Abstract:
A power distribution system comprises a DC bus supplied by an electrical generator and a resistive load connected to the DC bus. A switching device connects to the DC bus and the load and is configured to periodically open the circuit between the resistive load and the DC bus for a reoccurring period of time. The switching device senses the load on the DC bus and changes the length of the period of time based on the sensed voltage.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/064,283, filed Feb. 26, 2008. The foregoing provisional application is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates generally to the field of power systems for a sailing vessel. More specifically, the disclosure relates to a power distribution system including a DC bus and an appliance with a resistive element powered by the DC bus. 
         [0003]    Sailing vessels, for example, may generally include an on-board power system with a 240 volt direct current (DC) power bus. When the vessel is underway, the DC power bus is generally powered by an on-board DC generator. Electric appliances that are configured to operate on AC power may be applied to the DC bus through an inverter. It may be advantageous to couple an electric appliance that is configured to operate on AC power directly a DC bus. 
       SUMMARY 
       [0004]    One embodiment of the invention relates to a power distribution system comprising a DC bus supplied by an electrical generator and a resistive load connected to the DC bus. A switching device connects to the DC bus and the load and is configured to periodically open the circuit between the resistive load and the DC bus for a reoccurring period of time. The switching device senses the load on the DC bus and changes the length of the period of time based on the sensed voltage. Another embodiment of the invention relates to a power distribution system for a marine vessel comprising a DC bus supplied by an electrical generator or a rectified output from an AC power source. The DC bus is configured to supply power to an appliance including a resistive heating element. The appliance has an electromechanical switch including two contacts, wherein the electromechanical switch is designed to operate with an AC power supply. A switching device is connected between the DC bus and the appliance, wherein the switching device is configured to periodically open the circuit between the resistive load and the DC bus for a reoccurring period of time in order to quench an arc forming between the contacts of the electromechanical switch. The switching device senses the load on the DC bus and changes the length of the period of time based on the sensed voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         [0005]      FIG. 1  is a block diagram of an electrical system for a sailing vessel according to one exemplary embodiment of the present invention. 
           [0006]      FIGS. 2A and 2B  are graphs showing the voltage over time applied to an appliance coupled to the electrical system of  FIG. 1  according to two exemplary embodiments. 
           [0007]      FIG. 3  is a block diagram of a protection device for the electrical system of  FIG. 1  according to one exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION  
       [0008]    Referring to  FIG. 1 , a block diagram of an electrical system  100  for a sailing vessel is shown according to exemplary embodiments. The sailing vessel includes an on-board power source to power a motor-driven propeller. According to an exemplary embodiment, power is supplied by a diesel engine  105  (preferably 40 HP) turning a permanent magnet alternator  106  (preferably 25 kW). The alternator  106  uses a rectifier  107  to power, for example, a DC bus  110  for the vessel. The voltage of the DC bus  110  is preferably 240V. As shown in  FIG. 1 , the vessel may include a high voltage rechargeable energy source  112 , such as a battery bank, to store energy from the engine  105 . The high voltage rechargeable energy source  112  source can be coupled to the DC bus  110 . The electrical system  100  can be designed without the high voltage rechargeable energy source  112 , as shown in  FIG. 2  of U.S. Provisional Application No. 61/064,283. 
         [0009]    An electric motor  113 , such as a brushless DC permanent magnet motor, can be coupled to the DC bus  110  with a motor controller  111 . As shown in  FIG. 1 , two electric motors  113   a  and  113   b  with differing levels of horsepower can be coupled to the DC bus  110  by motor controllers  111   a  and  111   b.  One of the motors  113  can drive a propeller (not shown). If the electrical system  100  includes a high voltage rechargeable energy source  112 , one of the motors  113  may receive power from the high voltage rechargeable energy source  112  when the engine  105  and generator are turned off. 
         [0010]    An inverter  115  may be coupled to the DC bus  110  to convert the 240 V DC power to provide power to, for example, a 120 V AC power bus  116 . Various common appliances  117  such as televisions, microwaves, hair driers and other appliances may be coupled to the AC bus  116 . 
         [0011]    Some appliances  120  having a simple resistive heating element (e.g., water heaters, stovetops, etc.) may be able to be coupled directly to the DC bus  110 . However, such appliances  120  generally include electromechanical switches that require the current to drop to zero frequently (as occurs with an alternating current) to extinguish the arc that develops at the electromechanical switch&#39;s contacts. The arc generally needs to be extinguished at 100-120 Hz. If the current never drops to zero, the sustained arc at the contacts burns the contacts and may weld the contact closed. A sustained arc may also deposit an excessive amount of slag on the contact, preventing the contact from closing properly. 
