Single Phase or Polyphase?

Status
Not open for further replies.

jclint07

Member
Location
south missouri
I've been told over years Polyphase is not as common as use to be. Never had any experience with such, so bare with me as I post this question.
OK, say 3 phase 120/208Y is available in a store. If I install a single pole circuit, being one ungrounded conductor, one grounded conductor, and one grounding conductor(of course), THAT circuit is considered SINGLE PHASE, even though it is fed off a 3 PHASE Branch Circuit Panel. But what if I install a two-pole circuit, for instance a dryer, two ungrounded conductors, one neutral, and one grounding conductor(of course), off of same 3 PHASE Branch Circuit Panel, would that circuit be considered SINGLE PHASE or POLYPHASE? I've been taught that it is STILL a SINGLE PHASE circuit. Just curious. :)
 

mivey

Senior Member
You only have one voltage waveform so you only have one phase.

When referenced to the neutral, you start out with one waveform from the neutral to "A" and a waveform from the neutral to "B". When you connected across "A" and "B", the voltage is the difference from "A" to "B" (or "B" to "A"). When you subtract two voltages, you wind up with a single resultant voltage, i.e. one phase.

Add: I should add that the waveforms have to have different angles to be counted separately.
 
Last edited:

mivey

Senior Member
...two ungrounded conductors, one neutral, and one grounding conductor(of course), off of same 3 PHASE Branch Circuit .
FWIW, if you use single phase loads both line-line and line-neutral from that feed, you are using multiple single phase systems: The 120 volt is part of a system of voltages and 208 is part of a system of different voltages. In a multi-phase system, each system is comprised of voltages having the same magnitude.

So, a 3-phase 120/208 supply can deliver more than one type system: 120 volt single phase, 208 volt single phase, and 208 volt three phase to name a few.
 

gar

Senior Member
100311-1734 EST

Using a more liberal definition of a multiple phase system I could say:

With three wires from a wye source I have a three phase system. Any two of the wires would be single phase. By using A, B, and N this would not be a normal three phase system, but each pair of wires has a significantly different phase angle from the others. Appropriately use two transformers from these three wires and I could get back to a balanced 120 deg three phase system.

.
 

mivey

Senior Member
By using A, B, and N this would not be a normal three phase system, but each pair of wires has a significantly different phase angle from the others.
You would have a two phase system. This is labeled a "network" system in some texts. Maybe to distinguish it from the old two phase system with a 90 degree phase difference between the voltages.

Changing from 90 degrees to 120 degrees does not mean it is no longer 2-phase. Intentional angles of 89 or 91 or 119 or 121 would be examples of other two phase systems. A historic label does not re-define what it is. We might eventually run out of unique labels for different configurations, but the basic definition will remain the same.

Add: The pairing of A & B would be part of a separate system of voltages because it has a different magnitude.
 
Last edited:

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Maybe to distinguish it from the old two phase system with a 90 degree phase difference between the voltages.

Changing from 90 degrees to 120 degrees does not mean it is no longer 2-phase. Intentional angles of 89 or 91 or 119 or 121 would be examples of other two phase systems. A historic label does not re-define what it is. We might eventually run out of unique labels for different configurations, but the basic definition will remain the same.

This is from the 'New Catechism of Electricity by N. Hawkins' copyrighted in 1896, taken from http://antiquesockets.com/NEC-LIB.

Multiphased currents are two or more separate and distinct [my emphasis] derived from an ordinary "single phase" alternator. Their peculiarity lies, not in the nature of the currents themselves, but in the fact that they have different strengths at a given instant of time.

In the two-phased system, for instance, when one of the currents is at zero value, the other has its maximum value, or the currents are displaced in phase, whence the expression two-phased.

