B+C= Be-tal+ j+ ) e- /Lta1

ta= tar L L

C giving the first term in (1). The effect of the second term showsB(1-eOlat)+r (1-er/Lla)=ieT/Lta the reduction in arcing time ta.

Since r/L is small, (but not ra/L,, i.e., a), we can write byexpanding to the first approximation References

-.-rta C . / r 1. Bailey, and Cleghornie, Some phenomenon of commutation,B(_l a)+_ ta=J 1- L-ta J. Inst. Elec. Engrs. (London), vol 38, 1907, pp 162-174

2. Binder, Contribution to the dynamics of brush contact, Elektro-Compared with 1, r/Lta is small, and again tech. Z., vol 82A

3. Holm, R., Electric Contacts. Berliin: Springer-Verlag, 1958C 4. Holm, R., Theory of the sparking during commutation onB(1-eala)+- ==j (+1 ) dynamos, Trans. AIEE (Power Apparatus and Systems), vol 81,L Dec 1962, pp 588-594

If we have just assumed that after arcing5. Shobert, E. I., II, and J. E. Diehl, A new method of investigating

commutation as applied to automotive generators, ibid., vol 73,

di1954 (Feb 1955 section), pp 1592-1602

L - 12 6. Srinivasan, A., and K. Padmanabhan, Radio interference durinigdt commutation, J. Inst. Engrs. (India), Jun 1961

7. Kaseav, Minimum potentials of arc discharge and two types ofthen di= -(12/L)dt, and integrating arc with a cold cathode, Phys. Rev., Jun 1961

Dependence of the Conduction Mechanism onPolarity in Stationary and Sliding Contacts WhenHigh-Resistivity Film Is Present in the ContactElse Holm

Abstract: High-resistivity films thicker than 50 angstroms, resistance to the constriction resistance. Terms such aswhich are present in a sliding contact between a graphite brush contact and constriction resistance, contact voltage, tunneland a copper ring, are fritted at very small currents. Two r a vkinds of fritting are observed: 1) when the copper is anodic,metallic bridges through the film between members are formed be familiar concepts [1]. In this paper, an "a spot" refers to(the minimum field strength is approximately 4X104 volts per a contact spot which conducts either metallically or quasi-cm); and 2) when the copper is cathodic, the film is pulled metallically.away by electric forces to make place for a smaller spot whichconducts more current. Bridge formation during sliding requires I. Backgroundovervoltages because the time is too short for equilibrium states.The consequence on anodic copper is oxidation (limited by wear) In 1937, U. B. Thomas found that contact between a plat-of the clean cuts of broken bridges and fritting of overvoltages inum-iridium ball and a tarnished silver plate fritted to1-1.5 volts above a basic level U,. However, the contact spots contact voltage U between 0.035 and 0.05 volt when silveron cathodic copper are protected against oxidation by a tunnelconducting film, and U remains constant at Uo. The metallic was the anode. When silver was the cathode, U fluctuatedbridges are responsible for the great wear on the cathodic brush. between 0.8 and 1 volt.

Thomas' findings were not followed up until recently, whenA sliding contact in normal air at atmospheric pressure in- investigations of sliding between a graphite brush and avolves the presence of a high-resistivity film (20-150 angstrom copper ring (p 437 of [2]) showed that the U across the contactunits thick) when at least one member is a tarnishing metal. with the cathodic brush became high and fluctuated (see Fig.If such a film is thin enough, the conduction electrons tunnel I of [2] and Fig. 5 of [3]) above a constant UO, after a shortthrough it. If the film is thicker than approximately 50 period of sliding. Across the counter contact, U was seenangstroms, it is fritted at a voltage which depends upon the to be nearly constant at U0 during "smooth" sliding (Fig. 1).polarity. "Fritting" (p 130 ff of [1]) is that process which The aim of this paper is to explain the difference in U,performs a "sudden" irreversible increase in electrical conduc- for the two polarities by investigating the conduction mecha-tion through relatively thin films (to a few 1000 angstroms). nism through different kinds of initially almost insulatingAfter fritting, the conduction is essentially metallic but can be films. It will be emphasized that there is an essential sim-impaired by a thin remaining film. This film adds a tunnel ilarity between stationary and sliding contacts. Conse-

quently, the difference between the Us can not be based onPaper 31 TP 65-115, recommended and approved by the Rotating peoeawihd o peri iia a nbtMachinery Committee of the IEEE Power Group for presentation stationary and sliding contacts.at the IEEE Winter Power Meeting, New York, N. Y., January 31- A detailed investigation of a symmetric silver contact, in-February 5, 1965. Manuscript submitted December 24, 1963; eldn th frtigvlaeU,wscnutd. elmade available for printing December 1, 1964. ldn th frtngvtaeU wscdue.Awl-ELSE HOLM, formerly with the Stackpole CaWrbon Company, St finished fine silver rod was sulfided at room temperature to aMarys, Pa. has retired. blue-gray appearance (the film was 200-300 Angstrom thick),

