Why generator bearings insulation are provided ?

There is a phenomenon called shaft currents which arise in all turbo machinery. The shaft voltage and currents may be both electrostatic or electromagnetic in nature. the principle of all electrical generators is that when a conductor travels through a magnetic field , it generates voltage at two ends , if the two ends are connected then current flow will occur. if the above conditions are met then any machine will generate internal currents. here the conductor is the shaft and as the shaft and most commonly generator shafts rotate in a field environment. Thus the shaft forms a closed path short circuited where shaft currents are induced. This is mostly liberated as heat, but if these currents travel across interface materials like bearing, bearing lube oil film which offers electrical resistance sparks, surface/welding damage may occur. The most commonly observed reasons for shaft currents are

a. AC induced due to the formation of residual magnetic poles. This phenomenon is seen in a brushless DC rotor. The DC supplied has harmonic components which induces residual poles , and are self exciting in nature. This will cause current to flow in the shaft.

b. DC induced, due to permanently magnetized stator interacting with eddy currents in the rotor.

So how do we avoid such shaft currents from damaging the interface paths ?? well, just break the flow of current. Providing insulation in any one or more of the bearings to the machine base will restrict the current flow through the interfaces. yes there will still be localized shaft currents but they will will not now damage the beatings and oil piping. Usually it is a practice to give the insulation on non drive end of the generator. They will mostly split type so that insulation can me monitored.

Note - This type of shaft currents are also possible in all turbo machinery not just the generator.