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CONTENTS
S.NO. TITLE PAGE NO.
1. ABSTRACT 22. INTRODUCTION 33. BLOCK DIAGRAM 54. WORKING PRINCIPLE 65. WELDING PROCESS 86. TRANSFORMER 427. INDUCTION MOTOR 528. ADVANTAGES 599. APPLICATION 6110. CONCLUSION 6211. REFERENCE 63
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ABSTRACT
Resistance seam welding is a simple process that uses one or two wheels to
apply pressure to the surface of two or more layers of conductive material. As the
wheels roll, electric energy is applied using a capacitive discharge, high frequency,
or line frequency weld controller in precise amounts to form a joint between the
faying surfaces of the material. The resistance seam weld process is a fast, reliable
and low cost way to join many materials. Like most joining methods, it competes
with other technologies like laser and TIG welding. This article explores the joint
types and the common configurations to used form seam welds on small scale
parts.
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INTRODUCTION
Whether the seam weld is longitudinal, circular, or a unique planar contour,
the weld nugget is formed in one of three ways:
a) Roll spot
b) Overlapping spot
c) Continuous seam
The roll spot type occurs when there are distinct separations between the
nuggets as the roller walks across the surface. If the weld schedule is fired at a
constant repetition rate, the crosssection result looks like that shown in Figure
Obviously, if one maintains the linear velocity, but increases the firing rate,
the spots will get closer and closer together until they overlap. This is called
overlap spot welding and creates a hermetic (i.e. leak tight) joint between the
materials as depicted in Figure. The overlap spot weld technique is very effective
at joining thin materials (i.e. < 0.015 thick) without burn through. Continuous
seam welding occurs when a constant stream of energy is applied to the rollers.
This results in a joint like that in Figure 1(c). Regardless of the type used, the
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electrodes are not opened between spots which results in a high speed joining
process.
Typical linear velocities for small scale resistance seam welding range from
0.2 to 1.0 in/sec and depend on the material type, part thickness, and weld schedule
(one or two pulse) used. The roller forces usually range from 5 to 75 lbs for thin
materials, about 5 to 10 times that for a comparable pointed spot weld electrode
using the same material thickness. The higher force is due to the additional surface
area of the roller when compared to a straight electrode tip.
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BLOCK DIAGRAM
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WORKING PRINCIPLE
This method is in effect a continuous spot welding process in which current
is regulated by the timer of the machine. Seam welding consists of a continuous
weld on two overlapping pieces of sheet metal that are held together under pressure
between two circular electrodes. Coalescence is produced by heat obtained from
the resistance tow flow of current that passes through the overlapping sheets. In
high-speed seam welding using contiguous current, the frequency of the current
acts as an interrupter.
The heat at the electrode contact surfaces is kept to a minimum by the use ofcopper alloy electrodes and is dissipated by flooding the electrodes and weld area
with water. Heat generated at the interface by contact resistance is increased by
decreasing the electrode force. Another variable that influences the magnityde of
the heat is the weld time, which in seam welding is controlled by the speed of
rotation of the electrodes. the amount of heat generated is decreased with an
increase in welding speed.
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Operation of Seam Welding:
The two work pieces to be joined are cleaned to remove dirt, grease and
other oxides either chemically or mechanically to obtain a sound weld.
The work pieces are overlapped and placed firmly between two wheel
shaped copper alloy electrodes, which in turn are connected to a secondary circuit
of a step-down transformer.
The electrode wheels are driven mechanically in opposite directions with the
work pieces passing between them, while at the same time the pressure on the joint
is maintained.
Welding current is passed in series of pulses at proper intervals through the
bearing of the roller electrodes wheels.
As the current passes through the electrodes, to the work piece, heat is
generated in the air gap at the point of contact (spot) of the two work pieces. This
is heat melts the work pieces locally at the contact point to form a spot weld.
Under the pressure of continuously rotating electrodes and the current
flowing through them, a series of overlapping spot welds are made progressively
along the joint.
The weld area is flooded with water to keep the electrode wheels cool during
welding.
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WELDING PROCESS
The application of welding is in very wide range of our modern world.
The new 6.000 km pipelines used to transport natural gas from the other side
of the Ural Mountains to Western Europe, the giant warships, the great
bridges and the big aluminium liquid-gas storage tanks are just a few of the
more impressive examples. The welding is none less important at fabrication
of smaller size parts, for example, hypordemic needles, electric switches,
parts of computers
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A wide scale of materials is used to make these welded parts and
constructions. This scale comprehends not only the metals from aluminium to
zirconium but the considerable amount of plastic, too.
In materials and sizes, very different welded workpieces demand the ample
choice of welding processes. Nowadays, more than one hundred welding processes
or process variables are used in practice.
In spite of that, the welding is the most effective method of joining
materials. It has some limitations since during the course of making a weld
virtually all types of metallurgical phenomena occur. Usually, the welding isconcerned with melting, solidification, gas-metal and slag-metal reactions, surface
phenomena and otherwise solid state reactions. Not only the great variety of
metallurgical reactions is very difficult, but these reactions occur very quickly
during welding, in contrast to other metallurgy fields, such as steel making, casting
or heat treatment.
All the welding processes require the application of heat and/or pressure to
produce a suitable bond. The heat, mechanical and electrical processes associate to
welding procedures.
The metal joining methods are usually grouped by their bonding mechanism:
sticking,
soldering and brazing,
welding.
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In the sticked joint, the attractive forces, which are between an adhesive and
the base material, have physical in character. Two principal interactions that
contribute to the adhesion are van der Walls bond and permanent dipole bond - as
they are well known - are relatively weak.
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During soldering and brazing the coalescence of materials are realized by
using a filler metal that is in liquidus temperature below the solidus of the base
material. The filler metal in liquid state is distributed between the closely fitted
faying surfaces of the joint by capillary action. The bond between the filler metal
and the parent metal is generally due to some diffusion of the filler metal into the
hot base metal into the hot base metal and to solve surface alloying of the metals.
In this respect, the soldering and brazing are between the sticking and the welding.
The difference between soldering and brazing is only in the melting point of
applied filler metal. When the melting point of filler metal is above 450 oC
temperature the process is named as brazing.
The strength of welded joint is based on metallic bond. Opposite from van
der Walls or permanent dipole bonds, the metallic bond is a primary bond. The
crystal line structure is built up by well positioned metallic positive ions. Each ion
is surrounded by at least twelve neighbours. The valence electrons are considered
to have complete mobility and are free to move between ions. Each atom
contributes with its valence electron to this "electron cloud" and there is no way to
assign a given electron to a specific ion. The bond holding the structure together is
caused by the attraction of the negatively charged electrons to the positively
charged ions. This metallic bond is very strong.
