Series-versus parallel-connected generators (or multiple-engine D-C diesel-electric ship-propulsion systems

THE USE of electric equipment in a Diesel propulsion system for ship drive serves as a nonmechanical means of connecting one or more relatively high-speed engines to an inherently slow-speed propeller. Such an installation is much more flexible than a mechanical connection since the prime movers can be physically distributed to best utilize the available space and can also be instantly connected to or disconnected from the propulsion circuit. The use of synchronous a-c machinery has a number of attractive features. The motors and generators are smaller and lighter, have higher efficiencies, lower maintenance, and lower first cost than equivalent d-c machines. However, there are also some disadvantages. The speed range of the propelling motor is limited to that of the prime mover because there is a fixed ratio between generator and motor speed. Also, there is no means of varying the motor speed-torque ratio such as can be obtained by using d-c machinery. Thus, the use of a-c Diesel-electric drive is limited to vessels making relatively long voyages at a more or less fixed speed and where maneuvering is of relatively small importance. Direct current, on the other hand, has almost ideal characteristics for use in supplying propelling power to tugboats, ferries, dredges, and ice-breaking vessels, which are almost continuously maneuvering and operating under varied conditions of loading.