10-08-2012, 10:44 AM
Transformers
ELECTRICALMACHINES-I.pdf (Size: 2.08 MB / Downloads: 120)
Introduction
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. This
made the world look for other efficient methods for bulk power generation and transmis-
sion. During the second half of the 19th century the alternators, transformers and induction
motors were invented. These machines work on alternating power supply. The role of the
transformers became obvious. The transformer which consisted of two electric circuits linked
by a common magnetic circuit helped the voltage and current levels to be changed keeping
the power invariant.
Constructional features
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. These can be broadly divided into
1. Core construction
2. Winding arrangements
3. Cooling aspects
3.1 Core construction
Transformer core for the power frequency application is made of highly permeable
material. The high value of permeability helps to give a low reluctance for the path of
the flux and the flux lines mostly confine themselves to the iron. Relative permeability μr
well over 1000 are achieved by the present day materials. Silicon steel in the form of thin
laminations is used for the core material. Over the years progressively better magnetic prop-
erties are obtained by going in for Hot rolled non-oriented to Hot rolled grain oriented steel.
Insulation
The insulation used in the case of electrical conductors in a transformer is varnish
or enamel in dry type of transformers. In larger transformers to improve the heat transfer
characteristics the conductors are insulated using un-impregnated paper or cloth and the
whole core-winding assembly is immersed in a tank containing transformer oil. The trans-
former oil thus has dual role. It is an insulator and also a coolant. The porous insulation
around the conductor helps the oil to reach the conductor surface and extract the heat. The
conductor insulation may be called the minor insulation as the voltage required to be with-
stood is not high. The major insulation is between the windings. Annular bakelite cylinders
serve this purpose. Oil ducts are also used as part of insulation between windings. The oil
used in the transformer tank should be free from moisture or other contamination to be of
any use as an insulator.
Cooling of transformers
Scaling advantages make the design of larger and larger unit sizes of transformers
economically attractive. This can be explained as below. Consider a transformer of certain
rating designed with certain flux density and current density. If now the linear dimensions
are made larger by a factor of K keeping the current and flux densities the same the core and
conductor areas increase by a factor of K2. The losses in the machine, which are proportional
to the volume of the materials used, increase by a factor of K3.The rating of the machine
increases by a factor of K4.
ELECTRICALMACHINES-I.pdf (Size: 2.08 MB / Downloads: 120)
Introduction
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. This
made the world look for other efficient methods for bulk power generation and transmis-
sion. During the second half of the 19th century the alternators, transformers and induction
motors were invented. These machines work on alternating power supply. The role of the
transformers became obvious. The transformer which consisted of two electric circuits linked
by a common magnetic circuit helped the voltage and current levels to be changed keeping
the power invariant.
Constructional features
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. These can be broadly divided into
1. Core construction
2. Winding arrangements
3. Cooling aspects
3.1 Core construction
Transformer core for the power frequency application is made of highly permeable
material. The high value of permeability helps to give a low reluctance for the path of
the flux and the flux lines mostly confine themselves to the iron. Relative permeability μr
well over 1000 are achieved by the present day materials. Silicon steel in the form of thin
laminations is used for the core material. Over the years progressively better magnetic prop-
erties are obtained by going in for Hot rolled non-oriented to Hot rolled grain oriented steel.
Insulation
The insulation used in the case of electrical conductors in a transformer is varnish
or enamel in dry type of transformers. In larger transformers to improve the heat transfer
characteristics the conductors are insulated using un-impregnated paper or cloth and the
whole core-winding assembly is immersed in a tank containing transformer oil. The trans-
former oil thus has dual role. It is an insulator and also a coolant. The porous insulation
around the conductor helps the oil to reach the conductor surface and extract the heat. The
conductor insulation may be called the minor insulation as the voltage required to be with-
stood is not high. The major insulation is between the windings. Annular bakelite cylinders
serve this purpose. Oil ducts are also used as part of insulation between windings. The oil
used in the transformer tank should be free from moisture or other contamination to be of
any use as an insulator.
Cooling of transformers
Scaling advantages make the design of larger and larger unit sizes of transformers
economically attractive. This can be explained as below. Consider a transformer of certain
rating designed with certain flux density and current density. If now the linear dimensions
are made larger by a factor of K keeping the current and flux densities the same the core and
conductor areas increase by a factor of K2. The losses in the machine, which are proportional
to the volume of the materials used, increase by a factor of K3.The rating of the machine
increases by a factor of K4.