Electrical Energy Utilisation And Electric Tranaction
Heating is required for domestic purposes (such as cooking and heating of buildings) as well as for industrial purposes (such as melting of metals, hardening and tempering, case-hardening, drying and welding). Practically all the heating requirement cab be met by some form of electric heating equipment. Electric heating is advantageous over other systems of heating (coal, oil or gas heating) as it is economical.
Heating Element Material The material used for heating element should be of high resistivity, high melting point, low temperature coefficient and should be such that it may withstand the required temperature without getting oxidized.
The materials commonly used for heating elements for low and medium temperature services are either alloy of nickel and chromium (nickel 80% and chromium 20%) or alloy of nickel, chromium and iron (Ni 65%, Cr 15% and Fe 20%) The addition of iron to the alloy reduces the temperature at which oxidation takes place but the cost of the product is also reduced.
1. Resistance heating Resistance heating is based upon I2 R effect and has wide application such as heat treatment of metals (annealing hardening etc.) drying and baking of potteries, stoving of enamelled ware and commercial and domestic cooking. Temperature up to about 1,000ºC can be obtained in oven using wire resistance for heating elements. There are two methods of resistance heating.
(a) Direct Resistance heating. In this method of heating current is made to pass through the body to be heated. This method has high efficiency and widely used in salt bath furnaces and in electrode boiler for heating water.
(b) Indirect Resistance Heating. In this method of heating, the current is passed through heating element and heat produced is delivered to the charge by one or more of the modes of heat transfer. Normally this method is used in immersion heaters, resistance ovens, domestic and commercial cooking and heat treatment of metals.
2. Infrared or Radiant Heating In this method of heating , heat energing from incandescent lamps is focused upon the body to be heated up in the form of electromagnetic radiations.
This method of heating possesses the advantages of
(i) rapid heating
(ii) compactness of heating units
(iii) flexibility and
(iv) safety and finds applications in paintstoving, drying of radio cabinets and wood furniture, preheating of plastics prior to moulding, softening of thermoplastic sheets and drying of pottery, paper, textile etc where moisture content is not large.
3. Arc Heating The arc drawn between two electrodes develops high temperature (about 3,000-3,500ºC) depending upon the electrode material. The electric arc may be used in the following different ways.
By strucking the arc between the charge and the electrode (s). In this method of heating, heat is directly conducted and taken by the charge. The furnaces operating on this principle are called the direct arc furnaces and are commonly used for production of steel.
By strucking the arc between the two electrodes and transferring the heat produced to the charge solely by radiation. The furnaces operating on this principle are called the indirect arc furnaces and are used for melting non-ferrous metals.
4. Induction Heating In induction heating, effect of current induced by electromagnetic inducting in the charge is used. The power drawn, consequently depends upon the voltage and resistance of the charge
Hence for developing sufficient heat, the resistance of the charge must be low, which is possible only with metals, and voltage must be higher, which is obtained by using higher flux density and higher frequency magnetic materials, therefore, can be easily treated than non-magnetic material because of their higher permeability.
5. High Frequency Eddy Current Heating In this method of heating, the article to be heated is placed a high frequency current carrying coil, alternating magnetic field is set up, eddy currents are induced in the article and heating is, therefore, affected.
The eddy currents loss is primarily responsible for the production of heat, but hysteresis loss also contribute to it, though to a little extent, in the case of magnetic materials. The important application are surface hardening, annealing and soldering.
6. Dielectric Heating In dielectric heating, the material to be heated is placed as a slab between metallic plates or electrodes connected to high frequency ac supply. The heat is produced within the material itself and so uniform heating is obtained. Some of the application are preheating of plastic performs, gluing of wood, baking of foundry cores, diathermy, sterilization, textile industry, electronic sewing, food processing etc.
It is process of joining two pieces of metal at faces rendered plastic or liquid by th application of heat with or without application of pressure and addition of filler material.
