US2169031A

US2169031A - Electric discharge apparatus

Info

Publication number: US2169031A
Application number: US161069A
Authority: US
Inventors: Slepian Joseph
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1937-08-26
Filing date: 1937-08-26
Publication date: 1939-08-08
Anticipated expiration: 1956-08-08

Description

- Aug. 8, 1939. J. SLEPIAN ELECTRIC DISCHARGE APPARATUS Filed Aug. 26, 1937 3 Sheets-Sheet l WITN sszs: My

INVENTOR Jasep/z j/ep/b/r.

k2, ATTORNE 3 Sheets-Sheet 5 .Filed Aug. 26, 1937 Brush Brash CdnTacT/hg Brush Confadlhg Con'fa :fm

Brush Cbnfacf/ny Patented Aug. 8, 1939 UNITED STATES PATENT OFFICE Joseph Slepian, Pittsburgh, Pa., assignor to Westinghouse Electric & Manufacturing Company,

East Pittsburgh, Pa.,

sylvania Application August 26,

21 Claims.

My invention relates to electric discharge apparatus and it has particular relation to converting apparatus incorporating discharge devices.

Primarily my invention relates to apparatus for 5 supplying power to a load requiring 50 amperes or more and operating at potentials of from 50 to 3000 volts.

In accordance with the teachings of the prior art, alternating current of arc frequency is converted into alternating current of another frequency or direct current, or direct current is converted into alternating current by the operation of mechanical commutating equipment or by the operation of discharge devices such as rectifiers of the mercury-pool or hot-cathode type. In the mechanical commutating apparatus of the prior art, there is deleterious arcing at the contacts. When the brush of a commutator disengages a conducting segment it interrupts current flow and an arc is produced between the brush and the segment at the point at which the disengagement takes place. The arcing may be of such intensity as to burn away the contacts in a comparatively short time. The problem of arcing at the contacts is not involved in converting systems in which electric discharge apparatus is utilized. However, discharge devices have a limited resistance to high potential and back-fire discharges with their attendant complications arise when potentials of even moderate magnitude are involved.

It is accordingly an object of my invention to provide power converting apparatus incorporating mechanical commutating equipment wherein 5 arcing between the brushes and the conducting segments shall be suppressed.

Another object of my invention is to provide power converting apparatus incorporating both discharge devices and mechanical commutating equipment that shall be cooperatively associated in such manner as to eliminate both arcing at the contacts and backfire.

A more general object of my invention is to provide apparatus for opening and closing a circuit through which current of considerable magnitude is transmitted by the engagement and disengagement of mechanical contacts, wherein arcing in the region of disengagement of the contacts shall be suppressed.

More concisely stated, it is an object of my invention to provide apparatus of simple and relatively inexpensive structure for converting alterhating-current power into direct-current power or vice versa, or for changing the frequency or phase number of alternating current without ena corporation of Penn- 1937, Serial No. 161,069

countering arcing at the contacts or backfire in discharge apparatus.

According to my invention, I provide a system of discharge devices of suitable structure and current-carrying capacity which are connected in series with the elements of a mechanical commutating device. The connection is such that the discharge devices prevent arcing between the brushes and the segments and the commutating device functions to disconnect the discharge de vices from the power circuit during the times that they normally would be subjected to backfire potentials- In accordance with an embodiment of my invention, the power converted is supplied directly through the discharge devices and the commutator parts at all times. In accordance with a modification of myinvention, the discharge devices are only connected to the load during the transition interval when the brush is moving from one segment of the commutator to the other, and during this interval, they operate to prevent arcing between the brush and the segments. The discharge devices in the former case must be designed to carry the full load current; in the latter case comparatively small discharge devices may be utilized inasmuch as they carry current only during the short transition interval.

In the first-mentioned embodiment of my invention, the brush of the commutator, when it is leaving a segment, for a short interval of time simultaneously engages that segment and the next one. Current flow is during this interval initiated in the discharge device associated with the latter segment and a negative potential applied through the last ignited discharge device and the two segments to extinguish the discharge device that has been carrying current just before the corresponding segment is completely disengaged by the brush.

In the modification of my invention, the commutator is provided with main segments and auxiliary segments. Power is normally supplied to the load through the main segments directly. When the brush passes from one main segment to the successive one, it engages an intermediate auxiliary segment, first interconnecting the main segment with which it has been engaged with the auxiliary segment, and then interconnecting the auxiliary segment and. the main segment with which it is to be engaged. The auxiliary segment is connected to the source through a suitable discharge device which is normally non-conductive. A potential impulse is impressed in the circuit of the discharge device when the first main segment and the auxiliary segment are interconnected and the discharge device is energized. During the interval when the segments are interconnected the discharge device after being energized diverts the current from the circuit of the main segment to the circuit of the auxiliary segment. The brush is disconnected from the first main segment at a time that there is no current flow through the main segment. When the brush interconnects both the auxiliary segment and the succeeding main segment, the discharge device is extingui shed by the potential impressed through the last-mentioned main segment and, therefore, the brush is disconnected from the auxiliary segment without arcing.

