Single Phase/Two Phase Discussion

[winnie]

I finally got it through my head a key point that Jim is making.

A three wire 120/208 service is _defined_ by the documents that he references as a _single phase_ service. It isn't single phase service because there are no phase angle differences; it is a single phase service because that is what it has been named.

Jim has agreed, and I think that we've beaten to death the fact that there are phase angle differences between the two line-common voltages present in such a service.

In general practise, no active use is made of the phase angle differences present in a three wire 120/208 service. As this service is _used_ it is single phase.

To understand the _theory_ behind this service, it must be treated as _two_ legs of a three phase service, and this will show up in things like calculation of neutral current and the like.

We call a three wire 120/208 service a single phase service, even though there are _3_ conductors and thus _3_ different voltage phase angles. It is called this because of the way it is used.

We call a 3 wire 480V delta service a _three phase_ service, and it has _3_ different voltage phase angles.

But we call 4 wire 208Y/120 service a _three phase_ service. This is the name that we give it, even though there are _4_ conductors, _6_ conductor pairs, and thus _6_ different voltage phase angles.

We call a 3 wire 480V corner grounded delta service a _three phase_ service, although it only requires _two_ poles on the breakers, and could be provisioned using a suitable 480V "single phase" panel.

-Jon

 

[Smart $]

The answer is indeed yes, but the service itself is not a single-phase service...

Yes...

...but I'm not among those that need convincing :grin:

 

[Smart $]

...it is a single phase service because that is what it has been named.

That's the clincher (until it is changed) regardless of all else, including its correctness (or incorrectness ).

 

[dnem]

How can you get a higher current on the grounded leg ? . Where does the 1.732 figure come from ? . What would be going on in the delta that would cause an increase on just one leg ?

Just to be clear, I was describing a strange beast: a corner grounded delta, but at only 120V, serving single phase loads. That is why I noted that such an installation is not actually used anywhere.

D?oh !
120v corner grounded delta with single phase loads.
I overlooked the obvious and confused myself.

If it helps, picture carrying the collapsing phase-angle difference to the extreme, where the two ungrounded conductors are in phase: the neutral current is the sum of those of the two ungrounded conductors.

An example of this is the grossly-overloaded shared neutral when someone mistakenly connects the two ungrounded conductors to the same phase on a multi-wire circuit: the currents add instead of cancelling.

Carrying it to the extreme is helpful. . It illustrates the concept more dramatically.

 

[jim dungar]

The multi-wire circuit supplying them is poly-phase, because there is a difference in timing between the two voltage peaks, relative to the reference point (the neutral).

Heavy sigh.

If the definition of 1-phase or 3-phase is number line-line voltages available, then you cannot redefine it as line-neutral simply because a neutral point is available. The amount of current carried in the neutral conductor is not part of any system definition.

 

[kingpb]

All of this discussion has been interesting. I think it brings to light how important it is for all individuals within the electrical industry, whether it be engineers, electricians, technicians, or sales, to be more concise when discussing projects, because things can become easily misunderstood. Especially in this case, we must all be willing to go back to the standards from which things come.

The 120/208Y, open wye, single-phase service is derived from a 208Y/120V three phase, four wire system. As noted in C84.1, ?A modification of this three-phase, four-wire system is available as a 120/208Y-volt service for single-phase, three-wire, open-wye applications.?

Distinguishing between service voltage and system voltage in this case becomes readily apparent because it sets the point of reference. IEEE/ANSI C84.1 defines them separately as follows:

Service voltage: The voltage at the point where the electrical system of the supplier and the electrical system of the user is connected.

System voltage: The root-mean-square (rms) phase-to-phase voltage of a portion of an alternating-current electric system. Each system voltage pertains to a portion of the system that is bounded by transformers or utilization equipment.

Additionally, quoting from C84.1, in reference to secondary transformer configurations, ?Single-phase services and single-phase loads may be supplied from single-phase systems or from three-phase system. They are connected phase-to-phase when supplied from three-phase, three-wire systems and either phase-to-phase or phase-to-neutral from three-phase, four-wire systems."

