Carbon brush related failures.

           Failures of traction machines (TA, TG, Companion alternator, TMs) have been reported due to premature wear and other carbon brush related failures.

SN	Date of failure	Loco No.	Shed	Cause
1	27.3.11	20082		Companion alternator negative slip ring found blackish and carbon brush uneven.
2	3.5.11	20015		Companion Alternator one slip ring blackish. Sparking and excessive wear on carbon brush
3	20.5.11	14763		Traction Alternator negative slip ring carbon brush worn out and holder spring broken.
4	25.5.11	13011		CCEM commutator sparking.
5	26.5.11	12176		Carbon brush of CA found shortened due to sparking and slip ring surface became blackish.
6	4.6.11	17307		CCEM sparking on commutator.
7	27.6.11	13000		Both carbon brushes are in condemning limit. One slip ring damaged and groove created due to heavy sparking.
8	30.8.11	16086		TM No.1 button brush holder middle carbon brush’s 3 nos. pig tail up uprooted.
9	2.11.11	14966		TA carbon brush damaged.
10	17.12.11	17887		ECC carbon brush holder wires desoldered due to heat generation by bad commutation.

                    These failures are eminently avoidable, if analysis for released carbon brushes is carried out before fitment of new carbon brushes.

Carbon brush is an electrical contact which makes a connection with a moving surface. Performance of motor, TGs & TAs and other moving contact applications will be attained only when carbon brushes, brush holders and contact surface are properly maintained.

Inspection and maintenance guidelines for carbon brush and brush gears are outlined below:

·        All released carbon brushes of one machine duly tagged location wise should be kept at one place & examined for wear, colour, sign of sparking etc.

(contd..2)

(2)

·        If all carbon brushes are uniformly worn out then new carbon brushes can be fitted.

·        If one or two carbon brushes show excessive wear compared to rest then corresponding brush holder should be examined for pocket condition, spring tension, free movement of brush

·        If brushes show sign of sparking/discolouration then commutation should be checked.

·        If rapid wear is observed in all carbon brushes then grade & quality of carbon brushes and spring tension etc needs to be checked. It is essential that benchmark service life for different type of carbon brushes must be judiciously fixed.(not less than 80% of best ever value)

Colour of film, deposition rate, sign of spark, type of hairline, biased wear all have a tale to tell. We must train workmen to read those signals and take appropriate action before it is too late.

Appearance of the brush sliding face: Branch formation is due to extraneous dust. Breakage is due to oscillations. Burn mark is observed in case of additional current conduction and sparking. A brief guideline on deciphering tale tell signals is given below:

·          Shining sliding face or  slight porous sliding face is sign of  normal operation

·          Fine hairlining – is sign of normal operation under slight dust influence

·          Hairlining ,tracking with hairlines and grooves– is sign of Under load, influence of dust, oil or grease

·          Ghostmarks are due to communication problems e.g. false or incorrect position of the neutral zone or inter pole

·          Burning edges of the leaving or trailing edge are caused by difficult commutation, heavy sparking, interruption of contact due to out of roundness of commutator or insufficient brush holder spring pressure.

·          Eroded brush face is due to  Electrical overload or interruption of contact

·          Lamination of sliding face is due to Burned segments of the sliding face caused by a winding fault giving voltage surge during commutation

·          Copper nests on carbon brushes are due to Pick up of copper particles, often following copper drag.

·          Broken edges are caused by: High raised lamination, commutator seriously out of round, brush chatter by low load and idle running.

·          Strong brush sparkings are caused by Out of round commutator or slip ring, insufficient brush pressure, stuck carbon brushes in holder or oil/dirt between segments

Commutator appearance:

In addition to the physical appearance of the surface of the commutator, the skin or patina is of equal importance for the good running of the carbon brushes. Each carbon brush builds a characteristic patina which is affected by operating and ambient conditions.

(contd..3)

(3)

The patina consists mainly of copper oxides, graphite deposits and absorbed water and its appearance is of importance for the assessment of the running behaviour of the commutation set.

·          When a machine runs well, the patina or skin on a commutator will be even, slightly shiny and coppery brown to black in colour. Evenness of skin formation is parameter to critically look for.

·          Streaky or torn patina having some wide and narrow tracks of different colour are caused by High humidity, oil vapour, aggressive gases in the atmosphere, low electrical load on the brushes. When these conditions have been maintained for a longer period, commutator damage takes place.

·          Smutty patina, uneven skin having patchy colours and random spots are due to uneven commutator or unclean operating conditions.

·          Patina with dark areas, regular or irregular patches covering one or more commutator segments are due to Out of round commutator, vibrations of the motor causes by badly adjusted shaft or damaged bearings.

·          Dark patchy patina having definite edges is due to Raised segment or group of segment causing the brush to bounce.

