Page 201 - Power Electronics Handbook

192 Phase-controlled rectification and inversion
free-wheeling path for load current. At rol thyristor THl is fired and load
current flows via TH1 and D2 as before. At rl the current free-wheels
through D1 and D2 so that thyristor THl goes off, the system behaving as in
Figure 9.15(c), where all conducting thyristors are commutated at the end
of half cycles, except that now the conducting period for the bridge
thyristors and diodes are 180" - a and 180O + a respectively, so that they
could be unequally loaded depending on the load duty cycle.
Since thyristors are more expensive than diodes the converter given in
Figure 9.15(e) sometimes proves economical. Free-wheeling diode DS is
not necessary if the load is purely resistive. As seen, the system consists
essentially of providing a fully rectified wave at CD and then regulating this
with thyristor TH1. Since the waveform across the thyristor therefore falls
to zero only briefly every half cycle, a free-wheeling diode is essential, even
for slightly inductive loads, to ensure successful device commutation. The
operation of the circuit is readily followed by reference to the associated
load voltage waveform.
Figure 9.16 shows the load voltage and current waveforms obtained from
unidirectional circuits for various delay angles. Several features of these
systems are evident from this figure and the above discussions:
(i) The load voltage has a lower ripple due to the absence of negative
portions of the waveform.
(ii) The mean load voltage varies from a maximum to zero as the delay
angle changes from 0" to 180".
(iii) The power factor angle + changes proportionally to the delay angle a,
as for bi-directional converters, although it does not equal it.
(iv) Whereas for bi-directional converters the load current waveshape was
unchanged as the delay angle varied, for unidirectional converters the
load current period decreases with delay angle increase, so that at a
= 90" the load current is unchanged in value from that at zero delay
angle. However, since d.c. load voltage is zero there is now no net
input power and all the ax. current is quadrature component or
wattless. For a unidirectional converter, on the other hand, the a.c.
input current at very low d.c. output voltages is also very small, so
that the quadrature component of the current has been reduced.
(v) As mentioned above, unidirectional converters are often cheaper
than bi-directional ones.
(vi) There are no regenerative periods, so that a unidirectional converter
cannot pass power from the d.c. to the a.c. side.
It is seen from items (i), (iv) and (v) that there is an advantage to be
gained from using unidirectional converters in two-pulse systems, hence
their popularity in applications which do not require regeneration. For
systems with more than two pulses it will be seen later that the d.c. ripple
frequency increases between bi-directional and unidirectional systems by a
factor of two, so that, depending on the control range, when d.c. voltage
filtering requirements are stringent bi-directional converters are sometimes
preferred.
Bridge circuits can be converted from bi-directional to unidirectional
operation by changing half the devices from thyristors to diodes, although
the same rule does not apply to push-pull converters. The circuit of Figure