Three-Phase Uncontrolled Diode Rectifier

§ 01

Introduction

This experiment converts three-phase AC to DC using a six-diode bridge rectifier. The three-phase topology produces a 300 Hz output ripple (vs. 100 Hz for single-phase), yielding lower ripple factor and superior load performance.

⚡

Key Advantage

Lower Ripple, Higher DC Output Voltage

Three-phase rectifiers produce \(V_{dc} = 2.34 \cdot V_{phase,rms}\), with a ripple factor of only 0.042 — far superior to single-phase rectifiers.

§ 02

Theory

The three-phase bridge consists of six diodes in two groups: D1, D3, D5 in the positive group and D2, D4, D6 in the negative group. At any instant, one diode from each group conducts. The output voltage follows the highest line-to-line voltage, producing a six-pulse waveform per cycle.

Three-phase bridge rectifier circuit and waveforms — Fig. 1 — Three-phase bridge rectifier: circuit topology and six-pulse output waveform.

Three-Phase Bridge Rectifier Metrics

\[V_{dc} = \frac{3\sqrt{3}}{\pi}\cdot V_{m,phase} = 2.34\cdot V_{phase,rms},\quad \text{RF} \approx 0.042,\quad \text{Ripple frequency} = 6f = 300\text{ Hz}\]

For \(V_{LL,rms} = 61.2\text{ V}\) (phase-to-phase): \(V_{phase,rms} = 61.2/\sqrt{3} = 35.35\text{ V}\), \(V_{dc} \approx 82.7\text{ V}\).

§ 03

Simulation — R and RL Loads

Problem Statement

(a) Implement the 3-phase uncontrolled full-wave rectifier with R = 100 Ω. Input: \(V_{LL,rms} = 61.2\text{ V}\), 50 Hz.
(b) Add L = 6 mH in series with R. Observe changes in waveform and FFT analysis.

Simulink model three-phase bridge rectifier — Fig. 2 — Simulink model: three-phase source, six-diode bridge, R load, measurement blocks.

Calculation:

Form Factor \(= V_{rms}/V_{dc}\)
Ripple Factor \(= \sqrt{\text{FF}^2 - 1}\)
FFT fundamental at 300 Hz (6th harmonic of 50 Hz)

Expected improvement over 1φ:

Lower ripple factor (~0.042 vs ~0.482)
Higher mean output voltage
Continuous load current even for moderate L values

§ 04

Hardware Implementation

Hardware three-phase bridge rectifier — Fig. 3 — Hardware wiring: three-phase bridge rectifier with R/RL load.

Procedure — R Load (Case 1)

Connect circuit as in Fig. 3 (R = 100 Ω). Switch ON 3φ supply MCB.
Switch ON POWER MODULE and SCR–Diode module MCBs. Increase to 61.2 V RMS.
Connect CRO probes across R load. Observe output voltage and FFT plot (fundamental at 300 Hz).

Procedure — RL Load (Case 2)

Add L = 6 mH in series with R = 100 Ω. Connect CRO probes across RL load.
Observe and compare the smoother current waveform with the R-load case.

§ 05

Results

Required waveforms: Output Voltage, Output Current, Input Line Voltages, Input Line Currents, Diode voltages (D1 and D4), FFT lists — all in Simulink.

Performance Parameters

Parameter	R Load — Sim	R Load — HW	RL Load — Sim	RL Load — HW
V_RMS (V)
I_RMS (A)
V_AVG (V)
I_AVG (A)
Form Factor
Ripple Factor
THD (%)
V_fundamental (RMS)
V 2nd Harmonic
V 3rd Harmonic

In dB

\[20\log(V_{fundamental,rms}) = \underline{\hspace{4cm}}\text{ dB}\]