186 THE APPLICABILITY OF ELECTRICAL PROPULSION TO 



would be in the motors used. Since the Jupiter's installation was made, a new 

 type of induction motor has been developed which makes it possible to do away with 

 the water-cooled resistances used on the Jupiter; this simplifies the wiring some- 

 what, and also simplifies the operation, as it eliminates the use of the short-circuit- 

 ing levers for throwing in and out the water-cooled resistances. 



This motor is a double squirrel-cage motor; it has two independent squirrel 

 cages on the rotor, the outer one having conductors of high resistance and the 

 inner one conductors of low resistance. At high frequencies, such as would obtain 

 in the rotor when starting up or backing, the inner winding takes practically no 

 current, due to its high reactance under these conditions ; therefore the high resist- 

 ance winding on the outside is the only one in operation and this insures large 

 torque for starting or backing. The torque for backing conditions would also be 

 improved by automatically establishing maximum excitation before backing. The 

 connections for reversing would be so arranged that maximum excitation would 

 be established automatically during the act of reversal, no matter what the excita- 

 tion may have been before reversal. When the motor is running close to synchro- 

 nous speed, the reactance of the inner conductors is very much reduced, and they 

 then operate like any ordinary squirrel-cage motor. At intermediate speeds be- 

 tween synchronous speed and standstill or back, both the inner and outer conduct- 

 ors would take some current, and the torque characteristic curve would be the re- 

 sultant of the two windings. 



The motors for a battleship would differ in one other particular from those 

 used on the Jupiter. The stators would be provided with pole-changing switches so 

 that there would be two speed reductions between generators and motors. This 

 would enable the generators to be run at full speed both at the full speed of the 

 ship and also at some intermediate speed nearer the cruising speed of the ship. 

 The normal cruising condition of the ship would be to have one generator running 

 and to have the pole-changing switches on the motors thrown so as to give them 

 the larger number of poles and consequently the greater speed reduction. This 

 combination would be used till the speed limit of the turbine is reached ; this point 

 would probably be about 15 knots in a 21-knot ship. From this point on to about 

 19 knots, one generator would be used driving four motors with the pole-changing 

 switches thrown to give them the smaller number of poles. For speeds above this 

 both generators wodd be used, each driving two motors with pole-changing 

 switches thrown to give them the smaller number of poles. 



In choosing a method of propulsion for battleships, there are six points to 

 be considered. They are, in order of their importance :— 



1. Reliability. 



2. Maneuvering qualities. 



3. Economy. 



4. Space occupied. 



5. Weight. 



6. Care and upkeep. 



