Consider the performance of a 3-phase VSI. Then1.180° conduction provides higher fundamental output than 120° conduction.2.120° conduction is prone to short-circuiting the DC bus if "dead-time" is not provided.3.In 180° conduction, the potential of the load neutral is always zero for a balanced star load.Which of the statements given above is/are correct?

Correct Answer: A - 1 only

Quick Concept Explanation

In 180° mode three devices conduct at a time; in 120° mode only two conduct and one phase is disconnected during each step. Waveform/fundamental comparison confirms 1. The 60° same-leg off interval of 120° mode contradicts 2, and switching-state neutral displacement contradicts 3. Key relation: Typical fundamental phase RMS: 180° mode ≈0.4502Vdc; 120° mode ≈0.3898Vdc. Thus 180° gives the higher fundamental component.

Exam focus: 120° mode: two switches ON, one phase floating. 180° mode: three switches ON. Common trap: Assuming a balanced load forces the floating neutral to stay at zero under a non-sinusoidal switching waveform.

Statement-wise Verification

• Statement 1 - Correct.
  180° conduction provides higher fundamental output than 120° conduction.
  Standard six-step analysis gives higher fundamental phase/line output in 180° mode than in 120° mode.
• Statement 2 - Incorrect.
  120° conduction is prone to short-circuiting the DC bus if "dead-time" is not provided.
  For the stated comparison: in 120° conduction each device conducts 120° and there is a 60° interval when neither device in the same leg conducts; this inherent blank interval reduces the overlap/shoot-through issue that is more critical in 180° complementary switching.
• Statement 3 - Incorrect.
  In 180° conduction, the potential of the load neutral is always zero for a balanced star load.
  For a balanced star load with floating neutral, the neutral point shifts with the switching state in 180° mode; it is not fixed at the DC midpoint/zero potential.

Core Concept

Both modes use six 60° steps, but device conduction duration changes the phase voltage waveform. In 180° mode three devices conduct at a time; in 120° mode only two conduct and one phase is disconnected during each step. That difference affects fundamental voltage, neutral potential and commutation safety.

Formula / Key Relationship

Typical fundamental phase RMS: 180° mode ≈0.4502Vdc; 120° mode ≈0.3898Vdc. Thus 180° gives the higher fundamental component.

Step-by-Step Check

Waveform/fundamental comparison confirms 1. The 60° same-leg off interval of 120° mode contradicts 2, and switching-state neutral displacement contradicts 3. Therefore A.

Why the Other Options Are Wrong

Option B is incorrect because it includes incorrect statement(s) 2. Option C is incorrect because it includes incorrect statement(s) 3 and omits correct statement(s) 1. Option D is incorrect because it includes incorrect statement(s) 2, 3 and omits correct statement(s) 1.

Answer-Key Verification Note

The uploaded provisional key marks C . After independent technical verification, the defensible answer is A . This discrepancy is stated explicitly rather than silently copying the provisional key.

Exam Shortcut / Approach

120° mode: two switches ON, one phase floating. 180° mode: three switches ON.

Common Mistake

Assuming a balanced load forces the floating neutral to stay at zero under a non-sinusoidal switching waveform.

Quick Revision

120 versus 180 Degree Conduction is linked with Three-Phase VSI, Inverters and Power Electronics. 120° mode: two switches ON, one phase floating. 180° mode: three switches ON.