ABSTRACT
Dynamic modelling of a DC-DC chopper for improved performance is carried out. Modern power electronic systems require high quality, small, lightweight, reliable and efficient power supplies. High frequency (HF) electronic power processors are used in the direct current to direct current (DC-DC) power conversions for functions such as: conversion of DC input voltage to DC output voltage; regulation of the DC output voltage against load and line variations; reduction of voltage ripple on the DC output voltage below the required level; provision of isolation between the input source and the load; and protection of the supplied system and the input source from electromagnetic interference. Two controller types of Class A DC-DC choppers have been presented using two basic element (thyristor and Metal oxide silicon field effect transistor or metal oxide semiconductor field effect transistor (MOSFET) with difference converter configurations – half, semi, full, and dual arrangements- with a view to ascertaining ones with better performance. These configurations were studied and results obtained from implemented model in Simulink MATLAB. Model for DC drive used in the proposed system in form of Laplace transformation equations which was equally implemented in MATLAB/Simulink was presented. Brief comparisons between thyristor-based and MOSFET-based controllers showed thyristor converters consistently showed higher revolutions per minute (2230) and speed, but lesser efficiency in terms of speed control. However, MOSFET-based controllers showed greater efficiency in speed control but lower revolution per minute (1520), implying better speed control between actual and reference speed.
TABLE OF
CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Abstracts vi
Table of Contents viii
List of Tables x
List of Figures xi
Abstract
CHAPTER 1:
1INTRODUCTION 1
1.1 Background of
the Study 1
1.2 Problem
Statement 4
1.3 Objectives of
the Study 4
1.4 Scope and Limitation of the Study 5
1.5 Significance
of the Study 5
CHAPTER 2: LITERATURE
REVIEW 6
2.1 The DC Motor 6
2.2 Speed Control
of DC Motor 8
2.2.1 Armature
control of DC series motor 9
2.2.2 Field control
method of speed control 11
2.3 Fuzzy Logic
Controller (FLC) 11
2.4 Proportional-Integral
Controller 12
2.5 Proportional
Integral Derivative Controller 12
2.6 Speed Control DC Motor Using Microcontroller 13
2.6.1 Power rectifier
(bridge) 13
2.6.2 The full-wave
bridge rectifier 13
2.6.3 Filter circuit 14
2.7 Chopper
Controller 15
2.7.1 Applications of
chopper 16
2.7.2 Chopper
operation 16
CHAPTER 3: MATERIALS
AND METHODS 22
3.1 Methodology
Overview 22
3.1.1 Separately
excited DC motor (field and armature equations) 22
3.1.2 Basic torque
equation 23
3.1.3 Steady state
torque, speed and power drawn 23
3.2 Modelling of
DC Motor for Drive System 24
3.3 Thyristor-Based
Techniques of DC Motor Speed Control 28
CHAPTER 4: RESULTS AND
DISCUSSION 37
4.1 Results 37
4.1.1 Output of converter
drives 37
4.1.2 Output of
proposed systems 41
4.2 Discussion of Comparison of
Thyristor-Based and MOSFET-Based
DC-DC Converter 46
CHAPTER 5: SUMMARY,
CONCLUSION AND RECOMMENDATIONS 49
5.1 Summary 49
5.2 Conclusion 49
5.3 Recommendations 50
References 51
LIST OF TABLES
2.1 Advantage
and disadvantages of various DC motor (Hoft, 2012) 8
4.1 Summarised comparison of thyristor-based
and MOSFET-based
Converter 47
LIST OF
FIGURES
2.1 Flux
control circuit diagram (Mohan and Undeland, 2007) 9
2.2 Armature
and Rheostatic control (Mohan and Undeland, 2007) 10
2.3
(a) The full-wave bridge rectifier;
(b) output waveform (Hoft,
2012) 14
2.4 Filter
circuit (a) input waveform (b) Filter circuit (c) Output waveform (Hoft, 2012) 15
2.5 Diagram of Chopper First Quadrant (Dubey, 2009) 17
2.6 Diagram of Chopper Second Quadrant (Dubey, 2009) 17
2.7 Diagram of Chopper Two Quadrant (Dubey,
2009) 18
2.8 Two Quadrant Type B chopper or D Chopper
Circuit (Dubey, 2009) 19
2.9 Positive first quadrant operation and
negative fourth quadrant operation
(Dubey, 2009) 19
2.10 E-type chopper circuit diagram with load emf
E and E reversed
(Dubey, 2009) 20
3.1 Block diagram of chopper controller 22
3.2 Single phase half-wave
converter drive 28
3.3 Semi-converter drive
(single phase) 29
3.4 Full wave converter
(single phase) 30
3.5 Dual converter drive
(single phase) 30
3.6 Half converter drive
(single phase) 31
3.7 Semi-converter drive
(single phase) 32
3.8 Full wave DC-DC
converter (single phase) 32
3.9 Simulink MATLAB
implementation of thyristor-based DC-DC controller
(half-wave) 33
3.10 Simulink MATLAB
implementation of thyristor-based DC-DC controller
(full-wave) 34
3.11 Simulink MATLAB
implementation of MOSFET-based DC-DC controller
(half-wave) 35
3.12 Simulink MATLAB
implementation of MOSFET-based DC-DC controller
(full-wave) 36
4.1 DC output voltage for half
wave converter drive (single phase) at 90°
firing
angle 37
4.2 Output voltage waveform of
half wave converter drive (single phase)
