Abstract:
An improved fusible element for use within a circuit protection device is provided which includes a double wound fusible element configured to withstand high surge current associated with inductive and capacitive loads. The fusible element includes an insulated core having a longitudinal axis, a first wire wound about the core along the longitudinal axis of the core, and a second wire wound substantially orthogonally about a longitudinal axis of the first wire such that the fusible element is configured to withstand an over-current surge condition.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the invention relate to the field of circuit protection devices. More particularly, the present invention relates to a fuse employing a double wound fusible wire element configured to withstand high surge current associated with inductive and capacitive loads. 
         [0003]    2. Discussion of Related Art 
         [0004]    Fuses are typically used as circuit protection devices and form an electrical connection with a component in a circuit to be protected. The fuse is designed to protect the circuit or circuit component by being the intentional weak link in the circuit. One type of fuse includes a housing consisting of a plastic base and a plastic cap with a pair of conductors or terminals which extend through the base and are connected via a fusible element that forms a bridge between the terminals inside the housing. In order to fix the terminals inside the base portion of the housing, a portion of each terminal and/or the base is deformed in order to pinch the base around the terminals, thereby clamping the base around the respective terminals. The fusible element is attached to ends of each of the two conductors projecting above the base. The fusible element is typically a conductive wire which is soldered to the ends of the two terminals. The fuse is placed in a circuit to be protected such that the fusible element melts when an abnormal overload condition occurs. 
         [0005]    In certain circuit protection applications (e.g. motors, etc.), a surge current or short term current overload situation may typically occur until a steady state condition for the device is achieved. Fuses employed in these types of circuits must be designed to permit this short term surge to pass through the fuse without melting the fusible element. This high-surge condition is defined in terms of current and time (I 2 t) where it is desirable to avoid an open circuit unless the current exceeds a specific percentage of the fuse&#39;s rated current. 
         [0006]    One type of fuse used in these applications employs a spiral wound fuse element. In particular, the fuse element comprises a core of twisted yarn fibers with a fuse wire or wound around the core in a spiral pattern. The yarn that comprises the core is typically a ceramic material that is void of any material that could become conductive when the fuse is blown. The wound wire may include a plurality of wire strands configured to provide increased heat absorption indicative of, for example, a slow-blow or time-delayed fuse. 
         [0007]    When a circuit overload is encountered, the passage of the excess current through the fuse element causes it to generate heat and thereby elevate the temperature of the fuse wire. In other words, the core acts as a heat sink to draw this heat away from the fuse wire, thereby lowering the temperature of the fuse wire. In this manner, the transfer of heat from the fuse wire to the core lengthens the time required before the fuse wire melting temperature is reached. For higher current-rated fuses, a larger diameter fuse wire is used to withstand higher current passing through the wire and therefore higher temperatures. However, the wound fuse wire is limited in size, thereby limiting the amount of excess current the wire can withstand as well as the amount of heat transfer between the wound wire and the core. Accordingly, there is a need for a fuse that utilizes a wound fusible wire element and a fuse employing the same configured to provide high I2t characteristics on the fuse element that will withstand high surge current associated with inductive and capacitive loads to protect particular types of circuit components and associated circuits. 
       SUMMARY OF THE INVENTION 
       [0008]    Exemplary embodiments of the present invention are directed to an improved fusible element for use within a circuit protection device having a double wound fusible element configured to withstand high surge current associated with inductive and capacitive loads. In an exemplary embodiment, the fusible element includes an insulated core having a longitudinal axis; a first wire wound about the core along the longitudinal axis of the core, and a second wire wound substantially orthogonally about a longitudinal axis of the first wire such that the fusible element is configured to withstand a plurality of overcurrent pulses without melting. 
         [0009]    In another exemplary embodiment, a fuse includes a housing defining a cavity therein, a first end cap attached to a first end of the housing, a second end cap attached to a second end of the housing and a fusible element disposed in the cavity. The fusible element has a first end electrically connected to the first end cap and a second end electrically connected to the second end cap. The fusible element comprises an insulated core having a longitudinal axis, a first wire wound about the core along the longitudinal axis of the core, and a second wire wound substantially orthogonally about a longitudinal axis of the first wire. 
         [0010]    In another exemplary embodiment, a fuse includes a housing defining a cavity therein, a first end cap attached to a first end of the housing, a second end cap attached to a second end of the housing, and a fusible element disposed in the cavity. The fusible element has a first end electrically connected to the first end cap and a second end electrically connected to the second end cap. The fusible element comprises an insulated core having a longitudinal axis, a first wire wound about the core along the longitudinal axis of the core and a second wire wound substantially orthogonally about a longitudinal axis of the first wire. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates an exemplary fuse in accordance with an embodiment of the present disclosure. 
           [0012]      FIG. 2  is a perspective view of a fusible element in accordance with an embodiment of the present disclosure. 
           [0013]      FIG. 2A  is a cross-sectional view taken along the longitudinal axis of the fusible element of  FIG. 2  in accordance with an embodiment of the present disclosure. 
           [0014]      FIGS. 3A and 3B  illustrate an exemplary process for forming a double wound fusible element in accordance with an embodiment of the present disclosure. 
