PROJECT2014 - 2015

Step-DownTransformer

by Muhammad Jassim

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 myphysics mentor Mr. Nizar, for his vital support,guidance and encouragement, without which thisproject would not have come forth. I would alsolike to express my gratitude to the staff of theDepartment of Physics at Birla Public School fortheir support during the making of this project.

INTRODUCTION

The transformer is a device used for convertng a low alternatng voltageto a high alternatng voltage or a high alternatng voltage into a lowalternatng voltage (transfers energy between two or more circuitsthrough electromagnetc inducton).

Transformers range in size from small RF transformers a cubiccentmeter in volume to large units interconnectng the power gridweighing hundreds of tons. A wide range of transformer designs isencountered in electronic and electric power applicatons. Since theinventon in 1885 of the frst constant potental transformer,transformers have become essental for the AC transmission,distributon, and utlizaton of electrical energy.

PRINCIPLE

It is based on the principle of mutual induction. A varying currentin the transformer's primary winding creates a varying magneticfux in the core and a varying magnetic feld impinging on thesecondary winding.

This varying magnetic feld at the secondary induces a varyingelectromotive force (EMF) or voltage in the secondary winding. Theprimary and secondary windings are wrapped around a core ofinfnitely high magnetic permeability[e] so that all of the magneticfux passes through both the primary and secondary windings. Withvoltage source connected to the primary winding and loadimpedance connected to the secondary winding, the transformercurrents fow in the indicated directions.

STEP-DOWN TRANSFORMERIn our project step-down transformer is used: Thistransformer converts high voltage at alternatng currentinto low voltage alternatng current. Instep-downtransformer the number of turns in primary coil remainslarge as compare to secondary coil.

Step down transformers are designed to reduce electricalvoltage. Their primary voltage is greater than theirsecondary voltage. This kind of transformer "steps down"the voltage applied to it. For instance, a step downtransformer is needed to use a 110v product in a countrywith a 220v supply.

Step down transformers convert electrical voltage fromone level or phase confguraton usually down to a lowerlevel. They can include features for electrical isolaton,power distributon, and control and instrumentatonapplicatons. Step down transformers typically rely on theprinciple of magnetc inducton between coils to convertvoltage and/or current levels.

CONSTRUCTION

The transformer consists of two coils. They are insulated with each other byinsulated material and wound on a common core. For operaton at low frequency,we may have a sof iron. The sof iron core is insulatng by joining thin iron stripscoated 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.

THEORYSuppose, the number of turns in the primary coil is NP and that in the secondary coil is NS. The resistance of the coil is assumed to be zero. Let dq /dt be the rate of change of fux in each turn of the primary coil. If Ep be the e.m.f. in the primary circuit then, EP = –NP (1)

We suppose that there is no loss of fux between the primary and secondary coils. Then, the induced e.m.f. in the secondary coil will be: ES= –NS (2)

From equatons (i) and (ii), we fnd: Ns/Np= K is called transformer rato or turn rato.

For step up transformer K>1

For step down transformer K < 1

That is for step-up transformer NS> NP, therefore ES>EP.

For the step down transformer NS< NP therefore ES< EP

Efciency:

The efciency of the transformer is given by:

If Ip and Is be the currents in the primary and secondary circuits.

For ideal transformer = 1 = 100%.Therefore ESIS = EPIP

Or

Therefore, for step up, transformer current in the secondary is less than in the primary (IS<IP). And in a step-down transformer we have IS > IP.

Energy Losses in TransformerIn practce, the output energy of a transformer is always lessthan the input energy, because energy losses occur due to an u m b e r o f r e a s o n s a s e x p l a i n e d b e l o w .

1 . Loss of Magnetc Flux: The coupling between the coils isseldom perfect. So, whole of the magnetc fux produced by theprimary coil is not linked up with the secondary coil.

2. Eddy Current Loss: In actual iron cores inspite of laminaton,Eddy currents are produced. The magnitude of eddy currentmay, however be small. And a part of energy is lost as the heatproduced in the iron core.

3 . Heat Loss: In practce, the coils of the transformerpossess resistance. So a part of the energy is lost due tothe heat produced in the resistance of the coil.

4 . Hysteresis Loss: The alternatng current in the coilt a p e s t h e i r o n c o r e t h r o u g h c o m p l e t e c y c l e o fmagnetzaton. So Energy is lost due to hysteresis.

INDEX1. Acknowledgements2. Introduction3. Principle4. Step-Down Transformer5. Construction6. Theory7. Energy Loses in Transformer

8. Bibiliography

BibliographyBIBLIOGRAP

- Internet- www.wikipedia.com, www.encylopedia.com

– NCERT Physics Textbook