How Magnetic Induction lamps work
Introduction:
Thomas Edison is generally credited with the invention of the commercially viable electrical lamp we are familiar with. He was building on work done by early pioneers, where the conversion of electricity to light was demonstrated in laboratories as early as 1801 by Sir Humphrey Davy who is also credited with the invention of the electric arc lamp.
Interestingly, Canadians Henry Woodward and Matthew Evans filed a patent in 1874 for a light bulb which used a carbon filament in a nitrogen atmosphere. They were unsuccessful in commercializing the lamp but caught the interest of Edison who considered this Canadian technology so intriguing, he bought their Canadian and US patents [Canadian Patent CA 3738 and U.S. Patent 181,613] in 1875 for the then princely sum of $5,000 US dollars.
Edison continued this line of development and improved upon the Woodward and Evans patent by using a metal filament in a vacuum eventually producing the first practical and commercially successful light bulb in 1880.
Incandescent Lamps:
The most common form of electrical lighting we are all familiar with is the incandescent lamp. This consists of an evacuated glass envelope, which generally has two electrodes protruding through the wall of the glass vessel at the bottom, and sealed in place, to bring the electrical current into the interior of the lamp.
There is a thin filament, usually made of tungsten, suspended between the electrodes. More than two electrodes may be present, for example in a “3-way” lamp. There may also be other non-electrically connected wires provided for mechanical support of the filament.
The incandescent lamp works by passing an electrical current through the filament, typically made of tungsten, which then glows white hot emitting light. This is not an efficient process as approximately 95% of the energy supplied to the lamp is emitted as heat. The filament must be contained in an evacuated bulb, or a bulb filled with an inert gas, as any contact with oxygen will cause the heated tungsten filament to evaporate and break the electrical circuit, thus rendering the lamp useless.
Other Lamp Types:
There are many other types of lamps ranging from xenon arc lamps used in movie projectors, to metal halide, mercury vapour and sodium types, to fluorescent types, to light emitting diodes [LEDs]. It is beyond the scope of this article to cover all of these types in detail but I will cover fluorescent lamps as the Induction lamps are a modified form of the fluorescent lamp.
For details on other types of lamps, the reader is referred to http://en.wikipedia.org/wiki/List_of _light_sources which has a list of many different types of lamps.
Fluorescent Lamps:
A fluorescent lamp is a type of gas discharge tube where an electrical current excites mercury vapour in an inert gas producing UV light, typically at the 253.7 nm and 185 nm wavelengths. The UV light is up-converted, by a coating of phosphors on the inside of the glass tube, into visible light.
At each end of the typical fluorescent lamp, there are small tungsten filaments which are usually coated with a blend of metallic salts such as barium, strontium and calcium oxides. The filaments are provided to bring the electric current into the lamp, and the metallic salts are designed to promote the emission of electrons, in order to stimulate the mercury ions, which are released by a tiny drop of liquid mercury in the tube.
Fluorescent lamps are a negative resistance device [as more current flows, the resistance decreases allowing even more current to flow] so the lamps require a ballast to control the current to the lamp. The most common and simple type of ballast is a magnetic or “core and coil” ballast. This is a form of current limiting transformer which provides the lamp with the correct current needed for operation. These ballasts are cheap but inefficient as they emit heat [wasted energy] - typically between 12% and 15% of the energy consumed by the lamp is wasted in the ballast. Newer types of fluorescent lamps use high frequency electronic ballasts. While these are more costly to manufacture, they are much more energy efficient typically only wasting between 5% and 8% of the energy consumed by the lamp.
The choice of phosphor, or combination of phosphors, used in the coating on the inside of the tube influences the perceived colour of the light emitted. Certain phosphors emit red, green or blue light when excited by the UV light inside the tube. By combining various phosphors, manufacturers can offer “warm white”, “cool white” and “daylight” types of lamps (where these designations refer to the approximate colour temperature of the lamp) by mixing and matching the phosphors used in the lamp coating.
"Surely, my system is more important than the incandescent lamp, which is but one of the known electric illuminating devices and admittedly not the best. Although greatly improved through chemical and metallurgical advances and skill of artisans it is still inefficient, and the glaring filament emits hurtful rays responsible for millions of bald heads and spoiled eyes. In my opinion, it will soon be superseded by the electrodeless vacuum tube which I brought out thirty-eight years ago, a lamp much more economical and yielding a light of indescribable beauty and softness." -Statement by Nicholas Tesla published in “The World” in 1929.
Electrodeless Lamps:
Almost all of the light sources currently in use have one thing in common, metal electrodes sealed into the walls of the bulb to bring the electrical current inside the lamp chamber/envelope.
Unsurprisingly, the main failure mechanisms in these typical lamps with electrodes [other than breakage] is:
Failure of the filament due to depletion of the filament material over time as atoms are stripped off by the electrical current;
Vibration which breaks the filament, especially when it is hot;
Failure of the seal integrity of the lamp; typically caused by thermal stresses in the area where the electrodes go through the glass walls. The failure of the seal can either be sudden and complete, or a “slow leak” over time allowing the entry of atmospheric gasses which contaminate the interior.
