US2034126A - Electric valve converting system - Google Patents

Description

Max-Eh Q H, U

ELECTRIC VALVE CONVERTING SYSTEM Filed Nov. 15, 1933 3 Sheets-Sheet 1 Vi I. g MWEWW 1 nventor:

Ciodius H. Wiliis,

by E /Q17 law/MAM His Attorney.

March 17, Q HI vyg ug ELECTRIC VALVE CONVERTING SYSTEM Filed Nov. 15, 1953 3 Sheets-Sheet 3 uw m H. o Ams U m VMEA mm l M m. m

Patented Mar. 17, 1936 UNITED STATES ELECTRIC VALVE CONVERTING SYSTEM Clodius H. Willis, Princeton,

General Electric Company,

New York N. J., assignor to a corporation of Application November 15, 1933, Serial No. 698,120

13 Claims.

My invention relates to electric valve converting systems and more particularly to such systems suitable for transmitting energy from a source of current to a relatively high frequency alternating current load circuit.

Heretofore there have been devised numerous electric valve converting apparatus for transmitting energy between direct and alternating current circuits, direct current circuits of different voltages, or independent alternating current circuits of the same or different frequencies. The use of valves of the vapor or gaseous electric discharge type has found particular favor in such apparatus because of the relatively large amounts of energy which may be handled at ordinary operating voltages. It is well understood that satisfactory operation of a valve of this type is dependent upon the condition that its control electrode shall regain control of its conductivity during each interval after current has been interrupted in the valve, by forcing its anode negative with respect to its cathode, and before the poten tial across the valve reverses polarity. In order for the control electrode, or grid, to regain control of the conductivity of the valve it is necessary that it shall have become substantially completely deionized during this interval while its anode potential is negative. A definite time is required for this deionization of an electric valve, that is, the recombination of contained ions into an electrically neutral vapor or gas, and the majority of the circuits of the prior art place a definite limit on the interval during which the anode potentials of the electric valves are maintained negative. Thus, this deionization time places a definite limit upon the upper frequency at which the apparatus utilizing these valves may operate, while in certain cases it is desired to operate the apparatus at a frequency above this upper limit.

It is an object of my invention, therefore, to provide an improved electric valve converting system utilizing valves of the vapor or gaseous electric discharge type which will overcome the above mentioned disadvantages of the arrangements of the prior art and which will be simple, economical and reliable in operation.

It is another object of my invention to provide an improved electric valve converting system utilizing valves of the vapor or gaseous electric discharge type, by means of which energy may be transmitted from a source of current to an alternating current load circuit of relatively high frequency, and in which adequate deionization time is provided for the several electric valves.

It is a further object of my invention to provide an improved excitation circuit for the control electrodes of the electric valves of an electric valve converting system which, while particularly suitable for the excitation of valve converting systems as embodied in my present invention, are of general application to electric valve converting systems utilizing vapor or gaseous electric valves.

In accordance with my invention, a source of supply current is interconnected with a relatively high frequency alternating current load circuit through an inductive winding and a plurality of groups of vapor or gaseous electric valves. The several valves of each group are interconnected through means for producing a commutating potential effective to transfer the current between the valves and the valves are rendered conductive and nonconductive in the proper sequence. The control electrodes of the valves are excited to render successively conductive a single valve from each of the several groups, the valves within each group being rendered successively conductive. Thus, at any particular instant only one electric valve is conductive, all of the other electric valves being idle. It has been found that with such an arrangement, the commutating means is effective to maintain a negative anode potential on each of the electric valves for a large portion of each cycle of operation, so that the apparatus is enabled to operate at a substantially higher frequency than the arrangements of the prior art. Another feature of my invention consists in an improved excitation circuit for the control elec trodes of the several electric valves which is particularly suitable for the electric valve converting apparatus briefly described above, but is of general application to self-excited electric valve converting apparatus, that is, apparatus the excitation circuits of which are not energized from an independent source of electromotive force for determining the frequency of operation.

