PERPUSTAKAAN UMP

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SINUSOIDAL PWM CONTROLLER OF VOLTAGE SOURCE INVERTER FOR BETTER EFFICIENCY OF THREE PHASE INDUCTION MOTOR

CHEW HONG PING

Thesis submitted in partial fulfillment of the requirements for the award of the degree of Bachelor of Mechatronics Engineering

Faculty of Manufacturing Engineering UNIVERSITY MALAYSIA PAHANG

JUNE 2013

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ABSTRACT

Induction motors are widely used in industry nowadays. Induction motor can be controlled by adjusting voltage terminal, pole changing and frequency as scalar control. In this study, induction motor control by using SPWM technique that is better control than other scalar control methods. To generate this SPWM pulses signal, triangle wave as a carrier signal is compared with the sinusoidal wave, whose frequency is the desired frequency. The widths of these pulses signal are modulated to obtain inverter output voltage control. SPWM duty cycle control technique enable better efficiency of three phase induction motor to provide flexible control and novel cyclic operation as well as better protection schemes for the motor and control circuit. SPWM have improved the speed control and reduce the power losses in the system and is chosen as a better alternative. This paper focuses on step by step development SPWM implemented on a three phase Induction motor. Simulation model of SPWM is obtained using Multisim software. Simulation results are also provided.

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ABSTRAK

Motor induksi digunakan secara meluas dalam industri pada masa kini. Motor induksi boleh dikawal dengan melaraskan terminal voltan dan kekerapan kawalan skalar. Dalam kajian mi, kawalan motor induksi dengan menggunakan teknik SPWM yang kawalannya yang lebih baik berbanding kaedah kawalan skalar lain. Untuk menjana isyarat denyutan SPWM mi, gelombang segi tiga sebagai isyarat pembawa dibandingkan dengan gelombang sinusoidal, yang kekerapan frekuensi yang dikehendaki. Lebar isyarat denyutan mi dimodulat supaya inverter output dapat mengawal voltan yang dihantar ke motor induksi. SPWM kitar tugas kawalan teknik membolehkan kecekapan yang lebih baik untuk motor induksi tiga fasa bagi menyediakan kawalan fleksibel dan operasi kitaran novel serta skim perlindungan yang lebih baik untuk motor dan juga litar kawalan. SPWM telah meningkat mutu kawalan kelajuan yang baik dan juga mengurangkan kehilangan kuasa dalam sistem dan dipilih sebagai alternatif yang lebih baik. Kertas mi memberi tumpuan kepada langkah demi langkah pembangunan SPWM dilaksanakan pada motor induksi tiga fasa. Model simulasi SPWM diperolehi menggunakan perisian Multisim. Keputusan simulasi juga disediakan.

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TABLE OF CONTENTS

Page

SUPERVISOR'S DECLARATION in

STUDENT'S DECLARATION iv

ACKNOWLEDGEMENTS v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xiv

LIST OF ABBREVIATIONS xv

CHAPTER 1 INTRODUCTION

1.1 Project Background 1

1.2 Problem Statement 2

1.3 Objectives of the Study 3

1.4 Project Scope 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.2 Pulse Width Modulation (PWM) 4

2.3 Sinusoidal Pulse Width Modulation (SPWM) 5

2.4 Characteristic of SPWM 7

2.5 Carrier Wave Ratio and Modulation Index 7

2.6 Optocoupler 8

2.7 Gate Driver 9

2.8 Voltage Source Inverters (VSI) 9

2.9 Structure of Three Phase Voltage Source Inverter and How It Works 10

2.10 SPWM Control Voltage Source Inverter 11

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2.11 Squirrel Cage Induction Motor 12

