BEST PRACTICE: Regular inspection critical to avoiding collector-ring fires (video)

Collector-ring fires on generator exciters should not occur, but they do, and more often than most generating plant personnel would believe. Regular inspections of brush gear are your first line of defense against damage. Cutsforth Inc’s Steve Thompson, a frequent participant in user-group meetings, suggests monthly inspections. Click here for a backgrounder on exciter brush systems. Another resource: How to prevent collector-ring fires.

Visual inspections with the generator in operation can reveal problems such as short brushes, lead discoloration from carrying too much current, and grooving of the collector ring. But arming the technician with an ammeter and vibration probe makes him or her more effective. These instruments can help identify impending issues earlier than would a visual-only check. 

A short video shows current flow through brush leads made with a digital ammeter. Thompson said Cutsforth technicians believe readouts of from about 25 to 100 amps are indicative of good brush contact with the collector ring. Note that the first, second, and fourth readings are acceptable, but the third registers a zero, meaning the brush is not contacting the ring. When this happens, ring wear and tear is a likely result. Specifically, current passes through an arc, eroding the ring and creating low spots which contribute to an increase in brush vibration. The desired ring profile can be restored by grinding, but the cost for the skilled technician to do this is avoidable.

Brush hang-up in the brush box or guide often is caused by carbon deposits on the box walls; sometimes a weak brush spring is the culprit. Proof is presented in a second video. The maintenance tech pushes the vibration probe against the ineffective brush, freeing the restriction, and the amperage on the digital readout jumps quickly and significantly. Be aware that deposits can be difficult to remove and the task requires care and proper tooling to avoid marring the brush box wall. Recommendation: Clean brush boxes of residual carbon during your annual outage, or more often if necessary. Alternatively, replace your brush holders with ones requiring less maintenance.

Checking for brush vibration using the probe shown in the video is another element of a proper inspection. Readings of about 6 mils or more generally suggest that the collector ring is out of round and requires immediate attention. Truing can be done without a forced outage; traditional grinding also is an option. If you don’t have a suitable vibration probe at your plant, one can be ordered for a few hundred dollars via sales@cutsforth.com and connected to most commercially available vibration meters.

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