step down transformer

1. LORD MAHAVIRA
ACADMEY
10+2 English Medium
(Affiliated to CBSE, New Delhi)
Chilkana Road, Saharanpur-247001
Report on Project
STEP DOWN TRANSFORMER
SESSION: 2017-18

2. Acknowledgements
I would like to express my sincere gratitude to my physics mentor
MR. PANKAJ SIR for his vital support, guidance and
encouragement, without which this project would not have come
forth. I would also like to express my gratitude to the OTHER staff of
the Department of Physics for their support during the making of this
project.
(TEJASH BANSAL)

3. CERTIFICATE
This is to certify that TEJASH BANSAL, student of Class XII,
LORD MAHAVIRA ACADEMY, SAHARANPUR has
completed the project titled Transformer during the academic year
2017-2018 towards partial fulfillment of credit for the Physics
practical evaluation of CBSE 2017, and submitted satisfactory report,
as compiled in the following pages, under my supervision.
Principal Physics lecturer
Mrs Priya Jain PANKAJ SIR

4. CONTENTS
1. INTRODUCTION
2. OBJECTIVES
3. PRINCIPLE
4. CONSTRUCTION
5. THEORY
6. EFFICIENCY
7. CIRCUIT DAIGRAM
8. ENERGY LOSSES IN TRANSFORMER
9. USES
10. APPLICATIONS
11. PRECAUTIONS
12. BIBLIOGRAPHY

5. INTRODUCTION
The transformer is a device used for converting a low alternating
voltage to a high alternating voltage or a high alternating voltage into
a low alternating voltage.
A transformer which increases the a.c. voltage is called a “step up
transformer”

6. A STEP DOWN TRANSFORMER
A step-down transformer is one whose secondary voltage is less than
its primary voltage. It is designed to reduce the voltage from the
primary winding to the secondary winding. This kind of transformer
“step down” the voltage applied to it
As a step-down unit, the transformer convert high-voltage, low-
current power into low-voltage, high-current power.
OBJECTIVE
Prepare a project on “STEP DOWN TRANSFORMER”
PRINCIPLE
A Transformer based on the Principle of mutual induction according
to this principle, the amount of magnetic flux linked with a coil
changing, an e.m.f is induced in the neighbouring coil.
CONSTRUCTION

7. A transformer consists of a rectangular shaft iron core made of
laminated sheets, well insulated from one another. Two coils p1 & p2
and s1 & s2 are wound on the same core, but are well insulated with
each other. Note that the both the coils are insulated from the core,
the source of alternating e.m.f is connected to p1p2, the primary coil
and a load resistance R is connected to s1 s2, the secondary coil
through an open switch S. thus there can be no current through the
sec. coil so long as the switch is open.
For an ideal transformer, we assume that the resistance of the
primary & secondary winding is negligible. Further, the energy loses
due to magnetic the iron core is also negligible.
THEORY
When an altering e.m.f. is supplied to the primary coil p1p2, an
alternating current starts falling in it. The altering current in the
primary produces a changing magnetic flux, which induces altering
voltage in the primary as well as in the secondary. In a good-
transformer, whole of the magnetic flux linked with primary is also
linked with the secondary, then the induced e.m.f. induced in each
turn of the secondary is equal to that induced in each turn of the

8. primary. Thus if Ep and Es be the instantaneous values of the e.m.f.’s
induced in the primary and the secondary and Np and Ns are the no.
of turns of the primary secondary coils of the transformer and
Dфь / dt = rate of change of flux in each turnoff
the coil at this instant, we have
Ep = -Np dфь/dt ---------------- - (1)
Es = -Ns dфь/dt ----------------- (2)
Since the above relations are true at every instant, so by dividing 2
by 1, we get
Es / Ep = - Ns / Np ----------------(3)
As Ep is the instantaneous value of back e.m.f induced in the primary
coil p1, so the instantaneous current in primary coil is due to the
difference (E – Ep ) in the instantaneous values of the applied and
back e.m.f. further if Rp is the resistance o, p1p2 coil, then the
instantaneous current Ip in the primary coil is given by
Ip = E – Ep / Rp

9. E – Ep = Ip Rp
When the resistance of the primary is small, Rp Ip can be neglected so
therefore
E – Ep = 0 or Ep = E
Thus back e.m.f = input e.m.f
Hence equation 3 can be written as
Es / Ep = Es / E = output e.m.f / input e.m.f = Ns / Np = K
Where K is constant, called turn or transformation ratio.