The various welding processes used in general engineering are given below:
1. Gas welding :-
(ii) air acety lene and
2. Resistance welding :-
(iv) seam and
3. Arc welding :-
(i) carbon arc
(ii) metal arc
(iii) gas metal arc
(iv) gas tungsten arc
(v) atomic hydrogen arc
(vi) Plasma arc
(vii) sub merged arc
(viii) flux-cored arc and
4. Thermit Welding
5. Solid state welding:-
(iii) diffusion and
6. Newer welding :-
(i) electron beam and
(ii) la ser.
Resistance Welding By definition, resistance welding is that process in which a sufficiently strong electric current is sent through the two metal pieces in contact to the welded which melts the metal pieces by the resistance they offer to the flow of electric current. Resistance weld-ing includes butt welding spot welding, projection welding, seam welding and percussion welding. All arc alike in the principle of resistance welding is employed mainly for mass production. It is easily adapted to those components which can be moved to the machine and are light. The operation is extremely rapid and simple. This is the only process where heat can be controlled and which permits a pressure action at the weld.
1. Butt Welding It is of two types namely upset butt welding and flash butt welding. Upset butt welding includes end to end welds, lap welds and butt welds.
Flash-butt welding creates a joint which has practically the full strength of the parent metal under static loading conditions and a strength slightly less than the parent metal under dynamic loads. Flash butt welding is considered superior to upset butt welding and is used for welding chains, rail ends, rolled sections, shift axles etc.
2. Spot Welding In spot welding the parts of pieces are joined in spots, accompanied by heated relatively small sections of the parts or pieces between suitable electrodes under pressure. Such a welding only provides mechanical strength and is neither airtight nor watertight. It is applicable for joining components made from plate material, the plate thickness being limited (usually to 10 to 12 mm) by the pressure and current capacity of the available spot welding machines.
3. Projection Welding It is a modified form of spot welding and has several advantages over spot welding such as simple welding process, more output, increased electrode life, good finished appearance etc. This type of welding us usually used on punched, formed or stamped part , where the projections automatically exist.
4. Seam Welding Seam welding can be defined as series of continuous spot welds. This process is employed for making a continuous joint between two overlapping pieces of sheet metal. This type of welding is very important as it provides pressure tight or leak proof joint, Seam welding is employed for welding pipes, conduits, tanks, transformers, refrigerators, gasoline tanks, aircraft and various types of containers.
5. Percussion Welding This is the recent development in the field of welding which depends on the arc effect for heating and not on the resistance. This is a self timing spot welding method. In this process a current impulse is obtained by the discharge from a capacitor or from a magnetic field. The action of process is so rapid that there is little heating effect in the material adjacent to the weld.
Electric Arc Welding Arc welding is that process in which the pieces of the metal to be welded are brought to the proper welding temperature at a point of contact by the heat liberated at the arc terminal and in the arc stream so that the metal pieces are completely fused into each other, forming a single solid homogeneous mass, after it solidifies. In this process an electric arc is produced by bringing two conductors (electrode and metal piece) connected to a suitable source of electric current. momentarily in contact and then separating by a small distance. The current continues to flow across the small gap and gives intense heat. developed is utilized to melt the part of the work piece, and the filler metal and thus form the joint. So arc welded joint is a union of metal parts made by localized heating without any pressure. That is why sometimes this type of welding is known as the non-pressure welding. The heat developed by arc is also used for jointing.
The arc voltage varies from 20 to 40 voltage and current from 50 a in sheet metal work to 1,000 a in heavy automatic welding.
1. Carbon Arc Welding There are two methods of carbon arc welding. In one method no flux is used and in the other method flux either in the form of powder or paste is used to prevent the weld from oxidation. Former method is confined to non-ferrous metals and the later method is usually used for ferrous metals. For this type of welding only dc can be used. Carbon arc welding is use for welding sheet steel, copper alloys, brass, bronze and aluminum.
