Three phase from 2 transformers?

In a current thread, gfretw told a poster that one way to indentify 3 phase service is if it's connected to two or three transformers. That brought back a question that has stumped me for a very long time.
I know that it's possible to get 3 phase service with just two transformers. Apparently that is used for cost savings on lighter 3 phase loads. But what I don't understand is how you can get true 3 phase with only TWO transformers. Coming down the road you have 3 high voltage lines. Each of those has an AC waveform and each is 120deg off in phase from the other. If you have three step-down transformers, then you have one transformer connected to each high voltage line and obviously you get 3 waveforms out that have the same 120deg phase relation as those on the high voltage side.
But how do you get 3 waveforms at 120deg shift relative to each other with only two transformers connected to only two of the 3 high voltage lines?
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snipped-for-privacy@optonline.net wrote:

Hi, I can see ut, it depends how the windings are wound and how they are connected. I can visualize Delta 3-phase.
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On 1/14/2013 9:31 AM, snipped-for-privacy@optonline.net wrote:

Would be a lot easier to explain with a pencil and piece of paper.
The usual delta connection has a 120/240V transformer, and then adds 3-phase to that. A diagram is about halfway down http://en.wikipedia.org/wiki/Three-phase_electric_power where the transformers are connected in a triangle.
L1-N-L2 is a single phase 120/240V Edison circuit. Two transformers add L3 - L1-L2-L3 is 3 phase 240V. The voltage from N-L3 is 208V and is called the "high" leg (or several other names). This is "high-leg delta".
But it still works if the L1-L3 transformer is not used. It is then called open delta and only uses 2 transformers. This used to be moderately common and is still around. It is probably not used for anything new. It is pretty easy to identify because of 2 utility transformers, only one of which has a secondary neutral connection.
======================================For a wye connection, a Scott (or T) transformer connection can be used. See the diagram about halfway down labeled "Scott T transformer 3ph to 3 ph" http://en.wikipedia.org/wiki/Scott-T_transformer X1-X2-X3 are 3-phase with X0 being the neutral. This gives a wye 3-phase. A rather common use would be 480V 3-phase connected to H1-H2-H3 and 277/120V coming out X1-X2-X3-X0.
This is used on some relatively small "dry" transformers in buildings with a 480V service. The advantage is you have 3 phase with only 2 transformers. The disadvantage is you screw up the power factor on both transformers and have to derate them to 87% (if I remember right). (And if I remember right, the delta transformers above also have to be derated for the 3-phase power.)
I don't think utilities ever use this connection.
The transformers actually run at real 2-phase at 90 degrees. You can use a Scott connection to actually use the 2-phase.
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But the question is, when using only two transformers tied into two of the 3 phases, what generates the missing third phase that is 120deg away from the other two? Does a special transformer give the phase shift? A typical transformer that you would use in a 3 phase installation using 3 transformers, tied to all 3 high voltage phases gives outputs that are not phase shifted, or at least by only a little, right?
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On Tue, 15 Jan 2013 06:11:52 -0800 (PST), " snipped-for-privacy@optonline.net"

With the open delta, usually the only time you have phase problems is if you connect 240v line to line loads across the "missing" transformer. Then you will see voltage differences phase to phase.
The other thing you need to know is the line to neutral voltage on that 3d phase is 208v. It should be on the "B" phase and identified with orange tape in the panel. I still see it around here a lot, particularly in light industrial bays. It gives the user 3 phase and also some 120-240. Generally it is always done with 2 transformers, a big one and a smaller one. If you are ever in Key West at the Hog's Breath saloon, take a look at their transformers. That is the only place I have ever seen that has 3p center grounded delta with 3 transformers. I am guessing they grew into that, adding single phase 240v loads.
There is also another, rarer, delta, corner ground. They use that when there are no 120v loads. The usual place is a pumping location like a sewer lift pump on the side of the road. It can still be 2 transformers but the neutral connection is between the two transformers, not the mid point of one. When you physically look at the panel it is wired like 120/240 with 2 hots and a neutral but you have 240/0/240 instead of 120/0/120 and they are 120 degrees out of phase since the transformers are fed from multiple 3p primaries. The equipment looks exactly like regular 120/240v equipment except it carries a "delta" rating. (240v breakers) The grounded phase will be white.
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On Jan 15, 12:03pm, snipped-for-privacy@aol.com wrote:

