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om"n 15', 1946. O. E. BOWLUS ELECTRICAL CONTROL SYSTEM Filed Nov. 30, 1944' 5 Sheets-Sheet 2 Í L ( K ' `L Il 4 1 l /4 ’2/0 Í , ' /lz „4 ____ *___ ___. á INVENTOR. „rra/Mirna’. o@ 15, 1946. 2,409,534 O. E. BOWLU'S> ELECTRICAL CONTROL SYSTEM Filed Nov. 30, 1944 L-c. 5 Sheets-Sheet 3 > BY OCL 15, 1946. „ Q_ E_ BOWLUS I 2,409,534 ELECTRI CAL CONTROL SYSTEM ' Filed Nov. so, 1944 ` f I l f z.” L :40 f Í f 1 ¿te ‘5v î] 244m -g K - L f\ zal ` f4” 5 sheets-sheet 4 l' n w Jhr ¿ef \ ' m [i3/‘4 Í/Z Oct. l5, 1946. o. E. BowLus ~ 2,409,534 v ELECTRICAL CONTROL ASYSTEM Filed Nov. 30, 1944 5 Sheets-Sheet 5 wäw1@ 17772/7175 YS. 2,4Ü9,534 Patented Oct. 15, 1946 UNITED STATES PATENT OFFICE 2,409,534 ELECTRICAL CONTROL SYSTEM Omer E. Bowlus, Detroit, Mich., assignor to Chrysler Corporation, Highland Park, Mich., a corporation of Delaware Application November 30, 1944, Serial No. 565,954 zo onims. (Cl. 172-281) 1 2 The present invention relates to electrical con circuit in parallel with each other. In reading the drawings, Figure 1B may be placed imme diately to the right of Figure 1A, Figure 1C may be placed immediately below Figure 1A, and Fig ure 1D may be placed immediately below Figure trol systems and is particularly directed to the provision of improved apparatus which functions as a combination converter-inverter for deriving alternating current energy of a desired adjust able frequency from a source of alternating cur rent the frequency whereof may be randomly 1B. When the drawings are so arranged uncon-> nected terminals on the various sheets will line up with correspondingly designated unconnected variable over a range which is above, below, or terminals on the adjacent sheets, thereby com which includes, the output frequency. in its here in illustrated embodiments the present invention 10 pleting the circuits which extend from one sheet is particularly designed for aircraft purposes, and' to another; and Figure 2 is a 'series of curves depicting various serves to translate the variable frequency output operating characteristics of the system. of one or more engine driven generators into a It will be appreciated from a complete under three phase alternating current output of adjust ably ñxed frequency. In certain of its aspects 15 standingv of the present invention that in their broader aspects, the improvements thereof may the present invention is directed to improvement be embodied in widely differing systems, arranged upon the inventions disclosed and claimed in the for widely differing specific purposes. The sys copending application of Nims, Serial No. 565,955, tem specifically disclosed herein is particularly ñled November 30, 1944, and certain features dis closed but not claimed herein are claimed in thel 20 designed for use on multi-engine aircraft, to fur copending application of the present applicant and Nims, Serial No. 565,S56, iiled’November 3G, nish three phase alternating current for various control and operating purposes. The disclosure herein of the invention with particular reference to this application is, however, to be regarded in 1944, both 0f which applications are assigned to the assignee of this application. Principal objects of the present-l invention are to provide a system of the aforesaid type which is simple in arrangement, requires a minimum num ber of structural elements, is relatively light in weight, and is reliable and eflicient in operation; to provide such a system in which multi-phase alternating current input energy is translated into an illustrative and not in a limiting sense. As is indicated above, it is desirable, in con nection with modern aircraft, to provide self contained generating systems of the alternating current type, which are adapted to deliver alter nating current at an adjustably ñxed frequency and voltage, and which utilize, as a prime source of power, alternating current generators which multi-phase alternating current output energy; to provide such a system embodying improved means for timing the operations of the control apparatus associated with the several. output f» phases; to provide such systems in which the multi-phase output circuits of the several units may be connected in parallel, and embodying im proved means for synchronizing the control ap paratus of the several units; and to generally 40 improve and simplify the construction and ar ’ rangement of systems of the above generally indi cated type. . , are driven by the aircraft engines. Since the air craft engine speeds vary rather widely in opera tion, the frequency of the alternating current gen erators also vary, making it desirable to provide apparatus which is effective to translate alter hating current of a variable frequency into cur rent having a frequency which is adjustably fixed, and which may fall below, within, or above the frequency range of the generator. The aforesaid copending application of the present applicant Nims, Seria1 No. 565,955, filed November 30, 1944, With the above as well as other and more de discloses and claims certain features of such sys tailed objects in View which appear in the fol 1 tems, which, as specifically disclosed, are effec invention is shown in the accompanying drawings tive to deliver single phase alternating current. The present system on the other hand is arranged to deliver multi-phase alternating current, a throughout the several views of which corre three-phase output being specifically shown sponding reference characters are used to desig nate corresponding parts and in which: herein. Figures 1A and 1B show the power circuits for two substantially identical units, each comprising lowing description and in the appended claims, a preferred but illustrative embodiment of the Figures 1A, 1B, 1C, and 1D collectively provide a diagrammatic arrangement of power and con a main generator adapted to be driven for eX trol circuits for two units of the present inven tion, arranged to supply a multi-phase output ample by a corresponding engine of the associated aircraft and arrangements are shown for con 2,409,534 3 4 necting the output circuits of the two units in parallel. Similarly, Figures 1C and 1D show vir tually identical control circuits for the power units of Figures 1A and 1B respectively and also show the synchronizing interconnections between the primary winding te of associated transformer such control systems. For these reasons a de scription of one powei` and one control unit will suffice for a description of both, except in the re transformer l0. Primary windings 0G, till, and 53 cathodes of the main valves associated with out put phase I2 are connected to the terminals of 06, and the cathodes of the main valves associated with output phase I0 are connected to the ter minals of the primary winding 53 of an associated are provided with center taps which are con~ spects hereinafter noted. 20--22-24--26-28--30; 32-«34--35--38-li0m 42; and 44--66-48--50-52-54. nected through corresponding reactors l2, 14, and l5 to the center tap 'I3 of the generator it. Re actors '.'2, '14, and 'I6 may be and preferably are magnetically independent of each other, which relation is indicated by the dashed lines appear~ ing therebetween. Transformers 62, S0, and îil are provided wi grounded secondary windings Bâ, Bil, and B4, whi n may Generator be arranged I6 may to be of driven usualeither construction directly or, as shown, are directly connected to the corre~ Referring first to Figure 1A, power is arranged to be delivered to the three phase output con ductors l0, | 2, and I 4, from an alternating current generator I6, through a combination converter inverter comprising three series of main valves spending load conductors I4, I2, and and preferably, through suitable change-speed which load directly conductors connected. for the two parallel , units gearing, by a corresponding engine of the air~ 20 craft. Generator IE is provided with a usual Commutating condensers S0, and 90 con* direct current field winding 55 which, as described nected directly across the corresponding transi in the aforesaid Nims application, may be pro formers 62, t6, and l0 and serve control the vided with regulating apparatus which serves to conductivity of the associated valves, in the herc maintain the voltage of generator It' at a sub stantially uniform value through the