Page 45 – Multiple Questions with Explanation
Subject: Power Electronics
Level: GATE / PSU Concept + Numerical Practice
🔹 Q1. In SPWM, maximum fundamental line voltage is:
A) Vdc
B) 0.5 Vdc
C) 0.866 Vdc
D) 1.2 Vdc
Correct Answer: C) 0.866 Vdc
Explanation:
In SPWM, modulation index mₐ ≤ 1 in linear region.
Maximum achievable line voltage = √3/2 × Vdc ≈ 0.866 Vdc.
🔹 Q2. SVPWM improves DC bus utilization by approximately:
A) 5%
B) 10%
C) 15%
D) 25%
Correct Answer: C) 15%
Explanation:
SPWM max = 0.866 Vdc
SVPWM max = Vdc
Improvement ≈ (1 − 0.866)/0.866 ≈ 15%.
🔹 Q3. A three-phase inverter has how many switching states?
A) 6
B) 8
C) 4
D) 12
Correct Answer: B) 8
Explanation:
3 legs → each leg has 2 states.
Total states = 2³ = 8.
6 active vectors + 2 zero vectors.
🔹 Q4. If α = 75°, the reference vector lies in:
A) Sector 1
B) Sector 2
C) Sector 3
D) Sector 4
Correct Answer: B) Sector 2
Explanation:
Each sector = 60°.
Sector 1 → 0°–60°
Sector 2 → 60°–120°
75° lies in Sector 2.
🔹 Q5. If Vdc = 600 V and mₐ = 0.8 (SPWM), find fundamental line voltage.
VLL = 0.866 × mₐ × Vdc
= 0.866 × 0.8 × 600
= 0.866 × 480
≈ 415.7 V
Final Answer: ≈ 416 V
🔹 Q6. Why must upper and lower switches of same leg never be ON together?
Correct Answer: Shoot-through fault.
Explanation:
It directly short-circuits the DC supply, causing large current and device damage.
Dead-time is introduced to avoid this.
🔹 Q7. In 180° conduction mode, how many switches conduct simultaneously?
A) 1
B) 2
C) 3
D) 6
Correct Answer: C) 3
Explanation:
In 180° mode, each switch conducts for 180°.
At any instant, 3 switches are ON.
Quick Exam Reminder
✔ SPWM → 0.866 Vdc
✔ SVPWM → Vdc
✔ 6 sectors (60° each)
✔ 8 switching states
✔ Avoid shoot-through fault
Power Electronics Exam Series – Shaktimatha Learning
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