 4-wire delta (one winding is center tapped) has these 3-phases: 2@180?, and 1@90? while ignoring the 3@120?? If you mention them all is this a 6-phase transformer?
It doesn't just appear to be two currents. There are two different currents in the secondary winding. These are actually two different fluxes seen by the primary winding.

More later. I have to go.

#### Hameedulla-Ekhlas

##### Senior Member
Long Discussion, I dont want to interfere you but I have some link regarding Single phase, two phase and three phase. These topics hope to help you

See this tipic from one of these links

During the early 1890s, understanding the behavior of simple single-phase ac was enough of a challenge. It was not until Charles P. Steinmetz, the legendary General Electric scientist, developed the concept of the use of the "j" operator (unity magnitude at a 90? phase angle) and complex numbers for ac circuit calculations that the behavior of voltages and currents in ac circuits and machines was truly understandable. Likewise, it was not until the introduction of what eventually came to be known as "symmetrical components," during the early 20th century, that the calculation of three-phase voltages and currents became relatively straightforward. This technique utilized an "a" operator that was of unity magnitude at a 120? phase angle (?0.5 + j0.866). This operator was of significant value since, in a balanced 3 phase system, the voltages and currents are at 120? phase relationships to each other.

Symmetrical components actually facilitated calculations in unbalanced 3 phase circuits. They were originally known as "Fortescue components" since the method was introduced in 1918 by Charles L. Fortescue of the Westinghouse Electric Corporation. Significant additional work in this area was later contributed by Edith L. Clarke of the General Electric Company. During the late 19th century, however, this calculation tool did not exist, and the fact that changes in voltage or current magnitudes in one phase of a three-phase system affected the voltages and currents in the other two phases contributed to the difficulty in understanding 3 phase circuits.

Thus, the first ventures into the realm of polyphase (multiple phases) electric power used only two alternating current phases rather than three. The two phases were generated with a 90? phase difference between them, and the system that resulted was called 2 phase power. In fact, the first two-phase generators employed during the early 1890s were merely two single-phase machines coupled together with their rotors carefully set relative to each other so as to achieve the required quadrature phase relationship. Each generator, then, really fed a separate two-wire, single-phase circuit. Since the two phases were completely electrically isolated from each other, there were no interactions between voltage and current magnitudes in one phase with those quantities in the other phase. Therefore, from a theoretical standpoint, the 2 phase system was more easily understood than was the three-phase system.

The two phases were used together in a four-wire system to enable the operation of the new Tesla (or induction) motor that had been developed by Nikola Tesla. In order to be self-starting, the Tesla motor required some form of rotating magnetic field that had to be produced by a polyphase type of supply. The two-phase system was adequate for this purpose. The Westinghouse Electric Corporation supplied the power plant and lighting for the Colombian Exposition in Chicago in 1893. 2 phase power, produced by pairs of coupled single-phase generators, was used throughout this installation.

Page number 84 Topic Three phase/Two phase

http://www.3phasepower.org/2phasesystems.htm