Page 153 - Power Electronics Handbook

146 Power semiconductor circuits - a &sum6
6.6 and shown again in Figure 6.9(c). At to all four thyristors are
maintained non-conducting. At tl thyristors TH1 and TH2 are turned on.
At t2 all thyristors are again off until TH3 and TH, are fired at t4. It must
again be emphasised that this description applies for a resistive load only,
which has been adopted to explain the principles of operation. D.C. link
frequency changers are described in greater detail in Chapter 13.
The differences between direct a.c. and d.c. link frequency changers can
again be summarised as foliows:
(i) In the direct a.c. converter the semiconductor switches are all
naturally commutated, whereas for a d.c. link converter they must be
turned off by their gate or base control, or forced commutated.
(ii) The d.c. link converter operates from a d.c. source so that its output
is independent of the frequency and waveshape of the input, whereas
the direct a.c. converter follows the envelope of the a.c. input.
(iii) The d.c. link converter can give an output range of frequencies which
is infinitely variable. The direct a.c. converter, on the other hand, can
only change the frequency in discrete steps of the input, and can only
operate up to a maximum frequency equal to that of the input.