US2594890A - Contact protection arrangement - Google Patents

Description

April 29, 1952 w. B. ELLWOOD 2,

CONTACT PROTECTION ARRANGEMENT Filed Aug. 16, 1950 lNl/E/VTUP W B. ELLWOOD Patented Apr. 29, 1952 Walter B.'Ellwood, NewYork,N. Y., assignor to Bell Telephone Laboratories;Incorporated, New York,- N 'Y.,- a corporation of New York Application August 16, 1950, .SerialNo. 179,817

(CL 175-4294) I v "8-Claims. 1 This invention relates to electrical make-andbreakcontacts, andv more particularly to circuit arrangements for "protecting said contactsfrom deleteriouserosion caused by electrical arcing and/or energy dissipationduring contact operations.

Contactsofthe'type which make and break energized electrical circuits-develop. anumber of faults in service. For example, the repeatedmalb ing and breaking of a circuit with current flowing therethrough often results in' irregular. arcing attended withtransfer of contact metal from one cooperating 1 contact to the other, with consequent .pitting of one contact surface and building up of projections -on the other. Also, the continued transfer of material from one-contactto another often brings about a. localization of the contact area witha resultantincrease in current density at such-restrictedarea of contact. Thislocalization condition rendersmore pronounced the deteriorating-action cf the contactarc'ing. The transfer ofmaterialunder these circumstances sometimes becomes so great that a sufficient amount-thereof-is built up on one of the makeand-break contactstopreventthebreaking of the circuit by iseparation of the contacts.

In addition, arcing often forms thinfilms of oxides brother compounds on contact faces thereby increasing thecontact resistance which in turn tendsto aggravatethearc creating.con

-dit-ions until finally the contacts burn away or will otherwise no longer pass current.

In order to alleviate-the eifects of arcing there exist in the prior art several circuit arrangements for-minimizing contact erosion. Various-resistorcapacitornetworks have been used; however, in

additionto being space consuming and difficult to install in existing plant, such arrangements haveusually proved to be ineffective on both the .make and-break contactoperations. One such arrangement, which has beenwidelyused, whereina series resistor-capacitor subcombination shunted" a contact -tobe protected proved to be very effective on the-break contact operation but tended to be harmful on the make contact operation. 'For complete contact protection this circuit arrangementwas preferably employed with additional components which were to minimize contact arcing on themake contact operation.

.As the amplitude of the vtransientsoccurring atcontacts openingand nosing circuits are inf'luencecltoagreat extent by both the. inductance and the distributed capacitance to ground of the conductors connected v to the contacts, and as alarge. part of the-energy in these. transients wiring connected thereto. 'ness of "such choke coils was demonstrated in heated by copper.

-is-atfrequencies-about 500,000 cycles per second, it is obvious that modification of the high frequency or characteristic impedance of the-conductors connectinga contact to its associated circut "will affect the arcing created and theenergy dissipated at the contacts during make-andbreak operations.

Ithas long been knownin the art that arcing occurring during contact operations can be minimized or-eliminated by'connecting an inductor which has a high impedance at contact are transient frequencies in series with the contact energizing circuit at a point close to said con- :tact. *In'early experiments high frequency choke coils" were used. to eliminate arcing. For example, in 1918 it was found that the radio frequency interference radiated by a make-and-break spark ignition system-on a submarine chaser could be greatlyreduced by inserting an iron core-single layerchoke coil between the spark points and the In 1935 the usefulreducing erosion oftelephone relay contacts.

In 1938 a choke coil was suggested which comprised a permalloy plated wire wound on a permalloy shell of a relay structure and in the discussions of thisdevice, the usefulness of permalloy plated or iron-wire as conductors in lieu of the usual copper was debated. It was believed that the self conductance of permalloy plated wire or solid iron wire was sufficient to prevent arcing without the inclusion of a lumped inductor close to the contacts. Oscillographic observations showed little difference between'the voltage transients of contacts connected to their load by iron or permalloy-plated wire as com- .pare'd'to copper wire butrevealed that the current surges were'much weaker'than in the case "of contacts connected bycopper wire. Life tests indicated'that differences in surge characteristics gave corresponding differences in erosion and it'was conclusively shown that the'erosion was distinctly less on the contacts connected by iron and permalloyplated'wire than on those con- During these tests it was noticed that the contacts connected by copper wire could easily be distinguished from those con- "iiected by iron or permalloy plated wire as the "arc was distinctly brighter with the former.