10. In a step down transformer
Es < E so K < 1, hence Ns < Np
If Ip = value of primary current at the same instant t
And Is = value of sec. current at this instant, then
Input power at the instant t = Ep Ip and
Output power at the same instant = Es Is
If there are no losses of power in the transformer, then
Input power = output power Or
Ep Ip = Es Is Or
Es / Ep = Ip / Is = k

12. Efficiency:-
Efficiency of a transformer is defined as the ratio of output power to
the input power. i.e.
η = output power / input power = Es Is / Ep Ip
Thus in an ideal transformer, where there is no power losses, η = 1.
But in actual practice, there are many power losses, therefore the
efficiency of transformer is less than one.
CIRCUIT DIAGRAM
ENERGY LOSSES:-

13. Following are the major sources of energy loss in a transformer:
1. Copper loss is the energy loss in the form of heat in the copper
coils of a transformer. This is due to joule heating of conducting
wires.
2. Iron loss is the energy loss in the form of heat in the iron core of
the transformer. This is due to formation of eddy currents in iron
core. It is minimized by taking laminated cores.
3. Leakage of magnetic flux occurs inspite of best insulations.
Therefore, rate of change of magnetic flux linked with each turn of
S1S2 is less than the rate of change of magnetic flux linked with each
turn of P1P2.
4. Hysteretic loss is the loss of energy due to repeated
magnetization and demagnetization of the iron core when A.C. is fed
to it.
5. Magneto striation i.e. humming noise of a transformer.
USES OF TRANSFORMER

14. A transformer is used in almost all a.c. operations
1· In voltage regulator for T.V., refrigerator, computer, air
conditioner etc.
2. In the induction furnaces.
3. A step down transformer is used for welding purposes.
4. A step down transformer is used for obtaining large current.
5. In transmission of a.c. over long distance.
APPLICATIONS

15. A major application of transformer is to increases voltage before
transmitting electrical energy over long distance through wires.
Wires have resistance and so dissipate electrical energy at a rate
proportional to the square of the current through the wire. By
transforming electrical power to a high-voltage (and there for low-
current) form for transmission and back again afterward,
transformers enable economical transmission of power over long
distance. Consequently, transformers have shaped the electrical
supply industry, permitting generation to be located remotely from
points of demand. All but a tiny fraction of the world’s electrical
power has passed through a series of transforms by the time it
reaches the consumer.
Transformer are also used extensively in electronic products to step
down the supply voltage to a level suitable for the low voltage
circuits they contain.
The transformer also used electrically isolates the end user from
contact with the supply voltage.
Transformer range in size from a thumbnail-sized coupling
transformer hidden inside a stage microphone to huge unit weighing
hundreds of tons used in power stations, or to interconnect portions
of power grids. All operate on the same basic principles, although the
range of designs is wide. While new technologies have eliminated

16. the need for transformers in some electronic circuits, transformers
are still found in nearly all electronic devices designed for household
(“mains”) voltage. Transformer are essential for high-voltage
electrical power transmission, which makes long-distance
transmission economically practical.
Single and audio transformer are used to couple stages of amplifier
and to match devices such as microphones and record players to the
input of the amplifiers. Audio transformer allowed telephone circuit
to carry on a two-way conservation over a single pair of wires. A
balun transformer converts a single that is referenced to ground to a
signal that has balanced voltage to ground, such as between external
cables and internal circuits.
PRECAUTIONS

17. 1. The loss of power in the transmission lines is I2
R, where I is
strength of current and R is the resistance of wires. To reduce
the power loss, a.c. is transmitted over long distance at
extremely high voltages. This reduces I in the same ratio.
Therefore, I2
R becomes negligibly low.
2. Permeability of magnetic material of transformer core must be
high.
3. Oiling in transformer provides insulation as well as cooling.
4. The transformer core must be laminated to minimize loss of
energy due to eddy currents.
5. Safety from high voltage is maintained.

18. BIBLIOGRAPHY
1. Physics textbook for class XII ,1st
Edition NCERT (2007).
2. Concepts of Physics, H.C.VERMA, 1ST
Edition, Bharti
bhawan(1993).
3. Fundamentals of Physics , David Halliday, Robert resnick and
Jearl walker, 7th
Edition, Wiley-India(2004)
4. Practical physics for class XII,
J.S.Jaiswaland Dr.Rajendra singh ,3rd
Edition ,Laxmi
Publications(2009).