Abstract:
An apparatus for providing auxiliary power to a lighting system for heavy equipment during interruptions of power from a 250 volt direct current (DC) power supply is provided. The DC power supply is the only power source available to discharge lamps provided on the heavy equipment. The energy storage banks are provided between the power supply and a ballast for operating a gas discharge lamp. The energy storage banks store energy and provide the reserved energy to the ballast when the supply voltage to the ballast decreases below a level necessary for sustaining operating of the discharge lamp. The energy storage banks can comprise capacitors arranged in various series and parallel circuits and a blocking rectifier to prevent non-lighting loads on the heavy equipment from draining the energy storage banks.

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus for providing auxiliary power to a lighting unit for heavy equipment during interruptions of power from a direct current power supply. 
     BACKGROUND OF THE INVENTION 
     Heavy equipment is often used in harsh environments characterized by severe temperatures, poor air quality, the handling of dangerous materials, among other conditions. For example, the large electromagnetic cranes that are used in a steel mill for handling ingot and containers of molten steel can be subjected to high temperatures, as well as sparks from the steel manufacturing process. 
     An exemplary electromagnetic crane  10  is depicted in FIG. 1. A crane is generally transported on rails  12  and  14  provided on the floor  16  or elevated above the floor of the steel mill. The rails  12  and  14  provide power from a direct current (DC) power supply  18  (e.g., a 250 volt DC (VDC) power supply) to a power bus  20  in the crane. The power bus  20  (e.g., a 250 VDC bus) is provided in one of the rails  12 . The other rail  14  can provide the common or ground connection. The crane  10  has a horizontal section  26  which is provided with shoes  22  or other means that cooperate with the rails to guide the section  26  along the rails  12  and  14  and to prevent derailment of the crane. A section of the interior of the horizontal section  26  is illustrated to depict the contacts or brushes  24  provided on the crane for conducting a voltage provided via the power bus  20 . Components on the crane that require power such as a luminaire  30 , motors and control circuits are connected to the power bus  20 . These types of cranes are generally only powered by a 250 VDC power supply  18  and therefore provide the only available power source for loads such as a luminaire  30 . In addition, these cranes are typically not provided with uninterruptible power supplies (UPSs) because UPSs are regarded as too costly and not able to withstand the harsh conditions in which the cranes are used. 
     To ensure the safety of steel mill workers, many of the cranes used in the mill are automated or remotely controlled. Some cranes, however, can be manually operated by human operators located in a cab on the crane. Lighting is important to avoid mishandling of the steel, the crane and the various devices used during the manufacture of steel products (e.g., cauldrons for molten steel), particularly when the crane is manually operated by a human operator (i.e., controlled remotely or from within a cab on the crane). A number of existing cranes use either incandescent or high intensity discharge (HID) lamps which are subjected to intermittent power outages. For example, supply voltage to the crane can be interrupted by intermittent brush connections between the crane and the powered rails when the crane is in motion. In addition, electromagnets used on the crane to operate a boom, winch, grasping tool or other tool draw sufficient energy from the power bus to decrease, for varying periods of time, the system voltage provided to the crane by more than two-thirds. For example, the system voltage can decrease to 90 VDC or lower in a crane or other system using a 250 VDC power source. In the case of a conventional alternating current or AC-driven ballast, a voltage drop of this magnitude would cause the lamp to be extinguished. Direct current ballasts for HID lamps have been employed that have some degree of energy storage capability and can therefore withstand some interruptions in the supply voltage. These DC ballasts, however, are not able to prevent the lamp from being extinguished by the types of power interruptions that are common in the environments in which cranes and similar heavy equipment are used. While incandescent lamps do not cease operating as a result of voltage drop-off, they do not provide as much output and have a shorter operational life. 