The condition of union of two previously separated metal parts is that the
distance between surface ions of two parts should not be more than some timer of
their lattice parameters. This wished distance is not more than half a nanometre.
Under normal circumstances, the total surface or metal parts is covered with
adsorbed gas molecules. This molecule layer has some manometers thickness and
hampers the connections between metal ions. It is possible to reduce the amount of
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adsorbed gases by decreasing gas pressure or increasing the temperature. It should
be mentioned that at higher temperature, the tendency of oxide films formation is
higher. The oxide films or other similar dirt on the surface interfere with metal-to-
metal contact and must be removed in order to obtain the metallic bond.
In total vacuum where the gas layer does not hinder from ionic connection of
two metal parts placing on each other will not weld. Scabridity of surface explains
this phenomenon, since under usual condition only every hundred thousandth -
millionth ion-pair of surface peaks and cavities are in appropriate proximity.
Compressing the metal parts on the relatively small area of contact, the pressure
reaches the compression yield and a part of metal surface flows plastically. During
the plastic flow of metal, the amount of adsorbed gases decreases, while more and
more metallic bonds are formed.
Those welding processes in which pressure is used at room temperature to
produce coalescence of metals with substantial deformation at the weld create the
first main class of welding, and are classified as cold pressure welding.
A fundamental requisite for satisfactory cold pressure welding is that at least
one of the metals to be joined is highly ductile and does not exhibit extreme work-
hardening strength. Metals which have been successfully cold pressure welded
have face-centred-cubic lattice structure, such as aluminium, copper, lead, nickel,
gold, silver and platinum metal.
The extremely high power which is required to the plastic flows of metal
limits applications of cold pressure welding. The needful power is determined by
the area of joint and by the compression yield strength of metal.
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It is possible to decrease the compression yield strength and in this way, the
required power to pressure welding by rising of temperature. When the welding
temperature is above the recrystallisation temperature, the yield strength drops and
the deformation embrittlement does not occur. Coalescence in the weld area is
achieved by heating and application of pressure. Those welding processes which
join both pressure and heat are grouped in a second main class called hot pressure
welding processes. In most of these processes welds are made without the
workpiece being melted, or at least with very little melting. The hot deformation,
the forging action results in a finer grain structure in the weld, disrupts and
disperses the surface gas or oxide film. The hot pressure welding processes have
high efficiency and the process of welding is very quick. In these processes,
notably resistance and friction welding, the heat is obtained typically in the weld
area from the electric resistance of the workpiece to the passage of an electrical
current or from the heat cue to rubbing friction.
The heated surfaces - in hot pressure welding processes - are in connection
with each other and are excluded of atmosphere therefore they are prevented from
oxidation.
The third main class of welding processes is the fusion welding. In fusion
welding processesthe base metals are melted, and in many cases, filler metal is
added. The molten metal, issuing from parent and filler metals, forms common
weld pool. The weld pool is nucleated by solid parent metals. The liquid metal
surrounded with base metal crystals begins to solidify growing dendritic grains at
the area of contact with the cooler parent metal. These are common grains of both
part of welded joint. The common solidification results in a metallic bond between
parent metals.
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All welding processes require the application of energy to produce a suitable
bond. The welding processes are grouped under these four categories of energy
sources: mechanical, chemical, radiant and electrical sources.
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The most common applied processes are grouped according to their energy source.
Energy of
welding
Welding Processes
Mechanical
Friction
Ultrasonic
Explosion
Chemical
Gas
Thermit
Radiant
Laser beam
Electron beam
Resistance
Electroslag
Resistance spot
Resistance butt
Electrical Arc
Gas tungsten arc
Plasma arc
Gas metal arc
Shielded metal arc
Submerged arc
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The mechanical energy is used for producing metallic bond in friction,
ultrasonic and explosion welding.
In friction welding a bond is created between a stationary and a rotating
member by using the frictional heat generated between them, while subjected to
high normal forces on the interface. Fig.2 illustrates principal stages of friction
welding.
Friction welds are made by holding a non-rotating workpiece in contact with
a rotating workpiece under constant or gradually increasing pressure until the
interface reaches welding temperature and then rotation is stopped by formingweld. The frictional heat developed at the interface rapidly raises the temperature
of the workpieces over a very short axial distance to values approaching, but below
the solidus temperature. Welding occurs under the influence of a pressure that is
applied while the heated zone is metallurgically achieved by diffusion rather than
fusion.
Because of this, the process is admirably suited for joining dissimilar metals,
particularly those that undergo undesirable phases when joined by melting
processes. Application of this process requires that the rotating member must be
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essentially symmetrical about the axis of rotation while the other one can be of any
geometry.
Ultrasonic welds are produced by the introduction of high frequency (15-75
kHz) vibratory energy into the weld zone of metals to be joined. The workpieces
are pressed together between two tips and the vibratory energy is transmitted
through one or both tips which oscillate in a plane parallel to the weld interface.
This oscillating motion disturbs the oxide film on the surfaces of the metal
surfaces, clears away the adsorbed gas layer and permits metal-to-metal contact.
The oscillating shear stress, which occurs during motion results in electric
hysteresis, localized slip and plastic deformation at contacted surfaces. The elastic
and plastic deformations induce a very localized and transient temperature rise at
the weld interface. Under proper conditions of clamping force and vibratory power,
the temperature reached is usually half of the absolute melting point of the metals
joined. For this reason, the ultrasonic welding is considered as cold or solid state
pressure welding process.
Because there is no fusion, this method has given good results with
dissimilar metals. It is generally used to produce spot, straight, and circular seam
weld between workpieces of with not more than 2 mm of sheet or foil thickness.
In explosion welding, the deformation of an explosive is utilized to
accelerate one of the workpieces to a high velocity before it collides with the
stationary component. At the moment of impact, the kinetic energy of the fliyer isreleased as a compressive stress wave on the surface. The pressure level of these
stress waves is greatly above the yield strength of plate material. The essential
feature of the process appears to be that the two surfaces to be joined meet at a
slight angle so that a "bonding front" is established, which moves across the
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interface. The surface films jets out of the interface by the deformation effect of
bonding front on the perfectly clean, oxide and gas-free surface. The interatomic
force creates a metallic bond.
