## WINDING

An electrically good conducting material is used in winding a transformer to perform its purpose of altering the voltage units. the transformer consists of two coils called WINDING which are wrapped around a core.

The transformer operates when a source of ac voltage is connected to one of the winding and a load device is connected to the other. The winding that is connected to the source is called the PRIMARY WINDING. The winding that is connected to the load is called the SECONDARY WINDING.

 TYPES OF WINDING AND DETAILED EXPLANATION

Types of winding

Broadly there are seven types of winding generally used in transformer, they are.
1.      Spiral Winding (Only LV)
2.      Helical Winding(Only LV)
3.      Edge Winding(Only HV)
4.      Layer Winding(Both HV & LV)
5.      Disc Winding(Both HV & LV)
6.      Cross-over Winding(Only HV)
7.      Foil Winding

### 1.     Spiral Winding

In this type of winding even number (4,6,8,12,16,18,24) of conductor(copper) string film are arranged in rectangle ( rows and columns) order and are wrapped around farmer, some cardboard blocks are used to complete the winding, Spiral winding is usually for low voltage power supply.

### 2.      Helical Winding

We use helical winding low voltage, high capacity transformers, where current is higher, at the same time winding turns are lesser.
The output of transformervaries from 160 – 1000 kVA from 0.23-15 kV. To secure adequate mechanical strength the cross-sectional area of the strip not made less than 75-100 mm square. The maximum number of strips used in parallel to make up a conductor is 16.
There are three types:
·         Single Helical Winding
·         Double Helical Winding
·         Disc-Helical Winding
Single Helical Winding consist of winding in an axial direction along a screw line with an inclination. There is only one layer of turns in each winding. The advantage of Double Helical Winding is that it reduces eddy current loss in conductors. This is on account of the reduced number of parallel conductors situated in the radial direction.
In Disc-Helical Winding, the parallel connected strips are placed side by side in a radial direction to occupy total radial depth of winding.

### Multi-layer Helical Winding

We use it commonly for high voltage ratings for 110 kV and above. These types of winding consist of several cylindrical layers concentrically wound and connected in series.

We make the outer layers shorter than inner layers to distribute capacitance uniformly. These winding primarily improve the surge behavior of transformers.

### 3.      Edge Winding

Edge Winding Coil is made by Edge Winding of the strips with multi turns which are cut as per design to gain required resistance values. The Coil are inserted on porcelain grooves, which are in turn supported by a Metal Plate. These types of elements are fitted on Metal Trays of the Resistor Box.

### 4.      Layer Winding

Layer (barrel) winding are among the simplest of winding in that the insulated conductors are wound directly next to each other around the cylinder and spacers. Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination.

### 5.    Disc Winding

Primarily used for a high capacity transformer. The winding consist of a number of flat coils or discs in series or parallel. The coils are formed with rectangular strips wound spirally from the center outwards in the radial direction.

The conductors can be a single strip or multiple strips in a parallel wound on the flat side. This makes robust construction for this type of winding. Discs are separated from each other with press-board sectors attached to vertical strips. The vertical and horizontal spacers provide radial and axial ducts for free circulation of oil which comes in contact with every turn. The area of the conductor varies from 4 to 50 mm square and limits for current are 12 – 600 A. The minimum width of oil duct is 6 mm for 35 kV. The advantage of the disc and continuous winding is their greater mechanical axial strength and cheapness.

### 6.      Cross-over Winding

We use these winding for high voltage winding of small transformers. The conductors are paper covered round wires or strips. The winding are divided into a number of coils in order to reduce voltage between adjacent layers. These coils are axially separated by a distance of 0.5 to 1 mm. The voltages between adjacent coils should not be more than 800 to 1000 V.

The inside end of a coil is connected to the output side end of the adjacent one. The actual axial length of each coil is about 50 mm while the spacing between two coils is about 6 mm to accommodate blocks of insulating material. The width of the coil is 25 to 50 mm. The crossover winding has a higher strength than cylindrical winding under normal condition. However, the crossover has lover impulse strength than the cylindrical one. This type also consumes more labor cost.

### 7.      Foil Winding

A 12cm long wide conductor is used for foil winding. Usually foil winding is used for LV and foil winding is employed for dry type transformer, where less area is required to the power operation foil winding is used.

### MATERIAL USED FOR WINDING

The used for the winding depends on the application of transformer. Small power transformers are wound with solid copper wire insulated usually with enamel.
Larger power transformer may be wound with wire, copper, or aluminium rectangular conductors and strip type conductors are used for heavy currents.
Larger power transformer use multiple-stranded conductor as well. Each strand is insulated from the other and strands are arranged so that at certain point in the winding or throughout winding, each portion occupies different relative positions in the complete conductor.
This transposition equalize the current flowing in each strand of the conductor and reduce eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size. Winding both primary and secondary of power transformer may have external connections to intermediate points on the winding to allow adjustment of the voltage ratio.
Mostly copper is used as conductor because of its good electrical conductivity (after silver) and availability of material at lowest cost.

#### INSULATION

Enamel is used to insulate the winding of low voltage transformers. However, for power transformers paper insulation is used for winding wire as well as in the form of strips of paper insulation between winding layers. Basic insulation of wires whether using enamel or paper or otherwise should be readily understood as we want to create turns of wire which can build emf on each turn contributing to the total emf between the end terminals. Basic insulation need not by excessive because the voltage difference between a winding turn and its neighboring turn (in the same layer) is small in value and its the voltage across one winding turn. However, the voltage difference between a winding turn and its neighboring winding turn one layer above or below it can be excessive and equals the voltage drop across a large number of turns in one winding layer. Therefore an extra insulation layer using paper strip is used between winding layers.
Paper is used as the base for insulation in transformers because:
1) It is widely available
2) It is cheap
3) Can be easily applied to winding
4) Impregnated with oil makes good insulating medium of good electrical and thermal           properties
5) Is durable compared to any other material used for insulation.

### TOOLS AND FIXTURE

Tools generally used for winding are as follows
1.      Farmer
2.      ID caliper
3.      OD caliper
4.      Ruler
5.      Wire bender
6.      Scissor
7.      Glue
8.      Brazing kit to join conductors
9.      Wire cutter
TYPES OF WINDING
Fixtures used in winding are listed below
1.           Winding machine
2.           Crane
3.           bobbin stand
4.           Fork trolley
5.           Cardboard blocks

### QUALITY CHECK

During the process of winding time to time inspection is need to be conducted, an small imperfection in the quality of winding may cost whole performance of transformer which is need to be observed and alerted if any deviation in the winding is occurred. Inner and outer diameter of winding is indeed to check to satisfy the specification of the design so the winding can adjusted in the core and the cooling oil be circulated around the winding to avoid lamping.
TYPES OF WINDING