In every brushed DC motor, carbon brushes provide the sliding contact between the fixed terminal and the rotating commutator. They continuously wear down in operation and are therefore one of the few parts that wear by design.
Function of carbon brushes
The brushes conduct armature current through sliding contacts to the commutator, which switches the current according to rotor position. Only this interaction keeps the armature turning continuously. More on the interplay of brush and commutator in Brushed or Brushless Motor?.
Brush material composition
Carbon brushes are made from different carbon and graphite materials depending on the application, which differ markedly in electrical resistance, hardness, friction behaviour and permissible current density. The material choice depends on operating voltage, current, commutator surface speed and the operating environment.
| Material | Properties | Typical use |
|---|---|---|
| Hard carbon | High cleaning action, high resistivity, mildly abrasive | Universal motors, power tools, household appliances |
| Natural graphite | Resilient, vibration-damping, quiet running | Low operating voltage, automotive, high-speed slip rings |
| Electrographite | Very good electrical, mechanical and thermal properties, withstands high short-circuit currents | Most DC motors, traction motors, turbo-generators |
| Metal-graphite (copper-graphite) | High conductivity, low contact resistance | High current density at low voltage: starters, alternators, slip rings |
| Resin-bonded graphite carbon | High resistance, good damping | Generators, commutator motors, variable-frequency-drive applications |
Permissible current density ranges roughly from 7 to 30 A/cm² and depends heavily on the manufacturer and exact formulation. Resin or metal-salt impregnation further improves friction behaviour, spark suppression and oxidation resistance. JoMo selects the right brush grade based on your application's actual voltage, current and commutator surface speed.
The neutral zone and commutation
For sparkless commutation, a DC motor's brushes must sit exactly on the neutral zone - the region of the armature where no voltage is induced during commutation. If a brush sits outside this zone, the short-circuited armature coil is not voltage-free when the brush changes contact, causing sparking at the brush edge.
Under load, the cross-field created by armature current (armature reaction) shifts the magnetic neutral zone away from the geometric axis, more so as armature current increases. From roughly 1 kW of machine output, commutating poles (interpoles) automatically compensate for this cross-field, since their current is proportional to armature current - so brushes need not be readjusted across the load range.
During overhaul, the neutral position is found either statically (millivoltmeter between two brush arms with the armature stationary and externally energised) or - for the position actually effective at rated load - dynamically using a measuring strip on the commutator. Incorrectly set brushes show up as sparking, discolouration and accelerated wear on commutator and brush, as well as noticeable loss of torque and efficiency.
Signs of wear
Wear, sticking, or incorrect spring pressure on the brushes typically show up as increased sparking at the commutator, overheating, rough running and a visibly shortening brush length. If these signs are ignored, wear at the commutator accelerates further, in the worst case leading to motor failure.
Carbon brush holders
The brush holder guides the carbon brush radially onto the commutator and presses it against the surface with a defined spring force. Trailing holders tilt the brush in the direction of rotation, reaction holders against it - both designs reduce vibration and are chosen depending on rotation direction and reversing duty.
Common spring systems are simple coil springs, whose pressure drops as the brush wears, and constant-force (clock-spring) designs, which keep pressure almost constant over the full wear length. Depending on brush material, recommended spring pressure is roughly 18 to 55 kPa; pressure should not vary by more than 10-15% between the brushes of one machine.
The gap between brush holder and commutator surface is typically about 2.5 to 3 mm on most machines - too large a gap lets the brush tilt and wear unevenly in the holder, too small a gap risks mechanical collision. The pigtail carries current from the brush to the holder arm at low resistance; the spring itself should not carry significant current, or it will overheat and fatigue prematurely.
Maintenance measures
Routine maintenance of brushed DC motors includes inspecting the brushes and commutator and cleaning them of carbon dust and other debris. High-quality, correctly matched carbon brushes typically achieve a significantly longer service life than generic or ill-fitting products.
