Page 189 - Power Electronics Handbook

Bi-directional converters 181

fa) a - 0 (bl O0cac0Oo

Figure 9.3 illustrates the circuit waveforms for progressively increasing
delay angles, the following being noted from these curves:
6) For delay angles up to 90" more power is fed into the load than is
received back from the load, and beyond this point there is a net
transfer from the load to the supply. This cannot be maintained
continuously unless the load is provided with a suitable d.c. power
source.
(ii) Each thyristor conducts for a 180" period and the shape of the input
current waveform is unchanged. However, the phase angle
between the fundamental component of this current and voltage
input changes such that c$ = a. That this is necessarily true can be
verified at a = W, where there are now equal periods of power flow
in both directions between the load and the supply. As far as the
supply is concerned, therefore, the load is purely inductive and the
phase angle + must be 900. A converter, even though itself consisting
of a fixed value of resistance and inductance, will therefore present a