US3792369A

US3792369A - Variable reactance controls for ac powered heating magnetrons

Info

Publication number: US3792369A
Application number: US00288714A
Authority: US
Inventors: M Levinson
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-09-13
Filing date: 1972-09-13
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12

Abstract

A variable reactance combined in series with a magnetron where, when said variable reactance is a variable capacitive reactance, means are provided for DC operation of said magnetron and AC operation of said capacitive reactance; where variable reactance can be a fixed reactance and variable reactance combination; and where said variable reactance provides the regulation required by said magnetron over normal voltage variations of an electric utility service.

Description

United States Patent 91 [111 3,792,369 Levinson Feb. 12, 1974 VARIABLE REACTANCE CONTROLS FOR 3,396,342 8/1968 Feinberg 328/262 AC POWERED HEATING MAGNETRONS 3,684,978 8/1972 Otaguro 331/185 X Inventor: Melvin L. Levinson, 1 Meinzer St.,

Avenel, NJ. 07001 Filed: Sept. 13, 1972 Appl. No.: 288,714

Related US. Application Data Continuation-impart of Ser. Nos. 739,778, June 25, 1968, and Ser. No. 841,507, July 14, 1969, and Ser. No. 202,314, Nov. 26, 1971.

[52] US. Cl 331/71, 219/1055, 328/171, 328/253, 331/86, 331/185 [51] Int. Cl n 0359/10, 1105b 9/00 [58] Field Of Search 219/1055; 315/284, 285; 323/90, 91; 328/169-171, 253, 258, 262; 331/71, 8691,185, 186

[56] References Cited UNlTED STATES PATENTS 2,921,171 1/1960 Long.., 219/10.55

FOREIGN PATENTS OR APPLICATIONS 1,072,336 12/1959 Germany 331/86 Primary Examiner-l-lerman Karl Saalbach Assistant Examiner-Siegfried H. Grimm 9 Claims, 8 Drawing Figures variable reoctonce 1 'u-ti Iity service PATENTEDFEBIZIHM 3.792.369

SHEEIIDFB 'utility service utility service Fieoi 4 PIC-302 variable 10 3 variable 2 H 1 reactance I 2 1 q'recictcince] uti| ity 'utility se rv ice service 12 6 5 7 1 l 1 S J I utility utility service service VARIABLE REACTANCE CONTROLS FOR AC POWERED HEATING MAGNETRONS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of my copending applications: System to Power a Microwave Oven, U.S. Ser. No. 739,778, filed June 25, 1968; Power Supply Circuit for a Heating Magnetron, U.S. Ser. No. 841,507, filed July 14, 1969; and, Power Supply for a Heating Magnetron, U.S. Ser. No. 202,314, filed Nov. 26, 1971.

BACKGROUND OF THE INVENTION High power microwave furnaces are in competition with conventional gas and electric furnaces. The gas and electric furnaces both have had a major advantage since, they, by simple control, can be made to operate in small increments from low to high. This invention provides a magnetron-powered, microwave furnace with a variable reactance control which is as simple and as useful a control as the variable gas control on a conventional gas range. As the variable gas control, said variable reactance control does not expend power even while it is controlling high power. Said variable reactance, also similar to said gas control, provides complete control of a microwave generator from no output to its maximum design output.

With simple, complete variable power control a processor is able to select a microwave power output level which both complements an individual workloads characteristics and which allows normal internal conducted and convected heat transfer to assist in equalizing microwave spot and selective heating. The ability to select the proper microwave power level can mean: holding a liquid batch at a simmer vs. full-on uncontrolled boiling; the ability to use a microwave furnace, at full power, first to rapidly heat an article than to hold it, at reduced power, in a hot ready state; drying wet clay by microwaves at the highest power level by which water vapour can normally exit vs., at full power, the wet clay exploding. This invention is most advantageously employed in high-voltage, high-power industrial processes, other competing controlling means are available for low power applications, for example, my copending application, U.S. Ser. No. 202,314.

Various systems are employed to adjust the power output of a high-power magnetron power supply. In prior art, variable inductances, variacs, saturable reactors and transformer taps have been used to adjust the power output of a massive, high-voltage, magnetron plate transformer and means for adjusting a magnetrons magnetic field, to adjust its power output, have been employed. An improved system is required, since a variable inductor, variac or saturable reactor aggregated to a transformer which in turn is aggregated to a magnetron is not suitable to control the full gamut of said magnetrons power output and, initially and in operation, a transformer is costly and space consuming; transformer taps are of no use in a transformerless power supply; and adjusting the magnetrons magnetic field has limited utility.

