Incandescent lamps produce light as a result of the heating effect of an electrical current flowing through a filament wire. Both visible light and infra-red energy are radiated if the temperature is high enough.
Tungsten is uniquely suitable for use as a filament material on account of its high melting point and relatively low rate of evaporation at high temperatures. It is because of these considerations that tungsten is used for filaments in all present day incandescent lamps.
In order to get the tungsten filament wire in a form which can be conveniently fitted inside a lamp envelope it is customary to coil the wire into either a single of double helix. Coiling is achieved by winding the tungsten wire on a mandrel of either steel or molybdenum. This is then heat treated in order to relieve coiling strain, and the mandrel removed by dissolving in acid.
Right is illustrated a standard type of general lighting service (GLS) incandescent lamp with a coiled coil tungsten filament (A).
The filament is supported at intermediate points by molybdenum support wires (B). The electrical connections to the filament are made through the clamps on the end of nickel plated lead in wires (C) whire are part of a subassembly comprising usually of three or more components. The inner wire (C) is welded to a section of a dumet wire (D). The lower end of the dumet wire is connected to a protective fuse which is usually a thin monel wire connected directly to the solder contact of the cap, or a copper nickel alloy wire encapsulated inside a glass sleeve (F), filled with small glass beads (ballotini).
Additional form of protection is glass lined caps. All quality lamps will have both fuse and cap type protection. The connection between the fuse and soldered external contact on the cap is made by the copper lead wire (G) which is terminated through an eyelet in the cap and connected by solder at the contacts (H).
The molybdenum support wires (B) are inserted into an inner glass rod or tube extending from the pinch (J). The exhaust tube (K) is kept open at (J) to allow the air to be pumped from the envelope (L) after sealing, and in gas filled lamps for the introduction of the final filling gas. The metal cap (M) is fixed to the bulb by heat cured cement. The cap is usually constructed from either aluminium or brass.
The only practical material for incandescent lamp filaments is tungsten. It has a very high melting point of 3450oC and its evaporation rate is relatively low, lower than carbon for example. The coiled coil form gives a higher efficiency when used with gas filling. Single coils are used for their robustness in rough service types.
Gas filling with argon / nitrogen mixtures is used to suppress evaporation and reduce the change of flash failure. The ratio of argon to nitrogen varies.
90/10 is used for single coil lamps.
85/15 is used for coiled coil filaments
70/30 for high wattage lamps or lamps with high operating temperatures
Heat loss by convection from the filament is prevented by tight coiling. A film of stationary gas, (Langmuir Layer) forms around the filament restricting heat loss.
The part of the lamp body immediately above the filament gets very hot, and therefore gas filled lamps cannot be subjected to thermal shock such as rain.
Crompton Lighting strongly recommends that only vacuum lamps are used for external use.
Krypton mixtures can be used in special cases to either increase life or to increase light output. The filament has to be redesigned to meet whichever is the requirement. These improvements are possible because the molecular weight of krypton is approximately twice that of argon. It should also be noted that because krypton is a rare gas it is very expensive to produce krypton filled lamps.
For very fine filaments heat losses to a gas filing would be too great, so these filaments are designed for vacuum lamps only. Such lamps, apart from being generally lower wattage types, distribute the heat equally around the bulb and do not run so hot. All Crompton external lamps are vacuum lamps for use outdoors.
The pear shaped 60mm bulb has proved successful over the years for the standard GLS lamp. They are made on very high speed machines which preclude frequent resetting for other shapes. Minimum dimensions of GLS lamps are governed by BS EN60064 and limits for cap adhesion and cap temperature are governed by BS EN60432. For decorative lamps and display lighting where effect is more important, many interesting bulb shapes are available.
THE LIFE OF A TUNGSTEN LAMP
The nominal life of a GLS lamp has been 1000 hours almost since its introduction. Queries have arisen as to whether this is the optimum figure. Such are the variations in user requirements and the costs of replacing lamps, that there can be not universal optimum life. The committee responsible for BS EN 60064 is representative of a wide cross section of interests and include government departments, electrical contractors, professional institutions, organisations representing large user and the lighting industry. This committee reviews all the factors involved and has concluded many times that the long established life of 1000 hours should remain the standard rating. Lamp life can be increased either at the expense of light output i.e. reducing the light output to achieve extended life or by using a different filling gas to reduce the rate of tungsten evaporation i.e. krypton or halogen mixtures.
POSITION OF OPERATION
GLS Lamps can be burnt in any position but the optimum position for gas filled lamps is cap up. Some reduction life may occur due to the overheating of the glass in other operating positions, also some consideration is usually needed for the lamp holders in luminaries with regard to heat resistance. For optimum burning position of other types of tungsten lamps the manufacturers recommendations must be consulted.
Switching and Lamp Life
There is little effect on rated life by frequency of switching on and off under normal conditions. Where lamps are used in flashing equipment, the time interval should allow complete extinction of the filament.
Only in rare cases do vacuum lamps radiate interference with reception in the VHF range 40 to 70 MHz referred to as Barkhausen oscillation. It is due to electron emission and the space charge setting up oscillations which resonate with the stray inductance and capacitance of the filament and support wire configuration. Gas filled lamps are free of this phenomenon. All Crompton lamps comply with the EMC directive, and are fully CE marked.
CAUSES OF LAMP FAILURE
Owing to its high operating temperature the filament will slowly evaporate. It is not possible to produce a filament of totally uniform diameter throughout its length, and in consequence evaporation prevails at the thinnest point. As current density is at maximum and produces a “hot spot”. Eventually the filament melts following contraction and cooling at switch off and then at switch on it will break. This is the NORMAL failure of a lamp.
HOW TO RECOGNISE PREMATURE FAILURE
Airleak – Lamp has a milky white deposit inside the bulb (tungsten oxide). Cracks sometimes extend round the cap / glass junction causing complete separation of the bulb.
Moisture in the bulb – Lamp has a smoky blue appearance and blackening is prevalent on the lead in wires and supports.
Vibration – A tangled and stretched filament indicates that the lamp has been under excessive vibration whilst alight or has received a direct blow.
Overheating – A heat tarnished cap, which may be loose and charred or discoloured cement indicate that the lamp has been operating at excessive temperature possibly due to an inadequately ventilated fitting or abnormally high ambient temperatures, where the maximum cap / glass temperature of 210oC has been exceeded.
Arcing – Lead wires melted near the filament or support wires melted at the pigtail loop and fuses either blown if nomel or the ballotini sheath blackened.
Overvoltage – A sparkling or crystallised filament is typical of a lamp which has served a normal useful life. If this characteristic is observed in early failure it is evidence of over voltage.
Incandescent lamp life is dependent on the voltage. The higher the voltage the shorter the life and vice versa. UK electricity companies have a tolerance on supply of 230v, plus 10% or minimum 6%, meaning supply can be 253v or sometimes higher in some areas. All lamp characteristics but particular life, are affected by relatively small changes in voltage. This is shown graphically in the figure below.
Simple mathematical relationships over practical limits are:-
- Each 1% voltage variation produces:
- 0.5% amps and resistance change
- 1.5% watts change
- 2% lumens per watt change
- 4% lumens change
- 14% inverse change in lamp life
So whilst power consumption and light output rise due to an increase in voltage, more importantly life is significantly reduced.
e.g. 240v 100w GLS lamps on a 250v supply. Percentage voltage increase (250-240)/240 x 100% = 4% approx
Reduction in life (4x 14/100) x 1000 = 560 hrs.
So new life is 1000 – 560 = 440hrs.
Source : Crompton Lighting