Answer:

Introduction:
A 3-phase 6-pulse converter is a type of rectifier circuit used in power electronics. It consists of three sets of SCR (Silicon Controlled Rectifier) connected in a bridge configuration. The SCR conducts in a controlled manner to convert AC voltage to DC voltage. Each SCR operates for a specific duration during each cycle of the AC waveform.

Explanation:
The question asks about the conduction angle of each SCR in a 3-phase 6-pulse converter. Here, the term "conduction angle" refers to the angle for which the SCR conducts during each cycle of the AC waveform.

1. Conduction Angle of each SCR:
In a 3-phase system, each phase is 120 degrees apart from the other phases. Therefore, in a 6-pulse converter, each SCR conducts for a specific duration of the AC waveform. The conduction angle of each SCR can be determined as follows:

- The converter has 6 SCRs in total, with each SCR conducting for a specific duration during the positive half-cycle of the AC waveform.
- The conduction angle of each SCR is determined by dividing the positive half-cycle duration by the total number of SCRs.
- In a 6-pulse converter, the positive half-cycle is 180 degrees (since it is a 3-phase system) and the total number of SCRs is 6.
- Therefore, the conduction angle of each SCR is 180 degrees / 6 = 30 degrees.

2. Calculation of Conduction Angle:
To calculate the conduction angle of each SCR, we can use the formula:

Conduction Angle = Positive Half-cycle Duration / Total Number of SCRs

In this case, the positive half-cycle duration is 180 degrees and the total number of SCRs is 6.

Conduction Angle = 180 degrees / 6 = 30 degrees

Therefore, the correct answer is option 'A' - 120 degrees.

Conclusion:
In a 3-phase 6-pulse converter, each SCR conducts for 30 degrees during each cycle of the AC waveform. Since there are 6 SCRs in total, the total conduction angle for all the SCRs is 6 * 30 = 180 degrees, which covers the entire positive half-cycle of the AC waveform.