CERTIFICATET h i s i s t o c e r t i f y t h a tUMANG PARASHAR, s t u d e n t o f   C l a s sXI I -B,  BIRLA VIDYA NIKETAN has completed theprojectTO INVESTIGATE THE RELATION BETWEEN:i) OUTPUT AND INPUT VOLTAGEii) NUMBER OF TURNS IN THE SECONDARY COILAND PRIMARY COILOF A SELF DESIGNED TRANSFORMERduring the academic year 20012-2013 towards partialfulfillment of credit for the Physics practical evaluationof CBSE 2013, and submitted satisfactory report, ascompiled in the following pages, under my supervisionand guidance.(Mr.ASHISH BHATTACHARYA)Department of physicBirla Vidya Niketan

 ACKNOWLEDGEMENT 

Acknow l e d g emen t sI would like to express my sincere gratitude to my PhysicsteacherMrs. Nidhi Mehra, for her vital support,guidance and encouragement, without which this projectwould not have come forth. I would also like to expressmy gratitude to the staff of the Physics for their supportduring the making of this project.

INTRODUCTION 

A transformer is an electrical device which is used forchanging the A.C. voltages. A transformer is most widelyused device in both

low and high current circuit. As suchtransformers are built in an amazing strength of sizes. Inelectronic, measurement and control circuits,transformer size may be so small that it weight only afew tens of grams where as in high voltage powercircuits, it may weight hundred of tones.In a transformer, the electrical energy transfer from onecircuit to another circuit takes place without the use of moving parts.A transformer which increases the voltages is called astep-up transformer. A transformer which decreases theA.C. voltages is called astep-down transformer.Transformer is, therefore, an essential piece of apparatusboth for high and low current circuits

PRINCIPLE 

PRINCIPLE A Transformer based on the Principle of mutual inductionaccording to this

principle, the amount of magnetic fluxlinked with a coil changing, an e.m.f is induced in theneighbouring coil that is if a varying current is set-up in acircuit induced e.m.f. is produced in the neighboringcircuit. The varying current in a circuit produce varyingmagnetic flux which induces e.m.f. in the neighboringcircuit.

CONSTRUCTION The transformer consists of two coils. They areinsulated with each other by insulated material andwound on acommon core. For operation at lowfrequency, we may have asoft iron. The soft iron core isinsulating by joining thin ironstrips coated with varnish toinsulate them to reduce energy losses by eddycurrents.The input circuit is called primary. Andtheoutput circuit is called secondary.

THEORY AND WORKING OF A TRANSFORMERWhen an altering e.m.f. is supplied to the primary coilp1p2, an alternating current starts falling in it. Thealtering current in the primary produces a changingmagnetic flux, which induces altering voltage in theprimary as well as in the secondary. In a good-transformer, whole of the magnetic flux linked withprimary is also linked with the secondary, then theinduced e.m.f. induced in each turn of the secondary isequal to that induced in each turn of the primary. Thus if Epand Es be the instantaneous values of the e.m.f.’sinduced in the primary and the secondary and NpandNs

are the no. of turns of the primary secondary coils of the transformer andDфь / dt= rate of change of flux in each turnoff the coil at this instantwe haveEp= -Npdфь/dt----------------- (1) andEs= -Ns dфь/dt----------------- (2Since the above relations are true at every instant, so bydividing 2 by 1, we getEs/ Ep= - Ns/ Np----------------(3)As Epis the instantaneous value of back e.m.f induced inthe primary coil p1

, so the instantaneous current inprimary coil is due to the difference (E –Ep) in theinstantaneous values of the applied and back e.m.f.further if Rpis the resistance o, p1p2coil, thenthe instantaneous current Ip in the primary coil is givenbyIp= E –Ep/ Rp E –Ep

= IpRp When the resistance of the primary is small, RpIpcan beneglected so thereforeE –Ep= 0 or Ep= EThus back e.m.f = input e.m.f Hence equation 3 can be written asEs/ Ep= Es/ E = output e.m.f / input e.m.f = Ns/ N

p= KWhere 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< N

p If Ip= value of primary current at the same instant tAndIs= value of sec. current at this instant, thenInput power at the instant t = EpIpandOutput power at the same instant = EsIs If there are no losses of power in the transformer, thenInput power = output powerOrEpIp= EsIs

 Or

 Es/ Ep= Ip/ Is= KIn a step up transformer As k > 1, so Ip> Isor Is< Ipi.e. current in sec. is weaker when secondary voltage ishigher.Hence, whatever we gain in voltage, we lose in current inthe same ratio.Similarly it can be shown, that in a step downtransformer,

whatever we lose in voltage, we gain incurrent in the same ratio.Thus a step up transformer in reality steps down thecurrent & a step down transformer steps up the current.EFFICIENCYEfficiency of a transformer is defined as the ratio of output power to the input power. i.e.η= output power / input power = EsIs/ EpIp 

Thus in an ideal transformer, whereTHERe is no powerlosses, η= 1. But in actual practice, there are manypower losses; therefore the efficiency of transformer isless than one.PROCEDURE ) 

Demonstrating the principle of transformer by winding primary and secondary coil on a steel rod 1. Take a soft iron rod of cm and cm diameter.Wrap thick paper on it. 2. Wind a coil P of enameled copper wire 200 turns. 3. Wind another coil S of thick enameled copper wirewith 400 turns. 4. Both coils are wound over same length of the rod,so that almost the entire flux produced by current inone is linked to the other. 5.

