US1965416A - Electric valve circuits - Google Patents

Description

July 3,1934.

POTENTIAL A. HOWARD ELECTRIC VALVE CIRCUITS Filed 001;. l, 1951 Inventor: Alan Howard His Attorney Patented July 3, 1934 ELECTRIC VALVE cmom'rs Alan Howard, Schenectady, Y., assignor zto General Electric Company,

New York a corporation of Application October 1, 193 1, Seria l No."566,376 3 Claims. (o1. 17 5-3 63) My invention relates to electric valve circuits and more particularly to such circuits including valves of the vapor electric discharge type.

,I-Ieretofore there have been devised numerous 1 apparatus including electric valves for transmitting energy between direct current circuits, direct and-alternating current circuits, or alternatingcurrent circuits of different frequencies, voltages, lphases, etc. In such electric valve convert- 0 ing apparatus, iithas been found particularly advantageous to use valves of the vapor electricdischa'rge type because of the relatively large amounts of power which may be handled at ordinaryoperating voltages. In the majority of the arrangements of .the prior art, however, the several electric valves are adapted to be rendered conductive periodically for short intervals of time and 'to'be maintained non-conductive periodically for other'intervalsof time. In such apparatus it "aches been customary to maintain the valves nonconductive during the desired intervals by .impressing a negative potential upon the control grids, this potential being either aperiodic potentiaLeachcycleof which has an appropriate nega- 'tive-perio d, or a unidirectional negative bias 'potential upon which is superimposed a periodic potential, theDCsitive periods of which aresufiicientto renderthe grid positive during the'intervals in which the valve is to be made conductive.

"50 Inmany of theelectric valve converting appara- -tus,.however, ithas been foundthat periodic transients of very steep wave form are setup in the .apparatus due to the sudden transfer of theload current between the several electric valves. In

"" .case these transients are impressedacrossa nonconducting electric valve with such a polarity that the anode is made positive with respect to thecathode, it has 'been found that the .internal capacity between thegrid and anode of the elec- 40 .tric valve and the capacity between the grid and :anode circuits is often sufiicient to permit a potential to be electrostatically induced upon the grid of such a magnitudeas to overcome the grid bias and render the valve conducting, that is, the '45 grid of the electric valve loses control. Such an occurrence usually causes a short circuit upon'one or more of the parts of the apparatus with .consequent damage thereto and interruption'to service. '60 It is .an object of -my'invention, therefore, to

provide an improved excitation circuit fora vapor electric discharge valve which will overcome the above mentioned disadvantages of the arrangements of the prior art and which will "be simple and reliable in operation.

7 ...It is anotherLcbject of my invention to provide I an improvedgrid.excitationcircuit for vapor electric discharge valves which will avoid the possibility .of thegrid losing control of the conductivityof the valve upon the occurrenceof transients in the valve circuit. I

In accordance'with my invention, the effect of transient positive voltages in the anode circuit of a vapor electric discharge valve on the grid of I, the device, tending to deprive the grid of control over'the conductivityof the valve, is substantially eliminated by connecting between the grid and cathode :of the discharge device a condenser having a :capacitysubstantially-greaterthan the in- *ternal-capacitybetween the gridand anode of the device and/or that between the grid and anode circuits. By this expedient, the potential electrostati'cally'induced upon the control grid of the device tending tosdeprive it of control, is reduced to. such :2. value :that its effect is negligible.

:For a -:better understanding of my invention together .withother and further objects thereof, reference is 'had to the following description taken in connection with the accompanying drawingand itsscope will be pointed out in the appended 'claims. Referring now to the draw- -ing, there is'illus'tratedin Fig. l a vapor electric discharge'valve with'the internal capacities between the electrodes indicated in dotted lines; Fig. 2'shows certain operating characteristics of thearran'gement of Fig. l to aid in the under- "stahdingof the 'invention; "Fig. 3 is-a modification of the diagrammatic sketch of Fig. 1 to aid in the explanation of my invention; Fig. 4 illustrates my invention as applied to an electric power 'converting apparatus, while Fig. 5 is a 'copy 'ofian oscillogram illustrating certain operatingcharacteristics of the apparatus of Fig. 4.