The discharge devices that are preferably utilized in the practice of my invention are of the so-called Ignitron tube type. Such a discharge device comprises an anode consisting of a metal such as nickel or carbon, a cathode consisting of mercury and an ignition electrode consisting of a high resistance element such as silicon carbide or boron carbide, which dips into the mercury. The mercury may be replaced by another metal such as lead, bizmuth, zinc or tin for example. I shall hereinafter refer to a discharge device having a cathode of the mercury-pool type, meaning thereby a discharge device in which the cathode is either mercury or any other suitable metal, as mentioned above. Moreover, when I refer herein to an ignition electrode of the silicon carbon type,.I mean thereby an electrode composed of silicon carbide or any other suitable high-resistant material. For purposes of brevity, I shall also refer to discharge devices of the Ignitron tube type as discharge devices of the immersed-ignition-electro'de type.

It is to be noted that while the discharge device of the type mentioned hereinabove is greatly preferred in the practice of my invention, discharge devices of other types may be utilized. For example, in lieu of a high-resistance ignition electrode dipping into the mercury-pool, an ignition electrode spaced a short distance from the mercury pool or a metal collar mounted on the outside of the container in the region of the mercury. A discharge initiated between such an ignition electrode and the mercury pool, produces an are between the anode and the mercury. When I refer herein to an ignition electrode without mentioning that it is of the silicon carbide type, I mean thereby generically an ignition electrode of any general type. It may be a conductor separated from the mercury, a high-re- -l sistant element dipping into the mercury or even a ring mounted externally on the container in the region of the mercury.

Finally, it is to be noted that while discharge devices of the mercury-pool type are preferred in the practice of my invention, because they are capable of delivering currents of the magnitude in which I am primarily interested, discharge devices of the hot-cathode type may also be utilized, particularly Where the load requirements are more moderate. Such discharge devices are gen erally provided with control electrodes or control grids. Control grids as such may also be utilized with discharge devices of the mercury-pool type. In using the expression ignition electrode herein, I intend to include within the scope thereof what is generally understood by the expression control electrode or grid, whether such an element is incorporated in a discharge device of the mercury-pool type or in a discharge device of the hot-cathode type.

The novel features" that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Figure 1 is a diagrammatic view showing an embodiment of my invention.

Fig. 2 is a diagrammatic view showing a modification of my invention;

Fig. 3 is a graph illustrating the operation of the modification of my invention shown in Fig. 2; and,

Fig. 4 is a diagrammatic view showing the principal features of an element utilized in the practice of my invention in accordance With Fig. 2.

While my invention is applicable to conversion of power in general involving both rectification and inversion, it is illustrated herein as applied to rectifying systems. The apparatus shown in Fig. 1 comprises a polyphase transformer 5, the

primary 7 of which is connected in delta and supplied in the usual manner from a three phase network 9. The secondary ll of the transformer is connected in star and its windings |3, l5, l1, I9, 2| and 23 are so arranged that it supplies six-phase power. The neutral point 25 of the secondary H is connected to one direct-current load conductor 27 through a suitable smoothing reactor 29. The other load conductor 3| is connected to the brush 33 of a commutator which is driven from the three-phase line 9 through a synchronous motor 31 and successively engages a plurality of commutating segments 39, 4|, 43, 45, 4'l and 49. Each of the commutating segments 39, 4|, 43, 45, 41 and 49 is connected to the external terminal of a corresponding winding l3, l5, l1, l9, 2| and 23 of the secondary through a suitable discharge device 5|, 53, 55, 51, 59 and 6|, respectively, of the mercury-pool type. Each discharge device has an anode 63, a cathode 65 of the mercury pool type, and an ignitron electrode 61 of silicon carbide type. The segments 39 to 49 are each connected to the cathode 65 of the corresponding discharge devices 5| to 6| and the anode 63 of the discharge devices are connected to the external terminals of the respective secondary windings l3 to 23. The ignition electrodes 61' of the discharge devices 5| to 6| are connected to the anode 63 of the discharge devices through hot-cathode rectifiers 69, 7|, i3, 75, 11 and 19. The cathode 8| of each rectifier 69 to 19 is connected to the corresponding ignition electrode 6! and its anode 83 to the anode of the corresponding discharge device 5| to 6|.

