9.1.
INDUCTORS
All conductors when
carrying A.C. are supposed to have inductance. But, when a conducting wire is
given the shape of a component by winding it like a coil,
it is called an inductor. An inductor is commonly known as a choke. The word choke implies the meaning 'to check', hence, the component used for checking the flow of A.C. is called a choke. The main three types of chokes are as follows
it is called an inductor. An inductor is commonly known as a choke. The word choke implies the meaning 'to check', hence, the component used for checking the flow of A.C. is called a choke. The main three types of chokes are as follows
1. Low frequency
choke
2. Audio frequency
choke
3. Radio frequency
choke
 |
| Fig 9.1 L.F. and A.F. chokes |
1.
L.F. Choke. A choke designed to work in the frequency range
of 50 of 60 Hz is termed as If choke. It is made upto 100 henrys. It is used in
the filter circuits of rectifiers for smoothing the pulsating D.C. Iron core is
used in it. See Fig. 9.1(i) for its construction.
2.
A.F. Choke. A choke designed to work in the frequency range
of 20 to 20,000 Hz is termed as of choke. It offers a high impedance to al
currents but a very low impedance to D.C. It is also made upto 100 henrys. It
is used in impedance coupled amplifiers for coupling and in H.T.D.C. lines for
blocking the flow of al currents. Iron core is used in it also. See Fig. 9.1
for cores its construction.
Note.
The iron core used in L.F. and A.F. choke concentrates the magnetic field
inside the coil instead of allowing it to spread outside the coil. It also
increases the inductance value of a choke.
3.
R.F. Choke. A choke designed to work in the
frequency range of 20 kHz and upwards is termed r.f. choke. It is made to have
a low inductance value of the order for micro-henrys. Commonly, its inductance
value does not exceed 125 mH. It offers a high impedance for r.f. currents but
a very low impedance for a.f. currents and D.C. It is used in impedance coupled
a amplifiers for coupling and in H.T.D.C. lines for blocking the flow of r.f.
currents.
 |
| Fig. 9.2 R.F.choke |
On the basis of core
used, the types of chokes are as follows:
(i)
Air Core Type. It is simply a coil of fine copper
wire wound on a hollow cylindrical spool of an insulating material. It is made
in the inductance range of the order of micro and milli henrys, hence, it does
not require an iron core.
(ii)
Dust Core Type. It consists of a core inside the
coil. The core is made of iron dust and a suitable binding material. There are
two advantages of using dust cores: (a) It has relatively high inductance
value, (b) Dust core losses are less in comparison to iron cores.
iii)
Variable Dust Core Type. In this type of coil, threaded
core and Spool are used so that the inductance of the coil. More the core is
inside the coil, more is the inductance the coil.
In the coils working at high frequencies,
ferrite cores are used. Ferrite is a mixture of oxides of iron, barium and strontium.
The construction of a R.F. choke is shown in Fig, 9,2(i).
Besides above
explained types of inductors, many other types of inductors are also used in
electronic circuit, e.g., voice coil of headphone or loudspeaker, current coil
of moving coil meter, deflection yoke coil of a T.V. receiver etc.
9.2.
NON INDUCTIVE COILS
Though the inductance
is a very useful property of a.c. circuits, but there are some electronic
circuits in which the inductance is not desired at all. For example, when a
wire wound resistor is used in a.c. circuits, it should have a non inductive
winding; see Fig. 9.3.
 |
| Fig. 9.3 Non-inductive coil |
Besides non-inductive
coil, the feeder line used for joining a transmitter or a T.V. receiver to the
antenna and the wires used for connecting indirectly heated filaments to the
source of supply are designed to have minimum inductance. A twisted wire line
of two parallel conductor is used as feeder line.
9.3.
INDUCTOR'S WINDING
Various methods are
used for winding the inductors so as to minimise the undesired capacitance
between its turns. Principal winding methods are as follows:
1.
Solenoid. A solenoid is a single layered coil whose length
A is nearly 10 times of its diameter.
2.
Toroid. It is a solenoid which is turned into a loop
shape.
3.
Honeycomb Winding. In this method the layers of winding
take the shape of a honeycomb. The advantage of this method is this that the
size of the coil can be made pretty small.
4.
