US2516089A - Thermionic valve apparatus - Google Patents

Description

y 18, 1950 s. G. KING 2,516,089

THERMIONIC VALVE APPARATUS Filed July 24, 1945 2 Sheets-Sheet l d I nve ztor y @2124, @411, M vw,

Attorney July 18, 1950 as. KING 2,516,089

THERMIONIC VVALVEI APPARATUS Filed July 24, 1945 .2 Sheets-Sheet 2 Attorney Patented July 18, 1950 THERMIONIC VALVE APPARATUS Stanley George King, London, England, assignor, by mesnc assignments, to The Ohio Crankshaft Company, Cleveland, Ohio, a corporation of Ohio Application July 24, 1945,'Serial No. 606,816 In Great Britain May 8, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires May 8, 1964 1 Claim. (Cl. 323-89) v The present invention relates to thermionic valve apparatus for controlling the value of an alternating electrical quantity in accordance with fluctuations of another quantity. One application of the invention is to controlling the voltage across a pair of load terminals in accordance with fluctuations of voltage of the alternating current supply from which the load is fed. The arrangement may be such that the load voltage (or other controlled quantity) follows some predetermined law, but an important application of the invention is to means for reducing fluctuations of voltage across a pair of load terminals due to fluctuations of voltage of the supply,

According to one feature of the present invention the apparatus comprises a thermionic valve controlled in accordance with the controlling quantity or its fluctuations, and a variable reactance choke having a control winding fed from the said valve and serving to vary the permeability of the core and hence the reactance of the choke winding in accordance with the said fluctuations, the choke winding being connected in the circuit of the controlled quantity so as to produce the desired control.

According to a. further feature of the present invention electrical control means .for reducing fluctuations of voltage across a pair of load terminals due to fluctuations of voltage of the alternating current supply from which the load is fed, comprises a thermionicvalve controlled in accordance with fluctuations of supply or load voltage, and a choke having a main winding connected in series between the load and the supply'and a control winding carrying a current controlled by the valve and serving to vary the permeability of the core and hence the reactance of the main winding.

In one form of the invention for obtaining a substantially constant output voltage from a varying input voltage the input voltage is applied across two opposite junctions of a bridge having lamps in two opposite arms and resistances in the other two arms, and the grid of the valve is controlled in accordance with the voltage across the other two opposite junctions.

Whilst the controlling and controlled quantities must themselves b electrical quantities it is not necessary that the former should be the original source of the fluctuations or that the ultimate purpose should be to control the latter. Thus the controlling quantity may merely be an electrical quantity such as current, voltage, or the like which is used to represent a non-electrical quantity such for example as temperature,

pressure or the like, while the controlled quantity may in turn control the rate of flow of fuel or other fluid or the speed of a mechanism or the like.

A further feature of the invention is particularly concerned with, although not limited to, valve oscillators for industrial heating such as dielectric or induction heating.

Normally in such apparatus the valve is not expected to deliver its maxium output for more than a proportion of the time, and one feature of the present invention comprises the provision of an impedance in the filament heating circuit for reducing the filament current when the maximum output is not required, and means for automatically cutting out or reducing the impedance when the maximum output is required.

By this means a great increase of valve life can be achieved since a small reduction of filament voltage below the figure specified by the makers for efllcient operation will greatly increase the life of the filament. On the other hand even if no output at all is required switching off the filament completely has the disadvantage that it takes some time to heat up again and the output will therefore not be available promptly, say within a second or two, when it is required,

In one form of the invention the variable impedance is arranged to be automatically varied in accordance with the output current of the valve. For example the impedance may be constituted by a choke having one or more control windings to enable the permeability of the core to be varied by partial saturation to vary the impedance. Such a choke may have acore comprising three limbs with the windings arranged so that the flux due to the filament current circulates through two of them but notthe third on which the control winding is carried. Alternatively it may be constituted by a pair of transformers connected in series one pair of windings being opposed so that E. M. F.s induced in the control windings by the filament current balance out.

Instead of or in addition to being employed to prolong the life of the valve the control of filament current in accordance with the present invention may be employed to improve the emciency of operating conditions of the valve. 'l'o this end the current in the control winding may be subject-to a combined control or a number of control windings may be provided on the choke.

As already indicated the arrangement may be arranged to reduce the filament current when a the output demanded is reduced as determined by the load for example on an oscillator. In this case a control winding may be fed with a current proportional to the output current in the anode circuit.

A control winding may also be supplied with current which is adjustable manually at will. This will enable the best operating point on the magnetisation curve to be chosen and also enables the apparatus to be adjusted for slight variations between difi'erent valves which though nominally similar may actually require slightly difierent filament voltages.

