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Carbon BrushesA Comprehensive Guide,
for Industrial and Railway Technology
Specifications • Installation • Troubleshooting8900 West Tower Avenue • Milwaukee, WI 53224
800.962.4851 Fax 800.365.3113www.helwigcp.com
The name “brushes” comes frombundles of tiny copper wires, whichwere used as elastic contacts atthe starting period of the electricalengineering industry.
The term “carbon brush” appearedwith the change to materials madeout of graphite and carbon. Carbonbrushes have been developed forseveral decades: they are alsoindispensable hardware forelectrical machines in themicroelectronic era.
The production parameters permita wide variation of the physicalcharacteristics and the applicationpossibilities. Following DIN EN60276 we subdivide our carbonbrush grades into 5 main groups:• Electrographite brushes• Metal graphite brushes• Resin bonded graphite brushes• Carbon graphite brushes• Graphite brushes
Carbon Brushes - Very Important Functional Parts ofCarrying Current in Electrical Machines
Standards for carbon brushes, material and accessories
Below are given the most important standards for industrial andtraction carbon brushes.
DIN IEC 60136-3 Dimensions of brushes and brush holdersfor electrical machinery.
DIN EN 60276 Definitions and nomenclature for carbon brushes, brush holders, commutators andsliprings.
DIN IEC 60413 Test procedures for determining physical properties of brush materials used for electrical machines.
DIN IEC 60467 Test procedures for determining physical properties of carbon brushes for electricalmachines.
DIN 43021 Carbon brushes for traction motors. Dimensions and tolerances.
DIN 46224 Stamped cable sockets for flexibles of carbon brushes.
DIN IEC 60760 Flat plug contacts.
DIN 46438 Copper flexibles.
2 3
Information in this handbook courtesy of E-Carbon Partner:Pantrac GmbH, Berlin, Germany
t = tangential Brush dimension in direction of rotation of commutator or slipring
a = axial Brush dimension parallel to the axis of rotation
r = radial Brush dimension perpendicular to the axis of rotation
Dimensions of carbon brushesare specified according to DINIEC 60136-3 in the sequencet x a x r. In order to avoidmisinterpretation we suggest tospecify the dimensions in thissequence.
The cross section of the flexibleis determined according to a.m.norm and DIN 46438.
The length (l) of the flexible is thedistance between the top of the
brush and the center of theterminal (see sketch).
When ordering brushes withspecial terminals (e.g. plug-contact) the length (l) of theflexible should be measured inthe accordance to the sketch.
In standard DIN 46224 are givenrecommendations for basic typesand standards for dimensioningof plug types.
1. Solid brushesThe simplest type of brush used onbasic machines without electricaland/or mechanical (challenges).
2. Split brushesSplit bushes are formed byassembling 2, 3, or more carbonsections into one unit to createbetter electrical and mechanicalcontact conditions.
First, the (splitting) of the brushleads to a large number ofelectrical contact points betweenthe brush surface and thecommutator; it also increases theresistance in the transverse circuitof the brush because of theadditional contact resistancebetween the brush parts. Thecutting of the brush causes smalleracceleration forces over the brushparts which enables better dynamicproperties.
Split brushes give satisfactoryresults (mainly) on machines withreversing operations, since there isa faster adaptation to the runningsurface. In addition, tops made ofrubber, laminate, or both cause auniform pressure distribution, aswell as a bigger damping ability.
The tangential dimension of thebrush sections must not be lessthan 4 mm for mechanicalreasons.
Dimensions and Design of Carbon Brushes
commutator brush slipring brush
54
rr
a at t
cushioningelement
cushioningelement
P
P
P
1
2a
2b
- - -
3. Spread brushes
The spread brush is a specialtype of split brush in which thetops of both sections arechamfered at a certain angletowards their dividing line.Pressure to the brush is appliedvia a top piece with accordinglychamfered bottom face. The toppieces are generally made frombrush or insulation material thathas a cushioning effect.
4. Tandem brushes
Tandem brushes are special pairsof brushes, where each brushhas its own box within thetandem brush holder, pressedagainst the commutator byseparate pressure fingers. Theresult is a symmetrical brushpressure and current distribution.
5. Sandwich brushes
When the segments of splitbrushes are bonded together,they are called sandwich brushes.The bonding layer can be aninsulating material. From amechanical point of view this isnow a solid brush with anadditional polishing effect causedby the bonding layer. From anelectrical point of view, the brushhas an increased crossresistance. By using differentbrush materials for the singlewafers, it is possible to influencethe commutation properties ofthe carbon brush.
