Logic and Triggering Circuit
Logic and Triggering Circuit:
Fig: Various waveforms of logic circuits for one phase – (a) supply voltage of a three phase half wave (three pulse) cycloconverter showing output voltage of half cycle (b) modulating signal for positive converter and reference voltage (100%) (c)output of converters used in positive cycle (d) clock pulse for positive group (e) gate pulse to thyristor.
- Two similar blocks − one with the reference voltage, erb , and the other with the reference voltage, erc , are used for the other two phases, B & C of the output, in the case of a three-phase to three-phase cyclo-converter circuit.
- The reference voltage, era is compared with the modulating voltages, ea ,eb and ec , corresponding respectively to the supply voltages, eA , eB and eC .
- The comparators produce short pulses. These pulses drive clock-pulse generator (CP ), which is a positive edge-triggered mono-stable multi-vibrator.
- Therefore, six pulses are obtained (4, 4`, 5, 5`, 6,6` ). Now, these short pulses act as clock pulses to the flip-flops. The clearing of the pulses is done, when they are not required.
- This clearing is done in the sequence, 6 → 5 → 4. This limits the presence of pulses only when that thyristor is supposed to conduct, otherwise pulse is blocked.
- The pulse output of the flip-flop is ANDed with the input coming from the blanking circuit. The output of the AND gate is given to the driver circuit, which amplifies and isolates the pulses and drives the respective thyristors.
- With the circulating current mode of operation, no blanking circuit is needed. Hence, this input may be set at the logic ‘1’ permanently. ′6′.
- The block diagram of the logic circuit is general in the sense that the reference signals and modulating signals could be of any wave-shape.
- In most applications, the sinusoidal reference signals and co-sinusoidal modulating signals are commonly employed. The basic principle of the co-sinusoidal modulation is the same as that of cosine wave crossing-pulse timing control employed for firing of the thyristors in phase-controlled converters.
- The waveforms are drawn for one half cycle of the output voltage for the positive converter with load having unity power factor.
- For the negative converter, different modulating signals have to be taken into account. This is taken care of by using an inverter at the output of each comparator.