David Nebenzahl wrote:
I'm posting this to try to settle an argument going on in another
newsgroup (alt.home.repair) about phases in electrical power.
Over there, someone posted a question about GFCI breakers that morphed
into a discussion of multiphase electrical power. A disagreement arose
thereafter about whether a center-tapped transformer actually delivers
two separate phases of electricity or not. I'd like to get opinions
here, since at least some folks here have engineering backgrounds: over
there (a.h.r.), not so much. People there tend to be more electrician
types, rather than EEs and such.
The discussion started with a mention of 2-phase power. Turns out that
in the world of electrical power, this has a specific meaning. It refers
to a now-obsolete system of generating power in 2 phases that were 90°
apart, and was used at Niagara Falls:
http://www.3phasepower.org/2phasesystems.htm
And of course there's 3-phase power, widely used today.
The problem is this: several people, myself included, contend that the
two "legs" of power produced by a center-tapped transformer do, in fact,
constitute two separate phases of power, 180° apart. (This is how
household power is delivered in North America, with a step-down xfmr at
the power pole delivering 240 volts in the form of 120-0-120.)
Now it's true that in the electrical industry, this is called
"split-phase" power, and if you tried to tell the guy behind the counter
at the electrical supply house that it's 2-phase, he'd look at you funny.
However, I (and others) say that this is, in fact, true 2-phase power,
even if it's not called that. It just happens to be trivially easy to
generate it from a single phase, as it only involves inversion. (Unlike
3-phase, which requires rotary converters or electronic devices to
generate from single-phase power.)
Take, for example, any push-pull amplifier with a phase inverter or
phase splitter in front of it: it generates two separate phases out of a
single phase.
So, what do y'all say?
I do industrial EE work and terminology can really
get screwed up at times.
You have a CT (center Tap) this gives you 2 phases, using the
CT as the common, which of course are 180 degrees apart. How ever,
if you need 230 volts, you would need to use the 2 outer legs which
will only give you a single phase for that circuit. Hence, only
one winding over all. CT means nothing here.
To qualify for a phase, it must has a common or an opposing leg.
Lets look at a floating Delta 3 phase system, even though it has
no common to qualify it as 3 circuits 120 degree's out of phase, it
does have 3 separate windings, each 120 degree's out and can supply
a circuit on each winding if it wished.
Looking at a STAR (WYE), one end of each phase (winding) is connected
to a common point.. This common point does not need to be used but is in
some cases.
The point is, there are 3 individual windings that are out of phase
with each other. Makes it 3 phase...
In the case of residential, the CT will give you 2 phases of 180
degree's when you only need 115 volt circuits how ever, because the
transformer here is really only a single winding with a CT, when you
need full voltage for 230 appliances, that circuit ends up being
a single phase.. How ever, most appliances like that also have the
CT/Neutrual so it can use half of that to operate the control electronics.
Some people call it split phase, only because you are taking a single
phase and splitting it in the middle. But that does give you a 2 phase
source if you to use the CT as the common for both.. It would be the
same as 2 transformers with their secondaries joined on one side only to
form a CT.
Remember "Split Phase" only means a single winding with a CT it,
hence, you have split the phase in half!
Now here's a tricky one.. Just think of the 3 phase system with a CT
in each winding. Now we're talking !
Jamie