US2431247A - Mercury rectifier - Google Patents

Description

Nov. 18, 1947. c. w. HANSELL MERCURY RECTIFIER Filed Nov. 4, 1944 INVENTOR Zarerwe WiianseZZ ATTORNEY Patented Nov. 18, 1947 MERCURY RECTIFIER Clarence W. Hansell, Port Jeiferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 4, 1944, Serial No. 561,992

This invention relates to rectifiers and analogous devices having a mercury cathode.

In connection with pool type mercury arc rectifiers and D. C. to A. C. converters, one of the problems requiring solution has been that of anchoring or controlling the position of the cathode spot on the mercury pool. It is an observed fact that the spot on the surface of the liquid mercury, which is acting as the cathode terminal of the arc, i subject to a continual motion over the surface. In a metal clad mercury arc device in which metal forms the boundary of the active surface of the mercury pool, an arc cannot be maintained to the mercury surface because the cathode spot, in wandering about, quickly reaches the metal boundary surface, leaves the mercury surface to travel up on the metal container, or on mercury condensed on the metal surfaces of the container, and is either quickly extinguished or continues to burn with excessive heating and' rapid permanent disintegration of the metal container.

To restrain the motion of the cathode spot to a liquid mercury surface it is common practice in large mercury arc rectifiers to enclose a portion of the mercury surface With a short vertical cylinder of insulating material, and to strike the arc to the surface of th mercury within the area enclosed by the cylinder. In practice the arc frequently moves up to and away from the side of the insulating cylinder and, when close to it, subjects the cylinder to intense heating. Consequently, the insulator must be capable of withstanding intense thermal shock without breaking or disintegrating. For this reason insulators of fused quartz are commonly used, although less expensive insulators made of porcelain enameled steel have also come into limited use. Even with the arc confined by such an insulator to an area of the mercury surface, there still remains a large amount of motion of the cathode spot which causes variations in distribution of ionization in a mercury discharge device and considerable variation, instability and tendency for flash-back, or conduction in the wrong direction. In an ideal case the cathode spot should always be anchored at one point, usually at the center of the active mercury surface. Once the spot is anchored, it is easy to design shields and baffles and mercury cooling and condensing arrangements which greatly improve the voltage and current rating and the reliability of the device. It is therefore an object of the invention to provide means for definitely anchoring the source of electron emission, commonly called the cathode spot, of a rec- 2 Claims. (Cl. 2250-4215) tifier or analogous are at one point on a mercury or similar cathode.

Another object is to provide means for fixing the cathode spot by directing ions of relatively high velocity continually to the area where the spot is to be located.

Another object is to project ions onto a mercury cathod from an auxiliary anode located within another anode for anchoring the cathode spot.

Another object is to provide a blast of ions through a small constriction in an auxiliary anode to bombard an area of the liquid cathode for anchoring the cathode spot in that area.

Another object is to anchor the cathode spot through generation of secondary electrons by high velocity ionic bombardment of the cathode pool.

Other objects will appear in the followin specification, reference being had to the drawing, in which:

Figure 1 is a section through a three-phase mercury arc rectifier with a diagramof the circuits.

Figure 2 is a section through the end of the first and second auxiliary anodes.

Referring to Fig. l of the drawing, the container or casing of the mercury arc rectifier comprises a steel tank I having a steel base 2 separated therefrom by appropriate insulation 3. The base is secured to the tank by usual means, not shown. The tank has atop 4, which also may be "steel, through which the six power anodes of the three alternating current phases extend. Only four of these anodes, namely 6, 1, 8 and 9 of phases I and II, are shown, as the two anodes of the phase III are located in the front half of the rectifier cut off by the section plane. The first auxiliary anode ill enclosing the second auxiliary anode I] is suspended from the center of cover 4. These two auxiliary anodes in my invention serve as the means for anchoring the cathode spot and may be made of iron, having tips l2 and I3, preferably made of metal of high melting temperature, such as tungsten. Tip l2 of anode ID has a constriction l3 (Fig. 2) therethrough, flaring slightly outward toward the mercury pool l4 confined within insulation ring l5, of quartz or Pyrex, for example.

