2014-2015
Certificate This is to certify that student of Class XII, Kendriya Vidyalaya No.2 Uppal, has completed the project titled ‘‘Transformers’’ during the academic year 2014-2015 towards partial fulfilment physics practical examination conducted by AISSCE, New Delhi and submitted satisfactory report, as compiled in the following pages, under my supervision.
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I would like to express my special thanks of gratitude to my teacher Mr N.V.N.G.K Rao who gave me the golden opportunity to do this wonderful project on the topic Transformers , which also helped me in
doing a lot of Research and I came to know about so many new things I am really thankful to them. Secondly I would also like to thank my parents and friends who helped me a lot in finalizing this project within the limited time frame.
P.DHARMA TEJA
The transformer is a device used for converting a low alternating voltage to a high alternating voltage or a high alternating voltage into a low alternating voltage. It is a static electrical device that transfers energy by inductive coupling between its winding circuits. Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used in power plant substations or to interconnect portions of the power grid. All operate on the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in many electronic devices. Transformers are essential for high-voltage electric power transmission, which makes longdistance transmission economically practical. A transformer is most widely used device in both low and high current
circuit. 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.
Close-up of single-phase pole mount transformer.
It is based on the principle of mutual induction that 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.
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 s 1 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. For operation at low frequency, we may have a soft iron. The soft iron core is insulating by joining thin iron strips coated 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.
An ideal voltage step-down transformer. The secondary current arises from the action of the secondary EMF on the (not shown) load impedance.
The ideal transformer as a circuit element
When an altering e.m.f. is supplied to the primary coil p 1p2, 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, and 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 N p and Ns are the no. of turns of the primary secondary coils of the transformer and, Dфь / d t = rate of change of flux in each
turn
of
the
coil
at
this
Ep = -Np Dфь/dt
(1)
Es = -Ns Dфь/dt
(2)
instant,
we
have
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 R p is the resistance o, p1p2 coil, then the instantaneous current Ip in the primary coil is given by I =E – Ep / Rp
E – Ep = Ip Rp When the resistance of the primary is small, R p 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 E s / Ep = E s / 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 N s > Np
In a step down transformer
Es < E so K < 1, hence N s < 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 / Ep
Es Is =
Ip / Is
Or =
K
In a step up transformer
As k > 1, so I p > 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.
BASIC IDEA OF STEP DOWN TRANSFORMER
BASIC IDEA OF STEP UP TRANSFORMER
Efficiency of a transformer is defined as the ratio of output power to the input power i.e. η = output power / input power = E s 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.
In practice, the output energy of a transformer is always less than the input energy, because energy losses occur due to a number of reasons as explained below. 1. Loss of Magnetic Flux: The coupling between the coils is seldom perfect. So, whole of the magnetic flux produced by the primary coil is not linked up with the secondary coil. 2. Iron Loss: In actual iron cores in spite of lamination, Eddy currents are produced. The magnitude of eddy current may, however be small. And a part of energy is lost as the heat produced in the iron core. 3. Copper Loss: In practice, the coils of the transformer possess resistance. So a part of the energy is lost due to the heat produced in the resistance of the coil. 4. Hysteresis Loss: The alternating current in the coil tapes the iron core through complete cycle of magnetization. So Energy is lost due to hysteresis. 5. Magneto restriction: The alternating current in the Transformer may be set its parts in to vibrations and sound may be produced. It is called humming. Thus, a part of energy may be lost due to humming.
1.Take thick iron rod and cover it with a thick paper and wind a large number of turns of thin Cu wire on thick paper (say 60). This constitutes primary coil of the transformer. 2. Cover the primary coil with a sheet of paper and wound relatively smaller number of turns (say 20) of thick copper wire on it. This constitutes the secondary coil. It is a step down transformer. 3. Connect p1, p2 to A.C main and measure the input voltage and current using A.C voltmeter and ammeter respectively. 4. Similarly, measure the output voltage and current through s1and s2. 5. Now connect s1and s2to A.C main and again measure
voltage and current through primary and secondary coil of step up transformer. 6. Repeat all steps for other self made transformers by changing number of turns in primary and secondary coil.
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 XRays 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.
1. Values of current can be changed due to heating effect. 2. Eddy current can change the readings.
1. The output voltage of the transformer across the secondary coil depends upon the ratio (Ns/Np) with respect to the input voltage 2. The output voltage of the transformer across the secondary coil depends upon the ratio (Ns/N p) with respect to the input voltage 3. There is a loss of power between input and output coil of a transformer.
1. Keep safe yourself from high voltage. 2. While taking the readings of current and voltage the A.C should remain constant.
A Big Transformer
NCERT Textbook Class 12 NCERT Physics Lab Manual Class 12 Google Website
The End
