GATE Electrical Engineering - Complete Master Library

 

Page 39 – Sinusoidal PWM (SPWM) Inverter

Subject: Power Electronics
Topic: Sinusoidal Pulse Width Modulation
Level: GATE / PSU Very Important

                                        

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🔹 Why PWM is Needed?

• Square wave inverter has high THD
• PWM reduces lower order harmonics
• Improves output waveform quality
• Better control of output voltage

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🔹 Basic Principle

Compare:

• High frequency triangular carrier wave
• Low frequency sinusoidal reference wave

Switch ON when sine > carrier.

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🔹 Modulation Index

m_a = V_m / V_carrier

Where:

• m_a = Amplitude modulation index
• m_a ≤ 1 → Linear region
• m_a > 1 → Overmodulation

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🔹 Output Fundamental Voltage

V₁ = m_a × V_dc / 2

(For full bridge SPWM inverter)

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🔹 Frequency Modulation Index

m_f = f_carrier / f_reference

Higher m_f → Better harmonic performance.

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🔹 Numerical Example

Given:
V_dc = 400 V
m_a = 0.8

V₁ = 0.8 × 400 / 2

= 160 V (fundamental peak)

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🔹 GATE Important Points

• SPWM reduces lower order harmonics
• Linear control when m_a ≤ 1
• Overmodulation distorts waveform
• Used in motor drives & UPS

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Quick Revision

SPWM → Compare sine & triangle
V₁ = m_a V_dc/2

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PWM Series – Shaktimatha Learning