US2918616A - Current rectification utilizing interphase transformers - Google Patents

Description

Dec. 22, 1959 E. J. Dil-:BOLD 2,918,616

' CURRENT RECTIFICATION UTILIZING INTERPHASE TRANSFORMERS Filed Sept. 26, 1957 4 Sheets-Sheet 1 F/aZ.

HH #f3-4 3f INVENTOR. [a una J. D/Ea 0L D Arron/ver Dec. 22, 1959 E. J. DIEBQLD 2,918,616

CURRENT RECTIFICATION UTILIZING INTERPHASE TRNSFORMERS Filed Sept. 26, 1957 4 Sheets-Sheet 2 7i F V/ F/Ka4.

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ArroR/ve-Y Dec. 22, 1959 E. J. DIEBOLD 2,918,616

CURRENT RECTIFICATION UTILIZING INTERPHASE TRANsFoRMERs Filed Sept. 26, 1957 4 Sheets-Sheet 3 8 8 l mVENToR.

[c] G. 7 Epu/,4R0 J- 0/E80D 14T roR/vEY Dec. 22, 1959 E. J. D|EB0LD 2,918,616

CURRENT REcTIFIcA'rIoN UTILIZING INTERPHASE TRANsFoRMERs Filed sept.. ze, 1957 4 Sheets-Sheet 4 INVENTOR. EDWARD J /Eao/.o

@6W/4wd A Trop/vs Y vUnited States Patent Office 2,918,616 Patented Dec. 22, 1959 CURRENT RECTIFICATION UTILIZING INTERPHASE TRANSFORMERS Edward J. Diebold, Palos Verdes Estates, Calif., assignor to International Rectifier Corporation, El Segundo, Calif., a corporation of California Application September 26, 1957, Serial No. 686,362

Claims. (Cl. 321-11) 'I'his invention relates to the rectification of electric current, and more particularly to rectifying systems for providing relatively high current output.

An object of the invention is to provide such a rectifying system which is efficient at very high current outputs even though the output voltage be low.

Systems for rectifying alternating current to direct current commonly produces a D.C. output having superimposedy thereon an A.C. ripple voltage, the ripple being due to the form of the A.C. voltage wave impressed on the rectifier. The ripple voltage is ordinarily undesirable, and the effect of the ripple voltage may be minimized by the use of a multi-phase A.C. input instead of a single phase input. A difficulty commonly experienced in the use of A.C. inputs for such rectifying systems has been that when the rectified outputs from the several phases are combined to produce the final D.C. output, the A.C. ripple voltage which remains with the D.C. has produced undesirable short circuit currents, from the rectifierof one phase through the rectifier of the other phase; These short 4circuit currents can be very high and can'produce such severe power losses as to limit the amount of D.C. power output which can be obtained from the system.

The undesirable short circuit currents arise from the fact that the ripple voltages from the several phases are out of phase with each other, so that at an instant when the ripple: voltage at one phase is relatively high, the ripple voltage at another phase may be relatively low; and these differences in instantaneous values of ripple voltages produce the short circuit currents.

"In accordance with the present invention, the effect of these ripple voltages is compensated by use of interphase transformers. These interphase transformers each comprise a pair of conductors orwindings through which the rectified output from two of the phases are carried respectively; so that these two windings or conductors are in the samev magnetic field of the respective transformer. yThus, each of the interphase transformers carries the current from two of the phases. By this arrangement, when the instantaneous value of the ripple voltage from one of the phases builds up to a high value in one winding of an interphase transformer, while the instantaneous value of the ripple voltage of the other phase which isv being passed through the other winding ofthe same transformer is of a lower value, the phase having the higher instantaneous voltage causes a counterelectromotive force to be built up across the conductor or winding of the other phase of that transformer. The value'of'the counter electromotive force is substantially equal to the difference of instantaneous voltages of the two phases at the transformer. Thus, the counter electromotive force is of the proper value to prevent the iiow of short circuit current from any of the phases to another. By this arrangement, a large number of phases can be utilized, thereby reducing undesirable ripple effects; because thel short circuit currents which previously 2 made the use of many phases practically impossible, have been eliminated.

