The project presents here is on design and construct a single phase auto-transformer of rating 1kva, and step down voltage of 240v and voltage taps of 120, 48, 24v, 12v.An autotransformer is an electrical transformer with only one winding. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but autotransformers have the disadvantage of not providing electrical isolation.
An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points. The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits therefore have a number of windings turns in common since the volts-per-turn is the same in both windings; each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding. One end of the winding is usually connected in common to both the voltage source and the electrical load. The other end of the source and load are connected to taps along the winding. Different taps on the winding correspond to different voltages, measured from the common end. In a step-down transformer the source is usually connected across the entire winding while the load is connected by a tap across only a portion of the winding. In a step-up transformer, conversely, the load is attached across the full winding while the source is connected to a tap across a portion of the winding.
TABLE OF CONTENT
Title Page
Approval Page
Dedication
Acknowledgement
Abstract
Table of Content
CHAPTER ONE
1.0 Introduction
1.1 purpose of the study
1.2 significance of the study
1.3 statement of problem
1.4 application of an autotransformer
1.5 general application of transformer
CHAPTER TWO
2.0 literature review
2.1 review of transformer history
2.2 Review of types of transformer
2.3 review of classification of transformer
2.4 review of importance of different kinds of transformer cooling system
2.5 review of maintenance of transformer cooling system
CHAPTER THREE
3.0 constructions
3.1 autotransformer basics
3.2 operation of autotransformer
3.3 autotransformer design
3.4 autotransformer construction
3.5 construction procedure
CHAPTER FOUR
4.0 Result analysis
4.1 assembling of section and testing
4.2 testing of system operation
4.3 problems encountered
5.4 cost analysis
CHAPTER FIVE
5.1 Conclusion
5.2 Recommendation
5.3 Bibliography
CHAPTER ONE
1.0 INTRODUCTION
An autotransformer has its primary and secondary connected to each other electrically. A portion of the energy in an autotransformer comes from this connection while the balance comes directly from the supply. Building inspectors often object to auto transformers because they do not isolate one circuit from the other. One ground may be at a considerably higher voltage than the ground in another section of the same circuit. Local inspectors and utility companies should be consulted before installing autotransformers. Where the use of autotransformers is not objectionable, they do represent a considerable saving in price over that of a regular separate winding transformer. This saving varies as the ratio of windings changes. After the ratio of windings reaches approximately 4:1 or 5:1, there is very little economy in using an autotransformer. Autotransformers are most practical where a small percentage of voltage raising or lowering is required and isolation between the two circuits is not required. Autotransformers can be single phase or three phase. In neither case is any isolation provided. |
In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but autotransformers have the disadvantage of not providing electrical isolation.
Autotransformers are often used to step up or step down voltages in the 110-115-120 volt range and voltages in the 220-230-240 volt range—for example. Providing 110 or 120V (with taps) from 240V input, allowing equipment designed for 100 or 120 volts to be used with a 240 volt supply. But in this project a step down voltage of 240v as the primary voltage and 120, 48, 24v, 12v was designed.
1.1 PURPOSE OF THE STUDY
The purpose of this study is to the working principle, and the design and construct an autotransformer step down voltage of 240v/110, 80, 12v, 6v.
1.2 SIGNIFICANCE OF THE STUDY
The purpose of an auto transformer will be one of two things. The auto transformer can increase the voltage a bit. The auto transformer can also decrease the voltage by a bit if that is what is needed. But in this work, an auto transformer was designed to reduce 240VAC to 110, 80, 12v, 6v.
1.3 STATEMENT OF PROBLEM
An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output). These are important safety considerations when deciding to use an autotransformer in a given application.
Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is usually more economical.
In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of ground fault currents. A large three-phase autotransformer may have a "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents.
In practice, losses mean that both standard transformers and autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if it is used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.
Like multiple-winding transformers, autotransformers use time-varying magnetic fields to transfer power. They require alternating currents to operate properly and will not function on direct current.
1.4 APPLICATIONS OF AUTOTRANSFORMER
Power distribution
Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application in industry is to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100V-130V and 200V-250V). The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.
On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.
A special form of autotransformer called a zig zag is used to provide grounding on three-phase systems that otherwise have no connection to ground. A zig-zag transformer provides a path for current that is common to all three phases (so-called zero sequence current).
Audio
In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.
Railways
In UK railway applications, it is common to power the trains at 25 kV AC. To increase the distance between electricity supplies Grid feeder points they can be arranged to supply a 25-0-25 kV supply with the third wire (opposite phase) out of reach of the train's overhead collector pantograph. The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. At frequent (about 10 km) intervals, an autotransformer links the contact wire to rail and to the second (anti-phase) supply conductor. This system increases usable transmission distance, reduces induced interference into external equipment and reduces cost. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.
1.5 GENERAL APPLICATION OF TRANSFORMER
Transformers are used to increase voltage before transmitting electrical energy over long distances through wires. Wires have resistance which loses energy through joule heating at a rate corresponding to square of the current. By transforming power to a higher voltage transformers enable economical transmission of power and distribution. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.
Transformers are also used extensively in electronic products to step-down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.
Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground, such as between external cables and internal circuits.
1.6 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 work is on the introduction to a single phase autotransformer. In this chapter, the background, significance, objective limitation and problem of single phase autotransformer were discussed.
Chapter two is on literature review of single phase autotransformer. 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.
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