If two identical simple alternators have their armature shafts coupled in such a manner, [that] when one is directly under a field pole, the corresponding coil on the other is midway between two poles of its field, the two currents generated will differ in phase by a half-alternation, and will be two-phased currents; similarly, three-phased currents could be generated by coupling the armatures of three simple alternators so that the corresponding coils on each are equally "staggered" with respect to each other.
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. {paragraph 1}

Ah, you say, but what if I can describe multiple current directions?
Adding a neutral point to a circuit does not create multiple phases, it simply creates an arbitrary, although familiar, reference point, because one current is not at a maximum value when the other is at zero. {paragraph 2}

[My soap box.]
Our electrical industry chose to discuss our primary electrical system as Alternating Current not alternating voltage. So, the correct phrase should be 'the voltage from A-phase conductor to B-phase conductor'. But because familiarity breeds laziness, we dropped the word conductor and we say 'the voltage from A-phase to B-phase', and in the process we began to think of a 'phase' as a touchable, measurable, item and not a concept or description.[/My Soapbox]
 
Last edited:

jclint07

Member
Location
south missouri
Wow. Where do you come up with these very explanative answers, so I could study up ( and sound smart, too :D ) Not to add too much fuel to fire, but I have to ask, then what exactly is Polyphase AC? (since I've yet to come across it.) I apologize if it was defined in prior post, I must have missed it.
 

mivey

Senior Member
Wow. Where do you come up with these very explanative answers, so I could study up ( and sound smart, too :D ) Not to add too much fuel to fire, but I have to ask, then what exactly is Polyphase AC? (since I've yet to come across it.) I apologize if it was defined in prior post, I must have missed it.
The canonical rule for the number of phases is that the # of phases equals the number of equal magnitude and equal frequency waveforms that have a different phase angle and where waveforms of opposite polarity (180? phase angle difference) are counted as separate phases. Waveforms are taken from any two points as long as the set of points is only used once.

If they are not evenly displaced, then it is not a canonical polyphase system but a system with multiple phases.

Add: For example, the historic 2-phase system was not a canonical poly-phase system but was just a subset of a canonical 4-phase system.
 
Last edited:

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
...what exactly is Polyphase AC? (since I've yet to come across it.) I apologize if it was defined in prior post, I must have missed it.

Polyphase means more than one phase.
Multi-phase also means more than one phase.

So for most purposes polyphase=multi-phase.
 

mivey

Senior Member
This is from the 'New Catechism of Electricity by N. Hawkins' copyrighted in 1896, taken from http://antiquesockets.com/NEC-LIB.
If you think the source you quoted is saying that that his example of a specific two-phase current configuration defines all two phase current configurations from then to eternity then you are not understanding what he said.

Allow me to paraphrase your source: "As an example, in the type of two-phased system we currently have, when one of the currents is at zero value, the other has its maximum value, or the currents are displaced in phase, whence the expression two-phased."
Neither a 120/240V nor a 120/208V circuit meets the above definition of two-phase,
That is why you are not understanding the general definition. You focus on a specific system that was used, not the general case involving more than one phase. You quoted an example, not a definition.
as there is only a single L-L current, therefore the L-N currents are not distinct from each other. {paragraph 1}
The line-line system is a single phase system with loads connected line-line. The neutral has nothing to do with it until you blend it with a line-neutral single phase system.

The line-neutral system with different loads connected line-neutral can be seen as having two phases. If you use the neutral as the common tie point, you have combined the two single phase systems in such a way to make a two-phase system.
Ah, you say, but what if I can describe multiple current directions?
Adding a neutral point to a circuit does not create multiple phases, it simply creates an arbitrary, although familiar, reference point, because one current is not at a maximum value when the other is at zero. {paragraph 2}
And that is where you fail to grasp the general definition of a multi-phase system. You just can't seem to get past that point. Until you can see that concept, you will be able walk the path right up to the point where the road makes that final one-degree change, and then you have a waveform that is 180 degrees different from another waveform, and you will always hit that wall.

However, maybe it will help you if you will look in your history books at the historic 2-phase system that you are comfortable with. This system was used as two single-phase systems as well as being combined to form one two-phase system. You will find that they made what they called a four-phase system by tying the center points of the two phase system. In other words, they split the coil of the two-phase system to make four single-phase systems and combined them in a manner that gave them a four-phase system.