404 Hoim -Dependence of Conduction Mechanism on Polarity MAY 1965

Fig. 1. Simultane- IeIous Ut records OHM MTIafter 1 hour of slid- MATRiXAing on oxidized 0Eg TARNISHED A.j VS CLEAN.copper ring: U -- FILM -Z00Xconstant=0.4 volt l20= UO; 2 graphite l_brushes in series

C/OSSED RODS

then placed crosswise on a clean and polished silver rod.A "cross-rod" circuit was used. Then, starting with U=0, 10the voltage (and current) was increased in steps until the highresistance of the sulfide was fritted and, ultimately, until achosen maximum current was reached.The RU curves in Fig. 2 are typical of many such measure-

ments. The film resistance preceding fritting is high for .1-x-1_x l Uboth polarities. It is reversible until fritting voltage Uf is .01 .1 I VOLTreached: Ur+-.0.05 volt with matrix silver the anode but Fig. 2. Typical RU curves (resistance-voltage); sulfidized sil-Uf--4.9 volt with matrix silver the cathode. In a dry ver rod and clean silver rod, crossed: P - 9.5 gramsvacuum of mercury, U+0.05 volt and Uf--9 10-4 mmvolts.

After fritting, small contact spots with metallic or quasi-metallic conduction are produced for both polarities; see,for instance, the backward RU curves of Fig. 2. Thesea spots have no relationship to the conduction mechanism(apparently semiconduction) through the film before fritting.The findings of C. Tubandt [4] play a significant role in

the interpretation of the low fritting voltage when matrixsilver is the anode. Tubandt showed that positive ions aremoved by an electrical field through halogens such as AgI,AgBr, and AgCl and through sulfides such as Ag2S and Cu2S, Fig. 3. Typical Ul oscillograms; steady I variation between 0in agreement with Faraday's law. At the same time, he and 0.5 ampere; U+ when matrix silver Is anodefound the negative component in the lattice to be unmovable;the ions are delivered from the anode. Tubandt also dis-covered that the moving ions can conglomerate and formmetallic bridges between the members, as was indicated by melting temperature of the metal. This elevation lendselectronic conduction. strength to the conception that, as the current is increased,Tubandt's theory forms the basis to explain the 1000 times the film only recedes and is not burnt away (see Fig. 2 of

increased conduction at Ur+=0.05 volt in Fig. 2 (Ag+) by [5]). It is therefore believed that primary or A fritting ismetallic bridges which form after the positive ions from the connected with a film withdrawal.matrix metal moving through the almost insulating film, have Electrical forces pull the film away from a very smallreached the cathode. The metallic bridges are surprisingly better-conducting spot, which enlarges or adapts to thestrong and stable immediately after fritting, obviously momentary current and mechanical load. The latter processbecause of repair. The bridges are produced at a temperature seems to need a somewhat elevated temperature which maymuch lower than the softening temperature of silver. loosen the bonds between the layers of the surrounding filmWhen the matrix silver is the cathode, the same film re- and, at a smaller field, facilitate further withdrawal or adapta-

quires Ur-'4.9 volts to be fritted. It is evident that, in this tion through which a stronger bonded layer immediately oncase, the field prevents the positive ions at the matrix silver the metal may remain in the a spot. The first a spot estab-from moving into the film. The clean silver rod, now the lished after fritting is unstable and alternates between metallicanode, is incapable of delivering an effective number of posi- conduction and insulation. This indicates that the filmtive ions into the film; this is indicated by the fritting voltage, oscillates just after the fritting process. The spot stabilizeswhich is 100 times higher than its value for bridge formation with time, particularly at a slightly increased current.when the matrix silver was the anode. The same high U, It is possible to distinguish two kinds of fritting: A frittingis measured with a graphite rod. which is supported by diffusing ions through the film within

