May I ask a question about the single vs two phase stuff

[Sahib]

Read post #1. There seems to be a question to some of is there a 180 degree difference present.

My point is you can take two equal magnitude voltages in phase or two equal magnitude voltages with a 180 degree displacement away from a center tapped transformer. It is a fundamental concept that to define a voltage you can pick any reference point you want. You can pick the end of the winding or the center but it is up to you.

That some people don't get that the reference can be away from center is interesting to me. You can try to talk through it but if someone disagrees with that then you really go nowhere because that is just simple physics and not really a debate. Not much to discuss really other than try to help someone understand.

But that is not the part that causes most of the discussion.

Fundamentals aside, the fun with phases begins when you look at the definition of phase. Depending on the definition used, you can say there is one phase or you can say there are two phases. Both are valid because both definitions are valid in there own right.

To me, the error is to dismiss one or the other. I support both definitions.

Physically there is only one phase i.e single phase for 240/120V. But mathematically, center point is assumed 0 potential and then there are two phases with respect to center point. Still there is no restriction to confine 0 potential to center point alone: any number of points along the winding may be assumed at 0 potential and number of phases may be constructed accordingly but of no use .

 

[Sahib]

Still there is no restriction to confine 0 potential to center point alone: any number of points along the winding may be assumed at 0 potential and number of phases may be constructed accordingly but of no use .

Sorry there is restriction to confine 0 potential to one point only in the winding. Otherwise it would be a short.:slaphead:

 

[GoldDigger]

Once again, there is a distinct qualitative difference between the two "phases" present in a 120-0-120 scenario and any other phase count:
From a single phase (two terminal source of sine wave voltage) you cannot derive any other phase angles for your voltage phasor using only transformers. (Not allowing use of capacitors or inductors).
From two synchronized (same frequency) voltage sources at any relative angle except 0 or 180 degrees you can derive any phase angle from 0 to 360 using only transformers. Those two phasors form a basis for the phase space.
From two synchronized voltage sources at a relative angle of 180 degrees, you cannot derive any other phase angle using transformers.

This critical difference means that a 120-0-120 source is more similar to a simple single phase source than it is to a generalized two (or more) phase source.
One way of summing up this similarity is to just call 120-0-120 single phase in the first place.

 

[Besoeker]

This critical difference means that a 120-0-120 source is more similar to a simple single phase source than it is to a generalized two (or more) phase source.
One way of summing up this similarity is to just call 120-0-120 single phase in the first place.

On that basis, would you call hexaphase three phase?

 

[GoldDigger]

On that basis, would you call hexaphase three phase?

Probably. Just as in the US we call two crossed center tapped coils with a 90 degree phase difference two phase.

 

[mivey]

Physically there is only one phase i.e single phase for 240/120V.

Depending on which definition of phase you use, you can claim either. In the canonical definition of polyphase systems there are two. You can physically show two or one depending on how you define it.

How you define the term "phase" makes the difference. Both are valid in their own right.

This is all covered in prior posts.

 

[mivey]

Physically there is only one phase i.e single phase for 240/120V. But mathematically, center point is assumed 0 potential and then there are two phases with respect to center point. Still there is no restriction to confine 0 potential to center point alone: any number of points along the winding may be assumed at 0 potential and number of phases may be constructed accordingly but of no use .

Sorry there is restriction to confine 0 potential to one point only in the winding. Otherwise it would be a short.:slaphead:

Using the canonical definition of phases proves otherwise.

Physically, you can center-tap a winding and take one phase in one direction and one phase in the other and physically make use of it. This is also covered in a prior post: my post on an open-wye-wye transformer bank. Besoeker makes use of it in his high phase order inverters.

The math is just a model of the physical.

 

[Besoeker]

Probably. Just as in the US we call two crossed center tapped coils with a 90 degree phase difference two phase.

In a full wave three phase rectifier each semiconductor conducts for 120 degrees. In a hexaphase rectifier, each conducts for 60 degrees. Surely you can't reasonably call them by the same name?

 

[kwired]

Depending on which definition of phase you use, you can claim either. In the canonical definition of polyphase systems there are two. You can physically show two or one depending on how you define it.

How you define the term "phase" makes the difference. Both are valid in their own right.

This is all covered in prior posts.

Exactly what this topic of discussion is really all about. What is "in phase" with something else is dependent on the conditions set forth in the particular application. Even a simple two wire AC source we have + at one terminal at the same time we have - at the other terminal, so it depends on exactly what you are applying the term 'phase' to.

 

[Besoeker]

Exactly what this topic of discussion is really all about. What is "in phase" with something else is dependent on the conditions set forth in the particular application. Even a simple two wire AC source we have + at one terminal at the same time we have - at the other terminal, so it depends on exactly what you are applying the term 'phase' to.

The difference in timing when two or more wave forms cross zero from the negative half cycle to the positive half cycle with respect to a common reference, usually a neutral or star point.

 

[GoldDigger]

In a full wave three phase rectifier each semiconductor conducts for 120 degrees. In a hexaphase rectifier, each conducts for 60 degrees. Surely you can't reasonably call them by the same name?

You are trying to mix two concepts under the same name. I would indeed call a six half wave rectifier set driven from a crossed three phase supply (six hot conductors plus neutral) hexaphase. I would still call the supply itself three phase. Just as I would call the delta that drives your three full wave rectifiers (three hot and no neutral) a three phase supply. I guess to maintain consistency I would call the rectifier set itself triphase.

 

[kwired]

The difference in timing when two or more wave forms cross zero from the negative half cycle to the positive half cycle with respect to a common reference, usually a neutral or star point.

