ELECTROMAGNETISM – PART – 02 - IMPORTANT TERMS ELECTROMAGNETISM

A magnet can be broken into many pieces, and each piece becomes a new magnet with its own North Pole and South Pole.

MAGNETIC FLUX (Φ)

The invisible lines of force that make up the magnetic field are known as magnetic flux. It is denoted by Φ and its unit is weber.

MAGNETIC FIELD

A magnetic field is the region around the magnet where force of attraction and repulsion takes place.

MAGNETIC POLE

This is a region where the external magnetic effects of magnet are concentrated or it is the point where the strength of magnet is maximum.

UNIT MAGNETIC POLE

The unit of magnetic pole strength equal to the strength of a magnetic pole that repels a similar pole with a force of one dyne, the two poles being placed in a vacuum and separated by a distance of one centimeter.

MAGNETIC FLUX DENSITY (B)

Magnetic flux density at any point in a magnetic field is the magnetic flux passing per unit area at that point. It is denoted by B and its unit is Tesla or Weber / metre²

MAGNETIC FIELD INTENSITY (H)

The magnetizing force or the magnetic field intensity is defined as the magnetic motive force per unit length of the magnetic flux path. It is denoted by H and its unit is ampere-turn / metre.

MAGNETO MOTIVE FORCE (M.M.F)   

The force which drives the magnetic flux through a magnetic circuit is called the magnetic motive force. It is produced by passing electric current through a coil of wire have number of turns. It is measured in ampere-turns.

RELUCTANCE (S)

The property of a magnetic circuit which restricts the establishment of magnetic flux is called reluctance. It is denoted by S and its unit is ampere-turns / weber.

PERMEANCE (P)

The reciprocal of reluctance of a magnetic circuit is called as permeance. It is denoted by P and Its unit is weber / ampere-turn.

MAGNETIC CIRCUIT

A combination of media, mainly comprising ferromagnetic substance, forming a close circuit though which a flux of magnetic induction may pass.  

SIMPLE MAGNETIC CIRCUIT

A simple magnetic circuit is made up of a single magnetic material. Thus such a circuit reflects the magnetic properties of the materials used.

COMPOSITE MAGNETIC CIRCUIT

It is a magnetic circuit which comprises of two or more elements whose magnetic properties are different.

PERMEABILITY (µ)

It is the ratio between flux density to flux intensity. It is denoted by µ. µ = B / H. it has no unit, as it is a ratio.

ABSOLUTE PERMEABILITY (µ₀)

Absolute permeability of a medium is given by the ratio of flux density to magnetizing force at a point in the medium µ₀.

RELATIVE PERMEABILITY (µ_R)

Relative permeability of a medium is given by the ratio of absolute permeability of the medium to the absolute permeability of vacuum. It is also equal to the ratio of flux density produced in a medium to the flux density produced in vacuum by the same magnetizing force µ_r.

SERIES MAGNETIC CIRCUIT

A closed magnetic path consisting of different sections of varying permeabilities and physical dimensions but having the same magnetic flux is called a series magnetic circuit.

PARALLEL MAGNETIC CIRCUIT

The flux created by the Magneto Motive Force (M.M.F) acting in the magnetic circuits gets divided into two or more branches/portions of the magnetic core. The fluxes in these branches may be different, but the magnetic potential drops across the branches may be different, but the magnetic potential drop across the branches remains the same. Such a magnetic circuit is called parallel magnetic circuit.

 MAGNETIC CORE

A part of magnetic circuit surrounded by a coil.

MAGNETIC POTENTIAL DROP

When a magnetic flux Ф is established in magnetic circuit of reluctances S there is a always a magnetic potential drop M along direction of the flux. 

The magnetic potential drop is the product of the flux and reluctance. M = S x Ф.

MAGNETIC LEAKAGE

A small part of the flux completes its path through air rather than through the core, which is called as leakage reactance or magnetic leakage.

MAGNETIC LEAKAGE REACTANCE

Reactance of the transformer due to leakage (primary flux not linking secondary) is called as leakage reactance of transformer.

MAGNETIC LEAKAGE CO-EFFICIENT

The flux that follows in an undesired path is called leakage flux. The ratio of total flux to useful is called leakage flux co-efficient.

RETENTIVITY

A magnetic material can retain the magnetism even after the removal of the magnetising source. This property of the magnetic materials is called retentivity.

