Construction and working of single phase Transformer
A transformer is a static device, which transfers electric power from one electrical circuit to another electrical circuit through medium of magnetic field without a change in the frequency. It can raise or lower the voltage with corresponding decrease or increase in current, keeping the power of transformer as same. It works on the principle of electromagnetic induced in a coil if it links a changing flux.
Construction: Transformer essentially consists of two windings, which are would on a common laminated magnetic core as shown in fig. The core is made with soft iron or silicon steel laminations to provide a path of low reluctance for the magnetic flux. The windings are insulated from both the core and each other. The winding which is connected to supply mains is knows as “secondary winding”. There is no electrical or mechanical connection between the two windings, but there is a magnetic linkage between the two. For the sake of simplicity and for analyzing transformer theory the two windings are wound one over the other.
Working: when the primary winding
is connected to an A.c. supply source of voltage ‘v1’ and frequency ‘f’ hertz,
a current ‘I’ will flows through it. This current produces an alternating flux ‘
’ in the core which
varies with time. This flux first links with primary winding hence produces
self induced e.m.f. some times knows as back e.m.f. of the primary. The flux ‘’ passes through the
magnetic core, and links with secondary winding also, so induces an e.m.f.
called mutual induced e.m.f. ‘E2’ in the secondary winding. The magnitude of
e.m.f. induced is proportional to the rate of change of flux linkages and
number of turns. The frequency of the e.m.f. induced in the secondary winding
is same as that of the flux or that of supply voltage. The e.m.f. in the
secondary winding will able to circulate the current ‘I2’ in the load circuit. Thus
the energy is transferred from primary winding to the secondary winding through
the medium of magnetic field by electromagnetic induction principle without any
change in frequency.
Step-up and step-down
Transformers
Fig shows the two
winding transformer with primary winding having ‘N1’ turns and secondary
winding having ‘N2’ turns. When the primary winding is connected to an a.c.
supply mains, as alternating flux ‘’ is produced in the
core. Since this flux is alternating and links with N1 and N2 turns, so induces
an e.m.f. or self-induced e.m.f. and the e.m.f. E2 is known as secondary e.m.f.
or mutual induced e.m.f.
and secondary e.m.f. E2 = -N2 formula 
= 
The ratio of primary to
secondary e.m.f. is the same as the ration of primary to secondary winding
turns. Hence, the magnitude of E1 and E2 depends on the number of turns N1 and
N2, since the 
is common for both the windings. By selecting
the values of N1 and N2, property, we can transform the required power from one
voltage level to another. If N2>N1, then E2>E1 (or) V2>V1 i.e.
secondary voltage will be more than primary voltage and such a transformer is
known as step-up transformer. If N2<N1, then E2<E1 (OR) V2<V1 i.e. ,
the secondary voltage will be equal to primary voltage and such a transformer. Is
known as step-down transformer. If N2 = N1, then E2 = E1 (or) v2 = v1 i.e., the
secondary voltage will be equal to primary voltage and such a transformer is
known as one-to-one ratio transformer.
The winding connected
to higher voltage is known as high-voltage (H.v) winding and the winding
connected to lower voltage is known as low-voltage (L.V) winding. In case of
step-up transformer, Low-voltage winding is the primary and High-voltage
winding is the primary.
The step-up transformers are used at generation satations to increases the voltage from lower level to higher level for economical transmission purpose. The step-down transformers are used to decrease the voltage from higher lower level for consumers use. The one-to-one ratio transformers are used to electrically isolate the two parts of a circuit.
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