U.S. patent number 3,586,904 [Application Number 04/817,900] was granted by the patent office on 1971-06-22 for off-switching of liquid-metal arc switching device by auxiliary arc liquid-metal starvation.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Wilfried O. Eckhardt.
United States Patent |
3,586,904 |
Eckhardt |
June 22, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
OFF-SWITCHING OF LIQUID-METAL ARC SWITCHING DEVICE BY AUXILIARY ARC
LIQUID-METAL STARVATION
Abstract
An enclosed vessel containing one or more main anodes, one or
more auxiliary anodes, a cathode, and means for maintaining a low
pressure in the tube comprises a liquid-metal arc switching device.
The cathode is fed with a metal which is liquid at convenient
temperatures, so that limited quantities of the metal are present
and available on the cathode for arcing. The interior of the vessel
is maintained at a low background pressure so that during
nonconduction, vacuum space insulation is provided between the
anodes and the cathode. Arc initiation is accomplished by any
convenient initiator, and the arc runs upon the small amount of
liquid metal fed at an appropriate rate to the cathode. Arc
extinction is accomplished by causing current to flow from the
cathode to the auxiliary anodes at rates substantially higher than
the main anode current. This causes consumption of the liquid metal
at a higher rate than the liquid-metal feed rate, with consequent
arc extinction by cathode starvation.
Inventors: |
Eckhardt; Wilfried O. (Malibu,
CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
25224146 |
Appl.
No.: |
04/817,900 |
Filed: |
April 21, 1969 |
Current U.S.
Class: |
315/382; 313/163;
313/7; 315/111.01 |
Current CPC
Class: |
H01J
13/06 (20130101); H01J 13/50 (20130101); H01J
13/52 (20130101); H01J 2893/0088 (20130101) |
Current International
Class: |
H01J
13/00 (20060101); H01J 13/06 (20060101); H01J
13/50 (20060101); H01J 13/52 (20060101); H01j
013/04 () |
Field of
Search: |
;313/34,7,163,170
;315/111 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Demeo; Palmer C.
Claims
What I claim is:
1. A switching device, said switching device comprising:
an enclosing envelope, a main anode and an auxiliary anode within
said envelope, a cathode within said envelop;
electrical connection means on said main anode and said cathode for
connection to an external serially connected load and a first
source of electric power;
electric connection means on said cathode and said auxiliary anode,
a second source of electric power positioned externally of said
envelope and electrically connected to said cathode and said
auxiliary anode, said second source of electric power being of such
current capacity as to cause an arc current between said cathode
and said auxiliary anode at least as great as the arc current
between said cathode and said main anode;
said cathode comprising high arc voltage metal retaining walls,
means for feeding lower arc voltage liquid metal to rest against a
portion of said retaining walls at an adequate rate for supplying a
current-conducting arc between said cathode and said main anode and
not in excess of such a rate so that upon additional arcing between
said cathode and said auxiliary anode caused by current flow from
said second source of electric power, the lower arc voltage liquid
metal available for arcing becomes exhausted to result in arc
termination between said cathode and said main anode to stop
current conduction therebetween even in the presence of an electric
field therebetween.
2. The switching device of claim 1 wherein means are connected to
said envelope for maintaining the pressure in the envelope
sufficiently low to establish a vacuum of high insulative property
after arc cessation.
3. The switching device of claim 2 wherein said means for
maintaining the pressure in the envelope maintains the pressure in
the envelope below 10.sup..sup.-2 Torr during arcing.
4. The switching device of claim 2 wherein said main anode is
positioned to face said walls on said cathode so as to intercept
the high velocity jet stream emitted from the arc spots at the
junction between said lower arc voltage liquid metal and said high
arc voltage metal retaining walls.
5. The switching device of claim 4 wherein said auxiliary anode is
positioned to protrude into and partially intercept the arc jet
stream issuing from said cathode to said main anode.
6. The switching device of claim 5 wherein there are a plurality of
auxiliary anodes, each protruding into and partially intercepting
the arc jet stream.
7. The switching device of claim 5 wherein the number of auxiliary
anodes projecting into and partially intercepting the jet stream at
least equals the number of phases of said second source of electric
power.
