Three-Phase Fully Controlled Converter

§ 01

Introduction

This experiment converts three-phase AC to controlled DC using a six-SCR fully controlled bridge converter. This topology is widely used in high-power industrial drives and HVDC applications where precise voltage control is essential.

🏭

Industrial Application

High-Power DC Drive Control

Three-phase fully controlled converters power large DC motors in steel mills, paper mills, and traction systems. Firing angle control provides smooth, continuous speed regulation.

§ 02

Theory

Three-phase fully controlled bridge circuit — Fig. 1 — Three-phase fully controlled bridge converter: SCRs S1–S6 in two groups.

The bridge has two groups: positive group (S1, S3, S5) and negative group (S2, S4, S6). SCRs are triggered in sequence S1, S2, S3, S4, S5, S6 with 60° intervals. At any time, one SCR from each group conducts simultaneously.

Positive Group (S1, S3, S5)

Each SCR conducts for 120°. S1: 30°–150° + α; S3: 150°–270° + α; S5: 270°–390° + α.

Negative Group (S2, S4, S6)

Each SCR conducts for 120°. S4: 210°–330° + α; S6: 330°–450° + α; S2: 90°–210° + α.

Average Output Voltage — Three-Phase Fully Controlled

\[V_{dc} = \frac{3\sqrt{3}}{\pi}\cdot V_{m,phase}\cdot\cos\alpha = V_{dc0}\cos\alpha\]

where \(V_{dc0} = 3\sqrt{3}V_{m,phase}/\pi = 2.34V_{phase,rms}\) is the uncontrolled output. For \(V_{LL,rms} = 61.2\text{ V}\): \(V_{dc0} \approx 82.7\text{ V}\).

§ 03

Simulation

Problem Statement

Implement the 3-phase fully controlled full-wave converter with R = 100 Ω. Input: \(V_{LL,rms} = 61.2\text{ V}\), 50 Hz. Observe output at α = 0°, 45°, 90°.

Simulink 3-phase fully controlled converter — Fig. 2 — Simulink model: three-phase fully controlled bridge converter.

Fig. 3 — Three-phase input voltages (R, Y, B phases).

Output voltage at alpha=0 — Fig. 4 — Output voltage at α = 0° (maximum).

Output voltage at alpha=45 — Fig. 5 — Output voltage at α = 45°.

Output voltage at alpha=90 — Fig. 6 — Output voltage at α = 90° (zero average).

Gate triggering waveforms — Fig. 7 — Gate triggering pulses for SCRs with 60° phase shift.

Output current and SCR currents — Fig. 8 — Output current and individual SCR current waveforms.

§ 04

Hardware Implementation

Hardware three-phase controlled converter — Fig. 9 — Hardware wiring: three-phase fully controlled bridge converter with R load (100 Ω).

Connect circuit as in Fig. 9 (R = 100 Ω). Connect CRO probes across R load. Switch ON 3φ supply MCB.
Switch ON POWER MODULE and SCR–Diode module MCBs. Set voltage to 61.2 V RMS.
Switch ON driver power switch. Vary firing angle as per observation table.
Observe Output voltage waveforms in DSO. Record average output voltage.

§ 05

Results

Required: Circuit diagram, gate triggering sub-circuit, waveforms of Output Voltage, Output Current, Input Voltages, Input Currents at α = 45° (Simulink and DSO).

Simulation Observation Table

S.No	Firing Angle (time)	Firing Angle (degrees)
1.	0 ms	0°
2.	2.5 ms	45°
3.	5 ms	90°

Hardware Observation Table

S.No	Firing Angle (time)	Firing Angle (degrees)
1.	0 ms	0°
2.	2.5 ms	45°
3.	5 ms	90°