US2707742A

US2707742A - Methods of and apparatus for dielectric heating

Info

Publication number: US2707742A
Application number: US258015A
Authority: US
Inventors: Juhola Carl
Original assignee: United Shoe Machinery Corp
Current assignee: United Shoe Machinery Corp
Priority date: 1951-11-24
Filing date: 1951-11-24
Publication date: 1955-05-03
Anticipated expiration: 1972-05-03

Description

y 1955 c. JUHOLA 2,707,742

METHODS OF AND APPARATUS FOR DIELECTRIC HEATING Filed Nov. 24, 1951 3 Sheets-Sheet l Rventor: Car/ Mafia/a I .55 Mi Alto; e:

y 3, 1955 c. JUHOLA 2,707,742

METHODS OF AND APPARATUS FOR DIELECTRIC HEATING Filed Nov. 24, 1951 I5 Sheets-Sheet 2 May 3, 1

Filed Nov.

OSCILLATOR VOLTAGE OSCELLATUR VOLTAGE c. JUHOLA 2,707,?42

METHODS OF AND APPARATUS FOR DIELECTRIC HEATING 3 Sheets-Sheet 3 TIME jfivem far:

Car! duke/a United States Patent Cifiice METHODS F AND APPARATUS FOR DIELEQTRIC HEATING Carl Juhola, Manchester, Mass, assignor to United Shoe Machinery Corporation, Fiernington, N. 5., a corporation of New Jersey Application November 24, 1951, Serial No. 258,015

1i) Claims. (Cl. 2l9---10..77)

This invention relates to the treatment of materials by high-frequency electric energy, and more particularly to improved methods of and apparatus for controlling the application of the highfrequency electric energy responsively to operating conditions during the heating cycle to enhance the rapidity of the heating operation and to protect the oscillator and work from damage by arcs.

"i he voltage at which an arc will form in a dielectric heating system under load is likely to vary widely from load to load even though the loads are apparently similar. The breakdown point may also vary during the course of heating of a given load. Accordingly, the usual procedure in dielectric heating has been to avoid arcs by limiting the voltage of the high-frequency energy to a value low enough so that no breakdown is likely to occur. This means tempering the voltage to a value which the most sensitive load will support without arcing. This value will be referred to herein as a safe value. However, since the rate of heating of a load varies approximately as the square of the voltage gradicnt, such operation represents a considerable loss in the potential rapidity with which those loads which could withstand higher voltage could be heated.

Accordingly, an object of the invention is the provision of a method of dielectric heating in which the voltage of the high-frequency energy is varied in accordance with the breakdown conditions of the individual load so that the rate of heating of loads is not limited to that provided by the safe voltage.

In accordance with a feature of the invention, a method of dielectric heating is provided wherein the highfrequency energy is applied throughout the heating cycle in sub-cycles wherein the voltage is progressively increased until are forms, the arc is quenched, for example, by momentarily dropping the voltage, and the next sub-cycle commenced by re-application of the highfrequency electric energy at a voltage below that at which the are formed. Where the load can support a voltage above the safe voltage, the average value of a voltage of a series of such sub-cycles will be greater than the voltage for continuous operation at the safe value, and accor ingly the rapidity of the heating of such loads will be correspondingly increased.

Another object or". the invention is the provision of apparatus adapted to carry out the method of the invention.

in accordance with a feature of the invention, dielectric heating apparatus is provided for supplying high-frequency electric energy to a load throughout the heating cycle in sub-cycles wherein the voltage of the energy is progressively increased. By a feature of the invention should an arc occur during such progressive increase, the voltage is dropped to quench the arc, returned at a value below that at which the are formed, and another sub-cycle commenced.

The apparatus features improved means for sensing the formation of an arc, which means is here employed for controlling the supply of the high-frequency energy.

Cat

d it has previously been proposed to sense the formation of an arc by means responsive to the magnitude of a D. C. voltage derived from the oscillator feed-back or grid excitation, for example, from the oscillator grid current.

