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The First electric power generation station,
for commercial purpose, was established by
famous Thomas Elva Edison. On September
4, 1882 Edison switched on his Pearl Street
electrical power generating station and
started a new era of electric power supply.
Nikola Telsa, served as an employee under
Edison, He invented the Alternating Current
Electricity. After a long battle; the A.C.
electricity ruled out the Edison’s D.C.
electricity power generation and distribution
system. In this battle one electrical device“Transformer” place very important role.
The history of electricity starts with
the experiments of “Benjimin
Frenklin”, and later on Alsendro de
Volta had developed “Voletic cell”,
this cell was source of D.C.
electricity. In later phase Has Orsted
Christian discovered the magnetic
property of electric current, moving
one step ahead Michal Faraday and
other scientist established close
relation between electricity and
Magnet. A branch of
“electromagnetism” begins from these
discoveries and studies. Michal
Faraday developed the concept of
electric Generator and Motor.
A transformer is an electrical device which transfers electrical
energy from one circuit to another circuit through inductively
coupled conductors (the transformer's coils) without changing
frequency.
Basic Working Principle of Transformer:
The working of transformer can be understand by studying two principles/ effects
related with electricity
1. Electromagnetism (Magnetic effect of
electric current): When, an electric current
flow through a conductive wire a magnetic
field develops around the wire. In 1819, Hans
Christian Oersted, a Danish physicist and
chemist, and a professor discovered this
property of electric current. This effect of
electric current is known as electromagnetism.
The strength of developed magnetic field can be increased by
a.
Making a coil of long wire and
b.
Placing a soft
iron core in coil (iron is
magnetic substance; it
concentrates and amplifies
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the magnetic field created by the current in the coil).
2. Mutual Induction:
When a loop or coil comes close to a magnetized coil; an Electro Motive
Force (e.m.f.) develops in the in loop or coil. This phenomenon is known as
“Mutual Inductance”.
Consider two coils placed near each other as
shown in figure.
When current is passed through the coil (1),
magnetic flux is produced. This magnetic
flux is also linked with the loop or coil (2).
If the current is changed in the coil (1)
circuit, the produced magnetic flux also
changes. As this changing flux is linked with
the loop or coil (2), hence an e.m.f. will
induce in loop or coil (2).
This phenomenon of inducing emf in a coil by changing current in another
coil is known as mutual inductance.
The experiments related to electromagnetic and mutual induction was
carried out by Michal Faraday and Henry. There are two Faraday’s laws
related to EMI.
a. First Law: Whenever the amount of magnetic flux linked with a
circuit changes, an e.m.f. is induced in the circuit.
b. The magnitude of e.m.f. induced in a circuit is directly proportional to
the rate of change of magnetic flux linked with the circuit.
Basic Construction of Transformer:
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The Transformer is consists of three main parts
1. Primary Coil or Winding
2. Secondary Coil or Winding
3. Core
Primary winding:
The winding of transformer which remains connected with the source of power
supply, is know as primary winding. It may be either the high- or the low voltage
winding, depending upon the application of the transformer.
Secondary Winding:
The secondary winding is the winding of the transformer which delivers power to
the load. It may be either the high- or the low-voltage winding, depending upon the
application of the transformer.
Core:
The core is the magnetic circuit upon which the windings are wound.
Working of Transformer:
When an electric current flows in Primary Winding; a strong magnetic field
develops around it. The primary and secondary coils are wrapped around an iron
core, so most of the magnetic flux developed by the primary winding passes
through secondary winding.
The A.C. electricity has changing magnitude (of the voltage and current), When
the changing magnitude current flows in the primary winding the developed
magnetic flux also keeps changing. This changing magnetic flux induces a voltage
in the secondary winding.
The voltage induced across the secondary coil may be calculated from Faraday's
law of induction, which states that:
where Vs is the instantaneous voltage, Ns is the number of turns in the secondary
coil and Φ is the magnetic flux through one turn of the coil.
As the same magnetic flux passes through both the primary and secondary coils in
an ideal transformer, the instantaneous voltage across the primary winding equals
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Taking the ratio of the two equations for Vs and Vp gives the basic equation for
stepping up or stepping down the voltage
Np/Ns is known as the turns ratio, and is the primary functional characteristic of
any transformer. Ns/Np. Turns ratio is commonly expressed as an irreducible
fraction or ratio: a transformer with primary and secondary windings of,
respectively, 100 and 150 turns is said to have a turn’s ratio of 2:3.
Ideal power equation
When the secondary coil is attached to a load a current flows, electrical power is
transmitted from the primary circuit to the
secondary circuit. In ideal transformer, we
assume that the resistance of primary and
secondary windings is negligible and circuit
is perfectly efficient.
All the incoming energy is transformed
from the primary circuit to the magnetic
field and from magnetic field to the
secondary circuit. In ideal condition the
input electric power must equal the output
power:
giving the ideal transformer equation
This formula is a reasonable approximation for most commercial built transformers
today.
If the voltage is increased, then the current is decreased by the same factor. The
impedance in one circuit is transformed by the square of the turns ratio.For
example, if an impedance Zs is attached across the terminals of the secondary coil,
it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This
relationship is reciprocal, so that the impedance Zp of the primary circuit appears to
the secondary to be (Ns/Np)2Zp.
Types of Transformer:
The transformers are designed to perform specific function. Different types of
transformers
are
constructed
differently;
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The differences in construction may involve the size of the windings, the
relationship between the primary and secondary windings or the coolant of
transformer etc.
There are different categories of transformers on the basis of their winding,
purpose, use, construction etc.
