Download In other words, for a transformer there is no direct

HAMMAD
SHAKIL
109
HAMZA ZAIN
RAMSHA AHMED
92
76
Transformer,
working,
construction
& associated
terms
GROUP #11
Transformers
A transformer is a static electrical device that transfers
energy by inductive coupling between its winding circuits.
This means that it transforms electrical energy from one
circuit to another without any direct electrical connection
and with the help of mutual induction between the
windings. It transforms power from one circuit to another
without changing its frequency but may be in different
voltage level. A transformer makes use of Faraday's
law and the ferromagnetic properties of an iron core to
efficiently raise or lower AC voltages. A transformer is a device which is used to
convert high alternating voltage to a low alternating voltage and vice versa. The
transformer is based on two principles: firstly, that an electric current can produce a
magnetic field (electromagnetism) and secondly that a changing magnetic field within
a coil of wire induces a voltage across the ends of the coil (electromagnetic
induction). Changing the current in the primary coil changes the magnitude of the
applied magnetic field. The changing magnetic flux extends to the secondary coil
where a voltage is induced across its ends. The principle behind the operation of a
transformer, electromagnetic induction, was discovered independently by Michael
Faraday and Joseph Henry in 1831. On the right is an image of a normal pole
mounted transformer.
Working Principle:
The working principle of a transformer simply depends upon Faraday’s laws of
Electromagnetic Induction, which states that:
"Rate of change of flux linkage with respect to time is directly proportional to the
induced EMF in a conductor or coil".
Induction law:
When an alternating voltage V1 is applied across the primary coil of N1 turns of
winding, a magnetic flux produced in the primary coil due to the phenomenon of
mutual inductance it results an induced emf in secondary coil of N2 turn of winding, if
a load is connected to the secondary winding a current I2 will flows through the
secondary circuit. The voltage induced across the secondary coil may be calculated
from Faraday's law of induction.
Working:
A basic transformer consists of two sets of coils or windings. Each set of windings is
simply an inductor. AC voltage is applied to one of the windings, called the primary
winding. The other winding, called the secondary winding, is positioned in close
proximity to the primary winding,
but is electrically isolated from it.
The alternating current that flows
through the primary winding
establishes
a
time-varying
magnetic flux, some of which
links to the secondary winding
and induces a voltage across it.
The magnitude of this voltage is
proportional to the ratio of the
number of turns on the primary
winding to the number of turns on the secondary winding. This is known as the
“turn’s ratio.”
To maximize flux linkage with the secondary circuit, an iron core is often used to
provide a low-reluctance path for the magnetic flux. The polarity of the windings
describes the direction in which the coils were wound onto the core. Polarity
determines whether the flux produced by one winding is additive or subtractive with
respect to the flux produced by another winding. Transformer works on the principle
of mutual induction of two coils. When current in the primary coil is changed the flux
linked to the secondary coil also changes. Consequently an EMF is induced in the
secondary coil.
Construction:
A single phase voltage transformer basically consists of two electrical coils of wire,
one called the "Primary Winding" and another called the "Secondary Winding" that
are wrapped together around a closed magnetic iron circuit called a "core". This soft
iron core is not solid but made up of individual laminations connected together to
help reduce the core's losses. These two windings are electrically isolated from each
other but are magnetically linked through the common core allowing electrical power
to be transferred from one coil to the other.
In other words, for a transformer there is no direct electrical connection between the
two coil windings, thereby giving it the name also of an Isolation Transformer.
Generally, the primary winding of a transformer is connected to the input voltage
supply and converts or transforms the electrical power into a magnetic field. While
the secondary winding converts this magnetic field into electrical power producing
the required output voltage as shown.
Laminated magnetic cores are made of thin, insulated iron sheets, lying, as much as
possible, parallel with the lines of flux. Using this technique, the magnetic core is
equivalent to many individual magnetic circuits, each one receiving only a small
fraction of the magnetic flux (because their section is a fraction of the whole core
section.
Parameters of a Transformer:
Flux Leakage
In ideal transformer all the flux will link with both
primary and secondary winding but in reality it is
impossible to link all the flux in transformer with
both primary and secondary windings. Although
maximum flux will link with both winding through
the core of transformer but still there will be a small
amount of flux which will link either winding not
both. This flux is called leakage flux which will pass
through the winding insulation and transformer
insulating oil instead of passing through core. Due to this leakage flux in transformer,
both primary and secondary winding have leakage reactance. This reactance of
transformer is nothing but leakage reactance of transformer. This phenomenon in
transformer is known as Magnetic Leakage. It is impossible to eliminate all leakage
flux because it plays an essential part in the operation of the transformer. The
combined effect of the leakage flux and the electric field around the windings is what
transfers energy from the primary to the secondary.