It is an object of this invention to combine a variable reactance with a magnetron to, at one time, enhance the operation of said magnetron with regulation over normal voltage variations presentin an electric utility service, fusing, wattless variable power control, simplified off-on switching, and, with a capacitive reactance, an effective, higher power output. And, to produce in combination a circuit, which over prior art aggregations, is relatively simple and inexpensive.

It is a further object of this invention to create a compact, high-voltage, high-power, high-efficiency, wellregulated microwave power supply with full variable power-output control in a system that does not require either a line transformer by an electric utility service to step down voltage or a high-voltage, plate transformer by a processor to step up voltage where both transformers, at high-voltage, high-power, are expensive, massive and incur internal power losses.

It is still another object of this invention to cause a flow of selected, desired power output from a heating magnetron by means of combining a controllable, variable reactance directly in series with said heating magnetron.

It is still another object of this invention to create a compact, high-power power-supply for microwave heating whose output can be fully controlled by an operator in response to temperature and changing characteristics of a workload.

SUMMARY OF THE INVENTION Broadly the invention is combined with a circuit for varying the power output of a magnetron discharging across an alternating, public electric utility service and consists of one mesh where said one mesh combines, as two terminal elements, a magnetron and a variable controllable reactance, and where, when said reactance is a capacitance, means are provided across said magnetron to conduct in a direction opposite said magnetron.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a variable capacitance power supply circuit for a magnetron.

FIG. 2 is a variable inductance power supply circuit for a magnetron.

. FIG. 3 is a full-wave, variable-reactance power supply circuit for a magnetron.

FIG. 4 is a variable-reactance power supply circuit for two magnetrons.

FIG. 5 is a graph of the resultant impedance of various size capacitive reactances added vectorially to the resistance of one magnetron and relates to FIG. 1.

FIG. 6 is a view of an embodiment of the invention whose circuit diagram is FIG. 1.

FIG. 7 is a fixed capacitance, variable inductance power supply circuit for a magnetron.

FIG. 8 is a fixed inductance, variable capacitance power supply circuit for a magnetron.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, a series-parallel circuit, consisting of a variable capacitance 5 in series with a parallel circuit of magnetron 2 and a conducting means 7 disposed to conduct in a direction opposite magnetron 2, is connected across high-

voltage power line

3, 4 of a conventional, alternating, public electric utility service 1, as a 4,160v, 13,200v or 26,400v power line. Magnetron 2 is operated half wave and variable capacitance 5 is operated full wave. Means 7 (shown as a diode) is added to conduct in a direction opposite and in parallel to that of magnetron 2 to make said half wave and full wave operation compatible. Other means 7 can be employed, for example, (FIG. 3) a four diode bridge-full wave rectifier or (FIG. 4) a second magnetron 11 to operate variable capacitance 5 as an AC device and magnetrons 2 and II as DC devices. A reactance-varying means 6 is provided, constructed and disposed as to make the reactance controllable by a microwave furnace operator.

In operation, in FIG. I, when the half cycle of the alternating electric utility service l, applied across magnetron 2, is plate negative, cathode positive, no current flows through magnetron 2, but current flows through conducting means 7 to charge capacitance 5. On the alternate half cycle, plate positive, cathode negative, magnetron 2 discharges across electric utility service 1 through the capacitance of variable capacitance 5 set by reactance-varying means 6 as well as discharging from the electric charge accumulated on capacitance 5 from said previous half cycle.

In FIG. 2, a series circuit, combining a two terminal variable inductance 8 in one mesh with a magnetron 2, is connected across a high-

voltage power line

3, 4 of a conventional, high-voltage, alternating, electric utility service 1. A reactance varying means 6 is provided, constructed and disposed as to make the reactance controllable by a microwave furnace operator further described in my copending application, U.S. Ser. No. 841,507.

In operation, in FIG. 2, when the half cycle of the voltage of alternating utility service 1, applied across magnetron 2, is plate negative, cathode positive, magnetron 2 blocks any series current flow and variable inductance 8 lies dormant. n the alternating half cycle of voltage, plate positive, cathode negative, magnetron 2 discharges across electric utility service 1 through the inductance set by varying means 6 of variable inductance 8.

