US1845841A - Method and apparatus for preventing back-fires in metal vapor rectifiers - Google Patents

Description

Feb. 16, 1932. JONAS ET AL 1,845,841

METHOD AND APPARATUS FOR PREVENTING BACK FIRE IN METAL VAPOR REGTIFIERS Filed May 29, 1926 a b v Patented Feb. 16, 1932 UNITED STATES PATENT OFFICE JULIUS JONAS, OF BADEN, AND ERVI'IN KERN, OF VIETTINGEN, NEAR BADEN, SWITZER- LAND, ASSIGNORS TO AKTIENGESELLSCI-IAFT BROWN BOVERI & CIE., OF BADEN, SWITZERLAND, A JOINT-STOCK COMPANY OF SWITZERLAND METHOD AND APPARATUS FOR PREVENTING BACK-FIRES IN METAL VAPOR RECTIFIERS Application filed May 29, 1926, Serial No. 112,665, and in Germany June 2, 1925.

This invention relates to the prevention of certain undesirable phenomena known as back-iircs in apparatus for rectifying alternating current of the metallic vapor type, of which mercury vapor rectifiers are an example.

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 of a 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 further objects will be pointed out or indicated hereinafter, or be obvious to one skilled in the art upon an understanding of the invention.

In the drawings forming a part of this specification we show various arrangements of. apparatus, the same being presented for the purpose of illustrating the invention. These are not to be construed in any fashion as having the efl'ect of limiting the claims short of the true and most comprehensive scope of the invention in the art.

In the drawings,

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

Fig. 2 is a curve diagram illustrating the variation of the anode and shield potentials using the above apparatus,

Fig. 3 is a diagram of another form of ap paratus illustrating the invention, and

Fig. 4: is a curve diagram showing the variation of anode and shield potentials using the apparatus of Fig. 3.

The operation of metal vapor rectificers is liable to be interrupted by the occurence of baclefires (really internal short-circuits) 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 shutdown. 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 clear that a backfire 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 is not carrying a load current.

As a general rule the potential taken up by an insulated anode shield lies somewhere between the potentials of its associated anode and the cathode respectively. If an arc is struck from the anode, however, this forms a conducting path between the anode and its shield and the corresponding potentials will then be approximately equal.

Attempts have been made to suppress the back-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 causes the anode shields to become positive with 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 vapor rectifiers 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 lib 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 struc- 5 ture or shields during the'period of maximum tendency to back-fire.

Fig. 2 shows the variation of anode potential during a complete A. C. cycle. If the rectifier has six anodes for example, then 10 each carries the load current for one sixth of a cycle. This period is shown by t, and in Fig. 2. During this period the anode is positive with regard to the cathode. The horizontal line 11 represents the constant potential of the cathode. It will be seen that outside the period t t the anode is negative to the cathode and hence there will be tendency to back-fire. If, however, the anode shield is maintained negative to the 20 anode during the danger period the shield will prevent an arc passing from the cathode to the anode, since the signs of the potentials in anode and shield respectively allow the passage of current in one direction only, i. e.

from anode to shield. The curve 6 in Fig. 2 shows the variation in the potential of the anode shield which should be obtained to secure the effect of the invention. In the example given it occupies a dehnlte position relative to the curve a, and may be arrived at by subtracting a constant amount 0 from the ordinates of a. The anode shleld poten-.

tial may be made to vary in the desired way by the following means :T he shielos are all, 3 connected to a speclal star-connected sec- 'ondary winding of the transformer feeding the rectifier. This special secondary may be called an auxihary secondary to distinguish it from the mam winding. The aux- Qiliary secondary may, for example, be wound with regard to their respective anodes, a relativelv low uni-directional potential drop is applied between the respective neutral points of the main and auxiliary secondaries, the sign of this potential drop being such that i -tl1e neutral point of the main winding will be positive to the neutral point of the auxiliary winding. The necessary potential drop may be obtained from a small motor generator, rectifier, or battery, and it should be noted that only very small currents will be required.

This embodiment of the invention is illus trated inFig. 1.