The foregoing and other features of the invention will be fully understood from the following detailed description and the accompanying drawings, of which:

Fig. l is a circuit drawing illustrating a 3 phase rectifier system incorporating this invention;

Fig. 2 is an end View, partly in cross-section, showing an interphase transformer from Fig. l;

Fig. 3 is a circuit drawing illustrating a 6 phase rectifier system incorporating this invention;

Fig. 4 is a circuit drawing showing a portion of Fig. 3 in greater detail;

Fig. 5 is a circuit drawing showing a portion of Fig. 4 in greater detail;

Fig. 6 is an elevation showing the physical construetion of a portion of the circuit of Fig. 5;

Fig. 7 is another elevation of the same portion of the circuit of Fig. 5 as shown in Fig. 6, this view being taken at line 7-7 of Fig. 6;

Fig. 8 is a detailed cross-section of one of the current transformers from Fig. 7;

Fig. 9 is a cross-section taken at line 9 9 of Fig. 8;

Fig. 10 is a cross-section taken at line 10-10 of Fig. 9;

Fig. 11 is a graph showing instantaneous voltages of phases in t'he circuit of Fig. 1;

Fig. 12 is a graph showing the resultant voltages obtained by algebraic addition of the phases in Fig. l1;

Fig. 13 is a bottom view taken at line 13-13 of Fig. 2;

Fig. 14 is a cross-section of a current transformer suitable for use in the circuit shown in Fig. 7;

Fig. 15 is a cross-section taken at line 15-15 of Fig. 14; and

Fig. 16 is a top view taken at line 16-16 of Fig. 14.

Referring to Fig. 1, there is shown a 3-phase system in accordance with the present invention. The power input to the system is supplied by three primary transforming windings 10, 11 and 12, connected in the delta arrangement; although it will be understood that some other connection arrangement could be used, such as a star arrangement. Secondary windings 10s, 11s, and 12s are magnetically related to the respective primary windings 10, 11 and l2.

The midpoints of the three secondary windings are tapped at 13, 14, 15, respectively and the leads 16, 17 and 18 are brought to a negative bus Lar 19.

Each secondary winding is arranged for full-wave rectification by provision of rectifier units which are connected at the respective ends of each secondary winding. Secondary winding 10s has rectifiers 20 and 21; secondary winding 11s has rectifiers 22 and 23; and secondary winding 12s has rectifiers 24 and 25 connected in this manner. The terminals of rectifiers 20 and 21 which are remote from the winding 10s are tied together and to lead 26. Similarly the sides of rectifiers 22 and 23 remote from winding 11s are tied together and to lead 27 and the sides of rectifiers 24 and 25 remote from winding 12s are tied together and to lead 28. Leads 26, 27 and 28 are carried to a positive bus bar 29.

lf the rectifying system were to consist only of the elements thus described and without additional compensating elements, short circuit currents would circulate from one phase to the other. ,This would result from the fact that wave forms of any are displaced by Fig. 11 shows the form of the rectified voltages which are developed in each of the phases. The wave form E1 shown in full line represents the rectified voltage from the secondary winding 10s. The wave form E2 shown in dotted lines, shows the form of the rectifier voltage from secondary winding 11s. The wave form E3 shown in y i two of they phases broken lines repre-` sents the form of the rectified voltage from secondary winding 12s.

It is immediately apparent that the instantaneous values of the rectified voltages E1, E2 and E3 are not the same; hence, because all of the phases are interconnected at positive bus bar 29, rectied voltage developed in one phase will force current to flow through the elements of the other phases. Since the impedances are low in the forward direction through the rectifier elements 20-25, very large short circuit currents can result.

The undesirable effects caused by these instantaneous voltage differences between the several phases is avoided in accordance with the present invention by the provision of the interphase transformers 3i), 31 and 32. These inter-phase transformers are constructed as voltage transformers; that is, so that the product of the secondary turns and the primary voltage are substantially equal to the product of the primary turns and the secondary voltage. Leads 26 and 17 are respectively connected to the primary and the secondary windings of transformer 30. Inter-phase transformers 31 and 32 are similarly connected to leads 27 and 18, and leads 28 and 16, respectively.

Each inter-phase transformer may be constructed as shown in Figs. 2 and 13. Fig. 2 shows inter-phase transformer 30 in detail and it will be understood that this transformer, and its connections are representative of inter-phase transformers 31 and 32 and their connection. This transformer has but a single turn `or loop to con stitute each winding and the loops essentially consist of leads of opposite polarity from the secondaries of two different phases. Thus the leads 26 and 17, of opposite polarity with respect to their separate rectifier systems, are caused to pass straight through a transformer core. For this purpose the leads 26 and 17 merge into flat plates or strips 26' and 17 respectively, held parallel to each other in a central position within a core of magnetic material by spacers 33 and 34. These spacers are embraced by a pair of U-shaped magnetizable core pieces 35, 36 which are placed in opposition to each other, with a pair of air gaps 37 between their ends to form the core. A :layer of insulation 38 is wrapped around the core pieces and then a strap 39 with a buckle 40 is tightened down to hold the transformer together. The two windings, that is, the flat plates 26' and 27 of the inter-phase transformer are incorporated directly into the leads 26 and 17 respectively and in fact a single strip can constitute both the lead and its transformer loop.

