Figure 1: Equipment

3-Phase Diode-Bridge Rectifier Box

The 3-Phase Diode-Bridge Rectifier Box is made up of three sections which can be used individually or together to construct different diode rectifier circuits. These three sections are as follows and discussed in more detail in the following sections.

Note

The 3-Phase Diode-Bridge Rectifier Box also has a chassis ground connection available on the top of the box which allows you to ground the enclosure for safety. The connection uses a green safety banana jack that should be connected to an earth ground which is typically available on one of the power supply boxes.

3-phase Reactor

The six connections to the 3-phase reactor are available on the far left side of the box as is shown in the picture at the top of the page. The 3-phase reactor is simply three inductors that share a common magnetic core. A picture of the 3-phase reactor is shown in the picture below, note the 3 separate coil winding’s on the 3 different limbs of the iron core. The phases of the reactor are color coated (red, black, blue) and are connected as shown on the box’s diagram. Each phase of the reactor has an inductance of something close to its rated value of 5mH and it is also rated for a maximum of 600V and 8A.

In the lab we use this 3-phase reactor for both single-phase and three-phase circuits by just connecting to the winding’s that we wish to use.

figure 2. 3-phase reactor that is mounted inside the enclosure.

3-phase Diode-Bridge Rectifier

The five connections to the 3-phase diode-bridge rectifier are available near the center of the box as shown in the picture at the top of the page. The red, black and blue terminals to the left of the rectifier are the AC input connections and are fused with 10 amp fuses on each phase for protection. The fuses are located in the fuse holders which are accessible at the top of the box. Please do not check or change the fuses without the supervision of a lab instructor. The red and black connections to the right of the rectifier are the DC output connections with the polarity of the voltage indicated on the diagram.

A picture of the 3-phase diode-bridge rectifier package that is used in the box is shown below. Note that it is connected on a heatsink inside the box to assist the package with cooling. This particular package is rated to output 600 VDC and upto 25 Arms if sufficient cooling is provided.

In the lab we use this 3-phase diode-bridge rectifier for both single-phase and three-phase circuits by just connecting to the diodes in the package that we intend on using.

figure 3. 3-phase Diode-Bridge Rectifier that is mounted inside the enclosure.

Electrolytic Capacitor

The two connections to the electrolytic capacitor are available of the far right side of the box as is shown in the picture at the top of the page. This large capacitor is used to filter out the ac component of the output waveform so the output voltage of the rectifier can be mainly a DC voltage with a small as possible ac ripple. The capacitors nominal capacitance is 1000uF and is rated to take up to 450 VDC.

Note that the capacitor shown in the picture below has a brown resistor across its terminals. This resistor is known as a bleed resistor, its purpose is to slowly discharge the capacitor when the circuit is turned off as a capacitor this size can take a very long to discharge if left completely unconnected. Which could lead to safety issues. The resistance of this bleed resistor is typically something very large (>10 kΩ)as to not significantly effect the efficiency of the rectifier when in operation.

figure 4. Electrolytic Capacitor that is mounted inside the enclosure.

One problem with such a large capacitor is that it can store a significant amount of energy. To initially charge the capacitor from 0 volts to the operating voltage of the rectifier, which can be up to approximately 300 VDC, the capacitor can draw a significant amount of current from the power supply if there isn’t a significant amount of impedance between it and its source. Therefore an inrush limiting resistor (shown below) is inserted in the circuit during start up to limit the maximum current charging the capacitor. Once the capacitor is nearly charged the resistor needs to be removed from the circuit by shorting it with the switch available on the box. If the resistor wasn’t shorted it would cause undesirable losses in the circuit which we want to avoid.

figure 5. Inrush limiting resistor and its controlling switch.