On Sat, 15 Jan 2011 13:04:40 -0500, Jeff Thies wrote:

On 1/15/2011 11:22 AM, zzzzzzzzzz wrote:
On Sat, 15 Jan 2011 08:10:53 -0500, Jeff wrote:

On 1/13/2011 4:36 PM, David Nebenzahl wrote:
On 1/12/2011 7:22 PM Ralph Mowery spake thus:

"Dean 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Apparently that's one reason that 3-phase superseded 2-phase power.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.

In particular starting torque. All motors need a push in the right
direction to get them going. Often this is an artificial phase made by
the starting cap. You really want 3 phase for the big motors. In fact
the power company generates 3 phase power (sort of the reverse of a
synchronous motor), barring electronic means it is hard to get otherwise.

Not hard to get. A rotary phase converter will do it. Many woodworkers use a
three-phase induction motor as a rotary phase converter to power big tools
(often bought at auction from commercial installations).


I hadn't seen those before, using just a 3 phase idler motor. The Rotary
Phase Converters I had see were more traditional, and more involved
pieces of machinery (not to mention expensive). But it is not necessary
for a woodworking motor to have perfectly balanced 3 phase. Clever and
useful, nonetheless.

HVDC doesn't seem to have much impact in the US, but my rough
understanding is that converting back is all solid state these days.


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.

Technically, it's not two phase at all.

Call it what you want, but the two 120V lines are 180 out of phase
relative to neutral. Certainly not useful for starting a motor. I'll not
quibble over symantics.

No they, in fact, aren't. One is the negative of the other.