When the brush 33 of the commutator 35 is in engagement with any particular segment 39 to 49, say 41, current is transmitted to the load through the corresponding discharge device 59, the segment 41 and the brush. The movement of the brush 33 relative to the segments 39 to 49 should be so timed that at the instant that the potential supplied by the phase winding (2| in this case) whichis connected to the load is becoming equal to and smaller than the potential supplied by the succeeding phase winding (23 in this case) the brush 33 should engage the succeeding segment (49 in this case) and disengage the segment 4'! with which it has been in contact. During a short transition interval the brush interconnects the segment 41 which it is disengaging and the segment 49 which it is engaging, and when this occurs, potential is supplied through the ignition electrode 61 and the rectifier I9 associated with the succeeding lastmentioned segment from the corresponding phase winding 23 and the corresponding main discharge device BI is energized. Since the brush 33 interconnects the two segments 41 and 49 substantially all of the potential of the phase winding 23 in which the potential is increasing is impressed across the discharge device 59 in the circuit of the commutator segment 41 from which the brush is being disengaged, and the polarity of this potential is such as to oppose the potential of the phase winding 2| connected to the latter segment. When the potential of the latter phase winding 2| becomes substantially smaller than the potential of the former phase winding 23, the net potential impressed between the anode 63 and the cathode 65 of the discharge device 59 connected to the commutator segment 41 that is being disengaged is negative and the discharge device 59 is disengaged from the commutator segment 47 and it thus leaves the segment when there is no current between the brush and the segment. The same situation occurs as the brush passes from segment to segment and in each case, there is no sparking between the brush and the segments because there is no current flow between the brush and the segment which it is leaving.

It is to be noted that ignition circuits 83, 8 I, 61 for the respective discharge devices 5i to 6| are in parallel with the cathode circuits. Accordingly, the ignition circuits would, if symmetrically conductive, provide a shunt path around the corresponding discharge device for the flow of current from a commutator segment being engaged (as 49 for example) through a commutator segment being disengaged (as 4'! for example) to the transformer. It is for this reason that the rectifiers 69 to I9 are inserted in the ignition circuits. Their function is to prevent the flow of current between the interconnecting segments and their associated windings 23 to I3.

In the apparatus shown in Fig. 2, the power is applied directly through conductors connected to the commutator segments rather than through discharge devices such as 5| to El. However, the transfer of the current from one segment to another takes place through discharge devices and the latter operate to prevent arcing between the brush and the segments.

In addition to the

main segments

85, 81, 89, 9|, 93 and 95 which are directly connected to the external terminals of the secondary windings I 5 to 23, respectively of the transformer 5, the commutator 9T utilized in the aparatus shown in Fig. 2 is provided with short auxiliary segments 99, IGI, I93, I05, I91 and I09. An auxiliary segment 99 to I99 is associated with each main segment 85 to 95, respectively, and disposed adjacent thereto; each of the auxiliary segments is thus disposed between two main segments. The auxiliary segment 99 to I09 associated with any main segment 85 to 95, respectively, is connected to the same external terminal of the secondary windings 33 to 23, respectively, as the main segment. However, the connection in each case is not direct but one of the discharge devices I I I, H3, H5, H1, H9 and I2I and through one of the output windings I23, I25, I21, I29, I3I and i33 of a generator I35.

The discharge devices III to I2I are preferably of the same type as those utilized in the apparatus shown in Fig. 1, but may be of considerably smaller current-carrying capacity, inasmuch as they carry current only during the transition period as the brush I37 of the commutator 91 passes from one of the main segments 85 to 95 to the other. As in the aparatus shown in Fig. 1, the ignition electrode iii of each of the discharge devices III to Hill is connected to the anode 53 of the same discharge device through one of the rectifiers I39, MI, I43, I45, I47 and I49.

The generator I35 is of the induction type and potential is induced in its coils I23 to I33 by the operation of a suitable rotor I5i provided with field windings I53 and supplied with field current at all times from the transformer 5 through the main and auxiliary commutator segments 85 to 95, and 99 to I99, the brush I31, a conductor I55 connecting the brush to the windings I53, another conductor I5! connecting the windings I53 to the conductor SI of the direct-current load, the other conductor 2? of the direct-current load, and the smoothing reactor 29. Moreover, the rotor i5I is driven from the same synchronous motor 31 as the brush i3? and may, as a matter of fact, be mounted on the same shaft as the brush.

The angular positions of the brush it? and the rotor I5i are so adjusted relative to each other and to the position of the segments 85 to 95 and 99 to I99 and the coils i2? to 533 when the brush !3? simultaneously engages one of the main segments 85 to 95 and one of the auxiliary segments 99 to I99 connected to the same terminal of the secondary ii, a potential impulse is impressed by the coil E23 to E33 connected to the latter auxiliary segment. The potential thus impressed first acts in the circuit consisting of the conductor I59, iGI, ifii, ifil or i653 which connects the main segment 85 to 55 engaged by the brush 53? to the corresponding winding $3 to 23 of the secondary II, the last-n1entioned main segment, the brush, the auxiliary segment 99 to I69 engaged by the brush, the corresponding coil I23 to i333, the cathode of the corresponding discharge device ii i to i the ignition electrode 6'! of the same device, and the corresponding rectifier 599 to Current is then supplied through the corresponding one of the rectifiers I39 to I41 and the ignition electrode iii in question and a discharge initiated in the corresponding one of the discharge devices iii to I2I and thus current is transmitted through that discharge device and the corresponding auxiliary segment 99 to l99. Inasmuch as the auxiliary segment 99 to I99 which is involved is connected to one of the windings I3 to 23- of the secondary II as the corresponding main segments 55 to 95, the flow of curr nt through the discharge device diverts the current from the main segment. The motion of the commutator brush I3? is such that it leaves the main segment when the current "flowing therein is substantially zero.

