CHAPTER XI 



DESIGN OF ALTERNATORS FUNDAMENTAL 

 CONSIDERATIONS 



65. Introductory. In the continuous-current dynamo the func- 

 tion of the commutator is merely to rectify the armature cur- 

 rents in order that a machine with alternating e.m.fs. generated 

 in its windings shall deliver unidirectional currents at the ter- 

 minals. It may therefore be argued that the design of alter- 

 nating-current generators should be taken up before that of D.C. 

 dynamos, the changes caused by the addition of the commutator 

 being considered in the second place. There are, however, many 

 matters of importance to be considered in connection with an 

 alternating-current generator, which have no part in the design of 

 a continuous-current dynamo. Among these may be mentioned 

 the effects due to changes in wave shapes of e.m.f. and current; the 

 importance of the inductance, not only of the armature itself, but 

 also of the circuit external to the generator; and the fact that the 

 voltage regulation depends not only on the IR drop, but also on 

 the power factor of the load, i.e., on the phase displacement of the 

 current relatively to the e.m.f. The problems to be solved being 

 somewhat less simple than those connected with continuous-current 

 machines, the writer believes that the arrangement of the subject 

 as followed in this book is justified. 



It is proposed to treat the design of A.C. machines as nearly 

 as possible on the lines followed in the D.C. designs. In order 

 to avoid unnecessary repetitions, references will be made to pre- 

 vious chapters and stress will be laid on the essentials only, 

 particular attention being paid to the points of difference between 

 A.C. and D.C. machinery. 



The design of asynchronous generators will not be touched 

 upon. This type of machine is essentially an induction motor 

 reversed, the rotor, with its short-circuited windings, being me- 

 chanically driven. The writer has explained elsewhere the prin- 

 ciples underlying the working of these machines, 1 and since they 



1 ALFRED STILL: "Polyphase Currents," WHITTAKER & Co. 



237 



