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Full Project – Repair of single phase variac for laboratory use

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CHAPTER ONE

1.0 INTRODUCTION

1.1 Background of Study

A Variac is a variable auto-transformer. They are used extensively to regulate the voltage for tool testing, laboratories and universities. Variac’s usually have a toroidal auto transformer winding and a moving arm that gives the required voltage at different points along the surface of the toroid. The moving arm can either be manual or motorized. Variac’s are usually air-cooled or oil cooled. Oil cooled Variac’s can operate close to their maximum current rating for prolonged periods of time without over-heating.

A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer’s core, which induces a varying electromotive force (EMF) across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits. Faraday’s law of induction, discovered in 1831, describes the induced voltage effect in any coil due to a changing magnetic flux encircled by the coil.

Transformers are used to change AC voltage levels, such transformers being termed step-up or step-down type to increase or decrease voltage level, respectively. Transformers can also be used to provide galvanic isolation between circuits as well as to couple stages of signal-processing circuits. Since the invention of the first constant-potential transformer in 1885, transformers have become essential for the transmission, distribution, and utilization of alternating current electric power. A wide range of transformer designs is encountered in electronic and electric power applications. Transformers range in size from RF transformers less than a cubic centimeter in volume, to units weighing hundreds of tons used to interconnect the power grid.

A transformer consisting of only one coil which is shared by the primary and the secondary side of the circuit is known as an autotransformer. A variable autotransformer is known generically as a Variac, which is what I’ve dissected here. The ratio of the primary to the secondary windings is variable, which means that the ratio of the secondary voltage to the primary voltage is variable.

The inside of a variac looks like a giant rheostat. There is a single winding which is partially exposed so that the movable wiper can make an electrical connection. The primary connection of the transformer is made to both ends of this winding. The secondary connection is made to one end of the winding, called the common connection, and to the movable wiper. As the wiper moves, and ratio of the transformer changes.

Some people use variacs to bring long dormant electronic equipment back to life gradually. Variacs are also used in experiments and testing to simulate various voltage and line conditions. Electrical equipment designed for a voltage other than the 120V or 240V supplied domestically can be powered at the intended voltage level with a variac.

1.2 Statement of Problem

The main problem is that the single phase Variac gets faulty due to long period of usage and the other problem is lack of proper maintenance.

1.3 Research Objectives

The aim of this project is to repair a single phase Variac for laboratory use.

The objectives are as follows:

To ensure that the single phase Variac gives out the required voltage needed for student’s practical.
To bring the single phase Variac back into proper normal usage.

1.4 Contribution to Knowledge

To demonstrate practically the knowledge acquired so far in electrical troubleshooting, repairs and maintenance.

1.5 Limitation of the Project

To repair a faulty single phase Variac so that it can be put to usage in laboratory practicals.

1.6 Methodology

The method needed to carry out this project are as follows:

Physical inspection of the single phase Variac.
Troubleshooting of the single phase Variac circuit to find out the likely faulty components.

Procurement of replacement for faulty components.

Fixing of the procured replacement.
Testing of the repaired single phase Variac

1.7 Definition of Terms

Variac: A variac is a variable auto-transformer. They are used extensively to regulate the voltage for tool testing, laboratories and universities.
Laboratory: A laboratory is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physicians’ offices, clinics, hospitals, and regional and national referral centers.

Electromotive Force: electromotive force is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical transducers provide an EMF by converting other forms of energy into electrical energy.

Transformer: A transformer is a device that transfers electric energy from one alternating-current circuit to one or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage.

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