This indicates that, in this case, the members no longer the load-bearing area (bridge fritting) and A fritting whichserve as ion resources, but solely as terminals for the electron leads to film withdrawal, exclusively.current. Therefore, the current increase at 4.9 volts must The field strength producing A fritting is a material con-mean that a path with a considerably reduced film is offered stant (p 134 of [1 ]) of about 106 volts per cm. Evidently, U,-to the electrons in this moment. can be used to compute the film thickness before breakdown.The resistance drop in Fig. 2 (Ag-), when I is increased after A thickness of about 250 angstroms requires Uf--4.5 volts

the first a spot has been established, results in fluctuations according to R. Holm's measurements.between 0.08 and 0.16 volt. The contact temperature, The oscillograms of Fig. 3 were taken under the sameno doubt, is elevated (about 300°C), but is far below the conditions as Fig. 2, except for a steady I increase, and illus-

MAY 1965 Holm-Dependence of Conduction Mechanism on Polarity 405

oil 9 Fig. 4. Typical Ul This may be the reason why U increases only to the extentoscillogram with that it suffices to tunnel the electrons through the remainingrods used in Fig. 2 film. This, in turn, would explain why, after the jump, thewhen matrix silver UI curve is very similar to the upper part of the reversiblebris anodge; also, in Fig. 5 (upper right side) where a tunnel conducting film

bridgovervoltage: then apparently remained in the spot after fritting.11 I increased, in If the polarity is reversed again, the current does not use

steps to 1 ampere the path where the film had previously receded because it isand decreased to 0 easier to form a new path with bridges which require less than

0.1 volt (0.1 volt is an overvoltage because of the steady risein I).

11. Activation Energy Necessary to Move Silver lons bymHIM ~~~~Electrical Field Through a Silver Sulfide Film About 250

Angstroms Thick

It has been proved that metallic bridges are formed rathersuddenly through silver sulfide films at Ur+-0.05 volt. Witha waiting time of approximately 2 minutes, the fritting appearsat 0.04 volt: with a 6-minute wait, it appears at 0.03 volt.No breakdown at U smaller than 0.03 volt was observed,even after 25 minutes (measured on a vibration-eliminatingsupport). The equilibrium voltage of -0.05 volt for bridge

Fig. 5. Typical Ul oscillograms with polarity change for given fritting and adaptation is demonstrated in Fig. 2 (Ag+).contact between sulfidized silver and graphite; I is steadily It was also proved, with the same film but with the matrix

varied from 0 to 1 ampere: P 6.4 grams silver as the cathode, that now Uf--5 volt. It was con-

cluded that a film approximately 250 angstroms thick musthave been forced to recede instantaneously, thus making placefor a quasi-metallically conducting a spot.

trate overvoltages for the formation of metallic bridges and From the defined and distinct average value of Uf+0.04enlargement of their cross sections, because U+ is about 100 volt and a film about 250 angstroms thick, the field strengthper cent higher than its value in Fig. 2 (Ag+). This indicates for bridge formation is estimated atthat bridge formation requires time and that overvoltages will 0.04be characteristic whenever there is insufficient time for an 2.5 1 22X 104 volts per cmequilibrium state to be attained, as will be explained inSection II. This means that a silver ion with a diameter of 4 angstroms,

If, once fritting is completed, I is increased in small steps for example, would need an activation energy ofoccurring at 20-s intervals, then the overvoltages disappear 2X 04X4X410-s0.001 eVand the equilibrium voltages are measured according to thepoints shown in Fig. 4. Fritting with matrix silver as the to be moved by the field when referred to the elementarycathode is much less dependent on time. charge unit.The backward Ul curves in Figs. 3 and 4 arc reversibles. 1ll. Contacts Between Tarnished Metals and Graphite:

The reversible after bridge fritting is characteristic of a Dependence of the FriTting Process on Polaritymetallically conducting a spot between metal membersbecause R decreases with decreasing 1, in agreement with the CONDITIONS OF MEASUREMENTcalculation. After A fritting with U-, the reversible is Well-finished rods and rings of silver and copper were sul-usually straighter. This characteristic difference is inter- fided at room temperature (the film on the ring '150 ang-preted by the presence of a thin tunnel connecting film on the stroms thick): copper was also oxidized at 2 hours at 130°Cmetal after film withdrawal. (films on the ring -100 to 150 angstroms thick). TheA polarity change at a given contact between a sulfidized counter member is always a rod or brush of electrographite

