US2127232A - Rectifier transformer - Google Patents

Description

1938- L. c. NICHOLS 2,127,232

RECTIFIER TRANSFORMER Filed July 25, 1936 3 Sheets-Sheet l Aug. 16, 1938. L. c. NICHOLS. v f 2,127,232

RECTIFIER TRANSFORMER Filed July 25, 1936 I 3 Sheets-Sheet 2 Aug. 16, 1938.

L. C. NICHOLS RECTIFIER TRANSFORMER Filed July 25, 1936 5 Sheets-Sheet 3 Patented Aug. 16, 1938 UNITED STATES PATENT OFFiCE RE GTIFIER TRANSFORMER.

Application July 25, 1936, Serial No. 92,472

10 Claims.

This invention relates in general to improvements in rectifier transformers, and more particularly to transformers adapted to supply current to groups of electric valves operating in 5 parallel circuits.

It is well known that electric valve converting systems are occasionally subject to disturbances, such as the so-called backfires resulting from momentary failure of the valve action of a valve utilized as part of the system. In systems utilizing a transformer connecting one or more valves with an alternating current circuit, such failure particularly results in effectively short circuiting the alternating current circuit through 15 the transformer, and the latter is then caused to carry an excessive current which may damage the windings thereof by overheating and by the electromagnetic action thereon of the magnetic leakage fields thereof. It is therefore desirable 20 to so design the transformer as to reduce the magnitude of any backfire currents flowing through the windings thereof. In particular, when the system presents a plurality of parallel circuits, the transformer is preferably provided 25 with a secondary winding divided into parallel sections each connected in one of such circuits through a separate valve, such sections being severally associated with serially connected sections of the transformer primary winding. Dur- 30 ing a backfire some of the sections of the primary winding are thus associated with secondary winding sections which are without current, and such primary winding sections then are linked with extensive magnetic leakage fields which materially reduce the flow of current through the transformer. With such arrangement, current fiow if also forcibly initiated simultaneously through all parallel secondary winding sections, during normal operation of the transformer, and

40 such sections are forced to carry equal amounts of current at every instant.

It is, therefore, one of the objects of the present invention to provide a transformer for an electric valve converting system having windings 45 so arranged that the flow of backfire current therethrough is reduced to a relatively low value.

Another object of the present invention is to provide a transformer for an electric valve converting system having a secondary winding form- 50 ing parallel circuits and soarranged that the flow of current is forcibly initiated simultaneously through the several circuits.

Another object of the present invention is to provide a transformer for an electric valve con- 55 verting system having a secondary Winding forming parallel circuits and so arranged that such circuits are forced to carry equal currents at every instant.

Objects and advantages other than those above set forth will be apparent from the following de- 5 scription when read in connection with the accompanying drawings in which:

Fig. l diagrammatically illustrates the arrangement of the windings and the connections of a transformer of the shell type forming an embodiment of the present invention and comprising a three-phase primary winding and a six-phase secondary winding divided into three parallel sections supplying a group of eighteen valves of an alternating current rectifying system;

Fig. 2 diagrammatically illustrates another embodiment of the present invention differing from the embodiment illustrated in Fig. l in being provided with two parallel primary windings and with a six-phase secondary winding divided into four parallel sections;

Fig. 3 diagrammatically illustrates a further embodiment of the present invention consisting of a transformer of the core type having a threephase primary winding and a six-phase secondary winding divided into two parallel sections.

It will be understood that some of the features of any one of the embodiments illustrated may also be utilized in combination with features of the other embodimens to form further embodiments of the present invention.

Referring more particularly to the drawings by characters of reference, reference numeral 6 in Fig. 1 generally designates a transformer of the so-called shell type adapted to be utilized in conjunction with a plurality of electric valves l to form a converting system such as the system illustrated by way of example, which is adapted to rectify alternating current supplied thereto from a three-phase supply line 8 and to deliver the rectified current to a direct current output line 9. Valves l each comprise an anode Hi, the anodes being either each associated with a separate cathode, or being associated with a lesser number of cathodes or even with a single cathode connected with one conductor of line 9. It will be understood that by providing valves I with suitable control electrodes and with a suitable control system therefor, the transformer and the valves may also be caused to transmit energy from line 9 to line 8, or to transmit energy between line 8 and another alternating current line suitably connected with the transformer and with the valves.

