CA1123881A - Solid brush current collection system - Google Patents

Abstract

46,156I

ABSTRACT
SOLID BRUSH CURRENT COLLECTION SYSTEMS
A solid brush current collecting system for dynamoelectric machines which selectively uses compatible materials in different applications for the moving and stationary contact members. Brushes of different composi-tions are disclosed and the current transfer and collecting members are operated in an inert gas atmosphere, such as carbon dioxide, containing a vaporous organic substance to achieve operation of the current collecting system at higher temperatures, higher velocities and higher current, densities than is possible with conventional systems. Low friction and low wear rates are in part achieved by operating the contact members in a low temperature environment.

Description

;i;~238~

This invention relates to systems for collecting and transferring electrical current between relatively moving parts, as in a dynamoelectric machine.
The use of solid brushes for collecting or trans-ferring current in systems involving relatively moving parts, such as motors or generators, has been pro~en reliable and reasonably e~flcient for many commercial and industrial appli~
cations. With the advent of the more recently advanced elec-trical machinery desi~ns, particularly those involving super-conducting excitation coils and high power rated homopolarmachines, the need has arisen for improved systems capable of more efficlently collecting and transferring machlne current. This need is prlmaril~ due to the much greater electrical currents and speeds required to generate more power or transmit more torque than was previously possibl~.
Present designs of solid brush current collectors operating with sliprings or commutator systems adequately handle current densities of about 10 amperes per square centimeter and brush lifetimes of 0.5 - 2 years are typical.
These current densities apply to machines operating in ambient air and at conventional speeds but it is well known that brush li~etimes can De quadrupled by operating carbon ~ ..

