single vs. 3 phase

[rattus]

Ask Winnie:

Ask Winnie:

What is the difference? If you can use a transformer to manipulate the sources into deriving a higher order of phases (say 3 to 6), why can't you do the same for 1 to 2 or is it just a coincidence that they are inverses?

Winnie just answered your question rather eloquently.

 

[winnie]

I just wonder why it is valid to use transformer manipulations to derive higher-order phase systems when you have 3-phases, but it becomes invalid when you have only 1-phase. It would seem to me that in both cases you are shifting reference points around to create something different.

If you have a single phase system, with a center tap or not, the only thing you can create using transformer manipulation is another single phase system. This derived system may or may not have a center tap.

The available power which may be delivered to a load in either a single phase or single phase center tapped system varies with the AC cycle, and falls to zero twice a cycle.

Contrast this with any true polyphase system, where power is continuously delivered by the combination of phases, and a suitable transformer arrangement can create _any_ other number of phases without non-linear elements or energy storage. You can go from 90 degree 2 phase to 3 phase 4 phase (with either 90 degree or 45 degree separation), 5 phase, or even 17 phase should you wish.

If you choose to call a single phase center tapped system '180 degree two phase', you are still limited in that you can only transform it to another single phase system.

-Jon

 

[mivey]

From my point of view, the 360/n rule doesn't work all that well.

In an electric motor, what matters is the direction of current flow in the slots. I can always get a 180 degree phase difference by simply running the wire in the opposite direction; if you look at an ordinary 3 phase, 2 pole motor, you will find _6_ phase bands; A, C', B, A', C, B'. The difference between 'primed' and the 'not primed' phase bands is the direction of the wire in that part of the winding. So with a conventional _three_ phase supply, I can define _6_ different stator phase angles.

If you were to use the 360/n rule for 6 phases, then you would have 6 phases spaced 60 degrees apart. Call these phases A, B, C, D, E, F. If you wound such a '6 phase' motor, then you would find that the phase D belt was redundant with the A' belt. In other words, a 360/n 6 phase motor would be no different than a normally wired 3 phase motor.

IMHO 'legs' 180 degrees apart should be considered the same phase.

In the 18 phase motors that we've built, we used the phase angles 0,10,40,50,80,90,120,130,160,170,200,210,240,250,280,290,320,330.

On the other hand, I recall that EPRI did some research on high phase order power transmission; and they did use 360/n phase displacement for n=12 and n=18. This had something to do with pushing more power through the same right of way without raising voltage or current.

-Jon

I'm not promoting calling 180 degrees "2-phase", just discussing it, as well as the 360/n rule. Since the 3-phase motor has a pole pair (north-south) for each phase, wouldn't a 6-phase motor have 12 poles?

The armature of a 4-pole generator only has to rotate 1/2 turn to complete a cycle. If we are going to use the rotor rotation or physical degrees around the motor, we should use a different rule to define the degree separation: degrees = 720/(2n)

[edit: given the 18 phase motor, it does not appear that it matters if the poles are evenly spaced. Wouldn't it run smoother if they were? Why not 0,20,40,60,80...?]

[edit: changed 18-pole typo to 18 phase]

 

[mivey]

If you have a single phase system, with a center tap or not, the only thing you can create using transformer manipulation is another single phase system. This derived system may or may not have a center tap.

The available power which may be delivered to a load in either a single phase or single phase center tapped system varies with the AC cycle, and falls to zero twice a cycle.

Contrast this with any true polyphase system, where power is continuously delivered by the combination of phases, and a suitable transformer arrangement can create _any_ other number of phases without non-linear elements or energy storage. You can go from 90 degree 2 phase to 3 phase 4 phase (with either 90 degree or 45 degree separation), 5 phase, or even 17 phase should you wish.

If you choose to call a single phase center tapped system '180 degree two phase', you are still limited in that you can only transform it to another single phase system.

-Jon

Of course, a linear system can only produce another linear system (see #518). Maybe we should say that a polyphase system must be non-linear in that any two voltage differences, measured using at least three different supply conductors, are non-linear.

[edit: this would, of course, knock out the previous coined "4-phase" and reduce it back to "2-phase". But, you could have 4-phase with something like 0,80,180,260 degrees]

 

[winnie]

I'm not promoting calling 180 degrees "2-phase", just discussing it, as well as the 360/n rule. Since the 3-phase motor has a pole pair (north-south) for each phase, wouldn't a 6-phase motor have 12 poles?

I believe that you are misunderstanding the use of poles and phases in a motor.

