THE INDUCTION MOTOR. 



263 



torque is the same as before. Therefore the rotor speed which 

 corresponds to maximum torque depends upon the resistance of 

 the rotor windings, and the value of the maximum torque is inde- 

 pendent of the rotor resistance. This is shown by the curves I, 

 2, 3, 4 and 5 in Fig. 236. These curves show the relation 

 between speed and torque of an induction motor whose rotor 

 windings are short-circuited through an adjustable resistance ; 

 curve I shows the relation between speed and torque when the 



0.4 0.5 0.6 

 rotor speed (t\-=^ 



Fig. 236. 



0.7 



0.8 



0.9 



rotor resistance is very small, and curves 2, 3, 4 and 5 show the 

 relations between speed and torque for greater and greater rotor 

 resistance. 



A certain value of rotor resistance gives the maximum torque 

 at zero rotor speed, as shown by curve 5 in Fig. 236, and in hen 

 an induction motor is started it is desirable to increase the rotor 

 resistance so as to produce the maximum torque at starting ; this is 

 especially desirable if the motor starts under load. 