' 38B~

brushes at current densities of about 10 amperes/cm2 in an inert gas atmosphere, such as the hydrogen environment utillzed ln large synchronous condensers.
Although the actual mechanisms or phenomena associated with current transfer across sliding surfaces are incompletely unders~ood, it is known that the interface res-istance (electrical base) and friction (mechanical base) between a slipring or a commutator bar and brushes and wear rates between the contacting members, are grossly affected by the type and reactivity of the gaseous atmosphere in which they operate, the temperature at which the contact members operate, particularly at the interface, and the properties of the contact`ng materials.
Concerning operation in a gaseous atmosphere~ it is known that an unavoidable metal oxide film is deposited on collector surfaces during brush operation in atmospheric air. These brittle oxide films are semiconducting at best, and are physically hard and abrasive when disrupted during normal sliding operations. Because of this, they lead to relat:Lvely high unstable contact volta~e drops and prevent achievement of optimum low brush friction and wear.
The deposition of such films on the collector surfaces can be.minimized by operating the system in an inert gas atmosphere~ rather .than air. These oxygen free environments which include carbon dioxide, sulfur hexa-fluoride and hydrogen, are effective in extending the carbon brush lifetimes and in lowering the contact voltage drops since the insulating and abrasively-hard tarnish fllms are avoided. However, the demand now exists for high current 3 density brushes and the above gas environments were known to 38E~l produce good results only at prevailing current densities, i.e. about 10 amperes per square centimeter. Also with regard to the envlronmental factor, the pressure and compo-sitlon of ambient gases, including additives such as water vapor, contribute to the reduction of brush friction and wear. High friction and very high wear (dusting) occur when sliding contact pairs operate in vacuum or in dry gas am-bients, such as at high altitudes.
The temperature at the brush-slipring interface lC also directly affects brush life since dusting will occur at predetermined temperatures for different carbon brush materials. It appears that desorption of moisture ~rom the contacting surfaces becomes excessive as the critical temperature is reached for each humidity condition, and this condition must be eliminated for high current density applications.
It is therefore apparent that the need exists for an improved current collection system which wlll operate ~or greater lifetimes while simultaneously transferring current through the brushes in a magnitude 10 to 15 times greater than that posslble in present designs.
According to the present :Lnvention a solid brush current collecting system comprises a dynamoelectric machine having a stator and a rotor supported therein and arranged for electrodynamic cooperation therewith; at least one current collector on said rotor which collects current during machine operation; solid current collecting brushes mounted in brush holders on said machine, said brush holders being positioned to permit brush contact with said current collector; means enclosing said current collector and 31~23881 brushes in a substantlally fluld-tigh~ cavlty closed to the atmosphere; means for circulating a pressurized gas through said cavity; said gas havin~ mixed therewith an additlve which comprises a vaporous organic substance permitting a voltage drop at the brushes o~ no more than 0.2 volts and a brush wear rate no greater than 20 cublc mllllmeters of wear per megameter of slipring travel whlle runnlng at a temper-ature ~ust less than the critical brush bulk temperature.
The solid brush current collecting systems of the present invention selectively use compatible materials in different application~ for the moving and stationary contact members. The materials are chosen with regard to whether the applicatlon requires commutation as in heteropolar machines or merely transfers current as in homopolar ma-chines. These contact members are operated in an atmospheric environment which utilizes non-oxidizing gases, such as carbon dioxide, having the vaporous organic substance, for the purpose of operating the collecting system at higher temperatures, higher current densities, and higher velocities than ls possible wlth conventlonal systems. To transfer current in an arcless manner, the solid brush collector system undergoes forced contact cooling to maintain the temperature at the contacting members interface at relatively low values to thus achieve low friction and low wear rates of the sliding contact members.
In order that the invention can be more clearly understood3 a convenient embodiment thereof will now be descrlbed, by way of example3 with reference to the accom panying drawings in which:
3 Figure 1 is a view in elevation, partly in section, generally lllustrating a solid brush current collecting system;
Figure 2 is an enlarge~ view of the current col-lectlng system of Figure l;
Figure 3 illustrates the arrangement used to effect cooling of brushes by trans~errlng heat by conductlon from a brush holder to a heat exchanger on the brush holder surface;
Figure 4 lncludes curves which show electrograph-ite brush volume wear in alr or carbon dioxide;
Figure 5 includes curves which show average brush ring mechanlcal and electrical ener~y losses;
Figure 6 illustrates curves whic~h show brush wear characteristics for sllver-graphite, copper-graphlte and graphlte brushes;
~igure 7 includes curves w~lch show the effect of vapor additives on brush voltage drop; and Figure 8 includes curves which show the e~ect of vapor additi~es on brush wear.
Referring now to the drawings whereln like refer-ence characters designate like or correspondin~ parts throughout the several views, there is shown in Figures 1 and 2, a homopolar generator of advanced design havlng a base 10 which supports stator 12 and a rotor 14 arranged for electrodynamic cooperation therewith. Although the inven tion is useful with any kind of dynamoelectric machine, the machine components and construction not bearing directly on the invention are only generally illustrated. The rotor 14 is supported on bearings 16 located on opposite ends of the 3 machine and a coupler 17 is used ~or connecting the rotor to 1~23B~

prime mover, such as a motor. To provide for proper cool-ing, the rotor is equipped with an inlet 18 whlch supplies a low temperature coolant through a central passageway 20 and radially through ducts 22 prior to returning through dls~
charge passages to the space 24 and outlet 26.
The stator contains a pair of field coils 28 (only one shown) which are energized through appropriate conduc-tors and are cooled through coolant supply and discharge tubes 30. Current generated by the machine durlng operation is supplied through a conductor sleeve 32, circumferentially disposed on the rotor, to a commutator or sliprings 33 and current collecting apparatus 34 mounted in opposite ends of the stator. The collector brushes 44 are ~connected to cylindrical conductors 36 mounted on the stator inner sur-face whlch supply current to a load through terminals 38.
Referring to Figure 2 which sho~s the current col-lectors in greater detail, a circumferentially disposed cavity 40 is formed in the stator core 12 which is closed at the air gap by an insulated brush holder 42. The brush holder is appropriately bored or milled to pro~ide openings which house brushes 44 and each brush is ur~ed into contact with the rotor sliprings 33 by constant tension springs 46.
The brush shunts 48 are bolted or otherwise secured to the stator conductor 36 in the usual manner. In order to properly cool the brush holders located on both ends o~ the machine, separate coolant supply pipes 50 extend through opposite ends of the stator into cavity 40. These pipes are either embedded in or secured to the brush holder surface and extend circumferentially therearound before leaving the 3 brush holder cavi~y on the other side of the machine.

