Ok, We all know 2 X-formers and 3 hots can do an open delta, but.......
Can 3 X-formers and 2 hots be configured into some sort of 3 phase arrangement?

According to one source , this is called a 'scotch-something...'

You are referring to a Scott connected configuration, in which a 3 phase primary is connected to two transformers in such a manner that 2 phase power is availableon the secondary. This information is available in McGraw-Hill's "American Electricians Handbook"

hmm;
i see it in the index, but not in the book, go figure....

Sparky,
What edition do you have? I'm home and my 11th edition discusses this beginning on page 5-55. There is a diagram on page 5-57. When I go in tomorrow, I'll check my other(newer) edition.

well I have a 13th, the index says it's at 5-92 to 5-94, which is just a lot of stuff about drying X-formers

I have the 13th, also.
The index is confusing. It refers to sections, not pages. Look at SECTION 5-92 on PAGE 5.67

Designed to replace the Edison 3 wire DC hook up when swapped over to AC. Unless you work around coal mines (DC equipment runs on AC or batteries) you'll never see one. coal mine info is old, don't know if they still do it that way.

Look at SECTION 5-92 on PAGE 5.67
Dohh! Homer Simpson....
Ok, so I can't use my own book!

anywho,
there it be, stange animal that it is. Could this be used in reverse? I.E.- get 3 phase 208V out of 2--4160V high legs?
that was the description I got from my 'source'

According to my engineering books the Scott connection is normally used for 2-phase to 3-phase conversion. If used in a 3 to 2 conversion the transformer output needs to be reduced to 86.6% of the combined single-phase ratings.

JBD,
can you elaborate?
the particular situation here was a linesman stating that 2 out of 3 phases were available at a job, and 3 ph was created via this 'Scott' configuration.

We are getting into complicated areas where the slightest imconsistent word usage can change the whole meaning of the discussion.

There are true 2-phase systems where the phases are 90 deg apart from each other. Motors on these systems can create a large amount of torque, similar to DC. A (2) transformer Scott-Tee connections is used to convert 3-phase to 2-phase and 2-phase to 3-phase.

There is also a standard Tee transformer connection which is used for normal 3-phase to 3-phase voltage conversion.

In your case you are describing using 2 hot lines of a 3-phase system (The engineering equivalent circuit of 2 hot lines is a 1-phase system). You need some type of phase convertor system to go from 1-phase to 2-phase and finally to 3-phase. A "static" system would contain 1-phase capacitors to create a 4-wire 2-phase system and then a Scott-Tee transformer arrangement to provide the 2-phase to 3-phase conversion.

This type of phase conversion has some definite drawbacks (including effciencyand regulation) but it is better than nothing.

Hope this helps.

Utilities use a connection with two transformers in an open delta fed with two hots and a ground to provide 3 phase power. I don't know what it is called but I have one right outside my construction trailer. The two primary hots feed the ends of the open delta. The ground is connected to the point of the open delta.
Don(resqcapt19)

Steve,

Is it anything like the 2 phase schematics I have posted???

I think Don has hit on this one, where one Primary feeder is a grounded conductor and it's also common to the secondary. Kind of strange, but I have seen such Animals in Las Vegas and Oregon.
A 3 phase transformer bank was fed [primaries] with only two conductors that were suspended by Insulators. These two were on the "ends" of the open delta. In the "middle" [the paralleled connections], a bare conductor [which was run like the ground bonding conductors found on top of Steel Towers for long distance power distribution] made connection to the "middle" of the primary delta AND was bonded to ground at the pole, plus to the secondary's grounded conductor.

Though I was seeing things, so after having a long look [plus a few others to verify it existed], that was the setup!

It has benifits! It just looks really odd!

Scott SET

Utility transformer connections are not much different than ours. Some utilities may use a grounded phase system (same features as our Grounded B phase), they may even call it an "earth return" or a "common neutral". I believe low voltage grounded-B systems are much more common than similar medium voltage utility arrangements.

25 years ago I saw a 1-phase utility system that had only 1 wire on the pole, the earth was the only return path on the utility side.

