DECLARATION
We hereby declare that this is our work and that it has not been submitted before anywhere for the purpose of awarding a degree to the best of our knowledge.
…………………………………………
…………………
JUDE CHINOSO VIOLET
DATE
…………………………………………
…………………..
AJAYI TEMITOPE RACHEAL
DATE
…………………………………………
…………………
WALKER ENAEFE HILARY
DATE
…………………………………………
…………………..
GONU SHOLA
DATE
i
CERTIFICATION
This
is
to
certify
that
the
DESIGN
AND
CONSTRUCTION
OF
A
3KVA
MICROCONTROLLER BASED INVERTER WITH CONTROLLED OUTPUT was carried out by JUDE CHINOSO VIOLET, WALKER ENAEFE HILARY, AJAYI TEMITOPE RACHAEL and GONU SHOLA under the guidance of Engr. C. K. Igbinoba and has been duly approved for acceptance by the department of Electrical and Electronic Engineering, College of Technology Federal University of Petroleum Resources Effurun Delta State, Nigeria for the award of the Bachelor of Engineering Degree(B.Eng) in Electrical and Electronics Engineering.
…………………………………………
…………………
ENGR. C. K. IGBINOBA
DATE
(Project Supervisor)
…………………………………………
…………………..
ENGR. DR. J. E. OKIAIFOH
DATE
(Head of Department)
ii
DEDICATION
We dedicate this project to all al l the lovers of Engineering in the world.
iii
ACKNOWLEDGMENTS
First, we are grateful to Almighty God for His wisdom, and the grace to learn as quickly as possible on this project, and that He has seen us through to the end. We grateful to our supervisor Mr. C. K. Igbinoba for his guidance up to know and through the length of the project. And to Engr. Dr. G. Ofualagba for his support through the project, to Mr. Josiah for his expert advice during the length of the project. Lastly, to all the lecturers of the department for the way they have equipped us with things we should know about till now.
iv
ABSTRACT
The aim of this project is to design and construct a 3 KVA microcontroller based pure sine wave inverter with controlled output. A system that will convert the DC voltage from a battery source to 230 VAC at a frequency of 50Hz. The controlled output will be used to shut down heavy loads when the battery voltage reaches 22 VDC. This project work was realised by programming a PIC18f2550 microcontroller chip to produce a PWM which is used to drive the gates of a MOSFET H-BRIDGE to invert the DC voltage from the battery to a 24 VAC voltage output. The 24 VAC output is then passed through a transformer to realise the 230 VAC output. The PIC18f2550 chip is programmed to perform all the auxiliary functions such as low battery cut-off, over-load protection and control of the inverter display LCD. The PIC16f873a chip was also programmed to display the state of the inverter in every operating mode. The system was constructed and tested by connecting a 24 VDC battery to the inverter input, and the inverter gave 230 VAC at the output. A load of 1 KVA was connected to the controlled output and the output was cut-off when the battery level dropped to 22 VDC leaving the other output ON. The entire system was cut-off when the battery voltage dropped to 20 VDC.
v
TABLE OF CONTENTS
DECLARATION ........................................................................................................................i CERTIFICATION .....................................................................................................................ii DEDICATION ......................................................................................................................... iii ACKNOWLEDGMENTS ........................................................................................................iv ABSTRACT............................................................................................................................... v LIST OF FIGURES ..................................................................................................................ix LIST OF TABLES ..................................................................................................................... x ABBREVIATIONS .................................................................................................................. xi CHAPTER ONE ........................................................................................................................1 INTRODUCTION ..................................................................................................................... 1 1.1 Background of the study ................................................................................................. 1 1.2 Statement of problem ...................................................................................................... 2 1.3 Aim and Objectives ......................................................................................................... 2 1.4 scope/limitations of the project ....................................................................................... 2 1.5 Applications of project .................................................................................................... 3 1.6 Outline of the remaining chapters ...................................................................................3 CHAPTER TWO .......................................................................................................................4 LITERATURE REVIEW .......................................................................................................... 4 2.1 Review of exiting systems............................................................................................... 4 2.2 Review of principles behind this project ......................................................................... 4 2.2.1 Pulse Width Modulation:......................................................................................... 4 2.2.2 H-Bridge Configuration ........................................................................................... 5 2.2.3 Analogue to Digital Conversion .............................................................................. 6 2.2.4 Computer Programming .......................................................................................... 7 2.3 Components and their rellevance to the design ...............................................................7 2.3.1 MOSFET Drivers .................................................................................................... 7
vi
2.3.2 Micro-controller.......................................................................................................8 2.3.3 Transformer ............................................................................................................. 9 2.3.4 Voltage regulator ................................................................................................... 10 2.3.5 Relay ......................................................................................................................10 2.3.6 MOSFETs .............................................................................................................. 12 2.3.7 Octo-coupler .......................................................................................................... 13 CHAPTER 3 ............................................................................................................................ 15 METHODOLOGY AND SYSTEM DESIGN ........................................................................ 15 3.1 METHODOLOGY........................................................................................................15 3.1.2 Block diagram........................................................................................................ 15 3.2 theory and calculations ..................................................................................................16 3.2.1 signal generator ...................................................................................................... 16 3.2.1a PWM GENERATION TECHNIQUE AND REGULATION............................. 18 3.2.1 b WHY PIC MICROCONTROLLER ................................................................... 20 3.2.2 H-bridge.................................................................................................................22 3.2.2 a IGBTs Versus MOSFETs ................................................................................... 22 3.2.2 b Enhanced N-channel versus Enhanced P-channel MOSFETs ........................... 23 3.2.2 c MOSFET, GATE RESISTOR AND DC-LINK CAPACITOR SELECTION... 23 3.2.3 MOSFET DRIVER................................................................................................ 26 3.2.3a BOOTSTRAP CAPACITOR .............................................................................. 27 3.2.3b BOOTSTRAP DIODE ........................................................................................ 28 3.2.4 TRANSFORMER CALCULATIONS .................................................................. 29 3.2.5 Output Filter ..........................................................................................................30 3.3.1 Program Flowchart ................................................................................................ 31 3.3 mode of operation..........................................................................................................33 CHAPTER 4 ............................................................................................................................ 35 CONSTRUCTION, TESTING AND RESULTS .................................................................... 35
vii
4.1 IMPLEMENTATION ................................................................................................... 35 4.1.1 Transformer construction ......................................................................................35 4.1.2 H-bridge construction ............................................................................................ 37 4.1.3 circuit construction ................................................................................................ 38 4.2 Tests .............................................................................................................................. 39 4.2.1 Continuity Test ...................................................................................................... 39 4.2.2 Voltage Test ........................................................................................................... 39 4.2.3 No load test ............................................................................................................39 4.3 RESULTS......................................................................................................................40 4.3.1 signal generator output .......................................................................................... 40 4.4 BILL OF ENGINEERING MEASUREMENT AND EVALUATION........................ 43 CHAPTER 5 ............................................................................................................................ 44 CONCLUSION AND RECOMMENDATIONS .................................................................... 44 5.1 CONCLUSION .............................................................................................................44 5.2 RECOMMENDATIONS .............................................................................................. 44 REFERENCES ........................................................................................................................ 45 APPENDIX.............................................................................................................................. 47 PROGRAM SOURCE CODE ............................................................................................ 47 LCD DISPLAY SOURCE CODE ........................................................................................... 55
viii
