US3071667A

US3071667A - Vacuum-type circuit interrupter

Info

Publication number: US3071667A
Application number: US833182A
Authority: US
Inventors: Thomas H Lee
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1959-08-12
Filing date: 1959-08-12
Publication date: 1963-01-01
Anticipated expiration: 1980-01-01

Description

Jan. 1, 1963 'r. H. LEE 3,071,667

VACUUM-TYPE CIRCUIT IbiTERRUPTER Filed Aug. 12, 1959 2 Sheets$heet 1 TIME V01. THGE' flND CURRENT Inventor: Thomas H. Lee,

His Attorney.

Jan. 1, 1963 T. H. LEE 3,071,667

VACUUM-TYPE CIRCUIT INTERRUPTER Filed Aug. 12, 1959 2 Sheets-Sheet 2 Inventov: Thomas H. Lee,

His Attorn e5.

M mnelvzr/c C FIELD United States Patent Ofiice 3,071,667 Patented Jan. 1, 1963 3,071,667 VACUUM-TYPE CIRCUIT HNTERRUPTER Thomas H. Lee, Media, Pm, assignor to General Electric Company, a corporation of New York Filed Aug. 12, 1959, Ser. No. 833,182 12 Claims (Cl. 200-444) contact gap. Reignition of the arc is prevented only if the instantaneous dielectric strength across the gap continuously exceeds the instantaneous voltage that is being applied by the recovery voltage transient.

The maximum amount of voltage (V) that an interrupter can interrupt at a given contact separation can be defined as the peak value of the highest recovery voltage transient that can be withstood by the inter-contact gap at this particular separation during a circuit-interrupting operation. In a gas-type interrupter, the maximum amount for voltage V that can be interrupted at a given contact separation is only a small percentage of the voltage (V that that particular inter-contact gap could have withstood in the absence of prior arcing, e.g., on the order of about 5% or less. In a vacuum-interrupter,

however, the maximum amount of voltage that can be inu terrupted at a given contact separation is a much higher percentage of the voltage V that the gap could have withstood in the absence of prior arcing. For example, for low current interruptions with a vacuum interrupter, this ratio of V to V has approached a value of 1, though for high current interruptions it has been somewhat less. It has generally be accepted, however, that the very highest voltage interrupting capacity that one could expect to attain would be a value of voltage equal to V the noprior-arcing breakdown voltage.

While it has been recognized heretofore that the current-interrupting capacity of a vacuum interrupter could be increased by magnetically moving the terminals of the are at high speed along suitable arc-running surfaces, such arc movement has not raised the previously-accepted upper limit for voltage interrupting capacity. For low current interruptions, this are movement has had little effect on the voltage interrupting capacity V of the vacuum interrupter inasmuch as the interrupter was already capable, even without arc movement, of interrupting a voltage approximating the noarcing breakdown voltage V For high current interruptions, arc movement has resulted in some improvements in voltage interrupting capacity V, but even with such improvements, the voltage interrupting capacity still has fallen short of the no-arcing breakdown voltage V It is therefore an object of my invention to raise the voltage-interrupting capacity of a vacuum circuit interrupter to a level materially exceeding the no-prior arcing breakdown voltage of the inter-contact gap.

Another object is to attain such an improvement in voltage-interrupting capacity without producing any harmful electron bombardment of the interrupter parts adjacent the inter-contact gap by electrons emitted from the contacts of the interrupter.

In carrying out my invention in one form, I provide a vacuum interrupter that comprises a pair of contacts that are separable to establish a circuit-interrupting arc across the resulting inter-contact gap. In combination with this interrupter, I provide magnetic means that acts to establish a magnetic field that has its lines of force traversing the inter-contact gap in a direction generally perpendicular to most of the electric lines of force between the two separated contacts. The magnetic means is so constructed that when the current flowing across the inter-contact gap reaches the current zero point and the recovery voltage transient builds up across the gap, the minimum flux density B of this magnetic field is at least as high as about:

webers per square meter, where s is the separation between the contacts in meters at an instant 120 degrees after the contacts have parted, and E is the electric field intensity in the portion of said gap that has the highest field intensity in volts per meter based upon the peak value of the recovery voltage transient. This relatively intense magnetic field traversing the inter-contact gap at the instant of current zero interacts with the electric field across the inter-contact gap to cause electrons emitted from the cathode contact to follow a cycloidal path having a radius of curvature less than the length of the inter-contact gap. Thus, these electrons, instead of traveling across the inter-contact gap to bombard and thus vaporize the anode material, return to the cathode where they are recaptured without ever having reached the anode. By preventing these electrons from bombarding and hence vaporizing the anode, ionization of the intercontact gap is effectively prevented during the critical interval of current zero when the recovery voltage transient is building up, thus rendering the recovery voltage transient incapable of reestablishing the arc.

For a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a sectional view of a vacuum-type circuit interrupter embodying one form of my invention.

FIG. 2 is a diagrammatic representation illustrating certain current and voltage relationships occurring during the interruption of an inductive circuit.

FIG. 3 is a perspective view of the contact or electrode structure or" the circuit interrupter shown in FIG. 1 illustrating the coaction of the magnetic field and the electric field.

FIG. 4 is a plan view of the interrupter shown in FIG. 1 illustrating in more detail its magnetic structure.

FIG. 5 illustrates a modified form of my invention.

1 FIG. 6 illustrates another modified form of my invention.

FIG. 7 is a side elevation View of the electrodes of FIG. 1 illustrating in greater detail the configuration of the electric field between the electrodes.

FIG. 8 is a sectional view taken along the line 8-8 of FIG. 7.

FIG. 9 is a view similar to that of FIG. 4 illustrating a typical configuration of the magnetic field in the interrupter of FIG. 1.