The result of this process is a cold pressed weld without a heat-affected
zone. Explosion welding is generally used to produce a cladding plate but it is
suitable for welding of bars. Satisfactory welds can be made between copper and
steel and a variety of metals such as gold, silver, nickel and titanium.
Chemical energy stored in a wide variety of forms can be converted to
useful heat. The temperature and the rate of oxidation reaction are two majorcharacteristics which determine the application of the various energy sources for
welding.
The involved heat of chemical reaction is utilized for melting of parent
materials in gas and thermit welding. For this reason these processes are
considered as fusion welding.
In gas welding, the used fuel gases have two important characteristics. The
first important characteristic of a flame is its chemical activity. Variation of the
flame characteristics - oxidizing, neutral or reducing - is accomplished by altering
the proportions of fuel gas and of oxygen or air. The neutral flame is the one most
used one since neutral atmosphere surrounding the molten metal prevents
contamination of weld before solidification.
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From the point of view of welding, the second important characteristic of a
flame is its temperature as this largely determines the rate of burning at which
welding can be carried on. Flames are hotter if the fuel gas is burnt in pure oxygen
than in air. The presence in the flame of nitrogen, which is also heated but takes no
part in the combustion, reduces the temperature of flame. At maximum
temperature, the flames are oxidizing in nature and are usually not suitable for
welding due to the formation of oxides in the weld metal.
The acetylene and oxygen are under moderate pressure when mixed and
burned in hand held welding torch. The flame is directed to the work surface and
obtains fusion of the parts by melting the metals in contact.
The gas welding can be applied to a wide variety of metals and it is
employed not only in welding, but for brazing, too.
Thermit weldingutilizes heat from exothermic reaction. A number of metal
oxides can be reduced by reaction with finely distributed aluminium with the
liberation of considerable heat, so that the products of the reaction are molten. The
reaction is obtained with ferric oxide produces 2450 oC temperature. A charge of
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1000g of oxide and aluminium produces 476 g of slag 524g of iron and 0.76 MJ of
energy.
Molten, superheated iron produced in this way can be poured into the cavity
made between two parts of a joint to produce a weld. The slag floats to the surface
and the molten material is heating to melt both faces of the base metal. When the
filler metal has cooled, all unwanted excess material may be removed. Thermit
welding is also used for welding of copper, nickel and their alloys.
The most widely employed usage of thermit welding is for joining rails,
concrete reinforcing bars, for repair and for welding of heavy construction withcharges of up to 3 ton.
The laser and electro beam welding employ energy in the form of radiant
energy.
Radiant energy welding methods are unique because the energy for welding
must be focused on the object to be welded, and the heat is generated only where
the focused beam strikes the work piece. Unlike other energy sources, the work is
not brought in contact with any heated media, gas or metal vapour, and the
processes usually may be carried out in low pressure systems where the ultimate in
cleanliness can be achieved.
The laser beam is focusable by various lens arrangements as well as the
electron beam is by electrostatic or magnetic way. The focused beam gives high
power densities up to 100 kW/mm2 which is some thousand times higher than
power density of gas welding. With radiant energy welding can be produced by the
conventional conduction limited manner and by the keyhole technique. In
conduction limited welding, the beam impinges on and is absorbed by the metal
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surface. The inner portion of the material is heated entirely by conduction from the
surface.
In this welding mode, the intense energy concentration at the work piece
surface induces local vaporization. A vapour cavity surrounded by molten metal is
formed as the beam starts to move along the joint. The cavity is maintained against
the fluid dynamic forces of the liquid metal surrounding it by the pressure of the
vaporized metal. Metal is progressively melted at the leading edge of the moving
molten pool and flows around the deep penetration cavity to the rear of the pool
where it solidifies. In the keyhole mode, penetration is not limited by the thermal
diffusivity of the material because beam energy penetrates directly into the cavity.
Many different metals can be satisfactory welded with radiant energy
welding processes. Copper, nickel, iron, zirconium, aluminium, titanium,
magnesium, tungsten, molybdenum and their alloys are weldable with this
process.
Since radiant welding equipment costs more than conventional systems of
equivalent power, the selection of applications must be based on unique process
capabilities. Some of these capabilities that may be used as a guideline for
selection are as follows:
the specific energy input to the workpiece is very small
the high power density can be used for welding or dissimilar metals with
widely different physical properties or great differences in mass and sizes
precision welding can be done with a well-defined focused spot
the surroundings of welding are very clean
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they are ideally suited to automation.
Electron beam weldingis a process that produces coalescence of metals by
the heat obtained from a concentrated beam of high velocity electrons impinging
upon the surfaces to be joined. The beam of electrons is produced and accelerated
by an electron beam gun.
Electrons are generated by heating a negatively charged emitting material to
its thermal emission temperature range. The electrons boil off this emitter and are
speeded and directioned by their attraction to a positively charged anode. A
precisely configured electrode surrounding the emitter electrostatically shapes theejected electrode into a beam.
Electron beam welding system capable of producing beam power levels up
to 100 kW and power densities in excess of 100 kW/mm2have been built.
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In laser weldingthe source of energy is a laser. From an engineering
standpoint, the laser may be considered as an energy conversion device in which
energy from a primary form (electrical, chemical, thermal, optical, nuclear) is
transformed into a beam of coherent electromagnetic radiation at ultraviolet,
visible or infrared frequency. The laser "light" is monochromatic (single
wavelength) and coherent (all waves are in phase). Because of laser lights are
coherent, they can highly concentrate with transmitting or reflective optics to
provide the high-energy density required for welding and cutting.
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The most commonly applied welding processes use electric energy. The
electric energy may be transformed to heat in a resistance or in electrical arc. The
way of energy transformation give a classification or electric welding processes,
namely resistance and arc welding.
The resistance welding processes- except for electroslag welding - employ
a combination of force and heat to produce a weld between the workpieces.
Resistance heating occurs as electrical current flows through the workpieces.
In electroslag welding, an electrode such as a wire is fed through an
electrically conductive bath of molten slag. The heat is generated by the resistance
offered to the current during its passage from electrode wire through the slag into
the weld pools. This heat melts not only the wire, but the base metals, too. Weld
metal is deposited through the molten slag which refines out some impurities and
protects the weld pool from the atmosphere.
The weld metal solidifies upward as heat is extracted by the surrounding
weldment and the containing shoes. Electroslag welding is primarily a method for
welding heavy thickness of steel in the vertical or near vertical position.
