Existential Angst wrote:
The vast majority of 3 phase power in the US (that supplies 120V loads) is
208V (wye connection). New 240V (delta connection) systems are probably
quite uncommon.
Well, on the various ng concerned with machines all over the country, over
about 10 yrs I think I'm the ONLY one to reference 208 V, via NYC. Maybe
208 dominates in big industrial cities?
Even Long Island (NY) which perhaps has one of the highest concentration of
"small" machine shops ( 10,000 sq ft) in the country, uses 240 V.
120/208V dominates in general. Machine shops may want 240V because
machines may be commonly made for 240V. That may be historical - it may
be how early machines were made and was continued for compatibility with
older equipment. I have seen some machine tools in use that probably
predate Columbus.
But here's my Q:
For a given voltage, what difference would the end user see in terms of a
delta or wye connection?
And why is 208 wye, and 240 V delta?
208V has 3 - 120V transformers. One endpoint of each transformer is
connected together to form a "neutral". You have 120V from each of the
phase conductors to the neutral. That is a major advantage when you are
supplying 120V loads (compare to 240V delta). The voltage between phase
conductors is 208V. A 3-phase motor would be 208V. A diagram of the
transformer connections looks like a Y-wye (or star).
The major power distribution in a large building is likely 277/480V wye.
The 3 transformers are 277V with one end connected to a common neutral.
The voltage between phase conductors is 480V. Higher voltage means less
copper is used in wiring. A lot fluorescent (and non-incandescent)
lightning is 277V .For motors 480V 3-phase is nice. Stepdown
transformers to 120/208V wye are installed in electrical rooms where
necessary. [You could also get 240V delta.]
A 240V delta system starts out with a 120/240 transformer like is used
for a residential service. The center tap is the neutral, just like with
a residential service. For the 3-phase, 2 transformers are added, one
end of each connected to the ends of the original transformer and the
other end connected together to be the 3 phase "high leg". A diagram of
the transformer connections looks like a triangle or delta. The voltage
from the high leg to neutral is 208V. There may only be 2 transformers
(open delta). I suspect this system came from original 120/240V single
phase systems where some 3-phase load had to be added. You can add a
transformer, which can be much smaller than the original one (depending
on the 3 phase load). And there used to be a "delta breaker" (may still
exist) which I believe allowed 3 phase to be kludged into original
single phase services. I suspect this is how 3 phase came to many
machine shops. You have 120V from only 2 phases to the neutral and it is
much harder to "balance" the current in the 3 legs. If not balanced, the
voltages between phases may be different which increases motor heating.
If there are 3 transformers imbalance also causes "circulating" currents.
And, is each leg of the 240 V delta 3 ph separated by equal 120 deg shifts,
like the 208?
Yup.
I argue that it is not, that two of the phases *must* be 180 deg out of
phase, as that's the only way you could get 240 from two 120 legs. The 3rd
phase must be 90 deg to these two.
Not how delta works.
There did used to be 2-phase power (90 degrees). Niagra, which was
probably the first large hydro generation, was originally 2-phase.
Visavis 208/120 V systems, which is exactly consistent with 120 phase angle.
I argue this, but others hotly disagree, but without really being able to
tell me wye. 
If the above is correct, I surmise the reason is that the 208 3 ph is
supplied right from the generator, whilst 240 3 ph comes off of pole
transformers.
Nope.
You could look at machine shops and see if there are only 2 transformers
(open delta) and one is much smaller (most of the load is single phase).
For delta, one of the transformers has a 3rd connection (neutral).
--
bud--