silver rod and a graphite rod leads to a corresponding change material with pO about 5X10-3 ohm-cm and koc about 0.28in the conduction mechanism, as demonstrated by the oscillo- watt per cm per degree (heat conductivity). The contactgrams in Fig. 5: The fresh contact, with silver as the anode, hardness [1] is HC-2 tons per cm2 and the modulus ofshows a reversible with constant R-0.5 ohm which is com- elasticity, E, is -53 tons per cm2. The rods have a 0.317-cmparable to the reversible in Fig. 3 (U+). Reversing the polar- diameter and are crossed.ity at U= 0 and making silver the cathode results in the same The current I is varied between zero and Im, in two differ-R, only up to apl)roximately 0.25 ampere. ent ways. First, it is varied with a delay between the I steps;U increases at higher currents slowly at first but, finally, for every step, U is noted and R is calculated. R is plotted

with a jump to the normal trace of the reversible which is against U in RU curves. Second, I is varied with a steady,traced in the upper right side after a fresh contact has been though slow, increase which is traced against U in an oscillo-fritted. Figure 5 is interpreted in the following way: The scope as UI curves. It has already been proved that over-conducting bridge path is used by the reversed current in the voltages are produced with a steady I increase and that, withbeginning. However, this soon fails because the available the metal as the anode, this indicates that the formation offield cannot repair bridge damage which begins at 0.25 ampere. metallic bridges through the film and, also, the enlargementIt happens, though, that electrons tunnel through the film of their cross sections do not have sufficient time to proceedwhich often remains after A fritting in the a spot with U-. at lower voltages.

406 Holm-Dependence of Conduction Mechanism on Polarity MAY 1965

-0

OHM

7Ale TARNISHED Aa VS GRAPHITETARNISHED AVS iGRAPHITE LOHM FILM IZOA

5 FILM -o0A A,+ CROSSED RODS

ba2- @ CROSSED RODS10 _

10 -lLI0

0. I * I VOLT

Fig. 6. Typical RU curves with sulfidized silver rod and graph- I I |Lite rod, crossed: P - 6.4 grams .1 1 .5 v

(A)

The load-bearing area Ab with the well-finished rods(finished with 1/4 micron of diamond paste) is essentially RTANISHED A1 VS GRAPHITEelastically formed at P=5 to 50 grams and can be computed FILM - IZOAby means of Hertz' formula, in the case of a single Ab. CROSSED RODS

All measurements presented in these figures have been A schosen as the most typical from among a great many tests 9_(deviation 4 10 per cent, concerning Uf and U). The con-tact device for cross-rod and quasi-flat contacts at rest wasmounted on a vibration-eliminating support. The rings were la 51Ipolished with 1 micron of diamond paste.

CRoss-ROD CONTACTSRU curves (Fig. 6) of contacts between a sulfidized silver

rod and a graphite rod show that frittings for the two polaritiesare very similar to those in Fig. 2 where U, after fritting, is 10 _typical of a symmetric metal contact. With a graphite rod,the resistance is essentially within the carbon; thus, U- andU+ should be much higher than their corresponding values I. 1.0 VOLTin Fig. 2. That is the case after A fritting in Fig. 6 with Ag-.

After bridge fritting, however, contact voltage U+ is as (B)low as in Figs. 2 and 6 (Ag+) for metal members. In other Fig. 7. Typical RU curves with sulfidized silver rod andwords, carbon constriction is in some way decreased. It is graphite rod, crossed: P-31 and 51 gramsassumed that the bridges make roots into the porous carbonand that these roots have large areas combined with a small A-With silver anodeconstriction resistance which essentially is located within the B-With silver cathodecarbon. At higher currents, the roots do not continue to growand U soon rises as the current exceeds a certain value: Fig.2 (U+) demonstrates this point.The contact temperature at -0.05 volt is far below the

softening temperature of silver with po and kc, for the graphiteas given in section III (p. 88 of [1]). In Fig. 7 (U+), Uf+l-0.2 volt illustrates the overvoltage for bridge fritting (steadyI increase).When silver is the cathode, overvoltages do not seem

necessary for frittings; for example, compare the Ur- in Fig.7 with that of Fig. 8 (U-). Here, frittings result in a spotswith relatively high T, as indicated by U-2.5 volts afterfritting, but there is no indication that the melting tempera-ture of the metal is reached. In the contact surface T'-300'C is calculated at 2.2 volts with pi2.6- 10-3 ohm-cm and Fig. 8. Typical Ul oscillograms with same rods used in Fig. 6;k,0.15 watt per cm per degree for temperatures 300-500° C. P 51 grams

MAY 1965 Holm-Dependence of Conduction Mechanism on Polarity 407

The finding of a relatively low T supports the conception offilm recession, rather than the possibility that it burns away.

Oxidized Copper RodPrincipally, there is no difference between tarnished silver

and tarnished copper (either sulfidized or oxidized), withrespect to frittings for the two polarities [compare Figs. 9(overvoltage U+ with copper anode) and 8]. This provesthat copper ions are also moved through oxide film and thatthey can conglomerate much as silver ions do when they movethrough silver sulfide.

