Physics Project

Certificate This is to certify that UMANG PARASHAR, student of Class XII-B, BIRLA VIDYA NIKETAN has completed the project TO INVESTIGATE THE RELATION BETWEEN: i) OUTPUT AND INPUT VOLTAGE ii) NUMBER OF TURNS IN THE SECONDARY COIL AND PRIMARY COIL OF A SELF DESIGNED TRANSFORMER during the academic year 20012-2013 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 and guidance.

(Mr.ASHISH BHATTACHARYA) Department of physic Birla Vidya Niketan

Acknowledgements I would like to express my sincere gratitude to my Physics teacher Mrs. Nidhi Mehra, for her 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 Physics for their support during the making of this project.

INTRODUCTION A transformer is an electrical device which is used for changing the A.C. voltages. A transformer is most widely used device in both low and high current circuit. As such transformers are built in an amazing strength of sizes. In electronic, measurement and control circuits, transformer size may be so small that it weight only a few tens of grams where as in high voltage power circuits, it may weight hundred of tones. 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.

PRINCIPLE A Transformer based on the Principle of mutual induction according to this principle, the amount of magnetic flux linked with a coil changing, an e.m.f is induced in the neighbouring coil that is if a varying current is set-up in a circuit 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.

CONSTRUCTION The transformer consists of two coils. They are insulated with each other by insulated material and wound on acommon core. For operation at low frequency, we may have asoft iron. The soft iron core is insulating by joining thin ironstrips coated with varnish to insulate them to reduce energy losses by eddy currents.The input circuit is called primary. And theoutput circuit is called secondary.

THEORY AND WORKING OF A 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 goodtransformer, 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 = Or

Es Is

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.

Efficiency :Efficiency 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.

PROCEDURE i)

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 wire with 400 turns. 4. Both coils are wound over same length of the rod, so that almost the entire flux produced by current in one is linked to the other. 5. Connect the coil S with an AC voltmeter. Connect an identical voltmeter across P also. 6. Switch on the current in P and note voltage across the two coils 7. Find the ratio Vp to Vs

OBSERVATIONS 1. We will find that ratio of Vp and Vs across the two coils is equal to the ratio of number of turns in the coil 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 is induced) is called the secondary. 3.Since coil S is placed very close to the coil P,the power in the primary is transferred into the secondary through mutual induction. 4. It is clear from equation 1, that by appropriate choice of the turn ratio i.e., Np/Ns, we can obtain a

higher voltage or lower voltage in S compared to that in P.

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: humming noise of a transformer.

USES OF TRANSFORMER 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 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.

DIFFERENT TYPES OF TRANSFORMERS Instrument transformers Instrument transformers are used for measuring voltage and 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 an instrument, it can be scaled down to a standardized low value. Instrument transformers isolate measurement, protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled.

A current transformer is a transformer designed to provide a current in its secondary coil proportional to the current flowing in its primary coil.[88] Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are designed to have an accurately known transformation ratio in both magnitude and phase, over a range of measuring circuit impedances. A voltage transformer is intended to present a negligible load to the supply being measured. The low secondary voltage allows protective relay equipment and measuring instruments to be operated at a lower voltages.[89] Both current and voltage instrument transformers are designed to have predictable characteristics on overloads. Proper operation of over-currentprotective relays requires that current transformers provide a predictable transformation ratio even during a shortcircuit.

RF transformers There 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 a core means very low inductance. Such transformers may be nothing more than a few turns of wire soldered onto a printed circuit board. Ferrite-core transformers Widely used in intermediate frequency (IF) stages in superheterodyne radio receivers. are mostly tuned transformers, containing a threaded ferrite slug that is screwed in or out to adjust IF tuning. The transformers are usually canned for stability and to reduce interference. Transmission-line transformers For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique. The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil (called a tickler winding) to

inject feedback into an earlier (detector) stage in antique regenerative radio receivers. Baluns Baluns are transformers designed specifically to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar orcoaxial cable and are similar to transmission line transformers in construction and operation.