On 1/14/2013 11:22 AM, bud-- wrote:
On 1/14/2013 9:31 AM, wrote:
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?

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.


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



=======================================
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.