Since there is only a single lead or loop constituting each of the two windings passing through the magnetic field of the interphase transformer, the turns ratio is 1:1. In consequence the voltage across each transformer loop is equal to the voltage across the other transformer loop. Thus, when, as a single example, the strip 26 passing through the transformer has a significant instantaneous voltage across it at the same time that the strip 17 has a zero voltage across it, then the current in the rst mentioned strip 26 will induce across the second mentioned strip 17 an instantaneous counter electromotive force, or voltage, equal to that across the strip 26. Such a counter voltage is induced wherever the instantaneous voltage across one transformer loop differs from that across the other; and its effect is to equalize the voltages across the respective loops of the transformer.

Each of the other interphase transformers is arranged in a similar manner, that is, its leads or loops are taken from the secondaries of two different phases and from terminals of opposite polarity relative to the rectiers.

The voltage relationship in the circuit of Fig. 1 on account o-f these transformers is shown by the equations given below. In these equations, the terms El, E2, and E3 are the voltages between leads 16 and 26, 17 and 27, and 18 and 28, respectively, and represent the voltage from the taps 13, 14 and 15, respectively, across the rectifiers. The terms EA, EB and Ec, represent the voltages across the windings or loops of interphase transformers 30, 31 and 32, respectively; it being understood that there are equal but opposite voltages across the Equations 1, 2, and 3 are true under any circumstances. Equations 4 and 5, which are obtained by adding together Equations 1, 2 and 3, are true for instantaneous values in the system and also are true for the average value. While the voltage ED may be low, the current `which may be drawn from the system may be considerable. This follows from the fact that circulating currents between phases, which if present cause loss of power, are substantially eliminated, by action of the interphase transformers.

Fig. 3 shows another system according to the invention which permits the development of extremely high D.C. currents. This system shows a four-wire, threephase system in which the conductors or wires 50, 51 and 52 are for the respective three phases, and the conductor or wire 53 is neutral. This arrangement is equivalent to a six phase system, for voltage may be taken not only between each pair of phase conductors, but also from the neutral to each phase conductor.

In the arrangement of Fig. 3, there are shown six rectifying units 54, 5S, 56, 57, 58 and S9, one unit for each phase of the six phase rectifying system. Each of these rectifying units may be constructed as shown in Fig. 4. Fig. 4 shows rectifying unit 54 as exemplary of each of the other rectifying units, these units being shown in Fig. 3 in block form for clarity in disclosure. As can be seen from Fig. 3, leads 60 and 61 connect with rectifying unit 54 from wires 50 and 51. Leads 62 and 63 connect to rectifying unit 55 from wires 51 and 52, respectively. Leads 64 and 65 connect to rectifying unit 56 from wires 50 and 52, respectively. Leads 66 and 67 connect to rectifying unit S7 from wires 50 and 53, respectively. Leads 68 and 69 connect to rectifying unit 58 from wires 51 and 53, respectively. Leads 70 and 71 connect to rectifying unit 59 from wires 52 and 53, respectively.

In Fig. 4, leads 60 and 61 areshown connected to opposite ends of the primary winding 72 of a transformer 73 which has a core 74. It will be understood that the unit 54 shown in Fig. 4 is used in tive other places in the system of Fig. 3. vIn those places, leads 62 and 63, 64 and 65, 66 and 67, 68 and 69 and 70 and 71, respectively, will be connected to the ends of a primary winding 72 in the rectifying unit provided for that pair of lead-s.

Each such rectifying unit is provided with a pair of secondary windings 75 and 76, which have center taps 77 and 78, respectively. The center taps are both joined to a lead 79. Leads 80 and 81 are connected to opposite ends of secondary winding 75 with the center tap 77 connected to the secondary winding between them. Leads 82, 83 are connected to the ends of secondary winding 76, and the center tap 78 is connected to the secondary winding between them.