Referring now to the vacuum-type interrupter of FIG. 1, there is shown a highly evacuated envelope 10 comprising a cylindrical casing 11 of insulating material and a pair of end caps 12 and 13 closing oii the end of the casing. The casing 11 is of an imperforate vacuum-tight construction and is joined to the end caps by means of suitable seals 14 forming a vacuum-tight connection between the end caps and the casing 11.

The normal pressure within the envelope in under static conditions is lower than 10' mm. of mercury and is preferably in the range of 10' to 10* mm. of mercury. As is well known, at these low pressures, the vacuum has a a very high dielectric strength because there are so few molecules of gas remaining in the envelope that electrons can travel across the various gaps between the high voltage parts of the interrupter with little probability of col liding with the gas molecules that are present. It is these collisions which are primarily responsible for ionization nd resultant electrical breakdown in gases at higher pressures. If pressures substantially higher than l mm. of mercury Were to be utilized, then at least some of the interrupters potential breakdown paths would be longer than the average distance which the electrons could travel without colliding with a gas molecule. This average dis tance is commonly called the mean free path. Only with pressures less than about mm. of mercury is there a reasonable assurance that the mean free path of an electron will be longer than the potential breakdown paths in the interrupter. It is only under this latter condition that one achi ves the high level of dielectric strength that is generally required in a commercial vacuum interrupter, and the present invention is therefore concerned with interrupters that rely upon pressures of less than about 10* mm. of mercury.

Located within the envelope 10 are a pair of separable contacts, or electrodes, 17 and 18 shown in their opencircuit position. These contacts are formed of a suitable non-rnagnetic conducting material such as copper. The upper contact 17 is a stationary contact suitably secured to a conductive rod 17a, which at its upper end is united to the upper end cap 12. The lower contact 13 is a movable contact suitably joined to a conductive operating rod 18a that is suitably mounted for vertical movement. The operating rod 18:! projects through an opening in the lower end cap 13, and a flexible metallic bellows 2% provides a seal about the rod 18a to allow for vertical movement of the rod 18a. without impairing the vacuum inside the envelope it). The bellows 21 is secured in sealing relationship at its respective opposite ends to the operating rod 18a and the end cap 13.

Coupled to the lower end of the operating rod 18a suitable actuating means (not shown) is provided which is capable of driving the movable contact 13 upwardly into engagement with the upper contact 17 in order to close the interrupter. The closed position of the movable contact 18 is indicated by the dotted lines of FIG. 1. The actuating means is also capable of returning the contact 18 to its illustrated solid-line position in order to open the interrupter. When the contact 18 is driven downwardly from its dotted-line closed position to open the interrupter, a circuit interrupting arc is established across the resulting inter-contact gap. Assuming that the circuit in which the interrupter is connected is an alternating current circuit, the arc maintains itself until about the time a natural current zero is reached, after which the arc is prevented from reigniting by the high dielectric strength of the vacuum, as will be explained in greater detail hereinafter.

The arc, though quickly extinguished, vaporizes some of the metal of the contacts. In order to prevent this metallic vapor from condensing upon the internal walls of the insulating casing 11, there is provided a metallic shield 25 which is of a generally tubular configuration and extends along the length of the insulating casing ll on opposite sides of the inter-contact gap. This shield 25 corresponds to a similarly designated shield shown and claimed in Patent No. 2,892,911, Crouch, assigned to the assignee of the present invention.

This shield 25 is electrically isolated from both of the contacts and from ground by means of the insulating casing 11. in this regard, the shield 25 is provided with a suitable supporting flange 26 that is positioned in sealed relationship between the two tubular components making up the casing 11. When an arc is established between the two

contacts

17 and 18, the metallic vapors liberated from the contacts by arcing travel outwardly from the arcing gap. The shield 25, which surrounds the arcing gap, provides cool metallic surfaces which act to inter cept and condense these metallic vapors before they can reach the casing 11. Thus the shield 25 acts to prevent the build-up of undesirable metallic coatings along the internal wall of the casing 11.

The interrupting process in a vacuum interrupter is dififerent in a number of fundamental respects from the interrupting process in a gas-type interrupter. One of these fundamental differences, and the one that I am particularly concerned with in the present application, is the difference in the mechanism by which electrical breakdowns take place due to the usual recovery voltage transient occurring immediately following the point at which current Zero is reached, To illustrate this difference, consider first the mechanism by which the recovery voltage transient initiates arc-reignition in a gas or liquidtype interrupter. As an aid in such consideration, referonce may be had to FIG. 2, which is a graphical representation of the current and voltage conditions occurring during the interruption of a typical inductive circuit. When the contacts of the interrupter are initially separated at a point I), an arc will be established, and current I will flow through the arc until a first current zero is reached at 0. At this instant, the arc vanishes, and the usual recovery voltage transient, which is designated R.V.T., rapidly builds up across the separated contacts. Assuming that no breakdown across the contacts occurs, this recovery voltage transient can reach a peak value considerably in excess of the normal system voltage, which is indicated by the curve V This is illustrated in FIG. 2, where the recovery voltage transient is shown rising to a peak value V and thereafter oscillating about the normal system voltage V in the usual manner.

In a gas type interrupter, electrons emitted from the cathode contact as the recovery voltage transient builds up are accelerated toward the anode contract by the electric field between the separated contacts. Because a gas or vapor is present in the inter-contact gap, these electrons can move only a very short distance before they collide with the gas or vapor atoms or molecules in the inter-contact gap. Such collisions will ionizc these gas or vapor articles, thus producing more electrons, which electrons in turn will be accelerated in the electric field, thereby colliding with and ionizing additional gas or vapor particles. In other words, in a gas interrupter, the recovery voltage transient initiates sort of an avalanche of electrons that is accompanied by ionization of the gas and vapor particles within the inter-contact gap. When this ionization process has reached a certain critical point, i.e., a point at which the ions reaching the cathode continuously create more and more electrons, breakdown occurs and the arc is thus reignited.