2.12 Induction Motor Operation with Non-Sinusoidal Supply Waveform 14

CHAPTER 3 METHODOLOGY

3.1 Introduction 16

3.2 Project Flow Chart 16

3.3 Project Block Diagram 17

3.3.1 Block Diagram Process Flow 18

3.4 Generating SPWM signal 18

3.5 Simulation Using Multisim for SPWM Circuit 20

3.6 Optocoupler Circuit 23

3.7 Driver Circuit 25

3.8 IGBT Circuit 26

3.9 Power Supply for Control Signal Circuit 27

CHAPTER 4 RESULTS AND DISCUSSION

4.1 Introduction 28

4.2 Overall Implementation of System 28

4.3 Experiments on Sinusoidal Generating Circuit 30

4.4 Experiments on Triangle Generating Circuit 31

4.5 Experiments on Comparator Circuit 31

4.6 Experiments on Regulating Of Peak Voltage of Sinusoidal Wave 32

4.7 Experiments on Optocoupler Circuit 34

4.8 Experiments on Driver Circuit 34

4.9 Experiments on Driver after Connected with IGBT 37

4.10 Experiments on Speed of Induction Motor 38

4.11 Experiments on Efficiency of Induction Motor Control Method 41 between SPWM Techniques with Direct Terminal Voltage Supply

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CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Introduction 44

5.2 , Conclusion 44

5.3 Recommendation 45

REFERENCES

APPENDICES

Al LM 324 Op-amp Datasheet 50

A2 TL 082 Op-amp Datasheet 52

A3 LM 311 Analog Comparator Datasheet 54

A4 4N25 Optocoupler Datasheet 55

A5 Driver IR 2130 Datasheet 56

A6 Seminkron 200GB123D Datasheet 58

Bi Final Year Project 1 Gantt Chart 60

B2 Final Year Project 2 Gantt Chart 61

LIST OF TABLES

Table No. Title Page

4.1 Experiment Results of Current Drawn By the Induction 42

Motor with Different Voltage among Direct Terminal

Voltage Supply and with SPWM Technique

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LIST OF FIGURES

Figure No. Title Page

2.2 Pulse width modulation signal 5

2.2 Sinusoidal pulse width modulation 6

2.3 Optocoupler 8

2.4 Structure of three phase voltage source inverter 12

2.5 The modulation wave of PWM 12

3.1 Project flow chart 16

3.2 Project block diagram 17

3.3 Three phase sinusoidal wave (modulating wave) 18

3.4 Triangle wave (carrier wave) 18

3.5 A comparator configured as shown produces aPWM output 19

3.6 Sinusoidal PWM signal after comparing of triangle wave with 19 sinusoidal wave

3.7 SPWM generating circuit diagram 20

3.8 Three phase sinusoidal wave with 120 degree of phase different 21

3.9 Simulation result of SPWM generator for 0 degree phase 21 different of sinusoidal wave.

3.10 Simulation result of SPWM generator for 120 degree phase 21 different of sinusoidal wave

3.11 Simulation result of SPWM generator for 240 degree phase 22 different of sinusoidal wave

3.12 Triangle wave generator circuit 22

3.14 Optocoupler circuit from Multisim software 23

3.14 Simulation result for optocoupler 24

3.15 Gate driver circuit 25

3.16 One of the three high side drivers of 1R2130 25

3.17 Six IGBT in the Voltage Source inverter circuit 26

3.18 Power supply for control signal circuit 27

4.1 The overall implementation of the system 28

4.2 The Implementation of control signal and IGBT circuit 29

4.3 Three phase sinusoidal wave from sinusoidal wave generator 30 circuit

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44 One of the three phase sinusoidal wave from sinusoidal 30 wave generator circuit

4•5 The triangle wave from triangle wave generator circuit 31

4.6 Three phase output SPWM signal from comparator . 31

4.7 One of the SPWM signal output from analog comparator 32 circuit

4.8 The sinusoidal wave and output SPWM signal when peak 32

voltage is regulated to 6V

4•9 The sinusoidal wave and output SPWM signal when peak 33

voltage is regulated to 2.11 V

4.10 One of the non-inverting and inverting three phase SPWM 34 signal from optocoupler

4.11 One of the three high and low outputs of driver circuit 34

4.12 The view of dead time occurred between high and low 35

output signal

4.13 Detail view of the dead time that occurs when the high 35 signal is turned on and the low signal is turned off

4.14 Detail view of the dead time that occurs when the high signal 36 is turned off and the low signal is turned on

4.15 One of the three phase driver high and low output signal from 37 gate driver after connected with IGBT.

4.16 One of the three phase IGBT output to the induction motor 38

4.17 The speed of induction motor with 37V 38

4.18 The speed of induction motor with 40 V 39

4.19 The speed of induction motor with 52 V 39

4.20 The maximum speed of the induction motor 40

4.21 Experiment results of current drawn by the induction motor 41 at 30 V Using direct terminal voltage supply technique