After A fritting at U-, however, the backward UI curves Fig. 9. Typical Ul oscillograms, oxidized copper rod andwith copper oxide indicate that a film of considerable thick- graphite rod: P - 6.4 grams; I varied as In Fig. 8ness, but still capable of tunnel conducting, may be left onthe a spot (see Figs. 111-18 and III-19, of [1] where it isshown that tunnel resistance increases with decreasingvoltage).The resistance assumes high values when U approaches

0.3 volt on the backward UI curve, but these values are notso high as before A fritting. Even cycling the current 10times does not result in a perfect reversible, as the UI curveat the left side of Fig. 9 (U-) shows. Similar indications ofremaining films have also been observed on tarnished wolframand steel, after A fritting. Also, in sliding contacts (thebrush is on the copper ring), films are apparently left on thea spots under the anodic brush during "smooth" sliding. Fig. 10. Typical Ul oscillograms with thicker copper oxide(This will be discussed in Section IV.) film: steady variation I from 0 to 0.025 ampere and back;The copper ions evidently are not capable of forming backward Ul curves show differences

continuous metal bridges through thick films of copper oxide(about 400 angstroms). The oscillograms in Fig. 10, for bothpolarities, show frittings of such films at approximately 6volts. Apparently, the film also withdraws at the anodic r lcopper, before the moving ions can conglomerate into metal ; TARNISHED A VS GAPHITEbridges. OHM FLAT CONTACT

After fritting, however, U is usually high (about 2 volts) l° Actwhen copper is the cathode; thus T is also high (about300°C). Furthermore, the curved shape of the reversible IC?characteristic is enhanced by a remaining tunnel conducting Agfilm. With copper as the anode, R on the reversible is con-stant and relatively small. The difference can be explainedby assuming that, after A fritting for both polarities, a filmof the same kind indicated in Fig. 9 (U-) remained on the a IO \spot. This film is quickly short circuited by a metal bridgefor the copper anode.

QUASI-FLAT CONTACTS BETWEEN TARNISHED RINGS I(SILVER AND COPPER) AND A GRAPHITE BRUSH IThe films on the metal rings are the same kind as those O- I VOLT

investigated with tarnished rods; the brush (0.4 cm' face) Fig. 11. Typical RU curves of quasi-flat contacts at rest, sulfid-is of electrographite material. In flat contacts, the load is lized silver ring and graphite brush; P - 230 grams: I variedcarried by many parallel spots. Investigations seem to in- as in Fig. 6dicate that, when all the spots insulate, many are fritted at acertain voltage.

In flat contacts, fritting voltages and conduction afterfritting are also polar dependent for all three kinds of film Figure 11 (Ag-), with the silver ring as cathode against the(silver sulfide, copper sulfide, and copper oxide). The ef- anodic brush, shows a property which also appears duringfects are similar to the results with a single contact spot. sliding between flat members, i.e., adaptation of contact spots(Compare the RU curves of Fig. 11 with those in Fig. 6; to the maximum current by cycling I up and down severalinclude the interpretation of the low voltage across the con- times with the consequence that U declined from 1.6 to abouttacts between the metallic bridge and the carbon member, for 0.7 volt, where it usually remained. Adaptation in flatsmall currents.) The fact that, in flat contacts, a current of 1 stationary contacts by training the a spots as described ap-ampere (or even 5 or more) is carried with the same ease as parently consists of a further reduction of the tunnel con-0.05 ampere in a single contact spot (Fig. 6) indicates that ducting fihms which have remained on the a spots after Amany of the load-bearing spots must be fritted so that 1 fritting, as indicated by the curved backward RU characteris-ampere is divided, perhaps into 0.1 ampere per spot. tics which straighten out more and more. The author found

408 Holm-Dependence of Conduction Mechaniem on Polarity MAY 1965

Fig. 12. Typical oscillograms of quasi-flat contacts at rest; Ivaried steadily from 0 to 1 ampere and back; U + (anodic cop-

per) apparently illustrates overvoltages

\,rWW --X W ~~~~~PROTECTION

that contacts between a brush and oxidized (or sulfidized) p

copper peerform, in principle, similarly to those with a sulfid- SCALE / /' VACUUized silver ring. Compare Fig. 12 with Fig. 11.

