Question

The triggering pulses for 1-\(\phi\), full wave converter using Zero Crossing Detector, pulse amplifier and gate pulse isolation transformer, are obtained during

• A. \( \text{Ramp voltage with negative slope is less than control dc voltage} \)
• B. \( \text{Ramp voltage with negative slope is more than control dc voltage} \)
• C. Missing Option
• D. Missing Option

Hint:

For negative-slope ramp synchronization: - Firing happens when the falling ramp crosses below the threshold: \( V_{\text{ramp}} < V_c \). - Raising \(V_c\) causes the intersection to happen earlier, which decreases the firing angle \(\alpha\) and increases the converter's output voltage.

Answer 1

The Correct Option is A

Concept: Phase-controlled thyristor converters vary their output voltage by adjusting the firing angle ($\alpha$). To generate these trigger pulses reliably, systems often use a cosine-ramp firing circuit or a synchronized ramp comparator circuit. This circuit operates based on several key stages:
• A Zero Crossing Detector (ZCD) senses when the AC input voltage crosses $0\text{ V}$ and resets a ramp generator to sync it with the line frequency.
• A comparator circuit continuously compares this synchronized ramp voltage against an adjustable control DC voltage ($V_c$).
• When the comparison condition is met, the circuit generates a pulse, which is boosted by an amplifier and sent through an isolation transformer to trigger the thyristor gates.

Step 1: Operating principles of a ramp comparator circuit.
In a standard inverse-ramp firing configuration, a negative-going ramp voltage ($V_{\text{ramp}}$) starts at a high initial value at the beginning of the half-cycle and decreases linearly over time. This decreasing ramp is compared directly against an adjustable positive DC control voltage ($V_c$). At the start of the cycle, $V_{\text{ramp}}$ is greater than $V_c$, so the comparator output remains low.

Step 2: Determining the intersection point.
As time progresses, the ramp voltage drops. The firing pulse needs to be triggered at the exact instant the ramp crosses below the control threshold: \[ V_{\text{ramp}} < V_c \] Once the ramp voltage drops below the control DC voltage, the comparator switches states, producing a sharp rising edge that triggers the pulse amplifier and gate isolation transformer. Changing the level of the control voltage $V_c$ moves this intersection point, allowing smooth adjustment of the firing angle $\alpha$.

Step 3: Conclusion.
Thus, the trigger pulses are generated when the ramp voltage with a negative slope becomes less than the control DC voltage, which corresponds to option (1). Hence, the correct choice is option (1).