2. They are so important in our lives
that without them even the electric
bells fitted in our homes won’t
This is to certify that Tanuj Gupta, student of Class XII, Dasmesh Parivar International
School, has completed the project titled Transformers during the academic year 2012-
2013 towards partial fulfillment of credit for the Physics practical evaluation of CBSE
2013, and submitted satisfactory report, as compiled in the following pages, under my
Department of Physics
Dasmesh Parivar International School
"There are times when silence speaks so much more loudly than words of praise to
only as good as belittle a person, whose words do not express, but only put a
veneer over true feelings, which are of gratitude at this point of time."
I would like to express my sincere gratitude to my physics mentor 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 staff of the
Department of Physics at Dasmesh Parivar International School for their support
during the making of this project.
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. It is a static electrical device
that transfers energy by inductive coupling
between its winding circuits. Transformers range in
size from a thumbnail-sized coupling transformer
hidden inside a stage microphone to huge units
weighing hundreds of tons used in power plant
substations or to interconnect portions of the
power grid. All operate on the same basic
principles, although the range of designs is wide.
While new technologies have eliminated the need
for transformers in some electronic circuits,
transformers are still found in many electronic
devices. Transformers are essential for high-
voltage electric power transmission, which makes
long-distance transmission economically practical.
A transformer is most widely used device in both
low and high current circuit. In a transformer, the
electrical energy transfer from one circuit to
another circuit takes place without the use of
moving parts. A transformer which increases the
voltages is called a step-up transformer. A
6. transformer which decreases the A.C. voltages is
called a step-down transformer.
Transformer is, therefore, an essential
piece of apparatus both for high and low current
Close-up of single-phase pole mount transformer.
It is based on the principle of mutual induction that
is if a varying current is set-up in a circuit then
induced e.m.f. is produced in the neighboring
circuit. The varying current in a circuit produce
varying magnetic flux which induces e.m.f. in the
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. For operation at low frequency, we
may have a soft iron. The soft iron core is
insulating by joining thin iron strips coated with
varnish to insulate them to reduce energy losses
9. by eddy currents. The input circuit is called
primary. And the output circuit is called secondary.
An ideal voltage step-down transformer. The secondary current arises from the
action of the secondary EMF on the (not shown) load impedance.
The ideal transformer as a circuit element
10. THEORY AND WORKING
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, and then the induced e.m.f. induced in
each turn of the secondary is equal to that induced
in each turn of the 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 turn of the coil at this instant, we have
Ep = -Np Dфь/dt (1)
11. 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
I =E – Ep / Rp
E – Ep = Ip Rp
12. 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
In a step up transformer
Es > E so K > 1, hence Ns > Np
In a step down transformer
Es < E so K < 1, hence Ns < Np
If Ip=value of primary current at the same instant t
13. 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 / E p = Ip / Is = K
In a step up transformer
As k > 1, so Ip > Is or Is < Ip
I.e. current in sec. is weaker when secondary
voltage is higher. Hence, whatever we gain in
voltage, we lose in current in the same ratio.
Similarly it can be shown, that in a step down
transformer, whatever we lose in voltage, we gain
in current in the same ratio.Thus a step up
14. transformer in reality steps down the current & a
step down transformer steps up the current.
BASIC IDEA OF STEP DOWN TRANSFORMER
BASIC IDEA OF STEP UP TRANSFORMER
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.
16. ENERGY LOSSES
In practice, the output energy of a transformer is always
less than the input energy, because energy losses occur
due to a number of reasons as explained below.
1. Loss of Magnetic Flux: The coupling between the
coils is seldom perfect. So, whole of the magnetic flux
produced by the primary coil is not linked up with the
2. Iron Loss: In actual iron cores in spite of lamination,
Eddy currents are produced. The magnitude of eddy
current may, however be small. And a part of energy is
lost as the heat produced in the iron core.
3. Copper Loss: In practice, the coils of the transformer
possess resistance. So a part of the energy is lost due to
the heat produced in the resistance of the coil.
4. Hysteresis Loss: The alternating current in the coil
tapes the iron core through complete cycle of
magnetization. So Energy is lost due to hysteresis.
5. Magneto restriction: The alternating current in the
17. Transformer may be set its parts in to vibrations and
sound may be produced. It is called humming. Thus, a
part of energy may be lost due to humming.
USES OF TRANSFORMER
A transformer is used in almost all a.c. operations
In voltage regulator for T.V., refrigerator,
computer, air conditioner etc.
In the induction furnaces.
A step down transformer is used for welding
A step down transformer is used for obtaining
A step up transformer is used for the
production of X-Rays and NEON advertisement.
Transformers are used in voltage regulators
and stabilized power supplies.
18. Transformers are used in the transmissions of
a.c. over long distances.
Small transformers are used in Radio sets,
telephones, loud speakers and electric bells
A Big Transformer
The data used in this project was taken from the
Sears and Zemansky’s University Physics