Variable inductance 8 is a current sensitive device which responds in proportion both to the rate of change in current and the amount of current discharging through magnetron 2. Variable inductance 8, by its nature, first resists the discharge of current through magnetron 2 and then resists the cessation of said discharge.

The action of variable inductance 8 combined with a discharge device, as magnetron 2, is different than the same variable inductance in series combination with a conventional load. Witha conventional load, a variable inductance starts to build up a charge, a voltage drop, from the beginning of each cycle in competition with said conventional load with which it is in series. In contrast to a conventional load, sufficient voltage must appear across magnetron 2 to enable it to discharge and, only as a result of magnetron 2s discharging, can variable inductance 8 become effective.

FIG. 3 illustrates the combination of a variable reactance 9, a magnetron 2, and a full wave rectifier bridge 10.

FIG. 4 illustrates how two magnetrons, magnetron 2 .and magnetron 11, are connected in parallel so as to conduct on alternate half cycles and are combined with and controlled by variable reactance 9.

A magnetron representative of the family of cross field microwave generating tubes which includes amplitrons is a device capable of converting electrical power into electronic power (e.g. radio waves) at projected 98 percent efficiency. Individual magnetrons have been designed to handle one Megawatt of continuous power output (many Megawatts of pulse power output). More than one magnetron can be ganged together to provide many Megawatts of power output. A magnetron is an enclosed device which incorporates a high intensity magnetic field, which is provided by magnets. High frequency oscillation is produced in a magnetron by an electron (vs. ionized gas in a conventional arc discharge) beam emitted by a heated cathode, moving in curved or spiral paths within lobes or cavities because of said high intensity magnetic fields. The frequencies produced are dependent on the geometry of the magnetron and the strength of the magnetic field. The high frequency energy of the magnetron is picked up by a probe extending into the interior of the magnetron in the path of the spiralling electrons and is transmitted to the load by means of wave guides, coaxial cables and the like. Energy is emitted to the load by means of a suitable horn or antenna in a manner well known in the art.

Magnetrons are energized by means of pulsed direct current at high voltages, for example 5,500 volts. When said voltage is applied between the anode and the cathode of a magnetron, it requires a considerable potential to start the magnetron oscillating, and thereafter -current flows with a non-linear characteristic. Although a non-linear device, the magnetron reacts as a positive resistance device in that an increase in voltage results in an increase in current, but the problem of operating and controlling a magnetron is complicated by the fact that small increments of voltage change will result in rather high variations in current.

The higher voltage a magnetron operates at, the longer the projected life of the tube. 50,000 volts may mean 60,000 hours while 100,000 volts may mean 80,000 hours. The physical size of the small working cavity of a magnetron 'is' fixed by the frequency generated. A magnetrons power output is a function of current and magnet strength. With super cooled magnets, high power magnetrons can be discharged across very high voltage alternating electric utility service lines.

A magnetron is a discharge device, but it is different from common discharge devices. The discharge of the carbon arc furnace, the fluorescent light and mercury vapour rectifier tube is initiated by a voltage level that exceeds, for example, I30 percent, the sustaining voltage. The discharge of a magnetron starts gently (e.g. 60 percent) and builds up exponentially to crest at full power (e.g. 100 percent) of the applied voltage. Magnetrons also differ from conventional discharge devices for, to prevent their operating in an improper mode, they are purposely turned off each cycle. This unique, purposeful turning off of magnetron 2 permits variable inductance 8, when combined in the same series mesh with magnetron 2, to unsaturate and prepare to respond to the next maximum change in circulating current when magnetron 2 again turns from off to on as well as responding to the amount of circulating current, per se.

The capacitive reactance 5 and inductive reactance 8 of FIGS. 1 and 2, both are out of phase with the circuit resistance offered by magnetron 2. The discus- In my copending application,.U.S. Ser. No. 739,778,

I describe how a capacitor can be employed to provide,

without the expenditure of power, the necessary regulation of magnetron needs over normal power line variations. In FIG. 5 and Tables I and 2, I have used values taken from the manufacturers data of an Amperex Model DX 206 magnetron representative ofother magnetrons. Table 1 lists some unregulated characteristics of the DX 206 magnetron for comparison with the regulation set forth in Table 2. Table 2 provides values for FIG. 5. While FIG. 5 is a resistive plot, in the discussion that follows, voltage, current and power values that exist at resistive points are inferred. FIG. 5 is a chart of the capacitive reactance X of a capacitor on the y axis and the resistance R, of said representative magnetron, plotted on the x axis. The resultant impedance, Z, to 2, appears across utility service 1 and determines the series current that will flow through capacitance 5 and magnetron 2.