at
30° firing angle 38
4.3 Load current of full wave
converter (single phase) at 30° firing angle 38
4.4 DC output voltage for full
wave converter drive (single phase) at 30°
firing
angle 39
4.5 Output voltage waveform of
full wave converter drive (single phase)
at
30° firing angle 39
4.6 Load current of full wave
converter (single phase) at 30° firing angle 39
4.7 DC output voltage of
semi-converter drive at 30° firing angle 40
4.8 Output voltage waveform of
semi-converter drive at 30° firing angle 41
4.9 Load current of
semi-converter drive at 30° firing angle 41
4.10 DC output voltage for full
wave converter drive (single phase) at 30°
firing
angle 41
4.11 Output voltage waveform of
full wave converter drive (single phase)
at
90° firing angle 42
4.12 Load current of full wave
converter (single phase) at 90° firing angle 42
4.13 Speed response at reference
speed same as rated speed (half wave
thyristor-based
converter) 43
4.14 Electrical torque of half
wave thyristor-based converter 43
4.15 Speed response at reference
speed same as rated speed (full wave
thyristor-based
converter) 44
4.16 Electrical torque of full
wave thyristor-based converter 44
4.17 Speed response at reference
speed same as rated speed (half wave
MOSFET-based
converter) 45
4.18 Electrical torque (black)
and field current (red) of half wave MOSFET-
based
converter 45
4.19 Speed response at reference
speed same as rated speed (full wave
MOSFET-based
converter) 46
4.20 Electrical torque (black)
and field current (red) of full wave MOSFET-
based
converter 46
4.21 Actual vs reference speed
(MOSFET) 47
4.22 Actual vs reference speed
(thyristor) 48
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
OF THE STUDY
Modern power electronic systems
require high quality, small, lightweight, reliable and efficient power
supplies. High frequency (HF) electronic power processors are used in the
direct current to direct current (DC-DC) power conversions. Functions of DC-DC
converters includes the: conversion of DC input voltage to DC output voltage;
regulation of the DC output voltage against load and line variations; reduction
of voltage ripple on the DC output voltage below the required level; provision
of isolation between the input source and the load; and protection of the
supplied system and the input source from electromagnetic interference (EMI) (Singh and Nirmal, 2014).
The DC-DC
converters can be divided into two main types, the hard-switching pulse width
modulated (PWM) converters, and the resonant and soft-switching
converters. The PWM DC-DC converters are
widely used at all power levels, and its properties are well understood (Rodrigues et al.,
2010). Some benefits of PWM converters include low component count, high
efficiency, constant frequency operation, relatively simple control and
commercial availability of integrated circuit controllers, and ability to
achieve high conversion ratios for both step-down and step-up applications. A
major drawback of PWM DC-DC converters is that the rectangular voltage and
current waveforms cause turn-on and turn-off losses in semiconductor devices.
This limits the practical operating frequencies to hundreds of kilohertz.
Rectangular waveforms also inherently generate electromagnetic interference
(EMI) (Mohan and Undeland, 2007).
The development of high performance
motor drives is very essential for industrial applications.
DC motors provide excellent
control of speed for acceleration and deceleration. The power supply of a DC
motor can connect directly to the field of the motor, which allows for precise voltage control, and is necessary for speed and
torque control applications (Talavaru et al., 2014). Its importance is
due to simplicity, ease of use,
reliability, inexpensive for low horsepower ratings, as well as being less
complex compared to the alternating current (AC) drive system. DC motors are
preferably used as adjustable speed
machines, and a wide range of options have evolved for this purpose. AC drives with this capability would be
more complex and expensive (Afrasiabi and
Yazdi, 2013).
A series field DC motor is capable of
providing starting and accelerating torques in excess of 400 % of rated values, DC motors have long been the primary
means of electric traction (Okoro, 2004). Other applications include for mobile
equipment such as golf carts, quarry
and mine winders. DC motor is considered a single input single output (SISO) system possessing
torque/speed characteristics compatible with most mechanical loads. This makes a DC motor controllable over a
wide range of speeds by proper adjustment
of the terminal voltage. Nowadays, induction motors, brushless DC motors and synchronous motors have gained
widespread use in electric traction
systems (Okoro, 2004). DC motors are always good options for advanced control algorithm because the theory of DC motor
speed control is extendable to other
types of motors as well (Okoro, 2004).