expected generator operating speed range, which in the case of aircraft systems, may be from 4,000 to 10,000 R. P. M. Such regulating apparatus may also be arranged as described in the Nims appli 30 cation to maintain a proper division of the load between two or more of the present power units when such units are operated in parallel with each other. Each of the aforesaid main valves may be of any conventional type. Preferably and as indi |06, |08, and H0, and are also provided respec ||4; ||6 and H3; and |20 and |22. The ter minals of windings ||2 and H4 are connected, respectively in series with current limiting re~ sistors |251, between the grids and cathodes of valves 20-22-24 and 25-«20-30. Windings | i5 and Ilíl, and |20 and |22 in turn areJ correspond~ ingly connected between the grids and cathodes of the remaining associated valves. In accordance with this embodiment of the present invention, alternating voltages of ajc« B in turn is connected to the anodes of valves 22, 38, and M associated respectively with output phases I4, I2, and I0. Corresponding comments apply to the other generator phases, it being noted that each generator phase is associated with and is effective to supply current, under the conditions hereinafter stated, to one phase of the output circuit. The cathodes of the main valves associated with output phase I4 are connected to the respec» tive terminals of the primary winding 60 of an associated transformer 62. proximately square wave form are induced in the secondary windings ||2--| ill-_I |6~| |8-ií10 |22, the voltages in the two windings of each pair being 180 degrees out of phase with each other, and the voltages of the respective pairs being 120 degrees out of phase with each other. The con trol circuits for eiîecting such energization of the secondary windings of the control transformers t. associated with output conductor I4, valve t2 associated with output conductor I2, and valve 48 associated with output conductor I0. Phase Similarly, the These control transformers are provided tively with pairs of secondary windings |I2 and ... nu which are directly connected to the anodes of the corresponding main valves. That is to say, phase A is directly connected to the anodes of valve 2??, ( vided. cated they are usual three element gas-ñlled Generator I6 is illustrated as having 'six star connected phase windings A, B, C, D, E, and F, successive positive and negative half cycles of the output phases I4, |2, and i0, and to determine th displacement, in electrical degrees, between th voltages induced in such phases, correspondin control transformers I 00, |02, and |04 are pro- respectively with center tapped primary windings grid controlled Valves of the so-called discon~ tinuous type. That is to say, each of these valves, though normally non-conductive, may be ren dered conductive, when their anodes are surfV ciently positive with respect to their cathodes to sustain a discharge, by rendering their grids sufii ciently positive with respect to their cathodes. When so rendered conductive, the grids lose con trol and the valves remain conductive until the anodes are either negative with respect to their cathodes or are not sufficiently positive to sustain a discharge. It will be noticed that the cathodes of related groups of these valves are directly inter connected so that, although structurally separate valves are illustrated, multi-anode structures may be used instead. That is to say, for example, valves 20, 22, and 24 may be combined into a single multi-anode structure. inafter described manner. To control the initiation and duration of vthe are shown in Figures 1C and 1D, and are de scribed hereinafter, As will be appreciated, and as is diagrammatically shown in portions 1V, V, and VI of Figure 2, the above-described control voltages render the grids of the associated main valves alternately positive and negative with re~ spect to their cathodes, so as to cause successive positive and negative half cycles of voltage to be induced in the respective output windings 80* 82-84. It is believed that the operation of the above described power circuits will be apparent from a description of the operation of the converter inverter circuits associated with, for example, the control output phase windingI4. H2Atbecomes the timeeffective ti in Figure to posi~ tively bias the grids of Valves 20-22-24, and winding |l4 becomes effective to negatively bias the grids of the associated valves 25-2E2~-30. ils is described below, the latter action prevents any discharges in the last-mentioned valves, and it will beA understood that the former action tends 2,409,534 6 re-establish a discharge through any thereof in view of the negative bias applied to the grids. It the time t1, phase A is most strongly positive and, because of the common cathode connections of is to be noted-that the time required for the dis charge of condenser 86 is longer than the de the last-mentioned valves, this fact renders the cathodes of valves 22 and `24 positive with re Oi ionization time of the just-mentioned valves. to- render the valves 20--22-24 conductive. At spect to their anodes and prevents the initiation of a discharge therethrough. Valve 20 is, how ever, conductive, and current may flow there through from phase A through the left-hand half Consequently, the grids of these valves are en abled to obtain control thereof and maintain them non-conductive as aforesaid. It is believed to be evident that at the time t5, the flow of cur . of the associated primary transformer winding 10 rent through winding 60 transfers from phase B0, and through the corresponding reactor> 12. At the time tz in Figure 2, the potential of phase B rises to a value surliciently in excess of the voltage of phase A to render valve 22 conductive, and initiate a flow of current from phase B F and valve 25, to phase D and valve 28. Fur ther, it is believed to be evident that at the time te, at which time the original polarities of wind ings II2 and VII4 are restored, valve 28 is ex tinguished, and the next succeeding positive half cycle of output phase I4 is “initiated” from phase B through valve 22. It will be noticed from por tion I of Figure 2f that with the assumed fre quency relations between the input and the out through valve 22 and the left-hand half of pri mary winding E0. Conduction through valve 22 elevates the potential of the cathode of valve 29 to a value above that of its anode, and extin guishes the discharge through valve 20. Simi~ 20 put circuits, that during the ñrst above described larly, at approximately the time t3, the voltage of phase C rises to a Value above that of phase B, initiating a discharge through valve 24 and positive half cycle of the output phase I4, phases A, B, and C successively deliver current, through valves 2ll-22-24. During the ñrst described negative half cycle on the other hand valves 2S The aforesaid flow of current from the source 25 and 28 carry the current, which is derived from phases F and D. During the positive half cycle charges up the associated commutating condenser represented by the interval ist?, valves 22 and 24 86, bringing its left-hand terminal to a positive potential and its right-hand terminal to a nega conduct current from phases B and C. It will tive potential. This full charge is preferably ob thus be apparent that the number of phases and tained just before each commutation point is 30 valves which supply current to output phase I4 varies during successive half cycles of the same reached, in this case, the time t4. Throughout polarity and during successive half cycles of dif this interval, phases D, E, and F are, to vary» ing degrees, positive,` and the negative potential ferent polarity. The full lines in portion I of Fig established for their cathodes by winding 60 and ure 2` indicate the time intervals throughout which condenser 86, tends to cause ñow of current the correspondingly designated phases are ef through valves 26-28-30. Such current flow is fective to supply current to output phase I4. extinguishing the'discharge through valve 22. prevented, however, by the strong negative bias Considering now the general form of the volt applied to the grids of these valves by control age wave induced in the secondary winding 80 winding I I4. of transformer 62, and the phase relation of this At the time t4 in Figure 2, the