Seam Welding Machine

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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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