Page 181 - Rashid, Power Electronics Handbook

170 J. Rodrõ Âguez and A. Weinstein

Area A T1
v 1
v i d v Area A
+ L - d R 2 i s v 1 v T1 i T1
R
i + L 0 v ,v d wt v + N 2 i d
+ G + p 2p s N
v R v v v R v
s v d - v d d s 2 T2 i d
- N 1 T2
- i G v
0 wt T2
a) i i
s s
Area A
1
v i v Area A
+ L - d d 2 v d
i + L i d E d i N 2 N 2
+ G + 0 wt i = i · i = i ·
s
T2
s
T1
v E v 2p N 1 v N 1
s v d - d d v d
- s V max
- i G
0 wt 0 wt
b)
v -v
s s
i
FIGURE 11.2 Single thyristor recti®er with: (a) resistive-inductive load; g1 a
and (b) active load. 0 wt
i g2
When an inductive-active load is connected to the recti®er, 0 wt
as depicted in Fig. 11.2b, the thyristor will be turned ON if the i T1
®ring pulse is applied to the gate when v > E . Again, the 0 wt
s d
thyristor will remain in the ON state until A ¼ A . When the
1 2
thyristor is turned OFF, the load voltage will be v ¼ E . i T2
d d
0 wt
11.1.2 Biphase Half-Wave Rectifier
i s
The biphase half-wave recti®er shown in Fig. 11.3 uses a
center-tapped transformer to provide two voltages v and v . 0 wt
2
1

These two voltages are 180 out-of-phase with respect to the
mid-point neutral N. In this scheme, the load is fed via a FIGURE 11.3 Biphase half-wave recti®er.
thyristor in each positive cycle of voltages v and v and the
1 2
load current returns via the neutral N.
T L =0
With reference to Fig. 11.3, thyristor T can be ®red into the
1 a=0º î =V /R
ON state at any time provided that voltage v > 0. The ®ring i d t)( R max T L =1ms
T1 1.0
pulses are delayed by an angle a with respect to the instant R i ˆ T L = 3.2 ms
0.8 2
where diodes would conduct. Figure 11.3 also illustrates the p T L =10ms
current paths for each conduction state. Thyristor T remains 0.6
1
T L
in the ON state until the load current tries to go to a negative 0.4
value. Thyristor T is ®red into the ON state when v T2 > 0, 0.2
2
which corresponds in Fig. 11.3 to the condition at which t
0
v > 0.
2
The mean value of the load voltage with resistive load is FIGURE 11.4 Effect of the load time constant over the current ripple.
given by

®ring angle a ¼ 0 . The ripple in the load current reduces as
ð p
1 V max the load inductance increases. If the load inductance L !1,
V dia ¼ p a V max sin otdðotÞ¼ p ð1 þ cos aÞ ð11:4Þ then the current is perfectly ®ltered.
The ac supply current is equal to i ðN =N Þ when T is in the
1
2
T1
1
on-state and ÿi ðN =N Þ when T is in the on-state, where 11.1.3 Single-Phase Bridge Rectifier
1
T2
2
2
N =N is the transformer turns ratio.
2
1
Figure 11.4 shows the effect of the load time constant Figure 11.5a shows a fully controlled bridge recti®er, which
^
T ¼ L=R on the normalized load current i ðtÞ=i ðtÞ for a uses four thyristors to control the average load voltage. In
L
d
R