What is carbon brush in motor?

Carbon brush is a key component in the motor, mainly made of graphite, metal powder and binder, is blocky structure. It plays an indispensable role in the operation of the motor, through close contact with the commutator or collector ring. To achieve current conduction and commutation function, to ensure the stable operation of the motor.

Appearance and basic composition of carbon brush

Appearance and form

The appearance of the carbon brush is usually a block object, and the shape is mostly cuboid or cube. Its size varies depending on the type and power of the motor. Generally, the carbon brush of a small motor may be only a few centimeters long. The carbon brush used by large motors will be relatively large, the length may reach ten centimeters or even longer. And it is wider and thicker. The two ends of the carbon brush are usually flat. This is to better contact with the commutator or collector ring in the motor to ensure the stable conduction of current. In the corner part, some carbon brushes will do some round treatment. It can reduce the wear of other parts during the operation of the motor.

Carbon brush material composition

Carbon brush is mainly composed of graphite, metal powder and binder. Graphite is the main component of carbon brush, which has good electrical conductivity and self-lubrication. This self-lubricity allows the carbon brush to reduce friction and wear when it comes into contact with the rotating parts inside the motor. It adds metal powders to enhance the electrical conductivity and wear resistance of the carbon brush. The role of the binder is to firmly combine the graphite and metal powder together. So that the carbon brush will not disintegrate during the operation of the motor because of external forces such as centrifugal force and friction force. And ensure the structural integrity of the carbon brush.

 Position and function of carbon brush in motor

Installation position

The carbon brush is installed on the brush holder of the motor. And the brush holder is generally fixed in the stator part of the motor. In position, the carbon brush is near the commutator of the motor (for DC motors) or the collector ring (for AC motors). Moreover, through the spring device of the brush holder, you can apply the carbon brush to a certain pressure. So that it can be tightly attached to the surface of the commutator or collector ring. This tight fit state is the key to ensure the normal operation of the motor.

Current conduction effect

The most basic role of carbon brushes is to conduct current. When the motor is running, the current from the external power supply is transmitted through the wire to the carbon brush. It then passes the electricity to the armature winding of the motor (in DC motors) or the rotor winding (in AC motors).

Carbon brush function

In addition to conducting current, carbon brushes have other important functions in the motor. In DC motor, carbon brush and commutator cooperate to achieve commutation function. The carbon brush will change the direction of the current in the armature winding. So that the direction of the electromagnetic force received by the armature winding remains unchanged. So as to ensure that the motor can continue to rotate stably.

In the AC motor, the function of the carbon brush is mainly to ensure that the current can be stably input into the rotor winding, maintain the effective interaction between the magnetic field and the rotor. And ensure the normal operation of the motor.

Types of carbon brush

Carbon brushes can be classified according to different criteria.

According to the material composition classification, there are graphite carbon brush, metal graphite carbon brush and electrochemical graphite carbon brush. Graphite carbon brush with natural graphite as the main raw material, its self-lubrication is good. It is suitable for some current requirements are not high, low speed motor, such as small fan motor. Metal graphite carbon brush contains more metal components, strong current carrying ability. You can often use  it in high current, low speed motor, such as large industrial DC motor. Electrochemical graphite carbon brush is synthetic, its comprehensive performance is better. And you can widely use it in general industrial motors.

According to the type of application motor, there are DC motor carbon brush and AC motor carbon brush. DC motor carbon brush needs to have good commutation performance. While AC motor carbon brush pays more attention to stable current transmission and good contact with the collector ring.

In addition, according to the different working environment, it can also be divided into high temperature resistant carbon brush, moisture resistant carbon brush and dustproof carbon brush. The high temperature resistant carbon brush can work normally in a high temperature environment. The moisture resistant carbon brush is suitable for a high humidity environment. And the dustproof carbon brush can ensure the normal operation of the motor in a more dust environment.

