CHAPTER \a 



39. Dynamo used as alternator 40. Constructional differences between dynamos 

 and alternators 41. Standard type of alternator 42. Construction of turbine- 

 driven alternator 43. Armature windings of alternators 44. Calculation of 

 armature e.m.f. Effect of varying length of polar arc 45. Effect of varying 

 number of slots 46. General formula for e.m.f. of alternator 47. Armature 

 reaction in alternators 48. Walker compounded alternator. 



39. Dynamo used as Alternator 



ALTHOUGH the machines employed for the generation of alternating 

 currents differ somewhat in details of construction from continuous - 

 current dynamos, the principle of action is the same the motion of 

 a conductor across a magnetic field and, in fact, any continuous- 

 current generator may be easily modified so as to act as an alternator. 

 The only addition necessary is the fitting of a suitable number of 

 insulated contact-rings or " slip-rings " which are in permanent 

 connection with certain points in the winding. By means of brushes 

 making contact with the slip-rings, alternating currents may be sup- 

 plied to an external circuit. If the commutator is retained, then the 

 same machine may be made to supply at the same time both con- 

 tinuous and alternating currents, the continuous current being obtained 

 as usual from the commutator, and the alternating currents from the 

 slip-rings. Some very large generators of this kind are in use in the 

 United States, and are known as double-current generators. 



The construction of the typical modern alternator is, however (for 

 reasons which will be explained presently), different from that of the 

 typical modern continuous^current dynamo. We shall commence our 

 study of alternators by considering the action of a dynamo as an 

 alternator when fitted with slip-rings as explained. 



In the diagrams of Fig. 59 is shown a two-pole armature fitted 

 with two slip-rings, diagram (a), connected to two diametrically 

 opposite points, C and D, of the winding. For the sake of simplicity, 

 the winding itself is indicated by a simple circle. Let the arrange- 

 ment of the winding and the direction of rotation be such that e.m.f. 's 

 are induced in the two halves ACB and AJDB of the armature having 

 a direction from A to B inside the winding, as indicated by the 

 arrows, and hence producing a p.d. from B to A across an external 

 circuit connected to these two points. In the diagrams, the points 

 B and A arc supposed to be fixed in space, and to correspond to the 