IV. Two Graphite Brushes, in Series, Sliding Smoothly SEAL RINGon Differently Tarnished Metal RingsRagnar Holm's sliding device is sketched in Fig. 13: The

eccentricity of the ring is no greater than 0.0001 inch. The SHAFTbalance of the rotating apparatus is so good that vibrations Fig. 13. Friction deviefrom the motor are negligible. The brushes are free to movearound the pivots. (See Section III for material of brush.)Smooth sliding means no interference by wear and stick-

slip. The films are the same as those investigated in theprevious section with the brushes at rest. During sliding,the same phenomena are met as appear when the brushes areat rest, but additional complications are produced by thesliding action, whose components can now be analyzed bycomparing them with the simpler conditions in Section III.

SULFIDIZED SILVER RINGOvervoltages are measured for bridge formation as demon-

strated in the Ut records in Fig. 14 (Ut) during the com-mencement of sliding when I IS increased from 0 to 1 ampere _inles nth to seconds, is ia sed and

1thenat Fig. 14. Simultaneous Ut records of both polarities during in-in less than two seconds, first at low speed andl thlen at6ota ldn ihbuhs nsliie ivrrnmeters per second. Here, U+ indicates overvoltages; U-indicates A frittings at 1-1.8 volts in the beginning and, atthe end of the record, adaptation voltage by film withdrawalat U--0.4 volt.When a multitude of bridges has apparently formed on the

elevations of the ring, U stops the longer time at about 0.03volt. Thus, larger a spots form by adapting their areas Fig. 15. S i m u I -to the maximum current more and more with the number of t a n e o u s U trevolutions; note the thick lower border at the end of the records continuedrecord in Fig. 14 (U+). Overvoltages then disappear be- from Fig. 14: U _cause sliding occurs on cuts of broken silver bridges, which 0.4 volt = UO forevidently are covered with a graphite film since stick-slip U-is negligible. Figure 15 shows Ut records after 1/2 hour ofsliding at 5 amperes.With the cathodic ring, the sulfide film is almost instan-

taneously A fritted at 1 to 1.8 volts: see Fig. 14 (U-). OXIDIZED COPPER RINGDuring high speed, a great number of well conducting a spots In principle, sliding with the oxidized copper ring is veryhave formed. Many of these a spots are met again after similar to sliding with the sulfidized silver ring, during theevery revolution for long periods of sliding, as Fig. 15 (U-) first hour: the overvoltages, up to 1.5 volts for bridge frittingsshows. Here, again, stick-slip did not interfere. with copper as the anode, also disappear after a relatively

After the silver ring is cleaned from the remaining sulfide short time. The voltage remains at a basic level of Uo'-0.4films, U is approximately 0.12 volt for both polarities during volt. During this time, the initial deliberately producedsliding at 5 amperes. Here a thin visible graphite film oxide film of about 150 angstroms is worn down to a smallerevidently adhered to the sliding surface of the clean silver thickness, as indicated by the color changing to a morebecause sliding was smooth and wear was small. Table I metallic one (this also happened with the silver sulfide filmgives the wear numbers. on the anodic sliding track).

MAY 1965 Holm-Dependence of Conduction Mechanism on Polarity 409

However, after a few hours of sliding, voltage fluctuations(-+AU) develop above the UO level; see Fig. 1 (U+), whereU+ indicates the fluctuations of overvoltages between 0.4and 1 volt needed for frittings of developing oxide films.The only difference in sliding with sulfidized silver is that theclean cuts of broken copper bridges rapidly oxidize by fric-tional oxidation assisted by the temperature.The more oxide that accumulates, the higher the overvolt-

ages become for instantaneous frittings. Accumulation ofoxide continues until its further growth is balanced by wear[6], as indicated by AU first fluctuating at low voltages andthen increasing in number and amplitude until the amplitude Fig. 16. Ul oscillograms: 1) during sliding and, 2) with brushesremains 1-1.5 volts above the basic level (U0-0.4 volt) to at rest: U+ (anodic copper) indicates fluctuations at randomwhich -AU sometimes drops. for all currentsThe drop -AU to U. under the cathodic brush certainly

means that moments exist when all the a spots have maximumadaptation. Usually, contaminated contact spots appear Table I. Wear of Cathodic and Anodic Brushes on Differentalmost steadily under the cathodic brush after every revolu- Metal Ringstion indicated by the continuously fluctuating overvoltages. Distance WearAt overvoltages of 1 volt, however, frittings and adapta- P. I, of Travel,tions occur instantaneously. It is thus concluded that Ring Grams Amperes km (-) (+)the size of the a spots which are also under the cathode (600 5 5360 4.4 0.61