When a well-designed vacuum interrupter interrupting currents within its current rating, an entirely different set of conditions is present when the current reaches the current zero point. Although metallic vapor had been present in the inter-contact gap during arcing, most of this vapor has condensed or has been otherwise removed from the gap by the time the current zero is reached. My studies of this matter indicate that when the recovery voltage transient begins building up immediately following the point at which current zero is reached, the metallic vapor has almost completely disappeared from the gap and the original vacuum has been almost completely restored. Accordingly, there are no significant number of gas or vapor particles present to collide with the electrons which are emitted from the cathode as the recovery voltage transient builds up. Thus, the electric field developed by the recovery voltage transient accelerates most of these cathode emitted electrons across the inter-contact gap without their experiencing a single collision prior to their reaching the anode contact. This follows from the fact that the mean free path of an electron in a vacuum corresponding to that of the original vacuum is many times greater than the length of the total contact gap, as has been explained hereinabove. Having acquired considerable energy from the electric field by the time the anode is reached, the electrons bombard the anode with suflicient energy to vaporize some of the anode material. The effect of such vaporization is a decrease in the mean free path of the electrons traversing the inter-contact gap and a corresponding ionization of the vapor particles by the electrons. If this ionization process is allowed to continue to the abovedescribed critical point, a breakdown across the contacts will be initiated.

My invention contemplates turning back the cathodeemitted electrons to the cathode prior to their reaching the anode, and, in this way, preventing vaporization of the anode material by such electrons, thus preventing the breakdowns that had resulted from ionization of the vaporized anode material. I accomplish this result by providing a magnetic field that traverses the inter-contact gap from the instant that current zero is reached and, more specifically, a magnetic field that has a flux density B at least equal to and preferably exceeding a value of about:

webers per square meter, where:

m is the mass of an electron in kilograms and is equal to 9X 10* kg.;

E is the electric field intensity in volts per meter;

2 is the electronic charge in coulombs and is equal to 1.59 X 10-1 coulombs; and

s is the separation between the contacts in meters,

Simplfying this equation by substituting the constants set forth above, B should equal at least about:

In calculating the required flux density, the separation between the contacts should be the length of the intercontact gap that will be established within 120 electrical degrees after the contacts part (for reasons soon to be explained), and B should be the electric field intensity based upon a gap of this particular length and upon a voltage equal to the peak value of the recovery voltage transient. For example, in one typical vacuum interrupter, a contact separation of one centimeter is attained within 120 degrees after the contacts part. Assuming that it is desired to prevent this inter-contact gap from breaking down in response to a recovery voltage transient of 100 kv. peak value, then the flux density should be at least equal to about:

100,000 volts 1 X 100 era/meter 1 cm.

-s 33x10 0.1 meter A magnetic field having a minimum flux density of at least this value and having its lines of force extending generally perpendicular to the electric lines of force extending generally perpendicular to the electric lines of force will cause the cathode-emitted electrons to follow a cycloidal path that will return the electrons to the cathode before they can reach the anode. This is illustrated in the perspective view of FIG. 3 where the electric lines of force extending between the contact 18 (assumed to be the cathode) and the contact 17 (assumed to be the anode) are indicated at 30. The magnetic lines of force are indicated at 32, and a cathode-emitted electron at 34. The electron 34 instead of traveling completely across the gap of the anode will follow the cycloidal path indicated at 36. As will be apparent from FIG. 3, this path will return the electron to the cathode 18 where it will be recaptured. In this manner, the electron 34 is prevented from bombarding the anode 17 and thus vaporizing anode material. Most other cathode-emitted electrons are forced to follow a similarly-shaped path, and, thus, the gap is maintained essentially free of ionizable vapors that would have resulted from bombardment of the anode by the cathode-emitted electrons. In this way, the inter-contact gap is prevented from being broken down by the recovery voltage transient. It is to be understood that, as a general rule, the more the magnetic field intensity exceeds the minimum value set forth hereinabove, the shorter is the radius of the cycloidal path 36, and, hence, the lower is the probability that the cathode-emitted electrons will reach the anode or will have ionizing collisions in the gap.

The reason that the contact separation s used in the above equation is based upon the gap length attained after 120 degrees of contact separation is that a vacuum interrupter should be able to interrupt its circuit at the first current zero after the contacts part, providing the contacts have had about 120 degrees in which to separate before current zero is reached. If an appreciably shorter period of time has elapsed before the first current zero, then continued current flow through the next half cycle can ordinarily be tolerated. If the circuit application is such that current flow on the next half cycle can be tolerated only if the arcing interval during the previous half cycle had been less than electrical degrees or some other predetermined number of degrees, then the contact separation used in the equation should be based upon the gap length attained after this predetermined number of degrees. Ordinarilyy, however, a contact separation based on electrical degrees or slightly more is sufficient. In most vacuum interrupters, including the disclosed embodiments, the full contact separation will have been attained within 120 electrical degrees.

