Skip to main content

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.

Comments

Post a Comment

Popular posts from this blog

ADVANTAGES AND DISADVANTAGES OF CORONA

Corona has many advantages and disadvantages. In the correct design of a high voltage overhead line, a balance should be struck between the advantages and disadvantages. ADVANTAGES (i) Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors. (ii) Corona reduces the effects of transients produced by surges. DIS-ADVANTAGES (i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line. (ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. (iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
Read more

ADVANTAGES OF PER UNIT SYSTEM

PER UNIT SYSTEM The per-unit system expressed the voltages, currents, powers, impedances, and other electrical quantities basis by the equation: Quantity per unit (pu) = Actual value/ Base value of quantity ADVANTAGES OF PER UNIT SYSTEM While performing calculations, referring quantities from one side of the transformer to the other side serious errors may be committed. This can be avoided by using per unit system. Voltages, currents and impedances expressed in per unit do not change when they are referred from one side of transformer to the other side. This is a great advantage. Per unit impedances of electrical equipment of similar type usually lie within a narrow range, when the equipment ratings are used as base values. Transformer connections do not affect the per unit values. Manufacturers usually specify the impedances of machines and transformers in per unit or percent of name plate ratings. Transformers can be replaced by their equivalent series impedances. ...
1 comment
Read more

ABSOLUTE AND SECONDARY INSTRUMENTS

The various electrical instruments may, in a very broad sense, be divided into (i) Absolute Instruments (ii) Secondary Instruments. Absolute Instruments are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is necessary in their case. The example of such an instrument is tangent galvanometer, which gives the value of current, in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used and the horizontal component of earth’s field.  Secondary Instruments  are those, in which the value of electrical quantity to be measured can be determined from the deflection of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments, which are most generally used in ev...
Post a Comment
Read more