6. Carbon brushes withwear sensor
Carbon brushes with wearsensors signal when the wearlimit is reached and enable alower maintenance supervision.An insulated sensor cable isglued in the carbon brush and thewarning occurs when theinsulation of the contact is worndown through the wear of thecarbon brush.
The warning is electricallyevaluated and optically andacoustically recorded.
P3
4
5
6
P P
P bonding layer
flexible
insulatedsensor cable
6 7
Here are the required conditionsfor perfect carrying and for blackcommutation:• good roundness of the
commutator• no lamination protrusions• no flat points on the
commutator• a symmetrical undercutting of
the commutator insulation, and chamfered laminate edges
• very slight roughness of the commutator or slipring surface
• uniform brush pressure• good seating of the brushes to
the commutator/slipring surface.
Commutator and slipringmachining
A new or reconditionedcommutator should have an outof roundness of not more than0.02 mm. Lamination protrusionsbetween neighboring laminatesover 0.002 mm and the flatpoints must be eliminated.Depending on the applicationconditions and the carbon brushmaterial, generally the rotorsshould be reconditioned if thereis long-wave unroundness of morethan 0.30 mm and short-waveroundness of more than 0.15 mm.
Directions for Installation and Operation
Commutators with flush micaagainst the copper segmentsrequire very hard brush materialwith the consequence of highcommutator wear. For a betterlifetime, it is necessary toundercut the insulation (seesketches).
A new commutator should havea surface of roughness Rzbetween 4 and 8 µm.
Brush pressure
The brush pressure must beset to the operating conditions.It must also represent acompromise between themechanical and electrical wear.
Undercut to the correctwidth and depth
Undercut andbeveled correctly
Offset undercut(wrong)
Too thin an undercut(wrong)
90
0,2...0,4
brush pressure
brus
hw
ear
V total
V mechanical
V electrical
Recommendations for brush pressure
Brush pressure in kPaType of machine mounting
stable swing frame
DC machines up to 1500 r.p.m. 15-20 20-30
DC machines over 1500 r.p.m. 20-25 25-35
Three-phase commutator machines 18-20 25-30
Slipring motors 20-25 25-35
Turbo generators 15-25 -
Traction motors - 30-50
The tolerance of the brush should be limited to ± 10%
8 9
Seating in of the carbonbrushes
The interface of brushes to theslipring or commutator surfaceoccurs with the seating in thebrushes. Here with are severaltechniques used:
1. The carbon brushes areplaced in the brush holder. Astrip of abrasive linen(recommend garnet paper of 80-100 granulation) is pushedbetween the brush and thesurface of the commutator orslipring: This strip is then drawnin a tangential direction.
After the running surface hasapproximately taken the shapeof the commutator or slipring’ssurface the contact mean isdrawn in the direction of rotationof the machine to finish thegrinding. In order to withdraw theabrasive linen, the brushes haveto be raised. In this way, it isguaranteed that the brushestake the same position in thebrush holder as in the futureoperation of the machine.
2. A strip of abrasive linen isfitted around the commutator orslipring, and fixed with adhesivetape. Then the brushes areplaced in the holder and therotor of the machine is turned
over in the operating direction(particularly at smallermachines).
3. The machine is completelyequipped with carbon brushesand it is put into operation inidle running (possibly with areduced speed of rotation). Apumice stone is pressed in frontof the brushes on thecommutator. The dust of thepumice stone which resultsreaches the brushes and grindsthem in.This method of grindingis particularly suited to large DCmachines.
The seating in can be finishedwhen about 70% of the runningsurface has contacted with thecommutator/slipring. After theseating in the carbon brusheshave to be taken away from theholder and the machine has tobe cleaned by means of oilfreecompressed air.
Attention,the dust must not reach thecoils or the machine bearing.
The carbon brushes have to becleaned with a clean rag (withoutoil or fat) and their runningsurface have to be cleaned bymeans of a glass brush in orderto avoid infiltrated grains.
Mounting of the brush holdersand brushes
To ensure correct operation ofbrushes, the holders andbrushes must be fitted exactlygeometrically on the commutatorin accordance with the design ofarmature and windings, forinstance the distance betweenbrushes of different polaritiesmust be equal. To make certainthat this is the case, a strip ofpaper is put around thecommutator under the brushes,and the distance between theimpressions of the brushes ofeach pole can then bemeasured correctly. This strip ofpaper can be used as a recordfor checking the positions at alater stage. A variation of aslittle as 0.5 mm between thebrushes of different polaritiescan cause considerableproblems with regard tocommutation and currentdistribution.
abrasive linen
Figure to 1
pumice stone
Figure to 3
10 11
The distance between thecommutator and the lower edgeof the brush box should be setat between 1.5 and 2.0 mm. Ifthe commutator has beenremachined, the brush holderswill have to be reset to maintainthis distance. The setting of thebrush holders should be donewith a distance piece.
