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INTRODUCTION
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CHAPTER-1
INTRODUCTOIN 1.1 Introduction
Efficiency and cost are the major concerns in the development of low-power motor
drives targeting household applications such as fans, water pumps, blowers, mixers, etc.
The use of the brushless direct current (BLDC) motor in these applications is becoming
very common due to features of high efficiency, high flux density per unit volume, low
maintenance requirements, and low electromagnetic-interference problem. These BLDC
motors are not limited to household applications, but these are suitable for other
applications such as medical equipment, transportation, HVAC, motion control, and many
industrial tools. A BLDC motor has three phase windings on the stator and Permanent
magnets on the rotor. The BLDC motor is also known as an electronically commutated
motor because an electronic commutation based on rotor position is used rather than a
mechanical commutation which has disadvantages like sparking and wear and tear of
brushes and commutator assembly.
Brushless DC electric motor (BLDC motors, BL motors) also known as
electronically commutated motors (ECMs, EC motors) are synchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply,
which produces an AC electric signal to drive the motor. In this context, AC, alternating
current, does not imply a sinusoidal waveform, but rather a bidirectional current with no
restriction on waveform. Additional sensors and electronics control the inverter output
amplitude and waveform (and therefore percent of DC bus usage/efficiency) and
frequency (i.e. rotor speed). The rotor part of a brushless motor is often a permanent
magnet synchronous motor, but can also be a switched reluctance motor, or inductionmotor [citation needed].
Brushless motors may be described as stepper motors; however, the term stepper
motor tends to be used for motors that are designed specifically to be operated in a mode
where they are frequently stopped with the rotor in a defined angular position. This page
describes more general brushless motor principles, though there is overlap. Two key
performance parameters of brushless DC motors are the motor constants Kv and Km.
Electrical equipment often has at least one motor used to rotate or displace an object from
its initial position. There are a variety of motor types available in the market, including
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induction motors, servomotors, DC motors (brushed and brushless), etc. Depending upon
the application requirements, a particular motor can be selected. However, a current trend
is that most new designs are moving towards Brushless DC motors, popularly known as
BLDC motors.
The proposed BL buck – boost converter-based VSI-fed BLDC motor drive is shown
in the fig. The parameters of the BL buck – boost converter are designed such that it
operates in discontinuous inductor current mode (DICM) to achieve an inherent power
factor correction at ac mains. The speed control of BLDC motor is achieved by the dc
link voltage control of VSI using a BL buck – boost converter. This reduces the switching
losses in VSI due to the low frequency operation of VSI for the electronic commutation
of the BLDC motor. The performance of the proposed drive is evaluated for a wide range
of speed control with improved power quality at ac mains.
1.2 TYPES OF SYSTEMS
The power conversion systems are classified based on the type of input and output
power as follows:
AC to DC (rectifier)
DC to AC (inverter)
DC to DC (DC to DC converter)
AC to AC (AC to AC converter)
DC/AC converters (inverters)
An AC output waveform from a DC source is produced in DC to AC converters.
Applications of DC/AC converters are adjustable speed drives (ASD), uninterruptable
power supplies (UPS), active filters, photovoltaic generators, voltage compensators,and
flexible AC transmission systems(FACTS).
Topologies used in these converters are divided into two different types:
Voltage source inverters
Current source inverters.
In Voltage source inverters (VSIs) voltage waveform is a output which is
independently controlled. In current source inverters (CSIs) current waveform is the
controlled AC output. The power switching devices which are fully controllable
semiconductor power switches results the DC to AC power conversion. Fast transitions
than the smooth ones are produced as the output waveforms are made up of discrete
values. By the controlling of modulation technique it is detected when the power valves
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are on and off and for how long around the fundamental frequency the ability to produce
near sinusoidal waveforms is dictated. space-vector technique, and the selective-harmonic
technique ,the carrier-based technique, or pulse width modulation, are included in the
common modulation techniques.
We can use voltage source inverters in both single-phase and three-phase
applications. Single-phase VSIs are widely used for power supplies, single-phase UPSs,
and utilize half-bridge and full-bridge configurations, and when used in multicell
configurations, elaborate high-power topologies. In applications where sinusoidal voltage
waveforms are required, such as ASDs, UPSs, and some types of FACTS devices such as
the STATCOM. Three-phase VSIs are used. In the case of active filters and voltage
compensators where arbitrary voltages are required these are used. To produce an AC
output current from a DC current supply Current source inverters are used. This type of
inverter is practical for three-phase applications where high-quality voltage waveforms
are required.
There was a widespread interest on a relatively new class of inverters, called
multilevel inverters. Because of the fact that power switches are connected to either the
positive or to the negative DC bus, operation of CSIs and VSIs can be classified as two-
level inverters. A sine wave could better approximated by the AC output if more than
two voltage levels were available to the inverter output terminals. The multilevel
inverters, although more complex and costly, offer higher performance due this reason.
Whether or not they require freewheeling diodes, each inverter type differs in the DC
links used. Depending on its intended usage it can be made to operate in square wave or
pulse width modulation mode. We can implement PWM in many different ways and
higher quality waveforms are produced where simplicity is offered in square wave mode.
The output inverter section is fed from a constant-voltage source in Voltage Source
Inverters (VSI). The modulation technique which is to be selected for a given application
is determined by the wanted quality of the current output waveform. The VSI output has
discrete values. At the selective harmonic frequencies the loads should be inductive for a
smooth current waveform to be obtained. Capacitive load causes a choppy current
waveform with large and frequent current spikes to be received by load if the load and
source has no inductive filtering between them.
AC TO AC CONVERTERS
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The control of voltage, phase, and frequency of the waveform allows the converting
of AC power to Ac power applied to a load from a supplied AC system. Based on the
frequency of the wave form there are two main categories that can be used to separate the
type of converters. The converter which does not allow the user to modify the frequencies
are known as AC regulators or AC voltage Controllers. The converters which allow the
user to change the frequency are known as frequency converters for AC to AC
conversion.
There are three types of converters in frequency converters. They are matrix
converter, cycloconverter, DC link converter (aka AC/DC/AC converter).
AC Voltage Controller: The AC voltage Controller is used to vary the RMS voltage
across the load at constant frequency. Pulse Width Modulation AC Chopper Control
(PWM AC Chopper Control), ON/OFF Control, Phase-Angle Control is the three control
methods that are generally accepted. In Three-phase circuits as well as Single-phase
circuits, all the three methods can be implemented.
ON/OFF Control: For speed control of motors or for heating loads this method id
typically used. This control method involves turning the switch off for m integral cycles
and turning the switch on for n integral cycles. Undesirable harmonics are to be created
because of turning the switches off and on. During zero-current conditions (zero-
crossing) and zero-voltage conditions the switches are turned off and on, effectively
reducing the distortion.
Phase-Angle Control: To implement a phase-angle control on different waveforms
there are various circuits exits such as full-wave or half-wave voltage control. SCRs,
Triacs and diodes are typically used power electronic components. The user can delay the
firing angle with the use of these components in a wave which will only cause part of the
wave to be outputted.
PWM AC Chopper Control: The other two control methods often have poor
harmonics, output current quality and input power factor .PWM can be used instead of
other methods to improve these values. Turning the switches off and on several times
within alternate half-cycles of input voltage can be done by this PWM AC Chopper.
