PHYSICS PROJECT REPORT:

TRANSFORMERS

By Akhil Govind

Class 12

Roll Number

Indian Educational School

BONAFIDE CERTIFICATE

Certified to be the bona fide report of the original project work

done by AKHIL GOVIND with registration number ______________

of Class XII in Physics during the year 2012-2013

Date Physics Teacher

Principal School Seal Department of Physics

Submitted for AISSCE 2013 in Physics at the Indian Educational

School, Kuwait

Date External Examiner

ACKNOWLEDGEMENT:

I wish to express my sincere gratitude to my Physics teacher Mr.

Austin Antony, for his vital support, guidance and encouragement,

without which this project would not have come forth. I would

also wish to thank the members of my group who helped me with

the project.

AIM:

To study a Transformer in detail and construct a working

transformer.

INDEX:

SL NO. TOPIC PAGE NO.

1. Introduction

2. Principle of working of a transformer

3. Construction of a simple transformer

4. Applications of transformers

5. Energy losses in a transformer

6. Power distribution: Step up and Step Down transformers in details with equations

7. Bibliography

1. INTRODUCTION:

A transformer is an electrical device which is used to change A.C

voltages. A transformer transfers electrical energy from one circuit

to another by using inductively coupled conductors- the

transformer coils. An AC current in the first set of coils creates a

varying magnetic flux in the transformer’s core and thus a varying

magnetic field through the secondary winding. This varying

magnetic field induces a varying Electromotive force in secondary

winding.

Transformers are built in varying sizes and shapes. In some circuits,

they may weigh only a few grams, while in high voltage circuits,

they weigh hundreds of tonnes.

The transfer of electrical energy in transformers occurs without the

use of moving parts.

Transformers are of different types,

classified on the basis of parameters

like power capacity, frequency range,

voltage class, application type, cooling

class, purpose, basic magnetic form

etc.

Transformers are an essential part of

power distribution as they are used to

step up and step down the voltage to

minimise power dissipation and heat

loss while transmission.

Transformers work on the

principle of electromagnetic

induction, discovered

independently by Michael

Faraday and Joseph Henry in

1931.

(Faraday’s experiment with induction between coils)

(Faraday’s ring transformer)

2. Principle of working of a

transformer:

A transformer is based on two principles : 1) Electromagnetism and 2)

Electromagnetic Induction.

By changing current in primary coil, the magnetic flux developed also

changes and this changing magnetic flux induces a voltage in the secondary

coil.

A model of an ideal transformer is shown below:

The current passing through the primary coil creates a magnetic field. The

primary and secondary coils are wrapped around a material of high

magnetic permeability, such as Iron, so that magnetic flux passes through

both the primary and secondary coils.

When a load is connected to the secondary winding, the load current and

voltage will be in the directions indicated if the primary current and voltage

in the directions as shown.

INDUCTION LAW:

The voltage induced across the secondary coil may be calculated from

Faraday’s Law of Induction , which states that :

Vs = Ns . d Φ/d t

Where Vs is the instantaneous voltage, Ns the number of turns in the

secondary coil and Φ the magnetic flux through one turn of the coil.

If the turns of the coil are oriented perpendicularly to the magnetic field

lines, the flux is the product of the magnetic flux density B and the

area A through which it cuts. The area is constant, being equal to the cross-

sectional area of the transformer core, whereas the magnetic field varies

with time according to the excitation of the primary.

Since the same magnetic flux passes through both the primary and the

secondary coils in an ideal transformer, the instantaneous voltage across the

primary winding equals :

Vp = Np . d Φ/d t

By taking the ratios we get the basic equation for stepping up or stepping

down the voltage:

Vs / Vp = Ns / Np

Np/Ns is known as the turns ratio, and is the primary functional characteristic

of any transformer.

In the case of step-up transformers, this may sometimes be stated as the

reciprocal, Ns/Np.

Turns ratio is expressed as an irreducible fraction or ratio like 2:3 and not

0.667 or 100/150.

IDEAL POWER EQUATION:

If the secondary coil is attached to a load that allows current to flow,

electrical power is transmitted from the primary circuit to the secondary

circuit.

Ideally, the transformer is perfectly efficient. All the incoming energy is

transformed from the primary circuit to the magnetic field and into the

secondary circuit.

