Environmental Engineering Reference
In-Depth Information
where
I
a
,
I
b
and
I
c
are the RMS values of the three-phase load currents. Hence, the RMS
value of the current drawn from the supply is
I
=
I
a
cos
θ
+
I
b
cos
θ
b
+
I
c
cos
θ
c
.
θ
=
θ
b
=
θ
c
and
I
a
=
If the loads are balanced, i.e. when
I
b
=
I
c
,
then the supply current is
I
=
3
I
a
cos
θ.
According to Kirchhoff's current law, there are
i
s
=
i
−
i
a
,
(14.1)
and
i
r
=
i
−
i
a
−
i
b
−
i
c
=
i
s
−
i
b
−
i
c
.
(14.2)
As a result, the phasor diagram of the system can be obtained as shown in Figure 14.5, where the
cases with linear balanced and unbalanced three-phase loads are illustrated. The current
I
s
is
I
2
I
s
=
+
I
a
−
2
I
a
I
cos
θ.
θ
θ
θ
(a) With linear unbalanced three-phase loads
θ
θ
θ
(b) With linear balanced three-phase loads
Figure 14.5
Phasor diagrams of the system
Search WWH ::
Custom Search