constraction details of Transformer

 Construction of a Transformer

The transformer is simple in construction. Since there are no rotating parts. The transformer has the following important parts.

1.       Core

2.       Windings

3.       Tank

4.       Transformer oil

5.       Conservator

6.       Breather

7.       Terminals and bushings

8.       Exhaust pipe, oil gauge, temperature gauge

9.       Cooling system

1. Core:

The purpose of core is to provide a path of low reluctance for the magnetic. It is made with silicon steel laminations as shown fig. the purpose of laminating the core is to reduce eddy current loss. These laminations are made with silicon steel to reduce hysteresis loss, because silicon steel has low hysteresis co-efficient. The laminations are insulating from each other by means of varnish or impregnated paper laminations are insulating from each other by means of varnish or impregnated paper or enamel. The thickness of each lamination varies from 0.35mm for 50Hz frequency to 0.5mm for 25Hz frequency. The core laminations are joined in such a manner that the butt joints avoids air-gap and therefore the reluctance of the magnetic circuit decrease and mechanical strength of the core increase.

The core is divided into two parts namely yoke and limb or leg. The vertical portion of the core on which winding is wound is know as limb or leg. The top and bottom horizontal portions are called yoke of the core. Yoke and limb together provides a closed path for magnetic flux.

The laminations are cut in the form of log strips of different shapes like L, E, I, U, C etc. as show in fig. For small transformers ‘L’ sections are used. Generally rectangular strips of laminations of suitable width and thickness are placed together to form the core of suitable cross-section. The cross-section of the core of a small transformer is rectangular, but large size transformer cores are approximately circular in section since circular section will have the smaller perimeter for a given area and therefore require less copper in coil. The cores of the transformer limbs are clamped between iron frames with bolts and nuts.

In small size transformer, the simple rectangular cores can be used with either circular or rectangular coils as shown in fig. as the size of the transformer increases, it becomes wasteful to employ rectangular coils, hence circular coils are usually preferred. For this purpose square core may be used as obviously a considerable amount of useful space is wasted, hence a common improvement on square core is to employ cruciform core which demands as least two sizes of core strips. For very large transformers further core-steppings may factor but also results in reduced length of the mean turn consequent IR2 loss.

Depending upon the type of construction of cores, the transformers are classified as,

i)                    Core type

ii)                   Shell type and

iii)                 Berry type

1. Core Type Transformer:                                                                

In core transformer the core is surrounded by the winding as it has two limbs. In this type the magnetic flux has legs and non-magnetic material surrounding the core. This flux is called as leakage flux that may links with one winding and not with the other. It is necessary to minimize the leakage of flux to improve the performance of transformer.

In core-type transformer, leakage flux can be minimized by placing half of both low voltage (L.V) and high voltage (H.V) windings Over one leg or limb and other half of L.V and H.V. windings over the second leg as the first low voltage (L.V) winding is placed over the core and high voltage (H.V) winding is wound over L.V winding in order to minimise the amount of insulation required. The two windings are insulated from each other and core by paper, cloth or cooling ducts. Core type construction is used for high voltage, high power transformers.

2. Shell-Type Transformer:

In shell type transformer the winding is surrounded by the core as show in fig. it has three limbs, two side limbs and one central limb. The width of central limb is twice that of the side limbs. In this type, the magnetic flux has two magnetic paths. The total flux passes through the central limb and half the flux is passing through the side limbs. The L.V. and H.V windings are placed on the central limbs,

hence the winding is surrounding by the core. Shell type construction is used for low voltage, low power transformers. For a give output and voltage rating, shell type transformer requires more iron but less conductor material as compared to core-type transformer. This construction is commonly used for small transformers.

3. Berry type Transformer:

In this case the distributed paths of the magnetic field are used. One Limb of all the cores passes through the centre of the windings are show in fig. the width of the core inside the coils is less than the width of limb outside the winding. These are used to obtain the variable voltage.

Comparisons Between core-type and shell type transformers:

S.NO	Core type Transformer	Shell type Transformer
1.	In this core is surrounded by the winding	In this type winding is surrounded by the core.
2.	Cylindrical type winding is used	Sandwiched type winding is used
3.	The core has two limbs or legs and two legs are equal in size	The core has three limbs or legs and the size of central leg is twice the outer legs.
4.	Magnetic flux has only the magnetic path	Magnetic flux has two magnetic paths
5.	Used for high voltage, high power levels	Used for low voltage, low power levels
6.	Average length of core is more	Average length of core is less
7.	Cross section of core is less, hence more turns are required	Cross section of core is more, so less turns are required
8.	The half of L.V. and H.V. winding are wound on both limbs	The L.V. and H.V. windings are wound on central limb
9.	The shape of core Laminations are rectangular ‘L’ type	The shape of core laminations are ‘E’ type
10.	Because most of the portion of winding is visible hence it is easy to insulate and repair	Most of the winding is enclosed by the core hence difficult to insulate and repair

Windings 

The windings are made with copper conductors are placed on the cores. The winding which is connected to supply is known as primary and the winding which is connected to load is known as primary winding. The winding which is connected to load is known as secondary winding. The winding which is connected to high voltage (H.V) is known as high-voltage winding.

According to the construction and arrangement, the windings are mainly classified into: 

(i)                 Cylindrical type winding      Sandwich type winding.

1. Cylindrical type winding

This winding is layered type and is made up of turns helically wound round with turns close to each other. Hence, the height of the winding is equal to the height of the limb. Since, the low voltage (L.V) winding is easier to insulate, it is first wound near and over the core and then high voltage winding is wound over the L.V. winding. For improving the cooling of the winding layers, cooling ducts are providing between the two windings. This type of winding is most commonly used in core-type transformers as shown in fig above.

2. Sandwich winding

In this type of winding the L.V and H.V windings are placed one over the other alternatively as show in fig above. This arrangement reduces the leakage reactance. The two low voltage coils at the ends have half the turns of a normal low voltage coil. This type of winding is most commonly used in shell-type transformers. As compared to concentric winding they have several drawbacks such as more labor consuming in manufacture, less stable in respect of short-circuits and more difficult to insulate from each other and from the yoke.

Tank

Transformers are generally housed in a tightly-fitted sheet-metal tanks filled with special insulating oil. The core and windings are completely immersed in the oil inside the tank, in case of small transformers, the tanks are made with iron sheets or sheet steel, whereas in case of large transformers, the tanks are made with plain boiler plates or cast-aluminum plates. The sheets or plates are properly welded and a waterproof gasket being used at the joints, so that oil may not come out of the tank. When the transformer is working, heat is produced in the tank, hence needs cooling.

For cooling purpose, cooling tube are welded around the tank as show in below fig. In case of large transformers separate radiators are welded to the tank which provides better cooling. Eye-blot is welded to the tank at the top for lifting purpose and wheels or rollers are provided at the bottom for moving the transformer into position.  A tank must be able to withstand the stresses developed inside and has provision for connecting to the load and supply.

Transformer oil