Abstract:
A method and apparatus are provided to retrofit a gas discharge lamp ballast designed for use with a first lamp having a first power rating to be used with a second lamp having a second power rating. The ballast has a core, a coil and a serially-connected ballast capacitance device and is characterized by a first net impedance of the ballast reactance to allow operation of the first lamp in an operating range in which the first lamp is designed to operate. When the first lamp is replaced with a second lamp having a different lamp impedance than the first lamp, the capacitance of the ballast capacitance device is selected so as to change the first net impedance of the ballast to a second net impedance which allows the ballast to maintain proper operation of the second lamp in accordance with its specifications and rating.

Description:
BACKGROUND OF THE INVENTION 
     High intensity discharge (HID) lamps such as metal halide (MH) lamps, high pressure sodium (HPS) lamps and high pressure mercury vapor lamps have increasingly gained acceptance over incandescent and fluorescent lamps for commercial and industrial applications. HID lamps are more efficient and more cost effective than incandescent and fluorescent lamps for illuminating large open spaces such as construction sites, stadiums, parking lots, warehouses, and so on, as well as for illumination along roadways. Commercial HID lighting installations generally employ luminaires which are complete lighting units, each of which comprises a ballast and its housing, a lamp socket, and a lamp. 
     Additional savings with regard to further reducing energy consumption can be achieved by replacing an HID lamp of a particular type and wattage with a lower wattage HID lamp. For example, an entire luminaire having a conventional fluorescent lamp and its ballast can be replaced with a luminaire having a lower wattage HPS lamp and ballast therefor. Replacing the entire luminaire, however, is costly since all of the luminaire parts are being replaced. 
     As an alternative, a substitute lamp having a lower rated lamp wattage can be used with the existing ballast in a luminaire. This approach, however, is disadvantageous. Although a reduction in lamp wattage can result in an energy savings, the substitute lamp and the ballast are not matched so as to yield the most efficient performance. 
     A consequence of a mismatched lamp and ballast can be an increase in lamp current. The increased lamp current which occurs upon the substitution of a lamp having a lower rated lamp voltage into an existing luminaire is addressed in U.S. Pat. Nos. 5,606,222, to Cottaar et al. 5,606,222, to Cottaar et al, relates to a current-reducing device for reducing current through a lamp and ballast to reduce system wattage in a gas discharge lamp lighting system. The current-reducing device is described as a capacitor connected in parallel with the discharge lamp in a system having a lead-type ballast (e.g., a constant-wattage auto transformer (CWA)). The impedance of the parallel capacitor is selected to be between ten and twenty times the impedance of the lamp such that the capacitor is configured to take a substantial amount of current. This parallel arrangement is disadvantageous because it merely diverts energy that would normally flow through the lamp when no such arrangement is used. The reduced current through the lamp is unacceptable because it deteriorates the waveform provided to the lamp, thereby decreasing the operating life of the lamp. For example, the current crest factor increases, among other undesirable waveform changes, and prevents the lamp from operating optimally and in accordance with the lamp characteristics with which the lamp was designed to operate, including but not limited to open-voltage and sustaining voltage requirements, ignition and starting current requirements, lamp regulation requirements, and so on. Accordingly, a need exists for an apparatus and method to reduce system wattage in retrofit applications for gas discharge lamps (i.e., substitution of a typically lower wattage lamp using an existing ballast coil and core) which does not significantly shorten the operating life of the substituted, lower wattage lamp. 
     SUMMARY OF THE INVENTION 
     The above-described problems with retrofitting HID lamps with reduced wattage HID lamps are overcome by the present invention. Advantages are also realized with regard to substituting lamps in the same or different lamp-type family as the original lamp and having a higher or lower power rating. A method and apparatus are provided for converting an HID ballast for a particular lamp type and wattage to a ballast for use with a different lamp having a different power rating, thereby avoiding replacement of the core and coil of the original ballast. 
     In accordance with an aspect of the present invention, the series-connected ballast capacitor of a lead-type ballast is changed to a value which is selected to maintain the net impedance of the resulting ballast reactance at a correct magnitude for operating a different wattage lamp. 
     In accordance with an aspect of the present invention, an ignition circuit is connected across the terminals of the different lamp, if the substitute lamp requires an ignition circuit. 
     In accordance with an aspect of the present invention, the capacitance device is moved from between a terminal of the ballast and a first terminal of the substitute lamp to between a second terminal of the ballast connected to a neutral or grounded line and a second terminal of the substitute lamp. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The various aspects, advantages and novel features of the present invention will be more readily comprehended from the following detailed description when read in conjunction with the appended drawings, in which: 
     FIG. 1 is a schematic diagram of a conventional lead-type ballast connected to a lamp; 
     FIG. 2 is a schematic diagram of a lead-type ballast connected to a lamp power level converter in accordance with a first embodiment of the present invention for use with a substitute lamp having a different power rating; 
     FIG. 3 is a schematic diagram of a lead-type ballast connected to a lamp power level converter and an ignitor in accordance with a second embodiment of the present invention for use with a substitute lamp having a different power rating; 
     FIG. 4 is a schematic diagram of a lead-type ballast connected to a lamp power level converter and an ignitor in accordance with a third embodiment of the present invention for use with a substitute lamp having a different power rating; and 
     FIG. 5 illustrates an exemplary luminaire with which a lamp power level converter can be used in accordance with an embodiment of the present invention. 
    
