THE ALTERNATING CURRENT GENERATOR 233 



or direct-connected), but in large stations there is generally 

 a set of exciter bus bars on the switchboard and two or 

 more exciters (each equipped with its own driver) furnish 

 power to these busses. The field circuit of all alternators 

 are connected to this set of exciter bus bars. The exciters 

 are sometimes driven by alternating current motors and 

 at other times by small steam turbines or engines. 



The field winding of an alternator is generally designed 

 for 125 volts and this is, of course, independent of the 

 voltage generated in the armature winding, which may 

 be several thousand volts. 



59. Armature Winding. In general, the armature wind- 

 ing of an alternator resembles that of a continuous current 

 machine but is much simpler; it has comparatively few coils 

 and they are generally connected all in series. It was shown 

 in Chapter II that the winding of a c-c. armature might 

 be rather complex, using fractional pitch and being either 

 of the wave or lap type. But no such complexities are 

 encountered in alternator armatures; there are generally 

 not more than six coils per pair of poles and seldom more 

 than twelve and the arrangement and connections of these 

 coils are very simple. In the continuous current armature 

 it was necessary to use many coils of few turns each in 

 order to have the commutation take place without spark- 

 ing, but the alternator has no commutator and therefore 

 this limitation on its winding does not exist. 



Single-phase and Polyphase Winding. Sometimes all 

 the coils on an armature are connected in series with one 

 another so that there is only one set of ceils on the armature; 

 this is said to be a single-phase winding and the machine 

 is called a single-phase generator. There are only two 

 ends to the armature winding so the machine has but two 

 slip-rings and two brushes. Sometimes the coils of the 

 armature are divided in a certain way, to form two equal 

 independent groups and this is called a two-phase winding. 

 More often the coils are divided into three equal groups 