A control winding may also be supplied with current which builds up gradually or only after a delay when the valve is first switched on from cold. When the filament is cold and its resistance low comparativel heavy filament currents can flow; perhaps several hundred amperes in a large valve, and such currents may damage the seal where the current passes into the evacuated envelope, or the filament supporting structure while the filament itself may be damaged by the high electromagnetic forces produced. The delay in building up the control current may be achieved by the use of a condenser, and/or the control current may be supplied through a small valve rectifier of indirectly heated type which introduces a delay of perhaps 30 seconds. V

A control winding may also be supplied with current which varies with the grid voltage but in the opposite direction, to take into account any reduction in grid current that may occur especially during overload conditions or when grid emission takes place, assuming the oscillator is of the self-excited type.

The invention may be carried into eflect in various ways but certain embodiments will be described by way of example with reference to the accompanying drawings in which Figures 1 and 2 are circuit diagrams respectively of two different arrangements.

In the arrangements shown the invention is applied to the control of the cathode heating current of a thermionic valve oscillator for high frequency heating.

In the arrangement shown in Figure 1, a main oscillator valve A comprises an anode A a grid A and a filament A The loadcircuit for the valve A maybe of any conventional type such as an oscillator circuit of the conventional "Hartley type. Such circuits normally comprise a tapped inductance in parallel with a capacitor and in the figure shown, the outer terminals of the circuit would be connected to the terminals B and B The terminal B would be connected to the midtap on the inductance.

The anode A is connected through a condenser C to terminal B of the load circuit and through a high-frequency choke C to a positive terminal ('3 of a high-tension supply (not shown). The grid A is connected through a condenser C .to the second terminal 13 of the high-frequency load circuit. The filament A is connected across the outer terminals of a center tapped secondary winding D of a filament heating transformer D. The grid A is also connected through a grid resistance, C to the center tap of the filament transformersecondary winding D The third terminal B of the high-frequency load circuit is connected to earth or a common ground which is also connected to the minus terminal of the high-voltage power source.

The voltage supplied to the primary winding 1') of the filament transformer is controlled in accordance with the present invention. For this purpose it is connected to an alternating current supply E 12. in series with the main or alternating current windings F of a variable choke F of the saturable core reactor type. Thus as the core of the choke is more or less saturated, by current passing through the direct current or control windings about to be described, the reactance of the choke will be varied and the voltage applied to the filament of the main oscillator will be controlled. The choke may be of any of various known forms for example a core having three limbs of which the outer limbs carry the main choke windings, arranged so that the flux due to them does not pass through the middle limb,

while the centre limb carries the control windings. Alternatively the choke may consist of two identical transformers having their primary windingsand their secondary windings connected in series oppositely so that the E. M. F.s produced in the control circuit by th alternating current in the main windings, cancel out.

The control referred to is eflected simultaneously in a number of difierent ways in the arrangement shown but it will be appreciated that it is not essential to employ all these ways simultaneously and where one or more forms of control are not required one or more others may be employed independently.

In the first place the filament voltage is controlled in accordance with variations of supply voltage so as to reduce fluctuation of the voltage applied to the filament due to fluctuations of the supply voltage. For this purpose the choke F is provided with a control winding F connected in the anode circuit of a control valve G, shown as being fed through a rectifier G and transformer G connected to the supply E li-' For controlling the grid of the control valve a bridge circuit is connected across the supply E E' The arms of this bridge are constituted so that their resistances vary with the applied voltage to different extents, for example two opposite arms may be constituted by metal filament lamps H while the other two are constituted by constant resistances H Two opposite corners of the bridge being connected to the supply terminals the other two corners are connected through an isolating transformer J to a potentiometer J. The supplyis also connected through a transformer K to a potentiometer K and the tappings of the potentiometers are connected through a rectifier L and a potentiometer L. The tapping and one end of the latter potentiometer are connected respectively to the grid and filament of the control valve G.

'I-Ience in the operation of this control if the supply voltage should increase, for example, the resistances of the lamps H will increase while those of the resistances H will increase less or not at all, and the unbalance current will be applied through the transformer J to the potentiometer J A proportion of this voltage superimposed on a proportion of the supply voltage will then be rectified and a proportion of the result applied by the potentiometer L to the grid of the control valve.

By a nice choice of the characteristics both of the choke and of the control valve, and by adjustment of the circuit characteristics as by the potentiometers in the grid circuit, it is possible very largely to reduce or eliminate variations of the load voltage. A wide choice of valve charauaoso acteristic may be obtained by using a triode, tetrode, or pentode.