You are trying to mix two concepts under the same name. I would indeed call a six half wave rectifier set driven from a crossed three phase supply (six hot conductors plus neutral) hexaphase. I would still call the supply itself three phase. Just as I would call the delta that drives your three full wave rectifiers (three hot and no neutral) a three phase supply. I guess to maintain consistency I would call the rectifier set itself triphase.

Back to exactly what you are comparing to what?

 

[mivey]

Probably. Just as in the US we call two crossed center tapped coils with a 90 degree phase difference two phase.

But recognized in the engineering field as a subset of four phase. And with two center-tap windings bonded together more correctly a four-phase system but still named two-phase because of history.

Historically, the two phases were separate. Today they are provided as a sub-set of a four-phase system from bonded transformers. You could provide the two voltages from two generator windings rotated 90 degrees from each other but it is not cost effective.

You can also generate two voltages from two separate generator windings rotated 180 degrees from each other. Practically we just center-tap a transformer because it is more cost effective.

Both are similar in execution and result.

Take the case of 120/208 with a 120 degree separation and begin to step the separation gradually towards 180 degrees. Nothing magical happens at the last 0.00001 degree step. As for physical nit-picking, the two winding halves, due to the windings and physical construction of the two sides having slight physical differences, are not perfectly in phase.

So the two winding halves are not perfectly in phase (again, nit-picking). Three terminals vs. two terminals can matter. A polarity change with two terminals is exactly opposite because we are using the same two voltage points. Not so with three terminals.

The number of terminals matter when defining systems of voltages. The method is also in a prior post but I don't feel like typing it again.

 

[mivey]

Exactly what this topic of discussion is really all about. What is "in phase" with something else is dependent on the conditions set forth in the particular application. Even a simple two wire AC source we have + at one terminal at the same time we have - at the other terminal, so it depends on exactly what you are applying the term 'phase' to.

Yes. A two wire source can only have one phase by definition.

You need at least three terminals to have two phases. Then let the fun with definitions begin.

 

[GoldDigger]

...
Take the case of 120/208 with a 120 degree separation and begin to step the separation gradually towards 180 degrees. Nothing magical happens at the last 0.00001 degree step. As for physical nit-picking, the two winding halves, due to the windings and physical construction of the two sides having slight physical differences, are not perfectly in phase.
...

In theory, something magical does indeed happen with the last .00001 degree step, namely you lose the ability to derive arbitrary phase angles from that source.
In practice it becomes harder and harder to derive other phase angles the closer you get to 180, and at some point you would not make the effort.

 

[jaggedben]

I'll just remind people (yes, this has already been discussed) that standard industry nomenclature (at least in the US) is that a phase is a combination of two line conductors with a voltage. A neutral does not count. That's why even a 120/208 service is referred to as single phase by utilities. Two phase originally had four line conductors, with only two combinations having a voltage. All versions of three phase have three line conductors with three combinations having a voltage.

You can discuss the adjectival 'in phase' and 'out of phase' and 'anti-phase' as much as you like, but a 'phase' (noun) actually has a consistent industry meaning in this country.

 

[kwired]

I'll just remind people (yes, this has already been discussed) that standard industry nomenclature (at least in the US) is that a phase is a combination of two line conductors with a voltage. A neutral does not count. That's why even a 120/208 service is referred to as single phase by utilities. Two phase originally had four line conductors, with only two combinations having a voltage. All versions of three phase have three line conductors with three combinations having a voltage.

Still comes down to what aspects you are comparing with your definition of "phase".

You mentioned 208/120 single phase service - you can't directly power a three phase motor from that supply, which has to originate from a three phase source to have a 120 volt neutral on 208 line to line, or be derived by solid state switching means, but you still can transform this arrangement to three phase with an open delta transformer configuration.

 

[Sahib]

Depending on which definition of phase you use, you can claim either. In the canonical definition of polyphase systems there are two. You can physically show two or one depending on how you define it.

How you define the term "phase" makes the difference. Both are valid in their own right.

This is all covered in prior posts.

The definiton 240/120V is single phase supply is used by POCO in practice. Who uses the other definition in practice? Answer: None.

 

[Besoeker]

You are trying to mix two concepts under the same name. I would indeed call a six half wave rectifier set driven from a crossed three phase supply (six hot conductors plus neutral) hexaphase. I would still call the supply itself three phase. Just as I would call the delta that drives your three full wave rectifiers (three hot and no neutral) a three phase supply. I guess to maintain consistency I would call the rectifier set itself triphase.

I would usually call it a six pulse full wave.
To reduce harmonics we used 12-pulse and 24-pulse for some applications.

Sorry mods - wandering somewhat off topic.

 

[mivey]

In theory, something magical does indeed happen with the last .00001 degree step, namely you lose the ability to derive arbitrary phase angles from that source.
In practice it becomes harder and harder to derive other phase angles the closer you get to 180, and at some point you would not make the effort.

Definitions again. If there are more than two phases present in a circuit then you have multiple phases. this can apply to both voltages and currents.

Inside the two wire transformer circuit, there is only one phase present. The same electrons, fields, etc. even at the atomic level. No matter which way you take the two terminals.

Inside the three wire transformer circuit, there can be multiple phases present. A different set of electrons, fields, etc. Now it matters how you define the system of currents and voltages.

Define them one way and they are twin sets so would not be counted as unique. This uses two different reference points for the two waves but when shifted and overlaid they "match".

Define them a different way and they are twin sets so could be counted as unique. This uses one reference point (positive away from center) for the two waves and they are opposites.

The two transformer halves do not have to act the same. We usually use the full winding or one of the halves for a circuit. A circuit that uses both halves (rectifier, pulse, etc.) can have different electrons, fields, etc. Indeed, one half may have load current and the other have none. In the rectifier/pulse case, one half sends a positive signal but the other half conducts nothing, then 180 degrees later the other half sends a positive signal while the first half conducts nothing.

add: cut & paste typo.