GATE Electrical Engineering - Complete Master Library

 

Page 34 – Three Phase Fully Controlled Bridge Rectifier (6-Pulse)

Subject: Power Electronics
Topic: 3-Phase Fully Controlled Converter
Level: GATE / PSU Core

                                        

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🔹 Why Three Phase Converter?

• Higher DC output voltage
• Lower ripple
• Ripple frequency = 6f
• Used in HVDC, large drives

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🔹 Average Output Voltage

V_avg = (3√2 V_LL / π) cos α

Where:
V_LL = Line-to-Line RMS Voltage
α = Firing Angle

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🔹 Important Observations

• Maximum output at α = 0°
• Zero output at α = 90°
• Inverter mode when α > 90°
• Ripple frequency = 6f

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🔹 Numerical Example

Given:
V_LL = 400 V
α = 30°

V_avg = (3√2 × 400 / π) cos 30°

= (3 × 1.414 × 400 / 3.14) × 0.866

= (1696.8 / 3.14) × 0.866
= 540.4 × 0.866
= 468 V (approx)

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🔹 GATE Trick Points

• 6 SCRs used
• Each SCR conducts for 120°
• Continuous conduction assumed
• Better power factor than single phase

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 Quick Revision

V_avg = (3√2 V_LL / π) cos α
Ripple frequency = 6f

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Three Phase Converter – Shaktimatha Learning