When an are forms, the additional loading imposed thereby reduces the excitation and hence the grid current. 1 have found, however, that arcs may occur under circumstances where the resulting grid current after the arc has formed is still Within the range covered by normal loading requirements. For example, a small arc may occur during a lightly loaded portion of a heating cycle which includes a portion normally more heavily loaded than the loading imposed by the arc. However, i have noticed that the drop in grid current caused by the formation of an arc is much sharper than changes in c= rent during a normal heating cycle, although the niagni" de of the grid current may remain within normal limits. Accordingly, a feature of the invention comprises the provision of means responsive to a negative rate of change of the oscillator grid current, or of a voltage derived from the oscillator high-frequency excitation, for sensing the formation of arcs. Tris means may be employed to accompl sh various ends, for example, to drop and then recycle the oscillator anode voltage or to shut it oil entirely.

Since a negative rate of change o grid current is not present during starting of an osci ator, there is no need, such as there is in apparatus which cuts out below a pie-ietern'iined magnitude of grid current, to provide time delay or time constant devices to permit starting. Since any changes in the magnitude of grid current during normal operation are :dual, with consequent low rate of change, the circuit may readily made insensitive to these.

Other features and advantages of the invention will best be understood from cons ration of the following description taken in connection with the accompanying drawings, in which Fig. l is a schematic diagram of an apparatus enibodying the invention;

Fig. 2 is a schematic diagrmn of apparatus embodying the invention in another form.

Fig. 3 is a time-voltage graph of the sub-cycles of the heating cycle provided by the apparatus of Fig. 1; and

Pig. 4 is a time-voltage graph of the sub-cycles of a heating cycle provided by the apparatus of Fig. 2.

In Fig. 1 there is shown a dielectric heating system comprising oscillator tubes 1i) Whose anodes 12 are grounded and Whose cathodes 14 are connected to a tank circuit comprising an inductance 16 and heating electrodes 13 coupled to the inductance to by blocking condensers 2G. The cathodes of the tubes 15 may be indirectly heated as shown, or they may be directly heated filaments. For simplicity the connections for heating the cathodes are not shown, since they form no part of this invention.

The grid circuit of the oscillator comprises the inductance 22 connected between the grids 2d and having a center-tap connected to a center-tap on the inductance 16 through

resistors

26, 27 and 23 which constitute the grid resistance of the oscillator.

High voltage direct current for the anode circuit of the oscillator is supplied by a thyratron rectifier comprising tubes 39 having their anodes 32 connected to the secondary of a transformer 34- Whose primary is adapted to be supplied from a power line. The cathodes 36 are connected for grounding the positive terminal of the rectifier, While a center-tap of the secondary transformer 34, constituting the negative terminal of the rectifier, is connected to the cathodes M of the oscillator via a resistance 28 and the inductance 16. Filtering for this high negative voltage is provided by a filter comprising an inductance ll) and a condenser 42. The voltage drop across resistor 28 is employed to energize the anode circuit of an arc-sensing device hereinafter described.

It will be understood that the voltage of the highfrequency alternating electric energy supplied to the electrodes 18 is determined largely by the voltage of the direct current supplied to the oscillator by the thyratron rectifier. This direct current voltage is controlled by shifting the phase of alternating current supplied to the grids 44 with respect to the voltage supplied to the anodes 32. For this purpose, an alternating current derived from the power mains through a transformer 50 is applied to the grids 44 through a conventional phase-shifting circuit comprising the

resistors

52, 54 and

condensers

56 and 58. The resistor 52 has a shunt circuit comprising a variable resistor 6t? and contacts 62 of a relay 64, while the resistor 54 has a shunt circuit comprising a variable resistor 66, contacts 68 and a rheostat '70 having a slider 72. The parameters of the phasing and shunting circuits are so established that the rheostat 70 toward its high-voltage position. Accordingly', the oscillator plate voltage rises progressively until either the slider 72 reaches its limit or an are forms. In the latter case, relay opens its contacts 90 deenergizing the motor 78 with its clutch and the relay 4-, thus dropping the oscillator voltage and permitting the slider 72 to return to its original position from which it reascends for another sub-cycle of the heating cycle when the contacts 94 are closed.

Fig. 3 shows the resulting form of the oscillator voltage. A typical sub-cycle starts at a, an are forms b, and is quenched 0. Although in the four sub-cycles shown the time during which the voltage is dropped for rheostat 70 provides a means of controlling the voltage not go to zero. The rheostat range may advantageously I be from safe to maximum.

