Discussion
A six pulse thyristor rectifier bridge is connected to a balanced 50 Hz three-phase ac source. Assuming that the dc output current of the rectifier is constant, the lowest harmonic component in the ac source line current is :
Answer: C
The lowest harmonic in the line current is 250 Hz.
With constant DC current, each line of a six-pulse bridge carries a quasi-square wave of 120° positive and 120° negative blocks. Its Fourier series has no even harmonics (half-wave symmetry) and no triplen harmonics (the three phases are balanced and 120° apart), so only harmonics of order
\[ h = 6k \pm 1 = 5, 7, 11, 13, \ldots \]
remain. The lowest is the 5th:
\[ 5 \times 50 = 250\ \text{Hz} \]
100 Hz would be the 2nd harmonic and 150 Hz the 3rd, both absent. 300 Hz is the 6th harmonic, the lowest ripple frequency on the DC side of a six-pulse bridge, not in the AC line current.
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