Design and Construction of a 2KVA Variac (0v – 240v) Range

ABSTRACT

A variac is also known as a variable autotransformer. A single primary winding wrapped around a magnetic core is used as in the autotransformer but instead of being fixed at some point, the secondary is tapped continuously over the output voltage range. In the construction, a toroidal core was used so as to allow for easy variation of output voltage. All the details that are required to help in constructing the variac to ensure that it gives the desired results were also determined. The appropriate insulation materials as well as the electrical accessories were selected taking the power, voltage and current rating of the variac into consideration. The construction was divided into three segments: magnetic circuit, electrical circuit and casing construction. The magnetic circuit construction gives details of the magnetic core and how it was prepared for winding along. The electrical circuit construction gives details of the whole winding along with insulation and leads termination. The constructed work was tested. The tests carried out include continuity test, insulation resistance test, mechanical and running tests.

TABLES OF CONTENTS

Title Page – – – – – – – – – – i
Certification – – – – – – – – – ii
Dedication – – – – – – – – – – iii
Acknowledgement – – – – – – – – – iv
Abstract – – – – – – – – – – v
Table of contents – – – – – – – – – vi
List of figures – – – – – – – – – x
Index of tables – – – – – – – – – xi

CHAPTER ONE

1.0.0 Introduction – – – – – – – – – 1
1.0.1 Background History of the Project – – – – – 3
1.0.2 Block Diagram – – – – – – – – 4
1.0.3 Description of Block Diagram – – – – – – 4
1.0.3a Input (Ac Source) – – – – – – – – 4
1.0.3b Winding – – – – – – – – – 4
1.0.3c Carbon Brush – – – – – – – – 5
1.0.3d Variable Output (0v- 240v) – – – – – – 5
1.0.4 Areas of Application – – – – – – – 5
1.0.5 Objectives of This Project – – – – – – – 6

CHAPTER TWO

2.0.0 Literature Review – – – – – – – – 7
2.0.1 Transformer – – – – – – – – – 7
2.0.2 Operating and Working Principle Of A Transformer – – – 7
2.0.3 E.M.F Equation of a Transformer – – – – – 10
2.0.4 Constructional Features of a Transformer – – – – 12
2.0.5 Types of Transformer – – – – – – – 13
2.0.6 Transformer Core Materials – – – – – – 14
2.0.7 Core Shapes – – – – – – – – – 15
2.0.7.1 Toroidal Core – – – – – – – – 15
2.0.8 Autotransformer – – – – – – – – 18
2.0.9 Variac – – – – – – – – – – 22
2.1.0 Losses in Transformer- – – – – – – – 25
2.1.0a Copper Loss- – – – – – – – – 26
2.1.0b Iron Loss – – – – – – – – – 27
2.1.1 Cooling Transformers – – – – – – – 28
2.1.2 Carbon Brush – – – – – – – 30
2.1.3 Electrical Measuring Instrument – – – – – – 31
2.1.4 Electronic Instrument – – – – – – – 32
2.1.5 Operating Circuit Diagram – – – – – – 33

CHAPTER THREE

3.0.0 Construction, Mode of Operation and Testing – – – 34
3.0.1 Construction – – – – – – – – – 34
3.0.2 Core and Winding Analysis – – – – – – 35
3.0.3 Determination of Area of Core – – – – – – 37
3.0.4 Calculation of Core Parameters – – – – – – 38
3.0.5 Dimension of Core – – – – – – – – 39
3.0.6 Determination of Number of Turns – – – – – 40
3.0.7 Determination of Number of Turns For input 220v – – – 40
3.0.8 Magnetic Circuit – – – – – – – – 41
3.0.9 Electrical Circuit – – – – – – – – 42
3.0.9a Winding Construction – – – – – – – 42
3.0.9b Insulation – – – – – – – – – 43
3.0.9c Carbon Brush and Terminals Construction – – – – 43
3.1.1 Casing Construction – – – – – – – 44
3.1.2 Mode of Operation – – – – – – – – 46
3.1.3 Testing – – – – – – – – – 47
3.1.3a Continuity Test – – – – – – – – 47
3.1.3b Insulation Resistance Test – – – – – – 47
3.1.3c Mechanical Test – – – – – – – – 48
3.1.3d Running Performance Test – – – – – – 48
3.1.4 Safety Precaution – – – – – – – – 48

