Detailed
Solutions D- 04 JUNE
2004
1. a. B Silver.
b. B Superconductivity.
c. A Copper
d. A Insulators
e. A Dielectrics
f. B Curie point.
g. B Hard magnetic materials.
h. A P-type semiconductor.
PART- I
2a) Materials, which are commonly used in electrical and electronics engineering, can be classified as conductors, insulators, semiconductors.
Conductors: These are the material, which allow the current to pass through them. These have very low electrical resistance and are available in a large variety having different properties. Also the number of valence electrons is less than four. The, valence- band and the conduction- band overlap each other. Examples are copper, brass, aluminium, silver, gold, bronze, etc.
Semiconductors: These are the materials, which possess the electrical resistivity in between that of conductors and insulators. They are used for the manufacture of diodes and transistors. Also the number of valence electrons is equal to four. There is a small forbidden energy gap of about 1eV between the conduction and the valence band. Examples: germanium, silicon, selenium, etc.
Insulators: These are the materials, which do not allow the current to pass through them without any appreciable loss. They have very high electrical resistance and are also available in a large variety to cover different applications. Some of the specific insulating materials are used for the purpose of storing of an electrical energy and are called dielectric materials such as mica, ceramic, paper etc. These materials are used as a dielectric in capacitors. Also the number of valence electrons is more than four. The energy gap between valence and conduction band is very large (more than 5-6 eV). Examples: Mica, rubber, ceramics, glass, diamond etc.
2b) Advantages of aluminium as compared copper as a conductor of electricity: The electrical conductivity of aluminium is next to that of copper. Its resistivity is 2.8X10-8 ohm-m, i.e. about 1.6 times higher than copper. Its density is 2.68 which means that aluminium is much lighter than copper. Its melting point is 6550C. Like copper, it can be easily drawn into thin wires. Aluminium is soft metal but when alloyed with some other materials like magnesium, silicon or iron, it acquires higher mechanical strength and can be used for overhead transmission lines. Like copper, aluminium also forms an oxide layer over its surface when exposed to atmosphere and that layer prevents the material from further oxidation and acts as a resistance layer to corrosion.
Disadvantages: As aluminium is a soft material, there is always a possibility of loose contacts. Due, to the insulating property of aluminium oxide formed on the surface, it is difficult to solder aluminium wires. However for applications like winding of electrical machines and transformers, it is difficult to substitute aluminium for copper. This is because aluminium wires have lower tensile strength than that of copper.
3a)
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Thermoplastic materials: The properties of these plastic materials do not change considerably if they are melted and then cooled and solidify. They can be repeatedly melted or dissolved in various solvents. They are more elastic, less brittle and do not lose elasticity when subjected to prolonged heating. They are less apt to age thermally. They can be remoulded again and again in any shape after heating. Many of them possess extraordinary high insulating properties and are water repellent. They are polymers of linear structure, i.e. their molecules are elongated and are thread like. This, type of structure is fusible, soluble, highly plastic, capable of forming thin flexible threads and films. Examples are Polytetra Flouroethylene (P.T.F.E. or Teflon), Polyvinyl Chloride (P.V.C.).
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Thermosetting Plastic Materials: They undergo great changes when subjected to high temperatures for quite sometime. They are said to be baked and no longer can melt or be dissolved. They are less elastic, more brittle and lose their elasticity when subjected to prolonged heating. So they cannot be remoulded in different shapes once they are set and hardened. They are used, when the insulation is to withstand high temperatures without melting or losing its shape and mechanical strength. Thermosetting plastic substances are space-polymers and the molecules branch off in various directions during polymerisation. This structure makes them very rigid, poorly soluble, fusible and incapable of forming elastic threads and films. Examples are Phenol formaldehyde (Bakelite), Epoxy resins.
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3b) (i) Superconductivity- The resistivity of most metals increases with increase in temperature and vice-versa. There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 00Kelvin (-2730C). At this stage such metals or compounds are said to have attained superconductivity. Example – mercury becomes superconducting at approximately 4.5 Kelvin (-268.50C). Superconductivity, was discovered by Heike Kamerlingh Onnes. The transition from normal conductivity to superconductivity takes place almost suddenly; it occurs over a very narrow range of temperature about 0.050K. The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called transition temperature.