Referring now to Fig. 1 of the drawing, there is' conventiona'lly illustrated an electric valve of the vapor electric --discharge type comprising an envelope 10, an anode'll, a control grid 12, and a cathode 1 3. 'It willbe understood that the envelope 10 is filled with an ionizing vapor at low pressure; such for example as mercury vapor, argon, neon, 'etc. as is well understood lay-those skilled in the art. -'It will-also be understood by those-skilledin theart 'that'the cathode 13 con- 'ventionallyillustrated as of'the hot cathode type may be of "the mercury "pool type if so desired, it '105 onlyfbeingneces'sary that the cathode 13 shall be cOntinuOuSIy eXcited to forin a source of free electrons. The inter-electrode capacities of the anode'and grid'a'nd-the grid and cathode are indic'ated in dotted 'liriesbythe capacitors Op and Cr, respectively. In such a device, it can be mathematically demonstrated that, if a variable potential Ep be impressed upon the anode 11, the voltage Eg electrostatically induced upon the These characteristics are represented by the curves of Fig. 2 in which the curve Ep represents a variable potential impressed upon the anode and the dotted curve Eg represents the potential induced thereby upon the control grid. From the above formula it is apparent that if the capacity C: is very large relative to the capacity Cp, the potential Eg induced upon the control grid will be correspondingly small with respect to the anode potential Ep. In the majority of the vapor electric discharge valves commercially available, however, the control grid comprises a perforated cup shaped member closely surrounding the anode so that the capacity Cp is large relative to the capacity Cr.

In the arrangement of Fig. 3, however, an additional capacity Cr has been connected in parallel to the internal capacity Cr and the equation now becomes:

' {L EII EW cp+cf+clf Thus, by properly selecting the capacity 0 1", the potential Eg electrostatically induced upon the control grid by variations in the plate potential, may be limited to any desired value and may be made negligible compared with the magnitude of the normal grid potential or the negative grid bias potential.

In Fig. 4 there is illustrated an electric valve converting apparatus embodying my invention of the type known in the art as a parallel inverter, adapted to transmit energy from a direct current supply circuit 16 to an alternating current load circuit 17. This apparatus comprises a transformer 18 having a secondary winding connected to the alternating current circuit 1'7 and a primary winding provided with an electrical midpoint connected to one side of the direct current circuit 16, and, a pair of end terminals connected to the other side of the direct current circuit 16 through vapor electric discharge valves 19 and 20. A commutating capacitor 21 is connected between the electric valves 19 and 20 and, if desired, a smoothing reactor 22 may be connected in the direct current circuit. The vapor electric discharge valves 19 and 20 are each provided with an anode, a cathodeand a control grid for controlling the starting of current in the anode-cathode circuit. The control grids of the valves 19 and 20 are connected to their common cathode circuit through opposite halves of the secondary winding of a grid transformer 23, a negative bias battery 24,- and a current limiting resistor 25. The primary winding of the grid transformer 23 may be energized from any suitable alternating current circuit 26 of the frequency which it is desired to supply to the circuit 1'7, or in case the circuit 17 is connected to an independent source of electromotive force for determining its frequency, the primary winding of the transformer 23 may be energized therefrom through a suitable phase advancing condenser, as will be well understood by those skilled in the art. The grids of the devices 19 and 20 are also connected to their respective cathodes through condensers 27 and 28 anodes of the valves 19 by means of which the grids of the valves are enabled to maintain control of the conductivity of the valve even during the occurrence of positive transients on the anodes of the respective valves. The general principles of operation of the above described apparatus will be well understood by those skilled in the art or may be found explained in detail in United States Letters Patent No. 1,800,002, granted April '7, 1931, upon the application of E. F. W. Alexanderson. In brief, if one of the electric valves, for example valve 19, is-initially made conductive by a positive half cycle of grid potential from the transformer 23 of such a magnitude as to overcome the negative bias of battery 24, current will flow from the positive side of the direct current circuit, through the left hand portion of the primary winding of the transformer 18 and electric valve 19 to the other side of the direct current circuit. During this interval the capacitor 21 will become charged to a potential substantially twice that of the direct current circuit and, when electric valve 20 is made conductive, the potential of this capacitor iseffective to transfer the current from the valve 19 to the valve 20. Current will now flow 100 from the positive side of the direct current circuit, through the right hand portion of the primary winding of the transformer 18 and electric valve 20 to the other side of the direct current circuit. In this manner, the current is successively commutated between the electric valves 19 and 20, flowing in opposite directions through the primary winding of the transformer 18 and thus supplying an alternating potential to the circuit 17. In case the alternating current circuit .11 17 is connected to an independent source of electromotive force, it will be understood that the capacitor 21 may be omitted and the primary winding of the grid transformer energized from the alternating current circuit 17, in which case the electromotive force of the transformer 18 is effective to commutate the current between the electric valves.

