An alternator driven by the main engine to supply power to the mains. The shaft alternator can be arranged in a number of different ways. The appropriate configuration for medium-speed main engines is the drive of the shaft alternator by power take off (PTO) of the reduction gear. In case of low-speed main engine, the poles of the shaft alternator are mounted directly on the shaft which is designed in the poles area with an enlarged diameter. This confi guration with a large air gap of 7.5mm between the stator and rotor, is the most widely used assembly.
On ships with fixed pitch propeller, torque respective power is set via the propeller speed. If using CP propeller shaft speed and propeller pitch are adjusted simultaneously in order to achieve the optimum propeller efficiency in so-called combinator mode. The classical shaft alternator systems consist of a shaft alternator for active power generation, a synchronous compensator for reactive power and short circuit current generation, a frequency converter with rectifi er, DC intermediate circuit and linecontrolled thyristor inverter.