Acknowledgements

I would like to express my sincere gratitude to my

physics mentor MR. V.K PATHAK, 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.

I can’t forget to offer my sincere thanks to my class

mate who helped me to carry out this project work

successfully and for their valuable advice and support

which I received from them time to time.

SUSHIL NATH GUPTA

Certificate

THIS IS TO CERTIFY THAT SUSHIL NATH GUPTA OF CLASS 12 ‘C’ HAS

COMPLETED HIS PROJECT ON THE TOPIC TRANSFORMER.

HE HAS SUCCESSFULLY PREPARED THIS PROJECT REPORT IN THE

PHYSICS LABORATORY OF OUR SCHOOL.

THIS PROJECT MAY BE CONSIDERED AS PARTIAL FULLFILMENT OS

AISSCE 2014-2015 CONDUCTED BY THE CENTRAL BOARD OF

SECONDARY EDUCATION, DELHI.

……………………………………………….. …………………………………………………

INTERNAL EXAMINER EXTERNAL EXAMINER

INTRODUCTION

A transformer is an electrical device that transfers energy

between two or more circuits through electromagnetic induction.

A varying current in the transformer's primary winding creates a

varying magnetic flux in the core and a varying magnetic field impinging on the secondary winding. This varying magnetic

field at the secondary induces a varying electromotive force (emf) or voltage in the secondary winding. Making use of Faraday's

Law in conjunction with high magnetic permeability core properties, transformers can thus be designed to efficiently

change AC voltages from one voltage level to another within power networks.

Transformers range in size from RF transformers less than a

cubic centimetre in volume to units interconnecting the power grid weighing hundreds of tons. A wide range of transformer

designs is encountered in electronic and electric power applications. Since the invention in 1885 of the first constant

potential transformer, transformers have become essential for the AC transmission, distribution, and utilization of electrical energy

PRINCIPLE

IT IS BASED ON THE PRINCIPLE OF MUTUAL INDUCTION THET IS IF A

VARYING CURRENT IS SET UP IN A CIRCUIT THEN INDUCED E.M.F. IS

PRODUCED IN THE NEIGHBOURING CIRCUIT. THE VARYING CURRENT IN

A CIRCUIT PRODUCE VARYING MAGNETIC FLUX WHICH INDUCES E.M.F.

IN THE NEIGHBOURING CIRCUIT.

CONSTRUCTION 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.

VP - is the Primary Voltage NP - is the Number of Primary Windings

VS - is the Secondary Voltage NS - is the Number of Secondary Windings

Φ (phi) - is the Flux Linkage

THEORY AND WORKING OF TRANSFORMER

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 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) And

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

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.

ENERGY LOSSES:-

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.

EFFECIENCY OF TRANSFORMER

A transformer does not require any moving parts to transfer energy. This means that there are no friction or windage losses associated

with other electrical machines. However, transformers do suffer from other types of losses called “copper losses” and “iron losses” but generally these are quite small. Copper losses, also known as I2R loss is the electrical power which is lost in heat as a result of circulating the currents around the transformers copper windings, hence the name. Copper losses represents the greatest loss in the operation of a transformer. The actual watts of power lost can be determined (in each winding) by squaring the amperes and multiplying by the resistance in ohms of the winding (I2R). Iron losses, also known as hysteresis is the lagging of the magnetic molecules within the core, in response to the alternating magnetic flux. This lagging (or out-of-phase) condition is due to the fact that it requires power to reverse magnetic molecules; they do not reverse until the flux has attained sufficient force to reverse them. Their reversal results in friction, and friction produces heat in the core which is a form of power loss. Hysteresis within the transformer can be reduced by making the core from special steel alloys. The intensity of power loss in a transformer determines its efficiency. The efficiency of a transformer is reflected in power (wattage) loss between the primary (input) and secondary (output) windings. Then the resulting efficiency of a transformer is equal to the ratio of the power output of the secondary winding, PS to the power input of the primary winding, PP and is therefore high. An ideal transformer is 100% efficient because it delivers all the energy it receives. Real transformers on the other hand are not 100% efficient and at full load, the efficiency of a transformer is between 94% to 96% which is quiet good. For a transformer operating with a constant voltage and frequency with a very high capacity, the efficiency may be as high as 98%. The efficiency, η of a transformer is given as: Transformer Efficiency

where: Input, Output and Losses are all expressed in units of power. Generally when dealing with transformers, the primary watts are called “volt-amps”, VA to differentiate them from the secondary watts. Then the efficiency equation above can be modified to:

It is sometimes easier to remember the relationship between the transformers input, output and efficiency by using pictures. Here the three quantities of VA, W and η have been superimposed into a triangle giving power in watts at the top with volt-amps and efficiency at the bottom. This arrangement represents the actual position of each quantity in the efficiency formulas.

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.

BIBLIOGRAPHY

1. Physics for class XII by pradeep

2. www.yahoo.com

3. www.google.com

4. www.live.com

5. www.rediffmail.com

SESSION: - 2014-2015

PHYSICS PROJECT ON

TRANSFORMER

NAME – SUSHIL NATH GUPTA

CLASS – XII ‘C’

BOARD ROLL –

ROLL - 56