THEORY OF THE INDUCTION MOTOR. 157 



The expression gives the torque in terms of the slip, the resist- 

 ance and reactance of the rotor coils, the E.M.F. developed per 

 turn by the rotating field at full frequency, the number of turns 



Slip 



FIG. 63. 



per circuit on the rotor, and the angular velocity of the rotating 

 field. 



The relation between torque and slip is shown graphically in 



Fig. 63, which curve has its maximum value when K = - 



s 



Equation (14) shows that the starting torque is (putting 



K = 1)- 



T m 



~ 



which is greater the less the reactance of the rotor-windings, and 

 the less the angular velocity of the rotating field. For variations 

 of r> 2 the starting torque is the greatest when r% = s 2 , and then 

 becomes 



which varies inversely as the resistance. Thus, to produce a great 

 starting torque, the rotor resistance and reactance should be equal 

 to each other, and each as small as possible. 



Also, since e is proportional to the intensity of the rotating 

 fields, it follows that a great torque can only be obtained if, in 

 addition to the other favourable circumstances, the air-gap is made 

 as small as possible, and magnetic leakage as nearly as possible 

 eliminated. 