 Connect the coil S with an AC voltmeter. Connect anidentical voltmeter across P also. 6. Switch on the current in P and note voltage acrossthe two coils 7. Find the ratio Vp to Vs

 

OBSERVATIONS

1. We will find that ratio of Vp and Vs across the twocoils is equal to the ratio of number of turns in thecoil P to that in the coil S.i.e., Vp/Vs = Np/Ns ---------------(1)2. The coil P (to which AC voltage is applied) is Called the primary and coil S (in which AC isinduced) is called the secondary.3.Since coil S is placed very close to the coil P,thepower in the primary is transferred into thesecondary through mutual induction.4. It is clear from equation 1, that by appropriatechoice of the turn ratio i.e., Np/Ns, we can obtain ahigher voltage or lower voltage in S compared tothat in P.

ENERGY LOSSES:-

Following are the major sources of energy loss in atransformer:1.Copper loss:is the energy loss in the form of heat inthe copper coils of a transformer. This is due to jouleheating of conducting wires.2.Iron loss:is the energy loss in the form of heat in theiron core of the transformer. This is due to formation of eddy currents in iron core. It is minimized by takinglaminated cores.3. Leakage of magnetic flux:occurs inspite of bestinsulations. Therefore, rate of change of magnetic fluxlinked with each turn of S1S2is 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 repeatedmagnetization and demagnetization of the iron corewhen A.C. is fed to it.5.Magneto striation:humming noise of a transformer.

USES OF A TREANSFORMER:A transformer is used in almost all a.c. operations In voltage regulator for T.V., refrigerator,computer, air conditioner etc. A step down transformer is used for weldingpurposes. A step down transformer is used for obtaininglarge current.

DIFFERENT TYPES OF TRANSFORMERSInstrument transformers Instrument transformers are used for measuring voltageand current in electrical power systems, and for  power  system protection   and control. Where a voltage or current is too large to be conveniently used by aninstrument, it can be scaled down to a standardized lowvalue. Instrument transformers isolate measurement,protection and control circuitry from the high currents orvoltages present on the circuits being measured orcontrolled.A  current transformer   is a transformer designed toprovide a current in its secondary coil proportional to thecurrent flowing in its primary coil.

[88]  

Voltage transformers   (VTs), also referred to as "potential 

transformers" (PTs), are designed to have an accuratelyknown transformation ratio in both magnitude andphase, over a range of measuring circuit impedances. Avoltage transformer is intended to present a negligibleload to the supply being measured. The low secondaryvoltage allows protective relay equipment and measuringinstruments to be operated at a lower voltages.[89] Both current and voltage instrument transformers aredesigned to

have predictable characteristics onoverloads. Proper operation of over-currentprotective  

relays   requires that current transformers provide a 

predictable transformation ratio even during a short-circuit.RF transformersThere are several types of transformers used in radio frequency

 (RF) work. Steel laminations are not suitable

 

for RF. Air-core transformers

These are used for high frequency work. The lack of acore means very low  inductance . Such transformers maybe nothing more than a few turns of wire soldered ontoa  printed circuit board . 

Ferrite-core transformersWidely used in  intermediate frequency   (IF) stages 

in  superheterodyne   radio receivers . are mostly tuned 

transformers, containing a threaded ferrite slug that isscrewed in or out to adjust IF tuning. The transformersare

usually canned for stability and to reduceinterference.Transmission-line transformersFor  radio frequency   use, transformers are sometimes 

made from configurations of transmission line,sometimes  bifilar   or  coaxial   cable, woundaround  ferrite   or other types of core. This style of transformer gives an extremely wide  bandwidth   but onlya limited number of ratios (such as 1:9, 1:4 or 1:2) can beachieved with this technique.The core material increases the inductance dramatically,thereby raising its  Q factor . The cores of suchtransformers help improve performance at the lowerfrequency end of the band. RF transformers sometimesused a third coil (called a tickler winding) to

 

inject  feedback into an earlier (detector) stagein  antique   regenerative   radio receivers .   

BalunsBaluns are transformers designed specifically to connectbetween  balanced   and  unbalanced   circuits. These aresometimes made from configurations of transmissionline and sometimes  bifilar   orcoaxial   cable and are similarto transmission line transformers in construction andoperation.