4.22 Experiment results of current drawn by the induction motor 41 at 30 V using SPWM technique

4.23 Graph of current versus voltage 42

LIST OF SYMBOLS

D Duty cycle

f1 Desired fundamental frequency of the inverter

Switching frequency of the inverter

M Amplitude modulation index

Ma Modulated ratio

Mf Frequency modulation ratio

N Carrier wave ratio

P Number of pulses per cycle of the sine wave

Rms Root mean square

T Period of rectangular waveform

Ton Pulse duration

Ts Period of rectangular waveform

V Volt

V fl Peak amplitude of the control signal

Vd Voltage input

Vmax Voltage maximum

Vmin Voltage minimum

Vout Voltage output

Peak amplitude of the triangle waveform

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LIST OF ABBREVIATIONS

IC Internal circuit

PWM Pulse width modulation

SPWM Sinusoidal pulse width modulation

EMI Electromagnetic interference

CSI Current source inverter

DC Direct current

AC Alternating current

IGBT Insulated gate bipolar transistor

IGCTs Insulated gate commutated transistor

IEGTs Injection enhanced gate thyristor

MTTF Mean time to failure

SCIM Squirrel cage induction motor

ASD Adjustable speed drive

PPU Power processing unit

Op-amp Operational amplifier

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CHAPTER 1

INTRODUCTION

1.1, PROJECT BACKGROUND

The advantages of the utilization of the three phase induction motor are

increasing day by day. This lead the induction motor has found enormous applications

in industries these days. The proper functionality of induction motor is important to

achieve high efficiency control of motor and better use of electric power, since

improving efficiency in electric drives is important, mainly for economic saving and

reduction of environmental pollution (W. Leonhard, 1996). In order to attain proper

functionality, regulating of the speed of the induction motor is a need. Controlling the

speed of induction motor by adjusting the voltage supply, rotor resistance control and

pole changing method are ineffective. So, in order to fulfill the requirement for better

efficiency, a better technique of control method which uses sinusoidal pulse width

modulation technique is implemented.

The system consist of three phase power supply circuit, three phase diode

bridge rectifier, IGBT, Gate driver, optocouplers, three phase pulse width modulated

generator circuit and an three phase induction motor.

In this project, the carrier wave and the modulated wave are compared using the

comparator IC. Low pulses generated when the amplitude of the carrier signal is lower

than the modulated signal, otherwise, high pulse is produced. The output frequency is

controlled by the controlled signal. The switches speed is determined by frequency of

carrier signal. The control circuit is then protected from higher voltage of three phase

power supply by optocoupler circuit. The driver is use to drive the lower voltage output

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of control circuit to suit the requirement gate voltage of voltage source inverter, IGBT.

IGBT is acted as switching devices to control the voltage output to the three phase

induction motor. The driver has two types of pulse signal to switch the IGBT. Low

signal directly switch the lower part of IGBT leg. The upper part is switched by the

voltage provided by Bootstrap circuit.

1.2 -. PROBLEM STATEMENT

Controlling the induction motor by adjusting the voltage supply, rotor resistance

control and pole changing method are ineffective, the PWM technique is apply to

replace these methods.

Today industry is using electricity for motor. As the Power dissipation of

induction motor is large or high power consumption using the conventional method If

the conventional method still applied to the industry, this may cost them the bill of

electricity.

Besides, problem encountered in inverter drives is the non-sinusoidal nature of

the supply voltage, which results in increased motor losses and harmful torque

pulsations producing undesirable speed oscillations. Torque pulsations and speed ripple

may be appreciable at low frequency, wore they may result in abnormal wear of gear-

teeth or torsional shaft failure. Hence, in applications where constant or precise speed

controlled of induction motor is important.

