2 to 3 phase

*IF* you had a 4-wire system with a neutral, yes. Without that, I think the answer is no (with just transformers).
daestrom

Yes.

On 10 Jul 2004 02:46:45 -0700, JP put forth the notion that...
If what you're asking, is can you use two of the three legs from a 3- phase system to make 3-phase power with a transformer, the answer is no. If you only have two legs, you only have one phase. A transformer can't take one sine wave and turn it into three sine waves spaced 120 degrees apart.

IF i understand the question correctly: maybe. in california, at least in the area i was in, it is common to find open delta 3 phase power where one node of the secondary is grounded and only 2 transformers are used.
an open delta schematic can be found here then again there are devices called rotary converters which convert single phase power to 3 phase. in my like of work they are used on mountain top and rural locations to run 3 phase transmitters when to power co. only supplies single phase. for example: i am curious as to what prompted the question. do you have a specific problem you are trying to solve?

| IF, in a 3-phase 3-wire system, one phase is disconnected can we | create three phases again from rest of the two phases by using | transformer?
If you only have 3 wires, then when one is disconnected, you now only have 2 wires. You essentially have a single phase system at this point. You cannot derive three phase from that.
If you also have the neutral wire of a 4-wire WYE system, then you can do it.
You wanted this:
* \ \ *-----* / / *
But all you got was this:
* \ \ * / / *
With two 120 volt to 120 volt transformers, you can wire up this:
* * \ / \ \ / \ * * / / *
You can also derive the lost leg of 240 volt delta center tapped if you still have the high leg (lost one of the 120 volt legs). A 240/120 volt autotransformer can be used to re-derive the lost leg.
In both of these cases, a lot of derating of capacity is needed.

yes, but you don't have full current capability.

Such an 'open-delta' system requires three-wires, carrying the three-phase power. The question is how to get three phase with only two wires.
A rotoray converter *could* be used, but the OP specifically asked '... by using transformers.'
daestrom

Okay, John, I'll bite. How do you do this with only two wires coming into your setup? The OP mentioned 3-phase, 3-wire with one of the 3-wires disconnected. How would you re-generate the third phase? Transformers only, no rotory converter.
daestrom

Sorry, I read too quickly. I ASSumed that it was your basic "wye" type system.

i guess it depends on how you "disconnect one phase" could be one 'pole pig' opens up and you still have 3 wires.

er
It appears that the open delta you are referring to does have a 3 phase 3 wire primary supplying two transformers. You get a 3 phase output from the secondaries (whether the common point is grounded or not) This is a not uncommon approach in light load areas as a 3rd transformer can be added when load increases. Its capacity is about 58% of that of a full 3 phase bank. However, the primary would still be 3 phase.

Open-wye primary?open-delta secondary is a common utility-distribution configuration,.served from 2 phases and a neutral of a 4-wire wye system. See Cooper Power bulletin R201-90-2 figure 21
--s falke

Yes. But did you read the OP where he specifically asked about 3-phase, *3-wire*??? With no neutral?? Kind of makes your comment pointless.
daestrom

Well, the OP asked, "IF, in a 3-phase 3-wire system, one phase is disconnected can we create three phases again from rest of the two phases by using transformer?"
If he just lost one phase transformer in a delta system, why would he ask how to re-create three phases? As you say, an 'open-delta' doesn't actually lose any phase (just capacity), so there is no need to 'create three phases again from rest of the two phases'. Perhaps we read the question differently, but *I* read it as have 3-phase, 3-wire lead in and you lose one phase for some reason. With only two wires left, you can't recreate the 3-phase again with just transformers. You need either an open-delta with 3 wires, or wye connection with neutral.
daestrom

Thanks everybody for your replies. I am from an Asian country where electricity is scarce. So when farmers' require electricity for agricultural activities, the Electricity supply agencies disconnect one phase, from distribution point, of electrical lines which go to factories so that they can not use electricity. One of my friend wanted to run a 3 phase furnace (the furnace works on 23VAC and current consumption is 55 Amps.). He asked me the question, which I posted on forum.
BTW The following link mention about "Scott T connection of transformers is a method used for changing 3 phase to 2 phase or vice versa. is no mention of a neutral wire here. Just "3 phase to 2 phase or vice versa". Can this type of transformer(s) solve our problem?
We have "wye" system and do have a neutral wire, which is not disconnected. " I am very sorry for not mentioning it earlier."
Because of some reason my mind was fixed on the words of my friend i.e. " 2 to 3 phase" which he used when he asked me the question.
When so many experts are here to answer I want to take liberty of asking another question which is related to this topic. Can we create neutral of "wye" system again at our end if it is disconnected by electricity supply agencies assuming the all the 3 phases are present?
Best regards

