Abstract:
A technique for passive suppression of arcs within a microwave frequency waveguide section. The waveguide is configured to have a bend at a point where the naturally, relatively high location occurs within the run. The bend at the high point causes arcs to be trapped as heat naturally collects within the waveguide at such predictable locations. Vent holes formed in the exterior portion of the waveguide at this point allow trapped hot air gases to escape, and cause the arc to be drawn towards the sidewall of the waveguide at a point where the voltage approaches zero. Presenting this region of zero voltage to the arc causes the arc to extinguish itself.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a technique for suppressing arcs in an electromagnetic waveguide, and more particularly to a passive technique that introduces vent holes at a high point in a waveguide run. 
     Waveguides have been used for some time as an efficient way to carry microwave frequency energy over distances in a predictable manner. However, waveguides in some instances have a tendency to experience unpredictable behaviors such as internal arcing. In particular, even though a waveguide is sized to be capable of operating safely at the expected power levels without introducing a voltage breakdown, certain events or faults may occur to cause an energy discharge within the waveguide itself. Such faults may happen when dust, dirt or other ambient conditions introduce an abnormal voltage condition inside the waveguide. Such arcing is of concern since it may actually continue after the fault is no longer in existence. The arc not only partially blocks transmission of energy through the waveguide, but also may damage other system components. 
     For example, electromagnetic energy normally travels within the waveguide from an electromagnetic energy source through the waveguide towards a system that makes use of the microwave energy, such as a microwave oven cavity. Once an arc occurs, it tends to travel backwards within the waveguide, back towards the power source. The arc acts to reflect at least some electromagnetic energy back to the power source. This causes a decrease in power levels at points in the waveguide beyond the arc, meaning that the system in turn receives electromagnetic energy at a reduced power level. 
     A number of methods have been used in the past to detect and deal with the occurrence of an arc within a waveguide. For example, detectors may be attached to the waveguide which are responsive to the vibratory and electromagnetic disturbances resulting from an arc. The detectors can be arranged not only to determine the existence of an arc but also its location and velocity. 
     Upon detection of an arc, electronic control circuits can then be used to shut off the microwave power source or reduce its level so that the arcing will eventually cease. After a suitable delay, to allow any ionization caused by the arc within the waveguide to dissipate, the power source is then brought back on line again. 
     SUMMARY OF THE INVENTION 
     Arcing can be especially problematic in certain end uses such as microwave ovens. For example, in industrial process type microwave ovens that are used in large scale cooking applications, continuous and predictable microwave energy levels are required to produce a predicable end result of the cooking process. Any need to shut down the oven to extinguish an arc can therefore be very undesirable. 
     Consider that an arc tends to heat the air in its immediate vicinity within the waveguide. Since this hot air naturally rises, an arc will also tend to rise due to the heat in the ionized gases of the arc. When an arc traveling backwards towards a power source, encounters a bend in the waveguide, certain behavior is therefore observed under certain conditions. In particular, when the arc moves into a section of the waveguide where further travel backwards towards the source would involve moving downward in elevation, the arc will often become trapped by the rising effect of the hot air associated with the arc. At such a point, the force of the rising hot air on the arc actually opposes the electromagetic force that urges the arc to travel backwards. 
     Such arcs may therefore tend to set up in a stationary or stable location within the waveguide at a bend where further backwards travel would involve downwards movement. This not only reduces the electrical effectiveness of the microwave source but indeed may caused physical damage of the waveguide as such standing arcs actually may create enough heat and energy to deform or even burn through the waveguide itself. 
     The present invention seeks to eliminate these difficulties through a passive arc suppression technique. The invention is applied to a waveguide section that has a relatively high point in a waveguide run between the oven cavity and the power source, preferable in an unpressurized waveguide run, where backward electromagnetic movement of the arc would involve a downward movement in elevation. 
     In a preferred embodiment, an H field bend is formed at or near this position in the waveguide. By forming small vent holes in the upper portion of the H-bend at this point, the heat associated with the arc is allowed to rise and escape through the vent holes. The action of the escaping arc gasses tends to draw the arc upward toward the side wall of the H-bend at this point in the waveguide. The side wall of the H-bend at this point, however, presents a voltage of zero volts. This reduction in voltage at the location of the arc allows the arc to in turn naturally extinguish itself 
     The arc is therefore naturally extinguished as the heat escapes, without the use of arc detectors, power source controllers and the like that would otherwise interrupt the continuous operation of the microwave power source. 
     The invention can be used with many different types of microwave systems. For certain classes of industrial microwave ovens that use hot air processing as well as microwave processing, the introduction of hot air into the microwave oven cavity tends to exacerbate the arcing problem, since hot air is more readily ionized than ambient temperature air. The inclusion of vent holes in such systems is therefore effective in increasing their microwave heating efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a microwave cooking system that makes use of a passive arc suppression technique according to the invention. 
     FIG. 2 is a smaller scale batch oven which may also make use of the invention. 
     FIG. 3 is a partially cut away perspective view of a waveguide section having a high point formed therein that tends to trap arcs, showing the location of the vent holes. 
