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Three-phase fully controlled bridge converter: the effective firing sequence (pulse) frequency is how many times the line frequency?

Difficulty: Easy

Correct Answer: 6 times the line frequency

Explanation:


Introduction / Context:
Pulse frequency determines the ripple frequency in the DC output and the gating pattern of a 3-phase fully controlled rectifier. Understanding its relationship to mains frequency is crucial for filter design and control timing.


Given Data / Assumptions:

  • Topology: 3-phase full bridge (six SCRs, one per 60° conduction window).
  • Balanced three-phase supply.
  • Continuous conduction assumed for standard operation.


Concept / Approach:
In a 3-phase 6-pulse bridge, a device is fired every 60 electrical degrees, resulting in six firing events each line cycle. Therefore, the pulse frequency f_p = 6 * f_line. This also sets the dominant DC ripple at 6f_line for continuous current operation.


Step-by-Step Solution:

Line cycle = 360°.One gate command every 60° → 6 pulses per cycle.Thus, f_pulses = 6 * f_line.


Verification / Alternative check:

The converter is known as a '6-pulse' rectifier; harmonic spectra show ripple at 6f, 12f, 18f, etc.


Why Other Options Are Wrong:

3f corresponds to a 3-pulse (half-controlled) system; 9f/12f do not match the standard 6-pulse bridge.


Common Pitfalls:

Confusing device conduction angle (120°) with the firing interval (60°) and resultant pulse count.


Final Answer:

6 times the line frequency
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