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TitleGATE Machines Books
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Total Pages12
Table of Contents
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Module Test
Reference book
Document Text Contents
Page 2



Electrical Engineering


Page 6

Chapter 1 Machines

THE GATE ACADEMY PVT.LTD. H.O.: #74, KeshavaKrupa (third Floor), 30

Cross, 10

Main, Jayanagar 4

Block, Bangalore-11
: 080-65700750,  [email protected] © Copyright reserved. Web: Page 1




Single Phase Transformer (Synopsis)

 It has no moving parts and contains two circuits.
 Electrically isolated, magnetically coupled.
 Transformer is a static device.
 A transformer has no rotating or moving parts.
 A transformer is NOT an energy conversion device.
 There is no change in frequency.
 Voltage and current change simultaneously, depending on turns ratio.
 Transformer has two or more windings depending on application.
 Transformers require very little care and maintenance because of their simple, rugged

and durable.
 The efficiency of a transformer is high because there are no rotating parts, it is a static

 The efficiency of a 5 KVA transformer is of the order of 94-96%.
 The efficiency of a 100 MVA transformer is of the order of 97-99%
 Transformer is responsible for the extensive use of a.c. over d.c.

Constructional Details

 Core: Silicon steel or sheet steel with 4% silicon is used.
 The core plates of a transformer are made of silicon steel or sheet steel.
 The sheets are laminated to reduce eddy current losses.
 The sheets are laminated and coated with an oxide to reduce iron losses.
 The thickness of lamination is 0.35 mm for 60 Hz operation.
 The thickness of lamination is 5 mm for 25 Hz operation.
 The core provides a path of low reluctance.
 The relative permeability for the core material is of the order of 1,000.
 For a given value of flux, the primary AT required are less if the reluctance is low.
 A spiral core is assembled using continuous strip of transformer silicon steel wound in the

form of a circular or elliptical cylinder.
 In a spiral core transformer higher flux densities can be used.
 A spiral core transformer has Lower loss per Kg. Weight.


 Conventional transformer has two windings.
 The winding which receives electrical energy is called primary winding.
 The winding which delivers electrical energy is called secondary winding.
 Windings are made of High grade copper.

Page 7

Chapter 1 Machines

THE GATE ACADEMY PVT.LTD. H.O.: #74, KeshavaKrupa (third Floor), 30

Cross, 10

Main, Jayanagar 4

Block, Bangalore-11
: 080-65700750,  [email protected] © Copyright reserved. Web: Page 2

 Stranded conductors are used for windings carrying higher currents to reduce eddy
current loss.

 Additional insulation is provided for line end connections, because during disturbances
(switching over voltages and lightning), 80% of the voltage appears across the first 10%
of turns from the line end.

 For large power and distribution transformers, an oil-filled tank is necessary for cooling
the windings and the core.

 Two types of losses: Core and copper, occur during operation.
 Heat produced is roughly proportional to the volume of the material in which losses

 Heat dissipation is proportional to the surface area of the same material and the tank.
 The surface is CORRUGATED to increase the surface area. Radiators are also used.

Methods of Cooling

a) Natural Radiation-------- low voltage and output ratings. (500V, 5 KVA).
b) Oil filled and self cooled--------- large sized transformers. (132 KV, 100 MVA).
c) Forced cooling with air blast------ Transformers with ratings higher than 33 KV and

100 MVA.

Conservator Tank

 Due to variations in load and climatic conditions, the oil in oil-filled, self-cooled
transformers expands or contracts.

 In the absence of a conservator tank, high pressures are developed which may burst the


 To provide proper insulation to the output leads to be taken from the transformer tank.
 Porcelain type bushings are used up to 33 KV.
 Condenser type and oil-filled type bushings are used beyond 33 KV.


 Absorption of oil and dust by oil must be prevented.
 To prevent moisture and dust from entering the-conservator tank oil, breather is provided.

Types of Transformer

 Core type: Copper windings surround core.
 Shell type: Iron core surrounds the copper windings.
 To reduce the eddy currents induced in the core, thin laminations are used.
 To reduce the hysteresis loss, heat treated grain oriented silicon steel laminations are

 Generally, distribution transformers are of the Core type.

Core-Type Transformers: There are two types of core-type transformers, they are

1. Core-type
2. Distributed core type.

Page 11

Chapter 1 Machines

THE GATE ACADEMY PVT.LTD. H.O.: #74, KeshavaKrupa (third Floor), 30

Cross, 10

Main, Jayanagar 4

Block, Bangalore-11
: 080-65700750,  [email protected] © Copyright reserved. Web: Page 6

(d) Similarly, the secondary winding is idealized as shown in the figure.

r2 and x2 and are the resistance and leakage reactance of the secondary winding,

II) The actual secondary winding of T2 turns is replaced by an equivalent winding of turns T1,
such that the electrical characteristics remain unaltered. Let r

and x
be the resistance and

leakage reactance of the equivalent secondary winding. Let E
be the voltage induced in the


Then E E⁄ = V V ⁄ = T T ⁄ = ⁄

Let I2 and V2 be the equivalent secondary current and voltage, respectively.

The copper losses in them, i.e., actual and the equivalent secondaries must be the same.

r2 = (I2 )2 r2 so that r2 = (I2/ I2 )2 r2 = (I2/ I1)2 r2 = (T1/ T2)2r2

Again, the p.u. reactance voltage drops are to be the same.

i.e., (I2x2/E2) = (I2 x2 /E2 ) so that x2 =(I2/ I2 )( E2 /E2)x2 = (T1/T2) (T1/T2)x2 =x2(T1/T2)2

Thus, the equivalent resistance of the secondary winding referred to the primary,

r2 = r2 (T1/T2)2. Similarly, x2 = x2(T1/T2)2

III) Since the primary and secondary idealized windings are the seats of the induced e.m.f’s E1
and E21 satisfying the relation (E1/E2 ) = (T1/T2) – 1, the winding can be dispense with and
circuits representing the two sides can be joined together, as shown in the figure below.

The above circuit is the EXACT equivalent circuit of the transformer referred to the primary

In a similar manner, the equivalent circuit referred to the secondary side can be obtained.
(Actually, the roles of the primary and secondary windings are to be interchanged.

So, it is preferable to say that the equivalent circuit is referred to the H.Vside or L.V side)

= ( )


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