262 



ELEMENTS OF ELECTRICAL ENGINEERING. 



doubled rotor currents would be acted upon by a doubled driv- 

 ing force. 



As a matter of fact, however, the flux <> decreases with increase 

 of load because of the demagnetizing action of the rotor currents, 

 and this decrease of 4> causes a two-fold decrease of torque inas- 

 much as (a) it causes a decrease of induced electromotive forces 

 and therefore a decrease of rotor currents at a given rotor speed, 

 and () these decreased rotor currents are acted upon by the 

 reduced flux to produce a greatly reduced torque. A true speed- 

 torque curve of an induction motor is shown in Fig. 235. The 



torque 



2O 30 40 50 60 

 rotor speed n r 



70 80 90 



1 10 120 13 



Fig. 235. 



torque is zero at synchronous speed (n f = n\ and the torque 

 increases, reaches a maximum, and then falls off as n r is de- 

 creased. 



Break-down of the induction motor. If the load on an induc- 

 tion motor is increased until the motor develops its maximum 

 torque, then any further increase of load will cause the motor to 

 ccme to a standstill. This phenomenon is called the breaking 

 down of the motor. , 



Use of a starting resistance in the rotor windings. Imagine 

 the rotor resistance of an induction motor to be doubled, then, if 

 the relative speed, n n r , of the stator magnetism and rotor is 

 doubled, the electromotive forces induced in the rotor windings 

 are doubled, the rotor currents are the same as before, and the 