Slnce machine eff:Lciency requlres that the brushes operate at a temperature level where dustlng and substantial arclng will not occur, it is necessary to provlde a heat sink ~or the brushes to permlt transfer of heat therefrom by conduction. To accomplish this3 the brushholders supporting the brushes facilltate the exchange of heat between the brush holders and a coolant which flows through the cooling fluid inlet 52 and outlet 54 supported by the stator.
Preferably, the brush holder contains internal circuitous passageways which lie adjacent or close to the brushes in contact with the commutator or slipring sur~aces.
In the alternative arrangement shown in Figure 3, the brush holder ~2 is supported relatlve to slipring 33 in the manner of Figure 1, but additionally, a circular or rectangular pipe 50 is welded or otherwise affixed to the brush holder surface.
As indicated above, the making of this invention has been prompted by the recent design changes made in dynamoelectric machines, particular homopolar generators, which require brushes capable of continuous operation at current densities of 155 amperes per square centimeter and higher, at higher sliding velocities and at substantially decreased wear rates. These desirable performance charac-teristics are achieved by minimizing the brush-slipring inter~ace resistance which appears as electrical losses, by minimizing the friction between the brush and slipring which appears as mechanical losses, and by minimiz~ng the brush wear rates. To accomplish reduction in the electrical and mechanical losses~ and in wear rates, specific combinations 3~ of materials are selected for particular applications, the llZ3B81 operating environment is chan~ed from present practices and the contact members are positively cooled.
Considerlng the combination of materials 3 pre-limlnary brush-slipring test data showed that when metal was added to the graphite brush matrix, substantial reduction in the electrical component of the total energy loss was made possible. Conflrmation of this effect is shown in the following Table I, and by the curves of Figure 4.
TABLE I
BRUSH-RING TEST DATA

Cu Ring, 13 meters/second Moisture Additive 2 Single Brush Area: ~ cm 2 Loads: 7 newtons/cm ; 78 amperes/cm ~ric. Contact Energy~Loss Brush Brush GasCoe~.~ Drop, J/(cm .m) We~r, rade ~mbient ~ V Mech. Elec. Tbtal nm /Mm *EGl Air 0.17 o.66 0.34 o.48 0.822.32 **EGl Air O.05 0.78 0.35 2.38 2.7312.28 20EGl C02 0.05 0.41 0.35 2.50 2.850.81 SGl C02 0.23 0.00 1.59 0.00 1.592.Ll5 *Loads: 2 N/cm2~ 9 A/cm2 *NLoad: 39 A~cm2 These results show that silver-graphite grate SGl brushes which contain 80% silver by welght display an elec-trical contact loss of substantially zero but at the expense of increased mechanical loss. The total energy loss however was desirably reduced to 56% of that for the EG brushes, both grades operating under the same load conditions and in carbon dioxide. Although the total contact energy loss was ; reduced with SGl brush9 the brush wear rate was much higher.

