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ELECTRIC LAMPS:
Three types of artificial or electric lamps:
- Arc lamp
- Incandescent Lamp
- Discharge Lamps
- ARC LAMPS:
In arc lamps, electrodes are in contact with each other and are separated by some distance apart, the electric current is made to flow through these two electrodes. The discharge is allowed to take place in the atmosphere. Generally used arc lamps are, I. Carbon arc lamps II. Flame arc Lamps III. Magnetic arc Lamps I. Carbon arc lamps
Carbon arc lamp consists of two hard rod type electrodes made up of carbon. Two electrodes are placed end to end and are connected to DC supply not less than 45V. The positive electrode is of a large size. The two edges of the rods become incandescence due to the high resistance offered by rods. It is observed the carbon particles transfer from the positive rod to the negative one. Hence the positive electrode is of twice the diameter than that of the negative electrode. In case of AC supply, the rate of consumption of the two electrodes is same( Cross section of the two electrodes is same. A resistance R is connected in series with the electrode for stabilizing the arc. As current increases,
o Vaporizing rate of carbon increases o Decreases the resistance o Voltage drop across the arc decreases
So to maintain the arc between the two electrodes, series resistance should be necessarily connected. For maintaining the arc, the necessary voltage required is V=(39+2.8 L) V Where L=length of the arc II. Flame arc Lamps: The electrodes used in flame arc lamp are made up of 85% of carbon and 15% of fluoride. Normally the core type electrodes are used and the cavities are filled with fluoride. The principle is same as that of carbon arc lamp. When the arc is established between the electrodes, both fluoride and carbon get vaporized and give out vary high luminous intensities. The color output of the flame arc lamps depends upon the flame materials. The luminous efficiency of such lamp is 8 lumens/W. Resistance is connected in series with the electrodes to stabilize the arc.
III. Magneticarc Lamps
The Principle of the operation is same as that of carbon arc lamp. This lamp consists of positive electrode that is made up of copper and negative electrode that is made up of magnetic oxide of iron. Light energy radiated out when the arc is struck between the two electrodes.
- INCANDESCENT LAMP
These lamps are temperature dependent sources. When electric current is made to flow through a fine metallic wire, which is known as filament, its temperature increases. At low temperature, it emits only heat energy, but at very high temperature, the metallic wire emits both heat and light energy. These incandescent lamps are also known as temperature radiators. Choice of materials for filament The materials commonly used as filament for incandescent lamps are carbon, tantalum, tungsten and osmium. Properties of the incandescent lamp materials are
- The melting point of the filament material should be high
- The temperature coefficient of the material should be low
- It should be high resistive material
- The material should possess good mechanical strength to withstand vibrations
- The materials should be ductile
Construction
It consists of an evacuated glass bulb and an aluminum or brass cap is provided with two pins to insert the bulb into socket. The inner side of the bulb consists of a tungsten filament. A glass button is provided in which the support wired are inserted. A stem tube forms as air-tight seal around the filament whenever the glass is melted.
Operation
When electric current is made to flow through the fine metallic tungsten filament, its temperature increases. At very high temperature, the filament emits both heat and light radiations. The tungsten filament can be operated efficiently beyond 2,000˚C it can be attained by inserting a small quantity of inert gas nitrogen with small quantity of organ. The single coil filament is used in vacuum bulbs upto 25W and gas filled bulbs from 300 to 1,000 W.
3.DISCHARGE LAMPS
Discharge lamps are developed to overcome the drawbacks of the incandescent lamp.
Principle of Operation
In all discharge lamps, an electric current is made to pass through a gas or vapor, which produces its illuminance. In the process of producing light by gaseous conduction, the most commonly used elements are neon, sodium and mercury. The wave length of the electromagnetic radiation depends upon the nature of gas and the gaseous pressure used inside the lamp. The production of light in the gaseous discharge lamps is based on the phenomenon of excitation and ionization of gas or metal vapor present between the two electrodes of a discharge tube. A choke or ballast is provided to limit high currents and to a safe value. Here choke serves two functions
- It provides ignition voltage initially
- Limits high currents
The use of choke will reduce the power factor of all the gaseous lamps so that all the discharge lamps should be provided with a condenser to improve the power factor.
