240V Open Delta

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jim dungar

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"Center-Tapped Transformer and 120-/240-V Secondary Models" William H. Kersting:


Just because it got published in a book does not make it an industry standard. There are just as many books that do not have the same opinion.
 

mivey

Senior Member
Just because it got published in a book does not make it an industry standard. There are just as many books that do not have the same opinion.
Sure, and I have references for both, but the idea is to be able to move beyond the norm that we might have traveled under most of our lives. There is nothing wrong with trying to see a broader picture of the world around us. As with most things, gather the available knowledge that you can and see if you can find the common ground. Instead of clinging only to one piece of information, one could use their reasoning abilities to sort through the different pieces of information at hand.

We spend most of our lives trying to put the world as we know it into a neat little box and wrap it up with a nice bow. This makes our day-to-day life much easier. The assumptions and rules we lay out work for the majority of the stuff we do. They can greatly simplify our work & lives but we must never forget that there might be a broader picture.

That is not to say we should throw out our neat little box, as it serves us well for most things. But some people who travel through this life have a quest for knowledge that makes them question how the box was made, why it was made, and if there is a world beyond the box.

Standards are great but sometimes it might be worth looking into the rules that govern those standards, how those standards came to be, and to see if there are places those standards might not always apply. If nothing else, the quest to look beyond the standard brings a better understanding of the standard itself.

It is not for everyone, but some just have a desire to understand things better. For me, I saw the standard naming conventions we use have certain discontinuities and I was not content to blindly accept a pre-drafted list of names, especially since there seemed to be more than one list. I was willing to step back and investigate where the names might have come from, and why they are applicable to the systems we have even when they are not consistent.

For those that don't care one way or another: the historic names are fine if you can find the set you are happy with. But we often find fellow travelers that come here with questions. They notice the abnormalities in the names and want to know why they look inconsistent. Many choose to tell these poor souls to just accept the names as they are and move along without question (the ol' "just because" answer). Others try to bend the physics of the systems themselves to make it look like the name-set they have picked is perfect as-is.

I would rather be honest about the situation and try to help them understand better. If it is too technical for them, then they have the option to go back to the pre-defined list of names and just move along. But I think it is sad to just send someone on their way without at least offering to help them step beyond the norm.
 

jim dungar

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Standards are great but sometimes it might be worth looking into the rules that govern those standards, how those standards came to be, and to see if there are places those standards might not always apply.

Yes standards are necessary.
Given a 3-wire circuit (2 hots and a grounded common), what formula(s) do you use for determing KVA, when you are given Volts and Amps?
 

mivey

Senior Member
Yes standards are necessary.
Yep. They certainly simplify things and make our normal lives much easier.
Given a 3-wire circuit (2 hots and a grounded common), what formula(s) do you use for determing KVA, when you are given Volts and Amps?
:-? was there another part to the question? Generally, for a circuit, kVA = volts * amps / 1000.

What kind of circuit? What voltage and amps were you given? Were you talking about a single-phase 3-wire circuit? If the 3-wire circuit is unbalanced (i.e., you have some current on the common wire), you would have more than one voltage-current set to calculate.
 

jim dungar

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:-? was there another part to the question? Generally, for a circuit, kVA = volts * amps / 1000.

What kind of circuit? What voltage and amps were you given? Were you talking about a single-phase 3-wire circuit? If the 3-wire circuit is unbalanced (i.e., you have some current on the common wire), you would have more than one voltage-current set to calculate.


There are only three wires, why does anything else make a difference? But if you want ,assume there is equal current in each line to common conductor. What else do you need and why?

Have you ever discussed 'naming' open-wye circuits with any power engineers from Europe and India where they are extremely prevalent?
 

mivey

Senior Member
There are only three wires, why does anything else make a difference? But if you want ,assume there is equal current in each line to common conductor. What else do you need and why?
It appears you mean a simple circuit so I am going to assume 1st order voltages and currents and simple loads. Makes it easier since I'm traveling and have limited time anyway. I'll look at balanced and unbalanced for my answer because even for a simple circuit, it does make a difference.