Attempts were made to eliminate the difficulties and ex ense encountered in using the afore- ,-mentioned iron and/or permalloy plated wire in contact circuitzarrangements. Iron wire introduced an ,objectionably high direct-current resistancein many of the relay circuits connected therewith and also produced small iron particles due to filing at the time of making connection therewith which lodged in contact air-gaps. Whereas, although permalloy coated copper had a satisfactory resistance value, the plating of copper with permalloy was expensive and also the permalloy coating was difficult to make soldered connections to. Furthermore, a length of several feet of iron or permalloy plated wire was required to effectively limit contact arcing. To overcome these difficulties the usual connecting copper conductors were passedthrough permalloy beads or pieces of permalloy tubing adjacent the make-and-break contacts so that they provided a lumped series inductance near said contacts. However, it was found that the eddy currents generated at the high contact arcing transient frequencies created a magnetic skin effect on the surface of the beads or tubing. As a consequence, the magnetic lines of flux in the beads created by the transients tended to concentrate on the surface of the bead or the tubing and the inner portion of the bead or tubing was ineffectual for the purpose of adding to the series inductive effect necessary to minimize arcing or energy dissipation at the contacts. Therefore, beads or tubing of ordinary magnetic materials of a practical or convenient size are not effective as contact protection due to their small value of inductance.

Accordingly, it is an object and feature of this invention to minimize contact erosion by employing one or more bead type inductors having sufficient inductance at the transient frequencies present in contact arcs to minimize or eliminate contact arcing.

Another object and feature of this invention is a bead-type inductor which is small in size,.economical in cost, and which presents suflicient inductance and effective alternating-current resistance in a transient make-and-break contact circuit to minimize or eliminate arcing on both the make-and-break contact operations without increasing the direct-current resistance of said circuit.

To fulfill the objects of this invention a recently discovered magnetic material called ferrite in the form of a bead or the like surrounds'a conductor connecting to a contact of a relay or switch at or near the point where the conductor connects to the contact. This arrangement performs the function of imparting to the surrounded portion of conductor a high impedance at high frequencies so as to slow down and dissipate the discharge of the distributed conductor capacitance on a make contact operation, and during a break contact operation the bead inserts a dissipative resistance component to the contact connecting conductor 0 that a large'part of the stored energy of the conductor which causes arcing is dissipated in the loading bead instead of the contact.

The use of ferrite beads for contact protection is advantageous because this material possesses the propertie of high permeability and high electrical resistivity which provides a high effective inductance without lamination at frequencies in excess of a megacycle. A solid slug of the usual magnetic materials has little effective inductance at these frequencies because of the shielding or skin effect caused by induced eddy currents; however, the electrical resistivity of ferrites, ferrites being ceramic in nature, prevents the generation of high amplitude eddy currents at high frequencies by presenting a high resistance path to the flow of these currents and at the same time maintaining a permeability of about 50 to 2500 depending upon the ferrite used at a flux density of 10 gausses.

A ferrite has the general. formula MFezOr Where M represents a bivalent metal. When this bivalent metal is iron, nickel, manganese, magnesiurn, cadmium or zinc, a cubic lattice structure is formed. With the exception of cadmium and zinc ferrites, ferrites of these metals are ferromagnetic. The resistivity values range from 1O ohm-centimeter to 10 ohm-centimeter compared to a value of 55x10" ohm-centimeter for 4-79 molybdenum permalloy, for example. Only one of them, ferrous ferrite (Fell 6204), or magnetite, has had any appreciable use as a magnetic core material in the prior art. This is a well-known ferrite which occurs in nature. In its natural and synthetic form, it has been applied commercially in radio frequency cores for a number of years.