 

[jpweygandt]

Jerry,

You?re quite right about the lack of delta secondarys in Medina County. . My time spent as an inspector has been of no use in understanding or experiencing the configurations involved in this discussion. . I need to fall back on schooling and also years spent as a L38 wireman in Cuyahoga County and projects in Cuyahoga and Ashland Counties during my years as a contractor.

Since you have background with older corner grounded delta systems, I have a question for you.
?During World War I and II when copper was really scarce many three phase motors were fed with just two wires and the third was obtained at the location by attaching the third wire to the building steel. Thus the early Delta systems were less expensive and easier to install.?
The third grounded phase was delivering its current thru the building steel. . Now if the connection to the building steel would be lost on either the equipment end or the service end, I can imagine the danger to the worker if he was to touch that grounded phase while in contact with the building steel. . Up until the moment that the connection is lost, the worker could have touched the grounded phase wire without incident. . But then suddenly that same touch becomes lethal.

But as long as there is no lose of connection on the grounded phase, are you aware of any symptoms experienced by contact with the building steel or anywhere else along the grounded phase path ? . Since technically the workers body could be seen as a parallel path to ground, but obviously one of a much higher resistance, I can imagine a theoretically small amount of current taking the path thru the workers body resulting in a tingling or even a little shock possibly comparable to static discharge. . What was actually experienced by physical contact ?

David

To David,

I think everyone would agree that these installations weren't the safest. Some of them were installed before there was a NEC. These installations were probably were just as safe at the time as all conductors in a conduit are now. Once we were no longer at war and installations had to meet the NEC this form of installation went away. Corner grounding of Delta services is required both by the State of Ohio and the NEC. The installation of 480/277 and 208/120 volt is the norm now. A Delta service is not found very often anymore.

I did a long term installation for F.E. Myer Company in Ashland, Ohio. I can point to many electrical installations in Medina County. Plaza 71 on route 18, Kindercare Learning Centers, McDonald Restaurants and Wendys Restaurants along with many more. Some 15 years of installations. I'm pleased to see that inspections have become much more important with your expertise at doing the inspections.

Jerry

 

[LarryFine]

Heavy sigh.

I hate to see a grown man sigh. :roll: Okay, how about a compromise?

I certainly agree that a 3-wire 208/120 feed will supply single phase voltage wherever it is taken; either line to line or line to neutral. Every 120v load and every 208v load will receive a single, symmetrical, sine wave.

However, electrically speaking, the two ungrounded conductors do not have their respective voltage peaks (relative to the grounded conductor) at the same time; there will be a 60 degree difference in time.

Will this be apparent in any way? Absolutely not in any way that matters. You'd get 120v 1-phase and 208v 1-phase. I used the term 'poly-phase' only to point out the timing difference, not an electrical difference.

The only manifestation of the source's 3-phase characteristics is that the line-to-line voltage does not equal twice the line-to-neutral voltage. While 3 - 1 does not equal 1, losing a third phase does indeed result in 'single phasing'.

Why? Because a genuinely 3-phase load (or the strictly 3-phase part of a combination load) has no neutral connection, so a 3-phase supply dropping a phase results in the load receiving only one phase at 208v.

Feel any better, Jim? Are we in agreement now?

 

[jim dungar]

Feel any better, Jim? Are we in agreement now?

Almost.

But, you need to stop counting phases by taking line-neutral measurements. As Kingp has pointed out this is just plain wrong based on every ANSI and IEEE reference.

And using your method of the open-wye as poly-phase, what do you call the 120/240V MWBC fed by a "wild-leg" center tapped delta system? After all just add 1 wire and you have 3-phase.

Part of the problem is that our industry has taken short cuts in terminology for a long long time.

As an example: the standard terminology of 120/240V really doesn't tell us much actual about the system. Changing this to 120/240 1-Ph 3-Wire helps a lot, but we still don't know a lot about the service that supplies it. For example is the transformer a single winding with a center tap (like most pole top units) which gives us a single voltage which is cut in half, or two windings connected in series (like most dry type units) which is two voltages added together. This is important from a circuit analysis aspect but the connected equipment doesn't care at all.

