Rectifier with Capacitor Filter

§ 01

Introduction

This experiment analyses the effect of a smoothing (filter) capacitor on rectified output voltage for both single-phase and three-phase uncontrolled rectifiers. Adding a capacitor across the load significantly reduces ripple and raises the mean DC output voltage.

§ 02

Smoothing Capacitor Theory

Full-wave bridge rectifier with capacitor filter — Fig. 1 — Full-wave bridge rectifier with smoothing capacitor across the load.

The capacitor charges to the peak voltage when diodes conduct and discharges through the load when diodes are reverse-biased. Two key selection parameters:

Working Voltage

Must be higher than the no-load peak output voltage of the rectifier to prevent dielectric breakdown.

Capacitance Value

Larger capacitance ⇒ lower ripple voltage. Target: ripple < 100 mV peak-to-peak for practical DC supplies.

Ripple Metrics with Filter

\[\text{FF} = \frac{V_{rms}}{V_{dc}},\quad \text{RF} = \sqrt{\text{FF}^2-1}\]

A well-designed capacitor filter reduces RF from ~0.482 (unfiltered single-phase) to well below 0.1. The 3φ bridge with filter achieves RF < 0.01.

Ripple Frequency: Single-phase full-wave: 100 Hz (2× supply). Three-phase bridge: 300 Hz (6× supply). Higher ripple frequency means smaller capacitor is needed for same ripple.

§ 03

Simulation — Single-Phase with C Filter

Problem 1

Implement 1-phase uncontrolled full-wave rectifier with C = 5000 μF in parallel with R = 100 Ω. Input: 50 V peak (35.35 V RMS), 50 Hz. Observe output voltage changes.

Single-phase rectifier with capacitor filter Simulink model — Fig. 2 — Simulink model: single-phase bridge rectifier with 5000 μF capacitor filter.

§ 04

Simulation — Three-Phase with C Filter

Problem 2

Implement 3-phase uncontrolled full-wave rectifier with C = 5000 μF and R = 100 Ω. Input: \(V_{LL,rms} = 61.2\text{ V}\), 50 Hz. Observe and compare with single-phase results.

Three-phase rectifier with capacitor filter Simulink model — Fig. 3 — Simulink model: three-phase bridge rectifier with 5000 μF capacitor filter.

§ 05

Hardware — Single-Phase with C Filter

Hardware single-phase rectifier with capacitor — Fig. 4 — Hardware wiring: single-phase bridge rectifier with C = 5000 μF, R = 100 Ω.

Connect circuit as in Fig. 4 (R = 100 Ω, C = 5000 μF). Switch ON 3φ supply MCB.
Switch ON POWER MODULE and SCR–Diode module MCBs. Set voltage to 35.35 V RMS.
Connect CRO probes across R load. Observe output voltage waveforms and FFT.

§ 06

Hardware — Three-Phase with C Filter

Hardware three-phase rectifier with capacitor — Fig. 5 — Hardware wiring: three-phase bridge rectifier with C = 5000 μF, R = 100 Ω.

Connect circuit as in Fig. 5. Switch ON 3φ supply MCB.
Set voltage to 61.2 V RMS. Connect CRO probes and observe waveforms.

§ 07

Results

Attach waveforms: Output Voltage, Output Current, Input Voltage, Input Current for both single-phase and three-phase cases (Simulink and DSO).

Performance Parameters — Simulation

Parameter	Single Phase	Three Phase
V_RMS (V)
I_RMS (A)
V_AVG (V)
I_AVG (A)
Form Factor
Ripple Factor

Performance Parameters — Hardware

Parameter	Single Phase	Three Phase
V_RMS (V)
V_AVG (V)
Form Factor
Ripple Factor