Page 240 - Power Electronics Handbook

230 Direct a.c. frequency converters
and Figure 10.9 gives a system in which the load has a common terminal, so
that the individual bridge inputs need to be supplied from isolating
transformers. Isolated load circuits are preferred owing to the complete
.absence of any input transformer. In the isolated supply system each
secondary winding in effect feeds a separate single-phase load, so that the
total secondary rating exceeds that of the primary by over 20%. Extension
of the six-pulse bridge to twelve pulses, by the use of series-connected
converters, is shown in Figure 10.10.
The circuits described so far can be referred to as symmetrical
cycloconverters, since each phase of a three-phase output is made up of a
single-phase unit, but this is not always necessary. For instance, Figure
10.11 shows a three-phase load, supplied from an asymmetrical
cycloconverter, which has only two single-phase modules. Although the
component content of this converter has been reduced by one third
compared to symmetrical circuits, the utilisation of the thyristors and

r-
Converter I Converter 2 Converter 3
_.

3 phase
s
arsupply I
I-
I
3

-r

1
1

Figure 10.10 Bridge twelve-pdsc cydmnvcrter