polarities of 40 induced voltage relative to the output voltages of windings I I2 and I I4 are reversed, which reversed control transformer Illß, it will be appreciated that relation is maintained until the time te. The so long as valves 20-22-24 are conductive they negative polarity of control winding I I2 negativa tend to cause a ñow of current through the left ly biases valves ZIJ-22-24 which action is with hand half of the primary winding 60 of the as out effect on valves ‘20 and 22 since these valves sociated output transformer, resulting in, for ex are not conducting at the time t4. The negative ample, a positive half cycle of induced voltage bias applied to the conducting valve 24 tends to in secondary winding 89. Conversely, when valves extinguish the discharge therethrough, and may, 26-«23--30 are conductive they tend to cause a with certain classes of valves, be effective to do so. flow of current through the right-hand half of The positive bias applied to valves 26-28-30 ‘ winding 6U and induce a half cycle of voltage of negative polarity in winding 89. The impedances by control winding II4 tends to render all of these valves conductive. At the time t4, however. in the converter-inverter network delay the in phase ’F'is more positive than phases D and E duced voltage in winding 86 by a phase angle and consequently valve 26 is the only one of the equal to a fraction of a half cycle of the output just-mentioned three valves which becomes con~ ` frequency. This delay or phase shift may, in a general sense, be explained as follows: At each ductive. This action “initiates” in the sense dis» cussed below, the negative -half cycle of voltage commutation point, such as the time t4 in Figure of output phase I4. As soon as valve 26 becomes 2, the commutating condenser 86 is charged, as conductive, it elevates the right-hand terminal aforesaid. When, at time t4, valve 26 becomes of condenser 86 to a value which is lower than conductive, condenser 48 is enabled to elevate the the voltage of phase F by only the amount of potentials of the cathodes of valves 26-22-24 the relatively small voltage drop through valve as aforesaid. Condenser 86 also elevates the po 26. By virtue of the charge then existing on con tential of the center tap of winding 6D, These denser 86, this action immediately elevates the changes in potential are of course enabled by cathode potentials of all of valves 20, 22, and the associated reactor '1;2. The energy stored in 24 to values well above their anode potentials condenser 86 prevents an immediate reversal of and extinguishes any discharges existing therein. the induced voltage in winding Sil, such induced The reversal of the charging voltage applied to voltage falling to zero only after the expiration condenser 86 when valve 26 becomes conductive of an interval determined in part at least by the enables the initial charge to dissipate itself characteristics of the previously described discharge circuit for condenser 86. Similar com ments apply to the delayed reversal of the induced reverse polarity renders the cathodes of valves voltage in winding 8a which is initiated at each 20-22-24 strongly negative with respect to their other commutation point such as the time te. anodes, which action is, however, ineffective to 75 The form of the induced voltage wave in wind through verse charge winding to be huilt 6G and up on further condenser enables 85. a.This 2,409,534. 7 8 ing 80 is of course determined by the relative im viously described accurately timed relation, com pedances in converter-inverter network as a whole prises generally an oscillator circuit |33, which and it is preferred to so proportion these imped serves as a source of periodic voltage; a counter ances as to produce an induced voltage of ap proximately the wave form shown in the afore said Nims application. network |32, which serves to segregate successive Cil ' impulses from the oscillator circuit and appro priate them in proper order to the respective output phases; and a series of three inverter net It will be observed accordingly that although the transfer action between valve groups which works I34 which respond to the counter-network takes place at each commutation point, does not and control the delivery of energy to the respec necessarily result in an immediate reversal of 10 tive control transformers lili), W2, and |54. the induced voltage in the corresponding output The oscillator circuit I3@ may, in general, be winding, such as 8G, such transfer action does of any usual type and as illustrated, comprises a “initiate” or result in such a reversal. usual grid controlled gas-filled valve I4@ of the The operation of the valve banks associated previously mentioned discontinuous control type. with output phases I2 and I0 is the same as that Valve |40 is connected across terminals |42 and described above with the exception that the con |44 of an illustrative source of power, in series trol voltages applied to these banks are displaced with the primary winding oi a synchronizing 120 degrees with respect to each other and with transformer |46, a timing condenser |48, and a respect to the control voltages for output phase potentiometer resistor I 5E). Usual gas-filled volt I4. Portions II and III of Figure 2 indicate, in 20 age regulating glow tubes |52 and I5-’I are inter full lines, the intervals, with respect to the cor posed between terminals |42 and Illll and serve, as responding control voltages (portions V and VI will be understood, to maintain the voltage be respectively) during which the indicated phases tween these just-mentioned terminals at a sub are eifective to supply current to the correspond stantially uniform value, terminal M4 being ing output phases. grounded and terminal |42 being indicated, for By examination of portions I, II, and III oi illustrative purposes, as having a potential ci 24D Figure 2, it will be noticed that at any given time, volts. N eglecting the action of the synchronizin for example, the time t1, phase A is effective to transformer illû, it will be appreciated that when deliver current for each of output phases I4 and valve Idil is conductive, current is enabled to flow I2- Beginning at the time tz, in turn phase D therethrough and charge up condenser |48, which is effective to deliver current to each of output current flow is surge-like in character. By vir phases I2 and Il?. A particular generator phase, tue of the inductance in the plate circuit of therefore, serves to deliver current to a plurality the valve, the completion of this charging action of output phases at the same time. . is accompanied by a momentary reversal of the So long therefore as the above-mentioned syn voltage across the valve which temporarily ren~ chronously timed control voltages are developed ders its cathode positive with respect to its anode. by the control transformers IM, |02, and |84, This action blocks the valve and enables the en the six phase variable frequency input power is ergy stored in condenser |43 to dissipate itself translated into a three phase output having a through resistor |56. As this charge is progres frequency equal to that of the energy applied to 40 sively dissipated, the potential of the cathode of the control transformers. It will further be ap valve |40 is correspondingly lowered, thereby pro preciated from the foregoing that the input and output :frequencies are virtually independent of each other, thus accommodating the system to the relatively unusual case in which the input and output frequencies are identical, as well as to the more usual case in which they differ. It should be noted that the loading of the indi vidual phases of the generator has a substan gressively increasing the anode-cathode voltage across the valve. When the latter voltage reaches a critical value valve MB again breaks down and passes an impulse of current. Valve |40 is thus rendered conductive and non-conductive period ically, at a frequency determined primarily by the characteristics of the discharge circuit for condenser |48, each conductive period being a tially uniform average value, for any given output 50 very minor fraction of each non-conductive peri load, although at certain frequencies, the average od. During each conductive period the potential loading of the individual valves is not uniform. of the adjustable tap §56 on