THE INDUCTION MOTOR. 



397 



Attention may be called to one further point in con- 

 nection with the starting curves, Fig. 191. For a given 

 value of the starting torque there are evidently two 

 possible values of the rotor resistance. In designing the 



S = 100 per cent. 



Slip, 

 torque. 



. . . current in rotor. 

 Curves I. Rotor R = -1 ohm. 

 II. = '05 

 III. = '025 



FIG. 192. ROTOR CURRENT AND TORQUE COMPARED WITH SLIP. 



starting resistance, the greater value of the resistance 

 should be chosen, since this corresponds to a smaller 

 starting current. Consequently the rotor will start upon 

 the flatter right-hand limb of the torque curve (Fig. 191). 

 It will run when at full speed on the steeper limb of 

 the running curves shown in Fig. 190. 



Determination of Losses and Efficiency. From the tests 

 already given, it will be apparent that the losses in an 

 induction motor may be determined in a manner very 

 similar to that employed in the case of a transformer. 



Corresponding to the determination of the copper 

 losses in a transformer, we have the test on the motor 

 while held stationary, described on page 370. The power 

 supplied to the motor when locked is nearly all C-E loss, 

 since the iron losses are small at the low saturation 

 employed. The losses due to the resistance of the 

 windings increase with the load, and are thus easily 

 determined for any value of the stator current. 



The core losses and friction los es are practically 



