ELECTRIC PROPULSION OF SHIPS. 113 



B. Stopping: 



1. Move turbine speed control lever to idling position, 



2. Move field lever to "off" position, 



3. Move reverser to "off" position. 



C. Starting Back: 



1. Close reverser in "back" position, 



2. Close field and establish excitation, 



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



D. Reversing {from Ahead Operation) : 



1. Move turbine speed control lever to idling position, 



2. Move field lever to "off" position, 



3. Move reverser from "ahead" to "back" position, 



4. Move field lever to full excitation, 



5. Bring up turbine speed after motor has pulled into step. 



The above description applies to a single-screw^ drive having but one turbine set. For a 

 multiple-screw ship having generating sets for port and starboard sides, the operation would 

 be the same for each side of the ship, as just described. 



The above operations are based upon switching when the current in the circuits has 

 been reduced to very low values, or nearly dead circuit conditions. This arrangement is not 

 really necessary, as with such small powers it is not essential to open the generator field; 

 however, it may be justified in the light of conservatism. In any event, the reversers are so 

 designed that they are entirely capable of opening full power. 



The salient features of the wound secondary induction motor drive are its inherent 

 torque characteristics and the ease of handling. While the secondary control necessitates a 

 few additional switches, this is fully compensated for by the fact that the propeller energ}' 

 of reversal and the slip energy of reversal and starting are dissipated in resistors external 

 to the motor. The importance of this is dependent upon the amount of reversing that is 

 done, particularly from full speed. This system, therefore, represents the most conserva- 

 tive arrangement of turbine-electric drive. 



The squirrel cage motor system is shown in Fig. 2, Plate 13, and it will be noted that 

 the electrical connections are the same as for the wound secondary motor system, except 

 that there is no secondary control. This system is therefore somewhat simpler than the 

 wound secondary motor system from an electrical connection standpoint, both inside and 

 outside the motor. It also has a slight advantage in cost in that the motor is less expensive 

 to build. The squirrel-cage motor is shorter because of the absence of collector rings. It 

 has a disadvantage in conservatism and torque characteristics. 



The power factor of either of the induction motor systems is less than unity, the exact 

 value depending upon the number of poles and certain design features. For the ordinary mer- 

 chant-ship requirements these motors would have power factors of approximately 70 per cent, 

 the wound secondary motor having the higher power factor of the two by a small amount. The 

 induction motor drive therefore requires a generator with KVA. capacity in excess of its 

 KW. capacity, which adds somewhat to its weight and size, and which detracts a very small 

 amoimt from its efficiency. 



The efficiency of the squirrel-cage induction motor of ordinary design is slightly less 

 than that of the wound secondary motor, for the reason that sufficient permanent resist- 

 ance must be incorporated in the rotor windings to obtain the required torque for reversal 

 without excessive current. In motors with special double secondary windings, the required 



