Page 41 – Three Phase SPWM Inverter
Subject: Power Electronics
Topic: Three Phase Sinusoidal PWM
Level: GATE / PSU Advanced
🔹 Introduction
Three phase SPWM uses three sinusoidal reference waves (120° phase shifted) compared with a high-frequency triangular carrier.
🔹 Basic Principle
- Three sine waves: Va*, Vb*, Vc*
- Phase difference = 120°
- Compared with common triangular carrier
- Produces PWM gating signals
Switch ON when sine reference > carrier.
🔹 Modulation Index
ma = Vm / Vcarrier
- ma ≤ 1 → Linear region
- ma > 1 → Overmodulation
🔹 Line Voltage Fundamental
VLL1 = (√3 / 2) × ma × Vdc
Important for GATE numericals.🔹 Advantages
- Lower harmonic distortion
- Better voltage control
- Suitable for motor drives
- Improved power quality
🔹 Numerical Example
Given:
Vdc = 600 V
ma = 0.9
VLL1 = (√3 / 2) × 0.9 × 600
= 0.866 × 0.9 × 600
≈ 468 V (Fundamental Line Voltage)
🔹 GATE Important Points
- Line vs Phase voltage difference
- Linear modulation region
- Carrier frequency selection
- THD comparison with square wave
Quick Revision
Three sine waves (120° shifted)
Common triangular carrier
VLL1 = (√3 / 2) ma Vdc
Power Electronics Series – Shaktimatha Learning
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