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