But I'm not talking about phase problems. I'm talking about what creates the missing third phase period. Coming down the road we have 3 high voltage phases. One is at 0deg, one is at 120deg, one is at 240deg. If I hook up 3 seperate transformers to get 3 phase, which I see a lot, then coming out of those 3 transformers are lower voltage waveforms that are at 0deg, 120deg, 240deg, neglecting any unintentional shift. So, you have 3 phase just like on the high voltage side, just stepped down. It makes sense.
But now if they only use 2 transformers, I can see where you directly get any two of those 3 waveforms. Let's say I hook one of the two transformers to the 0 deg high voltage line. I hood the other to the 120deg high voltage line. I can see how out of that I get 0deg and 120deg for the 3 phase service, just like I did before. The question is, what generates the 240deg phase? Phase shift from one of the transformers?
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On Tue, 15 Jan 2013 09:52:19 -0800 (PST), " snipped-for-privacy@optonline.net"

You only need 3 points (phase A,B,C) to get three phases. In a delta the third (BC) winding is not necessary.
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On 1/15/13 11:03 AM, snipped-for-privacy@aol.com wrote:

Some cut.
One of the local REAs did the 480 volt version of that for irrigation well hookups. The largest well motors in this area are 100 hp. (124A) Add about 15 amps for the irrigation system.

A couple utilities do that for 3 well motors. We use fuses so we slug the grounded phase. They still use three transformers if there is a well motor. I think they'll use two transformers if it's only the irrigation system needing power. Those usually don't take over about 15 amps at 480.
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On 1/15/2013 8:11 AM, snipped-for-privacy@optonline.net wrote:

Crappy ASCII art (requires fixed width font):
B \ \ \ \ \ A ___________C N
This is a 240V open-delta secondary connection diagram (all diagrams are show pseudo-phasor relationships). A center-tapped 240V transformer is shown between A and C. A 240V transformer is shown between B and C.
Transformer connections on the primary are different voltage but connect to the 3 phase supply the same way (with no neutral). *The primary connections use all 3 phases*. The primary connections to the 3-phase supply put transformer AC and transformer BC at a 120 degree phase relationship. As a result you have 3-phase at secondary A-B-C just like you have 3-phase where the transformer primaries are connected.
=============================================Any modern 3-phase is likely to be 208/120V wye, or 480/277V wye for larger buildings and industrial.
A | | | | N / \ / \ / \ / \ B C
There isn't a simple trick like open delta for wye (because you need to create the neutral), but the Scott connection is about as simple.
B | | | |N | A____________C Q
On the primary side one transformer connects between phases A and C. The other transformer connects from phase B to the centertap (Q) of transformer AC. As a result the transformers are electrically at 90 degrees.
The secondary connections are similar and convert the 2-phase from the transformers back to 3-phase. The critical addition is a tap at N to supply the neutral.
For a 208/120V secondary you have AB 8V. AN=BN=CN0V (BQ0V)
===========================For both open-delta and Scott the transformer primaries connect to all 3 phases, which determines the phase relationships between the 2 transformers, and the phase relationship of the transformers creates the 3-phase at the secondary connections.
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On 1/14/2013 11:22 AM, bud-- wrote:

Open delta is our fairly large utilities standard implementation for light commercial loads that need 3 phase.

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On 1/14/2013 9:31 AM, snipped-for-privacy@optonline.net wrote: ...

Look up "open delta" ...
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