Matrix Converters and CycloConverters For ac to ac conversion in industries
CycloConverters are widely used, because they are able to use in high-power
applications. They are commutated direct frequency converters that are synchronized by a
supply line. The output voltage waveforms of the cycloconverters have complexharmonics with higher order harmonics, machine inductance can filter the harmonic
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which causes the machine current to have fewer harmonics, losses and torque pulsations
can be caused by the remaining harmonics. There are no inductors or capacitors in a
cycloconverter unlike other converters i.e. any storage devices. For this reason the
instantaneous output power and the input power are equal.
Single-Phase to Single-Phase Cycloconverters: Because of the decrease in both the
power and size of the power electronics switches these started drawing more interest
recently. The single-phase high frequency ac voltage can be either sinusoidal or
trapezoidal. For control purpose these might be zero voltage commutation or zero voltage
intervals.
Three-phase to Single-Phase cycloconverters: These three-phase to single-phase
cycloconverters are two kinds: 3φ to 1φ half wave cycloconverters and 3φ to 1φ bridge
cycloconverters. Either at polarity both the negative and positive converters can generate
voltage, resulting positive current supplied by the positive converter and the negative
current supplied by the negative converter.
New forms of cycloconverters are being developed with recent advances such as
matrix converters. The matrix converter utilize bi-directional, bipolar switches is the first
change first noticed in them. Matrix of 9 switches connecting the three input phases to the
three output phases are present in a single phase to a single phase matrix converter. At
any time without connecting any two switches any output phase and input phase can be
connected together from the same phase at the same time, otherwise a short circuit of the
input phase will be caused. Matrix converters are more compact, lighter and versatile than
other converter solutions. As a result to regenerate energy back to the utility they are able
to achieve higher temperature operation, higher levels of integration, natural bi-
directional power flow and broad output frequency.
1.3 ELECTRICAL MACHINES
Conversion of Electrical energy into Mechanical energy can be done by the electric
machine. A machine that converts mechanical energy to electrical energy is a Generator,
A machine which converts electrical energy to mechanical energy is a Motor, and the one
which changes the voltage level of an alternating current is a Transformer.
1.3.1Generator
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Fig 1.3.1.Electrical generator
An electric device which converts mechanical energy to electrical energy is called
a Generator. It forces electrons to flow through an external electrical circuit. It is similar
to a water pump. A water pump does not create the water inside but creates a flow of
water. The prime mover which is the source of mechanical energy may be a turbine
steam engine or reciprocating, an internal combustion engine, water falling from a water
wheel or turbine, a hand crank or any other source of mechanical energy.
Mechanical or Electrical terms are used to describe the two main parts of the
Electrical machine. The rotating part is the rotor; the stationary part is the stator in
mechanical terms. The power producing component is the Armature and the magnetic
field component is the field in Electrical terms. The armature can be on either the stator or
the rotor. The permanent magnets or the electromagnets are mounted on either the stator
or the rotor provides the mechanical energy.
The generators are divided into two types: AC Generators and DC Generators
AC Generator:
A machine which converts the mechanical energy into alternating current
electricity is the AC generator because the power transferred into the armature current is
greater than the power transferred into the field current. They always have the rotor with
field winding and the stator with armature winding.
Several types of AC Generators are there:An induction generator in which the currents in the rotor are induced by the stator
magnetic flux. The rotor can be drived above the synchronous speed by the prime mover
which causes the opposing rotor flux. This rotor flux cuts the stator coils by producing
active current in them, thus sending power back to the grids. An induction generator
cannot be an isolated source of power because it draws reactive power from the connected
system.
In a synchronous generator (Alternator) the separate current source provides the current
for the magnetic field.
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DC Generator:
It produces direct current electrical energy from mechanical energy. With in
mechanical limits it can operate at any speed and always output a direct current
waveform. The direct current generators known as dynamos works on exactly the same
principle as the alternators but have a commutator on the rotating shaft which converts the
armature alternate current into direct current.
1.3.2 Motor
Fig 1.3.2 Electric motor
An electric device which converts electrical energy into mechanical energy is
electric motor which is a reverse part of the generator. The electric motor operates
through interacting magnetic fields and current carrying conductors to generate rotational
force. The motors and generators have many similarities and many motors can work as
generators and vice versa. These are used in industrial fans, machine tools, house hold
appliances, disk drives and power tools. They may be powered by Alternate current and
Direct current.
Motors are divided into two types: AC motors and DC motors.
AC Motor:
It converts alternating current into mechanical energy. It consists of two basic
parts an outside stationary stator having coil with an alternating current to produce a
rotating magnetic field and an inside rotor attached to the output shaft that is given a
torque by the rotating field. By the type of rotor used the ac motors are divided into two
types.
Induction (asynchronous)motor: the induced current creates the rotor magnetic
field. To provide the induced current the rotor must turn slightly slower than the stator
magnetic field. Squirrel-cage rotor, solid core rotor and the wound rotor are the three
types of induction motor rotors.
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Synchronous motor does not base on induction so it can rotate exactly at the
supply frequency or sub-multiple. The direct current or a permanent magnet generates the
magnetic field of the rotor.
DC Motor:
A brushed dc motor uses rotating electrical magnets, internal commutation, and
stationary permanent magnets to produce torque from dc power supplied to the motor.
The electric current from the commutator to the spinning wire windings of the rotor inside
the motor can be carried by the brushes and springs. Brushless dc motors uses a rotating
permanent magnet in the rotor and stationary electric motors in the motor. A motor
controller converts d to ac. The complication of transferring power from outside the
motor to the spinning rotor eliminates in this brushless motors. A stepper motor is an
example of brushless synchronous dc motor and it can divide a full rotation into large
number of steps.
1.3.3 Transformer
A static device which converts alternating current from one voltage level to
another level (lower or higher) or the same level without changing the frequency is called
a Transformer. Through inductively coupled conductors (transformer coils) the
transformer transfers electrical energy from one circuit to another. A varying magnetic
flux in the transformer core is created by a varying electric current in the first or primary
winding and thus a varying magnetic field through the secondary winding. A varying
Electro Motive Force (EMF) or ―Voltage‖ in the secondary winding induces by the
varying magnetic field. This effect is called Mutual Induction.
There are two types of transformers
Step-Up transformer
Step-down transformer
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POWER ELECTRONIC DEVICES
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CHPTER-2
POWER ELECTRONIC DEVICES
2.1 MOSFET:
The metal – oxide – semiconductor field-effect transistor (MOSFET, MOS-
FET, or MOS FET) is a type of transistor used for amplifying or switching electronic
signals. Although the MOSFET is a four-terminal device with source (S), gate (G), drain
(D), and body (B) terminals,[1]
the body (or substrate) of the MOSFET is often connected
to the source terminal, making it a three-terminal device like other field-effect transistors.
Because these two terminals are normally connected to each other (short-circuited)
internally, only three terminals appear in electrical diagrams. The MOSFET is by far the
most common transistor in both digital and analog circuits, though the bipolar junction
transistor was at one time much more common.
The main advantage of a MOSFET over a regular transistor is that it requires
very little current to turn on (less than 1mA), while delivering a much higher current to a
load (10 to 50 times or more).
