1. Acknowledgement
s
"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 my friends for their support during
the making of this project.
2. Introduction
They are so important in
our lives that without them
even the electric bells fitted
in our homes won’t work.
3. 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
4. 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
5. transformer. A 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
circuits.
PRINCIPLE
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
6. circuit. The varying current in a circuit produce
varying magnetic flux which induces e.m.f. in the
neighboring circuit.
CONTRUCTION
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
7. with each other. Note that the both the coils are
insulated from the core, the source of alternating
e.m.f is connected to p1 p2, the primary coil and a
load resistance R is connected to s1 s2, the
secondary coil through an open switch S. Thus, no
current can be drawn through the secondary coil as
long as the switch is open.
For an ideal
transformer, we assume that the resistance of the
primary & secondary windings is negligible.
Further, the energy loses due to magnetic flux and
iron core is also negligible. For operation at low
frequency, we may use a soft iron. The soft iron
core is insulating by joining thin iron strips coated
with varnish to insulate them to reduce energy
losses by eddy currents. The input circuit is called
primary and the output circuit is called secondary.
8. 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
THEORY AND WORKING
9. 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 induced e.m.f in the
primary and the secondary coil and Np and Ns are
the no. of turns of the primary secondary coils of
the transformer and, dфB/dt = rate of change of
flux in each turn of the coil at this instant, we have
Ep = -Np dфB/dt …. (1)
Es = -Ns dфB/dt …. (2)
Since the above relations are true at every instant,
so by dividing (2) by (1), we get
10. 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 of p1 p2
coil, then the instantaneous current Ip in the
primary coil is given by,
I =E – Ep / Rp
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.
In a step up transformer
Es > E so K > 1, hence Ns > Np
11. 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 t = 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
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
12. transformer in reality steps down the current & a
step down transformer steps up the current.
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
13. Thus in an ideal transformer, where there are no
power losses, η = 1. But in actual practice, there
are many power losses; therefore, the efficiency of
transformer is less than one.
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
14. magnetic flux produced by the primary coil is not
linked up with the secondary coil.
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
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
15. In voltage regulator for T.V., refrigerator,
computer, air conditioner etc.
In the induction furnaces.
A step down transformer is used for welding
purposes.
A step down transformer is used for obtaining
large current.
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.
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 etc.