Leads 80, 81, 82 and 83 connect respectively with current equalizing units 84, 85, 86 and 87, respectively. One of these current equalizing units is shown partly in schematical notation in Fig. 5. Current equalizing unit 84 has been selected as an example, it being understood that each of the current equalizing units is contransformer 104. Lead` structed thesanie as the others.` In Fig. 5, lead 80 is shown connected to unit 84and it will be understood that leads 81, 82 and'83 will each be connected to other similar units 85, 86 and 87, respectively. Leads 88, 89, 90 and 90a are connected respectively to current equalizing units 84, 85, 86 and 87 and all of these are joined to another lead 91. In Fig. 5, lead 88 is shown departing from the current equalizing unit 84.

With reference to Fig. 3, leads 79 and 91 will be seen leaving the rectifying unit 54, and similar pairs of leads are shown connected to the others of the rectifying units. Leads 92 and 93 are connected to rectifying unit 55. Leads 94 and 95 are connected to rectifying unit 56. Leads 96 and 97 are connected to rectifying units 57. Leads 98 and 99 are connected to rectifying unit 58. Leads 100 and 101 are connected to rectifying unit 59. A

Current equalizing unit 84 will now be described in fuller detail with particular reference to Fig. 5. Lead 80 branches so as to be connected to a pair of current transformers 102, 103. A current transformer comprises a pair of windings arranged in a magnetic circuit in such a manner that the products of the current and the number of turns in one winding is maintained substantially equal to the product of the current and the number of turns of the other winding. Each of the current transformers 102 and 103 has two of its terminals connected to the said lead 80. A suitable coustruction for these current transformers will be described with reference to Fig. 8, but for the present it is sufficient to note that with respect to current transformer 102, for example, the rectangular line represents a current transformer core, while the lines passing therethrough represent straight-through conduits which serve as the windings of a 1:1 ratio transformer. Current transformers 104 and 105 are connected in the system as follows: A lead 106 interconnects one of the windings of current transformer 102 to one winding of current transformer 105. Lead 107 connects the other winding of the current transformer 102 with one winding of current 108 connects one winding of current transformer 103 to the other winding of current transformer 104, and lead 109 connects the remaining winding of current transformer 103 to the remaining winding of y current transformer 105. It will be observeclthat4 the windings in these current transformers are so'` connected that current travelling through one of -the windings is opposite in direction to current travelling through the other of said windings. v

:Four additional current transformers 110, 111, 112

and 113 are also provided. Transformers 110 and 111 receive current from current transformer 104, while transformers 112 and 113 receive current from current transformer 105.

'A lead 114 from one winding of current transformer 104, divides so as to connect to both windings of current transformer 110. Currenttherefore passes through thetwo windings of thiscurrent transformer in opposite directions. Lead 115 receives current from the other winding of current-transformer 104, and connects to both windings -of current transformer 111. One of the windings of current transformer 105 is connected by lead 116 'to both windings of current transformer 112, while'a similar arrangement is created by connecting lead 117 from `the other windingy of current transformer 105 to the two windings of current transformer 113. In all of the said current transformers, the current direction through each winding is opposite to current direction in the other winding, `so that the magnetic luxes due to current in each winding are opposed.

Leads 118 and 119 are connected, one to each of the two windings Aof current transformer 110. Similarly, leads 120, 121; 122, 123; and 124, 125, are connected, one to each of the two windings of current transformers '111,112 and 113, respectively. As will be further described below, these current transformers provide a means for equalizing the current which flows through leads 118-125. Each of these leads incorporates a rectifier which may be of any desired type such as germanium, silicon, or the like. Rectiers 126-133, inclusive, are incorporated in leads 118-125, respectively, for the purpose of rectifying current provided to these leads. These leads all connect to lead 88 on the other side of the rectifier, and lead 88 therefore receives rectilied current which is in turn provided to lead 91 as can be seen in Figs. 3 and 4.

Since each of the rectifying units 54 through `59 is constructed as shown in Figs. 3, 4, and 5, it will be appreciated that the system of Fig. 3 includes 192 rectifiers and 192 current transformers.

Broken line 135 in Fig. 5 outlines a portion of the system whose physical construction is shown in detail in Figs. 6 and 7. It will be understood that this desirable physical construction is a module which will be repeated as many times as necessary in order to make up the desired system. Fig. 7 shows the connection between leads and 88 for the rectifers 126-129, inclusive. As can be seen from Fig. 6, leads 80 and 81 are preferably provided in the form of flat plates to which two terminals of the current transformers'102 and 103 can be bolted. Center tap 77 is also a flat plate and these plates 77, 80 and 81 are preferably placed parallel to each other so that there is a minimum inductive voltage drop in the system.