It is characteristic of the cubic ferrites discussed above that they can be made to form solid solutions with each other in all proportions. In the recent developments in this field, certain of these solid solutions have been found to exhibit strong ferromagnetic properties, with permeabilities many times higher and hysteresis losses much lower than those of the constituent ferrites. This is brought about through proper choice and proportions of components and control of heat treatment to produce the conditions necessary for high permeability and low hysteresis loss; namely, low crystal anstropy and low magneto-striction. These characteristics are obtained in part through the use of a non-magnetic zinc ferrite component to reduce the Curie point; i. e., the temperature at which the material loses its ferromagnetism. In this way the elevated portion of the initial permeability temperature curve which occurs just below the Curie point can be brought within a useful temperature range.

Although the conditions for highest permeability and lower hysteresis loss do not correspond to conditions for highest resistivity, the resistivity of the high permeability ferrite is still of the order of ohm-centimeters or higher than that of metals by a factor of 10 a The process most used for producing ferrites consist of mixing the various finely powdered oxides in proper proportions, pressing them into the desired shape and heat treating at approximately 1200 C. in an appropriate atmosphere. Since a linear shrinkage of around 15 per cent occurs during heat treatment, uniform density throughout the unfired parts is necessary in'order to prevent the development of mechanical imperfections during firing. Thi tends to restrict the forming of parts from dry or semidry powders to simple shapes, such as rings, discs, bars, cylinders and shallow cups. It is possible that other pressing or extruding processes can be de veloped for these in more complicated form. However, for the purpose of making lumped inductors for contact protection simple geometrical shapes are quite adequate.

Because of the high resistivity of ferrites, conventional eddy current losses are usually low so that hysteresis losses and high frequency residual losses become controlling. Residual loss is not fully understood but in one recent explanation it is attributed to an electronic resonance. It is a function of frequency, the loss increasing much more rapidly above a certain critical frequency. This rapid increase in loss is accompanied also by a decrease in permeability. For

essence purposes of analysis the critical frequency has been-taken as that at which the permeability drops to super cent of the direct-current or-low frequency permeability. The low-frequency permeabilityand critical frequency bear' -a'n inverse relationship. Therefore, for an application over a given frequency range, a ferrite material having an appropriate low frequency'permeability should be a chosen. For example, manganese-zinc ferrite wane permeabilityof 1200 is useful up to about '3 'megacycles' while nickel-zinc ferrite with a permeability of 50-has usable properties up to 4'0 mega'cycles. The

ferrite compounds which have'been'foundto be best suited for contact protection are the manganese-zinc .series, the nickel-zinc series, copper-zinc series andmagnesium-zinc-series. For further information concerning ferrites, reference is herein made to publications by J. L.

Snoek, Non-metallic Materials for High -Frequencies, Philips Technical Review, December 1946, volume 8, No. 12, pages 353 to 384; New-Developments in Magnetic Materials, 1947, Elsevier Publishing Company.

In order that the invention may be clearly understood and readily carried into effect-it will now be fully described with reference to the accompanying drawing, in which:

Fig.1 shows a ferrite bead having a cylindrical shape with a hole therethrough for'pas'sing a :single straight wire through said bead;

Fig. 2 shows an alternate arrangement in which a ferrite bead is used in conjunction with a single turn coil of wire so that a greater inductance value may be obtained for improvedcontact protection;

Fig. 3 shows an embodiment wherein ferrite beads protect switch contacts from arcs generated by the distributed capacitance of the connecting conductors during operation of said switch;

Fig. 4 shows how ferrite beads areapplied to switch contacts whose circuit connection includes a twisted or cable pair of conductors; and

Fig. 5 shows ferrite bead protection directly incorporated into a switch.