Transformer 6 is provided with a three-phase 55,

core I I on which are arranged the phase portions I2, I3, Id of a three-phase primary winding, each portion consisting of a plurality of sections such as I211, I22), I20, I2cl serially connected across a pair of conductors of line 8. Means are provided for varying the connections between the sections of each primary winding phase portion to thereby adjust the secondary voltages of the transformer. Although such result could be obtained by changing taps of only one or of less than all the sections of each phase portion, it is generally preferred to provide all such sections with taps connected to no load tap changers equal in number to the number of interconnections between primary: winding sections. In the present embodiment, each primary winding portion comprises four sections and therefore presents three connections between sections, which connections may be changed by means of three tap changers. Only tap changers I6, I1, I8 associated with winding portion I2 are illustrated in detail, the tap changers associated with winding portions I3 and I5 being similarly arranged and connected. Tap changer I6, for example, is provided with a plurality of contacts I9, of which alternate contacts are severally connected with the taps and with one terminal of winding section I2a, the remaining segments being severally connected with the taps and with one terminal of winding section I2b. Each pair of adjacent taps may be bridged by a brush 2|, the brushes of the several tap changers being mounted on a common shaft 22 in order that they may be moved simultaneously over the same number of steps of the respective tap changers.

The polyphase secondary winding of transformer 6 comprises a plurality of phase portions 23, 24, 25 each divided into a plurality of interconnected parallel sections such as 23a, 23b, 23c interleaved with the sections of the associated primary winding phase portion I2. Both the primary and secondary winding sections consist of stacks of disk shaped coils, also called pancake coils, of which only the coils of winding portions I2 and 23 are shown in cross section, the supporting means therefor being omitted to clarify the drawings. Winding portions I3, I4, 24, 25 are shown in elevation and are represented as being insulated from each other and from core II by means of insulating spacers 26. Such spacers also serve to brace the windings against each other and against core II for preventing displacement of the winding sections by the electromagnetic action thereon of the magnetic leakage fields thereof during flow of backfire or short circuit current through the transformer. In addition, it is generally preferable to arrange the taps of each primary winding end section, such as I id, at the end of such section remote from the adjacent secondary winding section such as 23a, and to arrange the terminals and the taps of each intermediate primary winding section, such as I21), near the center of such section to thereby remove the taps, which constitute weak points of the winding, from the magnetic leakage fields present in the gaps between adjacent primary and secondary winding sections.

In order to obtain six-phase current from the transformer secondary winding, each section thereof, such as 23a, is divided into two portions which are symmetrically arranged with respect to the adjacent primary winding sections, such as I2a, I2b, and which may also be interleaved to reduce the leakage reactance thereof with respect to the associated primary winding portion. Each primary winding section is thus arranged adjacent both portions of at least one of the secondary winding sections and has a relatively low leakage reactance with respect thereto, and is also arranged remote from the remainder of the primary and secondary winding sections and, therefore, has a relatively high leakage reactance with respect thereto. The two portions of a sec ondary winding section, such as 23a, are adapted to alternately transmit unidirectional current impulses to the associated anodes, such as Hm, I 0d, of valves 1 in the course of alternate half cycles of the voltage of line 8. The operation of the secondary winding sections is preferably controlled by means of an-interphase transformer 27 connected therewith and forming therewith, in effect, a unitary transformer structure, as is well known in the art.

In operation, line 8 being energized, current is drawn from line 8 sequentially through winding portions I2, 13 and I4, and the currents induced thereby in winding portions 23, 24 and 25 are converted, by the action of valves 1, into successive unidirectional impulses which combine at the common cathode of valves I to form a flow of direct current supplied to line 9. At every instant, three valves receive current from the associated parallel sections of one of winding portions 23, 24 and 25, the action of interphase transformer 21 causing the flows of current which are successively initiated in the different groups of valves to overlap by one-half of their duration, so that current is carried at every instant by at least six valves. Considering such operation in greater detail, at a particular moment of the voltage cycle of line 8, the voltage conditions of such line are such that three valves 1 cease carrying current, and a flow of current is then established simultaneously through three valves I such, for example, as the valves comprising anodes Illa, I fig and Him respectively associated with parallel portions of winding sections 23a, 23b and 230. The three parallel circuits established through such winding portions are similar to one another and receive equal voltages, so that normally the flow of current is established therethrough simultaneously.