CHAPTER FOUR

4.0.0 Observation and challenges – – – – – – 51
4.0.1 Observation – – – – – – – – – 51
4.0.2 Challenges – – – – – – – – – 52
4.0.3 Experienced Gained – – – – – – – – 52
4.0.4 Conclusion – – – – – – – – – 53
4.0.5 Recommendation – – – – – – – – 53

REFERENCE – – – – – – – – – 55

LIST OF FIGURES

Figs 1.1 Block Diagram of A Variac – – – – – – 4
Fig 2.1 Induced E.M.F Variations in a Magnetic Core – – – 8
Fig 2.2 Diagram of a Transformer e.m.f Equation – – – – 10
Fig 2.3 Physical Diagram of a Toroidal Core- – – – – 17
Fig 2.4 Tapped Autotransformer – – – – – – 18
Fig 2.5 Autotransformer Physical Arrangement – – – – 19
Fig 2.6 Autotransformer Design – – – – – – 20
Fig 2.7 Variable Autotransformer – – – – – – 24
Fig 2.8 Winding Diagram of a Variac – – – – – – 25
Fig 2.9 Physical Diagram of a Carbon Brush – – – – 30
Fig 2.10 Operating circuit diagram – – – – – 33
Fig 3.1 Core parameter – – – – – – – 39
Fig 3.2 Casing Before Assembling of Parts – – – – – 44
Fig 3.3 Casing After Assembling of Parts – – – – – 45
Fig 3.4 Mode of Operation of a Variac – – – – – 46

INDEX OF TABLES

Table 3.1 Bill of Engineering Material – – – – – – 50

CHAPTER ONE

INTRODUCTION

Electric power is the backbone of modern industrial society. Electricity extraordinary versatility means it can be put in almost limitless set of applications which include transportation, heating and communications etc.
Electricity is usually generated, transmitted before it is distributed to the final consumer. During this process, voltage generated is transformed from one level to another with the aid of a transformer. The transformer which transforms the voltage has been proved to be the easiest means through which voltage can be transformed from one level to another.

Without transformers the electrical energy generated at generating stations won’t probably be sufficient enough to power up a city. In cases where there are no transformers, the number of power plants needed to be set up in order to power up a city will be so numerous. It is expensive.

Numerous power plants have to be set up in order to have sufficient power. Transformers help by amplifying the Transformer output (stepping up or down the level of voltage or current).

When the number of turns of the secondary coil is greater than that of primary coil, such a transformer is known as step up transformer.

Likewise when the number of turns of coil of primary coil is greater than that of secondary transformer, such a transformer is known as step down transformer.

A transformer therefore, is a static machine used for transforming power from one circuit to another without changing frequency. Since there is no rotating or moving part so transformer is a static device. It is usually made up of two winding called the primary winding (input) and the secondary winding (output). Transformer works on the principle of mutual induction. It operates on AC supply.

The ability of the transformer to be able to transform voltage from one level to another means it can serve different classes of consumers.

Unlike the previous voltage transformer which have two electrically isolated windings called: the primary and the secondary, An autotransformer is an electrical transformer which has only one single voltage winding which is common to both sides i.e. the primary and the secondary winding are linked together both electrically and magnetically. The winding has at least three terminals. i.e. it consists of a single copper wire, which is common in both primary as well as secondary circuit. The copper wire is wound a laminated silicon steel core, with at least three tapings taken out. The Secondary and primary circuit share the same neutral point of the winding.

The VARIAC to be constructed is a special type of transformer. It has become generic and commonly referred to as any variable autotransformer. It is called variable autotransformer because they have the same as that of autotransformer but the autotransformer has a fixed and tapped secondary that produces a voltage output at a specific level while the variable autotransformer is designed such that it can be varied continuously over the entire voltage range for which it is designed.

BACKGROUND HISTORY OF THE PROJECT

In the 1880s, Around 50 years before that in 1830 property of induction which is the working principle of transformer was discovered. Later the transformer design was improved resulting in more efficiency and lesser size. Gradually the large capacity of transformers in the range of several KVA, MVA came into existence. In the year 1950, 400KV electrical power transformer was introduced in high voltage electrical power system. In the early 1970s, unit Rating as large as 1100 MVA was produced and 800KV and even higher KV class transformers were manufactured in year of 1980.