(ii) Resistivity: The resistance R of a wire having cross-sectional area A and length L have the relationship, as R a L and R a 1/A ; resulting Ra L/A or R = r L / A ; Where r is the constant of proportionality and is called resistivity of a material and is defined as the resistance between the two opposite faces of a meter cube of that material. The unit of resistivity is ohm-m. Factors affecting resistivity are temperature, alloying, mechanical stressing, ageing.
4a) Electrical properties of an insulating material are:
Insulation resistance-It is the property, by the virtue of which, a material resists flow of electrical current. It should be high as possible. Insulation resistance is of two types: I) Volume resistance; ii) Surface resistance.
The resistance offered to the current, which flows through the material is called volume resistance. The resistance offered to the current, which flows over the surface of the insulating material is called surface resistance. Factors that affect the insulation resistance are-temperature variations, exposure to moisture, voltage applied, aging.
Dielectric Strength- It is the minimum voltage which when applied to an insulating material will result in the destruction of its insulating properties. It can also be defined as the maximum potential gradient that the material can withstand without rupture or without loosing dielectric properties. This value is expressed in volts or kilovolts per unit thickness of the insulating material. This value is greatly affected by the conditions under which the material is operated. Factors affecting the dielectric strength are temperature and humidity.
Dielectric Constant- Every insulating material has got the basic property of storing charge (Q), when a voltage (V) is applied across it. The charge is proportional to the voltage applied i.e. Q a V, or Q = CV. Where C is called the capacity or capacitance of the material across which the voltage is applied. Every insulating material behaves as a capacitor. Capacitance is different for different insulating material. The property of insulating materials that causes the difference in the value of capacitance, with the physical dimensions remaining the same is called dielectric constant or permittivity (Î) and Î= C/CO, where C is the capacity in presence of dielectric and CO is the capacity in air or vacuum or in the absence of dielectric.
Dielectric loss and loss angle: When a perfect insulation is subjected to alternating voltage, it is like applying alternate voltage to a perfect capacitor. In a perfect capacitor the charging current would lead the applied voltage by 900 exactly. This means that there is no power loss in the insulation. In most insulating materials this is not the case. There is a definite amount of dissipation of energy when an insulator is subjected to alternating voltage. This dissipation of energy is called dielectric loss. Factors affecting dielectric loss are – Frequency of applied voltage, humidity, temperature rise and voltage.
The dielectric phase angle is q and d = 900 - q is the dielectric loss angle.
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Also I is the phasor sum of Id & Ic, where Ic is the conduction current which is in phase with the applied voltage and Id is the displacement current which is in quadriture phase with applied voltage.
4b) An insulator is designed to withstand certain amount of heat. But when an insulator is overheated, dielectric losses will increase. Also overheating will affect the various important properties as electrical properties, mechanical strength, hardness, viscosity, solubility etc.
5) (i)Brass – It is an alloy of copper and zinc with 60% copper and 40% zinc. Its electrical resistivity is 7.0X 10-8 ohm-m, which is higher than the copper resistivity. It is ductile and can be drawn into different shapes. It melts at 8900C.Its specific gravity is 3.3.It has got an excellent corrosion resistance. It has got good mechanical properties. Brass is used as a structural and current carrying material in power switches, plugs, sockets, lamp holders, fuse holders, knife switches, sliding contacts for starters and reheostats, wave- guide components.
Bronze - Alloy of copper and tin. This alloy is very hard and brittle. Its corrosion resistance is better than brass. It is ductile and can be drawn into different shapes. In bronzes, which are used as electrical conductor, the content of tin and other metal is usually low as compared to the bronzes, which are used for mechanical applications. It is used for making structural elements for equipments. It is also used for making current carrying springs, sliding contacts, knife switches.
