22 TREATISE ON ALTERNATING CURRENTS. 



from the S. pole before it had finished cutting those issuing from 

 the N. pole, or vice versa according to the direction of rotation of 

 the armature, with the result that there would be a differential 

 effect and the E.M.F. developed would be less than it should be 

 for the number of conductors on the armature. 



Of the different types of alternators it is difficult to say which 

 is the best. There is one consideration, however, which is worth 

 while bearing in mind. Alternators are usually employed for 

 generating high E.M.F.s, more especially where energy has to 

 be transmitted for a considerable distance ; E.M.F.s up to 20,000 

 volts being not uncommon. Such being the case, it is advisable 

 that the armature coils in which the high pressure is generated, 

 and which are subjected to great electrical strain, should not be 

 subjected to mechanical strain. This consideration leads us to 

 prefer those types in which the armature is stationary, so that 

 where high pressures are to be generated machines of the stationary 

 field type or inductor alternators are to be preferred. 



We shall for the present assume that the E.M.F.s of all 

 alternators are capable of representation as either a single sine 

 function of the time, or as the sum of a series of sine and cosine 

 functions of the time. In either case we can base our calculations 

 on the representation of the E.M.F. as a single sine function (see 

 Chap. IX.). 



ARMATUEE EEACTIOX. 



18. The E.M.F. generated in the armature of an alternator 

 is due to a combination of two distinct causes. First, an E.M.F. 

 is generated by the rotation of the armature in a magnetic field, 

 and, secondly, there is an E.M.F. set up in the armature by the 

 variations of the armature current itself. This latter may be called 

 the E.M.F. of self-induction of the armature. Its effect is to 

 change the intensity and the direction of the resultant magnetic 

 field through which the armature rotates. It is sometimes termed 

 Armature Reaction. We shall show in a later chapter 

 (see Chap. XIII.) that the field strength of an alternator may, 

 according to circumstances, be either increased or diminished by 

 armature reaction. 



The number of lines of force passing through the armature 

 due to its own current i is Li, where L is its coefficient of self- 