The second control winding F on the choke serves to effect control in accordance with the load on the anode circuit of th main oscillator valve so that when the current taken from it is low the filament current will be automatically reduced. In this way the life of the filament can be considerably prolonged and yet when current is required in the load circuit little delay is involved in slightly increasing the filament tem perature to that required for full load emission. Accordingly the control winding F is connected in series with the load circuit of the main valve A conveniently between the filament of the latter and the negative terminal C6 of the high tension supply.

The third control winding F on the choke serves for manual control andin addition provides a time delay when the apparatus is first switched on from cold. For this purpose the control winding F is energised from the supply through 'a small indirectly heated valve rectifier M provided with. a suitable transformer M which need not be described in detail, and a manually adjustable resistance M By means of the latter the flux in the cor of the choke can be adjusted so that it works on a suitable part of its saturation curve, while the time taken by the rectifier M to warm up when first switched on from cold automatically restricts the filament current of the main valve until after the lapse of a certain time which can be to some extent adjusted by suitable choice of the rectifier,

The various controlls referred to, and especially that corresponding to the winding F may enable the filament emission current-for periods when the maximum output is wanted, to be greater than would be possible for continuous operation, and the efliciency to'be raised at the same time, provided precautions are taken, such as the provision of relays for example direct-acting overcurrent trip relays and/or thermal relays, to prevent overloads which are excessive in magnitude or duration.

For example in one case a valve with a nominal filament voltage of 16 gives 50% extra filament emission if this is increased by 5% while a decrease of "7 reduces the emission current to half its normal value. Increase of filament voltage to l'l or even 18 volts is permissible for short periods, while the life is greatly increased by reducing the voltage to 15 or even 14. Hence the choke may be set so that between no load and but by applying to the grid of the control valve a voltage made up of a number of components, actually three, added together. The single control winding F of the variable choke is connected in the anode circuit of the control valve G.

The first component of grid voltage is dependent on the voltage of the supply. Thus a voltage corresponding to that of the supply is fed through a transformer N and a rectifier N to a potentiometer N", In general if the fluctuation 6 of voltage across this potentiometer were sufficient to give the required control of the grid of the control valve G the total voltage across it would afford an excessive negative bias for the valve. Accordingly it is necessary to offset this voltage partially by a substantially constant though adjustable voltage. This may be obtained in any of a variety of ways for example as indicated in the drawing a voltage corresponding to that of the supply, and therefore liable to fluctuation. may be fed through a transformer P and a rectifier P through an adjustable resistance P to a potentiometer P shunted by a gas discharge voltage stabiliser P4, which keeps the voltage across the potentiometer substantially constant in known manner.

Even if the circuit is completely compensated to prevent changes of voltage across the controlled load, in this case the filament of the main valve, due to changes'of supply voltage, some changes of voltage across the load may arise for other reasons, for example due to gradual warming up of various windings, and means may be provided to minimise such changes, whatever their origin. To this end, as shown in the figure, the grid of the control valve is also subject to a control in accordance with the load voltage. Thus the voltage across the load is fed through a transformer Q and a rectifier Q to a potentiometer Q The portions of the potentiometers N P and Q in each case between the tapping and one end, are connected in series with each other and with a grid current limiting resistance R, between the grid and cathode of the control valve G. By appropriate adjustment of these potentiometers it is possible greatly to reduce fluctuations of load voltage due to fluctuation of supply voltage as well as due to other causes, In particular the component of grid voltage due to the potentiometer Q tends to reduce any fiuctuation of load -voltage due to fluctuation of frequency of the supply. If desired, however, a separate control may be provided for this purpose either instead of or in addition to that repre sented by the potentiometer Q If such control is not required the transformer N may be connected directly across the supply, but if it is to be included this transformer is connected, as shown, to a frequencv-sensitive potential .divider comprising a condenser S in series with a resistance S connected across the supply, the transformer N being shunted across the condenser.

If the frequency should increase, for example,

' the impedance of the main valve filament will choke will be increased and the current will be not be substantially affected, but that of the control winding of the choke, is provided by furnishing the control valve anode current from an indirectly heated valve rectifier T instead of from a metal rectifier. Until this warms up no anode current can fiow and the choke retains its maximum impedance;

In some cases it may be desirable to provide what may be termed a two stage control b providing a second variable reactance choke controlled by the first, In such a case the main windings of the first choke are connected in series with the primary winding of a rectifier feedtransformer across the mains, while the secondary winding of this transformer is connected to a rectifier. The rectified output is then connected to the control winding of the second stage variable choke the main windings of which are connected in the circuit of the current it is desired to control. In this case the circuit characteristics will not be arranged to sup l a constant rectified output but an output varying appropriately to give the desired control in the final stage.

