Patent Application
20060076864
This invention relates to electric lamps and, more particularly, to electrodeless fluorescent lamps which operate at frequencies from 20 kHz to 20 MHz and power levels in a range of 50 watts to 500 watts.
Electrodeless, inductively-coupled fluorescent lamps have been recently introduced into the market for indoor, outdoor, industrial and commercial applications. An advantage of electrodeless lamps is the absence of internal electrodes and heating filaments which are life-limiting factors in conventional fluorescent lamps. The life of electrodeless fluorescent lamps is substantially higher than that of conventional fluorescent lamps and can reach 100,000 hours.
A low and medium power (10-200 watts) electrodeless fluorescent lamp operated at a frequency of 20-1000 kHz is disclosed in U.S. Pat. No. 6,081,070, issued Jun. 27, 2000 to Popov et al. A bulbous lamp envelope with a reentrant cavity on its axis is fabricated of glass and is filled with inert gas (Ar, Kr, Xe) and mercury vapor. An inductively-coupled discharge is ignited and maintained in the lamp envelope by an azimuthal electric field induced in the envelope by a magnetic field. The magnetic field is generated by an induction coil wrapped around a ferrite core which is positioned in the reentrant cavity.
An exhaust tubulation is located on the cavity axis. A mercury amalgam is held in the tubulation by several glass pieces. The position of the amalgam is selected to keep the amalgam temperature between 80° C. and 120° C. within the ambient temperature range from −10° C. to +60° C. This provides in the lamp envelope an optimum mercury vapor pressure about 6×10−3 Torr.
To remove heat from the ferrite core so as to keep its temperature below the Curie point, a cooling structure is utilized. The cooling structure includes a cooling tube of high thermal conductivity metal or ceramic positioned inside the ferrite core, and a heat sink of a high thermal conductivity material located at the bottom of the lamp envelope. The cooling tube and the heat sink are thermally and electrically connected.
Such an arrangement maintained temperature of the ferrite core and induction coil wire below 200° C. at a lamp power up to 260 watts and an ambient temperature of +60° C. A further increase in lamp power leads to an increase in the power deposited on the walls of the reentrant cavity and transferred to the coil wire and ferrite core via convection and light radiation. Also, the increase in power absorbed by the inductive plasma results in an increase in power density on the cavity walls. As a result, the coil wire and ferrite core are heated to temperatures above 200° C., which causes deterioration of the coil wire and hence a reduction in lamp lifetime. To improve heat management, the thickness of the cooling tube inserted in the ferrite core can be increased. This can be achieved by reducing the cooling tube inner diameter. However, the decrease of the cooling tube inner diameter is limited by the finite diameter of the exhaust tubulation.
Accordingly, there is a need for improved electrodeless high power fluorescent lamps.
According to a first aspect of the invention, an electrodeless lamp is provided. The electrodeless lamp comprises a bulbous lamp envelope enclosing an inert gas and a vaporizable metal fill, the lamp envelope having a reentrant cavity, an electromagnetic coupler positioned within the reentrant cavity, and a spacer structure disposed between the electromagnetic coupler and an inside wall of the reentrant cavity.
According to a second aspect of the invention, an electrodeless lamp is provided. The electrodeless lamp comprises a bulbous lamp envelope enclosing an inert gas and a vaporizable metal fill, the lamp envelope having a reentrant cavity, a magnetic core positioned within the reentrant cavity, an inductive coil disposed on said magnetic core, a cooling structure disposed inside said magnetic core, and a spacer structure between the magnetic core and an inside wall of the reentrant cavity.
The spacer structure may comprise two or more spaced-apart, thermally insulating rings positioned between the magnetic core and the inside wall of the reentrant cavity. The thermally insulating rings may have protrusions to reduce thermal conduction from the cavity wall to the magnetic core. The spacer structure may include a portion positioned between an end of the magnetic core and a closed end of the reentrant cavity. The spacer structure defines a thermally insulating gap between the magnetic core and the inside wall of the reentrant cavity.
According to a third aspect of the invention, a method is provided for enhancing performance of an electrodeless lamp including a bulbous lamp envelope having a reentrant cavity and an electromagnetic coupler positioned within the reentrant cavity. The method comprises controlling a spacing between the electromagnetic coupler and an inside wall of the reentrant cavity with a thermally insulating spacer structure.
