Most power plant operators are familiar with generator maintenance problems such as collector rings going out of round and photographing on the ring surface. The primary cause of collector ring wear is improper contact pressure between the brush and the ring. Too much pressure causes excessive mechanical wear. Too little pressure causes electrical wear.
Collector rings typically go out of round as low spots develop in the surface of the ring. This is actually electrical erosion, similar to what happens in electrical discharge machining. Photographing generally occurs when a brush does not make good contact with the ring and the arc that is created imprints a gray or white photograph of a portion or the entire face of the brush on the surface of the ring. Once photographing has started, the situation usually gets progressively worse and arcing becomes more and more evident.
“The brush is often blamed for collector ring wear, but the real source of the problem is typically the brush holder, which restricts the brush’s movement one way or another,” said Dave Cutsforth, President of Cutsforth Products (www.cutsforth.com) “Almost always, too little brush contact pressure is the problem, which is related to the brush holder and/or spring. The brush holder contributes to insufficient contact pressure by restricting free movement of the brush.” Cutsforth has developed a new product, the EASYchange Brush Holder System, that eliminates the various factors leading to ring damage while making it easier to change brushes online.
The most common type of restriction in brush movement is the formation of small mounds of carbon on the inside of the brush box. These deposits can become so large that they completely stop the brush’s movement. TECO Energy in Tampa, Fla., faced such a situation on their Bayside Unit 1 240 MW Westinghouse generator and sought a brush holder that would ensure proper operation and permit on-line maintenance. The EASYchange brush holder box is made of stainless steel and incorporates a special non-porous finish to resist adherence of carbon deposits. The brush box is also removable, making inspection and cleaning very easy. Since installing the new brush holders at Bayside, carbon deposits have been all but eliminated and brush life has dramatically increased, according to Cutsforth.
EASYchange Brush Holder System. Photo courtesy of Cutsforth Products.
A second common type of holder-related restriction is brush binding, predominately caused by side pressure from the constant-force spring. This side pressure can cause the brush to bind to some degree if the brush box is shorter than the brush it holds. In some cases, the binding is so severe that a ledge can develop in the brush and cause it to “hang up.” At the MidAmerican Energy Neal North plant in Salix, Iowa, the GE Alterex generator had developed burn spots in the rings due to brush binding and carbon deposits. After truing the rings and installing new EASY Change brush holders, brush life has more than doubled and carbon deposits have been eliminated, according to Cutsforth. Because the brush holder system was designed with a brush box that is the same height as the new brush, binding is not an issue.
A third type of holder-related restriction is surface roughness, including pockets caused by electrical erosion. This problem develops when current passes through the side of the brush box because of a poor terminal connection, a poor shunt wire connection, or a brush that is too long. Side pressure from the spring or rotational forces from the ring pressing against these pockets or rough surfaces creates friction and reduces the brush’s contact pressure, which in turn causes the ring to erode.
The brush box of the EASYchange brush holder system is insulated from the current path and the terminal connection is designed so that the pressure member of the connection is automatically refreshed every time a brush is changed to ensure good contact. These measures eliminate the possibility of electrical erosion.
Carbon deposit buildup in brush holder box. Photo courtesy of Cutsforth Products.
The spring is the other potential contributor to insufficient contact pressure. Constant-force springs will eventually lose some of their tension over time or may get damaged and need to be replaced. In many cases, because it is difficult or just inconvenient, springs don’t get changed and this leads to electrical erosion of the collector ring or commutator. The EASYchange brush holder system addresses this issue by using a QUICKchange spring that is supplied with every brush. The QUICKchange spring assures proper tension and yet does not increase the cost over properly maintained, “spring clip” style spring assemblies.
The EASYchange brush holder system also eases the process of changing brushes. The brush and brush box along with the spring are all removed as a unit, with the brush held in the pocket. This allows brush changing and inspection to be performed away from the unit in a more comfortable environment.