Incandescent Lamp – Construction, Working and Advantages:
For sometimes the arc lamps were used for general lighting purpose but as the carbon-arc lamps were complicated so these have been superseded by filament lamps for general lighting.
When an electric current is passed through a fine metallic wire, heat is produced and the temperature of the wire increases. At low temperatures the wire radiates heat energy, as the temperature of the wire increases due to heating it radiates heat as well as light energy. The higher the temperature of the wire, higher is the amount of light energy radiated. A black body when heated to 6,250°C emits the maximum energy in the visible spectrum range.
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 can 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 the filaments in incandescent lamps are carbon, osmium, tantalum and tungsten. Though the carbon has a melting point of 3,500°C but its main drawback is that it starts vaporizing at very fast rate if it is operated beyond 1,800°C, moreover its temperature coefficient of resistance is negative, i.e., its resistance decreases with the rise in temperature and, therefore, it takes more current from mains. Because of its low operating temperature its efficiency is quite low and is of the order of 3.5 lumens per watt.
Osmium is very rare and expensive metal. The melting point of osmium is 2,600°C. Average efficiency of osmium lamp is of the order of 5 lumens per watt. The melting point of tantalum is only 2,800°C and, therefore, it cannot be operated at higher temperature to give more efficiency. The efficiency of tantalum filament lamps is about 5 lumens per watt. Nowadays tungsten is the most commonly used metal for filament due to its high melting point (3,400°C), high resistivity, low temperature coefficient (0.0051), low vapour pressure, being ductile and mechanically strong to withstand vibration during use. The hot resistance of tungsten filament is about 15 times the cold resistance and, therefore, there is a inrush of current at the switching instant. However, the inrush current is not beyond 15 times the normal current. Since the filament heats up immediately and resistance increases almost instantaneously, so the inrush current attains a maximum value in 0.003 second and falls to normal operating value in 0.2 second. The average efficiency of tungsten filament lamp is about 10 lumens per watt. The light spectrum of an incandescent lamp is continuous, i.e., it contains all the colours, but contains relatively excess of red and yellow radiations and less of blue and violet radiations.
Aging Effects of Incandescent Lamp:
The light output of an incandescent lamp decreases gradually. As a tungsten lamp ages, its light output decreases for two reasons. Evaporation of the filament tends to cause the bulb to blacken. Also, evaporation makes the filament slowly decrease in diameter, which means that the resistance of the filament increases. Therefore, an old filament draws less current and operates at a lower temperature, which reduces its light output. In turn the efficiency of the lamp (lumens output/watt input) also decreases with the passage of time. The current drawn and the power consumed by the filament decrease at the same rate as the lamp ages. However, the efficiency decreases about four times as fast, and the light output decreases approximately five times as fast. The total depreciation of light output is roughly 15% over the useful life range.
Effects of Voltage Variations:
The operating characteristics of an incandescent lamp are materially affected by departure from its normal operating voltage. An increase of 5% in operating voltage increases the lumens output by 20% but shortens the life of the lamp by 50%. On the other hand, reduction of 5% in operating voltage causes a reduction of approximately 20% in lumens output but doubles the life of the lamp. The efficiency of a lamp (lumens/watt) increases with the increase in voltage owing to increase in temperature and is proportional to the square of the voltage. 1% change in applied voltage modifies lamp wattage by 1.5%, efficiency by 2%, lumens output by 3.5%. The normal life of a filament lamp is about 1,000 working hours. The variations in power consumption, lumens output, efficiency and life of incandescent lamps with the variation in voltage are shown in Fig. 7.29.
Analytically these relationships are given as below :
- Lumens output ∞ (V)3.35
- Power consumption ∞ (V)1.55
- Luminous efficiency ∞ (V)2
- Life ∞ (V)-13 for vacuum lamps;
- ∞ (V)-14 for gas-filled lamps
Advantages of Incandescent Lamp:
- Operating power factor unity.
- Direct operation on standard distribution voltage.
- Availability in various shapes and shades.
- Good radiation characteristic in the luminous range.
- No effect of surrounding air temperature.
Filament Dimensions:
Depending upon the voltage and wattage the diameter of a tungsten lamp filament may be as small as 10 microns (about one-sixth of diameter of human hair).
The influence of the diameter of the filament on the heating is found easily if it is assumed that the heat lost by convection is negligible compared with that lost by radiation.
The power absorbed by the filament
where I is filament current in amperes, ρ is the resistivity of filament material at the operating temperature, l is length of the filament and d is diameter of the filament.
The power radiated by the filament
where e is the emissivity of the surface, K is a constant, T1 is the temperature of filament and T0 is the temperature of the surrounding medium.
Since
For two filaments of same material operating at the same temperature the diameter d is proportional to I2/3.
Gas Filled Lamps:
A metal filament can work in an evacuated bulb up to 2,000°C 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 working temperature more than 2,000°C 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, such as miners lamps. The tungsten filament can safely be burn at temperature of 2,400°C to 2,750°C according to the size of the lamp. However, due to presence of gas there is heat loss due to convection currents. This loss depends upon the surface area of filament. As such coiled coil filaments, which take much less space compared with coiled filaments are used with such lamps.
A coiled coil filament is made by winding tungsten wire on a fine iron wire to form a spiral which is again wound on to a thick iron wire to form a coiled coil. (iron is later on dissolved out by acid). Helical filaments also have a slower rate of tungsten evaporation. This evaporation eventually causes bulb blackening because the tungsten vapour condenses as a black film on the inner surface of the bulb. In a gas filled lamp, the hot gas carries the tungsten vapour upward. Therefore, a black spot forms at the top of the bulb instead of spreading over the entire inner surface, as in a high-vacuum bulb. Chemicals called ‘getters‘ are often placed inside the bulb to capture tungsten vapour and thereby reduce the rate of blackening. A piece of wire mesh called a collector grid may also be attached to each lead-in-wire to attract the particles of tungsten vapour. Efficiency of coiled-coil lamp is about 12 lumens/watt.
For low wattage lamps, however, the heat loss due to introduction of gas is more than in medium wattage lamps, so for low wattage (up to 40 watts) vacuum type lamps are used.
Halogen Lamp:
The halogen lamp is the latest member in the family of incandescent lamps. It possesses numerous advantages over the ordinary incandescent lamp. As already stated, the life and efficiency of an incandescent lamp fall off with use—partly due to slow evaporation of the filament and partly due to black deposit formed on the inside of the bulb. The addition of a small amount of halogen vapour to the filling gas restores part of the evaporated tungsten vapour back to the filament by means of a chemical reaction, i.e., there is a sort of “regenerative cycle‘.
Halogen lamps possess the following advantages :
- No blackening of lamp, hence no depreciation of lumens output.
- High operating temperature with increased luminous efficiency varying from 22 lumens per watt to 33 lumens per watt.
- Reduced dimensions of lamps–miniature size.
- Long life-2,000 hours.
- Better colour rendition.
Halogen lamps, which are being manufactured in sizes up to 5 kW, are suitable for outdoor illumination of buildings, playing fields, large gardens, fountains, car parks, airport runways etc. and for lighting of public halls, factories, sport-halls, photo film and TV studios etc.