If the distance between thebrush holder and commutator istoo high, this can lead to brushvibration, because the brush willbe tilted at a greater angle. Onthe DC machines, brushesshould be installed in the sametrack for each pole pair. Thisensures that the patina/filmbeing built is not adverselyaffected by the brushes on theplus and on the minus poles.On machines with high number
of poles, the brushes will bestaggered across thecommutator axially so that thereis even wear across thecommutator.
If the commutator is longenough, the best staggerarrangement would be asfollows:v = z = a/(p-1)
v = Staggerz = Distance between brush pair
of same polaritya = Axial brush dimensionp = Number of brush pole pairs,
i.e., half the number of poles of the machines
The fitting of brushes on thecommutator must be carried outeither exactly radial or at acertain angle, which is thencalled either a trailing or areaction position, according tothe direction in which thecommutator rotates.The trailing and reactionposition are used in order toreduce vibrations.
v
a
z
distance piece
trailingposition
radialposition
reactionposition
b
b
b
b
12 13
Assessment of Performance of Carbon Brushes
Appearance of the brush sliding face
The following pictures showtypical brush-sliding faces. Foreasy identification, we suggestyou use the symbols S1, S3 etc.
S1, S3 and S5 are satisfactorysliding faces, indicating thatthere are no mechanical orelectrical problems. Depending
on the carbon material, thesliding surface appears denseor porous, and shiny, dull ormatte. If there is dust in the
circulating air, fine hairliningmay occur, as shown in S5.
S1 Dense, shining sliding face
S3 Slight porous sliding face
S5 Fine hairlining
S7 Hairlining
S9 Tracking with hairliningand grooves
Normal operation
Normal operation
Normal operation, slight dust influence
Causes: Underload, influence of dust, oil, or grease, weak spring pressure
Causes: Like S7, but stronger
S1
S3
S5
S7
S9
14 15
S11 Ghostmarks, difficult commutation
S13 Burning edge of the leaving or trailing edge
S15 Eroded brush face
S17 Lamination of sliding face
S19 Double facing here for a twin brush
S21 Copper nests
S23 Broken edges
Causes: Commutation problems, e.g., false or incorrectposition of the neutral zone or interpole
Causes: Difficult commutation, heavy sparking, interruption ofcontact due to out of round of commutator or insufficientbrush holder spring pressure
Causes: Electrical overload, interruption of contact
Causes: Burned segments of the sliding face, caused by awinding fault giving voltage surge during commutation
Causes: Tilting of the brush in dual direction machine
Causes: Pick up of copper particles, often following copper drag
Causes: High raised lamination, commutator seriously out ofround, brush chatter by low load and idle running
S11
S13
S15
S17
S19
S21
S23
16 17
Commutatorappearance
In addition to the physicalappearance of the surface ofthe commutator, the skin orpatina/(film) is of equalimportance for the goodrunning of the carbon brushes.Each carbon brush builds acharacteristic patina/(film)which is affected by operatingand ambient conditions. Thepatina/(film) consists mainly ofcopper oxides, graphite depositsand absorbed water, and itsappearance is of importance forthe assessment of the runningbehavior of the commutationset. The following pictures areused by carbon brushmanufacturers and users ofbrushes as a guide to assist injudging the operation of carbonbrushes.
P2, P4 and P6 are examples ofnormal skin or patina/filmformation. When a machineruns well, the patina/film orskin on a commutator will beeven, slightly shiny and copperybrown to black in color.
Electrical, mechanical andatmospheric influences on thepatina/film appearance.
P12 Streaky patina/film having some wide and narrow tracks of different color. No commutator wear.
P14 Torn patina/film, general appearance as in P12, but with commutator wear
P16 Smutty patina/film, uneven skin having patchy colors and random spots
There may be appearance ofgreyish, blueish and reddishhues, but of importance is theevenness of the skin formationand not its color.