In Fig. 5 there is a copy of an oscillogram of the potential appearing across one of the electric valves in a circuit of the type illustrated in Fig. 4. The portions a of this curve represent the intervals during which the valve is conducting current, so that the potential across the valve is only the arc drop of the valve, which remains substantially constant. The intervals b of this curve represent the periods during which the valve is In case the above described circuit is operating at relatively high voltages, or

in case the capacities between the grids and and 20 or their respective circuits are sufficiently large, it is seen that there is some danger of the valves being rendered conductive during this initial portion of each period when they should be nonconducting, due to the potentials electrostatically induced upon their control grids, as described above, which may be 'sufiicient to overcome the negative bias of the battery 24. In case one of the valves should be rendered conductive during this initial portion of the period b, there would result a direct short circuit upon the direct current circuit 16 with consequent damage to the apparatus. However, with the addition of capacitors 2'1 and 28, which preferably have capacities very large with respect to the capacities between the grids and anodes of the valves, the effect of this inter-electrode capacity is substantially eliminated and the grids of the valves are enabled to maintain control of their conductivities.

While I have described my invention as applied to an electric valve converting apparatus of the type known in the art as a parallel inverter, it will be understood that it is in no wise limited thereto, but is equally suitable to any electric valve circuit in which the anode of the valve is subjected to periodic or transient potential Waves.

While I have described what I at present consider the preferred embodiment 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. In an electric translating circuit, the combination of a plurality of vapor electric discharge devices each provided with an anode, a cathode, and a control grid, the anode and grid of each valve forming a substantial internal capacity, a control circuit for rendering said devices alternately conductive and nonconductive in a predetermined sequence, an electric coupling between the anode-cathode circuits of said devices whereby the interruption of current in one device impresses potential transients on the anode of the nonconductive devices, and capacitive means interconnecting the grid and cathode of eachof said devices to eliminate substantially the effect on the grids of the nonconductive devices of the interruption of the current in a conductive device.

2. In combination, a vapor electric discharge device provided with an anode, a cathode, and a control grid, the anode and grid and grid and cathode forming internal capacities of Cp and Cr, respectively, an anode-cathode circuit for said device including a source of high potential transients, having a maximum value of Ep, means for applying a variable control potential to said grid, and capacitive means C: interconnecting said grid and cathode and so proportioned that the term C11 D+ 1+ is substantially less than the normal range of variation of said grid control potential.

3. In combination, a vapor electric discharge device provided with an anode, a cathode, and a control grid, the anode and grid and the grid and cathode forming internal capacities of Cp and Cr, respectively, an anode-cathode circuit for said device including a source of high potential transients having a maximum value Ep, means for applying to said grid a negative potential during certain intervals to maintain said device non-conductive, and a capacitor connected between said grid and cathode and having such a value C'r that the term D H" is negligible compared to the normal value of said negative potential.

ALAN HOWARD.

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