THE INDUCTION MOTOR. 



26l 



sufficient electromotive forces may be induced in the rotor con- 

 ductors by the more rapidly moving stator magnetism. 



It is at once evident from the foregoing statement that the 

 speed of the rotor at full load will be but little less than syn- 

 chronous speed if the short-circuited rotor windings have a very 

 low resistance, for, in this case, but little electromotive force need 

 be induced in the rotor conductors to produce sufficient current 

 to enable the stator magnetism to exert the necessary driving 

 torque. If, however, the resistance of the short-circuited rotor 

 windings is large, then the rotor speed will fall off greatly with 



Torque 



rotor speed 



Fig. 234. 



increase of load. That is, low rotor resistance is necessary for 

 good speed regulation of an induction motor. 



If the magnetic flux 4> which enters the rotor from one of 

 the polar regions of the stator were constant in value, that is if 

 it did not decrease in value with increase of load, then the differ- 

 ence between synchronous speed n and rotor speed n' would 

 be exactly proportional to the torque to be exerted on the rotor, 

 that is to say the speed-torque curve of the induction motor 

 would be a straight line AB, as shown in Fig. 234. This is 

 evident if we consider that to double the value of n n' would, 

 if 3> were constant, double the induced electromotive forces in 

 the rotor windings, this would double the rotor currents, and the 