     FIGS. 4A,  4 B and  4 C show more detailed views of an H-bend waveguide section having vent holes in an area of zero voltage. 
     FIG. 5 is another view of the H-bend showing how a voltage vector is created within the waveguide. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning attention now to the drawings more particularly, FIG. 1 illustrates an oven system  10  that may be used in a continuous feed industrial type application. The oven system  10  includes a number of cabinets  1 I 1  that enclose microwave energy sources  12 . Waveguide runs  14  of various types act as conduits for carrying microwave energy generated by the energy sources to the interior of a number of oven cavities or enclosures  15 - 1 ,  15 - 2 ,  15 - 3  (collectively, the enclosures  15 ). The present invention is related in particular to how the waveguides  14  may be structured to suppress the generation of arcs within them. 
     Shown is a continuous feed oven system  10  in which a series of three oven enclosures  15 - 1 ,  15 - 2  and  15 - 3  are provided. A door assembly  16  may be included on one or more of the enclosures  15  through which access may be provided to facilitate cleaning of the ovens. 
     The waveguide runs  14  are only partially shown for clarity. For example, the waveguides  14  above enclosure  15 - 1  appears to be open in the drawing, whereas they actually form a continuous connection between the microwave energy sources  12  and the enclosures  15 . It can also be seen that multiple energy sources  12  and waveguides  14  can be used to feed a given one of the enclosures  15 . 
     In addition, although the illustrated system  10  provides for cooking by microwave energy, the system  10  could also provide for cooking through hot air heating by convection. 
     Of particular interest in FIG. 1 is a bent waveguide section  20 - 1  which forms a part of waveguide run  14 -W. As more fully explained below, the bent waveguide section  20 - 1  is at a location in the waveguide run  14 -W at which an arc might be expected to set up in a stable position. The present invention eliminates or supresses the arc through a passive arc suppression technique. The invention can typically be applied to a bent waveguide section  20 - 1  that is located in a relatively high point in the waveguide run  14 -W between the oven enclosure  15  and the power source  12 . 
     In a preferred embodiment, the bent waveguide section  20 - 1  is an H field bend located at or near this relatively high position of the waveguide  14 -W. Vent holes (not shown in FIG. 1) are formed in the H-bend waveguide  20 - 1  in an appropriate location. These vent holes assist in suppressing an arc located the particular section of the waveguide  14 -W in which the bent waveguide section  20 - 1  is located. 
     A similar vented bent waveguide section  20 - 1  is used in the oven system shown in FIG.  2 . This figure illustrates a smaller batch type oven  22  that contains a single cabinet  11  having placed therein a microwave energy source  12 . A control panel  13  may be accessed by an operator to control the operation of the batch oven  22 . 
     The batch oven  22  makes use of a circularly polarized feed assembly  30  to couple microwave energy to its respective enclosure  15  such that energy originating from the rectangular waveguides  14  are presented to the cavity with a generating circularly polarized orientation. This prevents the supplied microwave energy from coupling to fixed modes internal to the enclosure  15 . For more information on the type of polarizing assembly  30  and the batch oven  22  more generally, reference can be made to U.S. Pat. No. 6,034,362 issued Mar. 7, 2000 to Alton. 
     Feeding the polarizing assembly  30  is a waveguide run  14  that consists of a series of rectangular waveguide sections including H-bend waveguide sections  20 - 1 ,  20 - 2 , and  20 - 3 , and straight waveguide sections  21 - 1  and  22 - 2 . Of interested in this particular arrangement is the H-bend waveguide section  20 - 1  which is located in a relatively high point in the waveguide run  14 . As can be seen in FIG. 2, this particular waveguide section  20 - 1  has vent holes  40  formed in an upper portion thereof. 
     To understand how the placement of vent holes  40  assists with the suppression of arcs within the waveguide run  14 , turn attention now to FIG.  3 . Shown here is a simple waveguide run  14  made up of a pair of H-bend waveguide sections  20 - 1  and  20 - 3 . The waveguide run  14  normally carries electromagnetic energy in a forward direction from the microwave power source  12  towards the enclosure cavity  15 . (It should be understood that the arrangement in FIG. 3 is a simplification of the waveguide runs  14  shown in FIGS. 1 and 2; in practice it is often necessary because of mechanical constraints to have multiple straight and bent waveguide sections in any given waveguide run  14 , such as was shown in FIG.  1 .). 
     FIG. 3 also illustrates how the waveguide run  14  presently has an arc  35  formed therein. The arc  35  is represented schematically in FIG. 3 as a low impedance short between the two major side surfaces  25 - 1  and  25 - 2  of the waveguide  14 . In a common scenario, the arc  35  has originated in a section of the waveguide run  14  near or in the cooking cavity  15 , such as in a place below the waveguide section  20 - 2 . Because the power source  12  represents a region of lower impedance, the arc  35  then tends to travel backwards through the waveguide run  14  towards the power source  12  in a reverse direction The arc  35  acts to reflect at least some electromagnetic energy back to the power source  12 . This causes a decrease in power levels at points in the waveguide  14  beyond the arc  35 , resulting in a situation where the cavity  15  in turn receives electromagnetic energy at a reduced power level. 