It is the same method used to split a winding today. Only today it seems to cause some people to stumble because they can't get beyond the labels and terms they have become accustomed to. It is a just matter of simple reasoning. Once you can see that, you can see the bigger picture of how these labels we use today were derived as well as their limitations.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The line-neutral system with different loads connected line-neutral can be seen as having two phases. If you use the neutral as the common tie point, you have combined the two single phase systems in such a way to make a two-phase system.And that is where you fail to grasp the general definition of a multi-phase system. You just can't seem to get past that point. Until you can see that concept, you will be able walk the path right up to the point where the road makes that final one-degree change, and then you have a waveform that is 180 degrees different from another waveform, and you will always hit that wall.

You cannot grasp that you are changing historical definitions to suit your needs, when you argue that the presence or absence of a neutral defines the number of phases created by a single winding in a single magnetic field.

You only seem to get two "180? different" waveforms because you chose to change your reference point from an outside point of the winding to its midpoint. This is nothing more than "fun with math".

And, what you call a 4-phase system (two 2-phase systems combined) I learned as a 2-phase 5-wire circuit, again because we did not depend on the presence of a neutral to define our systems.
 

mivey

Senior Member
You cannot grasp that you are changing historical definitions to suit your needs, when you argue that the presence or absence of a neutral defines the number of phases created by a single winding in a single magnetic field.
The result would be the same with two windings in two magnetic fields. The resultant system is the same.

Also, specific system configurations do not constitute a definition for all configurations. Suppose we set up a form 9S metering configuration and we all agree that it is a 3-phase metering configuration. That does not mean that the form 5S configuration is not a three phase metering configuration.
You only seem to get two "180? different" waveforms because you chose to change your reference point from an outside point of the winding to its midpoint. This is nothing more than "fun with math".
No, I have a different system configuration. If you think both configurations are exactly the same, then stop using polarized plugs and you may find that it is about definitions, not fun.
And, what you call a 4-phase system (two 2-phase systems combined) I learned as a 2-phase 5-wire circuit, again because we did not depend on the presence of a neutral to define our systems.
Then you only learned part of the story and are letting that be your limitation.
 

mivey

Senior Member
The key to understanding any system is understanding how it is defined. That does not mean that the same system can't be reconfigured and defined a different way. I can take a corner-grounded delta and move the ground to a center-tap. In doing so, I haven't changed any transformers but have reconfigured the system and the way it is defined.

To get beyond common labels and configurations, you have to look at things from a broader perspective. They cause confusion because in our tendency to generalize, we have dropped the reasonings that made the label fit a particular case. If someone doesn't understand the broader picture, they may not understand the limitations of the labels they are using.

We try to take the existing labels and use them to encompass more than the specific case they actually fit. The logic behind why the label works for the specific case is lost and you are left with a partial formula. If someone tries to use the partial formula to apply the label elsewhere the result may not fit and you have confusion.

If you can understand the logic that shows why the label works for the first case, you might could use the label in another place and form the proper caveats for the second case. If you understand a train of logic that works for the general case, you can create the caveats for any label.

First, let's follow a train of logic for the 90 degree displaced system as it has been well documented historically. The arrows help identify the defined reference angles I am using for this walk-through. Follow the pictures left-to-right, top-to-bottom:

90deg2-phaseand4-phase.jpg



Now let's use the same logic with 120 and 180 degree systems:

120and180deg2-phase.jpg
 

mivey

Senior Member
again because we did not depend on the presence of a neutral to define our systems.
And I would say it would be better to count the number of equal magnitude but phase-displaced waveforms. I admit when I first heard you say it, it sounded good as I had not given the labeling a whole lot of thought. I have since found out that there are holes in your methodology. Refer to the graphic that follows and let's go through the results of the configurations depicted:

Using the graphic below (problems noted in parenthesis):
#1: center-tapped winding = single phase
#2: grounded wye = three phase
#3: ungrounded delta = three phase
#4: historic 2-phase = two phase
#5: grounded open-wye = single phase
#6: 5-wire historic 2-phase = four phase (does not agree with your previous assertion)
#7: 3-wire historic 2-phase = single phase (does not agree with your previous assertion)
#8: corner grounded delta = single phase (obviously does not work)


What attempts could you make to fix the problems?
To fix #6 you will have to say that a center-tapped winding does indeed double the number of phases and that it is indeed four phases, not two. That would create issues with #1 because it would become two-phase.

