Page 250 - Power Electronics Handbook

240 Direct ax. frequency converters
as that of the anode of TH3. This places a reverse voltage, that on C1, equal
to twice that of the instantaneous a.c. supply at this instant, directly across
TH3, and it turns off. Load current is now supplied by TI& and the load
voltage has been reversed. Capacitor C1 need not be connected across the
transformer primary, the same effect being obtained with the capacitor on
the secondary side.
Cycloinverters are not commonly used, since they do not give any
significant advantage over forced commutated inverter systems. They have
found limited application for induction heating systems where, for
example, the transformer in Figure 10.20 is replaced by a centre-tapped
heating coil with a capacitor connected across it. The combination of coil
and capacitor forms a resonant circuit which turns the conducting thyristors
off at the required times.

10.7 Cycloconverter control circuits

As described in earlier sections, the mean voltage from a cycloconverter
must be able to oscillate about zero, moving from a maximum positive to a
maximum negative value in each cycle of the output frequency. To obtain
this the firing delay is usually biased at an angle of 90" and the firing angle
delay is then oscillated by a further 90" about this point, in both the positive
and negative directions. This causes the firing point of the positive group of
thyristors, making up the cycloconverter circuit, to be advanced and the
firing angle of the negative group to be retarded by the same amount,
during the positive half cycle, the roles being reversed during the negative
half cycle. At all times the sum of the positive and negative delay angles is
such that equation (10.5) is satisfied, as described earlier, so that the mean
output voltages from the two groups are equal in magnitude but opposite in
phase.
% + ar, = 180" (10.5)

The block diagram of Figure 10.21 illustrates functionally the basic
system for the thyristor-firing circuit of a cycloconverter, most of these
functions being available within integrated circuits. The reference voltage is
ideally a steady d.c. level, which produces a firing angle delay of 90", on
which is superimposed a cosine voltage. The frequency of this voltage will
determine the load frequency and the amplitude of the cosine curve, relative
to the d.c. level, fixes the modulation depth of the load voltage and hence its
magnitude. The distributor circuit feeds thyristors in both the positive and
negative converter group, but a feedback signal from the load current is
shown, in the system illustrated, to be used to blank pulses to the group
which is not conducting load current, so that circulating currents are
prevented. The reference wave generated is compared to a sample of the low-
voltage output and the firing angle is then adjusted such as to minimise the
harmonics by causing a delay over successive cycles. The delay is
continuously variable from 0" to 180" in response to the control inputs and
the reference, the pulses for each group being 120" apart.