LIST OF FIGURES Figure 2.2: H-Bridge Configuration using N-Channel MOSFETs ............................................5 Figure 2.3: MOSFET symbol .................................................................................................... 7 Figure 2.5: A simple electromechanical relay ........................................................................ 11 Figure 2.6: cross section of a power MOSFET........................................................................ 12 Figure 2.7: MOSFET characteristics ....................................................................................... 13 Figure 2.8: Schematic diagram of an opto-isolator .................................................................. 13 Figure 3.1: Block diagram of pure sine wave inverter............................................................. 15 Figure 3.2: graph of sinewave against duty cycle .................................................................... 16 Figure 3.3: internal structure of the PIC18F2550 chip ............................................................ 20 Figure 3.4: internal structure of the PIC16F873A chip ........................................................... 21 Figure 3.5: Series Gate Resistance vs. Amplitude of Negative Voltage Spike and Turn-off time .......................................................................................................................................... 24 Figure 3.6: PWM ripple current flow ......................................................................................25 Figure 3.7: IR2110 connection ............................................................................................... 27 Figure 3.8: H-bridge and MOSFET circuit .............................................................................. 29 Figure 3.9: LC filter ................................................................................................................. 30 Figure 3.10: program flowchart ............................................................................................... 31 Figure 3.11: complete circuit diagram ..................................................................................... 32 Figure 4.1: Bobbing cuttings.................................................................................................... 35 Figure 4.2: bobbing after cutting and gluing pieces together .................................................. 36 Figure 4.3: transformer while wearing in the laminations ....................................................... 36 Figure 4.4: MOSFETs and transformer connection with casing .............................................37 Figure 4.5: PCB diagram of inverter work .............................................................................. 38 Figure 4.6: Proteus simulation diagram for inverter oscillation .............................................. 40 Figure 4.7: Proteus simulation for charging oscillation ........................................................... 40 Figure 4.8: The inverter when on ............................................................................................. 41 Figure 4.9: The inverter when on ............................................................................................. 41 Figure 4.10: The output sinewave ............................................................................................ 42
ix
LIST OF TABLES
Table 2.1: Valid H-Bridge Switch State .................................................................................... 6 Table 3.1: duty cycles for PWM .............................................................................................. 17 Table 3.2: duty cycle in terms on of time on ........................................................................... 18 Table 4.1 Results table ............................................................................................................. 42 Table 4.2: Bill of Engineering measurement and evaluation ................................................... 43
x
ABBREVIATIONS
MCU
Microcontroller unit
ADC
Analog to digital converter
IC
Integrated circuit
KB
Kilobyte
VDD
Microcontroller supply voltage
VSS
Microcontroller ground
CPU
Central processing unit
ROM
Read only memory
RAM
Random access memory
AC
Alternating current
DC
Direct current
VDC
Direct current voltage
VAC
Alternating current voltage
LCD
Liquid crystal display
GND
Ground
CPU
Central processing unit
NO
Normally open
NC
Normally closed
AVR
Automatic voltage regulator
KVA
Kilo Volt Ampere
Qg
Gate charge of high-side FET
f
frequency
ICbs (leak)
bootstrap capacitor leakage current
Iqbs (max)
Maximum VBS quiescent current
VCC
Logic section voltage source
Vf
Forward voltage drop across the bootstrap diode
VLS
Voltage drop across the low-side FET
VMin
Minimum voltage between VB and VS
Qls
level shift charge required per
xi
CHAPTER ONE INTRODUCTION 1.1 BACKGROUND OF THE STUDY
Power instability in our environment pushes us to generate our own power. Developing countries which form a majority in African continent have the problem of unstable power supply, the Nigerian power grid also faces this problem. Nigeria has some major challenges in its power system: these include Insufficient power generation, overloading of distribution transformers, vandalism, power infrastructure problems, and so on. Which brings us to the importance of generating power on the sides to compliment the power supply. Of the several methods that can be employed in generating self-power, Renewable energy sources form the largest part. In renewable energy literature, the words “alternative” and “sustainable” are being used to emphasize the possibility of using sources other than the fossil ones which will never finish. Whatever they are called, “renewable”, “alternative” and “sustainable”, these energy systems are planned as the parts of the solution to the energy problem by substituting the fossil sources with the replenished ones. Renewable energy is the energy obtained from the continuous or repetitive currents of energy recurring in the natural environment. To assume any energy flow as “renewable” it should be replenished at least at the same rate as it is used. A renewable energy system is an “alternative” one, if it is able to provide some or whole part of the energy needs which is met by the fossil ones. Such a system is also sustainable because the energy supply will sustain continuously, as the sun continues to shine, gravity applies on objects and the earth rotates. Fossil fuels, on the other hand, like coal, petroleum and gas are the conventional ones having no sustainability. Although problem arise from the type of power generated from renewable energy sources; in the form of its inconsistency and its output form (direct current) hence the need of an inverter to be able to convert the DC energy formed to a steady constant frequency Alternating current form needed for home and industrial applications. Power inverters are used to convert direct current (DC), such as that from a battery, to alternating current (AC), such as what can be drawn from a standard wall outlet. These devices are used in a large number of applications, including conditioning energy derived
1
from solar cells into usable power and supplying electricity to household appliances during power outages.
1.2 STATEMENT OF PROBLEM
Due to the unstable nature of the electricity supply received by consumers of electricity in Nigeria, many consumers depend on the use of motor generating set as an alternative source of electricity supply which is noisy, expensive to maintain and causes a lot of pollution.
This research work propose the use of Inverter which is preferred to motor generating sets because it is noiseless, relatively small in size, maintenance free and pollution-free.
1.3 AIM AND OBJECTIVES
The aim of this project is to design and implement a pure sinewave single phase 3kva inverter with controlled output which can produce AC power outputs at high efficiency. The objectives of this project is to design an inverter that will produce an output up to 3 KVA with input from a 24 V battery and can be used to power AC loads. The Objectives of the system are
Generation of a pure sine wave output.
Use of microcontroller as the main circuit control component hence reducing the complexity of the circuit.
Having a controlled output.
To design a system that will make electric power available for 24 hours.
To design a system that will automatically switch of heavy loads when the back-up battery drops to a certain level.
To design a system that is noiseless.
To design a system that is maintenance free.
To design a system that is pollution free.
1.4 SCOPE/LIMITATIONS OF THE PROJECT
This project work focuses on the design of a single phase inverter which can deliver a maximum power of 3 KVA including the losses by converting the 24 VDC voltage from a battery bank to an output voltage of 230 (±30V).
2
1.5 APPLICATIONS OF PROJECT
This design is suitable for domestic applications, for inductive loads like air conditioners, refrigerators, air conditioners, pumps, etc. for loads which are very sensitive.
1.6 OUTLINE OF THE REMAINING CHAPTERS
This Project has five (5) chapters, each having its own importance. Chapter
1:
Introduction
-
Justifies
the
need
for
the
project
and
states
the
requirements/specifications of the project. Chapter 2: Literature review - Reviews some existing Inverter systems and the principles behind the design implemented in this project. Chapter 3: Methodology and system design - Gives a detailed account of the design process. Chapter 4: Construction, Tests and Results – Discusses the hardware implementation of the design, the results and problems encountered. The Bill of Measurement/Evaluation is also given in this chapter . Chapter 5: Conclusion and Recommendations - Summarizes the observations and inferences made in the course of the project. Recommendations for further work is also given in this chapter.
3
CHAPTER TWO LITERATURE REVIEW 2.1 REVIEW OF EXITING SYSTEMS
The design of a dc to ac voltage source inverters can be done using several energy conversion topologies. Suryawanshi et al. (2006); Abolarinwa and Gana (2010), Sekar and Baladhandapani (2013) designed and inverter circuit using Push pull, half-bridge and full bridge converters. Push-pull converters are restricted to low DC supply voltage applications because of their tendency to develop magnetic flux walking problems that can lead to failure. They are however commonly employed because of their rather simple structure compared to that of full-bridge converter circuits. Wuidart (1999) in his work reported that Full-bridge converters are used in high power applications where their higher component count and the associated cost are of secondary importance. Voltage source inverter systems can also be classified based on their output waveform type. The earliest power inverters produce square wave output waveforms. Square waveforms are easily and efficiently produced at high power levels. However, the high harmonic content implies that they cannot be used to power harmonic sensitive loads. A version of square wave inverters is the modified sinewave inverters. Inspite of the term, a ‘modified sinewave’ inverter produces a waveform that looks more like a square wave than a sinewave. Pure sinewave inverters are by far the best, because their output waveforms have very little harmonic content, though they are somewhat more complex to design compared to square wave and modified squarewave inverters. Consequently, they are much more costly. Kolla et al. (2013) provides a detailed explanation on the performances and characteristics of the various inverter types.
2.2 REVIEW OF PRINCIPLES BEHIND THIS PROJECT 2.2.1 PULSE WIDTH MODULATION:
Pulse width modulation (PWM) is a powerful technique for controlling analogue with a processor’s digital outputs. It is also known as pulse duration modulation (PDM). The leading edge of the carrier pulse remains fixed and the occurrence of the trailing of the pulses varies. PWM signals find a wide application in modern electronics. Some of these reasons are:
Easy to Generate – PWM signals are quite easy to generate. Many modern microcontrollers include PWM hardware within the chip; using this hardware often takes very little attention from the microprocessor and it can run in the background 4
without interfering with executing code. PWM signals are also quite easy to create directly from a comparator only requiring the carrier and the modulating signals input into the comparator.
Digital to Analogue Conversion – pulse width modulation can function effectively, as a digital to analogue converter, particularly combined with appropriate filtering. The fact that the duty cycle of a PWM signal can be accurately controlled by simple counting procedures is one of the reasons why PWM signals can be used to accomplish digital-to-analogue conversion.
A digital microcontroller PWM requires a reference signal, sometimes called a modulating or control signal, which is a sinusoidal in this case; and a carrier signal, which is a triangular wave that controls the switching frequency. Microcontroller modules are used to compare the two to give a PWM signal. The applications of PWM are wide variety used like ranging from measurement and communications to power control and conversion. In PWM inverter harmonics will be much higher frequencies than for a square wave, making filte ring easier. In PWM, the amplitude of the output voltage can be controlled with the modulating waveforms. Reduced filter requirements to decrease harmonics and the control of the output voltage amplitude are two distinct advantages of PWM. Disadvantages include more complex control circuits for the switches and increased losses due to more frequent switching. 2.2.2 H-BRIDGE CONFIGURATION
A H-Bridge or full-bridge converter is a switching configuration composed of four switches in an arrangement that resembles a H. By controlling different switches in the bridge, a positive, negative, or zero potential voltage can be placed across a load. When this load is a motor, these states correspond to forward, reverse, and off. The use of an H-Bridge configuration to drive a motor is shown
5
As shown the H-Bridge circuit consists of four switches corresponding to high side left, high side right, low side left, and low side right. There are four possible switch positions that can be used to obtain voltages across the load. These positions are outlined in Table 1. Note that all other possibilities are omitted, as they would short circuit power to ground, potentially causing damage to the device or rapidly depleting the power supply.