In arriving at the minimum flux density set forth hereinabove, it has been assumed that both the electric field and the magnetic field across the intercontact gap are generally uniform. Slight non-uniformities in either of these fields will not appreciably change this minimum flux density value (providing the field intensity E that is used is the field intensity in the region of the gap having the highest field intensity). In those devices in which either of these fields is highly non-uniform, then appreciable changes in this minimum value will become necessary, the extent of these changes depending primarily upon the degree of non-uniformity in these fields. It is diificult to mathematically establish minimum values which would be applicable to widely different highly nonuniform fields conditions, but it is a relatively simple matter to establish by tests the minimum value of flux density needed across a given gap in order to attain the desired result of turning the electrons back to the cathode. For example, in an interrupter Where full contact separation is anticipated within 120 electrical degrees, the interrupter is tested in its fully-open position. Across the fully-open inter-contact gap a magnetic field of a given intensity and configuration is established, and an alternating test voltage is applied in incrementally increasing steps. If the test voltage breaks down the inter-contact gap at a value equalling or below the static breakdown voltage in the absence of a magnetic field, this is an indication that the magnetic field is below the value of minimum density needed to return electrons to the cathode. This series of steps is then repeated with magnetic fields of the same configuration having progressively higher densities until a magnetic field intensity is arrived at sutficient to produce a significant increase in the breakdown voltage as compared to the breakdown voltage without the magnetic field. This value is the minimum magnetic field intensity needed for this particular gap with this particular magnetic field configuration.

The magnetic field 32 is produced in the embodiment of my invention shown in FIG. 1 by means of a U-shaped electromagnet having its opposite pole pieces 40 and 41 disposed at diametrically opposed locations relative to the gap. The usual coil 43 of the electromagnet is shown in the sectional view of FIG. 4 energized by direct current from an independent source so that the required magnetic field is present when current flowing between the

contacts

17 and 18 reaches its current zero point. Although I have shown an electromagnet, it is to be understood that the required magnetic field could instead be obtained by a suitable permanent magnet, or by means of an alternating-current-energized electromagnet, providing the flux produced by the magnet is sufiiciently out of phase with the main power current to provide a magnetic field of the required density at current zero.

Assuming uniform magnetic and electric fields, a magnetic field with a density substantially below that set forth by the above equation would be incapable of returning the cathode-emitted electrons to the cathode and would merely act to lengthen the path that the electron would be required to follow in order to reach the anode. This lengthening of the path would actually result in a higher probability of collisions between the electron and any gas or vapor particles that might be present and, as a result, would actually increase the probability of a breakdown, thus, in some cases, actually lowering the voltage-interrupting capacity of the interrupter.

It should be apparent from the above comparison of the interrupting process in a gas and in a vacuum that even the slightest travel of a cathode-emitted electron through the gas or vapor filled inter-contact gap of a gas interrupter would almost certainly result in ionizing collisions. Even if the electrons could be caused to follow a path such as 36 of FIG. 3, the mean free path in a gas is so short that it would be virtually impossible to return the electrons to the cathode before ionizing collisions with gas particles had occurred. While it is true that the presence of a magnetic field before current zero in a gas interrupter aids interruption by moving and cooling the arc and thus lessening the amount of contact vapors generated, little is gained from the presence of a magnetic field at current zero in a gas interrupter inasmuch as the action of the magnetic field at this instant can have little effect on the number of collisions that a cathode-emitted electron will have with the gas or vapor particles that are present in the inter-contact gap.

In a gas interrupter, the voltage interrupting ability V of a circuit interrupting gap is only a small fraction of the voltage V that the gap could have withstood in the absence of prior arcing. For example, in hydrogen, which is one of the best interrupting gases known, a one inch gap at 300 psi. can withstand a voltage V of about 200 kv., whereas after arcing for one half cycle at 400 amperes, its voltage interrupting capacity V is only kv. under typical recovery voltage conditions following current zero, or a ratio of V to V of about 1 to 20. In contrast, this ratio of V to V in a well-designed vacuum interrupter frequently is very close to a value of 1 and is greater than 1 to 2 even for high currents many thousands of amperes in magnitude. By using a magnetic field in a gas interrupter to move the arc, the voltage interrupting capacity V can be increased as a result of increased cooling of the arc and decreased vaporization of the contacts, but even with such improvements in voltage interrupting ability, the resulting voltage interrupting ability is still only a very small fraction of the no-prior-arcing breakdown voltage V of the gap. In contrast to this small fraction, the magnetic field of my invention enables me to interrupt with my vacuum interrupter voltage V not only equal to but even in excess of the no-prior-arcing voltage V in the absence of the magnetic field.

In order that the magnetic field of my invention be capable of returning cathode-emitted electrons to the cathode without a significant number of ionizing collisions, it is necessary that the inter-contact gap be essentially free of gas or vapor particles at the time the recovery voltage transient begins building up across the gap. For low current interruptions, e.g., up to a few hundred amperes, this freedom from gas and vapor particles at current zero is attained with very litle difficulty. For example, in interrupting several hundred amperes even with a pair of simple butt contacts and no magnetic field, my tests indicate that the gap is essentially free of vapors during the time the recovery voltage transient is building up. For higher currents, however, special arrangements must be provided in order to provide reasonable assurance that the inter-contact gap will be essentially free of metallic vapors. One way of providing such assurance for moderately high currents is to impel the arc to a location at which the voltage gradient across the gap is relatively low and to force the arcing products away from the portion of the gap where the voltage gradient is high. As a result, when the recovery voltage transient builds up after the current zero point is reached, the region of highest voltage gradient is essentially free of vapors and is capable of withstanding the recovery voltage transient without breakdown. The region that has residual vapors therein is subject to a relatively low voltage gradient and is able to withstand the recovery voltage transient in spite of the residual vapor particles.

The above principle is utilized to some extent in the simple butt contact arrangement of FIG. 1. In this regard, the magnetic field 32 acts to force any are established between the contacts from the central region of the contacts, where the highest voltage gradient is present, to the outer peripheral region of the contacts, where the lowest voltage gradient is present, and to impel the arcing products radially outward from the outer peripheral region so as to lessen contamination of the central or high volt-age gradient region with the vapors.

