4 DIRECT CURRENT GENERATORS. [Exp. 



4. Field magnets are usually* energized by direct current 

 passed through the field windings ; permanent magnets being used 

 only in small machines, called magneto-generators, used for bell- 

 ringers, etc. A generator is separately-excited or self-excited 

 according to whether the current for the field is supplied by an 

 outside source or by the machine itself. Alternators are sepa- 

 rately excited; direct current generators are usually self-excited. 



5. A direct current machine (either generator or motor) may 

 ber^ 1 ) Series wound, with the field winding of coarse wire in 

 series with the armature and carrying the whole armature current ; 



(2) Shunt wound, with a field winding of fine wire in shunt with 

 the armature and carrying only a small part of the whole current ; 



(3) Compound wound, with two field windings, the principal one 

 in shunt and an auxiliary one in series with the armature. 



The compound generator is in most general use, being best 

 suited for all kinds of constant potential service, both power and 

 lighting; the shunt generator performs similar service but not 

 so well. The characteristics of these machines will be studied 

 fully in Exp. i-B. 



The series generator is of interest because: (i) It is one of 

 the earliest types and of historical importance; (2) It is the 

 simplest type and illustrates in a simple manner the principles 

 which underlie all dynamo-electric machinery, both generators 

 and motors; (3) In a compound wound generator or motor, the 

 series winding is an important factor in the regulation of poten- 

 tial or speed. In itself the series generator is of relatively small 

 importance, because neither current or voltage stay constant; it 

 is used only in some forms of arc light machines with regulating 

 devices for constant current. 



In direct current motors, all three types of winding are em- 

 ployed : series wound motors for variable speed service in traction, 

 crane work, etc.; shunt and compound wound (including differ- 



*The induction generator, to be studied in a later experiment, does not 

 come under this classification. 



