TYPES OF ALTERNATORS. 



21 



north and south poles are placed 90 apart, there will be two 

 complete alternations per revolution of the armature. If six alter- 

 nately north and south poles are placed 60 apart, there will be 

 three complete alternations per revolution, and so on. Machines 

 having more than two poles are called multipolar alternators. 

 Except for special, or experimental, purposes, multipolar alternators 

 are universally used for commercial work. Fig. 5 gives a diagram- 

 matic representation of a 6-pole alternator. We do not propose 

 to enter into an exhaustive description of the various types of 

 alternators, as such would be beyond the scope of this treatise. 

 We will, however, give a general idea of the various types of 

 machines met with in 

 practice. 



17. Types of 

 Alternators. 

 There are, broadly 

 speaking, three types 

 of alternators: (1) 

 those in which separ- 

 ately excited poles or 

 field magnets 

 are stationary and in- 

 ducing coils or ap- 

 mature rotate; (2) 

 those in which the 

 armature is stationary 

 and the field magnets 

 rotate; and (3) In- 

 duetop Altepna- 

 tOPS, or those in 



which both the field magnets and armature are stationary and the 

 flux through the armature coils is varied by masses of iron 

 rotating in the gap between the field coils and the armature coils. 



In alternators giving a single alternating E.M.F. the magnet 

 poles occupy about one half the circumference of the machine, 

 as shown in Fig. 5, as also do the coils on the armature. The 

 reason for this is seen by reference to Fig. 5, in which the poles 

 are the same width as the spaces between them. If the armature 

 coil A were of greater angular width than the distance between 

 two consecutive poles N t S, it would begin cutting lines issuing 



FIG. 5. 



