CHAPTER V 

 SYNCHRONOUS MACHINERY 



136. Alternator. An alternator consists essentially of an 

 open coil of wire revolving at uniform speed in the magnetic 

 field between a pair of unlike poles. (Fig. 190.) The field m.m.f. 

 is produced by windings excited by direct current. 



Between the slip rings a and 6 an alternating e.m.f. is gener- 

 ated of instantaneous value 



e = n^lO- 8 volts, . . (225) 



at 



where n is the number of turns in the coil and -,- is the rate of 



at 



change of the flux interlinking with the coil or the rate at which 

 the coil is cutting the flux. The result is the same if the coil is 

 stationary and the field revolves. 



137. Types of Alternators. There are three principal types 

 of alternators, 



(a) revolving armature, 



(b) revolving field, 



(c) inductor. 



Type (a) is illustrated in Fig. 190. The field poles are sta- 

 tionary and the armature revolves between them. The ends of 

 the winding are brought out to two slip rings in single-phase 

 machines and to three or more slip rings in polyphase machines 

 and the current is collected by copper or carbon brushes. 



The armature is necessarily of small size since the peripheral 

 speed is limited and there is very little space for insulation. The 

 armature conductors are also acted upon by centriiugal forces 

 which tend to throw them out of the slots. The revolving arma- 

 ture is therefore only suitable for machines of small size and low 

 voltage. It is, however, necessary in the case of rotary converters 

 where the same armature winding carries both alternating and 

 direct currents. 



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