     The arc  35  tends to heat the air in its immediate vicinity within the waveguide  14 . Since hot air rises, an arc will also tend to rise due to the heat in the ionized gases of the arc. When an arc, traveling backwards towards the power source  12 , encounters a bend in the waveguide, such as within bend  20 - 1 , certain behavior is observed under certain conditions. In particular, when the arc  35  moves into a bend  20 - 1  where further travel backwards towards the source  12  would involve moving downward in elevation, the arc  35  will become trapped by the rising effect of the hot air opposing the backwards movement of the arc  35 . 
     Such an arc  35  may therefore tend to set up in a stationary or stable location within the bent waveguide  20 - 1  where further backwards travel towards the source  12  would involve a downwards movement in elevation. This not only reduces the electrical effectiveness of the microwave source  12  but indeed may caused physical damage of the waveguide run  14 , as such standing arcs  35  actually may create enough heat and energy to deform or even burn through the waveguide  14  itself. 
     Such an arc is therefore normally an extremely undesirable situation within the waveguide run  14  because the ionization created by the arc  35  not only substantially reduces the power handling capacity of the waveguide  14 , but may also lead to physical damage of the waveguide section  20 - 1 . 
     However, in accordance with the invention, vent holes  40  are formed in a suitable upper portion  38  of the waveguide section  20 - 1  near where the arc  35  tends to become trapped. The vent holes  40  serve as a mechanism for passive suppression of the arc  35  through a combination of physical results. In the preferred embodiment, these vent holes  40  are optimally located at a point in the waveguide  14  where the arc would tend to normally become trapped, and have to travel downward to continue its motion back towards the power source  12 . 
     By appropriately configuring the holes  40 , the hot air (which initially caused the arc  35  to be trapped within the waveguide section  20 - 1 ), will eventually escape through the holes  40 . As this release of the heated air occurs, the arc also tends to physically be drawn upwards towards the upper sidewalls  25 - 3  and  25 - 4  of the waveguide section  20 - 1 . If the waveguide section  20 - 1  is appropriately designed at this point from an electromagnetic perspective, such that the sidewalls present a region of zero voltage to the arc  35 , as the arc  35  is drawn towards the upper sidewalls  25 - 3  and  25 - 4 , it will extinguish itself naturally. 
     In a more complicated waveguide run  14  consisting of several such bent sections  20 - 1  that present an arc trap point, the vent holes  40  are preferably located at the trap point located closest to the cavity enclosure  15  where the arcs  35  originate. This prevents standing arcs occurring closest to the enclosure from damaging such waveguide sections. 
     One particular type of bent waveguide section  20 - 1  that can be used is shown in more detail in FIGS. 4A,  4 B and  4 C. This bent section illustrated is an H-bend type waveguide section  20 - 1  previously shown as  20 - 1  in FIG.  1  and  20 - 2  in FIG. 2. A so-called H-bend section has the axis of its bend along its respective H-plane. The H-bend section  20 - 1  consists of an upper flange  42  and lower flange  44  to enable coupling of the H-bend section  20 - 1  to other sections of waveguide  14 . The H-bend section  20 - 1  is formed preferably of aluminum one-eighth of an inch thick with a chromate golden finish per, for example standard MIL-C-5541 Class  3 . 
     The H-bend section  20 - 1 , generally rectangular in cross section, has vent holes  40  formed in an upper portion  45  thereof such as at the upper walls  25 - 3  and  25 - 4 . For  5  operation at an intended microwave frequency of approximately 900 MegaHertz (MHz), the waveguide section  20 - 1  may have a length dimension, D1, of approximately 9.75 inches and width dimension, W1, of approximately 4.8 inches. 
     The holes  40  formed in the upper portion  45  of the H-bend  20  are large enough to permit hot air gas to escape there through but small enough to prevent the escape of microwave energy in the operating frequency band. For operation at approximately 900 MHz, the holes  40  may typically be 0.25 inch in diameter and located on a grid spacing, S 1 , of approximately 1 inch in the narrow dimension of the waveguide, and a grid spacing, S 2 , of approximately 1.4 inches along the wide dimension. The space between the adjacent columns, along dimension S 3 , is typically one-half of the dimension S 2 , or as illustrated is 0.7 inches. 
     Although not shown in the drawings, it can be useful in practice to attach a fine mesh screen over the holes  40  to prevent objects from clogging the vent holes or entering the waveguide section  20 - 1 . 
     Turning attention to FIG. 5 there is seen another view of the H-bend section  20 - 1  with a schematic view of the voltage vector V displayed adjacent to it. The voltage vector V reaches a peak value within the interior of that section  20 , tapering to approximately zero volts at outer edges thereof. The zero voltage region with vent holes  40  along the outer bend  50  tends to draw the arc  35  towards it, causing the arc  35  to extinguish itself as the hot air ionized gas escapes through the vent holes  40 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. For example, other shapes of H-bends can accomplish the same results.