What Is Transformers? Definition Of A Simple Transformer | Core Type Transformer | Shell Type Transformer
INTRODUCTION:-
Economical and technologically feasible voltage levels at which large chunks of electric power can be generated are typically 11-37 kV, while the most convenient utilization voltages are 230/400 V for industrial, commercial and domestic purposes.
Large industrial motors may be run at 3.3, 6.6 or 11 kV. It is impossible to transmit directly, even over modest distances, the electric power as it is generated (11-37 kV). Unacceptably large power losses and voltage drops would result. As a rule of thumb economical transmission voltage is 0.625 kV/km line-to-line, e.g. 400 kV for a line of about 640 km. It is therefore essential to step-up voltages at the sending (generating) end and to step-down at the receiving end. Usually more than one step of Step-down may be necessary. Step-up and step-down of voltage levels is accomplished by means of static electromagnetic devices called Transformers.
It was seen in alternating flux is set up in a core by a coil excited with ac voltage, which in turn induces coil emf (The magnitude of flux is determined by the fact that the coil emf must equal the excitation voltage (KVL) ) of excitation frequency proportional to the number of coil turns.
If another coil is wound on the same core, the mutual flux (alternating) would induce emf in it also of the same frequency and of magnitude proportional to its coil turns. The ratio of the voltage of the two coils can be easily adjusted by means of their turn-ratio.
Such a device, which indeed is a mutually coupled circuit, is called a Transformer and is exhibited diagrammatically in Fig. Below:-
PIC:- 1 - "A SIMPLE TRANSFORMER"
The coil excited from the ac source is called the primary and receives electric power from the source. The other coil is called the secondary and the voltage induced in it could be used to feed a load. The subscript '1' will be associated with the primary and '2' with the secondary. Primary and secondary roles in a transformer are easily reversed by the prevailing electrical conditions at the two ports. To avoid confusion in practice the two transformer coils are known as HV (high-voltage) and LV (low-voltage) windings.
Also shown in are the mutual and leakage flux paths. Since a significant part of the leakage flux paths is through air, leakage fluxes Φ11 and Φ12 are less than the mutual flux Φ .
The dots indicate on the two coils (windings) are the polarity marks. As the mutual flux alternates, these coil ends simultaneously acquire the same polarity. Also current into the dot in one coil and out of the dot in the other coil would tend to produce core flux in the opposite direction.
The transformer shown in PIC:- 1 is an iron-core transformer. Transformers operated at 25-400 Hz are invariably of iron-core construction.
In special cases (particularly at high frequencies), the core may be made of nonmagnetic material in which case it is called an air-core transformer. Application range for air-core transformers are radio devices and certain types of measuring and testing instruments.
Since the transformer core carries alternating flux, it is made of laminated steel (0.35 mm thickness for 50 Hz transformers). The transformer core is constructed of rectangular sheet steel strips. Two types of core constructions are adapted for single-phase transformers-core and shell type as below:-
PIC:- 2 - (a) Core Type Transformer (b) Shell Type Transformer
The core type construction has a longer mean length of flux path and a shorter mean length of coil turn.
Flux linking only one winding of the transformer (leakage flux) is detrimental to transformer performance in terms of voltage drop. To reduce leakage flux half-LV and half-HV are wound on each limb of the core type transformer as shown in PIC:- 2 - (a).
For economical insulation, the LV coils are placed inside (next to core) and HV coils are placed on the outside. In a shell type transformer reduced leakage flux is achieved by sandwiching HV and LV coil packets.
To prevent ingress of moisture and deterioration of winding insulation, the built-in core and windings are placed in a steel tank filled with transformer oil.
Oval or circular tubes are provided on the outside surfaces of the transformer tank, aiding in natural circulations of oil, which removes the heat of core and winding (I2R) losses and transports it to the tank surfaces for cooling purpose. Oil circulations also removes the heat generated by iron losses in the core.
To prevent the coil from absorbing moisture from air and from being oxidized, the tank must be sealed and connected to the atmosphere through a narrow passage for breathing purposes. Inside this passage is placed silica gel for drying the air that the transformer breathes in.
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