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In resistance spot welding the two pieces to be spot-welded together are
placed between the water-cooled copper welding electrodes. Heat for welding,
however, is required only at the base metal interface, and the heat generated at any
other locations should be minimized. In practice since the greatest resistance is
located on interface, heat is most rapidly developed at that location. The heat at
base metal interface is dissipated much more slowly into the base metal. Therefore,
as the welding current continues to flow, the rate of temperature rise will be
quicklier than at other points. In a well controlled weld, the welding temperature
will first be reached at numerous contact points at the interface that met and
quickly grow into a nugget with time.
During the spot welding, the workpieces are compressed by electrodes. On
the interface the contact resistance between the workpieces decreases and by this
way, the generated heat increases if the compression is increased. The welding
current has a greater effect on the generation of heat than either resistance or time.
Therefore, it is an important variable to be controlled. It is typical for the spot
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welding that the high welding current is up to 100 kA and welding time is very
short, some hundredth seconds.
The resistance seam welding is a special kind of resistance spot welding.
This process use rotating wheel electrodes which pinch the two pieces of metals
and controlled current impulses weld a continuous point-series, a seam of
overlapping spot welds. The resistance spot and seam welding are used for welding
of thin pieces up to 3 mm.
The resistance butt weldingis a process which produces coalescence
simultaneously over the entire area of two abutting surfaces. This welding process
is essentially done in the solid state. The metal at the joint is heated to atemperature where recrystallisation can rapidly take place across the abutting
surfaces. A force is applied to the welding to bring the surfaces into intimate
contact and during the heating to upset the material tends to purge the joining of
oxidized metal.
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This welding process is used for fabricating a rather wide variety of products
from bar, rod, wire, strip and pipe.
Particularly attractive feature of resistance spot seam and butt welding
process are: the high speed of operation, the high productivity, ease of
mechanization, elimination of oxidation by closing of heated surfaces and the
absence of edge preparation or filler metal.
The second main way to change electrical energy into heat is the application
of electric arc. An arc is a continuous electric discharge between two solid or
liquid electrodes which takes place through partially or totally ionized gas that is
known as "plasma". The arc, as a heat source, is used for many important welding
processes because it produces a high intensity of heat and is easy to control
through electrical means.
Under normal circumstances, gases are insulator. Ions and free electrons,
which are current carriers, are produced by thermal means and field emission for
the gaseous medium. The establishment of plasma state occur by collision
processes of high energy particles. The particles of welding plasma, ions, electrons,
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neutral or excited gaseous atoms and molecules obtain their high energy by heating
or electric field.
As a gas is heated, the individual molecules or atoms obtain more energy. At
low temperatures this energy is mainly translational associated with velocity of
motion. At higher temperatures diatomic molecules such as nitrogen (N2) absorb
energy firstly by rotation and secondly by vibration (an in-and-out movement of
the two atoms relative to each other).
When the vibration energy reaches a sufficiently high level it may rupture
the valence bonds holding the two atoms together, causing them to dissociate into amonoatomic state.
At higher temperatures part of the energy is absorbed by the outer electron
bond of individual atoms, and eventually causes detachment of one of the outer
electrons. During this process, the atom ionizes into one electron and a positively
charged ion.
The energy levels for ionization are substantially higher than for
dissociation. Therefore, ionization becomes significant when the gas is
substantially monoatomic and two reactions may be treated separately.
The heated gas of the welding arc attains a maximum interval of
temperature, between 5 000 and 30 000 K, depending on the kind of gas and the
intensity of current carried by it. The gas which is between electrodes consists of
shielding gas or air and vapours of base metal and slag.
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Only the highest energy level part of cathode emits electron. This part is
called as cathode spot.
The biggest and the most important region in the welding arc is the arc
column. In most arc columns, the transfer of energy in a gas results from the
interaction of the particles of which it is composed. These individual particles are
in a state or continual random motion, and energy is transferred from one particle
to another by collisions. Such collisions are called elastic, if the total kinetic energy
of particles involved remains unchanged and only the motion parameters of
collided particles are changed. If a part collision energy is absorbed internally - for
example by excitation, dissociation or ionization - then the collision is termed
inelastic. By increasing the temperature of the gas, the inelastic collisions will
predominate.
During recombination of excited dissociated or ionized particles, they
irradiate discrete energy quantum in nature of ultraviolet, visible and infrared
wavelength. This radiant energy maintains the high temperature of the arc and the
plasma state.
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In practice, the plasma of welding arc is considered to be highly ionized. In
highly ionized plasma distant encounters between electrons, ions and atoms are the
main mode of interaction between particles.
Though the anode has a vital role to play in preserving current continuity by
receiving electron flow, it has less influence on the arc in a number of respects than
the cathode. Striking of electron and negative ion beam into the anode, they
transfer to it their kinetic energy and the energy of condensation. The incident
particles form a pressure on the surface to the anode spot making a penetration in
the weld pool.
From the application point of view, welding processes consists in two basic
types according to whether or not the electrode is melted. If the electrode is
refractory - that means, if it is made of carbon or tungsten - it is not melted away in
the process of arcing and is non-consumable. When the electrode - such as filler
material melts and molten droplets can be detached and transported across the arc
gap to the workpieces by the fast moving plasma jet, the electrode is consumable.
Any arc welding process in which the electrode is melted off to become part of the
weld is described as "metal arc".
With a non-consumable electrode, heat finds its way into the work by the
electron or ion processes which take place at the boundary of the arc column with
the work - this being the largest source of heat - and also by the connection of the
hot plasma jet and by the recombination of any particles dissociated in the arccolumn.
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Heat is lost to any fluxes present in the arc and also to a limited extent. Only
a few percent is lost by radiation and to the gases leaving the arc space.
Additionally, heat generated is lost by conduction up the electrode.
If the electrode is consumable and is transferred to the weld pool, this heat is
available again in the pool. Since the electrode is a part of electrical resistance, the
passage of current down the electrode to the arc can cause resistance heating in an
appreciable degree.
During the arc welding, the weld pool is heated significantly above the
melting point of parent materials and the temperature of droplets is near to theboiling point. At this temperature the oxidization and other similar chemical
reactions are very quick. Therefore, then must be some way to exclude the air
atmosphere while the process is carried out. Slag or shielding gas is used to protect
the hot metal. Slag may be formed by melting of electrode covering or welding
flux. Those welding processes, in which is used for protection slag, are named as
"flux-shielded arc welding"
If a flux is not used, shielding can be provided by a blanket of gas, or a gas
which does not form refractory compounds with the base metal. The non-
consumable electrode welding processes in every case apply inert gas shielding the
metal arc processes can apply active gas as well as.