If one of the circuits, such as the circuit including anode Illa and the associated winding portion 23a, fails to carry current for any reason such as non-uniform conditions of pressure and temperature in the valves, the primary winding portions I2a and I2b adjacent to section 23a cause the establishment of a magnetic leakage flux therethrough, such leakage flux closing itself about winding section I2a through a. portion only of core II and through winding section 23a. Winding section In accordingly takes an increased fraction of the voltage of line 8 and, as a result of the series connection of winding portion I2, winding sections I22), I2c, I2d thereof receive a lesser fraction of the voltage of line 8 and link with a reduced magnetic flux, whereby the voltages induced in adjacent winding sections 23b and 230 decrease while the voltage induced in section 23a rises to a value sufficient to overcome the cause of the failure of section 2311 to carry current, thus forcing the substantially simultaneous initiation of the flow of current through all three parallel sections. Such action would also take place in a similar manner, if two of the three parallel circuits tended to remain without current, to increase the voltages impressed on the two momentarily inoperative circuits and thus restore normal operating condition.

By a process similar to that above described, if the currents in the parallel circuits become unbalanced, the voltage impressed on the circuit carrying the smallest current is increased by an increase of the magnetic flux linking therewith, while the voltage impressed on the circuit carrying the highest current is decreased by a decrease of the magnetic flux linking therewith; such adjustment of the magnetic fluxes being obtained automatically by the distribution of the leakage fluxes produced by the serially connected primary winding sections.

The occurrence of a backfire in valves 1 would result from the loss of the valve action of one of the valves, of which the anode may then become a cathode. Assuming that anode lfla, for example, becomes a; cathode, current may flow thereto from all other anodes except the anodes normally operating in parallel therewith whereby line 8 is short circuited through the transformer. As is well known, a backfiring anode may receive current simultaneously from several of the secondary phase circuits of the transformer at every instant. For example, at a particular moment of the voltage cycle of line 8, current may flow from the upper terminal of interphase transformer 21 through one portion of each of the parallel sections of winding portion 24 and the associated anodes to anode Illa and the associated portion of winding section 23a back to the upper terminal of interphase transformer 21 and also from the lower terminal of the interphase transformer through one portion of each of the parallel sections of winding portion 25 and the associated anodes through anode Illa, the associated portion of winding section 2311, and through the interphase transformer. As is usual in the operation of transformers, such currents are induced in the secondary winding portions by the associated primary winding portions. During such operation, winding sections 231) and 230 are without current. Primary winding sections I20 and lZd may thus establish extensive magnetic leakage fields about themselves and through secondary winding sections 23d and 230 and, as the result of the serial connection of winding sections Her and I21) with sections I20 and l2d, the latter function in the same manner as windings of independent reactors connected in series with winding sections l2a, I221. The flow of current through winding portion [2 is thereby considerably reduced, and the magnitude of the flow of backfire current between winding portions 23, 24 and 25 is thus reduced to a material extent.

If generators or converting systems other than the systems shown are connected to supply current to line 8, the occurrence of a backfire at anode l a also causes direct current to flow from line 9 through the common cathode of valves 1, then functioning as an anode, through anode Illa, the associated portion of winding section 2311 and one winding of interphase transformer 21 to the other conductor of line 9. Such current is not established instantly at a constant value and, during the rise of the value thereof, the flow of such current through winding section 23a tends to induce a similar rising flow of current through winding portion I2. The magnetic leakage fields established about winding sections [20 and [2d, however, oppose the establishment of such current in winding portion I2, thereby also reducing the rate of increase of the current in winding section 23a. Such current may therefore be interrupted by means of suitable switching means before hav-- ing reached the maximum value thereof, such maximum value being determined by the resistance of the circuit traversed thereby. The flow of backfire current caused by failure of the valve action of anodes other than anode lOa would be reduced in the manner above set forth as a result of the serial connection of the sections of winding portions l2, l3 and I l. The heating and stressing of the conductors of the transformer windings are thus considerably reduced, and the thrusts impressed on the winding sections by the electromagnetic action of the magnetic leakage fields thereof are reduced to values such that the sections may be held in place by means of spacers 26.