resistor LED abrupt More particularly, in the event the input and out ly rises and during each non-conductive inter put frequencies are identical, certain of the main val, such potential gradually falls to a normal valves remain inactive if certain input and out 55 value. The potential of tap |55 is thus of the put phase relations exist. This circumstance usual saw-tooth wave form, as indicated in por `makes it desirable, of course, to utilise valves of tion VII of Fig. 2. sufficiently large capacity to reliably handle any The corresponding oscillator for the companion unbalanced loading conditions. unit (Figure 1D) duplicates the unit just de It will be appreciated that in the broader as 60 scribed, it being noted that the secondary wind pects of the invention, the main generator may ing of each synchronizing transformer is tied to be provided with a different number of phases the grid of the oscillator valve |43 for the other than the illustrated six phases. For example, a unit. More particularly, the grid cathode circuit three phase generator may be used, as disclosed for valve |49 of the unit shown in Figure lC in the aforesaid Nims application. In utilizing a extends from the ground terminal IM through three phase generator it will be appreciated that the corresponding valve Idil, conductor |58, each phase winding is connected to twice the thence through- the secondary winding IED of the number of anodes as in the present case. That companion synchronizing transformer |46 to the is, a particular phase winding would be con corresponding ground terminal IM (Figure 1D). nected to all of the anodes to which, for example, 70 It will be appreciated accordingly that each time phases A and D of the present generator are valve |453 ci one unit breaks down, a voltage im connected and so on. pulse is transmitted through the secondary wind Referring now to Figure 1C, the illustrated ing of the corresponding winding synchronizing arrangement for energizing the above-described transformer |45, which voltage impulse breaks control transformers |00, |02, and |04 in the pre down the valve |40 associated with the other 2,409,534 10 9 and extinguish valve |66. More particularly, op unit. The oscillator circuits for the two units are thus caused to operate in synchronism with eration of the counter-network is as follows: As suming that valves |62 and |64 are conductive, it will be appreciated that terminals |96 and |86 have potentials which are above ground by only the amount of the voltage drops through valves |62 and |64, the balance of the voltage between ground and conductor |14 being consumed in re sistors |68 and |10. The grids of valves |62, |64, and |66 are connected through resistors 208, 2|0, each other. It will be appreciated that the output frequency of each oscillator circuit is determined primarily by the desired output frequency of the power cir cuit and by the number of phases of the power circuit. In the present arrangement three output phases are provided and two impulses per out put phase are required from the oscillator circuit. Accordingly, assuming a 400 cycle output fre quency, it will be appreciated that the oscillator and 2|2, to terminal 2|4, the potential whereof 1's somewhat below ground, for example, 35 to 50 volts below ground. The ratio of the resistors 204 and |90 (which are connected to the aforesaid terminals |96 and |86) to resistor 2|2 is such that under the conditions stated, the grid of circuits are adjusted to have a frequency of 2,400 cycles per second. > ’ Each counter-network comprises primarily a series of three Valves, |62, |64, and |66, which valve |66 is negatively biased, which action ren ders valve |66 non-conductive. At the same time, the grids of valves |62 and |64 are connected preferably, but not necessarily, are of the high vacuum continuous control type. The cathodes of these valves are connected together and to the ground terminal |44. The anodes of these valves 20 through resistor |80 on the one hand and resistor |82 on the other hand, to terminal |16. Since are connected, through control resistors |68, |10, and |12, to a supply conductor |14, which is maintained, by regulator valves |52 and |54, at an intermediate potential, of the order, for example, valve |66 is non-conductive, terminal |16 is at substantially the potential of conductor |14, which potential is very materially higher than that of terminals |96 and |86, and the last-men tioned connections thus serve to maintain the grids of valves |62-|64 at potentials which are slightly above ground and at which these valves |66, which grid circuits include resistors |18, |80, are in wide-open condition. and |82. The grid circuit for valve |62 also in t will be noticed that under the above described cludes a delaying condenser |84, which functions 30 as hereinafter described. Similarly, the grid cir conditions the blocking condensers |84 and |98 receive charges, of the indicated polarities, the cuits oi valves |62 and |66 are tied to terminal charge on condenser |84 attaining a value equal |86, which is intermediate resistor |10 and the to the drop across resistor |80 and the charge on anode of valve |64. These grid circuits include of 150 volts. The grids of valves |62 and |64 are continuously tied to terminal |16, which is in termediate resistor |12 and the anode of valve resistors |88, |90, and |92 and the just-mentioned condenser |98 attaining a value equal to the drop grid circuit for valve |'66 includes a delaying con across resistor 202. denser |94. Finally, the grid circuits for valves Assuming now that the potential of terminal |56 associated with the oscillator circuit is |64 and §86 are tied to terminal |96, which is in termediate resistor |68 and the anode of valve |62. These grid circuits include as indicated a-delaying abruptly elevated, as described above, by the flow of a surge current through the oscillator valve |40, it will be appreciated that this action applies a peaked positive impulse, also as aforesaid, to the grids of each of the counter-valves |62, |64, and |66, This action of itself, is without effect on valves |62 and |64, in view of the fact that the condenser |98 and resistors 200, 202, and 204. The grids of all of the counter-tubes are tied, in parallel with each other, to the previously de scribed oscillator terminal |56. Each such grid circuit includes a small blocking condenser 206. It will be appreciated accordingly that each time a positive impulse is applied to terminal |56, such impulse is transmitted to the grids of the three counter-valves and correspondingly elevates the grids thereof are already at wide-open positive values. This action does, however, positively bias the grid of valve |66 and renders this valve fully conductive. respect to their cathodes. The small blocking condensers 206 are charged up very quickly and consequently cause each such impulse to be of the sharply peaked form shown in portion VIII of Figure 2, the potential of terminal |56 being , shown in portion VII of such figure. The functioning of this counter-circuit, in gen eral, is such that, at any given time, two of the three counter-valves are conductive and the re maining counter-valve is non-conductive. Each 60 time a positive impulse is delivered from the oscil lator circuit to the grids of these valves, the non conductive valve is fired or rendered conductive. This action does not alter the conductivity of one of the remaining two valves but it does extinguish the remaining valve. Thus, for example, during As soon as valve |66 becomes fully conductive, it immediately lowers the potential of terminal |16 to a value which is above ground only by the amount of the relatively small voltage drop through valve |66, which potential is substantially potentials of these grids to a positive value with , the same as the previously described potentials of terminals |96 and |86. The drop in potential of terminal |16 tends to but does not negatively bias the grid oi valve |64, since this tendency is op posed by the impulse from oscillator circuit. The drop in potential of terminal |16, however, does immediately drive the grid of tube |62 to a nega tive potential, relative to its cathode, because of the charge on condenser |84. As soon as this action occurs, valve |62 becomes non-conductive and elevates the potential of terminal |96 to a “ value corresponding