    
     Throughout the drawing figures, like reference numerals will be understood to refer to like parts and components. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a lead-type ballast  20  (e.g., a constant-wattage autotransformer (CWA) or a peaked-lead autotransformer (PLA)) is shown connected to a lamp  30 . Such lead-type ballasts account for a large portion of the installed base of the HID lamp market. The illustrated, exemplary CWA ballast  20  comprises a primary winding  22 , a secondary winding  24 , a core  23  and a series-connected ballast capacitor  26 . The CWA ballast  20  is connected to an AC power source  28  and an HID lamp  30 . With regard to the lamp  30 , the ballast  20  is designed for a particular lamp-type and wattage. 
     In accordance with the present invention, the ballast  20  can be converted for use with a different lamp having a different power rating such as the lamp  34  in FIG.  2 . As stated previously, energy savings and therefore cost savings can be realized by using more efficient HID lamps in place of many existing lamp-types. In the illustrated example, the lamp  30  is a 400 watt (W) MH lamp and the substitute lamp is a 320 W MH pulse-start lamp. It is to be understood that the present invention can be used with different existing lamp-types and substitute lamp-types. For example, the lamp  34  can be a 350 W lamp being substituted for the 400 W lamp  30 , or a 200 W lamp being substituted for a 250 W lamp  30 , and so on. 
     With reference to the conventional lamp circuit depicted in FIG. 1, the magnitude of the capacitor  26  is chosen such that the net impedance of the resulting ballast reactance allows for proper operation of the lamp within the specifications or ratings the for which the lamp  30  was designed when driven by the ballast secondary voltage. The specifications and ratings of lamps discussed herein are promulgated by lamp manufacturers and standards organizations such as the American National Standards Institute (ANSI) and the like. The net impedance is the vectoral sum of the core and coil secondary magnetic reactance and the capacitive reactance contributed by the ballast capacitor  26 . Altering the magnitude of the capacitor  26  affects the net impedance and therefore the lamp operating parameters. 
     With reference to the lamp circuit depicted in FIG. 2, the capacitor  26  has been replaced with a capacitor  32 , and the lamp  30  has been replaced with a lower wattage lamp  34 . A number of factors are considered in selecting the capacitor  32  such as OCV requirements, starting current requirements, lamp power regulation requirements, lamp current crest factor requirements, ignition requirements and sustaining voltage requirements. A number of these factors are presented in Table 1 for the 400 W MH lamp  30  and the 320 W MH lamp  34 . 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Parameter 
                 400 W MH lamp 
                 320 W MH lamp 
               
               
                   
               
             
             
               
                 Lamp Operating Voltage 
                 120-150 V rms   
                 120-150 V rms   
               
               
                 Lamp Operating Current 
                 3.25 A rms  (Typ.) 
                 2.63 A rms  (Typ.) 
               
               
                 Min. Starting Voltage 
                 280 V rms /504 V pk   
                 245 V rms /465 V pk   
               
               
                 10° C. Start) 
               
               
                 Lamp Starting Current 
                 3.2-50 A rms   
                 2.6-4.1 A rms   
               
               
                   
               
             
          
         
       
     
     A comparison of the lamp requirements listed in Table 1 reveals that a ballast  20  designed to meet the specifications of the lamp  30  also meets the corresponding specifications of the substitute lamp  34 . The lamp operating voltage is the same for both of the lamps  30  and  34 . The minimum starting voltage provided by the ballast  20  is more than adequate for starting and sustaining the lamp  34 . Since the lamp starting and operating requirements for the lamp  30  exceed those required by the lamp  34 , the core  23  and coils  22  and  24  are capable of providing sufficient current to operate the substitute, lower-wattage lamp  34 . The ballast impedance is modified in accordance with the present invention to correct the current in the lamp to maintain the lamp operating voltage. In the illustrated example, a 21 microfarad (μf) capacitor  32  is selected to replace the 24 μf capacitor  26 . As indicated by the following Table 2, the smaller capacitor  32  is selected to facilitate a change in the ballast  20  to accommodate a different lamp  34  in the lamp circuit having a different lamp impedance from the lamp  30 . 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Operating Parameter 
                   
                   
               
               
                 (Advance #71A6041 core 
                 400 W MH lamp 
                 320 W MH lamp 
               
               
                 and coil) 
                 (24 μf capacitor) 
                 (21 μf capacitor) 
               
               
                   
               
             
             
               
                 Nominal Supply Voltage 
                 480 V rms   
                 480 V rms   
               
               
                 Nominal Input Current 
                 1.02 A rms   
                 0.80 A rms   
               