Table 2.1: Valid H-Bridge Switch State
High Side Left
High Side Right
Low Side Left
Low Side Right
Voltage
Across
Load On
Off
Off
On
Positive
Off
On
On
Off
Negative
On
On
Off
Off
Zero Potential
Off
Off
On
On
Zero Potential
The switches used to implement an H-Bridge can be mechanical or built from solid state transistors. Selection of the proper switches varies greatly. The use of P-Channel MOSFETs on the high side and N-Channel MOSFETs on the low side is easier, but using all N-Channel MOSFETs and a FET driver, lower “on” resistance can be obtained resulting in reduced power loss. The use of all N-Channel MOSFETs requires a driver, since in order to turn on a high-side N-Channel MOSFET, there must be a voltage higher than the switching voltage. This difficulty is often overcome by driver circuits capable of charging an external capacitor to create additional potential. 2.2.3 ANALOGUE TO DIGITAL CONVERSION
Analogue signals are usually fundamentally different from those the microcontroller understands (ones and zeros) and have to be converted therefore into values understandable for the microcontroller. An analogue to digital converter is an electronic circuit which converts continuous signals to discrete digital numbers i.e. into a binary number and passes it to the CPU for further processing (Verle, 2009). Similarly, the outputs are always in digital form, thus when a digital system such as the microcontroller is used to monitor and/or control a physical process as in the case of this project, where it monitors the input voltage and controls the switching of relays connected to taps of an autotransformer, we must bridge the
6
gap between the analogue and digital nature of the system variables using Analog-to-digital converters (ADC) and digital-to-analogue converters (DAC) to create an interface. 2.2.4 COMPUTER PROGRAMMING
Programming of a computer device is the developing and implementing of specific instruction sets that will enable the device perform desired tasks. It is getting increasingly easy to do this with the development of several programming languages, compilers and simulation software for developing and testing a program. A microcontroller is a special purpose computer, possessing a central processing unit, several memory units and a bunch of peripheral devices (Verle, 2009). It is not a stand-alone device, it is normally embedded into another device and must be programed accordingly. Without a program, the microcontroller is idle and useless. In this project, the microprocessors PIC18F2550 and PIC16F873A has been used for the control of the inverter. The programs for the microcontrollers were written in C language and compiled using a mikroC PRO compiler for PIC.
2.3 COMPONENTS AND THEIR RELLEVANCE TO THE DESIGN 2.3.1 MOSFET DRIVERS
When utilizing N-Channel MOSFETs to switch a DC voltage across a load, the drain terminals of the high side MOSFETs are often connected to the highest voltage in the system. This creates a difficulty, as the gate terminal must be approximately 10V higher than the drain terminal for the MOSFET to conduct. Often, integrated circuit devices known as MOSFET drivers are utilized to achieve this difference through charge pumps or bootstrapping techniques. These chips are capable of quickly quickl y charging the input capacitance ca pacitance of the MOSFET (Cgiss) quickly before the potential difference is reached, causing the gate to source voltage to be the highest system voltage plus the capacitor voltage, allowing it to conduct. A diagram of an N- channel MOSFET with gate, drain, and source terminals is shown in Figure 3.
7
There are many MOSFET drivers available to power N-Channel MOSFETs through level translation of low voltage control signals into voltages capable of supplying sufficient gate voltage. Advanced drivers contain circuitry for powering high and low side devices as well as N and P-Channel MOSFETs. In this design, all MOSFETs are N-Channel due to their increased current handling capabilities. To overcome the difficulties of driving high side NChannel MOSFETs, the driver devices use an external source to charge a bootstrapping capacitor connected between Vcc and source terminals. The bootstrap capacitor provides gate charge to the high side MOSFET. As the switch begins to conduct, the capacitor maintains a potential difference, rapidly causing the MOSFET to further conduct, until it is fully on. The name bootstrap component refers to this process and how the MOSFET acts as if it is “pulling itself up by its own boot strap”. 2.3.2 MICRO-CONTROLLER MICRO-CONTROLLER
A microcontroller (also microcontroller unit, MCU or μC) is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog, serial and analogue I/O etc. Neither program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a, typically small, read/write memory. Microcontrollers are designed for small applications. Thus, in contrast to the microprocessors used in personal computers and other high-performance applications, simplicity is emphasized. Some microcontrollers may operate at clock frequencies as low as 32 kHz, as this is adequate for many typical applications, enabling low power consumption (milliwatts or microwatts). They will generally have the ability to retain functionality while waiting for an event such as a button press or other interrupt; power consumption while sleeping (CPU clock and most peripherals off) may be just nanowatts, making many of them well suited for long lasting battery applications. Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, remote controls, office machines, appliances, power tools, and toys. By reducing the size and cost compared to a design that uses a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to digitally control even more devices and processes. In order to use the H-bridge properly, there are four MOSFETs that need to be controlled. This can be done either with analogue circuits or a microcontroller. In this case, we chose the microcontroller over the analogue system for several reasons: 8
It would be simpler to adapt. With an analogue system, it would be difficult to make changes for the desired output. In many cases, this is a desired trait, as it would be designed for a single purpose and therefore a single output. However, as this is something that is designed to be available all over the world, it needs to be adjustable to different standards of frequency and voltage. With an analogue circuit, this would require a different circuit that it would have to switch over to, while with a microcontroller, it merely requires a change c hange in the program’s code.
It can allow for easy feedback to control the power flowing through the load. One of the problems that can occur with systems like this is that the variances in load can cause variances in the supplied current and voltage. With a microcontroller, it is possible to have it “look” at the power output and change the duty cycle based on whether or not the load requires additional power or is being oversupplied.
2.3.3 TRANSFORMER
A transformer is an electrical device that transfers energy between two circuits through electromagnetic induction. A transformer may be used as a safe and efficient voltage converter to change the AC voltage at its input to a higher or lower voltage at its output. Other uses include current conversion, isolation with or without changing voltage and impedance conversion. A transformer consists of two windings of wire that are wound around a common core to provide tight electromagnetic coupling between the windings. The core material is often a laminated iron core. The coil that receives the electrical input energy is referred to as the primary primar y winding, the output coil is the secondary winding. The ideal transformer induces secondary voltage VS as a proportion of the primary voltage VP and respective winding turns as given by the equation
= = = a
2.1
Where, a is the winding turns ratio, Vp is the primary/source voltage, Vs is the secondary voltage, Ns is the number of secondary turns Np is the number of primary turns
9
2.3.4 VOLTAGE REGULATOR
A voltage regulator is designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages. Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. Linear regulators are based on devices that operate in their linear region (in contrast, a switching regulator is based on a device forced to act as an on/off switch). In the past, one or more vacuum tubes were commonly used as the variable resistance. Modern designs use one or more transistors instead, perhaps within an integrated circuit. Linear designs have the advantage of very "clean" output with little noise introduced into their DC output, but are most often much less efficient and unable to step-up or invert the input voltage like switched supplies. All linear regulators require a higher input than the output. If the input voltage approaches the desired output voltage, the regulator will "drop out". The input to output voltage differential at which this occurs is known as the regulator's drop-out voltage. Lowdropout regulators (LDOs) allow an input voltage that can be much lower (i.e., they waste less energy than conventional linear regulators). Entire linear regulators are available as integrated circuits. These chips come in either fixed or adjustable voltage types. 2.3.5 RELAY
A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. 10
Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal. Magnetic latching relays require one pulse of coil power to move their contacts in one direction, and another, redirected pulse to move them back. Repeated pulses from the same input have no effect. Magnetic latching relays are useful in applications where interrupted power should not be able to transition the contacts. Magnetic latching relays can have either single or dual coils. On a single coil device, the relay will operate in one direction when power is applied with one polarity, and will reset when the polarity is reversed.
electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two contacts in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the figure 5 above also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature. When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is 11
switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing. When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components. 2.3.6 MOSFETS
A power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, for example an insulated-gate bipolar transistor (IGBT) or a thyristor, its main advantages are high switching speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive. They can be subject to low gain, sometimes to degree that the gate voltage needs to be higher than the voltage under control.
Power MOSFETs have a different structure from the lateral MOSFET: as with most power devices, their structure is vertical and not planar. In a planar structure, the current and breakdown voltage ratings are both functions of the channel dimensions (respectively width and length of the channel). With a vertical structure, the voltage rating of the transistor is a function of the doping and thickness of the N epitaxial layer, while the current rating is a function of the channel width. This makes it possible for the transistor to sustain both high blocking voltage and high current within a compact piece of silicon.