to the previously described L potential of terminal |16; that is to a potential substantially equal to that of conductor |14. With terminal |96 at the relatively high potential, the grids of Valves |64 and |66 are held positive so that these valves are substantially wide-open, through circuits corresponding to those previously extinguishes valve |62, leaving` valve |64 con described in connection with valves |62 and |64. ductive. The next impulse from the oscillator cir Similarly, with both valves |64 and | 66 conduc cuit fires valve |62 and extinguishes valve |64, tive, the grid of valve |62 is negatively biased in leaving valves |62 and |66 conductive. The next impulse from the oscillator circuit fires valve |64 75 a manner analogous to that previously described an interval between two successive impulses of the oscillator circuit, valves |62 and |64 may be conductive and valve |66 may be non-con ductive. The next impulse fires valve |66 and 70 11 ¿2,409,534 12 in connection with the negative bias on the grid of valve líiß. The single described impulse from the oscillator circuit, therefore, serves to extin guish Valve |52, leaving valves |5ë and itâ con ductive. It is believed to be evident that in an analogous manner, the next impulse from the of usual form. Grids 252 and 2E@ are also con nected, through associated resistors 232 25M to conductor 28S which is continuously main~ tained at a potential well below ground; for ezi ample, at a potential of minus 240 volts. At any given time one of the inverter valves 220-222 is conductive and the companion in oscillator circuit is effective, by ñring valve |52, to extinguish valve |64, leaving valves |52 and |555 conductive. Similarly, a succeeding impulse is eiîective, by ñring valve §54 to extinguish valve |56, leaving valves |52 and |554 conductive. verter valve is biased to a non-conductive con dition. A feature of the present invention re sides in. utilizing the anode-cathode circuit of each inverter valve to supply the associated con Each on or conductive interval of each counter trol transformer IDD, through the above-mentioned connections; and in utilizing the screen valve is therefore equal in length to twice the period of the oscillator circuit, and each oil or non-conductive interval of each counter-valve is equal to one period oi" the oscillator circuit. State-.fl in another way, each “cycle” comprising grids of these inverter valves as auxiliary anodeo to provide an output circuit for each valve to produce the inverter action. The inverter ac~ tion may thus be described independently of one on and one oil interval of each counter-valve, the primary output circuits of these valves. is equal in length to three periods of the oscillator. More particularly, and assuming that valve 22d Moreover, the cycles of the respective counte 20 is fully conductive, it will be appreciated that a valves have a phase displacement of one period of substantial part, for example, two-thirds, oi" the the oscillator; that is, a phase displacement of one voltage difference between. conductor 25B and third of a full cycle of cach counter-valve. These the grounded cathode is consumed in resistor 24B, leaving terminal 255 at a potential which phase and XI,relations of Figureare 2. indicated Thus, assuming in portions an oscilla-for IX, is above ground only by the amount oi the vol* frequency of 2,400 cycles, each counter-valve has age drop through valve 222. a frequency of 8D!) cycles. The impedance of the network between termi In the present system, each change from a non nal 266 and the negative conductor 235, and conductive to a conductive condition of each comprising resistor 262, condenser 26d, and re counter-valve is utilized to trigger the correspond* 30 sistor 224 is such that terminal 238 of this net ing inverter network iM. Each such inverter work, to which grid 259 is connected, is at a comprises a pair of high vacuum valves, desig sufficiently negative potential with respect to nated 22€), 222, 224, 22S, 228, and 23%. Each such ground to completely bias valve 280 to a non~ valve comprises main and auxiliary anodes, a com conductive condition. Under these conditions, the only voltage drop through resistor 2518 is trol grid and an indirectly heated cathode. Usual screen grid valves are usuable and are indicated in the drawings, the screen grids serving as the auxiliary anodcs. Since these inverter networks are identical, a description ol one thereof will suilice for all. Considering the inverter network 40 associated with output phase I4, and which corn prises valves 229 and 222, the cathodes of these 'I'he valvesanodes are connected oí these to valves the ground are connected conductor to the corresponding terminals of the primary winding due to the current flowing in the network con~ nection between conductors 252 and 225, and comprising resistor 243, condenser 25d, resistor 255, and resistor 282. The impedance of this network is such that under the indicated condi tions terminal 29ll, to which grid 252 is con' nected, is maintained at a potential with re spect to the cathode of valve 22S, at which this valve is in a wide-open condition. Under the - above conditions, further, condensers 238 and |06 of the associated control transformer lllîl, 210 contain variable charges, depending upon the which winding has a center tap 23S which is con-~ stage of the inverter cycle then in progress. tinuously connected to supply conductor 2. 8, Each time counter-valve 152 changes from a which is continuously maintained at al potential conductive to a non~conductive condition, the of, for example, 300 volts above ground. A sta~ 50 potential of the associated terminal 292 abruptly bilizing resistor 21.0 is connected across the pri» rises, as will be clear from the previous descrip tion. This increase in voltage, except in negli of mary valves winding 220 and |06.222The arescreen continuously grids 2M connected., and gible part, is not communicated to terminal 214 through control resistors 2li@ and 248, toa supply of the inverter circuit, since under the indi~ conductor 250 which is continuously maintained. cated conditions, rectifier 28|] ailords virtually at a. potential somewhat below the potential of. a short-circuit between conductor 2’i2 and conductor 238. For example, conductor 25o may ground. Such increase in voltage does apply a be maintained at a potential of approximately potential to and charge up the small blocking 240 volts above ground. condenser 216. The control grid 252 of valve 22B is connected, 60 Each time counter-valve |52 becomes conduc through a network comprising a condenser 254 and tive, the potential of terminal 292 abruptly falls a resistor 256, to terminal 253 which is intermedi to a considerably lower value, as will be clear ate resistor 248 and the anode of the companion from the previous description. This action im valve 2'2. The control grid of valve 22 is mediately pulls terminal 293 down to a potential similarly connected, through. a network comprising Cb Ul which is below the potential of terminal 292 by resistor 262 and condenser 254, to terminal 2&5. Athe amount of the charge on condenser 215. Grids 252 and 26|) in turn are interconnected to The constants of the circuit, including terminals` gether through condensers 268 and 21o. Conduc n 290 and 292, are such that the just-mentioned tor 212, which is connected to the anode of the drop in the potential of terminal 290, produced corresponding counter-valve |52, is connected 70 by valve |â2, is transitory in character. to terminal 214, intermediate the last-mentioned The peaked negative impulse (portion XII, ‘condensers Conductor 212 includes a bloclringf Figure 2) thus applied to terminal 25B serves condenser 215, and is connected to the ground to reduce the positive bias of he grid of valve conductor 232 through a relatively high resistance 226. This action in turn decreases current flow 218 and a continuously conductive rectifier 233, 75 between its cathode and its auxiliary anode or 2,409,534 13 14` screen grid 242. The latter action in turn de-` creases the voltage drop across resistor 246, degrees, the frequency of such outputs being de termined by the adjustably fixed frequency of the thereby elevating the potentials of terminals associated oscillator circuit |30. A further feature of the present invention re sides in providing means to properly synchronize the counter-networks for the several units. Referring to Figures 1C and 1D together, it will be noticed that auxiliary valve 300 is provided. 