Depends on the 3 phase wiring:

208/120 wye (or "Y") is very common, provides 120V and sqrt(3)*120=208 V
with the center of the Y or star grounded to make a grounded neutral.
Similar is 480/277 wye, provides 480V 3 phase for big loads (say A/C) and
277V for fluoresent lighting (a little less than 300V, above which you must
use 600V rated wire, switches, etc., but near enough to reduce th ecurrent
needed for a given wattage load)
then there's 240V Delta (and also 480V Delta) (no 120V as the three
transformer winding are wired in a triangular or delta configuration not a Y
or star configuration, but sometimes you need a little 120V 1 phase too, so
you centertap one of the three windings, ground it to make a grounded
neutral and you also have 120/240 single phase -- but -- the phase opposite
the centertap (the "Wild Leg" or "Hot Leg" is at a higher voltage with
respect to your grounded neutral.

Single phase service is just a single centertapped 240V winding, yielding
120/240 single phase.

The higher voltage distribution wires are invariably delta as they don't
need a neutral, and could be 480V, 2400V, 4800V, 13,200V 13,800 etc.

For the compressor another option may be a static (electronic) or a rotary
phase converter, but a "230V" capacitor start 1 phase motor may be
cheaper -- an undersized motor, within reason, could be used too if the
pulley's changed to run the compressor slower, say a 2 HP instead of a 3HP,
etc.


"Chief McGee" wrote in message
news2t3c.510193$I06.5575395@attbi_s01...
Adam, Please expound on why one of the 3phase is higher voltage. The "hot"
leg? Thanks

"Cylon" wrote in message
...
In article ,
Anthony wrote:

Single phase (110V) is as you describe. 230V power, is actually 'two
phase' power, and they are also out of phase with each other, so that
you
get somewhat the same effect as 3 phase.

This is not accurate. Most residential areas are fed 230V single-phase
power from the pole transformer. It's not 2-phase power. The transformer
is center-tapped to provide 120V single-phase power for outlets. At both
voltages, you still have 1-phase, 2 conductor power. In apartment
buildings and commercial properties, the 120V outlets are often coming
from 2 conductors of 120/208 3-phase power. From any 2 legs, you have
single-phase power.

3-phase power is exactly that, 3 phases 120 degrees apart. The advantage
of this is that you have enough phases to create a rotating magnetic
field in a motor without the need for phase-shifting capacitors,
resistors or inductors (so cheaper, more reliable motors drawing lower
current per conductor). Also, flipping any 2 legs will reverse your
motor--not needed for a compressor but darned handy for a lot of machine
tools, especially when tapping. 3-phase motors do pulsate less than
single-phase motors also, because of the aforementioned lack of
zero-crossings in the supply voltage. This directly affects the finish
of some machining operations like high-precision surface grinding.

Lastly, there was 2-phase power once. It was 3 wires with voltages 90
degrees apart IIRC. That's going back a ways though, it was extinct long
before even my parents were born. It had, best I can tell, no advantage
over 3-phase (only disadvantages).

The answer to the original poster's question is to get a 1-phase motor.
Assuming the compressor is 5HP or less, this is the cheapest option. If
it's over 5HP, adding a big single-phase input VFD to the motor may
compete in price with a big 1-phase motor.

-Adam
adam at airraidsirens dot com