376 THE INDUCTION MOTOR. 



strength and rotor current. In order that this rotor 

 cm rent may be produced by such a weak field, the slip 

 is considerable. The energy component of the stator 

 current must always increase in proportion to the rotor 

 current, and is consequently also fairly high, although 

 the true magnetising current is low, and most of the 

 primary current is spent in neutralising the rotor ampere 

 turns. 



As the voltage increases, the strength of the rotating 

 field increases, and, as the torque on the shaft remains 

 constant, the rotor current decreases in practically the 

 same proportion as the field becomes stronger. The slip 

 required to produce this current decreases. Also the 

 portion of the primary current spent in neutralising the 

 field due to the secondary current diminishes, while the 

 true magnetising current increases. The primary current 

 is consequently seen to decrease on the whole, though 

 not so rapidly as the rotor current. The same process 

 is seen to continue, the rotor current decreasing as the 

 field becomes stronger. The stator current continues to 

 decrease with the rotor current and to increase with the 

 increased voltage and magnetising current. The two 

 tendencies neutralise one another, and then the stator 

 current, as a whole, begins to increase as the rotor current 

 becomes more and more nearly constant. 



The power absorbed is seen to increase gradually, being 

 almost entirely due to iron losses, except at the lower 

 voltages, when the saturation is low and the currents com- 

 paratively high. 



The power factor decreases as the voltage and magne- 

 tising current rise. It is seen from the power-factor curve 

 how large a part of the primary current at low voltages 

 must be spent as " energy current," in neutralising the 

 secondary ampere turns, since the power-factor is so high 

 with the large initial current. 



The curves in Fig. 183 were taken with the rotor held 

 stationary. In this case stator and rotor currents and 

 power absorbed all increase with the voltage, and assume 

 much higher values than when the rotor is free to rotate. 

 The power in this case increases on account of the heavier 

 currents, and very slightly because the rotor core is now 

 subjected to the magnetising action of the rotating field at 