8. The switching device of claim 1 wherein a source of electric
energy and a load are serially connected between said cathode and
said main anode, said source of electric energy being arranged to
operate not in excess of a maximum current, and said second source
of electric energy is connected to said cathode and said auxiliary
anode, said second source of electric energy being arranged to
operate at currents in excess of said maximum current of said first
electric energy source.
9. A switching device, said switching device comprising:
an enclosing envelope, a main anode and an auxiliary anode within
said envelope, a cathode within said envelope;
electrical connection means on said main anode and said cathode for
connection to an external serially connected load and first source
of electric power;
electric connection means on said cathode and said auxiliary anode,
a second source of electric power connected to said cathode and
said auxiliary anode, said second source of electric power having
sufficient current capacity for drawing an arc between said cathode
and said auxiliary anode having current in excess of an arc between
said cathode and said main anode;
said cathode comprising high arc voltage metal retaining walls;
feed means for feeding lower arc voltage liquid metal to rest
against a portion of said retaining walls at an adequate rate to
supply sufficient liquid metal to supply an arc between said
cathode and said main anode and insufficient to continually supply
arcs between said cathode and both said main anode and said
auxiliary anode so that, upon arcing between said cathode and said
auxiliary anode, said liquid metal supply against a portion of said
retaining walls is exhausted and the arc between said cathode and
main anode is extinguished.
10. The switching device of claim 9 wherein the means are connected
to said envelope for maintaining the pressure in the envelope
sufficiently low to establish a vacuum of high insulative property
after arc cessation.
Description
BACKGROUND
This invention relates to the field of liquid-metal arc rectifiers
and switches, such as mercury arc rectifiers.
Mercury arc rectifiers are well known in the art. They suffer from
numerous problems, which problems primarily stem from the fact that
a large mercury pool is present in the device, and this large
mercury pool maintains, through evaporation, a fairly high
background pressure within the rectifier vessel. In commercial
devices, the pool temperature is kept as low as is practical and as
is consistent with arc operation, in order to maintain this
background pressure as low as is possible. However, despite this,
at the pool temperature usually found in such devices, there is
sufficient mercury vapor within the tube that de-ionization is slow
upon voltage reversal. This in turn occasionally causes arcing back
from the anode to the cathode, which is destructive to the device
and highly objectionable regarding the circuit in which the device
is used.
To overcome this limitation, state of the art high voltage mercury
tubes are provided with grading electrodes. These grading
electrodes lead to another limitation: the current which can pass
to one anode through such a set of grading electrodes is limited to
such extent that for higher currents a number of parallel anodes
and sets of grading electrodes are required. These limitations then
lead to the need for complex multianode tubes (with
current-dividing transformers to divide the current uniformly
between parallel anodes) and grading electrodes with attendant
voltage dividers.
The voltage holdoff properties of the conventional mercury pool
liquid cathode devices are determined by a tradeoff between the
desired voltage holdoff, the peak current, the voltage drop across
the arc, and voltage recovery rates. These conflicting requirements
do not permit the device to be designed for high voltage holdoff
and high current without the complex grading electrodes and the
multiple anodes mentioned above.
Furthermore, prior mercury arc rectifiers and switching devices are
not capable of forced current interruption against a voltage
persisting in the forward direction between anode and cathode.
SUMMARY
In order to aid in the understanding of this invention, it can be
stated in essentially summary form that is directed to a switching
device including an enclosing tube or vessel in which are located
one or more main anodes, one or more auxiliary anodes, a cathode,
and means for maintaining a low pressure in the tube. The cathode
is fed with a low arc-voltage metal which is liquid at convenient
temperatures, so that a small quantity of this metal is available
on the cathode to permit a current-carrying arc to run between the
cathode and the main anode. When forced interruption of this
current is desired, the auxiliary anodes are connected to draw an
arc current from the cathode to the auxiliary anodes, in excess of
the main current from the cathode to the main anode. This causes
consumption of the low arc-voltage metal, which is fed at a rate
suitable for the main anode current, with resultant starvation of
the cathode and arc cessation.