Relative to conventional operation of EG brushes in air, 38R~

however, essentially equal ll~e was obtained when the SG
brushes were operated in a carbon dioxide envlronment, even wlth eight tlmes higher current denslty. Moreover, the SG
brushes show a five fold advantage in life over the EG
brushes when load current of the latter is increased to half that of the former. Comparlng only EG brushes, with these same differences in current loading, an advantage ln life of fifteen fold was achleved when operatlon was in carbon dioxide rather than in air.
These results show that silver graphite grade SG 1 brushes which contain 80% silver by weight display an electrical contact loss of substantially zero but at the expense of increased mechanical loss. Th~e total energy loss however was desirably reduced to 56% of that for the EG
brushes.
The results produced by operation of the above brushes showed the desirability of determining the per-formance characteristics of multiple brushes, as indicated in the following Table. Therefore, twenty-~our com~nercially available metal-graphite brush grade materials were tested.
Many of the chosen materials are frequently incorporated ln brushes utilized in industrial and commercial applications and have proven performance ability at conventional current densities in air operation. The brush materials included copper or silver as the main metallic addition. They were formulated by the powder metallurgy compaction/sinter technique and they represent a range in metal content from 60 to 97 w/o ~percent metal by weight).

3~81 Approx. Approx. Approx.
Brush % Brush % Brush %
Grade Metal GradeMetal Grade Metal *CM3B 74 SM551 80 M9lX 92 SG212 75 W795 85 **C0157 93 SG142 75 SG515 85 *CMO 95 10 **ANK 75 SG20~ 85 W405 97 ME1541 76 *CM15 go 728 97 *Morganite Carbon Co.
**National Carbon Co.
Others - Stackpole Co.
The brushes were evaluated in combinatlon with copper alloy sllprings in a humidified (20C dew point) carbon dioxide gas atmosphere, with operating conditions of 78 amperes per square centimeter through the brush which is equivalent to ten times the conventional brush current density. The brushes were exposed to a mechanioal load of 7-8 N/cm2 tnewtons/square centimeter), and 13-25 m/s (meters/
second) ring speed. The contact energy loss and brush wear responses for these brushes are plotted as functions of brush metal content in Figures 5 and 6. Although some asymmetry was found in the contact performance between opposite polarity brushesS the average loss and wear values for both are shown in these figures. Many of the points represent averages of a number of duplicate runs, and scatter in the data is believed caused by different graphite base materials and different brush manufacturing processes.

38~3~

It will be noted that the upper curve shown in Figure 5 indicates that the total contact energy loss ~energy density per unit slide dlstance) is minimal when the brush metal content is near 80 w/o. The curve also show~
that the total loss is dominated by the electrlcal component when the metal content is less than about 70 w/o, but by the mechanical component at larger percentages. This evidence clearly points up the need for ef~icient use of metal in the brush material. Sufficient metal must be employed to achieve high conductivity, but a large amount of graphite is required to achieve low friction or good lubrication. The performance of certain copper-and silver-graphite brush grades of comparable metal content is shown to be similar thus suggestlng that economies can be realized ~lth addi-tions of copper rather than silver. In general, however, the copper-graphite brush grades perform with lower mechan-ical loss, i.e., lower friction coefficient, than the silver-graphite grades, but with higher electrical, i.e., higher contact drop, and total energy losse~.
Figure 6 illustra~es that brush wear whlch is volume wear per unit slide distance, is very low for small addltions of metal. In the range up to 65 w/o, wear in-creases ~rom 0.5 mm3/Mm with no metal to about 1 mm3/Mm (cubic millimeters per megameter)O At higher metal percent ages brushwear increases sharply, being 3 mm3/Mm at 75 w/o and 30 mm3/Mm at 85 w/o.
Concernlng the effect of temperature on brushwear, it is known that electrographitic brushes will experience very high brushwear in the form of dusting i~ the brushes are run at too high temperatures. The critical brush bulk ~LZ38~3 .-~3--temperature~, i.e., the temperatures lnsi.de the brush, for electrographitic brushes range between 180 and 220C when runnlng ln carbon dloxide environment~ humidified at levels of 0 and 20C dew point~ respectively. On the other hand, brush life is very long if the brush temperature is main-tained below these critical levels. It is believed that the reason for dusting is that desorption of moisture from the contact counterface graphite sites becomes excessive as the critical temperature is reached for each humidity condition.
There are unsatisfied surface energies which result in excessive adhesion forces between the contact members thus causing increased friction and high wear. It therefore is clear that for high current density applic~ations, adequate cooling of the sliding brush contacts is essential and the design illustrated in Figures 1-3 is intended to perform this cooling function.
To properly assess the influence of slipring or commutator materials on the current transfer system, nine~
teen different slipring materials were evaluated in combina-tion wi.th copper-graphite brushes. The ring materials included copper, silver, high strength-high conductivity copper alloys, graphite, nickel, nickel alloys, high zinc brass, and steels. The followin~ Table summarizes the operating condltions and test performance results for each of the slipring materials evaluated:

