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RECTIFICATION

Rectifiers can be classified as uncontrolled and controlled rectifiers, and the controlled rectifiers can be further divided into semi-controlled and fully controlled rectifiers. Uncontrolled rectifier circuits are built with diodes, and fully controlled rectifier circuits are built with SCRs. Both diodes and SCRs are used in semi-controlled rectifier circuits. There are several rectifier configurations. The most famous rectifier configurations are listed below.

• Single-phase semi-controlled bridge rectifier,
• Single-phase fully-controlled bridge rectifier,
• Three-phase three-pulse, star-connected rectifier,
• Double three-phase, three-pulse star-connected rectifiers with inter-phase transformer (IPT),
• Three-phase semi-controlled bridge rectifier,
• Three-phase fully-controlled bridge rectifier,
• Double three-phase fully controlled bridge rectifiers with IPT.

Apart from the configurations listed above, there are series-connected and 12-pulse rectifiers for delivering high quality high power output. Power rating of a single-phase rectifier tends to be lower than 10 kW.

Three-phase bridge rectifiers are used for delivering higher power output, up to 500 kW at 500 V DC or even more. For low voltage, high current applications, and a pair of three-phase, three-pulse rectifiers interconnected by an inter-phase transformer (IPT) is used. For a high current output, rectifiers with IPT are preferred to connecting devices directly in parallel.

There are many applications for rectifiers. Some of them are:

• Variable speed DC drives,
• Battery chargers,
• DC power supplies and Power supply for a specific application like electroplating

DC to AC CONVERSION

The converter that changes a DC voltage to an alternating voltage, AC is called an inverter. Earlier inverters were built with SCRs. Since the circuitry required turning the SCR off tends to be complex, other power semiconductor devices such as bipolar junction transistors, power MOSFETs, insulated gate bipolar transistors (IGBT) and MOS-controlled thyristors (MCTs) are used nowadays. Currently only the inverters with a high power rating, such as 500 kW or higher, are likely to be built with either SCRs or gate turn-off thyristors (GTOs). There are many inverter circuits and the techniques for controlling an inverter vary in complexity.

Some of the applications of an inverter are listed below:

• Emergency lighting systems,
• AC variable speed drives,
• Uninterrupted power supplies, and,
• Frequency converters.

DC to DC CONVERSION

When the SCR came into use, a DC-to-DC converter circuit was called a chopper. Nowadays, an SCR is rarely used in a DC-to-DC converter. Either a power BJT or a power MOSFET is normally used in such a converter and this converter is called a switch-mode power supply. A switch-mode power supply can be one of the types listed below:

• Step-down switch-mode power supply,
• Step-up chopper,
• Fly-back converter,
• Resonant converter.

The typical applications for a switch-mode power supply or a chopper are:

• DC drive,
• Battery charger, and,
• DC power supply.

AC to AC Conversion

A cycloconverter or a Matrix converter converts an AC voltage, such as the mains supply, to another AC voltage. The amplitude and the frequency of input voltage to a cycloconverter tend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable especially in Adjustable Speed Drives (ASD). A typical application of a cycloconverter is to use it for controlling the speed of an AC traction motor and most of these cycloconverters have a high power output, of the order a few megawatts and SCRs are used in these circuits. In contrast, low cost, low power cycloconverters for low power AC motors are also in use and many of these circuit tend to use TRIAC in place of SCRs. Unlike an SCR which conducts in only one direction, a TRIAC is capable of conducting in either direction and like an SCR, it is also a three terminal device. It may be noted that the use of a cycloconverter is not as common as that of an inverter and a cycloinverter is rarely used because of its complexity and its high cost.

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