To fix #7 to get back to two-phase, you will have to concede that the common point is actually a point where you connect a phase conductor. When you make that concession, you also have to make the same concession for #5 and then you have created another issue because #5 becomes two-phase.

To fix #8, you will have to concede that a common point can indeed be a point where you would connect a phase conductor. Same issues as #7.

The problem is that you are trying to mix configurations, different systems, and load types. The methodology is flawed.


Let's try a more generalized method that will work for any case:
Now let's look at the general methodology and how we derive the labels we use. I will state the method again: The # of phases equals the number of equal magnitude and equal frequency waveforms that have a different phase angle and where waveforms of opposite polarity (180? phase angle difference) are counted as separate phases. Waveforms are taken from any two points as long as the set of points is only used once.

This identifies what systems of voltages are available, not necessarily how many are used or how they are used. That means that one bank of transformers can deliver multiple system configurations. We might also deliver multiple system types to loads. A lot of times the label we use is based on the type loads we have.


Let's look at a 3-phase wye for an example:
There are 4 types of systems delivered: #1) three L-L voltages, #2) three L-N voltages, #3) two L-N voltages #4) one L-N voltage. Let's look at some examples of what we have and what we could use for each type of system:

#1A) 3 phases available, 3 phases used per load, label = 3-phase

#1B) 3 phases available, 1 phase used per load, label = 3-phase. Note: Here is where the labeling has to be clear. We have a three phase system delivered but we are actually using it as a combination of three single-phase systems. As long as you understand it, it is OK.

#1C) 3 phases available, 2 phases used per load, label = 2-phase. Note: Loads that might use two phase could be an power supply, phase converter, or something else that requires two voltages with a phase displacement.

#2A) 3 phases available, 1 phase used per load, label = 3-phase. (See 1B note)

#2B) 3 phases available, 2 phases used per load, label = 2-phase. (see 1C note)

#3A) 2 phases available, 1 phase used per load, label = network or single-phase

#3B) 2 phases available, 2 phases used per load, label = 2-phase

#4) 1 phase available, 1 phase used per load, label = 1-phase


What have we discovered?
So you can see that although a system may actually be a higher order (the # of phases available), traditional labeling methods may use the load to pick a label with a lower order. The problem that shows up is that people try to use this load-determined label to label the system in general.

What gets lost in the translation is why the system was labeled the way it was. If you remove the restrictions placed by the load, the system may have a higher order than the label would indicate.


Two-phase example:
As another example, lets consider the 5-wire system with 90 degree displacements. There are four phases available so it is a 4-phase system. You can just use two phases and label it two-phase.

You can also deliver from one to four single phase voltages. As long as you understand the general system, you will have no trouble transitioning from one to the other. If you try to restrict yourself to one type configuration, you have turned a blind eye to what is really happening and you will have trouble trying to apply that restricted label method to other configurations and systems.


Finally:
You can refer to my previous post and graphic for some examples of how systems can combine and be reconfigured.

If you use the general method, the number of phases will agree with the historic accounting of poly-phase systems as well as the system orders used in analysis by symmetrical components. You will also be able to see how the different labels apply and what caveats go along with their use.


Graphic dealing with your methodology:
countingphasesbylinevoltage.jpg
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I never said to ignore grounded conductors, as you conveniently did in figure #8.

I have said that the use of an available neutral point should not be a requirement for determining the number of phases.

Given a street lighting transformer, with a single 2400V input and a 240/480 output (the NEC does not require this system to have its neutral point grounded).

