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Michael Faraday propounded the principle of electro-magnetic induction in 1831.
It states that a voltage appears across the terminals of an electric coil when the flux linked
with the same changes. The magnitude of the induced voltage is proportional to the rate of
change of the flux linkages. This finding forms the basis for many magneto electric machines.
The earliest use of this phenomenon was in the development of induction coils. These coils
were used to generate high voltage pulses to ignite the explosive charges in the mines. As
the d.c. power system was in use at that time, very little of transformer principle was made
use of. In the d.c. supply system the generating station and the load center have to be
necessarily close to each other due to the requirement of economic transmission of power.
Also the d.c. generators cannot be scaled up due to the limitations of the commutator.
As mentioned earlier the transformer is a static device working on the principle of
Faraday’s law of induction. Faraday’s law states that a voltage appears across the terminals
of an electric coil when the flux linkages associated with the same changes. This emf is
proportional to the rate of change of flux linkages. Putting mathematically.
Transformers used in practice are of extremely large variety depending upon the
end use. In addition to the transformers used in power systems, in power transmission and
distribution, a large number of special transformers are in use in applications like electronic
supplies, rectification, furnaces, traction etc. Here the focus is on power transformers only.
The principle of operation of these transformers also is the same but the user requirements
differ. Power transformers of smaller sizes could be air cooled while the larger ones are
oil cooled. These machines are highly material intensive equipments and are designed to
match the applications for best operating conditions. Hence they are ‘tailor made’ to a
job. This brings in a very large variety in their constructional features. Here more common
constructional aspects alone are discussed.
Earlier it is seen that a voltage is induced in a coil when the flux linkage associated
with the same changed. If one can generate a time varying magnetic field any coil placed in
the field of influence linking the same experiences an induced emf. A time varying field can
be created by passing an alternating current through an electric coil. This is called mutual
induction. The medium can even be air. Such an arrangement is called air cored transformer.
Indeed such arrangements are used in very high frequency transformers. Even though the
principle of transformer action is not changed, the medium has considerable influence on the
working of such devices.
An ideal transformer is useful in understanding the working of a transformer. But it
cannot be used for the computation of the performance of a practical transformer due to the
non-ideal nature of the practical transformer. In a working transformer the performance as-
pects like magnetizing current, losses, voltage regulation, efficiency etc are important. Hence
the effects of the non-idealization like finite permeability, saturation, hysteresis and winding
resistances have to be added to an ideal transformer to make it a practical transformer.
Conversely, if these effects are removed from a working transformer what is left behind is an
Testing of Transformers
The structure of the circuit equivalent of a practical transformer is developed earlier.
The performance parameters of interest can be obtained by solving that circuit for any load
conditions. The equivalent circuit parameters are available to the designer of the transformers
from the various expressions that he uses for designing the transformers. But for a user
these are not available most of the times. Also when a transformer is rewound with different
primary and secondary windings the equivalent circuit also changes. In order to get the
equivalent circuit parameters test methods are heavily depended upon. From the analysis of
the equivalent circuit one can determine the electrical parameters. But if the temperature
rise of the transformer is required, then test method is the most dependable one. There are
several tests that can be done on the transformer; however a few common ones are discussed