welding-electricity & arc physics

8/6/2019 Welding-Electricity & Arc Physics

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Because in practice it is common to find wide variations in the magnitude of electrical

quantities, electrical units often have metric unit prefixes that represent powers of

ten. The following chart shows how three of these prefixes are used to represent large

and small values of current.

Unit Symbol Equivalent Measure

kiloampere kA 1 kA = 1000 A

milliampere mA 1 mA = 0.001 A

microampere A 1 A = 0.000001 A

Direction of Current Flow

Some sources distinguish between

electron flow and current flow. The

conventional current flow

approach ignores the flow of

electrons and states that currentflows from positive to negative. To

avoid confusion, this book uses the

electron flow concept which states

that electrons flow from negative to

positive.

Voltage

The force required to make electricity flow through a conductor is called a difference

in potential , electromotive force (emf), or voltage . Voltage is designated by the letter

E or the letter V.

The unit of measurement for voltage is the volt which is also designated by the letter

V. A voltage can be generated in various ways. A battery uses an electrochemical

process. A cars alternator and a power plant generator utilize a magnetic induction

process. All voltage sources share the characteristic of an excess of electrons at one

terminal and a shortage at the other terminal. This results in a difference of potential

between the two terminals. For a direct current (DC) voltage source, the polarity of

the terminals does not change, so the resulting current constantly flows in the same

direction.


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Thefollowing chart shows how selected metric unit prefixes are used to represent

large and small values of voltage.Unit Symbol Equivalent Measure

kilovolt kV 1 kV = 1000 V

millivolt mV 1 mV = 0.001 V

microvolt V 1 V = 0.000001 V

Resistance

A third factor that plays a role in an electrical circuit is resistance . All material impedes

the flow of electrical current to some extent. The amount of resistance depends upon

the composition, length, cross-section and temperature of the resistive material. As a

rule of thumb, the resistance of a conductor increases with an increase of length or a

decrease of cross-section. Resistance is designated by the symbol R. The unit ofmeasurement for resistance is the ohm (W). Resistors are devices manufactured to

have a specific resistance and are used in a circuit to limit current flow and to reduce

the voltage applied to other components. A resistor is usually shown symbolically on

an electrical drawing in one of two ways, a zigzag line or an unfilled rectangle.

In addition to resistors, all other circuit

components and the conductors that

connect components to form a circuit also have

resistance. The basic unit for resistance is 1 ohm;

however, resistance is often expressed in

multiples of the larger units shown in the

following table.

Unit Symbol Equivalent Measure

megohm M 1 M = 1,000,000

kilohm k 1 k = 1000


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Ohms Law

Electric Circuit

A simple electric circuit consists of a voltage source, some type of load, and

conductors to allow electrons to flow between the voltage source and the load.Ohms law shows that current varies directly with voltage and inversely with

resistance.

Current (I) is measured in amperes (amps)

Voltage (E) is measured in volts

Resistance (R) is measured in ohms

There are three ways to express Ohms law.

Conductors

An electric current is produced when free electrons move from atom to atom in a

material. Materials that permit many electrons to move freely are called conductors.

Copper, silver, gold, aluminum, zinc, brass, and iron are considered good conductors.

Of these materials, copper and aluminum are the ones most commonly used as

conductors.


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Insulators

Materials that allow few free electrons are called insulators.

Materials such as plastic, rubber, glass, mica, and ceramic are good insulators.

An electric cable is one example of how conductors and insulators are used. Electrons

flow along a copper or aluminum conductor to provide energy to an electric device

such as a radio, lamp, or a motor. An insulator around the outside of the copper

conductor is provided to keep electrons in the conductor.

Semiconductors

Semiconductor materials, such as silicon, can be used to manufacture devices that

have characteristics of both conductors and insulators. Many semiconductor devices

act like a conductor when an external force is applied in one direction and like an

insulator when the external force is applied in the opposite direction. This principle is

the basis for transistors, diodes, and other solid-state electronic devices.

Magnetism

The principles ofmagnetism are an integral part of electricity. In fact, magnetism can

be used to produce electric current and vice versa. When we think of a permanent

magnet, we often envision a horse-shoe or bar magnet or a compass needle, but

permanent magnets come in many shapes. However, all magnets have two

characteristics. They attract iron and, if free to move (like the compass needle), a

magnet assumes a north-south orientation.


