344 THE INDUCTION MOTOR. 



The advantage of putting the winding in the position 

 shown in Fig. 172 is that the coil can be wound complete, 

 and then applied to the inside of the ring, the projecting 

 part of the winding being bent over, as indicated in the 

 diagram. 



Proof of the Constancy of the Rotating Field. The simplest 

 method of proving generally that a constant rotating field 

 is 'produced by a polyphase current flowing in equally 

 spaced coils is to resolve the field due to each phase into 

 two components in two fixed directions at right angles to 

 each other. The component fields in each of the two 



FIG. 172. FIELD IN. INDUCTION MOTOR. 



directions are then added together, and the resultant 

 of the two fields thus found is calculated as the square 

 root of the sum of the squares of these two mutually 

 perpendicular fields. 



The method is generally applicable to currents having 

 any number of phases. The following calculation applies 

 to a three-phase circuit. 



In this case there are three coils, situated at 120 from 

 each other, and carrying varying currents, which are 

 assumed to produce fields proportional at any moment to 

 the strength of the current in the coil. 



Let z,, z 2 , z 3 = the number of lines induced in the 

 three coils I., II., and III., at the 

 instant under consideration. 

 Z = max. value of the number of lines 

 induced in the three coils in 

 rotation. 



