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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
vi
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.
vi'
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.
viii
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
lx
2.11
Squirrel Cage Induction Motor
12
2.12
Induction Motor Operation with Non-Sinusoidal Supply Waveform
14
METHODOLOGY
CHAPTER 3 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
between SPWM Techniques with Direct Terminal Voltage Supply
41
x
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
xi
LIST OF TABLES
Table No. 4.1
Title
Experiment Results of Current Drawn By the Induction Motor with Different Voltage among Direct Terminal
Voltage Supply and with SPWM Technique
Page
42
xli
LIST OF FIGURES
Title
Figure No.
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
sinusoidal wave
19
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
different of sinusoidal wave.
21
3.10
Simulation result of SPWM generator for 120 degree phase
different of sinusoidal wave
21
3.11
Simulation result of SPWM generator for 240 degree phase
different of sinusoidal wave
22
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
circuit
30
xlii
44
One of the three phase sinusoidal wave from sinusoidal
wave generator circuit
30
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
circuit
32
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
signal from optocoupler
34
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
signal is turned on and the low signal is turned off
35
4.14
Detail view of the dead time that occurs when the high signal
is turned off and the low signal is turned on
36
4.15
One of the three phase driver high and low output signal from
gate driver after connected with IGBT.
37
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
at 30 V Using direct terminal voltage supply technique
41
4.22
Experiment results of current drawn by the induction motor
at 30 V using SPWM technique
41
4.23
Graph of current versus voltage
42
xlv
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
xv
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
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
2
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 gearteeth 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 opamp, 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).
5
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
I
yflujil
0 D.T
2T 2T+D.T
T T+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
7
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.
Vd
Vcon>
Vtri
Qi is on, V0 = --
Vcon<
Vtri
Q2 is on, V0
Vd
- 2-
( 2.1)
( 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.
8
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 inputto-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 onchip 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.
10
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
11
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).
12
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
14
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 derating 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