2. Metal Arc Welding In this type of welding a metal rod of the same metal as being welded forms one of the electrode and also serves as a filler and no filler rod is used separately. For this type of welding both ac and dc can be used. for dc supply 50-60 V and for ac supply 70-100 V are used for welding.
3. Automatic Hydrogen Arc Welding In this method of arc welding, an arc is maintained between the two tungsten electrodes while a stream of hydrogen gas under a pressure of about 0.5 kg/ cm2 is passed through the arc and around the electrodes. AC supply is used in order to obtain equal consumption of the electrodes. This process is capable of producing smooth, uniform, strong and ductile welds.
4. Inert Gas Metal Arc Welding It is a gas shielded metal arc welding process and makes use of intense heat of an electric arc between a continuously electrode wire and the material to be welded. This process is particularly employed for welding light alloys, stainless steel and non-ferrous metals such as copper, aluminum and their alloys.
Electric Welding Equipment The electric welding sets may be either dc or ac type. DC welding sets are of two types namely generator type welding set consisting of a differential compound wound dc generator, giving drooping volt-ampere characteristic, driven by any type of prime mover (a squirrel cage induction motor or a petrol or diesel engine) and dry type cage inducting motor or a petrol or diesel engine) and dry type rectifier (selenium rectifier) used in conjunction with a multiphase, high leakage transformer.
AC welding sets are single phase or 3-phase stepdown transformers which provide low-voltage (80-100 V on open circuit) power for welding with some means of output control. In the set of this type the current control is achieved by using
(a) magnetic shunt or
(b) a choke coil or reactor placed in series with primary or secondary winding or
(c) tap changing switch in the primary winding. The use of series resistance can also be made for current control but with reduced efficiency
Illumination: Illumination differs from light very much, though generally these terms are used more or less synonymously. Strictly speaking light is the cause and illumination is the results of the light on surfaces on which it falls.
Terms Used in Illumination
1. Light is defined as the radiant energy from a hot body which produces visual sensation upon the human eye. It is expressed in lumen-hours.
2. Luminous flux is defined as the total quantity of light energy emitted per second from a luminous body and is measured in lumens (or cd-sr)
3. Luminous intensity in any given direction is the luminous flux emitted by the source per unit angle, measured in the direction in which the intensity is required. It is measured in candela (cd) or lumens per steradian.
4. Lumen is the unit of luminous flux and is defined as the amount of luminous flux given out in space represented by one unit of solid angle by a source having an intensity of one candle power in all directions. The total lumens given out by a source of one candela is 4lumens.
5. Candle power is the light radiating capacity of a source in a given direction and is defined as the number of lumens given out by the source in a unit solid angle in a given direction.
6. Illumination is define as the luminous flux (number of lumens), falling on the surface, per unit area. It is measured in lumens/m2 or lux or metre candle.
7. Candela is the unit of luminous intensity and is defined as of the luminous intensity per cm2 of a black body radiator at the temperature of solidification of platinum (2, 043 K).
8. Mean Spherical Candle power (MSCP) is defined as the mean of candle powers in all directions and in all planes from the source of light.
9. Lamp efficiency is defined as the ratio of luminous flux to the power input and is expressed in lumens per watt.
10. Specific consumption is defined as the ratio of the power input to the average candle power and is expressed in watts per candela.
11. brightness or luminance is defined as the luminous intensity per unit projected area of either a surface source of light or a reflecting surface. It is measured in nit (candelas/ m2).
Bigger unit of luminance is stilb (candelas/ Cm2).
12. Glare any be defined as the brightness within the field of vision of such a character as to cause annoyance, discomfort, interference with vision or eye fatigue.
13. Space-height ratio is defined as the ratio of horizontal distance between adjacent lamps and their mounting height.
14. Utilisation factor or coefficient of utilisation is defined as the ratio or total lumens reaching the working plane to the total lumens given out by the lamp.
