A transformer has been defined as a static electrical machine that transfers AC electrical power from one circuit to the other circuit at a constant frequency, but the voltage can be increased or decreased depending on the requirement. This static device transfers electrical energy from one circuit to another through the process of electromagnetic induction. This device is most commonly used for decreasing (step down) or increasing (step up) of voltage levels between the circuits.
The Working Principle of Transformers
The working principle of the transformer is based on a simple principle of Faraday’s Law of Electromagnetic Induction, which states that “the magnitude of voltage is directly proportional to the rate of change of the flux”.
Let us understand the working principle of transformers in a simpler manner:
Imagine, you have one winding (also referred to as a coil) that is supplied by an alternating electrical source. Through the winding, the alternating current produces a continually changing and alternating flux, which surrounds the winding. If another winding is brought in close proximity of this winding, some part of this alternating flux will link with the second winding. As the flux experiences a continuous change in the amplitude and direction, there must be a changing flux linkage in the second winding or coil.
As per Faraday’s Law of Electromagnetic Induction, there will be an EMF induced in the second winding. If the circuit of this secondary winding is closed, then the current will flow through it. This is the basic working principle of a transformer.
Different Types of Windings
After understanding the basic working principle of a transformer, it is clear that windings play a very crucial role in a transformer. There are different transformer types available with Transformer Manufacturer Company in India that use different types of windings, such as core type transformer and shell type transformer.
The core type transformer is the one in which the wrapping of primary and secondary winding is done on the outside limbs. While the shell type transformer is the one in which the primary and secondary windings are placed on the inner limbs.
The core type transformer uses concentric type windings. A low voltage winding is placed near the core, although to reduce the leakage reactance, the windings can be interlaced. The windings used for core type transformers depends on many factors, such as current rating, limit of temperature rise, short circuit withstand capacity, transport facilities, surge voltage and many more.
Types of Windings Used in Core Type Transformers
The following mentioned windings are layered type and either rectangular or round conductor.
- Cylindrical Windings:
(a) Multi Layered Windings
- Helical Windings:
(a) Single Helical Windings
(b) Double Helical Windings
(c) Disc- Helical Windings
- Multi- Layer Helical Windings
- Cross- Over Windings
- Disc and Continuous Disk Windings
- Aluminum Windings
Cylindrical windings are windings with low voltage that use up to 6.6 kV for kVA up to 600-750 and have a current rating between 10 to 600A.
Multi Layered Cylindrical Windings
Cylindical windings are mostly used in its multi- layered form. It is used with rectangular conductors in the two- layered type as it becomes very easy to secure the lead- out ends. The oil ducts separate the layers of the windings, this arrangement facilitated by the cooling process through oil circulation in the winding.
In multi layered cylindrical windings, circular conductors are used; they are wound on vertical strips to improve the cooling conditions. The arrangement creates oil ducts which facilitate good cooling. These types of windings are used for high voltage ratings up to 33kV, 800 kVA and the current ratings up to 80 A. the maximum diameter that is used in this type of winding for a bare conductor is 4mm.
Helical windings are used for low voltage and high capacity transformers, where the current is higher and at the same time the winding turns are lesser. The output of transformer in this case varies from 160 to 1000 kVA from 0.23 to 15 kV. For securing the adequate mechanical strength the cross- sectional area of the strip is made not less than 75 to 100mm square. The maximum number of strips that are used to make up the conductor in parallel is 16.
Helical Windings have three types:
Single Helical Windings
The winding in axial direction along a screw line with inclination is referred to as Single Helical Windings. These windings consist of only one layer of turns in each of the windings.
Double Helical Windings
The Double Helical Winding provides an advantage of reducing the eddy current loss in conductors. This is due to the reduced number of parallel conductors that are situated in a radial direction.
Disc- Helical Windings
The Disc- Helical Windings is designed in a way that the parallel connected strips are placed side to side in a radial direction so as to occupy the total radial depth of the winding.
Multi-layer Helical Windings
The Multi- Layer Helical Winding is used commonly for high voltage ratings such as 110 kV and above. These windings include several cylindrical layers concentrically that are wound and connected in series.
The outer layers of these windings are made shorter than the inner layers for distributing the capacitance uniformly. These windings basically improve the surge behavior of the transformers.
These windings are used for high voltage windings of the small transformers. The conductors of these windings are paper covered strips or round wires. The windings help reduce the voltage between adjacent layers as they are divided into a number of coils. The coils are separated axially by a distance of 0.5-1mm. The voltages between adjacent coils must not be more than 800 to 1000V.
The inside end of the coil is connected with the output end of the adjacent one. The axial length of each coil in actuality is about 50 mm, while the spacing between the two coils is about 6 mm so that it can accommodate the blocks of insulating material. The coil width is 25 to 50 mm. Crossover winding has greater strength than cylindrical winding under normal conditions. Although, the crossover has lower impulse strength as compared to the cylindrical one. This type of winding also consumes a higher labor cost.
Disc and Continuous Disc Winding
The Disc and Continuous Disc Winding is basically used for high capacity transformers. These windings consist of numerous flat coils/ discs in a series or parallel formation. The coils of these windings are formed with the help of rectangular strips that are wound spirally from the centre outwards in a radial direction.
The conductors of these windings can be a single strip or multiple strips in a parallel formation that are wound on the flat side. This formation of the conductors makes the construction for this type of windings extremely strong. The discs are separated from each other with the help of press- board sectors that are attached to vertical strips. The vertical and the horizontal spacers for free circulation of oil provide radial and axial ducts that come in contact with each turn. The area of the conductor ranges from 4 to 50 mm square and limits for current are 12 to 600 A. The minimum oil duct width is 6 mm for 35 kV. The advantage of these windings is that they provide greater mechanical axial strength and cheapness.
Shell Type Transformer Windings
Sandwich Type Winding
The Sandwich Type Windings provide easy control over the reactance, as the nearer two coils are together on the same magnetic axis; the greater is the proportion of the mutual flux and the lesser is the leakage flux. The leakage flux can be reduced by sub- dividing the sections of the low and the high voltages. The end low voltages sections, in Sandwich Type Windings contain half the turns of the normal low voltage sections that are referred to as half coils.
For balancing the magneto-motive forces of the adjacent sections, each of the normal sections, either high or low voltage carries the same number of ampere turns. The higher the degree of subdivision of the voltages, the smaller will be the reactance.
Advantages of Shell Type Windings
The Shell Type Windings provides a lot of advantages, such as high capability of withstanding short- circuits, higher mechanical strength, higher dielectric strength, great leakage magnetic flux control, extremely efficient cooling capability, a flexible design with a compact size. All these advantages come with a highly reliable design.
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