Just an observation - and somebody correct me
if I'm wrong ..
>From what I have read and studied on power
generation, power transmission and power
distribution, the power companies extend out from
the central (generating) station to the various sub-
(step-down) stations all three phases of generated
voltages from the generator (speaking of AC
systems now! And I'm not addressing an inter-tied
tructure as the present 'power grid' is built and
operated either.).
This would seem to be done for several reasons,
not the least of which is, three phase power generation
provides for a more equal 'torque-loading' (on
account of the sinusoidal function involved when
a wire is rotated in a circle and made to 'cut' through
magnetic lines of force) of the generator's armature
than a single phase would, and this also translates
out into the user segment of the system when higher
amounts of power (e.g industrial users and greater
amounts of torque as say provided by three-phase
motors) is required.
It also seems that the power companies further
extend three phase distribution lines out into the
area to be served also for the purpose of being
able to load-balance all three phases; single phase
business and residential 'user' loads can easily
be swung from a more heavily loaded phase,
for instance, to a less loaded phase as an area
develops or gets built-out. At some point, out
towards the 'edge' of a service area single phase
distribution lines begin to appear (we used to have
an old REA line running along side one of our
major secondary roads about 10 years back, but that
'line' has upgraded to all three phases now!)
So, it is this generation/transmission/distribution of
all three phases would appear to the be basis for
the "divisable by three" rule.
Caveat: All this is the result of observation and some
self-paced study as I have never worked for, or worked
in, the power generation or power transmission or
power distribution industries.
Jim P // WB5WPA //
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