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INTRODUCTION TO POWER ELECTRONICS
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:
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Rectification referring to conversion of ac voltage to dc
voltage,
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DC-to-AC conversion,
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DC-to DC conversion and
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AC-to-AC conversion.
"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:
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Step-down switch-mode power supply,
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Step-up chopper,
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Fly-back converter and
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Resonant converter.
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 THE ONLINE TEXT ON POWER ELECTRONICS
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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