12
For the MOSFET to carry drain current Id (on state) a channel between the drain and source must be created. This occurs when drain to source Vgs voltage exceeds the device threshold (Vgs>Vth). Once the channel is induced the MOSFET can operate in either triode region (drain current proportional to channel resistance) or the saturation region for the MOSFET to carry drain current Id (on state) a channel between the drain and source must be created. This occurs when drain to source Vgs voltage exceeds the device threshold (Vgs>Vth). Once the channel is induced the MOSFET can operate in either triode region (drain current proportional to channel resistance) or the saturation region 2.3.7 OCTO-COUPLER
In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is a component that transfers electrical signals between two isolated circuits by using light. 13
Opto-isolators prevent high voltages from affecting the system receiving the signal. Commercially available opto-isolators withstand input-to-output voltages up to 10 KV and voltage transients with speeds up to 10 KV/s. A common type of opto-isolator consists of an LED and a phototransistor in the same opaque package. Other types of source-sensor combinations include LED- photodiode, LED-LASCR, and lamp-photoresistor pairs. Usually opto-isolators transfer digital (on-off) signals, but some techniques allow them to be used with analogue signals. An opto-isolator contains a source (emitter) of light, almost always a near infrared light-emitting diode (LED), that converts electrical input signal into light, a closed optical channel (also called dielectrical channel and a photosensor, which detects incoming light and either generates electric energy directly, or modulates electric current flowing from an external power supply. The sensor can be a photoresistor, a photodiode, a phototransistor, a silicon-controlled rectifier (SCR) or a triac. Because LEDs can sense light in addition to emitting it, construction of symmetrical, bidirectional opto-isolators is possible. An optocoupled solid state relay contains a photodiode opto-isolator which drives a power switch, usually a complementary pair of MOSFETs. A slotted optical switch contains a source of light and a sensor, but its optical channel is open, allowing modulation of light by external objects obstructing the path of light or refle cting light into the sensor.
14
CHAPTER 3 METHODOLOGY AND SYSTEM DESIGN 3.1 METHODOLOGY
The method employed for the design of the pure sine wave inverter involves converting the DC input voltage to AC through the use of PWM generated by the microcontroller, then the output AC is stepped up to the desired output. This project can be complex when looked at as a whole, but can be much easier to analyse when broken into smaller divisions. As will be done in the following sections. 3.1.2 BLOCK DIAGRAM
15
From the block diagram, the various circuits needed for the design are shown, the whole circuit can be divided into two parts as a whole: namely the control circuit and the oscillator circuit. While the oscillator circuit is responsible for the generation of the pure sine wave, the control circuit act as the brain behind the automation of the rest of the circuit. The ensuing sections show the individual circuit design and how every part is pieced together.
3.2 THEORY AND CALCULATIONS 3.2.1 SIGNAL GENERATOR
The PWM signal to be used in the circuit is produced by a microcontroller, most powerful microcontrollers have the ability to produce up to 4 PWMs, for instance the PIC18F2550 chip from pic has this ability. The PORT C of this microcontroller has 2 outputs that produces PWM, while another PORT could be used to produce the complimentary signal. For the production of the PWM 32 were picked for this project, for calculating the value of the duty cycle for the samples we use the formula
× ; The corresponding angles are gotten by dividing the angle for one half cycle by total number of samples.
16
Assuming our maximum duty cycle is given as 250 then the first 16 sample s are: Table 3.1: duty cycles for PWM
Angle
Formula
Duty cycle
0o
0 × 250 5.625 ×250 11.25 ×250 16.875 × 250 22.5 ×250 28.125 × 250 33.75 ×250 39.375 × 250 45 ×250 50.625 × 250 56.25 ×250 61.875 × 250 67.5 ×250 73.125 × 250 78.75 ×250 84.375 × 250 90 ×250 95.625 × 250
0
5.625o 11.25o 16.875o 22.5 o 28.125o 33.75o 39.375o 45o 50.625o 56.25o 61.875o 67.5 o 73.125o 78.75o 84.375o 90o 95.625o
25 50 75 96 118 139 159 177 193 208 220 231 239 245 249 249 250
The 16 samples are then repeated to give a total of 32 samples, to form a half wave at 50 Hz. The microcontroller is set to produce the PWM at a chosen frequency of 3kHz and also 2 other sine waves at 50 Hz each, for the PIC16F873A chip for instance: The CCP modules in built in the microcontroller were not used in this project, but a software PWM was written in other to have full control. This would mean that more than 4 PWM can be produced and the output pins could be any of the controller pins rather than from the designated CCP output pins. The advantage of this would be in charging the batteries, since only the low side MOSFETs need to be switched to give appropriate charging current. This will be explained in the ensuing sections.
17
3.2.1A PWM GENERATION TECHNIQUE AND REGULATION
The software PWM created uses the delay function in the microC compiler, to get a frequency of 50Hz each duty cycle has to last for 625µs. T (period of sine wave) =
= = 0.02s
3.10
. = 0.01s . T p (period of PWM) = = = 312µs
For a half cycle =
3.11 3.12
A duty cycle is in simple terms the ratio of the time on to period of the PWM; so each of the duty cycles previously calculated show how long the pins should stay high relative to how long they should stay low. Hence any pin of the microcontroller could be made to stay high for that period of time as calculated for each duty cycle value, and stay off for the rest of the period. Time period on is calculated by
× percentage time on
Table 3.2: duty cycle in terms on of time on
Duty cycle
Percentage time on
Time period on
0o
0
0
25
0.10 0.196 0.292 0.384 0.472 0.556 0.636 0.708 0.772 0.832 0.88 0.924 0.956 0.98 0.996 0.98
30
49 73 96 118 139 159 177 193 208 220 231 239 245 249 250
18
60 90 119 147 173 198 220 240 259 274 288 298 305 310 312
The designated PWM are then varied to be on at the following times as calculated in the table above and be, low for the rest of the period. The PWM signal generated is regulated by a feedback technique which is one of the most critical aspects of the PWM generation without the regulation the output voltage becomes excessively high at start and may fall to be very low when load is applied else the need for the regulation, the regulation is done by a feedback circuit whose function is to detect the output voltage and make adjustments to the duty cycles: if they should be increased or decreased. The circuit configuration for the feedback is shown in ensuing sections.
19
3.2.1 B WHY PIC MICROCONTROLLER
PIC18F2550 and PIC18f877a have the Harvard architecture. Harvard architecture is a newer concept than von Neumann. It rose out of the need to speed up the work of a microcontroller. In Harvard architecture data bus and address bus are separate. Thus a greater flow of data is possible through the central processing unit and of course a greater speed of work.
It is also typical for Harvard architecture to have fewer instructions than von-Neumann's, and to have instructions usually executed in one cycle. Microcontrollers with Harvard architecture
20
are also called "RISC microcontrollers". Figure 7 presents the internal block of the PIC18F2550. And Figure 8 represents the Internal block of the P IC16F873A
21
For this design two chips are chosen, one for the LCD display and the other for the oscillation and the control of the entire circuit.
PIC chips are readily available in the market so there will be no need to incur extra cost of shipping.
The relative ease of programming with the PIC compiler which uses the C language which we are familiar with.
The PIC18F2550 has a stable internal oscillator up to 8MHz, and has a large memory space of 64KBs
3.2.2 H-BRIDGE
In other to generate a sine wave with both the positive voltage half and the negative voltage half with a centre zero volt from a single source require the use of a four switches arranged in an H configuration. The switches chosen in this case is the MOSFET. 3.2.2 A IGBTS VERSUS MOSFETS
In designing this circuit, a choice had to be made on with switch would be more suitable for use. One is the power MOSFET the other is the insulated gate bipolar transistor (IGBT). Each has its advantages, and there is a high degree of overlap in the specifications of the two. IGBTs tend to be used in very high voltage applications, nearly always above 200V, and generally above 600W. They do not have the high frequency switching capability of MOSFETs, and tend to be used at frequencies lower than 29 kHz. They can handle high currents, are able to output greater than 5kW, and have very good thermal operating ability, being able to operate properly above 100 Celsius. One of the major disadvantages of IGBTs is their unavoidable current tail when they turn off. Essentially, when the IGBT turns off, the current of the gate transistor cannot dissipate immediately, which causes a loss of power each time this occurs. This tail is due to the very design of the IGBT and cannot be remedied. IGBTs also have no body diode, which can be good or bad depending on the application. IGBTs tend to be used in high power applications, such as uninterruptible power supplies of power higher than 5kW, welding, or low power lighting. Power MOSFETS have a much higher switching frequency capability than do IGBTs, and can be switched at frequencies higher than 200 kHz. They do not have as much capability for high voltage and high current applications, and tend to be used at voltages lower than 250V and less than 500W. MOSFETs do not have current tail power losses, which makes them more efficient than IGBTs. Both MOSFETs and IGBTs have power losses due to the ramp up 22
and ramp down of the voltage when turning on and off (dV/dt losses). Unlike IGBTs, MOSFETs have body diode. Generally, IGBTs are the sure bet for high voltage, low frequency (>1000V, <20 kHz) uses and MOSFETs are ideal for low voltage, high frequency applications (<250V, >200 kHz). In between these two extremes is a large grey area. In this area, other considerations such as power, percent duty cycle, availability and cost tend to be the deciding factors. Since this project is about design of a 600W inverter, with a 24DC bus, and a switching frequency of 3 kHz MOSFET is the ideal choice, in spite of MOSFET switches having high ON state resistance and conduction losses. Also MOSFET being a voltage controlled device, it can be driven directly from CMOS or TTL logic and the same gate signal can be applied to diagonally opposite switches since the gate drive current required is very low. 3.2.2 B ENHANCED N-CHANNEL VERSUS ENHANCED P-CHANNEL MOSFETS
The use of P-Channel MOSFETs on the high side and N-Channel MOSFETs on the low side is easier, but using all N-Channel MOSFETs and a FET driver, lower “on” resistance can be obtained resulting in reduced power loss. This requires a more complex circuit since the gate of the high side MOSFETs must be driven positive with respect to Vs bus voltage to turn on the MOSFETs. 3.2.2 C MOSFET, GATE RESISTOR AND DC-LINK CAPACITOR SELECTION
a.
