Reasons for Overheating in Slip Ring Brushes Due to Mechanical Friction

Regarding overheating faults in slip rings and brushes—specifically those caused by mechanical friction—a significant proportion stems from mechanical and frictional factors. If, during startup and prior to the application of excitation current, the slip ring temperature is already observed to be elevated—or if, during operation, the temperature becomes excessively high but subsequently drops after removing a few brushes—it can be concluded with reasonable certainty that the issue is attributable to mechanical and frictional causes.

The circumstances under which mechanical friction generates heat are complex and primarily involve the following three aspects:

(1) The contact area between the brush and the slip ring is too small; generally, this contact area should not be less than 75% of the cross-sectional area of ​​a single brush. This issue may arise from poor seating (grinding) of the brush contact surface or from replacing too many brushes simultaneously during operation. When replacing brushes during operation, only one or two brushes per brush holder should be replaced at any given time. The new brushes being installed should be pre-ground against a template matching the diameter of the slip ring; furthermore, the grade of the new brushes must match that of the old ones. If a large number of brushes are replaced simultaneously during a major overhaul, sufficient time must be allowed during the pre-commissioning spin-up phase for an oxide film to form on the brush surfaces. As previously noted, the contact area between the brush and the slip ring should generally not be less than 75% of the cross-sectional area of ​​a single brush.

(2) The brush wobbles or jams within the brush holder. The brush must be able to move freely up and down within the holder; the clearance between the brush and the holder should comply with the product specifications. In the absence of specific specifications, this clearance may be set between 0.10 mm and 0.20 mm. The brush itself must be square and true in shape, with dimensional tolerances between its upper and lower ends not exceeding 0.05 mm.

(3) The clearance between the brush holder and the surface of the slip ring is excessive. Since brush materials are inherently brittle, an excessive clearance between the holder and the slip ring surface prevents the brush from making full, uniform contact with the slip ring during operation. Instead, the brush makes contact at an angle (obliquely), significantly increasing the likelihood of the brush chipping or fracturing. The clearance between the brush holder and the surface of the slip ring must comply with the product's technical specifications; in the absence of specific requirements, this clearance may be adjusted to 2–3 mm. To adjust the clearance, place a rubber pad 2–3 mm thick against the surface of the slip ring, press the brush holder firmly against the pad, tighten the positioning screws, and then remove the rubber pad.