Page 157 - Power Electronics Handbook

150 Static switches

= 0.45Zms (7.3)
Therefore each thyristor must have a mean current rating of 0.45 times
the r.m.s. line load current. Equation (7.2) applies equally well to the
thyristors and diodes of the three-phase circuits in Figure 7.1, since these
also carry half a cycle of the line current each.

7.3 D.C. contactors

Once conduction has been initiated in a thyristor or a triac it remains on
until the current decays to zero. This is accomplished naturally in an a.c.
circuit, owing to the reversal of the supply voltage, but for d.c. contactors
this voltage reversal must be artificially provided across the power
semiconductor, for a time in excess of its rated turn-off time.

d.c. supply

ov
+V
Load

D3
d.c. supply I/(
+A
fH2
TH,
ov

Figure 7.2 D.C. contactors: (a) and (b) circuit arrangements; (c) waveform across capacitor C
in Figure 7.2(b)