Page 155 - Power Electronics Handbook

148 Static switches
through them falls below a certain value, static contactors using these
devices differ in circuit detail when operating on a.c. or d.c.

7.2 A.C. contactors

A.C. contactors operate from single-phase or three-phase lines. Generally,
the power semiconductors used for these applications are triacs or
thyristors, and although thyristors are shown in all the illustrations in this
section two back-to-back connected thyristors can be replaced by a triac, if
required.
Figure 7.l(a) shows the simplest and most frequently used arrangement
for a full-wave single-phase contactor. With thyristors 1 and 2

A

Single-phase Load
a.c. supply

B
(a)
A

3 phase
a.c. supply

C

(bl

A

B Load

C

(d
Figure 7.1 A.C. contactors: (a) single-phase; (b) three-phase unidirectional; (c) three phase
reversing