352 THE INDUCTION MOTOR. 



quantities! change their direction with the velocity cor- 

 responding to the alternating current supplied. It must 

 be remembered that this resultant field rotates only with 

 the velocity n n l (= the slip) relatively to the con- 

 ductors on the rotor. 



The nature of the change of field experienced by each 

 conductor of the rotor may be represented in the manner 

 shown in Fig. 175, where the vectors Om l Ow 2 represent 

 the portions of the resultant field due respectively to the 

 stator and rotor currents, and Om r is the resultant rotating 

 field. The wave form of the E.M.F. induced by these fields 

 in the conductors is shown by the curves annexed. 



These curves may be considered to represent two 

 distinct relations in the rotor. (1) They represent the 

 variations of the E.M.F. in each conductor- of the arma- 

 ture, and consequently the current in an inductionless 

 armature. (2) They also may be taken as representing 

 at a given instant the strength of the field round the air- 

 gap of the motor, and, consequently, the distribution of 

 electromotive force and current in the conductors. The 

 same diagram thus represents at a fixed point the varia - 

 tioii with time of electromotive force and current, and 

 at a fixed time the variation as regards position of the 

 voltage and current induced in the rotor -winding. 



Thus the direction of the resultant line Om r 

 represents the direction and magnitude of the resultant 

 rotating field relative to the two components, and also 

 the magnitude and phase of the E.M.F. induced by the 

 rotating field relative to the E.M.F. which would be 

 produced by either component field acting alone. 



Heating of Rotor and Turning Moment. The induction motor 

 consists primarily of two parts, viz., the means for 

 obtaining a rotating field called the stator, and a 

 rotor or armature, carrying conductors, which are cut 

 by the rotating field and have currents induced in them. 



In its essentials, it thus corresponds to the usual 

 types of direct or alternating current generator. 



In the case of an ordinary direct-current dynamo, 

 the turning effort which has to be applied to the armature 

 in order to produce its rotation, is the exact equivalent of 

 the electro-magnetic reactions of the armature currents 

 upon the field. 



