Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
;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~
--3--
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
--4--
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,
--6--
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.
23
--8--
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~
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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
W759 60 ME1540 80 SG510 90
SG156 64 SG520 80 SG201 90
5004 65 SG216 80 W933 92
*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~
-12-
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:
Z3881
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
-15-
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
-17-
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
--18--
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.
