Logic For Four Quadrant Operation
For four-quadrant operation, the scheme outlined in this page makes use of
two converters, called the positive converter and the negative converter.
It has also been shown that the sum of the firing angles of the two converters
should be p radians. Hence the synchronizing signals
for the SCRs in both converters can be obtained as shown in Fig. 11.
It is also necessary to find out when the switch from the positive converter
to the negative converter or vice-versa can be made. One possible method is
outlined in Fig. 12. Based on the polarity of current reference signal, a
logic signal, called W can be developed. A comparator can be designed to yield
an output of 1 (W =1 ) when the current reference signal is positive and an
output of 0 (W =0 ) when the current reference signal is negative. Along with
W, another signal can be derived based on the armature current. A signal,
called Enable, can be produced such that Enable is 1 when the armature current
is zero. When Enable is 1, the output of a flip-flop can be set. Output Q
takes on the polarity of W signal. When both W and Q are at logic 1, the positive
converter is allowed to be triggered. When both W and Q are at logic 0, the
negative converter is allowed to be triggered.
The rest of the control arrangement for generating the firing signals is
shown in Fig. 13. The block diagram shows that there are active limits on
the firing angle. The voltage output of the bridge can be sensed and when
it is at about 1.05 times the rated armature voltage, the firing angle may
not be allowed to become any smaller.
The variation of firing angle towards either 0o or 180o
can be blocked, avoiding further rise in the output voltage. It is better
to have a provision that would allow for varying the limiting values to accommodate
changes in source voltages. It would not be difficult to implement such a
scheme for a micro-controller based control system.
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