Current now flows through one of the auxiliary segments 99 to 59 to which the brush i3? is connected and the corresponding discharge device II I to I2I for a short interval of time, but as the brush rotates it shortly engages the succeeding one of the main commutator segments 85 to 95, at the same time maintaining engagement with the auxiliary segment. In the rectifier system shown in Fig. 2, the rotating of the brush I3! is timed so that when the brush leaves the first main segment the potential of the corresponding winding I3 to 23 of the secondary iI is decreas- 15 ing and becoming equal to and less than the potential of the secondary winding corresponding to the succeeding segment. Moreover, while the auxiliary segment and the main segment corre- V are separated by vertical broken lines III,

spending to the succeeding winding are in engagement with the brush I3I, the potential of the succeeding winding rises to such a magnitude that it is greater than the sum of the potentials supplied by the preceding secondary winding and the one of the output coils I23 to I33 in the circult. At this time, the net anode-cathode potential impressed across the one of the discharge devices III to IZI that is in the circuit becomes negative and the discharge device is extinguished so that the brush leaves the auxiliary segment without arcing. The above-described process is repeated for each main segment and in each case the brush en ages an auxiliary segment and disengages the corresponding main segment without arcing and then engages the succeeding main segment and disengages the auxiliary segment without arcing. V

The operation of the apparatus shown in Fig. 2 will be better understood from a consideration of the graphs shown in Fig. 3. In preparing the graphs shown in Fig. 3 one main segment with which the brush I3! is in initial engagement and which I may assume to be the segment 93 has been identified by the letter B, and the succeeding main segment 95 by the letter C. Time is plotted horizontally and potential or current vertically. The difierent potential and current properties of the system are represented in graphs a, b, c, d and e arranged vertically. The different events occurring as the brush rotates I13, I I5 and I'll.

In graph a, the potential variation of segments A and C with reference to the neutral 25 of the secondary II are plotted. The corresponding curves IIQ and I 8! may beassumed to be portions of sine waves or at least approximate sine waves displaced in phase by 60. When the brush is contacting main segment A the potential represented by curve I79 and corresponding to A is above the potential represented by curve IBI and corresponding to C. The difference between the two decreases as the time during which the'brush contacts both A and B approaches the'potentials represented by the curves become equal during the time that segment B alone is contacted. The potential corresponding to curve IBI, i. e., segment C, becomes somewhat greater than the potential corresponding to segment A when the brush contacts B and C become considerably greater when C alone is contacted.

In graph b the potential impressed across the discharge device II9 in the circuit of auxiliary segment B is plotted as a function of time. As can be seen from the curve I83 during the time that segment A alone is engaged, this potential is zero inasmuch as the discharge device is connected in an open circuit. When segments A and B are both engaged the potential rises sharply as an impulse is induced in the coil I3I of the generator I that is in series with segment B. The potential first acts in the circuit consisting of conductor I61, segment 93, brush I37, segment I97, coil I3I, cathode 65 of discharge device II9, ignition electrode 67 of the same discharge device and rectifier I 41. The rise is interrupted when suflicient current is transmitted through the ignition electrode I Bl to energize the discharge device II 9 and the latter carries current. At this time the potential reduces to the are drop of the discharge device I I9 which is represented by the horizontal portion I 84 of curve I8I, continues at this value during the time that contact B alone is engaged by the brush and a small portion of the time that segments B and C are both engaged. It is to be noted that a characteristic of the generator I35 must be such that during the time just mentioned the potential supplied by the generator coil I3I should be at least equal to the arc drop of the discharge device H9 and as a matter of fact should pref-- erably not be substantially greater than this value.

While the segments B and C are both engaged by the brush, a sudden decrease in the potential across the discharge device represented by the vertical portion I85 of curve I8I takes place since the potential of the winding 23 of the secondary I I to which segment C is connected is now superimposed on the other potentials impressed in the circuit of the discharge device I59. As shown by curve I8I, the potential impressed across the discharge device 'I I9 becomes negative and when this occurs, the discharge device is extinguished and the net potential impressed in the circuit including the discharge device, the winding 23, the coil I 3I and the winding 2i is impressed as a negative potential between the anode 63 and cathode SI of the discharge device. The negative potential continues to decrease as is indicated by the downwardly sloping portion I 81 of curve I8I as the potential supplied by the winding connected to segment C rises. Since the net potential supplied in the circuit is impressed across the deenergized discharge device M9, the potential impressed between the segment B and the brush is zero as the brush is disengaged from segment B.

