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.)


David et al

A specific that many are not aware of and are left wondering when
discussed is the following.

The transformer that feeds your home and many small businesses has a
single phase impressed across the primary winding. Usually called a HV
winding. No big deal for this discussion. The "secondary winding" is
actually two identical windings. Sometimes called tertiary windings.
But this term usually leaves most on the dark. Not a commonly used
term since we don't usually get into the actual electrical and/or
mechanical construction of transformers, let alone the ones feeding
our homes. To most they are simply an unknown on the pole in the alley
or in a pedestal box in the front yard of residential houses.

Back to the actual transformer. A primary winding and two identical
secondary windings. The seondary windings are wound identically within
and about the primary winding. Well insulated of course. The primary
winding impresses each winding identically, in this case, 120 volts.
So the operation of the secondary windings are identical.

The secondary windings are connected in an addtive fashion to give 240
volts across the ends of the connected windings. Their common
terminal, internally connected, is sometimes thought of as a "center"
tap. But the windings are not actually centertapped. They are only
connected in a additive fashion to give the desired 240 volts. 120
volts is of course is simply a connection to a hot lead and the
common terminal. When both windings are utilized we end up with two
"hot" leads which are used for the 120 volt loads. The same additive
principle could be used to give perhaps 360 or 480 volts.

But in no case, with the information presented here, is there any
method or device utilized to give any additional phases. One or two or
more. The hot legs are simply the same phase, transformed as needed,
added within the transformer, with the resultant two hot legs, 180
degrees out of phase with each other. Were the windings connected "in
phase" the available voltage would be 50% and the power, amps,
available doubled.

Three phase distribution starts with a generator with 3 sets of
windings that is distributed as three phase and utuilzed as needed.
Residential as this discussion continues, and large users with many
motor (inductive) loadings commonly found in motors 5 or more HP. The
5 HP figure is an industry norm that takes advantage of the inherent
advantages of 3 phase. Mainly distribution costs and three phase
motors.

Disclaimer: Transformers, power distribution, are not my usual area of
expertise. This would be several different lifelong careers followed
by others. I simply deal with it on an almost daily basis and have for
over 50 years.

Enough for a long day.

Bob AZ