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bud--

On 11/25/2015 2:48 PM, wrote:
On Wednesday, November 25, 2015 at 3:12:24 PM UTC-5, Ralph Mowery wrote:

I hope that damn 2 phase 220 house wire does not start up again. There is a
true 2 phase and then then there is the split phase 240 that is common in
most of the houses in the US that some try to claim is 2 phase.

OK - I live near Philadelphia - which, together with Baltimore, MD shares one of the
last areas in the US where one may still get actual 2-phase power. 2-phase, 4-wire
power was developed primarily for heavy motor use in that era when the battle of AC
vs. DC was not yet settled and 3-phase power was barely a gleam in Tesla's Eye.

All multiphase came from Tesla. And AC was widely thought to be useless
until Tesla. Tesla had patents for both 2-phase and 3-phase. Tesla
patents also covered essentially all possible AC induction motor designs.

It is common to use 480/277V 3-phase wye as the power distribution in
large buildings. Transformers from 480/277V to 208/120V are located
throughout the building in electrical rooms. Transformers have 3-cores
for the 3 phases. Small transformers may be simpler, with 2 cores
T-connected (Scott). These cores run at true 2-phase. The disadvantage
is the power factor on the cores is screwed up and they have to be derated.

(SIDE NOTE: The last DC building in NYC fed from the Pearl Street Station (for
~125 years) went down in 2007.) For whatever reason, both Philadelphia and Baltimore
accumulated an inventory of heavy freight elevators that use 2-phase motors to this
day, such that the local utilities continue to supply the power to a very few customers
in specific locations within each city. I had the privilege of working on several such
elevators and heavy water pumps in my youth while working my way through school as an
electrician.

NOTE ALSO: some few households at the time also got 2-phase. And both PA and Reading
railroad workers living along the rights-of-way got 25-Hz current - used for railroad
traction motors, also to this day.

Standard household power in the US is 1/3 of a Delta-connected 3-phase system, being
hot-to-Hot at a nominal 240V and hot-to ground at 120V. Single-phase as only 1/3 of the
total capacity is realized. All this is done in the single-phase transformer at the
sub-station, not the distribution transformer at the pole (vault) in the neighborhood.

For the houses in my area (and probably all of Minneapolis), a single
8kv L-N distribution leg is tapped off from the 13.8kv 3-phase wye
distribution. Something like 4 blocks are fed from each tap. A pole
mounted transformer feeds 240/120V to something like a block.