Accordingly, it is an object of this invention to provide a
liquid-metal arc switching device wherein only a small amount of
metal available for arcing is positioned on the cathode so that
during arcing the pressure in the tube is relatively low and upon
cessation of arcing the pressure between the anode and the cathode
is quickly reduced to high vacuum to provide high insulative value
therebetween. It is a further object of this invention to provide a
liquid-metal arc switching device wherein the liquid metal has a
low arc voltage and is fed to a cathode structure having a higher
arc voltage so that arcing is restricted to the low arc voltage
material and arcing ceases when the low arc voltage material
available in the cathode structure becomes exhausted. It is a
further object of this invention to provide an arc switching device
which has the capability of withstanding high voltages across a
single anode-cathode gap because of the low pressure between the
anode and cathode when the device is not arcing. It is a further
object of this invention to provide a liquid-metal arc switching
device which has a cathode which emits a plasma during arcing, and
two or more anodes positioned to electrically couple to the plasma
during conduction for efficient conduction of large currents to any
of these anodes, and means to rapidly reduce the pressure in the
tube between the anodes and the cathode upon cessation of arcing to
provide a rapid rise of holdoff voltage versus time. It is a
further object of this invention to use one or more of said anodes
as auxiliary anodes and the others as main anodes, in a
liquid-metal arc switching device which has liquid metal fed at a
rate proportional to the current passing from the cathode to the
main anodes so that upon initiation of an arc between the cathode
and auxiliary anodes at arc currents in excess of that of the main
anode, the liquid metal in the cathode is exhausted with consequent
arc cessation so that current stops flowing to the main anodes of
the switching device. It is a further object of this invention to
connect the auxiliary anodes in a separate electric circuit from
the main anodes in a liquid-metal arc switching device so that an
auxiliary arc can be run from the cathode to the auxiliary anodes
at currents in excess of the main anode current, and at voltages
substantially lower than the standoff voltage of the switching
device. Other objects and advantages of this invention will become
apparent from a study of the following portion of this
specification, the claims and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE of the drawing is a longitudinal section through
a liquid-metal arc switching device with six auxiliary anodes
therein for off-switching by starvation of the liquid metal on the
cathode, and with a schematic electric circuit shown in association
therewith.
DESCRIPTION
Referring to the drawing, the liquid-metal arc switching device is
generally indicated at 10. The general term "switching device" is
used to generally describe a device which is capable of initiating
and/or interrupting current flow in a circuit. This includes
circuits wherein the current reaches a natural zero by means
independent of and exterior of the switch tube, as well as devices
in which the current zero is accomplished by the switch tube by an
increase in voltage thereacross by arc cessation. The first case
includes rectification of alternating current, and the second case
includes DC circuit breakers and commutators for converting DC to
AC. Thus, the device itself inherently interrupts current flow upon
voltage reversal and can be operated to interrupt current without
current or voltage reversal. For these reasons, the term switch
device is used generically to define the scope of such devices.
The switch device 10 includes an envelope 12 which serves as a
vessel or a tube within which the primary components of this switch
device are located. Vacuum pump connection 14 is provided on the
envelope 12 to either originally pump down the original
noncondensable gases after manufacture, whereupon it is sealed, or
can be continuously pumped to maintain the desired low background
pressure of noncondensable gases.
Main anode 16 is positioned within the envelope and has an external
connector which extends externally of the envelope for electrical
connection. Anode 16 is shown as being in the shape of a flat
circular disc so that a maximum area is exposed to the electrically
conducting plasma. Anode 16 is preferably provided with heat
exchanger 20. Heat exchanger 20 controls the temperature of anode
16 to prevent extreme temperature excursions. Anode 16 is
preferably maintained at a temperature above the condensation
temperature of the liquid metal, hereinafter described, so that
condensation does not occur thereon. Such condensation leads to
arc-back situations, and is thus undesirable. Heating of the anode
is caused by its absorption of the kinetic energy of the plasma
particles, by the recombination energy of electrons and ions, and
by the I.sup.2 R drop of the current flow through the anode
material. Thus, at high currents, cooling may be necessary through
heat exchanger 20 to prevent the anode from reaching destructive
temperatures. On the other hand, at low loads, heating by means of
heat exchanger 20 may be necessary to maintain anode 16 above the
liquid-metal condensation temperature. Heat exchanger 20 is
conventionally externally connected and is controlled by any
convenient temperature sensing means responding to the temperature
of anode 16.