15. Maintenance factor is defined as the ratio of illumination under normal working conditions to the illumination when the things are perfectly clean. It is always less than unity.
16. Depreciation factor is merely the inverse of the maintenance factor and is defined as the ratio of initial metre-candles to the ultimate maintained metre-candles on the working plane. It is always more than unity.
Laws of Illumination There are two laws of illumination. These laws are:
1. Law of Inverse square. The illumination of a surface is inversely proportional to the square of the distance between the surface and the source is sufficiently large so that the source can be regarded as a point source.
2. Lambert's Cosine Law. According it this law the illumination at any point on a surface is proportional to the cosine of the angle between the normal at that point and the direction of luminous flux.
Various Types of Electric Lamps:- The various types of electric lamps in common useare:
1. Arc Lamps Electric discharge through air gives intense light.
This principle is used in arc lamps. In an arc lamp electric current is made to flow through two electrodes in contact with each other which are drawn apart.
The result is an arc being struck. The arc maintains the current and is very efficient source of light. There are various forms of arc lamps such as carbon arc, flame arc or magnetic are lamps.
Carbon arc lamp is the earliest type of lamps and is still used in cinema projectors and searchlights. The luminous efficiency of such a lamp is 12 lumens per watt.
Flame arc lamp operates on the same principal on which carbon arc lamp operates. Thought the arc is very efficient but owing to objection to its colours it has now been superseded by the electric discharge lamps. Its luminous efficiency is 8 lumens per watt.
Magnetic arc lamp make use of copper electrode as a positive electrode and iron magnetic oxide electrode as a negative electrode. such lamps are rarely used.
2. Incandescent lamps The incandescent or filament type lamp consists of a glass globe completely evacuated and a fine wire known as filament within it. The glass globe is evacuated to prevent the oxidization and convection currents of the filament and also to prevent the temperature being lowered by radiation. The material, which cab be used for the filaments of incandescent lamps, must possess the properties of high melting point, low vapour pressure, high resistivity, low temperature coefficient, ductility and sufficient mechanical strength to withstand vibrations during use. The materials which can be used for filaments in incandescent lamps are carbon, osmium, tantalum and tungsten. Because of low operating temperature (1,800ºC) of carbon filament lamp, its efficiency is quite low (of the order of 3.5 lumens/watt). The average efficiency of tungsten filament lamp is about 10 lumens per watt.
The light output of an incandescent lamp decreases gradually as it ages. the total depreciation of light output is roughly 15% over the useful life range.
3. Gas-filled Lamps A metal filament cab work in an evacuated bulb up to 2,000ºC without oxidation and if it is worked beyond this temperature it vaporizes quickly and blackens the lamp. For higher efficiency it is necessary to use operating temperature above 2,000ºC keeping down the evaporation, Which is possible by filling the bulb with an inert gas-argon with a small percentage of nitrogen. Nitrogen is added to reduce the possibility of arcing . Krypton is the best gas for this purpose but it is so expensive that it is used only in special purpose lamps.
4. Gaseous Discharge Lamps The incandescent lamps suffer from tow drawbacks low efficiency and colour light. The gaseous discharge lamps have been developed to overcome these drawbacks. Discharge lamps are of two types:
(i) The lamps which give the light of the same colour as produced by the discharge through the gas or vapour such as sodium vapour, mercury vapour and neon gas lamps.
(ii) The lamps which use the phenomenon of fluorescence and are known as fluorescent lamps. In these lamps, the discharge through the vapour produces ultraviolet was which cause fluorescence in certain materials called the phosphors. The inside of the fluorescent lamp is coated with a phosphor which absorbs ultraviolet rays and radiates visible rays.
Example is fluorescent mercury vapour tube.