FIG. 5 shows an arrangement which utilizes the abovedescribed principle to a much greater extent. In this figure, each of the contacts is provided with a centrally disposed contact making surface 44 and an annular arerunning surface 42 surrounding the contact making surface 44. The annular arc-running surfaces 42 diverge with respect to each other at their radially outer regions so that when the contacts are separated, a much longer gap is present at the outer periphery of the arc-running surfaces 42 than at the central contact-making region 44. Any are established by separation of the contacts is driven from the central region 44 to the outer peripheral region 42 of the contacts by the magnetic field 32, and the arcing products are impelled radially outward from the outer peripheral region by the magnetic field. This results in the central region being essentially free from metallic vapors when the current zero point is reached, thus rendering the magnetic field more efiTective in turning back electrons without ionizing collisions in this region of highest voltage gradient. The outer peripheral region is subject to a much lower voltage gradient and is able to withstand the recovery voltage transient in spite of any residual vapor particles that might still be present. The portion of the magnetic field in the outer peripheral region, assuming it is sufiiciently intense, also helps to prevent breakdowns in this particular region by reason of its ability to return cathode-emitted electrons to the cathode before they can reach the anode and before a significant number of ionizing collisions can occur.

FIG. 6 shows another embodiment of my invention in which a somewhat different principle is relied upon to assure that the arcing gap is essentially free of metallic vapors at the time current zero is reached so that the magnetic field can return cathode-emitted electrons to the cathode without ionizing collisions. More specifically, in this embodiment the arc is continuously driven at high speed over the arc-running surfaces for the interrupter so as to minimize the amount of vapor that is generated from the arc-running surfaces and so as to increase the degree of diffusion of the vapor that is generated. This 9 will be pointed out in greater detail as the description of the embodiment shown in FIG. 6 proceeds.

Referring now to FIG. 6, the illustrated interrupter is shown as comprising an evacuated envelope 50- formed from a pair of aligned tubular members 52, 53 of insulating material and a pair of

end caps

54 and 55 disposed at the ends of the envelope. Suitable seals 56 are provided between the tubular members and between the end caps, and suitable seals 59 are provided between the tubular members themselves so as to provide vacuumtight connections between these parts. The envelope 50, like the envelope 10 of FIG. 1, is evacuated to a pressure of at least as low as 10- mm. of mercury.

Disposed within the vacuum chamber are two

rod contacts

57 and 58 shown in their open-circuit position. The rod contact 57 is a stationary contact or electrode and is electrically connected at its respective opposite ends to the end caps 54 and 55. The rod contact 58 is a movable contact which engages a side wall of the stationary electrode 57, as is shown by the dotted-lines, and is movable in a plane perpendicular to the axis of the stationary electrode 57. The movable contact 58 projects through an opening in the side wall of the lower tubular insulator 53, and a flexible bellows 60 provides a seal about the movable rod 58 to allow for horizontal movement of the rod 58 without impairing the vacuum inside the envelope 50. The bellows 60- is secured in sealing relationship at its respective opposite ends to the contact rod 58 and to a tubular, laterally-projecting portion 62 of the lower insulating cylinder 53.

Surrounding the stationary rod contact 57 is a tubular electrode 65 of a suitable conductive material such as copper. This arc-runner or electrode 65 is supported within the vacuum chamber by means of an annular conductive disc 66 secured to the outer periphery of the tubular electrode 65 and projecting through the wall of the envelope ft in sealed relationship to the walls of the envelope. The disc 66 and hence the tubular electrode 65 are conductively connected to the movable contact 58 by suitable means such as a conductive braid 68 disposed externally of the envelope 50.

Preferably the two

end caps

54 and 55 are connected together externally of the envelope so that power can flow through either of the end caps into the stationary electrode 57. From the stationary electrode 57, the power path is through the movable contact rod 58 to the exterior of the envelope 50.

Coupled to the outer end of the contact rod 58, suitable actuating means (not shown) is provided which is capable of driving the movable contact 53 to the left into its dotted-line position in order to close the interrupter and is also capable of returning the movable contact to its solid line position in order to open the interrupter.

When the contact rod 58 is driven to the right from its dotted-line closed position into its solid-line open position, a circuit-interrupting arc is established across the resulting inter-contact gap. The right-hand terminal of this are is quickly transferred from the movable contact 58 onto the tubular arc-running electrode 65 by the action of a magnetic field 70, soon to be described in greater detail, which has its lines of force 72 extending transversely of the cylindrical vacuum gap between the

electrodes

57 and 65. After the arc is transferred to the tubular arc-running electrode 65, the magnetic field interacts with the magnetic field surrounding the arc to cause one terminal of the arc to move repetitively about the inner periphery of the tubular arc-running electrode 65 about the outer periphery of the inner electrode 57. In other words, the are between the

electrodes

57 and 65 revolves about the longitudinal axis of the inner electrode 57.

The magnetic field 79 is produced by means of a coil 74 surrounding the envelope 50 and extending longitudinally thereof beyond the opposite ends of the tubular arc-running electrode 65. The coil 74 is provided with suitable insulation (such as 75) which precludes external breakdowns to the coil. The magnetic field 7 0 generated by this coil 74 has most of its lines of force extending over loop-shaped paths linked with the coil and extend ing along the longitudinal axis of the central electrode 57 within the window of the coil 74. Many of these longitudinally-extending lines of force traverse the cylindrical vacuum gap between the

electrodes

57 and 65, as is illustrated in FIG. 6. Near the longitudinal ends of the coil these magnetic lines of force fringe radially outward and then extend around the longitudinal ends of the coil to the outer periphery of the coil. Adjacent the outer periphery of the coil, most of the lines of force extend generally parallel to the contact rod 57.