My two waveform measurement possibilities are X1-X23 and X23-X4.
If X1 is our reference point, the two waveforms are seen as being in phase
If X23 is the reference, the two waveforms are seen as being 180? apart
If X4 is our reference point, the two waveforms are seen as being in phase

This is simply 'sleight of hand', where the number of phases in a system depend on which reference point is chosen.
 

kwired

Electron manager
Location
NE Nebraska
I think OP question is really "what does polyphase mean"

simple answer - more than one phase

A three phase system is a polyphase system. and they are very common just the term polyphase is not very common in the field.
 

mivey

Senior Member
I never said to ignore grounded conductors, as you conveniently did in figure #8.

I have said that the use of an available neutral point should not be a requirement for determining the number of phases.
The methodology flaw is obvious isn't it:
Nothing convenient about it. This was based on your premise that the open wye of figure #5 was single phase. When you say that, then you are ignoring grounded conductors. I made that illustration to point out one of the flaws in your methodology. It makes it blatantly obvious doesn't it? I covered this in my previous post but I will cover it again with slightly more detail.


Displaced neutral vs true neutral point:
When you compare #7 and #5, it becomes obvious that there is a difference between a common or grounded point and a neutral point. As I have pointed out in previous posts, the center-tap grounding point of a 3-phase delta is the neutral point of the single-phase system but is not the neutral point of the three-phase system, although we commonly call it the neutral in general. It is really just a grounded conductor.

The distinction is that all voltage vectors from the true neutral point to any system voltage point will sum to zero. That means some of the points we have labeled as a "neutral" are displaced from the true neutral point, and are really just a common or reference point.


The fix & the consequences:
Now you can see why the 3-wire two-phase of #7 is still a two-phase system: a wire from the grounded point is a conductor. If you recognize this fact, you must also concede that 3-wire open-wye of #5 has a grounded conductor that comes from a "displaced neutral" and must be included in the count. With that, we have the fact that the 3-wire open-wye is also a 2-phase system. A fact that you have previously denied because it did not match the label that you were trying to force upon the system.


It is a classic example of the confusion caused by labeling conventions:
This is a classic example of why trying to adhere to some labeling convention without the proper understanding of the caveats causes confusion. Normally, the 3-wire open-wye system of #5 serves single-phase L-L loads as well as single-phase L-N loads. It is able to supply two different type single-phase systems. That does not mean the it is a single phase system, only that it is being used for a single-phase supply. The two-phase supply is there as well if we had a load that would make use of the phase-displaced voltages.


Given a street lighting transformer, with a single 2400V input and a 240/480 output (the NEC does not require this system to have its neutral point grounded).

My two waveform measurement possibilities are X1-X23 and X23-X4.
If X1 is our reference point, the two waveforms are seen as being in phase
If X23 is the reference, the two waveforms are seen as being 180? apart
If X4 is our reference point, the two waveforms are seen as being in phase

This is simply 'sleight of hand', where the number of phases in a system depend on which reference point is chosen.
OK, now that we solved the issue with the displaced neutral, let's consider the neutral that is located at the true neutral point.

A reference point is a must:
Any voltage measurement must have a reference point. Defining a reference point is not 'sleight of hand' but is something that we must do. Changing a reference point is not 'sleight of hand' but redefines the system we are investigating. That is because the system we define is relative to our reference. As I have said many times, it is the very definition that is the basis for what we are going to discover with our measurements.

An example of why it is nothing magical:
If I tell you I am operating a circuit using a 6 volt and 12 volt input, that does not mean it will not kill you if you reach out and touch it. The voltage may be hundreds of volts above your reference point. The reference point used for the system defines the system. I could also have a system that operates at +- 6 volts. This is not the same thing as a system that operates at +6 and +12 volts. One uses only a positive voltage input with a mid-point tap. The other uses a +- voltage input. Both can be ultimately be supplied by a single 12 volt center-tapped source. The distinction is the reference point. There is nothing magical about it.

