Page 182 - Rashid, Power Electronics Handbook

11 Single-Phase Controlled Recti®ers 171

i i
i T1 d d V
P P max
i g1 i g1
T T
T 3 T 2
1 1 0 wt
i T3
+ i + i
v s v Load v s v Load
s d s d
v -v
i s s
T T D D d
4 2 1 2
N N I
i T4 d
0 wt
a) b) i ,i a
g1 g2
FIGURE 11.5 Single-phase bridge recti®er: (a) fully controlled; and (b) 0 wt
i ,i
half controlled. g3 g4
0 wt
i ,i
T1 T2
addition, Fig. 11.5b shows the half-controlled bridge recti®er,
which uses two thyristors and two diodes. wt
0
Figure 11.6 shows the voltage and current waveforms of the i ,i
fully controlled bridge recti®er for a resistive load. Thyristors T3 T4
T and T must be ®red simultaneously during the positive 0 wt
2
1
half wave of the source voltage v so as to allow conduction of
s
current. Alternatively, thyristors T and T must be ®red i s
4
3
simultaneously during the negative half wave of the source 0 wt
voltage. To ensure simultaneous ®ring, thyristors T and T
1 2 FIGURE 11.7 Waveforms of a fully controlled bridge recti®er with
use the same ®ring signal. The load voltage is similar to the
resistive-inductive load (L !1).
voltage obtained with the biphase half-wave recti®er. The
input current is given by
Figure 11.7 presents the behavior of the fully controlled
i ¼ i T1 ÿ i T4 ð11:5Þ recti®er with resistive-inductive load (with L !1). The
S
high-load inductance generates a perfectly ®ltered current
and its waveform is shown in Fig. 11.6.
and the recti®er behaves like a current source. With contin-
uous load current, thyristors T and T remain in the on-state
2
1
v beyond the positive half-wave of the source voltage v . For this
d s
V reason, the load voltage v can have a negative instantaneous
max d
value. The ®ring of thyristors T and T has two effects:
3
4
0 wt
i) they turn off thyristors T and T ; and
1
2
v -v
s s ii) after the commutation they conduct the load current.
i ,i
g1 g2 a
This is the main reason why this type of converter is called a
0 wt ‘‘naturally commutated'' or ‘‘line commutated'' recti®er. The
i ,i
g3 g4 supply current i has the square waveform shown in Fig. 11.7
S
0 wt for continuous conduction. In this case, the average load
i ,i voltage is given by
T1 T2
wt
0
ð pþa
i ,i 1 2V max
T3 T4 V dia ¼ V max sin otdðotÞ¼ cos a ð11:6Þ
p a p
0 wt
i
s 11.1.4 Analysis of the Input Current
0 wt
The input current in a bridge-controlled recti®er is a square
waveform when the load current is perfectly ®ltered. In
FIGURE 11.6 Waveforms of a fully controlled bridge recti®er with addition, the input current i is shifted by the ®ring angle a
s
resistive load. with respect to the input voltage v , as shown in Fig. 11.8a. The
s