New brushes must be bedded in to match the commutator's curvature before continuous operation: fine sandpaper is placed between brush and commutator and the machine moved by hand until the full brush face bears; afterwards, all sanding and carbon dust is removed completely. Where fewer than about 20% of a machine's brushes are replaced, evenly distributed, pre-radiused brushes usually bed themselves in during operation.
Brushes should always be replaced as a complete, matching set - mixing brush grades on one machine creates uneven contact resistance and accelerates wear. When changing grades, the old commutator film must also be fully removed.
Commutator care alongside brush maintenance includes checking for out-of-round running, chatter marks, grooving and bluish discolouration from overheating, plus - on classic mica-insulated commutators - timely mica undercutting. A surface that is too smooth and glossy prevents the protective graphite film from forming and must be lightly roughened before commissioning.
For cleaning: remove carbon dust with an industrial vacuum first; if compressed air is used afterwards, it should blow dust out of the machine rather than into it, since conductive carbon dust can otherwise compromise winding insulation. An insulation-resistance measurement before and after cleaning shows whether it was effective.
Advantages and disadvantages of carbon brushes
Advantages
- Simple, robust technology proven over more than a century, with a wear part that is easy to replace in the field.
- Graphite is self-lubricating and forms a thin, conductive film on the commutator that reduces friction and wear.
- With a correctly matched material, low contact resistance and reliable electrical contact.
- Lower acquisition and repair cost than brushless, electronically commutated drives.
Disadvantages
- A wear part by design, with recurring maintenance and replacement needs.
- Generates carbon dust, which can be a problem in clean rooms, textile production or optical applications.
- Brush sparking at the commutation edge generates sparks and electromagnetic interference; hazardous (explosive) areas generally require additional protective measures.
- Permissible commutator surface speed is limited by centrifugal force, and the protective graphite film only forms reliably within a certain humidity range - wear increases in very dry or very humid air.
Where these drawbacks outweigh the cost advantage, JoMo also designs brushless drives - more in our article Brushed or Brushless Motor?.
Repair or replace?
Brush or commutator wear caught in time can usually be repaired. Where faults recur, or the commutator is already badly affected, a technically updated replacement may be more economical. Our article Repair or Replace a DC Motor? explains the decision criteria.
Frequently asked questions
What is the function of carbon brushes in a DC motor?
Carbon brushes mechanically transfer armature current to the rotating commutator, establishing electrical contact with the spinning armature.
How can you tell that carbon brushes are worn?
Typical signs are increased sparking at the commutator, overheating, rough running, a visibly shortened brush length, and unusual noise.
What happens if carbon brushes are not maintained in time?
Wear, sticking, or incorrect spring pressure lead to increased sparking, overheating and accelerated commutator wear, up to motor failure.
How often should brushes and the commutator be inspected?
This depends heavily on load, speed, dust and humidity in the specific application; a blanket interval cannot be stated responsibly. We define sensible inspection intervals together based on your operating conditions.
What material are carbon brushes made of?
Depending on requirements, from hard carbon, natural graphite, electrographite, metal-graphite (copper-graphite) or resin-bonded graphite carbon. The material choice depends on operating voltage, current and commutator surface speed.
What is the neutral zone in a DC motor?
The neutral zone is the region of the armature where no voltage is induced during commutation. Only when brushes sit there does the motor commutate without sparking; commutating poles keep this position correct automatically under varying load from roughly 1 kW of output.
How much pressure should be applied to carbon brushes?
Depending on the brush material, recommended spring pressure is roughly 18 to 55 kPa. Too little pressure causes sparking, too much causes accelerated wear - spring force should be checked regularly with a spring scale.
What are the pros and cons of carbon brushes versus brushless drives?
Carbon brushes are inexpensive, robust and easy to replace, but generate wear, carbon dust and brush sparking, and are limited in speed and humidity tolerance. Brushless drives avoid these drawbacks but need more complex electronic control.
Sources (selection): elektropraktiker.de, rgbelektronika.de, Mersen: Setting the Neutral, Mersen: carbon brush maintenance guide, schunk-group.com, elosal.de, Wikipedia: Wendepolwicklung (commutating-pole winding).
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