Referring to Table 2, FIGS. 1 and 5, a brief discus sion of design considerations and operation follows:

I. A magnetron 2 is selected which can deliver a desired mean, high power output, point F. Point H is magnetron 2s absolute-maximum, allowable, operating point.

2. For said desired mean-point F, a maximum-point G and minimum-point E high-power points are selected for normal operation.

3. There are normal operating voltage variations present in an electric utility service voltage. An alternating electric utility service is chosen whose low voltage variation is higher than one half the voltage whose application across magnetron 2 will result in the selected maximum, high power output point G (an inductive reactance differs the lowest voltage of the utility service must be higher than maximum voltage, point G). Now turning to the maximum voltage of said electric utility service 1 (a maximum fixed by both government laws and an electric utility companys equipment and policy), a capacitance is chosen whose capacitive reactance both fixes the maximum current that can flow and results in a predetermined, design regulation requirement. For example: The power output of magnetron 2 is'a function of its plate current. The current, that circulates through resistive magnetron 2 and capacitive reactance 5, is a result of the combined impedance they present to electric utility service 1. The resistance R of magnetron 2, at a predetermined, maximum, high-power output, point G, is a known design characteristic. A capacitive reactance is chosen whose reactance X added vectorially to resistance R, at point G, results in (again using the highest voltage of said electric utility service) the proper impedance Z to permit said desired design current to circulate which results in point G. It can be seen, in Table 2, that each higher-voltage, electric utility service voltage requires a smaller capacitive reactance and provides better regulation. The utility (A to D and higher) service 1 selected should be the one that most nearly results in, at mean and minimum line voltages, magnetron 2s corresponding mean high-power point F and minimum highpower point E. The higher the utility service voltage the better the regulation at the expense of the power fac- X.

4. After the proper size utility service and capacitor is selected, power is varied down to magnetron cut-off by physically reducing the size of said capacitor. This reduction in size of said capacitance raises the capacitive reactance X,; which increase in the impedance lowers the current flow in said series combination; and which lower current lowers said magnetron 2s output and increases the resistance R offered by said magnetron 2. Said increase in resistance by said magnetron 2 further lowers said current until a balanced final current is reached and the new desired lower magnetron 2s power output is achieved. It should be noted that since a magnetron requires a high voltage above which it can discharge, the variable capacitance need vary only over the top part of its capacitive reactance range to result in a fully variable power output and the effective cutoff of magnetron2. Since it need never vary, it is preferred that that part of variable capacitive reactance 5 (or its counterpart, variable inductive reactance 8), above which magnetron 2 discharges, be constructed as a fixed capacitance on which the required variable part of the capacitive reactance is appended. Since said fixed capacitive reactance is always present in the circuit with magnetron 2, it can act as a fuse (i.e. act as a means to fix the total current that can flow) if magnetron 2 or means 7 should fail in service.

.ll K tL @Dd ate .t ffs ent' r i;

trary ei'tf'ciitiiit services. Tiiing'hr the electric utility service s voltage, the better is the regulation over normal power line voltage variations service ll would have to vary an unreasonable +1636 percent above normal, in plot A, to reach absolute maximum operating point l-I. Utilizing even higher utility voltages than those illustrated will result in a smaller power factor and better regulation. If said utility voltage is slightly more than one half (in this example, 2,760 volts, point D) of said magnetrons design discharge voltage (in this example, 5,500 volts) regulation is poor, but variable control allows some regulation by manual control. Without good regulation, proper operation is more demanding of an operator and the results are not as dependable. Other forms of regulation can be added as the saturable reactor and transformer of my copending application, U.S. Ser. No. 739,778 to provide better regulation. But, it should always be kept in mind, that both high-power saturable reactors and transformers are initially costly and space consuming and incur high operating costs, for example, a 750,000 watt magnetron transformers normal operating losses can continuously waste 50,000 watts during operation.

Variable power is also useful in the off-on switching of a high-power magnetron. It is preferred to switch the magnetron on at its lowest power output (i.e. the variable component of the variable reactance at its highest reactance) and gradually turn up the power. This gradual turning on, permits the electric utility service to better accomodate high-power drain and avoids the sudden shock of sudden high-power loading. This gradual turning on of high-power differs from the turn on of a high-power, transformer-operated, discharge furnace, for example, when a carbon arc steel furnace is switched on and off, other customers connected across a common electric utility line must be protected from serious voltage and transient surges. This gradual turning on is useful in many microwave processes to aid in uniform heating.

