Tham khảo tài liệu 'marine auxiliary machinery 7e episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 230 Auxiliary power closing the pilot valve. This action again stops excessive movement of the power piston and fuel rack. As the engine speed drops the flyweights move back in towards their former position while oil leaks through the needle valve allowing the receiving compensating piston to return towards its old position. Again the two movements act on the floating lever without moving the closed pilot valve. UMS operation A survey conducted recently 1990 suggests that about 40 of ships in the world fleet operate with periodically unmanned machinery spaces UMS . Of the 60 with watchkeepers most are older ships some are passenger vessels or other vessel types where watchkeeping is justified for extra security. A few are ships with control equipment which is defective for various reasons. UMS machinery spaces have automatic engine change-over in the event of a fault developing on the running machine. Some have programmed control of generators with automatic starting and stopping of stand-by engines as the demand for electrical power rises and falls. Synchronization opening and closing of breakers is automatic and load sharing is a function of speed sensing or load sensing governors. The unattended installations require high dependability which demands intimate knowledge of the machines and strict attention to the maintenance schedule. Generators driven from the main propulsion system Generators can variously be driven from the propeller shaft through a gearbox or by being mounted on the engine itself. Assuming that residual fuel is used in the main engine then all electrical power at sea is provided at much lower cost in terms of fuel price and auxiliary generator running hours. The diesel driven generator is needed only while manoeuvring and in port. Shaft driven direct current generators Direct current generators are not as sensitive to speed variation as are alternating current machines where frequency has to be maintained. If a direct current generator has an
