Firing (gating) frequency in a three-phase semiconverter For a 3-phase semiconverter (three thyristors and three diodes), the required firing frequency of the thyristor gating pulses is:

Introduction / Context:
In three-phase rectifiers, ripple frequency and pulse number differ from the firing (gating) frequency per device. Knowing how often each thyristor must receive a firing pulse helps design gate drivers and synchronizing circuits.

Given Data / Assumptions:

• Three-phase semiconverter: 3 SCRs + 3 diodes.
• Balanced three-phase supply.
• Continuous conduction assumed for conceptual discussion.

Concept / Approach:

In a semiconverter, only the positive group (say, anode-connected to phases) uses controllable devices, each naturally commutated at a line-frequency rate. Each SCR is triggered once per cycle of its phase. Thus the per-device firing frequency equals the line frequency. Note that the output ripple frequency relates to pulse number p = 3 for a semiconverter (3f ripple), but this is distinct from gating frequency per thyristor.

Step-by-Step Solution:

Verification / Alternative check:

Conduction tables for three-phase semiconverters show one controlled firing per phase period, with diodes providing the complementary paths.

Why Other Options Are Wrong:

Options (b), (c), and (d) confuse ripple/pulse frequency with gate firing rate. Option (e) has no basis in natural-commutated operation.

Common Pitfalls:

Mixing up output ripple frequency and required gate trigger repetition per SCR.

Final Answer:

Equal to the line frequency