The coilP lo which electric energy is

Iu Maximum energy in the inductor the primary and the coil érom whichsupptied
is called
3. 2
or output is obtained is called the
energy s drawn
×10 1
1.5 x10xh 30 secondary. To
prevcnt encrgy losses due to eddy cuTenAs, A
2 10-6
60x 10I4x10 2 nated core is used, Because of high permcabsity otJam
'
1.5x iron, the entire magnetic uxdue tn the curtet in sot the
primary coil practically remains in the iton coe and
I,= 2 x100.2A hence passes fully through the srondary. Thás abo
1
1 prevents the stray currents being generated in the con
4. C= 4x9.87 x(10 ) x 10 x103 ductors ying, around and the consequent power oss.
- 2.53 x 10 F= 2,53 pF. Two types of arrangerments are generally used for
1 (Or
1 winding of primary and secondary coils in a tran
4n'LC former:
5. 2n VLC
(300)² 1. Core type. In the core type transformers, the
t4rC 4x9.87 x(3 x 10) x 2.4 *10-6 primary and secondary coils are wound on separate
limbs of the core so that the core is largely surrounded
=1.056 x 10 H by the coils. Many of the modern transsormers are of
closed core type as shown in Fig 7.48(a)
7.22, TRANSFORMER Soft
iron-core

32. What is a transformer ? Explain the principle, Primary wwndarv

How
construction, working and theory of a transformer.various
Current affected in a trarsformer? What are the
reduced ?
be Q
energy losses in a transformer ? How can they
Transformer. A transformer is an electrical device for
that at
converting an alternating current at low voltage intovoltage,
higlh voltage or vice nersa Ifitincreasesiftheit input
decreases the (a)
it is called step up transformer and Soft
input voltage, it is called step down transformer. iron-core Primary
9 Secondary

To
A.C. S load
SOurce

-Laminated iron core

(a)
(b)

Fig. 7.48 (a)Core type and (b)Sheil type transfoner.