The number of poles is the number of magnetic north and south poles. The number of phases describes the electrically separate coils that produce the poles. It is entirely possible to have 2, 3, or more phases yet only have a 2 pole motor.

The 18 _phase_ motor that we built was actually only a 2 _pole_ machine. The seemingly unequal spacing of the phase angles was to minimize the redundancy; note however that the system could be considered two balanced 9 phase systems slightly rotated.

-Jon

 

[winnie]

Of course, a linear system can only produce another linear system....QUOTE]

I'm sorry, I misread how you were using term linear. You are using it to describe a tapped single phase system, where if you plotted all of the output phasors, they would be in a straight line.

I was using the term linear in its common engineering sense to mean that the output of a device (such as a transformer) must follow certain rules relative to its input.

If you represent the available voltages produced by a system as phasors, if those phasors lie in a straight line (your use of linear), no amount of addition of those phasors can produce anything off that line. If they fall anywhere off that line, then by suitable combination you can get any other phasor.

-Jon

 

[mivey]

I'm not promoting calling 180 degrees "2-phase", just discussing it, as well as the 360/n rule. Since the 3-phase motor has a pole pair (north-south) for each phase, wouldn't a 6-phase motor have 12 poles?

I believe that you are misunderstanding the use of poles and phases in a motor.

The number of poles is the number of magnetic north and south poles. The number of phases describes the electrically separate coils that produce the poles. It is entirely possible to have 2, 3, or more phases yet only have a 2 pole motor.

The 18 _phase_ motor that we built was actually only a 2 _pole_ machine. The seemingly unequal spacing of the phase angles was to minimize the redundancy; note however that the system could be considered two balanced 9 phase systems slightly rotated.

-Jon

I was using the terminology from my westinghouse motor book (1967) where they depicted a three phase motor with 6 poles, each phase having a north and south pole (i.e., 2 poles/phase).

[edit: I guess I should have said "2 poles and 4 poles" instead of "2-pole and 4-pole"]

 

[mivey]

Of course, a linear system can only produce another linear system....QUOTE]

I'm sorry, I misread how you were using term linear. You are using it to describe a tapped single phase system, where if you plotted all of the output phasors, they would be in a straight line.

I was using the term linear in its common engineering sense to mean that the output of a device (such as a transformer) must follow certain rules relative to its input.

If you represent the available voltages produced by a system as phasors, if those phasors lie in a straight line (your use of linear), no amount of addition of those phasors can produce anything off that line. If they fall anywhere off that line, then by suitable combination you can get any other phasor.

-Jon

I couldn't think of a better term, I considered throwing "axis" in the mix but settled on "linear".

 

[mivey]

Oops, just noticed I typed "18-pole motor" instead of "18 phase motor" in #523.

Maybe that is what caused the confusion.

 

[mivey]

...The seemingly unequal spacing of the phase angles was to minimize the redundancy...

What does that mean, and what is the benefit?

...note however that the system could be considered two balanced 9 phase systems slightly rotated

As long as you don't rotate it 180 degrees because then you would just have an inversion and would be back to 9 phases.

 

[gar]

080423-0647 EST USA

The IEEE definitions that mivey quoted in post #36 seem adequate.

I have not read all of the posts in this thread so my following comments may be redundant.

A single phase source (service) can not produce a rotating magnetic field without some appropriate phase shifting means in the load, such as a "shaded pole", capacitor, or electronically synthesized phase.

Any polyphase source can produce a rotating magnetic field.

A single phase input to an ideal transformer can not produce a polyphase output no matter how the secondary is tapped.

Any two legs of a polyphase source produces a single phase output.

.

 

[mivey]

summary for gar

summary for gar

The IEEE definitions that mivey quoted in post #36 seem adequate.

But has problems because the methodology is not consistent

I have not read all of the posts in this thread so my following comments may be redundant.

All good points that have been made and are not in question using the term polyphase the way you have. There was some discussion of some references using the notion of 180 degree 2-phase. The IEEE noted that their 2-phase definition had a problem because it did not follow this pattern, which some references hold is the general case for polyphase circuits (i.e. a n-phase sytem has a 360/n degree displacement between the voltages).

Jim likes to use the number of L-L voltages but I'm not sure it works for the "5-wire 4-phase" case. You could have 4 L-L voltages. We have references that call this 2-phase, some call it 4-phase, some call it both.

