THE INDUCTION MOTOR. 347 



time of one period of the alternating current, rotating 

 with constant angular velocity. 



It may be shown that if the system is that of a p- 

 phase current, the magnitude of the resultant field would 



T) 



be -- x maximum strength of each individual field. 



With regard to the direction of rotation of the field, it 

 is evident that it will revolve in the direction in which 

 the current in the successive coils attains its maximum 

 value. For instance, if after the current in coil I. has its 

 maximum current, the current in coil II. increases to its 

 maximum value, the field will rotate in the direction from 

 I. to II., and then to III., &c. If, on the other hand, the 

 current in coil III. reached its maximum value next after 

 coil I., the direction cf rotation would be I., III., II., <fec. 

 Thus by interchanging the connections of coils II. and 

 III. the direction of the rotation of the field is reversed. 

 Consequently in a three-phase motor supplied with 

 current at three terminals, the direction of rotation will 

 be reversed by interchanging the connections of any two 

 terminals to the supply circuit. 



Rotation of Armature of Induction Motor. It has been 

 explained in the previous paragraph that the effect of 

 polyphase currents passing through a polyphase winding 

 on a circular magnetic field system is to produce a rapidly 

 rotating pole, so that the result is similar to the rotation 

 of a permanently magnetised ring with poles produced at 

 fixed and equal intervals along the circumference. 



In an induction motor, the field system surrounds an 

 armature consisting of an iron core carrying a number of 

 conductors round its circumference. These conductors 

 are usually all connected together, so as to form a 

 number of complete electrical circuits. The lines of force 

 produced by the exciting current supplied to the ring 

 pass fro.m the circular field into the armature or rotor, 

 and by this means complete their path to the nearest 

 magnet pole of opposite polarity, as indicated for the case 

 of a single pair of poles in Fig. 172. In passing from 

 magnet to armature, and again from armature to magnet, 

 the lines of force cross the layer of armature conductors. 

 If the armature remains stationary, the revolving poles 

 will generate a rapidly alternating electromotive force in 



