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Ahmedabad Institute of
Technology
1
SUB-DC MACHINES AND TRANSFORMER
TOPIC-CONSTRUCTION OF SINGLE AND
THREE PHASE TRANSFORMER
GUIDED BY-PROF.MAYALAKSHMI PILLAI
MADE BY-GROUP 3
SEM-3
Made by
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Makvana Mitesh
Makvana shivam
Urvish maru
Dinesh matalani
Parmeshwar meena
Ronak mishra
Nikhil pansuria
Introduction
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 Transformer is an electrical device that transfers
electrical power from one circuit to another
by magnetic coupling.
 It does so without change of frequency
and without any moving parts.
 Transformer works only with AC supply
Construction of transformer
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The transformer is very simple in construction
It consists of magnetic circuit linking with two
windings.
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 The simple element of transformer consist of two coil
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having mutual inductance and laminated steel core.
The two coils insulated from each other and steel
core.
Other necessary parts are:
A suitable container
Suitable bushing
suitable medium
breathers
Practical transformer
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 In all types of transformer the core is constructed of
transformer sheet steel lamination assembled to
provide a continuous magnetic path within a
minimum of air gap included.
 The steel used is of high silicon content.
 The eddy current loss is minimized by laminating the
core.
 The lamination being insulated from each other by
alight coat of varnish.
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The thickness of lamination varies from 0.35 to 0.5 mm .
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 Constructional transformer are of two types:
 Core type
 Shell type
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 The transformer called core type because the
winding surrounds considerable part of core.
 The transformer called shell type because core
surrounds a considerable part of winding.
 In diagram of core type transformers the primary
and secondary winding are shown located on
opposite legs of core but in actual these are
interleaved to reduce leakage flux.
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Core type transformer
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 The coil used are form wound and are of cylindrical
types.
 But for large size transformers round coils are used.
 Circular cylindrical coils are used because of their
mechanical strength these are wound in helical
layers with different layer insulated from each other
by paper.
 Since low voltage winding is easiest to insulate it is
placed nearest to core.
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 Because of lamination and insulation, the effective
core area is reduced.
 For large size transformer it is wasteful to use
rectangular cylindrical coils so circular cylindrical
coils are preferred .
 A common improvement on square core is to
employ cruciform core which demand at least two
size of core stripes.
Cruciform core
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Shell type
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 In this the winding are put around the central limb&
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the flux path is completed through two side limbs.
The coils are form wound but are multi layer disc
type usually wound I the form of pancakes.
Advantages of such construction.
A relatively more rigid core
Lesser weight and size per kVA rating
Low iron losses at higher operating flux densities
Low cost of manufacture.
Shell type transformer
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Three phase transformer
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Three phase transformer
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 Large scale generation of electric power is usually 3-
phase at generated voltage of 13.2kV or higher.
 For transmission purpose 3 phase transformer are
necessary to step up generated voltage to
transmission line.
 In olden days for this purpose 3 single-phase
transformer are interconnected.
 But nowadays due to improvement in design 3 phase
transformer are used.
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 Advantage of 3phase over single phase transformer
 It occupies less floor space for same rating.
 Weight less, cost about 15%less.
 Only one unit is to be handled and connected.
 Disadvantage:
 If any one phase become disabled then whole
transformer has to be removed for service.
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Cooling in transformer
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 Voltage below 25000V transformer are cooled by air
blast no need of oil.
 For large transformer core and winding are
immersed in insulating oil and conservative tank is
placed above it.
 For very large transformer the oil is regulated by
means of pump and heat is carried out by water
which is flowing in pipes.
Breathers
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 In large transformers where complete air tight
construction is impossible, chambers are known as
breathers are provided to permit thee oil inside the
tank to expand and contracts temperature changes.
 The atmospheric moisture is entrapped in these
breathers & is not allowed to pass in oil.
Bushings
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 All transformer leads are carried out by suitable
bushing.
 For moderate voltage porcelain bushing are used to
insulate the leads as they come out of the tank.
 They are look like insulators on transmission lines.
 In high voltage installation oil filled type bushing are
used.
Conservator tank
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 It is the most important part of transformer for
cooling purpose.
 Its size is depends on the type of transformer and
power specification.
 Insulating mineral oil is filled in the conservative
tank.
 The mineral is used for insulation purpose and for
providing cooling in transformer
References
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 1.A textbook of electrical technology, volume 2 AC
and DC machine, B.L. theraja A.K. theraja, S.chand
and company pvt.ltd.
 2.DC machine and transformer , Dr J.G. Jamnani,
Mahajan publishing house