Electric heating – Microwave heating – Radiation protection
Reexamination Certificate
2000-06-02
2001-07-24
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Radiation protection
C219S690000, C219S746000, C219S756000, C219S757000, C333S239000, C333S249000
Reexamination Certificate
active
06265703
ABSTRACT:
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.
REFERENCES:
patent: 3541289 (1970-11-01), Smith
patent: 3622733 (1971-11-01), Smith
patent: 3916317 (1975-10-01), Jurgensen
patent: 5438183 (1995-08-01), Hayami et al.
patent: 2-46692 (1990-02-01), None
patent: 2-302507 (1990-12-01), None
Hamilton Brook Smith & Reynolds P.C.
Leung Philip H.
The Ferrite Company, Inc.
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