SEMINAR REPORT
ON
AUTO TRANSFORMER
(B. Tech (CSE) Semester -2
Seminar- SE -201)
2020-2021
Submitted to:
Ms. Priyanka Sharma
(CC- Section F-Sem-2)
Submitted by:
Aditya Sethi
Class Roll No: - 6
University Roll No:2018111
Student Id.: 20021069
B. Tech -CSE, Sem:2
GRAPHIC ERA HILL UNVERSITY, DEHRADUN
CERTIFICATE
Certified that Aditya Sethi (Student Id.- 20021069) has
completed
MOOC
Seminar
on
the
topic
“AUTO
TRANSFORMER” for the fulfilment of B. Tech (CSE) II Semester
Seminar (SE-201) in Graphic Era Hill University, Dehradun.
Student has successfully completed this course as best of my
knowledge.
Date: 20-Aug-21
(Ms. Priyanka Sharma)
Class Coordinator
Section-F Sem:2
GEHU, Dehradun
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ACKNOWLEDGMENT
I would like to express our gratitude to the Almighty, the most Beneficent
and the most Merciful, for Successful completion of MOOC Seminar.
I wish to thank our parents for their continuing support and encouragement.
We also wish to thank them for providing us with the opportunity to reach this
far in our studies.
I also acknowledge to my Class coordinator and Subject Teacher Ms.
Priyanka Sharma Ma’am who help me to understand this course.
At last, but not the least I greatly indebted to all other persons who directly
or indirectly helped me during this course.
Aditya Sethi
Student Id- 20021069
Session:2020-2021
C.S.E
GEHU, Dehradun
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TABLE OF CONTENTS
1. Auto Transformer- Principle
2. Construction of Transformer
1.1 Introduction
1.2 Principle of Auto Transformer.
3.Core construction
4. Copper savings in Auto transformer
5. Limitations
6. Advantages of Auto transformer
7. Diadvantages of Auto transformer
8. Application of Auto transformer
9. Conclusion
10. Result
11. Reference
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AUTO TRANSFORMER – PRINCIPLE
The working principle of autotransformer and construction is similar to that of
conventional two winding transformers. However, it differs in the way in which
the primary and the secondary are inter-related.
CONSTRUCTION OF AUTO
TRANSFORMER
INTRODUCTION
An autotransformer (or auto transformer) is a type of electrical
transformer with only one winding. The “auto” prefix refers to the
single coil acting alone. An auto transformer is similar to a two winding
transformer but varies in the way the primary and secondary winding of
the transformer are interrelated.
An auto transformer consists of a single copper wire, which is common
in both primary as well as secondary circuit. The copper wire is wound a
laminated silicon steel core, with at least three tappings taken out.
Secondary and primary circuit share the same neutral point of the
winding. The construction is well explained in the diagram. Variable
turns ratio at secondary can be obtained by the tappings of the winding
(as shown in the figure), or by providing a smooth sliding brush over the
winding. Primary terminals are fixed.
Thus, in an auto transformer, you may say, primary and secondary
windings are connected magnetically as well as electrically.
PRINCIPLE
Autotransformer has single winding. When an alternating supply is given to the primary
circuit, because of Faraday’s Law of electromagnetic Induction, an emf is induced in the
primary part. Since the magnetic field is alternating in nature, and conductors are stationary.
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Autotransformer
The induced emf in primary produces a flux, which is called as primary winding flux. This
flux links the secondary winding and induces an emf on secondary winding due to mutual
induction. Hence emf is transferred in the secondary winding. Based on a number of turns on
the secondary side, the magnitude of induced emf is determined.
Auto Transformer Construction
A transformer basically consists of two parts


Conductors
Core
The conductors in the autotransformer are made up of copper. They are of low resistance.
The copper conductors are insulated with each other. The material used for insulation is
impregnated paper, mica, etc. The insulation also helps in reducing eddy current losses. The
winding is wound around the core. For a single winding transformer, the requirement of
copper is less as compared to two winding transformers.
auto-transformer-construction
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To transfer flux from primary to the secondary, core is used. The core is made up of magnetic
material like silicon steel, CRGO steel, etc. CRGO steel is the most efficient material for
core, as it has the least hysteresis losses. The role core is to transfer flux from one part of
winding to other parts.
Other important parts as shown in figure 3 are bearings, brushes, terminal boards, etc. The
parts shown are used for dimmer stat basically used for laboratory purposes.
Copper savings in auto transformer
Now we will discuss the savings of copper in auto transformer compared to
conventional two winding transformer.