RESIDUAL MAGNETISM

The flux that remains in a temporary magnet after, it is removed from a magnet field is called residual magnetism.

MAGNETIC SATURATION

A magnetic material is saturated when an increase in m.m.f. no longer increases the flux in the material.

EDDY CURRENT      

Eddy current are those which are produced or induced in the masses of metals, whenever these metals are moved in magnetic field, or the magnetic field moves through the metals. The direction of the eddy currents is always in opposite direction to the cause (motion) to produce them.

MAGNETIC HYSTERESIS

The lagging of the magnetic induction with respect to magnetizing force is called magnetic hysteresis.

MAGNETIC SHIELDING

Transformers and choke produce strong magnetic in the space around the equipment. These magnetic field affect the performance of the equipment which are situated in close vicinity. Such a magnetic field may cause humming in radio receivers and also affects the readings of the measuring instruments. A hallow piece of ferromagnetic material is used to enclose the equipment which acts a magnetic shielding. The ferromagnetic material has high permeability.  

SOLENOID

A coil usually of tubular form for producing a magnetic field.

ELECTROMAGNETISM - PART - 01 - HISTORY OF MAGNETISM & ELECTROMAGNETISM

MAGNETISM

The word “magnetism” originated from the city of Magnesia (now called Mania in Turkey) where iron ores were discovered which had the property of adhering to each other in lumps. 

ELECTRO MAGNETISM

Electromagnetism is the part of science which deal with the relation between electricity and magnetism.

In the year 1819, Professor, Hans Christian Oersted, university of Copenhagen, Demark, accidently discovered that on passing a wire carrying current parallel to a magnetic needle, there was a deflection in the needle, as if it was acted upon by a magnet. In this experiment he concluded that every conductor carrying current is surrounded by a magnetic field.

In 1845 Faraday showed with the help of very powerful electromagnets that almost all substances were influenced by a magnet to varying degrees. He suspended, by a long and fine suspension wire, small bars of various solid substances between the poles that of strong electromagnets, while switching on the current, he found that some of the substances arranged themselves with their lengths parallel to direction of the field, while the other set themselves in a direction at right angles to the direction of the field.

MAGNETISM & ELECTROMAGNETISM

Magnetism is a force filed that acts on some materials but not on other materials. Physical devices which posses this force are called magnets.

Electromagnetism is the part of science which deal with the relation between electricity and magnetism.

MAGNET

Magnet is the substance having the properties of attracting iron and its alloy.

GENERAL CLASSES OF MAGNET

Natural magnet

Lodestone (an iron compound) is a natural magnet which was discovered centuries ago.

Artificial magnet

The magnets are made from various alloys containing elements like copper, nickel, aluminum and cobalt.

These magnets are much, stronger than the natural lodestone magnet.

Artificial magnet again classified as

i) Permanent magnet – The magnet which retains the magnetic properties for a long period. It is available in different shapes as (a) Bar magnets (b) U-shaped (c) Horse shoe magnets and (d) Compass needle, etc.

ii) Electromagnet or temporary magnet – The magnet which loses its properties as soon as the magnetizing force is removed.

MAGNETIC MATERIALS

The most common magnetic materials are iron, iron components, and alloys containing iron or steel.

Materials such as nickel and cobalt are slightly magnetic. They are attracted by strong magnets. Compared with iron, however, they are only weakly magnetic.

NONMAGNETIC MATERIALS

Materials which are not attracted by the magnets are called nonmagnetic materials.

Examples are Copper, brass, aluminum, silver, zinc and tin.

Nonmetals like wood, plastic, paper, leather and rubber.

A nonmagnetic material does not stop magnetic flux. Flux goes through nonmagnetic materials about as readily as it goes through air.

PERMANENT MAGNETS

Many alloys of iron, especially those contain more than 0.8% carbon become permanent magnet.

Tools such as screwdriver, pliers and haw-saw blades, contain more than 0.8% carbon.

Most permanent magnets are made of alloys such as Alnico) which can be highly magnetized.

Alnico magnets are composed of iron, cobalt, nickel, aluminum and copper. Applications of permanent magnets are used to make loud speakers, electric meters and motors.

TEMPORARY MAGNETS

Materials such as pure iron, ferrite and silicon steel make temporary magnets. Applications of temporary magnets are used in great quantities in motors, generators, transformers and electromagnets.

