182 ARMATURE CONSTRUCTION 



Now it follows, from the relative positions of the conductors of 

 the three phases, that the electromotive forces generated in the 

 three windings are 120 degrees behind each other ; and if they are 

 sine waves they may be represented, as in Fig. C, by three sine 

 curves displaced 120 behind each other and coloured according to 

 the phases. 



If the three circuits are equally loaded, these curves may also 

 be considered to represent the corresponding instantaneous values 

 of the current. 



It will be noted that at every instant the algebraic sum of the 

 three currents is zero. 



For instance, at the time demarcated by the line xy in Fig. C, 

 which corresponds to the positions of the conductors in Fig. D, 

 the current in the black conductors has its maximum value, and 

 the current in each of the red and blue conductors has a value of 

 just one-half of the maximum and in an opposite direction with 

 regard to the black phase. Hence at this instant the current 

 flowing out of the black conductors could be returned one-half 

 through each of the red and blue conductors. This would be the 

 case when the winding is Y-connected, as will be seen from Fig. D, 

 where, at the instant, the current in the black phase is flowing 

 away from the neutral point, while in the red and blue phases it 

 is flowing to the neutral point. 



A safe and easily understood way of connecting the three 

 windings correctly to the three collector rings and the common 

 connection is to consider that the winding whose conductors 

 occupy the position in the middle of the pole piece is carrying the 

 maximum current, and to indicate its direction on the winding 

 diagram by an arrow. The currents at the same instant in the 

 conductors immediately next to it on the right and left are in the 

 same direction, and should be so marked by arrow-heads. Now, 

 from the curves given in Fig. C above, it will be seen that where 

 one curve has a maximum value the other two have a value half as 

 great, and in the opposite direction. Therefore, if we consider that 

 the current in the winding occupying the position at the middle of 

 the pole face is flowing away from the common connection, then 

 the currents in the other two windings, which are each of half the 

 magnitude of the former, must both be flowing into the common 

 connection. We must therefore join those ends of the three 

 windings to the common connection, which will bring about this 

 condition at this instant. The other three ends of the winding 

 are connected to the three lines. 