266 and 288 and opening up valve 222. rIChe opening of valve 222 increases the drop across resistor 248 and correspondingly lowers the po~ tentials of terminals 258 and 202. The lowering of the potential of terminal 290 still further re duces the conductivity of valve 220 which is This valve may be of usual three element high reflected as an increase in the conductivity of 10 vacuum type. The anode of valve 3&0 is continu valve 222. The above described negative im~ pulse accordingly serves to initiate a progressive swing of valves 220 and 222, which swing takes ously connected to the grid 302 of valve |66 in Figure 1D. The cathode of valve 300 is con-„ tinuously connected to terminal 2 I4 which as in place exceedingly rapidly, with respect to the dicated is somewhat below ground, and the grid frequencies involved in the present system, and 15 thereof is continuously connected to terminal 304, which is negative with respect to terminal 2 I4. It serves to completely open valve 222 and com will be appreciated from a previous description pletely block orlî valve 220. The next time counter-valve I 62 becomes non that while Valve IEB of Figure 1C is conductive, the potential of terminal 308 is relatively low. conductive, the positive impulse applied to ter minal 292 is suppressed as before, making no 20 Assuming control switch 306 is closed, with valve change in the. conductivities of the inverter |166 conductive, it will be appreciated that the difference in potential between terminals 308 and valves. On the other hand, the next time SIG is absorbed in condenser 3|2, leaving the grid counter-valve |62 becomes non-conductive, a peaked negative impulse is again applied to ter of valve 300 negatively biased. The connection minal 214. Since the inverter circuit is sym metrical, it will be appreciated that this negative between valve 300 and valve |66 of Figure 1D is thus without effect. As soon, however, as Valve #56 of Figure 1C is extinguished, the potential of impulse serves to block off valve 222 and render valve 220 fully conductive. Under the as-l terminal 306 is abruptly elevated, thereby posi sumed conditions of a frequency of 800 cycles tively biasing valve 300 and rendering it conduc~ for the action of counter-valve 162, it wiil be 30 tive. valve 300 is rendered conductive, it appreciated that each of the inverter valves is brings the potential of the grid 302 of valve |66 thus cycled by the conductive and non-conduc (Figure 1D) to a strongly negative value with re tive conditions at the rate of 800 times a second, spect to its cathode. If at the time this occurs, which corresponds to a frequency of the inverter such valve E66 is non-conductive (which is the circuit of 400 cycles. Considering now the principal output circuits of the inverter valves, it will be appreciated that so long as inverter valve 220 is conductive, cur rent iiow in the corresponding portion of thc primary winding of the associated control trans former |00 is in a direction to establish `one polarity for the secondary or output windings H2 and H4 of this transformer. So long as valve 222 is conductive, on `the other hand, an condition assuming the counter-circuits for the two units are in proper step with each other) Such negative biasing is without eiiect. If, however, the counter-circuits should be out of step with each other, such negative biasing would imme diately extinguish valve |66 of Figure 1D. Such extinguishment would have the same effect as though it had been caused by an impulse by the associated oscillator circuit. The just-men tioned synchronizing circuit thus serves to insure, opposite polarity is established for windings |I2 45 when the units are placed in operation, that they and H4. are in proper step with each other. As previously mention, it is preferred that the A further feature of the invention resides in output voltages of windings H2 and I|4 be of the provision of improved means for insuring that Accordingly, in the present the inverter networks for each unit are in proper system, the impedance of the main anode-cath ode circuits ofinverter Valves 220 and 2.22, are square wave form. such inverter networks for a plurality of units are such that when either of these valves is rendered in step with each other. The need for such syn step with each other, and to further insure that conductive, current through the corresponding chronization arises, as will be understood, from main anode circuit rises gradually and Substan the fact that a negative impulse from, for ex tially linearly to a maximum value, which is at 55 ample, counter-Valve |62, is effective to ñre one tained at approximately the same time that the or the other of the two inverter valves 220 and next inverter or flip-dop action occurs. When 222, depending upon which of these valves was such action occurs current flow’ in the just-men last fired. As shown, conductors 234 and , 235` tioned circuit is abruptly interrupted and a grad serve to connect the control grids of valves 220 ual rise in current flow through the main anode 60 and 224 respectively, to the auxiliary anode or circuit of the other inverter valve is initiated. screen grid 231 of valve 230, through small block Current flow in the primary winding portions of ing condensers 239 and 24 I. It will be appreciated the control transformer |00 is consequently of that in view of the phase relations established by saw-tooth form and results in the approximately the counter-network, three of the inverter' valves square wave form secondary outputs indicated in 65 1220, and so forth, are conducting at any given portion IV of Figure 2. time and moreover each time inverter valve 228 is It is believed to be evident that the inverter rendered conductive, valves 220 and 224 should networks comprising valves 224-226 and valves already be in a conductive condition. It will be appreciated that each time inverter valve 228 is 228-230 function in the manner described above in connection with valves 220-222, in response 70 rendered conductive, the potential of screen grid 231 of valve 230 rises sharply. This positive im respectively to the change from non-conductive to conductive condition of the associated counter pulse is communicated, through the blocking con densers 239 and 24| to the control grids of Valves valves |64 and |66. Consequently, transformers |00-I 02-| 04 deliver square wave secondary out 220 and 224. If these valves are already con puts having phase displacements of 120 electrical 75 ductive (which is the condition if the inverter 15 2,409,534 16 circuits are in proper' step with each other), these ing phase of said output circuit, and control positive impulses are without effect. Ii?, on the means for actuating said units in predetermined other hand, either of valves 220 and 224 should be phase relation to each other so that the respec non-conductive (which is the condition if the in tive phases of the output circuit have correspond verter networks are out of step) such positive im ing phase relations, said control means includ pulse would immediately render the non-conduc ing a source of periodic control voltage common tive valve conductive and bring the circuits into to said units, a network having a plurality of out step with each other. It will be noticed that the put circuits individual respectively to said units, above synchronizing circuits are provided for both means rendering said network responsive to said units, Figures 1C and 1D. l0 source of control voltage for energizing said out In addition, in order to insure that the inverter put circuits in rotation, an inverter network indi .networks for both units are in proper step with vidual to each unit and actuable in response to each other, the screen grid of valve 230 for the successive energizations to actuate the corre unit of Figure 1C, is arranged for connection, sponding control means, and means rendering through a small blocking condenser 243 and a said inverter networks operably responsive to suc normally opened manually operable synchroniz cessive energizations of the corresponding out ing switch 245, to the control grid 24'! of inverter put circuits of said inst-mentioned network. valve 228 