TABLR lI

PERFORMANCE OF SELECTED SLIPRING MATERIALS
Evaluation Test Conditions Single Brush Area 1 cm Carbon Dloxlde Atmosphere Two Brushes per Set 2 Moisture Additive (20C dew point) Current Denslty 78 A~cm Ring Velocity 15 m/s Load Pressure 8 N/cm Copper-Graphite Brushes Single Energy Brush Brush Friction Loss Holder Brush Drop, Goef., Densi~y, Temp., Wea~, Slipring Material Vl JJcm .m C mm /Mm Grade C Steel0.74 0.14 4.86 157 1.72 K Monel~S 0.82 0.09 4.82 169 ~ 0.15 316 S/Steel0.7~ O.lO 4.50 165 20.97 35 ~n Brass0.58 0.11 3.81 123 2.29 45 Ni/55 Cuo.5Ll 0.08 3.34 136 0.55 30 Ni/70 Cu0.53 0.08 3-31 136 0.59 #3 Tbol Steel0.39 0.15 3.15 119 13.20 Monel 0.49 -7 3-00 127 0.99 Nickel 0.38 0.10 2.73~ 108 0.25 Graphite 0.26 0.13 2.36 96 0.10 Ag Plated Cu0.14 0.18 2.17 97 0.20 Zr Cu 0.07 0.21 2.05 92 0.20 15 Ni/85 Cu0.12 0.17 2.00 90 0.40 Cu (Ag Bearing) 0.10 0.192.00 85 C0.15 8 Sn/4 Zn/Cu~.07 0.20 1.98 93 0.20 Cupaloy O.11 0.17 1.97 88 0,20 ~FH~ Cu -7 0.19 1.91 92 0.30 PD 135 Cu 0.06 0.19 1.85 90 <0.15 KR Monel 0.13 0.15 1.82 92 0.25 Generally, the test results show that the lowest net power loss and longest life were achieved when the copper containing graphite test brushes were run on copper, super-strength copper alloys 9 and silver surfaced collector rings. Although lower friction coefficients accompany operation on nickel, high nickel-containing, and steel metal rings, the associated higher contact resistances (voltage drops) result in relatively-high total energy losses. It will be noted that KR Monel appears to be an exception~
4 combining low contact drop with medlum friction to yield low energy loss and low brush wear. Wear of the brushes was significantly increased when they were combined with steel and high z:Lnc brass metal rlngs.
A number of conclusions can be reached ~rom t.he evaluations made of both brushes and sliprings ln a current transfer system.
1. The present state-of-the-art practice of 10 amperes per square centimeter brush current density is ex-tendible to at least eight times if brushes are operated in a humidified CO2 gas environment.

2. Based on evaluation test condltions, at least 15 times longer brush life was obtained by operating electro-graphitic brushes in a C02 environment as compared to air, even with eight~times the conven-tional current density.

3. Total contact energy loss is substantially reduced (44%) through the introduction of sil~er to a graphite brush matrix. Equal life was obtained wlth silver-graphite brushes operating in a CO2 environment compared to convention~l electro-graphitic brushes operating in air, e~en with eight times the conventional current density.

4. Based on the evaluations made, commercially avail-able graphite brushes containing 65 to 75 w/o silver represent the optimum combinatlon o~ brush materials for continuous operation in high current density machines.