As shown in Fig. 6, the units may be arranged backto-back. For example, at the right hand side of Fig. 6, the devices connected to lead 80 are shown while at the left side the devices connected to lead 81 are shown. Rectifier elements 126-133, which have fins for cooling, are mounted directly to lead 88, which is a large, at plate.

The physical construction of the device shown schematically in Fig. 5 will be better understood from Fig. 7, wherein the leads 106, 107, 108, 114 and 115 are shown as at'plates, which are bolted to the terminals of the appropriate current transformers. This construction itself offers sufficient rigidity that no additional supporting structure is necessary. The leads 118, 119 and 121 may conveniently be of flexible copper cable connected to the rectifers in a known manner, and attached at their free end to the appropriate terminals of the current transformers.

A current transformer suitable for use in the system of Figs. 3-7 is shown in fuller detail in Figs. 8-10. In Figs. 6 and 7 these current transformers are shown only in outline. In Fig. 8, current transformer 102 is shown in detail, it being understood that the other current transformers may be, and preferably are, identical to it. A primary winding and a secondary winding 141 comprise two parallel bars, each of which has an ear at each end for a terminal. For examp'e, primary winding 140 has terminals 142 and 143, and secondary winding 141 has terminals 144 and 145. Fins 146 are provided for cooling the device. A spacer 147, which may be of glass or other insulating material, is provided for holding the two windings out of contact with each other.

A wrapping 148 of insulating material surrounds the windings and spaces therefrom a pair of U-shaped core members 149, 150. These core members are held apart from each other to leave an air gap 151 therebetween to reduce the possibility of core saturation during the operation of this device. A strap 152 is wrapped around the core pieces, and a buckle 153 fastens the strap to hold the device together. A pair of insulating washers 154, 155, space the sides of the core members from the winding terminals. Current flowing from terminal 142 to terminal 143 flows in the opposite direction through the core from current travelling from terminal 144 and terminal 145. This is the feature which is shown schematically in Figs. 2-7.

With further reference to Fig. 7, terminals 142, 143, 144 and 145 are marked so as to show the sense of current flow. These same terminals are marked on Fig. 5. This current transformer is simple in construction and inexpensive of manufacture and has suicient capacity to handle the requirements of this system. The windings and the core pieces may conveniently be cast and the rest of the construction is simple. Since each winding of the current transformer comprises a straight bar, each Winding is composed of a single turn or loop, creating a 1:1 ratio. This will cause the current in both bars or loops to be maintained at substantially the same value.

It will be recognized that the terms primary and secondary as applied to these transformer windings are used for convenience although both windings are the same.

A suitable type of voltage transformer for use in rectifying unit 54 is shown in detail in Figs. 14-16. In this transformer, a primary winding 160 makes one loop through the eyes of a pair of cores. The cores each comprise a pair of opposed U-shaped iron members 161 and 162; and 163 and 164. Each loop includes an air gap. A secondary winding 165 is suitably spaced and insulated from both the cores and from the primary winding by insulating means which are not shown. The secondary winding Wrapped over the primary winding and passed through the eyes of both of the cores. The secondary winding is provided with two turns, there being a `center tap terminal 165a at the mid point of the winding So that there is a single turn on each side of the center tap terminal.

With respect to the installation of this transformer in the system as illustrated in Fig. 4, the primary winding 160 corresponds to primary 72, while the secondary winding 75 corresponds to the secondary winding 165. Leads 60 and 61 are connected to primary winding 160. Leads 70 and 71 are connected to the ends of the secondary winding 165 and lead 77 is connected to the center tap terminal 165a. In actual construction, the second two turn secondary winding 76 shown in Fig. 4 may also be wrapped around the primary winding 160, if desired, so that the same cores could be utilized for both secondary windings. The additional secondary winding 76 is not illustrated in Figs. 14-16.

