GATE EE 2014 Set 3 — Question 63
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Power Electronics → AC-DC Converters → Controlled Rectifiers (3φ)
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Question
A three-phase fully controlled bridge converter is fed through star-delta transformer as shown in the figure.
The converter is operated at a firing angle of . Assuming the load current () to be virtually constant at and transformer to be an ideal one, the input phase current waveform is
The converter is operated at a firing angle of . Assuming the load current () to be virtually constant at and transformer to be an ideal one, the input phase current waveform is
Correct answer
(B) [figure]
Solution
In a star-delta () transformer feeding a three-phase bridge converter, the primary line current is related to the secondary phase currents. For a 6-pulse bridge, the secondary line currents are quasi-square waves. Due to the delta connection on the secondary, the phase currents in the delta winding () have a 6-stepped waveform with steps at and of the peak value. Given the turns ratio (Phase primary : Phase secondary), the primary phase current (which is also the line current for a star connection) is:Since the load current , the steps in the primary current waveform will be at and relative to the base? No, the peak of the phase current in the delta is . Thus the steps are at and . This corresponds to the waveform shown in option (B).
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