In the embodiment illustrated in Fig. 2 core H is provided with a three-phase primary winding, of which each phase portion such as 28 comprises four sections such as 28a, 28b, 28c, 28d connected in series parallel connection through tap changers such as 29 and 3|. The tap changers are illustrated as being each provided with an odd number of segments to obtain an even number of steps, the two winding sections being connected with one tap changer which is then provided with numbers of taps differing by one. In the present embodiment, each secondary phase portion such as 32 is divided into four sections such as 32a, 32b, 32c, 3201 each divided into two portions supplying two valves 1, there being accordingly twenty-four valves which are preferably divided into two equal groups each provided with a separate cathode to form a separate structure or rectifier. It is generally preferable to control the operation of the winding sections associated with different rectifiers by means of separate interphase transformers such as transformers 33 and 34. The operation of the serially connected primary winding sections in causing the currents through the associated parallel secondary winding sections to be initiated simultaneously and to be maintained at equal values, and in limiting the flow of backfire current through the transformer, is similar to that set forth with respect to the embodiment illustrated in Fig. 1.

In the embodiment illustrated in Fig. 3, the transformer is assumed to be provided with a core 36 adapted to receive coextensive phase winding portions arranged side by side about different legs of the core to form a transformer of the so-called core type. Each primary winding phase portion, such as 31, is divided into a plurality of sections such as 31a, 31b each Wound about one of the sections such as 38a, 38b of a secondary phase winding portion 38. Each winding phase portion is shown as being divided into only two sections whereby the transformer is operable to supply current to a group of twelve valves operating in parallel in pairs, but it will be understood that a greater number of sections may also be utilized as in the embodiments illustrated in Figs. 1 and 2. The two sections of each primary winding phase portion may be connected through a single tap changer such as at 39. The operation of the serially connected primary winding sections in causing the currents in the associated parallel secondary winding section to be initiated simultaneously and to be maintained at equal values and in limiting the flow of backfire current through the transformer, is similar to that above set forth with respect to the embodiment illustrated in Fig. 1.

It will be observed that, in the present embodiment, the two winding sections such .as 31a, 31b of a primary winding phase portion as well as the two sections such as 38a and 38b of the associated secondary winding phase portion are alined end to end on a core leg and each consists of a plurality of nested cylindrical coils. It is well known that such coils cannot be braced satisfactorily against the core to withstand axial thrusts resulting from the action of the magnetic leakage field established thereabout during a backfire. For example, if an anode connected with winding section 38a loses its valve action, during a part of the cycle of the voltage of line 8 a large current flows from at least one of the secondary winding portions other than portion 38 and through the associated anode or anodes of valves 1 through anode Illa and the associated portion of section 38a, while section 3% remains without current. Both portions of primary winding portion 3'! necessarily carry equal currents at such time and, section 38a being arranged dissymmetrically with respect to the entire winding portion 31, the magnetic leakage field of winding portion 31 tends to exert a large axial thrust on winding section 38a which would displace such section bodily on core 36. To avoid such result, means are arranged between the winding sections for diverting the magnetic leakage field of each primary winding section and of the adjacent secondary winding section from all other winding sections. Such means are thus essentially magnetic shields which may consist of additional yokes 4| of core 36 separating the top and bottom winding sections or of other equivalent means such as disks of magnetic material inserted between the top and bottom winding sections Each winding section is thus subject to only magnetic leakage fields which are symmetrical with respect thereto and which do not exert .any axial thrust thereon.

Although but a few embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that various modifications and changes may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. A transformer for an electric valve converting system comprising a primary winding having a phase portion thereof divided into a plurality of serially connected sections arranged on a common core, and a secondary winding having a plurality of interconnected parallel sections arranged on said core and each divided into two portions adapted to alternately transmit unidirectional current impulses, each said primary winding section being arranged adjacent both portions of at least one of said secondary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said primary and secondary winding sections and having a relatively high leakage reactance with respect thereto.

2. A transformer for an electric valve converting system comprising a polyphase primary winding having each phase portion thereof divided into a plurality of serially connected sections, and a polyphase secondary winding having each phase portion thereof divided into a plurality of parallel sections, each primary winding section being arranged adjacent at least one of said secondary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said primary and secondary winding sections and having .a relatively high leakage reactance with respect thereto to thereby limit the fioW of backfire current between said secondary phase p0rtions.