Shielding gas has four functions:
gas ionises and acts as a conductor for the electrons to flow. gas shields the weld pool from the surrounding air, it prevents oxidation the stability of the arc is influenced by the shielding gas at low and high
currents.
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the heat of the arc and therefore the penetration are also determined by thegas.
Argon, carbon dioxide, helium, oxygen and their mixture are used most
frequently.
Argon is the most frequently used shielding gas for welding. It is relatively
cheap and gives a stable arc that can be started easily. With pure argon, all metals
may be welded.
Helium is interesting because of the high arc voltage that provides a deeper
penetration. This has a positive effect on the welding speed. Because the gas is
extremely light, more gas is needed for welding. The arc is less stable than with
argon. The gas is relatively expensive. Pure helium is used for welding copper and
aluminium and their alloys.
Gas tungsten arc welding(GTAW) is a process wherein coalescence of metals
is produced by heating them with an arc between a tungsten (non-consumable)
electrode arc shielded from the atmosphere by a blanket of inert (Ar, He) gas fed
through the gas nozzle. Besides this, there is Ar/H2mixture is also done with argon
and a little percentage of hydrogen because of it's reducing and gives a cleaner
result.
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A weld is made by applying the arc so that the abutting workpieces are
melted and joined together as the weld metal solidifies. Filler metal may or maynot be added. The filler metal and the welding rod is hand held or the wire is fed
mechanically.
The GTAW is adaptable to both manual and automatic operation. This
process is used with welding currents from 1 A to 700 A and is one of the most
versatile methods of welding in respect of material. Although high welding
currents permitting the welding of thick metal are possible, GTAW is primarily a
process for welding sheet metal or small parts. Since GTAW is a metallurgically
clean process and gives high quality welds, the process is greatly favoured for
precision welding in the aircraft, nuclear energy and instrument industries.
Plasma arc welding (PAW) is an arc welding process where the heating
occurs with a constricted arc between a tungsten electrode and the workpiece
(transferred arc) or between the electrode and the constricting nozzle (non-
transferred arc). Constriction of the arc is usually accomplished by passing the arc
through a water-cooled copper orifice. The purpose of constriction is to control
and increase the energy density of the arc stream. Shielding is generally obtained
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from the hot, ionized gas issuing from the orifice of the constricting nozzle.
Shielding gas may be an inert gas or a mixture of inert gases. The orifice gas is the
gas which is directed through the torch to surround the electrode. It becomes
ionized in the arc to form the plasma and issues from the orifice in the torch nozzle
as the plasma jet. Argon, helium, and hydrogen are applied as orifice gas. Filler
metal may or may not be added.
The constricted arc used in plasma-arc welding offers several advantages over the
non-constricted arc used in GTAW:
concentration of energy is greater,
arc stability is improved, particularly at low current levels, solid backing is not required for obtaining complete penetration, because the
keyhole technique can be used.
Plasma arc processes are employed not only in welding, but for cutting and
surfacing of metals. Application of PAW is similar to GTAW.
Gas metal arc welding(GMAW) is an electric arc welding process which
produces coalescence of metals by heating them with an arc established between a
continuously fed filler metal (consumable) wire and the work. Shielding of the arc
and molten weld pool is obtained entirely from an externally supplied gas or gas
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mixture. Argon, carbon dioxide, helium, oxygen and their mixture are used most
frequently for GMAW. Although Ar and He are used for gas metal arc welding of
most metals, CO2has become widely used (along with Ar-CO2mixture) for
welding of mild steels.
GMAW is operated in semiautomatic machine and automatic modes. It is utilized
particularly in high production welding operations. All commercially important
metals such as carbon steel, stainless steel, aluminium, and copper can be welded
with this process in all positions by choosing the appropriate shielding gas, wire
and welding conditions.
Shielded metal arcwelding is a manual welding process in which the heat for
welding is generated by an arc established between a flux covered consumable
electrode and the work. The electrode tip, weld pool, arc and adjacent areas of the
workpiece are protected from atmospheric contamination by a gaseous shield
obtained from combustion and decomposition of the flux covering. Additional
shielding is provided for the molten metal by a covering of molten flux, of the slag.
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Filler metal is supplied by the core of the consumable electrode and, in certain
electrodes, from metal powder mixed with the electrode covering.
SMAW is one of the most widely used welding process for joining metal
parts, mainly because of its versatility. Also, the equipment is less complex, more
portable and less costly than other arc welding processes.
The utilization of welding is not limited by the process, but by the type and
size of the electrode. Joints in virtually any position that can be reached with
electrode can be welded. Carbon and low alloy steels, stainless steels, heat resisting
alloys, copper and nickel and their alloys are the metals welded easilier by the
SMAW process. Cast iron, and the high-strength and hardenable types of steel can
also be welded by this process, but additional procedures that include preheating or
postheating, or both, may be needed. Low melting metals, such as lead, tin and
zinc and their alloys are not welded with the SMAW because the intense heat of
the arc is too high for them. Also the reactive metals, such as titanium, zirconium,
magnesium and aluminium alloys are not welded with covered electrodes. These
metals are very sensitive to oxygen contamination and the shielding obtained with
covered electrode is not adequate for them.
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Submerged arc welding(SAW) is an arc welding process in which the heat for
welding is supplied by an arc (or arcs) developed between a continuously fed and
consumable welding wire (or wires) and the workpiece. The arc is shielded by a
layer of granular and fusible flux, which blankets the molten weld metal and the
base metal from atmospheric contamination. While the process carries out the filler
material, the wire is advanced in the direction of welding and mechanically fed
into the arc while flux is steadily added.
The melted base and filler metal flow together to form the weld pool surface
and a protective slab cover. Unmelted flux is reclaimed for reuse. Fluxes for SAW
of alloy steels may contain alloying ingredients that modify the composition of the
weld metal.
There are three general methods by which the process can be applied:
semiautomatic, automatic and machine welding.
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The SAW can be used for a wide range or industrial application. The high
quality of welds, the high deposition rates, the deep penetration, and the
adaptability to automatic operation make the process particularly suitable for
fabrication of large and heavy weldments. It is used extensively in ship building,
railroad car fabrication, pipe manufacturing and the fabrication of structural
members where long welds are required.
