Nantong Jingrun Industry Co., Ltd

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Name: Jack
Tel: +86-513-55880140
Fax: 86-513-65860718
Mobile: WhatsApp:86-15862708570
Add: No.503,14 Buliding, Guoliyuan, Chongchuan District, Nantong city, Jiangsu Province, China
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Assessment of Performance of Carbon Brushes

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. The patina consists mainly of copper oxides, graphite deposits and adsorbed water, and its appearance is of importance when assessing the most suitable brush grade. The following pictures show typical appearances of commutation surfaces.
The pictures are not of an international standard specification but are used by carbon brush manufacturers and users of brushers as a guide to assist in judging the operation of carbon brushes.

Normal Skin or Patina Formation

P2 P4 P6 P2, P4 and P6 are examples of normal skin or patina formation. When a machine runs well, the patina or skin on a commutator will be even, slightly shiny and coppery brown to black in colour. There may be appearance of greyish, blueish and reddish hues. but of importance is the evenness of the skin formation and not its colour.

Appearances of Badly Formed Patina or Skin

P12 P12 • Appearance: Streaky patina having some wide and narrow tracks of different colour.
No commutator wear.
Causes: High humidity, oil vapour, aggressive gases in the atmosphere, low electrical load on the brushes.
P14 P14 • Appearance: Torn patina, general appearance as in P12, but with narrower tracks and commutator wear.
Causes: As in P12, but the condi-tions have been maintained for a longer period causing commu-tator damage.
P16 P16 • Appearance: Smutty patina, uneven skin having patchy colours and random spots.
Causes: Uneven commutator or unclean operating conditions.
P22 P22 • Appearance: Patina with dark areas, regular or irregular patches covering one or more commutator segments.
Causes: Out of round commutator, vibrations of the motor caused by badly adjusted shaft or damaged bearings.
P24 P24 • Appearance: Dark patchy patina having definite edges as in Tl2 and T14.
Causes: Raised segment or group of segments causing the brush to bounce.
P26 P26 • Appearance: Commutator segments having patches in the middle or at the edges.
Causes: Often due to faulty grinding of the commutator.
P42 P42 • Appearance: Alternating light and dark bar markings .
Causes: Uneven current distribution over two parallel windings caused by double windings crossing in the same slot.
P46 P46 • Appearance: Mat patches in double pole pitches .
Causes: Usually by faulty soldering of the risers or segment connections.

Bar Burning

B2 B6 B8 B2, B6, B8. Appearance: Bar edge burning or burning in the middle of bar.
Causes: Sparking caused by commutation problems.
B10 B10 • Appearance: Perforated patina, light, dense or distributed build-up spots.
Causes: Patina destruction caused by too large electrical resistance.

Bar Marking

T10 T10 • Appearance: Dark patches at edge of bars in direction of rotation.
Causes: Frequently caused by long periods with the motor being stationary without power or short stationary periods under load.
T12 T12 • Appearance: Burning of a trailing edge and the next leading edge of a bar.
Causes: Caused by protruding segment as in L2.
T14 T14 • Appearance: Dark markings.
Causes: Sign of a low segment, could also be caused by a flat spot on the commutator (see L4).
T16 T16 • Appearance: Clearly defined dark markings together with segment edges burnt.
Causes: Raised mica (see L6).
T18 T18 • Appearance: Dark markings.
Causes: Badly undercut segment edges (see L8).


L L2 • Protruding segment
L4 • Low segment
L6 • Raised mica
L8 • Ridge on the segment edge
Causes: Faulty commutator segments
L10 • Copper drag
Causes: Bumps or vibrations from various causes

Appearance of the Brush Sliding Face

The following pictures show typical brush-sliding faces. For easy identification we suggest you use the symbols S1, S3 etc.
S1, S3 and S5 are satisfactory sliding faces, indicating that there are no mechanical or electrical problems. Depending on the carbon material the sliding surface appears dense or porous, and shiny dull or matt.
If there is dust in the circulating air fine hairlining may occur as shown in S5.

S1 S1 • Dense, shining sliding face Problem free operation
S3 S3 • Slight porous sliding face Problem free operation
S5 S5 • Fine hairlining Normal operation, slight dust influence
S7 S7 • Hairlining Causes: Underloaded, influence of dust, oil or grease.
S9 S9 • Tracking with hairlining and groves Causes: Like S7, but stronger.
S11 S11 • Ghostmarks, difficult commutation Causes: Commutation problems,e.g. false or incorrect position of the neutral zone or interpole.
S13 S13 • Burning edge of the leaving or trailing
Causes: Difficult commutation, heavy sparking, interruption of contact due to out of round of commutator or insufficient brush holder spring pressure.
S15 S15 • Eroded brush face Causes: Electrical overload, interruption of contact.
S17 S17 • Lamination of sliding face Causes: Burned segments of the sliding face caused by a winding fault giving voltage surge during commutation.
S19 S19 • Double facing here for a twin brush Causes: Tilting of the brush in dual direction machine.
S21 S21 • Copper nests Causes: Pick up of copper particles, often following copper drag.
S23 S23 • Broken edges Causes: High raised lamination, commutator seriously out of round, brush chatter.