When a voltage source inverter is used, pulse width modulation (PWM)

techniques are usually employed, whereby the quasi square wave shape is modulated so

as to minimize or eliminate the low order harmonic voltage components and thereby

reduce the torque pulsations. Hence, better efficiency can be achieved using PWM

control method.

1.3 OBJECTIVES OF THE STUDY

i. To design and implement a SPWM generator to control VSI.

ii. To use the VSI to control three phase induction motor for better efficiency.

iii. To compare power loss between SPWM and direct terminal voltage control

techniques.

1.4] PROJECT SCOPE

After intensively reviewed on the PWM generator, there are many issues need to be

tackled in order to achieve a preliminary objective. The scopes of this thesis are used for

the guideline of the project. The project scopes are as follows:

i. In the project, VSI converter is used, it consist of PWM signal, optocouplers,

Gate driver, IGBT, three phase diode bridge rectifier and a three phase induction

motor.

ii. To generate PWM signal some of the components are used such as LM 324 op-

amp, TL 082 op-amp, and LM3 11 op-amp. To protect the control circuit for

power circuit, optocoupler isolator is used such as 4N25. JR 2130 is used as, the

Gate driver for the IGBT. IGBT SKM 200GB123D is used as the switching

•devices for VSI.

iii. The simulation circuit is designed by using Multisim software part by part

according to the theories and methods that gain from the literature review. Then

simulate and analysis the output waveform.

iv. Construct all the circuits related on protoboard based on collected data from

simulation part and test the circuits with induction motor.

/

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

In this chapter, the overview and theories of several fields involve in this

dissertation are discussed. These theories will help us to understand the upcoming

discussion on SPWM generator, optocoupler, driver, Voltage Source Inverter, induction

motor and its implementation.

2.2 PULSE WIDTH MODULATION (PWM)

•Due to advances in solid state power devices, switching power converters are

used in more and more modern motor drives to convert and deliver the required energy

to the motor. The energy that a switching power converter delivers to a motor is

controlled by Pulse Width Modulated (PWM) signals applied to the gates of the power

transistors. PWM signals are pulse trains with fixed frequency and magnitude and

variable pulse width. There is one pulse of fixed magnitude in every PWM period.

However, the width of the pulses changes from pulse to pulse according to a modulating

signal. When a PWIVI signal is applied to the gate of a power transistor, it causes the

turn on and turns off intervals of the transistor to change from one PWM period to

another PWM period according to the same modulating signal. The frequency of a

PWM signal must be much higher than that of the modulating signal, the fundamental

frequency, such that the energy delivered to the motor and its load depends mostly on

the modulating signal (Yu, Z. and Mohammed, A. 1997).

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Pulse-width modulation is a very efficient way of providing intermediate

amounts of electrical power between fully on and fully off. A simple power switch with

a typical power source provides full power only, when switched on. PWM is a

comparatively recent technique, made practical by modern electronic power switches.

.PWM can be used to reduce the total amount of power delivered to a load

without losses normally incurred when a power source is limited by resistive means.

This is because the average power delivered is proportional to the modulation duty

cycle (Baburao Kodavati, 2010).

YIUU

Iyflujil

0 D.T T T+D.T 2T 2T+D.T 3T 31'+D.T

Time

Figure 2.1: Pulse width modulation signal

Source: Baburao Kodavati 2010

Based on Figure 2.1 the duty cycle D is defined as the ratio between the pulse

duration ('r) and the period (T) of a rectangular waveform. Furthermore, the advantages

of using PWM based switching power converter over linear power amplifier are easy to

implement and control and lower power dissipation (Yu, Z. and Mohammed, A. 1997).

