Page 209 - Power Electronics Handbook

Phase-controlled rectification and inversion

1

1 80 60 40 20 0
Per cent of max. d.c. voltage

030 45 60 75 90 105 120 135150180
a Scale for uni-directional
11 I I I I I I
0 20 30 45 60 70 80 90
a Scale for bi-directional
Flgnre 9.24 Critical inductance requirements for three-phase systems

also very small at delay angles approaching 18V, so that the critical
inductance is a finite value. Figure 9.23 clearly shows the advantage of
unidirectional converters in requiring smaller load inductance, but Figure
9.24 illustrates that the unidirectional converter has a higher critical
inductance requirement for three-phase circuits except at low load
voltages. This is due to the different waveforms obtained with multiple
systems. As has been seen previously, a unidirectional circuit gives half the
pulse numbers compared to the same bi-directional system, so that the
choice between the two types of converters would now depend on the
output voltage range required. Comparison of Figures 9.23 and 9.24 shows