A voltage control-operating means is provided which comprises a motor 78 connected in shunt with the relay 64. The motor is mechanically connected to the slider 72 through an electromagnetic clutch 73 having terminals 74 electrically connected to the motor terminals, and when energized, the motor drives the slider progressively up to its maximum voltage position. When the motor '78 is not energized, the slider 72 is returned progressively to its low voltage position by a spring St). The motor, clutch and spring may conveniently be provided as a conventional timing device. The relay 64 and the motor 78 are arranged for connection to the power mains through the contacts 913 of a relay 1152 and either holding contacts 92, or contacts 94. When the holding circuit is broken by even momentary opening of the contacts 90, the relay 6 and motor 78 remain unenergized until the closure of contacts 9 coincident with the return of slider 72 to the low voltage position.

In order to open the contacts 99 responsively to the formation of an arc, an electron discharge tube 1%, having the relay 102 connected in its anode circuit, has its grid circuit connected for biasing in accordance with the rate of change of voltage across the resistor 27. For this purpose a condenser 108 and a resistor 110 are connected in series across the resistor 27, the resistor 110 having its ends connected respectively to the grid and the cathode of tube 1%. The condenser 103 and resistor 110 form what is commonly referred to as a difierentiating circuit. An abrupt change of the oscillator grid current in the direction of a drop in the magnitude thereof produces a drop in voltage across the ditferentiating circuit, and the resulting charging current in resistor 110 imposes a positive bias on the grid to increase the conductivity of tube 1%. A variable resistor 112 is connected in shunt relation with the relay 1% and is adjusted so that the conduction in the tube 160 during normal operation of the oscillator is insuflicient to cause the relay 192 to open its contacts.

In operation, after the tube cathodes have been brought up to operating temperature and the work disposed in operational relation to the electrodes, a main power switch 125 may be closed to apply power from the line to the rectifier anode and grid circuits and to the relay 6 'At this time the motor 78 is energized, together with its magnetic clutch, and moves the slider 72 of voltage is caused progressively to increase as before.

quenching the arc is exaggerated in comparison with the other times, it will be readily seen that the average voltage is well above the safe voltage indicated by the dotted line.

Fig. 2 illustrates an embodiment which is similar to that of Fig. l, with the exception that recycling is performed after the lapse of a predetermined period after an arc has formed rather than upon the return of the slider 72 to its original position, whereby the voltage at which a sub-cycle commences after an arc may be a predetermined voltage below the voltage at which the arc formed. For this purpose, the relay 102 is provided wtih a pair of holding contacts 130 by which the relay 102 is adapted to be energized from a condenser 132. When the relay 102 is not energized, the condenser 132 is charged through contacts 134 and a protective resistor 136. Provision is made for varying the length of the holding period by a variable resistor 13%. In this embodiment no holding contacts are provided for the relay 64.

in the operation of this embodiment, upon application of energy to the motor 78 and relay 64, the oscillator the occurrence of an arc, however, the relay 162 deenergizes the motor and drops the oscillator voltage by opening the contacts 99 for a period determined by the holding circuit of relay 102 as described above, after which the relay 102 again closes the contacts and another sub-cycle is initiated at a voltage corresponding to the position to which the slider 72 has returned during the interval. The amount by which the slider 72 returns at each cycle may be adjusted.

Fig. 4 shows the form of the resulting voltage of the oscillator during a cycle of heating. Again the voltage starts at an original voltage, a, which may be the safe voltage, and rises progressively until it is dropped at b upon the formation of an arc. However, the voltage is now returned to a voltage at below the previous breakdown potential but still well above the original voltage a. In the next cycle, the voltage rises to a higher level b before arcing and accordingly the next sub-cycle commences at a voltage higher than that of the preceding sub-cycle. Thus the average voltage will be somewhat higher than provided by the embodiment first described.

If desired in the embodiment of Fig. 2, the motor speed may be set at a very low figure or zero and the heating cycle started with the slider 72 in its upper position. Then the voltage will automatically he stepped down with each arc until no further arcs occur.

Where the Work can not tolerate any arcs, provision may be made, as by auxiliary electrodes, to confine the location of arcs to locations harmless to the Work.