           [0015]      FIG. 4  illustrates an exemplary fuse utilizing the fusible element in accordance with an embodiment of the present disclosure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0016]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0017]      FIG. 1  illustrates a fuse  5  comprising a housing  10  defined by a base  15  and a cap  18 . The housing  10  forms a cavity within which a fusible element  30  is disposed. The housing may be formed of plastic or electrically insulating material capable of withstanding heat generated when the fuse is blown. The base and cap may also be made from plastic or other suitable material. A pair of conductors or terminals  20 ,  25  pass through the base  15  and are electrically connected via fusible element  30  disposed inside the housing  10 . The upper ends of terminals  20  and  25  may include, for example, clips that retain ends of fusible element in contact with respective ends of the terminals. Solder portions  35  and  40  are used to connect ends of fusible element  30  to conductors  20  and  25  respectively. Fusible element  30  is shown as being configured in a parallel relationship to the longitudinal surface of base  15  and perpendicular to the longitudinal axis of each of the conductors  20  and  25 . When an occurrence of a specified over-current or surge current condition occurs, the fusible element  30  melts or otherwise opens to interrupt the circuit path and isolate the protected electrical components or circuit from damage. In addition, an arc quenching material  45  may also be included within housing  10  to absorb the effects of the arc which occurs when the fusible element  30  melts after, for example, an over-current condition. 
         [0018]      FIG. 2  is a perspective view of just fusible element  30  in accordance with an embodiment of the present disclosure. The fusible element  30  comprises a core  50  formed from an electrically insulating material such as, for example, glass yarn. A double wound wire is disposed around core  50 . In particular, the double wound wire is defined by a first wire element  60  wound longitudinally about the core  50  from a first end to a second end and a second wire element  70  wound substantially orthogonally about a longitudinal axis of wire element  60 . In other words, the wire element  60  has a longitudinal axis which corresponds to its position with respect to core  50  and second wire element  70  is disposed orthogonally to the longitudinal axis of wire element  60 . The combination of wire elements  60  and  70  are wound about core  50  a plurality of turns or windings. The wire elements  60  and  70  used to form the double wound fusible element  30  comprise electrically conductive material configured to melt at a predetermined temperature (i.e. current rating) to interrupt the electrical circuit in the event of an overload. The wounded wire  70  on wire element  60  reduces the associated resistance without affecting the heat energy needed to melt the fuse element  30  when a current cut-off threshold is met. 
         [0019]      FIG. 2A  is a cross sectional view taken along the longitudinal axis of a portion of fusible element  30 . Wire element  70  is wound about wire element  60  which is wound about core  50  to define the fusible element. Although this figure illustrates that wire element  70  is in contact with core  50 , in one embodiment the portions of wire element  60  in between the windings of wire element  70  may be compressed on core  50  depending on the tension employed when winding the combination of wire element  60  and  70  about core  50 . 
         [0020]      FIGS. 3A and 3B  illustrate an exemplary process for forming the double wound fusible element  30 . In particular,  FIG. 3A  illustrates the winding of wire element  70  about wire element  60  a plurality of windings. The winding of wire element  70  about wire element  60  forms a plurality of interstices  65  between the respective windings. The frequency of the windings of wire element  70  about wire element  60  and consequently the number of interstices  65  therebetween may vary depending on the desired rating of the fuse.  FIG. 3B  illustrates the winding of the combination of wire elements  60  and  70  about core  50 . The winding of the combined wire elements  60  and  70  about core  50  form a plurality of interstices  55  between the respective windings. The contact of the wire elements  60  and  70  about the core  50  provides heat transfer from the wire to the core. In addition, by utilizing this double wound configuration, the mass of the fusible element  30  is increased which significantly increases the I 2 t value. 
         [0021]    As noted briefly above, the I 2 t value is the measurement of energy required to blow the fuse element  30  which corresponds to the measurement of the damaging effect of an overcurrent condition on the protected device or circuit. In particular, I 2 t is a calculation of how many overcurrent pulses the fuse can withstand. This is done with the comparison of I 2 t of the pulse and the fuse which is referred to as “relative” I 2 t. By employing a double wound fusible wire ( 60 ,  70 ) configuration about core  50 , the mass of the fusible element  30  is increased. With this increased mass, the amount of heat that the fusible element  30  generates due to an overcurrent condition is increased. Based on testing, it is believed that the I 2 t value using the double wound configuration in accordance with the present disclosure is increased approximately 250%-300% as compared with a single wound configuration (i.e. only employing wire element  60 ). 
         [0022]      FIG. 4  is a perspective view (not drawn to scale) of an alternative fuse  100  employing the double wound fusible element shown with reference to  FIG. 2 . In particular, fuse  100  includes a housing  110  which may be referred to as a tube or cartridge. Housing  110  may be made from a ceramic or similar material. Each of a pair of electrically conductive end caps  120 ,  125  is positioned at the respective ends of housing  110  to contain fusible element  30  therein. In addition, the respective ends of fusible element  130  are electrically connected to end caps  120  and  125  usually by soldering. As noted above, fusible element  30  comprises wire element  170  wound orthogonally about a longitudinal axis of wire element  160  and the combination of wire elements  160  and  170  are wound about core  150  a plurality of turns or windings. The wire elements  160  and  170  used to form the double wound fusible element  130  comprise electrically conductive material configured to melt at a predetermined temperature to interrupt the electrical circuit in the event of a prolonged overload condition. 
         [0023]    While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claim(s). Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.