Generator Collector Brush Holder Testing and Design Improvements

Electric generators and synchronous motors with static excitation use rotating slip rings (also known as collector rings) and stationary carbon brushes to transfer the field current from the stationary exciter to the rotating generator field. The carbon brushes experience wear from both mechanical friction and electrical contact with the rings. Therefore, the brushes need to be periodically inspected and replaced. This is often the most frequent maintenance activity for an electric generator. It is generally recognized that if brushes are not changed when worn down, this can result in a damaging condition called a flashover that will usually force the generator offline. Several collector flashovers were investigated to look for other common characteristics with the aim of reducing the risk of flashover occurrence and improving generator reliability. Some features of the generator collector brush holders were identified as significant contributors to collector flashovers and also to other, more common maintenance problems. Several brush holder designs were evaluated with regard to these features and also with regard to feedback received from operators. In addition, an in-house test rig was developed and used to compare multiple, existing brush holder designs and new prototype concepts for brush wear rate and current selectivity. This work led to a new brush holder design that addresses these concerns and has subsequently been successfully tested in a laboratory and at a customer site. That new brush holder design is being applied to both new units and as a retrofit to in-service aftermarket generators.