Patent Publication Number: US-6704184-B2

Title: Arc suppression in waveguide using optical detector and forced air

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a technique for suppressing arcs in an electromagnetic waveguide, and more particularly to a technique that detects the arcs and suppresses them using forced air. 
     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 behavior 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 may actually continue after the fault is no longer in existence. An arc is of concern because it not only substantially blocks transmission of energy through the waveguide, but also may physically damage the system components. 
     For example, electromagnetic energy normally travels within the waveguide from an electromagnetic energy source through the waveguide toward a system that makes use of the microwave energy, such as a microwave oven cavity. Once an arc occurs, electromagnetic forces tend to cause it to travel in a reverse direction within the waveguide, back toward the power source. The arc typically absorbs almost half of the forward power, and reflects a similar amount of electromagnetic energy back to the power source. This causes a decrease in power levels at points in the waveguide beyond the arc to negligible levels. 
     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 the 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 temporarily 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. 
     In accordance with one embodiment of the invention, an arc suppression system is provided which includes a waveguide run for carrying microwave energy, a sensing device, such as a photodetector, for sensing an arc within the waveguide run, and a blowing device for blowing a gas, such as compressed air, into the waveguide run, in response to a sensed arc to suppress the arc. 
     A controller can be connected to the sensing device and the blowing device for opening a valve of the blowing device, in response to the sensed arc, to allow the gas to suppress the sensed arc. A second blowing device can also be provided for blowing a gas to clean a viewing surface of the sensing device. 
     A microwave source for producing the microwave energy is further provided wherein the blowing device preferably blows the gas in a direction away from the microwave source. In one embodiment, the compressed gas has a pressure in the range of about 125 psi to 175 psi, and preferably about 175 psi. 
     In one embodiment, the photodetector is positioned on a bend in the waveguide run, which can be either pressurized or unpressurized. The bend can include 90 degree round bends, H-bends, and E-bends. 
     In one embodiment, the waveguide run carries the microwave energy to an oven cavity which has articles to be heated continuously fed therethrough. The oven cavity can also be heated by convection heating. 
     A method of suppressing an arc is also provided which includes providing microwave energy from a microwave source in a waveguide run, sensing an arc formed within the waveguide run, and blowing a gas into the waveguide run, in response to a sensed arc, for suppressing the arc. The method can also include the step of circularly polarizing the microwave energy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a microwave cooking system that makes use of an arc suppression system according to the invention. 
     FIG. 2 is a smaller scale batch oven which may also make use of the invention. 
     FIG. 3 is a side view of an arc suppression system installed on an H-bend waveguide section. 
     FIG. 4 is a partial cut-away view of a control box that forms part of the arc suppression system shown in FIG.  3 . 
     FIGS. 5 and 6 are side views of an arc suppression system installed on a 90 degree round bend waveguide section and a E-bend waveguide section, respectively. 
    
    
     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  11  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 ). 
     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. 
     The waveguide runs  14  can be pressurized or unpressurized for the operation of this invention. Most systems operate unpressurized, but applications such as pasteurization or sterilization usually require pressurization. 
     Of interest in FIG. 1 is a bent waveguide section  20 - 1  which forms a part of waveguide run  14 -W. The H-bend section  20 - 1  consists of an upper flange  24  and lower flange  26  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 . As more fully explained below, the bent waveguide section  20 - 1  is located in the waveguide run  14 -W at a position where an arc might be expected to set up in a stable position. The present invention suppresses such an arc through a technique that utilizes an arc suppression system (described below) that detects the arcs and suppresses them using forced gas, such as air. 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 one embodiment, the bent waveguide section  20 - 1  is an H field bend located at or near a relatively high position of the waveguide  14 -W. In other embodiments, the arc suppression system can be applied at virtually any point in the waveguide, for example, at bend  17 . 
     A similar 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 sensor and gas input to the waveguide are typically placed in a bend so that they are both pointing down a length of straight waveguide. The velocity of an arc is a function of the cw power level, linearly increasing with power level. For 70 kw in WR975 waveguide the speed is about 5 feet per second. A sufficient length of straight guide should be chosen to allow time for arc detection and suppression by the invention. If the bend is in a vertical plane then the length of straight is less critical. The heated ionized gases created by, and part of the arc tend to rise and prevent the arc from moving downward. The arc is therefore trapped in the bend, and will not travel past the detection and suppression device. 
     Before discussing the manner in which such arcs are suppressed, it will be instructive to review various components of the system  10  to understand why and where such arcs are created. 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 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 interest 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 . An arc suppression system is preferably positioned at point  32  on waveguide section  20 - 1 . 
     Turning to FIG. 3, an exemplary arc suppression system is illustrated. Generally, a sensing device  34 , such as a photodetector, is preferably positioned on a bend  32  so as to be able to detect the photometric energy of an arc which occurs inside the waveguide  20 - 1 . The sensing device  34  provides feedback to a control box  36  which opens a valve  37  (FIG. 4) allowing a compressed gas, such as air, to be forced into the waveguide via nozzle  40  to suppress the arc. In one embodiment, the compressed gas is stored in a tank  38  at a pressure of between about 125 and 175 psi, preferably about 175 psi. Preferably, the gas forces the arc away from the microwave source  12  in the direction where the power is much less and unable to sustain the arc. It is believed that a sufficiently strong blast of gas disrupts the ions in the waveguide and helps extinguish the arc. A so-called H-bend section has the axis of its bend along its respective H-plane. 
     The arc suppression system can further include a blowing device adjacent the sensing device  34  to clean the viewing surface of the sensing device. More particularly, a nozzle  42  can be configured to direct a compressed gas, for example, from tank  38 , at the sensing device  34  to remove any debris that may have accumulated at or near a viewing surface of the sensing device. 
     FIG. 4 illustrates further details of the control box  36 . An electronic controller  44 , such as a microprocessor, controls operation of the suppression system including sensing device  34 . A relay  46  is operated by the controller  44  to open the valve  37  allowing compressed gas to flow from the tank  38  to the nozzle  40 . In one embodiment, the valve  37  is open until the sensing device  34  no longer detects an arc in the waveguide. A power source, such as typical 120 volt line, is supplied to the control box  36  at conduit  48 . 
     FIGS. 5 and 6 illustrate the arc suppression system installed in alternative waveguide sections. More particularly, FIG. 5 illustrates the arc suppression system installed on a 90 degree round bend waveguide section  20 - 4 . FIG. 6 illustrates the arc suppression system installed on a E-bend waveguide section  20 - 5 . 
     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 bends can accomplish the same results.