The drawing of Fig. 1 shows a cylindricalshaped ferrite bead with a hole therethrough which can be used for contact rotection by stringing said bead on a conductor connected to the contact to be protectedtata position preferably as close as possible to the contact. As the air surrounding a particular'conductor hasa' permeability less than that of the ferrite material,

'it is'obvi-ous that the center hole should have'the .smallest'diameter C which will allow the connecting conductor to pass therethrough. As ferrite is essentially a ceramic this hole may be made during the method steps preceding the firing of the ceramic constituents. The bead can be made into many other geometrical shapes; however, simple geometrical shapes are easier to form than complex shapes. It is believed that the cylindrical bead shape represents the best form of ferrite for contact protection. vThematerial is black, hard and brittle, resembling a ceramic in physical properties.

The inductance in henries'of a single conductorprovided with a cylindrical bead is-expre'ssed by the equation EHBiiBi where A represents the diameter of the cylinder in centimeters, B represents the length of the lit cylinder in centimeters,- C the diameter 'Of the I liole passing therethrough in centimeters-and the permeability" of the particularferrite used.

In Fig. 2a sin le-turn of wire I, whih is pref erably'insulated, is passed through the holeof a ferrite head 2. Such an arrangementincreases the inductance imparted to a conductor passed therethroug-h by-an amount which varies directly as the square of the-number ofloops used, provide'd'that the beadis not otherwise magneticallysaturated. 'Incertain contact' applicati'ons where space is at a premium it is desirableto increase the series inductance by having a plurality of turns on the bead in lieu of: using more than "a fixednu'mber' of beads. 'In many applications the cost of additionalturns may exceed the cost of'an equivalent number of simple beadson one "straight conductor.

'The'property'of these beads which is of particular importance in the make contact-operation-is th'e'combin'ation of high magnetic per- "me'a'bility and high electrical "resistivity which permits a high effective inductance to'be obcurrerit resistance component which'varies from approiiimately 0 to 5'0 ohms per bead'for 'afr'edu'eney-range of Ci to'3 megacycles'with a bead having a diameter A of approximately 03inch, and alength'B of approximately 0.5 inch. The resistance component isreflected into the'c'onductor connected 'tothe contact and effectively dissipates a portion of the electrical energy stored in the distributed capacitance and inductance of the conductors connected thereto. This reflected resistance component also minimizes Contact arcing "3n, the make contact operation but to a lesser extent as the inductive component 'predominates the resistive component in value. v

In Fig. 3 three ferrite beads are shown me circuit arrangement for protecting a set of switch contacts,- which, for the purpose of illustration, are enclosed in a dry reed switch vessel, said contacts controlling a relay winding energizing circult. Ferrite beads 5 are strung on a conductor 1 which is preferably insulated. The beads should be placed as close as possible to contacts 4 so a minimum of unprotected wiring will be to the contacts. When'a potential is applied .to terminals 3 the magnetic electrodes of the switch approach one another until the contacts 4 are made. During the -make contact operation the electrical energy storedin the'distributed components of the circuit will attempt to discharge through the switch thereby creating-an are at contacts-4 if :sufiicient series impedance does notpreventsuchanoccurrence. The resistive and inductive components of the ferrite beads 5 "delay and dissipate the transient energy thereby-preventing arcing at the contacts during the make contact operation. When the steady state condition is reached, battery 9 supplies energizing current to relay winding 8. During would otherwise be dissipated in the contact with corresponding contact arcing.

Experimental tests wherein make-and-break contacts of a particular type were connected to a battery supply over a 12-foot cable revealed a lengthening of the life of said contacts of 100 to 350 per cent when four small ferrite beads were employed adjacent each of said contacts.

The use of ferrite beads as contact protection is especially advantageous whenever a plurality of conductors are multipled to a particular set of contacts as the value of the distributed circuit components which cause troublesome arcing in creases. For example, if a plurality of conductors were connected to multiple strap 6 the increased capacitance to ground would aggravate the arcing at contacts 4 as the number of conductors connected thereto. is increased.

