US3731105A - Arc suppressor for dielectric apparatus - Google Patents

Abstract

Dielectric heating apparatus including spaced electrodes, power means for generating a high frequency electromagnetic field of predetermined magnitude between said electrodes, a fluorescent lamp which is energized in response to said electromagnetic field when at said predetermined magnitude, means responsive to deenergization of said fluorescent lamp for inhibiting operation of power means whereby, upon an arc being instituted between said electrodes, said electromagnetic field is reduced in magnitude, said fluorescent lamp is de-energized, said power means is interrupted and the arc is suppressed thereby avoiding the danger of fire and thereby preventing damage to both the electrodes and material being heated.

Description

United States Patent 1191 Preston 1 May 1, 1973 1541 ARC SUPPRESSOR FOR DIELECTRIC 3,412,227 11/1968 Anderson ..219 10.55 APPARATUS 3,461,443 8 19 9 Vasel.....= ..250/206 x 3,644,739 2 I972 W'lk t l ..250 2l7 X [75] Inventor: Mark Preston, New lpswich, N.H. I mson e a [73] Assignee: Litton Business Systems, Inc., New Primary Examinerwalter Stolwein York, NY. Att0rr1eyArthur T. Groeninger [211 App! 255297 Dielectric heating apparatus including spaced elec- Related us. Application Data trodes, power means for generating a high frequency electromagnenc field of predetermlned magmtude Continuation of 29959, 9701 between said electrodes, a fluorescent lamp which is aballdonedenergized in response to said electromagnetic field when at said predetermined magnitude, means respon- [52] Cl 250/217 250/215 219/1055 sive to de-energization of said fluorescent lamp for in- Cl. operation of powgr means y p an Field of Search arc being instituted between Said electrodes" Said elec 250/220 217 219/1069 1077 1055 tromagnetic field is reduced in magnitude, said fluorescent lamp is de-energized, said power means is [56] References Cted interrupted and the arc is suppressed thereby avoiding UNITED STATES PATENTS the danger of fire and thereby preventing damage to both the electrodes and materlal bemg heated. 3,181,030 4/1965 Weinstein ..250/207 X 3,281,567 10/1966 Meissner et al. ..2l9/l0.77 3 Claims, 2 Drawing Figures l9 22, AC POWER H 0C .12 PAD/O SOURCE 4 POWER FREQ.

i SUPPLY GM t l1 ;i 1- 1 11 i CO/fROL 29 i i i START- STOP 1 f GE sw/TcHEs 1 i 'SOURCE J i i r i l EL 46 7 v i Q i 1 1 471 1 E i 'j I 1/,

E N i r 3O i :9 i A I p-or0- 3 ELECTRIC l M RELAY L M 4 K Pat ented May 1, 1973 2 Sheets-Shet 1 F I G. 1

l l l 0 c POWER SUPPLY 4c POWER SOURCE SW/TCf/ES START- STOP A c CONTROL VOLTAGE SOURCE INVENTOR M A R K PR E N ATTORNEY Patentea May '1, 1973 v MARK INVENT PRES N ATTORNEY ARC SUPPRESSOR FOR DIELECTRIC APPARATUS This is a continuation, of application Ser. No. 29,959, filed Apr. 20, 1970, now abandoned.

BACKGROUND OF THE INVENTION The presentinvention relates to the heating of a dielectric material by passage of the same through a high frequency electromagnetic field. The electromag- 1O netic field is set up between spaced electrodes which are connected to a high frequency source.

A problem inherent in such heating operations is that an arc is often inadvertently instituted between .the electrodes. This results from causes such as the evolution of vapor, or a defect (such as a pinhole) in the material being heated. Such an arc can damage the electrodes, cause a fire, burn the material, etc.

Heretofore, the prior art has recognized that arcing caused more power to be drawn from the power source. This extra draw of power is accompanied by a rise in plate current of the oscillator used in the system. In order to minimize the damage caused by arcing, an overload relay was inserted in the oscillator to turn it off. The problem with this arc suppressing system is that arcing does not always result in a rise in plate current. Sometimes an arc-can increase the impedance of the load circuit thereby causing a reduction in plate current of the oscillator. Accordingly, such a system is not a reliable arc detector.

An improvement on the foregoing is described and shown in U. S. Pat. No. 2,763,758 to Fred Kohler wherein a device, responsive to the direct currentresistance of the material being heated, renders the oscillator supplying the radio frequency energy nonoperable when the direct current resistance of the material being heated becomes lower than a given value.

