Why three phase. Remember power is proportional to voltage squared. If you
look at the sum of the power for each of the three phases you will find that
it is constant. So motors that use three phase power are much smoother than
2 phase motors and smaller for the same horsepower I believe. Also large DC
power sources use three phase power as it requires much less filtering.

Notice high-voltage transmission lines are always in groups of three for the
same reason. The voltage of three phase power supplied in moderate settings
is 208 so that 120 can be obtained using a "Y" type configuration.


"Toller" wrote in message
...
I know 3 phase is 120v line-neutral, and 208v line-line, but what is it
when all three lines are used? Or is it just not referred to at all, since
voltage between three lines doesn't mean anything?

Can 208v be called 2 phase? I know 2 phase is properly an obsolete system
that involved 2 120v line, 180 degrees out of phase; but is the term now
ever used for 208v?