For a better understanding of my invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompany drawings and its scope will be pointed out in the appended claims. Referring to the drawings, Fig. l is a diagrammatic illustration of my invention as applied to an apparatus for transmitting energy from a source of direct current to a relatively high frequency single phase alternating current load circuit; Fig. 2 represents certain operating characteristics of the arrangement shown in Fig. 1; Fig. 3 shows an improved excitation circuit applied to the power converting apparatus of Fig. 1; Fig. 4 illustrates a modified form of my improved grid excitation circuit shown in Fig. 3; Fig. shows a modification of the arrangement of Fig. 1 for operation from a single phase alternating current source; while Fig. 6 represents a modification of the circuit of Fig. 1 to operation from a three phase or four phase alternating current supply circuit. 7

Referring now more particularly to Fig. 1 of the drawings, there is shown an arrangement embodying my invention for transmitting energy from a direct current supply circuit H] to a single phase alternating current circuit H of relatively high frequency. This apparatus includes an output transformer l2 provided with a secondary winding l3 connected to the load circuit I l and a primary winding M. The winding I4 is provided with an electrical midpointconnected to one side of the direct current circuit, while its end terminals are connected to the other side of the direct current circuit through the groups, or pairs, of electric valves l5 and I6, and H and [3, respectively. Each of the electric valves iii-48, inclusive, is provided with an anode, a cathode,

and a control electrode or grid, and may be of any of the several types well known in the art although I prefer to use valves of the vapor or gaseous electric discharge type. Interposed in the connection from the winding Hi to the pair ofelectric valves [5 and I6 is a commutating circuit including a transformer winding I?! having an electrical midpoint connected to the terminal of the winding I l and end terminals connected tocorresponding electrodes of the electric valves l5 and it. This circuit also includes a commutating capacitor 2% connected across the winding [9, the winding l9 acting as a series auto-transformer so that the capacitor 20 is effectively in series circuit relationship with the load current of the apparatus. Similarly, a commutating circuit comprising a transformer winding 2i and a capacitor 22 is interposed in the connection from the winding M to the electric valves H and IS. A filter circuit comprising a smoothing reactor 23, a capacitor 24 and a reactor 25 is preferably interposed between the apparatus and the direct current circuit iii to minimize the pulsations of current drawn from the supply circuit.

In order to render conductive the several elec: tric valves l5!8, inclusive, in the proper sequence, their control electrodes, or grids, are connected to their common cathode circuit through current limiting resistors 26, negative bias batteries 2? and secondary windings of control transformers 23. The primary windings of the control transformers 28 are energized from different phases of a source of polyphase alternating potential of a frequency which is a submultiple of that which it is desired to supply the load circuit H. In the apparatus just described, these transformers are energized from the two phases of a quarter phase auxiliary motor generator set 29, which is operated to supplya control frequency one-half that at which the circuit H is to operate. I

In considering the operation of the above described apparatus, it will be assumed that the electric valve I5 is initially rendered conductive by means of its associated control transformer 28. Under these conditions current will flow from the positive side of the direct current circuit through the left-hand portion of the winding Ii, the left-hand portion of the winding l9 and the electric valve IE to the other side of the direct current circuit, generating a half cycle of alterthe commutating circuits nating current in the transformer [2 which is supplied to the load circuit 1 I. Due to the high exciting impedance of the winding E9 the current flowing in its left-hand portion must be balanced by a substantially equal and opposite current flowing in the right-hand portion, the only path for which includes the commutating capacitor 26. Thus, the capacitor 2B is effectively in series circuit relationship with the load current of the apparatus and it becomes charged to a potential dependent in magnitude upon the load current and the time interval during which current flows in the electric valve l5.

Substantially 180 electrical degrees later with respect to the load circuit I l, or 90 electrical degrees later with respect to the control potential supplied by the motor generator 29, electric valve ll is rendered conductive and the potential which has built up on capacitor 26 is effective to substantially instantaneously ccmmutate the current from the electric valve 5 to the electric valve ll. This commutating circuit may be traced as follows: From the right-hand terminal of the capacitor 29 which is positive, through the right-hand portion of the winding E9, the full winding It, the left-hand portion of the winding 2i, electric valve ll and electric valve l5. During the succeeding interval, current flows through the right-hand portion of the winding M generating a half-cycle of alternating current of opposite polarity, which is supplied to the load circuit l l. At the same time it is effective to charge the capacitor 22 in a manner similar to that described above in connection with the charging of capacitor 25. Because of the fact that the exciting impedance of the transformer winding I9 is very high and because of the fact that electric valves l5 and i6 are now both non-conductive, the charge on the capacitor 20 is substantially retained during the succeeding interval in which current is flowing in electric valve ll.