(ii) Thermocouples: They are used for the measurement of temperature. Depending on the range of temperature to be measured, proper materials are to be chosen for a thermocouple. If one junction called the cold junction is held at a known constant temperature, the emf produced becomes measure of the temperature of the other junction. The emf produced by a thermocouple is very small but it can be measured with reasonable accuracy by a sensitive moving coil millivoltmeter, which can be calibrated in terms of temperature. Some of the materials used for thermocouples are copper/constantan, iron/constantan, nickel/ nickelchromium.
(iii) P – N junction: When a p- type semiconductor is suitably joined to an n-type semiconductor the contact surface so formed is called p-n junction. All the semiconductor devices contain one or more p-n junction. P-N junction is fabricated by special techniques namely growing, alloying and diffusion methods. The p-type semiconductor is having negative acceptor ions and holes. The n-type semiconductor is having positive donor ions and negatively charged electrons. When the two pieces are joined together and suitably treated they form a p-n junction. The moment they form a p-n junction, some of the conduction electrons from n-type material diffuse over to the p- type material and undergo electron – hole recombination with the holes available in the valence band. Simultaneously holes from p-type material diffuse over to n- type material and undergo hole-electron combination with the electrons available in the conduction band. This process is called diffusion. When a p-n junction is connected across an electric supply, the junction is said to be under biasing. The potential difference across the p- n junction can be applied in two ways, namely- forward biasing and reverse biasing. When the positive terminal of a dc source is connected to p-type, and negative terminal is connected n-type semiconductor of a p-n junction, the junction is said to be in forward biasing. When the positive terminal of a dc source is connected to n-type, and negative terminal is connected p-type semiconductor of a p-n junction, the junction is said to be in reverse biasing. With forward bias, a low resistance path is set up in the p-n junction, and hence current flows through the circuit. With reverse bias, a high resistance path is set up and no current flows through the circuit. This property is best suited for rectification of ac into dc.
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(iv) Fuse materials: Fuse is a protective device, which consists of a thin wire or strip. This wire or strip is placed with the circuit it has to protect, so that the circuit-current flows through it. When this current is too large, the temperature of the wire or strip will increase till the wire or strip melts thus breaking the circuit and interrupting the supply.
A fuse material should possess the following properties;-
Low resistivity – This means, thin wires can be used, which will give less metal vapour after melting of the wire. Less metal vapour in the arc gives lower conductivity and thus makes quenching of arc easier.
Low conductivity of the metal vapour itself.
Low melting point- This means that the temperature of the fuse material for normal currents stays at a low value. Originally lead was used as fuse material because of its low melting point. But as the resistivity of lead is high, thick wires are required. For rewirable fuses alloys of tin and lead or tinned copper wires are commonly used. In cartridge fuses silver and silver alloys are used in fuses of lower ratings and copper alloys are used in fuses of higher ratings.
6) Insulating materials, on the basis of their physical and chemical structure may be classified in various categories as follows:
1.Fibrous materials: They are derived from animal origin or from cellulose, which is the major solid constituent of vegetable plants. The majority of materials are from cellulose. This includes paper, wood, card- board, cotton, jute and silk.
2. Impregnated fibrous material: The fibrous materials are impregnated with suitable impregnated oil, varnish, and epoxy - resin to improve its thermal, chemical and hygroscopic properties.
3. Non-resinous materials: Solid or semisolid insulations which are directly available in nature and are organic based come under this class. These materials are mineral waxes, asphalts, bitumen and chlorinated naphthalene.
4. Insulating liquids: Apart from working as insulation, they fulfil other important requirements like they offer good heat dissipation media, they used for extinguishing arcs in certain applications like circuit breakers. They include vegetable oils, fluorinated liquids, mineral insulating oils and synthetic liquids.
5. Ceramics: They are materials made by high temperature firing treatment of natural clay and certain inorganic matters. They are used as dielectric in capacitors, as insulators etc.
6. Mica and mica products: It is an inorganic mineral and one of the best natural insulating materials available. Mica is used as a dielectric in capacitors, as insulator. Some of the mica products are glass- bonded mica, synthetic mica, mica paper, manufactured mica.