The above description sets forth the apparatus and circuit of Figs. 3-7 from wires 50-53 through current rectifying units 54-59 which units include both means for rectifying the current and also means for equalizing the current through the various rectiliers. There still remains to be described the apparatus which serves to reduce, and preferably to eliminate entirely, the interphase currents which tend to ow between the phases when their instantaneous voltages are unequal. For this purpose, the system of Figs. 3-7 utilizes the same type of interphase transformer as shown in Fig. 2 for use in the system of Fig. 1. Interphase transformers 169-174 of this type are connected into the system as shown in Fig. 3. The leads passing through each of these interphase `transformers are shown as dotted lines, inasmuch as the windings of these transformers are essentially straightthrough conductors passing through the core of the voltage transformers (see Fig. 2). Leads 79, 92, 94, 96, 9S and 100 are negative, and leads 91, 93, 95, 97, 99 and 101 are positive. These leads are connected in pairs, one positive and one negative, from the rectifying units, to and through the interphase transformers in the combinations shown. It should be particularly observed that the interphase transformers are connected so that the leads run the same direction. Because one lead is negative and the other is positive, the ux in the transformer coil generated by one lead is opposed to that generated by the other lead. In this manner, the interphase transformer runs at lesser liux densities than it would if both windings produced ux through the core in the same sense. The transformer is more effective at these lesser ux densities.

Interphase transformer 169 connects to leads 91 and 92; transformer 170 connects to leads 93 and 94; transformer 171 connects to leads 79 and 95; transformer 172 connects to leads 97 and 98; transformer 173 connects to leads 99 and 100; and transformer 174 connects to leads 96 and 101. These leads proceed from the interphase transformers to output buses 175 and 176, 175 being the negative bus and 176 being the positive bus, the potential across the buses being ED. The negative leads are connected to bus 175, and the positive leads are connected to bus 176.

The operation of the system of Fig. l may be deduced from the drawings, the three secondary windings having voltages induced therein by their corresponding primary windings. The output of the secondary winding is rectiiied by the rectiers therein and, in previously known systems would thereafter simply be Withdrawn at bus bars 19, 29 without benefit of any interphase transformers. Figs. l1 and l2 show the system voltages. For example, the lines El, E2 and E3 shown in Fig. ll are the rectified full wave outputs of the respective secondaries, there being a peak for each secondary every 180 inasmuch as this system is set up for full wave rectification. El is shown in full line, E2 in dotted line and E3 in broken line. The summation of these voltages is shown in Fig. 12 by line 200 which summation is a D.C. voltage with an A.C. ripple voltage superimposed thereon.

The interphase transformers in the system of Figs. 37 provide the same advantages, that is, the substantial elimination of circulating currents between phases, in the same way as those transformers which are used in Fig. l. In the system of Figs. 3-7, however, means are provided not only for rectifying the current, but in view of the high currents which are intended to be carried, means are also provided for equalizing the currents through the various rectiers so as to minimize rectifier failures which could be caused by overloading any of them.

The particular system shown in Fig. 3 is designed to deliver 135,000 amperes at 3.2 volts. Each of the rectifying units 54-59 is designed to carry 22,500 amperes. With such large currents, there is a significant chance, in the event of uneven cooling, of over-heating, or of breakdown of some of these rectiiiers, that one of more of the rectiers might ultimately carry more than it should of the load with a consequent burn-out. For this reason, the current equalizing units 84-87 are provided. With respect to unit 84 shown in Fig. 5, it will be seen that current from lead 80 would, if the inductive and resistive effects of all the rectifiers were equal, simply tend to divide equally between said rectiiiers even if the current transformers were not present. However, because of the extremely low forward resistance drop of the rectiers, particularly of germanium and silicon types, which are the most suitable for high current work, it has been found that matching of rectifers to secure equality of these properties is not a solution for dividing the current equally. This is because only a very tiny absolute difference in resistance drop may amount to a very large percentage difference in conductivity, and also because small temperature differences greatly alter the rectifying properties of cells and the same change in temperature does not give the same change in all cells. Also, localized temperature differences between rectiers cause a further tendency to imbalance. These effects are so cumulative and progressive that, even when using only a few carefully matched rectifiers of the metallic type, it has often been found that there is frequently an early breakdown due to overloading of one of the rectifiers. This is the reason that the current transformers are provided.

