Primary Functions of Carbon Brushes
Carbon brushes (also known as electrical brushes) are sliding contact components primarily installed between the stationary and rotating parts of motors or generators to transmit energy or signals. Typically made of carbon-based materials and shaped like small blocks, they are pressed tightly against the shaft or commutator by springs. They conduct current during motor rotation and are wear-prone parts that require regular maintenance and replacement.
Encyclopedia Entry
Primary Functions of Carbon Brushes
- Current Conduction: Transfers external current to the rotating rotor via the brush, or grounds electrostatic charges from the main shaft to earth.
- Current Direction Reversal: In commutator motors, carbon brushes work in conjunction with the commutator to reverse current direction, enabling continuous motor operation.
- Sliding Contact Provision: Establishes a stable electrical connection between the stationary and rotating parts of machinery to conduct signals or power current.
- Grounding Protection: Certain carbon brushes are used for rotor grounding protection and for measuring positive-to-ground and negative-to-ground voltages of the rotor.
Materials Used in Carbon Brushes
- Graphite Brushes: Made from natural graphite mixed with binders. They offer excellent lubrication properties and are commonly used in smoothly operating small-to-medium DC motors and high-speed turbine generator slip rings.
- Electrographitic Brushes: Produced from carbon black, coke, and graphite processed at high temperatures. They feature superior commutation performance and long service life, making them widely applicable across various AC and DC motors.
- Metal-Graphite Brushes: Composed of electrolytic copper, graphite, and silver powder. With high conductivity, they are suitable for high-load, low-voltage motors such as automotive starter motors.
- Resin-Bonded Graphite Brushes: Characterized by higher electrical resistance and good commutation performance, these are mostly used in AC commutator motors.
When to Replace Carbon Brushes
- Excessive Wear: Replacement is mandatory when wear exceeds two-thirds of the brush length, or when the remaining height is less than one-third of the original length.
- Poor Contact: Replace if the contact area between the brush and the slip ring or commutator falls below 70%.
- Abnormal Conditions: Prompt attention or replacement is required if sparking occurs, edges chip, brush shunt connections overheat due to poor contact, or the brush bounces or sticks within the brush holder.
- Pressure Issues: Replace when spring pressure becomes abnormal, deformed, or broken, resulting in unstable contact.