June 2011 Electrical Apparatus

June 2011 Electrical Apparatus

This is a summary of the Electrical Apparatus June 2011 featured technical article, by Richard L. Nailen, P.E.   

Managing the sliding contact between a rotating commutator and fixed brushes was among the earliest challenges for electric machinery technologists. Most of the development effort, and its application today, has dealt with d-c motors and generators.

Less attention has been paid to the slip ring assembly, or collector, connecting the secondary winding of a polyphase wound-rotor motor to the external circuit. Large motors of that type are common in such major industrial facilities a cement and metal-processing.

Most rings are of copper alloys. High currents and speeds call for steel rings, which withstand higher temperature (although brush wear increases). Steel rings at high surface speed usually contain helical grooves. One purpose is to add cooling surface. Another is to shift current flow axially back and forth across each brush face, promoting even distribution. Still another is to inhibit formation of the wedge of air that would otherwise tend to lift the brush faces of the rotating ring.

To minimize brush friction and wear, large machines can be supplied with brush-lifting devices to short-circuit the collector rings and lift the brushes out of contact when the motor is running at full speed.

Collector insulation must be rated, like the rotor winding itself, for voltages ranging from 600 to 3,000 for horsepowers in the 1,000-10,000 range (regardless of the stator voltage).

Collector mounting within the motor enclosure increases the minimum span between bearings and exposes motor windings to brush dust. Many large collectors are therefore located outside the motor enclosure and separately cooled.

Cooling is needed because brush friction, contact voltage drop, and current flow in the rings themselves produce significant heat loss. Collector temperature is typically limited to 100 degrees C. Infrared thermography is a useful monitoring technique.

Essential to temperature control and long brush life is ring surface smoothness and concentricity. In U.S. practice, a 16-microinch finish is common. Ring eccentricity causes brush “bounce,” and the allowable limit depends upon the peripheral speed of the ring. Hence, a logical criterion is the product of ring diameter and shaft RPM.

To order a back issue with the full article, “Collectors: how they’re made and how they work” call 312-321-9440 or visit our online webstore.

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