Variometer. A slight variation can be done with
the help of dust or ferrite core in the inductance value of a coil. But, when
major variation is desired in the inductance value, a variometer is used. It
consists of two coils wound on separate hollow cylindrical spools which are
placed perpendicular to each other. When the two coils are perpendicular to
each other, the inductance is minimum and when the two coils, are parallel to
each other, the inductance is maximum. A simple variometer is shown in Fig.
9.4.
 |
| Fig. 9.4. Variometer |
9.4.
TRANSFORMERS
A transformer is a
static device which is used to transfer electrical energy from one circuit to
the other. It works on the principle of mutual inductance. Since, there is no
mutual inductance in D.C. circuits, therefore, a transformer cannot work on D.G.
and thus it can work only on A.C.
A transformer
consists of two types of coils or windings:
(i)
Primary. The coil connected to the source of supply is
called primary winding or only 'primary'.
(ii)
Secondary. The coil connected to the load is called
secondary winding or only 'secondary'. A transformer has a single primary
whereas it may have one or more secondaries.
9.5.
ADVANTAGES OF TRANSFORMERS
A transformer has the
following advantages:
(i)The transfer of
electrical energy takes place in a static mode and no part of the transformer
moves at all. Hence, the transfer of electrical energy does not require any
mechanical energy or any operator for looking after it.
(ii)The transfer of
electrical energy is almost peaceful, whereas in mechanical methods the
operation of machines produces heavy noise.
(iii)Since, the
secondary circuit of the transformer rests isolated from the primary circuit,
therefore, the use of a transformer in electronic equipments reduces the
possibilities of a shock to the mechanic on touching the chassis etc.
(iv)A transformer can
increase or decrease either voltage or current as required and it is its main
advantage.
(v)Since the transformer
is a static device, therefore, its wear and tear are almost nil and it does not
require much maintenance.
9.6.
CLASSIFICATION OF TRANSFORMERS
Transformers can be
classified on the following grounds:
I.
As per core
(i) Core type (ii)
Shell type
(iii) Berry type
2.
As per output
(i) Voltage step-up
type (ii) Voltage
step-down type
3.
As per use
(i) Mains or power (ii) Auto
(iii) Battery
eliminator (iv)
Driver
(v) Output (vi)
Push pull
(vii) I.F. (viii)
R.F.
(ix) E.H.T.
4.
As per phase
(i) Single phase (ii)
Three phase
5.
As per power rating
(i) Power (ii)
Lighting
6.
As per cooling system
(i) Self cooled (ii)
Air cooled
(iii) Oil cooled (iv)
Oil pressure cooled
(v) Water cooled.
Out of the above
stated types of transformers, only transformers referred under topics 1, 2 and
3 are used in electronics equipments. Therefore, only these three types of
transformers are described in detail hereunder.
9.7.
TRANSFORMERS CLASSIFIED AS PER CORE
There are three types
of transformers on the basis of core used between the windings
1.
Core Type
(i)
Open Core Type. In this type of transformer, the
primary and secondary windings are wound on leatheroid paper spools of
cylindrical or cubical shape. Laminated iron cores are fitted-inside the spool,
see Fig- 9.5. These types of transformers are almost out of use now.
(ii)
Closed Core Type. In this type of transformer, L-Shaped
laminated cores are used which form a closed magnetic path, see Fig. 9.6. In
this way, the leakage of magnetic flux is greatly reduced in this type of
transformer which increases the magnitude of electrical energy transferred.
2.
Shell Type. In this type of transformers, E and
/-shaped laminated cores are used. The primary and the secondary windings are
wound one above the other on the central part of the core. In this way, the
magnetic flux is divided into two parts at the centre of the core and covers
both the windings all around, see Fig. 9.7. Most of the L.F. and A.F.
transformers are of shell type.
3.
Berry Type. This is an improved form of a shell type
transformer. The main core of the transformer is cylindrical and both the
windings are wound on it. 8 to 10 shells joined to the main core cover the
coils all around, see Fig. 9.8. In this way, the leakage of magnetic flux is
minimised and the efficiency of the transformer is maximised. The transformer
is used in special types of circuits.
9.8.
TRANSFORMERS CLASSIFIED AS PER OUTPUT
There are two types
of transformers on the basis of output obtained from the same.
1.
Voltage Step-up Type. In this type of transformer, the
voltage available at the secondary winding is greater than the voltage applied
at the primary winding. Its secondary winding consists of a greater no. of
turns in comparison to that of the primary winding. Let the transfer of
electrical energy be 100% then —
Energy in primary = Energy in
secondary
PP = PS
or IP x EP = IS
x ES
In this way, it is
noted that by increasing the secondary's voltage its current will be reduced.