300 5 2450 3.1 1.2brush may correspond to UO. Copper 300 10 1320 11.0 3.1The conducting spots on the cathodic copper track do not 180 5 2860 1.8 0.6

oxidize quickly during smooth sliding. They are evidently (76 5 1700 1.8 084protected against oxidation by tunnel conducting oxide film Copper heated 600 10 180 21.0 6.0which remained on them after A frittings. This film is not to 140 C 300 5 1650 15.0 8.0worn off during smooth sliding (see Fig. 7 of [7]). It covers 300 0 4500 0.1 0.130 5 1100 0.7 0.5a spots as large as necessary for the passage of 20 amperes Silver 300 10 1250 0.8 0.5after maximum adaptation. U does not fluctuate but is 300 20 400 0.5 0.5nearly constant at U0-0.4 volt. Silver sulfidized 300 5 115 5.0 3.4During smooth sliding, U. can vary between 0.3 to 0.5 0300 1600 0.1 0.130 5 1070 0.5 0.5

volt. It is characteristic for a wide range of currents and Gold 300 10 1020 0.4 0.4300 20 1423 0.3 0.4electrographites. Thus, U0 is a kind of equilibrium between 115 5 2300 0.05 0.1

slow oxidation and adaptation. It is just sufficient toproduce maximum adaptation of the conducting spots onboth metal tracks. With anodic copper, UO appears onlywhen frittings are not needed: then, the overvoltages drop clean metal surfaces (the dew point is about 5° C). The ap-to UO. pearance of the graphite films is similar for both polarities.

Sliding performance under the anodic brush is similar to The wear is small and independent of the polarity.sliding on tarnished silver, as demonstrated in Fig. 1 (U-) With a copper ring, the anodic ring track always has afor oxidized copper and Fig. 15 (U-) for sulfidized silver. heavier deposit than the cathodic one. Still, the cathodicThis also shows that oxidation on the cathodic copper track brush wears considerably more. It is assumed that thisis negligible, a fact which supports the conception (section track is finely roughened by the metallic bridges for whichI) that, after A frittings, a tunnel conducting film remains the copper anode must deliver the material. The clean cutsin the a spots and that maximum adaptations are seldom of broken copper bridges, particularly after oxidizing, abradecomplete during smooth sliding. the brush face and the ring: the worn graphite gathersThe UI curves in Fig. 16 show =AU during sliding at 5 mainly in the fine valleys of the ring where, apparently, it

amperes to zero only when copper is anodic. The a spots for is mechanically kept. Replicas of the brush faces afterboth polarities are adapted solely to 5 amperes with U0 sliding show that the cathod c brush face has no ordered0.4 volt. Currents of less than 5 amperes use these spots graphite deposit; on the other hand for the anodic brush(see Fig. 10 of [5]) with the result that R remains constant. face (see Fig. 10 of [7]) with film withdrawal on the corre-The UI oscillograms in Fig. 16 are general experiences sponding ring, track ordering of the surface graphite platelets

during smooth sliding on copper rings at 5, 10, and 15 amperes is so perfect that the platelets give rise to a birefringent effect(testing limit) with a great variety of Stackpole electro- of pink and blue colors in polarized light exchanging theirgraphite brush grades, having different speeds (up to 22 colors on stage turning.meters per second) and loads. See Figs. 8 and 9 of [9]. Table I represents wear numbers in 10-6 cm3/km, most of

which are averages of several tests:

V. Dependence of the Wear on the Polarity When the 1. With a copper ring (previously thoroughly cleaned) theBrushes Are Sliding Smoothly on Copper Rings In Indi- wear of the cathodic brush (-) is larger than the wear of anodicvidual Tracks at Atmospheric Air Pressure brush (+), particularly when the ring is heated to 140° C.

When the brushes are sliding smoothly on a clean silver (or 2. Brushes on silver and gold rings wear about the same, withWhen the brushes are sliding smoothly on a clean silver (or no distinct influence of the current between 5 and 20 amperes,gold) ring, the sliding surfaces on the metal are apparently if stick-slip is negligible. The mechanical load seems to do more.covered with a thin graphite deposit which adheres to the This clearly indicates that smooth sliding takes place mainly

410 Holm-Dependence of Conduction Mechanism on Polarity, MAY 1965

between the graphite brush and the graphite deposit adhering damage to the film, because the wear becomes considerablyto the sliding surface of the noble rings. greater.3. The sulfidized silver ring (with which the investigations werecarried out in the previous sections) resulted in only slightlyhigher wear of the cathodic brush, evidently because the cuts Acknowledgmentof the broken silver bridges did not contaminate. The author would like to thank D. Emmett for his helpIn all instances, the ring diameter was 7 cm and the voltage with the measurements.speed was 4.4 meters per second.

