US2623176A

US2623176A - High-frequency heating apparatus

Info

Publication number: US2623176A
Application number: US113312A
Authority: US
Inventors: Witsenburg Emilius Carolus; Teunissen Hendrik Adolf
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1948-09-02
Filing date: 1949-08-31
Publication date: 1952-12-23
Anticipated expiration: 1969-12-23

Description

1952 E. c WITSENBURG ETAL 2,623,176

HIGH-FREQUENCY HEATING APPARATUS Filed Aug. 31, 1949 INVENTORS HENDRIK ADOLF TEUNISSEN EMlLlUS CAROLUS WITSEN URG Patented Dec. 23, 1952 HIGHFREQUENCY HEATING APPARATUS Emilius Carolus Witsenburg and Hendrik Adolf Teunissen, Eindhoven, Netherlands, assignors to Hartford National Bank andTrust Company, Hartford, 001111., as trustee Application August 31, 1949, Serial No. ll-3,312 .Inithe. Netherlands September 2, .1948

'8 Claims .Ffor the .inductive heating of metal articles orfor the capacitative heating of articles consisting of non-conducting material, high-frequency heating apparatus is known, in which the high-frequency energy is taken from the output circuit of a regeneratively arranged electron discharge tube.

In order to obtain a satisfactory efiiciency it is known to use a class-C electron-tube oscillator, the output circuit of which is included in the anode-circuit and from which a positive feedback voltage is taken. The positive feedback voltageis supplied, by way of a grid condenser, to the control grid which is connected to the oathode by way of a grid-leak resistance.

To obtain optimum efficiency it is necessary to match the load resistance to the oscillator, in other words to ensure that the tube is maximally controlled with respect to voltage and at the same timemaximally controlled with respect to current. Both in the case of inductive and capacitative coupling between oscillator and load, the coupling. factor between oscillator and load may be variable for matching the load, limited matching being also possible by varying the positive feedback of the tube oscillator.

If the high-frequency heating apparatus is used for heating articles of different kind and size, the power supplied to the load should be adjustable. To this .end it is usual tomake the supply voltageiof the. oscillator adjustable.

To indicate the oscillator load, use. may be made of a device indicating anode current and grid current since, as is well known, the anode currentand grid current increase and decrease respectively with an increasein load.

The present invention has for its object to provide an indication ofload matching in highfrequency heating apparatus of the said type, to permit adjustment to optimum load matching and/or .to avoid detrimental overloading of the oscillator tube.

According to the invention, this is achieved in a simple manner inchigh-frequency heating apparatus ofthe aforesaid type. by using as an indicator of load-matching a difierential device which is controlled in opposite senses by the anode current and the grid current.

Thev 'difierential device preferably consists of apointer device with a central zero position.

lfa differential pointer device is chosen such that in a normal, but otherwise arbitrary Working condition and optimum load matching, the influences exerted by the anode current and the grid current compensate oneanother, it is surprisingly found that the indication obtained consistently indicates practically optimum load matching in Working conditions varying Within wide limits, such as occur upon variation of, say, the oscillator supply voltage, the coupling between oscillator and load, the oscillator backcoupling or a combined variation of'these factors. Upon wrong matching the direction of deviation of the pointer device indicates undermatching or overmatching.

The pointer device may-comprise scales which are calibrated in terms of the power supplied to the load for different oscillator supply voltages.

According to a further feature of the invention, a differential device may also be used, whilst retaining the aforesaid advantages, to indicate or prevent detrimental overloading of the oscil-. lator tube. For this purpose the differential device preferably consists of a differential relay functioning as a maximum relay having a normally closed contactplaced in the anode supply circuit. 7

In order that the invention may be more clearly understood and readily carried into effect, one example will now be described with reference to the accompanying drawing.

Fig. l is a schematic diagram of one preferred embodiment in accordance with they invention; and

Fig. 2 11s a schematic diagram of a modification of the circuit shownin Fig. 1.

In the circuit arrangement. shown in Fig. 1, the high-frequency energy is taken from a class- C connected electron tube oscillator comprising a directly heated triode I.

In the anode circuit of the electron tube oscillator, consisting of a Colpitts arrangement, the frequency determining oscillatory circuit comprising a coil 2 and series-connected .

condensers

3, 4 is connected to the anode 5 of the triode by way of a blocking condenser 5. The positive feedback voltage is taken from the capacitative voltage divider formed by the

.condensers

3, 4 and is supplied, by way of agrid condenser I, to the control grid l0 which is connected to the cathode 9 by way of a grid resistance 8.

The circuit current appearing in the oscillatory circuit upon oscillation of the circuit arrangement is used for heating a workpiece l I. For this purpose a part I? of coil 2 is inductively coupled with a coupling coil l3 which usually consists of a single turn and is connected to a work coil I 4.

For optimum load matching, the coil part I! is furnished with tapping points 12f. so that the turns ratio of the transformer constituted by coil part l2 and coupling coil I3 is adjustable in steps. Furthermore, load matching is obtainable by placing an auxiliary coil IS in the anode circuit of the oscillator tube, which coil, together with the parts of coil 2 not forming the coupling coil I2, constitute an autotransiormer having a variable coupling factor. This may be effected by arranging an axially movable coupling coil l5 concentrically in coil 2.

The supply voltage of the oscillator circuit arrangement is obtained by full wave rectification, by means of rectifiers l6, ll, of an alternating voltage of mains frequency which is taken from the secondary of a transformer 18. This direct voltage is supplied, by way of chokes I9, l9, to the anode 6 of triode I. The primary of the transformer i8 is provided with tapping points 20, so that the supply voltage is variable, in order that the power to be supplied to the load may be variable.