MOSFET selection
In the H-bridge design 4 MOSFETs are used on each side of the bridge therefore bringing it all to a total of 16 MOSFETs. The Maximum input current is given by: I=
= = 125A.
3.13
So each side of the bridge should be able to c arry a maximum current of 125A, for this design the MOSFET Irfp260n was chosen, having a maximum rating of 40A. four of the MOSFETs where paralleled together on each side, so that the input capacity is now 160A. This is enough to handle the current sufficiently.
b.
Gate resistor selection
Driving MOS-gated power transistors directly from the driver c an result in unnecessarily high switching speeds. Increasing the value of the series gate resistor, results in a rapid decrease of the amplitude of the negative spike, while the turn-off time is a linear function of the series gate resistance. Selecting a resistor value just right from the “knee” in Figure 3.10 provides a 23
good trade-off between the spike amplitude and the turn-off speed the di/dt may have to be reduced by reducing the switching speed by means of the gate resistor. A graph of the negative spike and the turn-off time versus series gate resistance is shown in Figure 13 The layout should also minimize the stray inductance in the charge/discharge loops of the gate drive to reduce oscillations and to improve switching speed and noise immunity, particularly the “dV/dt induced turn-on”. For this design resistor values of 100 ohms were chosen.
c.
DC-LINK capacitor selection
The bus link capacitor is used in DC to AC inverters to decouple the effects of the inductance from the DC voltage source to the power bridge. The bus link capacitor provides a low impedance path for the ripple currents associated with a hard switched inverter. The ripple currents are a result of the output inductance of the load, the bus voltage and the PWM frequency of the inverter. The ripple currents have been the primary factor in sizing the electrolytic bus link capacitor. The bus link capacitor also plays a role in reducing the leakage inductance of the inverter power bridge. Leakage inductance in an inverter power bridge leads to inefficiencies due to the voltage spikes they produce when the power devices ar e switched on and off at a high rate of dI/dt. If the leakage inductance gets too large, the switching time of the power switches must be increased to keep the voltage spikes from damaging the power devices. Increasing the switching time of the power devices increases the turn on and turn off losses in each of the power switches contributing to more switching losses which manifest themselves in extra heat dissipation in the switching devices. The first step in sizing capacitors for inverter bus link applications should be to understand how much bus link capacitance is required for a given inverter design. The biggest design limitation for electrolytic capacitors in inverter 24
applications has been the amount of ripple current that the electrolytic capacitor can sustain. This limits the design criteria of the designer to figuring out how many individual capacitors are required for a given design rather than the total amount of capacitance that is required. To calculate the ripple current: The source inductance is usually large enough to limit the high frequency ripple current. Therefore, the ripple current in the bus link capacitor is essentially the same as the ripple current in the phase leg as illustrated in Figure 14. When the top left and bottom right switches are turned on the current flows from the bus link capacitor through the load via top left and bottom right switches and returns to the bus link capacitor. Similarly, the current flows from the bus link capacitor through the load when the bottom left and top right switches are turned on. The bus link capacitance selection is based on the maximum bus ripple voltage desired.
The maximum ripple voltage is when the duty cycle is at 50%, so that maximum peak to peak ripple voltage is given as ΔV0.5t = V bus / (32
L
C
f 2).
3.14
Choosing an allowable 1% for the ripple voltage we can calculate the DC link capacitor value; 1% of the bus voltage is 0.2. Re-arranging C=
× × × .
3.15
The primary inductance Is calculated as 25
Transformer core area a = 0.00063m2 From the core used B = 1.3T So that flux linkage ɸ = ɸ = 1.3
3.16
×
3.17
× 0.00063 = 0.00819 ɸ
The secondary winding inductance L = N
3.18
Where I is the input current calculated earlier as 138.89A . = 943µH . Then C = × 0.00000943
L = 24 ×
×.×
= 441µF
The maximum ripple current is given as ΔI 0.5t = 0.25 ×
3.19
( × )
Hence the maximum ripple current is calculated as ΔI 0.5t = 0.25
= 2.12A × ( × .)
For this project a capacitance 4, 4700µF capacit ors were used in parallel to efficiently handle the ripple current and the ripple voltage. 3.2.3 MOSFET DRIVER
For the MOSFET switch to be turned on the voltage at the gate terminal must be 10V higher than the drain terminal voltage. the drain of the high side device is connected to 24V DC power which is to be inverted into the 230V AC power. This is a problem because the 24V is the highest voltage in the system therefore, to drive MOSFETs in the H-Bridge MOSFET driver IC is used with a bootstrap capacitor specifically designed for driving a half-bridge. For this design the IR2110 MOSFET driver was chosen, it is rated at 600V, with a gate driving current of 2A and a gate driving voltage of 10-20V. The turn on and turn off ti mes are 120ns and 94ns respectively. The MOSFET driver operates from a signal input given from the microcontroller and takes its power from the battery voltage supply that the system uses. The driver is capable of operating both the high side and low side devices, but in order to get the extra 10V for the high side device, an external bootstrap capacitor is charged through a diode from the 18V power supply when the device is off. Because the power for the driver is supplied from the low voltage source, the power consumed to drive the gate is small. When the driver is given the signal to turn on the high side device, the gate of the MOSFET has an extra boost in charge from the bootstrap capacitor, surpassing the needed 10V to activate the device and turning the switch on. 26
3.2.3A BOOTSTRAP CAPACITOR
The bootstrap diode and capacitor are the only external components strictly required for operation in a standard PWM application. Local decoupling capacitors on the VCC (and digital) supply are useful in practice to compensate for the inductance of the supply lines. The voltage seen by the bootstrap capacitor is the VCC supply only. Its capacitance is determined by the following constraints: 1. Gate voltage required to enhance MGT 2. IQBS - quiescent current for the high-side driver circuitry 3. Currents within the level shifter of the control IC 4. MGT gate-source forward leakage current 5. Bootstrap capacitor leakage current Factor 5 is only relevant if the bootstrap capacitor is an electrolytic capacitor, and can be ignored if other types of capacitor are used. Therefore, it was ignored since only nonelectrolytic capacitors were used. The minimum bootstrap capacitor value was calculated from the following equation:
C ≥
cbs(leak) [ qbs(max)
3.20
– – – min
Where: Qg = Gate charge of high-side FET = 63nC f = frequency of operation = 3000Hz ICbs (leak) = bootstrap capacitor leakage current = 250μA Iqbs (max) = Maximum VBS quiescent current = 230μA VCC = Logic section voltage source = 24V Vf = Forward voltage drop across the bootstrap diode = 0.4V 27
VLS = Voltage drop across the low-side FET = 1.8V VMin = Minimum voltage between VB and VS = 10V Qls = level shift charge required per cycle (typically 5nC for 500 V/600 V MGDs and 20nC for 1200 V MGDs) The values substituted into this equation were found either in driver datasheet for IR2110 IC or IRFP260N MOSFET datasheet. Using these numbers minimum bootstrap capacitance value was calculated as
≥
× [ × ×
× ×
– . – . –
≥ 0.025 The capacitor value obtained from the above equation is the absolute minimum required, however due to nature the bootstrap circuit operation, a low value of capacitor can lead to overcharging which could in turn damage the IC. Therefore, to minimize the risk of overcharging and further reduce ripple on the Vds voltage the capacitor value obtained is multiplied by a factor of 15 to get a capacitor value of 0.3μF, but the design a capacitor value
of 47 f was used. 3.2.3B BOOTSTRAP DIODE
The bootstrap diode must be able to block the full voltage seen in the specific circuit and is about equal to the voltage across the power rail. The current rating of the diode is the product of gate charge times switching frequency. The high temperature reverse leakage characteristic of this diode can be an important parameter in those applications where the capacitor has to hold the charge for a prolonged period of time. For the same reason it is important that this diode have an ultra-fast recovery to reduce the amount of charge that is fed back from the bootstrap capacitor into the supply. In order to improve decoupling a decoupling capacitors has to be connected directly across the VCC and COM pins as shown in figure 12. The diode chosen for this design is the In4148 diode which meets the specifications given above.