PARAMAGNETIC SUBSTANCES

Para-magnetic substances are attracted by magnets slightly. Its permeability is greater than unity. Examples: Aluminum, copper sulphate and platinum

DIAMAGNETIC SUBSTANCES

Diamagnetic materials are repelled by magnets. Its permeability is less than unity. Examples: gold, lead, copper and antimony

FERROMAGNETIC SUBSTANCES

These materials are strongly attracted by magnets. Its permeability is very much greater than unity ranging to several thousands. Examples: Iron, Nickel and Cobalt

PERMEABILITY (µ)

It is the ratio between flux density to flux intensity. 

It is denoted by µ. µ = B / H. it has no unit, as it is a ratio.

ELECTRIC WELDING PART – 08 – ARC WELDING PRINCIPLE AND PROCESS

ARC WELDING

Arc welding is one of several fusion processes for joining metals. 

By applying intense heat, metal at the joint between two parts is melted and caused to intermix - directly, or more commonly, with an intermediate molten filler metal.

PRINCIPLE

Electric arc is luminous electrical discharge between two electrodes through ionized gas. The arc is formed between the actual work and an electrode (electrode may be stick or wire) that is manually or mechanically guided along the joint.
MAJOR COMPONENTS OF ARC WELDING

1. Power supply (AC or DC)

2. Welding electrode

3. Work piece

4. Welding leads (electric cables) connecting the electrode and

work piece to the power supply.

ARC WELDING MACHINES

a) D.C. welding machines

(i) Motor Generator set            
(ii) Transformer with rectifiers

b) A.C. welding machines.

PROCESS

1. When the electrode being near to the work piece, an arc is developed which closes the electrical circuit.

2. The arc temperature may reach 5500°C which is enough for fusion of the work piece edges and joining them

3. Filling rod is used as an outside metal, which melts and fills the weld pool.

4. The outside metal may be a consumable electrode which fills the weld pool.

5. Filler metal and work piece are of similar chemical compositions.

6. Neutral shielding or inert gases such as argon, helium are used to prevent contamination by oxide and nitride when the molten reacts with the surrounding atmosphere.

7. Shielding is provided in the weld zone as a flux coating on the electrode.

8. When a long join is required the arc is moved along the joint line.

9. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint.

10. Solidification is the process of becoming hard or solid by cooling or drying or crystallization.

WELDING ELECTRODES

In arc welding an electrode is used to conduct current through a work piece to fuse two pieces together.

Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.

1. Electrode can be a rod or a wire which is used to carry current between the tip and the work piece.

2. It may be specially prepared rod or wire that carry current as well as melts and supplies filler metal to the joint.

Most welding in the manufacture of steel products uses the second type of electrode.

TYPES OF ELECTRODES

1. Bare electrodes

Bare welding electrodes are made of wire compositions required for specific applications.

2. Light coated electrodes

Light coated welding electrodes have a definite composition.

A light coating has been applied on the surface by washing, dipping, brushing, spraying, tumbling, or wiping.

The coatings improve the characteristics of the arc stream.

3. Shielded arc or heavy coated electrodes

Shielded arc or heavy coated welding electrodes have a definite composition on which a coating has been applied by dipping or extrusion. The electrodes are manufactured in three general types: those with cellulose coatings; those with mineral coatings; and those whose coatings are combinations of mineral and cellulose.

4. Tungsten electrodes

Non consumable welding electrodes for gas tungsten-arc (TIG) welding are of three types: pure tungsten, tungsten containing 1 or 2 percent thorium, and tungsten containing 0.3 to 0.5 percent zirconium.

5. D.C arc welding electrodes

Direct current shielded arc electrodes are designed either for reverse polarity (electrode positive) or for straight polarity (electrode negative), or both.

Many, but not all, of the direct current electrodes can be used with alternating current. Direct current is preferred for many types of covered, nonferrous, bare and alloy steel electrodes.

6. A.C arc welding electrodes

Alternating current is used in atomic hydrogen welding and in those carbon arc processes that require the use of two carbon electrodes. It permits a uniform rate of welding and electrode consumption.

In carbon-arc processes where one carbon electrode is used, direct current straight polarity is recommended, because the electrode will be consumed at a lower rate.

7. Non-consumable Electrodes

There are two types of non-consumable welding electrodes.

The carbon electrode is a non-filler metal electrode used in arc welding or cutting, consisting of a carbon graphite rod which may or may not be coated with copper or other coatings.