| Thanks everybody for your replies. | I am from an Asian country where electricity is scarce. | So when farmers' require electricity for agricultural activities, the | Electricity supply agencies disconnect one phase, from distribution | point, | of electrical lines which go to factories so that they can not use | electricity. | One of my friend wanted to run a 3 phase furnace (the furnace works on | 23VAC | and current consumption is 55 Amps.). He asked me the question, which | I posted on forum.
I assume you meant 230 volts.
| BTW The following link mention about | "Scott T connection of transformers is a method used for changing 3 | phase to 2 phase or vice versa. | | There is no mention of a neutral wire here. Just "3 phase to 2 phase | or vice versa". | Can this type of transformer(s) solve our problem?
The voltage between any two phases, compared to the voltage between the remaining third phase and neutral, in a wye/star system, is 90 degrees apart. Two transformers with the appropriate voltages can be used to produce three phase at another voltage this way. Ratings are reduced somewhat, but it works, and many transformer sets are built this way. But you need all 4 wires to do it. That is what a "Scott T" connection is. The vector diagram looks like a "T".
Your problem can be solved with two transformers in a different configuration as my previous post described.
| We have "wye" system and do have a neutral wire, which is not | disconnected. | " I am very sorry for not mentioning it earlier."
I suspected that. Often the neutral is there and is not counted by many people. Motors designed for delta configurations do work on wye configurations and do not need to be connected to the neutral. Outside of the North American region, delta is much less common as a power configuration.
Delta would also cause problems if just one phase were disconnected. The two working phases would energize the disconnected line. This does not happen with wye.
| Because of some reason my mind was fixed on the words of my friend | i.e. " 2 to 3 phase" which he used when he asked me the question.
When one looks at the vector diagrams for center-tapped single phase such as used in North America, and then wye (or star) three phase, it really makes one think that single phase could be called two phase. There are indeed two vectors from "zero". They just happen to be at 180 degress, and thus can be wound on the same core as the other, or just be one winding with a center tap. So that's why it is usually just called single phase (you can't derive any more phases from it whichever way you call it).
But if you have 2 phase angles that are 120 degrees apart, with the neutral in between, then you can derive the missing 3rd phase using a pair of transformers like I described in my previous post in this thread.
| When so many experts are here to answer I want to take liberty of | asking another question which is related to this topic. Can we create | neutral of "wye" system again at our end if it is disconnected by | electricity supply agencies assuming the all the 3 phases are present?
Yes, a neutral can be created from the 3 original phases. There is a transformer specifically designed for doing this called a "zig-zag". Variations on that transformer also do other things, so you have to be specific. The design of it is a bit complex, but it is sufficiently common that many transformer manufacturers market complete units for this purpose.

snip of good stuff --------- --------- Actually, in Europe (where this system is not used) it is often called "2 phase" However the center tapped 3 wire system was originally called the Edison system and works with DC, it got stuck with this name and then with the 3 wire single phase (center tapped) nomencalture in the regions of the world where it is prevalent (and 2 phase refers to balanced voltages at 90 degrees) Neither of which are actually logical in the sense that with n phases you have n voltages 360/n degrees apart.

The furnace is powered by a 3 phase transformer which steps down 380V to 23VAC -(twenty three) 3 phase @ 55Amp. (low voltage high current).
regards

| Actually, in Europe (where this system is not used) it is often called "2 | phase" However the center tapped 3 wire system was originally called the | Edison system and works with DC, it got stuck with this name and then with | the 3 wire single phase (center tapped) nomencalture in the regions of the | world where it is prevalent (and 2 phase refers to balanced voltages at 90 | degrees) Neither of which are actually logical in the sense that with n | phases you have n voltages 360/n degrees apart.
Suppose you take a transformer that gives you 240/120 volts center tapped out, as in the Edison system. Now take two more of them for a total of three. Connect their primaries so each is on a different phase of three phase coming in. Connect all the secondary center taps together and to ground. Would you call the result "three phase" or "six phase". I'd call it six phase and the logic behind that is how else can you have six phase but that. Well, you could if you have some inconsistent phase angles not equally arranged.
In terms of the number of phase angles relative to ground coming out, a "Scott T" connection is three phase. But in terms of the number of magnetic field phase angles, it's only two. It seems the common nomenclature is to refer to the number of magnetic field phase angles, which is less useful in understanding how you can wire it up.