28
3.2.4 TRANSFORMER CALCULATIONS
The following calculations were followed in designing the transformer, for a 3KVA design: The calculations were based on the following formulas: Primary winding calculations 1) Core area = 1.152
× √( × )
3.21
2) Calculating turns per volt (tpv) TPV = 1
(4.44 × 10 × × × )
3) Primary winding current =
( × ) × (.)
3.22 3.23
4) Number of turns = TPV × primary volts
3.24
5) Primary winding area = ⁄ .
3.25
Secondary winding calculation
1.04 × ( × ) Secondary winding area = ⁄ .
6) Secondary number of turns =
3.26
7)
3.27
Actual calculated values
× √(3000) = 1.152 × 57.44 = 63.1 cm 2 1 = 1 (4.44 × 10 × 63.1 × 1.3 × 50) = 1.82 = 0.5
1. Core area = 1.152 2. Turns per volt
3. Primary winding current =
() = 138.89 A ( × .)
4. Primary copper wire thickness = 8 SWG 5. Number of turns for primary winding= 2 4 / 0.5 = 48
29
6. Secondary winding current =
= 14.49 A ×.
7. Secondary number of turns =
1.04 × 2 × 230 = 478 turns
8. Primary copper wire thickness = 17 SWG 3.2.5 OUTPUT FILTER
The final component necessary to output a pure sine wave signal is an output filter. For our circuit we need a basic LC low pass filter with the following setup below in Figure
This will filter out all the excess noise above the critical frequency. The goal for this is to bring the critical frequency as close as possible to the desired frequency of 50 Hz, removing other harmonics that crop up within the system. The lower the cut-off frequency, the greater the capacitance and inductance required to properly create the filter. The output of the H bridge was ideally a 50Hz sine wave. Because it was encoded using a 3 kHz PWM signal it was to be filtered. Due to the high current expected to be sourced by the load of our output, the only option is a passive low pass filter, which is an inductor and capacitor in series, with the load connected across the capacitor designed for passing all signals under 50Hz. Fc =
√
3.28
Since the secondary side of the transformer has an inductance all that was needed was just a shunt capacitor. From the earlier calculations Transformer core area a = 0.00063m2 From the core used B = 1.3T So that flux linkage ɸ = ɸ = 1.3
×
× 0.00063 = 0.00819
The secondary winding inductance L = N L = 478
×
ɸ
. = 0.27H .
Hence the capacitor value needed from the formula above is calculated from the formula C=
= = 3.7 × × .
× 10 = 3.7µF 30
For the design the available 4.7µF capacitor was used 3.3.1 PROGRAM FLOWCHART
The program written for the microcontroller functions by executing instructions in a specified order, thus the program instructions must be organized in the desired order to avoid improper co-ordination of events during operation. While the source code of the program is given in Appendix I of this report the flowchart given below shows the core instructions in the program and the order in which they will be executed by the microcontroller.
31
32
3.3 MODE OF OPERATION
In free running mode (when the inverter is not on or when there is no mains), the battery voltage is used to power the circuit. It is connected through a 10 Ω choke resistor which acts as a fuse for circuit protection. Voltage is supplied to the circuit through the 5 V and
12 V
voltage regulators. The 5 VDC is fed to the microcontroller and to the MOSFET driver and to the LCD. The 12V regulator is used to power the relays and also act as a bias voltage for the MOSFET driver. In inverter mode, the microcontroller senses the battery voltage through the voltage divider circuit made up of 100 KΩ and 1 KΩ resistor. If the battery voltage is above the low battery value of 21 V, the microcontroller sends the oscillations to the H-bridge which in turn uses the oscillations to convert the battery DC voltage to an AC voltage, the AC voltage is then stepped-up by the transformer TR2 to a nominal voltage of 230 V. The output of the transformer TR2 is then passed through a filter capacitor to reduce the high harmonic component of the AC wave. The output from the filter is then stepped-down through another transformer TR1 to provide feedback to the microcontroller. The output of the step down transformer is rectified and then passed through a voltage divider circuit made up of 100 KΩ and 10 KΩ resistor. The function of the feedback is to ensure that the output voltage is relatively fixed at 230 VAC with ± 30 VAC variation. The microcontroller switches the controlled output relay to ensure that the load powered by the controlled output is ON as long as the battery voltage is above 23 VDC. Once the battery voltage falls below 23 VDC, the controlled output is disconnected and only the main output is allowed to be powered. Under the condition where with the battery falls below 21 VDC which is the stated low battery, the main output is cut off. The maximum current drawn is calculated as the total power divide by the input voltage, which is 3 KVA divided by 24 VDC, which gives a total of 125 AMPS. Therefore the overload current is set at 110 AMPS to allow for inaccuracy. Under the condition that the input current rises above 110 AMPS, the oscillation is cut off and the condition could either be read as an overload or a short circuit. In mains mode, the microcontroller senses the presence of a mains input through the help of an optocoupler U1. The 230 VAC rectified passes through a voltage divider so that a proper voltage could be used to bias the optocoupler U1. When there’s mains, 5 VDC is sent to the designated mains pin of a microcontroller through the optocoupler. This way the microcontroller is aware of the presence of mains and switches the outputs to be powered directly from mains while also switching mains into the transformer TR2, hence the 33
transformer acts as a step down transformer in this case and steps down the 230 VAC to the appropriate 24 VAC needed to charge the battery. In this mode the H-bridge is configured as a buck converter using the lower side MOSFET as a switch, the higher side MOSFET as a diode then using the transformer as both a source and an inductor while using the DC link capacitor as ripple capacitors. The low side MOSFET are switched at the same time with a frequency of above 7 KHz. The microcontroller monitors the input current through the current transformer TR3 to ensure that the charging current is always within 9 AMPS to 10 AMPS during normal charge. When the battery voltage rises up to 27 VDC, the microcontroller switches the low side MOSFET in the buck converter configuration to ensure that the charging current is within 0.7 AMPS to 1 AMP. Charging continues as long as the battery voltage is less than the fully charge threshold and stops once the battery voltage is equal to the fully charge value of 30 VDC.
34
CHAPTER 4 CONSTRUCTION, TESTING AND RESULTS 4.1 IMPLEMENTATION 4.1.1 TRANSFORMER CONSTRUCTION
The materials used in constructing the transformer for this project are
Laminations sheet of steel
The Winding bobbing made of forma
Copper wire gauge 8 and gauge 17
Insulation paper made of Mica
Masking tape
Bracket and screws
Sandpaper
Epoxy glue
The constructional procedure is sequenced below: 1. We cut the bobbing according to the shape of the E and I of the lamination so that the windings fit into the lamination winding window.
2. We glued the bobbing sides together with Epoxy glue and allowed it to set.
35
3. We then insulated the bobbing with Mica paper and started the winding. We started at the low voltage side, and wound the precise number of turns as calculated in our specifications. After which we wound the high voltage side, we made sure to insulate the low voltage side with the Mica paper first, and then we wound the number of turns as calculated in our specifications.
4. After all the winding were completed, we then insulated the whole with the mica paper to ensure that there was no fracture to the windings. Then we started to wear in the laminations, after which we locked the lamination in place with the bracket and screws to reduce vibration during the transformer operation.
36
4.1.2 H-BRIDGE CONSTRUCTION
Construction of the H-bridge required the following materials:
The heat sink
The MOSFETs
Link cable
Screw
Drilling machine
DC-Link capacitors
The constructional procedure is sequenced below: 1. We started by mapping out the MOSFETs on the heat sink, after which we drilled the sink so that the MOSFETs could be screwed in. 2. Next we stripped the gauge 8 wire to ensure that it was conductive so that we could solder the MOSFETs on them. 3. We then aligned the drains of the high side MOSFETs and soldered them together, and we aligned the source of the low side MOSFETs and we soldered them together; we soldered them using the stripped wire. We then linked the source of the first set of the high side MOSFETs together and connected them to the drains of the first set of the low side MOSFETs, and then repeated the same for the other set. 4. We then soldered in the DC link capacitors, and the gate resistor to conclude the whole set up. 5. And finally we then soldered the H-bridge set up with the transformer and the battery input and fastened it to the casing
37
4.1.3 CIRCUIT CONSTRUCTION
The electronic circuit was constructed on a printed circuit. All components on the hardware were connected as shown in the circuit diagram and permanently soldered to the board
38
4.2 TESTS
During and after the construction of the 3kVA microcontroller based pure sine wave inverter some of the following tests were conducted, these tests were conducted with a working digital multi-meter and also and an analogue meter to ensure correctness in the values measured. 4.2.1 CONTINUITY TEST
This test is done to check the validity of an electrical path; this test was carried out a lot during the assembling of the project components, particularly on the PCB. The test is performed by simply selecting continuity test using the multi-meter dial, and placing the probes on the points being tested. If connected the multi-meter buzzes a high pitched sound and reads a value, if not connected “1” which signifies infinity is displayed on the screen. 4.2.2 VOLTAGE TEST
Voltage tests were carried out severally to ascertain the voltages at different points in the network. A multi-meter was also used in carrying out this test. To carry out this test, the voltage type to be measured (AC or DC) is selected and the range of the multi-meter was set to match the expected voltage to be measured. The red probe was place on the positive terminal and the black probe on the negative terminal . 4.2.3 NO LOAD TEST
After the wounding of the transformer, no-load test was carried out. The purpose of this test is to determine no-load or core loss. Which in turn can be used in calculating the core loss and the iron loss. One of the winding; in our case the High voltage side is connected to a supply and the other side is left open, then the voltages and current is measured. The value of the no-load current we got was 0.3A, which was within reasonable limits, since the no-load current is within 2 – 10% of the rated current value.