Fig 2.1 Symbols of MOSFET
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2.2 IGBT:
An insulated-gate bipolar transistor (IGBT) is a three-terminal power
semiconductor device primarily used as an electronic switch which, as it was developed,
came to combine high efficiency and fast switching. It switches electric power in many
modern appliances: variable-frequency drives (VFDs), electric cars, trains, variable speed
refrigerators, lamp ballasts, air-conditioners and even stereo systems with switching
amplifiers. Since it is designed to turn on and off rapidly, amplifiers that use it often
synthesize complex waveforms with pulse-width modulation and low-pass filters. In
switching applications modern devices feature pulse repetition rates well into the
ultrasonic range — frequencies which are at least ten times the highest audio frequency
handled by the device when used as an analog audio amplifier. The IGBT combines the
simple gate-drive characteristics of MOSFETs with the high-current and low-saturation-
voltage capability of bipolar transistors. The IGBT combines an isolated-
Gate FET for the control input and a bipolar power transistor as a switch in a
single device. The IGBT is used in medium- to high-power applications like switched-
mode power supplies, traction motor control and induction heating. Large IGBT modules
typically consist of many devices in parallel and can have very high current-handling
capabilities in the order of hundreds of amperes with blocking voltages of 6000 V. These
IGBTs can control loads of hundreds of kilowatts. The IGBT is a semiconductor device
with four alternating layers (P-N-P-N) that are controlled by a metal-oxide-semiconductor
(MOS) gate structure without regenerative action.
Fig 2.2 Symbol of IGBT
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Comparison with power MOSFETs And IGBTs:
An IGBT features a significantly lower forward voltage drop compared to a
conventional MOSFET in higher blocking voltage rated devices. As the blocking voltage
rating of both MOSFET and IGBT devices increases, the depth of the n- drift region must
increase and the doping must decrease, resulting in roughly square relationship decrease
in forward conduction versus blocking voltage capability of the device. By injecting
minority carriers (holes) from the collector p+ region into the n- drift region during
forward conduction, the resistance of the n- drift region is considerably reduced.
However, this resultant reduction in on-state forward voltage comes with several
penalties:
The additional PN junction blocks reverse current flow. This means that unlike a
MOSFET, IGBTs cannot conduct in the reverse direction. In bridge circuits,
where reverse current flow is needed, an additional diode is placed in parallel with
the IGBT to conduct current in the opposite direction. The penalty isn't overly
severe because at higher voltages, where IGBT usage dominates, discrete diodes
have a significantly higher performance than the body diode of a MOSFET.
The reverse bias rating of the N-drift region to collector P+ diode is usually only
of tens of volts, so if the circuit application applies a reverse voltage to the IGBT,
an additional series diode must be used.
The minority carriers injected into the N-drift region take time to enter and exit or
recombine at turn-on and turn-off. These results in longer switching times, and
hence higher switching loss compared to a power MOSFET.
The on-state forward voltage drop in IGBTs behaves very differently from power
MOSFETS. The MOSFET voltage drop can be modeled as a resistance, with the
voltage drop proportional to current. By contrast, the IGBT has a diode-likevoltage drop (typically of the order of 2V) increasing only with the of the current.
Additionally, MOSFET resistance is typically lower for smaller blocking voltages,
so the choice between IGBTs and power MOSFETS will depend on both the
blocking voltage and current involved in a particular application.
In general, high voltage, high current and low switching frequencies favor the IGBT
while low voltage, low current and high switching frequencies are the domain of the
MOSFET.
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2.3DIODE:
In electronics, a diode is a two-terminal electronic component that conducts
primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance
to the flow of current in one direction, and high (ideally infinite) resistance in the other. A
semiconductor diode, the most common type today, is a crystalline piece of
semiconductor material with a p – n junction connected to two electrical terminals.[5] A
vacuum tube diode has two electrodes, a plate (anode) and a heated cathode.
Semiconductor diodes were the first semiconductor electronic devices.
Main functions:
The most common function of a diode is to allow an electric current to pass in one
direction (called the diode's forward direction), while blocking current in the opposite
direction (the reverse direction). Thus, the diode can be viewed as an electronic version of
a check valve. This unidirectional behavior is called rectification, and is used to convert
alternating current to direct current, including extraction of modulation from radio signals
in radio receivers — these diodes are forms of rectifiers.
However, diodes can have more complicated behavior than this simple on – off
action, due to their nonlinear current-voltage characteristics. Semiconductor diodes begin
conducting electricity only if a certain threshold voltage or cut-in voltage is present in theforward direction (a state in which the diode is said to be forward-biased ). The voltage
drop across a forward-biased diode varies only a little with the current, and is a function
of temperature; this effect can be used as a temperature sensor or as a voltage reference.
P- N Junction Diode:
A p – n junction diode is made of a crystal of semiconductor, usually silicon, but
germanium and gallium arsenide are also used. Impurities are added to it to create a
region on one side that contains negative charge carriers (electrons), called an n-typesemiconductor, and a region on the other side that contains positive charge carriers
(holes), called a p-type semiconductor. When the two materials i.e. n-type and p-type are
attached together, a momentary flow of electrons occur from the n to the p side resulting
in a third region between the two where no charge carriers are present. This region is
called the depletion region due to the absence of charge carriers (electrons and holes in
this case). The diode's terminals are attached to the n-type and p-type regions. The
boundary between these two regions, called a p – n junction, is where the action of the
diode takes place. When a higher electrical potential is applied to the P side (the anode)
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than to the N side (the cathode), it allows electrons to flow from the N-type side to the P-
type side. The junction does not allow the flow of electrons in the opposite direction when
the potential is applied in reverse, creating, in a sense, an electrical check valve.
2.4 Inverters:
In general, inverters are utilized in applications requiring direct conversion of
electrical energy from DC to AC or indirect conversion from AC to AC. DC to AC
conversion is useful for many fields, including power conditioning, harmonic
compensation, motor drives, and renewable energy grid-integration.
In power systems it is often desired to eliminate harmonic content found in line
currents. VSIs can be used as active power filters to provide this compensation. Based on
measured line currents and voltages, a control system determines reference current signals
for each phase. This is fed back through an outer loop and subtracted from actual current
signals to create current signals for an inner loop to the inverter. These signals then cause
the inverter to generate output currents that compensate for the harmonic content. This
configuration requires no real power consumption, as it is fully fed by the line; the DC
link is simply a capacitor that is kept at a constant voltage by the control system. In this
configuration, output currents are in phase with line voltages to produce a unity power
factor. Conversely, VA compensation is possible in a similar configuration where output
currents lead line voltages to improve the overall power factor. In facilities that require
energy at all times, such as hospitals and airports, UPS systems are utilized. In a standby
system, an inverter is brought online when the normally supplying grid is interrupted.
Power is instantaneously drawn from onsite batteries and converted into usable AC
voltage by the VSI, until grid power is restored, or until backup generators are brought
online. In an online UPS system, a rectifier-DC-link-inverter is used to protect the load
from transients and harmonic content. A battery in parallel with the DC-link is kept fullycharged by the output in case the grid power is interrupted, while the output of the
inverter is fed through a low pass filter to the load. High power quality and independence
from disturbances is achieved.
Various AC motor drives have been developed for speed, torque, and position
control of AC motors. These drives can be categorized as low-performance or as high-
performance, based on whether they are scalar-controlled or vector-controlled,
respectively. In scalar-controlled drives, fundamental stator current, or voltage frequency
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and amplitude, are the only controllable quantities. Therefore, these drives are employed
in applications where high quality control is not required, such as fans and compressors.
On the other hand, vector-controlled drives allow for instantaneous current and voltage
values to be controlled continuously. This high performance is necessary for applications
such as elevators and electric cars.
Inverters are also vital to many renewable energy applications. In photovoltaic
purposes, the inverter, which is usually a PWM VSI, gets fed by the DC electrical energy
output of a photovoltaic module or array. The inverter then converts this into an AC
voltage to be interfaced with either a load or the utility grid. Inverters may also be
employed in other renewable systems, such as wind turbines. In these applications, the
turbine speed usually varies causing changes in voltage frequency and sometimes in the
magnitude.
