DIFFERENTIAL ACTION 81 



40. Constructional Differences between 

 Dynamos and Alternators 



If, however, we want a machine which is only required to generate 

 alternating current, the above arrangement admits of considerable 

 improvement. A glance at the diagrams of Fig. 59 will show that 

 with a uniformly distributed winding such as is used in continuous- 

 I'urrent armatures, we have opposition of e.m.f.'s in the different parts 

 of the winding. Thus, in Fig. 59 (b) the e.m.f. in the section AC of 

 the winding is opposed by the e.m.f. in the section AD ; it is thus 

 evident that in this particular position of the armature the conductors 

 forming the section AD are not only useless, but positively harmful 

 in (1) unnecessarily adding to the resistance and reactance of the 

 armature, and (2) reducing its e.m.f. Such opposition of e.m.f.'s in 

 different parts of the winding is conveniently spoken of as differential 

 action. In order to eliminate differential action, the winding must 

 be arranged so that all the conductors forming a group of the winding 

 between two contact-rings shall always be moving simultaneously 

 across a field of the same polarity. In order to secure this result, 

 the width of a band of conductors forming such a group must evidently 

 not exceed the width of the interpolar space i.e. the distance between 

 the neighbouring polar horns of two consecutive pole-pieces (s in 

 Fig. 60 (&)). Now, in continuous-current machines the interpolar arc 

 is only about 30 per cent, of the pole-pitch (the pole-pitch is the 

 distance between the middle points of two consecutive pole-pieces, 

 which corresponds to half the armature circumference in a two-pole 

 machine), so that if the same ratio of length of interpolar arc to 

 pole-pitch were retained in alternators, we should, in a single-phase 

 machine, utilize only 30 per cent, of the available winding space on 

 the surface of the armature. In order to increase the width of the 

 available winding space without at the same time introducing any appre- 

 ciable amount of differential action, it is usual to make the lengths of 

 polar and interpolar arcs equal, so that each of them, as well as the 

 width of a group of the winding, is equal to half the pitch of the pole- 

 pieces. In this reduction of the polar arc, and increase of interpolar 

 space, we have one of the characteristic constructional differences 

 between single-phase alternators and continuous-current dynamos. 



Since with the arrangement indicated only 50 per cent, of the 

 available winding-space is utilized with a single-phase armature, we 

 have, as regards the armature winding, another important construc- 

 tional difference between continuous-current machines and single- 

 phase alternators; the winding in the latter, instead of being 

 uniformly distributed as it is in the former, is broken up into groups 

 separated by spaces devoid of conductors. 



If, however, our alternator is to be a two-phaser, we may fill up 



G 