39
4.3 RESULTS 4.3.1 SIGNAL GENERATOR OUTPUT
1. Proteus simulation outputs for inverter oscillation
2. Proteus diagram for charging
40
41
Table 4.1 Results table
S/N
Battery voltage
AC output
State of load Normal output
Controlled output
1.
27
230
ON
ON
2.
26.5
220
ON
ON
3.
26
220
ON
ON
4.
25.5
200
ON
ON
5
25
220
ON
ON
6
24.5
230
ON
ON
7
24
230
ON
ON
8
23.5
220
ON
ON
9
23
230
ON
OFF
10
22
220
OFF
OFF
42
4.4 BILL OF ENGINEERING MEASUREMENT AND EVALUATION Table 4.2: Bill of Engineering measurement and evaluation
S/N
MATERIAL
DESCRIPTION
QUANTITY
PRICE
TOTAL
(N)
(N)
1
Resistors
½ WATT
33
10
330
2
Resistors
5 WATT
3
50
150
3
Transformer
3000VA
1
40,000
40,000
4
MOSFETs
IRFP260N
20
450
9000
5
MOSFET driver
IR2112
2
500
1000
6
Microcontroller
PIC16F873A
1
3000
3000
7
Microcontroller
PIC18F2550
1
3000
3000
8
Relay
260V AC
2
500
1000
9
Transformer
230/12
2
500
1000
13
Capacitors
104
13
20
260
14
Crystal
8MHz
1
200
200
15
LCD
2 BY 16
1
1000
1000
16
Diode
IN4001
15
10
150
17
Diode
IN4148
8
10
800
18
Transistor
2N2222
4
50
200
19
Capacitor
47uF
10
50
500
20
Capacitor
4.7uf
1
200
200
21
Capacitor
4700Uf
4
200
800
22
Socket
13A
2
250
500
23
Patress box
1
50
50
24
Rg45 wire
10 yards
100
1000
25
Soldering lead
10 yards
100
1000
26
78XX
Voltage regulator
3
100
300
27
IC socket
40,16,14,4 pins
5
50
250
ESTIMATED
65,150
TOTAL
43
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 5.1 CONCLUSION
The objectives of this project is to design an inverter that will produce an output up to 3 KVA with input from a 24 V battery and can be used to power AC loads. Within the limit of correctness, the project worked as proposed in the objectives and according to the results gotten in the preceding chapter. This proposed pure Sine Wave inverter has various applications because of its key advantages such as operation with very low harmonic distortion and clean power like utility-supplied electricity, reduction in audible and electrical noise in Fans, fluorescent lights and so on, faster, quieter and cooler running of Inductive loads like microwaves and motors.
5.2 RECOMMENDATIONS
An extra improvement could be made to allow for short circuit protection, so that if there is a condition for a bridge in both the phase and neutral lines, the microcontroller produces no oscillations. This would be important as a short circuit conditions can lead to permanent damage to circuit components. Secondly the board could be a printed circuit board that would ensure the validity of connected links.
44
REFERENCES
Abolarinwa, J. and Gana, P. (2010). Design and Development of Inverter with AVR Using Switch Mode Square Wave switching scheme. Assumption University Journal , 13(4). AN-538. (n.d.). Microchip application note. Bond, M. S. (n.d.). Selecting Film Bus Link Capacitors. UNIVERSITY OF GAVLE, Electrical and electronics engineering. 526 Industrial Way Eatontown: Electronic Concepts Inc. how-to-calculate-inductance-of-transformer-primary-coil-magnetization. (n.d.). Retrieved from electro-tech-online: http://www.electro-tech-online.com/threads/how-to-calculateinductance-of-transformer-primary-coil-magnetization.130811/ Kolla, J., Devakumar, V. S., and Babu, B. K. (2013). A Comparison on Power Elec tronic Inverter Topologies. International Journal of Innovative research and development , Vol. 2, Issue 5. Leung, I. F. (2011). PWM Techniques: A Pure Sine Wave Inverter. Worcester Polytechnic Institute. Liquid-cyrstal display. (n.d.). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Liquid-crystal_display Omolara, O. A. (2015). Design of a Microcontroller-Based Push-Pull Inverter with Automatic Voltage Regulator. Internation Journal of Innovative Reseach and Development , Vol 4. Issue 4: 27 - 32. optocoupler . (n.d.). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/optocoupler Power Inverters. (n.d.). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Power_inverter rectifiers, I. (n.d.). Application note AN-978. rectifiers, I. (n.d.). IR2101/IR2102 MOSFET driver datasheet. rectifiers, I. (n.d.). IRFP260n MOSFET datasheet. relays. (n.d.). Retrieved from Wikepedia: https://en.wikipedia.org/wiki/Relay Sanjay Dixit, A. T. (n.d.). 800VA Pure Sine Wave Inverter’s Reference Design. Texas Instruments Application Report. Sekar, R. M. and Baladhandapani, R. (2013). PIC Based Seven-Level Cascaded H-Bridge Multilevel Inverter. International Journal of Engineering and Innovative Technology (IJEIT), Vol. 2, Issue 10,. Suryawanshi, H. M., Borghate, B. B., Ramteke, M . R., and Thakre, K. L. (2006). Electronic 45
Ballast using a Symmetrical Half-Bridge Inverter Operating at Unity-Power-Factor and High Efficiency. Journal of Power Electronics, Vol 6. Theraja, B. T. (2005). A textbook on Engineering Technology. Ram Nagar, New Delhi: S. Chand & company Ltd. Verle, M. (2009). PIC microcontrollers- Programming in C. MikroElectronika. Voltage regulators. (n.d.). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Voltage_regulator Wuidart, L. (n.d.). Topologies for Switched Mode Power Supplies. STMicroelectronics Application note.
