1. Field of the Invention
The present invention relates to electrodes used in an electrical circuit of a cooking appliance, such as, an oven or the like for enhancing heat and mass transfer. More particularly, the invention relates to a needle-like mechanical electrode having a current-limiting resistive material associated with each needle. The present application is particularly adapted for use in connection with a needle electrode circuit, such as is disclosed in my copending application entitled Enhanced Heat and Mass Transfer Apparatus, filed Apr. 25, 1983 under U.S. Ser. No. 488,556 and having the same assignee. To the extent appropriate to the present invention the disclosure of the above-identified copending application is incorporated herein by reference.
2. Description of the Prior Art
In an electrical circuit having two spaced apart electrodes defining an electrical field for establishing current contact between two electrodes, the problem of arcing across the field needs to be addressed.
In the above-identified copending application there are disclosed two pairs of electrodes that electrically participate to enhance heat and mass transfer of food being cooked in an oven appliance. One pair of electrodes comprises a food surface and an array of electrically conductive needle-like elements spaced opposite from the food surface to define an electrical field therebetween. A voltage supply is electrically coupled to the food so that the food supplies the voltage necessary for causing the tip of each needle to generate a corona current through the field to an aligned portion of the food surface for maximizing heat and mass transfer, as is disclosed in the mentioned copending application. In normal operation, the voltage supplied of the food is controlled so as to maintain constant corona current flow through the field and the voltage about each needle is sufficiently above a corona threshold voltage--that voltage necessary to generate corona current--and below a voltage causing arcing.
It is known that the highest electrical concentration in the electrical field is formed about the tip of each needle. Although the tip of each needle is properly spaced from the food surface and the voltage supplied from the food provides corona current operation without arcing, isolated incidences of arcing have been observed to occur between an individual needle tip and the food surface. A localized arcing incidence is believed attributed to a rise in the corona current driving potential concentrating about a needle tip due, for example, to the introduction of mass matter in the field causing the dielectric resistance of the field to be reduced, such as, may occur during the forming of a liquid droplet or the deforming of a food portion to within the field projecting towards the needles.
Arcing is a highly objectionable occurrence because an arc is distracting to the operator and, more importantly, an arc introduces a high voltage surge accompanied by increased current in the circuit that may force electrical components to their operating limit which without regard may damage the components or cause the circuit to momentarily "shut down" until the arc disappears. Accordingly, there is a need to provide a mechanism for rapidly suppressing an arc without exposing electrical components to any excessive electrical activity associated with arcing and without interrupting the electrical operation of the circuit.
A widely used mechanism to achieve arc suppression includes interposing a series resistor in one leg of the electrical circuit to locate the resistor on a line extending from the needle electrode. In operation, a high voltage surge from any one needle of the electrode due to arcing is restricted through the resistor and protection of subsequent electrical components is afforded. Unfortunately, this single resistor mechanism has the disadvantage of reducing the entire operating efficiency of the electrical circuit during an arc suppressing operation and, therefore the overall performance of the circuitry suffers. Another disadvantage of this mechanism is that individual needles are not electrically isolated among one another so that a relatively short arcing duration may still occur which would rob the corona current driving potential from surrounding needles and thereby render the adjacent needles corona inactive during arcing.
From the foregoing, it is seen that there is a need of further improvement in needle electrodes to make the performance of such electrodes less sensitive to an arcing condition at one needle so that the remaining needles may continue to operate to generate corona current without interruption and without any appreciable loss of their corona current driving potential.