5. The slipring materials evaluations show that high strength-high conductivity copper alloys are con-sidered the best candidates for the desired high current contact systems. Copper-graphite brushes -l6-comblned wlth rings of this type ylelded lower energy lo~s and lower wear characteristics than when combined with ring materlals such as nickel, high zlnc brass, and steel.
It is believed important to note that the film on the slipring contributes importantly to the very low wear character of electro-graphitic brushes in carbon dioxide environments. The copper slipring initially is cleaned and a very light graphlte film is deposited on the slipring by the brushes during the lirst ~ew rotations of the slipring.
The ~ilm ia difficult to detect visually and it does not perceptively change thereafter. Electrical conduction across the brush-slipring interface ls limited essentially by the brush constriction resistance which varies directly - with the resistivity. Since only minute wear occurred during the performance runs made~ it is con~ectured that solid-to-solid touching of the brush ring contact is pre-vented by adsorbed vapor and/or gas ~ilms, Graphite trans-ferred to the slipring and graphite in the brush face serve as high affinity adsorp~ion sites for the ambient gas va-pors. Thus, brush sliding occurs on very thin quasi-~luid films. Friction drag occurs as these films are sheared or as graphite crystallites are made to slip upon one another as relative motion between the ring and the brushes takes place.
In addition to brush and slipring materials, five different non-oxidizing gas atmospheres, including sulphur hexafluoride, on brush performance were evaluated. These included two silver graphite brushes in combination with a 3 copper slipring which were operated under similar conditions -r in each of the ~ases. Similar brushes were also operated ln air to provide the oxidizing gas comparison. Laboratory grade gases were used for the experiments, each wlth dew points less than~-68C prior to receiving deliberate addl-tions of moisture (0C dew point) ~ust before enterlng the brush rin~ test enclosure. The results are shown in the following Table III. It is to be noted that desired brush performances are characterized by low energy lo~s and low brush wear. The net effect of electrical loss (contact voltage drop) and mechanical loss (friction coef~icient) per unit distance travelled is reflected in the ener~y loss characteristics shown.
TABLE III
EFFECTS OF ENVIRONMENT GAS

SG 2 Grade Brushes (1 cm2/brush), Copper Ring (13 m/s) 2 2 Brush Loads: 78 A/cm , 8 N/cm Brush Bulk Temperature Range: 67-80C

Contact Friction Energy Brush Drop~ Coef., Los~, We~r, Gas* V ~ J/cm .m mm /Mm Air .00 .34 2.3 23.3 C2 '03 .18 1.6 3.2 SF6 .18 .10 1.9 2.2 N2 .17 .o6 1.6 1.5 He .26 .o6 2.1 1.3 Ar .17 .06 1.5 0.7 *Approx. 1 atmosphere total pressure.
Moisture additive partial pressure 600 Pa.
3 Brush performance, in terms of desired low energy loss and low wear~ is si~nificantly better in each of the five wetted non-oxidizing gas environments than in air. A