Envelope 12 is made at least partially of insulative material to
provide electrical separation of the envelope walls between anode
16, condenser 50, and cathode 22. The envelope extends downwardly,
as shown in the drawing, from anode 16 to below cathode 22. Offset
24 provides an internal offset adjacent which insulating disc 26 is
positioned. Envelope 12 continues downward below offset 24 as a
cylindrical tube, and curves inward and upward to form tubular
reentrant section 28.
Cathode 22 is made of a metal which is a high arc voltage material,
as compared with the liquid metal used in connection with it. The
term arc voltage is defined as arcing voltage in Proceedings of the
Institute of Electrical Engineers, Volume 110, No. 4, Apr. 1963,
pages 796--797, Section 4.6. Furthermore, the metal of the cathode
structure must be compatible with the liquid metal. Cathode 22 has
pool-retaining walls 30 against which the liquid metal is located
and against which the arc runs, at the juncture or junctures
between walls 30 and the liquid metal. Above the walls, cathode 22
has a front face 32, which is substantially in the form of a planar
disc. Below the front face 32, cathode 22 extends downwardly in a
reduced diameter tubular neck 34 which is sealed to tubular
reentrant section 28 of the envelope by means of an appropriate
seal joint 36. In the embodiment shown, cathode 22 has a shoulder
36 positioned between the larger diameter front face and neck 34.
Insulating disc 26 rests against this shoulder and is maintained in
place by retainer 38. Other geometries of wall 30 than the one
shown in the drawing can also be employed.
Heater 40 is located within the hollow interior of the cathode
behind front face 32, and extends around the outer periphery, down
inside of shoulder 36 and into neck 34 so that the entire exposed
area of the cathode away from walls 30 can be controlled in
temperature. The central portion of the cathode, below the recess
formed by walls 30, extends downward to form neck 42, and heat
exchanger 44 is positioned exteriorly of the neck. Both heater 40
and heat exchanger 44 have suitable connections extending downward
within tubular reentrant section 28 to permit external connection
and temperature control. If desired, temperature sensors can be
mounted so that accurate temperature determinations of the
respective portions of the cathode 22 can be made.
The liquid metal required on the surface of the cathode structure
for arcing can be provided either by feeding of metal in the liquid
state or by intermediate vaporization and recondensation.
The term liquid metal is used to define those metals which are
liquid at or just somewhat above room temperature. While called
liquid metal, the metal is not necessarily in the liquid state when
fed to walls 30 which define pool-keeping surfaces. Mercury is a
convenient liquid metal, because it is normally liquid at room
temperatures. Furthermore, it has a suitably low arc voltage. Thus,
the arc preferentially strikes upon the liquid metal when the
external surfaces of cathode 22 are formed of a relatively high arc
voltage material. Additional materials which are suitable to act as
the liquid metal fed to cathode 22 are exemplified by cesium,
lithium, and gallium. If necessary, liquid-metal feed line 46, and
associated equipment to provide the liquid metal thereto can be
heated to maintain the liquidity or vapor state of the liquid
metal.
As indicated, cathode 22, as well as anode 16, are preferably
formed of high arc voltage materials. When mercury is employed as
the liquid metal, molybdenum serves as a suitable material for the
anode and cathode. Liquid-metal feed line 46 extends from a source
47 of liquid metal of such nature as to provide the proper flow
rate of liquid metal to cathode 22, as hereinafter described.