For a balanced circuit, there would be no common current and we could just use the line-line voltage times the line current to get the kVA (it is just a single-phase load like you would have for a 2-wire circuit). For a circuit where one line current equals the common current, and the other line current is zero, we could just use the line-common voltage times the line current to get kVA (again, just like a 2-wire circuit). Depending on what you are looking at, you may not get the answer you are looking for. If you are trying to size a single-phase transformer, you may over-load it by having all the load on one side of the secondary winding. For both of these I would use kVA = volts * amps / 1000 (using rms meter readings).

For the unbalanced case with common current and two non-zero line currents, I would calculate the kW and kvar for each coil (or 1/2 coil if a single-phase transformer), then calculate the total kVA using the net kW and kvar. In this case, I would use the relationship S=VI* to get the kW and kvar components in a complex format (S=P+jQ) so I could just sum the P's and Q's. We also know kVA^2 = kW^2 + kvar^2.

Have you ever discussed 'naming' open-wye circuits with any power engineers from Europe and India where they are extremely prevalent?
Nope.
 

jim dungar

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For the unbalanced case with common current and two non-zero line currents, I would calculate the kW and kvar for each coil (or 1/2 coil if a single-phase transformer)...

Oh, that's right a single phase transformer can feed a 2-phase load.:roll:

So, from all of your discussions, it appears that any time the load is only 3-wires it must be called 2-phase. Because even in a corner grounded open-delta connection one conductor is carrying a current that is common to the other two conductors.

Strange, that it is not commonly taught this way across the globe (or even the US).
 

skeshesh

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As far as my experiences I've always heard a three-wire system being referred to as single-phase (sometimes three-wire, 2-pole, 1-phase when talking about a breaker, etc). This is not a "technical issue" in my opinion. Sure the name may not be accurate and stuck around since whenever, but that's just an issue of nomenclature. It's a misleading concept for a layperson but a basic one for an electrical engineer or most of the technicians/contractors who've done their homework. So I guess my question is, knowing the convention is to call out a 3-wire, 1-ph is anyone claiming they call for 3-wire, 2-ph systems because it may be more "technically correct"?
 

mcclary's electrical

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Location
VA
As far as my experiences I've always heard a three-wire system being referred to as single-phase (sometimes three-wire, 2-pole, 1-phase when talking about a breaker, etc). This is not a "technical issue" in my opinion. Sure the name may not be accurate and stuck around since whenever, but that's just an issue of nomenclature. It's a misleading concept for a layperson but a basic one for an electrical engineer or most of the technicians/contractors who've done their homework. So I guess my question is, knowing the convention is to call out a 3-wire, 1-ph is anyone claiming they call for 3-wire, 2-ph systems because it may be more "technically correct"?

That's pretty much Mivey's point.
 

mivey

Senior Member
Oh, that's right a single phase transformer can feed a 2-phase load.:roll:
We know a phase is a voltage. We can also see that if two voltages of the same magnitude have different phase angles, then they are said to be different phases in a system. If you have a load that uses two voltages of the same magnitude but with different phase angles then you can say the load uses two phases. By reasonable progression of logic, a load that uses two equal-magnitude voltages that require a displacement of 180? could technically be called a 2-phase load.

An example of a load requiring voltages of this sort would be a 2-diode, full-wave rectifier as shown here from "Electronics: Circuits & Devices", Ralph J. Smith:

The bridge circuit is disadvantageous because four diodes are required, and two diodes and their power-dissipating voltage drops are always in series with the load. The full-wave rectifier circuit of Fig. 3.31 uses a more expensive transformer to produce the same result with only two diodes and with higher operating efficiency. The second output winding on the transformer provides a voltage v2 that is 180? out of phase with v1; such a center-tapped winding serves as a phase inverter. While v1 is positive, current i1 is supplied through diode 1; while v1 is negative, no current flows through diode 1 but v2 is positive and, therefore, current i2 is supplied through diode 2. The current through the load resistance is i1 + i2 and Idc = 2Im/pi as in the bridge circuit.