' llZ381 very low frict~on coefficient (0.06), lowest ener~y loss [1.5 J/cm2.m (~oules per square centimeter tlmes meters)], and lowest wear (O.7 r~m3/Mm were measured when the ~est brushes were run in an argon gas envlronment. Brush contack drop was v~ry low (O.03 V) in the carbon dioxide gas en-vironment, but it was six to nine times higher ln the other gases. The low contact voltage achleved with carbon diox-ide, however, is offset by a higher coefficient of friction (0.18) and higher brush wear (3.2 mm3/Mm).
Dynamic brush performance evaluations were made on five different hydrocarbon vapor additives as examples to support the interface model, in terms of their effect on the contact drop (resistance) and wear per~or~mance o~ silver-graphite brushes operating on a copper slipring in a "bone dry" carbon dioxide gas atmosphere.
Organic vapors include members of the alkane, alcohol, ketone, aldehyde and cycloparaffinic classes of materials selected from paraffinic (alkane) hydrocarbons having from 7 to 16 car~on atoms per molecule, such as, for example, heptane C7H16, dodecane C12~I26' hexadecane C16H34 and the like; alcohols having from 7 to 16 carbons, such as for example, heptanol C7H15OH, decanol CloH21OH and the like: ketones having from 7 to 16 carbons, such as, for example, 2~heptanone (amyl-methyl ketone) CH3CO(CH~2)4CH3, 2-decanone (methyl-octyl ketone) CH3COC8H17 and the like;
aldehydes having from 7 to 16 carbons, such as, for example, n-heptaldehyde (enanthaldehyde) CH3(CH2)5CHO 3 n-decylalde-hyde (capraldehyde) CH3(CH2)8CHO and the like~ and the cycloparaffinic compound decalin (decahydro naphthalene) 3 CloH18, and mixtures thereof. While these materlals contain ~l238~1 many isomers, the straight chain, normal (n-) single carbon-carbon bond forms is preferred because they are thought to attach better to the ~raphite materlals having less than 7 or more than 16 carbons present problems o~ addition. The most preferred materials are n-paraf~inic hydrocarbons having from 7 to 16 carbons.
Water vapor is also included for reference pur-poses. All of the addltive hydrocarbons are liquid at room temperature. Vapors were lntroduced into the continuously supplied test gas (CO2) by bubbling it through the additive, held at either 0 or 25C. Other vapor concentrations were obtained by blending portions o~ wetted and dry streams of the test gas. The total ambient gas press~ure was maintained near one atmosphere. The operating conditlons, the test vapor additives, and the brush-ring performance character-istics are shown in Figures 7 and 8. These tests were run in a CO2 environment at about 1 atmosphere total pressure, using 1 cm2 silver-graphite brushes and 13 m/s copper rings.
The brush loads were 78 A/cm2 and 8 N/cm2 and the brush temperature range was 65-78C.

It is evident from Fig. 7 that a significantly higher brush contact voltage prevails when hydrocarbon vapor additives are substituted for water vapor in C02 atmos-pheres. This is so even for very low partial pressures of the hydrocarbon additive vapors. The brush voltage magni-tude tends to be relatively`constant for all of the hydro-carbons and over very wide ranges of vapor concentrations.
There is, however, a modest increasing voltage character-istic noted with increasing vapor pressure.

3 Non-dusting wear was achieved through separate ~38~
~20~
additions of each Or the hydrocarbon vapors to pure dry C02 atmospheres in which high current silver-graphite brushes were operated, Fig. 8. Moreover, brush wear may be reduced by lncreasing the hydrocarbon additlves' vapor pressure in the range investigated. A given brush life is also achiev-able with lower vapor concentrations as the hydrocarbon molecular weight is increased. For example, equal brush life is indicated for 670 and 0.2 Pa vapor pressures of heptane and hexadecane, respectively. A much higher concen-tration of water vapor, 3000 Pa, is required to achieve thesame brush life. Although not shown, the brush-ring fric-tion coefficient remained essentially constant ~0.16) regardless of the vapor addltive or its c~oncentration pressure.
Those tests show that a substantial lmprovement in brush performance (lower interface energy loss and lower wear) was found when operation was in each of five selected gases (C02, SF6, N2, He and A) as compared to similar opera-tion in air. All test gas environments contained water vapor at a partial pressure of 600 Pa. The best performance exhibited by silver-graphite brushes operating at 78 A/cm2 current density was obtained in an argon environment.
~ ive different hydrocarbons were tested as vapor additions to an otherwise dry carbon dioxide gas atmosphere.
These were found to be equally as effective as moisture in providing lubrication and low wear. Brush performance in these environments was found to be dependent on the hydro-carbon molecular weight (chain length) and upon the vapor concentration. Relative to moisture additions, equal brush life is achieved wi~h very low concentrations of the hydro-~l238~
-2:L-carhon m~ ri~ls selectPd. (`'ont~ct voltage ~r~p C.~il be affected by varying the partlal pressure of the hydrocarbon additive.
[-t; will be apparent that many modifications and variations are possible in light of the above teachings.
~he specific materlals used for the contact members, both stationary and rotating, will obviously need to be selected for each particular application where tradeoffs in regard to contact resistance, friction and wear rates, can be made.
It will occur to those skilled in the art that different materials' com~inations may be sultable depending on whether the application requires commutation, for example, hetero-polar machines which use commutators or s~egmented rlngs; or merely transfers current, as for example in homopolar machines which generally use continuous collector rings. ~s indicated in this disclosure, typical combinations include electro-graphitic carbon brushes on copper commutators, silver or copper-graphite brushes on copper alloy or steel sliprings, or carbon brushes on copper sliprings. '.rhe stationary and rotating material members are, of course, operated in an oxygen-free gas environment into which is incorporated a suitable vapor additive. Also, the cavity housing the brush holders and ad~acent current collectors may be located in a portion of the stator as disclosed herein, or axially outwardly therefrom, as in direct current machines.

