104 ELECTRICAL MACHINERY 



generator alternates in direction so that evidently the field 

 current of such a machine could not come from its arma- 

 ture. A small c-c. generator, called an exciter, is usually 

 employed to furnish the field current of an a-c. generator. 



Different Field Windings. As was pointed out in the 

 last chapter the required number of ampere- turns may be 

 supplied by using a large number of turns and a small 

 current, or a few turns with a large current. The field 

 winding may be connected in parallel with the armature 

 of the machine; the coils are then wound with many turns 

 of comparatively fine wire, possibly as large as No. 14 

 on large machines while small machines might be wound 

 with No. 20 or smaller. When the field of a c-c. generator 

 is so connected across the armature terminals, i.e., in 

 shunt, or parallel, with the external circuit the machine 

 is said to have shunt-excitation, and the winding is called 

 a shunt field. 



When but a few turns of heavy wire are put in the field 

 coils, the field winding is connected in series with the 

 external circuit and the field is called a series field. 



In most c-c. generators the field coil is wound in two 

 parts. The larger part of the coil is made up of com- 

 paratively small wire and forms the shunt field, and the 

 smaller part of the coil is wound of a few turns of large 

 wire or copper ribbon and forms a series winding for the 

 generator. Such a generator, having the two kinds of 

 field coils, is said to be compound wound. 



Diagrams of the three kinds of windings are shown in 

 Fig. 60; (a) being the shunt, (6) the series, and (c) the 

 compound winding. In the compound winding the shunt 

 field may be connected directly across the armature as in 

 the full lines of Fig. 60 (c) or it may be connected across 

 the armature and series field both as shown in the dotted 

 lines in the same figure. The first connection is called a 

 short shunt while the second (the one in dotted lines) is 

 called a long shunt, A coil for a compound wound gen- 