Hence, a voltage step-up transformer is a current step-down transformer as
well. It is used in rectifiers, electricity distribution stations etc.
2.
Voltage Step-down Type. In this type of transformer, the
voltage available at the secondary winding is lesser than the voltage applied
at the primary winding. Its secondary winding consists of a lesser no. of turns
in comparison to that of the primary winding. A voltage step-down transformer
is a current step-up transformers as well. It is used in electric welding,
electroplating, electricity distribution system etc. and purpose its size is
made quite large.
9.9.
TRANSFORMERS CLASSIFIED AS PER USE
There are various
types of transformers which are used in electronic circuits. The principal
types are follows
1.
Mains or Power Transformer. A transformer working at 230
volts, 50 to 60 Hz A.C. main is called or power transformer. It consists of a
primary and 2-3 secondary windings. The transformer used in a valve type radio
receiver or an amplifier consists of the following windings:
(i) Primary — 230/250
volts ; black, yellow and green coloured wires.
(ii) H.T. Secondary —
300 – 0 – 300 volts: red, white and red coloured wires.
(iii) L.T. Secondary
— 6 volts ; brown coloured wires.
(iv) L.T. Secondary —
6 volts ; orange coloured wires.
The above stated
transformer is a voltage step-up transformer as well as a voltage step-down
transformer too. see Fig. 9.9 (a).
2.
Auto Transformer. It is quite different type of
transformer which consists of only one winding. The winding has a common tap
terminal. One end of the winding and the common tap terminal together work as
primary, while the other end of the winding and the common tap terminal together
work as secondary. This of transformer can be used as voltage step-up as well
as step-down transformer. It works on A C see Fie-. 9.9 (b). The EHT
transformer used in T.V. receivers is a sort of auto transformer and it works
at 15625 Hz.
3.
Battery Eliminator Transformer. This type of
transformer also works on A.C. main supply, i.e., 230 volts 50/60 Hz. It is
used in the power supply unit designed to produce L.T.D.C. that is why it is
known as a battery eliminator transformer. If a transistor radio operates at 6
volts D.C. then a power supply unit is made with a B.F. transformer, a
rectifier and a filter circuit and the receiver can now be operated at 230
volts A.C. in place of a battery. In this way, a battery eliminator eliminates
a battery. The secondary winding of
a B.E. transformer is made for the following voltage ranges — (6 — 0 — 6), (9—0
- 9). (12 — 0 — 12), (1.5, 3.0, 4.5, 6.0, 7.5, 9.0, 12.0). A 6 — 0 — 6 volts
B.E. transformer is shown in Fig. 9.9 (c)
4.
Driver Transformer. The transformer used for coupling and
impedance matching of the two amplifier stages is called a driver or matching
transformer. For maximum energy transfer from one stage to the other, the
output impedance of the first stage should be equal to the input impedance of the
second stage. The two impedances may not be equal usually, hence a matching
transformer is used for impedance matching.
A driver transformer
may be of voltage step-up or step-down type. The transformer used in A.F.
circuits Wiled an A.F. driver transformer and that used in R.F. circuits is
called an R.F. driver transformer. An A.F. driver transformer is shown in Fig.
9.9(d).
5.
Output Transformer. It is a voltage step-down type
matching transformer which is used in the output stage of a radio receiver for
matching the impedance of amplifier stage to the loudspeaker. It is a sort of
A.F. transformer. see Fig. 9.9 (e).
6.
Push-pull Transformer. A push-pull amplifier circuit
requires two such driver transformers whose one winding has a centre tapped
terminal also, see Fig. 9.9(f). The primary winding of the output push-pull
transformer and the secondary winding of the input push-pull transformer have
centre tapped terminals. use transformers are of two types - A.F and R.F. A.F.
transformers are used in receivers and public address amplifiers and R.F.
transformers are used in transmitters. .
7.
Single End Push-pull Transformer. The newly developed
types of push-pull amplifiers, used in receivers, and P.A. amplifiers require
only input push-pull transformer. Such circuits are called single end push-pull
amplifiers circuits and the transformer used in them is called single end
push-pull transformer. It consists of a primary and two identical secondary
windings, see Fig. 9.9 (g).
8.
