Synchronizing Signals
To vary the output voltage, it is necessary to vary the firing
angle. In order to vary the firing angle, one commonly used technique
is to establish a synchronizing signal for each SCR. It has been
seen that zero degree firing angle occurs 30o degrees
after the zero-crossing of the respective phase voltage. If the
synchronizing signal is to be a sinusoidal signal, it should lag
the respective phase by 30o and then the circuitry
needed to generate a firing signal can be similar to that described
for single-phase. Instead of a single such circuit for a single
phase rectifier, we would need three such circuits.
When the 3-phase source supply connected to the rectifier is
star-connected, the line voltages and the phase voltages have
a 30o phase angle difference between them, as shown
below.
The line voltage can also be obtained as:
This line voltage lags the R-phase voltage by30o and
has an amplitude which is 1.732 times the amplitude of the phase
voltage. The synchronizing signal for SCR S1 can be
obtained based on vRB line voltage. The synchronizing
signals for the other SCRs can be obtained in a similar manner.
To get the synchronizing signals, three control transformers
can be used, with the primaries connected in delta and the secondaries
in star, as shown below.
For S1, voltage vS1 is used as the synchronizing
signal. Voltage vS2 is used as the synchronizing signal
for SCR S2 and so on. The waveforms presented by the
synchronizing signals are as shown below. The waveforms do not
show the effect of turns ratio, since any instantaneous value
has been normalized with respect to its peak value. For example,
let the primary phase voltage be 240 V and then its peak value
is 339.4 V. The primary voltage is normalized with respect to
339. V. If the peak voltage of each half of secondary is 10 V,
the secondary voltage are normalized with respect to 10 V.
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