The three-phase mains N supply the pri 69 mary P of the transformer T feeding rectitier G, the transformer including a main secondary Q connected as shown to the rectifier anodes (l -(1 and an auxiliary secondary connected as shown to anode shields in -7a,.

As will be seen from Fig. l, the neutral point 0 of winding Q is joined to the neutral point 0 of winding Q through the battery B, 0 being positive to 0 The negative wire of the D. C. system N starts from the rectifier cathode 0 and the positive from K. In the example illustrated the anode shields h to 72. are insulated. If the voltage at the terminals of winding Q is the same as that of winding Q then the potential of a shield ill always be lower than that of the asso ciated anode by an amount 0. Thus a back-fire cannot start. On referring to the Figure 1 it will be seen that resistances w, to 70 are inserted in the leads joining the anode shields with the terminals of the auxiliary winding Q. These resistances may be ohmic, inductive, or capacitive, and their function may thus be explained :The introduction of the potential drop 6 undoubtedly fixes the potential of the shields lower than that of the respective anodes. The shields however are bombarded by ions coming from the cathode and thus there will always be a reverse current flowing through the shields, outside the period t t which raises the potential of the same and may in certain circumstances neutralize the effect of the impressed voltage (1. By inserting resistances as w to 10,-, this reverse current can be reduced to a negligible value.

One disadvantage of the arrangement described is the necessity for a supply of direct current. Suitable low voltage supplies are not always available, and in any case complicate the plant. The necessity for a D. C. supply may be entirely avoided. however, by making the voltage of the auxiliary secondary greater than that of the main secondary and not connecting the neutral points. 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 t t (Fig. 2) therefore, the anode and associated shield are at approximately the same potential. The shields of the anodes notcarrying current are then at a potential different from that of their respective anodes owshield potentiallying below the anode poteni tial. The maximum divergence occurs at time if, when the curve a is at Its maximum negative value and the difference ma is twice the difierence between the voltages of Q and Fig. 3 illustrates this arrangement diagrammatically. The same reference letters are used as in Fig. 1. The difference lies in the fact of winding Q having more turns than winding Q so that the shields will be supplied with a higher voltage than the anodes, and also that the neutral points and 0 are not connected. The potential of winding Q: is therefore entirely dependent on whichever anode is carrying load current at the time. Assume that an arc passes from anode a, through the shield 72., to the cathode K. The shield h will then take up the same potential as the anode a The potentials of the other shields will then depend on the magnitude and sign of the potential drops existing between the shields. It will be easily seen that in the case under consideration 1, 1 it follows that the shield It, must be negative with regard to the anode a 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. WVith 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 between the neutral point 0 of the winding Q which supplies the shields, and the cathode. A small current then flows between the shields and the cathode, taking the form of an are which unites with the main are from the anodes. In this way the )resence of an electrically conducting path etween anode and shield is ensured.

In applying the foregoing method for preventing baclcfires 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 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 ineffective. 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 lead-in 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 Q The resistances ee -w may then be dispensed with as no current can flow 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.,1n 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 polyphase metal vapor rectifier comprising anodes and shields associated respectively therewith, of means for impressing upon each shield a potential negative with res ect to that on the respective associated ano e during the entire non-working period of said anode.

2. In an electrical system of the character described, a polyphase metal vapor rectifier comprising anodes and shields associated respectively therewith, a polyphase winding supplying said anodes, and means for impressing upon each shield a potential negative with respect to that on the respective associated anode during a period of time immediately following each normal operating period of said anode, said means including a second polyphase winding.

3. In an electrical system of the character described, the combination with a polyphase metal 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 shield voltage is negative with respect to the anode voltage. I

4. In an electrical system of the character described, a polyphase vapor rectifier comprising anodes and shields associated respectively therewith, a polyphase winding supplying said anodes and having a neutral point, means for impressing upon each shield a potential negative with respect to that on the respective associated anode during a period of time immediately following each normal operating period of said anode, said means including a second polyphase winding having a neutral point, and a source of D. C. potential connected between said neutral 10 points with the negative side of said source connected to the neutral point of said second polyphase winding.

In testimony whereof we have hereunto subscribed our names at Zurich, Switzerland 15 on the 18th day of May A. D. 1926.

JULIUS JONAS. ERWIN KERN.

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