In Fig. 4 ferrite bead protection is incorporated in a circuit arrangement for energizing a relay winding wherein part of the circuit connection includes a cable l4 connected to terminals [3 and relay winding l5 so that battery 16 will energize said relay winding when contacts H are closed by the application of a potential to terminals l0. Ferrite beads [2 function in the same manner as do the heads 5 of Fig. 3. However, with a cable arrangement or twisted pair arrangementit is necessary that only a single conductor pass through the ferrite beads as is shown in Fig. 4. If both conductors of cable 14 passed through each of the beads the inductance of the beads would be very small due to the canceling magnetic fields of the conductors.

In Fig. 5 a single ferrite bead is shown cooperating with an enclosed contact of a dry reed-type switch. Bead l6 forms a collar for stationary electrode I1 within glass switch envelope 2|. Seals l9 and 20 fix the position of the bead with respect to electrode H as well as provide support for said electrode. Bead I6 functions in the same manner as the leads 5 and E2 of Figs. 3 and 4, respectively.

Such an arrangement wherein the ferrite bead is positioned closely to the make-and-break contact electrodes l1 and i8 improves the efficiency of the contact protection in that the impedance reflected into the contact circuit is much closer to the arcing contact faces than is possible in the usual case.

It is to be understood that the above-described arrangements are illustrative of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the scope of the invention.

What is claimed is:

1. In an electrical circuit including a pair of contacts movable with respect to one another for opening and closing said circuit to the flow of electrical current therethrough, a contact arc minimizing arrangement comprising a conductor connected to said contacts, a mass of magnetic ferrite closely surrounding said conductor at a point immediately adjacent said pair of contacts whereby said ferrite mass imparts a reflected impedance into said conductor at the contact are transient frequencies so as to dissipate and otherwise slow down the flow of electrical energy to said contact during said contact make-and-break operations.

2. A make-and-break contact protection arrangement, as defined in claim 1, wherein said mass is a manganese-zinc ferrite.

3. A make-and-break contact protection arrangement, as defined in claim 1, wherein said mass is a nickel-zinc ferrite.

4. A make-and-break contact protection arrangement, as defined in claim 1, wherein said mass is a copper-zinc ferrite.

5. A make-and-break contact protection arrangement, as defined in claim 1, wherein said mass is a magnesium-zinc ferrite.

6. In an electrical circuit including a pair of contacts movable with respect to one another for opening and closing said circuit to the fiow of electrical current therethrough, a contact arc minimizing arrangement comprising an electrical conductor for carrying current to and from said pair of contacts, a mass or magnetic ferrite material having an electrical resistivity in the range of 10- ohm-centimeters to 10 ohm-centimeters at room temperature and a magnetic permeability in the rangeof 50 to 2500 at a magnetic flux density of 10 gausses at a magnetizing current frequency within the range of 0.5 to 3.0 megacycles, and said ferrite mass being magnetically coupled to said conductor by a close physical positioning of said mass to the conductor portion immediately adjacent said contacts.

7. In an electrical circuit including a pair of make-and-break contacts for controlling the flow of current in said circuit, a contact are minimizing arrangement comprising a conductor connected to said contacts and a head of magnetic ferrite with a hole therein through which said conductor is passed and said bead physically positioned adjacent said contacts whereby a change of magnetic flux density in said bead induces an electromotive force in said conductor during make-contact operations.

8. In an electrical circuit including a sealed switch having a pair of conducting electrodes with make-and-break contacts thereon, a contact are minimizing arrangement comprising a mass of magnetic ferrite magnetically coupled to one of said conducting electrodes by physically positioning said mass closely to an enclosed conducting electrode of said switch whereby said mass imparts a reflected impedance into said conducting electrode during contact operations so as to minimize contact arcing.

WALTER B. ELLWOOD.

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

UNITED STATES PATENTS Number Name Date 1,032,658 Clement July 16, 1912 1,773,938 Bishop Aug. 23, 1930 1,840,089 Gilbert Jan. 5, 1932 2,228,798 Wassermann Jan. 14, 1941 2,298,468 Curtis Oct. 13, 1942 2,313,809 Curtis Mar. 16, 1943 2,375,609 Zuhlke May 8, 1945 2,452,529 Snoek Oct. 26, 1948

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