associated with the unit of Figure 1D. 3. In a system for delivering mtiltiphase alter With this arrangement, it will be noted that each nating current energy to an output circuit from time inverter valve 228 of Figure 1C becomes con 20 a source of multiphase alternating current en ductive, the consequent rise in potential of the ergy, a plurality of translating units each individ associated screen grid 231 of valve 230, causes a ual to a corresponding phase of said output cir positive impulse to be transmitted to the control cuit, each unit comprising a pair of valve means grid of valve 228 associated with the unit of Fig each defining a plurality of discharge paths hav ure 1D. If this valve is already conductive (which ing a common cathode connection and a pltu‘al is the condition if the inverter networks for the ity of anodes coupled to corresponding phases of two units are in step), this positive impulse is said source, means coupling the valve means of Without effect. On the other hand, if such net each unit to the corresponding output phase so works are out of step, such impulse brings them that current flow through the individual means into step. of each pair tends to cause, respectively, current In the present instance no source of energy for ilow of respectively opposite polarity in the corre supplying the direct current power circuits has sponding phase of the output circuit, periodically been indicated. It will be understood that these actuable control means for each unit for suc power circuits may be supplied from any suitable cessively rendering the corresponding valve means source. For example, a portion of the three conductive and non-conductive in alternate re phase output of the system may be utilized for lation, and timing means for actuating the sev this purpose. Alternatively, and as is described eral control means in predetermined phase rela in more detail in the aforesaid copending Nims tion to each other, said timing means including a application, an auxiliary or pilot generator may source of periodic control voltage common -to said be provided to supply the control energy. LLO units, a network having a plurality of output Although only a single complete embodiment circuits individual respectively vto said units, of the invention has been described in detail, it means rendering said network responsive to said will be appreciated that various modifications in source of control voltage for energizing said out the form, number, and arrangement of the parts put circuits in rotation, and means rendering each may be made without departing from the spirit control means operably responsive to each ener and scope of the invention. gization of the corresponding output circuit of What is claimed is: said network. 1. In a system for supplying a multiphase al 4. In a system for delivering multiphase alter ternating current output circuit from a source of hating current energy to an output circuit from electric energy, the combination of a plurality a source of multiphase alternating current en of translating units individual to said phases, orgy, a plurality of translating units each indi each unit including means actuable to translate vidual to a corresponding phase of said output energy from the source into single phase alter circuit, each unit comprising a pair of valve hating current energy and deliver the same to a means each deñning a plurality of discharge corresponding phase of said output circuit, and i paths having a common cathode connection and control means for actuating said units in prede a plurality of anodes coupled to corresponding termined phase relation to each other so that the phases of said source, means coupling the valve respective phases of the output circuit have cor means of each unit to the corresponding output responding phase relations, said control means phase so that current Flow through the individual including a source of periodic control voltage (SO means of each pair tends to cause, respectively, common to said units, a network having a plu current now of respectively opposite polarity in rality of output circuits individual respectively to the corresponding phase of the output circuit, said units, means rendering said network respon periodically actuable control means for each unit sive to said source of control voltage for energiz for successively rendering the corresponding valve ing said output circuits in rotation, and means (lo means conductive non-conductive in alter rendering each circuit operably responsive to nate relation, and timing means for actuating each energization of the corresponding output the several control means in predetermined phase circuit of said network. relation to each other, said timing means includ 2. In a system for supplying a multiphase al ing a source of periodic control voltage common ternating current output circuit from a source of to said units, a network having a plurality of out electric energy, the combination of a plurality of put circuits individual respectively to said units, translating units individual to said phases, each means rendering said network responsive to said unit including means actuable to translate energy source of control voltage for energizing said out from the source into single phase alternating cur put circuits in rotation, an inverter network indi rent energy and deliver the same to a correspond vidual to each unit and actuable in response to 2,409,534 17 18 successive energizations to a'ctuate the corre sponding control means, and means rendering said inverter networks operably responsive tosuc cessive energizations of the corresponding output circuits of said Erst-mentioned network. 5. In a system for controlling iiow of current mentioned means including a source of periodic voltage, a network having a plurality of output circuits effective respectively to so energize said timing means, and means rendering said output circuits successively responsive to said source of periodic voltage. 11. In a system for controlling ñow of current to a multiphase load circuit, a plurality of net works each having an output circuit individual to said phases, a network having a plurality of 10 to each phase of the load circuit for controlling such phase, a valve for each output circuit, output circuits individual to such phases, means changes in conductivity of said valves serving to rendering said network responsive to said source control the corresponding output circuits and ef of control voltage for energizing said output cir fect said control, a pair of independently operable cuits in rotation, and means rendering said con control means so constructed and arranged that trol means operably responsive to energization of between a source and a multiphase lo-ad circuit, control moans individual to each phase of said load circuit, a source of periodic voltage common the corresponding output circuit. each network is controlled by a different one 0i’ said control means for successively altering the 6. In an inverter network, _the combination of conductivities of the valves of each network, and a pair of electric valves each having a cathode, a means responsive to one network for periodically control grid and main and auxiliary anodes, means including control means coupling the aux 20 correcting any out of phase of the other network iliary anode and grid circuits of said valves to so as to maintain the networks in step with each other. gether to control the conductivity of _said valves, and an output circuit coupled to the main-anode 12. Apparatus for supplying a multiphase load circuit comprising a plurality of systems each as cathode circuit of each valve. 7. In an inverter network, the combination of 25 deiined in claim l, means connecting the respec tive output circuits in parallel to said load cir a pair of electric valves each having a cathode, a cuit, and synchronizing means for causing the control grid and main and auxiliary anodes, means including control means coupling the aux~ control means for the respective systems to be ac tuated in predetermined timed relation to each iliary anode and grid circuits of said valves to gether to control the conductivity of said valves, 30 other. an output circuit coupled to the main-anode 13. Apparatus for supplying a multiphase load cathode circuit of each valve, and means for peri odically actuating said control means to ‘ ex tinguish one valve and render the other valve conductive. 