It is to be understood that my invention is not limited to the particular embodiments disclosed but that changes or modifications may be made Within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of dielectrically heating loads which may vary as to the voltage of the high-frequency electric energy which can be sustained without harmful arcing, which method comprises supplying the high-frequency energy to the load throughout the heating cycle in a plurality of sub-cycles wherein the voltage is progressively increased until an are just forms, the arc is quenched, and the voltage of the high-frequency electric energy is returned to an initial value for the next subcycle, which value is less than that at which the are formed during the previous sub-cycle.

2. The method of dielectrically heating loads, which method comprises supplying the high-frequency electric energy to the load in a plurality of sub-cycles of the heating cycle, each sub-cycle comprising a progressive increase or" the voltage of the high-frequency energy, a dropping of the voltage upon the formation of an arc to quench the arc, and a return of the voltage to an initial value for the next sub-cycle, said value being less than that at which the are formed during the previous subcycle.

3. in a dielectric heating system including an oscillator, an electron tube having a grid circuit and an anode circuit, a differentiating circuit having input terminals and output terminals, means for deriving from the oscillator grid excitation voltage a direct current voltage proportional thereto and for supplying this direct current vol age to the said input terminals, connections between the output terminals for supplying the output voltage of the difierentiating circuit to said grid circuit as a bias voltage, and means responsive to the current in the said anode circuit for controlling the output of the oscillator.

4. In a dielectric heating system including a self-biased oscillator having a grid circuit through which the oscillator grid current flows, an electron tube having a grid circuit and an anode circuit, means for applying to the grid circuit of said tube a voltage which varies according to the rate of change of the oscillator grid current, and means responsive to the flow of current in the anode circuit of said electron tube for controlling the energization of the oscillator.

5. In a dielectric heating system, a self-biased oscilla tor having a grid resistance through which the oscillator grid current flows, a resistance-capacitance differentiating circuit across a portion of the grid resistance, an electron tube having the resistance of the differentiating circuit in its grid circuit, and a relay in the anode circuit of the electron tube, said relay having contacts arranged to control the energization of the oscillator.

6. In apparatus of the class described, an oscillator, a motor-driven control for providing a progressive increase of the oscillator output voltage during a sub-cycle of the heating cycle, a relay responsive to a reduction of the oscillator grid excitation, and means controlled by said relay for abruptly dropping the oscillator output voltage and for interrupting the energization of said control for an interval and then initiating another sub-cycle of progressively increasing output voltage.

Cir

7. In apparatus of the class described in combination, an oscillator, means for controlling the magnitude of the output voltage of the oscillator, and an arc sensing device cooperative With said controlling means responsively to the formation of an arc of the oscillator output to drop the magnitude of the output voltage to extinguish the arc and thereafter to raise the said magnitude to a higher value but below that at which the are formed.

8. In apparatus of the class described, an oscillator, voltage control means for determining the voltage of the oscillator output, a motor cooperating with the voltage controlling means for providing a progressive increase of the oscillator output voltage, and a relay responsive to a reduction in the oscillator grid current produced by the formation of an are for dropping the oscillator voltage to quench the arc and for initiating another sub-cycle of the heating cycle by re-applying the oscillator output at a voltage below that at which the are formed.

9. In apparatus of the class described, an oscillator, voltage control means operable to vary the voltage of the oscillator output, voltage control operating means energlzable to operate said control means to provide a progressive increase of the oscillator voltage up to maximum voltage, said operating means including means for operating said control means progressively toward a minimum voltage position when not so energized, means responsive to a change of condition of the oscillator grid current produced by the formation of an are for deenergizing the operating means and for dropping the oscillator output voltage to quench the arc, and means for again energizing the operating means after a time interval.

10. In apparatus of the class described, a self-biased oscillator, a variable voltage supply for the anode circuit of said oscillator, means for operating said anode supply means to provide a progressively increasing anode supply voltage for the oscillator, relay means responsive to a change of condition of the oscillator grid current produced by the formation of an are for dropping the volt age of the anode supply to a value insufficient to sustain the arc, and means for recycling the operating means to initiate a further sub-cycle of progressively increasing anode voltage from a value less than that at which the are formed.

References Cited in the file of this patent UNITED STATES PATENTS 2,391,085 Crandell Dec. 18, 1945 2,467,285 Young et al. Apr. 12, 1949 2,470,443 Mittelmann May 17, 1949 2,545,997 Hagopian Mar. 20, 1951 2,548,246 Walstrom Apr. 10, 1951