Pardon my ignorance of distribution transformers. I see single phase transformers with 1 wire feeding the primary all the time. Can someone explain?

Can you provide a photo? Are you saying that there is only one wire on the supply pole?

I would like to see this!

I'm not into shutterbugging. But, I swear, I can only see 1 wire going into the top of the can. I've thought that maybe they were some sort of autotransformers with a common, single ground point coming down to earth.
Once, I even asked a utility worker who was working on one and his reply was "they're single phase, they only need 1 wire."

Scott,
I did search thru the technical references you have provided , but no soap, sorry.

Guys, Also do not forget the neutral sizing on a Scott (2phase) connection. Remember it is only 90 deg. out of rotation. Scott, where do you live ? I have seen some VERY WRONG utilities in PA using a single wire on rural connections years ago, utilizing a ground return path, which is EXTREMELY hazardous. I would hope they have all been changed by now. No, they can have a single energized (ungrounded) line, and a grounded line, but must have 2. You guys keep overlooking "Uglys", they used to have the schematic for old 2 phase Scott circuits, one reason is that a lineman used to have to connect it and 'splain it to pass his Class A test, I don't think anyone does this anymore.

That is what I'm wondering. Can the 2nd line just be the same wire that grounds the secondary. In other words getting the 2nd primary wire by just plugging into the earth?

would'nt that be kind of high impedance thru the earth??

Sparky, Not only is it high impedance, you don't know where it ends up. It was just such a scenario that made me wander out to the pole, fella kept getting really clobbered out at his well. He had a tin cup out there and used it to drink from, and this was through linemans boots. Black Valley PA. The cure of course, was to replace the galvanized pipe with plastic, at least that stopped him from getting hit, didn't ground the thing properly. It works if the load is not heavy, and his was not.

In the early days of the REA (rural electrification authority) it was common to run only the single hot out in rural area where there was often miles between customers. The earth was used as the second conductor for these systems. I don't know if there are any of these circuits still in use. I haven't found any in this area.
Don(resqcapt19)

>The earth was used as the second conductor for these systems.
Obviously they would have tied the GC of the low side to the GC of the high side. But what was the high side voltage? It is hard to imagine this working very well at 4000 V. I would guess that the voltage would vary with the rainfall.

well i'm certainly eyeballin' the utility poles around here now....

>I can only see 1 wire going into the top of the can.
There is another wire somewhere.

How many wires were going pole-to-pole?

3 wires running along pole to pole. Only 1 ever is feeding the xfmr. I found a few clear examples where the main run is on one side of the street, and a single wire crosses over the street to the top of a transformer on the other side.
Is everyone saying that in their area, this is not the case?
I will have to try to take some pictures!

geez, i'm lookin' up while driving here

... while surfing my laptop, while on my cell phone, eating a breakfast sandwich. I have become so obsessed with looking at every poletop, that I will soon have an accident.
BTW, what did you find?

would any one like to chat about electricity, transformers, motors,
nec requirements, successful electrical contracting traits,habbits,marketing etc....
I am a 15 year electrician and would like to learn more..I take my California State contracting exam on 9/13/01......... my icq and email should be visable to users....

Redsy;
no one-liner yet.....no accident either

O.K. I stopped the truck and took a good look. One of the 3 wire on the crossarm is tapped, and right before it connects to the transformer, it seems to split into two and connects to the primary. Is it shielded cable with a grounded shield providing the 2nd connection? I will have to get pictures.

>Is it shielded cable...?
Now I think you've hit on something!

man oh man... ive never ever heard of a single primary with no neutral feeding a tx... now what you bay be thinking of here is a can grounded TX. theres only one insulator on the top of the transformer, so one leg of the primary coil is tied to the primary, the other leg is tied directally to the transformer tank, and that its bonded to the pole ground, and then the system ground. if what your saying is true, and they do use the ground as a return path, imaginw what would happen if the #6 bare copper line running down the pole in to the ground was CUT! primary voltage at your finger tips.. i DOUBT this is likley.

ever hear of 40.1 miles of underground primary run? i found out that there is such an insulation in teh florida everglades... they said it faulted about once a week...

