Construction of Transformer

The transformer basically consists of two coils having mutual inductance and laminated steel core. The coils are insulated from each other and the steel core. Other necessary parts are:

Some suitable container for assembled core and windings.
a suitable medium for insulating the core and windings from the container.
suitable bushings for insulating and bringing out the terminals of windings from the tank.

In all types of transformers the core is constructed of steel sheet laminations assembled to provide a continuous magnetic path with a minimum of air gap included. The steel used is of high silicon content. The eddy current loss is minimized by laminating the core. The thickness of the laminations varies from 0.35mm for a frequency of 50 Hz to 0.5mm for a frequency of 25 Hz. The sheets are imbricated to avoid the narrow gaps through the cross section of the core.

Types of Transformers

On the basis of construction, transformers are of two types:

1. Core Type

2. Shell Type

On the basis function

1. Step up

2. Step down

Comments

Core type and Shell type Transformers

Constructionally, the transformers are of two general types, distinguished from each other merely by the manner in which the primary and secondary coils are placed around the laminated core. The two types are:- Core type Shell type Another recent development is spiral core or wound core type, the trade name being spirakore transformer. Core type transformer:- In the core type transformer, the windings surround a considerable part of the core. In the simplified diagram of the core type transformer, the primary and secondary winding are shown located on the opposite limbs of the core, but in actual construction they are interleaved to reduce leakage flux. As in the figure, half the primary and half the secondary winding have been placed side by side or concentrically on each limb, not primary on one limb and the secondary on the other. The coil used are form-wound and are of the cylindrical type. In small size core type transformers, a simple rectangular core

Skin Effect in Conductors

When a conductor is carrying dc current , it is uniformly distributed over the cross section of the conductor . However an alternting current flowing through the conductor does not distribute uniformly, rather it has the tendency to concetrate near the surface of the conductor. This is known as Skin effect. Thus, skin effect is the tendency of an alternting current to concentrate near the surface of the conductor. Due to skin effect the effective area of cross section of the conductor through which current flows is reduced consequently, the resistance of the conductor is slightly increased when carrying an alternating current. Cause of skin effect The cause of skin effect can be easily explained. A solid conductor may be thought to be consisting of a large number of strands, each carrying a small part of current. The inductance of each strand will vary according to its position. Thus, the strands near the centre are surrounded by a greater magnetic flux and hence

Proximity effect in conductors

In a conductor carrying alternating current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions. The resulting current crowding is termed as the proximity effect. This crowding gives an increase in the effective resistance of the circuit, which increases with frequency. A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor, by electromagnetic induction. When an alternating current (AC) flows through an isolated conductor, it creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them. The result is that the current is concentrated in the areas of the conductor furthest away from nearby conductors carrying curre

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