2.5 Converter:
It is power electronic device which converts the one form of energy into another
form. It means here we using AC to DC conversion.
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.
CONVERTER METHODS
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CHAPTER-3
CONVERTER METHODS
3.1 Introduction:
DC/AC converters (inverters) An AC output waveform from a DC source is
produced in DC to AC converters. Applications of DC/AC converters are adjustable
speed drives (ASD), uninterruptable power supplies (UPS), active filters, photovoltaic
generators, voltage compensators, and flexible AC transmission systems (FACTS).
Topologies used in these converters are divided into two different types:
Voltage source inverters
Current source inverters.
In Voltage source inverters (VSIs) voltage waveform is an output which is
independently controlled. In current source inverters (CSIs) current waveform is the
controlled AC output. The power switching devices which are fully controllable
semiconductor power switches results the DC to AC power conversion. Fast transitions
than the smooth ones are produced as the output waveforms are made up of discrete
values. By the controlling of modulation technique it is detected when the power valves
are on and off and for how long around the fundamental frequency the ability to produce
near sinusoidal waveforms is dictated. Space-vector technique, and the selective-
harmonic technique, the carrier-based technique, or pulse width modulation, are included
in the common modulation techniques.
We can use voltage source inverters in both single-phase and three-phase
applications. Single-phase VSIs are widely used for power supplies, single-phase UPSs,
and utilize half-bridge and full-bridge configurations, and when used in multicell
configurations, elaborate high-power topologies. In applications where sinusoidal voltage
waveforms are required, such as ASDs, UPSs, and some types of FACTS devices such asthe STATCOM. Three-phase VSIs are used. In the case of active filters and voltage
compensators where arbitrary voltages are required these are used. To produce an AC
output current from a DC current supply Current source inverters are used. This type of
inverter is practical for three-phase applications where high-quality voltage waveforms
are required.
There was a widespread interest on a relatively new class of inverters, called
multilevel inverters. Because of the fact that power switches are connected to either the
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positive or to the negative DC bus, operation of CSIs and VSIs can be classified as two-
level inverters.
A sine wave could better approximated by the AC output if more than two voltage
levels were available to the inverter output terminals. The multilevel inverters, although
more complex and costly, offer higher performance due this reason. Whether or not they
require freewheeling diodes, each inverter type differs in the DC links used. Depending
on its intended usage it can be made to operate in square wave or pulse width modulation
mode. We can implement PWM in many different ways and higher quality waveforms are
produced where a simplicity is offered in square wave mode.
The output inverter section is fed from a constant-voltage source in Voltage Source
Inverters (VSI). The modulation technique which is to be selected for a given application
is determined by the wanted quality of the current output waveform. The VSI output has
discrete values. At the selective harmonic frequencies the loads should be inductive for a
smooth current waveform to be obtained. Capacitive load causes a choppy current
waveform with large and frequent current spikes to be received by load if the load and
source has no inductive filtering between them
AC TO AC CONVERTERS
The control of voltage, phase, and frequency of the waveform allows the converting
of AC power to Ac power applied to a load from a supplied AC system. Based on the
frequency of the wave form there are two main categories that can be used to separate the
type of converters. The converter which does not allow the user to modify the frequencies
are known as AC regulators or AC voltage Controllers. The converters which allow the
user to change the frequency are known as frequency converters for AC to AC
conversion.
There are three types of converters in frequency converters. They are matrix
converter, cyclo converter, DC link converter (aka AC/DC/AC converter).
AC Voltage Controller: The AC voltage Controller is used to vary the RMS voltage
across the load at constant frequency. Pulse Width Modulation AC Chopper Control
(PWM AC Chopper Control), ON/OFF Control, Phase-Angle Control is the three control
methods that are generally accepted. In Three-phase circuits as well as Single-phase
circuits, all the three methods can be implemented.
ON/OFF Control: For speed control of motors or for heating loads this method id
typically used. This control method involves turning the switch off for m integral cyclesand turning the switch on for n integral cycles. Undesirable harmonics are to be created
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because of turning the switches off and on. During zero-current conditions (zero-
crossing) and zero-voltage conditions the switches are turned off and on, effectively
reducing the distortion.
Phase-Angle Control: To implement a phase-angle control on different waveforms
there are various circuits exits such as full-wave or half-wave voltage control. SCRs,
Triacs and diodes are typically used power electronic components. The user can delay the
firing angle with the use of these components in a wave which will only cause part of the
wave to be outputted.
PWM AC Chopper Control: The other two control methods often have poor
harmonics, output current quality and input power factor .PWM can be used instead of
other methods to improve these values. Turning the switches off and on several times
within alternate half-cycles of input voltage can be done by this PWM AC Chopper.
Matrix Converters and Cyclo Converters: For ac to ac conversion in industries Cyc
lo Converters are widely used, because they are able to use in high-power applications.
They are commutated direct frequency converters that are synchronized by a supply line.
The output voltage waveforms of the cycloconverters have complex harmonics with
higher order harmonics, machine inductance can filter the harmonic which causes the
machine current to have fewer harmonics, losses and torque pulsations can be caused by
the remaining harmonics. There are no inductors or capacitors in a cycloconverter unlike
other converters i.e. any storage devices. For this reason the instantaneous output power
and the input power are equal.
Single-Phase to Single-Phase Cycloconverters: Because of the decrease in both the
power and size of the power electronics switches these started drawing more interest
recently. The single-phase high frequency ac voltage can be either sinusoidal or
trapezoidal. For control purpose these might be zero voltage commutation or zero voltage
intervals.
Three-phase to Single-Phase cycloconverters: These three-phase to single-phase
cycloconverters are two k inds: 3φ to 1φ half wave cycloconverters and 3φ to 1φ bridge
cycloconverters. Either at polarity both the negative and positive converters can generate
voltage, resulting positive current supplied by the positive converter and the negative
current supplied by the negative converter.
New forms of cycloconverters are being developed with recent advances such as
matrix converters. The matrix converter utilize bi-directional, bipolar switches is the firstchange first noticed in them. Matrix of 9 switches connecting the three input phases to the
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three output phases are present in a single phase to a single phase matrix converter. At
any time without connecting any two switches any output phase and input phase can be
connected together from the same phase at the same time, otherwise a short circuit of the
input phase will be caused. Matrix converters are more compact, lighter and versatile than
other converter solutions. As a result to regenerate energy back to the utility they are able
to achieve higher temperature operation, higher levels of integration, natural bi-
directional power flow and broad output frequency.
3.1.1Buck Power Converter:
A buck converter is a voltage step down and current step up converter.The
simplest way to reduce the voltage of a DC supply is to use a linear regulator (such as
a 7805), but linear regulators waste energy as they operate by dissipating excess power asheat. Buck converters, on the other hand, can be remarkably efficient (95% or higher for
integrated circuits), making them useful for tasks such as converting the main voltage in a
computer (12 V in a desktop, 12-24 V in a laptop) down to the 0.8-1.8 volts needed by
the processor.
Theory of operation
Fig.3.1.1. Buck converter circuit diagram.
Fig. 3.1.2.The two circuit configurations of a buck converter: On-state, when the switch is
closed, and Off-state, when the switch is open (Arrows indicate current according to
the conventional current model).
http://en.wikipedia.org/wiki/7805http://en.wikipedia.org/wiki/File:Buck_operating.svghttp://en.wikipedia.org/wiki/File:Buck_circuit_diagram.svghttp://en.wikipedia.org/wiki/7805
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Fig. 3.1.3. Naming conventions of the components, voltages and current of the buck
converter.