Winding Arrangements
Transformer windings form another important part of a transformer construction,
because they are the main current-carrying conductors wound around the laminated
sections of the core. In a single-phase two winding transformer, two windings would
be present as shown. The one which is connected to the voltage source and creates
the magnetic flux called the primary winding, and the second winding called the
secondary in which a voltage is induced as a result of mutual induction. The type of
wire used as the main current carrying conductor in a transformer winding is either
copper or aluminum. While aluminum wire is lighter and generally less expensive
than copper wire, a larger cross sectional area of conductor must be used to carry
the same amount of current as with copper so it is used mainly in larger power
transformer applications. As copper have a higher mechanical strength and smaller
conductor size than equivalent aluminium types, it is used in low voltage electrical
and electronic circuits. The insulation used to prevent the conductors shorting
together in a transformer is usually a thin layer of varnish or enamel in air cooled
transformers. This thin varnish or enamel paint is painted onto the wire before it is
wound around the core. In larger power and distribution transformers the conductors
are insulated from each other using oil impregnated paper or cloth. The whole core
and windings is immersed and sealed in a protective tank containing transformer oil.
The transformer oil acts as an insulator and also as a coolant. The different types of
winding are Helical, Concentric and Sandwiched.
Dot Orientation
We cannot just simply take a laminated core and wrap one of the coil configurations
around it. We could but we may find that the secondary voltage and current may be
out-of-phase with that of the primary voltage and current. The two coil windings do
have a distinct orientation of one with respect to the other. Either coil could be wound
around the core clockwise or anticlockwise so to keep track of their relative
orientations "dots" are used to identify a given end of each winding.
This method of identifying the orientation or direction of transformers windings is
called the "dot convention". Then transformers windings are wound so that the
correct phase relations exist between the winding voltages with the transformers
polarity being defined as the relative polarity of the secondary voltage with respect to
the primary voltage.
Hysteresis Loss
Transformer Hysteresis Losses are caused because of the friction of the molecules
against the flow of the magnetic lines of force required to magnetize the core, which
are constantly changing in value and direction first in one direction and then the
other due to the influence of the sinusoidal supply voltage. This molecular friction
causes heat to be developed which represents an energy loss to the transformer.
Excessive heat loss can overtime shorten the life of the insulating materials used in
the manufacture of the windings and structures. Therefore, cooling of a transformer
is important.
Eddy Current Loss
Transformer Eddy Current Losses on the other hand are caused by the flow of
circulating currents induced into the steel caused by the flow of the magnetic flux
around the core. These circulating currents are generated because to the magnetic
flux the core is acting like a single loop of wire. Since the iron core is a good
conductor, the eddy currents induced by a solid iron core will be large. Eddy currents
do not contribute anything towards the usefulness of the transformer but instead they
oppose the flow of the induced current by acting like a negative force generating
resistive heating and power loss within the core.
Laminating the core
Eddy current losses within a transformer core cannot be eliminated completely, but
they can be greatly reduced and controlled by reducing the thickness of the steel
core. Instead of having one big solid iron core as the magnetic core material of the
transformer or coil, the magnetic path is split up into many thin pressed steel shapes
called "laminations".
The laminations used in a transformer construction are very thin strips of insulated
metal joined together to produce a solid but laminated core. These laminations are
insulated from each other by a coat of varnish or paper to increase the effective
resistivity of the core thereby increasing the overall resistance to limit the flow of the
eddy currents. The result of all this insulation is that the unwanted induced eddy
current power-loss in the core is greatly reduced, and it is for this reason why the
magnetic iron circuit of every transformer and other electro-magnetic machines are
all laminated. Using laminations in a transformer construction reduces eddy current
losses.
The loss of energy, which appears as heat due to both hysteresis and eddy currents
in the magnetic path, is commonly known as "transformer core losses". Since these
losses occur in all magnetic materials as a result of alternating magnetic fields.
Transformer core losses are always present in a transformer whenever the primary
is energized, even if no load is connected to the secondary winding. Also this
hysteresis and the eddy current losses are sometimes referred to as "transformer
iron losses"; as the magnetic flux causing these losses is constant at all load.
Ideal Transformer
In an ideal transformer, the induced voltage in
the secondary winding (Vs) is in proportion to the
primary voltage (Vp) as given by the above basic
equation:
If the secondary coil is attached to a load that allows current to flow, electrical power
is transmitted from the primary circuit to the secondary circuit. Neglecting losses, the
input electric power must equal the output power:
giving the ideal transformer equation
.
An ideal transformer would have no energy losses, and would be 100% efficient. The
transformer windings are perfect conductors (zero resistance). The core loss is zero.
The flux leakage is zero. On the other hand, in practical transformers, energy is
dissipated in the winding, core, and surrounding structures. That is why the highest
efficiency of transformer is 98%.
