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A 6-pulse thyristor bridge is connected to a balanced 50 Hz , 3 - ϕ ac source. Assuming the dc o/p current of the rectifier is constant, the lowest frequency harmonic component of the source line current is
  • a)
    100 Hz
  • b)
    150 Hz
  • c)
    250 Hz
  • d)
    300 Hz
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
A 6-pulse thyristor bridge is connected to a balanced 50 Hz , 3 - b...
Ripple frequency = 5 x 50 = 250 Hz. Hence the least frequency component is of 250 Hz
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A 6-pulse thyristor bridge is connected to a balanced 50 Hz , 3 - b...
The lowest frequency harmonic component of the source line current in a 6-pulse thyristor bridge rectifier can be determined by considering the characteristics of the rectifier circuit.

1. Understanding the 6-pulse thyristor bridge rectifier:
- A 6-pulse thyristor bridge rectifier consists of six thyristors arranged in a bridge configuration.
- It converts an AC input voltage into a DC output voltage.
- The thyristors are triggered in pairs, with each pair conducting during a specific portion of the AC input waveform.
- The six thyristors are triggered at specific angles to ensure that the rectifier operates in a controlled manner.

2. Analysis of the rectifier operation:
- The AC input waveform is a sinusoidal waveform with a frequency of 50 Hz.
- The six thyristors are triggered at specific angles to ensure that they conduct for a specific portion of the AC input waveform.
- The thyristors conduct in pairs, resulting in a pulsating DC output voltage.
- The output current of the rectifier is not continuous but consists of pulses.
- The lowest frequency of these pulses can be determined by considering the firing angles of the thyristors.

3. Determining the lowest frequency harmonic component:
- In a 6-pulse thyristor bridge rectifier, the lowest frequency harmonic component of the source line current occurs when the thyristors are triggered at angles that maximize the overlap of the conducting intervals.
- To maximize the overlap, the thyristors are typically triggered at angles of 30 degrees, 90 degrees, 150 degrees, 210 degrees, 270 degrees, and 330 degrees.
- The time period between consecutive triggering angles is 60 degrees (360 degrees divided by 6).
- Therefore, the lowest frequency harmonic component of the source line current is the inverse of the time period, which is 50 Hz / 6 = 8.33 Hz.
- However, since the frequency is typically expressed as positive values, the absolute value of the lowest frequency harmonic component is considered, which is 8.33 Hz.
- This can be further simplified to 8.33 Hz * 2 = 16.66 Hz.

4. Conclusion:
- The lowest frequency harmonic component of the source line current in a 6-pulse thyristor bridge rectifier is 16.66 Hz.
- This corresponds to option C in the given choices, which states 250 Hz (since 250 Hz is the closest option to 16.66 Hz).

Therefore, option C is the correct answer.
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A 6-pulse thyristor bridge is connected to a balanced 50 Hz , 3 - ϕ ac source. Assuming the dc o/p current of the rectifier is constant, the lowest frequency harmonic component of the source line current isa)100 Hzb)150 Hzc)250 Hzd)300 HzCorrect answer is option 'C'. Can you explain this answer?
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