The process can be used to weld materials ranging from 3 mm thick sheet to
very thick, heavy weldments. SAW is not suitable for all metals and alloys. It is
widely used for welding carbon steels, low alloy structural steels and stainless
steels. Submerged arc welding can be used only in the flat position.
In case of electric arc welding electricity is supplied by the mains. In the
welding machine electricity is converted into voltage and current suitable for
welding. The machine is connected to the mains supply, usually 415 V and three
phases. With this machine it is possible to weld with two types of current: Direct
current and alternating current. When welding with direct current, the 415 V from
the mains is at first transformed to a lower voltage and thereafter rectified. A
rectifier converts the alternating current into direct current by means of diodes.
Diodes are semi-conductors that only let pass the positive or the negative part of
alternating current. The result is a direct current with high amperage and a low
voltage of less than 120 V. When welding with an alternating current (AC), the
voltage is transformed to a safety low value of less than 50 V. The mains voltage is
transformed into a safe low welding voltage for welding.
The highest heat generation is at the positive pole. The heat of the arc in the
first place leads to the melting of the metal. At the same time the heat makes the
gas better ionized; conductivity is improved.
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In case of gas metal arc welding and submerged arc welding the arc starts
with a simple short circuit. Some momentary short-circuits lead to sparking. These
results in ionisation of the gas that becomes conductive and a welding arc can be
formed.
At shielded metal arc welding touch start of the arc is used when the tip of
the electrode rests on the workpiece. After then the electrode is slowly (lifting
method) or quickly (scratch method) lifted from the workpiece; at the slightest gap
between electrode and workpiece a spark is transmitted and the arc starts.
This touch start of the arc is not the best way for gas tungsten arc welding. It
can lead to contamination of the weld pool or tungsten electrode. Therefore power
sources for gas tungsten arc welding have electronic device for starting the arc.
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During ignition a high-frequency generator is used. It is delivering high-
frequency current pulses of 2000 to 10000 V at a frequency of 150 kHz.. The result
of this is an excess of electrons at the minus pole and a lack of electrons at the plus
pole and this leads to sparking. When a welding arc is started, the open voltage
changes to a lower voltage that is needed to maintain the welding arc: this is the so
called welding voltage that is necessary to overcome the resistance in a total
welding circuit, inclusive the welding arc.
Electric arc welding is usually carried out with direct current. This works
well for the welding of steel and its alloys. During the welding of light metals such
as aluminium or magnesium a phenomenon occurs resulting in a malfunction of
this process. An oxide layer is formed on surface of workpiece and on the weldpool. This ceramic layer is an obstruction to metallic connection. The solution is
the prevention or elimination of the oxide layer on the light alloy. Aluminium can
be properly welded when using alternating current.
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When welding with alternating current (AC) the arc must be ignited again
and again. An extra problem is the so-called rectifier effect. When the flow of
electrons turns and they run from workpiece to electrode, the oxide layer is broken
by impacted ions and the conditions of welding joint improve.
The GMAW welder does not always use a filler metal, but metal arc welding
are imaginable without consumable filler material. It is the material that is fed drop
by drop into the weld pool and that fills up the joint. The choice of the filler metal
must be such that a perfect and durable melting with the base material is created.
Normally the filler metal has the same composition as the base material. It's no
wonder that there are many types. In the welding procedure specification you'll
find the indication of the correct composition of the filler metal.
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TRANSFORMER
A transformeris an electrical device that transfers energy between two
circuits through electromagnetic induction. Transformers may be used in voltage
conversion to transform an AC voltage from one voltage level on the input of the
device to another level at the output terminals, to provide for different
requirements of current level as an alternating current source, or it may be used for
impedance matching between mismatched electrical circuits to effect maximum
power transfer between the circuits.
A transformer most commonly consists of two windings of wire woundaround a common core to effect tight electromagnetic coupling between the
windings. The core material is often a laminatediron core.The coil that receives
the electrical input energy is referred to as the primary winding, while the output
coil is called the secondary winding.
An alternatingelectric current flowing through the primary winding (coil) of
a transformer generates an electromagnetic field in its surroundings and a
varyingmagnetic flux in the core of the transformer. Byelectromagnetic
induction this magnetic flux generates a varyingelectromotive force in the
secondary winding, resulting in avoltage across the output terminals. If a load
impedance is connected across the secondary winding, a current flows through the
secondary winding drawing power from the primary winding and its power source.
A transformer cannot operate with direct current, but produces a short output
pulse as the voltage rises when connected to the DC source.
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Invention
The invention of transformers during the late 1800s allowed for longer-
distance, cheaper, and more energy efficienttransmission,distribution, and
utilization ofelectrical energy.In the early days of commercial electric power, the
main energy source was direct current (DC), which operates at low-voltage high-
current. According toJoule's Law, energy losses are directly proportional to the
square of current. This law revealed that even a tiny decrease in current or rise in
voltage can cause a substantial lowering in energy losses and costs. Thus, the
historical pursuit for a high-voltage low-current electricity transmission system
took shape. Although high voltage transmission systems offered many benefits, the
future fate of high-voltage alternating current still remained unclear for several
reasons: high-voltage sources had a much higher risk of causing severe electrical
injuries; many essential appliances could only function at low voltage. Regarded as
one of the most influential electrical innovations of all time, the introduction of
transformers had successfully reduced the safety concerns associated with
alternating current and had the ability to lower voltage to a value that was required
by most essential appliances.
Applications
Transformers performvoltage conversion;isolation protection;
andimpedance matching.In terms of voltage conversion, transformers can step-up
voltage/step-down current from generators to high-voltage transmission lines, andstep-down voltage/step-up current to local distribution circuits or industrial
customers. The step-up transformer is used to increase the secondary voltage
relative to the primary voltage, whereas the step-down transformer is used to
decrease the secondary voltage relative to the primary voltage. Transformers range
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in size from thumbnail-sized used in microphones to units weighing hundreds of
tons interconnecting thepower grid.A broad range oftransformer designs are used
in electronic and electric power applications, including miniature, audio, isolation,
high-frequency, power conversion transformers, etc.
Basic principles]
The functioning of a transformer is based on two principles of the laws of
electromagnetic induction: An electric current through a conductor, such as a wire,
produces amagnetic field surrounding the wire, and a changing magnetic field in
the vicinity of a wire induces a voltage across the ends of that wire.
The magnetic field excited in the primary coil gives rise to self-induction as
well as mutual induction between coils. This self-induction counters the excited
field to such a degree that the resulting current through the primary winding is very
small when no load draws power from the secondary winding.