     A need therefore exists for a device which provides a supply voltage to discharge lamps on heavy equipment when the supply of power between the heavy equipment and its power source is interrupted. A need also exists for a device which can supply a voltage to the heavy equipment lamps during power outages that does not require an auxiliary power source such as batteries or an AC power supply. 
     SUMMARY OF THE INVENTION 
     The above-described problems with lighting units for heavy equipment having only a DC power supply as a power source for the lighting units are overcome by the present invention. 
     In accordance with an aspect of the present invention, an energy storage bank is provided to ensure continued operation of the lighting unit during supply voltage drop-offs. 
     In accordance with another aspect of the present invention, a blocking rectifier is provided between the power supply and the energy storage bank to prevent bleedback to non-lighting loads in the power distribution system of the heavy equipment. 
     In accordance with yet another aspect of the present invention, the energy storage bank stores energy and provides the reserved energy to the ballast of a discharge lamp when the supply voltage to the ballast decreases below a level necessary for sustaining operation of the discharge lamp. 
     In accordance with still yet another aspect of the present invention, the energy storage bank comprises capacitors arranged in various series and parallel circuits. 
     In accordance with another aspect of the present invention, plurality energy storage banks can be arranged in parallel with respect to the ballast to increase the amount of power that is reserved to ensure continued operation of a discharge lamp following a sudden voltage drop-off. 
     A lighting system for machinery powered via a supply line connected to a direct current power supply is provided. The supply line provides a predetermined steady-state potential and the machinery is provided with a discharge lamp and ballast connected to the supply line. The lighting system comprises an energy storage bank connected in parallel with respect to the direct current power supply and the ballast. The energy storage bank comprises at least one capacitor and is operable to maintain a voltage across the ballast corresponding approximately to the steady-state potential of the supply line, and to discharge and provide an adequate voltage across the ballast to maintain operation of the lamp when power from the supply line to the ballast decreases below a selected voltage such as the rated voltage of a selected ballast. 
     In accordance with another aspect of the present invention, the selected voltage corresponds to a nominal operating voltage for the ballast to sustain operation of a gas discharge lamp. 
     A lighting system for machinery powered via a supply line connected to a direct current power supply is provided. The supply line provides a predetermined steady-state potential and the machinery is provided with a discharge lamp and ballast connected to the supply line and comprises non-lighting loads. The lighting system comprises an energy storage bank connected in parallel with respect to the direct current power supply and the ballast. The energy storage bank comprises at least one capacitor and is operable to maintain a voltage across the ballast corresponding approximately to the steady-state potential of the supply line, and to discharge and momentarily sustain a voltage across the ballast when power from said supply line to the ballast decreases below a rated voltage for a selected ballast. In addition, the lighting system comprises a bleedback device connected in series between the direct current power supply and the energy storage bank which is operable to prevent any of the non-lighting loads from draining power provided by the energy storage bank when power from the supply line to the ballast decreases below the rated voltage. 
    
