262 ALTERNATING CURRENTS 



changed in the ratio of three to two, the winding will probably 

 be designed for % pitch at the higher speed making it a full- 

 pitch winding for the lower speed. In such a motor the best 

 possible design is not usually obtainable at both speeds. That 

 is, desirable characteristics, such as high power-factor, etc., are 

 sacrificed at one speed in order that a reasonably good motor 

 may be obtained at the other speed. Sometimes the stator 

 connections are changed from delta to Y at the same time that 

 the pole connections are changed. This changes the voltage 

 per phase and makes possible a better motor at each speed. 

 Because of the complications involved in changing the connec- 

 tions, it is not desirable to obtain more than two speeds by chang- 

 ing the number of poles. To avoid these complicated switching 

 connections, induction motors sometimes have two distinct 

 windings, the two windings being connected for a different num- 

 ber of poles. The 7,500-hp., wound-rotor induction motors used 

 to drive the electrically-propelled battleship Tennessee have this 

 type of winding. One winding is connected for 36 poles and the 

 other for 24 poles. 



In the electrically-propelled battleship New Mexico, the motors 

 are direct-connected to the propeller shafts. The stators can 

 be connected for 24 poles or for 36 poles, giving a speed change 

 of three to two. In wound-rotor types of motors it is necessary 

 to change the rotor as well as the stator connections. Otherwise 

 negative torque will be developed by certain of the rotor con- 

 ductor belts. 



Speed Control by Concatenation. This method requires two 

 motors, at least one of which must have a wound-rotor. The 

 speed is changed by changing the slip of one motor, which changes 

 the frequency supplied to the other motor. The two rotors are 

 connected rigidly together as indicated in Fig. 247. Line fre- 

 quency is supplied to the stator of one motor, as No. 1, Fig. 247. 

 This first motor should have a one-to-one ratio of transformation 

 between stator and rotor. That is, at standstill, and with the 

 external circuit of the rotor open, the voltage across the rotor 

 slip-rings should be equal to line voltage. Assume that the two 

 motors are similar and that the rotors operate at slightly less 

 than half the synchronous speed of the first motor. The rotor 

 frequency of No. 1 motor is slightly greater than half line fre- 