3. A transformer structure for an electric valve converting system comprising a polyphase primary winding having each phase portion thereof divided into a plurality of serially connected sections, a polyphase secondary winding having each phase portion thereof divided into a plurality of parallel sections, each primary winding section being arranged adjacent at least one of said secondary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said primary and secondary winding sections and having a relatively high leakage reactance with respect thereto to thereby limit the flow of backfire current between said secondary phase portions, and interphase windings connected with said secondary winding for controlling the operation thereof.

i. A transformer for an electric valve converting system comprising a primary winding having a plurality of serially connected sections arranged on a common core, a secondary winding having a plurality of interconnected parallel sections arranged on said core and each divided into two portions adapted to alternately transmit unidirectional current impulses, each said primary winding section being arranged adjacent both portions of one of said seconary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said primary and secondary winding sections and having a relatively high leakage reactance with respect thereto, and means arranged between said winding sections for diverting the magnetic leakage fields of each said primary winding section and of the adjacent said secondary winding section from all other said winding sections.

5. A transformer for an electric valve converting system comprising a primary winding having a plurality of serially connected sections arranged on a common core, a secondary winding .1-

having a plurality of interconnected parallel sections arranged on said core and each divided into two portions adapted to alternately transmit unidirectional current impulses, each said primary winding section being arranged adjacent both portions of at least one of said secondary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said primary and secondary winding sections and having a relatively high leakage reactance with respect thereto, said primary and secondary winding sections being interleaved, and means for bracing said winding sections against each other and against said core for preventing displacement of said winding sections by electromagnetic action thereon of the magnetic leakage fields thereof during flow of backfire current through the transformer.

6. A transformer for an electric valve converting system comprising a primary winding having a phase portion thereof divided into a plLu'ality of sections arranged in series parallel connection and arranged on a common core, and a secondary winding having a plurality of interconnected parallel sections arranged on said core and each divided into two portions adapted to alternately transmit unidirectional current impulses, each said primary winding section being arranged adjacent both portions of one of said secondary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said pri mary and secondary winding sections and having a relatively high leakage reactance with respect thereto.

'7. A transformer for an electric valve converting system comprising a primary winding having a plurality of serially connected sections arranged on a common core, a secondary winding having a plurality of interconnected parallel sections arranged on said core and each divided into two portions adapted to alternately transmit unidirectional current impulses, each said primary winding section being arranged adjacent both portions of at least one of said secondary winding sections and having a relatively low leakage reactance with respect thereto and being arranged remote from the remainder of said primary and secondary winding sections and having a relatively high leakage reactance with respect thereto, and means for adjusting the voltages of said secondary winding comprising means for varying the connections between said primary winding sections.

8. In an electric current converting system, the combination with a source of electric current, and electric valve means comprising a plurality of anodes, of means for causing the flow of currents supplied from said source of predetermined relative magnitudes simultaneously through said anodes and for opposing changes in magnitude of the flow of current through any one of said anodes relative to the magnitude of the flow of current through another thereof comprising a transformer winding phase portion divided into a plurality of interconnected parallel sections severally connected with said anodes, and another transformer winding portion connected with said source of current and divided into a plurality of serially connected sections severally closely inductively coupled with at least one of said interconnected parallel sections and relatively loosely coupled with the remainder of the said winding sections.

9. In an electric current converting system,

the combination with a source of electric current, and electric valve means comprising a plurality of anodes, of means for causing the flow of currents supplied from said source of predetermined relative magnitudes simultaneously through said anodes and for opposing changes in the magnitude of the flow of current through any one of said anodes relative to the magnitude of the flow of current through another of said anodes comprising a transformer winding phase portion divided into a plurality of interconnected parallel sections severally connected with said anodes and positioned on a common core, and another transformer Winding phase portion connected with said source of current and divided into a plurality of serially connected sections severally positioned on said core adjacent at least one of said interconnected parallel sections and having relatively low leakage reactance with respect thereto and being positioned remote from the remainder of the first and second said winding sections and having a relative high leakage reactance with respect thereto.

10. In an electric current converting system, the combination with a source of electric current, and electric valve means comprising a plurality of anodes, of means for causing the flow of currents supplied from said source of predetermined relative magnitudes simultaneously through said anodes and for opposing changes in magnitude of the flow of current through any one of said anodes relative to the magnitude of the flow of current through another thereof comprising a transformer winding phase portion divided into a plurality of interconnected parallel sections severally connected with said anodes, and another transformer winding connected with said source of current having a phase portion thereof divided into a plurality of sections,

in series parallel connection severally relatively closely inductively coupled with at least one of said interconnected parallel sections and relatively loosely coupled with the remainder of said winding sections.

LOUIS C. NICHOLS.

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