It has been assumed above that the load is constant. Obviously if a constant output voltage is obtained by variation of a series reactance, the value of the voltage will be altered if the load alters. In such a case it may be desirable to control the reactance in accordance with output load as well as input voltage, for example by providing one or more additional control windings on one or each variable choke. If there are to be wide variations both in load and in mains voltage, it may be useful or even necessary to provide a dummy load in addition to the ordinary load. This dummy load may conveniently be a saturable core reactor, the control windings being suitably connected (in the proper sense) to the valve control circuit.

It will be appreciated that the different forms of control described may be applied independently, for example the grid of the valve G may be controlled only in accordance with fluctuations of load voltage, In the latter case of course it will be impossible to secure a completely uniform output since some fluctuation must remain to efi'ect the control, for example if the mains supply voltage varies plus or minus 10% the equipment can easily be arranged to keep the variation of the output below say plus or minus As to the quality of the output voltage it should be borne in mind that the saturable choke may introduce distortion in the current wave form and it might therefore be necessary in certain circumstances to specify whether the constancy of output occurs at root mean square voltage or at mean voltage. If the equipment is to supply a heating device (such as a valve filament) constant R. M. S. voltage would be desirable, but in other cases, for example a rectifier circuit, constant mean voltage might be required. The choice of circuit constants will determine the conditions obtained.

One use for the invention has already been suggested, namely the feeding of a controlled voltage to the filaments of large oscillator valves. Another use is the control of generator output voltage. In this case some voltage-sensitive circuit such as a resistance-lamp bridge is essential unless the output voltage is capable of direct comparison with some standard such as a standard cell. If the generator is an alternator a quantity corresponding to its output may be rectified and then applied to the lamp bridge mentioned above. This will enable a polarised feedback voltage to be obtained from the bridge and this voltage in turn may be used to feed the grid circuit of the control valve. The valve anode current may pass through the control windings of a saturable choke; this choke being inserted in series with the A. C. sourc feeding the alternator field windings through a suitable rectifier or exciter. The output voltage of the generator will depend, not only on the value of the field voltage, but also on the resistance of the field winding, and on the magnitude (and phase angle for A. C.) of the output current, The output voltage may thus be subject to considerable variation but the basic circuit of this invention will deal with the variation whatever the cause and will always tend to reduce it although it will never reduce the deviation to zero. It is to be noted that when the system is used for control of alternator output voltage by means of field current variation, there may be circumstances when it will be necessary for the voltage-sensitive circuit to be of a type which will be responsive to wave form as well as numerical value of the output voltage. The circumstances will depend on the kind of load to be fed by the alternator.

The invention makes it possible to obtain considerable flexibility of control in accordance with the characteristic desired whether it be rising. drooping or constant, since wide variation is possible in the saturation characteristics of the reactances, and the electrical characteristics of the valve as well as in the choice of valves and arrangement of the various other components of the circuit.

An advantage of the system is that it is not specially dependent on maintenance of constant mains frequency because no resonant circuit is employed, in fact, the mains frequency may vary over fairly wide limits, say 40 to 60 cycles, providing this is allowed for in the original design of the components.

What I claim as my invention and desire to secure by Letters Patent is:

In a high-frequency power source, including a valve having an anode and a filament, a source of power for heating said filament and means for limiting the filament current when initially turned on, for maintaining the filament voltage substantially constant despite line voltage variations during continuous operation, and for increasing the filament current in response to anode currents; said means including saturable core-type apparatus intermediate said filament and said source of power, time-delay means, line voltage-responsive means and anode currentresponsive means, all of said means having a winding associated with said saturable core-type apparatus for appropriately controlling the degree of saturation and, therefore, the filament current.

STANLEY GEORGE KING.

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

UNITED STATES PATENTS Number Name, Date 1,497,948 Shoenberg June 17, 1924 1,979,352 Scofield Nov, 6, 1934 1,994,076 Kuhle Mar. 12, 1935 2,001,567 Case May 14, 1935 2,015,556 Fountain Sept. 24, 1935 2,066,943 Philpott Jan. 5, 1937 2,067,143 Logan Jan. 5, 1937 2,079,206 GraiT ,et al May 4, 1937 2,149,080 Wolff Feb. 28, 1939 2,228,078 Gulliksen Jan. 7, 1941 2,230,558 Bowen Feb. 4, 1941 2,236,195 McKesson Mar. 25, 1941 2,372,104 Myers May 20, 1945

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