2.3 SINUSOIDAL PULSE WIDTH MODULATION (SPWM)

Sinusoidal PWM refers to the generation of PWM outputs with sine wave as the

modulating signal. It is called as sinusoidal PWM because the pulse width is a

sinusoidal function of the angular position in the reference signal (Sathish Kumar T,

Gowrishankar, 2012). The on and off instants of a PWM signal in this case can be

determined by comparing a reference sine wave (the modulating wave) with a high

frequency triangular wave (the carrier wave) as shown in Figure 2.2. Sinusoidal PWM

technique is commonly used in industrial applications. The frequency of the modulating

wave determines the frequency of the output voltage. The peak amplitude of modulating

wave determines the modulation index and in turn controls the rms value of output

voltage. The rms value of the output voltage can be varied by changing the modulation

index. This technique improves distortion factor significantly compared to other ways

of multi-phase modulation. It eliminates all harmonics less than or equal to 2p-1, where

flp is defined as the number of pulses per half cycle of the sine wave. The output

voltage of the inverter contains harmonics. However, the harmonics are pushed to the

range around the carrier frequency and its multiples (Yu, Z. and Mohammed, A. 1997).

The regular PWM modulation methods can be classified as an open loop and a

closed loop owing to its control strategy. The SPWM use open loop PWM techniques.

The fundamental frequency SPWM control method was proposed to minimize the

switching losses. (Rodriguez J., Kouro, S., Rebolledo, J., Pontt,2005; 0. Lopez, J.

Alvarez, J. Doval-Gandoy, F.D. Freijedo,2008; Jinghua Z., Zhengxi L,2008 and W.

Shireen, L. Tao, 2008 )

In order to implement Sinusoidal PWM using analog circuits, one has to use the

following building blocks - (1) High frequency triangular wave generator; (2) Sine

wave generator; (3) Comparator; and (4) Inverter circuits with dead band generator to

generate complimentary driving signals with required dead band.

jY4TAY7Y Figure 2.2: Sinusoidal pulse width modulation

Source: Yu, Z. and Mohammed, A. 1997

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In the inverter of Figure 2.4, the switch Qi and Q2 are controlled based on the

comparison of sinusoidal wave and the triangle wave which are mixed in a comparator.

When sinusoidal wave has magnitude higher than the triangle wave the output of the

comparator is high, otherwise it is low.

Vcon> Vtri Qi is on, V0Vd

= -- (2.1)

Vcon< Vtri Q2 is on, V0Vd

- - (2.2) 2

2.4 CHARACTERISTIC OF SPWM

The advantages of using SPWM technique are:

i. Reduction of harmonic

ii. Control of inverter output voltage

The disadvantages of using SPWM technique are:

i. Reduction of available voltage

ii. Increase of switching losses due to high SPWM frequency

iii. EMI problems due to high-order harmonic

Harmonic will be minimized if the triangle wave frequency is chosen to be an odd triple

multiple of the sinusoidal wave frequency that is 3,9,15 times the sinusoidal wave.

2.5 CARRIER WAVE RATIO AND MODULATION INDEX

Carrier wave ratio and modulation index are two important parameters in the

PWM technique. The carrier wave ratio is defined as N f/f, where, f1 is the desired

fundamental frequency of the inverter and fs is the switching frequency of the inverter.

The amplitude modulation index is defined as M = V 0 IVt1-, where, V 0 is the peak

amplitude of the control signal, and Vtrj is the amplitude of the triangular waveform.

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Modulation index can be in the range of 0 to 1 (H. Sarhan and R. Issa, 2005). There is a

key parameter to indicate the ability the inverter output voltage and the parameter is

utilization ratio of the DC link voltage. Usually it is defined as the ratio of fundamental

amplitude of line voltage of inverter and DC bus voltage. There is a linear relationship

between amplitude of line voltage and phase voltage, so the modulation index and DC

voltage utilization ratio have a same meaning essentially (Jingjing Han, Ruifang Liu and

Hui Huang, 2011)

2.6 ,OPTOCOUPLER

Figure 2.3: Optocoupler

In electronic, voltage isolator also calls an optocoupler or photocoupler which

transfer electrical signals between two isolated circuits by using light. The internal

circuit of optocoupler is shown in Figure 2.3. Optocoupler prevent high voltage from

disturbing the system receiving signal. Typically available optocoupler withstand input-

to-output voltage up to 10kV and voltage transient with speed up to 10kV per

microsecond. The optocoupler consists of a phototransistor and an LED in the same

package. Optocoupler can be used for both transmission of digital and analog (on/off)

signal.

