396 THE INDUCTION MOTOR. 



torque is only reached after the motor has attained some 

 speed. 



The lower the resistance of the rotor circuit the nearer 

 to synchronous speed is the point at which the motor can 

 exert its greatest torque. Since the greatest slip usually 

 occurring in practical working is about 5 per cent., it is 

 an advantage to have an extremely low resistance of the 

 secondary winding. In cases where a separate resistance 

 is not employed for starting that is, in the case of 

 squirrel-cage motors it is necessary to sacrifice some of 

 the starting torque which might have been attained when 

 running at full speed, in order to get a sufficient torque 

 at starting, and when running at the initial low speeds. 



In Fig. 190 will be seen a dotted curve of current. 

 This curve shows the value of the stator current 

 (measured on the horizontal scale) corresponding to 

 each value of the torque (measured vertically). It is 

 possible thus to draw a single curve representing the 

 relation between current and torque independently of the 

 rotor resistance. This is due to the fact, which has 

 already been alluded to, that for a given -torque the rotor 

 will always take constant current, the slip automatically 

 adjusting itself to enable this rotor current to be induced. 

 Since the stator current depends only on the rotor current 

 and the constant no-load current, there will be a definite 

 value of the stator current for each torque independent of 

 the rotor resistance. As seen from the current curve, Fig. 

 190, there are actually two values of the current instead of 

 a single one, as just stated. This is because there are two 

 values of the slip for each value of the torque, as seen 

 from the torque-slip curves. The running conditions 

 always correspond to the lower slip, and, consequently, 

 lower current. 



The variation of rotor current with torque may be 

 easily followed from Fig. 192, where rotor and current 

 and torque are plotted for three different values of the 

 armature resistance. 



If two points on corresponding arms of two of the 

 curves be taken, each corresponding to the same torque, 

 e. g., a, and 6, and the corresponding currents be noted 

 on the current curves, the currents are seen to have the 

 same value. 