FIG. 6 is one embodiment of the circuit of FIG. 1 where a two plate variable capacitor M is shown and in which one plate is an extension of a factorys over;- head, branch electric-service-feeder-line 62 as it arrives at a factory s substation 86. In FIG. 6, electric utility line 62 is illustrated arriving at factory 85s substation 86 via high voltage towers 63 and 64. Wire length 69 is that section of utility line 62 that is illustrated leaving high-voltage insulator 82 on tower 64 and terminating on high voltage insulator 75 located on a fixed ground anchor 73 in substation 86. A pivot 81 is secured to tower 64 by high-voltage insulator 67. A second plate of capacitor 61,

electric line

65, 65 is illustrated following branch electric line 69 to terminate in high-

voltage insulator

83, 83 located on

movable anchor

70, 70. From pivot 81 wire 74 connects wire 65 to magnetron 66. Other high-

voltage insulators

75,75 are illustrated where applicable.

Movable anchor 70 is designed to be moved easily anywhere between

stop

71 and 72. 1t is preferred to motorize and remotely control movable anchor 70.

Forming a capacitor from the simple paralleling of two ordinary high-voltage lines can be improved by making any part of said lines larger in any areas they are in juxtaposition. For example, making at least part of said lines into flat ribbons 84,84 to provide the same amount of capacitance in a shorter distance. Movable anchor 70s function is to

separate lines

65, 69 and thereby mechanically lower the capacity and represents one means (e.g. means 6 of FIG. 1) of adjustment for varying the power output of magnetron 66. Moving the

lines

65 and 69 farther apart would turn down the power output of magnetron 66 and provide simplified no current, off-on switching. In line 74, disconnect switch 78 and wattmeter 80 are shown. Switch 78 may be ganged to movable anchor 70 to disconnect disconnect switch 78 at the lowest capacitance, largest reactance of capacitor 61.

The variable capacitance may be made into the configuration of a coaxial cable (not shown) incorporating high dielectric material to shorten the length and reduce the spacing. Peeling back at least part of the outer conductor would vary the capacitance. The variable capacitance may be made of a number of wires surrounding and paralleling an electric utility feeder line and varying the capacitance by means to vary mechanically or electrically the number of paralleling lines.

A variable inductance can provide the same utility as a variable capacitance, but, at very high voltages, mechanical problems arise providing proper electrical insulation for said variable inductance. The higher the voltage the more practical an air dielectric variable capacitance becomes. High-voltage towers are in wide spread employment throughout the country and are still highly practical for high voltage lines since air is an excellent electrical insulator and an air dielectric is self healing. in lower, high-voltage, high-power applications, 3 fixed capacitance can be employed in place of the fixed parallel power lines or a variable inductance may be preferred, when, as the voltage falls, the required variable capacitance becomes mechanically too large.

FIG. 7 illustrates a fixed capacitance 12 in series with a variable inductance 8. A fixed capacitive reactance 12 is chosen to just cutoffa given magnetron tube operating across a given electric utility service 1 whose voltage exceeds magnetron 2s voltage handling capabilities. Sincean inductive reactance and a capacitive reactance are 180 out of phase, a variable inductive reactance 8 is then added and varied to buck the capacitive reactance 12 and reduce the resultant total circuit impedance. As the resultant circuit impedance across utility service 1 lowers, the circulating current through magnetron 2 rises with an accompanying increase in power output. In this case, low or no inductance results in low power and the highest inductance results in full power. FIG. 8 is an alternate to FIG. 7 where a fixed inductance l3 and a variable capacitive reactance 5 are employed.

Multiphase high-voltage lines may be used. These represent a multiplication of FIG. 1 where additional capacitors regulate each phase and can be ganged to vary the power output of a single magnetron.

Although this invention has been described with a certain degree of particularity it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention.