In order that, with greatly varying working conditions, optimum load matching is consistently possible and/or detrimental overloading of the oscillator tube may be avoided, a differential pointer device is used and is controlled in opposite senses by the anode-current and the gridcurrent.

This may consist of a differential device comprising two coils connected in the anode-circuit and grid-circuit respectively, but it is preferred to use, as in the present example, a combination of resistances 2| and 22, shunted by a pointer device 23 having a central zero position, and connected in the grid-circuit and anode-circuit respectively. The resistances 2| and 22 are chosen such that in the case of optimum load matching, the pointer of the device 23 is in the central zero position.

In the case of optimum load matching, the indication is consistently practically zero and independent of the working conditions. In the case of wrong matching a deviation to the one side indicates undermatching, whereas a deviation to the other side indicates overmatching also with respect to the value. The indication with respect to load matching is more accurate as the supply voltage increases.

In the circuit arrangement shown, the feedback factor of the oscillator, upon variation of the transformation ratio, varies in such manner as to counteract optimum load matching. This may be prevented by using a current dependent resistance acting as a grid-resistance 8.

Some data for circuit components which have been found satisfactory in practice, for use in the circuit shown in the drawing, are given below:

The electron tube oscillator comprises two parallel-connected Philips transmission triodes type TA 12/20 Maximum power supplied, 20 kilowatts Maximum anode voltage, 10 kilowatts Maximum anode current per tube, 1.9 amp.

Maximum grid current per tube, 0.5 amp.

Pointer device: voltmeter with central zero position and a maximum deviation at approximately 5 volts.

Value of the resistance 22 placed in the anodecircuit 1 ohm.

Value of the resistance 2| placed in the gridcircuit: 7 ohms.

The high-frequency furnace used in practice should have a great working security. For this purpose it is known to connect in the cathodecircuit a resistance 24 providing a negative gridbias by which overloading is counteracted. In order to reduce the possibility of detrimental overloading, however, in the case of abnormal handling of the heating installation, as shown in Fig. 2, a diiferential relay 25 functioning as a maximum relay may be used as a differential device, which relay comprises a normally closed contact 26 placed in the anode supply circuit.

A device of this type used for preventing overloading permits the use of a smaller cathode resistance or even the omission of this resistance, which results in a higher efficiency of the heating installation.

The invention may also be used in oscillator arrangements other than that shown in the drawing, for example in tube oscillators, in which the feedback voltage is taken from a feedback coil which is placed in the grid-circuit and is inductively coupled with the frequency determining oscillatory circuit included in the anode-circuit, and may also be used in oscillation arrangements with capacitative coupling of the load.

What we claim is:

1. A high-frequency heating apparatus comprising a class C electron tube oscillator, having a variable supply voltage and an output circuit included in the anode circuit, from which output circuit a positive feedback voltage is taken and supplied, by way of a grid condenser, to a control grid connected to the cathode by way of a grid leakage resistance, and furthermore comprising means for load matching, characterised in that a differential device controlled in opposite senses by anode current and grid current is provided for indicating load-matching.

2. A high-frequency heating appa-artus as claimed in claim 1, characterised in that the differential device comprises two coils which are connected in the anode-circuit and grid-circuit respectively.

3. A high-frequency heating apparatus as claimed in claim 1 characterised in that both the grid circuit and the anode circuit of the oscillator comprise a resistance, which resistances are shunted by the said differential device.

4. A high-frequency induction heating system comprising a class C oscillator including an electron discharge tube having a cathode, a grid and an anode, a resonant output circuit coupled to said anode, a condenser regeneratively coupling said output circuit to said grid, means to couple said output circuit to a load, and means to supply operatin potentials to the electrodes of said tube to sustain oscillations therein whereby an increase in load results in an increase in anode current and a decrease in grid current; and apparatus for indicating optimum matching of said oscillator to said load including means coupled to the grid of said tube to develop a first voltage proportional to said grid current, means coupled to the anode of said tube to develop a second voltage proportional to said anode current, a differential indicator, and means to apply said first and second voltages in opposing senses to said indicator.

5. An arrangement, as set forth in claim 4, wherein said differential indicator comprises first and second coils to which are applied said first and second voltages respectively, and a zero center pointer actuated by said coils and occupying a zero position when the voltages applied thereto are equal.

6. An arrangement. as set forth in claimli,

wherein said first and second Voltages are of equal amplitude in the optimum load matching condition of said oscillator.

7. An arrangement, as set forth in claim 6, wherein said indicator further includes a plurality of calibrated scales corresponding to predetermined supply potentials.

8. A high-frequency induction heating system comprising a class C oscillator provided With an electron discharge tube having a cathode, a grid and an anode, a resonant output circuit coupled to said anode, a condenser regeneratively coupling said output circuit to said grid, means to couple said output circuit to a load, and means to supply operating potentials to the electrodes of said tube to sustain oscillations herein whereby an increase in load results in an increase in anode current and decrease in grid current; and apparatus for indicating matching of said oscillator to said load including means coupled. to the grid of said tube to develop a first voltage proportional to said grid current, means coupled to the anode of said tube to develop a second voltage proportional to said anode current and equal to I said first voltage in the optimum matching condition of said oscillator, a differential relay, means to apply said first and second voltages in opposing directions to said relay whereby said relay is actuated When said second voltage exceeds said first voltage, and means responsive to the actuation of said relay to interrupt said supply potentials.

EMILIUS CAROLUS WITSENBURG. HENDRIK ADOLF TEUNISSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS