256 ALTERNATING CURRENTS 



cost of winding and connecting the rotor coils. The controller 

 and resistors further add to the cost. In the running position, 

 this type of motor has a greater slip than the ordinary squirrel- 

 cage motor, because it is not possible to secure the very low 

 resistance obtainable with the squirrel-cage winding. As has 

 been pointed out, such external resistance may be used to obtain 

 speed control at reduced efficiency and with poor speed regulation. 

 Hence, this type of motor has better starting characteristics, 

 but poorer running characteristics than the squirrel-cage motor. 



FIG. 243. Slip-ring induction motor, assembled. 



Wound-rotor induction motors are used where considerable 

 starting torque is required, and frequently where speed adjust- 

 ment is desired. Common applications of this type of motor are 

 in cranes, elevators, pumps, hoists, railways, calenders, etc. 

 Figure 244 shows a Westinghouse 1,200-hp., 580-r.p.m., wound- 

 rotor induction motor driving a Henry A. Worthington centri- 

 fugal pump at Minneapolis, Minn. 



Another recent use of these wound-rotor induction motors is 

 in the electric propulsion of battleships. The motors are con- 

 nected directly to the propeller shafts. Two synchronous speeds 

 are obtained by changing the number of poles. Intermediate 

 speeds are obtained by changing the frequency of the generator. 