The tungsten electrode is defined as a non-filler metal electrode used in arc welding or cutting, made principally of tungsten.

The following materials are commonly used for coating

1. Titanium oxide                    2. Ferro-manganese

3. Silica, flour                          4. Asbestos clay

5. Calcium carbonate               6. Cellulose with sodium silicate

ADVANTAGES OF COATED ELECTRODES

1. Due to increase in melting rate, the operation of welding becomes faster.

2. The coating contains compounds of potassium and sodium, which supports arc stabilization.

3. It produce slag (slag means the scum formed by oxidation at the surface of molten metals) over the weld which reducing the changes in brittleness, smooth surface and protects from atmospheric contamination.

4. During welding, sputtering of metal is prevented.

FACTORS TO BE CONSIDERED FOR SELECTION OF ELECTRODES

1. The type of the metal to be welded.

2. The welding position.

3. The type of electric supply (A.C or D.C.)

4. The polarity of the welding machine.

ELECTRIC WELDING PART – 07 - PERCUSSION WELDING – ADVANTAGES & DISADVANTAGES

A percussion weld is similar to flash weld in which the required pressure is provided by a hammer like blow.

A short application of high intensity energy instantly heats the surfaces to be joined. This rapid heating is almost immediately followed by a quick blow to make the weld.

Generally ‘percussion’ welding is the term used in electronics industry for joining wires, leads and other items to a flat surface.

It is a high-speed welding process which can be used to produce either a fusion or forge (solid phase) weld. An arc is struck between a wire and component (a fuse body for example).

In this process, coalescence is produced simultaneously over the entire area of surfaces to be joined by the heat obtained from the arc. The arc is produced by a short pulse of electrical energy. Pressure is applied during or immediately following the electrical discharge.

There are two methods of percussion welding (i) the capacitor discharge and (ii) magnetic force methods.

Capacitor discharge method is again classified into two types (i) high voltage, low capacitance system and (ii) low voltage and high capacitance system.

Magnetic force percussion welding machine usually has an independent power source.

Metals which can be percussion welded include, copper alloys, aluminum alloys, nickel alloys, low and medium carbon steel and stainless steel.

Gold, silver, copper-tungsten, silver-tungsten and silver-cadmium oxide are percussion welded to copper alloys for commonly used electrical contacts. 

The equipment generally consists of press type resistance welding machine with specially designed transformer to obtain low voltage.

To produce the weld force electro-magnetic system is used. An air cylinder provides the initial force to bring the parts together.

In the capacitor discharge method, parts to be joined are connected to the capacitor and then forced together by a spring or by compressed air.

As the gap between the parts closes to a distance which the voltage can break down, an arc of high intensity is initiated and superficial melting takes place before the parts impact together. 

PRINCIPLE

Heat is generated by a high current arc between the two points to be joined. The high current melts a thin layer of the metal and the surfaces are brought together in percussion manner to get welded.

PROCESS

1. Remove the dirt, oil, paint or grease on the surfaces.

2. Work pieces are clamped into the fixture or machine.

3. Light pressure is applied on the parts and bring the faces together.

4. The arc developed due to high current strike the faces to be joined with high voltage to ionize the gap between the parts or melt and vaporize the projections on one part.

5. The arc established heats the faces of two parts to the welding temperature.

6. Move the parts together with the force, which may be spring, gravity, pneumatic or electromagnetic.

7. It extinguishes the arc while the parts are welded together.

8. Release the force and unclamp the welded parts.

ADVANTAGES

1. Parts with different thermal conductivities can be easily welded. 

2. Very quick process time, usually takes less than 0.1 second.

3. It causes very little damage to the material which is close to the weld.

4. Without any danger of annealing, hardened surfaces may be welded.

5. Heat balance is not a problem because the heat is concentrated at the ends of the work pieces.

DISADVANTAGES

1. The process is limited to butt welded joint only

2. The joint used in limited to about 1.5 to 3 cm² because of control of paths of arc is difficult.

3. The process cannot be used for welding heavy sections larger than 600 mm².

APPLICATIONS

1. This process joins similar or dissimilar metals.

2. This process is used for welding fine wire leads to filaments in lamps and terminals.

3. This welding is useful to make large contact assemblies for relays etc.

4. It can join wires and rods by butt joints.                                    

5. This process is used to attach metal tips to valve stems.