CHAPTER TEN - Shaft-driven Generators

Abstract

This chapter discusses the advantages of shaft-driven generators. Auxiliary diesel-driven generators that run continuously for 24 hours a day both at sea and in port can be expensive in terms of the fuel cost and maintenance requirement. A generator drive taken from the main propulsion system provides the means of reducing maintenance by avoiding the use of auxiliary diesel at sea. It also furnishes a method of obtaining electrical power from the cheapest fuel. Additional advantages brought by the installation of a shaft generator are that fewer diesel generator sets are needed and the shaft-driven machine can be of large enough capacity to take the full at-sea electrical load. The chapter also discusses the working of shaft-driven generators. A generator positioned directly in the shaft line between the main engine and propeller can be built so that its shaft is flange coupled as a part of the intermediate shaft system or the rotor can be based on a split hub that is clamped to a section of the main shaft. The converter system serves the shaft generator of a ship with a fixed-pitch propeller and a large main-engine speed range. The shaft generator must supply full output over the permitted speed range, and to achieve this at the lower end, it is overrated for higher speeds. The a.c. shaft generator is a synchronous machine that produces alternating current with a frequency that is dictated by variations in engine speed. Alternating current from the shaft generator, when delivered to the three-phase rectifier bridge, passes through the diodes in the forward direction only, as a direct current. The smoothing reactor reduces ripple. The original frequency is unimportant once the supply has been altered to d.c. by the rectifier.