Fig.3.1.4. Evolution of the voltages and currents with time in an ideal buck converter
operating in continuous mode.
The basic operation of the buck converter has the current in an inductor controlled
by two switches (usually a transistor and a diode). In the idealized converter, all the
components are considered to be perfect. Specifically, the switch and the diode have zero
voltage drop when on and zero current flow when off and the inductor has zero series
resistance. Further, it is assumed that the input and output voltages do not change over the
course of a cycle (this would imply the output capacitance as being infinite).
Concept:
The conceptual model of the buck converter is best understood in terms of the
relation between current and voltage of the inductor. Beginning with the switch open (in
the "off" position), the current in the circuit is 0. When the switch is first closed, the
current will begin to increase, and the inductor will produce an opposing voltage across
its terminals in response to the changing current. This voltage drop counteracts the
voltage of the source and therefore reduces the net voltage across the load.
http://en.wikipedia.org/wiki/File:Buck_chronogram.pnghttp://en.wikipedia.org/wiki/File:Buck_conventions.svg
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Over time, the rate of change of current decreases, and the voltage across the
inductor also then decreases, increasing the voltage at the load. During this time, the
inductor is storing energy in the form of a magnetic field. If the switch is opened while
the current is still changing, then there will always be a voltage drop across the inductor,
so the net voltage at the load will always be less than the input voltage source.
When the switch is opened again, the voltage source will be removed from the
circuit, and the current will decrease. The changing current will produce a change in
voltage across the inductor, now aiding the source voltage. The stored energy in the
inductor's magnetic field supports current flow through the load. During this time, the
inductor is discharging its stored energy into the rest of the circuit. If the switch is closed
again before the inductor fully discharges, the voltage at the load will always be greater
than zero.
Continuous mode
A buck converter operates in continuous mode if the current through the inductor
( I L) never falls to zero during the commutation cycle. In this mode, the operating principle
is described by the plots in figure 4:
When the switch pictured above is closed (on-state, top of figure 2), the voltageacross the inductor is . The current through the inductor rises
linearly. As the diode is reverse-biased by the voltage source V, no current flows
through it;
When the switch is opened (off state, bottom of figure 2), the diode is forward biased.
The voltage across the inductor is (neglecting diode drop). Current IL
decreases.
The energy stored in inductor L is
Therefore, it can be seen that the energy stored in L increases during On-time
(as I L increases) and then decreases during the Off-state. L is used to transfer energy from
the input to the output of the converter.
The rate of change of IL can be calculated from:
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With VL equal to during the On-state and to during the Off-state.
Therefore, the increase in current during the On-state is given by:
Conversely, the decrease in current during the Off-state is given by:
If we assume that the converter operates in steady state, the energy stored in each
component at the end of a commutation cycle T is equal to that at the beginning of the
cycle. That means that the current IL is the same at t =0 and att =T (see figure 4).
So we can write from the above equations:
The above integrations can be done graphically: In figure 4, is proportional
to the area of the yellow surface, and to the area of the orange surface, as these
surfaces are defined by the inductor voltage (red) curve. As these surfaces are simple
rectangles, their areas can be found easily: for the yellow rectangle
and for the orange one. For steady state operation, these areas must be equal.
As can be seen on figure 4, and . Where D is a scalar called
the duty cycle with a value between 0 and 1. These yields:
From this equation, it can be seen that the output voltage of the converter varies
linearly with the duty cycle for a given input voltage. As the duty cycle D is equal to the
ratio between tOn and the period T, it cannot be more than 1. Therefore, . This iswhy this converter is referred to as step-down converter .
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So, for example, stepping 12 V down to 3 V (output voltage equal to one quarter of the
input voltage) would require a duty cycle of 25%, in our theoretically ideal circuit.
Discontinuous mode
In some cases, the amount of energy required by the load is too small. In this case,
the current through the inductor falls to zero during part of the period. The only difference
in the principle described above is that the inductor is completely discharged at the end of
the commutation cycle (see figure 5). This has, however, some effect on the previous
equations.
Fig.3.1.5. Evolution of the voltages and currents with time in an ideal buck converter
operating in discontinuous mode.
We still consider that the converter operates in steady state. Therefore, the energy
in the inductor is the same at the beginning and at the end of the cycle (in the case of
discontinuous mode, it is zero). This means that the average value of the inductor voltage
(VL) is zero; i.e., that the area of the yellow and orange rectangles in figure 5 are the
same. This yield:
So the value of δ is:
The output current delivered to the load ( ) is constant, as we consider that the
output capacitor is large enough to maintain a constant voltage across its terminals during
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a commutation cycle. This implies that the current flowing through the capacitor has a
zero average value. Therefore, we have :
Where is the average value of the inductor current. As can be seen in figure 5,
the inductor current waveform has a triangular shape. Therefore, the average value of
IL can be sorted out geometrically as follow:
The inductor current is zero at the beginning and rises during ton up to Imax. That means
that ILmax is equal to:
Substituting the value of ILmax in the previous equation leads to:
And substituting δ by the expression given above yields:
This expression can be rewritten as:
It can be seen that the output voltage of a buck converter operating in discontinuous
mode is much more complicated than its counterpart of the continuous mode.
Furthermore, the output voltage is now a function not only of the input voltage (V i) and
the duty cycle D, but also of the inductor value (L), the commutation period (T) and the
output current (Io).
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From discontinuous to continuous mode (and vice versa)
Fig.3.1.6: Evolution of the normalized output voltages with the normalized output
current.
As mentioned at the beginning of this section, the converter operates in
discontinuous mode when low current is drawn by the load, and in continuous mode at
higher load current levels. The limit between discontinuous and continuous modes is
reached when the inductor current falls to zero exactly at the end of the commutation
cycle. Using the notations of figure 5, this corresponds to :
Therefore, the output current (equal to the average inductor current) at the limit between
discontinuous and continuous modes is (see above):
Substituting ILmax by its value:
On the limit between the two modes, the output voltage obeys both the
expressions given respectively in the continuous and the discontinuous sections. In
particular, the former is
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So Iolim can be written as:
Let's now introduce two more notations:
the normalized voltage, defined by . It is zero when , and 1
when ;
the normalized current, defined by . The term is equal to the
maximum increase of the inductor current during a cycle; i.e., the increase of the inductor
current with a duty cycle D=1. So, in steady state operation of the converter, this means
that equals 0 for no output current, and 1 for the maximum current the converter can
deliver.
3.1.2Boost Power Converter:
The schematic in Fig. 6 shows the basic boost converter. This circuit is used
when a higher output voltage than input is required.
3.1.2.1. Boost Converter Circuit
While the transistor is ON Vx =Vin, and the OFF state the inductor current flows through
the diode giving Vx =Vo. For this analysis it is assumed that the inductor current always
remains flowing (continuous conduction). The voltage across the inductor is shown in
Fig. 7 and the average must be zero for the average current to remain in steady state
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………… (18)
This can be rearranged as
………. (19)
and for a lossless circuit the power balance ensures
……….. (20)
Fig 3.1.2.2.Voltage and current waveforms (Boost Converter)
Since the duty ratio "D" is between 0 and 1 the output voltage must always be higher than
the input voltage in magnitude. The negative sign indicates a reversal of sense of the
output voltage.
3.1.3Buck-Boost Converter:
The schematic in Fig shows the basic boost converter. This circuit is used when a
higher output voltage than input is required.