Substantially 180 electrical degrees later with respect to the load circuit ll, electric valve i5 is rendered conductive and the potential across the capacitor 2'2 is effective to transfer the current from electric valve H to electric valve it, in the manner described above. through electric valve it is now effective to discharge the capacitor 28 and to recharge it to an equal potential of opposite polarity so that it is effective, 180 electrical degrees later with respect to the load circuit l I, to commutate the current from the electric valve 55 to the electric valve l8. In this manner, the current is successively commutated, or transferred, between the several electric valves, current flowing in only a single electric valve in any given instant and the current being transferred between a valve from each of the pairs l5-i5 and ill8, and also being successively transferred between the electric valves of each pair. With this type of operation it is seen that each electric valve is idle for onehalf cycle of alternating current with respect to the load circuit l l and is idle for three periods.

The manner in which the above described circuit operates to increase the time interval during which a negative potential is impressed upon the anode of each of the several electric valves, and thus to increase the deionization time, will be best understood by reference to the several diagrams of Fig. 2, which represent conditions of equilibrium after initial starting conditions have become negligible. In this figure the curves A and B represent the potentials appearing across l-92ll and 2l-22.

Current flowing 7 Thus during the interval ti-tz, capacitor 20 is being discharged and charged to an opposite polarity. During the interval iz-ts both electric valves 15 and I6 are nonconductive and the charge on capacitor 20 remains substantially constant. During the interval i3l54 electric valve I5 is conductive and the capacitor 20 is again charged to an opposite polarity, etc. In the same figure the curve C represents the alternating current supplied to the load circuit ll and, in the case of a resistance load, the alternating potential across this circuit. The rectangular wave form is, of course, the result of the smoothing reactors 23 and 25 which maintain the unidirectional current substantially constant.

The curve D of Fig. 2 represents the potential appearing across one of the electric valves, for example, the electric Valve I5. During the interval t1t2 the valve is conductive and its terminal potential is merely the discharge potential, which is a negligible value. At the instant t2 the current is commutated from the electric valve IE to the electric valve ll, after which the potential across this valve is one half the potential of the capacitor 29, which is negative, plus the full potential of the winding [4 which is positive,

plus one half the potential across the capacitor 22, which is initially negative but reverses polarity during conductive period of the electric valve W. This charging operation of the capacitor 22 accounts for the slope of the curve D between the points tz-t3. At the instant is the current is commutated from the valve 53 to the valve 16 and the full potential of the capacitor 2e appears directly across the electric valve l5, as indicated in the curve. During the interval t3t4 this potential reverses polarity due to the charging of the capacitor 20 to an opposite polarity, as explained above. At the instant t4 the current is commutated from the valve IE to the valve 18 and the potential appearing across the valve 15 is one half that across the capacitor 25, which is positive, plus that of the full winding 1 which is positive, plus half that of the capacitor 22, which is negative. This produces the drop indicated at 154. During the interval 154-455 the potential across the capacitor 22 reverses polarity giving rise to the increase in the positive potential indicated between t; and is, after which the cycle of operations is repeated. Thus, from the interval s-id the potential upon the anode of the electric valve I5 is negative so that all of this time is available for deionization of the valve. This interval, it is seen, is greater than one half cycle of alternating current of the circuit 1 I, a deionization time substantially greater than is possible with any of the conventional valve converting apparatus of the prior art.

In Fig. 3 is shown an improved excitation circuit for the control electrodes of the several electric valves of an electric valve converting apparatus which is of general application, but which is particularly suitable for use in connection with the power converting apparatus shown in Fig. 1, to which it is illustrated as applied in Fig. 3. In this arrangement, the electric valves 15-48, inclusive, are replaced with electric valves l5'!8, inclusive, respectively, which are similar to the valves of Fig. l with the exception that they include an additional auxiliary electrode, as illustrated. The auxiliary electrode of each of the electric valves l5i 8, inclusive, is connected to its respective cathode through a negative bias battery 30 and a primary winding of one of the control transformers 3|-34, inclusive. The secondary windings of these control transformers are connected to excite the control electrodes, or grids, of the appropriate electric valve.

