Understanding the DC Output Voltage of a 3-Phase Full-Wave Rectifier

In electrical engineering, understanding the output voltage of a 3-phase full-wave rectifier is essential for designing and optimizing power systems. This article will guide you through the process of calculating the DC voltage output for a given AC input voltage, specifically 400 Vac, and explain the significance of different voltage calculations.

Formula and Calculation

The formula to calculate the DC output voltage of a 3-phase full-wave rectifier is given by:

V_{DC} sqrt{3} times V_{L-L} - V_{D}

Where:

( V_{DC} ) is the DC output voltage. ( V_{L-L} ) is the line-to-line RMS voltage of the AC supply. ( V_{D} ) is the voltage drop across the diodes, which is typically around 2V for silicon diodes in a full-wave rectifier.

Step-by-Step Calculation

Given V_{L-L} 400 V, and assuming V_{D} approx 2 V, let's calculate the DC output voltage:

Calculate (sqrt{3} times V_{L-L}):

(sqrt{3} times 400 V approx 1.732 times 400 approx 692.8 V)

Subtract the diode drop:

(692.8 V - 2 V approx 690.8 V)

Therefore, the approximate DC output voltage of a 3-phase full-wave rectifier with a 400 VAC input is about 690.8 V.

Capacitor Filter and DC Voltage Calculation

When a DC capacitor filter is employed, the DC voltage is affected in two ways:

Peak Value: The peak value of the rectified three-phase waveform can be approximated using:

(400 V times sqrt{2} approx 566 VDC)

Average Value: Considering the loading of the rectifier or small/non-existent filter capacitor, the average value is:

(400 V times 1.35 approx 540 VDC)

Note that these calculations do not account for the forward conduction voltage drop of the rectifiers. With two series rectifier diodes in the current path, this may add approximately 2VDC to the final voltage.

Controlled Thyristor Rectifier

For a controlled thyristor rectifier, the multiplication factor is slightly different. The DC output voltage can be calculated as:

400 Vac times 122 approx 488 VDC

This calculation is more accurate for thyristor-based rectification systems.

Cheers!