US1971718A - Method and apparatus for preventing backfires in metal vapor rectifiers - Google Patents

Description

Aug. 28, 1934. JONAS ET A 1,971,718

METHOD AND APPARATUS FOR PREVENTING BACKFIRES IN METIAL VAPOR RECTIFIERS Original Filed May 29, 1926 Patented Aug. 28, 1934 METHOD AND APPARATUS FOR PREVENT ING BACKFIRES IN METAL VAPOR RECTI- Julius Jonas, Baden, and Erwin Kern, Wettingen, Switzerland, assignors to, Aktiengesellschaft Brown Boveri & Cie., Baden, Switzerland, a

joint-stock company of'Switzerland Original application May 29, 1926, Serial No. 112,665. Divided and this application February 28, 1930, Serial No. 432,033. In Germany June 2, 1925 3 Claims. (01. 175-363) This invention relates to the prevention of certain undesirable, phenomena known as back-fires in apparatus for rectifying alternating current of the metallic vapor type, of which mercury vapor 6 rectifiers are an example. The present application is a division of my co-pending application Serial No. 112,665, filed May 29, 1926.

The general object of the invention is the prevention or elimination of such phenomena in a simple and reliable manner.

A particular object is the provision of a method and apparatus for the purpose stated, and which produces the desired preventive effect at such times when the tendency for the undesirable phenomena to take place is strongest.

Another object is the provision ofa method and apparatus for the purpose stated which does not introduce parts into the interior of the rectifier which might themselves be the source of undesirable phenomena.

Other and furtherobjects will be pointed out or indicated hereinafter, or be obvious to one skilled in the art upon an understanding of the invention.

In the drawing forming a part of this specificationwe show an arrangement of apparatusythe same being presented for the purpose of illustratingthe invention. This is not to be construed in any fashion as havingthe effect of limiting the claims short of the true and most comprehensive scope of the'invention in the art.

In the drawing,

Fig. 1 is a diagram of apparatus illustrating the invention, V

Fig. 2 is a curve diagram illustrating the variation of the anode and shield potentials using 40 (really internal short-circuits)v which are usually accompanied by a number of other harmful effects. Thus a back-fire which ends in a dead short-circuit may have such a destructive effect on the'material: of the rectifier as to necessitate a complete shut-down. When a back-fire takes place, the current flows from the original cathode (or from one of the anodes) to an. anode which has temporarily assumed'the functions of a cathode,

and it is therefore clearthat a back-fire will usually tend to start during the time when the potential of the anode concerned is negative to that of the cathode; This condition is fulfilled during that portion of the cycle when the anode isnot carrying a load current.

causes the anode shields to become positive with thesame voltage as the main secondary winding *of the transformer, but is quite independent of the latter and galvanically separated therefrom,

As a general rule the potential taken why an insulated anode shieldlies somewhere between 1 fires by subdividing the anode shields or giving them certain shapes, but without success, the reason being that the mere presence of the shields cannot prevent the current from passing from cathode to anode. If a tendency to back-fire exists, the increasing glow-discharge current regard to the anodes so that the flow of current towards them is assisted.

The object of the invention is therefore a means for preventing back-fires in metal vaporrectifiers 5 having insulated anodes, according to which insulated metal structures arranged in the path of the back-fire discharge (these may be the anode shields themselves) are maintained at a potential negative to their associated anode over the period within each A. C. cycle during which the said anode is not carrying load current. The potential of the metal structures referred to shall be applied in such a way that the anode is always positive to the metal structure or shields during the period of maximum tendency to back-fire.

The anode shield potential may be applied and varied in the desired way by various means. As shown, for examplain Fig.1 of thedrawing of the herein identified parent application, Serial No. 112,665, now Patent 1,845,841, dated February 16,

1932, the shields are all connected to a special auxiliary star-connected winding of the transformer supplying the rectifier. Such auxiliary secondary winding may, for example, be wound for and a suitable source of uni-directional potential is connected in circuit between the respective neutral points of the main and auxiliary windings in 100 i such manner that the neutral point of the main winding will be positive with respect to the neutral point of the auxiliary winding.

Such described arrangement, although providing satisfactory operation, requires a suitable 105 source of low voltage direct current, which is not always available, and many case will introduce complications, The necessity for a source of direct current supply may be entirely avoided, however,by winding the auxiliary secondary to pro- 110 duce a voltage greater than the voltage produced by the main secondary winding and not connecting the neutral points of such windings. It appears at first that the potential of the auxiliary winding and consequently that of the connected shields will still be indefinite, but as soon as an arc is struck between anode and cathode through the shield, the latter will take up the potential of the anode then carrying load current. During the period, therefore, that an anode is carrying load current, such anode and associated shield will be at approximately the same potential. The shields of the anodes not carrying current are then at a potential different from their respective anodes owing to the different voltages of the main and auxiliary secondary windings of the supply transformer, and the shields are maintained, therefore, at a potential negative with respect to the potentials of their associated anodes.

Fig. 2 gives the potential curves for this case, where a is the curve of the anode voltage and b the curve of the associated shield. Over the period t1-t2 the curves are co-incident, owing to the are forming a conducting path. Over the remaining parts, i. e. the anode non-operating period, of the cycle the curves diverge, the shield potential lying below the anode potential. The maximum divergence occurs at time t3 when curve a is at its maximum negative value and the difference mm is twice the difierence between the voltages of the main and auxiliary secondary windings of the supply transformer.

