Abstract:
A bus cooling fan that operates without an external electrical power supply. The fan is positioned proximate two bus bars of a polyphase electrical system. The fan includes a rotor supporting fan blades and mounted on an axle. The axle is supported by cross-braces in a shroud surrounding the rotor. The shroud is open at the ends. The fan is fabricated of non-ferrous, non-conducting material with the exception of the rotor. The bent rods which interact with the electromagnetic field created by the current-carrying bus bars of the polyphase electrical system to make the rotor function as an induction motor when the fan is placed proximate two bus bars.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention pertains to cooling systems for electrical panels. More particularly, this invention pertains to a bus cooling fan that is powered from the electromagnetic field generated by an electrical bus and that does not require external electrical connections. 
     2. Description of the Related Art 
     Electrical panels, such as switchboards and switchgear, are used in electrical power distribution systems. These panels typically contain circuit breakers, transformers, and instrumentation. The electrical connections between these internal components are made with bus bars, with one bus bar for each phase of the polyphase electrical system and another bus bar for the neutral conductor, if there is one. The current flowing in many electrical power distribution systems is oftentimes high, resulting in heating of the bus bars and other components in the electrical panels. It is common to cool these components with an externally powered, electric cooling fan, which forces air to circulate through the panel. Using an externally powered, electric cooling fan necessitates an external power source and wiring. 
     Externally powered, electric cooling fans are typically driven by single-phase induction motors. These motors typically have a wound stator and a squirrel cage rotor, which rotates and drives the fan blades. Single-phase induction motors require some means of applying torque to the rotor in order to overcome inertia and start rotating. Various methods are known for starting single-phase induction motors. For example, the stator may use a shaded pole, where one-half of each pole is surrounded by a short-circuited winding called a shading coil. Another example is a split-phase motor, where the stator has two windings, one of which is an auxiliary winding which has an out-of-phase current relative to the other winding. A capacitor can be wired in series with the auxiliary winding of the split-phase motor. If used, the capacitor can be switched out of the circuit after the motor reaches a certain speed, or the capacitor can be left in the circuit and used to improve the motor&#39;s power factor. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a cooling fan without external electrical connections, provides cooling for polyphase bus bars in an electrical panel. The cooling fan operates as an induction motor in which the fan blade supports act as a rotor and the bus bars act as a stator. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which: 
     FIG. 1 is a perspective view of an electrical, three-phase bus with two bus cooling fans constructed in accordance with the present invention; 
     FIG. 2 illustrates a section view of two bus bars of the three-phase bus showing an end view of a bus cooling fan; and 
     FIG. 3 is a perspective view of the bus cooling fan viewed from the opposite end from that illustrated in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A bus cooling fan that operates without an external electrical power supply is disclosed. The fan draws its rotational energy from a rotating electromagnetic field produced by the current-carrying bus bars of a polyphase electrical system inside an electrical panel. 
     FIG. 1 shows a perspective view of bus cooling fans  10 , constructed in accordance with the present invention, mounted adjacent to and between parallel bus bars  102  of a three-phase electrical power bus, such as would be utilized inside an electrical cabinet. It is well known that a magnetic field is generated around a current carrying conductor such that the direction of the magnetic flux is determined by the direction of current flowing in the conductor (right hand rule of magnetism). It is also well known that the instantaneous strength of the magnetic field is proportional to the amount of instantaneous current flowing in the conductor. In an AC power system the strength of the magnetic field at a particular point along the conductor varies with the sinusoidal current cycle as current passes the particular point. In a three-phase system of parallel bus bars  102  as shown, the peak current flowing through bus bar  102 A, at any particular instant in time, is 120 degrees out of phase with the peak current flowing in bus bar  102 B and 240 degrees out of phase with the peak current flowing in bus bar  102 C. Therefore, at an arbitrary point in a plane perpendicular to the parallel bus bars  102 , and the current flowing in them, the vector of the localized magnetic field at the arbitrary point will tend to rotate as the sinusoidal current in each electrical phase passes through the plane. The fan  10  is positioned in a plane generally perpendicular to the parallel bus bars  102  so as to force air along the longitudinal axis of the bus bars  102 . Those skilled in the art will recognize that the fans  10  may be individually located anywhere along the run of bus bars  102  without departing from the spirit and scope of the invention. 
