Quick Answer
The commutator surface is a record of how the brushes and the machine are behaving. A healthy commutator carries a uniform, smooth, satin film, light brown to dark grey depending on grade and load — the uniformity matters far more than the shade. Problems fall into three groups of causes: electrical (overload, light load, poor commutation, unequal current sharing between brushes), mechanical (commutator runout, high bars or high mica, flat spots, vibration, worn or misaligned brush holders, wrong spring pressure) and environmental (air too dry or too humid, silicone or oil contamination, corrosive gases, abrasive dust). The patterns point to them. Streaking without copper wear usually comes from humidity, oil vapour, gases or light load. Threading means copper transfer, typically from light load or low spring pressure, and grooving means abrasion. Slot bar marking that follows the coils per slot is often a harmless design effect, but eroded bars mean commutation trouble. Pitch bar marking, repeating with the number of brush arms, points to mechanical disturbances or equalizer faults. Copper drag points to a cold commutator, contamination or commutation faults, and burnt bar edges mean sparking. The rule that saves most money: find and correct the cause before turning the commutator or changing the grade — otherwise the problem returns on the new surface within weeks.
The call usually starts the same way: "the brushes are sparking." Sometimes they are. More often, the brushes are the messenger. They are the softest part of a sliding contact that also involves the commutator copper, the mica, the brush holders, the springs, the machine's electrical design and the air inside the enclosure — and when any of those goes wrong, the brushes and the commutator surface show it first.
Learning to read that surface is the most useful maintenance skill on a brush machine. This guide covers what a healthy commutator looks like, what the common patterns mean, and how to find the cause rather than treat the symptom.
The Commutator Film
Within hours of running, a thin film forms on the commutator surface under the brushes. It is made mainly of copper oxides, carbon and graphite particles transferred from the brush, and adsorbed water vapour. It is not dirt. It is what makes the sliding contact work:
- It lubricates, keeping friction and wear low
- It provides contact resistance, which helps the brush control the current reversal during commutation
- It protects the copper from direct carbon-on-copper wear
A good film is uniform, smooth and satin, with the same colour on every bar. Its shade depends on the grade, current density, temperature and atmosphere — light brown, chocolate, grey or near-black can all be normal. What is not normal is variation: bands, patches, bar-by-bar differences, bright copper, or a surface that changes from inspection to inspection.
The film needs warmth and a little moisture to form. On a commutator running below about 40°C, or in air holding less than about 2 g of water vapour per cubic metre, it does not build up properly, whatever the grade.
How to Inspect
- Isolate and lock out the machine; brush gear on DC machines and exciters can be live at dangerous voltage
- Use good light and look along the commutator, not just straight at it — patterns show better at a low angle
- Rotate the commutator slowly by hand and check every bar
- Photograph the surface at each inspection from the same position, so changes can be compared
- Remove a few brushes and examine their faces, sides and shunts
- Measure brush length to record wear rate
- Check spring pressure on every brush, and the brush fit in each holder
Reading the Commutator Surface
Pattern | What it looks like | Common causes |
|---|---|---|
Uniform film | Even colour and texture on all bars | Healthy — keep conditions as they are |
Streaking | Bands of lighter and darker film running round the commutator, with no copper wear | Excess humidity, oil vapour, aggressive gases, light load. Not harmful in itself, but can progress to threading |
Threading | Fine lines cut into the copper in the brush tracks, as copper transfers to the brush faces | Light load, low spring pressure, porous or unsuitable grade, contamination |
Grooving | Uniform circumferential wear, the width of the brush track | Abrasive dust, abrasive grade, spring pressure below about 10 kPa, lack of brush staggering on slip rings |
Copper drag | Copper smeared over the bar edges, sometimes bridging the slots | Copper transferred in a molten state: a cold or over-ventilated machine, silicone vapour, abrasive dust, uneven spring pressure, too smooth a surface, commutation faults. Chamfered bar edges help prevent it |
Pitch bar marking | Dark or burnt bars in a pattern that repeats with the number of brush arms | Cyclic disturbance: unbalance, bent or misaligned shaft, worn bearings, weak foundation; failed equalizers or poor riser connections. Can end in flashover if left |