Graph c shows the variation of the current passing through segment A as the brush rotates. As can be seen from the curve I89 the current is of substantial magnitude as long as the brush engages segment A alone. When segments A and B are both engaged the current decreases to zero as it is diverted by the current flow through the discharge device H9 and auxiliary segment B. It is to be noted that the current begins to decrease substantially at the same instant that the potential across the discharge device I I9 begins to decrease as is shown in the curve I8I of graph b. 'The angular adjustment of the brush I31 must be such that it leaves contact A at the instant that the current as shown in graph c becomes zero. 7

In graph d the current through segment B is plotted as a function of time. As can be seen from the curve I9I the current is zero until the discharge device becomes energized and then it rises substantially at the same rate as the current through segment A falls until it attains a certain. value. The rise of the curve I9I representing the current through segment B and the fall of the curve I89 representing the current through segment A should add up to substantially the same value inasmuch as both are supplied from the same winding and the segment B and its discharge device I I9 are merely diverting current flow from the main segment A. The current flow through segment B continues until the brush engages B and C. At this instant a negativepotential is superimposed on the discharge device I I9 through segment C and the current in segment B decreases until the discharge .device is extinguished at which point it becomes substantially zero.

As has been mentioned, the brush should leave segment A when the current through this segment zecomes zero. If it disengages the segment after this instant a portion of the current flowing through the discharge device II9 will also flow through the segment A and the brush I31. In graph c this condition is represented by dropping curve I89 below the time axis. The current is of opposite polarity to that which has been flowing to the load and its presence of course may produce arcing as the brush leaves segment A. Since the flow of current in the negative direction which has just been discussed is produced solely by the potential impressed by the generator coils I23 to I33, this situation may be considerably improved by so designing generator I35 that the characteristic of the potentials supplied by the coils is such as to suppress the negative current. In a properly designed generator, the potential delivered by the coils I23 to I33 should rise rapidly to a sufficient value to energize the corresponding discharge devices III to I2I and then should gradually fall to a value just sufficient to maintain the arc in the discharge devices.

In graph e the current through segment C is plotted as a function of time. As can be seen from the curve I93, this current increases as soon as segments B and C are engaged and then continues at a predetermined value after the current in segment B is extinguished.

In Fig. 4 a generator, such as I prefer to use for supplying the energizing impulses for the discharge devices III to I2I in circuit with the auxiliary segments 99 to IE9, is shown. The machine shown in Fig. 4 will function in the same manner as the generator I35 shown in Fig. 2. However, it is more practical for the purpose than is the machine shown in Fig. 2. The machine shown in Fig. 4 is provided with a four-pole rotor I95 suitably energized by field windings I9'I connected to rings I99 and 29I which in turn engage brushes 293 and 205 carrying current supplied as shown in Fig. 2. As the armature I95 rotates potential is induced in sets 20?, 209, 2I I, 2I3, 2I5 and 2!! of four coils each. The individual coils of each set are spacially displayed by 90 and connected together as shown. The sum of the potentials induced in any of the sets 29'! to 2H of four such coils is impressed in the circuit of the corresponding one of the auxiliary segments 99 to I99 and the corresponding .discharge devices III to IZI. Each set of four coils corresponds to a single one of the coils I23 to I33 shown in Fig. 2.

Inasmuch as the rotor I is of the four-pole type, the sets of four coils are displaced 30 around the periphery of the rotor when the generator is designed for a six-phase system. Since a coil corresponding to a phase winding is, in the machine shown in Fig. 4, located at the same point on the periphery as the coil corresponding to the opposite phase winding, two coils are arranged at each point, one behind the other. To cut down the size of the machine the rotor is provided with a short-circuited winding consisting of a plurality of bars 2I9 disposed in slots in the outer surfaces of the poles ZZI of the rotor I95 and interconnected by rings 223.

While my invention is shown herein as applied to rectifier systems, it should be emphasized that it may equally as well be utilized for inverting purposes. Such a use involves no more than the interchange of the power supply source and the load and the proper timing of the energization of the discharge devices. When the apparatus is used for inverting purposes, the power is supplied from a direct current source and is consumed by an alternating load. If such a load is of the polyphase type corresponding to the source in the apparatus actually shown and described, the potentials derived from the direct-current source are impressed on the phase windings of a primary which corresponds to the secondary II shown in Figs. 1 and 2 and induced in a suitable secondary. The potentials impressed on the phase windings are then in relative magnitudes, the converse of the potentials impressed in the rectifying system. In terms of curve a of Fig. 3, this means that in an inverting system curve I8I corresponds to segment A and curve I'I9 to segment C.

It is to be noted, moreover, that while I have shown the brushes of the commutators in Figs. 1 and 2 as rotating, it is equally as well possible to have the segments rotate and the brushes remain fixed. It is only important that the segments successively engage and disengage the brushes.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In combination, a first current path, a first discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, said anode and cathode and said ignition electrode and cathode being connected in parallel in said path, a first contact in said first path, a second current path, a second discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, said last-named anode and cathode and said last-named ignition electrode and cathode being connected in parallel in said second path, a second contact in said second path, a third current path, a third contact in said third path, said third contact being movable relative to said first and second contacts so that it may be engaged with and is engaged from said first and second contacts, and said contacts being so arranged that said third contact is being disengaged from the said first contact when it is being engaged with said second contact and in the transition interval is in engagement with both the first and the second contacts, means for producing current flow in said first and third paths when said first and third contacts are in engagement, and means for producing current flow in said second and third paths when said second and third contacts are in engagement, said discharge devices being so connected as to prevent current fiow between the first and second paths in the transition interval.

