Number of conducting devices in a 3-phase bridge inverter (120° conduction mode) At any instant during 120° conduction mode operation of a three-phase bridge inverter, how many thyristors (main devices) conduct simultaneously?

Introduction / Context:
Three-phase inverters can operate in 180° or 120° conduction modes. The instantaneous number of conducting devices affects line-to-line voltage waveforms and harmonic content.

Given Data / Assumptions:

• Voltage-source three-phase bridge with ideal switches and antiparallel diodes.
• Operation in 120° conduction mode.

Concept / Approach:
In 120° mode, each device conducts for 120° of the electrical cycle, and conduction intervals are arranged so that only one device from the upper group and one from the lower group conduct at any time, producing a quasi-square line-to-line voltage.

Step-by-Step Solution:
Assign sequential 120° conduction blocks to the six devices.Ensure that devices from the same leg do not conduct simultaneously.At any instant: exactly two devices ON (one on the positive bus, one on the negative bus), delivering current to a pair of phases.

Verification / Alternative check:
State tables and space-vector diagrams for 120° mode confirm two active devices at a time, yielding six non-zero voltage vectors per cycle.

Why Other Options Are Wrong:
1: Cannot form a closed current path for three-phase load.
3: Applies to 180° conduction mode.
4 or 6: Overconstrained, not typical for basic 120° mode.

Common Pitfalls:
Confusing device count in conduction with the number of active phases carrying current (which is also two in 120° mode).

Final Answer:
2