Current owing through leads 106 and 107 of current transformer 102 tends to be maintained equal, because if either of these leads tends to carry more than the other, a

19j counter 'electromotiv-force; will .be inducedv in-the other lead by l'flux induced in-jthe corevonaccountof, the current imbalance.y This willtend v,to maintain the 'equality of currentfinthe two windings, sincegthe4 Atransformer ratio is 1 :l. 1Thisfisza'consequenceofrtheflow ofqcurrent through these two windings in opposite directions. The same situation holds true' in the rest .of .the current -`transformers suchas. current transformer 103sothatthe currents lfrom betweenlea'ds 1.08 and.109 tend yto be maintained equal. Although the currentsinjleads, 1 '06va'nd 1,07 are equal, andthefcurrentstin leads 108 and f109are equal, therev is no guarantee that thecurrents in rallfour leadsare equal. Fotfthis, reason, lead 1.08 from the nrightvhand side enters current transformer 104 while lead 1 06 from the left hand sidei enters :current transformerr105. ,Leads 107 'and 109 are. 'Connectedas shownto.,transformersj104 and 105,

respectively; and this tends to make the current in leads 114,115, 116and 117 equal. Equal division of current into leads'v 11S-125 is causedby similar means in current transformers 110+113.

n It-will therefore-beseen that-means yar-e provided for creating an-equal division of'currentl to each of the individualrectifierswhich is self-correctingin the event that `one-fr'ect-ifieritendsto drawfmore or less currentv than the others. After this divisionfand;rectification, therectified current ,yissuppliedl to the positivefandnegative leads'as shown,;is3pass ed thrjough the interphase transformers with fthesamefaction'asrdescribed. in'connection with the system of Fig. l, and then the output can be withdrawn from buses175 and 176.

The special-physical constructions of lboth the system l, and the varioustransformers provide inexpensive and convenient expedients forthev purpose specified.

L'lhisinvention provides .means ,for securing high .power ."DtC otpil't'lfrom multi-phase rectifiersy'stems'by Vvirtue offtheelimi'nationof .interphase currents. Ithasbeen e found that very high efficiency and output can be obtained by this means, eveiatlf-vrylow voltages. gibier-.invention ignoti@ belimitsd. bythe embodiments shown in the drawing and described in the description, which are given-by .way of illustration and not of limitation, but only in accordance with the scope of the appended claims.

I claim: 1.y In amulti-phase rectifierl system wherein alternating current from a plurality of phases is rectified, and wherein a D.C. fv lead of a first polarity and a D.C.lead of a second polarity are provided for each phase to conduct therectified current therefrom, and in which the said leadsof first polarity are all connected to a first bus, and the said leads of second polarity are all connected to a second bus, means for reducing circulating currents between the phases, comprising: as many voltage transformers as there are phases, each of said voltage transformers having a primary winding and a secondary winding, the windings ratio of the voltage transformers being unity, one of said windings being connected in a lead of first polarity from one phase, and the other of said windings being connected in a lead of second polarity from another phase, whereby each voltage transformer is connected to a lead from each of two different phases, and each of the leads of each phase is connected to a different one of said voltage transformers, whereby inequality of current ow between the two windings of a given interphase transformer generates a tiux in said transformer which creates a counter electromotive force tending to equalize the currents liowing in said two windings.

'2. Apparatus according to claim l in which the windings are incorporated in said leads such that current ow Ain said windings is opposite in direction.

3. A multi-phase rectifier system comprising: a primary anda secondary coil in inductive relationship for each of a plurality of phases, the primary coil providing energy to the secondary coil, each said secondary coil hav- 10 ing a pair of ends; a rectifier connected to each end of each of said secondary coils; a D.C. lead of a first polarity connected to a mid-point of each of said secondary coils; a D.C. lead of a second polarity connected to both rectifiers connected to a single secondary coil; whereby a lead of first polarity and lead of second polarity are provided for each phase to conduct full-wave rectified current therefrom; and a plurality of voltage transformers equal in number to the number of phases, each of said voltage transformers comprising a primary winding and a secondary winding, and having a turns ratio equal to unity, a lead of tirstpolarity from one phase and a lead of second polarity from another phase being connected individually to the respective windings of each transformer, whereby ytheleads of different polarity of eachl phase each passes through a different one of the voltage transformers, and each voltage transformer is connected to a lead from each of two different phases, the leads being connected 'to the windings so that current ow is opposite in direction in the windings of each voltage transformer, said windings constituting continuations of the leads connected thereto; and a bus of a first polarity and a bus of a second polarity, all leads of. first polarity, and all leads of second polarity being connected to the bus of second polarity.

