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Introduction to Power Electronics
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Definition
Power electronics refers to control and conversion of electrical
power by power semiconductor devices wherein these devices operate
as switches. Advent of silicon-controlled rectifiers, abbreviated
as SCRs, led to the development of a new area of application called
the power electronics. Prior to the introduction of SCRs, mercury-arc
rectifiers were used for controlling electrical power, but such
rectifier circuits were part of industrial electronics and the
scope for applications of mercury-arc rectifiers was limited.
Once the SCRs were available, the application area spread to many
fields such as drives, power supplies, aviation electronics, high
frequency inverters and power electronics originated.
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Main Task of Power Electronics
Power electronics has applications that span the whole field
of electrical power systems, with the power range of these applications
extending from a few VA/Watts to several MVA / MW.
The main task of power electronics is to control and convert
electrical power from one form to another. The four main forms
of conversion are:
"Electronic power converter" is the term that is used to refer
to a power electronic circuit that converts voltage and current
from one form to another. These converters can be classified as:
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Rectifier converting an ac voltage to a dc voltage,
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Inverter converting a dc voltage to an ac voltage,
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Chopper or a switch-mode power supply that converts a dc
voltage to another dc voltage, and
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Cycloconverter and cycloinverter converting an ac voltage
to another ac voltage.
In addition, SCRs and other power semiconductor devices are used
as static switches.
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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 circuits rectifier configurations.
The popular rectifier configurations are listed below.
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Single-phase semi-controlled bridge rectifier,
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Single-phase fully-controlled bridge rectifier,
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Three-phase three-pulse, star-connected rectifier,
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Double three-phase, three-pulse star-connected rectifiers
with inter-phase transformer (IPT),
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Three-phase semi-controlled bridge rectifier,
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Three-phase fully-controlled bridge rectifier and
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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 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, 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:
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Variable speed dc drives,
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Battery chargers,
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DC power supplies and Power supply for a specific application
like electroplating
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DC - To - AC Conversion
The converter that changes a dc voltage to an alternating voltage
is called an inverter. Earlier inverters were built with SCRs.
Since the circuitry required to turn 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:
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Emergency lighting systems,
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AC variable speed drives,
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Uninterrupted power supplies, and
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Frequency converters.
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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 of one of the
types listed below:
The typical applications for a switch-mode power supply or a
chopper are:
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DC drive
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Battery charger and
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DC power supply.
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AC - To - AC Conversion
A cycloconverter or a cycloinverter 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. On the
other hand, the circuit that converts an ac voltage to another
ac voltage at the same frequency is known as an ac-chopper.
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 triacs 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.
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Additional Insights
into Power Electronics
There are several striking features of power electronics, the
foremost among them being the extensive use of inductors and capacitors.
In many applications of power electronics, an inductor may carry
a high current at a high frequency. The implications of operating
an inductor in this manner are quite a few, such as necessitating
the use of litz wire in place of single-stranded or multi-stranded
copper wire at frequencies above 50 kHz, using a proper core to
limit the losses in the core, and shielding the inductor properly
so that the fringing that occurs at the air-gaps in the magnetic
path does not lead to electromagnetic interference. Usually the
capacitors used in a power electronic application are also stressed.
It is typical for a capacitor to be operated at a high frequency
with current surges passing through it periodically. This means
that the current rating of the capacitor at the operating frequency
should be checked before its use. In addition, it may be preferable
if the capacitor has self-healing property. Hence an inductor
or a capacitor has to be selected or designed with care, taking
into account the operating conditions, before its use in a power
electronic circuit.
In many power electronic circuits, diodes play a crucial role.
A normal power diode is usually designed to be operated at 400
Hz or less. Many of the inverter and switch-mode power supply
circuits operate at a much higher frequency and these circuits
need diodes that turn ON and OFF fast. In addition, it is also
desired that the turning-off process of a diode should not create
undesirable electrical transients in the circuit. Since there
are several types of diodes available, selection of a proper diode
is very important for reliable operation of a circuit.
Analysis of power electronic circuits tends to be quite complicated,
because these circuits rarely operate in steady-state. Traditionally
steady-state response refers to the state of a circuit characterized
by either a dc response or a sinusoidal response. Most of the
power electronic circuits have a periodic response, but this response
is not usually sinusoidal. Typically, the repetitive or the periodic
response contains both a steady-state part due to the forcing
function and a transient part due to the poles of the network.
Since the responses are nonsinusoidal, harmonic analysis is often
necessary. In order to obtain the time response, it may be necessary
to resort to the use of a computer program.
Power electronics is a subject of interdisciplinary nature. To
design and build control circuitry of a power electronic application,
one needs knowledge of several areas, which are listed below.
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Design of analogue and digital electronic circuits, to build
the control circuitry.
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Microcontrollers and digital signal processors for use in
sophisticated applications.
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Many power electronic circuits have an electrical machine
as their load. In ac variable speed drive, it may be a reluctance
motor, an induction motor or a synchronous motor. In a dc
variable speed drive, it is usually a dc shunt motor.
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In a circuit such as an inverter, a transformer may be connected
at its output and the transformer may have to operate with
a nonsinusoidal waveform at its input.
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A pulse transformer with a ferrite core is used commonly
to transfer the gate signal to the power semiconductor device.
A ferrite-cored transformer with a relatively higher power
output is also used in an application such as a high frequency
inverter.
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Many power electronic systems are operated with negative
feedback. A linear controller such as a PI controller is used
in relatively simple applications, whereas a controller based
on digital or state-variable feedback techniques is used in
more sophisticated applications.
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Computer simulation is often necessary to optimize the design
of a power electronic system. In order to simulate, knowledge
of software package such as MATLAB and the know-how to model
nonlinear systems may be necessary.
The study of power electronics is an exciting and a challenging
experience. The scope for applying power electronics is growing
at a fast pace. New devices keep coming into the market, sustaining
development work in power electronics.
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Structure of this Online
Text
The text contains several chapters. Each chapter is divided
into sections. Each section is presented as a separate page.
Each page is on a separate topic or a separate circuit.
Each circuit is described in detail and in addition, a sufficiently
high level of mathematical analysis has also been presented.
It has also been how the circuit can be simulated using Pspice,
MathCad and Matlab. In addition, there would be an interactive
Java applet to illustrate how the circuit operates.
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