Design And Construction Of A 2KVA Core Type Single Phase Transformer

 ABSTRACT

Transformer generally transforms electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings. The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance.

This work focused on core type transformer, in which its winding surrounds a considerable part of its cores (or limbs), where the half turns of primary and secondary windings that are placed on each core. Usually, it uses a circular coil for the strength it brings—this type of coil is wound in a helical layer with the various layers insulated from each other by micarta board, cloth, paper or a cooling duct.

CHAPTER ONE

• INTRODUCTION

A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a varying electromotive force (emf) or “voltage” in a second coil. Power can be transferred between the two coils, without a metallic connection between the two circuits. Faraday’s law of induction discovered in 1831 described this effect. Transformers are used to increase or decrease the alternating voltages in electric power applications.

Since the invention of the first constant-potential transformer in 1885, transformers have become essential for the transmission, distribution, and utilization of alternating current electrical energy.^[3] 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 interconnecting the power grid weighing hundreds of tons.

There are two major types of transformer, that is, the shell type and core type transformer.The types of transformers differ in the manner in which the primary and secondary coils are provided around the laminated steel core. For the course of this work a core type transformer are discussed in this work.

In core-type transformer, the windings are given to a considerable part of the core. The coils used for this transformer are form-wound and are of cylindrical type. Such a type of transformer can be applicable for small sized and large sized transformers. In the small sized type, the core will be rectangular in shape and the coils used are cylindrical. The figure below shows the large sized type. You can see that the round or cylindrical coils are wound in such a way as to fit over a cruciform core section. In the case of circular cylindrical coils, they have a fair advantage of having good mechanical strength. The cylindrical coils will have different layers and each layer will be insulated from the other with the help of materials like paper, cloth, micarta board and so on.

For a single phase core type, the transformer need two coils having mutual inductance and a laminated steel core. The two coils are insulated from each other and from the steel core. The device will also need some suitable container for the assembled core and windings, a medium with which the core and its windings from its container can be insulated.

In order to insulate and to bring out the terminals of the winding from the tank, apt bushings that are made from either porcelain or capacitor type must be used.

In all transformers that are used commercially, the core is made out of transformer sheet steel laminations assembled to provide a continuous magnetic path with minimum of air-gap included. The steel should have high permeability and low hysteresis loss. For this to happen, the steel should be made of high silicon content and must also be heat treated. By effectively laminating the core, the eddy-current losses can be reduced. The lamination can be done with the help of a light coat of core plate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, the thickness of the lamination varies from 0.35mm to 0.5mm for a frequency of 25 Hertz.

1.1                                                   AIM OF THE PROJECT

The aim of this work is construct a single phase core type step down transformer of 2kva power. In this work, we chose the primary voltage of this work to be  220vac while the secondary is 110VAC

1.2                                             OBJECTIVE OF THE PROJECT

At the end of this work, students involved will be able to:

1. Use transformer equation to calculate the input and output voltage of t step-down transformer.
2. Know the difference between core type and shell type transformer

• Know how to wind transformer

1. How to calculate transformer winding.

1.3                                                 SCOPE OF THE PROJECT

This work is on core type transformer. This transformer is mutual inductance between two circuits which is linked by a common magnetic flux. This transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance.

The core laminations are joined in the form of strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core. These staggered joints are said to be ‘imbricated’. Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force.

1.4                                          APPLICATION OF THE PROJECT

Core type transformers are popular in High voltage applications like Distribution transformers, Power transformers, and obviously auto transformers. Reasons are, High voltage corresponds to high flux.  In fact LV (low voltage) winding itself acts as an insulation between HV (high voltage) winding and core.

1.5                                         SIGNIFICANCE OF THE PROJECT

The advantages of core type transformer are as below:

1. It offers good mechanical strength: The cylindrical coils used in core type transformers are wound over a symmetrical core section. The way they are constructed will give you the advantage of getting better mechanical strength than other types of coils. As mentioned above, a core type transformer is created with one half of every winding enclosed around each of the limbs of its magnetic circuit.
2. It has the advantage of preventing condensed flux leakage and iron loss: When it comes to its lamination, the one in a core type transformer is typically arranged to create an overlapping joint with an extra pair of laminations to make for an accurate core thickness. This stacking of laminations also provides this transformer the advantage of preventing condensed flux leakage and iron loss.
3. It is efficient for high frequencies: Because the core type transformer uses several steel laminations, which are protected by a non-conducting insulation material between layers, the eddy currents will be contained and the magnetizing effects will be reduced. Now, while these thin laminations are more difficult to manufacture and more expensive, they make the transformer efficient for high frequencies.

1.6                                              PROBLEM OF THE PROJECT

1. It is not good to use outdoors: As with other dry transformers, a core type transformer is not good for outdoor use. Unlike oil transformers, it is not protected from corrosion and other extreme weather elements that can slowly degrade its inner components, especially those made of metal. This type of transformer is good for household appliances and mid-size industrial appliances, such as indoor power centrals and plants.
2. It can be noisy: A core type transformer and other dry transformers can be noisy. It can produce audible noise emissions from the tin work or electrical noise that arises from arcing on the covers. Now, this is not good if you have sensitive appliances or equipment nearby.

1.7       DIFFERENCE BETWEEN SHELL AND CORE TYPE TRANSFORMERS

1. In core type transformer winding is placed on two core limbs, whereas in case of shell type transformer winding is placed on mid arm of the core.
   Other limbs will be used as mechanical support.

2 . Core type transformers have only one magnetic flux path but Shell type transformers have two magnetic flux path.

3. Core type has better cooling since more surface is exposed to atmosphere but in case of shell type transformer, cooling is not effective.
4. Core type is very useful when we need large size low voltage but shell type transformer is very useful when we need small size high voltage.
5. In core type output is less, because of losses. In shell type transformer output is high because of less loss, thus efficiency will be more in case of shell transformer.
6. In core type winding is surrounded considerable part of core whereas in shell type Core is surrounded considerable part of winding of transformer.
7. Shell type has less mechanical protection to coil but Core type has better mechanical protection to coil.
8. Core type is easy to repair and maintain but shell type is not easy to repair. We need a skilled technician to maintain it.
9. In core type transformer concentric cylindrical winding are used. In shell type transformer sandwiched winding are used.