ABSTRACT
New technology in digital excitation control provides a method to balance thyristor currents in parallel power rectifier bridges that provide current to a synchronous generator field. Balancing current between parallel bridges feeding a common load has traditionally been accomplished by using current balancing transformers in each bridge or adding inductance in series with the new thyristors or SCRs (silicon controlled rectifiers). This work introduces Active Current Balance, a new technique which modifies the average current in parallel leg thyristors to facilitate current balance between the bridges. This is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are fired carry more current during this interval, increasing their average current. The thyristors that are not fired do not carry current during this interval and thus their average current is reduced. Using this technique (referred to in this work as “skip firing”), the average current in parallel legs can be adjusted through feedback and control to equalize the average thyristor current loading. Additional feedback and control parameters are proposed, such as heatsink temperatures, which can be measured and utilized to program the “skip firing,” in order to balance thyristor heat-sink temperatures.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of Contents vi
List of Tables viii
List of Figures ix
List of Plates x
Abstract xi
CHAPTER 1: INTRODUCTION 1
1.1 Background of the Study 1
1.2 Problem Statement 6
1.3 Aim and Objectives of the Study 6
CHAPTER 2: LITERATURE REVIEW 7`
2.1 Speed Control of Conventional Induction Motor 9
2.2 Theory of Inverse Parallel Controller 20
2.3 Thyristor as a Switching Device 22
CHAPTER 3: MATERIALS AND METHODS 24
3.1 Method 24
3.2 Dynamic Modeling of the Three Phase Inverse Parallel Controller 24
3.3 vrms Expression for 3 Phase Inverse Parallel Thyristor Controller 27
3.4 Modelling of the Thyristor Controller 29
3.5 Modelling of the Thyristor Controller Parmeters 30
3.6 Sequential Switching of the Inverse Parallel Thyristors 32
3.7 Total Harmonic Distortion Analysis of the AC Controller 37
CHAPTER 4: RESULTS AND DISCUSSION 39
4.1 Construction and Implementation 39
4.2 Casing and Packaging 40
4.3 Inverse Parallel Controller Test with Induction Motor 41
4.4 Overall Performance Analysis 51
4.5 Bill of Quantity 51
4.6 Problem Encountered 52
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS 53
5.1 Conclusion 53
5.2 Recommendations 53
References 55
LIST OF TABLES
3.1: Pole voltage for the three phase inverse parallel controller 35
3.2: Phase-to-neutral voltage for the three-phase inverse parallel controller 36
3.3: Line-to-line voltage for three phase inverse parallel controller 36
3.4: Instantaneous Controller Voltage 38
4.1: Relationship of the controlled terminal voltage phase-A with speed of the induction motor at different delay angles obtained in the in the machine laboratory 47
4.2: Bill of quantity for the construction of 2KVA three phase inverse-parallel thyristor controller. 51
LIST OF FIGURES
3.1: Three phase ac full wave voltage controller 25
3.2: Gate drive signal for a six step mode of operation 34
4.1: Inverse-parallel controller power circuit test with an induction motor 41
4.2: Stator terminal voltage (V) against delay angle (αo). 48
4.3: Rotor speed (RPM) against delay angle (αo) 49
4.4: Rotor speed (RPM) against Stator terminal voltage (V). 50
LIST OF PLATES
4.1: Implemented inverse-parallel controller on vero board 39
4.2: Inverse-parallel controller inside black casing box 40
4.3: Inverse-parallel controller connected to induction motor and test equipment at the machine laboratory 42
4.4: Phase-A displayed stator voltage waveform on oscilloscope at 90O delay angle 43
4.5: Phase-B displayed stator voltage waveform on oscilloscope at 90O delay angle 44
4.6: Phase-C displayed stator voltage waveform on oscilloscope at 90O delay angle 45
4.7: Obtained phase A&B voltage waveform at the terminal of induction motor with 120O displacement as seen on the oscilloscope 46
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF THE STUDY
Over the past decades DC machines were used extensively for variable speed -applications due to the decoupled control of torque and flux that can be achieved by armature and field current control respectively. DC drives are advantageous in many aspects as in delivering high starting torque, ease of control and nonlinear performance. But due to the major drawbacks of DC machine such as presence of mechanical commutator and brush assembly, DC machine drives have become obsolete today in industrial applications Bose (1997). The robustness, low cost, the better performance and the ease of maintenance make the asynchronous motor advantageous in many industrial applications or general applications.
Squirrel cage induction motors (SCIM) are more widely used than all the rest of the electric motors as they have all the advantages of AC motors and are cheaper in cost as compared to Slip Ring Induction motors; require less maintenance and rugged construction. Because of the absence of slip rings, brushes maintenance duration and cost associated with the wear and tear of brushes are minimized. Due to these advantages, the induction motors have been the execution element of most of the electrical drive system for all related aspects: starting, braking, speed change and speed reversal etc Vasudevan et al (2005).
To reach the best efficiency of induction motor drive (IMD), many new techniques of control has been developed in the last few years. Now-a-days, using modern high switching frequency power converters controlled by microcontrollers, the frequency, phase and magnitude of the input to an AC motor can be changed, hence the motor speed and torque can be controlled. Today, it is possible to deal with the axis control of machine drives with variable speed in low power applications mostly due to joint progress of the power electronics and numerical electronics. The dynamic operation of the induction machine drive system has an important role on the overall performance of the system of which it is a part Takahashi and Noguchi (1986).
The advent of power electronics and therefore the means of controlling the speed of induction machines, has led to ac variable speed induction motor drives, which are by far the most popular and most used in meeting many applications nowadays. Venter et.al (2012).