Liquid-metal feed line 46 extends up through the reentrant section
28, and in the example of the drawings is connected centrally of
cathode 22 to feed the liquid metal to the space defined between
walls 30. If desired, a suitable flow restriction 48 can be
positioned at the lower juncture of walls 30 with the opening
therebelow connecting to feed line 46 to impede liquid-metal flow
to the space defined between walls 30. When liquid metal is fed to
the space in its liquid state, a porous flow restriction is
desirable for it prevents an arc from extending down into the feed
line when the pool between walls 30 is exhausted. However, when the
liquid metal is fed in vapor form, a flow restriction in the form
of a capillary passage is preferred, with the capillary somewhat
larger than the passages through the porous mass employed in the
example for liquid feeding.
Additionally, within envelope 12 condenser 50 is located. It serves
emitted one means for continuously removing from the tube metal
vapor emitted by the cathode. Condenser 50 is suitably externally
connected to maintain the desired temperature upon the condenser
surface, as described below. When condenser 50 is maintained at a
temperature to condense the metal to the liquid state, it is
collected by trough 52, and can be recirculated to the cathode. In
the case of recirculation of the liquid metal, it can be drained
out through line 54, and an appropriate isolator will be required
in the recirculation line.
Auxiliary anodes 56 are also located within envelope 12. Since the
plasma jet emerging from the cathode is substantially in the form
of a solid cone having its apex on the cathode, the auxiliary
anodes 56 are arranged as radial fins protruding into the plasma
cone. Thus, during normal conduction of current from the cathode to
the main anode 16, auxiliary anodes 56 do not substantially
interfere with the operation. However, auxiliary anodes 56 provide
sufficiently large surfaces within the plasma jet that they are in
good electrical coupling therewith. Six auxiliary anodes 56 are
illustrated.
The auxiliary anodes are connected through lines 58 to a source of
electric current 60. In the example given by the FIGURE, source 60
may be a DC source, and all six auxiliary anodes may be connected
in parallel, or source 60 may be a three or six phase AC source,
and the auxiliary anodes may be connected to the different phases.
source 60 in turn is connected through switch 62 to line 64 which
is connected to cathode 22. Thus, continuity between cathode 22 and
auxiliary anodes 56 is controlled by switch 62. Main anode 16 is
connected by line 66 serially through load 68 and source of
electric current 70. Source 70 is also connected to cathode 22 by
means of line 64. Since the source of electric current 70 is
driving a load, for example load 68, the voltage thereof may be
quite high, and this voltage is the voltage switched by switching
device 10. On the other hand, since the voltage against which
source 60 operates is the voltage drop through the arc between
cathode 22 and auxiliary anodes 56, source 60 can operate at fairly
low voltages and at greater currents than the current at the main
anode 16, and still be of considerably less power than that
supplied by a source 70. For example, if the source 70 supplies
voltage at 10.sup.5 volts, and at 1,000 amperes, the current
supplied by a source 60 can be 10,000 amperes at the voltage drop
across the arc, which is about 10 volts. If both currents flow for
equal periods, the power spent in the auxiliary discharge to the
auxiliary anodes 56 is 10.sup.-3 times the main circuit power
passing through main anode 16.
When mercury is the liquid metal, the liquid-metal feed requirement
for such an arc, drawn from a specific cathode may, for example, be
one cubic centimeter of mercury per hour per 100 amperes of
discharge current. Thus, in the example above, 10 cubic centimeters
of mercury are required per hour to supply sufficient mercury to
maintain a 1,000 amperes arc from the cathode to the main anode.
When arcing to the auxiliary anode takes place at 10,000 amperes,
the amount of mercury feed is only one-eleventh of that required to
maintain arcing. If the mercury available on the cathode for arcing
is 0.01 cubic centimeters, it is clear that the arc is extinguished
in about 0.3 seconds.
The pressure within envelope 12 is pumped down through a vacuum
pump connection 14, before the device is put into use. In some
cases, when the contents of the envelope have a minimum of
out-gassing, the envelope can be sealed by closure of connection
14. In other cases, it may be desirable to keep the connection with
a vacuum pump so that noncondensables can be pumped down when
operation of the device so indicates.
The arc is initiated by any convenient means. No specific arc
initiator is disclosed in the drawings, but those well known in the
art can be used. Examples of such are auxiliary electrode igniters,
semiconductor igniters, and the like. Alternatively, a laser
igniter directed onto the liquid-metal surface is suitable, as is
an igniter which emits a puff of liquid-metal plasma into the space
between the anode and the cathode to initiate arcing.
