Design And Construction Of A 1KVA Microcontroller Based Power Inverter System

The aim of this project is to design and implement a single phase inverter which can convert DC voltage to AC voltage at high efficiency and low cost. Solar and wind powered electricity generation are being favored nowadays as the world increasingly focuses on environmental concerns. Power inverters, which convert solar-cell DC into domestic-use AC, are one of the key technologies for delivering efficient AC power. A low voltage DC source is inverted into a high voltage AC source in a two-step process. First the DC voltage is stepped up using a boost converter to a much higher voltage. This high voltage DC source is then transformed into an AC signal using pulse width modulation. Another method involves first transforming the DC source to AC at low voltage levels and then stepping up the AC signal using a transformer. A transformer however is less efficient and adds to the overall size and cost of a system. Therefore the former method is the one used to implement this project.

To deliver such performance, the power inverters is driven by high-performance PIC 16F877A microcontroller units (MCUs) that can achieve high-level inverter control, and therefore this microcontroller is the heart of the system and controls entire system. The microcontroller is programmed using embedded c compiler and in specific mikroC pro to generate sine pulse width modulated (SPWM) pulses which are used to drive H-bridge. By alternate switching switches of two legs of H-bridge alternating 12V DC voltage is converted into 240V Ac voltage.

The design is essentially focused upon low power electronic appliances such as personal computers, chargers, television sets. To build the design it is first mathematically modeled then is simulated in Proteus and finally the results are practically verified.

TABLE OF CONTENTS

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

1.0      INTRODUCTION

1.1      OBJECTIVE OF THE PROJECT

1.2      PURPOSE OF THE PROJECT

1.3     SIGNIFICANCE OF THE PROJECT

1.4      LIMITATION OF THE PROJECT

1.5      PROBLEM STATEMENT

1.6      APPLICATION OF THE PROJECT

1.7      INVERTER RATING

1.8      IMPORTANT CONSIDERATION OF INVERTER

1.9     DIFFERENCE BETWEEN CONVENTIONAL GENERATOR AND INVERTER

1.10   PROJECT ORGANISATION

CHAPTER TWO

2.0     LITERATURE REVIEW

2.1      HISTORITICAL BACKGROUND OF AN INVERTER

2.2         TYPES OF INVERTER

2.3      SATETY OF INVERTER

2.4      INVERTER CAPACITY

2.5      REVIEW OF EARLY INVERTERS

2.6      HOW TO CHOOSE AN INVERTER

CHAPTER THREE

3.0     CONSTRUCTION

3.1      BASIC DESIGNS OF A PURE SINE WAVE

3.2     BLOCK DIAGRAM OF THE SYSTEM

3.3     DESCRIPTION OF PURE SINE WAVE INVERTER UNITS

3.4     SYSTEM CIRCUIT DIAGRAM

3.5      CIRCUIT OPERATION

3.6     DESCRIPTION OF COMPONENTS USED

3.7      HOW TO CHOOSE THE BEST INVERTER BATTERY

CHAPTER FOUR

RESULT ANALYSIS

4.0      CONSTRUCTION PROCEDURE AND TESTING

4.1      CASING AND PACKAGING

4.2      ASSEMBLING OF SECTIONS

4.3      TESTING OF SYSTEM OPERATION

4.4     COST ANALYSIS

CHAPTER FIVE

5.0      CONCLUSION

5.1      RECOMMENDATION

5.2      REFERENCES

CHAPTER ONE

1.0                                       INTRODUCTION

Electronic devices run on AC power, however, batteries and some forms of power generation produce a DC voltage so it is necessary to convert the voltage into a source that devices can use. Hence a need for power rating inverter to smoothly operate electrical and electronic appliances. Most of the commercially available inverters are actually square wave or quasi square wave inverters. Electronic devices run by this inverter will damage due to harmonic contents [1]. Available sine wave inverters are expensive and their output is not so good. For getting pure sine wave we’ve to apply sinusoidal pulse width modulation (SPWM) technique. This technique has been the main choice in power electronics because of its simplicity and it is the mostly used method in inverter application [2]. To generate this signal, triangular wave is used as a carrier signal is compared with sinusoidal wave at desired frequency.