·          Commutator segments having patches in the middle or at the edges  are Often due to faulty grinding of the commutator or commutating problems.

·          Alternating light and dark bar markings are due to uneven current distribution over two parallel windings causes by double windings crossing in the same slot.

·          Burning at the edge or in the middle of bar. Causes: sparking caused by commutation problems.

·          Perforated patina, light, dense or distributed build-up spots. Causes Patina destruction caused by too large electrical resistance.

Maintenance practices:

Directions for installation and operation:

·        Commutator must be fully round, free from lamination protrusions and flat points.

·        A symmetrical undercutting of the commutator insulation and chamfered laminate edges and Very slight roughness of the commutator or slip ring surface

·        Uniform brush pressure and Good seating of the brushes to the commutator/slip ring surface

·        A new commutator should have a surface of roughness Rz between 4 and 8 micrometer.(figs for TM,T.G/TA)

(contd..4)

(4)

·        A new or reconditioned commutator should have an out of roundness of not more than 0.02 mm.

Bedding in of carbon brushes:

·          The interface of brushes to the slip ring or commutator surface occurs with the bedding in of the brushes. Proper bedding must be ensured after change of new brushes.

·          After bedding, the carbon brushes have to be cleaned with a clean rag (without oil or fat) and their running surface have to be cleaned by means of a glass brush in order to avoid infiltrated grains. The carbon brushes should be removed from the holder and the machine has to be cleaned by means of oil free compressed air

Concentric running

Commutator and slip rings must have perfect concentric running. Otherwise  the case the brushes will lift from the contact area causing excessive sparking. This leads to pitting and consequent damage to slip ring or commutator surface.

          Smaller distortions can be rectified by hand with the help of commutator ground stones. However, with excessive distortion, commutator or slip ring has to be machined or ground off in its own bearing if possible at full speed.

 Carbon Brush Holder :

The fit of the carbon portion of the brush in the holder is critical for stable electrical contact. If there is inadequate space between holder walls and thickness and width and carbon brush, there is potential for binding the brush in holder particularly with increased temperature and contamination.

    

     On the other hand, an excess amount of space between holder and carbon will result in an unsuitable electrical contact as the brush face can move tangentially or axially within the holder.

     Over a long period, a holder can become worn out from brush movement or distorted from heat. Therefore, it is important to periodically measure the thickness and width dimensions on the top and bottom of the holders to ensure dimensional accuracy contact with the brush and the commutator.

The brushes should be equally spaced around the commutator. This spacing can be checked by wrapping a paper tape around the commutator, marking the location of the same edge of such brush and then measuring the distance between marks on the paper. A GO & NO GO gauge should be fabricated.

Important factors to check for proper functioning of the holder and brush are

·        Inside holder dimension

·        Holder spacing

(contd..5)

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·        Holder angle

·        Holder height

·        Spring face

·        Electrical connections

Use of digital graphical duplex type ovality measurement should be introduced.

TIR- total indicated run out (≤ 0.06mm)

MBTB- mean bar to bar height (≤ 0.005mm)

Service limits as laid down by OEM for different traction machines should be followed.

Carbon brush

To obtain satisfactory operation, it is essential that the correct care is given to carbon brushes and brush holders.

·        The clearance between the end of the brushes box and commutator should be between 1.5mm to2.0mm

·        No particle should be lodged between the commutator and bearing surface of the brush

·        Brushes must be correctly aligned so that in same brush arm they lie in the neutral axis circumferentially to ensure uniform wear over the commutator.

·        Brushes can be damaged when a motor works under abnormal conditions i.e. when the commutator surface is rough, contaminated or distorted. These abnormal conditions can create excessive sparking resulting in overheating of the brushes.

·        If the brush box is not located in the correct neutral axis or if there is electrical fault in the motor, brush will over heat and cause burning on edge of the brush with consequent damage to the commutator.

·        Vibration and chattering of the brush can be caused by incorrect brush tension, distorted surface of commutator or slip ring, raised insulation and high risers.

·        Maintain recommended uniform spring tension in all brush holders. The correct spring tension is of the most important for the patina to form on the surface of the commutator or slip ring, this will create the ideal film for carbon brushes to slide on and commutate with minimum wear. 

·        Clearance between brush and brush box is very important. Free movement of the brush must be available at all time. Dust and oil within brush box or on the brush surface can lead to sticking of brushes in the brush box or present

·        Carbon brush pig tail strength should be standardized by measuring force required to pull out pig tail from old carbon brushes. New carbon brushes should be tested at 90% of pull out force. Proper insulated sleeve/nylon tags should be used to arrest power ground cases.

A technical seminar should be planned  to explain the above concepts to shed supervisors and workmen