A. On the basis of windings: There are two types of Transformers on the basis
of winding
1. Single Winding or Auto transformer:
The single winding transformers are also known as the “Auto
Transformer”. This type of transformer consists of single winding or coil,
tapping the winding in between. It is cost effective to use Auto transformer
where the ratio between Input Voltage and Output Voltage is less than 2.
2. Windings insulated or Two Winding Transformer:
This type of transformer is consists of two separate windings known as
“Primary” and “Secondary winding”. Two winding transformers are
generally used where ratio between High Voltage and Low Voltage is
greater than 2.
B. On the basis of number of Output wires: The transformers can be
categories into three categories on the basis of numbers of output wires
(these transformers can be of step-up or step down nature.)
1. Two Output wire:
The two output wire transformer is the common
transformer. In this transformer two wires are
coming out from the Primary and Secondary
windings; as shown in diagram. This type of
transformer provides one voltage range in the
output.
For example: A transformer marked as 9V-0 or
0-9V, provides 9Volt potential difference or
Voltage across the two output wires.
2. Center Tapped Transformer:
In this type of transformer, three wires come
out of the secondary winding, the third wire is
tapped from the center of the wire used for
secondary coil or winding; as shown in
diagram.
In center tapped transformer the linked
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magnetic flux are divided into two equal parts. This type of transformer can
be used to get two different voltage ranges.
For example: A transformer marked as 12V-0-12V, can be used to get 12
Volt potential difference (Voltage between 1- 2= 12 Volt) while we can get
the 24 Volt potential difference between 1 – 3 = 24 Volt).
3. Multi Output Transformer:
This type of transformer provides multiple numbers of output wires for
different voltage ranges. To select specific output voltage range, a selector
switch is used. The diagram of a multi
output transformer is shown here.
The Voltage difference between wires (as
marked in their specification)
A to B
=
3.0 Volt
A to C
=
4.5 V
A to D
=
6 Volt
A to E
=
9 Volt etc.
C. On the basis of Output Voltage: On the basis of Output voltage (relative to
Input) there are three types of transformers.
Step-up Transformer
o The transformer which increases the output voltage or voltage of secondary
winding are known as “Step-up” transformer.
o The step-up transformer has a greater number turns in secondary windings
than number of turns in primary winding.
o It therefore increases voltage while reducing current.
o For step up transformer
NS>NP
Number of turns in secondary winding is
more than the number of turns in primary
winding. Hence the induced voltage in the
Secondary Winding is more than voltage
of Primary Winding.
Vs > Vp
o The Step-up transformers are largely use
in electric power generation and
transmission network.
o In our home Television or Computer uses
a cathode ray tube as screen that requires thousands of volts, though it's
running on a 220V wall socket.
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Step-down Transformer
o A transformer in which the output voltage or Secondary Voltage decreases is
known as “step-down” transformer.
o The Step-down transformer has lesser number of turns in the secondary
winding.
o For step-down transformer
NP > NS
Number of turns in secondary winding is
lesser than the number of turns in primary
winding. Hence the induced voltage in the
Secondary Winding is less than voltage of
Primary Winding.
Vp > Vs
o
A battery eliminator is an example; the
battery eliminator is a device that can be
used to operate a battery-operated device with wall outlet socket.
o Battery operated devices, such as Mobile, cassette player etc; can be
operated on 220V by an adapter with a step-down transformer inside.
Isolation Transformer
o Isolation transformers don't necessarily step up or step down voltage, though
they can. Isolation transformers can serve a number of purposes. They break
a circuit into a primary and a secondary, a break that won't allow directcurrent noise through. They prevent capacitance buildup between the
primary and secondary, which causes high-frequency noise. They prevent
unintentional ground connections between the primary and secondary.
(Ground loop hum occurs in speakers, for example.) It can isolate the
secondary circuit from the primary's current to prevent shock and
inadvertent grounding from high voltage discharge.
D. On the basis of use: On the basis of use of use there are different types of
transformers.
a. Large power
b. Distribution
c. Small power
d. Sign lighting
e. Control and signaling
f. Gaseous-discharge lamp transformer
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g.
h.
i.
j.
Bell ringing
Instrument
Constant-current
Series transformers for street lighting
E. On the basis of numbers of Phase: There two categories of transformers on
the basis of number of phase
1. Single Phase Transformer
2. Poly Phase or Three Phase Transformer.
F. On the basis of Mounting:
a. Pole Mounted
b. Platform Mounted
c. Subway
d. Vault
e. Special
G. On the basis of coolant: There are number of transformer types on the basis
of coolant. Some of the categories are
a. Self-air–cooled (dry type)
b. Air-blast–cooled (dry type)
c. Liquid-immersed, self-cooled
d. Oil-immersed, combination self-cooled and air-blast
e. Oil-immersed, water-cooled
f. Oil-immersed, forced-oil–cooled
g. Oil-immersed, combination self-cooled and water-cooled
Role of Transformer in electrical power transmission.
Generation of Electrical Power in low voltage level is very much cost effective.
Hence Electrical Power is being generated in low voltage level. Theoretically,
this low voltage leveled power can be transmitted to the receiving end. But if
the voltage level of a power is increased, the current of the power is reduced
which causes reduction in ohmic or I2R losses in the system, reduction in cross
sectional area of the conductor i.e. reduction in capital cost of the system and it
also improves the voltage regulation of the system. Because of these, low
leveled power must be stepped up for efficient electrical power transmission.
This is done by step up transformer at the sending side of the power system
network. As this high voltage power may not be distributed to the consumers
directly, this must be stepped down to the desired level at the receiving end with
help of step down transformer.
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