2.7 GATE DRIVER

Gate driver is a device that connects the control electronics up to the power

stage. It is actually a powerful amplifier that can power Mosfet and IGBT. It accept a

low power input from a controller IC and produce a high-current drive input for the gate

of a high-power transistor which are power Mosfet and IGBT. Besides, these driver help

to control electronics that need higher power than the PWM signals can generate. Gate

driver also help to increase the input capacitance of power semiconductor associated

with the circuit the driver is located on. This can be used in conjunction with several of

other discrete transistors. There are two type of gate driver can be used for the

electronics devices which are on-chip gate drivers and off-chip gate drivers. The on-

chip gate drivers can be used when application is for a low , voltage and low-current uses.

For the off-chip gate driver, which are referred to as discrete gate drivers. These are

ideal for high voltage and high current applications. Discrete gate drivers also increase

the galvanic isolation to help protect the device from electrical faults and mechanical

faults.

2.8 VOLTAGE SOURCE INVERTERS (VSI)

Motor drive systems fed by pulse-width modulation voltage source inverters

(PWM-VSI5) are widely used in industrial applications for variable-speed operation,

such as aeronautics, railway traction and robotics. The wide use of VSI is due to the

high switching frequency of the semiconductor (Baburao Kodavati et. Al, 2010;

Rodriguez J., Kouro, S., Rebolledo and J., Pontt, 2005) and the use of PWM speed

controllers.

Besides, based on (Aeron Vander Meulen and John Maurin, 2010), in the

medium voltage adjustable speed drive market, the various topologies have evolved

with components, design, and reliability. The two major types of drives are known as

voltage source inverter (VSI) and current source inverter (CSI). In industrial markets,

the VSI design has proven to be more efficient, have higher reliability and faster

dynamic response, and be capable of running motors without de-rating.

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VSI fully integrated design saves money with higher efficiencies, minimizing

install time, eliminating interconnect power cabling costs, and reducing building floor

space. Efficiencies are 97% with high power factor through all load and speed ranges.

Fast dynamic response for rapid changes in motor torque and speed allow a wide range

of applications. Minimum component count increases the mean time to failure (MTTF),

an important number in critical uptime applications. Also, new replacement motors are

not required for retrofit applications. All of these factors produce a high-quality, robust,

industrial design.

The voltage source inverter topology uses a diode rectifier that converts utility,

line AC voltage (60 Hz) to DC. The converter is not controlled through electronic firing

like the CSI drive. The DC link is parallel capacitors, which regulate the DC bus voltage

ripple and store energy for the system. The inverter is composed of insulated gate

bipolar transistor (IGBT) semiconductor switches. There are other alternatives to the

IGBT: insulated gate commutated thyristors (IGCTs) and injection enhanced gate

transistors (IEGTs).The IGBT switches create a PWM voltage output that regulates the

voltage and frequency to the motor.

A capacitive load in the VSIs will generate large current spikes, which can be

prevented by an inductive filter between the AC side of VSI and the load. The

efficiency parameters of an inverter such as switching losses and harmonic reduction

are principally depended on the modulation strategies used to control the inverter (E.

Babaei, S.H. Hosseini, G. Gharehpetian, 2010; A.H. Ghaemi, H.A. Abyaneh, K.

Maziumi, 2011; M. Ramasamy and S. Thangavel, 2012). The Inverter control

techniques are based on fundamental frequency and high switching frequency.

2.9 STRUCTURE OF THREE PHASE VOLTAGE SOURCE INVERTER

AND HOW IT WORKS

The structure of a typical three phase voltage source power inverter is shown in

Figure 2.4. Va, Vb and Vc are the output voltages applied to the windings of a motor.

Qi through Q6 are the six power transistors that shape the output, which are controlled

by a, a', b, b', c and c'. When an upper transistor is switched on, that is when a, b or c is

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1, the corresponding lower transistor is switched off, that is the corresponding a', b' or c'

is 0. The on and off states of the upper transistors Qi, Q3 and Q5 or equivalently, the

state of a, b and c are sufficient to evaluate the output voltage (Yu, Z. and Mohammed,

A. 1997).

2.10 SPWM CONTROL VOLTAGE SOURCE INVERTER

An inverter bridge that consists of six switches whose state of on or off is

controlled by the PWM drives is used in a most common way. The outputs are

connected to each motor terminal to supply the power for motor. The structure of the

voltage-source inverter (VSI) for three-phase induction motor system is shown in Figure

2.4. A SPWM scheme is used to control the switching devices to generate approximate

sinusoidal signals in the stator phases. Figure 2.5 shows the general principle of SPWM.