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Magnetic Lines of Flux

Every magnet has two poles, one north pole and one south pole. Invisible magneticlines of flux leave the north pole and enter the south pole. While the lines of flux are

invisible, the effects of magnetic fields can be made visible. When a sheet of paper is

placed on a magnet and iron filings loosely scattered over it, the filings arrange

themselves along the invisible lines of flux.

The density of these lines of flux is greatest inside the magnet and where the lines of

flux enter and leave the magnet. The greater the density of the lines of flux, thestronger the magnetic field.

Interaction Between When two magnets are brought together, the magnetic flux

Magnets around the magnets causes some form of interaction. When two unlike poles

are brought together the magnets attract each other. When two like poles brought

together the magnets repel each other.


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Electromagnetism

An electromagnetic field is a magnetic field generated by current flow in a conductor.

Every electric current generates a magnetic field and a relationship exists between the

direction of current flow and the direction of the magnetic field. The left-hand rule for

conductors demonstrates this relationship. If a current-carrying conductor is grasped

with the left hand with the thumb pointing in the direction of electron flow, the

fingers point in the direction of the magnetic lines of flux.

Current-Carrying Coil

A coil of wire carrying a current, acts like a magnet. Individual loops of wire act as

small magnets. The individual fields add together to form one magnet. The strength of

the field can be increased by adding more turns to the coil, increasing the amount of

current, or winding the coil around a material such as iron that conducts magnetic flux


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more easily than air. The left-hand rule for coils states that, if the fingers of the left

hand are wrapped around the coil in the direction of electron flow, the thumb points

to the north pole of the electromagnet.

An electromagnet is usually wound around a core of soft iron or some other material

that easily conducts magnetic lines of force. A large variety of electrical devices such as

motors, circuit breakers, contactors, relays and motor starters use electromagnetic

principles.

Alternating CurrentThe supply of current for electrical devices may come from a direct current (DC) source

or an alternating current (AC) source. In a direct current circuit, electrons flow

continuously in one direction from the source of power through a conductor to a load

and back to the source of power. Voltage polarity for a direct current source remains

constant. DC power sources include batteries and DC generators. By contrast, an AC

generator makes electrons flow first in one direction then in another. In fact, an AC

generator reverses its terminal polarities many times a second, causing current tochange direction with each reversal.

AC Sine Wave

Alternating voltage and current vary continuously. The graphic representation for AC is

a sine wave. A sine wave can represent current or voltage. There are two axes. Thevertical axis represents the direction and magnitude of current or voltage. The

horizontal axis represents time.


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2. Nature ofElectric

Arc, Arc

heat as a

Heat

Source, Arc

Power

When the waveform is above the time axis, current is flowing in one direction. This is

referred to as the positive direction. When the waveform is below the time axis,

current is flowing in the opposite direction. This is referred to as the negative

direction. A sine wave moves through a complete rotation of 360 degrees, which is

referred to as one cycle. Alternating current goes through many of these cycles each

second.

Polarity and Current Flow


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Conduction of Current in the Arc


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Arc Structure and Mechanism - The arc constitutes a mechanism by which electrons

are emitted from the cathode, they pass through the hot ionized arc column and are

transferred to the anode where they get condensed and absorbed.

It is presently well accepted that there are three distinct regions of a welding are,

namely cathode drop region or cathode fall space, arc column or arc plasma region

and anode drop region or anode fall space.

The arc column is situated in between anode and cathode drop regions which are

spread over an approximate distances of 10-2

mm and 10-1

to 10-2

mm respectively.

The cathode is negative, anode is positive and arc column is electrically neutral as it

contains equal number of ions and electrons.

The conditions in the arc column are quite different from the regions where the arc

comes in contact with the electrode, (i.e. cathode drop region in DCSP) and the

workpiece (i.e. anode drop region in DCSP). In the immediate vicinity of the electrode

or the job, the plasma can no longer maintain its high temperature because it comes in

contact with comparatively much colder workpiece and electrode.

High temperature gradients exist on both the ends of the arc column and naturally the

arc gets divided into three distinct zones. Out of these zones, the most concentratedsource of heat is the cathode spot, hottest region is the arc column and the largest

quantity of heat is produced at the anode.

In a welding arc, the electrons are emitted from the cathode, get accelerated in the

cathode drop region and gain energy. As they enter arc column, they lose their energy

by colliding with gas atoms and molecules which in turn get ionized, i.e. electrons and

positive ions are separated.