4. In a multi-phase rectifier system wherein alternating current from a plurality of phases is to be rectified and wherein current in each phase to be rectified is supplied tol a winding having a center tap and two ends, means for rectifying current in one of said phases comprising: alead of first polarity attached to said center tap; a lead of second polarity attached to each of said ends, 4and rectifier means incorporated in each of said leads of second polarity comprising eight rectifier cells electrically connected to said lead of second polarity, four first current transformers, each of said first current transformers having a primary and a secondary winding with aturns ratio equal to unity, each rectifier cell being connected to one and only one of said windings, each of said first current transformers being connected to a different pair of cells, one cell of each pair being connected to the primary winding and the other cell of each pair being connected to the secondary winding of the same first current transformer, two second current transformers each having a primary and a secondary winding and a turns ratio equal to unity, each of the windings of said second current transformer being attached to both windings of a different one of said first current transformers, and two third current transformers, each having a primary winding and a secondary winding with a turns ratio equal to unity, each of the windings of each of said third current transformers being connected to a winding of a different one of the second transformers, and all windings of said third current transformers being connected to said lead of second polarity, whereby a current through the lead of second polarity flows successively through the third, second and first current transformers so as to be equally divided into the rectifiers, and through the recti` fier cells.

5. Apparatus according to claim 4 in which each phase is provided with a primary coil and a pair of secondary coils, each secondary coil having the aforesaid center tap and pair of ends, there being provided four means for rectifying current as defined in claim 4, one for each of said positive leads.

6. A multi-phase rectifier system comprising: a primary and a secondary coil for each of a plurality of phases, the primary coil providing energy to the secondary coil, each secondary coil having a pair of ends; a lead of a first polarity connected to a midpoint of each of said secondary coils; a lead of a second polarity connected to each end of each of said secondary coils; and means for rectifying current for each of said phases owing through said leads of the second polarity, each of said means comprising: eight rectifier cells connected to said lead of second polarity, four first current transformers, each of said first current transformers having a primary and a secondary winding with a turns ratio equal to unity, each rectifier cell being connected to one and only one of said windings, each of said first current transformers being connected to a different one of the cells, one cell of each pair being connected to the primary winding and the other cell of said pair being connected to the secondary winding of the same first current transformer, two second current transformers each having a primary and a second-ary winding and a turns ratio equal to unity, each of the windings of said secondary current transformers being attached to both windings of a different one of said first transformers, and two third current transformers, each having a primary winding and a secondary winding with a turns ratio equal to unity, each of the windings of each of said third current transformers being connected to a winding of a different one of the second transformers, all windings of said third current transformers being connected to said lead of second polarity, whereby a current through the positive lead flows successively through the third, second and first current transformers so as to be equally divided into the rectifier cells, and thence through the rectifier cells to resume iiow through the lead of second polarity; a bus of second polarity to which leads of second polarity are connected; -a bus of first polarity to which leads of first polarity are connected; and means for reducing ripple in the output between the said buses comprising: as many voltage transformers as there are phases, each of said voltage transformers having a primary winding and a secondary winding and a turns ratio equal to unity, one of said windings being connected in a lead of second polarity from one phase and the other of said windings being connected in a lead of first polarity from `another phase, whereby each voltage transformer is connected to a lead from each of two different phases, and each phase has each of its leads connected to a different one of said voltage transformers, whereby inequality of instantaneous voltage between the two windings of a given voltage transformer generates a fiux in said transformer which creates a counterelectromotive force tending to equalize the currents owing in said two windings in the voltage transformer and thereby reduce interphase currents.

7. Apparatus according to claim 6 in which the windings of the interphase transformers are interconnected in said leads in such a manner that current flow in said windings is opposite in direction.

8. Apparatus according to claim 7 in which the lead of second polarity between the said coil and the third current transformers comprises a flat plate, and in which the lead of first polarity is a fiat plate parallel to the aforesaid lead of second polarity, and in which the lead of second polarity attached to the rectifier cells is a fiat plate.

9. Apparatus according to claim 7 in which the primary and secondary coils are included in transformers, each transformer comprising a core comprising a pair of magnetizable rings a single turn passing through the rings for a primary coil, and a double turn passing through said rings for a secondary coil, said double turn being center tapped by the lead of first polarity, its ends being attached to the said leads of second polarity.

l0. Apparatus according to claim 7 in which six phases are provided from four lines, there being six of the aforesaid means for rectifying current, and six voltage transformer's.

References Cited in the file of this patent UNITED STATES PATENTS 1,086,300 Kraus Feb. 3, 1914 2,289,090 Bedford July 7, 1942 2,856,577 Schmidt Oct. 14, 1958 FOREIGN PATENTS 612,131 France Oct. 18, 1926

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