I.F. or Intermediate Frequency Transformer. It is a sort
of R.F. transformer which is especially designed to transfer electrical energy
at a specified frequency. The I.F. value for radio receivers is kept between
450 to 470 kHz and for Tv. receivers it is kept at 33.4 MHz, see Fig. 9.9 (h).
Each winding of the transformer has a capacitor connected across the winding.
Either adjustable capacitors or threaded iron dust cored windings are used in
I.F. transformers for frequency tuning purpose, see Fig. 9.9 (j)
9.
R.F. or Radio Frequency Transformer. All transformers
working above 20 kHz are called R.E. transformers. The oscillator and antenna
coils of a radio receiver are also R.F. transformers, see Fig. 9.9(i).
Note.
(1) L.F. and A.F. transformers have laminated iron cores. The purpose of using
such cores is to achieve the required amount of inductance.
(2) I.F. and R.F.
transformers are made coreless or they have iron dust cores.
9.10.
TURNS RATIO
The ratio of the no.
of turns of the primary to that of the secondary winding is railed
turns ratio, or transformation ratio.
In an ideal
transformer (a transformer without any losses; though it is not possible but we
may assume so).
Where, NP = no.
of turns in the primary winding
NS
= no. of turns in the secondary winding
EP
= primary voltage, volts
ES
= secondary voltage, volts
K =
transformation ratio.
If the value of K is
larger than unity then the transformer is of voltage step-up type and if it is
lesser than unity then it is of voltage step-down type.
Since for an ideal
transformer —
where, Ip = primary current, amps.
IS
= secondary current; amps.
9.11.
TRANSFORMER EFFICIENCY
The ratio of the
input to the output power of a transformer is called transformer efficiency.
Its symbol is n.
where, PO = Power
output, watts
Pi
= power input, watts
n =
transformer efficiency.
9.12.
IMPEDANCE RATIO
The ratio of the
primary winding impedance to the secondary winding impedance of a transformer i
called impedance ratio.
Since for an ideal
transformer —
where, ZP =
primary impedance, ohms
ZS
= secondary impedance, ohms
Note.
Ratio RS/RP may also be used in place of ZS/ZP.
Example
9.1. What will be the turns ratio required for step-downing 110 volts to 23
volts ? If the secondary current is 10 A then calculate the primary current. [W/CAL-1971] Solution. Given : Primary voltage, EP
= 110 V
Secondary voltage, Es = 2.5
V
Example
9.2. A mains transformer of 230/110 volts can provide If the transformer
efficiency is 90%, then calculate the primary current.
Solution.
Given : Primary voltage, EP = 230 V
Secondary voltage,
ES = 110 V
Secondary power
Po = 1 kW = 1000 W
Efficiency = 90%
Example
9.3. The turns ratio of a transformer is 50 :1. If its primary voltage is 10 V
and primary resistance is 10 kΩ then calculate (a) power for a
10 Ω
secondary load, (b) value of load resistance for maximum power output.
[W/CAL-1978]
Solution
: Given : Turns ratio, NP :
NS = 50 : 1
Primary voltage, EP
= 10 V
Primary resistance, RP =
10 kΩ
Secondary load, RS
= 10 Ω,
For maximum output,
the secondary winding resistance should be equal to load resistance, hence
required load resistance
= 4 ohms Answer
(ii)
9.13.
TRANSFORMER ON LOAD AND NO-LOAD
(a)
Transformer on No-load. If a load is not connected to
the secondary winding of a transformer thee it is said to be in no-load state,
its secondary circuit will remain open and hence there will be no flow of
current in it.
In the above stated
condition, the magnitude of primary current will also be low because the back
e.m.f. induced in the primary (due to self induction) will oppose the flow of
primary current. Besides it, primary winding will consume sonic energy on
account of transformer losses. If there are no transformer losses and primary
circuit is purely inductive, then there will be no electrical energy
consumption but it is impossible. A transformer winding has copper loss, iron
loss, eddy current loss etc. and these losses consume electrical energy
unnecessarily.
(b)
Transformer on Load. When the secondary winding of a transformer
is connected to a load, the flow of current is started.
where IS =
secondary current, amps.
ES
= secondary e.m.f, volts
RS
= secondary coil's resistance, ohms
Rz = load
resistance, ohms.
 |
| Fig. 9.10 Transformer on load |
[Note. The terms Rs + RL may be replaced with ZS +
ZL , where, Fig. 9.10. Transformer on load ZS is
secondary impedance and ZL is load impedance]
As the current starts
to flow in the secondary winding's circuit, the voltage across the load gets
reduced.
