In a 3-phase VSI in 120° conduction mode,1.Each switch conducts for 120° in a cycle.2.Only two switches conduct at any given time.3.The phase voltage is zero for 60° in every half-cycle.Which of the statements given above are correct?

Correct Answer: B - 1, 2 and 3 only

Quick Concept Explanation

Three-phase voltage-source inverters can be operated with 120° or 180° conduction sequences. In 120° conduction, each switch is on for 120° and only two devices conduct at a time, leaving one phase disconnected for intervals. Key relation: Conduction duration: 120°→two switches at a time; 180°→three switches at a time.

Exam focus: Track pole voltage and load-neutral voltage separately. Common trap: Balanced load does not force the floating neutral to the DC midpoint at every switching state.

Statement-wise Verification

• Statement 1 - Correct.
  Each switch conducts for 120° in a cycle.
  In 120° conduction, each switch is on for 120° and only two devices conduct at a time, leaving one phase disconnected for intervals.
• Statement 2 - Correct.
  Only two switches conduct at any given time.
  Conduction duration: 120°→two switches at a time; 180°→three switches at a time.
• Statement 3 - Correct.
  The phase voltage is zero for 60° in every half-cycle.
  Three-phase voltage-source inverters can be operated with 120° or 180° conduction sequences.

Core Concept

Three-phase voltage-source inverters can be operated with 120° or 180° conduction sequences. In 120° conduction, each switch is on for 120° and only two devices conduct at a time, leaving one phase disconnected for intervals. In 180° conduction, three devices conduct at a time and the fundamental output is higher; the floating star neutral can shift relative to the DC midpoint.

Formula / Key Relationship

Conduction duration: 120°→two switches at a time; 180°→three switches at a time.

Why the Other Options Are Wrong

Option A is incorrect because it omits correct statement(s) 3. Option C is incorrect because it omits correct statement(s) 2. Option D is incorrect because it omits correct statement(s) 1.

Exam Shortcut / Approach

Track pole voltage and load-neutral voltage separately.

Common Mistake

Balanced load does not force the floating neutral to the DC midpoint at every switching state.

Quick Revision

120-Degree Conduction Mode is linked with Three-Phase VSI, Inverters and Power Electronics. Track pole voltage and load-neutral voltage separately.