Two-phase "magic":
Let's look at the "magic" produced by the 5-wire system with 90 degree displaced voltages. There are two larger L-L voltages: The voltages across a complete coil. But there are also four smaller L-L voltages. There was no 'sleight of hand' used to create these four L-L voltages. These are real L-L voltages with 90 degree displacements. To ignore this fact is to ignore what is really there: One system with large voltages and one system with smaller voltages.


In summary:
As I discussed previously, we have other sources that can supply more than one type system. How these sources are used may be the basis for how they are labeled. But, how they are labeled or how they are used does not change what is ultimately available at the source. You must understand that the limitation of the label does not translate into a limitation of the supply.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Do not obfuscate by including high-leg delta systems, to not get into open wye configurations. Do not belittle me, by inferring I do not know the difference between a neutral point and a grounded conductor.

Given the example of an single primary winding transformer and a two winding secondary (X1-X3 and X2-X4 connected in series).

The single waveforms are:
a) X1-X23
b) X1-X4
c) X23-X4
Do you agree that each one of these is a single phase waveform?

The possible paired waveform combinations are:
d) X1-X23 and X1-X4
e) X23-X1 and X23-X4
f) X4-X23 and X4-X1
Are you saying that all of these combinations are 'two-phase', even if some of the waveforms are identical except for magnitude?
 

mivey

Senior Member
Do not obfuscate by including high-leg delta systems, to not get into open wye configurations.
:-? The purpose of the illustration was to emphasize a point, not to cover up a point, or to distract.
Do not belittle me, by inferring I do not know the difference between a neutral point and a grounded conductor.
I have great respect for you and have no desire to belittle you. Don't make this personal as I am critiquing based on what you have said. My primary focus is on what is being said and your position. Let's keep this on a professional level.

We also must consider the other readers so I'm sure in my attempt to be thorough I have stated many things that you know already. I also cover some basic info because I want you to follow my thought process.
Given the example of an single primary winding transformer and a two winding secondary (X1-X3 and X2-X4 connected in series).

The single waveforms are:
a) X1-X23
b) X1-X4
c) X23-X4
Do you agree that each one of these is a single phase waveform?
Yes. Given the reference direction you have used: X1-X4 alone is a single phase system with a higher voltage. X1-X23 alone is a single-phase system with a lower voltage. X23-X4 alone is a single-phase system of a lower voltage.
The possible paired waveform combinations are:
d) X1-X23 and X1-X4
e) X23-X1 and X23-X4
f) X4-X23 and X4-X1
Are you saying that all of these combinations are 'two-phase', even if some of the waveforms are identical except for magnitude?
No.

X1-X23 and X1-X4:
X1-X23 and X1-X4 are two separate single-phase supply systems because they have different magnitudes.

If you have the X1-X4 system and want to know its relation to X1-X23, X1-X23 is just a tap in the single-phase X1-X4 system. Alone, X1-X23 can become a single-phase supply for a separate system, but it is not a separate phase in the X1-X4 system.

X23-X1 and X23-X4:
X23-X1 and X23-X4 can be two separate single-phase supply systems because they have different phase angles.

In what we normally see, these are used to supply single-phase loads so are being used as separate single-phase supplies. Combining the single-phase wiring allows us to save one conductor run. This combination is not being used as a two-phase supply, but as two single-phase sources. As a bonus we have the X1-X4 larger-voltage single-phase supply as well.

Because X23-X1 and X23-X4 have the same magnitude and separate phase angle, they can also be used together as a two-phase supply. This would require a load that needs two equal magnitude voltages with a phase angle displacement.

X4-X23 and X4-X1:
X4-X23 and X4-X1 is the same case as X1-X23 and X1-X4 but I'll state it for completeness.

X4-X23 and X4-X1 are two separate single-phase supply systems because they have different magnitudes. If you have the X4-X1 system and want to know its relation to X4-X23, X4-X23 is just a tap in the single-phase X4-X1 system. Alone, X4-X23 can become a single-phase supply for a separate system, but it is not a separate phase in the X4-X1 system.
 
Last edited:
Status
Not open for further replies.
Top