2-DiodeFull-WaveRectifier.jpg



But, what I posted had to do with Blondel's Theorem. Since you did not want to give any information other than the circuit was 3-wires, what I described was how to measure a 3-wire circuit using a common wire such that you only use two stators (or one stator for certain load restrictions/conditions). The number of phases is a separate issue. Blondel's Theorem (copied from the "Handbook for Electricity Metering"):

"If energy is supplied to any system of conductors through N wires, the total power in the system is given by the algebraic sum of the readings of N wattmeters, so arranged that each of the N wires contains one current coil, the corresponding voltage coil being connected between that wire and some common point. If this common point is on one of the N wires, the measurement may be made by the use of N-1 wattmeters."

So, from all of your discussions, it appears that any time the load is only 3-wires it must be called 2-phase. Because even in a corner grounded open-delta connection one conductor is carrying a current that is common to the other two conductors.

Strange, that it is not commonly taught this way across the globe (or even the US).

Strange that you would mention "all my discussion". I'm not sure if you are just not reading them, or still not listening:

...I wonder does he call a corner grounded open-delta, 2-phase at 60?? After all it is two phase conductors with one common conductor.

If you would start listening to what I am saying, maybe you would understand and you wouldn't have to wonder because you could figure it out for yourself. The source has three, equal-magnitude, phase-displaced voltages so it would normally be labeled as a 3-phase source.

Now for how it is used:
Let the corners of the delta be a, b, and c. Let's ground a.

If the load uses the voltage Vba OR Vca, then it is a single-phase load and I am using the source as a single-phase supply.

If the load uses the voltage Vba AND Vac AND Vcb, then it is a three-phase load and I am using the source as a three-phase supply.

If the load uses the voltage Vba AND Vac (or any other combination of two equal magnitude, phase-displaced voltages), then it is a two-phase load and I am using the source as a two-phase supply.

That's right: The 3-phase source can supply all three systems.
 

mivey

Senior Member
As far as my experiences I've always heard a three-wire system being referred to as single-phase (sometimes three-wire, 2-pole, 1-phase when talking about a breaker, etc). This is not a "technical issue" in my opinion. Sure the name may not be accurate and stuck around since whenever, but that's just an issue of nomenclature. It's a misleading concept for a layperson but a basic one for an electrical engineer or most of the technicians/contractors who've done their homework.
You got it. The historical names are not consistent across different systems and often lead people to ask: "Why do we call it 'name a' when it looks so much like another system we call 'name b' ?".

Looking at the different names we use and the different systems, it is obvious to all except those who turn a blind eye that the names did not all come out of the same mould. The puzzle pieces don't all fit together nicely. The reason is because the names are derived from the physical systems using constraints and assumptions that do not fit the same for every system. You simply can't take a name that is a stripped-down sub-set of a generalized system and re-create the general system because there are pieces of the puzzle missing.

So I guess my question is, knowing the convention is to call out a 3-wire, 1-ph is anyone claiming they call for 3-wire, 2-ph systems because it may be more "technically correct"?
No call-outs, at least not in the EC world. The purpose of the discussion here is to understand the names, how they were derived, and why they fit what we use them for. The discontinuities in the names can be explained by stepping back to look at the general system from which they were derived.

In the utility world, folks know what 2-phase grounded-wye primary distribution is because it has been called that for a very long time. As for the secondary & services, the only 3-wire secondary widely called something different is the 3-wire "open-wye", which is called a "network".
 

jim dungar

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The source has three, equal-magnitude, phase-displaced voltages so it would normally be labeled as a 3-phase source.

Isn't this what you don't want me to do, define the number of phases by counting line-line voltages? And now, when it is convenient you are moving your reference point when you measure your voltages.

The only difference between a corner grounded open delta load, an open wye load, and a center tapped single winding load, is the angle between the voltages. So 3-phase suddenly becomes 2-phase as soon as the angle becomes different than 60??

If you remove one 'phase conductor' from a 3-wire delta, you only have single phase (1 line-line voltage) even though you still have 2 'phase conductors'.

You claim that 2-phase is a common term in the utility industry, it may very well be among line workers. But it clearly is not common in the rest of the world. I state definitively it is not commonly used by electrical equipment manufacturers except as an adjective. I do not see it commonly used in the IEEE color book series. Utilities like Commonwealth Edison and Xcel Energies do not use it in their service rates books and metering manuals. The NEC only uses it to describe what you derisively call the 'historic' system.
 

mivey

Senior Member
Isn't this what you don't want me to do, define the number of phases by counting line-line voltages? And now, when it is convenient you are moving your reference point when you measure your voltages.
What I said was using ONLY line-line voltages was incorrect. Line-neutral voltages can also be phases.