The problem with the Kohler arc suppressing system is that the electrodes must be isolated from each other with no direct current passing between them. However, a common practice in dielectric heating is to shunt the electrodes with a tuning inductance for the purpose of obtaining a sufficiently high and more uniform voltage distribution. Since such an inductance would act as a short circuit for direct current, the use of the Kohler system is impractical for many dielectric heating systems.

the field is reduced in magnitude due to a lowering of electrode voltage.

A lightproof, dust tight enclosure surrounds that portion of the lamp extending on the outside of the enclosure, A photoelectric cell is mounted in'the enclosure.

The photoelectric cell, in response to de-energization of the light source, transmits a signal to a control unit to interrupt the power supply to the electrodes.

The present invention has the advantage of working with any type of electrode or any type load circuit. As hereinbefore noted, the Kohler dc superimposed type system used in the past cannot be used with electrodes which are not isolated from each other. Some of the other systems mentioned above can only work when the electrodes are very close together so that when an arc forms, .it is a good solid arc, easily distinguishable from spurious signals resulting from external inter- A third system used in the past may be described as as spurious frequency detection system. In this system, various frequencies, which are present when arcing is instituted, are detected by a receiver circuit. The receiver transmits a signal to interrupt power to the oscillator. The problem with this arc suppressing system is that spurious frequencies are caused by numerous causes other than electrode arcing and as a result, a great number of false alarms are detected.

BRIEF DESCRIPTION OF TI'IEINVENTION A fluorescent lamp is inserted through the enclosure which surrounds the electrodes and heating chamber of ferences or minor malfunctions unrelated to arc detection. v

Since the critical parts of the present invention are sealed in a dust tight, lightproof container, environmental conditions cannot affect its operation. Systems in the past, such as the Kohler system described above, often fail when dirt or the like cause a short circuit between the electrodes.

The are suppressing system of. the present invention is substantially failsafe. False alarms are only possible when the fluorescent lamp burns out. The system of the present invention isvfast acting and it is easily and simply adusted by adjustably positioning fluorescent lamp. Most of the systems discussed above are slow in operation because they require that the are be formed long enough so as to cause a change in theoperating system of the apparatus.

DRAWINGS FIG. 1 is a combined block and schematic diagram of I DETAILEDDESCRIPTION,

Referring to FIG. 1, the present invention isillustrated in connection with conventional dielectric heating apparatus 1 including a heating chamber 2 which is substantially fully enclosed by wall structure. The wall structure for chamber 2 is made generally from copper or aluminumwhich are good conductors and provide a low resistance path to ground and thus prevent the spread of the radiation beyond the chamber 2.

A web or cord of material 3 to be dried enters the heating chamber 2 through a slot 4 formed in top wall 5, passes through the heating chamberv 2 and exits through a slot 6 formed in bottom wall 7.

In order to generate an RF field, a series of electrodes 8 and 9 are mounted alternately in chamber 2 adjacent the moving web 3 and are connected by suitable conductors 10 and 11, respectively to a radio frequency generator 12 and to ground.

I A fluorescent lamp 13 is adjustably mounted in the sidewall 14 of heating chamber 2 so as to permit the same to be extended into the chamber a selected distance. If a lamp such as a fluorescent lamp is extended into a high frequency electromagnetic field of sufficient magnitude such as is used in dielectric heatthe ing, the gas in the lamp ionizes and the lamp lights up.

The extent to which the lamp is extended before it lights up will depend upon the length of the lamp and field intensity. Generally, a field intensity of more than 0.5 watts per inch of lamp at a frequency of 1 MHz is required to light a conventional 6 watt fluorescent lamp. By the arrangement of the present invention,

- only a portion of lamp 13 is exposed to the field while the remainder of the lamp is shielded by sidewall 14 from the field. As the lamp 13 is extended into heating chamber 2, more and more thereof is exposed to the field until a point is reached when there exists sufficient field exposure to fully ionize the gas in the tube. It is at this point that lamp 13 is held so that if the field cy range of 50 KHz to 3,000 MHzand is connected to suitable DC power supply 16 by conductors l7 and 18 The DC power supply comprises an AC to DC converter which is connected by suitable conductors 19 and 20 to AC power source 21.

Conductor 19 includes normally open contact 22. Upon closure of contact 22, the electrodes 8 and 9 are energized thereby creating an electromagnetic field in heating chamber 2 resulting in the heating of web 3 passing therethrough.