Fig 3.1.3.1. Buck-Boost Converter Circuit
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While the transistor is ON Vx =Vin, and the OFF state the inductor current flows through
the diode giving Vx =Vo. For this analysis it is assumed that the inductor current always
remains flowing (continuous conduction). The voltage across the inductor is shown in
Fig. 7 and the average must be zero for the average current to remain in steady state
………… (18)
This can be rearranged as
………. (19)
and for a lossless circuit the power balance ensures
……….. (20)
Fig 3.1.3.2.Voltage and current waveforms (Boost Converter)since the duty ratio "D" is
between 0 and 1 the output voltage must always be higher than the input voltage in
magnitude. The negative sign indicates a reversal of sense of the out put voltage
Operating Principle of PFC BL Buck – Boost Converter:
The operation of the PFC BL buck – boost converter is classified into two parts
which include the operation during the positive and negative half cycles of supply voltage
and during the complete switching cycle.
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A. Operation During Positive and Negative Half Cycles of Supply Voltage:
In the proposed scheme of the BL buck – boost converter, switches S w1 and
S w2 operate for the positive and negative half cycles of the supply voltage, respectively.
During the positive half cycle of the supply voltage, switch S w1 , inductor Li1
, and
diodes D1 and D p are operated to transfer energy to dc link capacitor C d as shown
in Fig. 2(a) – (c).
Similarly, for the negative half cycle of the supply voltage, switch S w2 , inductor
Li2 , and diodes D2 and Dn conduct as shown in Fig. 3(a) – (c). In the DICM operation
of the BL buck – boost converter, the current in inductor Li becomes discontinuous for
a certain duration in a switching period.
B. Operation During Complete Switching Cycle:Three modes of operation during a complete switching cycle are discussed for the
positive half cycle of supply voltage as shown hereinafter.
Mode I: In this mode, switch S w1 conducts to charge the inductor Li1 ; hence, an
inductor current iLi1 increases in this mode as shown in Fig. 2(a). Diode D p completes
the input side circuitry, whereas the dc link capacitor C d is discharged by the VSI-fed
BLDC motor as shown in Fig. 3(d) Mode III: In this mode, inductor Li1 enters
discontinuous conduction, i.e., no energy is left in the inductor; hence, current iLi1
becomes zero for the rest of the switching period. As shown in Fig. 2(c), none of the
switch or diode is conducting in this mode, and dc link capacitor C d supplies energy
to the load; hence, voltage V dc across dc link capacitor C d starts decreasing.
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Fig.3.1.3.3: Operation of the proposed converter in different modes (a) – (c) for a
positive half cycle of supply voltage and (d) the associated waveforms. (a) Mode I.
(b) Mode II. (c) Mode III. (d) Waveforms for positive and negative half cycles of supply
voltage.
Design of PFC BL Buck – Boost Converter:
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Fig.3.1.3.4: Operation of the proposed converter in different modes (a) – (c) for a negative
half cycle of supply voltage and (d) the associated waveforms. (a)Mode I. (b)Mode II.
(c)Mode III. (d)Waveforms during complete switching cycle.
Fig.3.1.3.5. Supply current at the rated load on BLDC motor for different values of input
side inductors with supply voltage as 220 V and dc link voltage as 50 V
Converter Comparison
The voltage ratios achievable by the DC-DC converters is summarised in notice
that only the buck converter shows a linear relationship between the control (duty ratio)
and output voltage. The buck-boost can reduce or increase the voltage ratio with unit gain
for a duty ratio of 50%.
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Fig:3.1.3.6.Comparison of Voltage ratio
Simulated performance of proposed BLDC motor drive :
The performance of the proposed BLDC motor drive is simulated in
MATLAB/Simulink environment using the Sim- Power-System toolbox. The
performance evaluation of the pro- posed drive is categorized in terms of the
performance of the BLDC motor and BL buck – boost converter and the achieved power
quality indices obtained at ac mains. The parameters associated with the BLDC motor
such as speed (N), electro- magnetic torque (T e ), and stator current (ia ) are analyzed
for the proper functioning of the BLDC motor. Parameters such as supply voltage (V s ),
supply current (i s ), dc link voltage (V dc ), inductor ‘s currents (iLi1 , iLi2 ), switch
voltages (V sw1 , V sw2 ), and switch currents (isw1 , isw2 ) of the PFC BL buck –
boost converter are evaluated to demonstrate its proper functioning. Moreover, power
quality indices such as power factor (PF), displacement power factor (DPF), crest factor
(CF), and THD of supply current are analyzed for determining power quality at ac mains.
A. Steady-State Performance
The steady-state behavior of the proposed BLDC motor drive for two cycles of
supply voltage at rated condition (rated dc link voltage of 200 V) is shown in Fig. 6. The
discontinuous induc- tor currents (iLi1 and iLi2 ) are obtained, confirming the DICM
operation of the BL buck – boost converter. The performance of the proposed BLDC
motor drive at speed control by varying dc link voltage from 50 to 200 V is tabulated in
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Table III. The harmonic spectra of the supply current at rated and light load conditions,
i.e., dc link voltages of 200 and 50 V, are also shown in Fig. 7(a) and (b), respectively,
which shows that the THD of supply current obtained is under the acceptable limits
of IEC 61000-3-2.
Fig.3.1.3.7. Steady-state performance of the proposed BLDC motor drive at rated
conditions.
B. Dynamic Performance of Proposed BLDC Motor Drive
The dynamic behavior of the proposed drive system during a starting at 50 V,
step change in dc link voltage from 100 in Fig. 3.1.3.7 A smooth transition of speed
and dc link voltage is achieved with a small overshoot in supply current under the
acceptable limit of the maximum allowable stator winding current of the BLDC motor.
The controller gains are given in the Appendix.
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C. Performance Under Supply Voltage Variation
The behavior of the proposed BLDC motor drive in practical supply conditions is
demonstrated, and the performance is also evaluated for supply voltage from 90 to 270 V.
Table IV shows different power quality indices with variation in supply voltage. The THD
of supply current obtained is within the limits of IEC 61000-3-2. Fig. 9(a) and (b)
shows the harmonic spectra of supply current at ac mains at rated conditions of dc link
voltage and load on the BLDC motor with supply voltage as
90 and 270 V, respectively. An acceptable THD of supply current is obtained for both the
cases which show an improved.
Fig.3.1.3.8. Dynamic performance of proposed BLDC motor drive during (a)
starting, (b) speed control, and (c) supply voltage variation at rated conditions
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3.1.4 Cuk Converter
The buck, boost and buck-boost converters all transferred energy between input
and output using the inductor, analysis is based of voltage balance across the inductor.
The CUK converter uses capacitive energy transfer and analysis is based on current
balance of the capacitor. The circuit in Fig. below (CUK converter) is derived from
DUALITY principle on the buck-boost converter.
Fig.3.1.4.1.CUK Converter
If we assume that the current through the inductors is essentially rippled free we can
examine the charge balance for the capacitor C1. For the transistor ON the circuit
becomes and the current in C1 is IL1. When the transistor is OFF, the diode conducts and
the current in C1 becomes IL2.
Fig:3.1.4.2.CUK "ON-STATE"
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Fig:3.1.4.3. CUK "OFF-STATE"
Since the steady state assumes no net capacitor voltage rise ,the net current is zero
…………… (24)
which implies
…….. (25)
The inductor currents match the input and output currents, thus using the power
conservation rule
………… (26)
Thus the voltage ratio is the same as the buck-boost converter. The advantage of
the CUK converter is that the input and output inductors create a smooth current at both
sides of the converter while the buck, boost and buck-boost have at least one side with
pulsed current.
Isolated DC-DC Converters
In many DC-DC applications, multiple outputs are required and output isolation
may need to be implemented depending on the application. In addition, input to output
isolation may be required to meet safety standards and / or provide impedance matching.
