What Is the Function of Carbon Brushes?

Carbon brushes are essential components in many brushed electric motors, generators, alternators and other rotating electrical machines. Their primary function is to transfer electrical current between a stationary electrical circuit and a rotating component while maintaining continuous electrical contact.

Unlike a fixed electrical connection, a carbon brush operates against a rotating commutator or slip ring. The brush remains stationary while its contact surface slides continuously over the rotating surface.

This simple operating principle allows electrical power to be transferred while the machine is rotating.

How Do Carbon Brushes Work?

A typical brushed motor contains four important components:

Carbon brush → Commutator → Armature → Carbon brush holder

In a DC motor, electrical current enters the motor through one carbon brush and passes into the rotating commutator. The commutator distributes the current to the armature windings.

As the armature rotates, the commutator continuously changes the electrical connection between the windings and the brushes.

The second brush provides the return electrical path.

The carbon brush therefore acts as the electrical interface between the stationary and rotating parts of the motor.

In a slip-ring motor or generator, the operating principle is slightly different. Instead of contacting a commutator, the carbon brush contacts a rotating slip ring.

Why Are Carbon Brushes Made from Carbon?

Carbon is particularly suitable for electrical sliding contacts because it combines several useful properties.

A carbon brush can provide:

• Electrical conductivity

• Controlled friction

• Wear resistance

• Good sliding characteristics

• Thermal stability

• Relatively low damage to the commutator or slip ring

Different carbon materials are used depending on the application.

Common brush families include:

• Electrographite

• Natural graphite

• Resin-bonded carbon

• Metal-graphite

• Silver-graphite

• Copper-graphite

The correct grade depends on factors such as motor speed, current, voltage, commutator or slip-ring material, humidity and mechanical operating conditions.

Therefore, two motors using the same physical brush dimensions may still require different carbon grades.

Carbon Brushes and Current Transfer

One of the most important functions of a carbon brush is maintaining continuous current transfer while the rotor is moving.

Consider a motor rotating at 3,000 rpm.

The rotor completes:

3,000 ÷ 60 = 50 revolutions per second

The brush therefore maintains a sliding electrical contact with a rapidly rotating surface while the motor is operating.

If the electrical contact becomes unstable, several problems can occur, including increased sparking, electrical noise, localized heating and accelerated brush or commutator wear.

For this reason, the brush must maintain an appropriate contact pressure throughout its working life.

The Function of the Carbon Brush Holder

The carbon brush does not normally operate alone.

A carbon brush holder keeps the brush in the correct position and allows it to move toward the commutator or slip ring as the brush wears.

A spring applies pressure to the brush.

The basic system is therefore:

Spring → Carbon Brush → Commutator / Slip Ring

The holder must provide enough clearance for the brush to move freely without allowing excessive movement.

Mersen's technical documentation identifies proper brush guidance and clearance as important factors in preventing brush sticking or excessive movement.

Spring Pressure

Contact pressure is another important factor in brush operation.

If the pressure is too low, the brush may lose stable contact with the rotating surface.

If the pressure is too high, mechanical friction and brush wear can increase.

As an engineering reference, Mersen gives approximately 180–250 g/cm² as a typical brush-pressure range for stationary electrical machines. For certain high-vibration applications, its published guidance gives approximately 350–500 g/cm².

These values are reference figures rather than universal specifications. The correct pressure depends on the motor or generator, brush grade, dimensions, speed and operating conditions.

Carbon Brushes Are Designed to Wear

One unusual characteristic of carbon brushes is that they are intentionally designed as a wear component.

The brush is softer and easier to replace than the commutator or slip ring.

During normal operation, a small amount of carbon material is gradually removed from the brush surface.

This means the brush becomes shorter over time.

The spring inside the brush holder compensates for this reduction in length and continues pushing the brush toward the rotating surface.

Eventually, the brush reaches its minimum usable length and needs to be replaced.

This makes carbon brushes relatively inexpensive maintenance components compared with major motor components such as the armature, bearings or stator.

What Happens When Carbon Brushes Wear?

Worn carbon brushes can cause several symptoms:

• Increased electrical sparking

• Motor noise

• Reduced motor performance

• Intermittent electrical contact

• Excessive carbon dust

• Higher commutator temperature

• Uneven brush wear

• Reduced motor reliability

Severe brush wear can eventually prevent the brush from maintaining sufficient contact pressure.

For this reason, carbon brushes in serviceable motors should be inspected periodically.

Carbon Brush Applications

Carbon brushes are used in many types of electrical machines.

Common applications include:

DC Motors

DC motors are one of the most common applications. The brushes transfer current to the armature through the commutator.

Generators

Carbon brushes can transfer electrical current between rotating and stationary components in certain generator designs.

Slip-Ring Motors

In wound-rotor AC motors, carbon brushes contact rotating slip rings.

Automotive Motors

Small brushed motors can use carbon brushes in applications such as:

• Window regulators

• Windshield wipers

• Seat adjustment motors

• Blower motors

• Sunroof motors

• Door-lock actuators

Industrial Equipment

Carbon brushes are also used in industrial motors, generators, cranes, machine tools and other rotating electrical equipment.

How Long Do Carbon Brushes Last?

There is no single service-life figure that applies to every carbon brush.

Brush life depends on:

Current density + speed + brush grade + spring pressure + commutator condition + operating environment

For example, a brush operating at high current density and high rotational speed may wear considerably faster than a brush operating under light electrical loading.

Humidity, contamination, vibration, commutator roughness and incorrect spring pressure can also accelerate wear.

Therefore, manufacturers normally specify brush grades according to the actual machine operating conditions rather than using one universal service-life number.

How to Select the Correct Carbon Brush

When replacing a carbon brush, the physical dimensions are important, but they are not the only factors.

Useful information includes:

• Brush length

• Brush width

• Brush height

• Carbon grade

• Lead type

• Terminal configuration

• Spring configuration

• Voltage

• Current

• Motor speed

• Commutator or slip-ring material

• Machine model

• Original part number

For a replacement project, an original brush or a clear photograph can also help identify the correct configuration.

For customized carbon brushes, an engineering drawing or physical sample provides even more useful information.

Vocarbon Carbon Brushes

Vocarbon supplies carbon brushes and carbon brush holders for industrial and automotive electrical applications.

The product range can cover carbon brushes for DC motors, generators, slip-ring motors and compact automotive motors.

For replacement requirements, customers can provide an original part number, drawing, sample, dimensions or application information.

The objective is not simply to reproduce the physical shape of a brush. The carbon grade, dimensions, electrical characteristics and mechanical configuration all need to be considered together.

A correctly selected carbon brush helps maintain stable current transfer, controlled wear and reliable operation of the complete electrical machine.

Conclusion

The fundamental function of a carbon brush is simple:

It transfers electrical current between a stationary circuit and a rotating electrical component while maintaining continuous sliding contact.

However, achieving reliable operation requires much more than conductivity alone.

The carbon grade, brush dimensions, spring pressure, brush-holder clearance, commutator or slip-ring condition and machine operating parameters all influence performance.

For this reason, carbon brushes should be regarded as precision electrical and mechanical components rather than ordinary consumable parts.

Vocarbon — Carbon Brushes, Carbon Brush Holders and Slip Ring Components for Electric Motors and Rotating Electrical Machines.