Unit -IV
SINGLE PHASE TRANSFORMER
5. Constructional features
Transformers used in practice are of extremely large variety depending upon theend use. In addition to the transformers used in power systems, in power transmission anddistribution, a large number of special transformers are in use in applications like electronicsupplies, 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 requirementsdiffer. Power transformers of smaller sizes could be air cooled while the larger ones areoil cooled. These machines are highly material intensive equipments and are designed tomatch the applications for best operating conditions. Hence they are ‘tailor made’ to ajob. This brings in a very large variety in their constructional features. Here more commonconstructional aspects alone are discussed. These can be broadly divided into
1. Core construction
2. Winding arrangements
3. Cooling aspects
5.1 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.
Later better laminations in the form of cold Rolled Grain Oriented (CRGO), -High B (HiB)
grades became available. The thickness of the laminations progressively got reduced from
over 0.5mm to the present 0.25mm per lamination. These laminations are coated with a thin
layer of insulating varnish, oxide or phosphate. The magnetic material is required to have
a high permeability µ and a high saturation flux density, a very low remanence Br and a
small area under the B-H loop-to permit high flux density of operation with low magnetizing
current and low hysteresis loss. The resistivity of the iron sheet itself is required to be high
to reduce the eddy current losses. The eddy current itself is highly reduced by making the
laminations very thin. If the lamination is made too thin then the production cost of steel
laminations increases. The steel should not have residual mechanical stresses which reduce
their magnetic properties and hence must be annealed after cutting and stacking.
In the case of very small transformers (from a few volt-amperes to a few kilo volt-
amperes) hot rolled silicon steel laminations in the form of E & I, C & I or O as shown in
Fig. 5.1 are used and the core cross section would be a square or a rectangle. The percentage
of silicon in the steel is about 3.5. Above this value the steel becomes very brittle and also
very hard to cut. The saturation flux density of the present day steel lamination is about 2
Tesla.
Broadly classifying, the core construction can be separated into core type and
shell type. In a core type construction the winding surrounds the core. A few examples of
single phase and three phase core type constructions are shown in Fig. 5.2. In a shell type on
the other hand the iron surrounds the winding.
In the case of very small transformers the conductors are very thin and round.
These can be easily wound on a former with rectangular or square cross section. Thus no
special care is needed for the construction of the core. The cross section of the core also
would be square or rectangular. As the rating of the transformer increases the conductor size
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| Figure 5.1: E and I,C and I and O Type Laminations |
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| Figure 5.2: Core and Shell Type Construction |
also increases. Flat conductors are preferred to round ones. To wind such conductor on a
rectangular former is not only difficult but introduces stresses in the conductor, at the bends.
From the short circuit force with stand capability point of view also this is not desirable.
Also, for a given area enclosed the length of the conductor becomes more. Hence it results in
more load losses. In order to avoid all these problems the coils are made cylindrical and are
wound on formers on heavy duty lathes. Thus the core construction is required to be such as
to fill the circular space inside the coil with steel laminations. Stepped core construction thus
becomes mandatory for the core of large transformers. Fig. 5.3 shows a few typical stepped core
constructions. When the core size increases it becomes extremely difficult to cool the same
(Even though the core losses are relatively very small). Cooling ducts have to be provided
in the core. The steel laminations are grain oriented exploiting the simple geometry of the
transformer to reduce the excitation losses. The iron losses in the lamination, when the flux
