DASMESH PARIVAR

INTERNATIONAL

SCHOOL

PROJECT REPORT

ON

TRANSFORMERS Submitted By:

TANUJ GUPTA

CLASS: XII

ROLL NO:

________________

They are so important in our lives that without them even the electric

bells fitted in our homes won’t work.

CertificateThis 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 supervision.

_________________Department of PhysicsDasmesh Parivar International School

Acknowledgements"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.

TANUJ

GUPTA

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. 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 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.

Close-up of single-phase pole mount transformer.

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 circuit. The varying current in a circuit produce varying magnetic flux which induces e.m.f. in the neighboring circuit.

CONSTRUCTIONA 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 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

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)   

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

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

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

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

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

EFFICIENCYEfficiency 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.

ENERGY LOSSESIn 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 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 theTransformer 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

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.

A Big Transformer

BibliographyThe data used in this project was taken from the following sources:

www.google.com www.wikipedia.com

www.scribd.com Sears and Zemansky’s University Physics

The End