The power anodes may have the usual heat radiators l5. These anodes and the two auxiliary anodes extend through openings in the cover 4 and are supported by appropriate insulators I6 fastened to the cover in the usual way. The enclosure is made vapor-tight and is evacuated, ex-

cept for mercury vapor, as is well known. Auxiliary electrode H is supported on the auxiliary electrode ill by insulation II. The auxiliary anode II is surrounded by an iron condensing cylinder l8 and each of the power anodes is surrounded by cylinders [9, having control grids [9' of known construction.

The rectifier may be ignited by mounting the auxiliary anode l so that it can move into and away from the mercury pool, as is often done in rectifiers, but for starting the arc I prefer to use an igniter spark plug 20 having a high resistance anode 2] of Thyrite, spaced from the pool about inch. The Thyrite anode and base 2 are connected to the terminals of the secondary 22 of transformer 23. This secondary has a high voltage, say 15,000 volts, and a high reactance and resistance. Thyrite, a product of the General Electric Company, is a, suflicient conductor to start the arc with this high voltage, but it is practically an insulator for the voltage applied across the power anodes and mercury cathode.

The three mains 24, 25 and 26 of the threephase power to be rectified are connected to overload circuit breaker 21, indicated by block diagram. The three lines leading from the circuit breaker are connected to the three transformers 28, 29 and 35 in delta, though, of course, these transformers may be connected in Y if desired. The secondary of transformer 28, designated as phase I, has its end terminals connected to power anodes 6 and I and its mid terminal connected through one arm of the equalizing reactor 3! to ground. The secondary of transformer 29, phase II, has its end terminals connected to the electrodes 8, 9 and its mid terminal connected through the second arm of the equalizing reactor to ground. The secondary of transformer 30, phase III, is connected to the remaining two anodes in the front section of th transformer, not visible in the drawing. The midpoint of this secondary is connected through the remaining arm of the equalizing reactor to ground.

A small control transformer 32 is tapped off one ofthe phases of the power supply to be rectified and the secondary of this transformer is connected through switch 33 to control lines 34, 35. Control line 3 3 is connected through a resistance to a small control rectifier, diagrammatically illustrated at 35, This rectifier may be of the copper oxide, vacuum tube, mercury arc, or any other type. The output of the rectifier is connected through the control coil of a relay 3! to the auxiliary anode U and also through resistance 38 to the first auxiliary anode It. Resistance 38 puts a lower operating voltage on auxiliary anode l e than thatimpressed on the auxiliary anode H, but its voltage may be obtained in any other way. The Voltage on anode I l is above 50 and below 250 volts, or thereabouts.

Auxiliary control line 35 is connected though a resistance to ground. A circuit is tapped off from control line 36 through closing coil 39 of the overload circuit breaker 2'! and make-contacts 40 of relay 3?. Control line 35 is connected through break-contacts 41 of relay 3? and the primary of igniter transformer 23 to control line 34, A spring 42 may be used-to bias the relay armature to close break-contacts 4|.

The operation of my improved rectifier is as follows:

Switch 33 is closed, which places potential between the auxiliary anodes i0 and II and the mercury cathode l4. As indicated, this may be between 50 and 250 volts. Simultaneously, a high secondary voltage, preferably about 15,000 volts, is placed across the Thyrite terminal of spark plug 20 and the mercury cathode M, which produoes a small arc. Electrons are formed by this arc and an arc immediately is struck between auxiliary anodes l8, H and the mercury cathode. The are to the anode ll must pass through the small constriction 13' in anode i0 and this reduces the current to such value that the arc voltage is above normal; that is, above the Value it would have were the constricted opening absent.

