Carbon BrushesA Comprehensive Guide,

for Industrial and Railway Technology

Specifications • Installation • Troubleshooting

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

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%

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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

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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

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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

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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

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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

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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

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