For the same reasons as pointed out in connection with PEG. 1, the minimum intensity of the magnetic field 70 in the cylindrical vacuum gap region between the

electrodes

57 and 65 is at least equal to 3.4x m /g webers per square meters, where s is the length of the gap between the

electrodes

57 and 65 in meters measured in the region where the electric field intensity is the highest and E is the electric field intensity in this region, specifically :in the portion of this region near the inner electrode where electric field intensity is the highest. E is expressed in volts per meter based upon the peak value of the recovery voltage transient. A magnetic field of at least this intensity will cause electrons emitted from the electrode 57, when it serves as the cathode, to be returned to the electrode 57 and recaptured by the electrode 57 before they can reach the anode electrode 65, in the same general manner as pointed out in connection with the arrangement of FIG. 1. Likewise, the intense magnetic field 70 will cause electrons emitted from the electrode 65, when it serves as the cathode, to be returned to the cathodic electrode 65 before they can reach the anode 57. The electric field in the interrupter of FIG. 6, though not uniform, is only slightly non-uniform, rather than highly non-uniform, and thus the above equation can be utilized for determining the approximate minimum flux density needed to return electrons to the oathode before they can reach the anode.

For reasons which will soon. appear more clearly, the coil 74 has more turns per unit of length at its longitudinal ends than at its central region. This results in a non-uniform magnetic field near the ends of the arerunning electrode 65. This non-uniformity may be explained by thinking of the extra end turns as generating local magnetic lines of force 76 extending only about the end turns. These local magnetic lines of force coact with the main magnetic lines of force to produce a magnetic field configuration in which the magnetic lines of force converge toward the central electrode 57 in the region adjacent the ends of the tubular electrode 65, as is shown at 7-8 in FIG. 6.

This converging configuration of the magnetic field at the ends of the tubular electrode 65 helps to insure that electrons emitted from the ends of the tubular electrode 65, when it is serving as a cathode, will be returned to the tubular electrode 65 rather than being forced to follow a path leading to the end caps 54 or 55 or to the insulators 52 or 53. This will be explained by first referring to FIG. 7 which is a side elevational view of the

contacts

17 and 18 of FIG. 1 showing the electric lines of force 30 between the

contacts

17 and 18 when they are separated. It will be noted from FIG. 7 that all of the electric lines of force starting from the fiat centrally disposed surfaces of the contacts are at right angles to the magnetic field. In other Words, the electric field in this particular region has practically no component along the magnetic lines of force. However, this is not true in those contact regions which are widely removed from the central flat region. This point is illustrated in FIG. 8

1 l which is a cross-sectional view along the line 8-8 of FIG. 7. Here, it can be seen that the electric lines of force at A and B are still perpendicular to the magnetic lines of force, but at C and D the electric lines of force are actually parallel to the magnetic lines of force. Thus, an electron starting from a region such as C or D will be accelerated along the magnetic lines of force. After this electron is moved along the electric line of force for a short distance, it begins to feel the influence of the component of the electric field which is at right angles to the magnetic field. The inter-action between these two fields will then cause the electron to begin to gyrate. However, by this time, the electron has already attained some velocity in the direction of the magnetic field, and in a magnetic field assumed to be uniform there would be nothing to change this velocity. Thus, if the magnetic field of the device of FIG. 1 were assumed to be uniform, the electron would eventually hit the shield 25 de icted in FIG. 1. This bombardment of the shield 25 by the electrons emitted from the particular electrode regions vertically aligned with the lines 6 and D can vaporize the material of the shield or can release gases absorbed Within the shield material, thus leading to a breakdown between the shield and one of the contacts or electrodes. If the shield is of a refractory material that is well degassed, such breakdown will occur only after the voltage applied across the contacts is very much in excess of the voltage that the gap could have withstood without the magnetic field.

In the arrangement of FIG. 1, the number of electrons reaching the shield 25 can be materially reduced by using a non-uniform magnetic field instead of a uniform field. Such a non-uniform field is shown in FIG. 9, which is a view taken along a line corresponding to the line 88 of FIG. 7. This field has, converging magnetic lines of force on opposite sides of the contact 17, but in the immediate region of the center line 80, the lines of force extend in parallel straight line paths. In the region Where there are converging lines of force, there is actually a component of force which is normal to the direction of the main field. If the magnetic field is sutficiently intense, this component is capable of changing the direction of the velocity of the electron moving along the main direction of the magnetic field. thus, returning the electron to the contact 17. The action of a converging magnetic field in changing the direction of the velocity of the electron is the well-known magnetic mirror effect by which a converging magnetic field acts to reflect electrons like a mirror. However, in the central region adjacent the center line 80, where the field is more uniform and there is no appreciable convergence of the magnetic lines of force, there is no appreciable magnetic mirror effect. Electrons emitted from this region are able to maintain their velocity parallel to the main magnetic field and finally to bombard the shield 25. Thus, the non-uniform magnetic field of FIG. 9, though it is more effective than a uniform magnetic field in preventing electrons from bombarding the shield 25, is still incapable of preventing those electrons emitted from the highly restricted central region, where the field is uniform, from reaching the shield 25.