7. Asbestos and asbestos products: These are strong and flexible fibres. It finds extensive use in electrical equipment as insulation because of its ability to withstand very high temperatures. Some of the asbestos products are: asbestos roving, asbestos paper, asbestos tapes and asbestos cement.
8.Glass: Glass is an inorganic material made by the fusion of different metallic oxides. It is normally transparent, brittle and hard. Glass finds its use in electrical industry because of its low dielectric loss, slow ageing and good mechanical strength. Glass is used in electrical- bushings, fuse bodies, insulators, radio and television tubes.
9. Natural and synthetic rubber: Natural rubber is obtained from the milky sap of rubber trees. It finds limited applications because it is rigid when solid, sticky when warm and gets oxidised, when exposed to atmosphere. Synthetic rubber are of various types such as butadiene rubber, butyl rubber, chloroprene and silicon rubber which are obtained by the polymerisation. Synthetic rubber, are used as insulating material for wires and cables. It is also used as jacketing material for cables.
10. Insulating resins and their products: Plastic or resins are of two types – one derived from plant and animals the other synthetic obtained from chemical reactions. Natural resins are used as binder material. It is used as thickening agent for manufacture of mineral insulating oils. Synthetic resins are used as insulation, manufacture of switches and instrument mountings, electrical bushings, radio and television cabinets etc.
11.Laminates, adhesives, enamels and varnishes: Laminates are multiple, thin layers or sheets of insulating materials like that of mica, paper, cloth, glass etc, bonded together. Adhesives, is a class of material compositions required to carry out bonding between two or more solid surfaces. Adhesives are used in the manufacture of laminated boards, coil winding cylinders, rods, tubes and special shaped insulators. Enamel is a fusible insulated coating of some organic base material, which is generally applied on conducting surface. Enamel finds extensive use in coating wires used for the windings of low rated motors, transformers, various types of instruments, etc. Varnish is a liquid, which when applied to a surface dries resulting in hard shining coating which is resistant to air and water. Lacquer is used for protecting wood and metal surface from external weather conditions.
PART- I I
7a) Soft and Hard magnetic materials: All ferromagnetic materials are divided into two broad groups - soft and hard magnetic materials.
Materials, which have, a steeply rising magnetization curve, relatively small and narrow hysteresis loop and consequently small energy losses during cyclic magnetization are called soft magnetic materials. They are easily magnetised and demagnetised Soft magnetic materials are therefore employed in building cores for use in alternating magnetic fields. Examples are nickel-iron alloy and soft ferrites. Fig below shows a typical narrow hysteresis loop for soft magnetic materials.
Magnetic materials, which have a gradually rising magnetisation curve, large hysteresis loop area and large energy losses for each cycle of magnetisation, are called hard magnetic materials. They are not easily magnetised and demagnetised. Such materials are used for making permanent magnets. Examples are carbon steel, tungsten steel alnico. Fig below shows a broad hysteresis loop for hard magnetic materials.
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7b) (i) Corrosion : The process of constant eating (destruction) up of metals (from the surface) by the surrounding is called as corrosion. The metals are corroded when exposed to the atmosphere. The metals are generally converted into their oxides. This oxide covers the surface of the metal, which results in the destruction of the metal. Rusting of iron is the most common example of corrosion in which iron makes iron oxide with reaction with the oxygen of the atmosphere. The iron oxide covers the surface in the form of brownish powder. Therefore the conducting material should be corrosion resistant.
(ii) Bimetals: A bimetal is made of two metallic strips of unlike metal alloys with different coefficients of thermal expansion. At a certain temperature the strip will bend and actuate a switch or a lever of a switch. The bimetal can be heated directly or indirectly. When heated, the element bends such that the metal with the greater coefficient of expansion is on the outside of the arc formed while that with smaller coefficient is on the inside. When cooled, the element bends in the other direction. Alloys of iron and nickel with low coefficients of thermal expansion are used as one element of the bimetallic strip. The other element consists of materials having high values of thermal expansion. Examples are iron, nickel, constantan, brass etc. Bimetallic strips are used in electrical apparatus and in devices such as relays and regulators.