 

[rattus]

Beg to differ:

Beg to differ:

However, electrically speaking, the two ungrounded conductors do not have their respective voltage peaks (relative to the grounded conductor) at the same time; there will be a 60 degree difference in time.

Larry, the separation is 120, not 60 degrees, which it must be in order to obtain 208V line to line.

 

[dnem]

For you guys that are still arguing
I still think there’s no reason to argue

I honestly think that if everyone would stop typing the word
"phase"
by itself without a qualifying word, the disputes would resolve and end.

If no one ever types the word "phase" but instead always types the words
"phase coil"
"phase load" or
"phase conductor"
I believe agreement could be found within a few posts.

David

All of this discussion has been interesting. I think it brings to light how important it is for all individuals within the electrical industry, whether it be engineers, electricians, technicians, or sales, to be more concise when discussing projects, because things can become easily misunderstood. Especially in this case, we must all be willing to go back to the standards from which things come.

The 120/208Y, open wye, single-phase service is derived from a 208Y/120V three phase, four wire system. As noted in C84.1, “A modification of this three-phase, four-wire system is available as a 120/208Y-volt service for single-phase, three-wire, open-wye applications.”

Distinguishing between service voltage and system voltage in this case becomes readily apparent because it sets the point of reference. IEEE/ANSI C84.1 defines them separately as follows:

Service voltage: The voltage at the point where the electrical system of the supplier and the electrical system of the user is connected.

System voltage: The root-mean-square (rms) phase-to-phase voltage of a portion of an alternating-current electric system. Each system voltage pertains to a portion of the system that is bounded by transformers or utilization equipment.

Additionally, quoting from C84.1, in reference to secondary transformer configurations, “Single-phase services and single-phase loads may be supplied from single-phase systems or from three-phase system. They are connected phase-to-phase when supplied from three-phase, three-wire systems and either phase-to-phase or phase-to-neutral from three-phase, four-wire systems."

Three different categories
"phase coil" refers to the system
"phase load" refers to the service
"phase conductor" refers to any current carrying conductors (whether grounded or ungrounded) except the neutral that carries only unbalanced load

current carrying
1. phase
2. neutral

non-current carrying
1. electrode earthing (grounding)
2. equipment bonding (grounding)

David

 

[Big4xDawg]

Single Phase vs. Two Phase

Single Phase vs. Two Phase

First, 240V single phase sine waves are 180 degrees out of phase. Second, all three phase voltages (ie. 208, 240) have 120 degree differences in their sine waves. True two phase has a 90 degree difference in the sine waves. 0

The first ventures into the relm of polyphase electric power used only two alternating current phases rather than three. The two phases were generated with a 90 degree difference between them, and the system that resulted was called two-phase power. The first two phase generators in the early 1890's were two singel 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 seperate two wire, single phase circuit. The two phases were used together in a four wire system.

 

[jim dungar]

First, 240V single phase sine waves are 180 degrees out of phase. Second, all three phase voltages (ie. 208, 240) have 120 degree differences in their sine waves.

You are not being consistent in your method of viewing your sine waves.
In your 3-phase 240V example you are looking at your L-L voltages and comparing the waves of the three combinations AB, BC, and CA. But in your 1-phase example you refer to a single L-L voltage of 240V but then you talk about multiple waves.

It is possible to have a single phase 120/240 3 wire system created by connecting (2) 120V transformers in series and therefore have (2) L-N waves added together for 240V. But if you take (1) 240V winding and center tap it there can only be (1) wave cut in half for the 120V.

 

[jugband1]

2 phase misconception

2 phase misconception

My experience as a former utility designer in Philadelphia is that a 2 phase system consists of 5 wires on the secondary side. 2 sets of 240 with 170 volts across phases.

I would design 208 wye for an apartment building for example. This would allow the apartment complex 3 phase for the elevators, fire pumps etc. The tenants would then receive 120/208 single phase 3 wire for their individual units.

And yes there are still some 2 phase primary circuits in service today in older sections of the country.

Jugband1