    
     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 depicts heavy equipment having a luminaire and a direct current power source for use with the apparatus of the present invention; 
     FIG. 2 is a circuit diagram of a lighting system constructed in accordance with an embodiment of the present invention; 
     FIG. 3 is a circuit diagram of an energy storage bank constructed in accordance with an embodiment of the present invention; 
     FIG. 4 is a circuit diagram of an energy storage bank constructed in accordance with an embodiment of the present invention; 
     FIG. 5 is a circuit diagram of an energy storage bank constructed in accordance with an embodiment of the present invention; 
     FIG. 6 is a circuit diagram of a lighting system constructed in accordance with an embodiment of the present invention; 
     FIG. 7 is a circuit diagram of an external rectifier and energy storage bank in accordance with an embodiment of the present invention; 
     FIG. 8 is a circuit diagram of an energy storage bank having an internal rectifier in accordance with an embodiment of the present invention; 
     FIG. 9 is a circuit diagram of a rectifier used with plural energy storage banks in accordance with an embodiment of the present invention; and 
     FIG. 10 is a circuit diagram of plural rectifier and energy storage bank circuits 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. 2, a lighting system  38  constructed in accordance with an embodiment of the present invention operates in conjunction with the direct current power supply of a piece of heavy machinery. For illustrative purposes, the present invention will be described in connection with the 250 VDC power supply  18  of the crane  10  depicted in FIG.  1 . The power supply  18  is illustrated in FIG. 2 as a voltage source  40  for providing a substantially flat DC signal across the ballast  42  of the luminaire  30  provided on the crane  10 . The ballast is preferably an electronic HID ballast which is capable of operating from a 250 VDC power supply. Such a ballast is commercially available from WPI Electronics Inc., Warner, N.H. The luminaire  30  also comprises an optical assembly  44  and an HID lamp  46 . The optical assembly  44  is constructed to withstand the demanding physical environment associated with the operation of the crane. In accordance with an aspect of the present invention, an energy storage bank (ESB)  48  is provided which is parallel with respect to the voltage source  40  and the ballast  42 . 
     The ESB  48  is preferably a capacitance device that stores energy. As stated previously, heavy equipment is susceptible to interruptions in power due to intermittent brush connections with the power bus  20 , and voltage drop-offs due to the drain of electromagnets  34  on the system power supply when the electromagnets are used to move the crane  10  or operate a tool  36  on the crane. When such sudden supply voltage decreases occur, the ballast  42  may not be able to sustain operation of the lamp  46  and the lamp is extinguished. In accordance with the present invention, the ESB  48  provides temporary and sufficient power to the ballast to allow for the continued operation of the lamp during sudden supply voltage drop-offs. The ESB  48  floats at the steady-state potential of the supply line. For a particular amount of voltage drop-off, the ESB  48  supplies an adequate amount of energy to maintain operation of the lamp. The amount of current drawn by the load (e.g., a lighting load) is a function of the capacitive energy of the ESB  48  at a given time and of the amount of voltage drop-off. 
     The ESB  48  comprises at least one capacitor connected across the ballast  42 . A capacitor in the ESB  48  stores energy and is operable to discharge and provide energy to the ballast  42  when the supply voltage decreases below the rated voltage level of the ballast. The ESB  48  is preferably a set of capacitors arranged in parallel, in series, or in both parallel and series configurations. For example, the ESB  48  can be a battery of  55  microfarad capacitors, as described below. The capacitors are preferably rated at a higher voltage than the rated voltage of the ballast (e.g., 300 V for a rated ballast voltage of 250 V). FIG. 3 depicts an exemplary ESB  48  comprising several capacitors  50   a,    50   b,    50   c  and  50   d  connected in parallel with respect to each other and the ballast. FIG. 4 depicts another exemplary ESB  52  comprising several capacitors  54   a,    54   b  and  54   c  connected in a series circuit  56 . The series circuit  56  is connected in parallel with respect to the ballast. Alternatively, the ESB  48  can be configured as the ESB  58  depicted in FIG. 5 which comprises a number of parallel capacitors  60   a,    60   b,    60   c  and  60   d.  These parallel capacitors are connected in series with parallel capacitors  61   a,    61   b,    61   c  and  61   d.  The series circuit, in turn, is connected in parallel with respect to the ballast  42 . 
     As shown in FIG. 6, more than one ESB  48  (i.e., ESB  48   a,  ESB  48   b,  . . . . ESB  48   n ) can be connected in parallel with respect to the ballast  42 . The more ESBs  48  that are used in this configuration, the more energy that is stored and reserved for supply to the ballast  42  when the supply voltage decreases below a rated voltage level for the ballast, that is, the nominal voltage level required at the ballast for operating the lamp  46 . For example, when the input voltage supplied to the ballast  42  in the lighting system  38  decreases below 120 VDC, a nominal HID lamp  46  will extinguish. For a ballast  42  which can operate from a 250 VDC power source, the input voltage is 250 VDC (nominal) and the input current is 1.9 amperes DC (ADC) at 250 VDC. For a situation wherein total voltage drop-off occurs, the following calculation provides an approximate indication of how long a lamp can be sustained by an ESB in accordance with the present invention. Power is determined to be (1.9 A)(250V) or 475 watts. If the current draw of the ballast  42  is considered to be inversely proportional to the input voltage, then the period of time t that an ESB  48  can maintain operation of the lamp is defined as follows: 
     