Presuming that a suitable voltage is applied across the anode to
cathode space, and liquid metal is available on cathode walls 30,
the ignition of the device will cause conduction. The amount of
liquid metal available on walls 30 for evaporation is kept
purposely low so that the electron to atom ratio is high. As a
consequence, the pressure in the envelope will remain relatively
low, even during conduction. A typical range for mercury as the
liquid metal is from 10.sup..sup.-2 to 10.sup..sup.-6 Torr.
Heater 40 maintains the face of the cathode sufficiently hot that
liquid-metal condensation cannot occur there. Similarly, the
temperature of the pool-keeping walls 30 is controlled by heat
exchanger 44. In cases where a liquid-metal pool is desired, heat
exchanger 44 cools the pool-keeping walls to prevent excessive
evaporation, especially in the case of high currents where the arc
introduces a considerable amount of heat into the pool-keeping
structure. In cases where the arc current is low, it may be
necessary to heat the pool-keeping walls, especially in the case
where the liquid metal is fed in vapor form to the walls 30. In
such a case, the walls 30 are preferably slightly above the
equilibrium condensation temperature of the liquid-metal vapor at
that pressure. Thus, only transient condensation can occur.
Transient condensation, in this sense, is a situation wherein
slightly superheated metal vapor deposits upon the wall in a thin
film over a portion of the area, with such condensation occurring
for a short time. However, since the surfaces are above the
equilibrium condensation temperature, no condensation of droplets
or masses of metal occurs. Instead, liquid metal is continuously
deposited and evaporated off, and occupies only a portion of the
wall area at any one time.
As stated above, heat exchanger 40 keeps the remainder of the
cathode above condensation temperature to prevent metal pools from
forming thereon to prevent the arc from acting on any surface but
the walls 30. In addition, the heating prevents metal condensation
at the juncture between neck 34 and tubular reentrant section 28,
for arcing at this juncture would be destructive. Additionally,
insulating disc 26 prevents any appreciable quantity of the metal
vapor from reaching that juncture in order to prevent the
destructive arcing.
Condenser 50 is positioned to condense metal vapor in order to
prevent a buildup of vapor pressure within envelope 12. In the case
of mercury, in order to maintain the metal vapor pressure within
the tube in the absence of arcing below 10.sup..sup.-5 Torr, when
it is desired to retain the condensed mercury in the liquid state,
the temperature of condenser 50 is maintained at about 240.degree.
K. When a lower pressure is desired, a lower condenser temperature
can be used, resulting in solidification of the condensed mercury.
In the latter case, the condenser can be periodically warmed to
permit liquid mercury to drain out of the bottom of trough 52
through drain 54.
In operation, switch 62 is open and the main circuit electron
current is passing from cathode 22 to main anode 16 and thence
through load 68 source 70. Liquid metal is supplied to the cathode
at a suitable rate to maintain a sufficient quantity to feed the
arc, and thus maintain continuity. However, when off-switching is
desired, switch 62 is closed. Thereupon, the current supplied by
source 60 also draws additional arc current. In the example given,
this current is 10 times the current to the main anode 16. This
auxiliary anode current consumes the liquid metal available on the
cathode, and without increase in liquid-metal feed, the liquid
metal on the cathode soon becomes exhausted and the arc terminates
by starvation. In a switching device capable of switching the
10.sup.5 volts at 10.sup.3 amperes, as described above, and with an
auxiliary anode current of 10.sup.4 amperes, the arc will be
starved in approximately 300 milliseconds. With arc starvation, the
source 70 builds up voltage across the switching device 10. With
mercury as the liquid metal and molybdenum as the cathode structure
material, a rate of voltage rise not exceeding 10 kilovolts per
microsecond has been found permissible.
This invention having been described in its preferred embodiment,
it is clear that it is susceptible to numerous modifications and
embodiments within the ability of those skilled in the art and
without the exercise of the inventive faculty. Accordingly, the
scope of this invention is defined by the scope of the following
claims.
* * * * *