THE INDUCTION MOTOR. 



375 



they do not, of course, represent the usual running con- 

 ditions. To obtain them, the voltage was raised very 

 gradually from zero, readings on the instruments being 

 taken as rapidly as possible at points where the motor 

 tended to increase its speed to the values of the upper 

 curve. In order to get the overlapping part on the higher 

 speed curves, the voltage was gradually reduced, care 



1300 



1200 



1100 



1000 



900 



800 



43 700 







600 



500 



400 

 300 

 200 



100 

 



50 



40 



30 



20 



10 







60 



10 20 30 40 50 



Volts. 

 FIG. 183. CURVES OF STATOR AND ROTOR CURRENTS AND WATTS IN STATIONARY MOTOR. 



being taken to avoid sudden changes of any kind, which 

 would result in the speed of the motor falling to its lower 

 value when n earing the lower end of the curve. 



It is interesting to trace the variation in the normal 

 current curves. At low voltages the strength of the 

 rotating field is very weak, and in order to overcome the 

 resistance to turning of the rotor, the rotor current is 

 necessarily high, since the torque is the product of field 