Slot bar marking | Every second, third or fourth bar darker, repeating with the number of coils per slot | Slot bar filming is a design effect and usually harmless. Slot bar burning, with the bars eroded, means commutation trouble; a grade with better commutating ability is the usual remedy |
Burnt bar edges | Pitting and burning at bar edges, usually trailing edges | Commutation sparking, overload, brush bounce, failed components, electrical asymmetry, a poorly commutating grade |
Raw copper | Bright copper with little or no film | Abrasive contamination, very dry air, silicone, excessive pressure, very light load, a commutator running too cold |
Heavy, thick or patchy black film | Film built up unevenly, sometimes flaking | High temperature, overload, oily or chemical contamination; patchy film also from an out-of-round or dirty commutator |
Ghosting | A dark imprint of the brush face on the commutator | Long standstill with the brushes in place, or momentary stalling under voltage |
Flat spots or high bars | Individual bars or zones burnt or worn differently, often with brush chatter | Runout, loose bars, mechanical damage, previous flashover |
Reading the Brushes
Brush condition | Common causes |
|---|---|
Polished, glazed face | Light load, low humidity, oil or silicone contamination |
Etched or pitted face | Sparking, overload, poor commutation |
Edge burning | Commutation sparking at the leaving edge; brush position or interpole fault |
Chipped edges | Brush bounce from runout, high bars, high mica or vibration; weak springs; brush loose in holder; chatter at light load |
Cracked or split brush | Mechanical shock, overheating, holder too tight |
Copper particles embedded in face | Threading in progress, copper pick-up |
Uneven wear between brushes | Unequal spring pressure, unequal current sharing, mixed grades |
Discoloured or burnt shunt | Excessive current through that brush; poor contact elsewhere forcing current through it |
Brush sticking in holder | Holder dirty or worn, brush oversize, high temperature |
Sparking
Some visible sparking at the brush trailing edge is normal on many DC machines under load. What matters is its type, size and whether it is changing.
Commutation sparking appears at the trailing edge of the brushes and grows with load. Causes:
- Overload or rapid load changes
- Brushes not set on the magnetic neutral
- Interpole (commutating pole) strength wrong for the load — too strong or too weak
- Brush grade with too low a contact drop for the machine
- Unequal brush spacing around the commutator
Where the interpole setting is in doubt, a black-band test settles it: the interpole current is boosted and bucked at several loads to find the band of sparkless commutation. IEC 60773 describes the method.
Mechanical sparking appears irregularly, at any load, often with noise. Causes:
- Commutator runout, flat spots, high or low bars
- High mica — the mica insulation standing proud of the copper
- Brush bounce from low spring pressure or worn holders
- Vibration from the drive or foundation
Ring fire — sparks travelling round the commutator — or a flashover between brush arms is serious and needs immediate shutdown and investigation.
Mechanical Checks
These are the checks that find most of the causes:
- Commutator runout — measured with a dial indicator on the commutator surface while rotating slowly. A new or refurbished commutator should be within about 30 µm total indicated runout (TIR). Regrinding or turning is normally due when TIR exceeds about 75 µm, or the step between adjacent bars exceeds about 7.5 µm.
- Surface finish — a fine, uniform turned or ground finish, not a mirror; the film forms better on a slightly textured surface. Brush makers recommend a roughness of about Ra 0.9–2.0 µm for industrial commutators and Ra 0.5–1.0 µm on machines under 1 kW. Below about Ra 0.4 µm, problems are likely.
- Mica undercut — the mica between bars must sit below the copper surface, with clean, chamfered bar edges. A common rule is an undercut depth about equal to the slot width, with the bar edges chamfered about 0.2–0.5 mm at 45°.
- Brush holder clearance — the gap between the holder's bottom edge and the commutator. Brush makers recommend about 2–3 mm, typically 2.5 mm; reset the holder if the gap is under about 2 mm or over 3 mm. Too large a gap lets the brush tilt and chatter.
- Brush fit in holder — free sliding without rocking. IEC 60136 sets the clearance between brush and holder box, from a few hundredths of a millimetre on the smallest brushes to a few tenths on the largest. A worn holder box lets the brush tilt even if the brush is the correct size.
- Holder alignment and spacing — brush arms equally spaced round the commutator, holders parallel to the bars.
- Spring pressure — measured on every brush with a spring balance at the working position, at the grade's nominal value (20 kPa for EG4), with the spread between brushes kept within about 15%.
- Machine vibration — bearing condition, alignment, foundation.
