Phase Alternating Current
The power of alternating current (AC) fluctuates. For domestic
use for e.g. light bulbs this is not a major problem, since the
wire in the light bulb will stay warm for the brief interval
while the power drops. Neon lights (and your computer screen)
will blink, in fact, but faster than the human eye is able to
perceive. For the operation of motors etc. it is useful, however,
to have a current with constant power.
Variation for Three Phase Alternating Current
is indeed possible to obtain constant power from an AC system
by having three separate power lines with alternating current
which run in parallel, and where the current phase is shifted
one third of the cycle, i.e. the red curve above is running one
third of a cycle behind the blue curve, and the yellow curve
is running two thirds of a cycle behind the blue curve.
we learned on the previous page, a full cycle lasts 20 milliseconds
(ms) in a 50 Hz grid. Each of the three phases then lag behind
the previous one by 20/3 = 6 2/3 ms.
you look along the horizontal axis in the graph above, you will
find that the sum of the three voltages is always zero, and that
the difference in voltage between any two phases fluctuates as
an alternating current.
the next page you will see how we
connect a generator to a three phase grid.