Page 186 - Power Electronics Handbook

Chapter 9
Phase-controlled rectification and
inversion

9.1 Introduction

Controlled rectifier circuits had been in use long before power
semiconductors were invented, the control element being the mercury arc
rectifier, although these have now almost entirely been replaced by the
semiconductor switch, primarily the thyristor.
Phase-controlled rectification and inversion was introduced in Chapter
6, and although the principle is fairly simple there are a bewildering
number of different combinations which exist in practice, some of these
being described in this chapter. All these circuits can usually be divided as
follows:
(i) Bi-directional or unidirectional converters.
(ii) Each of the above converters can then be either push-pull or bridge
in configuration, the push-pull arrangement requiring an input
transformer.
(iii) All these converters can have any number of output pulses, the pulse
number usually being related to the number of phases of the input
a.c. supply. The higher the pulse number, the lower the ripple
content in the output d.c. voltage.
Bi-directional converters are capable of inversion in addition to
rectification, and this is often a desirable feature. Where it is not required
unidirectional converters can be used, the resulting circuit being cheaper
and having a smaller d.c. voltage ripple and input ‘wattless’ current
content.
Push-pull circuits have the advantage that there is only one conducting
device in series with the load, which was much more of an important
consideration when mercury arc rectifiers were used, since they could have
arc drops of 30V or more. However, modern thyristors lose only a volt or
two, so that this is important only when working from abnormally low
supply voltages. Generally, bridge circuits have better transformer
utilisation and are more frequently used.
The present chapter first describes the principles of unidirectional and
bi-directional converters, followed by the effects of discontinuous load
current and source reactance on their operation. The performance factors
used in the analysis of these converter circuits are then obtained, and the

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