A single bearing alternator is an AC generator that has a single bearing on the main shaft which holds the main rotor and excitor rotor. A two bearing alternator has two one at either end of the shaft. Single bearing alterators, are easier, faster and cheaper to assemble and have therefore replaced the two bearing alternators across most applications. the two bearing type is used primarily in larger (>2MW) sets, mobile and marine applications.
In a two bearing alternator the shaft is supported at both ends by an independent bearing, whereas in a single bearing design the alternator is only supported at the non-drive end by a single bearing. The single bearing alternator has its drive end supported by the engine flywheel to which it is coupled and therefore effectively uses the bearing on engines crank shaft for support.
Engine manufacturers require a torsional vibration analysis (TVA) to ensure the alternator is compatible with the engine and may specify the use of single or double bearing arrangements in some instances.
To couple a two bearing alternator to an engine requires a flexible coupling, a single bearing alternator is normally directly coupled to the engine.