         [0012]    As shown in  FIG. 1 , a protection device  130  may be provided between an appliance  120  and the DC bus  110  to allow the appliance  120  to operate using DC power. According to one exemplary embodiment, the appliances  120  are normally configured to operate on 240 V AC power. The appliances  120  are adapted for use with 240 V DC power with a protection device  130 . As shown in  FIGS. 2A and 2B , the protection device  130  periodically (e.g., between 100 Hz and 120 Hz) interrupts the current to the appliance  120  to extinguish the arc that develops at the contacts of the electromechanical switch, allowing the switch to open without damaging itself. 
         [0013]    Referring to  FIG. 3 , a block diagram of a protection device  130  is shown according to one exemplary embodiment. The protection device  130  includes a switch  132 , an oscillator  138 , and a voltage sensor  135 . The switch  132  may be, for example, a single insulated-gate bipolar transistor (IGBT) that is driven by a simple oscillator circuit  138 . The switch  132  holds the current off for a period of time for each cycle. The amount of time the current is held off is determined by the DC voltage, sensed by a voltage sensor circuit  135 . The oscillator  138  and voltage sensor  135  circuits can be any number of a different designs well known to those skilled in the art. An exemplary detailed circuit diagram of the protective device  130  is shown in  FIG. 5  of U.S. Provisional Patent Application No. 61/064,283. 
         [0014]    Operation of the system is now described with reference to  FIGS. 2A and 2B . Under normal operation the DC bus  110  is at 240 V DC and the appliance  120  is configured to operate at 240 V AC. In this normal situation, the protection device  130  simply has to interrupt the current for a short period (e.g., 800 μS at 120 Hz) to extinguish the arc, as described above and shown in  FIG. 2A . Thus, an effective voltage seen by the device is actually less than 240 V due to the period of time when the voltage is zero. However, the electrical system  100  of the vessel may occasionally be powered by outside sources, such as shore power  108  when the vessel is docked. Typical European shore power operates at 240 V AC. As shown in  FIG. 1 , a 240 V AC shore power source  108   a  or a 120 V AC shore power source  108   b  may be coupled to the vessel&#39;s 240 V DC bus via a dual voltage input isolation transformer  109  and rectifier  107 . 
         [0015]    Certain capacitive loads may be coupled to the DC bus  110 . When coupled to 240 V AC shore power, 240 V is the root-mean square (RMS) value of the AC voltage rectified via a rectifier  107 . However, the rectified voltage may actually reaches peaks of approximately 340 V. The capacitive loads attached to the DC bus will store a charge at the maximum voltage to which the capacitors are exposed. As a result, the capacitors effectively make a 240 V DC bus into a 340 V DC bus. To compensate for the increased voltage, and the potential for damage resulting from the higher than normal voltage, the protection device  130  is configured to sense the voltage of a DC bus  110  and increase the amount of time the current is held off, as shown in  FIG. 3B . Increasing the off time allows the RMS voltage applied to the resistive element of the appliance  120  to remain relatively constant and helps prevent the resistive element from overheating and becoming damaged. Thus, even though the switch is open for different periods of time in  FIGS. 2A and 2B , the effective voltage applied to the appliance is the same. 
         [0016]    Because resistive heaters do not require a voltage that reverses itself, the protection device  130  does not need to function as an inverter and can instead be a simplified device that periodically interrupts the current to the appliance  120  with a single switch. The protection device  130  allows a predominantly DC electrical system on a vessel to be adapted for use with conventional off the shelf AC appliances. The protection device  130  is provided as a separate component so the appliances  120  themselves do not need to be altered, which could potentially void any warranties on the appliances. The protection devices  130  may be added to existing vessels, adapting the electrical systems without physically altering the existing system. 
         [0017]    While the electrical system  100  described above is generally referred to as a 240 V DC system, it should be understood that the protection device  130  is equally as effective at other voltages, such as 120 V. 
         [0018]    It is important to note that the construction and arrangement of the power distribution system as shown in the various exemplary embodiments is illustrative only. Although only a few embodiments of the present application have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the application. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present application. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present application. 
         [0019]    The foregoing description of embodiments of the application has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the application to the precise form disclosed, and modifications and variations are possible in light of the above teachings, or may be acquired from practice of the application. The embodiments were chosen and described in order to explain the principles of the application and its practical application to enable one skilled in the art to utilize the application in various embodiments and with various modifications as are suited to the particular use contemplated. 
         [0020]    Although the description contains many specificities, these specificities are utilized to illustrate some of the preferred embodiments of this application and should not be construed as limiting the scope of the application. The scope of this application fully encompasses other embodiments which may become apparent to those skilled in the art. All structural, chemical, and functional equivalents to the elements of the above-described application that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present application. A reference to an element in the singular is not intended to mean one and only one, unless explicitly so stated, but rather it should be construed to mean at least one. Furthermore, no element, component or method step in the present disclosure is intended to be dedicated to the public.