Before the advent of
modern silicon control rectifier (SCR) controllers, speed control of DC
machines were achieved using passive devices
such as bank of resistors, mercury arc rectifiers, magnetic amplifiers
or Ward Leonard speed control schemes (Ogata, 2011). The Ward Leonard system is an AC motor - DC
generator set that feeds a variable voltage to the armature of a shunt wound DC
motor to vary the motor's speed. While the Ward-Leonard system has good speed
and torque control with a speed range of 25:1, it was phased out due to the
excessive cost of purchasing three separate rotating machines, as well as the
considerable maintenance necessary to keep the brushes and commutators of two
DC machines in proper operating conditions. Today's SCR controlled DC drives
have numerous advantages over previous electrical drive systems, such as the
Ward Leonard drive (Kiran et al., 2014). In the first method, variable resistance
inserted between the fixed- voltage DC source and the motor. This method is
inefficient because of loses in the resistance. In the second method, the motor
-generator set is used to supply the power to the motor whose speed is to be
controlled. A variable DC output voltage of the generator is obtained by
controlling the field current of DC generator which is driven by a constant
speed DC motor. This system is still used in some industrial drives; therefore
the system is bulky, costly, slow in response and less efficient. In 1960 high
power thyristor device became available to make the solid-state DC power
converter practical. These converters offer greater efficiency, fast response, smooth
operation, smaller size and lower weight and cost.
The chopper circuit of
force commutated thyristors is another effective method of controlling the
armature voltage and speed of a DC machine. The chopper is a static power
electronic device that converts fixed DC input voltage to a controlled
(variable) DC output voltage. It can be used to obtain a variable output
voltage for varying the speed of a DC motor by changing the mark period ratio
of the chopper (Mohan and Undeland, 2007).
A DC Chopper comprises series connections of DC input voltage source,
controllable switch and load resistance. In most cases, the switch has
unidirectional voltage-blocking and current-conduction capabilities. Power
electronic switches are usually implemented with power metal-oxide-semiconductor field-effect transistors (MOSFETs), insulated-gate bipolar transistors (IGBTs), MOS-controlled thyristors (MCTs), power Bipolar Junction Transistors (BJTs) or gate turn-off
thyristors (GTOs).
The thyristor-controlled chopper performs a switching action
between the supply and the load. The
increasing cost of fuel for operation of internal combustion engines, rapid rate of depletion of
energy sources and the possibility of scarcity has necessitated the need to
find alternative means of driving electric
cars, trolleys and forklifts using the DC-DC chopper. A Chopper may be
considered as the DC equivalent of an AC transformer since they behave in an
identical manner. As choppers involve one stage conversion, they are considered
more efficient. Choppers are now being used all over the world for rapid
transit systems. These are also used in trolley cars, marine hoist, forklift
trucks and mine hauliers (Hoft, 2012;
Ayasun and Karbeyaz, 2007).
This project aims
to model
a DC-DC chopper that provides an improved and
alternative switching performance. The performance of the chopper in a DC motor system is
to be evaluated via computer simulation such as the MATLAB/SIMULINK software.
1.2 PROBLEM
STATEMENT
DC motors play vital
roles in every academic and research laboratory, technical workshops or industries;
hence, the necessity to study and suggest remedies for DC motor speed control.
Prior to this project, people may have encountered difficulty in getting an
efficient, reliable, durable and relatively inexpensive DC-DC controller. This
project will provide a procedure for adapting a suitable model to control DC
motor speed, and thus indicate an improved performance of existing DC-DC
chopper.
1.3 OBJECTIVES
OF THE STUDY
The main objective of this project is to obtain a dynamic
model of a DC-DC chopper for improved switching performance.
The specific objectives of this project are to:
i.
Investigate the existing type of SCR DC motor controllers;
ii.
Obtain
mathematical models for MOSFET and Thyristor -based DC-DC choppers;
iii.
Model
and simulate the chopper controller using MATLAB/Simulink; and
iv.
Carry-out
comparative analyses of modelled DC-DC choppers to ascertain the chopper which
yields a better performance.
1.4
SCOPE AND LIMITATION OF THE STUDY
The scope
of the work is to comparatively evaluate the performances of the MATLAB/Simulink
modelled DC-DC chopper. This project is limited to thyristor and MOSFET –based
choppers.
1.5 SIGNIFICANCE
OF THE STUDY
A chopper is an electronic switching
device that switches voltage ON and OFF in a remarkably high speed on a motor
in a process called chopping. Hence, the following significance:
- Systems
containing chopper have smooth control capability and are highly efficient
and fast in response.
- Since it
operates on the pulse width modulation (PWM) principle, it is fast in
response (no time delay in its operation) as it takes a fixed DC input
voltage and gives variable DC output voltage.
- The size
and cost of the system are reduced as a chopper is used to step-down or
step-up the fixed DC input voltage in the absence of a transformer.
iv.
DC
motor speed control experimentation in schools is possible since the chopper is
a relatively inexpensive and simple to use.
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