-m

>there's only one insulator on the top of the transformer
Yes, that is common.

>imagine what would happen if the #6 bare copper line running down the pole in to the ground was CUT! primary voltage at your finger tips.. i DOUBT this is likley.

The situation you describe is not as bad as if the poco grounded conductor were broken for a transformer primary wired to the poco grounded conductor.

I guess you are saying that the center tap (X0) of the secondary would be in series with the now otherwise ungrounded end of the primary (P2).

Cut the ground wire on the pole and it is still grounded at the service.

But for a system relying on the poco grounded conductor, the ground on the pole and at the service are presumed neglible.

The real answer is that current can't flow in two directions at the same time through the transformer.

The is no complete circuit.

First let's consider the set up with no magnetic field. In this case, X0, X1, and X2 all have the same potential. Even if that is primary voltage, it obviously has nowhere to go. So the measured voltage to ground or anything else is zero.

If it is not, then a magnetic field will arise in the primary and secondary coils. Say that the tranformer is a 16 to 1 step down.

Now look at the circuit with the magnetic fields standing. In order for energy to flow from P1 to P2, it must pass by X0. To the extent that it goes into the earth, 16 times as much current can flow between X1 and X2 at 1/16th the voltage -- as expected.

However, to get primary voltage to your finger tips, it must somehow travel from X0 to X1 or X2. Let's suppose that somehow it manages to get to X1. Where can it go next? It is already the same potential as X0, so it can't flow there. If it tries to flow to X2, it has to flow from X2 to X0. But it can't flow from and to X0 simultaneously. So the path is shaped like a "6". It's a dead end. There is no way to get primary voltage into the house through an undamaged transformer. Aren't we lucky?

>they said it faulted about once a week...
What kind of fault? A lightning strike?
That seems likely. But the kind of fault that requires dispatching a repair crew and digging up the wire seems unlikely.
Do you know who "they" are?


[This message has been edited by Dspark (edited 10-05-2001).]

This message is being posted for Allan Hildenbrand.

---Quote---


[- Word Wrap was driving me crazy]

[This message has been edited by Webmaster (edited 10-10-2001).]

Thanks, Scott. I knew I wasn't crazy. (am i?)

It gets even better!!

After I sent my comment on this thread to Scott35 yesterday, I took a walk down the alley to the pole I describe above. And what to my wondering eyes should appear but the absence of the bottom five feet of the ground wire!!!

The end of the ground wire is oxidized black, which, to me, means the cut end has had a LONG time to weather.

The xformer is a 50 KVA unit which, with an 8 KV primary voltage and 150 percent overload means the primary current is about 9.5 amps.

Al Hildenbrand

[This message has been edited by ElectricAL (edited 10-10-2001).]

hey dspark,

a guy i work with, he does primary fault locating, hes worked with FPL for like 40 years.. he said they ran 1ø aluminum 40.1 miles from a riser to a few pad mounts.. radial feed... the guy i work with said it took something like a 45 amp fuse just to energize the cable.. anything lower and it would overload due to resistance or something.. the fuse ended up being around 100a with the pots at the end... he said there was a splice pit every 500 or so feet... thats like over 400 segments of cable... this is in the everglades too, so the cable is constantly surrounded by water... he said a few years after it was installed, it started faulting regularly once every week or 2.. and yes, a crew had to some out dig and repair...

-m

Looking back up the thread to 8.31.01 and Don's (resqcapt19) question:

Utilities use a connection with two transformers in an open delta fed with two hots and a ground to provide 3 phase power. I don't know what it is called but I have one right outside my construction trailer. The two primary hots feed the ends of the open delta. The ground is connected to the point of the open delta.
Don(resqcapt19)

Sounds to me like the primary is two legs of a grounded wye. With the secondary hooked up as open delta (and presumably a grounded center tap for 120/240) one gets 240 3-phase.

The full name of the hookup is "open wye - open delta". The two transformers can deliver 86.6% of their combined KVA.