A.C. To
he
2. Shell type. In the shell type transformers,
S
sOurce load
primary and secondary coils are wound one over
coils are
another on the same limb of the iron core. The
core. Trans
Laminated iron core
very largely surrounded by the iron
transmitters and
(6) formers used in radio and TV
7.48(b).
receivers are of shell type, as shown in Fig.
Fig. 7.47 (a) Step-up and (b) step-down, transformer. Working, As the alternating current flows
through
Principle. It works on the principle of mutual the primary, it generates an altemating magnene i
induetion, ic, when achanging current is passed hrough in the core which also passes through the sevvaay
one of the tuo ináuctiely caupled cails, an induced enfis Thischanging flux sets up anenfindued emt n h
in the primary. If
set up in the other coil. secondary, also a self-induced
Construction, A transformer essentially consists of there is no leakage of magnetic flux, then tlu hnked
WO COIs of insulated copper wire having different with each turn of the primary will be equat to hat
number of turns and wound on the same soft iron core. linked with each tum of the secondary.
 Theory. Consider the situation when no load is Hence
connected to the secondary, i.e., its terminals are open. 1,E N, .2)
Let N, and N, be the number of turns in the primary Thus astep up transformer steps up the voltage.
and secondary respectively. Then but steps down the current exactly in the same ratio
do
Induced emf in the primary coil, E=-N dt
Similarly, a step down transformer steps down the
voltage but steps up the current exactly in the same ratio.
do
Induced emf in the secondary coil, E, =- N, dt The efficiency of a transformer is defined as
Power output x 100%
where is the magnetic flux linked with each turn of Power input ..3)
the primary or secondary at any instant. Thus The efficiency of real transformers is fairly high
E, N, (90 -999%) though not 100%.
E, N Energy losses in transformers. The mafn causes for
Let Ebe the emf applied to the primary. By Lenz's energy loss in transformers are as follows:
law, self-induced emfE,opposes Ein the primary coil. 1. Copper loss. Some energy is lost due to heating
:.Resultant emf in the primary =8 -8 of copper wires used in the primary and secondary
This emf sends current I,, through the primary coil windings. This power loss (=IR) can be minimised by
using thick copper wires of low resistance.
f resistance R.
2. Eddy current loss. The alternating magnetic flux
E-¬, = RI, induces eddy currents in the iron core which leads to
But Ris very small, so the term RI, carn be neglected. some energy loss in the form of heat. This loss can be
Then E=¬ reduced by using laminated iron core.
ThusE, may be regarded as input emf andE, as the 3. Hysteresis loss. The alternating current carries
utput em. the iron core through cycles of magnetisation and
Output emf N; demagnetisation. Work is done in each of these cydes
..(1) and is lost as heat. This is called hysteresis loss and can
E, Input emf N, be minimised by using core material having narrow
The ratio N, / N,, of the number of turns in the hysteresis loop.
secondary to that in the primary, is called the turns 4. Flux leakage. The magnetic flux produced by the
ratio of the transformer. It is also called primary may not fully pass through the secondary.
transformation ratio. Some of the fluX may leak into air. This loss can be
Ina stey up transformer, N, >N, ie, the turns ratio coils
minimised by winding the primary and secondary
over one another.
is greater than 1 and therefore E, >E,. The output
voltage is greater than the input voltage. 5. Humming loss. As the transformer works, its
In astep down transformer, N, < N, i.e., the turns core lengthens and shortens during each cycle of the
ratio is less than 1and thereforé E, <E,. The output alternating voltage due to a phenomenon alled
gives rise to a humming sound.
voltage is less than the input voltage. magnetostriction. This
electrical energy is lost in the form of
It may be noted that equation (1) has been derived So some of the
by using the following three assumptions : humming sound.
1. The primary resistance and current are small. For Tour Knowledge
2. The same flux links both with the primary and A step-up transformer changes a low-voltage into a
secondary windings as the flux leakage from high-voltage. This does not violate the law of
the core is negligibly small. conservation of energy. The current decreases by the
3. The terminals of the secondary are open or the same proportion. When voltage increases n times, the
current taken from it is small. current reduces to 1/n times.
A Atransformer is essentially an a.c. device. It cannot
Currents in primary and secondary. Assuming the work on d.c. It changes alternating voltages/currents.
transformer to be ideal one so that there are no energy It does not affect the frequency of a.c.
losses, then
A The small transformers are self-cooled, which
Input power =Output power or transfer heat directly to the surroundings. Large
where I, and I, are the currents in the primary and transformers are cooled by placing them in oil tanks
secondary, respectively. to prevent overheating.
23
USSES OF TRANSFORMERS stations in a region deliver power to a common
a state some of the important uses of transfomers. regional pool, called the grid. From the grid, the power
Some uses of transformers: is fed to the cities at 33000 V, the stepping down is
done outside the city. Then again at a sub-station, the
Cmalltransformers are used in radio receivers, supply is stepped down to 6600 V. For domestic
telephones, loud speakers, etc.
purposes, the voltage is again stepped down to 220 V.
.In voltage regulators for TV, refrigerators, Step up Step down Step down
air-conditioners, computers, etc.
: In stabilised power supplies. 6600 V 132,00 V 2200 V
4 Astep-down transformer is used for obtaining
large current for electric welding. Generating Area Consumer
s Astep-down transformer is used in induction station sub-station sub-station
furnace for melting nmetals.
A step-up transformer is used for the Fig. 7.49 Schematic diagram of adistribution system.
production of X-rays. Examples Based on
7. In the transmission of electric energy from the Transformers and Long Distance
generating stations to the consumers. Power Transmission
FoRMULAE UsED
24 LONG DISTANCE TRANSMISSION
OF ELECTRICAL POWER 1. The voltages and currents in a transformer are
related as
34. Explain the use of transformers in long distance
-mSTmission of electric power.
Use of transformers in long distance transmission
tdlectric power. The most importarnt application of where suffix 1 refers to primary coil, 2 to secon
-nsformers is in the transmission of electrical power dary coil and kis the transformation or turns ratio.
om a power station to far away areas where it is 2. E,4 (Power in primary coil)
tually used. Following are the disadvantages of =¬,l, (Power in secondary coil)
Fansmitting the electrical power at low voltage : 3. Efficiency of a transformer,
1. Large length of transmission cables have Output power 100%
preciable resistance. Hence a large amount of energy n=
Input power
'R) will be lost as heat during transmission.
4. Power is transmitted from power stations to
.2' Large voltage drop (R) occurs along the line sub-stations at very high voltages to redue cost
wTe. Hence the voltage at the receiving station will be and reduce losses.
zuch smaller than that at the generating station. UNITS USED
3. To carry large currents and to keep the resistance Voltages E, E, are in volts ; currents ,.
Étransmission wires low, thick wires have to be used. ampere ; and k and nhave no units.
he cost of installing thick wires will be extremely
igh. EXAMPLE 67/The prfmarycoil of an ideal step-ur tran
Thus the long distance power transmission at low The
formerinput
has 100tumsand the transformation ratio is ziol
voltage and the power are 220 V ai 11001
roltage and high current is neither efficient nor
Konomical. If Iis the currernt in the cable, and R its respectively. Calculate :
resistance, the power wasted in the cable is I'R. The () number of turns in he secondary
power loss can be reduced by reducing I or R. The (ü) the current in the primary
power supplied by the generator is given by P= VI, (i) voltage across the secondary
where Vis the voltage across its terminals. Since (iv) the current in the secondary
l=P/V, for agiven amount of power P, the power (v) power in the secondary. ICBSE D 06)
oss is less if I is less or Vis high. Solution. Here N, =100, E, =220 V, P =1100 W
In actual practice, as shown in Fig. 7.49, a typical
POwer station generates 1000 kW åt 6600 volts. This (1) Transformation ratio, k =100
Voltage is first stepped up to 132000 volts before N,
tansmission. Transmission lines from different power .:. N, =100 N, =100 x 100=10,000.