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
A simplified cross-sectional diagram of a lamp assembly in accordance with an embodiment of the invention is shown in
Lamp envelope 30 may be made from glass and may have a bulbous shape, as shown in
A mercury amalgam 60 is positioned in exhaust tubulation 42 and controls the mercury vapor pressure in the lamp envelope 30. Several glass pieces 62 hold the amalgam 60 in a fixed position that is optimum to provide mercury vapor pressure in lamp envelope 30 within a wide range of ambient temperatures.
Electromagnetic coupler 32 is located in reentrant cavity 40 and includes a magnetic core 70, an induction coil 72 and a cooling structure 74. The coupler 32 is connected thermally and electrically to the base fixture via a lamp base 76.
Induction coil 72 may be made from multiple strand wire, such as Litz wire, wound around magnetic core 70. The number of strands may be in a range of 7 to 470. The number of coil turns may be in a range of 10 to 100. In a preferred embodiment, the number of strands is 19 and the number of turns is 32. The magnetic core 70 may be made from a ferrite material, such as MnZn material, that has very low power losses at frequencies of 20 to 400 kHz, and has good thermal contact with cooling structure 74. Additional details of the ferrite core are provided in published U.S. application No. 2002/0067129 A1, which is hereby incorporated by reference. In a preferred embodiment, the outer diameter of magnetic core 70 is 32 mm the inner diameter is 16.5 mm. The length of magnetic core 70 may be in a range of 20 mm to 300 mm and in a preferred embodiment is 100 mm. The magnetic core 70 and induction coil 72 are positioned along cavity axis 44 so that the center of core 70 is approximately positioned where the diameter of the lamp envelope 30 is maximum. This location of the core 70 and coil 72 provides a low plasma electric field and hence a low magnetic field and associated core power losses.
Cooling structure 74 may include a cooling tube 80, an extension tube 82 and a coil spacer 84. The cooling tube 80 is made of a material having high thermal conductivity, such as Cu, Al, Al2O3, boron nitride, etc., and is disposed along cavity axis 44. In a preferred embodiment, cooling tube 80 is made of copper. The inner diameter of cooling tube 80 is larger than the outer diameter of exhaust tubulation 42. Cooling tube 80 is thermally and electrically connected to extension tube 82. In a preferred embodiment, cooling tube 80 and extension tube 82 are made as one piece, as shown in
The outer diameter of cooling tube 80 is slightly smaller than the inner diameter of magnetic core 70 so as to provide good thermal contact between magnetic core 70 and cooling tube 80. In a preferred embodiment, the outer diameter of cooling tube 80 is 16.4 mm, and the outer diameter of extension tube 82 is 36 mm. The inner diameter of both cooling tube and extension tube 82 is 9.5 mm in the preferred embodiment.
The top of cooling tube 80 is preferably positioned below the top of magnetic core 70. The distance between the core top and the cooling tube top is preferably greater than 5 mm. The positioning of the cooling tube 80 inside core 70 increases the coupler Q-factor. In a preferred embodiment, the distance between the core top and the cooling tube top is 10 mm.
To limit propagation of visible light through the wall of reentrant cavity 40 and heating of electromagnetic coupler 70, a reflective coating 110 may be deposited on the atmospheric side of cavity wall 40a of reentrant cavity 40. The visible light is reflected from the cavity wall into lamp envelope 30 and eventually radiates from the lamp envelope surface, thereby increasing the total light output.
Electrodeless lamp 12 further includes a spacer structure disposed between electromagnetic coupler 32 and an inside wall 40a of reentrant cavity 40. In the embodiment of
Spacer 90 may extend above magnetic core 70 by a few millimeters so as to provide a gap between the top of magnetic core 70 and a closed end 40b of reentrant cavity 40, as shown in
A first embodiment of a spacer suitable for spacers 90 and 92 is shown in
A second embodiment of a spacer suitable for spacers 90 and 92 is shown in
An embodiment of a spacer structure in accordance with the invention is shown in
A variety of different spacer structures and configurations may be utilized within the scope of the invention. The primary function of the spacer structure is to establish a gap between the electromagnetic coupler and the inside wall of the reentrant cavity.
To obtain a high Q-factor for coupler 32, magnetic core 70 and extension tube 82 of the cooling structure should be separated. Experiments have shown that to exclude the effect of the copper of extension tube 82 on the coupler Q-factor, the distance between the bottom of the magnetic core 70 and the extension tube 82 should be larger than 10 mm. In a preferred embodiment, coil spacer 84 is a glass cylinder with a length of 12 mm and an outer diameter of 25 mm, and is positioned between magnetic core 70 and extension tube 82.