       
           C ( dV/dt )=−475 /V   
       
     
     
       
           V ( dV )=−475 /C ( dt ) 
       
     
     
       
           V   2 /2=−475/( C*t )+ K   
       
     
     
       
           V=( 2 K −(950 /C*t )) ½   
       
     
     
       
         For  V (0)= V   o =250=(2 K ) ½ , then  K =31,250. 
       
     
     
       
         Thus,  V ( t )=(62,500−(950 /C*t )) ½ . 
       
     
     Accordingly, for V=120, t=50.63C seconds where C is the equivalent ESB capacitance. Thus, the larger the value of C, that is, the more ESBs that are employed in the lighting system  38 , the longer the operation of the lamp  46  is sustained following a significant decrease in the supply voltage. 
     Another consideration when using the lighting system  38  of the present invention is the system drain on the ESB(s)  48  from other devices in the lighting system. As stated previously, the direct current power supply  18  provides power via a power bus  20  to non-lighting loads in the crane  10  such as motors and control devices, in addition to one or more luminaires  30 . When a decrease in the supply voltage occurs (e.g., due to intermittent brush contact with the power bus  20 ), energy from the ESB(s)  48  can be drained by any non-lighting loads connected to the power bus  20 . This situation is hereinafter referred to as bleedback. In accordance with another aspect of the present invention, a blocking rectifier  62  is provided in series with the input of the ESB  48 , as shown in FIG. 7, to allow current flow in only one direction relative to the power source  40  FIG.  2 ). A blocking rectifier  62  can be arranged integrally in an ESB  48 , as illustrated in FIG.  8  and referred to generally as ESB  68 . The ESB  68  in FIG. 8 therefore has dedicated inputs  64  and outputs  66  for connection to the supply side and the ballast side, respectively, of the lighting system  38 . Thus, a single ESB  48  can be used in conjunction with a single rectifier  62  which can, but need not be, internal to the ESB of the present invention. 
     In addition, a single rectifier  62  can be used in conjunction with a plurality of ESBs  48   a,    48   b,    48   c,  and so on, as illustrated in FIG.  9 . Only one rectifier  62  is necessary to prevent bleedback into the power distribution system of the crane  10 , regardless of the number of ESBs  48  that are used in the lighting system  38 . The rectifier  62  is connected externally with respect to the ESBs  48 , or the ESB  48   a  that is most proximal with respect to the power supply  40  contains a rectifier, as illustrated by the ESB  68  in FIG.  8 . Thus, two types of ESBs are used, that is, one or more ESBs  48  and at least one ESB  68 , which is located so as to be most proximal to the power supply  40 . Alternatively, a plurality of the ESBs  70   a,    70   b  and  70   c,  which each have a rectifier  62 , can be used, as illustrated in FIG.  10 . Thus, only one type of ESB  70  is used. The redundancy of the rectifiers  62  in the ESBs  70   b  and  70   c  does not significantly affect the performance of the lighting system  38 . In addition, when one of the redundant rectifiers fails, the remaining rectifiers between the failed rectifier and the ballast allow for some level of bleedback control. This maintenance of bleedback control is not available in the circuit depicted in FIG. 9 following failure of the only rectifier  62 . The rectifier(s)  62  described with reference to FIGS. 7 through 10 is preferably selected to have a sufficiently high reverse breakdown voltage to withstand transients in the system. For example, 3000 V rectifiers can be used. 
     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.