Advances in microcontroller technology have made it possible to perform functions that were previously done by analog electronic components. With multitasking capability, microcontrollers today are able to perform functions like comparator, analog to digital conversion (ADC), setting input/output (I/O), counters/timer, among others replacing dedicated analog components for each specified tasks, greatly reducing number of component in circuit and thus, lowering component production cost. Flexibility in the design has also been introduced by using microcontroller with capability of flash programming/reprogramming of tasks [3].

The proposed approach is to replace the conventional method with the use of microcontroller. In this project PIC16F877A microcontroller was used. It has low cost and reduces the complexity of the circuit for the single phase full bridge inverter [4]. The focus of this report is on the design and prototype testing of a DC to AC inverter which efficiently transforms a DC voltage source to a high voltage AC source similar to the power delivered through an electrical outlet (240Vrms, 50Hz) with a power rating of approximately 1000W.

The method in which the low voltage DC power is inverted is completed in two steps. The first being the conversion of the low voltage DC power to a high voltage DC source, and the second step being the conversion of the high DC source to an AC waveform using pulse width modulation. Another method to complete the desired outcome would be to first convert the low voltage DC power to AC, and then use a transformer to boost the voltage to 240 volts

[5]. This paper focused on the first method described and specifically the transformation of a high voltage DC source into an AC output.

This project builds upon the work of another project which mandated to build the DC to DC boost. In this report, it is detailed how the inverter’s controls are implemented with a digital approach using a microprocessor for the control system and how effective and efficient a 3- level PWM inverter can be. The inverter device will be able to run more sensitive devices that a modified sine wave may cause damage to such as: laser printers, laptop computers, power tools, digital clocks and medical equipment [1]. This form of AC power also reduces audible noise in devices such as fluorescent lights and runs inductive loads, like motors, faster and quieter due to the low harmonic distortion.

1.2                                            OBJECTIVE OF THE STUDY

The Objectives of this project is to design an inverter that can be derived by 24V battery and can be used to operate AC loads while minimizing the conventional inverter cost and complexity using Microcontroller.

1.2                                                PURPOSE OF THIS WORK

The purpose of this work is to design an electrical device that converts direct current (DC) to alternating current (AC); the resulting AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. The output of this work produces a sine wave-form of the mains / utility voltage which is rated 1000W which can be powered from the source of 12V battery.

1.3                         SIGNIFICANCE OF THE PROJECT

• Some electronic devices may pick up inverter noise while operating with modified sine waveform. Using fluorescent lighting can be problematic when using modified sine wave inverters. Most of the equipment on the market is designed for use with sine waves. Some appliances, such as microwaves, drills, clocks or speed motors will not produce full output if they don’t use sine wave current, moreover they may damage the equipment. Some loads, such as light dimmers will not work without sine wave at all. It’s safe to say any electronic device that requires sensitive calibration can only be used with pure sine wave inverters
• Pure Sine Wave output is the most compatible AC power from an inverter, and it is the best waveform for all AC electrical appliances.
• Pure Sine Wave output eliminates interference, noise, and overheating.
• Reduces audible and electrical noise in fans, fluorescent lights, electronics gear and magnetic circuit breakers.
• Prevents glitches and noise in monitoring equipment.
• It can be efficiently electronically protected from overload, over voltage, under voltage, and over temperature conditions.
• Inductive loads like microwave ovens and variable-speed motors operate properly, quieter and cooler. Some appliances will not produce full output if they do not use Pure Sine Wave power.
• Some appliances, such as variable speed drills and bread makers, will not work properly without Pure Sine Wave power.