We know that weight of copper of any winding depends upon its length and crosssectional area. Again length of conductor in winding is proportional to its number of
turns and cross-sectional area varies with rated current.
So weight of copper in winding is directly proportional to product of number of
turns and rated current of the winding.
Limitations
An autotransformer does not provide electrical isolation between its
windings as an ordinary transformer does; if the neutral side of the
input is not at ground voltage, the neutral side of the output will not
be either. A failure of the isolation of the windings of an
autotransformer can result in full input voltage applied to the output.
Also, a break in the part of the winding that is used as both primary
and secondary will result in the transformer acting as an inductor in
series with the load (which under light load conditions may result in
near full input voltage being applied to the output). These are
important safety considerations when deciding to use an
autotransformer in a given application.
Because it requires both fewer windings and a smaller core, an
autotransformer for power applications is typically lighter and less
costly than a two-winding transformer, up to a voltage ratio of about
3:1; beyond that range, a two-winding transformer is usually more
economical.
In three phase power transmission applications, autotransformers have
the limitations of not suppressing harmonic currents and as acting as
another source of ground fault currents. A large three-phase
autotransformer may have a "buried" delta winding, not connected to
the outside of the tank, to absorb some harmonic currents.[4]
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In practice, losses mean that both standard transformers and
autotransformers are not perfectly reversible; one designed for
stepping down a voltage will deliver slightly less voltage than
required if it is used to step up. The difference is usually slight
enough to allow reversal where the actual voltage level is not critical.
Like multiple-winding transformers, autotransformers use timevarying magnetic fields to transfer power. They require alternating
currents to operate properly and will not function on direct current.
Because the primary and secondary windings are electrically
connected, an autotransformer will allow current to flow between
windings and therefore does not provide AC or DC isolation.
Advantages of using Auto Transformers
The advantages of an auto transformer include:
1. For transformation ratio = 2, the size of the auto
transformer would be approximately 50% of the corresponding
size of two winding transformer. For transformation ratio say
20 however the size would be 95 %. The saving in cost of the
material is of course not in the same proportion. The saving of
cost is appreciable when the ratio of transformer is low, that is
lower than 2. Thus auto transformer is smaller in size and
cheaper.
2. An auto transformer has higher efficiency than two winding
transformer. This is because of less ohmic loss and core loss
due to reduction of transformer material.
3. Auto transformer has better voltage regulation as voltage
drop in resistance and reactance of the single winding is less.
Disadvantages of Using Auto Transformer
The disadvantages of an auto transformer include:
1. Because of electrical conductivity of the primary and secondary
windings the lower voltage circuit is liable to be impressed
upon by higher voltage. To avoid breakdown in the lower
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voltage circuit, it becomes necessary to design the low voltage
circuit to withstand higher voltage.
2. The leakage flux between the primary and secondary windings
is small and hence the impedance is low. This results into
severer short circuit currents under fault conditions.
3. The connections on primary and secondary sides have
necessarily needs to be same, except when using
interconnected starring connections. This introduces
complications due to changing primary and secondary phase
angle particularly in the case of delta/delta connection.
4. Because of common neutral in a star/star connected auto
transformer it is not possible to earth neutral of one side only.
Both their sides should have their neutrality either earth or
isolated.
5. It is more difficult to maintain the electromagnetic balance of
the winding when voltage adjustment tappings are provided.
It should be known that the provision of tapping on an auto
transformer increases considerably the frame size of
the transformer. If the range of tapping is very large, the
advantages gained in initial cost is lost to a great event.
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Applications of Auto Transformer
The applications are











It increases the voltage drop for the distribution cable
It is used as a voltage regulator
It is used in audio, distribution, power transmission and railways
Autotransformer with several tappings is used to start
the motors like induction as well as synchronous.
It is used in laboratories to obtain a varying voltage continuously.
It is used like regulating transformers in voltage stabilizers.
It increases the voltage in AC feeders
It is applicable in electronics testing centers wherever frequently
changing voltages are required.
It is used where high voltages are necessary like boosters
or amplifiers
It is used in audio devices like speakers to match the impedance
as well as to adjust the device for nonstop voltage supply.
It is used in power stations where the voltage needs to step down
and step up to equal the voltage at the receiving end which is
necessary for the device.
CONCLUSION
From above study we conclude that Auto transformer have less amount of
copper loss required. High efficiency, possible to get smooth and continuous
variation voltage.
RESULT
These are the constructional features of the transformer and the main points in the
design of the transformer.
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REFERENCE
1. Textbook of Electrical Technology by Dr. B.L. Theraja
2. Electrical engineering fundamentals by Vincent Del Toro
3. Https://google.com
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