               
                 Nominal Input Power 
                 449.3 W 
                 371.3 W 
               
               
                 Input Power Factor 
                 0.92 
                 0.97 
               
               
                 Lamp Voltage 
                 122 V rms   
                 131 V rms   
               
               
                 Lamp Current 
                 3.57 A rms   
                 2.77 A rms   
               
               
                 Lamp Power 
                 379.6 W 
                 320.6 W 
               
               
                 Ballast Losses 
                 69.7 W 
                 50.7 W 
               
               
                   
               
             
          
         
       
     
     As stated above, prior art arrangements which employ current-diverting devices across a lamp, and which do not alter ballast characteristics, merely reduce current through the lamp. This approach is unacceptable because the waveform provided to the lamp deteriorates, thereby decreasing the operating life of the lamp. The method of retrofitting a ballast in accordance with the present invention by changing the current provided through the ballast, as opposed to merely diverting the current, maintains the integrity of the waveform at the lamp. The prior art arrangements, on the other hand, compromise various waveform characteristics such as the crest factor. 
     The method of the present invention is generally used to replace a lamp  30  in a lamp circuit with a lamp having a lower power rating or operating wattage. Thus, the ballast is processing less energy, and therefore current, which allows for cooler ballast temperatures due to reduced ballast losses. The lamp  34  can have a higher power level than the lamp  30 . Accordingly, the ballast capacitance can be increased. Thermal issues, however, require consideration to ensure against ballast core and coil failure and the possibility of overheating surrounding components. 
     The ballast modification of the present invention is particularly effective when the substitute lamp  34  is from the same family of lamps (e.g., HPS or MH, among others) as the original lamp  30  and ballast  20 , and has a wattage rating that is close to and less than the wattage rating of the original lamp  30 . By using a substitute lamp from  34  from the same family as the original lamp  30 , the lamps typically have similar operating requirements. Substituting a lamp  34  from a different family of lamps, however, can also desirable. For example, a metal halide-type lamp  34  can be substituted for an original lamp  30  from the HPS family, which used with an HPS ballast, if the MH lamp color is preferred over the HPS lamp color. 
     As stated previously, substituting a lamp  34  having a higher power rating than the original lamp  30  can increase ballast losses and, depending on the nominal operating temperature, overheat the ballast and cause the premature failure thereof. If, however, the thermal characteristics of the ballast are known, and ballast modification in accordance with the present invention will not cause ballast limitations to be exceeded, then a lamp  34  having a higher wattage can be used. 
     A number of HID lamps require external high-voltage ignition circuits. If an existing ballast  20  is not equipped with an ignition circuit and the substitute lamp  34  requires such a circuit, a two-lead ignition circuit can be installed during the ballast modification or retrofit process of the present invention. As shown in FIG. 3, an ignition circuit  36  is provided across the lamp  34  and is operable to provide the required starting pulse for the substitute lamp  34 . 
     The lamp circuit depicted in FIG. 4 is similar to the circuit depicted in FIG. 3, except that the capacitor  32  is provided after the lamp  34  and ignition circuit  36 . This placement of the capacitor  32  in the lamp circuit is advantageous when the neutral line supplying the ballast  20  is grounded, or when an ignition circuit is introduced to a ballast which had no prior ignition circuit. When an ignition circuit is introduced into a PLA ballast housing, for example, the metal case of the capacitor  32  needs to be grounded to the luminaire housing. Thus, if the capacitor  32  is placed in series with the ballast common or neutral line, as shown in FIG. 4, then the dielectric stress between the capacitor electrodes and ground is minimized. 
     The ability to use an existing ballast in an existing luminaire with a different lamp having a different power ratio is advantageous for a number of reasons. For example, a different lamp can be substituted for an original lamp in a luminaire because it is more energy efficient, increases lamp life, provides improved lamp color or lumin maintenance, among other performance factors. In addition, the present invention realizes other advantages in terms of retrofit installations which will be illustrated using an under-canopy luminaire, as depicted in FIG.  5 . The under-canopy luminaire  40  comprises an optical assembly  42  mounted on door  44 , which is removably mounted to a housing  46 . The housing contains a number of lamp circuit components (not shown) such as a ballast and a power source. Replacing a lamp in the optical assembly with a more efficient lamp was previously accomplished by mounting new ballast components to a new door having a new optical assembly for a substitute lamp. The new door replaced the existing door  44  and the lamp components in the housing  46  such as the ballast were disconnected. The ballast modification process of the present invention is advantageous when retrofitting a luminaire for a different power-rated lamp because the expense and weight of the new door and the components mounted thereon is eliminated. A new core and coil is not required since the existing ballast in the housing  46  can be used. In addition, the use of the existing core and coil takes advantage of the preferred thermal location for the magnetic devices in the housing  46 . By placing a new core and coil on a new door assembly, some difficulty in transferring heat away from these new magnetic devices is presented. Also, the significant weight of the new door is avoided, as well as the additional cost for components such as a new core and coil, which are rendered unnecessary by the retrofitting process of the present invention. 
     Although the present invention has been described with reference to a preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various modifications and substitutions have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. All such substitutions are intended to be embraced within the scope of the invention as defined in the appended claims.