1.4                                         LIMITATION OF THE PROJECT

• More expensive than Modified Sine Wave power inverters. Physically larger than their Modified Sine Wave counterparts.
• The built-in circuit becomes far more complex due to multiple conversions from AC (Alternating Current) to DC (Direct Current) and back to AC (Alternating Current). 3-DC, 4-D or All DC inverter ACs have even more conversions taking place as there are more components working on DC.
• Repair costs increase as components are more sophisticated and as a result, more expensive. They require more effort to build or repair.
• Response Time: The inverter shall respond to any line voltage variation in 1/2 cycle while operating linear or non-linear loads, with a load power factor of 0.60 of unity. Peak detection of the voltage sine wave shall not be permitted to avoid inaccurate tap switching due to input voltage distortion.
• Operating Frequency: The inverter shall be capable of operating at +10% to -15% of the nominal frequency, 50Hz.
• Rating: this device shall be rated at 1000VA.
• Access Requirements: The inverter shall have removable panels on the front, rear and sides as required for ease of maintenance and/or repair.
• Metering: An input meter is provided to display line voltages.
• Ventilation: The inverter isolation transformer shall be designed for convection cooling. Fan cooling is required for the MOSFET used.

1.5                                                  PROBLEM STATEMENT

Electricity is the major source of power for country’s most of the economic activities. But in our country Kenya, we have been suffering due to electricity crisis for a long time. To reduce this problem, there are some alternative ways which can help in this purpose. But among all of the methods solar system may be an easy and effective one especially in the rural areas where the electricity has not reached yet.

This solar energy is a renewable energy which is inefficiently exploited. The importance of solar energy is that it’s free, clean and with very high potentials in the future [2]. Photovoltaic systems (PV) are used to convert the solar energy into electrical energy using photovoltaic panels which can then be used into domestic electrical applications.

An important piece of solar power supply is the DC to AC inverter which converts the DC voltage from a battery to an AC voltage that is necessary to operate electronic components. Due to the delicate nature of this equipment, an inverter which is capable of producing a pure sine wave is necessary to avoid noise and wear on delicate and expensive gear. Many of these devices are very expensive so it is the goal of this project to design a DC/AC inverter capable of producing a pure sine wave for use with domestic equipment. In this project, an inverter circuit was designed that can supply an electrical load of up to 1000 watts, but due to the high ratings of the 1000 watts load, the unavailability and high cost of the components, and for safety reasons, a 125 watts application system was implemented and realized.

1.6                                       APPLICATION OF THE PROJECT

This study exposes me the applications and uses of a pure sine wave inverter which are as follows:

DC power source utilization

Inverter designed to provide 220 VAC from the 12 VDC source provided in an automobile. The unit shown provides more than 20 amperes of alternating current, or enough to power more than 1000W load.

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.

Uninterruptible power supplies

An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when main power is not available. When main power is restored, a rectifier supplies DC power to recharge the batteries.

Induction heating

Pure Sine wave Inverters convert low frequency main AC power to higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.

HVDC power transmission

With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC. The inverter must be synchronized with grid frequency and phase and minimize harmonic generation.

Variable-frequency drives

A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters.

Electric vehicle drives

Adjustable speed motor control inverters are currently used to power the traction motors in some electric and diesel-electric rail vehicles as well as some battery electric vehicles and hybrid electric highway vehicles. In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.

Air conditioning

An inverter air conditioner uses a variable-frequency drive to control the speed of the motor and thus the compressor.

Electroshock weapons

Electroshock weapons and tasters have a DC/AC inverter to generate several tens of thousands of V AC out of a small 9 V DC battery. First the 9VDC is converted to 400–2000V AC with a compact high frequency transformer, which is then rectified and temporarily stored in a high voltage capacitor until a pre-set threshold voltage is reached. When the threshold (set by way of an air gap or TRIAC) is reached, the capacitor dumps its entire load into a pulse transformer which then steps it up to its final output voltage of 20–60 kV. A variant of the principle is also used in electronic flash and bug zappers, though they rely on a capacitor-based voltage multiplier to achieve their high voltage.

