Scott-Tee Transformer Connection

JAL

There have been questions of what is a 2 phase 5 wire system. Well it is really called a Scott-Tee transformer connection. The basic design is that 3 phase high voltage feeds the line side of the transformer and you get a load side of 240/120 volt 2 phase 5-wire. The 240/120 volt 2 phase has two individual phases, which are 90° out of phase with each other – unlike 3 phase, which has three individual phases, which are 120° out of phase with each other. A Scott-Tee transformer connection is an antiquated polyphase power system, which was made obsolete by the present single and 3 phase systems.
Just be careful out there, you never know what you will run into.

Sparky Joe

wow, interesting

JAL

If somebody out there has a schematic it would make it more informative. This stuff is real old and there doesn’t seem to be much information on this subject.

Sparky Joe

So it sounds like the secondary would be 4 phase(because of 90 dgrees) though only 2 are used. Your first 3 wires are hots A,B,C and the second 2 are neutrals, center tapped between A-B and B-C. Wonder what you'd get from A to B-C neutral or from A to C for that matter. Not sure if I even know what I'm talking about, a drawing sure would be nice.

JAL

No the transformer is really a 3 phase primary to (2) 2 phase secondary with a common neutral (5 wires). From what I understand there are variations on this where you can have more than (2) 2 phase secondarys.

About 1900 this transformer configuration was used to step up the voltage and convert from 2 phase to 3 phase for long distance transmission at the generator station. Then at the other end the 3 phase would be converted back to a lower voltage and 2 phase for the end user.

You need to remember that there is a difference between single phase (present day residential service which is 180 deg. out of phase) and this old 2 phase( which is 90 deg. out of phase).

I know this is hard to understand but I am having to think real hard about it too.

It would be real helpful if there was a schematic for this.

My brain is going into meltdown. AHHHHHHHHH

Sparky Joe

I just assumed it was 4 phase, though I never heard of such a thing, because of the 90 degree phase shift. Saw lots of junk when I searched for a picture, though none of them look like they relate. But I saw that a Tee connection is 2 coils, with one tapped from the center of the other.
Do you know what kind of voltage variations you get from one of these? or is it simply 120/240 and nothing else?

frenchsparky

i try to post the url link but this forum refuse let me to do this so if you want it i will ask other person here to help me and i will provide the link so we can show the display here


merci , marc

Sparky Joe

Just type it in without the link Frenchy, I'll take the time to look it up.
Very curious to see how this works

JAL

I found it. The red lines were added where I thought the neutral would go. Strange beast ain't it?

Attachment 2153

Sparky Joe

So its a Tee primary with 4 taps and a common neutral off the secondary, kinda tough to imagine in practical application, stuck thinking inside the box I guess, I still wonder if you'd get any odd voltages from '1' to '3', for example.

JAL

I was thinking the same thing. With the neutral that I added the voltage from 1 to 3 would be 240 volts maybe. Because of the phase angle involved maybe not? We need to hear from somebody that has actually worked on the stuff in order to be sure.

I read somewhere that Mexico uses this transformer connection a lot.

frenchsparky

http://electrical-contractor.net/ubb...ML/000054.html

this is the part of photo describing the scott tee transformer what it did show from 3 phase to 2 phase

most area in usa 2 phase is about history allready but there are few exsting servcie still using that

merci, marc

RobertWilber

The Scott connection is an alternative source from which to derive acceptable 2 phase power from a three phase source. In Philadelphia, many areas actually still have real 2 phase 5 wire distribution available. New high tension services can only be 3 phase. Go to my website and find excelon on the links page. select PECo and search "Blue book" then type in 2 phase. There are diagrams available.

I have seen guys miswire three phase motors into 2 phase systems and have other loads work like a Scott winding [until they burned up ...]

It is like having 2 normal services with a common neutral, but the pairs of hots 90 degrees out of phase [at right angles] It only gets ugly if you pick up the wrong 2 hots as a pair [funny voltages]

When I was learning the trade, I worked for a contractor who told me he had a transformer blow up because of wiring to a piece of 2 phase switchgear that was manufactured improperly. He said the conduits buried under the concrete blew right up out of the floor.

JAL

Thanks Robert. I knew there was somebody out there that knew about this. Did I draw the neutral in the right place? I just guessed as to where it would go and if both neutrals were supposed to be connected together to make it 2 phase 5 wire. The only diagrams I could find were 2 phase 4 wire.
From looking at the transformer connection it did occur to me that there could be some real high fault currents on the secondary. Your comment "conduits buried under the concrete blew right up out of the floor" sort of confirmed what I thought.

RobertWilber

there is so much of 2 phase that has been replaced that I don't hit much Scott stuff anymore, but it looks right [without checking].
I had a 2 phase service fault because of improperly installed grounding that fused [melted] all the way under 16th street to the main runs.
Like a puddling furnace.
didn't even trouble the transformer.
Just sat there spitting molten copper out of the end box
really exciting
first time I ever had a fire truck move so I could park