If this condition is met, the incoming power must equal the outgoing power:

Pincoming = Poutgoing = IpVp = IsVs

Giving the Ideal transformer equation:

Vs / Vp = Ns / Np = Ip / Is

The changing magnetic field induces an electromotive force across each

winding. Since the ideal windings have no impedance, they have no voltage

drop and so the voltages Vp and Vs measured at the terminals of the

transformer, are equal to the corresponding EMFs. The primary EMF, acts in

opposition to the primary voltage, and is termed the Back EMF. This is in

accordance with Lenz’s law, which states that the induction of EMF always

opposes the development of any such change in the magnetic field.

3. CONSTRUCTION OF SIMPLE

TRANSFORMER:

MATERIALS REQUIRED:

1. Some pieces of soft iron, preferably in U shape.

2. Plastic coated wire.

3. Some 6 V bulbs in suitable bulb holder.

4. Connecting wires with 4mm plugs.

5. Alligator clips.

6. A 4 V A.C supply fitted with 5A trip switch.

METHOD:

Transformer setup

Strip the insulation off the end of a long piece of copper wire and wrap it 30 times around a cylinder of soft iron, to make a solenoid. Connect the ends of the wire into the power supply using crocodile clips or other suitable connectors. This is called a primary coil. Connect a 6 V bulb in parallel with the primary coil.

Now take another long piece of wire. Strip the ends, and either: wrap it around the same soft iron core 60 times or if you have U-shaped soft iron cores; use a separate core to put the 60 turns on. This is called the secondary coil .

You should now have two pieces of bare ends from the second piece of wire. Connect them to a 6 V bulb so that you have a loop that is independent of the power supply.

Turn on the power supply and join the two u-shaped soft iron cores together.

Compare the brightness of the two bulbs. The secondary will be much brighter.

Repeat with 15 turns on the secondary coil. This time the secondary will be much dimmer.

(Simple diagram of a transformer)

4. APPLICATIONS OF

TRANSFORMERS:

A major application of transformers is to increase voltage before transmitting electrical energy over long distances 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 therefore low-current) form for transmission and back again afterward, transformers enable economical transmission of power over long distances. Consequently, transformers have shaped the electricity 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 transformers by the time it reaches the consumer.

Transformers 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 electrically isolates the end user from contact with the supply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground, such as between external cables and internal circuits.

The principle of open-circuit (unloaded) transformer is widely used for characterisation of soft magnetic materials.

5. ENERGY LOSSES IN

TRANSFORMERS:

In practice, the output energy of a transformer is less than input energy due

to the following reasons :

1) Flux Leakage: There is always some flux leakage; that is, not all of the flux due to primary passes through the secondary due to poor design of the core or the air gaps in the core. It can be reduced by winding the primary and secondary coils one over the other.

2) Resistance of the windings: The wire used for the windings has some resistance and so, energy is lost due to heat produced in the wire (I2R). In high current, low voltage windings, these are minimised by using thick wire.

3) Eddy currents: The alternating magnetic flux induces eddy currents in

the iron core and causes heating. The effect is reduced by having a laminated core.

4) Hysteresis: The magnetisation of the core is repeatedly reversed by

the alternating magnetic field. The resulting expenditure of energy in the core appears as heat and is kept to a minimum by using a magnetic material which has a low hysteresis loss.

6. Power distribution: Step up and

Step Down transformers in

details with equations:

STEP UP TRANSFORMER: The secondary voltage is higher than the primary voltage. Thus this transformer “steps up” the voltage supplied to it.

STEP DOWN TRANSFORMER: The secondary voltage is lower than the primary voltage. This transformer “steps down” the voltage supplied to it.

STEPPING UP AND STEPPING DOWN: Electrical energy is changed from that of low voltage to higher voltage at a step up transformer at the generating station. This energy is transmitted at high voltage thus minimising the power loss as heat and at the receiving station, the step down

transformer changes voltage from higher to lower. This low voltage AC current is transmitted to domestic circuits. EQUATIONS ASSOCIATED:

We know that:

Vs / Vp = Ns / Np = Ip / Is

The ratio Vs / Vp = Ns / Np is called the transformation ratio K.

For a step up transformer Ns > Np and for a step down transformer Ns < Np

Thus the number of turns in primary is more in step down type and less in

step up type of transformers. Thus the turns ratio Ns / Np is less than 1 in the

step down type and more than one in step up type.

7. BIBLIOGRAPHY:

1. Comprehensive Physics Class 12 2. www. Wikipedia.org 3. www. Wiki-how.com