The above discussed DC-DC topologies can be adapted to provide isolation between
input and output.
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3.1.5 Fly back Converter
The fly back converter can be developed as an extension of the Buck-Boost
converter. Fig (a) shows the basic converter; Fig (b)(replacing inductor by transformer)
replaces the inductor by a transformer. The buck-boost converter works by storing energy
in the inductor during the ON phase and releasing it to the output during the OFF phase.
With the transformer the energy storage is in the magnetization of the transformer core.
To increase the stored energy a gapped core is often used.
In Fig (c) the isolated output is clarified by removal of the common reference of
the input and output circuits.
Fig: 3.1.5.1.Flyback converter re-configured
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3.1.6 Forward Converter
The concept behind the forward converter is that of the ideal transformer
converting the input AC voltage to an isolated secondary output voltage. For the circuit in
Fig. (forward converter), when the transistor is ON, Vin appears across the primary and
then generates
………… (27)
The diode D1 on the secondary ensures that only positive voltages are applied to the
output circuit while D2 provides a circulating path for inductor current if the transformer
voltage is zero or negative.
Fig .3.1.6.1.Forward Converter
The problem with the operation of the circuit in Fig above(forward converter) is
that only positive voltage is applied across the core, thus flux can only increase with the
application of the supply. The flux will increase until the core saturates when the
magnetizing current increases significantly and circuit failure occurs. The transformer can
only sustain operation when there is no significant DC component to the input voltage.
While the switch is ON there is positive voltage across the core and the flux increases.
When the switch turns OFF we need to supply negative voltage to reset the core flux. The
circuit in Fig. below shows a tertiary winding with a diode connection to permit reverse
current. Note that the "dot" convention for the tertiary winding is opposite those of the
other windings. When the switch turns OFF current was flowing in a "dot" terminal. The
core inductance act to continue current in a dotted terminal.
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Fig. 3.1.6.2.Forward converter with tertiary winding
3.2 Power Factor Corrected (PFC):
In electrical engineering, the power factor of an AC electrical power system is
defined as the ratio of the real power flowing to the load, to the apparent power in the
circuit,[1][2] and is a dimensionless number in the closed interval of -1 to 1. Realpower is
the capacity of the circuit for performing work in a particular time. Apparent power is the
product of the current and voltage of the circuit. Due to energy stored in the load and
returned to the source, or due to a non-linear load that distorts the wave shape of the
current drawn from the source, the apparent power will be greater than the real power.
A negative power factor occurs when the device (which is normally the load)
generates power, which then flows back towards the source, which is normally considered
the generator. In an electric power system, a load with a low power factor draws more
current than a load with a high power factor for the same amount of useful power
transferred. The higher currents increase the energy lost in the distribution system, and
require larger wires and other equipment. Because of the costs of larger equipment and
wasted energy, electrical utilities will usually charge a higher cost to industrial or
commercial customers where there is a low power factor.
Linear loads with low power factor (such as induction motors) can be corrected
with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers,
distort the current drawn from the system. In such cases, active or passive power factor
correction may be used to counteract the distortion and raise the power factor. The
devices for correction of the power factor may be at a central substation, spread out over a
distribution system, or built into power-consuming equipment.
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Definition and calculation:
AC power flow has three components: real power (also known as active power) (P),
measured in watts (W); apparent power (S), measured in volt-amperes (VA); and reactive
power (Q), measured in reactive volt-amperes (vary).
[6]
The power factor is defined as:
In the case of a perfectly sinusoidal waveform, P, Q and S can be expressed as vectors
that form a vector triangle such that:
If is the phase angle between the current and voltage, then the power factor is equal to
the cosine of the angle, , and:
Since the units are consistent, the power factor is by definition a dimensionless
number between −1 and 1. When power factor is equal to 0, the energy flow is entirely
reactive, and stored energy in the load returns to the source on each cycle. When the
power factor is 1, all the energy supplied by the source is consumed by the load. Power
factors are usually stated as "leading" or "lagging" to show the sign of the phase angle.
Capacitive loads are leading (current leads voltage), and inductive loads are lagging
(current lags voltage).
If a purely resistive load is connected to a power supply, current and voltage will
change polarity in step, the power factor will be unity (1), and the electrical energy flows
in a single direction across the network in each cycle. Inductive loads such as
transformers and motors (any type of wound coil) consume reactive power with current
waveform lagging the voltage. Capacitive loads such as capacitor banks or buried cable
generate reactive power with current phase leading the voltage. Both types of loads will
absorb energy during part of the AC cycle, which is stored in the device's magnetic or
electric field, only to return this energy back to the source during the rest of the cycle.
For example, to get 1 kW of real power, if the power factor is unity, 1 K VA of
apparent power needs to be transferred (1 kW ’ 1 = 1 K VA). At low values of powerfactor, more apparent power needs to be transferred to get the same real power. To get
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1 kW of real power at 0.2 power factor, 5 K VA of apparent power needs to be transferred
(1 kW ’ 0.2 = 5 K VA). This apparent power must be produced and transmitted to the load
in the conventional fashion, and is subject to the usual distributed losses in the production
and transmission processes.
Electrical loads consuming alternating current power consume both real power
and reactive power. The vector sum of real and reactive power is the apparent power. The
presence of reactive power causes the real power to be less than the apparent power, and
so, the electric load has a power factor of less than 1.
A negative power factor (0 to -1) can result from returning power to the source, such as in
the case of a building fitted with solar panels when their power is not being fully utilized
within the building and the surplus is fed back into the supply.
Power quality:
Power quality determines the fitness of electric power to consumer devices.
Synchronization of the voltage frequency and phase allows electrical systems to function
in their intended manner without significant loss of performance or life. The term is used
to describe electric power that drives an electrical load and the load's ability to function
properly. Without the proper power, an electrical device (or load) may malfunction, fail
prematurely or not operate at all. There are many ways in which electric power can be of
poor quality and many more causes of such poor quality power. The electric power
industry comprises electricity generation (AC power), electric power transmission and
ultimately electric power distribution to an electricity meter located at the premises of the
end user of the electric power. The electricity then moves through the wiring system of
the end user until it reaches the load. The complexity of the system to move electric
energy from the point of production to the point of consumption combined with variations
in weather, generation, demand and other factors provide many opportunities for the
quality of supply to be compromised.
While "power quality" is a convenient term for many, it is the quality of
the voltage — rather than power or electric current — that is actually described by the term.
Power is simply the flow of energy and the current demanded by a load is largely
uncontrollable
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3.3 Bridgeless Buck-Boost Converter:
Fig. 3.3.1 Bridgeless Buck-Boost Converter
BLDC motor drive with the concept of variable dc link voltage. This reduces
the switching losses in VSI due to the fundamental switching frequency operation for the
electronic commutation of the BLDC motor and to the variation of the speed by
controlling he voltage at the dc bus of VSI. A CCM operation of the Cuk converter has
been utilized which re quires three sensors and is not encouraged for low cost and low
power rating. For further improvement in efficiency, bridgeless (BL) converters are used
which allow the elimination of DBR in the frontend [13] – [21]. A buck – boost converterconfiguration is best suited among various BL converter topologies for applications
requiring a wide range of dc link voltage control.
These can provide the voltage buck [13] or voltage boost [14],[15] which limits
the operating range of dc link voltage control. Wei et al. [16] have proposed a BL buck –
boost converter but use three switches which is not a cost-effective solution. A new
family of BSEPIC and Cuk converters has been reported in the literature [17] – [21] but
requires a large number of components and has losses associated with it. This paper
presents a BL buck boost converter-fed BLDC drive with variable dc link voltage of VSI
for improved power quality at ac mains with reduced components.