8a) Soft ferrites and their applications: These are non- metallic compounds consisting of ferric oxide and one or two bivalent metal oxides such as Nickel oxide, Manganese oxide or Zinc oxide. They have the resistivity of about, 109 ohm-cm, which reduces eddy current losses at high frequency. The magnets made out of it have high coercive force and square hysteresis loop. Magnetic permeability of these materials is as high as 10,000 to 30,000. These materials are fabricated into shape such as- E, U, I, beads and self - shielding pot cores.
Applications – High frequency power transformers operating at 10 to 100 kHz, pulse transformers up to 1000 MHz, adjustable air gap inductors, recording heads make use of cores made of soft ferrites.
8b) (i) Eddy currents: Magnetic materials placed in alternating magnetic fields also have eddy currents induced in them. This is because the material is subjected to rate of change of flux linkages and in accordance with Faraday’s Law of electromagnetic induction, emfs are induced in the material causing currents, called eddy currents, to flow in the material. These currents cause loss of energy. This results in the heating up of the material. Eddy current loss is proportional to the square of the frequency and the square of the thickness of the material and inversely proportional to the resistivity of the material. In order to reduce this eddy currents loss thin sheets called laminations are used instead of solid core.
(ii) Magnetostriction: When ferromagnetic materials are magnetized a small change of dimensions of the material takes place. There is a small extension with corresponding reduction of cross-section of the crystals of which the material is made. When subject to rapidly alternating magnetic fields there is a rapid and continuous extension and contraction of the material. This is called magnetostriction. Magnetostriction is the major cause of hum in transformers and chokes.
(iii) Permeability – It is defined as the capability of the material to conduct flux. It is defined as the ratio of magnetic flux ‘B’ in a medium to the magnetic flux intensity ‘H’ at the same location in the medium, i.e. m = B/H, where B is plotted against H, a curve is obtained, called magnetization curve or B-H curve. Also m =mo mr where mo = absolute permeability of air and mr = relative permeability for air, mr = 1. The permeability of any material is not a constant. The permeability at low value of H is called initial permeability. The common core materials such as low carbon steel, silicon steel have low initial permeability.
9a) Diamagnetic Materials: These are the materials whose atoms do not carry permanent magnetic dipoles. If an external magnetic field is applied to a diamagnetic material, it induces a magnetization in the direction opposite to the applied field intensity. For these materials the relative permeability is negative. These are hardly used as magnetic material in electrical/ electronic engineering applications. Examples- Aluminium oxide, copper, gold, barium chloride, superconductors.
Paramagnetic Materials: The atoms of these materials contain permanent magnetic dipoles. Individual dipoles are oriented in random fashion such that resultant magnetic field is zero or negligible. For these materials relative permeability is slightly greater than unity and it is independent of magnetizing force. In presence of external magnetic field, paramagnetic materials get weakly magnetised in the field direction and the susceptibility is given by c= C/T where C is a curie constant and T is the temperature. Ex. Chromium chloride, chromium oxide, manganese sulphate, air.
Ferromagnetic Materials: These are materials in which magnetic dipoles interact in such a manner that they tend to line up in parallel. A ferromagnetic substance consists of a number of regions or domains, which are spontaneously magnetized. The direction of magnetization varies from domain to domain. In the presence of external magnetic field, the ferromagnetic materials get strongly magnetised and exhibit hysteresis loop. The susceptibility of these is given by c = C/ T-TC where C is Curie constant, TC is the Curie temperature above which the ferromagnetic material exhibits paramagnetic behaviour. The resultant magnetization is zero or nearly zero. The relative permeability is very high. The ferromagnetic materials are widely used in industries. Ex. Iron, nickel, cobalt.