Diagnosis Table
Symptom | Probable causes | What to check first |
|---|---|---|
Rapid brush wear, glazed brushes, streaky film | Light load, humidity too low or too high, oil vapour or other contamination | Current density at actual load; humidity; silicone or oil sources |
Threading in brush tracks | Light load, low spring pressure, copper pick-up | Current density and spring pressure; remove brushes symmetrically or change grade |
Grooving | Abrasive dust, abrasive grade, low spring pressure | Air filtration, enclosure, grade, spring pressure |
Sparking increasing with load | Commutation: neutral, interpole, overload, grade | Brush position, interpole air gap and current, load profile |
Irregular sparking at any load, chipped brushes | Runout, high bars, high mica, bounce | Dial indicator, mica, spring pressure, holder wear |
Bar marking repeating with the coils per slot | Slot bar filming (design effect) or slot bar burning (commutation) | Whether the bars are eroded; interpole setting, grade |
Bar marking repeating with the number of brush arms | Pitch bar marking: cyclic mechanical disturbance, equalizer or riser fault | Balance, shaft, bearings, foundation; bar-to-bar resistance, equalizer and riser connections |
One brush or arm running hot, shunt discoloured | Unequal current sharing (selective action) | Spring pressures, brush fit, connections, mixed grades |
Copper drag | Cold or over-ventilated machine, silicone, abrasive dust, uneven pressure, commutation faults | Commutator temperature, ventilation, contamination sources, spring pressure; chamfer the bar edges |
Burnt bar edges after an event | Flashover, fault, severe overload | Insulation, protection settings, event record |
Maintenance Procedures
Cleaning. Remove brush dust by vacuum, or with clean, dry, low-pressure air directed so dust is carried out of the machine, not into the windings. Carbon dust on insulation is a tracking and flashover risk.
Stoning. A commutator stone held against the running commutator removes light surface irregularities and restores a uniform surface. It is a light correction, not a substitute for turning.
Turning or grinding. When runout, flats or deep grooving exceed what stoning can correct, the commutator is turned or ground — in place with a portable tool, or in a lathe during a rewind or overhaul — and then the mica is undercut and the bar edges chamfered.
Bedding in new brushes. New brushes must be shaped to the commutator curvature before full load:
- Wrap a strip of medium-grit abrasive cloth (about grade 80), abrasive side out, round the commutator, or draw it under each brush. Brush makers specify silicon carbide cloth; non-conductive garnet paper is also widely used
- Draw the abrasive in the direction of rotation for radial and trailing holders, and against the direction of rotation for reaction holders; on reversing machines, seat the brushes in both directions
- Never use emery cloth or emery paper — emery is conductive, and particles embedded in the brush face or lodged between bars cause sparking and short circuits
- A pumice bedding stone is for small corrections only, because its dust slightly wears the commutator copper
- Vacuum out all abrasive dust thoroughly afterwards
- Run at reduced load at first where possible, so the film can form
Ordering brushes with the contact face pre-radiused to the commutator diameter shortens bedding.
Record keeping. For each machine, record brush length, film appearance (with photographs), spring pressures, runout and any sparking at each inspection. A change in wear rate or film appearance is the earliest warning.
Common Troubleshooting Mistakes
Turning the commutator without finding the cause. The new surface threads or grooves again within weeks.
Prevention: Identify and correct the cause — load, environment, mechanical fault — before machining.
Changing grade to cure a mechanical fault. High mica or runout sparks with any grade.
Prevention: Complete the mechanical checks before changing grade.
Emery cloth for bedding. Conductive particles embed in brushes and commutator.
Prevention: Use medium-grit silicon carbide cloth or garnet paper, drawn in the right direction for the holder type, and clean thoroughly afterwards.
Replacing only the worn-out brushes. New and old brushes of different length, face shape and sometimes grade share current unevenly.
Prevention: Replace complete sets, bed them in, and never mix grades.
Polishing the commutator to a mirror finish. The film struggles to form on a too-smooth surface.
Prevention: Finish to the recommended surface texture after turning (about Ra 0.9–2.0 µm for industrial commutators), not to a polish.
Ignoring spring pressure. Weak springs cause bounce and burning; uneven springs cause uneven current sharing.
Prevention: Measure every spring with a spring balance at each inspection and replace weak springs as sets.
Blowing brush dust into the windings. Carbon dust on insulation leads to tracking and flashover.