46
APPENDIX PROGRAM SOURCE CODE
/*Pure sine wave inverter source code by Ajayi Temitope, Walker Enaefe, Shola Gonu and Jude Chinoso For PIC18F2550*/
sbit Ho1 at rc2_bit; // high side 2 pwm sbit Lo1 at rb1_bit; // low side 1 pwm sbit Ho2 at rc1_bit; // high side 1 pwm sbit Lo2 at rb0_bit; // low side 2 pwm sbit turn_on at rb3_bit; // inverter switch sbit mains_sense at rc7_bit; // mains sensor pin sbit main_relay at rd5_bit; // mains and charging relay controller pin sbit controlled_relay at rc6_bit; // controlled relay controller pin sbit bet3 at rd0_bit; // lcd control pin sbit bet4 at rd1_bit; // lcd control pin
const short max_count = 16; // the maximum time period of oscillation for pwm charger const short maxi_count = 31;//31; //maximum time period of oscillation const short keep_time = 3;//this should be 20us but due to the external oscillator its 3// keeps the track of time for oscillator const short keeping_time = 2;// should be 10us dut keeps track of time for charger const short signal_regulator = 32; //varies the different duty cycles in other to regulate the output voltage const short low_batt_level = 20; const short full_batt_level = 29; const short float_charge_level = 27; unsigned short duty_cycle, ascending, descending, count, adder, mode, delay;/* ascending keeps track of high part of duty cycle, descending keeps track of low part*/ int output_voltage_collect,input_current_collect,output_voltage,input_current,battery_collect,bat tery_voltage,i,j,k; 47
float AC_VOLT,BAT_VOLT,IN_CURRENT; char sum,sum1;
const unsigned char sample_number[]={ 0,1,1,2,3,3,4,4,5,5,6,6,6,7,7,7,7,7,7,6,6,6,5,5,4,4,3,3,2,1,1,0, //22% duty cycle
0,1,1,2,3,4,4,5,6,6,6,7,7,7,7,7,7,7,7,7,7,6,6,6,5,4,4,3,2,1,1,0, //24% duty cycle
0,1,2,3,4,5,5,6,7,8,8,8,9,9,9,9,9,9,9,9,8,8,8,7,6,5,5,4,3,2,1,0, //30% duty cycle
0,1,2,4,5,6,7,8,9,10,10,11,12,12,12,12,12,12,12,12,11,10,10,9,8,7,6,5,4,2,1,0, //40% duty cycle
0,2,4,5,7,9,11,12,14,15,16,17,17,18,19,19,19,19,18,17,17,16,15,14,12,11,9,7,5,4,2,0, //60% duty cycle
0,2,4,6,8,10,13,14,16,17,18,20,20,21,22,22,22,22,21,20,20,18,17,16,14,13,10,8,6,4,2,0, //70% duty cycle
0,3,6,9,12,15,18,20,23,25,26,28,29,30,31,31,31,31,30,29,28,26,25,23,20,18,15,12,9,6,3,0 //100% duty cycle }; //pwm duty cycles
void turn_off(); void check_battery_level(); void check_input_current(); void check_output_voltage(); void AVR(); void charging_algorithm(); void oscillator(); 48
void main() { adc_init(); mode = 1; osccon = 0b01100111; // initialize the chip frequency to 8MHz OSCCON2 = 0B00000011; OSCTUNE.PLLEN = 0; cm1con0 = 0; // turn off comparators cm2con1 = 0; cm2con0 = 0; ansele.f2=ansele.f3=ansele.f4 = 1; //configure port e as analogue anselb=anselc=anseld= 0 ;//configure the other ports as digital duty_cycle = 0; trisb0_bit = 0; trisb1_bit = 0; trisb3_bit = 1; trisc1_bit = 0; trisc2_bit = 0; trisc7_bit = 1; trisd5_bit = 0; trisc6_bit = 0; trisd0_bit = trisd1_bit = 0; portb.f0 = 0; portb.f1 = 0; portb.f3 = 1; portc.f1 = 0; portc.f2 = 0; portc.f7 = 1; portd.f0 = portd.f1 = 0; main_relay = controlled_relay = 0; adder = 32; i,j,k,sum,sum1 = 0; TRISA = 0xFF;
while(1) 49
{ check_battery_level(); check_input_current(); if((turn_on == 0) && (mains_sense == 0)) { if(((int)BAT_VOLT < 20) || ((int)IN_CURRENT > 80)) { turn_off(); }else if(((int)BAT_VOLT > 20) && ((int)IN_CURRENT < 80)) { oscillator(); } if((int)BAT_VOLT < 20) { bet3 = 1; bet4 = 0; } if((int)(IN_CURRENT > 80)) { bet3 = 0; bet4 = 1; } } else if((turn_on == 0) || (mains_sense == 1)) { if((int)BAT_VOLT >= 30) { turn_off(); bet3 = 1; bet4 = 0; }else if((int)BAT_VOLT < 30) { bet3 = 0; bet4 = 1; 50
charging_algorithm(); } }if (mains_sense == 1) { main_relay = 1; }else if(mains_sense == 0) { main_relay = 0; } if((mains_sense == 0) || ((int)BAT_VOLT <= 22)) { controlled_relay = 1; }else if((mains_sense == 1) || (int)(BAT_VOLT > 22)) { controlled_relay = 0; } } }
void oscillator() { output_voltage_collect = ADC_get_sample(1); output_voltage = output_voltage_collect * 4.888; AC_VOLT = (output_voltage*203.74); AC_VOLT = AC_VOLT/1000; if(j++>5) {j = 0; if((int)AC_VOLT < 140) { adder = adder + 32; } else if((int)AC_VOLT > 260) { 51
adder = adder - 32; } if (adder > 192) { adder = 192; } if (adder <32) { adder = 32; } } if (mode == 1) { count = 0; for(count = 0;count < 31; count++) { sum = count + adder; for(ascending = sample_number[sum];ascending > 0; --ascending) { Ho1 = 1; Lo2 = 1; Ho2 = 0; Lo1 = 0; delay_us(keep_time); } for(descending
=
((sample_number[sum])
maxi_count;descending++) { Ho1 = 0; Lo2 = 0; Ho2 = 0; Lo1 = 0; delay_us(keep_time); } mode = 0; } } if (mode == 0) 52
+
1);descending
<=
{ count = 0; for(count = 0;count < 31; count++) { sum1 = count +adder; for(ascending = sample_number[sum1];ascending > 0; ascending--) { Ho1 = 0; Lo2 = 0; Ho2 = 1; Lo1 = 1; delay_us(keep_time); } for(descending
=
(sample_number[sum1])
maxi_count;descending++) { Ho1 = 0; Lo2 = 0; Ho2 = 0; Lo1 = 0;// same goes here for low 1 delay_us(keep_time); } mode = 1; } } }
void charging_algorithm() { if((int)BAT_VOLT > 28 && (int)BAT_VOLT<30) { duty_cycle = 2; bet3 = 1; bet4 = 1; } else if((int)BAT_VOLT < 28) { 53
+
1;descending
<=
duty_cycle = 8; } //count = 0; for(count = 0;count < 64; count++) { for(ascending = duty_cycle ;ascending > 0; --ascending) { Ho1 = 0; Lo2 = 1; Ho2 = 0; Lo1 = 1; delay_us(keeping_time); } for(descending = duty_cycle + 1;descending <= max_count;descending++) { Ho1 = 0; Lo2 = 0; Ho2 = 0; Lo1 = 0; delay_us(keeping_time); } }
void check_input_current() { input_current_collect = adc_get_sample(0); input_current = input_current_collect * 4.888; IN_CURRENT = (input_current*100.0); IN_CURRENT = IN_CURRENT/1000; }
void check_battery_level() { battery_collect = adc_get_sample(2); battery_voltage = battery_collect * 4.888; BAT_VOLT = (battery_voltage*100.0); BAT_VOLT = BAT_VOLT/1000; 54
}
void turn_off() { Ho1 = 0; Lo2 = 0; Ho2 = 0; Lo1 = 0; }
LCD DISPLAY SOURCE CODE
sbit LCD_RS at Rb4_bit; sbit LCD_EN at Rb5_bit; sbit LCD_D4 at Rb0_bit; sbit LCD_D5 at Rb1_bit; sbit LCD_D6 at Rb2_bit; sbit LCD_D7 at Rb3_bit;
sbit LCD_RS_Direction at TRISb4_bit; sbit LCD_EN_Direction at TRISb5_bit; sbit LCD_D4_Direction at TRISb0_bit; sbit LCD_D5_Direction at TRISb1_bit; sbit LCD_D6_Direction at TRISb2_bit; sbit LCD_D7_Direction at TRISb3_bit;
sbit oscillator at rc5_bit; sbit mains_sense at rc6_bit; sbit bet3 at rd0_bit; sbit bet4 at rd1_bit;
int output_voltage_collect,input_current_collect,output_voltage,input_current,battery_collect,bat tery_voltage,k; float AC_VOLT,BAT_VOLT,IN_CURRENT;
55
void show_oss(); void show_mains(); void display_bat(); void display_volt(); void check_battery_level(); void check_output_voltage(); void main() { ADC_Init(); lcd_init(); cmcon = 0x07; adcon1 = 0x80; lcd_cmd(_lcd_clear); lcd_cmd(_lcd_cursor_off); trisa = 0xff; trisc.f5 = 1; trisc.f7 = 1; trisd.f0 = 1; trisd.f1 = 1; while(1) { if(((oscillator == 0) && (mains_sense == 0))||((oscillator == 0) && (mains_sense == 0))) { show_oss(); } else if((oscillator == 0) || (mains_sense == 1)) { show_mains(); } } }
void show_oss() { 56
check_output_voltage(); check_battery_level(); lcd_out(1,2, "Walker,Tope,Jude,Shola Inverter"); for(k=0; k<15; k++) { Lcd_Cmd(_LCD_SHIFT_LEFT); Delay_ms(500); } lcd_cmd(_lcd_clear); lcd_out(1,1, "Inverter mode"); display_volt(); if(bet3 ==1 && bet4 ==0) { delay_ms(500); lcd_out(2,1,"Low battery"); delay_ms(3000); } else if(bet3 ==0 && bet4 == 1) { delay_ms(500); lcd_out(2,1,"Over load"); delay_ms(3000); lcd_cmd(_lcd_clear); } else if(bet3 ==1 && bet4 == 1) { delay_ms(500); lcd_out(2,1,"Over load"); delay_ms(3000); lcd_cmd(_lcd_clear); } }
void show_mains() { 57
lcd_cmd(_lcd_clear); check_battery_level(); lcd_out(1,2, "Walker,Tope,Jude,Shola Inverter"); for(k=0; k<15; k++) { Lcd_Cmd(_LCD_SHIFT_LEFT); Delay_ms(500); } lcd_cmd(_lcd_clear); lcd_out(1,1, "Mains mode"); display_bat(); if(bet3 ==1 && bet4 ==0) { delay_ms(300); lcd_out(2,1,"Fully charged"); delay_ms(4000); } else if(bet3 ==0 && bet4 == 1) { lcd_out(2,1,"Charging"); delay_ms(1000); } else if(bet3 ==1 && bet4 == 1) { delay_ms(300); lcd_out(2,1,"Float charging"); delay_ms(3000); lcd_cmd(_lcd_clear); } }
void check_output_voltage() { output_voltage_collect = ADC_get_sample(1); output_voltage = output_voltage_collect * 4.888; 58