2. In combination, a first current path, a first discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, said anode and cathode and said ignition electrode and cathode being connected in parallel in said path, a first contact in said first path, a second current path, a second discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, said last-named anode and cathode and said last-named ignition electrode and cathode being connected in parallel in said second path, a second contact in said second path, a third current path, a third contact in said third path, said third contact being movable relative to said first and second contacts so that it may be engaged with and disengaged from said first and second contacts, and said contacts being so arranged that said third contact is being disengaged from the said first contact when it is being engaged with said second contact and in the transition interval is in engagement with both the first and the second contacts, means for producing current flow in said first and third paths when said first and third contacts are in engagement, said current-fiow-producing means including means for impressing a first alternating potential in said first and third current paths, and means for producing current fiow in said second and third paths when said second and third contacts are in engagement, said currentfiow-producing means including means for impressing a second alternating potential of the same frequency as said first-named alternating potential but displaced in phase by radians relative thereto, n being an integer, in said second and third current paths, said discharge devices being so connected as to prevent current flow between the first and second paths in the transition interval.

3. In combination, a first current path, a first discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, said anode and cathode and said ignition electrode and cathode being connected in parallel in said path, a first contact in said first path, a second current path, a second discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, said last-named anode and cathode and said last-named ignition electrode and cathode being connected in parallel in said second path, a second contaEt in said second path, a third current path, a third contact in said third path, said third contact being movable relative to said first and second contacts so that it may be engaged with and disengaged from said first and second contacts, and said contacts being so arranged that said third contact is being disengaged from the said first contact when it is being engaged with said second contact and in the transition interval is in engagement with both the first and the second contacts, means for producing current flow in said first and third paths when said first and third contacts are in engagement, said current-fiow producing means including means for impressing a first potential in said first and third current paths, and means for producing current flow in said second and third paths when said second and third contacts are in engagement, said currentfiow-producing means including means for impressing a second potential in said second and third current paths, said first potential being greater than said second potential when said first and third contacts are in engagement, and said second potential being greater than said first potential when said second and third contacts are in engagement, and said discharge devices being so connected as to prevent current flow between the first and second paths in the transition interval.

4. In combination, a first current path, a first discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, means for connecting said anode and cathode in said current path,

means for connecting said ignition electrode and said cathode in parallel With said anode and cathode, the last said means including current rectifying means arranged to pass current in the same direction as said discharge device, a first contact in said first path, a second current path, a second discharge device having an anode, a cathode of the mercury-pool type and an ignition electrode of the silicon carbide type, means for connecting said anode and cathode of said second discharge device in said current path, means for connecting said ignition electrode and said cathode of said second discharge device in parallel with said anode and cathode, the last said means including current rectifying means arranged to pass current in the same direction as said second discharge device, a second contact in said second path, a third current path, a third contact in said third path, said third contact being movable relative to said first and second contacts so that it may be engaged with and disengaged from said first and second contacts, and said contacts being so arranged that said third contact is being disengaged from the said first contact when it is being engaged with said second contact and in the transition interval is in engagement with both the first and the second contacts, means for producing current fiow in said first and third paths when said first and third contacts are in engagement, and means for producing current fiow in said second and third paths when said second and third contacts are in engagement, said discharge devices being so connected as to prevent current flow between the first and second paths in the transition interval.

5. In combination, a plurality of sets of terminals from which a polyphase potential is to be derived or between which a polyphase potential is to be impressed, a current path connected at one end to a corresponding terminal of each of said sets of terminals and at the other end to a contact, additional contact means arranged to successively connect each of said contacts to the other terminal of said sets of terminals, said contact means operating to connect the successive contacts together in the transition interval between the separate connection of successive terminals, an electric discharge path having an anode, a cathode of the mercury-pool type and an ignition electrode of silicon carbide type corresponding to each current path, means for connecting the anode and cathode of a discharge device in the corresponding path, said anodes and cathodes being so connected as to block the flow of current between successive current paths in the transition interval when the corresponding contacts are interconnected, and means for connecting said ignition electrodes and cathodes in parallel with the corresponding anodes and cathodes.

6. Apparatus in accordance with claim 5 characterized by means, in the connections of the ignition electrodes and the cathodes, for preventing the fiow of current between ignition electrode connections corresponding to successive current paths in the transition interval when the corresponding contacts are connected to each other.

'7. In combination, a first current path, a first discharge device having an anode, a cathode and an ignition electrode, said anode and cathode and said ignition electrode and cathode being connected in parallel in said path, a first contact in said first path, a second current path, a second discharge device having an anode, a cathode and anignition electrode, said last-named anode and cathode and said last-named ignition electrode and cathode being connected in parallel in said second path, a second contact in said second path, a third current path, a third contact in said third path, said third contact being movable relative to said first and second contacts so that it may be engaged with and disengaged from said first and second contacts, and said contacts being so arranged that said third contact is being disengaged from. the said first contact when it is being engaged with said second contact and in the transition interval is in engagement with both the first and the second contacts, means for producing current flow in said first and third paths when said first and third contacts are in engagement, and means for producing current fiow in said second and third paths when said second and third contacts are in engagement, said discharge devices being so connected as to prevent current fiow between the first and second paths in the transition interval.