The efficiency of a transformer is given as:
Types of transformers according to application:
Autotransformers
Autotransformers
are
different
from
traditional
transformers
because
autotransformers share a common winding. On each end of the transformer core is
an end terminal for the winding, but there is also a second winding that connects at a
key intermediary point, forming a third terminal. The first and second terminals
conduct the primary voltage, while the third terminal works alongside either the first
or second terminal to provide a secondary form of voltage. The first and second
terminals have many matching turns in the winding. Voltage is the same for each
turn in the first and second terminal. An adaptable autotransformer is another option
for this process. By uncovering part of the second winding and using a sliding brush
as the second terminal, the number of turns can be varied, thus altering voltage.
Polyphase Transformers
This type of transformer is commonly associated with three phase electric power,
which is a common method of transmitting large amounts of high voltage power,
such as the national power grid. In this system, three separate wires carry alternating
currents of the same frequency, but they reach their peak at different times, thus
resulting in a continuous power flow. Occasionally these “three-phase” systems have
a neutral wire, depending on the application. Other times, all three phases can be
incorporated into one, multiphase transformer. This would require the unification and
connection of magnetic circuits so as to encompass the three-phase
transmission. Winding patterns can vary and so can the phases of a polyphase
transformer.
Leakage Transformer
Leakage transformers have a loose binding between the primary and secondary
winding, which leads to a large increase in the amount of inductance leakage. All
currents are kept low with leakage transformers, which help prevent overload. They
are useful in applications such as arc welding and certain high-voltage lamps, as well
as in the extremely low-voltage applications found in some children’s toys.
Resonant Transformer
As a type of leakage transformer, resonant transformers depend on the loose pairing
of the primary and secondary winding, and on external capacitors to work in
combination with the second winding. They can effectively transmit high voltages,
and are useful in recovering data from certain radio wave frequency levels.
Audio Transformer
Originally found in early telephone systems, audio transformers help isolate potential
interference and send one signal through multiple electrical circuits. Modern
telephone systems still use audio transformers, but they are also found in audio
systems where they transmit analog signals between systems. Because these
transformers can serve multiple functions, such as preventing interference, splitting a
signal, or combining signals, they are found in numerous applications. Amplifiers,
loudspeakers, and microphones all depend on audio transformers in order to
properly perform.
Types of transformers according to construction:
1.
Core type Construction.
2.
Shell type Construction.
1- Core-type transformers:
The core-type transformer enclose the core i.e. the
primary and secondary windings were wound on a
separate lamps and around a closed iron ring so
that the windings are well visible, but they hide the
core limbs, Only the upper and lower yoke of the
core are visible. The axis of the core type windings
is normally vertical. Core-type (or three limbs) is
the most commonly used method of construction,
the smaller core means less weight and expense.
2- Shell-Type Transformers:
The shell-type transformer is considered the most
efficient. Such transformers are used in transmitting
commercial power. The core of the shell-type
transformer is made of laminated silicon steel
sheets placed on top of one another. The coils are
wound around the central section of the core; the
core of a shell-type encloses the windings and the
core hides the major part of the windings. The axis
of the shell-type windings can be horizontal or
vertical in a shell-type.
s/n
Core-type Transformers
Shell-type Transformers
1
enclose the core
Enclose the windings
2
the windings are well visible, but the core hides the major part of the
they hide the core limbs
windings
3
The coils are wound around all The coils are wound around the
core lamps
central section of the core
4
The axis of the core type windings The axis of the shell-type windings
is normally vertical.
can be horizontal or vertical in a
shell-type
Core-type (or three limbs) is the The shell-type transformer is
most commonly used method of considered the most efficient and
construction, the smaller core used for larger transformers
means less weight and expense
because they can be made with a
reduced height.
The cylindrical types of coils are Generally, multi-layer of disc type
used
or sandwich coils are used
5
6
7
8
The coils can be easily removed for Large number of laminations must
maintenance point of view
be
removed
for
making
maintenance for any winding
As windings are distributed, the As windings are surrounded by the
neutral cooling is more effective
core , there is no neutral cooling
9
It is preferred for low voltage It is preferred for high voltage
transformers
transformer
10
It has a single magnetic circuit
11
In a single phase type, the core has In a single phase type, the core has
two limps
three limps
12
In a three phase type, the core has In a three phase type, the core has
three limps
five limps
It has double magnetic circuits
References:
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html
http://qcelectricalsvc.com/images/transformer.jpg
http://wiki.answers.com/Q/What_is_the_working_principle_of_a_transformer
http://ecmweb.com/archive/basics-transformers
http://www.electrical4u.com/what-is-transformer-definition-working-principle-oftransformer/
http://www.electronics-tutorials.ws/transformer/transformer-basics.html
http://www.electrical4u.com/resistance-leakage-reactance-or-impedance-oftransformer/
http://www.electronics-tutorials.ws/transformer/transformer-construction.html
http://www.thomasnet.com/articles/electrical-power-generation/transformer-types
http://www.electrical-knowhow.com/2011/11/ep-3-course-types-of-transformers.html