References

Conclusion 1. Holm, R., Electric Contacts Handbook. Berlin, Germany:Springer Verlag, 1958, pp 130, 358, 134, 410, 88, 436, 4372. Holm, E., Contribution to the theory of the brush-collectorU0, defined as the contact voltage between 0.3 and ap- contact, Trans. AIEE (Power Apparatus and Systems), vol 78,

proximately 0.5 volt, is determined by the equilibrium be- pt III-A, Aug 1959, pp 431-438tween7adaptationanoxidation, i.e., producing ada n3. Holm, E., Specific friction force in a graphite brush contact as atween adaptation and oxidation, i.e., producing adaptation function of the temperature, J. Appl. Phys., vol 33, 1962, pp

(seldom complete) of the a spots to the current leaving a thin 156-163protecting film on them. This is true for a wide range of cur- 4. Tubandt, C., Uber einseitige ionen und gemischte stromleitungin kristallen, Z. Elektrochem., vol 26, 1920, pp 358-363 (inrents. U, is a consequence of smooth sliding, demonstrated German)here when the brushes slide in their individual paths. The 5. Holm, R., Remarks about fritting during commutation, Proc.

appearance ofU < 0.3 volt leads to increased friction and Internat'l Research Symp. on Electric Contact Phenomena, Orona,appearance of U < 0.3 volt leadls to increasedl friction andt Me., 1961, pp 41-47wear, evidently because portions of the protective film in the 6. Kerridge, M., Metal transfer and the wear process, Proc. Phys.contact spots are damaged and because sliding partly occurs Soc. (London), vol 68B, 1955, pp 400-407on small bare copper spots. 7. Holm, E., Temperature effect on the specific friction force in

contacts where at least one member is graphite, 1961 Proc.When sliding takes place in a common path, periods exist Internat'l Research Symp. on Electric Contact Phenomena, pp

when the contact spots are not the same under both brushes 71-908. Shobert, E. I., Jr., Electrical resistance of carbon brushes onand U for both polarities is similar to the Ut records in Fig. copper rings, Trans. AIEE (Power Apparatus and Systems), vol

1 for sliding in individual tracks: U is nearly constant at 73, pt III-A, Aug 1954, pp 788-799U0 across the anodic brush contact. Usually, however, an 9. Holm, E., Influence of the current direction on the contact

voltage in the sliding contact graphite-copper, presented at theexchange and sharing of contact spots occurs, apparently with Internat'l Conf. on Contacts, Graz, Austria, 1964

Sakuma Frequency Converter ProjectIsao Takei

Abstract: Power system frequency has developed on a 50-cyclebasis in northern Japan and on a 60-cycle basis in the south. Fig. 1. FrequencyPast efforts to unify the frequency have been unsuccessful. .i JBoth systems have now developed to the extent that this is now areas In Japanvirtually impossible. To obtain the advantages of intercon-nection, despite this handicap, modern high-voltage dc techniques 60-CYCLE FREO. AREAare used. In a single station, 50-cycle power is converted todirect current and direct current to 60 cycles (or vice versa). 50-CYCLE FREQ. AREA LIIThis is, in effect, a de transmission system without a line. How-ever, location of both rectifiers and inverters in the same stationpermits some simplification over a conventional dc tie.

The Japanese power system has been developed in two aseparate groups, one of 50 cycle and the other of 60 cycle,for historical reasons. The border line is approximately thecenter of the country, as indicated in F'ig. 1.ITo overcome the inconvenience of two different cycles,

several attempts at unification have been made; however,none of them was successful. The rapid expansion of power ssesi eei er ae tams mosbet ovr

Paper 31 TP 65-182, recommended and approved by the Substations one cycle system into the other.Committee of the IEEE Power Group for presentation at the IEEE The hig;h-voltage de power transmission technique de-Winter Power Meeting, New York, N. Y., January 31-February 5, veloped in 'Sweden and utilized in their Gotland submarine1965. Manuscript submitted May 28, 1963; made available for lieadteEgshCnelcoiglneovrsatraigprinting December 16, 1964. leadteEgihCanlcosn iecnet lentnI5A0 TAKEI is with the Electric Power Development Company, current to direct current, and vice versa. It is also suitableTokyo, Japan. for frequency conversion. Frequency conversion by thisMAY 1965 Takei-Sakuma Frequency/ Converter Project 411