If I steal Jim's idea and expand it, we could get rid of the "inverted" voltages. We could say an n-polyphase system is defined by n non-linear L-L voltages. Non-linear in this sense means that the phase angle difference between any two defining L-L voltages, must have a phase angle difference other than 0 or 180. The "180 degree 2 phase" then becomes single phase, the "90 degree 4 phase" becomes two phase, etc.

This idea allows for n-phase systems that do not conform to the 360/n degree displacement. This does not seem to be a problem as we have already done that with the traditional 2-phase system.

[edit: I forgot to mention, this would clean up the definitions used by the "neutral doesn't matter" camp. However, there is another camp who refuse to call 3-wire 120/208 single-phase because of the non-linear neutral point. Some references call this a network service]

 

[megloff11x]

This comes up often enough and causes enough consternation that maybe a short article/newsletter snippet is in order to explain:

single phase
two-phase
three-phase

And while on the subject, all the flavors of transformers and hookups and why the started and what they give us.

Matt

 

[rattus]

Exception:

Exception:

Rattus, given the 360/n rule and taking the case where n=2, it would appear you would disagree with this description and would say that a 2-phase system cannot exist.

This 2-phase notion was the OP's original question. It would appear that we call it single-phase by convention, but how would the center-tap reference that we call single-phase be different than a "real" 2-phase system?

I would add the restriction that n must be odd. Clearly the real 2-phase system does not fall under this rule.

 

[rattus]

But my point has been I can only think of two reasons (oscilloscope tracings and manufacturing convenience)to chose the neutral as the basis of voltage direction. But I have listed more than 2 reasons why I think it shouldn't.

Jim, the reasons to a neutral reference are:

1. It is a logical, valid approach to the problem. The results are the same, and any problem can be solved with a neutral (if available) reference.

2. Some prefer to use a reference wherever possible. I would even say that it is conventional. Some, if not all, analysis programs require the use of a reference.

3. It is simply a matter of 'druthers. You 'druther do it one way. Others 'druther do it another. I 'druther use the neutral reference because that is the way I learned it.

 

[jim dungar]

Jim, the reasons to a neutral reference are:

1. It is a logical, valid approach to the problem. The results are the same, and any problem can be solved with a neutral (if available) reference.

2. Some prefer to use a reference wherever possible. I would even say that it is conventional. Some, if not all, analysis programs require the use of a reference.

3. It is simply a matter of 'druthers. You 'druther do it one way. Others 'druther do it another. I 'druther use the neutral reference because that is the way I learned it.

These reasons are applicable to any method and do not justify a particular position other than "because it can be done".

I am not against a reference, I just see no advantage to using the neutral in a 3-W circuit versus using the physical connection of the transformer(s)

 

[winnie]

I am not against a reference, I just see no advantage to using the neutral in a 3-W circuit versus using the physical connection of the transformer(s)

I would argue that the actual physically connection of the transformer has little relevance unless you are analyzing the transformer itself. Instead it makes sense to use a reference that best fits the load. That was my whole point in bringing up things such as 'T connections' and the like.

A transformer with a 'T' connected secondary may be used as a 'wye' source. In such a case, I would use the normal everyday representation of three phasors 120 degrees apart, unless I was actually analyzing the transformer, when I would need to use something to represent the fact that there are only _2_ coils involved with a _90_ degree phase angle difference.

-Jon

 

[coulter]

I would argue that the actual physically connection of the transformer has little relevance unless you are analyzing the transformer itself. Instead it makes sense to use a reference that best fits the load. ...

I absolutely agree

... That was my whole point in bringing up things such as 'T connections' and the like. ...

Good point. But the conversation is about 3W, 1ph - not 3ph. Just because mivey and I grew up around electricians connecting house wiring is no reason to continue a limiting view for mathematical analysis. If 1ph analysis weren't trivial, this insistance on using an arbitrary "N" reference point would needlessly complicate the math model.

carl

 

[jim dungar]

I absolutely agree


Good point. But the conversation is about 3W, 1ph - not 3ph. Just because mivey and I grew up around electricians connecting house wiring is no reason to continue a limiting view for mathematical analysis. If 1ph analysis weren't trivial, this insistance on using an arbitrary "N" reference point would needlessly complicate the math model.

carl

Well stated.

 

[rattus]

Who?

Who?

I absolutely agree


Good point. But the conversation is about 3W, 1ph - not 3ph. Just because mivey and I grew up around electricians connecting house wiring is no reason to continue a limiting view for mathematical analysis. If 1ph analysis weren't trivial, this insistence on using an arbitrary "N" reference point would needlessly complicate the math model.

carl

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