Single-Phase Semi-Controlled Converter

§ 01

Introduction

This experiment studies single-phase semi-controlled (half-controlled) converters in both symmetrical and asymmetrical configurations with RL load. These converters use a mix of SCRs and diodes, enabling unidirectional power flow with free-wheeling action.

§ 02

Theory

Symmetrical semi-controlled converter circuit — Fig. 1 — Symmetrical semi-controlled converter: 2 SCRs and 2 diodes in bridge arrangement.

Asymmetrical semi-controlled converter circuit — Fig. 2 — Asymmetrical semi-controlled converter: SCRs in one arm, diodes in the other.

In the positive half-cycle (0 < t ≤ π/ω): SCR T1 is triggered at angle α. T1 and D1 conduct, applying supply voltage to the load. After π/ω, the free-wheeling diode (FWD) conducts, short-circuiting the load (output voltage = 0), allowing inductive energy to freewheeled. SCR T1 turns off naturally.

Average Output Voltage

\[V_{avg} = \frac{V_m}{\pi}(1 + \cos\alpha)\]

For \(V_m = 50\text{ V}\): At α = 0°: \(V_{avg} = 31.83\text{ V}\). At α = 90°: \(V_{avg} = 15.92\text{ V}\). At α = 180°: \(V_{avg} = 0\text{ V}\).

Key Difference from Fully Controlled: Semi-controlled converters can only produce positive output voltage (single-quadrant). However, they have better power factor and simpler gate drive requirements compared to fully controlled converters.

§ 03

Simulation — Symmetrical Semi-Controlled

Problem Statement

Implement the 1-phase symmetrical semi-controlled converter with RL load (R = 12.5 Ω, L = 6 mH). Input: 50 V peak (35.35 V RMS), 50 Hz. Observe output at α = 0°, 45°, 90°, 120°.

Symmetrical semi-controlled Simulink model — Fig. 3 — Simulink model: symmetrical semi-controlled converter with RL load.

Waveforms symmetrical at 45 degrees — Fig. 4 — Waveforms at α = 45°: output voltage, current, FWD conduction.

§ 04

Simulation — Asymmetrical Semi-Controlled

Problem Statement

Implement the 1-phase asymmetrical semi-controlled converter with RL load (same parameters). Compare with symmetrical results — note differences in waveform shapes and harmonic content.

Asymmetrical semi-controlled Simulink model — Fig. 5 — Simulink model: asymmetrical semi-controlled converter with RL load.

§ 05

Hardware — Symmetrical Semi-Controlled

Connect circuit as in Fig. 6 (R = 12.5 Ω, L = 6 mH). Switch ON 3φ supply MCB.
Switch ON POWER MODULE and SCR–Diode module MCBs. Set voltage to 35.35 V RMS.
Switch ON driver power switch. Connect CRO probes across RL load.
Vary firing angle and record results.

§ 06

Hardware — Asymmetrical Semi-Controlled

Connect circuit as in Fig. 7. Repeat hardware procedure from symmetrical case.
Compare output waveforms at the same firing angles with the symmetrical results.

§ 07

Results

I) Symmetrical — Simulation

S.No	Firing Angle (time)	Firing Angle (degrees)
1.	0 ms	0°
2.	2.5 ms	45°
3.	5 ms	90°
4.	6.66 ms	120°

I) Symmetrical — Hardware

S.No	Firing Angle (time)	Firing Angle (degrees)
1.	0 ms	0°
2.	2.5 ms	45°
3.	5 ms	90°
4.	6.66 ms	120°

II) Asymmetrical — Simulation

S.No	Firing Angle (time)	Firing Angle (degrees)
1.	0 ms	0°
2.	2.5 ms	45°
3.	5 ms	90°
4.	6.66 ms	120°

II) Asymmetrical — Hardware

S.No	Firing Angle (time)	Firing Angle (degrees)
1.	0 ms	0°
2.	2.5 ms	45°
3.	5 ms	90°
4.	6.66 ms	120°