(a) sodium vapour lamp:- Principally sodium vapour lamp consists of a bulb containing a small amount of metallic sodium, neon gas and two sets of electrodes connected of a pin type base. The lamp operates at a temperature of about 300º C and in order to conserve the heat generated and assure the lamp operating at normal air temperature the discharge envelope is enclosed in special vacuum envelope designed for this purpose. The efficiency of a sodium vapour lamp under practical conditions is about 40-50 lumens/watt.
(b) High Pressure mercurryVapour Lamp:-The mercury vapour lamp in construction is similar to sodium vapour lamp. It gives greenish blue colour light, Which causes color distortion. The efficiency is about 30-40 lumens per watt. These lamps (MA type) are manufactured in 250 and 400 W ratings for use on 200-250 V ac supply.
(c) mercury Iodide Lamp:- These lamps are similar is construction to high pressure mercury vapour lamps but in addition to mercury, a number of iodides are added which fill the gaps in the light spectrum, and thus improve the colour characteristic of the light.
Their efficiency is also higher (75-90 lumens/watt).
Such lamps are suitable for application in the fields of floodlighting industrial lighting and public lighting.
(d) Neon lamp:- It is a cold cathode lamp and consists if a glass bulb filled with neon gas with a small percentage of helium. These lamps give pink coloured light. Their efficiency lies between 15-40 lumens/watt. such a lamp so of the size of an ordinary incandescent lamp. The power consumption is of the size of an ordinary incandescent lamp. The power consumption is of the order of 5W. These lamps are used as indicator lamps night sizes as neon tubes for the purpose of advertising.
(e) Fluorescent Tubes:- Fluorescent ilighting has a great advantage over other light sources in many application. This efficiency is about 40 lumens per watt. The tube contains small quanitity of argon gas at a pressure of 25 mm of mercury and tow drops of mercury. The inside surface of the tube is coated with a thin layer of fluorescent material in the form of a powder. The coating material used depend upon th colour effect desired and may consist of zinc silicate, cadmium silicate or calcium tungstate. The tube is provided with tow electrodes coated with electron emissive material. A starting switch is provided in the circuit to put electrodes directly across the supply mains at the time of starting so that electrodes may get heated and emit sufficient electrons.A stabilising choke is connected in series with it, which acts as a ballast in running condition and provides a voltage impulse for starting. A capacitor is connected across the circuit to improve the power factor. The normal life of fluorescent tubes is 7,500 hour. the average life is for three burning hours per switching operation.
(f) Compact Fluorescent Lamp (CFL):- A compact fluorescent lamp (CFL), also called compact fluorescent light, energy-saving light, and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent lamp; some types fit into light fixtures formerly used for incandescent lamps.
The lams uses a tube which is curved or folded to fit into the space of an incandescent bulb, and a compact electronic ballast in the base of the lamp.
Compared to general service incandescent lamps giving the same amount of visible light, CFLs use less power (one fifth to one third) and have a longer rated life (eight to fifteen times). Like all fluorescent lamps, CFLs contain mercury, which complicated their disposal
The fact that electrical energy can produce chemical changes and the processes based on it, called the 'electrolytic processes' are widely use for the extraction of pure metals from their ores (such as aluminium, zinc copper, magnesium, sodium etc). refining of metals (such as gold, silver, copper, nickel, lead, iron etc.) manufacturing of various chemical such as caustic soda, potassium permanganate, hydrogen, oxygen, chlorine etc.) electrodeposition of a metals including electroplating, electrotyping, electroforming, building up of worn out parts in metallurgical chemical and other industries. Though the various processes mentioned are different in apparent detail but fundamentally they are alike, being based on the principle of electrolysis. The mass of chemical deposition due to flow of electric current I though the electrolyte for time is given by the equation m = ZIt where Z is given by the equation is the electrochemical equivalent of the substance in kg/coulomb. Power supply required for electrolytic processes is direct current and at very low voltage. The power required for electro deposition is usually very small (say 100-200 A at 10-12 V). Power supply required for extraction and refining of metal and large scale manufacture of chemicals is in very large amounts.