In recent years, the field oriented control of induction motor drive is widely used in high performance drive system because of its advantages. Now a day’s there is a great demand in industry for adjustable speed drives. Even though investigations have been carried out for decades for the efficient control of the speed of induction motor. Sachin and Satya (2012).
Efficiency improvements of constant-speed, variable torque drives are different from those of constant speed and constant torque applications. During the past 20 years, beginning with the Nola Controller, attempts have been made to design simple, inexpensive thyristor/triac controllers. Such controllers are able to sense the torque of a drive and subsequently adjust the input voltage and current of single- and three-phase induction motors as they are used in appliances and commercial applications, where the torque required changes with load Ewald et al (2002). In order to accurately measure additional losses in controllers and induction motors, as well as power factors including the total harmonic distortions of currents and voltages at the input and output of the controller, computer-aided testing (CAT) circuits are used. Most single-phase controllers generate dc current components of the input and output currents through imperfect gating signals. Such dc components cause additional losses, vibration, and audible noise.
Three-phase induction motors are inherently more efficient than single-phase motors. Therefore, the percentage power savings due to a controller can be expected to be less for three-phase machines than for single-phase motors Ewald et.al (2002). Power electronic converters are used as interface between three-phase grid supply and the driving motor Wheeler et.al (2002).
In this thesis, an induction motor drive is analyzed. The focus here will be to model a power electronic control scheme for speed control of the induction motor.
The power electronic converter is the backbone of a variable speed drive Rodriguez et.al (2012). It is used to process the electrical power of utility grid and supply to the electric motor. The rapid growth in the semiconductor material and switching devices has led to tremendous improvements in the power converters and in the development of numerous species. Without proper control of the induction motor speed, it is practically impossible to achieve the desired task for a specified application Iqbal (2006).
The power electronics controller for induction motor is a group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults. On the other hand, the main advantages of modern controllers are; high efficiency, low weight and small dimensions makes it compact and portable, fast operation as a result of no moving part compared to mechanical switches and high power densities are gained using it in the switch mode operation. With the corresponding control, power electronics controllers are good and simple methods for changing the motor voltage. These electronics not only control the motor’s speed, but can improve the motor’s dynamic and steady state characteristics. In addition, power electronics can reduce the system’s average power consumption and noise generation of the motor Okoro (2013).
A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to control its speed. The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor. Electric drives play an important role in the field of power electronics since they are used in a wide range of applications. In this context it is important to match the correct drive to the application in accordance with its requirements. In the recent decades, a huge step had been taken in power semiconductors and microprocessors development. As a result, modern drive system technology had changed dramatically, and accordingly more studies were done on electric drive systems to fulfill the various needs of different application. The continuous improvements in power electronics field made it easier to develop modern switch mode inverters based on high speed power transistors, like MOSFET and IGBT Badran et al (2013).
Motion control is the backbone of automation system widely used in every section of industrial and commercial activities, the heart of this motion control is variable speed drives (VSDs). An induction motors (AC drives) are widely used VSDs as a result of its low maintenance cost while offering equal and often superior dynamic performance over their DC drives counterparts in all possible scales (Large, Medium and small) of motion control tasks such as production machine, industrial robots, proportional tasks in electric transit vehicles, electric elevators, pumps and similar Okakwu and Oluwasogo (2014).
Dynamic simulations play an important role in the pre-testing of motor drive systems. Pre-testing is conducted by engineers in industry as well as by researchers in academia. Pre-testing using dynamic simulations can help researchers to determine the experimental setup that will be used for a given set of experimental tests. The transient behaviour of an electric machine is of particular importance when the drive system is to be controlled. Many different methods and control algorithms are available in the literature for controlling the three-phase induction motor Vasudevan and Arumuyam (2004). A dynamic model of a machine leads to insight into the electrical transients Vasudevan et.al (2005).
There is an increasing trend of using fast switching devices for several industrial applications. Thyristors are known to have low switching frequency of the order of hundreds of Hz. Hence the alternative is to use fast switching devices such as MOSFETs and IGBTs. Thus a simulation model was developed by them incorporating a soft start system using IGBTs . The motor speed response to various conduction angles were also depicted. It was once again observed that the response settling time is inversely proportional to the conduction angle. Moreover, the speed response is smoother compared to the thyristor soft starts case.
The advantage of using IGBTs is that due to same extinction angle as that of firing angle, the fundamental current in this case is in phase with the voltage, hence displacement factor becomes unity. A plot of the power factor vs firing angle was also illustrated in their work. They revealed that the power factor varies significantly with change in the firing angle especially for IGBT based soft starter. Nevertheless, it was higher for thyristor based system Riyaz et.al (2009).
1.2 PROBLEM STATEMENT
The three phase induction motor drives have found application in the industries, but not without problems associated with efficient control, without reduction in system’s dynamic performance. Previous works on the dynamic models of inverse parallel controllers for speed control of induction machines have been developed using thyristor based controllers. There is need for the use of Thyristor based inverse parallel controllers to be adopted for the speed control of induction machines. The inverse parallel controller if designed and implemented will contribute a lot and be of immense help.
1.3 AIM AND OBJECTIVES OF THE STUDY
This project aims at the design and construction of a three-phase thyristor based inverse parallel controller trainer for laboratory applications improved performance.
The specific objectives for this research are;
• To critically review the current developments in controllers by adequate review of some relevant literatures.
• To critically review the current developments in inverse parallel controllers by adequate review of some relevant literatures.
• To model and simulate a MOSFET based inverse parallel controller.
• To laboratory tests to validate theoretical findings.
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