The lamp operates as follows. A high frequency voltage is applied to induction coil 72 from a signal generator and an amplifier via a matching network (not shown). The high frequency voltage is preferably in a frequency range of 20 kHz to 20 MHz. When the azimuthal electric field in the lamp envelope in an area adjacent to reentrant cavity 40 reaches its breakdown value, an inductively-coupled plasma appears around the reentrant cavity near the center of the magnetic core 70 and induction coil 72. As the plasma power heats amalgam 60 via convection and heat conduction through tubulation 42, the mercury vapor pressure increases and thereby causes an increase in ultraviolet radiation output. As a result, visible light generated from phosphor coating 52 excited by the ultraviolet radiation also increases until reaching a maximum value at a mercury pressure of about 6 milliTorr. The lamp consumes high frequency power in a range of 50 watts to 500 watts.
Referring to
The excellent lamp efficacy is achieved due to high plasma power efficiency, which is due to very low coil/core power losses. Referring to
Coil/core power losses are low due to the high plasma density and hence low plasma electric field at high plasma power, where the plasma power is greater than 200 watts. Coil/core power losses are also low because of low coil and core temperatures which, at a lamp power of 235 watts, are about 160° C. The power losses in the ferrite material (MN-80 or 2H8) are still low even at ferrite temperatures of 1600 to 180° C. Core and coil temperatures as a function of ambient temperature are shown in
The increase in the gap between the core and the cavity wall results in a decrease of the core and coil temperatures due to less effective transfer of heat from the cavity wall to magnetic core 70 and induction coil 72. This is illustrated in
The increase of the gap between the cavity wall and the coil/core decreases the coupling between the coil/core and the inductively-coupled plasma. To maintain the discharge and the plasma, the coil current generating the magnetic field to maintain the discharge is increased. As a result, the coil/core power losses increase. This phenomenon, as discussed in U.S. Pat. Nos. 5,621,265 and 6,081,070, causes a decrease of the lamp power efficiency and hence lighting efficacy. However, for a lamp operating at a power of 200 to 250 watts, an increase in coil power losses even by 50 percent, from 3 watts to 4.5 watts, does not have any noticeable effect on the lamp power efficiency. Thus, the efficacy of an electrodeless fluorescent lamp operated at a high power of 200 to 300 watts and a frequency of 100 to 400 kHz is not sensitive to the variation (up to 10-15 mm) of the gap dimension between the cavity wall and the core/coil.
A novel method is described for reducing coil and core temperatures without reducing lamp power efficiency and hence visible light efficacy. The method takes advantage of the decrease in inductive electric field in the plasma in high power (150-500 watts) electrodeless lamps, which have much higher plasma density than low power (20-50 watts) electrodeless lamps. The decrease in electric field causes a decrease of the magnetic field needed to maintain the plasma. As a result, the coil/core power losses substantially decrease to a level of 2 to 3 watts. For electrodeless lamps operating at a power of 150 to 500 watts, this power loss is negligible and constitutes less than 2 percent of the total lamp power. Even a two-fold increase in the coil/core power losses does not appreciably affect the lamp power efficiency.
The insensitivity of the high power lamp efficacy to variation of the coil/core power losses within a range of 2 to 6 watts enables an improvement of coil/core cooling at the expense of coil/core losses. This is accomplished by increasing spacing between the coil/core and the cavity wall and cavity top, which results in a reduction of heat transfer from the hot cavity wall and cavity top to the coil/core.
At the same time, the increase in distance between the cavity wall and the coil/core degrades the coil/core coupling with the inductive plasma. Indeed, the increase in spacing between the coil/core and the plasma causes a decrease of the coupling coefficient. It was shown in the prior art that the coupling coefficient k depends strongly on the plasma and the coil radii, as k equals Rc2/Rpl2. Consequently, the decrease of the coupling coefficient is accompanied by an increase of the coil current and voltage and hence, by an increase of the magnetic field through the magnetic core. This leads to an increase in coil losses (due to the coil current increase) and magnetic core losses (due to the magnetic field increase). However, since at high power Plamp is much greater than Ploss and is approximately equal to Ppl, the lamp power efficiency Ppl/Plamp is not sensitive to the increase of Ploss and is close to one.
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.