8. In an inverter network, the combination of a pair of electric valves each having a cathode, a control grid and main and auxiliary anodes, means including control means coupling the aux iliary anode and grid circuits of said valves to~ gether tov control the conductivity of said valves, translating means having a winding, the respec tive terminals whereof are connected tc the main anodes of said valves, said winding having an intermediate terminal, and a source of energy connected between said intermediate terminal and the cathode of said valves. 9. In an inverter network, the combination of a circuit comprising a plurality of systems each as deiined in claim 2, means connecting the respec« tive output circuits in parallel to said load circuit, 35 and synchronizing means for causing the control means for the respective systems in predetermined timed relation 14. In a system for delivering ternating current energy to an to be actuated to each other. multiphase al output circuit from a source of mt. aìphase alternating current energy, a plurality or translating units each in dividual to a corresponding phase of said Output circuit, each unit comprising a pair of electric valve means each defining a plurality of dis charge paths having a common cathode connec tion and a plurality of anodes coupled to corre sponding phases of said source, means coupling the valve means of each unit to the correspond ing output phase so that current flow through plurality of pairs of electric valves, each having a cathode, a grid and main and auxiliary anodes, 50 the individual means of each pair tends to cause, respectively, current iiow of respectively opposite means including control means coupling the aux iliary anode and grid circuits of the valves of each polarity in the corresponding phase of the out pair together to contro-l the conductivity of the put circuit, periodically actuable control means corresponding valves, timing means to periodi for each unit for successively rendering the cor cally actuate said control means to extinguish one valve of each pair and render conductive the other valve of such pair, an output circuit coupled across the cathode and main anode circuit of each valve of each pair, and means to energize said timing means of the respective pairs in prede termined phase relation to each other. 10. In an inverter network, the combination of a plurality of pairs of electric valves, each having a cathode, a grid and main and auxiliary anodes, means including control means coupling the aux iliary anode and grid circuits of the valves of each pair together to control the conductivity of the corresponding valves, timing means to peri odically actuate said control means to extinguish one valve of each pair and render conductive the other valve of such pair, an output circuit coupled across the cathode and main anode circuit of each valve of each pair, and means to energize said timing means of the respective pairs in prede termined phase relation to each other, said last responding valve means conductive and noncon ductive in alternate relation, a source of periodic control voltage common to said units, a counter network including an electric valve'individual t0 each phase of the aforesaid output circuit, means coupling said valves to said source of pe riodic control voltage so that the conductivities of said valves are altered in predetermined suc cession, and means rendering each control means operably responsive to the condition of the as sociated said valve. 15. In a system for supplying a multiphase al ternating current output circuit from a source of electric energy, the combination of a plurality of translating units individual to said phases and common to >said source, each unit including means actuable to translate energy from the source to single-phase alternating current energy and delivering the same to a corresponding phase of said output circuit, a source oi periodic control 75 voltage common to said units, a counter-network 2,409,534 19 including an electric 4valve individual to each phase of the aforesaid output circuit, means coupling said valves to said source oi' periodic control voltage so that the conductivities of said valves are altered in predetermined succession, lil and means including an inverter network indi vidual to each phase of the output circuit, each such inverter network being operably responsive to the condition of the associated electric valve. 16. In a system for delivering multiphase al ternating current energy to an output circuit from a source of multiphase alternating current energy, a plurality of translating units each in dividual to a corresponding phase of said output circuit, each unit comprising a pair of electric valve means each deiining a plurality of dis charge paths having a common cathode connec 20 condition of each valve for producing a control voltage for the corresponding phase. 18. In a system for producing multiphase con trol voltages, the combination of a source of periodic control voltage common to such phases, a counter-network including an electric valve in dividual to each such phase, means coupling said valves to said source of periodic voltage so as to alter the conductive conditions of said valves in rotation, and means including an inverter net work individual to each said valve and operably responsive to the condition thereof for producing a control voltage for the corresponding phase. 19. In `a system for controlling flow of current to a multiphase load circuit, a plurality of net works each having an output circuit individual to each phase of the load circuit for controlling such phase, said networks comprising a pair of tion and plurality of anodes coupled to corre sponding phases of said source, means coupling independently operable control means so con the valve means of each unit to the correspond 20. structed and arranged that each network is con ing output phase so that current flow through the trolled by a different one of said control means individual means of each pair tends to cause, re to actuate the corresponding output circuits in spectively, current flow of respectively opposite rotation, and means responsive to one network polarity in. the corresponding phase of the output for controlling the other network so as to correct circuit, pc iodically actuable control means for any out-of-phase operation of the other net each unit for successively rendering the corre work with respect to the one network whereby spending valve means conductive and noncon the networks will be brought periodically in step with each other. ductive in alternate relation, a source of periodic control voltage common to said units, a counter 20. ln a system for controlling flow of current network including an electric valve individual to to a multiphase load circuit, a plurality of net each phase of the aforesaid output circuit, means works each having an output circuit individual coupling said valves to said source or" periodic to each phase of the load circuit for controlling control voltage so that the conductivities of said such phase, said networks comprising a pair oi valves are altered in predetermined succession, independently operable control means so con and means including an inverter network indi structed and arranged that each network is con vidual to each phase of the output circuit, each trolled by a different one of said control means to such inverter network being operably responsive actuate the corresponding output circuits in rota to the condition of the associated electric valve. tion, means responsive to one network for con 17. In a system for producing multiphase con trolling the other network so as to correct any trol voltages, the combination of a source of pe 40 out-of-phase operation of the other network with riodic control voltage common to such phases, a respect `to the one network whereby the networks counter-network including an electric valve inwill be brought periodically in step with each dividual to each such phase, means coupling said other, and means for selectively rendering said valves to» said source of periodic voltage so as to alter the conductive conditions of said valves in rotation, and means operably responsive to the responsive means effective or ineffective. OMER E. BOVVLUS.