Causes: High humidity, oilvapor, aggressive gases in theatmosphere, low electricalload on the brushes
Causes: As in P12, but theconditions have beenmaintained for a longer period,causing commutator damage
Causes: Uneven commutatoror unclean operatingconditions
P2 P4 P6
P12 P14 P16
18 19
P22 Patina/film with dark areas,regular or irregular patches covering one or more commutator segments
P24 Dark patchy patina/film having definite edges as in T12 and T14
P26/P28Commutator segments having patches in the middle or at the edges
P42 Alternating light and dark bar markings
P46 Mat patches in double pole patches
Causes: Out of roundcommutator, vibrations of themotor caused by badly adjustedshaft or damaged bearings
Causes: Raised segment orgroup of segment causing thebrush to bounce
Causes: Often due to faultygrinding of the commutator orcommutating problems
Causes: Uneven currentdistribution over two parallelwindings caused by doublewindings crossing in the sameslot
Causes: Usually by faultysoldering of the risers orsegment connections
P22 P24
P26 P28
P42 P46
20 21
B2, B6, B8Burning at the edge or in the middle of bar
B10 Perforated patina/film, light, dense or distributed build-up spots
T10 Dark patches at edges of bars in direction of rotation
T12 Burning of a trailing edge and the next leading edge of a bar
T14 Dark markings
Causes: Sparking caused bycommutation problems
Causes: Patina/film destructioncaused by too large electricalresistance
Causes: Frequently caused bylong periods with the motorbeing stationary without poweror short stationary periodsunder load
Causes: Caused by protrudingsegment, as in L2
Causes: Sign of a low segment,could also be caused by a flatspot on the commutator
B2 B6 B8
B10
T10 T12 T14
22 23
T16 Clearly defined dark markings together with segment edges burnt
T18 Dark markings
Commutator wear
R2 Top view of a commutator
R4 Commutator bar showing abnormal metal abrasion
Causes: Raised mica (see L6)
Causes: Badly undercutsegment edges (see L8)
Causes: Trackwise normalmetal abrasion after longperiod of operation withcorrectly positioned brushes
Causes: Abnormal abrasion iscaused by incorrect brushalignment, inadequate brushmaterial or contamination, etc.
T16 T18
R2 R4
24 25
L2 Protruding segment
L4 Low segment
L6 Raised mica
L8 Ridge on the segment edge
L10 Copper drag
Causes: Faulty commutatorsegments
Causes: Bumps or vibrationswith various causes
L2 L4
L8
L10
L6
26 27
L2L4
L6
L8
L10
Cause
Out of round commutator orslipring
Insufficient brush pressure
Carbon brushes are stuck inholder
Oil or dirt between segments
Carbon brushes (not seated in)
Brush holder too far from thecommutator or slipring
Protruding insulation segments
Machine vibrating or chattering
Wrong positioning of brushbridge
Faulty installation of brush arms
Interpole too strong or too weak
Incorrect brush grade
Corrective measures
Turning or grinding (see“Directions for Installation andOperation”)
Increase brush pressure (seepage 9)
Carefully remove foreign bodiesand dust from brush holder.Dust grooves are recommended
Clean segments, filter coolingair, and possibly seal bearings
Repeat the seating in process
Adjust distance between holderand commutator to 2 mm
Undercut insulation and chamfersegments
If it is not possible to reducethe vibration of the machine,increase brush pressures or usea brush design fitted with fiberand rubber top
Establish neutral position andadjust brush arms accordingly
Adjust brush arms correctly
Machine manufacturer to correctfault, or install another brushgrade to compensate
Please, contact our technicalservice
Cause
Producing or low segments(L2,L4)
Raised mica insulation(T16, P24)
Out of round commutator or sliprings,i.e., badly out of balance(P16)
Bad soldering of risers(P42, P46)
Electrolytic deposit from brushto steel on stationary steelsliprings (galv. element)
Corrective measures
Retighten and turn thecommutator
Turning the commutator,undercut mica and possiblyretighten commutator
Rebalance and/or remachinecommutator or slipring
Resolder risers
In case of long standstillperiods, insert insulating stripunder the carbon brush
Instructions in Case of Operating Difficulties
Strong brush sparking Patches or burn marks
28 29
Cause
Overload on brush track due touneven current distribution
Dusty environment(P14)
Aggressive gasses or vapors(P12)
Grooving caused by lowelectrical load on brushes(P14)
Grooving caused by oil film oncommutator or sliprings
Material loss by anodic whenusing sliprings with DC current
Copper drag(L10)
Development of flat spots
Corrective measures
Adjust brush pressures to thecorrect level. Possibly usebrushes with a higher polishingeffect
Blow in clean air by installing afilter
Blow in clean air and usebrushes with a strongerpolishing effect
Reduce number of brushes perpole or change brush grade
Seal bearings and avoid oilvapor
Change polarity of sliprings fromtime to time
Because of complex nature ofpotential causes, please contactour Technical ServiceDepartment
Install starting current limits
Cause
Uneven current distribution
Bad connection of wire to brush
Mixed brush grades
Brushes stuck in holder
Corrective measures
Adjust brush pressure to correctlevel
Change carbon brushes
Use only one brush grade
Clean holder, brushes andcheck tolerances, use dustgrooves eventually
Excessive wear of commutatorand sliprings Uneven brush wear
30 31