8. In combination, a first current path, a first discharge device having an anode, a cathode and an ignition electrode, means for connecting said anode and cathode in said current path, means for connecting said ignition electrode and said cathode in parallel with said anode and cathode, the last said means including current rectifying means arranged to pass current in the same direction as said discharge device, a first contact in said first path, a second current path, a second discharge device having an anode, a cathode and an ignition electrode, means for connecting said anode and cathode of said second discharge device in said second current path, means for connecting said ignition electrode and cathode of said second discharge device in parallel with the last said anode and cathode, the last said means including current rectifying means arranged to pass current in the same direction as said discharge device, a second contact in said second path, a third current path, a third contact in said third path, said third contact being movable relative to said first and second contacts so that it may be engaged with and disengaged from said first and second contacts and said contacts being so arranged that said third contact is being disengaged from said first contact when it is being engaged with said second contact and in the transition interval is in engagement with both the first and the second contacts, means for producing current fiow in said first and third paths when said first and third contacts are in engagement, and means for producing current fiow in said second and third paths when said second and third contacts are in engagement, said discharge devices being so connected as to prevent current flow between the first and second paths in the transition interval.

9. In combination, a first current path, a first contact on said first path, a second current path, a second contact in said second path, said first and second contacts being engageable with and disengageable from each other, a third current path, a third contact in said third path, said second and third contacts being engageable with and disengageable from each other, and said second contact being engaged with said third contact when it is being disengaged from said first contact, said first, second and third contacts being in engagement with each other during a transition interval, an electric discharge device in said third path, means for causing current to fiow in said first and second paths when said first and second contacts are in engagement, means for so coupling the last said means to said third current path that it may also cause current to fiow through said third path when said third and second contacts are in engagement whereby current may be diverted from said first to said third path, and means for rendering the discharge device in said third path conductive during said transition interval so that during said interval current is diverted from said first to said third path.

10. In combination, a first current path, a first contact on said first path, a second current path, a second contact in said second path, said first and second contacts being engageable with and disengageable from each other, a third current path, a third contact in said third path, said second and third contacts being engageable with and disengageable from each other, and said second contact being engaged with said third contact when it is being disengaged from said first contact, said first, second and third contacts being in engagement with each other during a transition interval, an electric discharge device in said third path, means for causing current to fiow in said first and second paths when said first and second contacts are in engagement, means for so coupling said first and third paths that current may be diverted from said first to said third path, and generating means having an output circuit in said third path for impressing a potential to render the discharge device in said third path conductive during said transition interval so that during said interval current is diverted from said first to said third path.

11. Apparatus according to claim 10 characterized by the fact that the operation of the generating means is synchronized with the engagement and disengagement of said first, second and third contacts.

12. In combination, a first current path, a first contact on said first path, a second current path, a second contact in said second path, said first and second contacts being engageable with and disengageable from each other, a third current path, a third contact in said third path, said second and third contacts being engageable with and disengageable from each other, and said second contact being engaged with said third contact when it is being disengaged from said first contact, said first, second and third contacts being in engagement with each other during a transition interval, an electric discharge device having a control electrode and a plurality of principal electrodes in said third path, means for causing current to fiow in said first and second paths when said first and second contacts are in engagement, means for so coupling said first and third paths that current may be diverted from said first to said third path, and means for impressing a single potential between the electrodes of said. discharge device to render the discharge device in said third path conductive during said transition interval so that during said interval current is diverted from said first to said third path.

13. In combination, a first current path, a first contact on said first path, a second current path, a second contact in said second path, said first and second contacts being engageable with and disengageable from each other, a third current path, a third contact in said third path, said second and third contacts being engageable with and disengageable from each other, and said second contact being engaged with said third contact when it is being disengaged from said first contact, said first, second and third con tacts being in engagement with each other during a transition interval, an electric discharge device having an anode, a cathode of the mercury-pool type and an igni ion electrode of the silicon carbide type, means for connecting said anode and cathode in said third path, means for connecting said ignition electrode in parallel with said anode and cathode, means for causing current to flow in said first and second paths when said first second contacts are in engagement, means for so coupling said first and third paths that current may be diverted from said first to said third path, and means for impressing a potential across the parallel network consisting of said anode, said cathode and said ignition electrode render the discharge device in said third path, conductive during said transition interval so that during said interval current is diverted from said first to said third path.