1.7                              INVERTER RATINGS

The ratings that you should look at when buying an inverter (depending on the type) are:

1. Continuous Rating: This is the amount of power you could expect to use continuously without the inverter overheating and shutting down.
2. Half Hour Rating: This is handy as the continuous rating may be too low to run a high energy consumption power tool or appliance, however if the appliance was only to be used occasionally then the half hour rating may well suffice.
3. Surge Rating: A high surge is required to start some appliances and once running they may need considerably less power to keep functioning. The inverter must be able to hold its surge rating for at least 5 seconds. TVs and refrigerators are examples of items that require only relatively low power once running, but require a high surge to start.
4. IP rating – defines the ability of the inverter seals to prevent water and dust ingress. Although some inverter manufacturers claim high IP ratings suitable for outdoor installation, the quality and location of the seals and ventilation will greatly affect the ability of the inverter to outlast the many years solar installations are expected to work.
5. Peak efficiency– represents the highest efficiency that the inverter can achieve.

1.8                        IMPORTANT CONSIDERATION FOR INVERTERS

Before going into construction of an inverter, students must know the following:

OUTPUT FREQUENCY

The AC output frequency of a power inverter device is usually the same as standard power line frequency, 50 or 60 hertz

If the output of the device or circuit is to be further conditioned (for example stepped up) then the frequency may be much higher for good transformer efficiency.

OUTPUT VOLTAGE

The AC output voltage of a power inverter is often regulated to be the same as the grid line voltage, typically 220 VAC, even when there are changes in the load that the inverter is driving. This allows the inverter to power numerous devices designed for standard line power.

Some inverters also allow selectable or continuously variable output voltages.

OUTPUT POWER

A power inverter will often have an overall power rating expressed in watts or kilowatts. This describes the power that will be available to the device the inverter is driving and, indirectly, the power that will be needed from the DC source. Smaller popular consumer and commercial devices designed to mimic line power typically range from 150 to 3000 watts.

Not all inverter applications are solely or primarily concerned with power delivery; in some cases the frequency and or waveform properties are used by the follow-on circuit or device.

BATTERIES

The runtime of an inverter is dependent on the battery power and the amount of power being drawn from the inverter at a given time. As the amount of equipment using the inverter increases,  the runtime will decrease. In order to prolong the runtime of an inverter, additional batteries can be added to the inverter.

When attempting to add more batteries to an inverter, there are two basic options for installation: Series Configuration and Parallel Configuration.

Series configuration

If the goal is to increase the overall voltage of the inverter, one can daisy chain batteries in a Series Configuration. In a Series Configuration, if a single battery dies, the other batteries will not be able to power the load.

Parallel configuration

If the goal is to increase capacity and prolong the runtime of the inverter, batteries can be connected in parallel. This increases the overall Ampere-hour (Ah) rating of the battery set.

If a single battery is discharged though, the other batteries will then discharge through it. This can lead to rapid discharge of the entire pack, or even an over-current and possible fire. To avoid this, large paralleled batteries may be connected via diodes or intelligent monitoring with automatic switching to isolate an under-voltage battery from the others.

1.9     DIFFERENCE BETWEEN CONVENTIONAL GENERATOR AND INVERTER

1.10                                      PROJECT WORK ORGANISATION

The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows:

Chapter one of this works is on the introduction to pure sine wave power inverter. In this chapter, the background, significance, objective limitation and problem of pure sine wave power inverter were discussed.

Chapter two is on literature review of pure sine wave power inverter. In this chapter, all the literature pertaining to this work was reviewed.

Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed.

Chapter four is on testing analysis. All testing that result accurate functionality was analyzed.

Chapter five is on conclusion, recommendation and references.