Where, VL = voltage across the load, volts
ES
= secondary e.m.f. in no load condition, volts
IS
= secondary current, amps.
ZT
= total impedance (Zs + ZL), ohms
For obtaining maximum
electrical power (P) for the load, the load impedance should be equal to the secondary
winding's impedance.
9.14.
TRANSFORMER LOSSES
There are following
four types of transformer losses or a.c. losses :
1.
Eddy Current Loss. The use of iron core is necessary in
L.F. and A.F. transformers. The iron core increases the inductances of the
coils. If the core is not used, the amount of electrical energy transferred
from one coil to another will be quite small. Since, the iron core is situated
in the magnetic field of the primary winding, an e.m.f. is induced in it also
in accordance with Faraday's laws of electromagnetic induction. The e.mf
induced in the core produces eddy currents which circulate in the core.
The loss of magnetic
flux caused by the eddy currents is called the eddy current loss. The core gets
heated up due to this loss. The loss can't be eliminated completely, but it can
be reduced to a certain extent.
Remedy for reducing
the Eddy Current Loss. The core is made by joining together insulated iron
laminations instead of solid iron bar. The laminations are painted with varnish
on their one or both the sides. The use of laminated core reduces eddy current
loss, because the e.m.f. induced in the laminations cannot set up a flow of
current due to insulating layers of varnish between the laminations.
2.
Hysteresis Loss. It is evident that the magnetism
produced in a substance follows the magnetising current. On the completion of
each a.c. half cycle, the magnitude of current reaches zero but a certain
amount of magnetism is retained in the core. The loss of electrical energy in
counterbalancing the residual magnetism in each cycle is called hysteresis
loss. The loss cannot be eliminated completely but it can be reduced by using
mildsteel laminations.
3.
Copper Loss. Transformer windings are commonly
made with copper wire. The loss of electrical energy due to resistance of the
copper windings is called copper loss (I2R).
This loss also cannot
be eliminated completely because every matter has resistance either low or
high.
4.
Leakage Loss. The portion of the magnetic flux
produced by the primary winding, which does not pass through the secondary
winding causes a loss of electrical energy which is called leakage loss. The
use of mild steel laminations reduces leakage loss. If solid iron cores were
used in the transformers then there would have been more eddy current loss in
them.
9.15.
SKIN EFFECT
The conductors,
inductors and transformers working at high frequencies have an additional loss
(other than those stated above) which is called skin effect. Since, at high
frequencies the flow of current is restricted to the surface of the conductor,
thus, the central part of a conductor produces unnecessary resistance; the
effect is called skin effect.
Cause.
The magnetic field of a current carrying conductor is composed of concentric
lines of force. On high frequencies, these lines of force get more concentrated
at the central part of the conductor. In this way, the inductance and hence the
inductive reactance of the central part of the conductor is increased which
opposes the flow of current.
Remedy.
A litz wire is used for making inductors and transformers to be used at high
frequencies. The litz wire is composed of a number of very fine copper wires
twisted together. The outer and the inner part of a very fine copper wire are
almost the same, therefore, it has least skin effect. Besides it, hollow
conductors (copper pipes) are used in transmitters for minimising the skin
effect. Since there is no central part in such conductors, hence there exists
minimum skin effect.
9.16.
SHIELDING
When two or more r.f.
transformers are used in an electronic equipment, they may have undesired
coupling between them. The reason for this effect is the wide spread magnetic
fields of the coils at high frequencies.
The above stated
drawback is eliminated by covering the r.f. transformers with metallic covers
which are called shields. The shield is well connected to the earth terminal,
i.e., chassis of the equipment.
Any stray magnetic
field when passes through the shield, it induces an e.m.f in the same. The
e.m.f. sets up a flow of current in the shield and the flow of current in turn
sets up its own magnetic field. According to Lenz's law Aluminium the induced
magnetic field does not allow the original field Shield to spread outside the
shield.
 |
| Fig. 9.11. Shielding |
In this way, the
covering of r.f. transformers with metallic covers, for the elimination of
undesired coupling effect, is called shielding.
The size of a shield
should be kept fairly large because the eddy currents flowing in it causes loss
of electrical energy for the transformer. Usually, the shields are made with
aluminium sheets because aluminium is a good conductor, cheap and mechanically
rigid metal, see Fig. 9.11.
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