14. In combination, a first current path, a first contact on said first path, a second current path, a second contact in said second path, said first and second contacts being engageable with and disengageable from each o her, a third current path, a third contact in said third path, said second and third contacts being engageable with and disengageable from each other, and said second contact being engaged with said third contact when it is being disengaged from said first contact, said first, second and third con tacts being in engagement with each other during a transition interval, an electric discharge device in said third path, means for impressing a first potential to cause a current to fiow in said first and second paths when said first and second contacts are in en agement, means for so coupling said first and third paths that current may be diverted from said first to said third path, means for rendering the discharge device in said third path conductive during said transition interval so that during said interval current is diverted from said first to said third path, a fourth current path, a fourth contact in said fourth path, said fourth contact being engageable with and disengageable from said second contact and said second contact being disengaged from said third contact when it is being engaged with said fourth contact, said third and fourth contacts being in engagement with said second contact during a transition period, and means for impressing a second potential to cause current to flow in said fourth path when said second and fourth contacts are in engagement and to extinguish the current in the third path during the transition period.

15. In combination, a first current path, a

first contact on said first path, a second current path, a second contact in said second path, said first and second contacts being engageable with and disengageable from each other, a third current path, a third contact in said third path, said second and third contacts being engageable with and disengageable from each other, I and said second contact being engaged with said third contact when it is being disengaged from said first contact, said first, second and third oontacts being in engagement with each other during a transition interval, a unilateral conductive valve in said third path, means for impressing a first potential to cause a current to flow in said first and second paths when said first and second contacts are in engagement, means for so coupling said first and third paths that current may be diverted from said first to said third path, means for rendering said valve in said third path conductive during said transition interval so that during said interval current is diverted from said first to said third path, a fourth current path, a fourth contact in said fourth path, said fourth contact being engageable with and disengageable from said second contact and said second contact being disengaged from said third contact when it is being engaged with said fourth contact, said third and fourth contacts being in engagement with said second contact during a transition period, and means for impressing a second potential to cause current to flow in said fourth path when said second and fourth contacts are in engagement and to extinguish the current in the third path during the transition period.

16. Apparatus according to claim 15 characterized by the fact that the first and second potentials vary continuously, that the second contact is successively in engagement with the first, third and fourth contacts and that the first potential is greater than the second potential when the first contact is in engagement with the second contact and the second potential is greater than the first potential when the fourth contact is in engagement with the second contact.

17. In combination, a plurality of sets of terminals from which a polyphase potential is to be derived or between which a polyphase potential is to be impressed, a plurality of pairs of contacts corresponding to said pairs of terminals, a first set of current paths, each current path of said set connecting a first contact of each of said pairs of contacts to a terminal of a corresponding one of each of said pairs of terminals, a second set of current paths, each path of the lastsaid set connecting the second contact of each of said pairs of contacts to the same ter minal of said corresponding one of each of said pairs of terminals to which the corresponding current path of said first set of paths is connected, each of said second sets of paths having interposed therein a normally non-conductive asymmetric electric discharge path, contact means engageable with and disengageable from each one of said pairs of contacts in periodic sequence, for connecting the corresponding pairs of current paths to the other terminals of said pairs of terminals, said contact means operating to firstconnect said first contact of each of said pairs of contacts in its turn, then to connect the second contact of said pairs of contacts, connecting said first and second contacts of each pair together for a predetermined first transition interval, and then to connect the first contact of a succeeding pair, first connecting the second contact of the first pair and the first contact of the second pair together during a transition interval, and means for rendering the discharge path in the second current path being connected conductive during the first transition interval and non-conductive during the second transition interval.

18. Apparatus according to claim 17 characterized by the fact that the means for rendering the discharge paths conductive includes potential generating means operating in synchronism with the engagement and disengagement of the contact means and the pairs of contacts.

19. A commutating system in which segments are connected in pairs to the terminals of a polyphase winding energized from a poiyphase source, the first segment of each pair being directly connected to the winding terminal, and the second segment being connected to the same winding terminal through a valve or rectifier means, and means synchronous with said source for diverting the current from the first segment of a pair to the second segment.

20. A commutating system providing for the transfer of power between an alternating current system and another system and having main and auxiliary segments, a main segment and a corresponding auxiliary segment being connected to the same terminal of said system, the auxiliary segments being in each case associated with valve means, and means synchronous with said alternating-current system for diverting the current from the main segments to the auxiliary segments.

21. In combination, a first current path, a first contact in said current path, a second current path, a second contact in said second path, said second contact being movable relative to said first contact so that it may be engaged with and disengaged from said first contact, a third current path, a third contact in said third path, said second contact being movable relative to said third contact so that it may be engaged with and disengaged from said third contact and said first, second and third contacts being so disposed that when said second contact is being disengaged from said first contact it is being engaged with said third contact and in the tranition in terval, which transition interval is a short one, is in engagement with. both the first and third contacts, means for producing current flow in said first and second paths when said first and second contacts are in engagement, means for producing current flow in said third and second path when said third and second. contacts are in engagement and asymmetrically conductive electric discharge devices of the immersed-ignitionelectrode type in said first and third paths for preventing current flow between said paths when said second contact is in engagement with said first and third contacts.

JOSEPH SLEPIAN.