On 1/12/2011 7:22 PM Ralph Mowery spake thus:

"Dean Hoffman" wrote in message
...

Metspitzer wrote:

It is considered single phase. If you remove the center tap, you
have the same thing on the primary as you do on the secondary.

If you chose to put the secondary tap anywhere but the center,
you still have 240 total, but the fraction of 240 changes as you
move the center tap.

This sentence is the one that doesn't ring true. "The two insulated
wires each carry 120 volts, but they are 180 degrees out of phase
so the difference between them is 240 volts. "

The common 240 volt system in the US is only single phase. A true 2 phase
system will have the the voltages only 90 deg out of phase. In a 240 volt
single phase system , the center, neutral or whatever you want to call the
wire will carry only the unballanced currents and can be the same size as
the other two wires. A true 2 phase system usually has 4 wires, but it it
is wired up with only 3 wires, the 'center' wire has to be the largest wire.

There are always some on here that do not understand the differance in a
split phase 120/240 volts system ususally used in the homes and a true 2
phase system. I doubt that hardly anyone here has seem a true 2 phase power
system.

No, I had never really known what they were until now. Just looked it
up, and it's just as you say.

I'm wondering, though, just how effective or even useful 2-phase systems
really were. If you graph the waveforms, you see that there's a hole in
it, a "missing" phase, the one that would start at 180° that's present
in a 3-phase system. So what you have is current pulses that go
"bump-bump (pause)" instead of "bump-bump-bump", right?

Apparently that's one reason that 3-phase superseded 2-phase power. It's
true that 2-phase was better than single-phase for running certain types
of induction motors.

It's a matter of semantics, I know, but the 120+120=240 system we've
been discussing actually is a 2-phase system, even though it's not
really called that. One side is 180° out of phase with the other side,
so by definition you have a 2-phase system.

Hopefully the previous poster who brought this up and was confused by
this is less so now.


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