9b) The process of joining two or more metals is known as soldering. Alloy of two or more metals of low melting point, used for joining two or more base metals is known as soldering material or solder. The most common solder is composed of 50% tin. Its melting point is about 1850C. Many commercial solders, contain larger percentage of lead and some antimony with less tin, as the electrical conductivity of lead is only about half that of tin. For soldering flux is to be used. Solders are of two types- Soft solders and hard solders. Soft solders are composed of lead and tin in various proportions. Hard solders may be any solder with a melting point above that of lead-tin solders. Soft solders are used in electronic devices and hard solders in power apparatus for making permanent connections. Examples: Tin- Lead solders, Tin – Antimony – Lead solders, Tin – Zinc solders, Lead – Silver solders, Cadmium – Silver solders etc.
10) (i) Mica and Mica Products – Mica is an inorganic material. It is one of the best insulating materials available. From the electrical point of view, mica is of two types – Muscovite mica and Phlogopite mica.
Muscovite mica – Chemical composition is KH2Al3(SiO4)3. The properties are
i) Strong, tough and less flexible
ii) Colourless, yellow, silver or green in colour
iii) Insulating properties are very good
iv) Abrasion resistance is high
v) Alkalies do not affect it
Uses – Muscovite mica is used where electrical requirements are severe. Because of high dielectric strength, it is used in capacitors. It is also used in commutators, due to high abrasion resistance.
Phlogopite mica – Chemical composition is KH(MgF)8MgAl(SiO4)3. The properties are
i) Amber, yellow, green or grey in colour
ii) Greater structural stability, being tougher and harder than muscovite mica, less rigid
iii) Resistant to alkalies, but less to acids
iv) Greater thermal stability than that of muscovite mica and greater temperature resistant.
Uses – It is used when there is greater need of thermal stability as in domestic appliances like irons, hotplates and toasters.
Mica products
(i) Glass bonded mica – Ground mica flakes and powdered glass when moulded makes glass bonded mica. This material is impervious to water and chemically stable. This is used in high humidity and high ambient temperatures.
(ii) Mica paper & mica sheet – Mica is broken into small particles in aqueous solution. Out of this sheets of mica paper are produced which are used as insulation for armature and field coils of rotating machines. Also used as washers, spacers, sleeves, tubes etc.
(iii) Manufactured mica – Mica flakes held together with adhesives is called manufactured mica. It is used in commutators, electrical heating devices (heater plate), motor slot insulation, transformers, etc.
(ii) Teflon: It is obtained by the polymerisation of tetrafloourethylene.
Properties: It has good electrical, mechanical and thermal properties. It can tolerate very, high temperature, without damage. Dielectric constant does not change with time, frequency and temperature. Its insulation resistance is very high. It is highly resistant to water absorption. It melts at 3270C.Its maximum useable temperature is 3000C.
Uses: Teflon is used as dielectric material in capacitors. It is used as covering for conductors and cables, which are required to operate at high temperature. It is used as a base material for PCB’s.
(iii) Rubber: Rubber is a polymeric material with a high elastic yield strain. The different types of rubber materials are- Natural rubber, Hard rubber and Synthetic rubber.
Natural Rubber – Natural rubber is extracted from milky sap collected from special trees. Water is then evaporated. Additives like sulphur, oxidation inhibitors like aromatic amino compounds, softeners like vegetable oil and fillers like carbon black and zinc oxide are added to it. It is vulcanised, by adding sulphur and heating it. Vulcanization improves heat and frost resistance of rubber, making it mechanically strong. The permittivity and power factor varies depending on the sulphur content and temperature change.
Properties – This rubber is moisture repellent and has good insulating properties. It has good abrasion resistance, high strength, low hysteresis.
Applications – It is used for the manufacturing of protective clothing such gloves, boots. It is used as an insulation covering for wires and cables.
Hard Rubber – Hard rubber is obtained by addition of more sulphur and by extended vulcanization.
Properties – It has good electrical properties. Water absorption is less. Maximum permissible operating temperature is 600 C. It can’ t be continuously exposed to sun as it is harmful. It has high tensile strength.
