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CHAPTER ONE

INTRODUCTION

As blackouts roll through power-starved communities, the threat to you and your computer is not the lack of electricity, but the change in power. When the lights are off and you are about to start any industrial or computer-based projects,  all your efforts will be wasted. Even when your system acts as a server, a sudden shutdown could disrupt the processing of many others. You can make your work immune to the intransigence of rolling blackouts and protect against many other types of unexpected power disturbances. Your secret weapon is the uninterruptible power supply or uninterruptible power source. Commonly called the UPS, this devices is a cleaver threefold package-a set of battery, an inverter that transforms the low-voltage direct current of the batteries into the standard alternating current equivalent to your wall outlet, and a battery changer that assures that reserve power storage system (the batteries) with interfaces to mach it to utility power and your computer system.

A UPS differs from an auxiliary emergency power system or standby generator in that it will provide instantaneous or near-instantaneous protection from input power interruptions by means of one or more attached batteries and associated electronic circuitry for low power users, and or by means of diesel generators and flywheels for high power users. While not limited to protecting any particular type of equipment, a UPS is typically used to protect computers, data centers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption and/or data loss. UPS units range in size from units designed to protect a single computer without a video monitor (around 200 VA rating) to large units powering entire data centers, buildings, or even cities. The UPS is designed to project against changes, specifically a temporary loss of electrical supply. This project focuses on conversion of AC to DC and from DC to AC power inverters, which aim to efficiently transform a DC power source to a high voltage AC source, similar to power that would be available at an electrical wall outlet. Inverters are used for many applications, as in situations where low voltage DC sources such as batteries, solar panels or fuel cell must be converting electrical power from a car battery to run a laptop, TV or cell phone.

DC and AC Current

In the world today there are currently two forms of electrical transmission, Direct Current (DC) and Alternating Current (AC), each with its own advantages and disadvantages. DC power is simply the application of a steady constant voltage across a circuit resulting in a constant current. A battery is the most common source of DC transmission as current flows from one end of a circuit to the other. Most digital circuitry today is run off of DC power as it carries the ability to provide either a constant high or constant low voltage, enabling digital logic to process code executions. Historically, electricity was first commecially transmitted by Thomas Edison, and was a DC power line. However, this electricity was low voltage, due to the inability to step up DC voltage at the time, and thus it was not capable of transmitting power over long distances. V =IR P=IV = I2R   As can be seen in the equations above, power loss can be derived from the electrical current squared and the resistance of a transmission line. When the voltage is increased, the current decreases and concurrently the power loss decreases exponentially; therefore high voltage transmission reduces power loss. For this reasoning electricity was generated at power stations and delivered to homes and businesses through AC power. Alternating current, unlike DC, oscillates between two voltage values at a specified frequency, and it’s ever changing current and voltage makes it easy to step up or down the voltage. For high voltage and long distance transmission situations, all that is needed to step up or down the voltage of the transformer. Developed in 1886 by William Stanley Jr., the transformer made long distance electrical transmission using AC power possible. Electrical transmission has therefore been mainly based upon AC power, supplying most Nigerian homes with a 220 volt AC source. It should be noted that since 1954 there have been many high voltage DC transmission systems implemented around the globe with the advent of DC/DC converters, allowing the easy stepping up and down of DC voltages. Like DC power, there exist many devices such as power tools, radios and TV’s that run off of AC power. It is therefore crucial that both forms of electricity transmission exist; the world cannot be powered with one simple form. It then becomes a vital matter for there to exist easy ways to transform DC to AC power and vice versa in an efficient manner. Without this ability people will be restricted to what electronic devices they use depending on the electricity source available. Electrical AC/DC converters and DC/AC inverters allow people this freedom in transferring electrical power between the two.
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Inverters and Applications
Power inverters are devices which can convert electrical energy of DC form into that of AC. They come in all shapes and sizes, from low power functions such as powering a car radio to that of backing up a building in case of power outage. Inverters can come in many different varieties, differing in price, power, efficiency and purpose. The purpose of a DC/AC power inverter is typically to take DC power supplied by a battery, such as a 12 volt car battery, and transform it into a 220 volt AC power source operating at 50Hz, emulating the power available at an ordinary household electrical outlet. Power inverters are used today for many tasks like powering appliances in a car such as cell phones, radios and televisions. They also come in handy for consumers who own camping vehicles, boats and at construction sites where an electric grid may not be as accessible to hook into. Inverters allow the user to provide AC power in areas where only batteries can be made available, allowing portability and freeing the user of long power cords. However, most UPS units are also capable in varying degrees of correcting common utility power problems like:
1.           Power failure: defined as a total loss of input voltage.
2.           Surge: defined as a momentary or sustained increase in the mains voltage.
3.           Sag: defined as a momentary or sustained reduction in input voltage.
4.           Spikes, defined as a brief high voltage excursion.
5.           Noise, defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment. 6.           Frequency instability: defined as temporary changes in the mains frequency.
7.           Harmonic sinusoidal waveform distortion: defined as a departure from the ideal expected on the line Factors to consider when designing an inverter is
·        The input voltage
·        The output voltage
·        The frequency of the oscillators
·        The rating of the load to be powered by the inverter
·        The ampere range of the charging unit
·        The cost of the construction
·        The relative importance of the inverter
·        The ampere ranger of the charging unit    
·        The cost of the construction
·        The relative importance of the inverter at that time specification
·        D.C input voltage (12V-24V)
·        A.C output voltage (220v-240v)
·        Output frequency (50Hz-60Hz)
·        Output power (450-550)
·        Maximum power (550)
·        Continuous output power (500)
·        Overload shutdown
·        Low voltage shutdown  
1.2 AIMS AND OBJECTIVES  
 The objective of this project may be summarized as listed below  
·        To give a brief introduction on what UPS.
·        To highlight the basic components used in the construction of a UPS and their functions .
·        To describe the way in which there are designed and
·         Finally to highlight its application and relevance in the society    
1.3  SCOPE OF THIS PROJECT  
This project research encompasses the features of the variety of the components used in the construction of a UPS, their limitations and favourable working conditions, and the application of inverter in different field.  
1.4  AVAILABILITY OF DESIGN MATERIALS  
Materials used in the construction of this UPS are readily available in electronic stores across the country and are also very affordable depending on the rating of the UPS to be constructed.