This embodiment of the invention is illustrated in Fig. 1 of the drawing and shows the conductors NW of a three-phase alternating current circuit connected with the primary winding P1 of transformer T supplying the rectifier G. The transformer includes a main secondary winding Q1 connected, as shown, to the rectifier anodes m-ae and an auxiliary secondary winding Q2 connected, as shown, to the anode shields h1hs, through current limiting resistances w1-w6.

- As will be seen from the drawing, the negative and positive conductors Ng of the direct current output circuit start from the neutral point 01 of the main secondary winding Q1 and the cathode K respectively, and the neutral points 01 and 02 are not connected. In the example illustrated, the anode shields are insulated from all electrically conductive elements of the device and, as will be seen, the auxiliary secondary winding Q2 has more turns than the main secondary winding Q1 so that the shields hlh6 will be supplied with a higher voltage than the associated anodes a1a6.

Assuming now that an arc passes from anode (1.1 through the shield hi to the cathode K, the shield hl will then take the same potential as the anode cm. The potentials of the other shields will then depend on the magnitude and the sign of the potential drop existing between the shields. It will be readily seen, therefore, that in the case under consideration there must be a potential drop between the anode a4 and the shield hi which is given by the difierence of two potential drops, namely, the potential drop between the terminals 1 and 4 in winding Q2 and the potential between terminals 1 and 4 in winding Q1. Since the potential drop between 1' and 4' is greater than the potential drop between 1 and 4 and 4, 4' and negative with regard to 1, 1', it follows that the shield h! must be negative with regard to or.

It has been assumed hitherto that the arc (load current) leaving the anode forms a conducting path between anode and shield and thus equalizes the potentials of both. With shields of certain shapes or sizes it may happen, however, that the arc does not come into contact with the shield at all, and its potential will therefore be indefinite. To ensure that both parts have the same potential the shields are given a certain degree of activity by allowing them to work as anodes on to a loading resistance. For this purpose a relatively large resistance is inserted be tween the neutral point 02 of the winding Q2 which supplies the shields, and the cathode. A small current then fiows between the shields and the cathode, taking the form of an arc which unites with the main arc from the anodes. In this way the presence of an electrically conducting path between anode and shield is ensured.

In applying the foregoing method for preventing back-fires the effect of very small reverse currents on the shield potential must be taken into consideration. It must be assumed that a certain minimum reverse current is always passing. The effect of this reverse current is to raise the shield voltage and thus tends to counteract the externally impressed negative charge. There are various ways of rendering these reverse currents inefiective. For example, a smaller control shield can be fitted inside the anode shield proper and maintained at a potential somewhat lower than the anode. The outer shield then protects the control shield to a certain extent from radiation from the arc and thus prevents its potential being raised by the reverse current. Another way would be to provide either the inner or the outer shields, or both, with an insulating coating which must also surround the lead to the shield from the point where it enters the rectifier. This insulating covering would prevent reverse currents passing through the shields.

Finally, it is advisable to dispense entirely with all leads in the interior of the rectifier since back-fires are very liable to start from the leadin points. This may be done by adopting a known design in which the anodes are housed in tubular members of non-metallic insulating material which extend outside the rectifier. The shield is then placed round the lower part of the tubular member and connected directly to the corresponding terminal of winding Q2. The resistances w1w6 may then be dispensed with as no current can fiow through the shields.

The anode shields may be replaced by other devices which when given a potential opposing the flow of current towards the anode eliminate the reverse currents. Equivalent devices may take the form of gratings, grids, rings, etc., in short any insulated metal structure placed in the path of the back-fire discharge.

What we claim is:

1. In an electrical system of the character described, the combination with a vapor rectifier comprising anodes and shields associated respectively therewith, of means for impressing upon each anode and the associated shield similar sine waves of voltage, characterized by the fact that at any instant of time the voltages impressed by said means on the shields are greater in magnitude than the voltages impressed on the associated anodes, the relation of said anodes to the said shields and the connections of said means therewith being such that potential is impressed on said shields from the associated anodes during the operating period thereof of such sign and magnitude as to render the shields more negative than the associated anodes during the non-operating periods of the latter.

2. In an electrical system of the character described, the combination with a vapor rectifier comprising anodes and shields associated respectively therewith, of a transformer winding connected with and operable to'impress sine waves of voltage on each of saidanodes, and a second tial thereof is impressed on the associated shields to thereby render the potential of the respective shields more negative than the potential of the associated anodes during the non-operating periods of the latter.

3. In an electrical system of the character described, the combination with a metal vapor rectifier comprising a plurality of anodes and shields associated respectively therewith, of a transformer winding comprising a plurality of sections connected respectively with and being operable to impress voltages of varying sign and magnitude on said anodes, and a second transformer winding comprising a plurality of sections connected' respectively with and being operable to impress on said shields voltages corresponding in sign atevery instantof time to the sign of the voltages impressed on the associated anodes, the said sections of each of said windings having a star point connection and the voltages impressed on the said shields by the second said winding being at every instant of time greater in magnitude than the voltages impressed on the associated anodes by the first said winding, the position of the shields with respect to the associated anodes being such that during the operating periods of the latter vthe potential thereof is impressed on the shields associated with the nonoperating anodes by way of the said neutral point connection of the said second transformer winding to thereby render the shields more negative than the associated anodes during the nonoperating periods of the latter.

JULIUS JONAS. ERWIN KERN.

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