Motor Carbon Brushes: Function, Selection and Replacement

1. Function of Carbon Brushes

A carbon brush is pressed and sintered from graphite and metal powders (copper, silver). Under spring pressure it slides against the rotating slip ring or commutator, feeding external current into the rotor winding: wound-rotor asynchronous motors connect starting/speed-control resistors through slip rings; DC motors commutate armature current through the commutator; synchronous motors supply rotor excitation through slip rings.

2. Grades and Selection

Carbon brush grades (J164, J204, D374B, CH17, etc.) represent different material formulations that determine resistivity, contact voltage drop, friction coefficient and permissible current density:
- Graphite-based (D series): good lubrication for high-speed commutators (DC motors);
- Metal-graphite (J series): high copper content, low resistivity and high current capacity for wound-motor slip rings — J164 is common on crane and metallurgical wound-rotor motors;
- Electrographite: good commutation with small sparking.
Always select the grade specified on the motor nameplate or in the OEM manual; never mix different grades on the same motor.

3. Dimensions and Customization

Brush cross-section (width × thickness × height) must match the brush holder: excessive clearance causes chatter and sparking, too little causes sticking. Standard sizes are available off the shelf; special dimensions can be made to drawing — a typical example is a J164 20×50×50 mm custom brush. When ordering, provide: grade, dimensions, shunt length and terminal type, and whether a wear-alarm wire is required.

4. Replacement and Maintenance

• Replacement interval: when a brush wears to 1/3 of its original height (or the shunt reveals the warning mark), it must be replaced; always replace the complete set on one motor to avoid uneven current sharing between old and new brushes
• Seating: bed new brushes with fine glass-paper (grade 00; never use emery cloth) along the commutator curvature until contact area reaches 70%+
• Routine checks: check spark grade (normally faint pinpoint sparking below grade 1.25), holder spring pressure (typically 15–25 kPa), slip-ring surface finish and any burned grooves
• Slip-ring care: minor burns can be stoned with a fine oilstone; severe out-of-roundness requires machining; periodically blow away carbon dust with dry compressed air to prevent dust short circuits