This difliculty is overcome in the arrangement of FIG. 6 inasmuch as there is no significant region where the magnetic field is parallel to the electric field at the cathode surface. Thus, it is much more difiicult for electrons to escape from either of the

electrodes

57 or 65 to the adjacent parts of the interrupter. For example, when the electrode 57 is serving as the cathode, the transverse magnetic field merely returns the electrons toward the central axis of the interrupter where they will be recaptured by the cathodic electrode 57 without ever having hit the end caps 54 or 55, the insulators 52 or 53, or the opposite electrode 65. When the cylindrical electrode 65 is the cathode, the magnetic field at 78 prevents bombardment of the end caps 54 and 55 and the insulators 52 and 53 by electrons emitted from the ends of the tubular electrode 65. To illustrate this action in more detail reference may be had to the region at the lower right hand corner of the interrupter of FIG. 6. In this particular region, the electric lines of force are depicted at 79 and the magnetic lines of force at 72. It can be seen from this figure that the magnetic lines of force 72 have a component perpendicular to the electric lines of force 79 at the surface of the electrode 65 and, more specifically, a perpendicular component acting toward the center of the tubular electrode 65. These magnetic lines of force therefore operate by the magnetic mirror effect to cause electrons emitted in a downward direction from the outwardly flared inner-peripheral region at the end of the electrode 65 to follow a path which curves toward the central rod 57 and then extends upwardly and back to the tubular electrode 65. By forcing the electrons to follow such a path, they are prevented from bombarding the end cap 55 and the insulating tube 53. As explained hereinabove, such electron bombardment tends to release gases and vapors from the bombarded parts and therefore to lead to breakdown to these parts. By substantially preventing such bombardment, breakdowns to these parts are prevented with an even higher degree of eficiency than in the arrangement of FIG. 1.

It is to be understood that in order to produce the magnetic-mirror effect referred to hereinabove the field intensity must be considerably in excess of the minimum field intensity expressed by the equation inasmuch as only a component of the reflecting magnetic force lines is normal to the path of electron movement. Thus, to effectively utilize the magnetic-mirror effect for the end regions of the tubular electrode 65 of FIG. 6, the magnetic field intensity should be appreciably in excess of the value derived from the equation. For an interrupter such as shown in FIG. 6, this higher value can be readily determined by subjecting the interrupter of FIG. 6, while fully-open, to a series of simple tests involving increasing the flux density to a value which precludes breakdowns from the ends of the tubular electrode 65 for the voltage that it is desired to withstand. Still higher flux densities would produce an even more pronounced magnetic-mirror effect capable of raising the breakdown voltage to a still higher level.

It is to be understood that, in constructing the disclosed vacuum interrupters, the various parts inside the vacuum envelope should be freed of sorbed gases and other contaminants sufiiciently to avoid harmful impairment of the vacuum by any such gases or contaminants during operation of the interrupter. Conventional vacuum processing techniques can be used for attaining this desired end.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects, and I therefore intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An alternating current circuit interrupter of the vacuum type comprising an envelope evacuated to a pressure lower than 10- mm. of mercury, a pair of separable electrodes disposed within said evacuated envelope, means for separating said electrodes to establish an interelectrode gap across which an alternating current circuit interrupting arc is formed and across which a recovery voltage transient having a peak value of at least 36 kilovolts is developed immediately after the current zero point is reached, magnetic means for providing during the build-up of said recovery voltage transient a magnetic field having its magnetic lines of force extending transversely with respect to the electric lines of force between the separated electrodes, the minimum flux density of said 1.3 a magnetic field being sufliciently high at the time a recovery voltage transient builds up at current zero to cause electrons emitted from the cathodic electrode to be returned to said cathodic electrode without reaching the anodic electrode, the magnetic lines of force of said magnetic field being so oriented that the electric lines of force originating from substantially all potential breakdown surfaces of said electrodes are intersected adjacent the surfaces of each electrode by magnetic lines of force that have a component acting transverse to the electric lines of force and in a direction to return electrons emitted from the cathodic electrode to the cathodic electrode before said electrons can bombard adjacent parts of the interrupter.

2. The interrupter of claim 1 in which the intensity of said magnetic field is sufficiently high to return electrons to the cathodic electrode by the magnetic-mirror effect in those regions adjacent the cathode where the magnetic lines of force are disposed at appreciably less than ninety degrees relative to the intersected electric lines of force.

3. The vacuum-type circuit interrupter of claim 1 in which the minimum flux density is sufficiently high to enable the interrupter at a given electrode separation to interrupt an amount of voltage exceeding the amount of voltage that the inter-electrode gap at a corresponding separation can withstand Without prior arcing in the absence of said magnetic field.

4. The interrupter of claim 1 in which said inter-electrode gap includes a region of highest electric field intensity and a region of appreciably lower electric field intensity, means for rendering said region of highest electric field intensity substantially free of electrode vapor at the instant that a recovery voltage transient builds up at current zero comprising means for magnetically propelling said circuit-interrupting arc to said region of lower electric field intensity and for magnetically forcing the arcing products away from said region of highest field intensity.

5. An alternating-current circuit interrupter of the vacuum-type comprising an envelope evacuated to a pressure lower than 10- mm. of mercury, a pair of electrodes within said evacuated envelope that are spaced-apart during a circuit interrupting operation to provide an interelectrode gap across which circuit-interrupting arcs are developed and across which a recovery voltage transient having a peak value of at least 36 kilovolts is developed immediately after the current Zero point is reached, magnetic means for providing during the build-up of said recovery voltage transient a magnetic field having its magnetic lines of force extending transversely with respect to the electric lines of force between the spacedapart electrodes, the minimum flux density of said magnetic field in the interelectrode gap when the recovery votlage transient builds up being at least as high as about:

3.4x ro /i webers per square meter, where s is the length of the interelectrode gap in meters in the gap region of highest field intensity 120 electrical degrees after a circuit-interrupting arc is first established, and E is the electric field intensity in volts per meter in the portion of said gap region where the field intensity is the highest based upon a voltage equal to the peak value of the recovery voltage transient, the magnetic lines of force of said magnetic field being so oriented that the electric lines of force originating from substantially all potential breakdown surfaces of said electrodes are intersected adjacent the surfaces of each electrode by magnetic lines of force that have a component acting transverse to the electric lines of force and in a. direction to return electrons emitted from the cathodic electrode to the cathodic electrode before said electrons can bombard adjacent parts of the interrupter.

v 6. The interrupter of claim 5 in which said interrupter further includes restricted cathode regions where the magnetic lines of force are disposed at appreciably less than ninety degrees relative to the electric lines of force intersected thereby, the magnetic field intensity in said re stricted cathode regions appreciably exceeding the minimum flux density set forth in claim 5 and being sufficient to return electrons emitted from said restricted cathode regions to the cathode by the magnetic-mirror effect.