Claims (10)

What we claim is:

1. A solid brush current collecting system compris-ing:
a dynamoelectric machine having a stator and a rotor supported therein and arranged for electrodynamic co-operation therewith;
at least one current collector on said rotor which collects current during machine operation;
solid current collecting brushes mounted in brush holders on said machine, said brush holders being positioned to permit brush contact with said current collector;
means enclosing said current collector and brushes in a fluid-tight cavity closed to the atmosphere;
means for circulating a pressurized non-oxidizing gas through said cavity;
said gas having an additive mixed therewith, said additive comprising a vaporous organic substance selected from the group consisting of paraffinic hydrocarbons having from 7 to 16 carbons, alcohols having from 7 to 16 carbons, ketones having from 7 to 16 carbons, aldehydes having from 7 to 16 carbons, decalin, and mixtures thereof.

2. The system according to claim 1 wherein the brushes are graphitic type brushes having metal dispersed therein which ranges in content from 30 to 97 percent by weight.

3. The system according to claim 2 where the brushes defined therein have a wear rate which ranges between 3 and 30mm3/Mm when the metal content in the brush ranges between about 75 and 85 w/o.

4. The system according to claim 1 wherein the gas circulated through said housing comprises carbon dioxide.

5. The system according to claim 1 wherein the rotor and each of said brush holders contain internal passages;
means adapted for connection to a source of coolant supply connected to said passages and arranged to circulate a coolant therethrough for carrying away heat generated during machine operation.

6. The system according to claim 1 wherein each of said brush holders has a cooling pipe placed in heat exchange relationship therewith for carrying away generated heat.

7. The system according to claim 1, wherein said vaporous organic substance is a paraffinic hydrocarbon having from 7 to 16 carbons.

8. The system according to claim 1. wherein said vaporous organic substance is a n-paraffinic hydrocarbon having from 7 to 16 carbons.

9. A solid brush current collecting system compris-ing:

a dynamoelectric machine having a stator and a rotor supported therein and arranged for electrodynamic cooperation therewith;
at least one current collector on said rotor which collects current during machine operation;
solid current collecting brushes mounted in brush holders on said machine, said brush holders being positioned to permit brush contact with said current collector, and where-in the brushes are graphitic type brushes having a metal dis-persed therein ranging in content from 30 to 97 percent by weight;
means enclosing said current collector and brushes in a fluid-tight cavity closed to the atmosphere;
means for circulating a pressurized non-oxidizing gas through said cavity;
said gas having an additive mixed therewith, said additive comprising a vaporous organic substance selected from the group consisting of paraffinic hydrocarbons having from 7 to 16 carbons, alcohols having from 7 to 16 carbons, ketones having from 7 to 16 carbons, aldehydes having from 7 to 16 carbons, decalin, and mixtures thereof; and cooling means for carrying away heat generated by said machine during operation, said cooling means including coolant flow passages in the machine and in said brush holders through which a liquid coolant is circulated.

10. The system according to claim 9 wherein the non-oxidizing gas circulated through the cavity includes a gas containing water vapor and selected from at least one of carbon dioxide, sulphur hexafluoride, nitrogen, helium and argon.