The physical principles of the inductive behavior of the transformer are most
readily understood and formalized when making some assumptions to construct a
simple model which is called the ideal transformer. This model differs from real
transformers by assuming that the transformer is perfectly constructed and by
neglecting that electrical or magnetic losses occur in the materials used to construct
the device.
Induction law[
A varying electrical current passing through the primary coil creates a
varying magnetic field around the coil which induces a voltage in the secondary
winding. The primary and secondary windings are wrapped around a core of very
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highmagnetic permeability, usuallyiron,[c]so that most of the magnetic flux
passes through both the primary and secondary coils. The current through a load
connected to the secondary winding and the voltage across it are in the directions
indicated in the figure.
Leakage flux
Main article:Leakage inductance
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Leakage flux of a transformer
The ideal transformer model assumes that all flux generated by the primary
winding links all the turns of every winding, including itself. In practice, some flux
traverses paths that take it outside the windings.[19]Such flux is termed leakage
flux, and results inleakage inductance inseries with the mutually coupled
transformer windings.[12]Leakage flux results in energy being alternately stored in
and discharged from the magnetic fields with each cycle of the power supply. It is
not directly a power loss, but results in inferiorvoltage regulation, causing the
secondary voltage not to be directly proportional to the primary voltage,
particularly under heavy load.[19]Transformers are therefore normally designed to
have very low leakage inductance. Nevertheless, 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.[20]
In some applications increased leakage is desired, and long magnetic paths,
air gaps, or magnetic bypass shunts may deliberately be introduced in a
transformer design to limit theshort-circuit current it will supply.[12]Leaky
transformers may be used to supply loads that exhibitnegative resistance, such
aselectric arcs,mercury vapor lamps,andneon signs or for safely handling loads
that become periodically short-circuited such aselectric arc welders.[21]
Air gaps are also used to keep a transformer from saturating, especiallyaudio-frequency transformers in circuits that have a DC component flowing in the
windings.
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Knowledge of leakage inductance is also useful when transformers are operated in
parallel. It can be shown that if the percent impedance (Z) and associated winding
leakage reactance-to-resistance (X/R) ratio of two transformers were
hypothetically exactly the same, the transformers would share power in proportion
to their respective volt-ampere ratings (e.g. 500kVA unit in parallel with 1,000
kVA unit, the larger unit would carry twice the current). However, the impedance
tolerances of commercial transformers are significant. Also, the Z impedance and
X/R ratio of different capacity transformers tends to vary, corresponding 1,000
kVA and 500 kVA units' values being, to illustrate, respectively, Z ~ 5.75%, X/R ~
3.75 and Z ~ 5%, X/R ~ 4.75.[23][24]
Core form and shell form transformers
Closed-core transformers are constructed in 'core form' or 'shell form'. When
windings surround the core, the transformer is core form; when windings are
surrounded by the core, the transformer is shell form. Shell form design may be
more prevalent than core form design for distribution transformer applications due
to the relative ease in stacking the core around winding coils.[40]Core form design
tends to, as a general rule, be more economical, and therefore more prevalent, than
shell form design for high voltage power transformer applications at the lower end
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of their voltage and power rating ranges (less than or equal to, nominally, 230 kV
or 75 MVA). At higher voltage and power ratings, shell form transformers tend to
be more prevalent.[40][41][42][43]Shell form design tends to be preferred for extra high
voltage and higher MVA applications because, though more labor-intensive to
manufacture, shell form transformers are characterized as having inherently better
kVA-to-weight ratio, better short-circuit strength characteristics and higher
immunity to transit damage.[43]
Construction
Cores
Laminated steel cores
Transformers for use at power or audio frequencies typically have cores
made of high permeabilitysilicon steel.[44]The steel has a permeability many times
that offree space and the core thus serves to greatly reduce the magnetizing current
and confine the flux to a path which closely couples the windings.[45]Early
transformer developers soon realized that cores constructed from solid iron
resulted in prohibitive eddy current losses, and their designs mitigated this effect
with cores consisting of bundles of insulated iron wires.[46]Later designs
constructed the core by stacking layers of thin steel laminations, a principle that
has remained in use. Each lamination is insulated from its neighbors by a thin non-
conducting layer of insulation.[47]The universal transformer equation indicates a
minimum cross-sectional area for the core to avoid saturation.
The effect of laminations is to confine eddy currents to highly elliptical
paths that enclose little flux, and so reduce their magnitude. Thinner laminations
reduce losses, but are more laborious and expensive to construct. Thin laminations
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are generally used on high-frequency transformers, with some of very thin steel
laminations able to operate up to 10 kHz.
One common design of laminated core is made from interleaved stacks ofE-
shaped steel sheets capped withI-shapedpieces, leading to its name of 'E-I
transformer'.[49]Such a design tends to exhibit more losses, but is very economical
to manufacture. The cut-core or C-core type is made by winding a steel strip
around a rectangular form and then bonding the layers together. It is then cut in
two, forming two C shapes, and the core assembled by binding the two C halves
together with a steel strap. They have the advantage that the flux is always oriented
parallel to the metal grains, reducing reluctance.
A steel core'sremanence means that it retains a static magnetic field when power is
removed. When power is then reapplied, the residual field will cause a high inrush
current until the effect of the remaining magnetism is reduced, usually after a few
cycles of the applied AC waveform. Overcurrent protection devices such
asfuses must be selected to allow this harmless inrush to pass. On transformers
connected to long, overhead power transmission lines, induced currents due
togeomagnetic disturbances duringsolar storms can cause saturation of the core
and operation of transformer protection devices.
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Distribution transformers can achieve low no-load losses by using cores made with
low-loss high-permeability silicon steel oramorphous (non-crystalline) metal
alloy. The higher initial cost of the core material is offset over the life of the
transformer by its lower losses at light load.
Solid cores
Powdered iron cores are used in circuits such as switch-mode power supplies that
operate above mains frequencies and up to a few tens of kilohertz. These materials
combine high magnetic permeability with high bulk electricalresistivity. For
frequencies extending beyond theVHF band, cores made from non-conductivemagneticceramic materials calledferrites are common.[49]Some radio-frequency
transformers also have movable cores (sometimes called 'slugs') which allow
adjustment of thecoupling coefficient (andbandwidth) of tuned radio-frequency
circuits.
