90 ALTERNATING CURRENTS 



arranged to lie on two surfaces only (as with the two-phase winding), 

 the coils being alternately straight and bent-up. The three phases in 

 Fig. 76 are distinguished by full, dotted, and chain-dotted lines. It 

 will be noticed that the coils belonging to any one phase are alter- 

 nately straight and bent. 



In the two-phase winding, the distance apart of two neighbouring 

 coils belonging to different phases amounts to the pole-pitch 



7T 



(corresponding to a phase difference of ^> or 90), and in the three- 

 phase winding to $ the pole-pitch. Now, a distance of the 

 pole-pitch corresponds to a phase difference of only 60. Thus, 



* M ii\ 



(a) W 



FIG. 76. Three-phase Armature Winding. 



if we consider three consecutive coils, A, B, and C, there will be 

 a phase difference of 60 between the e.m.f.'s in A and B, and B 

 and C, and a phase difference of 120 between the e.m.f.'s in A and C. 

 But by reversing the connections of B we alter the phase of its e.m.f. 

 relatively to the other two by 180, and so obtain three e.m.f.'s differing 

 120 in phase, as required for a three-phase system. 



A little consideration will show that if the direction of rotation 

 of the field in Fig. 76 is counter-clockwise, or left-handed, the vector 

 diagram of e.m.f.'s in the three phases is as shown at (a). If now 

 the direction of rotation be reversed, the diagram assumes the form 

 shown at (&), the dotted and chain-dotted vectors having changed 

 places. Thus we see that a reversal of the direction of rotation of a 



