Page 154 - Power Electronics Handbook

Chapter 7
Static switches

7.1 Introduction

The most basic applications of a thyristor are those in which its use is
essentially that of a switch connecting a load to its supply, these circuits
generally being referred to as static contactors, due to the absence of
moving parts. In this chapter typical d.c. and a.c. contactor systems will be
examined and reference made to electronic control and protection circuits,
which often enable them to perform a variety of functions.
A static contactor has several desirable features when compared to its
mechanical counterpart. There are no moving parts to wear or bum out; it
is mechanically robust and noise free in operation; it can operate very
quickly, in less than 0.1 ms, compared to about 50ms for a mechanical
contactor; and it is readily adaptable to sophisticated electronic control.
This can include zero-level switching, such that the contactor closes at the
zero voltage point of the supply waveform and so prevents interference in
the lines; gradual build-up in voltage applied to the load to prevent current
surges; and rapid isolation and switch-off in the event of a fault.
However, the static contactor is not used as frequently as its mechanical
counterpart, and for some very good reasons. It is generally more
expensive and physically larger; it is more prone to failure due to current
and voltage overloads; and it does not provide complete isolation between
supply and load due to leakage current through the power semiconductors.
There are several applications where it is frequently used, for example in
explosive atmospheres where arcing of contacts would be dangerous; in
environments where a conventional contactor would generate intolerable
interference, such as in computer installations; when the contactor is
mounted in inaccessible conditions where it could not be maintained,
since a static contactor needs no maintenance; where it is required to
operate frequently, since static contactors do not wear out; and when the
contactor is required to respond to voltage signals generated by some other
process, such as a fault detector, since static contactors can be readily
controlled by electronic signals.
Contactors can be used to operate from a.c. or d.c. supplies. Generally,
mechanical contactors can have the same construction for both these
sources, only their ratings being affected. Since some power semiconduc-
tors, such as thyristors and triacs, can be turned off only when the current
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