     FIG. 2 illustrates a section view of two bus bars  102 A,  102 B with the bus cooling fan  10  situated between them. The center-point of the fan  10  is located at a position other than the midpoint between the bus bars  102 . As illustrated, the fan  10  is positioned nearer the first bus bar  102 A than the second bus bar  102 A. The fan  10 , as illustrated, has four fan blades  202   a ,  202   b ,  202   c  and  202   d , which, when rotated about an axle  214 , force air through a shroud  210 . The shroud  210  has front and back cross-members  212  which provide support for the axle  214 . The blades  202  are mounted to the axle  214  by bent rods  204   a ,  204   b . Those skilled in the art will recognize that the rods  204  serve as a rotor of an induction motor and that the rod configuration and shape may vary without departing from the spirit and scope of the present invention. The illustrated embodiment does not show the mounting of the shroud  210  to the bus bar  102 A; however, this can be accomplished through conventional means for fastening attachments to bus bars. Those skilled in the art will recognize that, alternatively, the shroud  210  can be attached to another support member, not shown, instead of attaching it to a bus bar without departing from the spirit and scope of the present invention. 
     FIG. 3 shows a rear, perspective view of the bus cooling fan  10  with a portion of the shroud  210  shown cut-away, exposing the fan blades  202 , the bent rods  204 , and the central portion of the axle  214 . In the illustrated embodiment, the shroud  210 , the front and back cross-members  212  (shown in FIG.  2 ), the fan blades  202 , the axle  214 , and conventional bearings (not shown) for rotationally securing the axle  214  to the front and back cross-members  212  are fabricated of non-ferrous, non-conducting, and non-magnetic material. In the illustrated embodiment, the bent rods  204  are fabricated of a ferrous, conducting material. Those skilled in the art will recognize that the materials used to fabricate the various components of the cooling fan  10  may vary without departing from the spirit and scope of the present invention. 
     The cooling fan  10  performs similar to an induction motor when placed proximate two of the bus bars  102  of a polyphase electrical system. The bent rods  204 , lying in the plane perpendicular to the bus bars  102 , act as the rotor and the localized rotating magnet field produced by sinusoidal current flowing in the bus bars  102  functions as the stator of the induction motor. The bent rods  204  are inductively coupled to the bus bars  102  such that a counter-electromagnetic field with respect to that generated by the current-carrying bus bars  102  is established in the bent rods  204 . The counter-electromagnetic field in the rods  204  continuously tries to align itself with the rotating magnetic field produced by the bus bars  102 , thereby causing the rotor to rotate about the axle  214 . By placing the center of rotation of the fan  10  slightly off-center in the space between the bus bars  102 , the rotor  204  develops enough starting torque to begin rotation. The power requirements for the fan  10  to perform its cooling function are not great. Accordingly, the rotor need not be very efficient and the slip speed can be relatively high for an induction motor. Those skilled in the art will recognize that the shape of the bent rods  204  may vary without departing from the spirit and scope of the present invention. However, in the illustrated embodiment, each of the two bent rods  204   a ,  204   b  which define the rotor are substantially Z-shaped; that is,, the rods  204  are supported by the axle  214  in the center of the rod  204  and the rod  204  perpendicularly extends away from the axle  214  and the distal ends of each rod  204  are bent in a plane substantially perpendicular to the axle  214 . It is also within the scope of the invention that the plurality of parallel bus bars  102  can be configured other than in a straight line and still produce the localized rotating magnetic field as described. 
     In operation, the cooling fan  10  rotational speed increases with increased current flow through the bus bars  102 . As the current flow increases in the bus bars  102 , so too does the heat generated by the bus bars  102 , thus, the increase in rotational speed of the fan  10  further aids in cooling the bus bars  102 . The shroud  210  directs the air flow from the fan  10  along the length of the bus bars  102 . In the illustrated embodiment, the shroud  210  is cylindrical, but those skilled in the art will recognize that other shapes can be used to control the air flow without departing from the spirit and scope of the present invention. 
     From the forgoing description, it will be recognized by those skilled in the art that a bus cooling fan that operates without an external electrical power supply has been provided. This bus cooling fan circulates the air inside an electrical panel and is not dependant upon an external power source. 
     While one embodiment has been shown and described, it will be understood that it is not intended to limit the disclosure, but rather it is intended to cover all modifications and alternate methods falling within the spirit and the scope of the invention as defined in the appended claims.