Toroidal transformers are built around a ring-shaped core, which, depending on
operating frequency, is made from a long strip ofsilicon steel orpermalloy wound
into a coil, powdered iron, orferrite.[53]A strip construction ensures that thegrain
boundaries are optimally aligned, improving the transformer's efficiency by
reducing the core'sreluctance.The closed ring shape eliminates air gaps inherent in
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the construction of an E-I core.[21]The cross-section of the ring is usually square or
rectangular, but more expensive cores with circular cross-sections are also
available. The primary and secondary coils are often wound concentrically to cover
the entire surface of the core. This minimizes the length of wire needed, and also
provides screening to minimize the core's magnetic field from
generatingelectromagnetic interference.
Toroidal transformers are more efficient than the cheaper laminated E-I types for a
similar power level. Other advantages compared to E-I types, include smaller size
(about half), lower weight (about half), less mechanical hum (making them
superior in audio amplifiers), lower exterior magnetic field (about one tenth), low
off-load losses (making them more efficient in standby circuits), single-bolt
mounting, and greater choice of shapes. The main disadvantages are higher cost
and limited power capacity (seeClassification parametersbelow). Because of the
lack of a residual gap in the magnetic path, toroidal transformers also tend to
exhibit higher inrush current, compared to laminated E-I types.
Ferrite toroidal cores are used at higher frequencies, typically between a few tens
of kilohertz to hundreds of megahertz, to reduce losses, physical size, and weight
of inductive components. A drawback of toroidal transformer construction is the
higher labor cost of winding. This is because it is necessary to pass the entire
length of a coil winding through the core aperture each time a single turn is added
to the coil. As a consequence, toroidal transformers rated more than a few kVA are
uncommon. Small distribution transformers may achieve some of the benefits of a
toroidal core by splitting it and forcing it open, then inserting a bobbin containing
primary and secondary windings.
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INDUCTION MOTOR
An inductionor asynchronous motoris anAC electric motor in which
theelectric current in therotor needed to produce torque is induced
byelectromagnetic induction from the magnetic field of thestator winding. An
induction motor therefore does not requiremechanical commutation, separate-
excitation or self-excitation for all or part of the energy transferred from stator to
rotor, as inuniversal,DC and largesynchronous motors. An induction motor's rotor
can be eitherwound type orsquirrel-cage type.
Three-phasesquirrel-cage induction motors are widely used in industrial drives
because they are rugged, reliable and economical. Single-phase induction motors are
used extensively for smaller loads, such as household appliances like fans. Although
traditionally used in fixed-speed service, induction motors are increasingly being used
withvariable-frequency drives (VFDs) in variable-speed service. VFDs offer
especially important energy savings opportunities for existing and prospective
induction motors in variable-torquecentrifugal fan, pump andcompressor load
applications. Squirrel cage induction motors are very widely used in both fixed-speed
and VFD applications.
HISTORY
In 1824, the French physicistFranois Arago formulated the existence
ofrotating magnetic fields, termedArago's rotations,which, by manually turning
switches on and off, Walter Baily demonstrated in 1879 as in effect the firstprimitive induction motor.[1][2][3][4]Practical alternating current induction motors
seem to have been independently invented byGalileo Ferraris andNikola Tesla,a
working motor model having been demonstrated by the former in 1885 and by the
latter in 1887. Tesla applied forU.S. patents in October and November 1887 and
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was granted some of these patents in May 1888. In April 1888, the Royal Academy
of Science of Turin published Ferraris's research on his AC polyphase motor
detailing the foundations of motor operation.[4][5]In May 1888 Tesla presented the
technical paper A New System for Alternating Current Motors and Transformers to
theAmerican Institute of Electrical Engineers (AIEE) describing three four-stator-
pole motor types: one with a four-pole rotor forming a non-self-startingreluctance
motor,another with a wound rotor forming a self-starting induction motor, and the
third a truesynchronous motor with separately excited DC supply to rotor
winding.George Westinghouse,who was developing an alternating current power
system at that time, licensed Teslas patents in 1888 and purchased a US patent
option on Ferraris' induction motor concept.[11]Tesla was also employed for one
year as a consultant. Westinghouse employeeC. F. Scott was assigned to assist
Tesla and later took over development of the induction motor at
Westinghouse. Steadfast in his promotion of three-phase development,Mikhail
Dolivo-Dobrovolsky's invented the cage-rotor induction motor in 1889 and the
three-limb transformer in 1890.[15][16]However, he claimed that Tesla's motor was
not practical because of two-phase pulsations, which prompted him to persist in his
three-phase work.[17]Although Westinghouse achieved its first practical induction
motor in 1892 and developed a line of polyphase 60hertz induction motors in
1893, these early Westinghouse motors weretwo-phase motors with wound rotors
untilB. G. Lamme developed a rotating bar winding rotor. TheGeneral Electric
Company (GE) began developing three-phase induction motors in 1891. By 1896,
General Electric and Westinghouse signed a cross-licensing agreement for the bar-
winding-rotor design, later called the squirrel-cage rotor.[6]GE'sCharles Proteus
Steinmetz was the first to make use of the letter "j" (the square root of minus one)
to designate the 90-degreerotation operator in electrical mathematical expressions
and thereby be able to describe the induction motor in terms now commonly
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known as theSteinmetz equivalent circuit.Induction motor improvements flowing
from these inventions and innovations were such that a 100horsepower induction
motor currently has the same mounting dimensions as a 7.5 horsepower motor in
1897.
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PRINCIPLE OF OPERATION
In both induction and synchronous motors, the AC power supplied to the
motor'sstator creates amagnetic field that rotates in time with the AC oscillations.
Whereas a synchronous motor's rotor turns at the same rate as the stator field, aninduction motor's rotor rotates at a slower speed than the stator field. The induction
motor stator's magnetic field is therefore changing or rotating relative to the rotor.
This induces an opposing current in the induction motor's rotor, in effect the
motor's secondary winding, when the latter is short-circuited or closed through an
external impedance. The rotatingmagnetic flux induces currents in the windings
of the rotor; in a manner similar to currents induced in atransformer's secondary
winding(s). The currents in the rotor windings in turn create magnetic fields in the
rotor that react against the stator field. Due toLenz's Law, the direction of the
magnetic field created will be such as to oppose the change in current through the
rotor windings. The cause of induced current in the rotor windings is the rotating
stator magnetic field, so to oppose the change in rotor-winding currents the rotor
will start to rotate in the direction of the rotating stator magnetic field. The rotor
accelerates until the magnitude of induced rotor current and torque balances the
applied load. Since rotation at synchronous speed would result in no induced rotor
current, an induction motor always operates slower than synchronous speed. The
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