Prevention: Vacuum, or direct low-pressure dry air so dust leaves the machine.
Silicone products near the machine. Unexplained rapid wear.
Prevention: Keep silicone sealants, sprays and greases out of brush machine areas.
No photographic record. Gradual change is invisible without comparison.
Prevention: Photograph the commutator from the same position at every inspection.
Supply from Kasko Makine
Kasko Mak branded EG4 carbon brushes
EG4 electrographite brushes for DC machines, slip-ring motors, generators and turbogenerator exciters — low contact resistance and a low friction coefficient for stable commutation and low commutator wear. Nominal current density 12 A/cm², maximum peripheral speed 60 m/s, contact drop 0.9–1.9 V per pair, friction coefficient 0.25 maximum, nominal spring pressure 20 kPa, dimensions and test methods to GOST 12232-89.
Produced in 25 × 32 × 40 mm and to drawing or sample, with single or twin tamped shunts, forked, ring or flat-blade terminals, and top cushion, contact shoe or radiused face options to shorten bedding.
Engineering support
Send photographs of the commutator surface and of brush faces, sides and shunts, with the machine data — current, voltage, speed, commutator diameter, brush arrangement, load profile and environment. We will review the likely causes and the grade match, and quote replacement brushes.
Send your photos and machine data for a diagnosis and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.
Continue Reading: Carbon Brush Series
- Carbon Brushes: Types, Grades and How They Work
- How to Select a Carbon Brush Grade
- Replacement Carbon Brushes: Dimensions, Shunts and Terminals
- Slip Ring and Exciter Brushes
Frequently Asked Questions
Q: What should a healthy commutator look like?
A: A healthy commutator carries a uniform, smooth, satin film with the same colour on every bar. The shade can range from light brown to dark grey depending on brush grade, current density, temperature and atmosphere, and any of those shades can be normal. Warning signs are variation — bands, patches, bar-to-bar differences, bright copper — or a surface that changes between inspections.
Q: What causes carbon brushes to spark?
A: Sparking that grows with load is usually commutation sparking, caused by overload, brushes off the magnetic neutral, incorrect interpole strength or a grade with too low a contact drop. Irregular sparking at any load usually has a mechanical cause such as commutator runout, high bars, high mica, weak springs or worn holders. Ring fire or flashover requires immediate shutdown.
Q: What causes threading on a commutator?
A: Threading is fine thread-like lines cut into the copper in the brush tracks. It starts when copper transfers to the brush faces and the embedded particles cut the surface. The usual causes are low current density, low spring pressure, a porous or unsuitable grade and contamination. Raising current density by removing brushes symmetrically or changing to a suitable grade, correcting spring pressure and removing the contamination source stops it.
Q: What is bar marking on a commutator?
A: Bar marking is a regular pattern of darker or burnt commutator bars. Slot bar marking, on every second, third or fourth bar, follows the number of coils per slot; as plain filming it is a design effect and usually harmless, but eroded bars mean commutation needs attention. Pitch bar marking repeats with the number of brush arms and points to unbalance, shaft, bearing or foundation problems, or faulty equalizer and riser connections.
Q: Why must emery cloth never be used to bed in carbon brushes?
A: Emery is conductive. Its particles embed in the brush face and lodge between commutator bars, causing sparking and short circuits between bars. Brush makers specify medium-grit silicon carbide cloth, about grade 80, or non-conductive garnet paper, drawn under the brushes in the direction of rotation for radial and trailing holders and against it for reaction holders. All abrasive dust must be removed before the machine runs.
Q: Should a commutator be polished to a mirror finish?
A: No. The commutator film forms and stays stable more readily on a fine, uniform turned or ground surface than on a mirror polish. Brush makers recommend about Ra 0.9–2.0 µm for industrial commutators, and problems are likely below about Ra 0.4 µm. After turning, undercut the mica, chamfer the bar edges and bed in the new brushes, so the film can build up under reduced load where possible.
Q: Why does one carbon brush run hotter than the others?
A: Brushes in parallel share current according to their contact resistance, so a brush with higher spring pressure, a better fit, a different grade or a cleaner contact takes more than its share and overheats, often with a discoloured shunt. Left alone, this selective action can burn off shunts, overload the remaining brushes and end in flashover. Unequal spring pressures, worn holders, mixed grades and poor connections are the usual causes.
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