Working principle of carbon brush

Commutation principle

In DC motors, the commutation principle is a key part of the work of the carbon brush. When the motor armature rotates, the direction of the current in the armature winding needs to be constantly changed to ensure the continuous rotation of the motor. The combination of carbon brush and commutator achieves this process. The commutator consists of a plurality of insulated commutators. As the armature rotates, the carbon brush touches the different commutators in turn. When the carbon brush slides from one commutator to another commutator, it will change the direction of the current in the armature winding.

Electromagnetic induction and energy conversion

When the current enters the armature winding (or rotor winding) through the carbon brush, the electromagnetic force will be generated in the winding under the action of the magnetic field. And this electromagnetic force will drive the rotor of the motor to rotate. So as to achieve the conversion of electrical energy to mechanical energy. At the same time, during the motor rotation process, the armature winding (or rotor winding) will cut the magnetic field line. And according to the law of electromagnetic induction, the induced electromotive force will be generated. The direction of this induced electromotive force is opposite to the direction of the applied supply voltage. And it is called the back electromotive force. The back electromotive force is fed back to the power supply through the carbon brush to complete a complete energy conversion and transfer process.

Carbon brush specification

Carbon brushes have several key performance indicators, including contact pressure drop, friction coefficient, wear rate and current density.

The contact voltage drop reflects the contact resistance between the carbon brush and the commutator (or collector ring). And the smaller the contact voltage drop. The smaller the energy loss during current transmission and the higher the efficiency of the motor.

The friction coefficient directly affects the wear of the carbon brush and the operating resistance of the motor. And the lower friction coefficient can reduce the wear of the carbon brush, extend its service life. And also reduce the energy consumption and heating of the motor.

The wear rate is an important indicator to measure the service life of the carbon brush. And the carbon brush with low wear rate can maintain good performance for a long time. Reducing the maintenance cost and downtime of the motor.

The current density represents the current size that the carbon brush can withstand per unit area. For high-current motors, it is necessary to select a carbon brush with a higher current density to ensure that it can work stably.

Performance advantages of carbon brushes

Carbon brushes have many performance advantages. First of all, its good electrical conductivity can effectively reduce the energy loss of the motor and improve the operating efficiency of the motor. Secondly, the self-lubricity of the carbon brush can reduce the friction and wear between it and the commutator (or collector ring). It not only extends its own service life, but also reduces the damage to the commutator (or collector ring).

Moreover, the carbon brush has high mechanical strength and stability. And can adapt to the centrifugal force and vibration generated by the motor at high speed. As well as various stresses under complex conditions, to ensure that the motor can operate reliably under different working conditions. In addition, the carbon brush has strong adaptability to the surface of the commutator (or collector ring). It can automatically adjust the fit between the contact parts to a certain extent. Ensure the stability of the electrical contact. And reduce the failure caused by poor contact, such as ignition and heating.

Common faults and maintenance of carbon brushes

Common Fault types

Excessive wear: The length of the carbon brush is significantly shortened, resulting in poor contact with the commutator (or collector ring). Reduce motor performance, and failure to start or interrupt operation in serious cases.

Spark phenomenon: the contact part of the carbon brush and the commutator (or collector ring) sparkles. This accelerates the wear of the carbon brush, affects the stability and efficiency of the motor. And may even lead to short circuit.

Poor contact: Due to insufficient pressure of the carbon brush spring, loose brush grip, or oil and oxide layer on the surface of the commutator (or collector ring), the current transmission is blocked. The motor speed is unstable, and increases the degree of heating.

Maintenance measures

Regularly check the wear of the carbon brush, replace it in time when it is worn to the original length of 1/3-1/2. And choose the same model and specifications of the product to ensure correct installation.

Keep the surface of carbon brush and commutator (or collector ring) clean. Regularly clean oil, dust and impurities, sandpaper or special tools can be used to lightly polish the oxide layer.

Check the pressure and elasticity of the carbon brush spring, adjust the position or replace the spring if insufficient. And tighten the brush grip to prevent the carbon brush from shifting and shaking.

Conclusion

With the continuous development of motor technology, the performance requirements of carbon brush will become higher and higher. And the future research direction of carbon brush will be to further improve the electrical conductivity, reduce the wear rate, optimize the commutation performance and enhance the environmental adaptability. So as to better meet the performance needs of the motor.