TABLE 1 Percent Percent PereentI I E E W W 5,400v -1.82 17.1511 980w -11 5,500V 0 15.70k 1,100w 0 5,600v +1.82 14.50k 1,200w +0 TABLE 2 Percent cos R R ohms Z ohms E. E. R/

E 17.151; 32.4k 5,120v 6. 8 .529 F 15.70k 31.4k 5,500v 0 5 G 14.50k 30.91! 5,960v +8. 37 475 H 13.90k 31.2k 6,400v +16. 36 446 E 17.151; 26.21! 4,140v 6. 55 655 F 15.701: 25.351: 4,430v 0 619 G 14.501; 24.71;? 4,770v +7. 8 .588 E 17 .15k 19.81; 3,140v 3. 22 867 F 15.701; 18.551; 3,245v 0 846 G 14.501; 17.61: 3,390? +4. 47 824 E 17.151: 17.211 2,720v 1. 45 998 F 15.70]! 15.75k 2,760v 0 .998 G 14.50k 14.55 2,850v +3. 26 998 I claim:

1. An operating circuit for a high-power heating magnetron which includes a magnetron; a variable reactance to vary the power output of. said magnetron; means for an operator of said circuit to vary the reactance of said variable reactance; and means to connect said variable reactance and said magnetron to an alternating-voltage, electric utility service so that said magnetron discharges across said utility service during at least one voltage alternation of said utility services alternating voltage, where said improvement in saidcircuit comprises in combination:

where said variable reactance is connected in series in the discharge circuit of said magnetron and, during said voltage alternation of said electric utility service in which said magnetron discharges, said magnetron blocks current flow to prevent a voltage drop across said variable reactance until said magnetron'discharges and, when said magnetron discharges, said variable reactance limits the plate current of said magnetron to a value below said magnetrons maximum allowable plate current notwithstanding normal variations in the voltage of said electric utility service.

2. In an operating circuit according to claim 1 where said variable reactance is a combination of a fixed reactance l5 and a variable reactance.

3. In an operating circuit according to claim 2 where said fixed reactance is a capacitor and said variable reactance is an inductor.

4. In an operating circuit according to claim 2 where said fixed reactance is an inductor and said variable reactance is a capacitor.

5. ln an operating circuit according to claim 1 where said variable reactance limits the change in said plate current of said magnetron, said plate current being maintained at a substantially constant current notwithstanding normal variations in the voltage of said electric utility service.

6. In an operating circuit according to claim 1 which includes a disconnect switch and where said disconnect switch is ganged to said means for an operator to vary the reactance of said variable reactance to disconnect said magnetron at the lowest magnetron output position of said means for an operator to vary the reactance of said variable reactance.

7. In an operating circuit according to claim I where said variable reactance is a'variable inductance, where the full voltage drop of said utility service appears across said magnetron up to its discharge voltage, and where voltages of said utility service which are higher than said magnetrons discharge voltage divide in proper phase across said resistive magnetron and said variable inductance.

8. In an operating circuit according to claim 1 which includes a second magnetron connected across said first magnetron, the anode of said first magnetron connected to the cathode of said second magnetron and the cathode of said first magnetron connected to the anode of said second magnetron, where said first and second magnetrons conduct on alternate alternations of said alternating-voltage electric utility service and where both said magnetrons discharge in response to said service and vary their power output in response to variations in said variable reactance.

9. In an operating circuit according to claim 1, where said variable reactance is a variable air capacitance of at least two plates and where means are provided to mechanically move said plates apart.

Claims

1. An operating circuit for a high-power heating magnetron which includes a magnetron; a variable reactance to vary the power output of said magnetron; means for an operator of said circuit to vary the reactance of said variable reactance; and means to connect said variable reactance and said magnetron to an alternating-voltage, electric utility service so that said magnetron discharges across said utility service during at least one voltage alternation of said utility service''s alternating voltage, where said improvement in said circuit comprises in combination: where said variable reactance is connected in series in the discharge circuit of said magnetron and, during said voltage alternation of said electric utility service in which said magnetron discharges, said magnetron blocks current flow to prevent a voltage drop across said variable reactance until said magnetron discharges and, when said magnetron discharges, said variable reactance limits the plate current of said magnetron to a value below said magnetron''s maximum allowable plate current notwithstanding normal variations in the voltage of said electric utility service.

2. In an operating circuit according to claim 1 where said variable reactance is a combination of a fixed reactance 15 and a variable reactance.

5. In an operating circuit according to claim 1 where said variable reactance limits the change in said plate current of said magnetron, said plate current being maintained at a substantially constant current notwithstanding normal vaRiations in the voltage of said electric utility service.

7. In an operating circuit according to claim 1 where said variable reactance is a variable inductance, where the full voltage drop of said utility service appears across said magnetron up to its discharge voltage, and where voltages of said utility service which are higher than said magnetron''s discharge voltage divide in proper phase across said resistive magnetron and said variable inductance.