The only difference between a corner grounded open delta load, an open wye load, and a center tapped single winding load, is the angle between the voltages.
The difference has to do with the magnitude and phase angle. Since a phase is a voltage, the number of equal-magnitude voltages with unique phase angles is what determines the number of physical phases. Some systems can supply more than one system of voltages. For example the 3-wire 120/208 wye: a one (1) single-phase 208v system, one (1) 2-phase 120v system, two (2) single-phase 120v systems.


So 3-phase suddenly becomes 2-phase as soon as the angle becomes different than 60??
Now that really is funny. How about: "So 3-wire 2-phase suddenly becomes 1-phase as soon as the angle becomes different than 90??". Given the historic names we use, the conventional name might change, but the physical system still has two phases.

In the delta case, when the angle suddenly becomes different, one of the voltages suddenly no longer has the same magnitude and can't be counted in the system of voltages (remember: equal-magnitude but different phase angles). Physically, that leaves us with two phases (voltages) in the system.

In the common-conductor case, the magnitude stays the same but the angle changes. So that voltage is still is part of the phase count (unless it suddenly matches a different voltage that has already been counted).

If you remove one 'phase conductor' from a 3-wire delta, you only have single phase (1 line-line voltage) even though you still have 2 'phase conductors'.
I agree with that and have said the same. But when counting phases in a system, we are counting voltages as phases, not counting wires.

You claim that 2-phase is a common term in the utility industry, it may very well be among line workers. But it clearly is not common in the rest of the world. I state definitively it is not commonly used by electrical equipment manufacturers except as an adjective.
You have to be able to distinguish between a conventional name and the physical system. The conventional name is derived from the system and may not represent the system's complete physical make-up. That is why some names appear to conflict between different systems. Just using common sense, it should be easy to see that if you have two phases then it is a 2-phase system (with a phase in this context being a voltage, not a piece of wire).

"Used as an adjective" is just your opinion. Because you refuse to see things from a different perspective, when I showed you an example of where it was used differently, you concluded that it must have been "used as an adjective". After that, I clearly showed you where the author meant two phases as in two voltages or emfs. Because of your refusal to be able to see a different perspective, you then concluded that the author must be some non-industry-standard anomaly.


I do not see it commonly used in the IEEE color book series.
Why quote standards as the definitive answer if you only agree with them when they match your own viewpoint?

Utilities like Commonwealth Edison and Xcel Energies do not use it in their service rates books and metering manuals.
We already have conventional names for service drops. What makes you think they would stop using them? The idea is to understand the relationship between the names being used and the physical system.

The NEC only uses it to describe what you derisively call the 'historic' system.
Derisively? You are way off base. Where have I scoffed at this system? My use of the term "historic" for the quarter-phase system was to try to distinguish between the different systems I was discussing.

If you have been reading my posts with that notion, maybe that is why you are not hearing me. Perhaps you would better understand what I am trying to say if you get rid of the idea that I am attempting to make fun of the existing system.

How about you consider that I am trying to to make a serious effort to explain the relationship between the names we use and the physical systems we have and then re-read my posts?

It is a simple fact that the conventional names we use are not completely descriptive of the physical systems we have. That is why the names appear to be in conflict. If you can step back and look at why the names work for what we are doing, you can understand why they are applicable even if they don't always exactly match the physical system.
 

mivey

Senior Member
and

and

Isn't this what you don't want me to do, define the number of phases by counting line-line voltages? And now, when it is convenient you are moving your reference point when you measure your voltages.
What I said was using ONLY line-line voltages was incorrect. Line-neutral voltages can also be phases.
And convenience has nothing to do with it. Take the 4-wire 208Y/120 system:

If I am considering a system made up of 120v phases, I have three (3) 120v line-neutral voltages: 3-phase 120v

If I am considering a system made up of 208v phases, I have three (3) 208v line-line voltages. 3-phase 208v

So there are two different physical 3-phase systems that this supply can deliver.
 
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