Proposed PFC BL buck – boost converter-Fed BLDC motor drive
The proposed BL buck – boost converter-base VSI-fed BLDC motor drive. The
parameters of the buck – boost converter are designed such that it operates in
discontinuous inductor current mode (DICM) to achieve a inherent power factor
correction at ac mains. The speed control
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Fig. 3.3.2. Proposed BLDC motor drive with front-end BL buck – boost converter
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TYPES OF INVERTERS
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CHAPTER-4
TYPES OF INVERTERS
4.1 Voltage Source Inverter:
If a voltage source inverter is used for mains feed in, the dc link voltage has to be
greater than the rectified line to line voltage [7]. So the low level fuel cell dc voltage has
to be increased towards the dc link voltage. This can be achieved by using an additional
boost converter, the whole system is shown in fig. 2.The dc link capacitor decouples the
voltage source inverter and the boost converter and keeps the dc link voltage ripple to an
adequate level. A feasible power flow control method could be to keep the dc link voltage
constant via the boost converter
Fig. 4.1.1: Circuit diagram of a voltage source inverter linked with a boost converter for a
fuel cell generation system.
4.2 Current Source Inverter:
The current source inverter, whose topology is shown in fig. 3, increases the
voltage towards the mains by, so the voltage of the fuel cell must be lower than the lowest
rectified line to line voltage [5] if the fuel cell is directly connected to the CSI. At the
CSI, similar to the VSI+BC system, the dc link inductor Ldat the fuel cell side yields to an
appropriate dc current ripple. The switches of the current source inverter have to be
reverse blocking. If IGBTs are used for the current source inverter, the reverse
Vfc[V]operating point at no load operating point at nominal load Fig. 1: Example of a characteristic
curve of a single fuel cell blocking capability can at present only be achieved with diodes
connected in series to the IGBTs. This yields to relatively high semiconductor conduction
losses. Another interesting semiconductor is the reverse blocking IGBT (RBIGBT). The
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development of RBIGBT is in progress [8] so it can be supposed that they will be
available in the foreseeable future also for the CSI.
Fig. 4.2.1: Circuit diagram of a current source inverter for a fuel cell generation system
4.3 Difference Between VSI and CSI:
VSI CSI
VSI is fed from a DC voltage source having
small or negligible impedance.
CSI is fed with adjustable current from a
DC voltage source of high impedance.
Input voltage is maintained constant The input current is constant but adjustable.
Output voltage does not dependent on the
load
The amplitude of output current is
independent of the load.
The waveform of the load current as well as
its magnitude depends upon the nature of
load impedance.
The magnitude of output voltage and its
waveform depends upon the nature of the
load impedance.
VSI requires feedback diodesThe CSI does not require any feedback
diodes.
The commutation circuit is complicated Commutation circuit is simple as it contains
only capacitors.
Power BJT, Power MOSFET, IGBT, GTO
with self commutation can be used in the
circuit.
They cannot be used as these devices have
to withstand reverse voltage.
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BLDC MOTOR
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CHAPTER-5
BLDC MOTOR
Brushless direct current (BLDC) motor:
Although efficiency is Brushless DC electric motor (BLDC motors, BL
motors) also known as electronically commutated motors (ECMs, EC motors)
are synchronous motors that are powered by a DC electric source via an
integrated inverter/switching power supply, which produces an AC electric signal to drive
the motor. In this context, AC, alternating current, does not imply a sinusoidal waveform,
but rather a bi-directional current ith no restriction on waveform. Additional sensors and
electronics control the inverter output amplitude and waveform (and therefore percent of
DC bus usage/efficiency) and frequency (i.e. rotor speed).The rotor part of a brushless
motor is often a permanent magnet synchronous motor, but can also be a switched
reluctance motor, or induction motor. Brushless motors may be described as stepper
motors; however, the term stepper motor tends to be used for motors that are designed
specifically to be operated in a mode where they are frequently stopped with the rotor in a
defined angular position. This page describes more general brushless motor principles,
though there is overlap.
Brushless vs. brushed motors
Brushed DC motors have been in commercial use since 1886. Brushless motors, on
the other hand, did not become commercially viable until 1962.Brushed DC motors
develop a maximum torque when stationary, linearly decreasing as velocity
increases.[5] Some limitations of brushed motors can be overcome by brushless motors;
they include higher efficiency and a lower susceptibility to mechanical wear. These
benefits come at the cost of potentially less rugged, more complex, and more expensive
control electronics.
A typical brushless motor has permanent magnets which rotate around a
fixed armature, eliminating problems associated with connecting current to the moving
armature. An electronic controller replaces the brush/commutate assembly of the brushed
DC motor, which continually switches the phase to the windings to keep the motor
turning. The controller performs similar timed power distribution by using a solid-state
circuit rather than the brush/commutate system.
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Brushless motors offer several advantages over brushed DC motors, including more
torque per weight, more torque perwatt (increased efficiency), increased reliability,
reduced noise, longer lifetime (no brush and commutated erosion), elimination of ionizing
sparks from the commutator, and overall reduction of electromagnetic interference (EMI).
With no windings on the rotor, they are not subjected to centrifugal forces, and because
the windings are supported by the housing, they can be cooled by conduction, requiring
no airflow inside the motor for cooling. This in turn means that the motor's internals can
be entirely enclosed and protected from dirt or other foreign matter.
Brushless motor commutation can be implemented in software using
a microcontroller or microprocessor computer, or may alternatively be implemented in
analogue hardware, or in digital firmware using an FPGA. Commutation with electronics
instead of brushes allows for greater flexibility and capabilities not available with brushed
DC motors, including speed limiting, "micro stepped" operation for slow and/or fine
motion control, and a holding torque when stationary.
The maximum power that can be applied to a brushless motor is limited almost
exclusively by heat too much heat weakens the magnets [6] and may damage the winding's
insulation.
When converting electricity into mechanical power, brushless motors are more
efficient than brushed motors. This improvement is largely due to the brushless motor's
velocity being determined by the frequency at which the electricity is switched, not the
voltage. Additional gains are due to the absence of brushes, which reduces mechanical
energy loss due to friction. The enhanced efficiency is greatest in the no-load and low-
load region of the motor's performance curve. Under high mechanical loads, brushless
motors and high-quality brushed motors are comparable in efficiency.
Environments and requirements in which manufacturers use brushless-type DC
motors include maintenance-free operation, high speeds, and operation where sparking is
hazardous (i.e. explosive environments) or could affect electronically sensitive
equipment.
Controller implementations:
Because the controller must direct the rotor rotation, the controller requires some
means of determining the rotor's orientation/position (relative to the stator coils.) Some
designs use Hall Effect sensors or a rotary encoder to directly measure the rotor's
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position. Others measure the back EMF in the undriven coils to infer the rotor position,
eliminating the need for separate Hall Effect sensors, and therefore are often
called sensorless controllers.
A typical controller contains 3 bi-directional outputs (i.e. frequency controlled three phase output), which are controlled by a logic circuit. Simple controllers employ
comparators to determine when the output phase should be advanced, while more
advanced controllers employ a microcontroller to manage acceleration, control speed and
fine-tune efficiency.
Controllers that sense rotor position based on back-EMF have extra challenges in
initiating motion because no back-EMF is produced when the rotor is stationary. This is
usually accomplished by beginning rotation from an arbitrary phase, and then skipping to
the correct phase if it is fou