Introdaction of single phase transformars

Working Principle of a Transformer
A transformer is a static (or stationary) piece of apparatus by means of which electric power in
one circuit is transformed into electric power of the same frequency in another circuit. It can raise or
lower the voltage in a circuit but with a corresponding
decrease or increase in current. The physical
basis of a transformer is mutual induction between
two circuits linked by a common magnetic flux. In
its simplest form, it consists of two inductive coils
which are electrically separated but magnetically
linked through a path of low reluctance as shown
in Fig. 32.1. The two coils possess high mutual
inductance. If one coil is connected to a source of
alternating voltage, an alternating flux is set up in
the laminated core, most of which is linked with
the other coil in which it produces mutually-induced
e.m.f. (according to Faraday’s Laws of Electromagnetic Induction e = MdI/dt). If the second
coil circuit is closed, a current flows in it and so electric energy is transferred (entirely magnetically)
from the first coil to the second coil. The first coil, in which electric energy is fed from the a.c. supply
mains, is called primary winding and the other from which energy is drawn out, is called secondary
winding. In brief, a transformer is a device that
1. transfers electric power from one circuit to another
2. it does so without a change of frequency
3. it accomplishes this by electromagnetic induction and
4. where the two electric circuits are in mutual inductive influence of each other.
32.2. Transformer Construction
The simple elements of a transformer
consist of two coils having mutual
inductance and a laminated steel core. The
two 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 its windings from
its container ; suitable bushings (either of
porcelain, oil-filled or capacitor-type) for
insulating and bringing out the terminals
of windings
from the
tank.
In all
types of
transformers,
the core is constructed of transformer sheet steel laminations assembled
to provide a continuous magnetic path with a minimum of air-gap
included. The steel used is of high silicon content, sometimes heat
treated to produce a high permeability and a low hysteresis loss at the
iron core
secondary
coil
secondary
coil
110/120
volts
220/240
volts
110/120
volts
primary
coil
220/240
volts
Principle of transformer
Fig. 32.2
Transformer 1117
usual operating flux densities. The eddy current loss is minimised by
laminating the core, the laminations being insulated from each other by a
light coat of core-plate varnish or by an oxide layer on the surface. The
thickness of laminations varies from 0.35 mm for a frequency of 50 Hz to
0.5 mm for a frequency of 25 Hz. The core laminations (in the form of
strips) are joined as shown in Fig. 32.2. It is seen that the joints in the
alternate layers are staggered in order to avoid the presence of narrow
gaps right through the cross-section of the core. Such staggered joints
are said to be ‘imbricated’.
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 known as (i) core-type and (ii) shelltype.
Another recent development is spiral-core or wound-core type, the
trade name being spirakore transformer.
In the so-called core type transformers, the windings surround a
considerable part of the core whereas in shell-type transformers, the core
surrounds a considerable portion of the windings