Output equations; Main Dimensions; kVA output for 1 & 3 phase transformers; Window space factor; Design of core and winding; Overall dimensions; Operating characteristics; No-load current; Temperature rise in Transformers; Design of Tank; Methods of cooling of Transformers.

1. This unit consists of
 Introduction
 Types
 Output equation
Design of cores
Overall dimensions
 Design of winding
 Design of tank with cooling tubes
 Efficiency and losses

2. The primary and secondary coils are wrapped around a core of
very high magnetic permeability, such as iron, so that most of the
magnetic flux passes through both the primary and secondary
coils.
Changing the current in the primary coil changes the magnetic
flux that is developed. The changing magnetic flux induces a
voltage in the secondary coil.
INTRODUCTION

3. TYPES
 Core Type

4.  Shell Type

5. COMPARISON OF CORE AND SHELL TYPE
TRNSFORMERS

6. OUTPUT EQUATION OF SINGLE PHASE
TRANSFORMER

7. L
V
L
V
L
V
L
V
H
V
H
V
H
V
H
V
1-phase core type transformer with concentric windings
Window
1-phase shell type transformer with sandwich windings
LV
HV
LV
HV
LV
LV
HV
LV
HV
LV
Window
CROSS SECTION VIEW OF SINGLE PHASE
TRANSFORMER
CORE
CORE

10. OUTPUT EQUATION OF THREE PHASE TRANSFORMER

11. L
V
L
V
H
V
H
V
L
V
L
V
H
V
H
V
3-phase core type transformer with concentric windings
L
V
L
V
H
V
H
V
Window
CROSS SECTION OF THREE PHASE TRANSFORMER

13. DESIGN OF CORES
For core type transformer the cross-section may be
(i) Rectangular
(ii) Square
(iii) Stepped
When circular coils are required for distribution and power transformers,
the square and stepped cores are used.
For shell type transformer the cross-section may be
Rectangular

14. The rectangular core is suitable for small and low voltage transformers.
 In square cores the diameter of the circumscribing circle is larger than the
diameter of stepped cores of same area of cross-section.
Thus when stepped cores are used the length of mean turn of winding is
reduced with consequent reduction in both cost of copper & copper loss.
However with a large number of steps a large number of different sizes of
laminations have to be used. This results in higher labour charges for shearing
and assembling different types of laminations.

17. 2-Step
Or Cruciform- Core
Ө
b a
b
a

18. Ratio Square
Core
Cruciform
Core
3 stepped
core
4 stepped
core
Gross Area / Area of circumscribing circle 0.64 0.79 0.84 0.87
Actual Iron Area / Area of circumscribing circle 0.58 0.71 0.75 0.78
Core Area factor Kc 0.45 0.56 0.6 0.62
Ratios of area of core and circumscribing circle
The Area of circumscribing circle is more effectively utilised
by increasing the number of steps.

19. DESIGN OF WINDING
The transformer has one low voltage winding and one high voltage
winding.
The design of winding involves the determination of number of
turns & area of cross-section of the conductor used for winding.
The number of turns is estimated using voltage rating & emf per turns.
The area of cross- section is estimated using rated current & current
density.
Usually the number of turns of low voltage winding is estimated first
using the given data & it is corrected to nearest integer.

20. Then the number of turns of high voltage winding are chosen
to satisfy the voltage rating of the transformer.
Number of turns in low voltage winding , T1=V1/Et=AT/I1
Number of turns in high voltage winding , T2=T1*V2/V1
V1,V2 = Voltage in low & high values
Rated current in a winding = (KVA per phase*103)/Voltage
rating of the winding

23. OVERALL DIMENSIONS OF THE TRANSFORMER

24. Vertical cross
section
Horizontal
cross section

28. TUTORIAL
Estimate the main dimensions including winding conductor
area of a 3 – phase , - Y core type transformer rated at
300kVA, 6600/440 V,50Hz. A suitable core with 3 – steps
having a circumscribing circle of 0.25m diameter and a leg
spacing of 0.4m is available. Emf per turn =8.5 V, current
density = 2.5 A/mm 2 , Window space factor= 0.28 , stacking
factor = 0.9

29. Determine the dimensions of the core , the number of turns , the
cross section area of conductors in primary and secondary
windings of a 100 kVA , 2200/480 V, 1 phase , core type
transformer to operate at a frequency of 50 Hz , by assuming
the following data. Approximate Volt / turn = 7.5 V. Maximum
flux density = 1.2 Wb/m2.Ratio of effective cross sectional area
of core to square of diameter of circumscribing circle is
0.6.Ratio of height to width of window is 2.Window space
factor = 0.28.Current density = 2.5 A / mm2

31. Ht
Wt
H
D D
l
l
b b
De
De
Lt

32. Methods of cooling of transformers