Environmental Engineering Reference
In-Depth Information
i o
D
+
~
v s
R
v o
-
(a) Topology
0
π
0
π
ωt/rad
ωt/rad
(b) Input voltage
(c) Output voltage
Figure 1.3 Uncontrolled rectifier with a diode
half cycle of the input voltage can pass the diode to reach the load and, hence, the output
voltage is of DC but with a significant amount of ripples. The input current is not sinusoidal
either so there is a significant amount of harmonic currents.
In order to reduce the ripples in the output voltage and to reduce the harmonics in the
input current, several diodes are often connected to form bridge rectifiers. Figure 1.4 shows
a single-phase bridge rectifier and its operation principle. Compared to the rectifier with one
diode shown in Figure 1.3(a), both half cycles of the input voltage are passed to the load. As a
result, the ripples in the output voltage are reduced and the harmonic components in the input
current are reduced as well. In this case, the DC output voltage is
2 2
π
π
2 V s sin
1
π
V o =
ω
t d(
ω
t )
=
V s
0
.
9 V s ,
0
where V s is the RMS value of the input voltage.
For three-phase applications, the bridge rectifier shown in Figure 1.5 can be adopted. The
pair of diodes with the highest instantaneous line voltage conduct, in the order of D 1 D 2
D 2 D 3
D 1 D 2 for 120 each time. Hence, the out-
put voltage is the envelope of the line voltages with six ripples, which further improves the
performance of the DC output voltage. In this case, the DC output voltage is
D 3 D 4
D 4 D 5
D 5 D 6
D 6 D 1
3 6
π
π/ 6
2
3 V s cos
1
π/
V o =
×
ω
t d(
ω
t )
=
V s
2
.
34 V s ,
6
0
where V s is the RMS value of the phase input voltage.
 
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