IEC 60136: Motor Brushes and Brush-Holders — The Current Bridge Between Rotation and Station

Motor Brushes and Brush-Holders — The Sliding Contact Standard for DC and Wound-Rotor Machines

IEC 60136 specifies brush and brush-holder dimensions for electrical machinery. Despite the rise of brushless motors, brushes remain irreplaceable in large DC machines (steel rolling mills, mine hoists), railway traction motors, and wound-rotor induction motors.

Brush selection triad: (1) Current density — carbon brushes typically 6–10 A/cm², electrographitic grades up to 12–15 A/cm². Too low: brush overheats, accelerated oxidation. Too high: uneconomically large brush. (2) Contact voltage drop — carbon-on-copper total drop ~1.5–2.5 V (nearly independent of current — a unique “constant-voltage-drop” characteristic of carbon materials). (3) Spring pressure — must maintain 150–250 g/cm² for stable contact; too low causes sparking, too high accelerates wear.

TN Lab — Brushes are the sole method of transferring current between rotating and stationary parts. Selection errors can severely damage slip rings.