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An Adjustable-Speed PFC Bridgeless Buck-Boost Converter-Fed BLDC Motor Drive

DEPT. OF EEE Page 1 AMRN

INTRODUCTION

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DEPT OF EEE Page 2 AMRN

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

http://en.wikipedia.org/wiki/File:Buck_chronogram_discontinuous.png

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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

http://en.wikipedia.org/wiki/File:Buck_continuous_discontinuous.svg

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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 proposed 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), electromagnetic 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 inductor 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

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