The current through the coil of relay 3! attracts the relay armature against tension of spring 42, which opens break-contacts 4i and closes make-contacts 40. This control current also energizes coil 39 and closes the overload circuit breaker 21. Such a remote control overload breaker is a well-known construction and the mechanism for accomplishing this action is not shown. The closure of the breaker 21 places the desired potential across each of the six power anodes and the mercury cathode Hi. Arcs are immediately struck between the positive power anodes and the cathode in three-phase order. The return circuit is, of course, completed to the mid terminals of the three secondaries through the equalizing reactor 3!. The coils of this reactor are magnetically coupled, so that the current in them is maintained substantially equal. On the next half cycle the current passes from the remaining three anodes to the cathode in threephase order, giving full-wave rectification. The rectified potential appears across D. C. lines &3, 44. When relay 3'1 operated, switch GI opened and current ceased to flow through the spark plug circuit.

In my researches leading up to the making of this invention, I found that the power are wandered erratically around over the mercury pool. When. the current through auxiliary anode H was gradually increased, the behavior remained the same until a critical value of current was reached, at which the voltage of the arc between this anode and the mercury pool began to increase. As soon as the are between anode II and the cathode pool reached about 50 volts, I found that the cathode spot on the mercury pool ceased to wander and became firmly anchored on the portion of the mercury surface directly under the small hole in auxiliary anode i0.

From my practical use of the invention, I consider an arc voltage of volts between anode H and cathode I i, with a current of about one ampere, sufiicient to positively anchor the cathode spot under usual working conditions, depending on mercury vapor pressure and the size of the opening in anode I0.

In explanation of the reasons for this eifective anchoringof the cathode spot, there are a number of probable phenomena which cooperate to produce the result. Due to current limiting in the hole in the first anode l0, brought about by electrical vacuum pumping and excessive percentage ionization of the gas and vapor in the path of the arc, there are ions and molecules issuing from the hole and striking the mercury surface with more than enough velocity to produce secondary electron emission. The high voltage are also produces intense ultra violet light which falls most intensely upon the mercury surface directly under the hole and this also produces electron emission. The high voltagehigh velocity arc, by bombardment, pushes away the mercury vapor in its path and produced a relatively high vacuum therealong. This gives a freer path to electrons moving away from the cathode and an increased electric field to move them.

It has previously been assumed that rapid evaporation of mercury at the small area constituting the cathode spot forces the spot to extinguish itself and new spots to form continuously and that this continual forming of new spots constitutes the motion of the cathode spot. With the high voltage arc, the ion blast from the hole seems to sweep away the vapor evaporated and to cause a much larger spot area which can remain stationary within the required limits.

Having the knowledge that a current-limited are through a hole, near the cathode, with 50 or more volts arc drop, can be used to anchor the cathode spot, it is relatively simple to add this feature to mercury arc rectifiers, Thyratron controllers, D. C. to A. C. converters and other pool type mercury are devices and further explanation herein is not required.

I claim:

1. A mercury arc rectifier comprising a sealed casing containing a mercury pool, a main power anode, spaced from the mercury pool at one side thereof, a tubular anode having an end with an aperture directly over and near said mercury pool, and an auxiliary anode within, and insulated from, said tubular anode, said aperture being sufficiently small to reduce the ar current between the pool and the auxiliary anode and circuit connections for placing greater voltage between the mercury pool and the auxiliary anode than between it and the tubular electrode.

A polyphase mercury arc rectifier comprising a sealed casing containing a mercury pool, main power anodes spaced from said pool around the outside thereof, a tubular anode having an end with an aperture directly over and near the center of said mercury pool and an auxiliary anode within, and insulated from, said tubular anode, said aperture being sufiiciently small to reduce the arc current between the pool and the auxiliary anode and circuit connections for applying at least volts at 1 ampere current between the auxiliary anode and the mercury pool and a lesser voltage between the tubular electrode and said pool.

CLARENCE W. HANSELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,097,473 Schaefer May 19, 1914 2,173,023 Mulder Sept. 12, 1939 2,179,929 Hansell Nov. 14, 1939 FOREIGN PATENTS Number Country Date 379,687 Germany Aug. 27, 1923

Images