posted 05-03-2010 08:14 PM ET (US)
In a permanent magnet alternator (which is what I assume is being discussed), the stator is a passive component. It is really just some copper wire wound on forms. The wire is insulated with an enamel-like insulation, sometimes called FORMVAR. The insulation is very thin compared with the usual polyvinyl-chloride insulation on hook-up wire. The stator is stationary.The most likely modes of failure of a stator are from either short circuit or open circuit. If the stator temperature rises to the point where the wire insulation becomes soft or melts, it is possible for coil turns in the stator to short out to each other or to the chassis of the engine. Really high temperature could lead to heating of the copper wire to the point where it might melt and cause an open circuit. Heat is the most likely cause of failure, although abrasion of the wire from mechanical interference could also occur. Mechanical damage to the wire could occur as a secondary effect from heat. The wiring could sag too much if heated.
We now look to sources of heat. The engine is a very good source of heat. If the engine is not getting proper cooling, the extra heat from the engine may cause the stator temperature to rise. Another source of heat is the stator itself. The flow of electrical current through the stator windings will generate some heat, and the more current flow the more heat generated. If the boat charging system is constantly required to produce a high current, this can lead to heating of the stator.
A stator could be damaged by excessive voltage. If running at very high speeds the stator will generate higher than normal voltages. Any weakness in the wire insulation might be overcome by an arcing, leading to a short circuit.
If you give us a description of how the stators have failed, we could use that information to form an opinion of the circumstances that lead to the failures.