In order to close and open contact 22, a control unit 23 is provided for initiating the operation of the apparatus and for automatically shutting it down when an arc occurs. The control unit 23 includes a start-stop switch 24. When the start button is pressed, a circuit is completed between AC voltage source 25 andelectromagnetic coil 26. Energization of coil 26 closes contact 22 thereby energizing the electrodes 8 and 9. Energization of coil 26 also simultaneously opens a normally closed contact 27 which connects source 25 through conductors 28 and 29 to electromagnetic coil 44. Deenergization of coil 44 opens a contact 45 which normallyconnects incandescent lamp 30, extending into lightproof enclosure 15, to source25 through conductors 46 and 47. Incandescent lamp 30 is used for purposes hereinafter more particularly described.

Also extending into lightproof enclosure 15 is a photo cell 31 which, upon de-energization of fluorescent lamp 13 as a result in arcing, transmits a signal by conductors 32 and 33 to a photoelectric relay 34. The photoelectric relay 34 is conventional and may be of the type described in College Physics, Page 705 I by Sears and Zemansky, 2nd Edition, published by Ad- 1 line with coil 26 and as a result, coil 26 is de-energized.

Upon de-energization of coil 26, contact 22is opened thereby cutting off power source 21 and de-energizing I the electrodes 8 and 9.

De-energization of coil 26 also results in the closing of contact 27, thereby turning on lamp 30 in container 15 following the cutting off of power to electrodes 8 and 9. Lamp 30 is turned on after de-energization of the electrodes due to the time delay caused by lamp 30 being operated through coil 44 and contact 45.

Photocell 31, in response to lamp 30 being turned on,

transmits a signal to photoelectric relay 34 which in response thereto closes contact 35 to make the apparatus again ready for start.

Upon restart of the system, fluorescent lamp 13 is energized by the field in chamber 2. Incandescent lamp 30 is turned off as hereinbefore described due to the opening of contact 27 thereby making ready photocell 31 for detection of a change in condition of lamp 13. Energizing lamp 30 through the coil and contact provides a time delay to assure that lamp 13 isturned on before lamp turns off. If lamp 30 de-energized before lamp 13 was energized, the dark condition would be detected by photocell 31 and the system would be disabled and restart would be prevented.

Referring to FIG. 2, the fluorescent lamp 13 is shown adjustably mounted in a rubber grommet 36 is sidewall 14 of chamber 2; The grommet 36 resiliently grasps thehereinbefore described, only that portion of the lamp 13 extending into chamber 2 is subject to the field and only when a sufficient length of lamp 13 is extended into chamber 2 will the lamp light up.

The incandescent lamp 30 and photocell 31 are mounted by brackets 38 and 39 and extend through apertures 40 and 41 in container 15.,Light and dust sealing rings 42 and 43 snugly engaging lamp 30 and photocell 3l,respectively, are provided in apertures 40 and 41 to assure that-the interior of container 15 is maintained lightproof.

The foregoing is considered illustrative only of the principles of the invention. It is not desired to limit the invention to the exact construction shown and described,'and accordingly, all suitable modifications and equivalents may be resorted to as falls within the scope of the invention.

What is claimed:

1. The combination comprising means for generating a high frequency field within a chamber in a frequency range of from 50 KH to 3000 MHz, an elongated light source at least part of which extends within said chamber and which is energized in response to said field, means responsive to de-energization of said light source upon a predetermined decrease of the strength of said field for inhibiting operation of said generating means, means for adjustably mounting said light source so that a selected portion thereof extends into said chamber and is exposed and energized by said high frequency field within said chamber.-

2. The combination as defined by claim 1, means including a second light source for de-activating said inhibiting means immediately after the generating means in inhibited.

3; The combination as defined by claim 2, means for deactivating said second light source after said generating means is started and the first mentioned means is energized.

Claims (3)

1. The combination comprising means for generating a high frequency field within a chamber in a frequency range of from 50 KHz to 3000 MHz, an elongated light source at least part of which extends within said chamber and which is energized in response to said field, means responsive to de-energization of said light source upon a predetermined decrease of the strength of said field for inhibiting operation of said generating means, means for adjustably mounting said light source so that a selected portion thereof extends into said chamber and is exposed and energized by said high frequency field within said chamber.

2. The combination as defined by claim 1, means including a second light source for de-activating said inhibiting means immediately after the generating means in inhibited.

3. The combination as defined by claim 2, means for deactivating said second light source after said generating means is started and the first mentioned means is energized.

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