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The generator at the
power station which
produces our AC mains
rotates through 360
degrees to produce one
cycle of the sine wave
form which makes up the
supply.
In the next diagram
there are two sine
waves.
They are out of phase
because they do not
start from zero at the
same time.
To be in phase they must
start at the same time.
The waveform A starts
before B and is LEADING
by 90 degrees.
Waveform B is LAGGING A
by 90 degrees.
The next left hand
diagram, known as a
PHASOR DIAGRAM, shows
this in another way.
The phasors are rotating
anticlockwise as
indicated by the arrowed
circle.
A is leading B by 90
degrees.
The length of the
phasors is determined by
the amplitude of the
voltages A and B.
Since the voltages are
of the same value then
their phasors are of the
same length.
If voltage A was half
the voltage of B then
its phasor would be half
the length of B.
All this has nothing to
do with "set your
phasors on stun".
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The voltages A and B
cannot be added together
directly to find the
resulting voltage,
because they are not in
phase.
The result of the two
voltages can be found by
completing the phasor
diagram as shown on the
right.
The resulting voltage is
slightly greater in
amplitude than A or B,
and leads B by 45
degrees and lags A by 45
degrees.
Since the two voltages
are 90 degrees apart,
then the resultant can
be found by using
Pythagoras, as shown.
In Fig. 1 above,
the two phasors are 180
degrees out of phase.
The resultant voltage is
found by subtracting B
from A.
The result is a voltage
in phase with A but
slightly smaller in
amplitude.
In Fig. 2 the two
voltages are in phase
and are added to find
the result, which is in
phase with A and
slightly greater in
amplitude.
In Fig.3 a
parallelogram must be
constructed to find the
resulting voltage.
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