With the circuit arrangement just described, the auxiliary electrode of each of the electric valves, because of the negative charge received from its associated negative bias battery, is effective to collect a positive ion current from its associated valve during those intervals in which the valve is conductive; that is, during those intervals in which the contained vapor is in an ionized state. Upon the interruption of current in a valve, this positive ion current is interrupted very suddenly, producing a surge of induced voltage in the secondary winding of the associated control transformer, which is efiective to excite the control electrode of another electric valve to which it is connected. Thus, the secondary winding of the control transformer 3!, the primary winding of which is included in the circuit with the auxiliary electrode of the electric valve i5, is connected to excite the control electrode of the electric valve H, while the secondary winding of the control transformer 33, associated with the auxiliary electrode of the electric valve 1?, is connected to excite the control electrode of the electric valve it, etc., in the same sequence as described in connection with the operation of the apparatus shown in Fig. l. The apparatus of the power circuit, perse, is in all respects similar to that described in connection with Fig. 1. This improved grid excitation circuit, however, enables the apparatus to operate Without any independent source of grid excitation. With such an arrangement, the operating frequency is dependent primarily upon the constants of the load circuit.

In Fig. 4 is shown a modification of the excitation circuit of Fig. 3 which avoids the use of electric valves of a special type employing auxiliary electrodes. In this arrangement, the grid of each of the electric valves 15-48, inelusive, is connected to their common cathode circuit through a current limiting resistor 28 and the secondary winding of one of the control transformers 3i34', inclusive. In this case, however, the primary windings of the control transformers 3 |'34, inclusive, interconnect the same control electrodes which are utilized for rendering the valves alternately conductive and nonconductive, and the common cathode circuit of the electric valves through a negative bias battery 35!, which also serves as a bias for the control circuits, as in the arrangement of Fig. 3. In this arrangement, included in series with the primary windings of the control transformers 3|'--34', inclusive, are the unilaterally conductive devices illustrated as electric valve rectifiers 3538, inclusive, respectively. Current limiting resistors 39 may also be included in the primary circuits of the control transformers.

The control grid of each electric valve is energized from the particular control transformer whose primary winding is energized from the auxiliary circuit associated with the sequentially receding valve, as in the arrangement of Fig. 3; that is, the control electrode of the electric valve ii is energized from the control transformer 3!, whose primary winding is energized through a circuit including the control electrode of the electric valve l5; the control electrode of the electric valve 55 is energized from the secondary winding of the control transformer 33, the primary winding of which is energized from the circuit including the control electrode of the electric valve l1, etc.

The operation of the circuit arrangement of Fig. 4 is substantially identical with that of Fig. 3. When the current is transferred from the electric valve IE to the electric Valve H, the interruption of current in electric valve l5 interrupts the positive ion current flowing in the circuit including its control electrode, the resistor 39, the primary winding of the control transformer 3|, auxiliary rectifier valve 35 and negative bias battery 30. The interruption of current in this circuit produces an impulse in the secondary winding of the control transformer 3|, which is effective to excite the control electrode of the electric valve I! to render it conductive. Similarly, the interruption of current in electric valve ll produces an impulse in the secondary winding of control transformer 33' which is effective to excite the control electrode of the electric valve l6, etc.

If the auxiliary rectifier valves 3538, inclusive, were omitted, it is seen that the positive impulse produced in the transformer 3i on the interruption of positive ion current in the electric valve l5 would produce an impulse not only upon the control electrode of the electric valve ll, but also through the control transformer 33, an impulse upon the control electrode of the electric valve l6 and perhaps, even through the control transformer 32 upon the control electrode of the electric valve 18. That is, positive impulses produced upon interruption of current in one valve, through the interconnection of the control electrode in both the control and excitation circuits of the electric valves might proceed progressively through all of the electric valves. This is prevented by means of the auxiliary unilaterally conductive devices or valve rectifiers 3538, inc. For example, the polarity of the electric valve rectifier 35 is such as to permit the flow of positive ion current in the primary circuit of the control transformer 3|. Upon the interruption of current in the electric valve l5, the impulse induced in the secondary windin of the transformer 3! is such as to impress a positive potential upon the control electrode of the electric valve l6. This positive potential tends to produce a flow of current in the circuit including the secondary winding of the transformer 3i resistors 25 and 39, and the primary winding of the control transformer 32, but no current can flow in this circuit as it is of a polarity opposite to the direction of conductivity of the electric valve 35. Thus, the positive impulse generated in the secondary winding of the control transformer Si is confined to the control electrode of the sequentially successive valve, so that only the proper electric valve is rendered conductive by this impulse.