The ions and electrons then move towards cathodic and anode respectively and get

concentrated over there. Because of this concentration of charge carriers (i.e.,

electrons and ions) in the anode and cathode drop zones, a nonlinear voltage

distribution is prevalent along the arc length, and high electric field strengths arcfound in cathode and anode drop zones.

Approximate potential drop in the cathode and anode drop regions is of the order of

10 volts and 12 to 1 volt respectively. In such an are, magnetic field strength, current

density and pressure all decrease from cathode drop region towards the arc column,

because arc crosssection increases rapidly in the arc column.

Depending upon certain conditions, the electric current flows, along the axis of the arc

or both along the axis and transversely. The current flow through the arc gives rise to

self induced magnetic field which compresses the arc plasma resulting in appreciable

axial and radial pressure gradients in the arc.

The radial pressure gradient constricts (pinch effect) the arc and raises the

temperature of the arc discharge whereas axial pressure gradient gives rise to plasma

streaming which transports material (metal and slag particles) and heat, from the

electrode to the workpiece.

Plasma streams stabilize the arc and exert a pressure on the molten pool which helps

increase penetration. Both these effects are proportional to the square of the arc

current.

The three regions of the welding arc will be discussed briefly as follows:

1. Cathode drop zone. It is contained within two imaginary planes, one just at the end

of cathode spot and the other at the beginning of arc plasma column. Cathodic tip


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appears darker as compared to arc column.

Cathode drop region is important because electrons are produced here, and the arc

stability depends upon a regular supply of electrons. The significance of cathode drop

zone still increases when, in AC welding, arc is to be reignited every half cycle (as the

current passes through zero in AC cycle)

The cathode drop mechanism for electrodes made up of high melting point and low

work function materials like tungsten, thoriated or Zirconiated tungsten and carbon

graphite is as follows:

At high temperature, the electrons are emitted from the cathode by THERMIONIC

EMISSION, get accelerated in cathode drop zone, gain kinetic energy, which is lost in

the arc column when the electrons collide with the gas atoms and molecules. These

gas atoms get ionized. The ions so produced travel towards the cathode (being

attracted by it), strike it and give up their kinetic energy.

This produces high heat and even with conduction and radiation losses from the

cathode, it is maintained at high temperature necessary for further emission of

electrons. In this case cathode spot is not well defined.Another cathode mechanism which can be explained through FIELD EMISSIONS is

associated with low melting point electrodes. There is a relatively larger cathode area

containing many active well defined small cathode spots which move around with a

velocity of about 104 cm/sec.

These cathode spots, constantly form, are vanished and get reformed elsewhere.

Perhaps, vaporisation of the electrode material accompanies this mechanism. A third

mechanism, PLASMA EMISSION, is related to high pressure and low current (though

high current density) arcs, In this case the cathode is stationary and well marked.

Arc Plasma Column

Arc column is that portion of the welding arc which is situated between anode andcathode drop regions. Arc column consists of a radiating mixture of electrons, ions (+)

and highly excited neutral atoms and molecules. In order to keep current flowing

between (the electrode and the job, or) cathode and anode (in DCSP) arc column

provides and maintains, a regular supply of ions and electrons.

The current carried by the ions because of their heavy mass and thus less mobility is

much lesser and can normally be neglected in comparison to the current carried by

electrons or the total current. Thus it is concluded that mainly the density of electrons

is responsible for maintaining current between electrode and job. Arc column may

have temperatures ranging from 5,000 to 50,000K.

Energy to reach this temperature is achieved from the catholically emitted electrons

which collide with gas atoms (in the arc column) raise their temperature and ionize

them to single, double or triple degrees of ionization, and in turn producing more

electrons which again collide with neutral atoms and thus the degree of ionization

increases.The approximate temperatures of electrons and ions are of the order of

15,000-50,000K and 6,000K respectively.

Anode Drop Zone

It is situated in between the anode spot and the place where the arc column finishes.

This region forms the electrical connection between the arc plasma column and the

anode. The potential drop in the anode drop region exists because of the

concentration of electrons which enter in this zone from arc column.


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Three phenomena occur in the anode drop zone, i.e., temperature falls (from that of

arc column), ions are produced, and accelerated towards arc column. The formation of

positive ions is influenced by temperature and anode plasma composition.

The chances of ion formation increase as the anode plasma temperature rises.

Depending upon anode material, the metal vapours formed exert a substantial effect

on the anode drop, because due to collisions with the electrons, vapours get ionized

and supply positive ions.

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