Applications – Hard rubber is used for construction of storage battery housing, panel boards, bushings of various types. It is also used as jacketing material for cables.
Synthetic Rubber – The different types of synthetic rubber are
(a) Butadiene rubber – its properties are greater resistance to ageing and oxidation, lower tensile and tear strength, lower water absorption and higher heat conductivity.
(b) Butyl rubber – its properties are excellent resistance to vegetable oils and alcoholic solvents, but it is easily attacked by petroleum oils and greases. It has high resistance to ozone, high thermal and oxidation stability but poor tensile strength. It is used as insulation for wires and cables.
(c) Chloroprene Rubber (Neoprene Rubber) – It has better resistance to thermal ageing, oxidation, sunlight and gas diffusion. These rubbers have better thermal conductivity and more flame resistance. They exhibit better adhesion to metals. They possess better resistance to attack by solvents like mineral and vegetable oils but poor resistance to aromatic hydrocarbon liquids. They are inferior in mechanical properties like tear and tensile strength and abrasion resistance. Neoprene rubber is used as insulating material for wires and cables. It is also used as jacketing material for cables.
(d) Chlorosulfonated Polyethylene (Hypalon) – It has better electrical properties, high resistance to degradation when exposed to high temperature and oxidation. It can be operated at temperatures as high as 1500 C. It has poor solvent resistance to hydrocarbons. It is mechanically less tough. It is used as insulating material for wires and cables and also as jacketing material for cables.
(e) Silicon Rubber – It has high thermal conductivity. Its tensile strength is low, has good flexibility at low temperatures and resistant to ozone, oxidation and severe atmospheric conditions. It can be used over a wide range of temperatures from –1000 to 1500 C. Silicon rubber is used as insulating material for wires and cables, in the manufacture of moulded parts, as an insulating tape and coating material.
(iv)Asbestos: It is inorganic fibrous material. Two types of asbestos are available.
(A) Chrysotile asbestos: It is hydrated silicate of magnesium. Its specific gravity varies between 2-2.8. It is highly hygroscopic. It has high dielectric losses and dielectric strength. The melting temperature is 15250C.
(B) Amphibole asbestos: It is found in Africa and Alaska. Its fibres cannot be woven easily as the fibres are too soft or too hard and brittle. It possesses good tensile strength. It is highly hygroscopic. Its electrical properties are poorer.
Uses: Asbestos is used in low voltage work as insulation in the form of rope, tape, cloth and board. It is impregnated with liquid or solid resin in all such applications to improve its mechanical and electrical properties. It is used as insulator in wires and cables under high temperature conditions, as conductor insulator and layer insulator in transformer, as arcing barrier in switches and circuit breakers.
11) (i) Cold rolled grain oriented steel: The grain orientation of silicon steel is obtained by a special technique called cold rolling. Sheet steel obtained in this process is called Cold Rolled Grain Oriented Steel. CRGO silicon steel is widely used for making transformer cores. The magnetising current required by transformers using CRGO steel is low. If CRGO sheet steel is used as core material for rotating machines, the core will be assembled from a large number of sections but this still will not result in grain orientations completely parallel with the flux path because of the circular nature of cores of rotating machines. Also assembling the core from sections of sheet steel will make the construction difficult and expensive.
(ii) Alnico: It contains 18% Ni, 10% Al, 12% Co, 6% Cu and balance 54% of Fe. It is an alloy of iron, cobalt, nickel and small amount of aluminium and copper.
Properties are -
- Saturation flux density is 1.2 Wb/m2.
- More expensive than Alni, but magnetic properties are better than Alni.
- Available in different grades, each having some different properties.
- Hard and brittle, so it cannot be machined but has to be cast to shape and finished by grinding.
- Magnets made with this alloy are smaller in size and lighter in weight than that made with tungsten steel.
- The hysteresis loop is more rectangular.
- Thermal and mechanical process, modify the crystal size and shape and thereby alter the shape of the hysteresis curve.
Uses: Alnico magnets find applications in loudspeakers, microwave devices, motors, generators, separators, vending machines and communication devices.