Types of Discharge Lamps:
Type 1: Those lamps in which colour of light is the same as produced by the discharge through the gas or vapour. E.g Sodium Vapour, Mercury vapour and neon gas lamps
Type 2: Those lamps which use the phenomenon of fluorescence, these are known as fluorescent lamps. E.g fluorescent mercury vapour tube
I. Sodium Vapour Lamp
This type of lamp is of low luminosity, so the length of this lamp is large. To get the required length it is made in the form of a U-tube. Two Oxide coated electrodes are sealed with the ends. The tube contains a little sodium and neon gas. The U-tube is enclosed in a double walled vacuum flask to keep the temperature within working range. Capacitor is connected to improve the power factor which will become low by using poor regulation transformer. Working: Before the lamp starts working, the sodium is in the form of a solid, deposited on the sides of the tube walls. In the beginning when the switch is on, it operates as a low pressure neon lamp with pink color. The lamp gets warmed, sodium is vaporized and it radiates yellow light and then, after sometime, about 10-15minutes, the lamp starts giving full light.
The lamp fails to operate when
- The filament burns out
- The cathode stops to emit electrons
- The sodium particles may concentrate on one side of the tube
- The lamp is blackened, the output will be reduced In order to start the discharge lamp, a striking voltage of
- 380v is required for 40W lamp
- 450v is required for 100W lamp
II. High Pressure Mercury Vapour Lamp
It consists of two bulbs
- Arc tube
- Outer bulb Arc tube is made of quartz, which containing the electric discharge. The arc tube contains a small amount of mercury and argon gas. Outer bulb is made of hard glass, which protects the arc-tube from changes in temperature. Working: When the supply is switched on, initial discharge for the few seconds is established in the argon gas between the auxiliary starting electrode and the neighboring main electrode and then argon between the two main electrodes. The heat produced due to this discharge through
- A choke is connected in series with the tube filament. It provides a voltage impulse for starting the lamp and acts as a ballast later
- The filament is connected to a starter switch
Working:
When the supply is given, full voltage acts on the starter. A glow discharge is set up in the starter which warms the electrodes and causes the bimetal strip to bend and touch the electrodes. The circuit becomes a complete series. Current flows and causes emission of free electrons from filaments. At the same time voltage at the starter falls to zero and the bimetal strip cools down. The electrodes of the starter switch then open and interrupt the current in the circuit. Its effort is to induce high voltage surge of about 1000 volts in the choke. This voltage produces the flow of electrons between the lamp electrodes and the lamp lights up immediately.
Stroboscopic Effect:
Because of the alternating nature of supply, it crosses zero two times in a cycle. For 50 – Hz supply of alternating current, a discharge lamp will be extinguished twice in a cycle and 100 times per second. A human eye cannot identify this extinguish phenomenon, because of the persistence of vision. This effect is called as ‘stroboscopic effect’. This effect can be avoided by employing any of the following techniques,
- If we have 3-phase supply, then the fluorescent lamps that are adjacent should be fed from different phases. Then no two lamps will not be in same phase at zero instant of AC supply, so light is present at any instant.
- If the available supply is single phase, then twin tube circuitry can be employed so that stroboscopic effect is eliminated.
8
S.No.
Aspect
Incandescent Lamp
Fluorescent Lamp
Mercury Vapour Lamp
Sodium Vapour Lamp
Starting
They have instantaneousstart and becomemomentarily off whensupply goes off
They have a reaction time ofone second or a little moreat the start. They go off andrestart when the supply isrestored
They take 5 to 6 minutesfor starting. They go offand cannot be restartedafter the recovery of thevoltage till the pressurefalls to normal
They take 5 to 6 minutesfor starting. They go offand cannot be restartedafter the recovery of thevoltage till its value fallsto normal value
Colour of light
Very near the natural
Varies with the phosphorcoating
They suffer from colourdistortion
Yellowish, colourdistortion is produced
Installation cost;Running cost
Minimum; maximum
Maximum; minimum
High but lesser than thatof fluorescent lamps;Much less thanincandescent lamps buthigher than fluorescent lamps
Maximum;Less than for filamentlamps but more than forfluorescent lamps
Average life
1000 hours
4000 hours
3000 hours
3000hours
Efficiency
10 lm/W
40 lm/W
40 lm/W
60 - 70 lm/W
Stroboscopic effect
No
Yes
Yes
Yes
Applications
Automobiles, Trains,Emergency lights,Aeroplanes, Signals forrailways, flood lights, etc.
Semidirect lighting,domestic, industrial,commercial, roads and halls,etc
Suitable for open spacelike yards, parks andhighway lighting, etc ;their position of workingis vertical
Very suitable for streetlighting purposes; theirposition of working ishorizontal (Not suitablefor local lighting)