The dream of lighting inventors has been to produce a lamp with no internal electrodes so as to eliminate these common failure modes. In an electrodeless lamp the envelope [bulb] is completely sealed and thus there is no chance of atmospheric contamination due to seal failure and no electrodes to wear out over time. On 23 June 1891, Nicholas Tesla was granted US patent 454,622 to cover a very early form of Induction lamp.
In an electrodeless lamp, the main failure mechanisms [other than breakage] are:
Depletion of the mercury amalgam inside the envelope [bulb]. When the mercury ions are excited and bombard the phosphors [which then emit the light we see], a small percentage of them are absorbed by the phosphor coating over time. Once the mercury ions inside the envelope are depleted, the lamp emits only a very dim light and has to be replaced.
Failure of the electronics [ballast] used to drive the lamp. This is not a catastrophic failure mode as typically the electronics [ballast] are external to the lamp and can easily be replaced.
So how do you get an electrical current inside the bulb (glass envelope) of a lamp to excite the mercury ions There are two types of practical electrodeless lamps available on the market today, microwave lamps and magnetic induction lamps.
A microwave lamp bombards a capsule of sulfur with radio frequency (microwave) energy which causes the sulfur to be heated, becoming a light emitting plasma. The capsule containing the sulfur has to be rotated to prevent uneven heating and must be cooled by a fan thus the lamps contain mechanical parts subject to failure. Typically the magnetron used in these lamps last between 15,000 and 20,000 hours and must then be replaced so maintenance costs are higher than conventional lighting. The major advantage of the microwave sulfur lamps is that they are the only artificial light source that has an output spectrum which closely approximates sunlight.
Magnetic Induction Lamps:
Magnetic induction lamps are basically fluorescent lamps with electromagnets wrapped around a part of the tube, or inserted inside the lamp. In external inductor lamps, high frequency energy, from the electronic ballast, is sent through wires, which are wrapped in a coil around the ferrite inductor, creating a powerful magnet.
The induction coil produces a very strong magnetic field which travels through the glass and excites the mercury atoms in the interior which are provided by a pellet of amalgam (a solid form of mercury). The mercury atoms emit UV light and, just as in a fluorescent tube, the UV light is up-converted to visible light by the phosphor coating on the inside of the tube. The system can be considered as a type of transformer where the inductor is the primary coil while the mercury atoms within the envelope/tube form a single-turn secondary coil.
In a variation of this technology, a light bulb shaped glass lamp, which has a test-tube like re-entrant central cavity, is coated with phosphors on the interior, filled with inert gas and a pellet of mercury amalgam. The induction coil is wound around a ferrite shaft which is inserted into the central test-tube like cavity. The inductor is excited by high frequency energy provided by an external electronic ballast causing a magnetic field to penetrate the glass and excite the mercury atoms, which emit UV light, that is converted to visible light by the phosphor coating.
The external inductor lamps have the advantage that the heat generated by the induction coil assemblies is external to the tube and can be easily dissipated by convention into the air, or conduction into the fixture. The external inductor design lends itself to higher power output lamps which can be rectangular or donut shaped. In the internal inductor lamps, the heat generated by the induction coil is emitted inside the lamp body and must cool by conduction to a heat-sink at the lamp base, and by radiation through the glass walls. The internal inductor lamps tend to have a shorter lifespan than the external inductor types due to higher internal operating temperatures. The internal inductor type looks more like a conventional light bulb than the external inductor type lamps which may be more appealing in some applications.
As with conventional fluorescent lamps, varying the composition of the phosphors coated onto the inside of induction lamps, allows for models with different colour temperatures. The most common colour temperatures of induction lamps are 3500K, 4100K, 5000K and 6500K.
Induction lamps require a correctly matched electronic ballast for proper operation. The ballast takes the incoming mains AC voltage [or DC voltage in the case of 12V and 24V ballasts] and rectifies it to DC. Solid state circuitry then converts this DC current to a very high frequency which is between 2.65 and 13.6 MHz depending on lamp design.
This high frequency is fed to the coil wrapped around the ferrite core of the external or internal inductor. The high frequency creates a strong magnet field in the inductor which couples the energy through the glass walls of the lamp and into the mercury atoms inside the tube.
The ballasts contain control circuitry which regulates the frequency and current to the induction coil to insure stable operation of the lamp. In addition, the ballasts have a circuit which produces a large “start pulse” to initially ionize the mercury atoms and thereby start the lamp. The induction lamps do not start at 100% output - they start at between 75% and 80% output. It takes between 60 and 120 seconds for the mercury bearing amalgam in the lamp to heat up and release enough mercury atoms for the lamps to reach 100% light output.
The close regulation of the lamp by the ballast, and the use of microprocessor controlled circuits allows the ballasts to operate at around 98% efficiency. Only around 2% of the energy is wasted in the induction lamp ballast compared to the 10-15% wasted in traditional “core and coil” type designs used with most high output commercial and industrial lighting.
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