An isosceles triangle carrier wave is compared with a sinusoidal modulating wave, and

the points of intersection determine the switching point of power devices. The output

waveform of the inverter controlled by SPWM is a series of rectangle pulses, whose

amplitudes are same and the widths are different.

The switching angle control is decided by the switching frequency and

modulation index, and the output fundamental wave amplitude has a linear relationship

with the amplitude modulation index. And the change of the switching frequency can

only change the centre of the fundamental wave frequency distribution, which does little

influence on the amplitude of each harmonic (Zang, C., Z. Pei, J. He, T. Guo, J. Zhu,

and W. Sun, 2009). It can be seen that the pulses in the output waveform have a sine

weighting equivalent to the reference waveform. This method was realized first with

analog circuits, and it can be modelled in the MATLAB/simulink to generate a series of

simulated PWM wave.

There are six power switches in total in the three-phase VSI and when one of the

switches in the upper half inverter bridge is opened, the corresponding one in the lower

half bridge will be closed. So there are actually only 8 switching modes (000, 001 ...1 11)

existing in three-phase VSI (Jingjing Han, Ruifang Liu and Hui Huang, 2011).

motor phases

Figure 2.4: Structure of three phase voltage source inverter

Source: Yu, Z. and Mohammed, A 1997

Figure 2.5: The modulation wave of PWM

Source: Jingjing Han, Ruifang Liu and Hui Huang 2011

2.11 SQUIRREL CAGE INDUCTION MOTOR

As the Squirrel Cage Induction Motor (SCIM) drives have been the traditional

alternating current workhorses in the industries, and their widespread acceptance makes

this study very important not only for its evaluation but to provide comparison with

other drive systems. The maximum efficiency point occurs when the SCIM magnetizing

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2.12 INDUCTION MOTOR OPERATION WITH NON-SINUSOIDAL

SUPPLY WAVEFORM

The developments in the power electronics field have lead to an ever-increasing

use of static switching devices to control the torque and speed of ac motors. Invariably,

the output voltage and current waveforms of these devices contain numerous harmonics

and these harmonics have detrimental effects on the motor performance in form of de-

rating and torque pulsation, especially at low speed. The order and magnitude of these

harmonics depend on the design as well as nature of load being supplied.

Prior to the advent of solid-state controllers for the speed control of induction

machines, the supply voltage used to be sinusoidal in nature, being practically free from

the time harmonics. The standard integral slot windings, having similar pattern of

conductor distribution for all the phases, have been used with these machines giving

reasonably good performance (P.L. Alger, 1975; B. Heller and V. Hamata, 1977). At

present time, the induction motors are widely supplied from several types of solid-state

adjustable voltage—frequency controllers with a wide range of operating features.

However, in any case, the motor has to be de-rated for the harmonic effects due to the

non-sinusoidal nature of the voltage supply.

The magnitude and the distribution of the additional losses and the related motor

de-rating, in steady state, depends on the harmonic contents of the applied voltage and,

in some way, on the motor design. The output voltage of the present day static

controllers deviates substantially from the sinusoidal form and contains wide spectrum

of time harmonics of which the lower order time harmonics in general, having

frequency closer to the wanted output frequency and the sub-harmonics in particular,

are found to be potentially objectionable in practice (L.A. Doggett, E.R. Queer, 1929;

G.C. Jam, 1964; E.A. Klingshirn and H.E. Jordan, 1968 ) and are at the same time

found difficult to be filtered off (B.R. Relley, 1964; B.D. Bedford and R.G. Hoft, 1970)

Hence, attempts are made to highlight the current and future issues involved in the

development of induction motor drive technology to impart good dynamic stability with

improved performance. Induction motors are the most used in industry since they are