Hello friends in this video we are going
to see equations waveforms and phasor
diagram of three-phase supply so I will
write a equation first so already we
have discussed that will having three
single phase
supplies having 120 degree phase
difference between them so I will
consider the first voltage VR equal to
VM sine Omega T then next voltage will
be VY and it will be lagging VR by 120
degree
hence phase difference of 120 degree
will be there so it's a VM sine Omega t
minus 120 degree and third voltage will
be VB and that is equal to VM sin Omega
t minus 240 degree so if you talk in
terms of angle I can say this minus 240
degree same as plus 120 degree so this
is VB so these three voltages will be
present simultaneously lets draw
waveform so right now I have written
equations
now I am going to draw waveforms
so first voltage is VR VM sin Omega T
a sinusoidal voltage having a phase
angle zero so starting from zero so this
will be VR VY so VY is VM sine Omega
t minus 120 degree so it will lag this
vr by 120 degree because of this minus
sin so it will start from here and it
will be like this
this is VY so if I extend it will be
like this VY VB VB maybe will be the
voltage which will lack VY by 120
degree so it will be like this
so this will be VB so the phase
difference between them is 120 degree
now if this is a zero obviously this is
120 degree so VY lacks VR by 120 degree
whereas if I continue this like this so
it is starting from this point from here
to here it is 120 degree so what I can
say is VY lags VR by 120 degree and VB
leads VR by 120 degree why we are adding
this statement because from this we are
going to draw a phasor diagram so the
phasor diagram VR is a reference because
the phase angle is 0 its the RMS value
VY VY lacks VR by 120 degree so it
will be like this and VB leads VR by 120
degree
so this is all about equations waveform
and phasor diagram now we will see some
important terms related to three-phase
system and those terms are first we will
say what is symmetrical system so what
does that mean if in a three-phase
system the three voltages are of same
magnitude and same frequency and display
from each other by 120 degree obviously
that is the phase angle between them it
is defined as symmetrical system it is
defined as symmetrical system and second
most important parameter is phase
sequence what does it mean for that I
will draw three phase waveform once
again and here I will show you a
simplest way to draw a three phase
waveform so what I will do I will first
draw triangles like this with different
colors because this is VR this will be
VY and this will be VB I will draw
x-axis
and now I will connect these ends
and y-axis I will draw like this so this
is voltage and this particular waveform
is your VR so this is VR if I connect
this
this will be VY and finally if I
continue a form it will be like this VB
this will look a proper waveform whereas
this is 120 degree and from here to here
its also 120 degree now with respect to
this waveform we will define what is the
phase sequence so see properly here we are
will attain the maximum value first then
VY and then VB and if I extend this
further it it will be again we are VY VB
 so this thing is happening in sequence
are first then Y and finally B so that
is called as a phase sequence so let us
define it so the phase sequence is
nothing but the sequence in which the
voltages in three phase system or you
can say three phase supply reach their
maximum positive values is called phase
sequence generally the phase sequences
is RYB see all these given by the color
that normally we prefer for connecting a
phase that means our phase normally you
connect with a red wire Y phase yellow R
and B phase blue wire so what is the
need of this phase sequence very
important to understand need of phase
sequence or you can say what is the
significance of phase sequence what will
happen instead of having RYB  I have
 RBY or something like that so what
is the significance of a phase sequence is
this the phase sequence is important in
determining the direction of rotation of
AC motors and parallel operations of
alternators it is very important to note
down in which direction motor is moving
and if you change the phase sequence the
direction of rotation of AC motor will
get reversed so for this reason knowing
the phase sequence and connecting motor
as per the phase sequence is must thank
you
