112 ELECTRIC PROPULSION OF SHIPS. 



dentally, in the case of practically all merchant ships this speed is lower than the most eco- 

 nomical speed of the turbine. As the propeller is most efficient when designed and oper- 

 ated at low speeds, it is advisable not to exceed 90 to 100 revolutions per minute for the 

 ordinary merchant ship. 



Assuming a propeller speed of 100 revolutions per minute, a 60-pole motor and a 2- 

 pole generator, the generator and consequently the turbine would operate at 3,000 revo- 

 lutions per minute (neglecting the slip in case of the induction motor). This gives a 

 reduction from the turbine to the propeller of 30:1, which is approximately the same ir- 

 respective of the type of motor used. 



Fig. 1, Plate 13, shows 'a diagrammatic scheme of connections for an induction-motor 

 drive of the wound secondary type. Power is supplied from the turbine-driven generator 

 to the induction motor through one of the reversers. The generator is excited from one 

 of the auxiliary geared-turbine D. C. sets. Whether the auxiliary set would supply gen- 

 erator excitation only or simultaneously furnish ship's auxiliary power depends princi- 

 pally upon the electrification of engine-room auxiliaries, winches, etc. 



The turbine is under the control of a governor capable of maintaining constant speed 

 over about 75 per cent of the entire speed range. The governor speed-control valve is regu- 

 lated from the control stand by an oil relay valve or system of rods and levers, depending 

 upon the type of governor used. The propeller speed is adjusted by throttling the turbine 

 as in the case of a geared-turbine drive. The turbine is started and brought up to its 

 "idling speed" by the throttle valve at the turbine. 



Similarly, the generator field switch and rheostat and the ahead and back switches are 

 controlled from the control stand by means of levers. These levers are mechanically inter- 

 locked, so that it is necessary to follow the proper sequence of operations in starting, stop- 

 ping and maneuvering. 



Usually the reversers are interlocked with the field lever so that the field lever must be 

 in the "off" position before the reversers can be opened or closed. Similarly, the field is 

 interlocked with the turbine speed-control lever so that the control valve must be set for 

 low speed before the field can be opened or closed. 



The secondary control is automatic, being either of the solenoid contactor or motor- 

 operated contactor type. The actuating means is energized through contacts on the field 

 lever near the end of its stroke. This arrangement insures the establishment of voltage 

 at the motor before closing the secondary accelerating switches. Similarly, moving the 

 field lever to the "off" position opens the secondary switches. 



The operating levers and a complete complement of electrical instruments and steam 

 gauges are arranged convenient to the operator, the instruments and gauges being mounted 

 on a panel directly above or in front of the levers. By observing the instruments, the oper- 

 ator will be kept informed at all times of the performance of all machines, even though the 

 machines be obscured from his view. 



By a suitable arrangement of the switches and control levers, the entire maneuvering 

 of a ship of any size and any number of screws can be under the complete control of one 

 operator. 



The interlocking system necessitates the following sequence of operation: 



A. Starting Ahead (T-urbine Idling) : 



1. Close reverser in "ahead" position, 



2. Close field and establish excitation, 



3. Adjust turbine speed to desired value after secondary is completely short-circuited. 

 (Indicated by pilot light.) 