In Fig. 5 is shown an extension of the power circuit illustrated in Fig. 1 suitable for operating from a single phase alternating current supply circuit 40 provided with an electrical neutral N. In this arrangement, the several circuit elements of Fig. 1 are duplicated, the duplicate elements being referred to by the similar reference numerals with the subscript a. With such an arrangement, the commutating windings l9 and '99. and 2| and 2 is are preferably inductively coupled, while the primary windings l4 and 4a. constitute independent primary windings of the output transformer [2. The operation of the circuit is substantially identical with that of Fig. 1, one of the duplicate inverting apparatus operating on half cycles of the alternating supply current of one polarity and the other operating on half cycles of alternating supply current of an opposite polarity. In case it is not important to secure a single cathode potential for the several electric valves, as shown in the several figures, the electric valves of the a apparatus may be connected oppositely to those of the other apparatus. and the electrical neutral and one of the outer terminals of the supply circuit 4i] may comprise an alternating current circuit without an electrical neutral. The operation in such a case will be identical.

In Fig. 6 is shown a further modification of the power circuit of my improved electric valve converting apparatus for operating from either a three or four phase alternating current supply circuit. In the system illustrated, the apparatus is energized from a Scott-connected transformer having a three phase primary network 4| connected to a three phase alternating current supply circuit 42 and a quarter phase secondary network comprising the phase windings 43, 44, 45 and 45. This circuit is similar to that of Fig. 5 with the exception that the midtapped primary windings l4 and I45 of the output transformer i2 comprise two separate, insulated portions l4, [4, Ma and I 4a" which are fed from the separate phase windings 43-46, inc., of the secondary network of the supply transformer. The connections are such that the phase winding supplying one half cycle of the alternating current output is displaced in phase from the winding supplying half cycles of the alternating current output of opposite polarity. The primary winding I4 is energized from the phase winding 43 while the primary winding I4" is energized from the phase winding 44 displaced in phase from the winding 43 by electrical degrees, referred to the supply circuit. Similarly, the winding Ma and 44a" are energized from the windings 45 and 46, also phase displaced by substantially 90 electrical degrees. The result of these connections is to minimize the modulation in the amplitude of the alternating current output at the supply frequency which would result if the several primary windings of the output transformers I2 were energized from alternating potentials of the same phase, due to the cyclical variation in the amplitude of the supply voltage. In this arrangement also the filtering reactors 25 and 25a of Fig. 5 are necessarily divided into two portions 25' and 25 and. 25a and 25;, each pair being inductively coupled as illustrated, while the filtering capacitors 24 and 24a. are also divided into separate units 24 and 24", and 24a and 248.. In other respects the operation of the apparatus of Fig. 6 is similar to that of the preceding figures.

While I have described what I at present consider the preferred embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electric valve converting system for transmitting energy from a source of current to a high frequency alternating current load circuit, comprising an inductive winding, a plurality of groups of electric valves interconnecting said source and said winding, means interconnecting the valves of each group for producing a series circuit commutating potential, said means being arranged to carry only alternating current, and means for rendering successively conductive a single valve from each of the several groups, the valves within each group being rendered successively conductive, said load circuit being energized from said inductive winding.

2. An electric valve converting system for transmitting energy from a source of current to a high frequency alternating current load circuit, comprising an inductive winding, a circuit interconnecting an intermediate point of said winding With a terminal of said source, a group of electric valves interconnecting each terminal of said inductive winding with another terminal of said source, means interconnecting the valves of each group for producing a commutating potential, and means for rendering successively conductive a valve from each of the several groups, only one valve of said system being conductive at any instant, the valves within each group being rendered successively conductive, said load circuit being energized from said inductive winding.

3. An electric valve converting system for transmitting energy from a source of current to a high frequency alternating current load circuit, comprising an inductive winding, a circuit interconnecting an intermediate point of said winding with a terminal of said source, a group of electric valves interconnecting each terminal of said inductive winding with another terminal of said source, commutating means interconnecting the valves of each group including capacitance means in series circuit relation with the load current of the system, and means for rendering successively conductive a valve from each of the several groups, the valves within each group being rendered successively conductive, said load circuit being energized from said inductive winding.

4:. An electric valve converting system for transmitting energy from a source of current to a high frequency alternating current load circuit, comprising an inductive winding, a. circuit interconnecting an intermediate point of said wind ing with a terminal of said source, a group of electric valves interconnecting each terminal of said inductive winding with another terminal of said source, an inductive network interposed in the connections of each group of said valves to a terminal of said inductive winding, each of said networks being provided with an electrical neutral connected to the respective terminal of said winding and with terminals connected to the electric valves of its respective group, commutating capacitors connected between the terminals of said networks, and means for rendering successively conductive a valve from each of the several groups, only one valve of said system being conductive at any instant, the valves within each group being rendered successively conductive, said load circuit being energized from said inductive winding.