7. In the interrupter of claim 5, means for rendering said region of highest electric field intensity substantially free of electrode vapor at the instant that a recovery voltage transient builds up at current zero comprising means for magnetically propelling said circuit interrupting arc to a region of relatively low electric field intensity and for magnetically forcing the arcing products away from said region of highest field intensity.

8. In the interrupter of claim 5, means for rendering said region of highest electric field intensity substantially free of electrode vapor at the instant that a recovery voltage transient builds up at current zero comprising means for propelling the terminals of a circuit-interrupting are at high speed over said electrodes so as to lessen the amount of vapors generated by said circuit-interrupting arc.

9. The interrupter of claim 5 in which said electrodes comprises arc-running surfaces spaced apart at a time electrical degrees after a circuit-interrupting arc is first established by a substantially greater distance in one region than in the region where the arc is initiated, means for driving said circuit-interrupting are from the arcinitiation region into said one region and means for propelling the arcing products away from said arc-initiation region.

10. An alternating-current circuit interrupter of the vacuum-type comprising an envelope evacuated to a pres sure lower than 10- mm. of mercury, a rod-shaped electrode disposed within said envelope, a tubular electrode disposed within said envelope and separated from said rod-shaped electrode by a cylindrical vacuum gap, means for establishing a circuit-interrupting are between said electrodes comprising a contact rod extending in a radial direction through one Wall of said tubular electrode and mounted for movement into and out of contact-making engagement with a portion of said rod-shaped electrode, magnetic means for producing a magnetic field that has magnetic lines of force extending longitudinally of said tubular electrode and traversing said cylindrical vacuum gap so as to intersect the electric lines of force between said electrodes, said magnetic field acting to revolve any are developed across said vacuum gap about said rodshaped electrode, the minimum flux density of said magnetic field in the inter-electrode gap when a recovery voltage transient builds up after the current zero point is reached being at least as high as about:

3.4x ro webers per square meter, where s is the length of the inter-electrode gap in meters in the region of highest field intensity, and E is the electric field intensity in volts per meter in said region of highest field intensity near said rod-shaped electrode based upon a voltage equal to the peak value of the recovery voltage transient.

11. The interrupter of claim 10 in which said magnetic means comprises means for rendering said magnetic field non-uniform near the ends of said tubular electrode, the magnetic lines of force in said region of non-uniformity being so oriented that substantially all potential breakdown services on the ends of said tubular electrode are' intersected adjacent the surface of said tubular electrode by magnetic lines of force that have a component acting in a direction to return electrons emitted from the ends of the tubular electrode to the tubular electrode before said electrons can bombard other parts of said interrupter situated near the ends of said tubular electrode.

12. The interrupter of claim 10 in which said magnetic means comprises means for rendering said magnetic field non-uniform near the ends of said tubular electrode, the magnetic lines of force in said region of non-uniformity being so oriented that substantially all potential breakdown services on the ends of said tubular electrode are intersected adjacent the surface of said tubular electrode by magnetic lines of force that have a component acting in a direction to return electrons emitted from the ends of the tubular electrode to the tubular electrode before said electrons can bombard other parts of said interrupter situated near the ends of said tubular electrode, the magnetic field intensity in the region at the ends of said tubular electrode appreciably exceeding the minimum flux density defined in claim 10 and being suflicient to return 15 electrons emitted from said tubular electrode to said tubular electrode by the magnetic-mirror effect.

References Cited in the file of this patent UNITED STATES PATENTS Re. 21,087 Rankin May 16, 1939 1,836,725 Proctor Dec. 15, 1931 2,027,836 Rankin et a1. Jan. 14, 1936 10 2,725,446 Slepian Nov. 29, 1955 2,864,998 Lee Dec. 16, 1958 OTHER REFERENCES Publication: Soviet Physics--Technical Physics, vol. 2, No. 5, pp. 909912.

Claims

1. AN ALTERNATING CURRENT CIRCUIT INTERRUPTER OF THE VACUUM TYPE COMPRISING AN ENVELOPE EVACUATED TO A PRESSURE LOWER THAN 10-4MM. OF MERCURY, A PAIR OF SEPARABLE ELECTRODES DISPOSED WITHIN SAID EVACUATED ENVELOPE, MEANS FOR SEPARATING SAID ELECTRODES TO ESTABLISH AN INTERELECTRODE GAP ACROSS WHICH AN ALTERNATING CURRENT CIRCUIT INTERRUPTING ARC IS FORMED AND ACROSS WHICH A RECOVERY VOLTAGE TRANSIENT HAVING A PEAK VALUE OF AT LEAST 36 KILOVOLTS IS DEVELOPED IMMEDIATELY AFTER THE CURRENT ZERO POINT IS REACHED, MAGNETIC MEANS FOR PROVIDING DURING THE BUILD-UP OF SAID RECOVERY VOLTAGE TRANSIENT A MAGNETIC FIELD HAVING ITS MAGNETIC LINES OF FORCE EXTENDING TRANSVERSELY WITH RESPECT TO THE ELECTRIC LINES OF FORCE BETWEEN THE SEPARATED ELECTRODES, THE MINIMUM FLUX DENSITY OF SAID