5. An electric valve converting system for transmitting energy from a source of current to a high frequency alternating current load circuit, comprising an inductive winding, a plurality of groups of electric valves interconnecting said source and said winding, each of said valves being provided with a control electrode, means interconnecting the valves of each group for producing a commutating potential, and means for exciting the control electrodes of said valves with a polyphase alternating potential of a frequency which is an even submultiple of that of said load circuit to render said valves conductive singly and in a predetermined sequence, said load circuit being energized from said inductive winding.

6. An electric valve converting system for transmitting energy from a source of current to a high frequency alternating current load circuit, comprising an n-phase inductive winding, 2n groups of m electric valves, each group interconnecting a terminal of said winding and said source, each of said valves being provided with a control electrode, means interconnecting the valves of each group for producing a commutating potential, and means for exciting the control electrodes of said valves with an rim-phase alternating potential of a frequency equal to nm times that of said load circuit to render said valves conductive singly and in a predetermined sequence, said load circuit being energized from said inductive winding.

7. In an electric valve converting system for transmitting energy from a source of current to a load circuit and including more than two vapor electric valves, each provided with a cathode and a control electrode, means for rendering said valves conductive in a predetermined sequence comprising an excitation circuit for the control electrode of each of said valves, and means for establishing a positive ion current between the control electrode and cathode of each valve and means responsive to the interruption of positive ion current in each valve for producing a positive impulse in the excitation circuit of the sequentially succeeding valve.

8. In an electric valve converting system for transmitting energy from a source of current to a load circuit and including more than two vapor electric valves each provided with a plurality of electrodes, means for rendering said valves conductive in a predetermined sequence comprising means for establishing a positive ion current between certain of the electrodes of each of said valves during their respective conductive periods, and means responsive to the interruption of the positive ion current in each valve for rendering conductive the sequentially successive valve.

9. In an electric valve converting system for transmitting energy from a source of current to a load circuit and including a plurality of vapor electric valves each provided with an anode, a cathode, a control electrode and an auxiliary electrode in contact with the ionized medium of the valve, means for rendering said valves conductive in a predetermined. sequence comprising means for establishing a positive ion current between the auxiliary electrode and another electrode of each of said valves, an excitation circuit for the control electrode of each of said valves, and means for energizing the excitation circuit of each valve in response to the interruption of the positive ion current in the circuit of the auxiliary electrode of the sequentially preceding valve.

10. In an electric valve converting system for transmitting energy from a source of current to a load circuit and including a plurality of vapor electric valves each provided with an anode, a cathode, a control electrode and an auxiliary electrode in contact with the ionized medium of the valve, means for rendering said valves conductive in a predetermined sequence comprising a circuit interconnecting the cathode and auxiliary electrode of each of said valves and including a control transformer and a source of negative bias potential, and an excitation circuit interconnecting the cathode and control electrode of each of said valves and energized from the control transformer associated with the sequentially preceding valve.

11. In an electric valve converting system for transmitting energy from a source of current to V a load circuit and including more than two vapor electric valves, each provided with an anode, a

' cathode, and a control electrode, means for rendering said valves conductive in a predetermined sequence comprising means for establishing a positive ion current between the control electrode and cathode of each of said valves during their respective conductive periods, and means responsive to the interruption of the positive ion current in each valve for exciting the control electrode of the sequentially successive valve.

12. In an electric valve converting system for transmitting energy from a source of current to a load circuit and including more than two vapor electric valves, each provided with an anode, a cathode, and a control electrode, means for rendering said valves conductive in a predetermined sequence comprising a circuit interconnecting the control electrode and the cathode of each of said valves and including a control transformer, a source of negative bias potential, and a unilaterally conductive device, and an excitation circuit interconnecting the control electrode and cathode of each of said valves and energized from the control transformer associated with the sequentially preceding valve.

13. An electric valve converting system for transmitting energy from a source of direct current to a high frequency alternating current load circuit comprising an inductive winding provided with an electrical midpoint connected to one side of said source, a plurality of groups of electric valves interconnecting the terminals of said winding and the other side of said source, means interconnecting the valves of each group for producing a series circuit commutating potential, and means for rendering successively conductive a single valve from each of the several groups, the valves within each group being rendered successively conductive, said load circuit being energized from said inductive winding.

CLODIUS H. WILLIS.

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